synthesis of inorganic nanoparticles using microfluidic...

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Synthesisofinorganicnanoparticlesusingmicrofluidicdevices

RazwanBaber

Athesissubmittedforthedegreeof

DoctorofPhilosophy

From

UniversityCollegeLondon

DepartmentofChemicalEngineering

Supervisors:

Prof.AsteriosGavriilidis

Prof.NguyenTKThanh

April2017

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DeclarationofAuthorshipandOriginality

I,RazwanBaber,confirmthattheworkpresentedinthisthesisismyown.Whereinformationhasbeenderivedfromothersources,Iconfirmthatthishasbeenindicatedinthethesis.

Signature:……………………………………………………………………………..Date:April2017

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Abstract

Thewell-knownbenefitsofusingmicro-scaleflowareassociatedwithcontrolledmasstransferbecause

of the reduced dimensions of microfluidic devices. The improvedmass transfer characteristics make

microfluidicdevicesanidealcandidateforsynthesisofnanoparticles(NPs).Inthisstudy,masstransfer

characteristicsare investigated formicrofluidicdevices suchas the coaxial flow reactor (CFR)and the

impingingjetreactor(IJR),throughtheuseoftheinteractionbyexchangewiththemeanmixingmodel,

Villermaux-Dushmanreactionscheme,highspeedcamera imagesandflowvisualizationusingdyeand

water. These reactors are contrasting in that the CFR has slowermixing because of its laminar flow

profilewhereasastheIJRhasmixingtimeintheorderofafewms.Theimportanceofmixingtimeas

wellasmanipulationofhydrodynamicsat themicroscale is shownthrough theuseof these typesof

reactorsforNPsynthesis.BothtypesofreactorshavebeneficialpropertiesforthesynthesisofNPs.NPs

were characterized using UV-Vis spectroscopy, DCS and TEM. Longer residence times and a higher

consumptionofreagents intheCFR ledtoreductionofsizeofsilverNPs, leadingtoreductions insize

from9.3±3nmto3.7±0.8nm.TheIJRshowedareductioninNPsizewithincreasingmixingefficiency,

withareductioninsizefrom7.9±5.8nmto3.4±1.4nmusingcitrateasaligandandfrom5.4±1.6nm

to4.2±1.1nmusingPVAasaligand.Asystemtocontrolnucleationandgrowthperiodswasdeveloped

bycombiningtheCFRwithacoiledflowinverter(CFI).SizecontrolovergoldNPswasachievedsimplyby

changingtheflowrateofreagents,withareductioninsizefrom23.9±4.7nmto17.9±2.1nm.Various

hydrodynamicswithin theCFRwerealso tested.Vortex flowathigherReynoldsnumbersdidnotgive

goodcontroloverNPsizeanddispersity,whilereducingtheinnertubeinternaldiameterresultedina

decreaseinNPsizefrom10.5±4.0nmto4.7±1.4nm.SilverandgoldNPsynthesiswasalsoperformed

inabatch reactor. Twodifferentmixing configurationsandvariation in theorderof reagentaddition

was investigated, and confirmed the importance ofmass transfer conditions in determining size and

dispersity.Inbothcases,fastandefficientmixingofthereducingagenttotheprecursorresultedinthe

smallestNPs.Therangeofsizesobtainedwere6.7±1.7nmto11.5±2.4nmand13.1±2.2nmto18.0±

4.8nmforsilverandgoldNPsrespectively.Alltheseresultsshowastronglinkbetweenmasstransfer

andNP sizeanddispersity for the systems tested, showing theneed forwell controlledandcarefully

consideredmasstransferconditions.

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AcknowledgementsI would like to express my deepest gratitude to my supervisors, Prof. Asterios Gavriilidis and Prof.

Nguyen Thi Kim Thanh,whomwithout thisworkwould not have been possible. Prof. Gavriilidis, has

taughtmea lotabout research,buthasalsogivenme invaluableadviceoutsideofworkwhich really

helpedmotivatemewhentheresearchwasnotgoingmyway.Prof.Thanhwasinvaluableforherinput,

and along with her group at the Royal Institute of Great Britain, helped immeasurably with their

expertiseonnanoparticles.AspecialmentionalsogoestoDr.LucaMazzei,whohasbeenamentorto

meduringthisresearch.Hetaughtmewhatitmeanstobecriticalwhenassessinganyresearch,andto

neverstopbeingcurious.

Therearemanypeople to thank fromProf.Gavriilidis’ labgroup.Special thanks toEnhong,who

never ceases to amaze me with his flair for solving engineering problems. Thanks to Noor, Gaowei,

Redza,Achilleas,Luigi,Damiano,Hendrik,Anand,Huang,Spyros,Conor,CharaandValentina.Youhave

allhelpedmeinonewayoranother,whetheritwastakingthetimetochataboutaresearchproblemor

makingme smilewitha joke.Roxanne,Niall, Luke,AansandKuan,wewent throughmany trials and

tribulationstogetherwiththeevertemperamentalTEMthroughtheyearsandIthankyouforsharing.

Thanks tomygood friends,AnayetandAkhbar.Youare the funniestpeople Ihavemet,each in

yourownway.YoualwayslightenedmymoodwhenIwasbeingtooserious.

Myparents,MohammedandChandni,mysincerestappreciationgoesouttoyou.Youalwayskept

things in perspective forme, and reassuredmewhen Iwas unsure. You always gavememotivation,

whenIwasunmotivated,tokeepgoing.Thankyouforraisingme,IhopeImakeyouproudandcontinue

todosoeveryday.Tomybrothers,Arshad,AsadandImran,thankyouforteachingmethelessonsof

lifeandlookingoutformeeverystepoftheway.Tomysister,Yasmin,thankyouforteachingmehow

tobestronginadversity.Tomysister,Rehksena,thankyouforallthesupportandsacrificeyoumake

everysingledaytomakesureIamdoingwell.

Thankyoutomybest friend, foryour love,supportandunderstandingthroughoutthisPhD.You

helpedtokeepmegoingwhenIwasreadytogiveup.Youhaveperhapsmadethebiggestsacrificeofall

forputtingupwithme.Youarealwaysinmymindandheart.

Thankyouallformakingthisapleasantandenjoyablejourney.

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ContentsAbstract........................................................................................................................................................3

Acknowledgements......................................................................................................................................4

Listoffigures................................................................................................................................................8

Listoftables...............................................................................................................................................16

Listofabbreviations...................................................................................................................................17

1 Introduction.......................................................................................................................................18

1.1 Motivation................................................................................................................................18

1.2 Nanoparticles............................................................................................................................18

1.3 Characterisationtechniquestodeterminethemorphologyofnanoparticles.........................19

1.3.1 Ultraviolet-visiblespectroscopy...........................................................................................19

1.3.2 Differentialcentrifugalsedimentation.................................................................................19

1.3.3 Transmissionelectronmicroscopy.......................................................................................19

1.4 Microfluidics.............................................................................................................................20

1.5 Mixing.......................................................................................................................................23

1.5.1 Diffusion...............................................................................................................................23

1.5.2 Advection.............................................................................................................................24

1.6 Objectives.................................................................................................................................24

1.7 Chapteroverview......................................................................................................................24

2 LiteratureReview..............................................................................................................................26

2.1 Microfluidicdevicesformasstransfer......................................................................................26

2.1.1 Parallellamination................................................................................................................26

2.1.2 Seriallamination...................................................................................................................26

2.1.3 Segmentedflow....................................................................................................................27

2.1.4 Injectionmicromixers...........................................................................................................27

2.1.5 Chaoticadvectionmicromixers............................................................................................27

2.2 Silvernanoparticles...................................................................................................................27

2.2.1 Batchsynthesisofsilvernanoparticlesviasodiumborohydridereduction.........................28

2.2.2 Microfluidicsynthesisofsilvernanoparticles......................................................................29

2.2.3 Mechanisticstudiesofsilvernanoparticlesynthesis...........................................................34

2.3 Goldnanoparticles....................................................................................................................39

2.3.1 Batchsynthesisofgoldnanoparticles..................................................................................39

2.3.2 Microfluidicsynthesisofgoldnanoparticles........................................................................43

2.3.3 Mechanisticstudiesofgoldnanoparticlesynthesis.............................................................48

3 Synthesisofsilvernanoparticlesinamicrofluidiccoaxialflowreactor............................................54

3.1 Experimental.............................................................................................................................55

3.1.1 Chemicals.............................................................................................................................55

3.1.2 ExperimentalSetup..............................................................................................................55

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3.1.3 Synthesisofsilvernanoparticles..........................................................................................56

3.1.4 Characterizationofnanoparticles........................................................................................57

3.2 Investigationonvarianceinnanoparticlemorphologyusingparametricstudies....................57

3.2.1 Effectoftotalflowrate........................................................................................................57

3.2.2 Effectoftrisodiumcitrateconcentration.............................................................................60

3.2.3 Effectofsilvernitrateconcentration....................................................................................63

3.3 Conclusions...............................................................................................................................66

4 Synthesisofsilvernanoparticlesusingamicrofluidicimpingingjetreactor.....................................68

4.1 Experimental.............................................................................................................................69

4.1.1 Chemicals.............................................................................................................................69


4.1.3 NanoparticleSynthesis.........................................................................................................70

4.1.4 CharacterisationofNanoparticles........................................................................................70

4.2 Mixingcharacterizationmethods.............................................................................................71

4.2.1 Dilution-basedmethods.......................................................................................................71

4.2.2 Reaction-basedmethods......................................................................................................71

4.3 Villermaux-Dushmanreactionscheme.....................................................................................72

4.4 Theinteractionbyexchangewiththemeanmixingmodel......................................................72

4.5 Experimentaldeterminationofmixingtimeintheimpingingjetreactors..............................74

4.6 Flowregimecharacterizationofimpingingjetreactorusinghighspeedcamera....................77

4.7 InvestigatingtheeffectofwebernumberonNPmorphology.................................................79

4.7.1 EffectofwebernumberonsilverNPsynthesis1(citratesystem).......................................79

4.7.2 EffectofwebernumberonsilverNPsynthesis2(PVAsystem)...........................................85

4.8 Conclusions...............................................................................................................................91

5 Effectofhydrodynamicsandmixingconditionsonthecontinuoussynthesisofgoldandsilvernanoparticlesinacoaxialflowdevice........................................................................................................93

5.1 Experimental.............................................................................................................................95

5.1.1 Chemicals.............................................................................................................................95


5.1.3 Nanoparticlesynthesis.........................................................................................................97

5.1.4 Characterizationofnanoparticles........................................................................................98

5.2 Resultsanddiscussion..............................................................................................................98

5.2.1 Effectofincreasingflowrateongoldnanoparticlesizeanddispersity...............................98

5.2.2 Effectoftemperatureongoldnanoparticlesizeanddispersity........................................103

5.2.3 Silvernanoparticlesynthesisusingacoaxialflowdevicefollowedbyasplitandrecombinemixer 107

5.2.4 SilvernanoparticlesynthesisusingacoaxialflowdeviceoperatedathighRe..................110

5.2.5 EffectofinnertubeinternaldiameteronsilverNPsynthesisusingtheCFR.....................112

5.3 Conclusions.............................................................................................................................114

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6 Masstransfereffectsinthebatchsynthesisofsilverandgoldnanoparticles................................116

6.1 Methodology..........................................................................................................................116

6.1.1 Chemicals...........................................................................................................................116

6.1.2 ExperimentalSetup............................................................................................................117

6.1.3 Nanoparticlesynthesis.......................................................................................................118

6.1.4 Characterizationofnanoparticles......................................................................................118

6.2 Resultsanddiscussion............................................................................................................119

6.2.1 MixingCharacterization.....................................................................................................119

6.2.2 Silvernanoparticlesynthesis..............................................................................................120

6.2.3 Goldnanoparticlesynthesis...............................................................................................125

6.3 Conclusions.............................................................................................................................130

7 Conclusionsandfurtherwork.........................................................................................................132

7.1 Conclusions.............................................................................................................................132

7.2 Furtherwork...........................................................................................................................133

AppendixA:MixingcharacterizationusingtheVillermaux-Dushmanreactions.....................................136

AppendixB:Synthesisofsilvernanoparticlesinamicrofluidiccoaxialflowreactor...............................142

AppendixC:Synthesisofsilvernanoparticlesusingamicrofluidicimpingingjetreactor.......................144

AppendixD:Effectofhydrodynamicsandmixingconditionsonthecontinuoussynthesisofgoldandsilvernanoparticlesinacoaxialflowdevice............................................................................................148

Listofpublications...................................................................................................................................152

Bibliography.............................................................................................................................................153

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Listoffigures

Figure1-1:Schematicrepresentationofabatchreactor..........................................................................20

Figure1-2:Schematicrepresentationofaflowreactor............................................................................21

Figure1-3:Schematicrepresentationofturbulentflowandlaminarflow................................................21

Figure2-1:Exampleofaparallel laminationmixer.49Reproduced fromRef.49withpermissionofThe

RoyalSocietyofChemistry.........................................................................................................................26

Figure2-2:Slugswithamixtureofbluedyeandwatershowingadvectionwithin.ReprintedfromRef.

57,Copyright(2006),withpermissionfromElsevier.................................................................................27

Figure 2-3: Experimental flow reactor setup of silver NP synthesis used by Lin et al. Reprinted with

permissionfromRef.73.Copyright(2004)AmericanChemicalSociety....................................................29

Figure 2-4: Micro flow setup to synthesise silver nanorods and TEM image of silver nanorods.

ReproducedfromRef.74withpermissionofTheRoyalSocietyofChemistry..........................................30

Figure 2-5: Experimental setup of microreactor used by Liu et al. Reprinted from Ref. 79, Copyright

(2012),withpermissionfromElsevier.......................................................................................................31

Figure2-6:SpiralflowreactorusedbyKumaretal.,80Wrepresentsreactingphase,Krepresentsliquid

keroseneinertphase,Arepresentsgasairinertphase.Brepresentsinletmicromixer(crossmixer)used

togenerateslugs,withPEEKandStainlessSteelbeingused(dimensionsgiven).CandDarePMMA(0.5

mm ID) and Stainless Steel (1mm ID) spiral reactors respectively. Reprinted from Ref. 80, Copyright

(2012),withpermissionfromElsevier.......................................................................................................32

Figure 2-7: Configuration of microfludic device with four inlets: 1) Carrier fluid inlet, 2) Precursor

solutioninlet,3)Solvent inlet,4)Reducingagent inlet.Righthandsideshowssegmentedflowof ionic

liquid(BMIM-Tf2N)withintheoilcarrierphase(PCTFE).Athigherflowrates,jettingoccursfurtheralong

thechannelasthedropletspinchandformthinstrandphases.ReprintedwithpermissionfromRef.81.

Copyright(2012)AmericanChemicalSociety............................................................................................33

Figure2-8:SilverNPsmadeusingA)Microfluidicdevice,B)usingabatchreactor,C)UV-Viscomparison

ofboth (microfluidicdevice is black, batch is red). Reprintedwithpermission fromRef. 81.Copyright

(2012)AmericanChemicalSociety.............................................................................................................33

Figure2-9:Suggestedgrowthmechanism for silverNPsynthesis.Reprintedwithpermission fromRef.

68.Copyright(2012)AmericanChemicalSociety......................................................................................34

Figure 2-10:Mechanism of silver NP synthesis proposed by Takesue et al. 82 Silver nanoparticles are

formed through (I) an induction period during which Ag+ ions are reduced to Ag0 atoms; (II) a

nucleation-dominant formation period during which Ag13 clusters are nucleated and simultaneously

consumed in silver nanoparticle formation; and (III) a growth period during which larger silver

nanoparticlesareformedbycoalescenceandaggregationofsmallersilvernanoparticles.Reprintedwith

permissionfromRef.82.Copyright(2011)AmericanChemicalSociety....................................................35

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Figure 2-11: Growth mechanisms proposed by Henglein and Giersig at low and optimal citrate

concentrationswithtwodifferentmechanismsoccurring.Atoptimalconcentrations,itissuggestedthat

asurfacereductionofsilverionsoccursonthesurfaceofformedNPsowingtothereductionpotential

of the citrate adsorbed onto the surface. At low concentrations, coalescence of smaller silver NP

clustersoccurs.34,83Ref.34publishedbyTheAmericanChemicalSociety...............................................36

Figure 2-12: Absorption spectra of Ag colloids prepared by γ irradiation of an aqueous solution (N2

purged)containing1mMAgNO3,1%methanol,and(a)1mM,(b)5mM,and(c)10mMsodiumcitrateas

stabilizer.84ReprintedwithpermissionfromRef.84.Copyright(2003)AmericanChemicalSociety........37

Figure 2-13: Growthmechanism through pulse radiolysis of amixture of silver nitrate and trisodium

citrate proposed by Pillai and Kamat: primary and secondary growth steps in the formation of silver

nanocrystallites.84ReprintedwithpermissionfromRef.84.Copyright(2003)AmericanChemicalSociety.

....................................................................................................................................................................37

Figure2-14:MechanismsofsilverNPgrowthusingdifferentbeamcurrentsunderaTEMmicroscope.86

ReprintedwithpermissionfromRef.86.Copyright(2012)AmericanChemicalSociety...........................38

Figure2-15:Microfluidicdeviceused tosynthesizehexagonalgoldNPs.40Reprinted fromRef.40with

permissionfromTheInstituteofPhysics...................................................................................................43

Figure2-16:SynthesisofgoldNPsusingsyringepumps,t-mixerandfusedsilicacapillarytubingsetina

hotplate.Reproduced(Adapted)fromRef.107withpermissionofTheRoyalSocietyofChemistry......44

Figure2-17:SplitandrecombinemixeremployedforthesynthesisofgoldNPs.ReproducedfromRef.

109withpermissionofTheRoyalSocietyofChemistry............................................................................45

Figure2-18:InterdigitatingmicromixerusedbyShalometal.forthesynthesisofgoldNPsviatheBrust-

Schiffrinmethod.110ReprintedfromRef.110,Copyright(2007),withpermissionfromElsevier.............46

Figure 2-19:Segmented flow in microchannel employed for synthesis of gold NPs by Cabeza et al.

ReprintedwithpermissionfromRef.111.Copyright(2012)AmericanChemicalSociety.........................47

Figure 2-20: Interdigitatingmixer used by Tsunoyama et al. for the synthesis of gold NPs. Reprinted

(adapted)withpermissionfromRef.112.Copyright(2008)AmericanChemicalSociety.........................47

Figure2-21:MechanismsuggestedbyJietal.forgoldNPsynthesis.Twopathwaysexistdependenton

pH.95ReprintedwithpermissionfromRef.84.Copyright(2007)AmericanChemicalSociety..................51

Figure 2-22: Mechanism of gold NP formation suggested by Polte et al. Reprinted (adapted) with

permissionfromRef.120.Copyright(2010)AmericanChemicalSociety..................................................51

Figure2-23:MechanismofgoldNPformationintheTurkevichsynthesissuggestedbyPolte.Ref.124–

PublishedbyTheRoyalSocietyofChemistry.............................................................................................52

Figure3-1:Schematicofthecoaxialflowreactorsetup. Insertshowsflowvisualizationof laminarflow

insidethecoaxialflowreactorwithbluedyeflowingthroughtheinnertubeandwaterflowingthrough

theoutertube............................................................................................................................................56

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Figure 3-2: TEM images and particle size distributions of silver NPs synthesized at different total

flowrates.A:1ml/min,B:2.5ml/min,C:8ml/minandD:14ml/min.Concentrationofsilvernitrate0.2

mM,trisodiumcitrate0.2mM,sodiumborohydride0.6mM.0.798mminnertubeI.D..........................58

Figure 3-3: TEM images andparticle size distributionsof silverNPs synthesized at different trisodium

citrateconcentrations.A:0.05mM,B:0.1mM,C:0.25mMandD:1.5mM.Concentrationofsilver

nitrate0.1mM,sodiumborohydride0.3mM.Totalflowrate2.5ml/min,0.556mminnertubeI.D......62

Figure3-4:AbsorbancepeaksofsilverNPssynthesizedatvarioustrisodiumcitrateconcentrationsinthe

range 0.025- 1.5 mM. Total flow rate 2.5 ml/min, concentration of silver nitrate 0.1 mM, sodium

borohydride0.3mM.0.556mminnertubeI.D.........................................................................................63

Figure3-5:TEMimagesandparticlesizedistributionsofsilverNPssynthesizedatdifferentsilvernitrate

concentrations.A:0.05mM,B:0.15mM,C:0.25mMandD:0.4mM.Concentrationoftrisodiumcitrate

0.5mM,sodiumborohydride0.3mM.Totalflowrate2.5ml/min,0.556mminnertubeI.D..................65

Figure 3-6: Absorbance peaks of silver NP synthesized at various silver nitrate concentrations in the

range 0.05-0.4mM. Total flow rate 2.5ml/min, concentrations of trisodium citrate 0.5mM, sodium

borohydride0.3mM.0.556mminnertubeI.D. Inset:Silvernitrateconcentrationvs.peakabsorbance

forresonancepeaks...................................................................................................................................66

Figure4-1:Representationoftheimpingingjetreactor............................................................................70

Figure 4-2: Segregation index vs. mixing time estimated using the IEM model for the Villermaux-

Dushmannreactionschemefor0.5mm(blueline)and0.25mm(redline)IJRs.Concentrationsofbuffer

solutionwereKI:0.032M,KIO3:0.006M,NaOH:0.045M,H3BO3:0.09MinbothIJRs.Concentrationof

H2SO4was:0.017Mand0.02Mfor0.5mmand0.25mmIJRrespectively..............................................74

Figure4-3:MixingtimecalculatedbytheVillermaux-Dushmannreactionschemevs.Webernumberfor

0.5mm(blacksquares)and0.25mm(red triangles) IJRs.Concentrationofacidsolution (input1)was

0.017Mand0.02MH2SO4for0.5mmand0.25mmI.D.tubingIJRrespectively.Concentrationsofbuffer

solution(input2)wereKI:0.032M,KIO3:0.006M,NaOH:0.045M,H3BO3:0.09MinbothIJRs............75

Figure4-4:Flowvisualizationof jetsemittedfrom0.5mmtubesattotal flowrate:A:32ml/min(We:

13),B:40ml/min(We:20),C:48ml/min(We:29),D:56ml/min(We:39),E:64ml/min(We:51)andF:

72ml/min(We:65).Picturesshowsideviewandfrontview....................................................................78

Figure4-5:Flowvisualizationofjetsemittedfrom0.25mmtubesattotalflowrate:A:18ml/min(We:

32),B:22ml/min(We:48),C:26ml/min(We:68),D:30ml/min(We:90),E:34ml/min(We:115),F:38

ml/min(We:144)andG:42ml/min(We:176).Picturesshowsideviewandfrontview.........................79

Figure4-6: TEM imagesandparticle sizedistributionsof silverNPs synthesisedatdifferent total flow

ratesusinga0.5mmI.D.tubingIJR.(A)32ml/min(We:13),(B)48ml/min(We:29),(C)64ml/min(We:

51)and(D)72ml/min(We:65).Concentrationofsilvernitrate0.9mM,trisodiumcitrate6mMininput

1andsodiumborohydride1.8mMininput2...........................................................................................82

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Figure4-7: TEM imagesandparticle sizedistributionsof silverNPs synthesisedatdifferent total flow

ratesusinga0.25mm I.D. tubing IJR. (A)20ml/min (We:40), (B)24ml/min (We:58), (C)28ml/min

(We:78)and(D)32ml/min(We:102).Concentrationofsilvernitrate0.9mM,trisodiumcitrate6mMin

input1andsodiumborohydride1.8mMininput2..................................................................................83

Figure4-8:Dependenceofpeakabsorbance(peakwavelength386-389nm)onWebernumberobtained

fromUV-VisspectroscopyofsilverNPssynthesisedatdifferenttotalflowratesusing: i)0.5mm(black

squares)andii)0.25mmI.D.tubingIJR(redtriangles).Concentrationofsilvernitrate0.9mM,trisodium

citrate6mMininput1andsodiumborohydride1.8mMininput2........................................................84

Figure4-9: TEM imagesandparticle sizedistributionsof silverNPs synthesizedatdifferent total flow

ratesusinga0.5mmI.D.tubingIJR.(A)32ml/min(We:13),(B)48ml/min(We:29),(C)64ml/min(We:

51)and (D)72ml/min (We:65).Concentrationofsilvernitrate0.9mM,PVA0.02wt% in input1and

sodiumborohydride1.8mMininput2.....................................................................................................88

Figure4-10:TEMimagesandparticlesizedistributionsofsilverNPssynthesizedatdifferenttotalflow

ratesusinga0.25mm I.D. tubing IJR. (A)18ml/min (We:32), (B)26ml/min (We:68), (C)34ml/min

(We:115)and(D)42ml/min(We:176).Concentrationofsilvernitrate0.9mM,PVA0.02wt%ininput1

andsodiumborohydride1.8mMininput2..............................................................................................89

Figure 4-11: Dependence of peak absorbance (peak wavelength 398-401 nm) on Weber number

obtained fromUV-Vis spectroscopyof silverNPs synthesisedat different total flow ratesusing: i) 0.5

mm (black squares) and ii) 0.25mm (red triangles) I.D. tubing IJR. Concentration of silver nitrate 0.9

mM,PVA0.02wt%ininput1andsodiumborohydride1.8mMininput2..............................................90

Figure4-12:DependenceofsilvernanoparticlediameterontheWebernumber.Datashownis for0.5

mmand0.25mmIJRsandforbothcitrateandPVAsurfactants...............................................................92

Figure5-1:Schematicrepresentationsofcomponentsusedinthemicrofluidicflowsetup:a:coaxialflow

reactor,b:coiledflowinverterandc:splitandrecombinemixer.............................................................94

Figure5-2:SchematicrepresentationofexperimentalsetupforgoldNPsynthesis.................................96

Figure 5-3: Schematic representation experimental setup for silver NP synthesis: a) CFR used in

conjunctionwithSARandb)CFRusedinisolation....................................................................................97

Figure5-4:TEMimagesofgoldNPssynthesizedusingtheCFRwithaCFIresidenceloopataflowrateof

A:0.25ml/min,B:0.5ml/min,C:0.75ml/min,D:1ml/min,E:1.5ml/min,F:2ml/minandG:3ml/min.

H:AveragediameterofgoldNPsvsflowrateofsynthesis(barsrepresentstandarddeviationofthesize).

Concentration of tetrachloroauric acid, 0.557 mM; concentration of trisodium citrate, 0.09 M;

volumetric flow rate ratio, 32.3: 1 (Qout:Qin, HAuCl4: Na3citrate); molar flow rate ratio, 1:5 (HAuCl4:

Na3citrate);temperature,80°C................................................................................................................101

Figure5-5:NormalizedDCScurvesofgoldNPssynthesizedusingtheCFRwithaCFI residence loopat

flow rates between 0.25 and 3ml/min. Inset: Diameter of gold NPs vs. flow rate of synthesis (bars

represent standard deviation of size). Concentration of tetrachloroauric acid was 0.557 mM and

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concentration of trisodium citrate was 0.09 M. Concentration of tetrachloroauric acid, 0.557 mM;

concentration of trisodium citrate, 0.09 M; volumetric flow rate ratio, 32.3: 1 (Qout:Qin, HAuCl4:

Na3citrate);molarflowrateratio,1:5(HAuCl4:Na3citrate);temperature,80°C.....................................102

Figure5-6:UV-VisspectraofgoldNPssynthesizedusingtheCFRwithaCFIresidenceloopatflowrates

between 0.25 and 3 ml/min. Inset: Peak absorbance (black squares) and peak wavelength (red

diamonds)ofgoldNPsvs.flowrateofsynthesisanalyzedusingUV-Visspectroscopy.Concentrationof

tetrachloroauricacid,0.557mM;concentrationoftrisodiumcitrate,0.09M;volumetricflowrateratio,

32.3:1(Qout:Qin,HAuCl4:Na3citrate);molarflowrateratio,1:5(HAuCl4:Na3citrate);temperature,80°C.

..................................................................................................................................................................102

Figure 5-7: TEM images of gold NPs synthesized using the CFR with a CFI residence loop at a

temperature of A: 60°C, B: 70°C, C: 80°C, D: 90°C, E: 100°C. F: Average diameter of gold NPs vs

temperature of synthesis (bars represent standard deviation of the size). Concentration of

tetrachloroauricacid,0.557mM;concentrationoftrisodiumcitrate,0.09M;volumetricflowrateratio,

32.3:1(Qout:Qin,HAuCl4:Na3citrate);molarflowrateratio,1:5(HAuCl4:Na3citrate);flowrate,1ml/min.

..................................................................................................................................................................105

Figure5-8:NormalizedDCScurvesofgoldNPssynthesizedusingtheCFRwithaCFI residence loopat

temperatures between 60 and100°C. Inset: Diameter of gold NPs vs. temperature of synthesis (bars

representstandarddeviationofsize).Concentrationoftetrachloroauricacid,0.557mM;concentration

oftrisodiumcitrate,0.09M;volumetricflowrateratio,32.3:1(Qout:Qin,HAuCl4:Na3citrate);molarflow

rateratio,1:5(HAuCl4:Na3citrate);flowrate,1ml/min..........................................................................106

Figure 5-9: UV-Vis spectra of gold NPs synthesized using the CFR with a CFI residence loop at

temperaturesbetween60and100°C.Inset:Peakabsorbance(blacksquares)andpeakwavelength(red

diamonds)ofgoldNPsvs.temperatureofsynthesis.Concentrationoftetrachloroauricacid,0.557mM;

concentration of trisodium citrate, 0.09 M; volumetric flow rate ratio, 32.3: 1 (Qout:Qin, HAuCl4:

Na3citrate);molarflowrateratio,1:5(HAuCl4:Na3citrate);flowrate,1ml/min....................................106

Figure 5-10: TEM images of silverNPs synthesized using the CFR followed by a SARmixer at a silver

nitrate concentration of A: 0.05mM, B: 0.15mM, C: 0.25mM,D: 0.4mM. Concentration of sodium

borohydride,0.3mM;concentrationoftrisodiumcitrate,0.5M;volumetricflowrateratio,1:1(Qout:Qin,

NaBH4: AgNO3); molar flow rate ratio, 0.5-4:3:5 (AgNO3:NaBH4:Na3citrate); flow rate, 2.5 ml/min;

temperature,22-24°C..............................................................................................................................108

Figure5-11:UV-VisspectraofsilverNPssynthesizedusingtheCFRfollowedbyaSARmixeratasilver

nitrateconcentrations rangingbetween0.05and0.4mM. Inset:Peakabsorbance (blacksquares)and

peakwavelength(reddiamonds)ofsilverNPsvs.silvernitrateconcentrationofsynthesis.Concentration

ofsodiumborohydride,0.3mM;concentrationoftrisodiumcitrate,0.5M;volumetricflowrateratio,1:

1 (Qout:Qin, NaBH4: AgNO3); molar flow rate ratio, 0.5-4:3:5 (AgNO3:NaBH4:Na3citrate); flow rate, 2.5

ml/min;temperature,22-24°C.................................................................................................................109

13

Figure5-12:Diametervs.silvernitrateconcentrationfortheCFRoperatedfollowedwithaSAR(black

squares) and without a SAR (red diamonds). Concentration of sodium borohydride, 0.3 mM;

concentrationoftrisodiumcitrate,0.5M;volumetricflowrateratio,1:1(Qout:Qin,NaBH4:AgNO3);molar

flowrateratio,0.5-4:3:5(AgNO3:NaBH4:Na3citrate);flowrate,2.5ml/min;temperature,22-24°C......109

Figure 5-13: TEM image of silver NPs synthesized using the CFR with inner tube I.D. of 0.798 mm

operatedunderhighRewithatotalflowrateofA:5.1ml/min,B:10.1ml/min,C:15.1ml/min,D:20.1

ml/min.Theoutertubeflowratewasfixedat0.1ml/min.Concentrationofsilvernitrate,0.1mM;of

sodiumborohydride,0.3mM;concentrationof trisodiumcitrate,0.5M (all concentrations statedare

thoseaftercompletemixing);molar flowrate ratio,1:3:5 (AgNO3:NaBH4:Na3citrate); temperature,22-

24°C..........................................................................................................................................................111

Figure5-14:FlowvisualizationofCFRwith innertubeI.D.of0.798mmoperatedunderturbulentflow

conditions.BasicBluedyewaspumped through the inner tubeat20ml/minandwaterwaspumped

throughtheoutertubeat0.1ml/min.Reis520intheinnertubeand212inthemainchannel.Picture1

is takenbeforedye ispumped through the inner tubeup,Pictures2-5asoperational time increased,

Picture6aftersteadystateisreached.....................................................................................................112

Figure5-15:TEMimagesofsilverNPssynthesizedusingtheCFRwithinnertubediametersofA:0.142

mm,B:0.345mm,C:0.447mm,D:0.556mm,E:0.701mm,F:0.798mm,G:Nanoparticlediametervs.

inner tube internal diameter. Concentration of silver nitrate, 0.1 mM; concentration of sodium


NaBH4:AgNO3);molarflowrateratio,1:3:5(AgNO3:NaBH4:Na3citrate);flowrate,1ml/min,temperature

22-24°C....................................................................................................................................................113

Figure5-16:UV-Vis spectraof silverNPssynthesizedusing theCFRwith inner tubediameters ranging

between 0.142 and 0.798 mm. Inset: Peak absorbance (black squares) and peak wavelength (red

diamonds)ofsilverNPsvs.innertubeinternaldiameterusedintheCFR.Concentrationofsilvernitrate,

0.1 mM; concentration of sodium borohydride, 0.3 mM; concentration of trisodium citrate, 0.5 M;

volumetric flow rate ratio, 1: 1 (Qout:Qin, NaBH4: AgNO3); molar flow rate ratio, 1:3:5

(AgNO3:NaBH4:Na3citrate);flowrate,1ml/min,temperature22-24°C..................................................114

Figure6-1:SchematicofexperimentalsetupforsilverandgoldNPsynthesisconsistingofanErlenmeyer

flaskwith precursor or reducing agent stirred via amagnetic stirrerwith a syringe pump reagent via

tubinga)silverNPsystemusingastainlesssteel0.5mmI.D.tubingandb)goldNPsystemusingaPTFE

0.5mmI.D.tubingwithaglycerinebathandtemperatureprobefortemperaturecontrolat80°C.Stirrer

speedwassetat500rpminbothcases..................................................................................................117

Figure6-2:Schematicrepresentationofinjectionpointswithinthebatchvesseltovarymixingefficiency

a) injectionisaddedfromabovethesolutioninthebatchvesselandb) injectionisaddedclosetothe

stirbartip.................................................................................................................................................119

14

Figure 6-3: Segregation index vs. mixing time estimated using the IEM model for the Villermaux-

Dushmann reaction scheme. Concentrations of buffer solution were KI: 0.0117 M, KIO3: 0.0023 M,

NaOH:0.0909M,H3BO3:0.1818M.ConcentrationofH2SO4was:0.07M...............................................120

Figure6-4:TEMimagesofsilverNPssynthesized inabatchvesselundervariousconditions.A)0.6ml

sodiumborohydride[63.3mM]addedviadropsto20mlofamixtureofsilvernitrate [0.38mM]and

trisodium citrate [0.383mM] at a flow rate of 0.5ml/min, B) 0.6ml sodium borohydride [63.3mM]

addednearthestirbartipto20mlofamixtureofsilvernitrate[0.38mM]andtrisodiumcitrate[0.383

mM]ataflowrateof50ml/min,C)0.6mlofamixtureofsilvernitrate[9.1mM]andtrisodiumcitrate

[9.1mM]addedviadropsto20mlsodiumborohydride[1.36mM]ataflowrateof0.5ml/minandD)

0.6mlofamixtureofsilvernitrate[9.1mM]andtrisodiumcitrate[9.1mM]addednearthestirbartip

to20mlsodiumborohydride[1.36mM]ataflowrateof50ml/min.Experimentswereconductedat

roomtemperatureandstirringspeedwassetat500rpm......................................................................124

Figure6-5:AveragepeakabsorbanceandaveragepeakwavelengthobtainedusingUV-visspectroscopy

ofsilverNPssynthesizedinbatchvesselundervariousconditions.A)0.6mlsodiumborohydride[63.3

mM]addedviadropsto20mlofamixtureofsilvernitrate[0.38mM]andtrisodiumcitrate[0.383mM]

ataflowrateof0.5ml/min,B)0.6mlsodiumborohydride[63.3mM]addednearthestirbartipto20

mlofamixtureofsilvernitrate[0.38mM]andtrisodiumcitrate[0.383mM]ataflowrateof50ml/min,

C)0.6mlofamixtureofsilvernitrate[9.1mM]andtrisodiumcitrate[22.7mM]addedviadropsto20

ml sodiumborohydride [1.36mM] at a flow rate of 0.5ml/min andD) 0.6ml of amixture of silver

nitrate [9.1 mM] and trisodium citrate [22.7 mM] added near the stir bar tip to 20 ml sodium

borohydride[1.36mM]ataflowrateof0.5ml/min.Experimentswereconductedatroomtemperature

andstirringspeedwassetat500rpm.Barsrepresentstandarddeviationofabsorbanceandwavelength

for3repeatedexperiments.....................................................................................................................125

Figure6-6: TEM imagesof goldNPs synthesized inabatchvesselunder various conditions.A)0.6ml

trisodiumcitrate[92.7mM]addedviadropsto20mloftetrachloroauricacid[0.556mM]ataflowrate

of 0.5 ml/min, B) 0.6 ml trisodium citrate [92.7 mM] added near the stir bar tip to 20 ml of

tetrachloroauricacid [0.556mM]ata flow rateof50ml/min,C)0.6mlof tetrachloroauricacid [18.5

mM]addedviadropsto20mltrisodiumcitrate[2.78mM]ataflowrateof0.5ml/minandD)0.6mlof

tetrachloroauricacid[18.5mM]addednearthestirbartipto20mltrisodiumcitrate[2.78mM] ata

flowrateof50ml/min.Experimentswereconductedat80°Candstirringspeedwassetat500rpm..128

Figure 6-7: Peak absorbance and peak wavelength obtained using UV-vis spectroscopy of gold NPs

synthesizedinbatchvesselundervariousconditions.A-Au)0.6mltrisodiumcitrate[92.7mM]added

via drops to 20ml of tetrachloroauric acid [0.556mM] at a flow rate of 0.5ml/min, B - Au) 0.6ml

trisodiumcitrate[92.7mM]addednearthestirbartipto20mloftetrachloroauricacid[0.556mM]ata

flow rate of 50ml/min, C - Au) 0.6ml of tetrachloroauric acid [18.5mM] added via drops to 20ml

trisodiumcitrate[2.78mM]ataflowrateof0.5ml/minandD-Au)0.6mloftetrachloroauricacid[18.5

mM] added near the stir bar tip to 20ml trisodium citrate [2.78mM] at a flow rate of 0.5ml/min.

15

Experimentswere conductedat 80°Cand stirring speedwas set at 500 rpm.Bars represent standard

deviationofabsorbanceandwavelengthfor3repeatedexperiments...................................................129

Figure6-8:NormalizednumberpercentagevsdiameterobtainedusingDCSofgoldNPssynthesizedin

batchvesselundervariousconditions.A-Au)0.6mltrisodiumcitrate[92.7mM]addedviadropsto20

mlof tetrachloroauricacid [0.556mM]ata flow rateof0.5ml/min,B -Au)0.6ml trisodiumcitrate

[92.7mM]addednearthestirbartipto20mloftetrachloroauricacid[0.556mM]ataflowrateof50

ml/min,C -Au)0.6mlof tetrachloroauricacid [18.5mM]addedviadrops to20ml trisodiumcitrate

[2.78mM] ataflowrateof0.5ml/minandD -Au)0.6mloftetrachloroauricacid[18.5mM]added

nearthestirbartipto20mltrisodiumcitrate[2.78mM]ataflowrateof0.5ml/min.Experimentswere

conductedat80°Candstirringspeedwassetat500rpm.Insetisanaveragediameterforeachcondition

obtainedfrom3repeatexperiments.......................................................................................................130

16

Listoftables

Table3-1:FWHMandpeakwavelengthofsilverNPsatvarioustrisodiumcitrateconcentrations inthe

range 0.025- 1.5 mM. Total flow rate 2.5 ml/min, concentration of silver nitrate 0.1 mM, sodium


Table3-2:FWHMandpeakwavelengthofsilverNPsatvarioussilvernitrateconcentrationsintherange

0.05 – 0.4 mM. Total flow rate 2.5 ml/min, concentrations of trisodium citrate 0.5 mM, sodium


Table 4-1: FWHM and peak wavelength of silver NPs synthesised in the 0.5 mm IJR using trisodium

citrateasastabilisingagentatflowratesbetween32and72ml/min.Concentrationofsilvernitrate0.9

mM,trisodiumcitrate6mMininput1andsodiumborohydride1.8mMininput2................................84

Table 4-2: FWHMand peakwavelength of silverNPs synthesised in the 0.25mm IJR using trisodium

citrateasastabilisingagentatflowratesbetween18and34ml/min.Concentrationofsilvernitrate0.9

mM,trisodiumcitrate6mMininput1andsodiumborohydride1.8mMininput2................................85

Table 4-3: FWHM and peakwavelength of silver NPs synthesised in the 0.5mm IJR using PVA as a

stabilisingagentatflowratesbetween32and72ml/min.Concentrationofsilvernitrate0.9mM,PVA

0.02wt%ininput1andsodiumborohydride1.8mMininput2..............................................................90

Table 4-4: FWHMandpeakwavelengthof silverNPs synthesised in the 0.25mm IJRusing PVAas a

stabilisingagentatflowratesbetween18and34ml/min.Concentrationofsilvernitrate0.9mM,PVA

0.02wt%ininput1andsodiumborohydride1.8mMininput2..............................................................91

Table 5-1: Diameter and dispersity of gold NPs synthesised at various flow rates with corresponding

Pecletnumber..........................................................................................................................................103

Table6-1:SummaryofconcentrationsforeachmixingconditionusedinthesilverNPsynthesis.........121

Table6-2:SummaryofconcentrationsforeachmixingconditionusedinthegoldNPsynthesis...........126

17

Listofabbreviations

CFD

Computationalfluiddynamics

CFI

Coiledflowinverter

CFR

Coaxialflowreactor

DCS

DifferentialCentrifugalSedimentation

ETFE

Ethylenetetrafluoroethylene

FWHM

Fullwidthathalfmaximum

IJR

Impingingjetreactor

NP

Nanoparticle

PTFE

Polytetrafluoroethylene

PVA

Polyvinylalcohol

RTD

Residencetimedistribution

SAR

Splitandrecombine

SAXS

Smallanglex-rayscattering

TEM

Transmissionelectronmicroscopy

UV-Vis

Ultraviolet-visible

18

1 Introduction

1.1 Motivation

In recent years, nanoparticles (NPs) have become a widely researched area to investigate themany

interestingpropertiesarisingfromallmannerofdifferentmaterials,shapesandsizes.Whenconsidering

the“bottom-up”chemicalsynthesisapproachtomakingNPs,moststudiesemploytheuseofreactors

consistingofavesselwhichismechanicallystirred. Inchemicalengineeringterms,thesereactionsare

carried out in a “batch” reactor. The benefits of using such reactors suit the aim of discovering new

methods involvingdifferentconcentrations, temperaturesand reagents ina relatively simplemanner.

More recently, microfluidic devices have been employed to synthesize NPs because of the various

benefitsofferedbythesetypesofreactors.Theseincludeimprovedheatandmasstransfer,efficiency

and safety.1 It is believed because of these enhanced characteristics (in particular heat and mass

transfer); improvements in the morphological characteristics of NPs synthesized using chemical

methodscanbemade(i.e.narrowsizedistributionsandcontrolofsize).

1.2 Nanoparticles

NPsarematerials,withasizerangefrom1to100nm,whichdifferfromtheirbulkcounterpartsbecause

they exhibit uniqueproperties due to their high surface area to volume ratios andquantumeffects.2

Importantly, thepropertiesof theNPsdependonthesize,compositionandshape,makingthemvery

different from their bulk counterpartswhich retain their properties irrespectiveof size.NPs exhibit a

largesurfaceareatovolumeratio(alargepercentageoftheiratomsarelocatedonthesurface),which

changes significantly with increasing size in the scale between 1-100 nm, resulting in changing

propertiesoftheNPs.Themostobviousexampleofthischangeinpropertieswithsizeisquantumdots

which exhibit different colours on exposure to ultraviolet light depending on the size of the NPs.3, 4

MorphologycontrolstheNPpropertieshencebeingabletocontrolthesizedistributionandsizeofthe

synthesized NPs precisely is beneficial. Traditionally batch reactors have been used to explore new

synthetic routes to obtain a variety of different NP shapes such as spheres,5 plates,6 disks,7 rods,8

flowers,9urchins,10octopods,11wires12andcoreshellstructures.13SuchawidearrayofNPshapeshave

beenthesubjectofinterestbecauseofthechangingpropertieswhichachangeintheshapeoftheNP

wouldafford.

Themain focusof the researchpresentedhereinhasbeenonsilverandgoldNPs, the literature

presented hereafter focuses on these materials. Silver NPs are used in many applications, such as

sensors,14-16optics,17-19electronics,20, 21 catalysis,22 23, 24biomedicine,25-27andsurfaceenhancedRaman

scattering.28 Gold NPs can be used in biological/biomedical,29, 30 electrochemical,31 catalytic,32 and

opticalapplications.19Withthevastamountofpotentialapplicationspresented,even justconsidering

goldandsilverNPs,thereisthenaneedtobeabletocontrolthesizeanddispersityofNPsforthemto

display suitableproperties forany intendedapplication.Besides this, it is alsodesirable tobeable to

produce NPs in suitable quantities in a reliable manner. Flow reactors have the advantage over

traditionally usedbatch reactors for the synthesis ofNPs, because they are less labour intensive and

Chapter1

19

more conducive to mass production of materials. Given that the size and dispersity of the NPs

determines their properties and hence application, it is important to consider the advantages and

disadvantagesofthecharacterisationtechniquesforNPanalysisusedwithinthiswork.

1.3 Characterisationtechniquestodeterminethemorphologyofnanoparticles

Thefollowingsectioncontainsabriefdescriptionalongwiththeadvantagesanddisadvantagesofthe

characterisation techniquesused in thiswork todetermine the size anddispersity of the synthesised

NPs.ThecharacterisationtechniquesusedwereUV-Visspectroscopy,DCSandTEM.

1.3.1 Ultraviolet-visiblespectroscopy

UV-Vis spectroscopy allows the measure of the interaction of NPs with the visible and ultraviolet

portions of the electromagnetic spectrum. This interaction is highly dependent on thematerial, size,

shapeanddispersityoftheNPsbeingmeasured.SmallchangesinthesizeoftheNPscancausechanges

intheirinteractionwithlightwaves.

1.3.2 Differentialcentrifugalsedimentation

DCSusescentrifugal force toseparateparticlesofdifferentsizebasedon theirdensity.SinceNPsare

very small, they do not settle in aqueous solutions very quickly and therefore a centrifugal force is

appliedusingaspinningdisctoallowsmallerparticlestosediment inareasonabletimeframe.Stokes’

Law is used to determine the distribution of sizes in a nanoparticle population by determining the

amountoftimerequiredforagivensizeofparticletosettleinasolutionofknownviscosityanddensity.

Theoperationof theDCS involves the injectionof a sampleofNPs into the centreof a spinningdisk

whichcontainsasolutionofsucroseinwaterwithadensitygradient.ThesizeoftheNPdeterminesthe

speedatwhichitreachesthelightsourceanddetector.Theintensityofthelightisrecordedovertime

and changes depending on the concentration of NPs passing through the light source and detector

region.ThetimecanberelatedtosizeusingStokes’LawandinthiswaythesizedistributionoftheNPs

canbedetermined.

1.3.3 Transmissionelectronmicroscopy

The TEM operation can briefly be described as follows. An electron gun generates electrons which

travel through a vacuum generated within the microscope. The electrons pass through an

electromagneticlenswhichfocusesthebeam.Thistravelsthroughthespecimen(athincarboncoated

coppergridwhichtheNPsampleisplacedon),wheretheNPsinteractwiththebeamandelectronsare

absorbedorscattered.Thebeamtravels throughthespecimenandthroughvarious lenses toreacha

detector (such as a fluorescent film)where the sample is visualised. Areaswhere the electron beam

interacted stronglywith the sampleandwereabsorbedand/or scatteredappeardark,whereasareas

wheretheelectronsdidnotinteractappearlight.Inthisway,theNPsarevisualised.

Introduction

20

1.4 Microfluidics

Microfluidicsisabranchofflowprocessesconcerningflowreactorswhichhavereactionchannelswith

diameters in themicrometre range (<1mm).Thishasbeenanactiveareaof research in recentyears,

uncoveringmanychallengesandbenefitsthatmicrofluidicspresent.33

In this study, various types of microfluidic devices have been employed to synthesize gold and

silverNPswiththeaimof investigatinghowdifferentdesignscanbeusedtomanipulatetheresultant

size and size distribution of the NPs. The size and dispersity of the NPs are largely controlled by a

chemicalreactionandsubsequentnucleationandgrowth.34Theseareinturncontrolledbyparameters

suchasconcentrationofreagents,pHandtemperature.Usually,achangeinmorphologicalparameters

is brought about by changing one of the above parameters. Currently,many studies in the literature

focusondoingexactlythis.Thisapproachenablesavarietyofshapes,sizesandcompositionsofNPsto

beobtainedinarelativelysimplemanner.Ontheotherhand,masstransferwithinthereactorhasbeen

thefocusofmuchfewerstudies.Themasstransfergovernstheconcentrationsatwhichthereactions

takeplace,andmicrofluidicdevicesoffertightercontrolovermasstransferwhencomparedtoalarger

volume flow or batch reactor. Another benefit of using microfluidic devices is the spatiotemporal

separationofthevariousstagesinthesynthesisincludingreaction,nucleationandgrowthwhichallows

foragreatercontroloversize,shape,compositionandsizedistributionoftheNPs.35-37Italsoallowsfor

anincreasedflexibilityinanalysingthemechanismsofeachofthesestages.38-40

Asmentionedpreviously,themajorityofresearchsynthesisingNPshasbeencarriedoutinbatch

processes,e.g.bringingthereactantstogetherandallowingthemtoreacttoformproduct,followedby

removaloftheproductsaftersomereactiontime.Themaindifferencebetweenabatchprocessanda

flow process reaction is that product is continuously withdrawn as reactants are pumped in a flow

process. Figure1-1 showsa schematicofabatch reactorandFigure1-2 showsa schematicofa flow

reactor.

Figure1-1:Schematicrepresentationofabatchreactor

Chapter1

21

Figure1-2:Schematicrepresentationofaflowreactor

AkeyconceptinmixingisthedimensionlessReynoldsnumber,describingtheratioofinertialforcesto

viscousforcesinafluid:

Re=inertialforcesviscousforces

=𝜌ud𝜇

where𝜌isthedensityofthefluid,uisthevelocityofthefluid,disthediameterofthechanneland𝜇is

theviscosityofthefluid.Thisdimensionlessnumber isuseful incharacterisingthetypeofflowwithin

thesystem.Therearetwodistinctcharacteristicflows,laminarflowandturbulentflow.Laminarflows

occur at lowReynolds numbers, typically at Re < 2000. Turbulent flow typically occurs at Re > 2000

(though in amicrochannel, turbulence can occur at lower Reynolds numbers). There is a transitional

phasebetweenthetwocharacteristicflows.Inthelaminarregime,mixingoccursthroughdiffusionand

thevelocityprofileisnon-uniformacrossthechannel(sloweratthewalls,fasterinthecentre).Inthe

turbulent regime, non-uniform velocity profiles form resulting in a stretching and thinning of fluid

lamellae. This improves the macromixing (time scale of bulk homogenisation) and also reduces the

diffusion distance as fluid lamellae are stretched resulting in faster mixing on the microscale. A

schematicrepresentationofthesetypesofflowwithinachannelisshowninFigure1-3.

Figure1-3:Schematicrepresentationofturbulentflowandlaminarflow

Introduction

22

Withinamicrochannel,Reynoldsnumbersareusuallyverylow(Re<100)becauseofthesmallchannel

diameterthefluidflowsthrough.Thismeansthatflowgenerallyfallsintothelaminarregimesomixing

primarilyoccursthroughdiffusion.Theperformanceofamixercanbemeasuredbytheamountoftime

ittakestomixreactantstogether(mixingtime).Mixingtimecanbeseenastheamountoftimeittakes

forreactantstoachieveahomogeneousconcentrationprofile.Dependingonthedegreeofturbulence

inthemicrochannel,mixingwilloccurthroughamixtureofchaoticadvectionanddiffusion.Diffusional

transport is described by Fick’s Law, and the characteristic mixing time can be deduced from this

relationshipasfollows:

t∝dt2

D

wheretisthemixingtime,dtisthelamellawidthandDisthediffusioncoefficient.33Itcanbeseenfrom

this relationship that mixing time is dependent on the distance across which a given species has to

diffuse tomixwith another species. Thus, to achieve quickmixing times it is necessary to efficiently

reduce the diffusion distance between different specieswhich are to bemixed.Ottino describes the

striationthickness,s,as:

s=12(sa+sb)

wheresaandsbarethicknessesofstreamAandstreamBrespectively,asthesimplestmeasureofthe

stateofmechanicalmixing.41Thisislinkedtothemixingtimebeingproportionaltothelamellawidthas

stated by Fick’s Law.Mixing inmicrofluidic devices is generally focused on this reduction of striation

thickness to improve mixing efficiency. Mixing efficiency is a key parameter when considering mass

transfer.Therearesomeotherpertinentdimensionlessnumbers,besidestheReynoldsnumber,when

consideringmixingwithinthemicrofluidicdevicesstudiedinthefollowingchapters.ThePecletnumber

isusedtodefinethedominantmechanismofmasstransportinafluidandisaratioofthetimescalesof

diffusionandadvection:

Pe=diffusiontimeadvectiontime

=( L

2

D )

( uL )=

LuD

whereListhecharacteristiclength,uistheflowvelocityandDisthediffusioncoefficient.AhighPeclet

number (>> 1) is indicative of an advection dominated mass transport mechanism whereas if the

number is low (<<1) it is an indicativeof a diffusiondominatedmass transportmechanism.Another

importantnumber is theWebernumber,which is a ratiobetween the inertial forcesand the surface

tensionforcesinafluid:

Chapter1

23

We=ρu2dσ

whereρ isthedensityoffluid,uisaveragevelocityoffluidbeforeimpact,disthediameterofthejet

(takenas the tube internal diameter) andσ is the surface tensionof the fluid. TheWebernumber is

useful when describing the behaviour of impinging jets (chapter 4), as it is useful in characterising

whetherthereisacontinuousjetafterimpingementorthereisdropletbreakupawayfromthejetafter

impingement.Thisisanimportantconsiderationwhenlookingatthemixingefficiencyofimpingingjets.

1.5 Mixing

Mixingoccursprimarilythroughtwomechanisms,advectionanddiffusion,whichcanbedescribedina

more encompassing term, convection. Diffusion describes themovement ofmolecules from areas of

highconcentrationtoareasof lowconcentrationand isrelativelyslow.Whenconsideringmicrofluidic

devices,theprimarymodeofmasstransport isthroughdiffusion(becausetheflowisgenerally inthe

laminar regime). However, the distance across which diffusion of molecules occurs in a microfluidic

device is small. Usually this distance is reduced further by manipulating the flow in themicrofluidic

devicestocreatethinnerlamellaeoffluid(differentmixingdesignsarediscussedinchapter2).Laminar

flow is a result of the viscous forces inmoving fluid dominating the inertial forces (described by the

Reynoldsnumber),resultinginasmoothflowwithstraightstreamlinesalongthedirectionoftheflow.

When the inertial forcesbegin to increase, eitherby increasing the fluid velocityor applyingexternal

force such as stirring of the fluid, the streamlines no longer maintain a smooth streamline in the

directionoftheflow.Themovementofthefluidcanthenenhancethemixingthroughadvection,where

thefluidelementsarestretchedandfolded,ultimatelyleadingtofastermixingbecausediffusioncanact

overashorterdistance.

1.5.1 Diffusion

Fick’s law of diffusion describes the flux of a species soluble in a fluid, where the specieswill travel

withinthefluidfromanareaofhighconcentrationtoanareaoflowconcentration.Consideringspecies

𝑎travelingthroughmedium𝑏inthe𝑦direction,thelawcanbestatedas:

𝑛)* = −𝐷).𝑑𝐶)𝑑𝑦

where𝑛 is the flux [mol/m2s],𝐷). is themolecular diffusivity of species a inmediumb and𝐶 is the

concentration [mol/m3]. Diffusion describes the movement of material at the smallest scale, and is

essentiallyhowallmixingoccurs.Thespeedatwhichthisprocessoccursisdependentonthediffusivity

of thematerial and the difference in concentration between the areas thematerial has to travel (as

describedbyFick’sLaw).

Introduction

24

1.5.2 Advection

Advection of a fluid enhances themass transfer through themotion of the bulk fluid, which can be

describedby its velocityprofile.Velocityprofileswithina fluid canbecomputedbyusing theNavier-

Stokes Equations. This enhancement in mass transfer occurs when fluid moves in such a way to

experience increased exposure to the surrounding bulk fluid. In laminar flow, each fluid element has

limitedinteractionwiththebulkfluid,becausethevelocityprofileconsistsofstraightstreamlineswhich

causethefluidelementstostayona linearpathwhichcannottravelthroughoutthebulkfluid.When

the flow transitions into a turbulent flow, the fluid elements followmore complex streamlineswhich

allow them to interactwithmore fluidelements in thebulk fluid, aswell as causinga stretchingand

thinning of these fluid elements to reduce diffusion distance, enhancing themass transport through

diffusion.

1.6 Objectives

Synthesis of NPs consists of reaction, nucleation and growth kinetics, also known as “reactive

crystallization”. Changes in size and dispersity of NPs can be achieved by changing reagent

concentrations,thusaffectingkineticsoftheprocessesinvolvedinreactivecrystallization.Thisapproach

isoftenusedfromachemistryperspective.Theaimofthisstudy is to investigatehowchangingmass

transfer conditions affect NP size and dispersity from a chemical engineering perspective, hence

considering flow hydrodynamics, mixing efficiency and concentration profiles within the microfluidic

devices.Thisisachievedbyusingvariouscharacterizationtechniquesonthemicrofluidicdevicessuchas

mixing time characterization using a competing parallel reaction scheme, high speed camera imaging

andflowvisualizationusingamicroscope.Thischaracterizationenablesanunderstandingofthemass

transferineachofthemicrofluidicdevicesusedinthiswork.SynthesisofsilverandgoldNPscanthen

beperformedunderavarietyofdifferentmasstransferconditionsaffordedbythemicrofluidicdevices

used.ThisallowsaconnectiontobemadebetweenthesizeanddispersityofNPssynthesizedandthe

masstransferconditionstheyweresynthesizedunder.

1.7 Chapteroverview

§ Chapter 2 contains a literature review concerning microfluidic devices, silver NPs and gold NPs.

Various designs of microfluidic devices used to achieve ‘passive’ mixing were reviewed (‘active’

mixerswerenotconsideredbecauseoftheir increasedcomplexity).SilverandgoldNPsynthesis in

batchreactors,flowreactorsandanoverviewofthemechanisticstudiesonnucleationandgrowth

foreachmaterialarealsocovered.

§ Chapter3 containsa studyon the synthesisof silverNPsusinga coaxial flow reactor (CFR). Silver

nitrate is reducedviasodiumborohydride inthepresenceof trisodiumcitratewithintheCFR.The

effectof flowrate,precursorconcentrationand ligandconcentrationonNPsizeanddispersityare

presented.

Chapter1

25

§ Chapter4containsastudyonthesynthesisofsilverNPsusinganimpingingjetreactor(IJR).TheIJR

ischaracterizedusingtheVillermaux-Dushmanreactionsandhighspeedcameraimages.Detailson

the Villermaux-Dushman reactions and the interaction by exchange with the mean (IEM) mixing

modelarediscussed.SilverNPsaresynthesizedbyreductionofsilvernitrateviasodiumborohydride

witheither trisodiumcitrateorPVAusedasa ligand.Theeffectof flowrateonsilverNPsizeand

dispersityusingIJRsoftwodifferentjetdiametersispresented.

§ Chapter5containsastudyonthesynthesisofgoldandsilverNPsusingaCFR inconjunctionwith

othermicrofluidicdevices(coiledflowinverterandsplitandrecombinemixer)aswellasusingthe

CFR in isolation with different types of hydrodynamics (vortex flow and variation of stream

thickness). GoldNPs are synthesized by reduction of tetrachloroauric acid via trisodium citrate at

elevated temperatures, silver NPs are synthesized by reduction of silver nitrate via sodium

borohydrideinthepresenceoftrisodiumcitrate.Theeffectofvariationsinmasstransferconditions

andtemperatureonNPsizeanddispersityarepresented.

§ Chapter6containsastudyonsynthesisofgoldandsilverNPsusingabatchreactor.GoldNPsare

synthesized by reduction of tetrachloroauric acid via trisodium citrate at elevated temperatures,

silverNPsaresynthesizedbyreductionofsilvernitrateviasodiumborohydride in thepresenceof

trisodiumcitrate.Theeffectofmixingefficiencyand theorderof reagentadditiononNPsizeand

dispersityarepresented.

§ Chapter 7 summarizes conclusions drawn from the work presented along with possible future

directions.

26

2 LiteratureReview

2.1 Microfluidicdevicesformasstransfer

The following section gives an overview of microreactor technologies, in particular micromixers,

currently inuse.This istoaid intheunderstandingofhoweachtechnologyworks,thefocusbeingon

themethodofmixing.

There exist awide variety ofmicromixers reported in the literature. These are divided into two

categories, passive and active micromixers.42, 43 Active micromixers require the application of an

externalforcetoenhancethemixingperformance,andaremorecomplextofabricateandpackagein

general than passivemicromixers. Passivemicromixers rely on the geometricalmanipulation of fluid

elements within to achieve mixing.44 This section focuses on passive micromixers, a simpler type of

micromixerwithmanymorestudiesreportedintheliterature.

2.1.1 Parallellamination

T-mixers, 45-47 Y-mixers,48 interdigitated streams49 and hydrodynamic focusing fall50 into this class of

micromixing.Thebasicconceptbehindthemixingistoreducethestriationthicknessbetweenstreams

tobemixed,whichinturnreducesdiffusiondistanceandspeedsupmixing.Figure2-1isanexampleof

aninterdigitatedparallellaminationmicromixerstudiedbyBessothetal.49

Figure2-1:Exampleofaparallellaminationmixer.49ReproducedfromRef.49withpermissionofTheRoyalSociety

ofChemistry.

2.1.2 Seriallamination

This type of micromixer consists of a number of splitting and recombination (SAR) steps a stream

consistingoftwocomponentstoproduceprogressivelythinnerlamellaetodecreasethediffusionpath.

This form consists of a number of elements which will perform successive SAR of the streams. The

differencebetweenparallellaminationandthistypeisthatstreamsthataresplit,arerecombinedafter

onesplit.Thisisthenrepeatedtoproduceprogressivelythinnerlamellae.Acharacteristicadvantageof

this type of mixer is that the flowrate is not the main factor in determining mixing performance.

Schonfeld et al.,51 Kashid et al.,52 Buchegger et al.53 and Chen et al.54 have all investigated serial

laminatorsofdifferentdesigns.

Chapter2

27

2.1.3 Segmentedflow

This type ofmicromixer functions by splitting a continuous fluid flow into smaller discrete segments,

similar to tiny individual reactors flowing through a larger flow reactor. These smaller segments are

generally dispersed in a non-reactingmediumwhich carry the smaller segments through the reactor.

Themovementofthesesegmentscreatesaninternalflowfield,42thusmixingisgeneratedwithineach

segment. Waelchi and Von Rohr,55 Kashid et al.,56 Tanthapanichakoon et al.57 have investigated

segmentedflowreactors.Figure2-2givesavisualdepictionoftheinternalflowfieldsinslugstypically

foundinsegmentedflow.57

Figure2-2:Slugswithamixtureofbluedyeandwatershowingadvectionwithin.ReprintedfromRef.57,Copyright

(2006),withpermissionfromElsevier.

2.1.4 Injectionmicromixers

Thebasicmixingprinciplebehindinjectionmicromixersisonereagentisinjectedintoanotherreagent

streamthroughanumberofsmallnozzleswhichreducethestriationthicknessofthestreams,resulting

inquickerdiffusion.Miyakeetal.58,59andVoldmanetal.60haveinvestigatedinjectionmicromixers.

2.1.5 Chaoticadvectionmicromixers

Chaotic advection can be described as themovement of fluid in such away that the interfacial area

betweenreagents tobemixed iscontinuously increased,accompaniedbya reduction in thediffusion

length between reagents (striation thickness).61 Usually some kind of internal structures within the

microchannelareusedtoinduceflowfieldstoincreasetheinterfacialareabetweenfluidstobemixed.

Stroocketal.62andKangandKwon63havestudiedthestaggeredherringbonemixer(SHM),which isa

typeofchaoticadvectionmicromixer.

2.2 Silvernanoparticles

ThissectionpresentssilverNPsynthesisstudiesusingbatchreactors,microfluidicdevicesandthosethat

investigatemechanismsinsilverNPformation.ThemainfocusofthebatchreactorsectionissilverNPs

synthesizedusingsodiumborohydrideasareducingagentgiventhisisthesystemthathasbeenstudied

in thiswork.Awider variety of systemshavebeen reviewed for studiesusingmicrofluidic devices to

highlight thepossibilitiesofcontrollingsizeanddispersityof synthesizedNPs.Studies that investigate

themechanismsofreaction,growthandnucleationofsilverNPsarealsoreviewed.

LiteratureReview

28

2.2.1 Batchsynthesisofsilvernanoparticlesviasodiumborohydridereduction

AvarietyofreducingagentsexistforreductionofsilverprecursorstoformNPs.Inthissection,thefocus

isonsodiumborohydrideasareducingagentgiventhatthisthereagentusedinthiswork.

Creightonetal.usedicecoldsodiumborohydridetoreducesilvernitratetoproduceNPsinquitea

wide range between 1-50 nm with some sols produced being in the range between 1-10 nm.64

Interestingly,thesolswerestableoveraperiodofseveralweeksintheabsenceofanyligand.

Shirtcliffeetal.usedsilvernitrate,sodiumborohydrideandsodiumhydroxideasreagentsforthe

synthesisofsilverNPsintherangebetween15-77nm.65TheyusedEppendorfpipettestomixreagents

withinacuvetteat2°Candfoundthatthemethodandorderofmixinghadasignificanteffect.Addition

ofsilvernitratetoamixtureofsodiumborohydrideandsodiumhydroxideproducedthenarrowestand

mostreproduciblepeaksusingUV-Visanalysis(thoughtheTEMimagesshowedquitelargepolydisperse

NPs,possiblybecauseofthelackofligandusedinthesystem).

Ryuetal. synthesizedhighlyconcentratedsilverNPsamples (upto40wt%)byreducing icecold

silvernitrateviaslowadditionofsodiumborohydrideand/orhydrazinemonohydrateinthepresenceof

polyelectrolytesintheMWrangebetween1200-30000.66NPswereintherangebetween1-35nmwith

themajorityofconditionstestedproducinglessthan10nmNPs.

Songetal.produced silverNPsbyadding silvernitrate toa solutionof sodiumborohydrideand

sodiumdodecylsulfate.67Theyobservedthatincreasedsodiumborohydrideresultedinlessaggregation

oftheNPs,pointingtothehydrolysisofsodiumborohydridecausingaggregationbutwhenahighmolar

ratio(sodiumborohydridetoprecursor)wasused,athicklayerofBH4- ionspreventedboronhydroxide

from attaching onto the surface and destabilizing the NPs. However another possibility is that the

hydrolysis reaction causes an increase in pH which ultimately suppresses the hydrolysis reaction,

preventing the switching point which results in a sudden shift to a higher size due to coalescence,

becauseoftheconsumptionofsodiumborohydride.68

Pinto et al. synthesized silver NPs in the size range between 3-10 nm by addition of sodium

borohydridetoamixtureofsilvernitrateandtrisodiumcitrate.69Onincreasingsodiumborohydrideto

silvernitratemolarratios,thesizeoftheNPsdecreasedandtheconcentrationofNPsincreasedbased

onUV-Vis spectroscopy analysis. The standard deviation of the absorbancemaximum increasedwith

increasingsodiumborohydridemolaramounti.e.alowerreproducibilitybetweenexperiments.

Thøgersenetal.synthesizedsilverNPsinthesizerangebetween6-19nmbyadditionoficecold

AgNO3slowlyviadropstoan icecoldsolutionofsodiumborohydride.70Theyfoundan increasingsize

whenthemolarratioofsilvernitratetosodiumborohydridewasincreased.

Agnihotrietal.synthesizedsilverNPsintherangebetween5-100nmbyaddingsilvernitratetoa

heatedsolutionofsodiumborohydrideandtrisodiumcitrate(60°C),followedbyfurtherheatingofthe

solution to 90°C.5 They were able to synthesize monodispersed NPs and they suggested a two-step

growthmechanism,where sodiumborohydrideproduced small seeds througha coalescenceof small

silver clusters followed by a surface reduction type growth via citrate to grow the NPs into a more

monodispersed sample. They were able to synthesize a wide range of sizes by varying the

concentrationsof thereagentsused in thesystem. Ingeneral,alteringtheratiosof reducingagent to

Chapter2

29

precursorwasthemainmethodtoachievecontrolofsizeoftheNPs.Thishighlightstheimportanceof

concentrationintheresultantNPsize.

2.2.2 Microfluidicsynthesisofsilvernanoparticles

Wagneretal.haveusedsplitandrecombinemicromixersinserieswithresidenceloopsinbetweento

allow the growth of silver NPs.71 They reduced silver nitrate using sodium borohydride. Synthesized

silverNPshadameandiameterof15nm.Thestudyshowedadependencyofthesizedistributionofthe

NPs on the flowrate, with an increased flowrate resulting in a narrower size distribution. A faster

flowratewouldresultinbettermixing,withanarrowersizedistributionpointingtoasharperdistinction

between nucleation and growth periods. This suggests mixing is extremely important in obtaining

monodisperseNPs.Kohleretal.usedasimilarsplitandrecombinemixeraswellasanumberofother

configurations, which included the use of t-mixers and segmented flow techniques to synthesize

compositegoldandsilverNPs.72Tetrachloroauricacidandsilvernitratewereusedasprecursors,while

ascorbicacidwasusedasareducingagent.

Linetal.mixedsilverpentafluoropropionateinisomethyletherinthepresenceoftrioctylamineto

forma singlephase.73 SilverNPswere formed through thermaldecompositionusinga simple tubular

microchannel which was immersed in an oil bath held at temperature between 100-140°C. The

experimentalsetupisshowninFigure2-3.

Figure2-3:ExperimentalflowreactorsetupofsilverNPsynthesisusedbyLinetal.Reprintedwithpermissionfrom

Ref.73.Copyright(2004)AmericanChemicalSociety.

Synthesizedparticlesrangedfrom7.4to8.7nminsizewithsizevariancerangingfrom10%to19%.The

study showedan increase in flowrate resulted ina smalleraveragediameterof theNPs, though they

weremorepolydisperse.Thestudysuggeststhisispossiblydowntowalleffectsplayingarole,resulting

inaparabolicvelocityprofilewithinthetube.Sincethissystemisonephase(reagentsarepremixed),

theoreticallymixingshouldnotbeanissue(assumingtheyarewellmixedbeforehand).Theactivationof

thereactionoccursthroughan increase intemperature.Therefore,toobtainmonodispersesilverNPs

the temperature profile must be uniform. Since microreactors have extremely high surface area to

volume ratios, they excel at heat transfer. Assuming the temperature profile is uniform, the other

possibilitythatleadstopolydispersityisabroadresidencetimedistribution(RTD).

Sonnichsenet al. used a simple setup involving a threeway valve followed by a residence loop

(tubing inner diameters of 0.75 mm for silver and 1mm for gold) shown in Figure 2-4.74 They

LiteratureReview

30

synthesised both gold and silver nanorods based on themethod by Jana et al.75 Though reasonable

resultswereobtainedforgoldnanorods,silvernanorodsweremoredifficult.Theresultsshowedawide

rangeofdifferent sizesand shapes inTEM images (Figure2-4). They reported residence timeswitha

spreadofabout12minuteswhentheyinjectedapulseofpre-madeNPsintothereactor.Thisleadsto

poorsizecontroloftheNPsbecauseresidencetimesdifferbetweendifferentNPsflowingthroughthe

reactor.Thiswouldresultinbroaderparticlesizedistributions.Similaraffectswereseenintheworkof

Linetal.73

Figure2-4:MicroflowsetuptosynthesisesilvernanorodsandTEMimageofsilvernanorods.ReproducedfromRef.

74withpermissionofTheRoyalSocietyofChemistry.

Knaueretal.employedamicrosegmentedflowtechniquewithinamicrochanneltosynthesisedouble

shellNPswithagoldcorewhichhasasilvershellandasubsequentgoldshell.76Tetrachloroauricacid

andsilvernitratewerereducedbyascorbicacidinthepresenceofcetyltrimethylammoniumbromide.A

narrowersizedistributionforthesynthesizedNPswasobtainedinthecaseofthemicrosegmentedflow

reactor in comparison to the batch synthesis. This supports the idea of better control over size

distributionswithabetterRTD.AnarrowRTDisacharacteristicofsegmentedflowtechniques.77

Koehleretal.alsosynthesizedAu/AgcompositeNPsusinganumberofsplitandrecombinemixers

in series to bring the reactants together.78 Tetrachloroauric acid and silver nitrate were reduced by

ascorbicacid.Theimportanceofmixingwashighlightedinthisstudy,indicatedbythediverserangeof

differentNPcompositesobtainedbychangingmixingorders,mixing timesand flow rates (whichalso

affectstherateofmixing).

Liuet al. studied the effects of various parameters on the size distribution of synthesized silver

NPs.79 The synthesis is a thermal reductionof silver nitrate using plant extract fromC.Platycladiwith

additionofsodiumhydroxide.ThesetupisshowninFigure2-5.

Chapter2

31

Figure2-5:ExperimentalsetupofmicroreactorusedbyLiuetal.ReprintedfromRef.79,Copyright(2012),with

permissionfromElsevier.

Particlesizeincreasedwhenflowratewasincreasedfrom0.2ml/minto0.4ml/min(3.5nmto7.2nm)

andthendecreasedwhentheflowratewasfurtherincreasedfrom0.4ml/minto0.6ml/min(7.2nmto

3nm).IncreasingC.Platycladiconcentrationresultedinincreasingparticlesizeanddispersity(4.7±1.9

nmto15±8.6nm).Themicroreactorpropertiesalsoplayedakeyrole.T-typemixersproducedsmaller

andmorenarrowlydistributedparticlesthanthatproducedbyY-typemixers(4.7±1.9nmagainst5.3±

2.9nmrespectively).Particlesproduced inPTFE reactorsweresmallerandmorenarrowlydistributed

than those produced in ETFE (4.2 ± 0.9 nm against 4.7 ± 1.9 nm respectively). Both reactors had a

channel diameter of 0.8 mm. Smaller channel diameters (0.5 mm, 7.0 ± 1.9 nm) produced larger

particles on average than larger channel sizes (0.8 mm, 4.7 ± 1.9 nm), however the flowrate was

changedtoproducesimilarresidencetimeswhichmayhavealsoaffectedflowhydrodynamics.

Kumaret al. synthesised silverNPs in a spiral flow reactorusing segmented flow techniques via

inert liquid kerosene and inert gas air.80 Stearic acid sophorolipid molecules were used as both a

reducingandcappingagent.Silvernitratewasreducedbythesemoleculesat90°C.Figure2-6showsthe

setupusedinthestudy.

LiteratureReview

32

Figure2-6:Spiral flowreactorusedbyKumaretal., 80Wrepresentsreactingphase,Krepresents liquidkerosene

inertphase,A representsgasair inertphase.B represents inletmicromixer (crossmixer)used togenerate slugs,

withPEEKandStainlessSteelbeingused(dimensionsgiven).CandDarePMMA(0.5mmID)andStainlessSteel(1

mmID)spiralreactorsrespectively.ReprintedfromRef.80,Copyright(2012),withpermissionfromElsevier.

They studied the effects of variations in the slug flow (liquid-liquid, gas-liquid, size), flow rate, inlet

micromixerdiameterandflowrateratioshaveonthesizedistributionoftheformedsilverNPs.Various

effectswerereported,themainonebeingthatsmallerandnarrowersizedistributionswereobtained

whenthereactivephasewascontinuousratherthaninslugform.Itissuggestedthatinternalcirculation

(mixing),flowvelocitiesanddifferencesinmaterialproperties(liquid-liquidandgas-liquid)contributeto

theparticlesizedistribution.Flowratesusedwere35μl/min,100μl/minand1ml/minwithratiosof

inert phase to reacting phase being 0.5, 1 and 2. At lower flowrates, smaller and more narrowly

distributedparticleswereformed,withair/waterat35μl/min(2:1airtowater)performingthebestin

termsofnarrowdistributionwiththesmallestsize.

Lazarusetal.studiedthesynthesisofsilverandgoldNPsusingsegmenteddropletflowusingionic

liquid.81 Droplet formation of viscous BMIM-Tf2N (1-butyl-3-methylimidazoliumbis-

(trifluoromethylsulfonyl)imide)withinacontinuousfluorocarbonoilphase,polychlorotrifluoroethylene

(PCTFE), was achieved by coating the PDMS microfluidic device with a fluoropolymer via initiated

chemicalvapourdeposition(iCVD).Thisallowsthecarrieroilphasetocoatthesurfaceofthechannel

walls to prevent the ionic droplets from contacting the wall (improving the residence time because

segmentedflowisknowntoimproveRTD,butfurtherimprovementswouldbeobtainedifwalleffects

areminimisedasinthiscase).Dropletformationatanumberofdifferentflowrateratioscanbeseenin

Figure2-7.

Chapter2

33

Figure2-7:Configurationofmicrofludicdevicewithfour inlets:1)Carrierfluidinlet,2)Precursorsolutioninlet,3)

Solventinlet,4)Reducingagentinlet.Righthandsideshowssegmentedflowofionicliquid(BMIM-Tf2N)withinthe

oil carrier phase (PCTFE).At higher flowrates, jettingoccurs further along the channel as thedroplets pinch and

formthinstrandphases.ReprintedwithpermissionfromRef.81.Copyright(2012)AmericanChemicalSociety.

In the study, 1-butyl-3-methylimidazolium borohydride (BMIM-BH4) is used as a reducing agent

preferentiallyover sodiumborohydride.1-methylimidazolewasusedasa stabilisingagent,because it

binds tometal surfacesand servesas anacid scavengerwhichwouldprevent thedestabilisationand

agglomerationofNPsinthecaseofacidbuild-up.AgBF4wasusedasaprecursor(HAuCl4wasusedfor

gold).

Figure2-8:SilverNPsmadeusingA)Microfluidic device, B) using abatch reactor, C)UV-Vis comparisonof both

(microfluidic device is black, batch is red). Reprinted with permission from Ref. 81. Copyright (2012) American

ChemicalSociety.

Figure 2-8 shows the Ag NPs synthesised in the study with a marked difference between the two

synthesismethodsemployed,batchreactorandmicroreactor.ThemeansizeoftheAgNPssynthesised

in the microfluidic device are 3.73 ± 0.77 nm whereas those in batch produced large coral like

assemblies.TheUV-VisdatapresentedsuggestsahighnumberofsmallerAgNPsoflessthan10atoms

presentinthebatchsynthesis.

LiteratureReview

34

Overall,avarietyofdifferentmicrofluidicdeviceshavebeenusedforthesynthesisofsilverNPsto

showcasethepotentialbenefitstheypresent.

2.2.3 Mechanisticstudiesofsilvernanoparticlesynthesis

Polteetal. studied themechanismsof silverNP synthesisusinga continuous SAXS (small angle x-ray

scattering)setup.68Silverperchloratewasusedasaprecursor,sodiumborohydrideasareducingagent

andPVPasastabilisingagent.System1usednostabilisingagentandsystem2showsthesynthesiswith

PVPas a stabiliser, showing similar trendsbut taking longer than system1 in termsof thegrowthof

silverNPs(Figure2-9).Itseemsthatsurfactantsmayslowthekineticsdown(PVPinthiscase).Inboth

casesthegrowthmechanismisthesame,however,thehydrolysisofNaBH4(whichissupposedlysped

upbythepresenceofmetalNPs)slowsdownwiththeadditionofPVPresultinginalongertimebefore

the switching point. The switching point is the point at which particle stability decreases massively,

resultinginparticlescoalescingfrom2-3nmupto6-8nminsize.Thehydrolysisofsodiumborohydride

resultsinachangeinthecolloidalstability,resultinginaninevitableincreaseinthesizeofNPsdueto

coalescence coupledwitha reduction inpolydispersity.An illustrationof the suggestedmechanism is

showninFigure2-9.

Figure2-9:SuggestedgrowthmechanismforsilverNPsynthesis.ReprintedwithpermissionfromRef.68.Copyright

(2012)AmericanChemicalSociety.

Takesueetal.performedmechanisticstudiesonsilverNPsynthesisusingsilvernitrateasaprecursor

withcitricacidasastabiliserandtannicacidasareducingagent.82SAXSwasusedtocharacterisethe

NPsonlineinaflowreactorsetup(Y-mixer).Themoleratiosofsilvernitrate:citricacid:tannicacidwas

1:5:0.02withsodiumhydroxideusedtoadjustthepHto12.Themaingrowthmechanismsuggestedby

thisstudyisthatAg13clustersareformedwhichthencoalesceandaggregatetoformlargerNPs.Thisis

evidencedbyaninitialformationofseedparticleswhichmatchthesizeofanAg13cluster,followedby

gradual increase in the sizeof theNPs toa final average sizeofaround7nm.Ag55 clusterswerenot

formedinsignificantnumbersbecausethesizeofthisNPwasonlypresentinverysmallamounts.The

proposedmechanismcanbeseeninFigure2-10.Theonsetofnucleationbecomesclearat0.59msin

Chapter2

35

thissynthesis.0.71nmAgclustersformthemajoritybetween0.59and0.79ms,correspondingtoAg13

clusters,howeversomeparticleslargerthan10nmexistandtheprocessofgrowthisessentiallyoverat

3.93ms.TheauthorsuggeststheprocessofgrowthisdictatedbyAg13particlescoalescingandgrowing

toformlargerparticles.

Figure2-10:MechanismofsilverNPsynthesisproposedbyTakesueetal.82Silvernanoparticlesareformedthrough

(I)aninductionperiodduringwhichAg+ionsarereducedtoAg0atoms;(II)anucleation-dominantformationperiod

duringwhichAg13clustersarenucleatedandsimultaneouslyconsumedinsilvernanoparticleformation;and(III)a

growthperiodduringwhichlargersilvernanoparticlesareformedbycoalescenceandaggregationofsmallersilver

nanoparticles.ReprintedwithpermissionfromRef.82.Copyright(2011)AmericanChemicalSociety.

HengleinandGiersigsynthesisedsilverNPsusingAgClO4solutionasaprecursorwhichalsocontains2-

propanol,N2Oandsodiumcitrateinvariousconcentrations.83Thereducingagentisthe1-hydroxyalkyl

radicalgeneratedthroughradiolysis.Themainfindingsofthepaperisthatatlowcitrateconcentrations

theNPssynthesisedarepartlyagglomeratedwithmanyimperfections,atanintermediateconcentration

theparticlesarewellformedandseparatedwithnarrowsizedistributionsandathigherconcentrations

thereiscoalescenceofparticlesduetotheinstabilitycausedbythehighionicstrengthofthesolution.

Themolar ratioof sodiumcitrate toprecursor in the study ranges from0.5 to15. Theypropose two

mechanisms of silver NP growth (Figure 2-11). Surface reduction growth which occurs via electron

transfer, reducing Ag+ ions onto the surface of the particles already present in the solution, or

coalescenceofsmallerNPstoformlargerNPs.Thestudysuggeststhatifthecitrateconcentrationistoo

low,thesilverNPsarenotsufficientlycoveredwithstabilisingcitrateandcoalescenceoccurs.Athigher

concentrations, the ionic strength of the solution destabilises particles. A very important growth

experimenttheycarriedoutwhichinvolvedtakingaseedsolutionofsilverNPsandaddingfurthersilver

precursorandreducingelectronstocheckhowtheseedparticleswereaffected.Theexperimentshows

LiteratureReview

36

aperfect surface reductionoccurringas all the silver reduces tomakeexistingparticlesbigger rather

than produce new particles. This occurs by introducing 5 ml of seed particles to another 35 ml of

solution,whichafterirradiationproducesparticleswhichhavegrownperfectlyinsizeaccordingtothe

amountofsilveradded.This is importantbecause itshowsthepotential fortuningNPsizesbutmore

importantlythatthemechanismofsurfacereductionforgrowthofsilverNPsdoesexist.

Figure2-11:GrowthmechanismsproposedbyHengleinandGiersigatlowandoptimalcitrateconcentrationswith

twodifferentmechanismsoccurring. Atoptimalconcentrations, it is suggested thata surface reductionof silver

ionsoccursonthesurfaceofformedNPsowingtothereductionpotentialofthecitrateadsorbedontothesurface.

At low concentrations, coalescence of smaller silver NP clusters occurs.34, 83 Ref. 34 published by The American

ChemicalSociety.

PillaiandKamatstudiedtheroleof thecitrate ion in reducingsilver ionsaswellas its stabilisationof

silverNPsandtheformationofcomplexeswithsilverions.84Theyreducedsilvernitrateusingcitrateat

elevatedtemperaturesaswellasγradiolysis.Usingcitrateasareducingagent,ahigherconcentration

allowed the reaction to proceed faster as well as resulting in increased absorbance at 420 nm

wavelengthon theUV-Vispeak.This is inagreementwitha stronger reducingeffectproducing faster

reactions,aswellasproducingamoreconcentratedsampleat420nm,indicatingmoreNPsinthatsize

rangeweresuccessfullyformedandstabilised.Inthecaseofusingγradiolysis,theauthorsuggeststhe

silvermaynotreactat lowcitrateconcentrationsfullybecauseofa lowerpeakabsorbance.However,

the peak absorbance may be lower because of the presence of aggregates; naturally the peak

absorptionwillbelowerifthereisahigherconcentrationofaggregateswhichcontributeabsorbanceto

longerwavelengths(indicatedbymoreprominentshoulderpeaks).Citrateconcentrationinthecaseof

γradiolysiswillaffectthestabilisationoftheNPsandastheconcentrationincreases,theshoulderpeak

absorbancedecreasesindicatingdecreasingconcentrationofaggregates(Figure2-12).Higherreducing

power points to smaller average particle size from the difference in size between the twomethods

Chapter2

37

(Turkevitch,50-100nmandγ radiolysis,5-10nm),where thecitratehas less reducingpower than the

radiolysis(resultinginalowersupersaturation).Agrowthdominatedmechanismisfavouredwhenthe

reducingpower is relatively low (citrate) resulting in fewernucleiwhichgrow to larger sizesandvice

versaforhigherrelativereducingpower(γradiolysis).Thegrowthmechanismtheysuggestisshownin

Figure2-13.

Figure 2-12: Absorption spectra of Ag colloids prepared by γ irradiation of an aqueous solution (N2 purged)

containing 1 mM AgNO3, 1% methanol, and (a) 1mM, (b) 5mM, and (c) 10 mM sodium citrate as stabilizer.84

ReprintedwithpermissionfromRef.84.Copyright(2003)AmericanChemicalSociety.

Figure 2-13: Growth mechanism through pulse radiolysis of a mixture of silver nitrate and trisodium citrate

proposed by Pillai and Kamat: primary and secondary growth steps in the formation of silver nanocrystallites.84


Zhangetal.synthesisedsilverNPsandnanorodsusingsilvernitrateasaprecursor,sodiumborohydride

asareducingagentandtrisodiumcitrateasstabilizingagent.85Thoughthisisnotaspecificmechanistic

study, there is useful information regarding mechanisms of silver NP formation. To obtain different

shapesofNPs,preferentialgrowthisrequired incertaindirections.Theevidence inthisstudysuggest

that nanocrystals formedwith little defects show a uniform coverage of citrate, whereas thosewith

defects are more suitable for nanorod growth because of the preferential adsorption of citrate on

different surfacesbecauseofdefects in the crystal structure. Thishas implicationsonmonodispersity

andshapecontrolofNPs.Moreover,thedifferenceindefectsofNPsformedarisesfromadifferencein

themixingofthereagents(allatonceanddropwise).

Woehletal.studiedtheeffectsofreducingpower(i.e.numberofelectronsavailableforreduction)

usingin-situTEMscanningonthenucleationandgrowthcharacteristicsofsilverNPs.86Reductionwas

achieveddirectlyfromelectronsacquiredfromtheTEMbeam,whicharetransferredtothesolutionand

reduce the silver nitrate. The authors use classical nucleation theory to explain the effects observed

LiteratureReview

38

pointing to a certain concentration of electrons (reducing power) needed to achieve the conditions

which induce nucleation (supersaturation) and observed there was no nucleation below a certain

threshold of electron concentration. They find at a dose rate much higher than this threshold, the

growthoftheNPsisdiffusionlimitedandiscontrolledbyhowquicklysilveratomscanfindthesurface

of the crystals from the bulk of the solution. In this case themorphology of crystalswas seen to be

spherical. At an electron dosage closer to that of the threshold for inducing nucleation, growth is

reaction limited in the sense that the surface reduction of silver atoms onto the NP is the slowest

process resulting inNPs that grow intodifferent shapes. Thesedifferencesareattributed todiffusion

limitationsinthecaseofsphericalparticles,wherethereisanabundanceofreducedsilverionswhich

can attach onto the surface of formed nanocrystals, and reaction limited in the non-spherical case

where there is a lack of electrons available near the surface of the particles leading to a surface

reductionreactionlimitingstep.ThesummaryoftheirinvestigationcanbeseeninFigure2-14.

Figure2-14:MechanismsofsilverNPgrowthusingdifferentbeamcurrentsunderaTEMmicroscope.86Reprinted

withpermissionfromRef.86.Copyright(2012)AmericanChemicalSociety.

Thevariousmechanistic studies in the literaturehaveperformed thesilverNPsynthesisusing slightly

different conditions (reaction vessels, chemicals, concentrations etc.), so must be considered when

makingcomparisons.ThestudiesbyPolteetal.andTakesueetal.bothuseSAXSandachievedsimilar

sizedNPs. 68, 82 They are in agreement that the overarchingmechanismof the silverNP formation is

coalescencewithnomentionofsurfacereduction(surfacereductionimpliedbythestudybyHenglein

andGiersig).83However,becauseofthelimitedconditionsatwhichtheSAXSstudiesarecarriedout,itis

possible that theremay be differentmechanisms depending on the conditions. Henglein and Giersig

andZhangetal.discussthepresenceofdefectsincrystals,whichissomewhatcorrelatedinthestudies

to differences in shapes and size distributions.83, 85 Different growth mechanisms depending on the

defectspresentonthecrystal surfacehavebeenhighlightedby thesestudies.Lastly,PillaiandKamat

and Jiang et al. discuss the complexes formed between citrate and silver ions,84, 87 which affect the

kineticsofthegrowth,addingmorecomplexitytothe issueofcontrollingthesize,distributionofsize

andshapeofthesilverNPs.

Chapter2

39

2.3 Goldnanoparticles

ThissectionpresentsgoldNPsynthesisstudiesusingbatchreactors,microfluidicdevicesandthosethat

investigatemechanisms ingoldNPformation.Themainfocusofthebatchreactorsection isgoldNPs

synthesized using the Turkevichmethod. Awider variety of systems have been reviewed for studies

usingmicrofluidic devices to synthesize gold NPs. Finally, studies that investigate themechanisms of

reaction,growthandnucleationofgoldNPsarereviewed.

2.3.1 Batchsynthesisofgoldnanoparticles

ThemostpopularmethodforsynthesisofgoldNPsisknownastheTurkevichmethod.Turkevichetal.

testedavarietyofdifferentmethodsbutfoundthesodiumcitratemethodtobethemostrepeatable.88

Thisfamousmethodinvolvesbringingastirredaqueoussolutionofchloroauricacidtotheboilbefore

addingasodiumcitratesolutiontotheprecursor.Subsequently,thecolourtransitionsthroughyellow,

clear,grey-blue,purpleandfinallyawinered.ThemethodproducesNPstypicallyintherangebetween

15-20nm.Thepopularityofthismethodisdowntoitseaseandreliabilityinproducingmonodispersed

gold NPs. The effect of temperature, sodium citrate concentration were studied by Turkevich et al.

Decreasing temperaturebelow100°Cresulted inslowingdownthereaction,butachange insizewas

observed.ThesmallestNPswereseenat80°Cbutincreasedslightlyinsizeifsynthesistemperaturewas

set to70°C.Reducing the sodiumcitrate concentration resulted in somemarkedchanges suchas the

colourchangesbeingmoredefinedwhile thegrey-bluewasmorebluewithdecreasedconcentration.

Theoverallsynthesiswasmorerapidatlowercitrateconcentrationswhiletheaveragediameterofthe

NPsincreased.Thecolourchangeofthesolution,fromtheinitialyellowofthechloroauricacidtothe

finalwinered,givesanideaofthemechanismstakingplacetoformtheNPs.

FrensworkedfurtheronthismethodandsynthesisandsynthesisedNPsintherangeof12-150nm

byvarying theamountofcitratewhichwas injected into theboilingsolutionof tetrachlorauricacid.89

Theconclusionof the studywas that the final sizeof theNPswasgovernedby thenumberofnuclei

formedwhichwascontrolledbytheamountofcitrateadded.Higheramountsofcitrateledtosmaller

NPs, and Goodman et al. observed a similar trendwith the same procedurewhen increasing citrate

concentrationwithNPsizefrom21to80nm.90

Chow and Zukoski synthesised gold NPs with sizes ranging between ca. 16-52 nm for

unagglomeratedparticles (very largeagglomeratedNPswereseen incaseswerethesynthesisofgold

NPswasnot complete).91Theyvaried temperaturebetween60-80°Cand found that the finalparticle

sizetendedtoca.20nmandittooklongertoarriveatthefinalsizewithdecreasingtemperature.

Suetal.synthesisedgoldNPsbysonicationoftetrachlorauricacidandtrisodiumcitratesolutions

at4°C.92Theyvariedthegoldprecursortocitratemolarratiofrom1:1to1:8andfoundthatnon-stable

largeaggregateswere formedat ratios atorbelow1:3andNPs in the size rangebetween17-22nm

wereformedatoraboveratiosof1:4.

Peietal.synthesisedgoldnanowireswithamodificationonthestandardmethodinwhich200ml

ofsodiumtetrachloroaurate isbroughtupto80°C inatemperaturecontrolledbathfollowedbyrapid

additionof trisodiumcitrate invarious controlledvolumes toadjust themolar ratioof citrate togold

LiteratureReview

40

from 0.1 to 2.7.93 Nanowireswere formed for ratios below 0.4, and particleswere formed for ratios

above0.4.Abluecolourwasassociatedwithwireformationwhichwasobservedatallratiosfrom0.2to

2.7,butthistransitionedintoaredcolourforratiosat0.4orhigherwhichisassociatedwithspherical

goldNPs.

Kimlingetal.synthesisedgoldNPsusingtheTurkevichmethod,UV-assistedreductionwithcitrate

and ascorbic acid reduction. The reaction volume for the Turkevich method was 100 ml, 2 ml in a

cuvettefortheUV-assistedreductionand50mlforascorbicacidreduction.94Theytestedgoldtocitrate

molarratiosofca.0.1-1.5forallthesyntheses.UsingtheTurkevichmethodtheysynthesisedgoldNPs

betweenca.17-130nmatconcentrationsoflessthan0.8mM,1mMand1.2mMtetrachlorauricacid.

They found that the size increasedwhen reducing the citrate concentration, attributed to the lackof

stabilizationofsmallerNPsbecauseof incompletecoverageofcitrate.Atconcentrations lessthan0.8

mM, the size of the NPs was independent of gold concentration and dependent on citrate

concentration. When gold concentration was increased above 0.8 mM the size of the NPs almost

doubledatlowercitratetogoldmolarratios.IncontrasttheUV-assistedreductionshowedasimilarsize

correlationforgoldconcentrationsof1and1.6mMwithNPsintheca.17-117nmrange.Theascorbic

acid reduction enabled synthesis of NPs between ca. 17-110 nm. This method showed an NP size

increasewhenincreasingthegoldconcentrationfrom0.5mMto1.6mMsimilartothatintheTurkevich

method,butwhenincreasingthepHto7,thesizeoftheNPswassimilarforbothgoldconcentrations

supposedlybecauseofareductionininstabilitiescausedbytheacidityatthehigherconcentrations.

Jietal. investigatedtheroleofcitrateinthestandardmethod.95Byvaryingthetrisodiumcitrate

concentration, they foundaminimumsizeofca.15nmtoexistatamolarratioof3.5to1trisodium

citratetotetrachloroauricacid, if theratiowas lowerorhigherthanthis therewas increase insizeof

NPs. They found that at higher citrate ratios, the reaction took longer to proceed. Nanowire like

structureswereobservedintheearlystagesofsynthesisatcitrateratiosinthelowerrange(<3.5)while

athigherratiostheNPswheresphericalinallstagesofthesynthesis.Theconsumptionofprecursorwas

foundtobeingeneralmorerapidatlowercitrateratios,inlinewiththeideathatreactivityandhence

nucleationrateishigheratlowerpH.BytuningthepHbutkeepingthecitratetogoldprecursormolar

ratio constant, the size was varied from small to large in the same way as increasing citrate

concentration would but the dispersity was reduced and more constant through the pH variation

methodfrom20-40nmNPsize.

Patungwasa and Hodak investigated the effect of pH on gold NPs.96 They found that NP size

decreasedwithincreasingpHintherangebetween4and6.5.Thiswassuggestedtobebecauseofthe

chargeonthecitrateadsorbedontotheNPsurface,atlowerpHthereisdecreasedchargeonthecitrate

surfaceresultinginreducedstabilitywhereasathigherpHthecitratehasahigherchargewhichwould

explain why smaller and less polydisperse NPs are seen at higher pH. The system tends to higher

averageNPsizeatlowpH.

Ojea-Jiménezetal.synthesisedgoldNPsusingthestandardmethodaswellasamodificationusing

deuteriumoxideasasolventinsteadofwater.97TheysynthesisedgoldNPsbyheatingtrisodiumcitrate

in either 100% H2O, 50% D2O/H2O or 100% D2O to which they added tetrachloroauric acid and

Chapter2

41

subsequentlyletthesolutionboilfor3.5min.TheNPsizewasintherangeof5.3±1.1nmforpureD2O

and9.0±1.2nm forpureH2O.The reduction in sizebyusingdeuteriumoxidewasattributed to the

increasedreducingstrengthofthecitrate(inD2O).

Uppal et al. synthesised gold NPs from 11 to 15 nm with trisodium citrate added to boiling

tetrachloroauric acid drop wise over a period of 6 min.98 The solutions were removed from the

temperaturebathusedtoheatthesolutionwhenthecolourofthesamplewentlilacandthesamples

wereobservedoveranumberofdays.Itwasfoundthatthepeakwavelengthdecreasedorblue-shifted

from540nmto522-523nmregardlessofwhetherthesolutionwasstoredindarkorlightconditions.

Uppal et al. also synthesised goldNPswith a number of different initiationmethods.99 This involved

initiating the reaction between tetrachlorauric acid and trisodium citrate using thermal heating,

sonolysis,microwaves andUV-light. These initiationmethodswere testedwith premixed solutions of

tetrachlorauricacidand trisodiumcitrate,where the trisodiumcitrateconcentrationwas increased to

varythereducingagent/stabilisingagentconcentration.11.0to11.9nmNPswereformedusingthermal

initiationwith little variance in sizewith increasing citrate concentration,11-17nmNPswere formed

usingmicrowaveswithadecreaseinsizewithincreasingcitrateconcentration,8nmNPswereformed

usingUV-lightinitiationwithanincreaseintheellipticalshapeoftheNPswithincreasingcitrateand17

to 18 nm NPs were formed using sonolysis and size changed minimally with increasing citrate

concentration.

Ojea-Jiménezetal.comparedthestandardmethodtoan inversemethod,which involvesadding

tetrachloroauric acid to hot sodium citrate.100 They found that theNPs synthesised using the inverse

methodwere smaller than those in the standard synthesiswhileNPs synthesized using the standard

methodatasodiumcitratetotetrachloroauricacidratioof13.6were36.6nmwhiletheinversemethod

gaveNPs9nminsize, loweringthesodiumcitratetotetrachloroauricacidratioto6.8gaveasmaller

disparity in size between the methods: 17.8 nm and 14.9 nm for the standard and inverse method

respectively.Byusingamixtureofsodiumcitrateanddicarboxyacetone(DCA)(90:10)theyfoundthat

thesurfaceplasmonresonancebandofthesolutiondevelopedquicker(from11minwithnoDCA,to5

minwithDCA)while therewasareduction from21.2to17.7nm insizewhenDCAwasaddedtothe

citratebefore addition to thehot tetrachloroauric acid. Byusing a 50:50mixtureofDCAand sodium

citrate,itwasfoundthattheresultantNPswerelargerandmorepolydisperse(25.9nm)andusingDCA

only resulted in no NPs being formed. Therewas a possibility of changes in size when changing the

sodiumcitrateDCAratiobecauseofchangingpH,andpHisaparameterknowntoaffectsizeofNPs.95

TocheckwhetherDCAhadarealeffectontheNPsize,thepHwasmodifiedusingsodiumhydroxide.A

decreaseinsizewasfoundwhenthepHwasadjustedtosimilarvaluestothatofexperimentswhichdo

notuseDCA,andtheresultantNPswerefoundtobesmallersuggestingthatDCAdoesindeedplayan

importantrole inthesynthesisofgoldNPs.Ojea-JiménezandCampanerasynthesisedgoldNPswitha

modificationofpHbyaddinghydrochloricacid to the tetrachloroauricacidbeforeadditionof sodium

citrate to themixtureat90°C,with thepHof6.5,5.6and4.7being tested.101TheNP sizedecreased

from36.6nmto16.1nmwithdecreasingpHvalue.

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42

Sivaramanetal.synthesisedgoldNPsusingtheinversemethodbyaddingtetrachloroauricacidto

boilingtrisodiumcitratesolution.102Theyfoundthatatcitratetogoldratiosof<5,themeansizeofthe

NPwasnotsignificantlyaffected(52nmforinversemethod,56nmforTurkevichmethodataratioof1

to1)whereasforratios>5,therewasasignificantreductioninthesizeoftheNPs(10and16.1nmata

5.2ratio,and7.2and13.6nmataratioof20.8)whenusingtheinversemethod.Theyinvestigatedthe

possibilityoftheoxidationofcitratetoDCAbyboilingcitratebeforeadditiontotetrachlorauricacidand

comparing it to the standardmethod (citrate is notboiledbefore addition toboiling tetrachloroauric

acid)andtheinversemethod.TheyfoundthattheinversemethodyieldedsmallerNPsasexpected,but

thefirsttwomethodsyieldedsimilarsizessuggestingthatorderofreactantadditionisthecausefora

changeinNPsize,ratherthantheoxidationofcitratetoDCA.

Schulz et al. synthesized gold NPs of ~ 3.5 nm with a low dispersity of 5-6% using an inverse

method.103 Themethod was optimised by the use of sodium citrate/citric acid buffer to control pH.

EDTA was used to remove potential trace contaminations of metals such as zinc or copper, it was

suspectedthatthesecausedtheformationofgoldtriangularplatessincethemetalionscanaffectthe

growth of the Au NPs. It was found that the addition of EDTA reduced the formation of triangular

particlestopracticallyzero.

Wuithschicketal.synthesizedgoldNPsusingtheTurkevichmethodandinvestigatedtheeffectof

temperature,pHandvariationofreactantconcentrationsandorderofaddition.104Itwasobservedthat

there was a minimum size achieved of 12.2 nm when the synthesis was carried out 60°C and the

synthesistimedecreasedwithincreasingtemperature.ByvaryingtheinitialpHofthetetrachloroauric

acid,apHwindowbetween2.8and4wasfoundtoproducethebestdispersityofaround10%withthe

minimumsizeachievedof10.8nm.Thecitrateconcentrationsupposedlyhadlittleeffectaslongasthe

concentrationwasenough tomaintainaneutralpHaftermixing,whereas increasing tetrachloroauric

acid concentrations led to a decrease in the sizeof theNPs. Changing theorderof reactant addition

showed that adding tetrachloroauric acid to a larger volume of citrate gave smaller NPs than the

standardmethod.

Piella et al. synthesised sub-10 nm gold NPs based on a modification on the inverse method,

involving the use of tannic acid together with sodium citrate as a reducing agent.105 Highly

monodispersed3.6nmseedsweresynthesised(505nmpeakwavelength)andweresubsequentlyused

infurthergrowthstepstoobtainarangeofsizeinthesub-10nmrange.Parametricstudiesonoptimal

tannicacidtogoldmolarratio,temperatureandpHwereconducted,andtheoptimalvalueswerefound

tobe0.01,70°CandpH8 respectively forobtaining the smallestNPs. Sodiumcitrateand tannicacid

werealsotestedinisolationwhere10.9±1.3nm(518nmpeakwavelength)and4-12nmNPs(527nm

peakwavelength)wereobtainedrespectively,suggestingthereisasynergisticeffectbetweenthetannic

acid and sodium citrate, although the resultant 4-12 nm NPs from using tannic acid alone may be

becauseofinsufficientstabilisation.

Kettemannetal.synthesisedgoldNPsusingthestandardmethodandamodificationto improve

reproducibilityofthesynthesis.106WiththeimprovedsynthesistheywereabletosynthesiseNPsinthe

11-18 nm with less than 0.6 nm deviation in all cases. Similar to Schulz et al.,103 in the improved

Chapter2

43

synthesistheyusedabufferofsodiumcitrateandcitricacidtoachieveapHbetween5.5and6,which

was found to be the area inwhich the synthesis ismost reproducible because of the stability of the

HCit2- species. This species was suggested as the form of the citrate which is responsible for the

reductionofthegoldprecursor.

2.3.2 Microfluidicsynthesisofgoldnanoparticles

Wengetal.synthesizedgoldNPsusingaTurkevichmethodinanovelmicrofluidicdevice(Figure2-15).40

HexagonalNPs of around 35 nmwere obtained at a temperature of 115°Cwith reaction time in the

orderof2 to5min.Theyobserveda coalescenceof smallerNPs to form largerNPsbyanalyzing the

sample at a reaction of 2min. Thismechanismwas possibly due to the relatively quickmixing time

obtained in the reactor (reaction volumes were ca. 30-40 μl) which may have led to the quick

hydroxylationoftheAuCl4- speciestoAuClxOH4-x

- species,whichisknowntoreducethereactivityofthe

goldprecursor,possiblyleadingtoalowernucleationrateandincreasedgrowthrate,particularlywhen

thecitratetogoldmolarratiosarebetweenca.4-9.Thequickermixingtime(relativetoabatchprocess)

wouldhavereducedthetimeforreactionwhiletheAuCl4- speciesexistsinsolution.

Figure2-15:MicrofluidicdeviceusedtosynthesizehexagonalgoldNPs.40ReprintedfromRef.40withpermission

fromTheInstituteofPhysics.

Ftounietal.synthesizedgoldNPswiththeTurkevichmethodusingafusedsilicacapillaryandat-mixer

tomixchloroauricacidtosodiumcitratepriortointroductionintotheheatedcapillary(Figure2-16).107

In this case neither reagent was preheated before being mixed, and the residence times within the

capillarywerevariedbetween35and94s. Inthisresidencetimerange,theyobtainedNPsinthesize

rangebetween1.5and3nm,with largerNPsbeingobtainedat longer residence times. They further

investigatedtheeffectofsodiumcitratetogoldprecursormolarratio,andfoundaminimumataround

3.15.Theexplanationgivenisthatatlowercitrateratios,thereisanexcessofgoldprecursorwhichis

suggestedtoleadtoanincreasedgrowthofNPswhereasathighercitrateratiosthesizeissuggestedto

increase because of an increased ionic strength in solution. No discussion on the balance between

nucleation, growthand stabilization in termsofpHand reactivityof the respective reagents ismade,

LiteratureReview

44

althoughthisislikelytoplayasignificantroleinthefinalsizeoftheNPs.Temperatureeffectwasalso

investigated,andthesizeof theNPsdecreasedwith increasingtemperature in therangebetween60

and100°C.

Figure2-16:SynthesisofgoldNPsusingsyringepumps,t-mixerandfusedsilicacapillarytubingsetinahotplate.

Reproduced(Adapted)fromRef.107withpermissionofTheRoyalSocietyofChemistry.

Suganoetal.synthesizedgoldNPsbetween10-45nmusingay-typemicromixerwithapulsedflowof

chloroauricacidandsodiumcitratetomixthereagentswithinasmallchannelatroomtemperature.108

They investigatedhowmixingaffected the synthesisbyaltering thepulsing ratebetween50,100and

200Hz.Analysisofthemixingshowedthat increasingthepulsingrate improvedthemixingefficiency.

Theyusedtwocollectionmethods,inthefirstmethodthefluidwasallowedtocollectattheoutletof

themicromixerfor5minbeforecollectionandinthesecondmethodthefluidwascollectedforatotal

of 5 min through a 0.5 mm tube which was connected to the outlet of the micromixer. The

polydispersity of the resultant NPs showed the following behavior, collection method 2 produced

relativelylowerpolydispersitythancollectionmethod1forallpulsingfrequencies.Thisisattributedto

thereasoningthatcollectionmethod1allowsfluidthathastravelledthroughthemixerinitiallytomix

andreactwithfluidthatistravellingandexitingthemixeraftertheinitialperiod,resultingininteraction

ofthefluidwithdifferentconcentrationsovera5minperiod.Collectionmethod2sawareduction in

polydispersity from 50 Hz to 100 Hz, but further increase in frequency showed no improvements

suggestingthatfastermixingreducespolydispersityonlyupuntil100Hzandfurtherreductioninmixing

timehasnoeffect.ThesizeoftheNPswasfoundtoincreasewithahigherpulsingfrequency,suggesting

fastermixingresultedinlargerNPs.Thisismostlikelyduetothereactionrateofprecursordecreasing

withmixingastheAuCl4- speciesbecomesmorehydroxylatedoncontactwithcitrate.

Köhleret al. used a split and recombine type reactor (Figure 2-17) tomix tetrachloroauric acid,

ascorbicacid,PVA,sodiummetasilicateandFe(II)sulfatetosynthesisegoldNPs.109Becausetheyused4

pumpswhichwereindividuallycontrolledtofeedreactantstothereactorwhichhad4inlets,theywere

abletopreciselycontroltheconcentrationssimplybyalteringtheflowrates.Theyobtainedawidearray

of goldNPs using thismethod including small 5 nmgoldNPs,NPs arranged in a circle like structure,

aggregatesandlargerhexagonalaggregates.

Chapter2

45

Figure 2-17:Split and recombinemixer employed for the synthesis of goldNPs. Reproduced fromRef. 109with

permissionofTheRoyalSocietyofChemistry.

Wagner and Köhler synthesized gold NPs in a split and recombine reactor (similar to that shown in

Figure 2-17, but with 2 inlets instead of 4 using tetrachlorauric acid, ascorbic acid and

polyvinylpyrrolidone.Withincreasingflowrateinthereactor,adecreaseinNPsizewasobservedfrom

35nmataflowrateof1ml/minto24nmataflowrateof8ml/min.Alargersizeatlowerflowrates

wasattributed toapreferentialnucleationat thewallsof the reactor,whereadarkviolet filmwhich

gradually deepened and became opaque was observed. Supposedly this effect is lessened with

increasingflowratebecauseofanincreasedsheeratthewallsofthereactor,thoughifanequalsample

volumewastakenforeachexperiment, thismayhavebeenbecausethereactorwasoperatedovera

shorterperiodoftimeatthehigherflowrateandhencetherewaslesstimeforthedarkvioletfilmto

buildup.AnotherreasonforthesmallerNPsizeathigherflowrateswassuggestedtobeanincreased

nucleation rate because of more efficient mixing, which would lead to a quicker contact of excess

reducingagentwith theprecursor ions.Meandiameteranddispersityalsodecreasedwith increasing

ascorbicacidconcentrations,duetotheincreasednucleationrateobtainedatahigherreducingagent

concentration.SmallerandlessdisperseNPswereobtainedathigherpH(21nmatpH2.8,8nmatpH

9.5), attributed to the increasing redox potential of ascorbic acid at higher pH resulting in increased

nucleationrate.IncreasingPVPconcentrationalsoresultedinsmallerNPs,attributedtothesuppression

of growth around newly formed nuclei which occurs because of the large PVPmolecule slowing the

approachofgoldprecursor to thesurfaceofanuclei stabilizedbyPVP.Reductionof foulingwasalso

attainedusingtwodifferentapproaches,elevatedpHandsilanizationofthereactorwallstoproducea

hydrophobic surface. Both approaches achieved similar results in obtaining much higher peak

absorbancethanthestandardapproachatlowerpH.Köhleretal.andWagneretal.alsousedthesplit

andrecombinereactorstosynthesizegoldNPsusingborohydride.71,72

Shalom et al. synthesized small gold NPs using the Brust-Schiffrin method within a micromixer

whichsplitstwostreamsinto16substreamsandsubsequentlyinterdigitatesthestreamsforimproved

mixing(Figure2-18).110Tetrachloroauricacidwasreducedviasodiumborohydridewith1-dodecanethiol

being used as a stabilizing agent. NPs between 3-4 nm were synthesized within the reactor and a

significantreductioninsizeanddispersitywasachievedascomparedtosimilarconditionsusingabatch

reactor.Thiswasproposedtobebecauseofmoreefficientmixing,andsupposedlytherateofsodium

LiteratureReview

46

borohydrideadditionaffectstheNPsize.NPswillbemorepolydispersewhenadditionisslowbecause

nucleigeneratedearlyinthereactionareexposedtoanincreasedthiolconcentrationandthosenuclei

generated later as more sodium borohydride is added grow to a larger concentration because of

decreased thiol concentration. However, it is not mentioned that borohydride has an abundance of

electronsavailabletoreducethegoldprecursor,andhenceifmixingisslowthismayleadtoareduction

of many gold ions to metal via a single borohydride molecule, which subsequently doesn’t provide

enough surface charge to stabilize the formed nuclei leading to aggregation because of poorly

distributed sodium borohydride. Therefore one could argue that the polydispersity observed in the

batchisduetopoorlydispersedborohydride,leadingtolargerNPsproducedinitiallyratherthansmaller

NPs because of an increased thiol concentration. However, this would still explain why dispersity is

reduced in themicrofluidicsystem,sincethere isabetterdispersionofsodiumborohydrideearlier in

themixingprocess.

Figure2-18: InterdigitatingmicromixerusedbyShalometal. for the synthesisofgoldNPsvia theBrust-Schiffrin

method.110ReprintedfromRef.110,Copyright(2007),withpermissionfromElsevier.

Cabezaetal.synthesizedgoldNPswithinamicrochannelinwhichsegmentedflowwasgeneratedwith

anaqueousphase thatwas segmentedwitheitherair, tolueneor siliconeoil (Figure2-19).111Sodium

borohydridewasmixedwith tetrachloroauric acid and tetradecyltrimethylammonium bromide in the

aqueous phase and then segmented by the inert fluid at room temperature before being introduced

intoaheatedchannelwitha temperatureof100°C.They found increasing residence time resulted in

largerandmorepolydisperseNPs,withsizeincreasingfrom3.8±0.3to4.9±3.0nmataresidencetime

from 10 to 40 s, for a water-toluene systemwhere toluenewas the dispersed phase andwater the

continuousphase.Smallerslugsoffluidwereobtainedatalowerresidencetime,indicatingthatthese

types are more beneficial for controlling the size and polydispersity of the resultant NPs in the

continuousphase.This isprobablybecauseofthedecreasedsizeofthecontinuousphaseslugswhich

would result in faster mixing and therefore decreased polydispersity. Considering this, they further

Chapter2

47

investigated the effect of changing the dispersed phase fluid on NPs synthesized in the continuous

aqueousphaseandfoundthatairasthedispersedphaseproducedthesmallestandmostmonodisperse

NPs(2.8±0.2nm)followedbytoluene(3.8±0.3nm)andsiliconeoilproducedabimodaldistribution

(7.8 ± 6.5 nm and 15.5 ± 3.1 nm). This was determined to be because of how well the continuous

aqueous slug phase recirculatedmaterial,with air producing the highest circulation rate followed by

tolueneandsiliconeoil.Thiswasdownthehydrodynamicswithinthemicrochannelandthedifference

inmaterialpropertiesoftheinertdispersedphase.

Figure2-19:Segmented flow inmicrochannelemployed for synthesisofgoldNPsbyCabezaetal.Reprintedwith

permissionfromRef.111.Copyright(2012)AmericanChemicalSociety.

Tsunoyama et al. synthesized small gold NPs of around 1 nm by reducing tetrachlorauric acid with

sodium borohydride in the presence of polyvinylpyrrolidone in an interdigitating micromixer (Figure

2-20).112Smallerclusterswereobtainedwhentheflowratewasincreasedandwhentheconcentration

oftheprecursorandreducingagentwasincreased.

Figure2-20:InterdigitatingmixerusedbyTsunoyamaetal.forthesynthesisofgoldNPs.Reprinted(adapted)with

permissionfromRef.112.Copyright(2008)AmericanChemicalSociety.

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48

ThegoldNPsynthesisinmicrofluidicdevicesstudiespresentedshowthewiderangeofpossibilitiesand

potential benefits that microfluidic devices offer for the synthesis of NPs, and also highlight the

importanceofmasstransferincontrollingsizeanddispersityoftheNPs.

2.3.3 Mechanisticstudiesofgoldnanoparticlesynthesis

Oneof themorecommonlyheldviewsof themechanismforgoldNP formation isaburstnucleation

process,asdescribedbyLaMerandDinegar,113inwhichthenucleiareformedinashortperiodofhigh

nucleationrate,causingthesupersaturationtodropbelowtherequiredlevelfornucleationandintoa

growthphasewhichresultsinmonodispersedparticles.Turkevichetal.conductedastudytoinvestigate

themechanisms involved in thenucleationandgrowthofgoldNPs in thechloroauricacidandcitrate

system.114Thenucleationmechanismwassuspectedtohavefourdistinctregions:inductionperiod,an

auto-acceleratingperiod,alinearperiodandadecayperiod.Briefly,theinductionperiodwasbelieved

to be caused by the oxidation of citrate ions into acetonedicarboxylate ions which when reaching a

sufficientlevel,inducesnucleationthroughthereductionofprecursor.Theauto-acceleratingportionis

suspected tobe causedby the increase inproductionof acetonedicarboxylate ionspast the speedat

whichtheycanbeutilisedforreduction.Thelinearproportioniswhennucleationachievesaconstant

rate and the limiting factor is the speedatwhich theprecursor is reduced to formnuclei. Thedecay

period occurs when nucleation gives way to the growth of the NPs, i.e. the precursor begins to be

utilisedatthesurfaceofexistingnucleisurfacesratherthannucleatingnewparticles.Itisimportantto

note thatat thedecaystage, itwasobserved that less than5%of thegoldprecursorandcitratehad

beenconsumed,sothedecayperioddoesn’tseemtobeduetotheexhaustionofreagents.

Biggsetal.investigatedthecolloidalstabilityofthegoldNPsandtheroleoftheAuCl4- ionandthe

citrateion.115UsingAFMmeasurements,itwasshownthatthecitrateionbindsontothesurfaceofgold

causingarepulsiveforce,butwhentheAuCl4- ionwasintroduced;therepulsiveforceisgreatlyreduced

suggestingthatthecitrateisdisplacedbyAuCl4- .ThepreferentialattachmentofAuCl4

- ontothesurfaces

of the gold NPs causes a reduction in the repulsive force of the NPs leading to flocculation. This

flocculation is thecauseof thepurplish-bluecolorusuallyobserved in theTurkevichsynthesis,andas

theAuCl4- isconsumedthroughsurfacereduction,thereisasubsequentincreaseinrepulsionforcesat

thesurfaceasmorecitrateattachesleadingtotheformationofawell-dispersedgoldsol.Injectionofa

mixtureof goldprecursorand citrateontoa goldplate surface thathadbeenpreviously equilibrated

withcitrateshowedthatthesurfacepotentialwentfromrepulsivetoattractiveandremainedattractive

becauseoftheslowreductionoftheAu(III)ontothesurfaceofthegoldplate.116Furtherworkwasdone

on this by Wall et al. using the same techniques but with a thiol coated cantilevers to prevent NP

formation, and the results revealed that the behaviorwas consistentwith amultilayer adsorption of

citrateontothesurfaceofthegoldandbecauseofthelowchargedensity,therewasmostlikelysome

counter-ionpairingwiththecitrate.117ThepossibilityoftheAuCl4- ionadsorbingwithacounter-ionor

withaCl-displacementwaslikelyaccordingtotheresultsofthestudy.

Chapter2

49

Chow and Zukoski suggest a growth mechanism were large fluffy NPs are formed in the initial

stages of the reaction and then subsequently these larger agglomerated NPs break down to form

smallerandmoremonodispersedNPs.91TheysuggestthattheconcentrationofsmallNPsincreasesand

larger NPs decreases through the course of the reaction. The proposed mechanism can be briefly

described as follows. Aurate ions are reduced by citrate to form gold atoms which form clusters

consisting of gold metal, with adsorbed citrate and aurate ions. Since aurate ions are preferentially

bindedovercitratetothesurfaceofthegold,thereisalowersurfacepotentialwhichresultsinaloose

aggregation.Over time the aurate ions are reducedonto the surface to grow theNP, resulting in an

increase in surfacepotential eventually leading to anelectrostatic repulsionwhich causes the loosely

aggregatedNPstobreakoff intosmallerNPs.ThisdescribeshowthelargefluffyNPseventuallybreak

downtoformamonodispersedandstablesol.

Pei et al. monitored the evolution of gold at various reaction times by monitoring the

concentration of the precursor species (AuCl4- ) and the total gold concentration, which allowed the

concentrationofmetallicgoldtobeinferred.93 ItwasfoundthatthereductioninAuCl4- ionwasfaster

withahighercitratetoprecursorratiowhich isnaturalsincethecitratebehavesasareducingagent.

NanowiresremainedinsolutioniftherewasAuCl4- ionsstillinsolutionandinthecaseofratios0.2and

0.3, nanowires persisted until at least 180 min, supporting the idea that the AuCl4- does lead to an

aggregationofNPsassuggestedbythestudiesabove.AssoonasAuCl4- wasconsumed,thebluecolorof

thenanowiresdisappearedandtheredcolorofsphericalNPswasobtained.HenceexcessiveAuCl4- ion

was important innanowire formation,and itwasobservedthatexposuretohydrogengascausedthe

breakdownofnanowires intoNPspresumablybecauseofa reductionofexcessAuCl4- . It is suggested

thatthemechanismisasfollows:nucleationofsmallgoldparticles,subsequentattachmentoffurther

AuCl4- ontotheparticlesurface,thisAuCl4

- ionresults inanattractiveforcebetweenNPswhichcauses

themtosticktogether,followedbygrowthofnanowiresandlarge2-Dstructuresofnanowires.

Kimlingetal.foundthatthecrystalsizeswithincitratepreparedgoldNPwereusuallysmallerthan

theparticlediameterindicatingapolycrystallinenature.94Thissupportstheideaofnucleationfollowed

by growth through coalescence. They propose a four step process described here briefly as: cluster

formation,agglomerationofclustersintoNPswithadefinedsurfaceplasmonresonance(SPR),fusionof

these NPs into larger entities and finally there is a significant amount of atoms in solution which

continuetoaggregateleadingtotheir incorporationintothelargerentitiesresultinginahomogenous

density and improved symmetry of NPs resulting in a blue shift of the SPR. This growthmechanism

appliedforUV-assistedNPformationandusingascorbicacidasareducingagent,andwasbasedonthe

movementof theSPRwhichstarts froman initialphasewhichblueshifts, followedbyaredshiftand

then finally a blue shift. This behavior could also be explained by a loose aggregation of NPs due to

unreduced AuCl4- on the surface , explaining the red shift, and eventual surface reduction leading to

uniformstableNPs,explainingthesubsequentblueshift.

LiteratureReview

50

Kumaret al. proposed amodel for the formation of goldNPs based on reactions they deduced

from evidence in the literature pertaining to stoichiometry of reactions and suggestions for the

oxidationofcitrateintotheDCAmolecule.118Briefly,themechanismisdescribedbyareductionofthe

Au3+ ion toAu+via theoxidationof thecitratemolecule toDCA,whichsubsequently formsacomplex

withtheAu+.TheDCAorganizestheAu+anddisproportionationofthegoldionsintogoldatomsoccurs.

Theseatomspickup furtherAu+ ionsto form largeraggregatesandfurtherdisproportionationoccurs

leading to larger gold clusters and subsequent formation of gold nuclei. Based on this, a model is

presentedwithmassbalancesforthespecies involved,whichaccounts forthemassofauricchloride,

aurouschloride,citrate,DCAandacetone(acetoneisalsoproposedasareagentwhichcanreduceauric

chloride to aurous chloride). A population balance model is also included so that the number

concentrationofparticles,whichplaysaroleintheconsumptionofprecursorthroughsurfacegrowth,

can be deduced. Kinetic rate constants for each step of the proposedmechanism are then obtained

throughfittingtotheexperimentaldatapresentedbyFrens.89Areasonablefittotheexperimentaldata

in the literature is obtained, suggesting that the proposedmechanism is able to account for at least

some of the physical mechanisms occurring in the synthesis of gold NPs in the Turkevich synthesis.

Interestinglythereisnosuggestionofparticleaggregationtoformwirelikestructuresandsubsequent

reduction intomonodisperseNPswhich is observedexperimentally bymany studiesdiscussed in this

section.

Jietal.proposeamechanismfortwodifferentsetsofratiosofcitratetogoldprecursor,onefor

lowcitratetogoldratios(<3.5)andtheotherforhighcitrateratios(>3.5),showninFigure2-21.95At

high citrate ratios,where the pH is closer to neutral there is nucleation followed by a slowdiffusion

controlledgrowth,evidencedbysmallsphericalNPsatthe initialstagefollowedbygrowth into larger

spherical NPs. At the lower citrate ratios, dense nanowires with fatter heads and thinner tails were

observedwhicheventuallyleadtosphericalNPs.Itissuggestedthisisfarmorelikelybecauseofintra-

particleripeningratherthanthroughthecleavingofmaterial fromthenanowireswhichthengrowto

largersphericalNPswhichissuggestedbyPongetal.119Thisisbecausetheevolutionfromnanowiresto

spherical NPs takes much longer than the reduction of gold precursor into gold metal, hence there

wouldbea lackofmonomersavailable forgrowthofanycleavedNPsandmostof theprecursorhas

alreadybeenutilizedtoformthenanowirestructures.Thereisalsoanobservedfatteningofthehead

andthinningofthetailovertimewhichsupportstheideaofintra-particleripening.Themechanismwas

found to be controlled by the reactivity of the precursorwhichwas determined by its speciation. By

tuningthepH, itwasfoundthatthereactivityofprecursorcorrespondedwiththeswitchingpointsof

thegoldioncomplexesi.e.reactivityfellasthepHincreasedasaresultofincreasingthehydroxylation

of thegold ion fromAuCl4- toAuCl3OH

- toAuCl2OH2-.This reactivity is suggested tobe the reason for

nanowirelikestructuresatlowerpHandsphericalNPsathigherpH.Sivaramanetal.alsofoundtherate

ofreductionofgoldspecieswashighestwhenthetetrachloroauricacidwaspresentinitsmostreactive

form(i.e.AuCl4- ).102Theextentofprotonation/deprotonationofcitratemaybeimportantalso,sincethe

pointatwhichaminimuminsizeoccurredwasatpHaround6.5,correspondingtothethirdpKavalueof

6.4forcitricacidwhereitiscompletelydeprotonated(Ct3-).

Chapter2

51

Figure 2-21:Mechanism suggested by Ji et al. for gold NP synthesis. Two pathways exist dependent on pH.95


Polteetal.observedthenucleationandgrowthofthegoldNPsusingtheTurkevichsynthesisat75and

85°CthroughtheuseofUV-Vis,SAXSandXANESanalysis.120Incontrasttomechanismswhichsuggest

theformationof largerNPswhicheventuallyformsmallerandmoresphericalandmonodisperseNPs,

therewerenolargerNPsobservedinthisstudy.Insteadthemechanismisproposedtobeconsistingof

fourmainsteps: the reduction followedbynucleationofgold, thecoalescenceofgoldnuclei to form

largergoldNPs,slowgrowthof the largergoldNPsthroughreductionofprecursorontoexistingNPs,

and finally amore rapid growth onto existingNPs possibly because of an autocatalytic effect (Figure

2-22). Polteet al. alsoobserved a coalescenceof smallerNPs to form largerNPswhenusing sodium

borohydrideasareducingagent.121GoiaandMatijevicusediso-ascorbicacidtoreducetetrachloroauric

acid and found a coalescence of gold NPs of a crystal size of 30 nm which formed much larger

micrometer sized particles.122 Park et al. modelled the process of burst nucleation followed by

aggregationofprimaryparticlestoformlargersecondaryparticlesandthemodelwasfoundtosensitive

tothesurfacetensionoftheparticlesandsinglet-singletaggregationrate.123

Figure2-22:MechanismofgoldNPformationsuggestedbyPolteetal.Reprinted(adapted)withpermissionfrom

Ref.120.Copyright(2010)AmericanChemicalSociety.

Wuithschicket al. investigated themechanism from the speciation of reagents aspect and answered

somequestionspertainingtothecontrolofthesizeoftheNPs.104Theypresentasuggestedmechanism

forgoldNPformationfortheTurkevichsynthesis,basedonthatproposedbyPolte(Figure2-23).124The

main suggestion is that the speciation of the AuCl4- controls the nucleation of the gold NPs which

LiteratureReview

52

ultimatelydeterminestheirfinalsize.ThedegreeofhydroxylationoftheAuCl4- affectsthereactivityas

highlighted by Jiet al.,95 and that the Turkevichmethod producesmonodisperseNPs because of the

change inspeciationoftheprecursorwhichcreatesaseparationofnucleationandgrowthasAuCl4- is

bothconsumedandtransitions intomorehydroxylated formsaspHgoes fromacidic toneutral.They

notethatoncethenucleation isover, thekineticsofgrowthhas littleeffectonthe finalsizewhich is

determinedinthenucleationphase,suggestedtobeoftheorderof30slongfortheconditionstested.

Theyemphasizethat,contrarytothemajorityofstudieselucidatingthemechanismofgoldNPsynthesis

usingtheTurkevichmethod,thereisnoaggregateoragglomerateformationleadingtotheformationof

wire like structures according to the SAXS analysis by Polte et al.120 The bluish-gray color which is

observed in the Turkevich synthesis is suggested to be due to an optical alteration of gold NPswith

precursor ions adsorbedonto the surface rather thanbecauseof an aggregationor agglomerationof

NPs. Plech et al. synthesised NPs using a modified Turkevich method in which X-rays were used to

inducenucleation.125They introducedamixtureofchloroauricacidandtrisodiumcitrate intoanX-ray

capillary, where it was exposed to X-rays and formed NPs. In the early stages of the process, they

suggestabimodaldistributioncentredaround7.8and3.9nmat38s,and9and4.6nmat83s.Following

this period, the data suggests that the particles are close in distance and that there is clustering of

particles to bigger aggregates (20-40 particles). Finally after 2h, the data can be fitted by isolated

particleswithafewclustersstillpresent.ThefinalsizeoftheNPswasnotdifferenttotheaggregates

detectedintheearlierstages,supportingtheideaofcoalescenceofsmallerNPstoformlargerones.

Figure2-23:MechanismofgoldNPformationintheTurkevichsynthesissuggestedbyPolte.Ref.124–Publishedby

TheRoyalSocietyofChemistry.

Kettemannetal. investigated the importanceof the speciationof thecitrate species in theTurkevich

synthesis.106 Depending on the pH, the protonation/deprotonation of the citrate species changes,

increasingpHfromacidictobasicdeprotonatesthespecies.TheydeducedthattheHCit2- wasthelikely

species which reduced the AuCl4- and if the synthesis is manipulated to ensure a reproducible

concentrationofHCit2- ,thesynthesisbecomeshighlyreproducible.HAuCl4

- wasalsopronetodifferences

Chapter2

53

from batch to batch, it was found that boiling and refluxing the precursor for 1 h enabled

reproducibility.

Muchwork has been carried out to understand themechanisms involved in goldNP formation,

with particular focus on the reduction of gold precursor using citrate at elevated temperatures. The

colorchangesobserved in thesynthesiscanbest summarize themechanisms. Initially there isacolor

change from the yellowofAuCl4- to clear,which ismost likely the transitionof theAu3+ ion to some

intermediatesuchasAu+assuggestedbyKumaretal.118 Followingthis,apurple-bluecolor isusually

observed.Theconsensusisthatthiscolorarisesfromflocculationorlooseaggregationofnewlyformed

goldnuclei,observedtobelargefluffyNPs,91orwirelikestructures.93,95Theexplanationofthisseems

to arise from thepreferential binding of gold precursor to the surface of gold nuclei over the citrate

molecule,ashighlightedbytheworkofBiggsetal.andWalletal.115-117Thisthenleadstothecolorof

thesolutiondeepeningandeventuallyturningrubyred,indicatingtheformationofmonodispersedgold

NPsof10-20nm.This is suggested tobecausedby theslowsurface reductionof thegoldprecursor,

whicheventually leads to increasing stabilityofNPsas citratebegins to attachonto the surface. This

increasedstabilitycausesarepulsion,whichenablestheflocculatedNPstobreakapart.Thereseemsto

beanagreementthatacoalescenceofnuclei inthe initialphasetoformsmallgoldNPs, indicatedby

their polycrystalline nature and the SAXS studies by Polte et al. and Plech et al.120, 125 There is a

suggestionthattheformationofwirelikestructuresorlargerNPsdoesn’texistinsolutionandthatthe

purplebluecoloriscausedbyanopticaleffectofgoldprecursorbindingontothesurfaceofgoldNPs.104

However, it is still seems like flocculation isa strongpossibilitybecauseof thepreferentialbindingof

goldprecursoronthesurfaceofgoldNPs,whichwouldcreateattractiverather thanrepulsive forces.

ThespeciationofthecomponentsintheTurkevichsynthesishavealsobeenidentifiedtobeimportant

indeterminingthemechanisms,reactionsratesandresultantNPsize.

54

3 Synthesisofsilvernanoparticlesinamicrofluidiccoaxialflowreactor

The disadvantage of using reactorswith a high surface area to volume ratio is that there is a higher

chanceoffoulingandaccumulationofmaterialonthewallsofthechannel.Hence,microreactorswhich

are not easily susceptible to accumulation ofmaterial on theirwalls are desirable. There are various

studieswhichimplementeddifferentstrategiestoavoidfouling,suchasthesegmented-flowapproach

and the reduction of wall interactions through pH alteration and surface silanization of the channel

walls.76, 126, 127One typeof design that alleviates theoccurrenceof fouling is the coaxial flow reactor

(CFR)whichallowsaninnerstreamofreagenttobesurroundedbyanouterstream,creatingareaction

interfacebetweenthestreams.35,36,128-130TheCFRislesssusceptibletofoulingbecausethereductionof

precursorionsandnucleationofmetalNPsoccurattheinterfacebetweentheinnerandouterstreams,

rather thannear the channelwalls. This reduction in fouling relieson the flowprofileof the channel

remaininginastablelaminarregime.Thishasadverseeffectswithregardstomixingefficiency,unlessa

reductionofdiffusiondistancesorsufficientcontacttimeisapplied.

ExamplesofCFRsarefoundinthestudiesbyTakagietal.whoproducedtitaniaNPsusinganaxial

dual-pipe microdevice.128 Using various flow conditions and tuning of solvent properties such as

viscosityandsurface tension, the flowcharacteristics in theirdevicewere tunedand led to increased

controlovermass transfer.Themass transferof the reagents in themicrodevicedetermined the size

and monodispersity of the NPs which was demonstrated by using different solvent types, tube

diametersandprecursorconcentrations.Makietal.employedaconcentric-axledual-pipemicroreactor

to synthesize zirconia NPs,128, 129 and showed that increasing residence time resulted in larger NPs.

Abou-Hassanetal.usedacoaxialflowdeviceoperatedunderlaminarflowconditionstosynthesizeiron

oxide, goethite and Fe2O3@SiO2 core/shell NPs.35, 36, 130 7 nm iron oxideNPswere synthesized in the

coaxialflowdeviceandpolydispersitydecreasedfrom35%inatypicalbatchsynthesistoca.20%inthe

coaxial flow device.130 Goethite NPs were synthesized using the coaxial flow device as a nucleation

sectionbeforealongresidenceloopinatemperaturecontrolledbath.ThegoethiteNPswereproduced

witharesidencetimeofaround15minasopposedtoseveralhoursusingtraditionalbatchmethods,

andwere also smaller than thosemade in batch vessels.35 Similarly, amultistep approach using the

coaxialflowdeviceswasusedtosynthesizeFe2O3@SiO2core/shellNPswithinamuchshortertimescale

ascomparedtobatch.36Limetal.operatedaCFRintheturbulentregimetoproducevariousNPssuch

asironoxidewithanabilitytotunethesizethroughchangingtheflowconditions.131Schüleinetal.used

amicrocoaxial-injectionflowmixerforthesynthesisofnickelNPs.Theprocessforsynthesizingnickel

NPsquicklyblockedcommerciallyavailablemicrostructuredmixersandthecoaxial-injectionmixerwas

usedinapilotplantscaleprocessmainlybecauseitcouldavoidfoulingandblockageissues.132

Synthesis of silver NPs using silver nitrate, sodium borohydride (with sodium hydroxide) and

trisodiumcitrateinacoaxialflowreactorinthisstudyhasnotbeenreportedintheliterature.Shirtcliffe

etal.reportedthesynthesisofsilverNPsusingsodiumborohydrideandsodiumhydroxideasareducing

agent for silvernitrateusingaPTFE flowchamberandalsousingEppendorfpipettes tomix reagents

directlyintoplasticcuvettes.65Wagneretal.synthesizedsilverNPsinasplitandrecombinemixerusing

Chapter3

55

borohydrideasareducingagentforsilvernitrateanddespitefoulingwereabletocontrolthesizeofthe

NPsbytuningtheborohydrideratio,obtainingNPs intherangeof10-20nmforborohydridetosilver

nitrate ratios ranging from 3 to 40.71 Further studies using borohydride as a reducing agent for the

reductionofasilverprecursor to formsilverNPs in theaqueousphaseweremainly inbatchreactors

withNPsizesrangingbetween1-100nm.5,64,67,69

Themainobjectiveofthiswork istostudyhowthesizeandmonodispersityofsilverNPscanbe

controlled by changing the operating parameters of flow rate and concentration of ligand (trisodium

citrate) andprecursor (silver nitrate). Themotivation for using a CFRwas to avoid fouling issues and

blockages,anissuewhichwasencounteredwhensynthesisingNPsusingthesamechemicalsystemina

split and recombine type micromixer (see Appendix B, Figure B1 for images of fouling in such a

micromixer).

3.1 Experimental

3.1.1 Chemicals

Silver nitrate (AgNO3, 0.01 M stock solution), trisodium citrate (HOC(COONa)(CH2COONa)2 ·2H2O,

powderform)andsodiumborohydridesolution(NaBH4,~12wt%in14MNaOHstocksolution),were

obtainedfromSigmaAldrichCompanyLtd.,UK.Allchemicalswereusedwithoutfurtherpurificationand

solutions were prepared with ultrapure water (resistivity 15.0 MΩ·cm) and diluted to the desired

concentrationsbeforebeingintroducedintotheCFR.

3.1.2 ExperimentalSetup

Syringe pumps (Pump 11 Elite OEM Module, Harvard Apparatus) were used in the experiments to

deliverthetwostreamsofsilvernitrate/trisodiumcitratesolutionandsodiumborohydridesolutionto

theCFR.TheCFRusedinthisstudyconsistedofasmallinnertubewhichwasinsertedintothecenterof

alargeroutertube(Figure3-1).Boththeinnerandoutertubesweremadeofglassandinnertubesof

twodifferentinternalchanneldiametersof0.556mm(0.8mmexternaldiameter)and0.798mm(1.09

mm external diameter) were used in separate experiments. The outer tube had an internal channel

diameterof2mm.ThedistancefromtheinnertubeoutlettotheoutletoftheCFRwas130mm.A1.59

mmPTFEtubesleevewithasuitable innerdiameter isplacedaroundtheinnertubetokeepthetube

stablewithinthelargeroutertube.ThecapillarieswereconnectedtogetherusingaT-piececonnector

(0.508mmthru-hole,UpchurchScientific)whichwasdrilledtoasizeof2mminternaldiameter(I.D.)to

allow the inner tube to be inserted into the outer tube. The flow ratio between the inner andouter

stream,Qout:Qin,wasfixedat1:1andtheCFRoperatedinthelaminarflowregimeforallexperiments.

Synthesisofsilvernanoparticlesinamicrofluidiccoaxialflowreactor

56

Figure3-1:Schematicof thecoaxial flowreactorsetup. Insertshows flowvisualizationof laminar flow inside the

coaxialflowreactorwithbluedyeflowingthroughtheinnertubeandwaterflowingthroughtheoutertube.

3.1.3 Synthesisofsilvernanoparticles

Atypicalsynthesiswasas follows:silvernitrateandtrisodiumcitrateweremixedtogether insolution

andintroducedviatheinnertubeoftheCFR,whilethesodiumborohydridesolutionwasintroducedvia

theouter tubeof the reactor. Fresh trisodiumcitrate solutionwasmadebydissolving thepowder in

ultrapurewater before each synthesis. All reactionswere carried out at room temperature (typically

between 22-24 °C). All concentrations, unless otherwise stated, are concentrations of the individual

streams being introduced into the CFR. Since the sodium borohydride was stored in 14 M sodium

hydroxide, the concentration of sodium hydroxide was 3.21 times higher than the stated sodium

borohydrideconcentrationinallcases.

The literature shows that borohydride can supply up to eightmolar equivalents of electrons to

reduce the silver ions to silver metal.133-135 The amount of electrons released from the borohydride

molecule depends on whether the formed H+ ions react with H- ions from the molecule to form

hydrogengas,whichwouldlimittheelectronsavailableforreducingmetalions.Highlybasicconditions

reducetheformationofhydrogengashencefreeingupmoreelectronsforreductionofthemetalion.In

this study sodium borohydride was stored and used under basic conditions which should produce a

higher amount of electrons for reducing the metal ions. Samples were stored for months without

noticeableformationofhydrogenbubbleswhereaswhensodiumhydroxidewasnotusedtoarrestthe

hydrolysisreaction,hydrogenbubblesformedwithinthesamplesafterashorttime.Theactualamount

ofelectronsutilizedtoreducesilverisunknown,thoughitissuggestedbyShirtcliffeetal.thatsixofthe

availableeightelectronsareutilizedtoreducesilverionsunderbasicconditions.65

Theprimarypurposeofsodiumborohydrideistoreducethesilverionstosilvermetal;however,it

alsoaffects thegrowthof theNPs.Thesurfacechargeof theparticles isaffectedbyborohydrideand

hydroxide ion adsorption and it is well established that the conversion of borohydride to borate in

aqueoussolutionresultsinchangesintheparticlecharge.68,136Underbasicconditionstheconversionof

borohydrideslowsdown.137,138However,trisodiumcitrateisusedastheprimarystabilizingagentinthis

study.Theconcentrationsofthereducingagent(sodiumborohydride)andligand(trisodiumcitrate)in

Chapter3

57

relation to the precursor concentration (silver nitrate) determine the nucleation, growth and

stabilizationof theNPs.Sincetheconcentration is important indeterminingthesizeanddispersityof

resultant NPs, the mass transfer during the reaction becomes important as this dictates the

concentration at which the reaction takes place at any given time during the reaction. The

reproducibility of the synthesis in the CFR over a range of flow rateswas assessed by repeating the

synthesis 4 times for each flow rate (Figure B2 in Appendix B showsUV-Vis analysis of the repeated

syntheses).

3.1.4 Characterizationofnanoparticles

ThesilverNPswereanalyzedwithinanhourofthesynthesisusingaUV-Visspectrometer(USB2000+

Spectrometer andDT-Mini-2-GS light source, OceanOptics). Transmission electronmicroscope (TEM)

imageswerecapturedusinga JEOL1200EX iimicroscopewitha120kVaccelerationvoltage.Carbon

coatedcopperTEMgridswerepreparedwithinanhourof synthesisbypipettingapproximately15µl

sampleontothegridandallowingittodryatroomtemperature.Itwasimportanttokeepthedroplet

smallenoughtoonlypartiallycoverthegrid.Thishastwobeneficialeffects.Firstly,thedryingtimeof

grid is shorter because of a lower volume of solution. The second reason relates to the well-known

‘coffee ring’ effect, where particles are pushed towards the edges of a droplet that is evaporating,

meaningthattheseareaswillcontainaggregatedNPswhicharenotrepresentativeofthesamplebut

rather an artefact of sample preparation. When the grid is prepared in such a way, the ring of

aggregatedNPscanbephysically identifiedonthesmallcoppergridandeasilyvisiblewhenanalysing

thesampleusingTEM.Thisallows thedifferentiationbetweenaggregates thatare in the sampleand

those that are artefacts of sample preparation. The TEM images have in the inset particle size

distributions, average diameter, d, standard deviation, δd, indicating polydispersity and number of

particlescountedtoobtaintheparticlesizedistribution,n.PEBBLESsoftwarewasusedforthecounting

andsizingofTEMimagesofthesynthesisedNPs.

3.2 Investigationonvarianceinnanoparticlemorphologyusingparametricstudies

3.2.1 Effectoftotalflowrate

Experimentswerecarriedout intheCFRwithan innertubechanneldiameterof0.798mmatvarious

flowrates.TheCFRwasusedtosynthesizesilverNPsatconcentrationsof0.2mMsilvernitrate,0.2mM

trisodium citrate and 0.6 mM sodium borohydride at total flow rates ranging from 1 ml/min to 14

ml/min.Themolarratioofsodiumborohydridetosilvernitrateis3:1,buttheratioofelectronstosilver

ionsismanytimesgreaterthanthis.Figure3-2showsTEMimagesofsilverNPssynthesizedat1ml/min,

2.5ml/min,8ml/minand14ml/minalongwiththeirrespectiveparticlesizedistributions.Theparticle

sizesynthesizedatthe lowerflowratesof1ml/min(3.1±1.6nm)and2.5ml/min(3.3±2.2nm)are

statisticallysmallerthanthoseatthehigherflowratesof8ml/min(5.6±2.7nm)and14ml/min(5.4±

3.4nm)while thepolydispersity steadily increases from±1.6nm to ± 3.4nm.However, theparticle

sizesat8ml/minand14ml/minwerenotfoundtobestatisticallydifferentusingat-test(p=0.209).

According toMie theory, forNPs in the rangebetween6-10nm thepeakabsorbance increases

with size (foragivenconcentration)but thepeakwavelengthdoesnot shift significantly.139Fromthe


58

UV-Visanalysis(seeAppendixB,FigureB3),thepeakwavelengthdoesnotchangesignificantly(between

392-393nmforallflowrates)inagreementwiththeory.However,thepeakabsorbanceofthespectra

decreasesminimally from1.35 to 1.27 (statistically different, p < 0.01)with increasing flowrate even

thoughTEMshowsanincreasingaveragesizeofNPsfrom3.1nmto5.4nm.Thiscanbeexplainedby

theincreasedpolydispersityobservedathigherflowrateswhereNPslargerthan10nmareformed.This

results in increasedfullwidthathalfmaximumoftheresonancepeaksathigherflowrates,andsince

themassofsilverusedineachexperimentisconstant,thepeakabsorbancedecreases.

Figure 3-2:TEM images and particle size distributions of silverNPs synthesized at different total flowrates. A: 1

ml/min,B:2.5ml/min,C:8ml/minandD:14ml/min.Concentrationofsilvernitrate0.2mM,trisodiumcitrate0.2

mM,sodiumborohydride0.6mM.0.798mminnertubeI.D.

The concentrations of precursor, reducing agent and ligand are important factors in controlling

nucleation and growth and hence size of the synthesized NPs. The evolution of the concentration

profiles along the length of the reactor is controlled bymass transfer, i.e. themixing characteristics.

Mixing in theCFRoccurs by diffusion, since it operates in the laminar flow regime for the flow rates

employed.Wecanuseasimplerelationship:140

Chapter3

59

t≈(x/2)2

2D

wheretistime,xisthethicknessofthestreamandDisthediffusioncoefficient,toestimatethetime

required for the reagent to diffuse significantly along the radial direction in the CFR (i.e. the mole

fractionbecomesapproximatelyuniformacrossthechanneldiameterintheabsenceofreaction).Using

adiffusioncoefficientof1.7x10-9m2/sforAgNO3,141andastreamthicknessof1mm(halfthediameter

oftheoutertube),atimeof73siscalculated.Usingadiffusioncoefficientof3.5x10-9m2/sforNaBH4

inNaOHsolution,142atimeof36siscalculated.Thevolumeofthereactoris0.4ml,yieldinganaverage

residencetimeof1.75-24.5sforflowratesbetween1-14ml/min.Hence,completemixingdoesnot

occur because the residence times are shorter than the characteristic diffusion times. At higher flow

rates,minimalmixing occurs in the channel as themolecules have very little time for diffusion. The

majority of the reagentmixing in this case is at the outlet of the CFR where droplets form and are

collected.Ahigh reactant concentration in the streams ismaintained in theCFRbecauseof the slow

mixing. This is confirmed when considering the Peclet number of the conditions tested. For a

characteristiclengthof2mm(innerdiameteroftheoutertube)thePecletnumbervariesbetweenca.

3000-42000 for sodiumborohydride and ca. 6000-87000 for silver nitrate. This indicates that for the

flowconditionstested,themasstransportisdominatedbyadvection.Thisisbecausetheresidencetime

withinthechannelisquiteshortcomparedtothetimeneededfordiffusionalongthecross-sectionof

thechannel.Evengiventheadvectiondominatedmasstransport(becausetheresidencetimeisquicker

thandiffusiontime),thesizeanddispersityoftheNPsisstillsignificantlyaffectedwithvaryingflowrate

whichhighlightsthespeedatwhichthereactionstakeplacewiththelimiteddiffusiontimeavailable.

Literature suggests that reaction and nucleation in silver NP synthesis have very fast kinetics

evidencedbythetimescalesofNPappearanceinstudiesbyPolteetal.whichsuggestsreductionoccurs

inlessthan200ms,68andTakesueetal. inwhichnucleationisobservedinlessthan1ms.82Thus,the

reactionislikelytooccuratorveryclosetotheinterfacebetweenthetwostreams,atleastintheinitial

partofthereactorandathighflowrates.Theinnerandouterstreamsbehaveasreservoirsofprecursor

and reducing agent respectively which supply the necessary reagents at the interface region for

reduction of silver ions to silver metal and subsequent clustering and stabilization. Comparing the

averageresidencetimeswithcharacteristictimesfordiffusion,wecanconcludethatathighresidence

time(24.5s)mostsilvernitrateintheinnerstreamwouldbeconsumed,becauseofthefastkineticsand

sufficientdiffusiontimeoftheborohydride(36s).Thisislikelynottobethecaseatlowresidencetime

(1.75s).

Higher flow rates resulted in the appearance of larger silver NPs. This may be rationalized as

follows.The reactionoccurringat the interfacewithin theCFRresults in silveratomsbeing formed in

thatregion.Theconcentrationofsilveratomsincreasesuntiltheyformclusters(synthesismechanisms

in the literature strongly suggest that the metal atoms form clusters which coalesce to form small

NPs).68, 82, 143 A smaller amount of silver precursor is consumed and smaller amounts of clusters are

formedwithintheCFRatlowresidencetime.Ahigheramountofsilvernitrateisleftunreactedatthe

outletoftheCFR,wheredropletsformandmixingandconsumptionofreagentscontinuewithinthem.


60

Mixing there is through recirculation patterns as the droplets form. Even though these convective

patterns promote faster mass transfer than diffusion, they result in non-homogenous concentration

areas. Thus, the larger amount of remaining silver ions reacts and moves to areas where the

concentration of silver nitratemay be high relative to borohydride. The resultant NPs would have a

lowersurfacechargebecauseofthelowamountofborohydrideionswhichadsorbontotheirsurface.

ThisinturnallowstheNPstoaggregateandgrowtoalargersize,sinceitisestablishedthatadecrease

ofsurfacechargeoftheNPs(mostlikelybecauseofborohydrideiondepletion)causesNPstogrowtoa

larger size.136 The appearance of higher amount of larger NPs, further gives rise to the increased

polydispersity observed. The increasing Peclet number (with increasing flow rate) also quantifies the

diminishing significance of the diffusionmechanism on themass transportwith increasing flow rate,

furthersupportingtheideathatthereislessreactionoccurringwithinthechannelathigherflowrates.

Aninterestingobservationwasthatalayerofsilverformedontheinnerwallneartheoutletofthe

innertubethroughwhichthesilvernitratewasflowing.Thisfoulingoccurredontheouterwallofthe

innertubewhensilvernitratewasflowingthroughtheoutertube.Foulingonlyoccurredonthesideof

theinnertubewheresilvernitratewasflowingi.e.asilvernitraterichzone(seeFigureB4andFigureB5

in Appendix B for images of fouling on the channel walls). The reason fouling did not occur on the

sodiumborohydride sideduringexperimentsmaybebecause silvermetal atomsproduced in sodium

borohydriderichzoneswouldhaveahighnegativesurfacechargeandhencecannotgrowanddeposit

on the negatively charged glass channelwall in the sodium borohydride stream.144 This is consistent

withthelargerNPsobtainedathigherflowrates,whichincreasesthepolydispersity.Anotherpointto

noteisthatcitratemoleculeswerealsoavailableinthesilvernitratestream,buttheydidnotprevent

thedepositionofsilvermetalontheinnertube.

3.2.2 Effectoftrisodiumcitrateconcentration

TheeffectofsurfactantconcentrationontheNPmorphologywasinvestigatedbyvaryingthetrisodium

citrateconcentration.Thesilvernitrateandsodiumborohydrideconcentrationswerekeptconstantat

0.1mMand0.3mMrespectively.Theroleofsodiumborohydrideandwhy itwas insuchhighexcess

wasdiscussedpreviously.Trisodiumcitrateconcentrationwasvariedbetween0.025mMand1.5mM.

Duetothesehighcitrateconcentrations,thesilvernitrateandsodiumborohydrideconcentrationwas

reduced (though still using a 3 to 1 ratio of sodium borohydride to silver nitrate) to keep the ionic

strengthaslowaspossible.Theflowrateofeachstreamwas1.25ml/minmakingatotalflowrateof2.5

ml/min. Figure 3-3 shows TEM images and particle size distribution of the silver NPs synthesized at

variouscitrateconcentrations (0.025mMto0.75mM)withaveragediameterandstandarddeviation

foreachconcentration.ItcanbeseenthatthepolydispersityandaveragesizeoftheNPsdecreaseswith

increasingcitrateconcentrationbecauseofthereductionintheamountoflargefusedNPspresent.

Figure3-4showstheUV-VisspectraofthesilverNPsamplesforeachcitrateconcentrationtested.

The resonance peaks at lower citrate concentration are characterized by a lower plasmon peak

absorbance and a higher absorbance at longer wavelengths; and as the citrate concentration is

increasedupto0.25mMtheplasmonpeak increasesandthepeakathigherwavelengths is reduced.

The tail in the resonancepeak is indicativeof thepresenceof larger fusedNPs,asconfirmedbyTEM

Chapter3

61

imaging at lower citrate concentrations, suggesting citrate plays a key role in reducing formation of

largerfusedNPs.Increasingtheconcentrationofcitrateabove0.5mMupto1.5mMdoesnotalterthe

synthesized silver NP characteristics. This shows that a minimum citrate concentration is needed to

obtain relativelymonodispersedNPs and to prevent the formation of larger fusedNPswith irregular

shapes.Henglein andGeirsig also showed in a studyusing radiolysis for the reductionofAgClO4 that

largerfusedNPswereformedwheninsufficientamountsofcitratewereused(theyalsoshowedlarge

fused NPs when the citrate concentration was too high).83 Further qualitative analysis of the TEM

images shows that at lower concentration, there are smaller NPs which surround the larger NPs.

BecauseofthehighcontrastofthelargerfusedNPs,thesesmallerNPSmaybehardertoidentifysince

theydonotyieldashighofa contrast.Table3-1 shows the fullwidthathalfmaximum (FWHM)and

peakwavelengthofthesilverNPssynthesisedatthevariouscitrateconcentrations,obtainedfromthe

UV-VISspectra.AsexpectedtheFWHMdecreaseswithincreasingconcentration(lowerFWHMindicates

lowerdispersity),inlinewiththeTEMimagesshowingdecreasedpolydispersitywithincreasingcitrate

concentration.Interestingly,thepeakwavelengthshowsincreasewithincreasingcitrateconcentration

(higherpeakwavelengths indicate increasingNP size). This is obviouslynot the caseaccording to the

TEMimagesaswellasthelargeshoulderpeaksintheUV-Vis,butitdoessupporttheideathatatlower

citrate concentrations, there is a possibility of smaller NPs existing (in particular, smaller than the

smallestNPspresentinthesynthesisathighercitrateconcentration).


62

Figure 3-3: TEM images and particle size distributions of silver NPs synthesized at different trisodium citrate

concentrations. A: 0.05mM , B: 0.1mM , C: 0.25mM andD: 1.5mM. Concentration of silver nitrate 0.1mM,

sodiumborohydride0.3mM.Totalflowrate2.5ml/min,0.556mminnertubeI.D.

Chapter3

63

Figure 3-4:Absorbance peaks of silver NPs synthesized at various trisodium citrate concentrations in the range

0.025-1.5mM.Total flow rate2.5ml/min, concentrationof silvernitrate0.1mM, sodiumborohydride0.3mM.

0.556mminnertubeI.D.

Table3-1:FWHMandpeakwavelengthofsilverNPsatvarioustrisodiumcitrateconcentrationsintherange0.025-

1.5mM.Totalflowrate2.5ml/min,concentrationofsilvernitrate0.1mM,sodiumborohydride0.3mM.0.556mm

innertubeI.D.

Citrateconcentration(mM) FWHM(nm) Peakwavelength(nm)

0.025 96 381

0.05 91 379

0.1 84 385

0.25 77 389

0.5 77 393

1 75 392

1.5 77 392

3.2.3 Effectofsilvernitrateconcentration

TheeffectofprecursorconcentrationontheNPsizeandpolydispersitywasinvestigatedbyvaryingthe

concentrationofsilvernitrate.Thetrisodiumcitrateandsodiumborohydrideconcentrationswerekept

constantat0.5mMand0.3mMrespectively.Thesilvernitrateconcentrationwasvariedbetween0.05

mMand0.4mM.Sodiumborohydrideisexpectedtoreduceallsilverionsevenatthehighestprecursor

concentrationusedsinceitisexpectedtosupplyanexcessofelectronsforreduction.Theflowrateof

eachstreamwas1.25ml/minmakingatotalflowrateof2.5ml/min.TheBeer-Lambertlawappliedfor


64

synthesizedAgNPshavingpeakabsorbance valuesbelow1.5. Therefore, sampleswerediluted if the

absorbancewasabovethisvaluefornormalization.

Figure 3-5 shows TEM images of the silver NPs along with their respective particle size

distributions.Thesizeincreasedfrom3.7±0.8nmto9.3±3nmwithincreasingsilverconcentration.It

can be seen that small and relatively monodisperse NPs form at the lowest concentration with a

transitionintoamorepolydispersebimodaldistributionoccurringat0.15mMsilvernitrate.Thehigher

polydispersitymayberelatedtoalargerextentofreductiontakingplaceinthedropletsattheexitof

the CFR, as discussed earlier. Figure 3-6 shows the UV-Vis spectra for each synthesis carried out at

various silver nitrate concentrations. The peak absorbance increases with increasing concentration

showingalineardependence.TheincreaseismostlikelybecauseofahigherconcentrationofNPsand

increasedaverage sizeof theNPs (absorbance increaseswith size forNPsbelow10nm).139Table3-2

shows the FWHM and peak absorbance of the silver NPs synthesised at various silver nitrate

concentrations,obtainedfromtheUV-Visspectra.Atthelowestconcentrationthespectraisdiffuseand

hasalargerpeakwavelength(whichindicateslargeandpolydisperseNPs).However,theNPSare3.7nm

accordingtoTEM.Atthissize,theNPswouldshowverylittleabsorbance,sotheabsorbanceshownin

thespectracouldbe fromsmallamountsof largerNPswhichwouldhaveamuchhigherabsorbance.

The largeFWHMcouldbebecause the largerNPs thatexist in thesolutionarepolydisperse.Thiscan

explain to an extent the FWHM and peak wavelength at a concentration of 0.15 mM also. With

increasing concentration after 0.15 mM, the FWHM and peak wavelength shows little variation

indicatingthatthereisminimalchangeinNPsizeandpolydispersity.

Asdiscussedpreviously,highamountsofsilvernitrateinthedropletsformingattheoutletofthe

CFRcanleadtosilvernitraterichzones(duetoimperfectmixing)whichleadtolargerNPsforming(as

wellasincreasedpolydispersity).Inthecaseofthelowestsilvernitrateconcentration(0.05mM)there

isalowamountofsilvernitrateinthedropletsattheoutletoftheCFR,leadingtoalowerpossibilityof

silver nitrate rich zones. At higher silver nitrate concentrations there is a larger amount within the

droplets at theoutlet of theCFR, increasing the likelihoodof reactionsoccurring in silvernitrate rich

zonesleadingtoanincreasedformationoflargerNPsandpolydispersity.

Chapter3

65

Figure 3-5: TEM images and particle size distributions of silver NPs synthesized at different silver nitrate

concentrations.A:0.05mM,B:0.15mM,C:0.25mMandD:0.4mM.Concentrationoftrisodiumcitrate0.5mM,

sodiumborohydride0.3mM.Totalflowrate2.5ml/min,0.556mminnertubeI.D.


66

Figure3-6:AbsorbancepeaksofsilverNPsynthesizedatvarioussilvernitrateconcentrationsintherange0.05-0.4

mM.Total flowrate2.5ml/min,concentrationsof trisodiumcitrate0.5mM,sodiumborohydride0.3mM.0.556

mminnertubeI.D.Inset:Silvernitrateconcentrationvs.peakabsorbanceforresonancepeaks.

Table3-2:FWHMandpeakwavelengthofsilverNPsatvarioussilvernitrateconcentrationsintherange0.05–0.4

mM.Total flowrate2.5ml/min,concentrationsof trisodiumcitrate0.5mM,sodiumborohydride0.3mM.0.556

mminnertubeI.D.

Silver concentration (mM) FWHM(nm) PeakWavelength(nm)0.05 88 402 0.15 76 399 0.2 67 397

0.25 64 395 0.3 68 396 0.4 66 395

3.3 Conclusions

TheperformanceofaCoaxialFlowReactor(CFR)wasinvestigatedforsilverNPsynthesis.Thereaction

wasconfinedtotheinterfacebetweentheinnerandouterstreamsresultinginthegenerationofsilver

clusterswhich increase in concentration along the length of theCFR.Hence the residence time is an

important parameterwhichdetermines the amount of the precursor consumedbefore the advective

mixingwithinthedropletsattheendoftheCFR.Residencetimedeterminestheamountofsilvernitrate

consumedintheCFR,anditisshownthatshorterresidencetimeswherelesssilvernitrateisconsumed,

results in larger NPs being formed. This is possibly because reactions in silver nitrate rich zones (i.e.

reactor outlet at high flow rates) lead to the formation of NPs with low surface charge which have

reducedstabilityandhencehaveatendencytogrowbycoalescence.Forthesamereason, increasing

Chapter3

67

silvernitrateconcentration,whilemaintainingaconstantresidencetimeresulted inan increaseofNP

size and polydispersity. Increasing citrate concentration reduced the amount of larger fused NPs

resulting in a smaller average size and reduced polydispersity. Such fused NPs disappeared above a

citrate concentration of 0.25 mM. Increasing the concentration of citrate past this amount did not

decrease the size of the NPs further, indicating that a threshold is reached. The CFR offers a simple

method tocontrol thesizeof theNPsbecauseof itsuniquedesign.Bykeepingadvectivemixing toa

minimum,thereactionismaintainedattheinterfaceregionbetweenthetwostreams.Thisavoidsthe

problem of blockage of the channels by containing the reaction away from the channelwalls, unlike

other types of microfluidic devices where the reaction can take place close to the channel walls.

68

4 Synthesisofsilvernanoparticlesusingamicrofluidicimpingingjetreactor

The synthesisof silverNPsusing sodiumborohydrideasa reducingagent typically leads to smallNPs

becauseborohydrideisastrongreducingagent,howeverittendstogivepolydisperseNPs.Ourprevious

studyusingacoaxialflowreactorproducedsilverNPsinthe4-10nmrangeandshowedthatmixingis

animportantparameterwhenusingborohydrideasareducingagent.145Splitandrecombinetypeflow

reactors71 and PTFE flow chambers65 have been utilized to synthesize silver NPs using borohydride

obtainingNPs in the rangeof 10-20nm for borohydride to silver nitrate ratios ranging from3 to 40.

BatchreactorsynthesesofsilverNPsusingborohydrideasareducingagentproducedNPsinthe1-100

nmrange.5,64,67,69

Anon-confinedimpingingjetreactor(IJR)withcharacteristicdimensionsinthemicrometerrange

isemployedinthisstudytoinvestigatehowmanipulatingmasstransferaffectstheNPcharacteristics,in

particularthesizeanddispersity.IJRisaparticularlyusefuldevicetouseinthesynthesisofNPssinceit

doesn’t clog because of absence of channel walls. Fouling is particularly problematic in microfluidic

devices since their small dimensions make them potentially vulnerable to blockage, and various

strategiesintheliteraturehavebeenusedtoavoidfoulingsuchassegmentedflow,126coaxialflow128,145

andsurfacesilanization.127

The IJR has foundmost common application in bipropellant liquid rocket engines146 but it also

presents some very distinct advantages for reactions which produce solids. Mahajan and Kirwan

characterizedthemixingtimesinanIJRusingtheBournereactionscheme.147Theysubsequentlycarried

out a precipitation reaction for the synthesis of lovastatin crystals and compared the size of crystals

obtainedwhen themixing timewas i) slowerand ii) faster than the induction time forcrystallization.

They concluded that amoremonodisperse particle size distribution (PSD)was obtainedwhenmixing

timewasfaster.Theyalsoconcludedthat ifthemixingtimeisfasterthantheinductiontime(elapsed

time period between achievement of supersaturation and the appearance of crystals),148 further

improvements to themixing time do not significantly affect the resultant crystals. Erni and Elabbadi

studiedthehydrodynamicpropertiesoftheIJRandcharacterizedvariousflowregimesbeforeusingthe

IJR for an acid induced crystallization of sodium benzoate. They found a decrease in both size and

dispersity with an increasing jet velocity.149 Kumar et al. employed an IJR for the synthesis of

nanocrystallineMgO,aprocesswhichyieldsarigidgelwhichcanquicklyclogmicrochannels.Achange

inimpingementangleandvelocityofthejetswasfoundtoaffectthesurfaceareaofthenanocrystalline

MgO formedand foreachparameter therewasanoptimumthatexisted toobtainmaximumsurface

area.Itissuggestedthatthisoptimumexistsbecauseofachangeinhydrodynamicsintheimpingement

zone,wheretoohighaflowrateorimpingementangleledtonon-optimalmixingconditionsresultingin

areducedcontacttimeforreaction.150Hosnietal.usedimpingingjetreactorsforthesynthesisofZnO

NPs,comparingresultsagainst those formed inabatchstirredvesselandaT-mixer typereactor.The

sizeoftheNPssynthesizedineachreactordependedontheenergydissipationusedformixing,withthe

Chapter4

69

IJRhavingintermediateenergydissipationversustheothertwotypesofreactorsandhenceproducing

anintermediatesizeofZnONPs.151

Confinedimpingingjetreactors(CIJR)areasimilartypeofreactorfortheuseinreactionsinvolving

precipitationofsolids.JohnsonandPrud’hommeemployedaCIJRfortheprecipitationofhydrophobic

organic activesNPs; importantly theCIJRallows forhomogeneous conditionsprior toprecipitationof

NPs (i.e. formation of block copolymer NPs and nucleation and growth of organic actives) which

ultimately separates the effect ofmixing on the final size of the NPs.152 The NP size decreasedwith

increasingmixing efficiency up until a ‘breakpoint’was reached, atwhich point the size of the block

copolymerNPs remained constant. They also observed the same effectwhen using a different block

copolymer,andthe‘breakpoint’correspondedtoaDahmköhlernumberof1.153Marchisioetal.studied

theprecipitationofbariumsulfateNPsinaCIJRandsimilarlyobservedareductioninparticlesizewith

increasingmixingintensity,butnofurtherimprovementswereobservedwhenthecharacteristicmixing

timescalereachedthereactiontimescale.154ReductioninsizeofprecipitatedbariumsulfateNPswith

increasingmixing intensitywasalsoobservedbySchwarzerandPeukertusingaT-mixer.155Siddiquiet

al.usedaCIJRforthesynthesisofironoxideNPswheretheyfoundareductionfrom800nmto200nm

in agglomerate size by increasing the flow rate under high concentration conditions.156 These studies

showthe importanceofmixingforreactionswhich leadtoformationofNPs,sincetheytypicallyhave

reactiontimesthataresmallandhenceneedverygoodmixingefficiencytoapproachmonodispersity.

According to the literatureabove, a reduction inNP size is reportedwith increasingmixingefficiency

attributed to increasing supersaturation levelswhichwould favor a higher nucleation rate andhence

smallerparticles.

The IJRused in thisstudypresentsaconvenientplatformfor thesynthesisofsilverNPsbecause

the reactor has no channel walls in the reaction zone, eliminating the need for lengthy cleaning

procedureswhich are needed in traditionalmicrochannels because of fouling. The IJR can synthesize

silverNPs inahighly repeatablemanner,associatedwith the lackof foulingwhichcanoccur inother

typesofreactorswheresurfacesarepresentnearthereactionzone.Themainobjectiveofthisworkis

toinvestigatetheeffectofmixingonthesizeanddispersityofsilverNPssynthesizedusingtwodifferent

ligands: i)trisodiumcitrateandii)polyvinylalcohol(PVA).Thehydrodynamicsandmixingefficiencyof

theIJRhavealsobeenstudied.

4.1 Experimental

4.1.1 Chemicals

Silver nitrate (AgNO3, 0.01 M stock solution), trisodium citrate (HOC(COONa)(CH2COONa)2·2H2O,

powderform,PVA(molecularweight89000-98000Da),sodiumborohydridesolution(NaBH4,~12wt%

in 14 M NaOH stock solution), potassium iodide (KI, 99%) potassium iodate (KIO3, 98%), boric acid

(H3BO3,99.5%)andsulfuricacid (H2SO4,3Mstocksolution)were obtainedfromSigma.Allchemicals

were usedwithout further purification and solutions were preparedwith ultrapure water (resistivity

15.0MΩ·cm).

Synthesisofsilvernanoparticlesusingamicrofluidicimpingingjetreactor

70


Syringepumps(Pump11EliteOEMmodule,Harvard)wereusedintheflowexperimentstodeliverthe

reagents to the IJR. Figure 4-1 shows a schematic representation of the IJR, which consisted of two

stainlesssteeltubeswithequal internaldiametersof0.25mmor0.5mmalignedatanangle,q=48°.

This was achieved by inserting the tubes into a perspex plate which had 1.5mm (depth andwidth)

channelsmilledata48°angleusingaRolandEGX-400engravingmachine.Theouterdiametersofthe

tubingwas1.59mm.Spacingbetweenthecentersofthefluidjetsatthepointoftubeoutlets,d,was3

mm.

Figure4-1:Representationoftheimpingingjetreactor.

4.1.3 NanoparticleSynthesis

Amixturewasmadebycombiningsilvernitrateandligand(trisodiumcitrateorPVA)intoasolutionand

diluting to the appropriate concentration for input 1 (Figure 4-1) and diluting sodium

borohydride/sodiumhydroxidesolutiontotheappropriateconcentrationforinput2.Sincethesodium

borohydridewas stored in 14M sodiumhydroxide, the concentration of sodiumhydroxidewas 3.21

times higher than the stated sodium borohydride concentration in all cases. The syringe pumps

deliveredreagentstoinput1andinput2attheappropriateflowratestoproducethedesiredjets.As

the two jets collide, the reagents mix followed by subsequent reaction and formation of silver NPs.

Reactionswerecarriedoutat roomtemperature (22-24°C).All concentrationsstatedare thoseat the

inletsbeforeanymixingofreagentsoccurs.

4.1.4 CharacterisationofNanoparticles

ThesilverNPswereanalyzedusingaUV-Visspectrometer(USB2000+SpectrometerandDT-Mini-2-GS

lightsource,OceanOptics)between45to60minaftersynthesis(thesignalofthesampleswerestable

in thiswindowof time). SilverNP sampleswere dilutedwith additional ultrapurewater to bring the

absorbanceintoasuitablerange(i.e.obeyingtheBeer-Lambertlawandavoidingsaturationofthelight

detector)andthedatawerenormalizedsothatthemaximumabsorbanceintheflowraterangetested

representedanabsorbanceof1.TransmissionelectronmicroscopeimageswerecapturedusingaJEOL

1200 EX ii microscope with a 120 kV acceleration voltage. Carbon coated copper TEM grids were

preparedbetween45to60minaftersynthesisbypipettinga5µlsampleontothegridandallowingit

todryat roomtemperature.Carewas takentonotcover theentireTEMgridwithsamplebut rather

only a portion of it (as discussed in Chapter 3). Particle size distributions (insets for each TEM image

Chapter4

71

presented)havethefollowingnomenclature:disaveragediameter,δdisthestandarddeviationofthe

NPdistributionandnisthenumberofparticlescountedtoobtaintheparticlesizedistribution.PEBBLES

softwarewasusedforthecountingandsizingofTEMimagesofthesynthesisedNPs.

4.2 Mixingcharacterizationmethods

For single phase mixing, two types of experimental technique can be considered for mixing

characterisation:dilution-basedmethodsandreaction-basedmethods.157

4.2.1 Dilution-basedmethods

Thesemethodsinvolvethecontactingofaclearliquidwithacoloureddyeandobservingthechangein

concentration of the coloured dye along the channel. Mixing is considered complete when the

concentration of the coloured dye is uniform across the diameter of the channel. There are two

limitations to this kind of characterisation. The channel must be optically transparent for visual

observations using amicroscope to bemade. Because the observation ismade perpendicular to the

channel, theconcentration that isvisuallyobserved isactuallyanaverageof theconcentrationof the

dyeoverthedepthofthechannel.Examplesofthistypeofcharacterisationare:Mixingconcentrated

dye with colourless water solution,158 and dilution of fluorescent species such as fluorescein and

rhodamineB.50

4.2.2 Reaction-basedmethods

Threetypesofreactionexistinthisbranchofcharacterisation:

1. Acid-basereactions:

2. Reactionsproducingacolouredspeciesproduct

3. Competingparallelreactions

Thefirsttwomethodsalsorelyonopticalobservationoftheproduct. Acid-basereactionsrelyonthe

presenceofapHindicatorsuchasmethylorange,bromothymolblueandphenolphthalein. Reactions

producing coloured species include the reductionofpotassiumpermanganate in alkalineethanol and

theironrhodanidereaction.Boththesemethodsgivesimilardisadvantagestodilution-basedmethods

(notbeingabletoobtainconcentrationsacrossthedepthofthechannel,butobtainanaverageofthe

concentrationoverthedepthofthechannel).

Competingparallelreactionsrelyonparallelreactionswhichcompeteforasinglereactant.Oneof

thesereactionswillbefasterthantheother.Thedegreetowhichtheslowerreactionoccursdepends

on the measure of mixing. There are two types of these reactions used for measuring the mixing

performance in microreactors: the Villermaux-Dushman reaction and the second Bourne reaction

scheme (the simultaneous diazo coupling of 1- and 2-napthol with diazotized sulfanilic acid). Both

methods involve measuring the concentration of specific products using UV-Vis spectroscopy, which

allowscertainparameterstoquantifymixingtobecalculatedsuchassegregationindex.Forthisstudy,

theVillermaux-Dushmanreactionswerechosentocharacterizemixing.


72

4.3 Villermaux-Dushmanreactionscheme

TheVillermaux-Dushmantestreactionsystemisawell-knownmethodtocharacterizemixingefficiency

inareactor.159Mixingtimewithinamicromixingdevicecanbeobtainedusingthisreactionsystemby

tuningtheconcentrationsappropriately.TheVillermaux-Dushmannreactionsystemcanbedescribedas

follows:Astoichiometricdefectofsulphuricacidisintroducedtoamixtureofborate,iodideandiodate

ions.Thefollowingreactionsoccur:

H2BO3-+H+↔H3BO3 (4.1) instantaneous

5I-+IO3-+6H+↔3I2+3H2O (4.2) veryfast

I2+I-↔I3- (4.3)

Reaction1ismuchfasterthanreaction2,andsincethereisastoichiometricdefectofacid,onlyreaction

1occurs if there isperfectlyefficientmixing. Inpractice, reaction2 increasesas themixingefficiency

decreasesbecauseiodideandiodatewillreactwithacid ifthelocalconcentrationoforthoborateions

hasbeendepleted(thisonlyoccursifthemixingisnotperfectlyefficient).Reaction2producesiodine,

whichfurtherreactswithiodideionstoformtriiodide(reaction3).Adefectofacidamountisneeded

becausethemeanpHmustbewithinanarrowrangeclosetothevaluefortheiodinedismutationpH

(pH*).IfthepHislowerthanpH*,iodinecanforminacidsolutionirrespectiveofmicromixingeffects,

while on the other hand the pH cannot be too basic since formation of iodine becomes

thermodynamicallyunstable.159Bymeasuringthetriiodideconcentration,theextenttowhichreaction2

occurs can be measured, hence allowing the mixing efficiency to be estimated. Triiodide exhibits a

strongpeakat353nmenablingitsconcentrationtobecalculated.Theextinctioncoefficientoftriiodide

was found to be 23209M-1·cm-1 at 353 nm,which is similar to the values reported in the literature

(detailsonthisprocedurecanbe found inAppendixAandcalibrationcurve isshown inFigureA2).159

The micromixing quality is quantified by the segregation index which is discussed in detail in other

studies.159, 160 The segregation index can be linked to the mixing efficiency through the use of a

phenomenologicalmixingmodel.

4.4 Theinteractionbyexchangewiththemeanmixingmodel

Theinteractionbyexchangewiththemean(IEM)modelwasusedtoestimatethemixingtimefromthe

experimentalresults,andcanbeexpressedasfollows:161,162

dCk,1dt

= <Ck>-Ck,1tμ

+Rk,1 (4.4)

dCk,2dt

= <Ck>-Ck,2tμ

+Rk,2 (4.5)

<Ck>=αCk,1+ 1-α Ck,2 (4.6)

Chapter4

73

whereCisconcentrationofspeciesk,<Ck>istheaverageconcentrationofthespecieskacrossacidand

bufferstreams,Risthereactionrateofspeciesk,tμisthemixingtimeandαisthevolumefractionof

theacidstream.Equations3.4and3.5representtheacidstream(orinjection)andthebufferstream(or

volumeinthetank)respectively,andhencethesubscriptfollowingkintheseequationsrepresenteither

the acid (1) or buffer (2) stream. These equations form thedifferentialmass balance equations that

need tobesolved toobtaina theoretical concentrationof triiodide (or segregation index) foragiven

mixingtimeaccordingtothemodel.Thesegregationindex,𝑋2,isdefinedas:

Xs=YYST (4.7)

whereYistheratioofacidmolenumberconsumedbyreaction2dividedbythetotalacidmolenumber

injected:

Y=2 I2 + I3

-

α(Ho+) (4.8)

andYSTisthevalueofYwhenthemixingprocessisinfinitelyslow(perfectsegregation):

YST=

6(IO3- )0

(H2BO3- )0

6(IO3

- )0

(H2BO3- )0

+1

(4.9)

A curve of mixing time vs. segregation index can be obtained by setting a mixing time and the

concentrationofreagents,followedbysolvingforarangeofmixingtimes.Thedifferentialmassbalance

equationswere solvedusinggPromsModelBuilder3.7.1.Anexample code fora flow reactor canbe

seen inAppendix A. By solving the IEMmodel for a range ofmixing times, segregation index can be

plottedagainstmixingtime.Byperformingtheexperimentsusingtheappropriatereactor,thetriiodide

concentration for the given set of conditions can be obtained and the segregation index can be

determined. The corresponding mixing time is then determined by using the plot generated by the

model i.e. seewhichmixing time correspondswith the segregation index determined experimentally

fromtheplot.Therelationshipbetweenmixingtimeandsegregation indexfor theconcentrationsets

usedtocharacterisemixingtimeintheIJRscanbeseeninFigure4-2.


74

Figure4-2:Segregationindexvs.mixingtimeestimatedusingtheIEMmodelfortheVillermaux-Dushmannreaction

schemefor0.5mm(blueline)and0.25mm(redline)IJRs.ConcentrationsofbuffersolutionwereKI:0.032M,KIO3:

0.006M,NaOH:0.045M,H3BO3:0.09MinbothIJRs.ConcentrationofH2SO4was:0.017Mand0.02Mfor0.5mm

and0.25mmIJRrespectively.

4.5 Experimentaldeterminationofmixingtimeintheimpingingjetreactors

TheVillermaux-Dushmannreactionsystemwascarriedoutinthe0.5mmand0.25mmIJRs.Forinput1,

0.017Mand0.02Mofsulphuricacidwereused(correspondingto0.034Mand0.04MH+)forthe0.5

mmand0.25mmIJRrespectively.Thedissociationofsulphuricacidisconsideredtobealmostcomplete

forpHabove4,whereasifthereactionstakeplaceatalowerpHthentheassumptionofcompleteacid

dissociationwouldresultinoverestimatingthemixingefficiency.CommengeandFalknotedthatthepH

of the solution (acid and buffer combined) will increase as mixing takes place resulting in more

dissociation, while suggesting that if the reaction takes place at pH 2.5, the mixing time will be

underestimated by 25%.160 Input 2, which is considered the buffer solution, had a concentration of

0.032Mpotassiumiodide,0.006Mpotassiumiodate,0.045Msodiumhydroxideand0.09Morthoboric

acid.Figure4-3showsmixingtimevs.Webernumberobtainedforthe0.5mmand0.25mmIJRs.

Chapter4

75

Figure 4-3:Mixing time calculated by theVillermaux-Dushmann reaction scheme vs.Weber number for 0.5mm

(blacksquares)and0.25mm(redtriangles)IJRs.Concentrationofacidsolution(input1)was0.017Mand0.02M

H2SO4 for 0.5mmand 0.25mm I.D. tubing IJR respectively. Concentrations of buffer solution (input 2)were KI:

0.032M,KIO3:0.006M,NaOH:0.045M,H3BO3:0.09MinbothIJRs.

TheWebernumberisdefinedas:163

We=ρu2dσ

whereρ isthedensityoffluid,uisaveragevelocityoffluidbeforeimpact,disthediameterofthejet

(takenasthetubeinternaldiameter)andσisthesurfacetensionofthefluid.Valuesoffluidproperties

usedare those forwaterat roomtemperature.TheWebernumber represents the ratioof inertial to

surface tension forces, and is useful in describing what regime the IJR is operating in. The Weber

number range for the 0.5 mm and 0.25 mm IJRs was 13-65 and 32-176 respectively. The Reynolds

numberisdefinedas:

Re=ρudμ

whereμisthedynamicviscosity.TheReynoldsnumberrepresentstheratioofinertialtoviscousforces,

anddescribeswhether the flow isa laminar, intermediateor turbulent regime.TheReynoldsnumber

rangeforthe0.5mmand0.25mmIJRswas680-1520and760-1770respectively.FromFigure4-3itcan

beseenthatthemixingtimeforbothreactorsissimilarwhenoperatingundersimilarWebernumbers.

ThisindicatesthattheWebernumberisakeyparameterindeterminingthemixingefficiencyofanIJR.

Erni and Elabaddi propose that the mixing time for an impinging jet is proportional to the Weber


76

numberaccordingtoapowerlawrelationship(tm∝We-0.75).149FittingthedatainourstudyforWe<

80, the following relationshipwas found, tm∝We-0.89.Mixing time decreaseswith increasingWeber

numberuptoavalueofca.90whereitreachesaminimumvalueofaround1ms.IncreasingtheWeber

numberpastca.90usingthe0.25mmIJRresults inagradual increase inmixingtime. Ingeneral, the

specificpowerdissipationwithinareactorcanbeusedtoestimatethemixingtimeandseveralstudies

have showna correlationbetween the two.52, 160, 161 The specificpowerdissipation for the IJR canbe

estimatedusingthefollowingrelationships:147

𝜖 = 456 (4.10)

𝑃 = 89(𝑚8𝑢89 + 𝑚9𝑢99) (4.11)

𝑉 = @AB

Cℎ (4.12)

where 𝜖 is overall energy dissipation per unit mass [W/kg], P is rate of overall mechanical energy

dissipation [W] assumed to be equal to the kinetic energies of the two impinging jets,𝜌 is the fluid

density[kg/m3],Visthevolumeoftheimpingementzone[m3],𝑚isthemassflowrateofajet[kg/s],𝑢

isthevelocityofajet[m/s](withsubscripts1and2denotinginput1or2),disthediameter[m]andhis

theeffectivethicknessofthecylindricalmicromixingregion.Itisdifficulttoestimatewhatthecylindrical

micromixingregionwouldbegiventhenon-uniformshapeoftheliquidsheetafterimpingementandits

changingdimensionsdependingontheflowrate.IfdisassumedtobethejetdiameteroftheIJRwhile

the value of h to be 10% of the internozzle distance,147 mixing time can be estimated using the

correlationsuggestedbyCommengeandFalk:160

𝑡F = 0.15𝜖KL.CM (4.13)

where 𝑡F is the mixing time [s]. Using the correlation, the mixing time estimate was found to be

between0.9-2.6msforthe0.5mmIJRand0.25-0.9msforthe0.25mmIJRfortheconditionsusedin

thisstudy.ThemixingtimesfoundusingtheIEMmodelwerebetween1-6.5msforthe0.5mmIJRand

0.9-3.5msforthe0.25mmIJR.ConsideringthatboththecorrelationandtheIEMmodelonlygivean

estimateofthemixingtime,theorderofmagnitudeagreementissatisfactory.Increasingspecificpower

dissipationbyincreasingflowratewithintheflowreactorshoulddecreasemixingtime.Howeverthisis

notthecasefortheIJRwherethemixingtimelevelsoffandbeginstoincreasepastaWebernumberca.

90, most likely because of droplet formation and breakup at the liquid rim of the sheet formed on

impingementofthejets.ErniandElabaddiobservedthatthereisatransitionfromtheliquidsheettoa

fragmentedsheet (unstable rim)ataWe~30ata surfactant concentrationof8.2μMsodiumdodecyl

sulfate(SDS),We~80ataconcentrationof0.82mMandWe~200at82mM.149Theyobservedthatthe

mixing quality deteriorateswhen there is excessive droplet fragmentation; droplets ejected from the

sheethavelittletimeforefficientmasstransferwhichishighestintheliquidsheet(theyestimatethat

droplets took 0.6ms to be ejected from the liquid sheet).Using the fourthBourne reaction scheme,

Chapter4

77

theyestimatedmixingtimeintheirsystemtobe4.9ms,whichisofthesameorderofmagnitudeasour

work.The flowregime inoursystemwassubsequently investigatedtoascertainwhere this transition

occurs.

4.6 Flowregimecharacterizationofimpingingjetreactorusinghighspeedcamera

Theflowregimeofthe0.5mmand0.25mmIJRwasinvestigatedbycapturingimagesoftheIJRduring

operationusingahighspeedcamera(PhotronFastcamSA1.1highspeedcamera).Figure4-4showsthe

flowregime for the0.5mmIJRsystem, for flowratesbetween32-72ml/min (Webernumber13-65).

Fromtheimages,itcanbeseenthattheflowpatternchangesacrosstheflowraterangeinvestigated.

Atthelowestflowratethejetscollideandcoalesce,formingasmallsheet.Astheflowrateincreases,

thesheetincreasesinlengthanddiameter.Furtherincreaseintheflowrateresultsinawavyliquidfilm

surrounding the liquid sheet. This behavior has been observed in other studies where a wavy film

occurs,althoughitisusuallyaccompaniedbydropletsleavingthestablesheetboundedbyaliquidrim.

ThiseffectarisesfromRayleigh-Plateauinstability.164,165Itispossiblethatthesewaves,observedonthe

liquid rim and sheet, arise from small oscillations in the flowdelivered from the syringe pumps. This

wavy nature of the liquid sheet is also transferred into the chain of fluid following the initial sheet

formedbythecollisionofthejets.Theliquidfollowingthecollisionofthejetsremainscontinuousinall

cases for the0.5mm IJR,withnodropletbreakup in the regionvisualized.Figure4-5shows the flow

regimeforthe0.25mmIJRsystem,for flowratesbetween18-42ml/min(Webernumber32-176).At

lowerflowrates,astablesheetandliquidrimareformedatthepointofimpact.Atflowrateabove26

ml/min,anunstable rimwithdropletsbreakingoff themainstructurebegins to form.Fromthe front

viewitcanbeseenthattheflowisalsodevelopingasprayinthatplane.Atthispointtheinertialforces

overcome the surface tension forces of the fluid resulting in droplet breakaway from themain liquid

sheet/rim.Alsoforthe0.25mmIJRsystem,thechainoffluidfollowingtheliquidsheetsafterimpactis

non-continuousafterthefirstfewsecondaryliquidsheetsareformed.

Takingthemixingtimeestimationsandflowregimeobservationsintoaccount,themixingbehavior

ofthe IJRsystemcanberationalized.Therearetwomodesofatomization:reflectiveandtransmitive.

Between these two regions there exists a well-mixed atomization condition.166 In the reflective

atomizationregime,whichoccursatrelativelylowjetvelocities,thejetsessentiallycollideandcoalesce

forming a thin liquid sheet immediately after collision. Deflection of each jet will be equal as they

bounceoneachotherbecauseofequalmomentum. Inthis region, theturbulence in themixing layer

following impingement increases as flow rate increases leading to an increasing mixing efficiency.

However, because of the turbulent nature of the jets therewill be a fluctuation in the trajectory of

droplets/sheet as velocity increases; there is an eventual breakup of the liquid sheet where smaller

droplets break away. At this point the atomization transitions into a transmitive mode, whereby

dropletsareejectedfromtheliquidrimofthesheetbecauseofincreasedmomentum.Thisisobserved

inFigure4-5atWe>90.Thedropletsthatareejectedfromtheliquidrimhavelittlecontacttimeinthe

relativelyefficientlymixedsheetandthemixingefficiencysuffersbecausehigheramountsofdroplets

areejected.TheresultsshowthatbelowaWe~90,weareinthereflectiveatomizationregime.Above

thisWebernumbertheregimetransitionsintoatransmitiveregime.AWe~90seemstoyieldoptimal


78

mixingas confirmedby themixing timeestimation. Theobserved transition is somewhathigher than

thoseobservedbyErniandElabaddiwhoatminimalsurfactantconcentrationobservedtransitionatWe

~30.149Howevertheyuse jetsatanangleof110°witha jetspacingof10mmwhichcouldaffectthe

transitionpoint.

Figure4-4:Flowvisualizationof jetsemitted from0.5mmtubesat total flowrate:A:32ml/min (We:13),B:40

ml/min(We:20),C:48ml/min(We:29),D:56ml/min(We:39),E:64ml/min(We:51)andF:72ml/min(We:65).

Picturesshowsideviewandfrontview.

Chapter4

79

Figure4-5:Flowvisualizationof jetsemittedfrom0.25mmtubesattotalflowrate:A:18ml/min(We:32),B:22

ml/min(We:48),C:26ml/min(We:68),D:30ml/min(We:90),E:34ml/min(We:115),F:38ml/min(We:144)and

G:42ml/min(We:176).Picturesshowsideviewandfrontview.

4.7 InvestigatingtheeffectofwebernumberonNPmorphology

4.7.1 EffectofwebernumberonsilverNPsynthesis1(citratesystem)

TheeffectthattheWebernumberhasonthesilverNPswasinvestigatedinthe0.5mmand0.25mm

IJRsusingtrisodiumcitrateasastabilizingagent.Concentrationsofinput1were0.9mMsilvernitrate,6

mMtrisodiumcitrateandforinput2were1.8mMsodiumborohydride.Totalflowratesforthe0.5mm

IJRrangedfrom32ml/minto72ml/minandforthe0.25mmIJRrangedfrom18ml/minto34ml/min

(repeatabilityofthesynthesisisshowninAppendixC,FigureC1).

Figure4-6showsTEMimagesforNPssynthesizedinthe0.5mmIJRat32,48,64and72ml/min

wheretheaveragediameteranddispersityoftheNPsare7.9±5.8nm,7.7±5.0nm,5.0±2.8nmand

3.4±1.4nm.AveragediameterandpolydispersitydecreasewithincreasingWebernumber(flowrate).

T-testsshowedp-values<0.001whencomparingallthePSDsagainsteachother.Thissuggeststhatthe

PSDsareallstatisticallydifferent.

Figure4-7showsTEMimagesforNPssynthesizedinthe0.25mmIJRat20,24,28and32ml/min


5.1±4.6nm.ItappearswithincreasingWebernumber(flowrate)theNPsreduceinaveragesizeupto

ca. 5 nm. T-tests showedp-values > 0.05when comparing thePSDsof 20 and24ml/min, 24 and32

ml/minand28and32ml/min(allotherp-valueswhere<0.001).ThesesuggestthatthePSDsarenot

statisticallydifferent;however,thisismostlikelydowntothelargepolydispersity.Visualanalysisofthe

TEMimagesrevealstwodifferentpopulationsofNPs,thosethataresmallerthan20nmandthosethat

arelarger.AlthoughthelargerNPswerepresentinsmallfrequencies(notlargerthan15countsforany

flow rate), there were enough present to affect the average size and dispersity calculations of NPs


80

synthesizedat20ml/minand32ml/min,butat24ml/minand28ml/minvirtuallyno largerNPsare

present. If NPs larger than 20 nm are removed from the t-test analysis, only NPs synthesized at 20

ml/minand24ml/minhaveap-value>0.05withallotherp-valuesbeing<0.001. FromthePSDs in

Figure4-7itcanbeseenthat,astheWebernumberisincreased,theaveragesizegetssmallerandthe

dispersionisreducedinthe<20nmrange.However,thepolydispersityishigheratthelargestWeber

numbereven though ithas the leastdispersecurve forNPs<20nm.This isbecausesynthesisat the

highestWebernumberproducedthehighestfrequencyoflargerNPs(>20nm),andthedisparityinsize

betweenthelargerandsmallerNPsishigherascomparedtoNPsproducedatlowerWebernumbers.

Figure4-8showsthepeakabsorbancevs.WebernumberobtainedfromUV-Visspectroscopyfor

the silverNPs obtainedusing the 0.5mm IJR (We= 13-65) and the 0.25mm IJR (We= 32-115). It is

worth noting that theUV signal did not change significantly 24 hours after synthesis (~1% change in

peak absorbance). For each jet diameter, the peak absorbance in general decreases with increasing

Weber number. Since the NP size identified from TEM images was primarily below 10 nm, the

decreasingpeakabsorbanceispossiblybecauseofadecreaseinsizeoftheNPs.ForsilverNPsthemolar

extinctioncoefficientincreaseswithsize,167andabsorbanceincreaseswithincreasingsizebelowthe10

nmrange,aspredictedbyMietheory.139Table4-1andTable4-2showtheFWHMandpeakwavelength

forsilverNPssynthesisedusing trisodiumcitrateasastabilisingagentatvarious flowrates in the0.5

mmand0.25mmIJRrespectively.TheFWHMforthe0.5mmIJRincreasessignificantlyafteraflowrate

48ml/minwhilethepeakwavelengthremainsfairlyconstant.ThereisagradualincreaseintheFWHM

with increasing flow rate for the 0.25mm IJR, while the peakwavelengths again stay approximately

constant.

TheTEMimagesandpeakabsorbanceoftheNPswouldsuggestthattheNPsdecreaseinsizewith

increasingWeber numberwhile the dispersity shows amore complex relationship. Firstly the results

obtainedfromthe0.25mmIJRwillbediscussed. IntermsoftheNPsynthesis, thesizeanddispersity

changes can be explained by taking into account mixing characteristics of the impinging jet at the

particularWebernumber.TheNPsbelow20nmarereferredtoassmallerNPsandthoseabove20nm

arereferredtoas largerNPs.AtlowWebernumbers,thesmallerNPstendtobeofalargerandmore

polydispersenature.AstheWebernumberincreases,theaveragesizeoftheseNPsdecreaseswhilethe

PSDsbecometighter.Thiscanbeexplainedthroughthemixingcharacteristicswithinthe liquidsheet,

whereincreasingturbulenceresultsinashortermixingtime.Borohydridehasastabilizingeffectonthe

NPs,136, 139, 145 and can also supply a high amount of electrons permole (up to amaximum of eight

electrons in theory).133-135Therefore, if there isefficientmixingwithinthe liquidsheet, therewillbea

sufficientsupplyofsodiumborohydrideinthelocalregionswherereactionistakingplace,resultingin

increasedstabilizationandsmallerNPs.Ifthereareinsufficientamountsofborohydridemolecules,less

stableNPs,whichcangrowrelativelyeasily,are formed.Poor localmixingefficiency,which results in

low local concentrations of borohydride,will lead to larger NPs, since borohydride can reducemany

times its own molar equivalent of silver ions. The formation of larger NPs can be compared to the

formation of iodine in the Villermaux-Dushmann reactions, which is caused by a local lack of

Chapter4

81

orthoborate ions. Hence, increasing themixing efficiency (Weber number) decreases the size of the

smallerNPs.

It issuggestedthatthe largerNPsareformedinsilvernitraterichregions.Thepresenceof large

NPswasmostfrequentataWebernumberof40and102.IntermediateWebernumbersof58and78

hadvirtuallynoNPsover20nm.Theappearanceof largerNPsandtheirchangeinsizeandfrequency

can be explained by mixing efficiency and the transition from a reflective to a transmitive type of

atomization. At the lowest Weber numbers, in the reflective atomization regime, there is lower

turbulencewithin themixing zone.This leads to thepossibilityof reactionsoccurring inpoorlymixed

regions,resulting intheformationof largerNPs.Atthe intermediateWebernumbers,veryfew larger

NPs are formed because of the increasing mixing efficiency. At large Weber numbers, there is a

transition into transmitiveatomization.At thispointanunstable rim is seenwithdroplets leaving the

mainbodyoffluid,resultinginanetdecreaseinmixingefficiency.Withinthesedroplets, it ispossible

thatlargerNPsareformedespeciallyiftheyaresilvernitraterich.

Thereductioninsizeanddispersityismoreobviousforthe0.5mmIJR.Thisreductionisobtained

as the mixing efficiency is increased within the liquid sheet, causing the average size of the NPs to

decrease andminimizing the formation of larger NPs. Again this is because the reactions are taking

placewithamoreefficientspreadofsodiumborohydride,whichincreasesstabilityofformedNPsand

decreasestheformationofNPsinsilvernitraterichregions.Thereisreductioninsizeandpolydispersity

whenthehydrodynamicsof the IJR transition fromauniform liquidsheet (We<29) toa liquidsheet

withwaves(We>39)withthesizeandpolydispersityultimatelyreducingfrom7.9±5.8nmto3.4±1.4

nm.

ForNPs less than10nm, thepeakwavelengthdoesn’tchangesignificantlywithsize.139This is in

linewiththeapproximatelyconstantpeakwavelengthsshowninTable4-1andTable4-2,andtheTEM

imagesshowNPswithanaveragediametersoflessthan10nm.TheincreasingFWHMcanpossiblybe

explained by the change in the larger NPs,where the size increases (particularly in the 0.25mm IJR

case).TheseNPswhicharelargerthan10nm,causeashiftinpeakwavelengthforthispopulationofthe

NPs,causingawideningoftheFWHM.Thisisonepossibleexplanationandtherecouldbesomeother

phenomenonwhichexplainsthewideningoftheFWHM.


82

Figure4-6:TEMimagesandparticlesizedistributionsofsilverNPssynthesisedatdifferenttotalflowratesusinga

0.5mmI.D.tubingIJR.(A)32ml/min(We:13),(B)48ml/min(We:29),(C)64ml/min(We:51)and(D)72ml/min

(We:65).Concentrationofsilvernitrate0.9mM,trisodiumcitrate6mMininput1andsodiumborohydride1.8mM

ininput2.

Chapter4

83

Figure4-7:TEMimagesandparticlesizedistributionsofsilverNPssynthesisedatdifferenttotalflowratesusinga


(We:102).Concentrationofsilvernitrate0.9mM,trisodiumcitrate6mMin input1andsodiumborohydride1.8

mMininput2.


84

Figure4-8:Dependenceofpeakabsorbance(peakwavelength386-389nm)onWebernumberobtainedfromUV-

VisspectroscopyofsilverNPssynthesisedatdifferenttotalflowratesusing:i)0.5mm(blacksquares)andii)0.25

mm I.D. tubing IJR (red triangles).Concentrationof silvernitrate0.9mM, trisodiumcitrate6mM in input1and

sodiumborohydride1.8mMininput2.

Table 4-1: FWHMand peakwavelength of silverNPs synthesised in the 0.5mm IJR using trisodium citrate as a

stabilisingagentatflowratesbetween32and72ml/min.Concentrationofsilvernitrate0.9mM,trisodiumcitrate6

mMininput1andsodiumborohydride1.8mMininput2.

Flowrate(ml/min) WeberNumber FWHM(nm) Peakwavelength(nm)32 13 66 38740 20 66 38648 29 67 38656 39 73 38764 51 75 38772 65 77 387

Chapter4

85

Table4-2:FWHMandpeakwavelengthof silverNPs synthesised in the0.25mm IJRusing trisodiumcitrateasa

stabilisingagentatflowratesbetween18and34ml/min.Concentrationofsilvernitrate0.9mM,trisodiumcitrate6

mMininput1andsodiumborohydride1.8mMininput2.

Flowrate(ml/min) WeberNumber FWHM(nm) Peakwavelength(nm)18 129 72 38920 160 72 38922 193 74 38724 230 77 38726 270 78 38928 313 79 38930 360 87 38632 409 91 38834 462 89 388

4.7.2 EffectofwebernumberonsilverNPsynthesis2(PVAsystem)

TheWebernumbereffectonsilverNPswasalsoinvestigatedinthe0.5mmand0.25mmIJRsusingPVA

asastabilizingagent.Concentrationsofinput1were0.9mMsilvernitrate,0.02wt%PVAandforinput

2were1.8mMsodiumborohydride.Totalflowratesforthe0.5mmIJRrangedfrom32ml/minto72

ml/minandforthe0.25mmIJRrangedfrom18ml/minto42ml/min(repeatabilityofthesynthesisis

showninAppendixC,FigureC2).PVAasasurfactantaffectsthesurfacetensionoftheaqueoussolution.

Thesurfacetensionforwateris72mN/manditisestimatedthatPVAreducesittoavaluebetween60-

65mN/m.168Althoughthere isasmall increase inWebernumberbecauseof reducedsurface tension

usingPVA, thehydrodynamicsobservedshowedminimalchangewhencompared tousingwateronly

(seeAppendixC,FigureC3).

Figure4-9showsTEMimagesforNPssynthesizedinthe0.5mmIJRat32,48,64and72ml/min,


4.2 ± 1.1 nm.Using this jet diameter, theNP size decreaseswith increasingWeber number (up to a

maximumofWe:65).T-testsyieldedp-values<0.001whencomparingthePSDs.Thedispersityofthe

NPs does not change significantly with increasing Weber number. One of the differences from the

trisodiumcitratestabilizedsystemisanotablelackoflargerNPs,resultinginlowerpolydispersity.This

difference can be attributed to the PVA molecules used as ligands which provide a bulkier steric

stabilization as opposed to the electrostatic stabilization of the smaller citratemolecule, leading to a

strongerbarriertopreventgrowththroughcoalescenceofNPs.

Figure4-10showsTEMimagesforNPssynthesizedinthe0.25mmIJRat18,26,34and42ml/min,


4.6±1.1nm.ThesizeanddispersitydonotseemtobecorrelatedwiththeWebernumberinthiscase.

T-test yielded p-values of 0.68when comparing 18 and 34ml/min, 0.22when comparing 26 and 42

ml/min and < 0.001 in all other cases. It appears that in the 0.25mm IJR there is not a discernible

changeofNPsizewhencompared to the0.5mm jets,especiallywithp-valuessuggesting there isno

differencebetween18and34ml/minand26and42ml/min.


86

Figure4-11showsthepeakabsorbancevs.WebernumberobtainedfromUV-Visspectroscopyfor

thesilverNPsobtainedusingthe0.5mmIJR(We=13-65)andthe0.25mmIJR(We=32-176).Intermsof

stability,theUVsignalchangedinthefirst15minaftersynthesis,butremainedstableafter30minand

up to 2 hr. After 24 hr therewas a 16% increase in peak absorbance. For the 0.5mm IJR, the peak

absorbance decreases with increasing Weber number. For the 0.25 mm IJR, the peak absorbance

decreasesminimallyastheWebernumberincreases.Thisindicatesthatinthe0.5mmIJRcasetheNP

sizedecreaseswithincreasingWebernumber,whichisinagreementwiththeTEMobservations.Inthe

0.25 mm IJR case, the peak absorbance trend indicates that the NP size changes minimally with

increasingWebernumber,againinagreementwithTEMobservations.Table4-3andTable4-4showthe

FWHMandpeakwavelengthforsilverNPssynthesisedusingPVAasastabilisingagentatvariousflow

ratesinthe0.5mmand0.25mmIJRrespectively.Similartothetrisodiumcitratesystem,theFWHMfor

the 0.5mm IJR increases significantly after a flow rate 48ml/min and the peakwavelength remains

fairlyconstant.Again,similartothetrisodiumcitratesystem,thereisagradual increaseintheFWHM

withincreasingflowrateforthe0.25mmIJR,whilethepeakwavelengthstayapproximatelyconstant

exceptforthehighestflowratewhereitdecreasesby4nm.

PVA has a much higher molecular weight than trisodium citrate and is expected to be more

effective at stabilising the NPs through adsorption onto the silver NP surface through its hydroxyl

groups,whicharenumerousalongthechain.169, 170Citratemolecules,whicharemuchsmaller,adsorb

onthesurfaceoftheNPsthroughtheircarboxylgroups(citratehasthreecarboxylgroupsasopposedto

themanyhydroxylgroupsonaPVAchain).171Anotheraspectofthetwoligandsistheeffecttheyhave

onthekineticsofthereductionreactionandsubsequentnucleationofNPs.Itispossiblethattheligands

interact with the precursor in different ways; trisodium citrate behaves as a buffer and affects the

solutionpH,whereasPVAdoesnotbehave inthismanner.Literature indicatesthatthePVAmolecule

slows down growth of silverNPs compared to reductionwith no ligand present, by investigating the

evolutionofabsorbanceovertime.172Thisreductioningrowthrateismostlikelybecauseofthebulky

nature of the PVA, which would make it more difficult for clusters of silver to approach a growing

nucleus.Itissuggestedthat,sincetrisodiumcitratemoleculesaresmallerthanPVA,theywouldallowa

closerapproachforsilverclustersinsolutiontotheNPsurface(literaturesuggestsvariousclustersizes

suchasformationofAg13andAg2+).82,84Thepresenceofhighlyconcentratedsilvernitratezonesinthe

liquidusinganIJRinthecaseofcitratecappedNPswilllikelyresultingrowthatthesurfaceoftheNP

throughcoalescenceofsilverclusters.ThepossiblereasonthatnolargerNPsareseenusingthePVAare

that itprovidesamuch largerbarrierontheNPsurface, inhibitingtheapproachofsilverclustersand

rapid growth in areas that are rich in silver nitrate. At the same time, the stabilising effect of

borohydridecanbetakenintoaccount.Thereisabalancebetweenhowquicklyreducedsilverclusters

canapproachandincorporateontotheNPsurfacevs.theabundanceofborohydrideinthevicinityof

the NP. As mixing efficiency increases, there is a better distribution of borohydride, resulting in a

suppressedgrowthrateandsmallerNPs,whichistheobservedeffectusingPVAasa ligandinthe0.5

mmIJR.Usingthe0.25mmIJR,whichyieldsamoreconstantsizewithincreasingflowrate,thesizeof

theNPsdoesnotfallbelowca.4nmwithincreasingmixingefficiency.ThesizeoftheNPsusingthe0.25

Chapter4

87

mmIJRfluctuatesbetween4.3and4.7nm.Thereasonforthisfluctuationseemstobeduetotheflow

regimeinthe0.25mmIJR,withtheunstablerimregioncontributingtoafluctuatingsizewhichisnotas

repeatable as a stable rim. The repeatability data for the PVA system (Appendix C, Figure C2) also

indicates that silver NPs synthesized in the 0.25 mm IJR show more fluctuation in absorbance as

comparedtothe0.5mmIJR,furthersuggestingthattheunstablerimcontributestothefluctuatingNP

size in the 0.25 mm IJR system. It is expected that improved mixing efficiency should result in a

reductioninsizeasexplainedaboveandobservedinthe0.5mmIJR.Howeverthisreductioninsizeis

notseenwithimprovingmixingefficiencyinthe0.25mmIJRbecausethePVAmaybeimposingalimit

on the size of NPs. The synthesis was repeated in the 0.5 mm IJR at a We=65 using double the

concentrationofPVA(0.04wt%).Thesizewasreducedfrom4.2±1.1nmto3.7±1.0nm,suggesting

thatthePVAconcentrationisoneofthefactorsthatdeterminetheNPsize(TEMimageandPSDforthis

experimentareshowninAppendixC,FigureC4).ThissuggeststhattoachieveNPssmallerthan4nm,an

increaseinPVAwouldberequired,otherwiseonceacertainmixingefficiencyisachieved;thesizeofthe

NPdoesn’tdecreaseatafixedPVAconcentration.TheincreaseinFWHMinthecaseofboththe0.5mm

IJR and 0.25mm IJR is interesting, given the peakwavelength doesn’t change. The decrease in peak

absorbanceandconstantpeakwavelengthinconsistentwithareductioninsizeofNPsbelowthe10nm

size and the dispersity doesn’t seem to significantly increase according to the TEM analysis.

Interestingly,thesameeffectwasseenintermsoftheFWHMtrendwhenusingtrisodiumcitrateasa

stabilisingagentandthiswarrantsfurtherinvestigation.


88

Figure4-9:TEMimagesandparticlesizedistributionsofsilverNPssynthesizedatdifferenttotalflowratesusinga


(We:65).Concentrationofsilvernitrate0.9mM,PVA0.02wt%ininput1andsodiumborohydride1.8mMininput

2.

Chapter4

89

Figure4-10:TEMimagesandparticlesizedistributionsofsilverNPssynthesizedatdifferenttotalflowratesusinga


(We: 176). Concentrationof silver nitrate 0.9mM,PVA0.02wt% in input 1 and sodiumborohydride 1.8mM in

input2.


90

Figure4-11:Dependenceofpeakabsorbance(peakwavelength398-401nm)onWebernumberobtainedfromUV-

VisspectroscopyofsilverNPssynthesisedatdifferenttotalflowratesusing:i)0.5mm(blacksquares)andii)0.25

mm (red triangles) I.D. tubing IJR. Concentration of silver nitrate 0.9mM, PVA 0.02wt% in input 1 and sodium

borohydride1.8mMininput2.

Table4-3:FWHMandpeakwavelengthofsilverNPssynthesisedinthe0.5mmIJRusingPVAasastabilisingagent

at flow rates between 32 and 72ml/min. Concentration of silver nitrate 0.9mM, PVA 0.02wt% in input 1 and


Flowrate(ml/min) WeberNumber FWHM(nm) Peakwavelength(nm)32 13 80 40040 20 81 40148 29 82 39856 39 86 40064 51 91 40072 65 93 400

Chapter4

91

Table4-4:FWHMandpeakwavelengthofsilverNPssynthesisedinthe0.25mmIJRusingPVAasastabilisingagent

at flow rates between 18 and 34ml/min. Concentration of silver nitrate 0.9mM, PVA 0.02wt% in input 1 and


Flowrate(ml/min) WeberNumber FWHM(nm) Peakwavelength(nm)18 32 93 40122 48 95 40026 68 93 39930 90 97 40034 115 99 40038 144 101 39942 176 100 395

4.8 Conclusions

The impinging jet reactor (IJR) was characterized visually and using the Villermaux-Dushmann test

reactionsystemtoinvestigatetheflowregimeandmixingefficiencyunderarangeofflowratesandjet

diametersof0.25mmand0.5mm.AmaximummixingefficiencywasidentifiedataWebernumberof

ca.90,anditwasfoundtocorrespondtothepointatwhichtheflowregimetransitionsfromastable

rim sheet/chain like pattern to anunstable rimaccording to visual observationof the impinging jets.

Thispointisidentifiedasthetransitionfromareflectivetoatransmitivetypeatomization.

Two different ligands for silver NP synthesis were investigated using the IJR: trisodium citrate,

whichelectrostaticallystabilizesNPs,andPVA,whichisa largechainmoleculethatstericallystabilizes

NPs.TrisodiumcitrateproducedmorepolydispersesilverNPs.Averagesizewasreducedwithincreasing

flowratebecauseof increasedmixingefficiency,reducingthesilvernitraterichzonesoccurringinthe

mixingzoneafterimpingement.IncreasingmixingefficiencyinthePVAsystempastaWebernumberca.

60didnotyieldsmallersizeNPssuggestingalimitofsizeachievableusingPVAasaligand,possiblydue

tothesmallamountofPVApresent.MoremonodispersedNPsareobtainedusingPVA,indicatingthatit

ismoreefficientatstabilizingtheNPssinceithasmorepotentialsitesavailableforadsorptionontothe

surface of the NP than the citrate molecule, and it kinetically hinders growth during NP formation.

Figure 4-12 shows the diameter of the synthesised silver NPs against theWeber number for all the

synthesis conditions tested. The overall trend shows that theNP diameter decreaseswith increasing

WebernumberwhentheWebernumberisrelativelylow,andthenthedecreaseislesspronouncedand

levelsoffasWebernumber increasestorelativelyhighnumbers.Thetrendmatcheswellwiththatof

thedependenceofmixingtimeontheWebernumber,howeveritmustbenotedthatthetrendisalso

influenced by the surfactant that is used (PVA concentration appears to limit the size that can be

achievedinthatsynthesis).ThedecreasingsizeofNPwithincreasingnumberwasmoreapparentwhen

citratewas used as a surfactant. The lack of fouling and the efficientmixing in the IJR are themain

benefitsforthesynthesisofNPs.ThesizeoftheNPscanbereducedwith increasingmixingefficiency

while also reducing polydispersity. The IJR is scalable through a 'scale-out' procedure of essentially

increasingthenumberofIJRsoperatingintandem,makingitsuitableforlargescalesynthesisofNPs.


92

Figure4-12:DependenceofsilvernanoparticlediameterontheWebernumber.Datashownisfor0.5mmand0.25

mmIJRsandforbothcitrateandPVAsurfactants.

93

5 Effectofhydrodynamicsandmixingconditionsonthecontinuoussynthesisof

goldandsilvernanoparticlesinacoaxialflowdevice

There aremany studies in the literature on the synthesis of gold and silverNPs. Twomore common

synthesismethodswereused inthisstudy:thewell-knownTurkevichsynthesisforgoldNPs88andthe

reductionofsilvernitrateviasodiumborohydrideinthepresenceoftrisodiumcitrateforsilverNPs.

TheTurkevichmethodforsynthesisinggoldNPsisperhapsthemostcommon,andtherearemany

studies in which batch reactors were used.89-106, 173 Flow syntheses of gold NPs using the Turkevich

methodare lesscommon.Ftounietal.synthesizedgoldNPswiththeTurkevichmethodusingafused

silicacapillaryandat-mixertomixchloroauricacidtosodiumcitratepriortointroducingthemintothe

heated capillary.107 Residence times within the capillary were varied between 35 to 94 s and they

obtained NPs in the size range between 1.5 and 3 nm, with larger NPs being obtained at longer

residence times. A minimum size of NPs was found at around 3.15 ratio of citrate to gold. The

temperatureeffectwas investigated,andNPsizedecreasedwith increasing temperature in the range

between60and100°C. Suganoetal. synthesizedgoldNPs in the size range10-45nmusinga y-type

micromixerwithapulsedflowofchloroauricacidandsodiumcitratetomixthereagentswithinasmall

channel at room temperature.108Mixing efficiencywas alteredby adjusting thepulsing rate between

50,100 and 200 Hz. The NP size increased with a higher pulsing frequency, suggesting faster mixing

resultedinlargerNPs.Wengetal.synthesizedgoldNPsusingtheTurkevichmethodrecipeusinganovel

microfluidicdevice.40HexagonalNPsofaround35nmwereobtainedata temperatureof115°Cwith

reaction time in the order of 2 to 5 min. There are also various studies in which gold NPs were

synthesizedwithmethodsotherthanthatofTurkevichinflowreactors.109-112,127

SilverNPsarecommonlysynthesizedinbatchwithsodiumborohydrideusedtoreducethesilver

precursor.5,64,66,67,69,70SARmixertypereactors71andPTFEchambers65havebeenutilizedtosynthesize

silverNPs inflowusingborohydride,obtainingNPs intherangeof10-20nmforborohydridetosilver

nitrateratiosrangingfrom3to40.WehavealsopreviouslyusedtheCFRandanimpingingjetreactor

forsynthesisofsilverNPsusingborohydrideasareducingagent.145,174

TheCFRhasbeenusedtosynthesizeavarietyofdifferentNPssuchastitania,128zirconia,129 iron

oxide,35, 36, 130 polymer,131, 175, 176 nickel,132 palladium177 and galliumnitride.178 The SARmixer uses the

principleofreducingthediffusiondistancebetweenmultiplestreamsthatarerequiredtomixandcan

becategorizedasaserial laminationdevice.SARmixershavebeenemployedforthesynthesisofgold

andsilverNPs.71, 109, 127Parallel laminationdevicesaresimilartoserial laminationbutthereduction in

diffusiondistanceisdoneinonestepratherthanmultipleones,andthesehavealsobeenusedforthe

synthesisofNPs.110,112TheCFIisprimarilyusedasaheattransferdeviceintheliterature,179,180andcan

beusedasaninlinemixer181buthasnotbeenusedspecificallyforthesynthesisofNPs.

Avarietyofdifferentconfigurationswereusedinthisstudy,withthefocalpointbeingtheCFR,for

the synthesis of gold and silver NPs were employed in this study. The CFR works on the basis of a

cylindricalinnerstreamofreagentwhichissheathedbyacylindricalouterstreamofreagent,creatinga

Effectofhydrodynamicsandmixingconditionsonthecontinuoussynthesisofgoldandsilvernanoparticlesinacoaxialflowdevice

94

reactioninterfacebetweenthestreams.Theconfigurationsusedeither:addedanextraflowcomponent

following the CFR for a specific purpose or, by altering the hydrodynamics within the CFR. Flow

components that were used in conjunction with the CFR were: coiled flow inverter (CFI) for the

synthesis of gold NPs using a Turkevich recipe where tetrachlorauric acid and trisodium citrate are

mixed and heated, and a split and recombine (SAR) mixer for the synthesis of silver NPs through

reduction of silver nitrate by sodium borohydride in the presence of trisodium citrate. Schematic

representationsoftheseflowcomponentscanbeseeninFigure5-1.

Figure5-1:Schematicrepresentationsofcomponentsusedinthemicrofluidicflowsetup:a:coaxialflowreactor,b:

coiledflowinverterandc:splitandrecombinemixer.

TheCFIwasusedasaresidenceloopfollowingtheCFRbecauseofitsmixingcharacteristics.Ithasideal

propertiesinsynthesizingNPsbecauseofanimprovedresidencetimedistribution.182Thisimprovement

arises from enhancedmass transfer because of a secondary flow developingwithin a helically coiled

channel,knownas‘Deanflow’.ThisischaracterizedbytheDeannumber,whichistheratiobetweenthe

squarerootoftheproductofinertialandcentripetalforcesoverviscousforces:

De= d2r 5udN= d

2rRe

wheredisthediameterofthechannel,ristheradiusofcurvatureofthecoil,𝜌isthefluiddensity,uis

themeanvelocityofthefluid,𝜇istheviscosityofthefluidandReistheReynoldsnumber,theratioof

inertialtoviscousforces.Themixingisfurtherimprovedwhenthissecondaryflowisinverted,asinthe

CFI,byintroducing90°bendsinthehelicallycoiledchannel.TheSARisadevicewhichimprovesmixing

byreducingthediffusiondistancebetweenstreamsbycontinuouslysplittingandrecombiningtheflow

Chapter5

95

intothinnerandthinnerinterdigitatedlamellae.HydrodynamicchangesintheCFRwhenoneoperates

atahigherRenumberasopposedtolaminarflowbyincreasingthevelocityoftheinnerstreamorwhen

onechangesthethicknessoftheinnerstreamtoreducethediffusiondistance.Thevariouscomponents

andhydrodynamicconditionswerechoseninanefforttodemonstratewhattheeffectofmasstransfer

isonthesynthesisofgoldandsilverNPsintheCFR.

NPs were synthesized under a variety of different hydrodynamics and mixing conditions to

understandhowthesecouldaffectsizeanddispersity.Asmentionedpreviously,understandinghowto

controlsizeanddispersityisimportantbecauseitallowsNPstobetailoredforspecificapplications.Two

systemswerestudiedinthiswork:thesynthesisofgoldNPsintheCFRinconjunctionwiththeCFIusing

the Turkevichmethod recipe and the synthesis of silver NPs in the CFR in conjunctionwith the SAR

mixer,aswellastheCFRinisolation,usingsodiumborohydridetoreducesilvernitrateinthepresence

of trisodium citrate. The effect of flow rate (or residence time) and temperature on NP size and

dispersitywere investigated for thegoldNP system.Theeffectof silver concentrationon theNP size

and dispersitywas investigated using the CFRwith SARmixer system.Using the CFR in isolation, the

silverNPsynthesiswasperformedathighRe,toinvestigatetheeffectofhydrodynamicssilverNPsize

and dispersity, and the effect of changing the internal diameter of the inner tube in a laminar flow

regime(toreducethediffusiondistanceoftheinnerstream)onNPsizeanddispersitywasinvestigated.

5.1 Experimental

5.1.1 Chemicals

Silvernitrate(AgNO3,0.01Mstocksolution),trisodiumcitrate(HOC(COONa)(CH2COONa)2·2H2O,powder

form,sodiumborohydridesolution(NaBH4,~12wt%in14MNaOHstocksolution)andgold(III)chloride

hydrate (HAuCl4.xH2O, powder form) were obtained from Sigma. All chemicals were used without

furtherpurificationandsolutionswerepreparedwithultrapurewater(resistivity15.0MΩ·cm).


Syringepumps(Pump11EliteOEMmodule,Harvard)wereusedtodeliverthereagents.Thesetupfor

goldNP synthesis using theCFR followedby aCFI acting as a residence time loop, as canbe seen in

Figure5-2.TheCFRconsistedofa0.798mminternaldiameter(I.D.)innerglasstube(1.09mmexternal

diameter) and a 2mm I.D. outer glass tubewith the length from theoutlet of the inner tube to the

outletoftheCFRof21mm.Upstream,thecapillarieswereconnectedusinganETFET-piececonnector

(0.5mmthru-hole,UpchurchScientific)whichwasdrilledtoadiameterof2mmtoallowtheinnertube

tobeinsertedintotheoutertube.Theinnertubewasinsertedintoa1mmI.D.PTFEtubesleeve(1.59

mmexternaldiameter)tokeepthetubestableswithinthelargeroutertube.Downstream,theCFRwas

connectedtotheCFIbyusinganETFEunion(0.75mmthru-hole,UpchurchScientific).TheCFIwasa3.5

m 1mm I.D. PTFE tube that had nineteen 90° bendswith 20 arms, each arm consisting of 5.5 coils

(lengthalsoincludesentranceandexitlengthoftubing).Thecoilshadanaxialpitchof3mm,whilethe

coil diameterwas 8mm. The coilswere fabricatedbydrilling 1.6mmholes (using aRoland EGX-400

engravingmachine)intoaPTFEplate(2mmthickness,DirectPlastics)andthePTFEtubingwasthreaded

throughtheseholeswhichwerearrangedtoresultincoilingwiththedimensionsstatedabove.TheCFR


96

andCFIwere immersed ina glycerolbath (GR150,Grant Instruments)with temperaturebeing varied

between 60-100°C. The tetrachloroauric acidwas fed through a preheating CFI consisting of four 90°

bendsandsimilaraxialpitchandcoildiameterastheCFIdescribedabovetoensureitcameintocontact

with the trisodium citrate at the desired temperature. The outlet of the CFI was split to a waste

container and a sample container. These containers were pressurized using nitrogen gas and a

backpressureregulator,whichwassetto2barstopreventvaporizationofwaterwithinthereactor.

ThesetupsusedforthesynthesisofsilverNPscanbeseeninFigure5-3.TheCFRusedwassimilar

tothatinthegoldNPsynthesis,butthelengthfromtheinnertubeoutlettotheoutletoftheCFRwas

130mm (a longer lengthwas used for additional residence time)while the inner tube I.D. usedwas

between0.142mmand0.798mm (between0.559mmand1.09mmexternal diameter). A glass SAR

mixer(Micromixerchip,Dolomitemicrofluidics)wasplaceddownstreamoftheCFRinordertoenhance

the mixing by laminating the stream into thinner striations. The SAR mixer had an internal channel

diameterof0.125mmx0.35mm(depthxwidth)ofthemainchanneland0.05mmx0.125mmofthe

secondary channels responsible for splitting the flow. The internal volume was 8 μl. The CFR was

connectedtotheSARusinganETFEunion(0.75mmthru-hole,UpchurchScientific).FollowingtheSAR,

a10cmlong1mmI.D.PTFEtubedeliveredthesilverNPstothesamplecontainer.

Figure5-2:SchematicrepresentationofexperimentalsetupforgoldNPsynthesis.

Chapter5

97

Figure5-3:Schematic representationexperimental setup for silverNP synthesis: a)CFRused in conjunctionwith

SARandb)CFRusedinisolation.

5.1.3 Nanoparticlesynthesis

For the gold NP synthesis, initially the reactor setupwas filled with tetrachloroauric acid before the

systemwaspressurizedusingnitrogengasandabackpressureregulatorsetto2bar.Thereactorsetup

was then immersed in the temperature bath (60-100°C). A trisodium citrate solution and

tetrachloroauricacidweredilutedtotheappropriateconcentrationsandpumpedthroughtheinnerand

outer streams of the CFR respectively. Itwas found that filling the reactorwith tetrachloroauric acid

prior to synthesis reduced the amount of fouling drastically. The reactor was then operated at the

appropriate flowrate,andone reactorvolumewasallowed topass throughathigh temperatureand

collectedinthewastecontainerbeforetheoutletoftheCFIwasswitchedtosamplecollectiontocollect

10mlofsample.Smallamountsoffoulingwereobservedwhenthereactorwasoperatedataflowrate

of 0.25ml/min towards the end of the CFI. Itwas suspected this is because the long residence time

enabled citrate to diffuse towards the channelwallswhichwould increasepH. The surface chargeof

PTFEisclosetozeroatpH3.5,whichiscoincidentallyaroundthepHofthetetrachloroauricacid.183This

wasthereasonforthereductioninfouling,sinceareactorfilledwithamixtureofcitrateandprecursor

prior to immersion in the temperature bath resulted in increased fouling at the channel walls. The

surfacechargewouldbenegativeinthepHrangenearneutral(whichisthepHwhenmixingcitrateand

tetrachlorauricacidattheseconcentrations),whichprobablyattractsNPsinthegrowthphasebecause

ofthepositiveAuionadsorbedontothesurfaceoftheNP.Thusitisbeneficialtoallowasmalllayerof

tetrachloroauricacidtoremainatthewalltokeepthesurfacechargeneutral.Inlaminarflowthelayer


98

willremainforsometimebeforecitratediffusesnearthewallbecauseoftheflowprofileandnon-slip

conditionatthewallsofthereactor.Thisisevidencedbysmallamountsoffoulingonlyoccurringat0.25

ml/minwheretherelativeresidencetimeislonger.

For the silver NP synthesis, silver nitrate and trisodium citrate solutions were premixed to the

appropriate concentration and pumped through the inner stream. Sodium borohydride/sodium

hydroxidesolutionwasdilutedtotheappropriateconcentrationandpumpedthroughtheouterstream

of the CFR. Sodium borohydride was stored in 14M sodium hydroxide; hence the concentration of

sodiumhydroxidewas3.21timeshigherthanthestatedsodiumborohydrideconcentrationinallcases.

SilverNPsyntheseswerecarriedoutatroomtemperature(22-24°C).Allconcentrationsarethoseatthe

inletsbeforeanymixingofreagentsoccurs,unlessstatedotherwise.


NPs were analysed using a UV-Vis spectrometer (USB 2000+ Spectrometer and DT-Mini-2-GS light

source,OceanOptics).SilverNPswereanalysedwithinanhourofsynthesis(thesignalofthesamples

were stable in thiswindowof time).GoldNPswere analysedover time andwere analysedonce the

signal was stable (the signal became stable usually after 1-2 days). NP samples were diluted with

additionalultrapurewatertobringtheabsorbanceintoasuitablerange(i.e.obeyingtheBeer-Lambert

lawandavoidingsaturationofthelightdetector)ifnecessaryandthedatawerenormalizedsothatthe

maximum absorbance in the particular set of experiments tested represented an absorbance of 1.

TransmissionelectronmicroscopeimageswerecapturedusingaJEOL1200EXiimicroscopewitha120

kVaccelerationvoltage.CarboncoatedcopperTEMgridswerepreparedwithinanhourofsynthesisfor

silverNPsandwhenthesamplewasstable forgoldNPsbypipettinga5µl sampleonto thegridand

allowing it todryatroomtemperature.Whenpreparingthesample, thedropletplacedonthegrid is

smalltoallowforquickerdryingandfortheringofaggregatedNPstobeidentifiedeasily(asdiscussed

in Chapter 3). Particle size distributions (insets for each TEM image presented) have the following

nomenclature:d is averagediameter,δd is the standarddeviationof theNPdistributionandn is the

number of particles counted to obtain the particle size distribution. Differential centrifugal

sedimentation analysis (CPS disc centrifuge UHR, Analytik) was carried out on gold NPs when the

samples were stable. PEBBLES software was used for the counting and sizing of TEM images of the

synthesised silver NPs. ImageJ software was used for the counting and sizing of TEM images of the

synthesisedgoldNPs.PEBBLESsoftwaretendstobebetterforlowercontrastimageswheretheoutlines

oftheNPsarehardertodistinguish,inthiscaseitwasmoresuitabletousethisprogramforsilverNPs.

TheImageJsoftwarecancountandsizeNPsfasterthanPEBBLES,butworksbetterwithhighercontrast

images.GoldNPimagesweresuitablyhighcontrasttoobtainaccuratesizedistributionsusingImageJ.

5.2 Resultsanddiscussion

5.2.1 Effectofincreasingflowrateongoldnanoparticlesizeanddispersity

TheeffectthattheflowratehasonthegoldNPswasinvestigatedintheCFRwithaninnertubeinternal

I.D.of0.798mmandanouter tube I.D.of2mm.The concentrationof trisodiumcitratewas0.09M

Chapter5

99

throughtheinnertube;theconcentrationoftetrachlorauricacidwas0.557mMthroughtheoutertube.

Thevolumetricflowrateratiowasfixedat32.3:1(Qout:Qin)andthetotalflowratewasvariedbetween

0.25and3ml/min.Themolarflowrateratiowas1:5(HAuCl4:Na3citrate).TheReynoldsnumbersvaried

between3and32intheCFRandbetween5and63intheCFI.TheDeannumberswererangedfrom1.9

to22.4intheCFIwhichisabovethethresholdof1.5requiredforfullydevelopedDeanflow.182

Figure5-4showsTEMimagesofgoldNPssynthesizedataflowrateof0.25,0.5,0.75,1,1.5,2and

3ml/minwheretheaveragediameteranddispersityoftheNPsare18.9±2.3nm,18.4±2.8nm,19.7±

2.4nm,17.9±2.1nm,20.6±7.2nm,21.7±3.3nmand23.9±4.7nmrespectively.Figure5-5shows

correspondingDCSmeasurements,wheretheaveragediameteris16.7±2.8nm,15.1±3.6nm,16.4±

3.0nm,14.9±2.3nm,18.5±3.1nm,22.5±3.8nmand26.4±4.0nmnmrespectively.Thetrendofthe

averagediameterof theNPsmatcheswith thatobserved fromTEM images. Figure5-6 showsUV-Vis

spectraofgoldNPssynthesizedataflowrateof0.25,0.5,0.75,1,1.5,2and3ml/min.Experimentsat

selected flow rateswere repeated three times and analyzedusingDCS (shown inAppendixD, Figure

D1).TheaveragesizeoftheNPsfrom0.25ml/minto1ml/minchangesminimallyaccordingtotheTEM

andDCSanalysis(althought-testsshowthatallPSDsarestatisticallydifferenttoasignificancevalueof

at least 0.05). Themolar extinction coefficient increaseswith size for goldNPs,184, 185 therefore for a

constantnumberconcentrationofNPs,anincreaseinpeakabsorbancewouldindicateincreasingsize.

However, the number concentration of NPs of changing size would change for a constant gold

concentration i.e. largerNPssynthesizedat thesamegoldconcentrationwouldhavea lowernumber

concentration.Hendeletal.showedthattheabsorbanceat400nmofcitratecappedgoldNPsinthe15-

30nmrangeincreaseslinearlywithincreasingsizeforaconstantgoldconcentration.186Thissuggestsa

largersizehasamoresignificanteffectontheabsorbancethennumberconcentrationofNPs.Figure5-6

showsthatthereisanincreaseintheabsorbanceat400nmwithincreasingflowratebutitisunlikely

thatthisisbecauseofincreasingsizesinceTEMandDCSshowamoreconstantsizeinthe0.25-1ml/min

range. The peak wavelength should redshift with increasing size,184 but the wavelength changes

minimally between 523 - 525 nm further suggesting there is aminimal change in size. One possible

explanation for the increase in absorbance up to 1 ml/min may be related to fouling. Fouling was

observed at the lowest flow rate suggesting an increase of fouling with decreasing flow rate. This

explainsthedecreaseinabsorbanceatlowerflowrates(despiteaconstantNPsize)sincesomeofthe

goldremainswithinthereactor.Increasingtheflowratefrom1.5ml/minto3ml/minshowsanincrease

in the size of the NPs from 20.6 to 23.9 nm according to TEM analysis, and from 18.5 to 26.4 nm

accordingtoDCSanalysis.Thepeakabsorbanceincreaseswithincreasingflowrateinthisrangeandthe

peak wavelength increases from 525 to 529 nm, which indicates an increase in the size of the NPs

consistentwithDCSandTEMdata.

The data shows that the size stays fairly constant between 0.25 and 1 ml/min and thereafter

increasesuptoaflowrateof3ml/min.Thiscanberationalizedasfollows.IntheCFRsection,thereisa

laminarflowwheretheouterflowoftetrachloroauricacidfocusesthe innerflowoftrisodiumcitrate.

Thereislittlemixinginthisregion.Masstransferoccursthroughdiffusionattheinterfacebetweenthe

twostreams.Inthemoreacidicconditionsofthetetrachloroauricacidstream,thereactionrateisfaster


100

andnucleationisfavored.Inthemorebasicconditionsinthetrisodiumcitratestream,thereactionrate

is slower. This is because of the speciation of the precursor, in acidic conditions the speciation is

favoured towards AuCl4-,95, 102, 104 whereas in more basic conditions the speciation passes through

various forms fromAuCl3OH- toAuOH4.As thegoldspeciesacquiremorehydroxyl ions, the reactivity

decreasesandthere isatransitionfromnucleationofgoldatomsintoclusterstogrowthofgoldonto

existing clusters. In the CFR region of the reactor, the tetrachloroauric acid and the trisodium citrate

remainwell separatedandonlyexchangematerial throughdiffusionat the interface. In this region,a

higherreactionrateandhencenucleationwilloccurinthetetrachloroauricacidstream.Therefore,the

CFRbehavesasanucleationsection.FurtheralongintheCFIresidenceloop,thestreamsaresubjected

toDeanflowandflowinversion.Theseenhancethemixingandlowerthereductionrateofgold ions,

hencetransitioningfromthenucleationsectionintheCFRtoagrowthsectionintheCFI.Giventhis,the

longernucleationcanbemaintained, the smaller the resultantNPs shouldbeobtained. In this study,

thereisamarkedincreaseinthesizeoftheNPsastheflowrateincreasespast1ml/min,supportingthe

hypothesisofnucleationdominatingintheCFR,sinceathigherflowrates,reactionsattheinterfaceof

streamsdonotpersistaslong.Thereisalimitationonsizereductionwhenoperatingbelow1ml/min,

which may be related to a decrease in citrate concentration. The decrease in citrate concentration

wouldresultinadecreaseinreductionofgoldprecursor,leadingtoasuppressionofnucleationwithin

theCFRsection.Finally,thePecletnumberforthesyntheseswas>>1atallflowrates, indicatingthat

masstransportwasadvectiondominated(Table5-1showsPecletnumberfortheCFRsectionwherethe

I.D.was2mmandthediffusioncoefficientusedwas1.4x10-9m2/s,187whichisfortetrachlorauricacid,

the Peclet number is doubled for those shown in the table for the CFI where the I.D. was 1 mm).

IncreasingPecletnumber indicateddecreasingsignificanceofdiffusionontheprocess, soagreeswith

theideasdiscussedabovetoexplainwhythegoldNPsizeincreaseswithincreasingflowrate.

Chapter5

101

Figure5-4:TEMimagesofgoldNPssynthesizedusingtheCFRwithaCFI residence loopata flowrateofA:0.25

ml/min ,B: 0.5 ml/min, C: 0.75 ml/min, D: 1 ml/min, E: 1.5 ml/min, F:2 ml/min and G: 3 ml/min. H: Average

diameter of goldNPs vs flow rate of synthesis (bars represent standard deviation of the size). Concentration of

tetrachloroauric acid, 0.557mM; concentration of trisodium citrate, 0.09M; volumetric flow rate ratio, 32.3: 1

(Qout:Qin,HAuCl4:Na3citrate);molarflowrateratio,1:5(HAuCl4:Na3citrate);temperature,80°C.


102

Figure5-5:NormalizedDCScurvesofgoldNPssynthesizedusingtheCFRwithaCFIresidence loopat flow rates

between 0.25 and 3 ml/min. Inset: Diameter of gold NPs vs. flow rate of synthesis (bars represent standard

deviationofsize).Concentrationoftetrachloroauricacidwas0.557mMandconcentrationoftrisodiumcitratewas

0.09M.Concentrationoftetrachloroauricacid,0.557mM;concentrationoftrisodiumcitrate,0.09M;volumetric

flowrate ratio,32.3:1 (Qout:Qin,HAuCl4:Na3citrate);molar flowrate ratio,1:5 (HAuCl4:Na3citrate); temperature,

80°C.

Figure5-6:UV-VisspectraofgoldNPssynthesizedusingtheCFRwithaCFIresidence loopatflowratesbetween

0.25and3ml/min.Inset:Peakabsorbance(blacksquares)andpeakwavelength(reddiamonds)ofgoldNPsvs.flow

rate of synthesis analyzed using UV-Vis spectroscopy. Concentration of tetrachloroauric acid, 0.557 mM;

concentrationoftrisodiumcitrate,0.09M;volumetric flowrateratio,32.3:1 (Qout:Qin,HAuCl4:Na3citrate);molar

flowrateratio,1:5(HAuCl4:Na3citrate);temperature,80°C.

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103

Table5-1:DiameteranddispersityofgoldNPssynthesisedatvariousflowrateswithcorrespondingPecletnumber

Flowrate(ml/min) PecletNumber Diameter(nm) Dispersity(nm)

0.25 1.9x103 18.9 ±2.3

0.5 7.6x103 18.4 ±2.8

0.75 11.4x103 19.7 ±2.4

1 15.2x103 17.9 ±2.1

1.5 22.7x103 20.6 ±7.2

2 30.3x103 21.7 ±3.3

3 45.5x103 23.9 ±4.7

5.2.2 Effectoftemperatureongoldnanoparticlesizeanddispersity

Theeffect that the temperaturehasongoldNPs synthesiswas investigated in theCFRwithan inner

tubeinternalI.D.of0.798mmandanoutertubeinternalI.D.of2mm.Theconcentrationoftrisodium

citratewas0.09M through the inner tube,while the concentrationof tetrachlorauric acidwas0.557

mMthroughtheoutertube.Thevolumetricflowrateratiowasfixed32.3:1(Qout:Qin)andthetotalflow

ratewasfixedat1ml/min.Themolarflowrateratiowas1:5(HAuCl4:Na3citrate).TheReynoldsnumber

intheCFRwas11andintheCFIwas21.IntheCFI,theDeannumberwas7.5.

Figure5-7showsTEMimagesofgoldNPssynthesizedatatemperatureof60,70,80,90and100°C

wheretheaveragediameteranddispersityoftheNPsare25.5±5.7nm,22.7±3.9nm,17.9±2.1nm,

18.6±2.1nmand19.4±2.4nm respectively. T-tests for thePSDs showedp-valuesof<0.001when

comparing all PSDs, indicating statistical difference to a significance level of 0.001. Figure 5-8 shows

correspondingDCSmeasurements,wheretheaveragediameteris31.1±5.5nm,18.4±3.8nm,14.9±

2.3nm,16.2±3.4nmand17.6±2.7nmrespectively.The trendof theaveragediameterof theNPs

matchesthatobservedintheTEMimages.Figure5-9showscorrespondingUV-Visspectra.

TheaveragesizeoftheNPsdecreasedfrom25.5nmto17.9nmwhenincreasingthetemperature

from60 to80°Cand then increased slightly to19.4nmwhen increasing the temperature from80 to

100°C. DCS characterization also shows similar trends. The peak absorbance is highest at 70°C after

whichitdecreaseswithincreasingtemperaturewhilethepeakwavelengthchangesfrom528to523nm

from60to80°Candfrom523to525nmfrom80to100°C.Thecharacterizationconfirmsthatthereisa

minimumat80°C.Turkevichetal.alsoobservedaminimumsizeof16.5nmat80°C.88ChowandZukoski

found thatNPshadapproximately the same size at synthesis temperaturesof 60, 70 and80°Cbut it

took longer for NPs at lower temperatures to arrive at the final size.91 Wuithschick et al. found a

minimum of NP size at 60°C using the Turkevich method,104 while Piella et al. found a minimum at

70°C.105 Increasing temperatureaffectsanumberof factors in thesynthesis.TheconversionofAuCl4-

speciestohydroxylatedspeciesspeedsupathighertemperaturesmeaningthatnucleationperiodalso

terminatesfasterasthemorereactivespecies isconvertedtolessreactivespecies.Notealsothatthe

equilibriumshiftstomorehydroxylatedformsathighertemperatures.104However,therateofreduction

ofAuCl4- athighertemperatureswouldalsospeedup,resultinginahighernucleationrateinashorter


104

timeperiod. Finally, the sizeof the stablenucleiwould alsobe slightly larger at higher temperatures

becauseofthelargerthermalenergyavailable.104Thus,thereisabalanceofcompetingeffects,resulting

inaminimumsizeatanintermediatetemperature.Afurtherpointtonoteisthattheheattransferand

mixingwouldalsoplaya role inhowthe temperatureaffects theresultantNPs.For instance inbatch

synthesis, the citrate is injected at room temperature to boiling citrate, and so, depending on the

amountandnatureoftheinjection,thereactioncouldtakeplaceatdifferenttemperaturesinlocalized

regionsaroundtheinjectionpoint,whichmayexplainthevarietyofresultsreportedintheliterature.

The repeatability of the synthesis was investigated by repeating the experiment at each

temperature three times and characterizing the size using DCS (Appendix D, Figure D2). The

repeatability in sizewas found to beworst at 60°C and improvedwith increasing temperature up to

80°C,afterwhichitremainedfairlyconstant.

Chapter5

105

Figure5-7:TEMimagesofgoldNPssynthesizedusingtheCFRwithaCFIresidenceloopatatemperatureofA:60°C,

B:70°C,C:80°C,D:90°C,E:100°C. F:Averagediameterof goldNPsvs temperatureof synthesis (bars represent

standard deviation of the size). Concentration of tetrachloroauric acid, 0.557 mM; concentration of trisodium

citrate,0.09M;volumetricflowrateratio,32.3:1(Qout:Qin,HAuCl4:Na3citrate);molarflowrateratio,1:5(HAuCl4:

Na3citrate);flowrate,1ml/min.


106

Figure5-8:NormalizedDCScurvesofgoldNPssynthesizedusingtheCFRwithaCFIresidenceloopattemperatures

between60and100°C.Inset:DiameterofgoldNPsvs.temperatureofsynthesis(barsrepresentstandarddeviation

ofsize).Concentrationoftetrachloroauricacid,0.557mM;concentrationoftrisodiumcitrate,0.09M;volumetric

flow rate ratio, 32.3: 1 (Qout:Qin, HAuCl4: Na3citrate);molar flow rate ratio, 1:5 (HAuCl4: Na3citrate); flow rate, 1

ml/min.

Figure5-9:UV-VisspectraofgoldNPssynthesizedusingtheCFRwithaCFIresidenceloopattemperaturesbetween

60 and 100°C. Inset: Peak absorbance (black squares) and peak wavelength (red diamonds) of gold NPs vs.

temperature of synthesis. Concentration of tetrachloroauric acid, 0.557mM; concentration of trisodium citrate,

0.09 M; volumetric flow rate ratio, 32.3: 1 (Qout:Qin, HAuCl4: Na3citrate); molar flow rate ratio, 1:5 (HAuCl4:

Na3citrate);flowrate,1ml/min.

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107

5.2.3 Silver nanoparticle synthesis usinga coaxial flowdevice followedby a split and recombine

mixer

ASARmixerwasattachedontotheCFRtoseetheeffectthatmulti-laminationwouldhaveonthesilver

NPsynthesis.TheCFRhadaninnertubeI.D.of0.556mmandanoutertubeI.D.of2mm.Thedistance

from the inner tube outlet to the outlet of the CFR was 130 mm. The effect that the silver nitrate

concentration had on the silver NPs was investigated using this configuration, previously we have

studiedtheeffectofsilvernitrateconcentrationusingtheCFRinisolation.145Silvernitrateandtrisodium

citrate solutionswere premixed before pumping through the inner tubewith concentration of silver

nitrate varied between 0.05 and 0.4 mM, the trisodium citrate concentration was fixed at 0.5 mM.

Concentrationof sodiumborohydridewas fixedat0.3mMandpumped through theouter tube. The

volumetric flow rate ratiowas fixed at 1:1 (Qout:Qin) and the total flow ratewas fixed at 2.5ml/min.

Experimentswerecarriedoutatroomtemperaturebetween22-24°C.

Figure5-10showsTEMimagesofsilverNPssynthesizedatasilvernitrateconcentrationof0.05,

0.15,0.25and0.4mM.TheaveragediameteranddispersityoftheNPswere5.5±2.4nm,4.7±2.8nm,

4.4±1.8nmand3.4±1.4nm.T-testsforthePSDsshowedp-valuesof<0.001whencomparingallPSDs

exceptwhencomparing0.15and0.25mMwherethep-valuewas0.15.Inthiscasethereisnostatistical

differencebetween0.15mMand0.25mMaccordingtothep-value.Figure5-11showscorresponding

UV-Vis spectra. Peak absorbance increases with increasing concentration, the peakwavelength stays

relativelyconstantbetween398-400nmbetween0.05-0.25mM,howeverat0.3mMthewavelength

decreases to395nmandat0.4mM thewavelength is 392nm.Althoughpeakwavelength shouldn’t

change according to Mie theory for silver NPs less than 10 nm,139 a blue shift in peak wavelength

generallyindicatessmallerNPs.

ThelaminarflowprofileinsidetheCFRcreatesaninterfacebetweenthesilvernitrateandsodium

borohydride,resultinginmixingthroughdiffusionratherthanenhancedbyastretchingandthinningof

the lamellae though turbulent flow for example. The laminar flow profile prevents reaction in silver

nitrate rich zones resulting in smaller NPs. Larger NPs occur in silver nitrate rich zones because the

sodium borohydride can reduce multiples of its own stoichiometric equivalent in silver ions,133-135

resultinginlargerandlessstableNPswhichareabletogrow.ThroughtheSARmixerthelaminarflow

conditionismaintainedbutthemixingefficiencyisincreasedsincethediffusiondistanceisreducedas

thestreamiscontinuouslysplitandrecombinedinawaytoseriallylaminatetheflowintothinnerand

thinnermultiplesof theoriginal stream,aswell as the creationof an increasednumberof interfaces

between reagents. In the experiments above, increasing silver nitrate concentration, led to higher

supersaturation levels and thus a higher nucleation rate resulting in smaller NPs. In previous

experimentswehaveconductedusingonlythecoaxialflowreactor,theoppositetrendwasobserved;

NPsizeincreasedfrom3.7±0.8nmto9.3±3nmwithincreasingsilverconcentrationfrom0.05-0.4mM

with the same sodium borohydride and trisodium citrate concentrations (a comparison is shown in

Figure5-12).145ThisoccurredbecausereactionwasnotcompleteintheCFRandtheadvectivemixingin

thedroplets at theendof theCFRallowed thepossibility of sodiumborohydride to travel into silver

nitraterichzones,whichasdiscussedpreviously,wouldleadtolargerNPs.Sincedownstreammixingis


108

morecontrolledwiththeSARmixer,silvernitraterichzoneswhichgiverisetolargerNPsarelesslikely

to occur. The above indicates that the mixing condition of the fluid at the exit of the CFR has a

pronouncedeffectonNPsynthesis.

Figure 5-10: TEM images of silver NPs synthesized using the CFR followed by a SAR mixer at a silver nitrate

concentrationofA:0.05mM,B:0.15mM,C:0.25mM,D:0.4mM.Concentrationofsodiumborohydride,0.3mM;

concentrationoftrisodiumcitrate,0.5M;volumetricflowrateratio,1:1(Qout:Qin,NaBH4:AgNO3);molarflowrate

ratio,0.5-4:3:5(AgNO3:NaBH4:Na3citrate);flowrate,2.5ml/min;temperature,22-24°C.

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109

Figure 5-11:UV-Vis spectra of silver NPs synthesized using the CFR followed by a SARmixer at a silver nitrate

concentrations ranging between 0.05 and 0.4mM. Inset: Peak absorbance (black squares) and peakwavelength

(reddiamonds)ofsilverNPsvs.silvernitrateconcentrationofsynthesis.Concentrationofsodiumborohydride,0.3

mM;concentrationoftrisodiumcitrate,0.5M;volumetricflowrateratio,1:1(Qout:Qin,NaBH4:AgNO3);molarflow

rateratio,0.5-4:3:5(AgNO3:NaBH4:Na3citrate);flowrate,2.5ml/min;temperature,22-24°C.

Figure5-12:Diametervs.silvernitrateconcentrationfortheCFRoperatedfollowedwithaSAR(blacksquares)and

withoutaSAR(reddiamonds).Concentrationofsodiumborohydride,0.3mM;concentrationoftrisodiumcitrate,

0.5 M; volumetric flow rate ratio, 1: 1 (Qout:Qin, NaBH4: AgNO3); molar flow rate ratio, 0.5-4:3:5

(AgNO3:NaBH4:Na3citrate);flowrate,2.5ml/min;temperature,22-24°C.


110

5.2.4 SilvernanoparticlesynthesisusingacoaxialflowdeviceoperatedathighRe

Intheprevioussection(5.2.3),wehaveobserveddifferencesinNPsynthesiswhenmixingconditionsare

altereddownstreamoftheCFR.InthissectionweinvestigatetheeffectofmixingconditionsintheCFR

itself.ThisisachievedbyusingaCFRwithlowI.Dandhighflowrateoftheinnertube,toincreasetheRe.

Even though turbulent conditions are not achieved,mixing is enhanced by recirculation vortices that

appearundertheseconditions.188TheCFRhadaninnertubeI.D.of0.798mmandanoutertubeI.D.of

2mm.ThedistancefromtheinnertubeoutlettotheoutletoftheCFRwas130mm.Theconcentrations

of silver nitrate, trisodium citrate and sodium borohydride were varied so that the concentration

(assumingfullmixingofthestreams)foreachflowratetestedwasfixedat0.1mM,0.5mMand0.3mM

respectively. Silver nitrate and trisodium citrate were pumped through the inner tube and sodium

borohydridewaspumpedthrough theouter tube.Thevolumetric flowrate ratiowasvariedbetween

1:50and1:200 (Qout:Qin) and the total flow ratewasvariedbetween5.1and20.1ml/min.Themolar

flow rate ratio was fixed at 1:3:5 (AgNO3:NaBH4:Na3citrate). Experiments were carried out at room

temperaturebetween22and24°C.

Figure5-13showsTEMimagesofsilverNPssynthesizedintheCFRwithvortexflow.TheReynolds

number intheinnertubewasintherange132-530andinthemainchannelwas intherange54-212.

Theaveragediameteranddispersitywas5.9±1.5nm,6.2±2.7nm,7.7±3.4nmand6.2±2.2nmfor

5.1,10.1,15.1and20.1ml/minrespectively.Figure5-14showsflowvisualizationoftheCFRataflow

rateof20.1ml/mintodemonstratetheturbulentnatureoftheflow.Flowvisualizationusingdyeand

wateratotherflowratescanbeseeninAppendixD(FigureD3).

Flow rates of 5.1, 10.1 and 20.1ml/min showed similar sizes, although 5.1ml/min had a lower

polydispersity. T-tests showed that p-valueswere < 0.001when comparing 15.1ml/min to anyother

flowrate,and<0.05whencomparing5.1and20.1ml/min.Allotherflowratecomparisonsshowedthe

NPpopulationwasnotstatisticallydifferenttoanydegreeofconfidence.

TheNPsobtainedusingtheCFRoperatedinvortexflowshowedthattherewereminimalchanges

and no observable trends as flow rate was increased. Other systems such as iron oxide NPs and

polystyrene NPswere synthesized successfully with low dispersity using a CFR operated in turbulent

flow,131inthiscaseincreasingmixingefficiencyseemedtohavenosignificanteffectonthesynthesized

NPs in terms of size and polydispersity. This suggests that increasing mixing efficiency doesn’t

necessarilyleadtoincreasedmonodispersitybecauseofaperceivedimprovementinmixingefficiency,

although the smallest and least disperse NPs were obtained at the lowest flow rate 5.1 ml/min.

Observingthehydrodynamicsneartheoutletoftheinnertube,theconcentrationofthedye(thiswould

representsilvernitrateintheNPsynthesis)spreadsacrosstheentirecrosssectionofthechannel.The

steadystateconcentrationofdyenearthechannelwalls inthesectionimmediatelyafter jetemission

fromtheoutlet the inner tube ismostdiluteat the lowest flowrateand thensubsequentlybecomes

moreconcentrateathigher flowrates.Thisareathenbecomesmoreconcentrated insilvernitrateas

theflowrate increases.Thiswouldexplainwhythere isa lowersizeanddispersityat thelowest flow

rate,wheretheratioofsilvernitratetosodiumborohydrideislowerandhencemorestableNPswhich

arelesspronetogrowththroughcoalescencearesynthesized.

Chapter5

111

Figure5-13:TEMimageofsilverNPssynthesizedusingtheCFRwith innertubeI.D.of0.798mmoperatedunder

highRewithatotalflowrateofA:5.1ml/min,B:10.1ml/min,C:15.1ml/min,D:20.1ml/min.Theoutertubeflow

rate was fixed at 0.1 ml/min. Concentration of silver nitrate, 0.1 mM; of sodium borohydride, 0.3 mM;

concentrationof trisodiumcitrate,0.5M (all concentrations statedare thoseafter completemixing);molar flow

rateratio,1:3:5(AgNO3:NaBH4:Na3citrate);temperature,22-24°C


112

Figure5-14:FlowvisualizationofCFRwithinnertubeI.D.of0.798mmoperatedunderturbulentflowconditions.

BasicBluedyewaspumpedthroughtheinnertubeat20ml/minandwaterwaspumpedthroughtheoutertubeat

0.1ml/min. Re is 520 in the inner tube and 212 in themain channel. Picture 1 is taken before dye is pumped

throughtheinnertubeup,Pictures2-5asoperationaltimeincreased,Picture6aftersteadystateisreached.

5.2.5 EffectofinnertubeinternaldiameteronsilverNPsynthesisusingtheCFR

TheeffectoftheinnertubeI.D.onthesynthesisofsilverNPsintheCFRwasinvestigatedbyvaryingthe

innertubeI.D.between0.147and0.798mm.TheoutertubeI.D.was2mmandthedistancefromthe

innertubeoutlettotheoutletoftheCFRwas130mm.Theconcentrationofsilvernitratewas0.1mM,

whilethetrisodiumcitrateconcentrationwasfixedat0.5mM.Thesetwocomponentswerepremixed

andpumped through the inner tube.Concentrationof sodiumborohydridewas fixedat0.3mM; this

component was pumped through the outer tube. The volumetric flow rate ratio was fixed at 1:1

(Qout:Qin)andthetotalflowratewasfixedat1ml/min.

Figure5-15showsTEMimagesofsilverNPssynthesizedataninnertubediameterof0.147,0.345,

0.447,0.556,0.701and0.798mmwheretheaveragediameteranddispersityoftheNPsare4.7±1.4

nm, 5.9 ± 2.4 nm, 5.9 ± 2.2 nm, 6.6 ± 3.7 nm, 8.8 ± 2.6 nm and 10.5 ± 4.0 nm. T-tests for the PSDs

showedp-valuesof<0.001whencomparingallPSDsexceptwhencomparing0.345and0.447mm(p-

value=0.91),0.345and0.556mm(p-value=0.0012)and0.447and0.556mm(p-value=0.0012). In

thiscasethereisnostatisticaldifferencebetween0.345and0.447mminnertubeI.D.accordingtothe

p-value.0.345and0.447mmarestatisticallydifferentto0.556mmtoasignificancelevelof0.01.Figure

5-16showscorrespondingUV-Visspectra.Thepeakabsorbanceshowedamaximumat0.447mm(UV-

visdataofrepeatscanbeseeninAppendixD,FigureD4)andthepeakwavelengthremainedrelatively

constantbetween392and394nmforalldiameters.

ThesizeoftheNPsincreasedwithanincreaseintheinnertubeI.D.TheinternaltubeI.D.controls

thethicknessoftheinnerstream,andhencethediffusiondistanceacrossthestream.Byusingsmaller

innertubingI.D.,thediffusiondistanceisreduced.Byreducingthediffusiondistance,moreofthesilver

nitrateintheinnerstreamcanreactwithsodiumborohydride.Asthediffusiondistanceincreaseswith

increasingdiameter,thereisalargeramountofunreactedsilvernitrateattheendofthechannel.When

droplets format theoutlet, this increases the likelihoodof reactionsanduncontrolledgrowthofNPs

occurringinsilvernitraterichregionswithinthedroplets.HencesmallerNPsareobtainedbyreducing

thediffusiondistanceofthesilvernitratestreamwithintheCFR.

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113

Figure5-15:TEMimagesofsilverNPssynthesizedusingtheCFRwithinnertubediametersofA:0.142mm,B:0.345

mm, C: 0.447 mm, D: 0.556 mm, E: 0.701 mm, F: 0.798 mm, G: Nanoparticle diameter vs. inner tube internal

diameter.Concentrationofsilvernitrate,0.1mM;concentrationofsodiumborohydride,0.3mM;concentrationof

trisodium citrate, 0.5 M; volumetric flow rate ratio, 1: 1 (Qout:Qin, NaBH4: AgNO3); molar flow rate ratio, 1:3:5

(AgNO3:NaBH4:Na3citrate);flowrate,1ml/min,temperature22-24°C.


114

Figure 5-16:UV-Vis spectra of silver NPs synthesized using the CFRwith inner tube diameters ranging between

0.142and0.798mm.Inset:Peakabsorbance(blacksquares)andpeakwavelength(reddiamonds)ofsilverNPsvs.

inner tube internal diameter used in the CFR. Concentration of silver nitrate, 0.1mM; concentration of sodium

borohydride,0.3mM;concentrationof trisodiumcitrate,0.5M;volumetric flowrate ratio,1:1 (Qout:Qin,NaBH4:

AgNO3);molarflowrateratio,1:3:5(AgNO3:NaBH4:Na3citrate);flowrate,1ml/min,temperature22-24°C.

5.3 Conclusions

Silver and gold NPs were synthesized in continuous flow mode in a Coaxial Flow Reactor (CFR) in

conjunctionwithothermicrofluidiccomponentssuchasaContinuousFlowInverter(CFI)asaresidence

tile loopforgoldNPsynthesisoraSARmixer forsilverNPsynthesis.Sizecontrolwasachieved inthe

synthesisofgoldNPsbyvarying the flowrate.TheCFIprovideda laminar region inwhichnucleation

occurredat the interfaceof the twostreamsand theCFIprovidedamixing region (due to secondary

flowcirculation)withanimprovedresidencetime(becauseofflowinversion)forgrowthtooccur.Inthis

waynucleationandgrowthperiodscouldbecontrolled,resultinginanNPsizerangeof17.9-23.9nmfor

aflowraterangeof0.25-3ml/min.Aminimumsizeof17.9nmexistedat80°Cwhenthesynthesiswas

testedintemperaturerangeof60-100°C.Inbatchsyntheses,achangeinsizeisachievedbyalteringthe

concentrationsororderofreagentadditionratherthanthemixingconditions.Usingthisapproach,the

systemoffersahigherprecisionofcontroloverthemasstransfer,asopposedtoabatchreactor, ina

relativelysimplemanner(changingtheflowrateofreagents)toachieveavariationinsizeofgoldNPs.

TheCFRusedinconjunctionwiththeSARmixerproducedadecreasingsilverNPsizefrom5.5to3.4nm

with increasing silver concentration,which is the opposite trend to using the CFR in isolation. This is

becausetheSARmixerprovidedveryefficientmixinginawell-controlledmannertopreventreactionin

silvernitrate rich regions.TheCFRalsoallows someof the reaction tooccurbeforeentering theSAR

Chapter5

115

mixer.ThisreducedthefoulingoccurringintheverysmallandcomplexgeometryoftheSAR(usingthe

SARinisolationresultedinahigheramountoffouling).AvortexflowregimeathigherRenumberwas

testedintheCFRwithsizerangeof5.9-7.7nmforaflowraterangeof5.1-20.1ml/min.Notrendinsize

or dispersity was observed using this type of flow regime, even though the mixing efficiency was

increasedwithincreasingflowrate.Interestingly,thesmallestandleastdisperseNPswereobservedat

thelowestflowrate,whichwasmostlikelybecauseoftheregionclosetotheoutletoftheinnertube.

Thespreadofsilvernitrateacrossthechannelundertheseconditionswasrelativelysmallcomparedto

higherflowratesandNPspronetolessgrowthwouldbeformedinregionsdiluteinsilvernitrateand

concentratedinsodiumborohydride.DecreasingtheinnertubeI.D.intheCFRresultedinadecreasein

NP size from10.5 to4.7nm,due to the reduceddiffusiondistanceof the inner streamresulting ina

fasterconsumptionofsilvernitrateinawell-controlledmasstransferregion(i.e.withintheCFRrather

thanat theoutlet),whichwas supportedbyCFDmodellingof the reactionwithin theCFR.The study

demonstrates the versatility of using microfluidic devices and the potential benefits arising from

manipulatingthemasstransferandhydrodynamicsonNPsizeanddispersitybyusingtheCFRand/or

othercomponentsinconjunctionwiththeCFRforsynthesis.Thislevelofcontrolovermasstransferis

noteasilyachievedusingbatchreactors.

116

6 Masstransfereffectsinthebatchsynthesisofsilverandgoldnanoparticles

TherearemanysyntheseswhichinvestigatehowconcentrationsaffectthesizeoftheNPsandnaturally

different conditions yield different sizes. One factor that is rarely investigated in depth in batch

synthesisstudiesisthemasstransfer,whichisanimportantfactorsinceitalsohasamajorroletoplay

indeterminingatwhatconcentrationsreactionsoccurinlocalizedregionswithinthereactor.

Thebatch reactorsused in the following chapter employ activemixingwhereexternal energy is

appliedtomixthefluidswithin(inthiscaseamagneticstirbarandstirrermotor).Thistypeofmixing

enhancesadvectioninthefluid,leadingtomoreefficientmixing.Themicrofluidicdevicesdonotrelyon

external energy being applied, but rather their small dimensions and manipulations of the channel

geometrytoenhancemixing.Asdiscussedintheintroduction(Chapter1),masstransporthastwomain

mechanismswhicharediffusionandadvection.Microfluidicdevices relymainlyondiffusion formass

transport,incontrastthebatchreactorsusedinthefollowingchapteralsoemployadvectionasamajor

componenttotheoverallmasstransport.

SilverNPsarecommonlysynthesizedusingsodiumborohydrideasareducingagentwithinabatch

reactorvessel(chapter2).5,64-70Ingeneral,alteringthemolarratiosofreducingagenttoprecursorwas

themainmethod toachieve controlover the sizeof the silverNPs. Thishighlights the importanceof

concentration intheresultantNPsize.SimilarlyforgoldNPs,theTurkevichmethodisaverycommon

method employed. This involves the reduction of tetrachloroauric acid via the citratemolecule at an

elevated temperature. Many studies investigate this method for synthesis of gold NPs using batch

reactors (chapter 2).88-98, 100-106 The batch syntheses studies on the Turkevich method investigate

changing theorderof reagentaddition,but there isa lackof studieswhich focusspecificallyonmass

transferandhowitaffectstheresultantNPsizeanddispersity.

ThisstudytakesthesetwopopularmethodsforthesynthesisofsilverNPsandgoldNPsinbatch

vessels, and investigates the role mass transfer plays in the resultant size and dispersity of the

respectiveNPsystem.ThemixingtimeischaracterizedusingtheVillermaux-Dushmanreactionscheme

and various mixing configurations and order of addition of the reagents is investigated to draw

conclusionsontheeffectofmasstransfer.Forallofthemixingconfigurations,thefinalconcentration

aftermixingresultedinthesameconcentrationofreducingagentandprecursor.

6.1 Methodology

6.1.1 Chemicals

Silver nitrate (AgNO3, 0.01 M stock solution), trisodium citrate (HOC(COONa)(CH2COONa)2·2H2O,

powderform,sodiumborohydridesolution(NaBH4,~12wt%in14MNaOHstocksolution)andgold(III)

chloridehydrate(HAuCl4.xH2O,powderform),potassiumiodide(KI,99%)potassiumiodate(KIO3,98%)

andboricacid(H3BO3,99.5%)wereobtainedfromSigma.Sulfuricacid(H2SO4,3Mstocksolution)was

Chapter6

117

obtained from Alfa Aesar. All chemicals were used without further purification and solutions were

preparedwithultrapurewater(resistivity15.0MΩ·cm).


Syringepumps (Pump11EliteOEMmodule,Harvard) connected toa0.5mm internaldiameter (I.D.)

316LstainlesssteeltubeforsilverNPsanda0.5mmI.D.PTFEtubeforgoldNPswereusedtodeliver

oneofthereagentstothebatchvessel.Thebatchvesselwasa50mlsmallneckglassErlenmeyerflask

(Fisher Scientific). A digital hotplate stirrerwith temperature control (Stuart)was used togetherwith

PTFEstirbars(15mmx1.5mm,VWRinternational)tomixthereagentswithinthevessel.Aglycerine

bathwasusedtogetherwiththetemperaturecontrollertoheatthereagentswithinthebatchvesselin

thecaseofthegoldNPsynthesis.Acapwasusedtocoverthebatchvesselandthetubewasinserted

throughthecapforadditionofreagents.Thelengthofthetubecouldbeadjustedtoallowadditionof

reagents fromabove the solutionornear the tipof themagnetic stirbar.Thiswasdone toallow for

variation inmixingconditions.Figure6-1showsaschematicof theexperimental setups for silverand

goldNPsynthesis.

Figure6-1:SchematicofexperimentalsetupforsilverandgoldNPsynthesisconsistingofanErlenmeyerflaskwith

precursororreducingagentstirredviaamagneticstirrerwithasyringepumpreagentviatubinga)silverNPsystem

usingastainlesssteel0.5mmI.D.tubingandb)goldNPsystemusingaPTFE0.5mmI.D.tubingwithaglycerine

bathandtemperatureprobefortemperaturecontrolat80°C.Stirrerspeedwassetat500rpminbothcases.

Masstransfereffectsinthebatchsynthesisofsilverandgoldnanoparticles

118

6.1.3 Nanoparticlesynthesis

InatypicalsilverNPsynthesis,20mlofreagentwereplacedwithinthebatchvesselandstirredat500

rpm.Tothis,0.6mloftheotherreagentwasaddedviathesyringepumpandtubeateither:0.5ml/min

fromabove thesolution indropletsor,50ml/minwith the tube inserted into thesolutionwithin the

batchvesselnearthestirbartip.Thereagentswereamixtureofsilvernitrateandtrisodiumcitrate,and

thesodiumborohydridesolution.Theorderofmixingwastestedbyswitchingwhichreagentwasplaced

inthebatchvessel,either20mlofsilvernitrateandtrisodiumcitrateor20mlofsodiumborohydride

(sodiumborohydridewasstored in14Msodiumhydroxide leadingtoNaOHconcentrationbeing3.21

timeshigherthanthesodiumborohydrideconcentrationinallcases).Similarly,inthegoldNPsynthesis

20mlof reagentwasplaced in thebatchvessel and0.6mlof reagentwasadded to this in a similar

mannertothesilverNPsynthesis.20mloftetrachlorauricacidor20mloftrisodiumcitratewereplaced

inthebatchvessel.Thissolutionwasplacedintheglycerinebathwhichwasheatedto80°C.Totestthe

lengthoftimerequiredtobring20mlofsolutionto80°C,temperaturewasmeasuredovertimeafter

inserting solutionat room temperature into theglycerinebathand itwas found that after7min the

temperature reached a steady state of 80°C. Hence the solution in the experimentswas heated and

stirredfor15minbeforeadditionof0.6mlofreagentfromthesyringepump.


ForUV-Visanalysis,NPswereanalyzedusingaUV-Visspectrometer(USB2000+SpectrometerandDT-

Mini-2-GSlightsource,OceanOptics).SilverNPswereanalyzedwithinanhourofsynthesis(thesignalof

thesampleswerestableinthiswindowoftime).GoldNPswereanalyzedovertimeandwereanalyzed

oncethesignalwasstable(usually1-2days).NPsamplesweredilutedwithadditionalultrapurewaterto

bringtheabsorbanceintoasuitablerange(i.e.obeyingtheBeer-Lambertlawandavoidingsaturationof

the lightdetector) ifnecessary.Transmissionelectronmicroscope imageswerecapturedusinga JEOL

1200 EX ii microscope with a 120 kV acceleration voltage. Carbon coated copper TEM grids were

preparedwithin anhourof synthesis for silverNPs andwhen the samplewas stable for goldNPsby

pipettinga5µlsampleontothegridandallowingittodryatroomtemperature.Carewastakentonot

covertheentireTEMgridwiththedroplet,sothattheaggregatedringaroundtheedgeofthedroplet

couldeasilybeidentifiedasanartefactofsamplepreparation.Particlesizedistributions(insetsforeach

TEM image presented) have the following nomenclature: d is average diameter, δd is the standard

deviation of the NP distribution and n is the number of particles counted to obtain the particle size

distribution. Differential centrifugal sedimentation analysis (CPS disc centrifuge UHR, Analytik) was

carriedoutongoldNPswhenthesampleswerestable.PEBBLESsoftwarewasusedforthecountingand

sizingofTEMimagesofthesynthesisedsilverNPsandImageJsoftwarewasusedforthecountingand

sizingofTEMimagesofthesynthesisedgoldNPs.

Chapter6

119

6.2 Resultsanddiscussion

6.2.1 MixingCharacterization

1mlof0.07Msulfuricacidwasaddedto20mlofabuffersolutionconsistingof0.1818Morthoboric

acid,0.0909Msodiumhydroxide,11.67mMpotassiumiodideand2.33mMpotassiumiodate.Stirrer

speedwas set to 500 rpm and the twomixing configurations used in the NP synthesis were tested.

Thesewere:acidwasinjectedfromthecentreabovethebuffersolutionornearthestirbartip.Figure

6-2showsaschematic illustratingthetwoconfigurations.Figure6-3showsthecomputedsegregation

indexvs.mixingtimefortheconcentrationsetoftheexperimentsusingtheIEMmixingmodel(details

can be found in Appendix A). Eachmixing configuration was repeated three times and the triiodide

concentration was measured (extinction coefficient was found to be 23209 l/mol.cm, details on the

procedure used to find this can be found in the Appendix A). Themixing time averaged over three

repeatexperimentsforthe injectionfromabovethesolutionwas209mswhilethe injectionnearthe

stirbartipwas46ms.Areductioninmixingtimewasexpectedsincethereisamuchhighershearrate

near the stir bar tip, resulting in improvedmixing efficiency. Since both experiments used the same

concentrationset,injectionnearthestirbartipwasapproximately4timesmoreefficienttheninjection

fromabovethebuffersolution.

Figure6-2:Schematicrepresentationofinjectionpointswithinthebatchvesseltovarymixingefficiencya)injection

isaddedfromabovethesolutioninthebatchvesselandb)injectionisaddedclosetothestirbartip.


120

Figure6-3:Segregationindexvs.mixingtimeestimatedusingtheIEMmodelfortheVillermaux-Dushmannreaction

scheme.ConcentrationsofbuffersolutionwereKI:0.0117M,KIO3:0.0023M,NaOH:0.0909M,H3BO3:0.1818M.

ConcentrationofH2SO4was:0.07M.

6.2.2 Silvernanoparticlesynthesis

SilverNPsweresynthesizedinthebatchvesselbyreductionofsilvernitrateviasodiumborohydridein

the presence of trisodium citrate. The effect of changing the mixing configuration and the effect of

changing theorderof reagentadditionon thesizeanddispersityof theNPswas investigated.Mixing

configurations were those described in the previous section i.e. injection of reagent from above the

solutionorinjectionnearthestirbartip,andtheorderofreagentadditionwasachievedbyswapping

thereagentthatstartedinthebatchvesseli.e.asilvernitrateandtrisodiumcitratemixtureorsodium

borohydride. The general procedurewas to inject 0.6ml of reagent via tubing into 20ml of reagent

whichwasbeingstirredinthebatchvessel.FourconditionsweretestedandwillbedescribedasA-Ag-R-

S,B-Ag-R-F,C-Ag-P-SandD-Ag-P-F.Theorderofreagentaddition isdenotedbythe lettersRandP,R

representstheadditionof0.6mlofreducingagenttothebatchvesselandPrepresentstheadditionof

0.6mlofprecursortothebatchvessel.ThemixingconfigurationsaredenotedbythelettersSandF,S

denotes the additionof 0.6ml of reagent via droplets at a rateof 0.5ml/min fromabove the20ml

mixturewhileFdenotestheadditionof0.6mlofreagentnearthestirbartipatarateof50ml/min(for

example, the condition A-Ag-R-S describes the addition of 0.6 ml of reducing agent at a rate of 0.5

ml/minto20mlofprecursor).Table6-1summarizestheconcentrationsforeachmixingcondition(3to

1molarratioofborohydridetosilvernitratewasusedinallcases).

Chapter6

121

Table6-1:SummaryofconcentrationsforeachmixingconditionusedinthesilverNPsynthesis

Condition ReducingAgent–NaBH4 Precursor–AgNO3/Na3citrate

A-Ag-R-S 63.3mM 0.38/0.38mM

B-Ag-R-F 63.3mM 0.38/0.38mM

C-Ag-P-S 1.36mM 9.1/9.1mM

D-Ag-P-F 1.36mM 9.1/9.1mM

Figure6-4showsTEMimagesofthesynthesizedsilverNPswithanaveragediameterof11.1±3.0nm,

6.7±1.7nm,9.5±1.4nmand11.5±2.4nmforconditionsA-Ag-R-S,B-Ag-R-F,C-Ag-P-SandD-Ag-P-F

respectively. The average peak absorbance and average wavelength obtained from 3 repeat

experimentsobtainedbyanalysing thesilverNPsusingUV-vis spectroscopycanbeseen inFigure6-5

(thepercentageofdeviation inpeakabsorbance foreach conditionwas:A-Ag-R-S=3.2%,B-Ag-R-F=

2.1%, C-Ag-P-S = 2.2% and D-Ag-P-F = 2.7%). The peak wavelength had increased deviation for

conditionsA andDon repeat experiments.Overall therewas little variation in the valueof thepeak

wavelengthacrossalltheconditionstested.SincetheNPsare lessthanaround11nm, it isconsistent

with thebehaviour of little variation in peakwavelength forNPs below10nm.139 ConditionB-Ag-R-F

produced the smallest NPs followed by condition C-Ag-P-S, while conditions A-Ag-R-S and D-Ag-P-F

produced NPs of comparable size. The smallest NPs were produced when a small volume of

concentratedsodiumborohydridewasaddedquicklyintothedilutesilvernitrateandtrisodiumcitrate

mixture, orwhendrops of a concentratedmixture of silver nitrate and trisodium citratewere added

slowlytoasodiumborohydridesolution(conditionBandCrespectively).Conversely, largerNPswere

synthesizedwhendropsofconcentratedsodiumborohydridewereaddedslowlytoadilutemixtureof

silvernitrateandtrisodiumcitrateorwhenaconcentratedmixtureofsilvernitrateandtrisodiumcitrate

wereaddedquicklyintoadilutevolumeofsodiumborohydride.Inthissynthesis,trisodiumcitrateisthe

ligandandsinceitismixedwithsilvernitratepriortoadditionofsodiumborohydride,itcanbeassumed

that the ligand is available in the vicinity where silver nitrate is reduced by sodium borohydride.

However it is known that sodiumborohydride also behaves as a stabilizing agent,where the surface

charge of the particles is affected by borohydride ion adsorption.136 The difference in sizes obtained

using the different conditions is due to the availability of sodium borohydride in the local region

surroundingareaswheresilvernitratehasbeenreducedtosilvermetal.Thesilvermetalsubsequently

produces small silver clusters which are stabilized by sodium borohydride. Silver NPs catalyse the

hydrolysis of sodium borohydride which produces hydrogen and normally this catalysis leads to a

switchingpointwheretheNPssuddenlyincreaseinsizeduetoaggregation,68whichoccursbecausethe

stabilizing presence of borohydride ions is depleted through the hydrolysis reaction which provides

evidencefortheirstabilizingeffect.NoswitchingpointisobservedinthisstudyduetoanelevatedpH

becauseofthepresenceofsodiumhydroxide,suppressingthehydrolysisreaction.137Anotherimportant

point to note is that sodiumborohydride can reducemore than its own stoichiometric equivalent of

silver ions since it can provide multiple electrons in the reduction reaction.133-135 Therefore a small

amountofsodiumborohydrideca3enreducemanysilverions(whichsubsequentlyformsilverNPs),but


122

there may be a deficient amount of sodium borohydride to provide stabilization of NPs resulting in

growthtoalargersize.Thiswouldoccurthroughcoalescenceofsmallersilverclustersintolargersizes,

thereducedsurfacechargeresultinginthepossibilityofclustersapproachingeachothercloseenough

for attractive forces tobring them together. Coalescencehasbeen identified inprevious studies as a

maingrowthmechanisminthesilverNPformationprocess.68,82,139

Consideringthepropertiesandroleofthesodiumborohydrideinthereaction,aswellasthemass

transportmechanismswithinthebatchreactor,canhelptoexplaintheresultsobtained.Thesizeofthe

NPs obtained under conditions A-Ag-R-S and D-Ag-P-F can explained as follows: the conditions

potentiallycreatemixingconditions inwhichsilvernucleiareexposedtodeficientamountsofsodium

borohydridewithanabundanceofprecursorleadingtotheirgrowthtoalargersize.ConditionA-Ag-R-S

has concentrated sodium borohydride which is added slowly drop by drop. Since there is enhanced

advectionwithinthebatchvesselbecauseofthestirringaction,eachdropwillbemixedwiththebulkof

thesilvernitrate/trisodiumcitratesolutionbeforesubsequentdropsareadded.Thefirstfewdropscan

reduce themuch largermassof silver nitrate in thebatch vesselwhich createsNPsprone to growth

becauseofdeficiencyinsodiumborohydride.ConditionD-Ag-P-Fhasconcentratedsilvernitrateadded

quickly to dilute sodium borohydride. Since there is a sudden influx of a largemass of silver nitrate

becauseaddition isquick (0.6mlof fluid takes0.72 seconds toaddata flow rateof50ml/min), the

reactionofsilvernitratecanoccurinlocalizedregionsinthemicroscalewhereitishighlyconcentrated.

Thiswouldagain createNPsprone togrowth since the reactionand subsequentnucleationoccurs in

areaswherethesilvernitratetosodiumborohydrideratioishigh,resultinginalargerfinalsize.Inthis

case,ratherthantheenhancedadvectioncausinggrowthoftheNPs(asisthecaseforconditionA-Ag-R-

S),theslowdiffusiononthemicroscaleaftertheinfluxofasmallconcentratedvolumeofsilvernitrate

wouldcausereactionsinareasthatarerichinsilvernitrate.Thisleadstothepossibilityoflargenuclei

forming at the very beginning of the reaction because of decreased stability (sodium borohydride

deficiency) and would result in fewer nuclei which would subsequently grow into larger NPs. The

smallest NPs are obtained under the condition B-Ag-R-F, when a concentrated volume of sodium

borohydride isaddedquicklytoadilutevolumeofsilvernitrate.Comparedtothecasewheresodium

borohydrideisaddedslowly,inthiscasealargemassofsodiumborohydridebecomesavailablequickly

to the dilute volume of silver nitrate. This would create a condition in which there is initially an

abundanceofsodiumborohydrideinlocalizedregionsinthemicroscalebeforetheenhancedadvection

causescompletemixinginthebulkofthefluidwithinthevessel,resultinginsilverNPswhicharevery

stableandhencenotpronetogrowth.Asthemixingproceedsandcompletesonthemacroscale, the

sodiumborohydrideconcentrationwouldreduceduetomixingwithalargerdilutevolumeandbecause

of reaction that has occurred during the mixing process. The reaction can be visibly seen as a dark

brownplumeonadditionofsodiumborohydride immediatelyafter injection,afterwhichthesolution

transitionsintoabrightyellowcolouroncethemixturehasbeenhomogenizedthroughmixing.Thedark

brownplumecouldbe indicativeofaggregatedsilverNPsandthe transition toabrightyellowcolour

(indicativeofnon-aggregatedsilverNPs)maybebecauseofrapidacquisitionofstabilisingborohydride

causingtheaggregatedNPstorepeleachotherandseparate.Asthesodiumborohydrideconcentration

Chapter6

123

isreduced,reactionmayoccurinsomeregionswherethesodiumborohydridetosilvernitrateratiois

lowerandhencethepossibilityofsomelargerNPsmaybeproduced.Infact,thisiswhatisseeninthe

particlesizedistributionforconditionB-Ag-R-F.ThereisarelativelylargeamountofsmallNPs(around6

nm)with relatively small amountsof largerNPs (around10nm). Finally, conditionC-Ag-P-Sproduced

the most monodispersed NPs and these were of an intermediate size. Concentrated drops of silver

nitratewereadded toadilutevolumeof sodiumborohydride. In this case,each silvernitratedrop is

exposedtoamuchhighermassofsodiumborohydridesoisthereforeexpectedtomakestableNPsthat

arenotpronetogrowth.However,sincethemicromixingtimeisslowerusingthismixingconfiguration,

it is expected that some reactionwill take place as the concentrated silver nitrate drop ismixed on

addition to the dilute volume of sodium borohydride. This explains the intermediate size obtained,

wherethelocalizedsilvernitrateconcentrationishighbutissubsequentlyexposedtoanabundanceof

sodiumborohydride.Thecombinationofdiffusionandadvectionandhowitcanoffercontroloverthe

NPsizeisdemonstratedwellintheseexperiments.Becausediffusionisslow,largernucleicouldformin

thesilvernitraterichdropasitentersthebulkfluid(wherereactionoccursinsilvernitraterichregion

withinthefluidintheimmediatevicinityofthedrop)butthisisquicklysuppressedasadvectionaidsin

mixingthesilvernitratedroplet intothebulksodiumborohydridefluidtostabilise it. Inessence,each

dropof silvernitrate reacts to formsilverNPswhicharequickly stabilizedas theyaremixed into the

sodiumborohydridesolution,andfurtherdropsthenproduceNPswhichdonot interactwithexisting

NPsinthesolutionbecauseoftheirstabilitywhichexplainsthelowerdispersityoftheresultantNPs.It

is theconfigurationwhichgives themostuniformsodiumborohydrideconcentrationexposure to the

silverNPs,andpointstothesodiumborohydridebeingcrucialindeterminingtheNPsizeanddispersity

in this synthesis. Shirtcliffe et al. also compared different orders of reagent addition and found that

adding silver nitrate to sodium borohydride produced the narrowest and most reproducible UV-VIS

spectra, pointing to lower dispersity.65 Song et al. also noted that increasing sodium borohydride

resulted in decreasing aggregation, again highlighting the importance of sodium borohydride to the

stabilityofthesilverNPs.67


124

Figure 6-4: TEM images of silver NPs synthesized in a batch vessel under various conditions. A) 0.6 ml sodium

borohydride [63.3mM] added via drops to 20ml of amixture of silver nitrate [0.38mM] and trisodium citrate

[0.383mM]ataflowrateof0.5ml/min,B)0.6mlsodiumborohydride[63.3mM]addednearthestirbartipto20

mlofamixtureofsilvernitrate[0.38mM]andtrisodiumcitrate[0.383mM]ataflowrateof50ml/min,C)0.6mlof

amixtureofsilvernitrate[9.1mM]andtrisodiumcitrate[9.1mM]addedviadropsto20mlsodiumborohydride

[1.36mM]ataflowrateof0.5ml/minandD)0.6mlofamixtureofsilvernitrate[9.1mM]andtrisodiumcitrate

[9.1 mM] added near the stir bar tip to 20 ml sodium borohydride [1.36 mM] at a flow rate of 50 ml/min.

Experimentswereconductedatroomtemperatureandstirringspeedwassetat500rpm.

Chapter6

125

Figure 6-5:Averagepeak absorbance and averagepeakwavelengthobtainedusingUV-vis spectroscopyof silver

NPssynthesizedinbatchvesselundervariousconditions.A)0.6mlsodiumborohydride[63.3mM]addedviadrops

to20mlofamixtureofsilvernitrate[0.38mM]andtrisodiumcitrate[0.383mM]ataflowrateof0.5ml/min,B)

0.6mlsodiumborohydride[63.3mM]addednearthestirbartipto20mlofamixtureofsilvernitrate[0.38mM]

andtrisodiumcitrate[0.383mM]ataflowrateof50ml/min,C)0.6mlofamixtureofsilvernitrate[9.1mM]and

trisodiumcitrate[22.7mM]addedviadropsto20mlsodiumborohydride[1.36mM]ataflowrateof0.5ml/min

andD)0.6mlofamixtureofsilvernitrate[9.1mM]andtrisodiumcitrate[22.7mM]addednearthestirbartipto

20 ml sodium borohydride [1.36 mM] at a flow rate of 0.5 ml/min. Experiments were conducted at room

temperature and stirring speed was set at 500 rpm. Bars represent standard deviation of absorbance and

wavelengthfor3repeatedexperiments.

6.2.3 Goldnanoparticlesynthesis

GoldNPsweresynthesizedinthebatchvesselbyreductionoftetrachloroauricacidviatrisodiumcitrate

atatemperatureof80°C.Theeffectofchangingthemixingconfigurationandtheeffectofchangingthe

order of reagent addition on the size and dispersity of the NPs was investigated. The same four

conditionstothoseusedforsilverNPsynthesisweretestedandwillbedescribedasconditionsA-Au-R-

S,B-Au-R-F,C-Au-P-SandD-Au-P-F.Table6-2summarizestheconcentrationsforeachmixingcondition

(3to1ratioofcitratetotetrachloroauricwasusedinallcases).


126

Table6-2:SummaryofconcentrationsforeachmixingconditionusedinthegoldNPsynthesis

Condition ReducingAgent–Na3citrate Precursor–HauCl4

A-Au-R-S 92.7mM 0.556mM

B-Au-R-F 92.7mM 0.556mM

C-Au-P-S 2.78mM 18.5mM

D-Au-P-F 2.78mM 18.5mM

Figure6-6showsTEMimagesofthesynthesizedgoldNPswithanaveragediameterof18.0±4.8nm,

13.1±2.2nm,16.3±3.3nmand16.2±2.1nmforconditionsA-Au-R-S,B-Au-R-F,C-Au-P-SandD-Au-P-F

respectively. The average peak absorbance and average wavelength obtained from 3 repeat

experimentsobtainedbyanalyzingthegoldNPsusingUV-visspectroscopycanbeseeninFigure6-7(the

percentageofdeviationinpeakabsorbanceforeachconditionwas:A-Au-R-S=0.8%,B-Au-R-F=1.6%,

C-Au-P-S=0.5%andD-Au-P-F=0.2%).ThepeakwavelengthofNPssynthesisedusingconditionB-Au-R-F

was slightly lower than the other conditions in line with the average size of the NPs being smaller.

Diameterwasfoundtobe14.3±3.3nm,13.8±1.7nm,14.2±2.8nmand14.1±2.1nmbasedona

numberpercentageobtainedfromDCSmeasurementsforconditionsA-Au-R-S,B-Au-R-F,C-Au-P-Sand

D-Au-P-Frespectively.ThecurvesanddiametervsconditioncanbefoundinFigure6-8.

The largestandmostpolydisperseNPswereobservedusingconditionA-Au-R-Sandthesmallest

NPs were observed using conditions B-Au-R-F. Conditions C-Au-P-S and D-Au-P-F produced NPs of a

similarsizethoughD-Au-P-FproducedslightlymoremonodispersedNPs.ThereactionsintheTurkevich

method are complex because of the many different species which can coexist in the solution as it

proceeds. The precursor can exist in increasingly hydroxylated form fromAuCl4- toAuOH4

- ,95with the

hydroxylatedformbeingformedatelevatedpH.Accordingtothe literature, increasinglyhydroxylated

formsresultinadecreaseinreactivity.104Thiswouldresultinalowernucleationrateandhencelarger

NPs.Thecitratespeciesalsochanges itsspeciationdependentonpH.At lowerpH itexists in itsmost

protonated form of H3Cit with increasing deprotonation to Cit3- as pH is elevated while the HCit2-

speciesissuggestedtobethemostlikelyformresponsibleforreductionofthegoldprecursor.106Hence,

themixingofthesetworeagentsbecomesimportantbecausethecombinationoftheacidicprecursor

withthebufferreducingagentresultsinanevolutionofpHasmixingproceeds.Ideally,toachievesmall

monodispersedNPs,thenucleationperiodshouldbemaintainedtoproducemanynuclei(resultingina

small size) and there should be a good separation between nucleation and growth (resulting in

monodispersity). Nucleation occurs when there is high reaction rate, which depends on less

hydroxylatedformsofprecursorandmonoprotonatedcitratebeingpresent.Aseparationofnucleation

andgrowthisachievedbyatransitionfromhighreactionratetoalowerreactionrate.Thisoccursinthe

Turkevichmethodbecausethebuffercapacityofthecitratesolutionneutralizestheacidicprecursoras

mixingproceedswhilethepHincreasesastheprecursorreacts.Consideringtherolesofprecursorand

reducing agent, the results obtained from each condition can be explained. Condition A-Au-R-S

produced the largest and most disperse NPs. Concentrated citrate is added drop wise to the dilute

precursor.RelativelylargeNPsareproducedinthiscasebecausecitrateisaddedslowly(0.6mltakes72

Chapter6

127

s toaddata rateof0.5ml/min), resulting in the reduction reactionoccurringata relatively lowrate

sincethecitrateconcentrationwillbeperpetuallylow.Thisisthecasebecauseoftheenhancementsin

mass transport throughadvectionbecauseof the stirringmotion,whichhastens the incorporationof

citrate into the bulk fluid which causes dilution of the concentration. A low reduction reaction rate

results ina lowernucleation rateandhence largerNPs.Thepolydispersenatureof the samplearises

from theevolving citrate concentrationas the rateof additionwasquite slow. Initially there is a low

citrate to gold ratio, resulting in larger and less spherical shapedNPs.Over time this ratio decreases

resultinginsmallerandmoresphericalNPs.ConditionB-Au-R-FproducedthesmallestNPsbecausethe

citratewasaddedatahigherflowratenearthestirbartip.Thisresultedintheprecursorbeingexposed

quicklytoalargeramountofreducingagentincomparisontoconditionA-Au-R-S,resultinginahigher

reaction rate and subsequent amounts of nucleation leading to smaller NPs. In this case, the

concentratednatureofthecitrateleadstoahighreductionrateinthelocalvicinity,resultinginsmaller

and more numerous nuclei. As mixing proceeds through the aid of advection, the increased

concentration of nuclei would ultimately grow to a smaller size assuming the remaining unreacted

precursor is spreadequallyacrosseachnucleus.ConditionC-Au-P-SandconditionD-Au-P-Fproduced

NPs of a similar size. The addition of 0.6 ml to a larger volume of 20 ml wouldn’t reduce the

concentration of the larger volume significantly. In this case, the length of time required to add the

precursorisnotsignificantsinceit isallexposedtoapproximatelythesameconcentrationofreducing

agentresulting insimilarreactionratesandnucleationrates,which iswhyconditionsC-Au-P-SandD-

Au-P-FproduceasimilarsizeofNP.Sivaramanetal.notethatthereisnosignificantreductioninsizeof

goldNPsusingtheinversemethodforcitratetogoldmolarratios<5,102andweobservenosignificant

reductionaccording to this studyatacitrate togoldmolar ratioof5.Thequickadditionofcitrate to

tetrachloroauricaciddidyieldsmallerNPs,butthemanner inwhichreagentsareaddedhasnotbeen

thespecificfocusofotherstudiesofgoldNPsynthesesinbatchreactors.Anotherpointtonoteisthat

thefasteradditionratenearthestirbartipingeneralhadalowerdispersitythantheslowadditionvia

drops.Consideringthatthecitratebehavesasareducingagentandstabilizingagenttheconditionscan

becomparedwithrespecttothisreductionindispersity.Incomparingtheconditions,thisdifferencein

dispersitycanbeexplainedwiththeevolutionofconcentrationswithinthestirredfluid.Fastadditionof

citratecreatesburstnucleationinconditionB-Au-R-F,afterwhichthesolutionpHincreasesresultingin

a transition to growth. Slow addition of citrate in condition A-Au-R-S results in nucleation initially,

subsequentadditionofcitratetheninteractsnotonlywithgoldprecursorbutalsogoldnuclei.Thisgives

the possibility of both nucleation and growth occurring at the same time. In fact, this results in

somewhatofanoverlapbetweengrowthandnucleation.Inessence,thereisasharperseparationusing

conditionB-Au-R-F than conditionA-Au-R-S, resulting in adecreaseofdispersitywith fast additionof

citrate.Similarly,withconditionC-Au-P-SandD-Au-P-F,addingtetrachlorauricacidslowlyresultsinthe

possibilityofanoverlapofnucleationandgrowth,whereasthefastadditionwouldresult inasharper

separationofnucleationandgrowthresultinginlowerdispersity.


128

Figure 6-6:TEM images of goldNPs synthesized in a batch vessel under various conditions. A) 0.6ml trisodium

citrate[92.7mM]addedviadropsto20mloftetrachloroauricacid[0.556mM]ataflowrateof0.5ml/min,B)0.6

mltrisodiumcitrate[92.7mM]addednearthestirbartipto20mloftetrachloroauricacid[0.556mM]ataflow

rateof50ml/min,C)0.6mlof tetrachloroauricacid [18.5mM]addedviadrops to20ml trisodiumcitrate [2.78

mM]ataflowrateof0.5ml/minandD)0.6mloftetrachloroauricacid[18.5mM]addednearthestirbartipto20

mltrisodiumcitrate[2.78mM]ataflowrateof50ml/min.Experimentswereconductedat80°Candstirringspeed

wassetat500rpm.

Chapter6

129

Figure6-7:PeakabsorbanceandpeakwavelengthobtainedusingUV-visspectroscopyofgoldNPssynthesized in

batch vessel under various conditions. A - Au) 0.6ml trisodium citrate [92.7mM] added via drops to 20ml of

tetrachloroauricacid[0.556mM]ataflowrateof0.5ml/min,B-Au)0.6mltrisodiumcitrate[92.7mM]addednear

the stir bar tip to 20 ml of tetrachloroauric acid [0.556 mM] at a flow rate of 50 ml/min, C - Au) 0.6 ml of

tetrachloroauricacid[18.5mM]addedviadropsto20mltrisodiumcitrate[2.78mM]ataflowrateof0.5ml/min

andD-Au)0.6mloftetrachloroauricacid[18.5mM]addednearthestirbartipto20mltrisodiumcitrate[2.78

mM]ataflowrateof0.5ml/min.Experimentswereconductedat80°Candstirringspeedwassetat500rpm.Bars

representstandarddeviationofabsorbanceandwavelengthfor3repeatedexperiments.


130

Figure6-8:NormalizednumberpercentagevsdiameterobtainedusingDCSofgoldNPssynthesizedinbatchvessel

undervariousconditions.A-Au)0.6mltrisodiumcitrate[92.7mM]addedviadropsto20mloftetrachloroauric

acid[0.556mM]ataflowrateof0.5ml/min,B-Au)0.6mltrisodiumcitrate[92.7mM]addednearthestirbartip

to20mlof tetrachloroauricacid [0.556mM]ata flowrateof50ml/min,C -Au)0.6mlof tetrachloroauricacid

[18.5mM]addedviadropsto20mltrisodiumcitrate[2.78mM]ataflowrateof0.5ml/minandD-Au)0.6mlof

tetrachloroauricacid[18.5mM]addednearthestirbartipto20mltrisodiumcitrate[2.78mM]ataflowrateof

0.5 ml/min. Experiments were conducted at 80°C and stirring speed was set at 500 rpm. Inset is an average

diameterforeachconditionobtainedfrom3repeatexperiments.

6.3 Conclusions

Mixingefficiency inthebatchvesselwascharacterizedusingtheVillermaux-Dushmanreactionsystem

fortwodifferentmixingconfigurations:injectionofreagentfromabovethestirredsolutioninthebatch

vesselandinjectionofreagentnearthestirbartipofthestirredsolution.Itwasfoundthatthemixing

timewas209mswheninjectionwasfromaboveand46mswheninjectionwasnearthestirbartip.

SilverNPsweresynthesizedbyreducingsodiumborohydrideinthepresenceoftrisodiumcitrate.

Variousmixing configurationswere testedand itwas foundadditionof sodiumborohydrideat a fast

ratenearthestirbartipproducedthesmallestNPs(6.7±1.7nm).Ingeneral,largermorepolydispersed

NPs were found when the mixing conditions encouraged areas rich in silver nitrate and deficient in

sodiumborohydride.GoldNPswere synthesized using the Turkevichmethod,where tetrachloroauric

acidisreducedbytrisodiumcitrateatanelevatedtemperature.Usingthesamemixingconfigurationsas

those for the silverNP synthesis, the smallestNPs (13.1±2.2nm)wereobtainedwhen the reducing

Chapter6

131

agent(trisodiumcitrate)wasaddedatafastratenearthestirbartiptotheprecursor(tetrachloroauric

acid).Theseweresignificantlysmallerthanifthetrisodiumcitratewasaddedataslowratefromabove

thesolution(18±4.8nm),showingthatthemasstransferconditionsandhowthetrisodiumcitrateis

administeredtotheprecursorintheclassicalTurkevichmethodisimportant.Themixingconfiguration

didnotsignificantlyaffect thesizewhenusing the inversemethod, similar towhat isobserved in the

literature for similar citrate to gold molar ratios. However, fast addition of reagent produced lower

dispersitythanslowadditionbecauseitproducesasharperseparationofnucleationandgrowth.

The study highlights the importance of mass transfer conditions in determining the size and

dispersityoftheresultantNPs,evenwhenthemolarratioofreducingagenttoprecursorisfixed.Often,

batchreactorstudiesstatethemolarratioofsyntheses,butthismaybeinsufficienttoreproduceresults

sincespecificinformationonthemannerinwhichreagentsaremixedmayalsobeneeded.Theresults

demonstrate that both diffusion and advection mass transport mechanisms are important in

determiningtheresultantsizeanddispersityofsynthesisedNPs.Masstransportconceptsofadvection

anddiffusioncanexplainapparentdifferences insynthesesusingsimilarconcentrationof reagentbut

differentapparatus.

132

7 Conclusionsandfurtherwork

7.1 Conclusions

Theworkpresentedinthisthesisfocusesontheroleofmasstransferonthesizeanddispersityofsilver

and goldNPs synthesized inmicrofluidic devices. To achieve this, characterization of themicrofluidic

devices in termsofmass transfer characteristicswasmade tounderstandhowconcentrationprofiles

would evolve during a reaction. The concentrations at which reaction, nucleation and growth of NP

formationtakesplaceareveryimportantindeterminingtheirsizeanddispersity.Theobjectiveistobe

abletocontrolsizeanddispersityofsynthesizedNPssincethiscontrolstheirproperties,andhencetheir

suitabilityforuseinanyapplication.TheconcentrationatwhichtheprocessesforNPformationoccuris

controlledbymasstransfer.Subsequently,avarietyofmicrofluidicdevicesweretestedwithgoldand

silverNPsystemstoinvestigatehowchangingmasstransferconditionsaffectedthesizeanddispersity.

ThegoldNPsysteminvestigatedwasthewell-knownTurkevichmethod,wheretetrachloroauricacidis

reducedviacitrateatelevatedtemperatures.ThesilverNPsystemwasthereductionofsilvernitratevia

sodiumborohydrideinthepresenceofaligand(trisodiumcitrateorPVA).Themicrofluidicdevicesused

werethecoaxial flowreactor (CFR), impinging jet reactor (IJR),coiled flow inverter (CFI)andsplitand

recombine (SAR) mixer. A batch reactor study for the synthesis of silver and gold NPs was also

performed.

The primary objective of this study was to investigate how a variety of different mass transfer

conditionscanaffectthesizeanddispersityofsynthesisedNPs.Theoverarchingconclusionofthestudy,

afterinvestigatingavarietyofdifferentmasstransferconditionsusingmicrofluidicdevicesandabatch

reactorvessel,isthatmasstransferisanimportantpropertytoconsiderwhenmanipulationofsizeand

dispersityofNPsisdesired.ThevarietyofreactorsusedforthesynthesisofNPsinthisstudyeachhad

their own unique mass transport characteristics which can be summarised as follows: the CFR

maintained a separation of reagents through the channel because of the laminar flow and lack of

geometry manipulation to reduce diffusion distance, the IJR presented an efficient mixing device as

demonstratedbythelowmixingtimesfoundwiththeconditionstested(<10ms),theCFIutilisedDean

flowtoaidthemixingofthereagentswhileimprovingRTD,theSARalsopresentedanefficientmixing

device through channel geometry manipulation which reduces the diffusion distance and the batch

reactor vessel used external energy through the action of amagnetic stirrerwhich enhancedmixing

throughanadvectionmechanismtospeedupthebulkmixingofthefluid.

Tomanipulate the size of synthesised NPs, controlling reaction and subsequent nucleation and

growthisthemainconsideration.Theseprocessesaredrivenbytheconcentrationsatwhichtheytake

place,which is ultimately determined by themass transport. Another important consideration is the

stabilisationwhenachievingsmallmonodispersedNPs isdesired.This increases thecomplexityof the

problemofNPsizecontrol,particularlywhenthestabilisingagentbehavesasareducingagent(asisthe

casewiththereactionsstudiedinthiswork).TheCFRpresentsapromisingmicrofluidicdevicetocontrol

sizeofsynthesisedNPsbecauseofrestrictionofreactionataninterfacebetweenreagentstreams(due

Chapter7

133

tolaminarflowandsimplechannelgeometry).Thisallowsforareservoirofreagentswhicharesupplied

to the reaction interface at a constant concentration at a fixed point along the channel length. The

concentrationofreagentsdropsasreactionproceedsalongthelengthofthechannel,whichresultsina

naturaltransitionfromnucleationtogrowth.TheCFRalsosuppressesfoulingbecausethesynthesisof

nanoparticlesisprimarilytakingplaceawayfromchannelwalls.ThemajordrawbackoftheCFRisthatit

would require very long channels to allow reagents to fully mix (because mass transport through

diffusiononlyisaslowprocess),orthroughtheutilisationofsmallerchanneldimensions.Althoughitis

possibletoimplementtheCFRwithlongchannels,thismaybeunfeasiblebecauseofspacelimitations

within a laboratory, and the use of smaller channel dimensions would present the need for more

powerful (andexpensive)pumps.The IJRperformedextremelywellwhenconsidering theproblemof

foulingofchannelwallswhensynthesisingNPs.SincetherewerenochannelwallsintheIJR,fouling(or

cleaning)waseliminated. ItdemonstratedagoodcontroloverNPsizeand iseasilyscaledupthrough

numbering up. However, the operation window in terms of flow rates lacks flexibility because a

minimumflowrateisrequiredtoobtainjetswhiletoohighaflowratewouldresultinadeteriorationof

mixingquality.Ontopofthis,itisunknownhowtheflowrateratio(iftheflowratesfromeachjetare

notequal)wouldaffect themass transportproperties.TheCFIpresentsan improvedRTDbut fouling

wasobservedundercertainconditions,andtheSARsufferedthemostfromfoulingproblem(becauseof

ahighsurfaceareatovolumeratio)eventhoughitisanefficientmixer.Thismakesthesereactorsless

suitable for synthesis of NPs since they produce a dynamic condition as fouling evolves within the

channel over the course of the reaction. The batch reactor is a simple and effective means of

synthesisingNPs,whichisperhapsthebiggestadvantageinthesensethatitistheeasiesttoimplement

within the laboratory setting. It offers a quick and easyway to test a variety of synthesis conditions,

whichcanthenbe further investigatedwitha focusonmass transportwithin themicrofluidicdevices

presentedinthisworkaswellasotherspresentedintheliterature.Thelowsurfaceareatovolumeratio

withinthebatchreactorvesselsusedinthisworkminimisestheissueoffoulingbutneverthelessthere

wassomefoulingobservedonthePTFEmagneticstirrersusedinthesyntheses.Ofthedevicesstudied

inthiswork,theCFRandIJRpresentthemostpromisingreactortechnologiesforcontrollingthesizeof

NPsusingmicrofluidicdevices. It is hoped that this studyprovides aplatform for furtherwork in the

areaofsynthesisofnanomaterialsusingmicrofluidicdevices.

7.2 Furtherwork

Therearesomeexcitingdirections inwhichworkonsynthesisofNPs inmicrofluidicdevicescantake.

Ultimately,thegoalwouldbetoproducelargequantitiesofNPsinareliablemannerwiththeabilityto

tunethesizeconveniently.

TheCFRhasshownthepotentialtocontrolsizethroughvariationinresidencetimeusinglaminar

flow.AvortexflowregimeathigherReynoldsnumberwasalsotested,butdidn’tproducegoodresults

in termsof increasingcontroloversize.Furtherworkusing theCFRshould includeoperating itunder

turbulentconditionstoachievemixingkineticsfasterthanthereaction.Thiswouldintheoryallowfor

reproducible results sincemass transferwouldno longerplay a factorbecause concentrationswithin

thereactorwouldbehomogenizedbeforereactionoccurred. Astudyonthekineticsof thesilverNP

Conclusionsandfurtherwork

134

system to discover the reaction rate equation and kinetic rate constant would be beneficial to

determinehowefficientthemixingneedstobe.Theoreticallythisshouldincreasemonodispersitysince

reaction,nucleation,growthandstabilizationcantakeplaceatthesameconcentrationacrossthebulk

ofthefluid,ratherthanatvariouslocalizedconcentrationswithinthereactor.Discoveryofthereaction

rateequationandrateconstantwouldalsoenablethesimulationofaccurateCFDmodels,enablingthe

potentialtotestavarietyofreactordesignsinavirtualenvironmentratherthanhavingtofabricateand

test them experimentally. However, CFD can be computationally expensive for accuratemodels and

would be dependent on designing models which reduce computational expense or the use of very

powerfulcomputers.

TheIJRoffersexceptionalcontinuousflowcharacteristicsforsynthesisofNPs,namelythelackofa

possibilityofblockagethroughfouling.Becauseofthisthereisnoneedtocleanthereactorbeforeruns

withaquaregiaforinstance,meaningthatitcanproducevarioussamplesofNPsatdifferentflowrates

quickly. This makes it ideal to rapidly obtain information on new NP systems when the idea is to

investigatehowmasstransferaffectssizeanddispersity.OnelimitationoftheIJRhoweveristhatithas

anarrowrangeofoperability intermsofflowrates,becauseitrequiresacertainflowratetoactually

generatea jetand ithasa limiton themaximumflowrate since toohigha flow ratewill generatea

spraythatdoesnotmixefficiently.OneinterestingpossibilitywouldbetooperatetheIJRwithunequal

volumetricflowrateratios,to investigatehowthemasstransfer isaffected.HydrodynamicsoftheIJR

wouldbeaffected,withthecombinedstreambeingskewedinonedirectiondependentonwhichside

the jet velocity was higher. By knowing the mass transfer characteristics under these conditions,

operationunderunequalvolumetricflowratesforeachinputwouldallowanincreaseintheconditions

thatonecouldtestquicklyforanewNPsystem.

Flow reactors in general also afford benefits over batch reactors that would make them ideal

candidatesforinvestigatingkineticsandmechanismsofvariousNPsystems.Thisisbecauseofconcept

of space time ina flow reactorasopposed to reaction time inabatch reactor.More specifically, the

lengthor residence time ina reactorallows the reactions tobestudiedatanyparticularmoment.By

conductingin-linecharacterizationsuchasUV-VisorSAXS,onecanstudytheprocessofNPformationat

very small time resolutions (similar to the studiesbyPolteetal. andTakesueetal.)82, 121 This system

wouldalsoallowforawiderangeofconditionstobestudiedquicklywhenoperatedinconjunctionwith

a microfluidic device which can be operated continuously for long periods. This would help obtain

informationtounderstandthemechanismsofNPformationinarelativelyrapidmannerasopposedto

usingbatchreactors.

Synthesis of NPs in the batch reactor also revealed that different results can be obtained by

changing themass transferconditions.There isaneed formorespecific informationwhendescribing

how reagents aremixedwhen consideringbatch reactor synthesis studies.Batch reactors still offer a

verysimpleandeffectiveroutetosynthesizeNPs,andareagoodtooltoinformresearcherswhowant

to translate syntheses to flow. Itwould be interesting to see how the batch reactor results could be

reproduced in a flow system, for instance using a distributed feed to mimic the droplet addition of

reagenttothebulkinthebatchreactor.

Chapter7

135

136

AppendixA:MixingcharacterizationusingtheVillermaux-Dushmanreactions

Triiodideextinctioncoefficientevaluation

Aknownconcentrationof iodineismadebymixingastoichiometricbalanceof iodideandiodate ions

usingthefollowingconcentrations:

[KI]=1.13mM

[KIO3]=0.226mM

Tothisweaddanexcessofsulphuricacidtoconvertalltheiodideandiodateintoiodineaccordingto

thereaction:

𝐼𝑂QK + 5𝐼K + 6𝐻T → 3𝐼9 + 3𝐻9𝑂

14.2mMH2SO4 isaddedtoamixtureof2.26mMKIand0.452mMKIO3withequalvolumes(i.e. the

volumeofacidisequaltothatoftheiodide/iodatemixture).Thisresultsinaconcentrationaftermixing

of 7.1mMH2SO4, 1.13mMKI and 0.226mMof KIO3. By combining these reagents in the quantities

above,oneobtainsthefollowingiodineconcentration:

5 𝐼𝑂QK = 𝐼K =53𝐼9 = 1.13𝑚𝑀

𝐼9 = 0.68𝑚𝑀

Diluting the resulting solution to 20%, 40%, 60%, 80% and 100% the original concentration and

measuringtheabsorbanceat460nm(wavelengthatwhich iodineabsorbs lightsignificantly),onecan

obtain theextinction coefficient for iodine. FigureA1 shows theobtained calibration curve for iodine

fromwhich an extinction coefficient of 626 l/mol.cm is obtained (literature value of 758 l/mol.cm is

reportedforiodineat460nm).189

137

FigureA1:Calibrationcurvefor iodine,absorbancevs.concentration,withstraight lineusedtoobtain

theextinctioncoefficientofiodine.

Followingthis,wepreparesolutionswiththenecessaryconcentrationfortriiodideextinctioncoefficient

calculations. To do this the saturationmethod is used. In thismethod, we take a known amount of

iodineandmixwithanexcessofiodideasperthefollowingreaction:

𝐼K + 𝐼9 ↔ 𝐼QK

By using a large excess of iodide ions (saturation), the equilibrium is driven largely in favor of the

triiodideion.Thus,1mlof0.321MKIwasaddedto1mlof92.88µMiodine.Thepreparedsolutionis

thendilutedtotheappropriateconcentrationsandtheabsorbanceismeasuredat353nmtoobtainthe

calibration curve and extinction coefficient for triiodide. A value of 23209 l/mol.cm is found for the

triiodideextinctioncoefficientshownFigureA2.

y=626.07x

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.00E+00 2.00E-04 4.00E-04 6.00E-04 8.00E-04

Absorban

ce@

460nm

Iodineconcentration(M)

138

FigureA2:Calibrationcurvefortriiodide,absorbancevs.concentration,withstraightlineusedtoobtain

theextinctioncoefficientoftriiodide.

y=23209x

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.00E+00 2.00E-05 4.00E-05 6.00E-05

Absorban

ce@

353nm

Triiodideconcentration(M)

139

gProms model for solution of differential mass balance equations for the Villermaux-Dushman

reactions:

Model:

PARAMETER

AlphaASREAL#Volumetricfractionofacid

k1ASREAL#reactionrateconstantforreaction1,l/mol.s

k3ASREAL#forwardreactionrateconstantforreaction3,l/mol.s

k3primeASREAL#backwardreactionrateconstantforreaction3,1/s

kASINTEGERDEFAULT6#chemicalspecies(1=H+,2=H2BO3-,3=I-,4=IO3-,5=I2,6=I3-)

sASINTEGERDEFAULT2#streamnumber(1=acidstream,2=reagentstream)

tmASREAL#mixingtime,s

VARIABLE

k2ASNoType#Reactionrateconstantforreaction2,whichvarieswithionic

strength,l/mol.s

CASARRAY(k,s)OFMolarConcentration#Concentrationofcomponentkinstreams,mol/l

CaASARRAY(k)OFNoType#Averageconcentrationofcomponentk,mol/l

r1ASARRAY(s)OFReactionRate#Reactionrateforreaction1instreams,mol/l.s



IASMolarConcentration#ionicstrength

EQUATION

#DifferentialMassbalanceequationsforeachspecies

#chemicalspecies(1=H+,2=H2BO3-,3=I-,4=IO3-,5=I2,6=I3-)

FORm:=1TOsDO

$C(1,m)=(Ca(1)-C(1,m))/tm-r1(m)-6*r2(m);

$C(2,m)=(Ca(2)-C(2,m))/tm-r1(m);

$C(3,m)=(Ca(3)-C(3,m))/tm-5*r2(m)-r3(m);

$C(4,m)=(Ca(4)-C(4,m))/tm-r2(m);

$C(5,m)=(Ca(5)-C(5,m))/tm+3*r2(m)-r3(m);

$C(6,m)=(Ca(6)-C(6,m))/tm+r3(m);

END

#averageconcentrationofspecies

FORj:=1TOkDO

Ca(j)=(1-alpha)*C(j,2)+alpha*C(j,1);

END

140

#reactionrateequations

FORz:=1TOsDO

r1(z)=k1*C(1,z)*C(2,z);

r2(z)=k2*C(1,z)^2*C(3,z)^2*C(4,z);

r3(z)=k3*C(5,z)*C(3,z)-k3prime*C(6,z);

END

Process:

UNIT

IEMASIEM_VMD

SET

WITHINIEMDO

tm:=1e-4;#mixingtime,s

k1:=1e20;#reactionrateconstantforreaction1,l/mol.s

k3:=5.6e9;#forwardreactionrateconstantforreaction3,l/mol.s

k3prime:=7.5e6;#backwardreactionrateconstantforreaction3,1/s

Alpha:=0.5;#Volumetricfractionofacid

END

EQUATION

#Equations

WITHINIEMDO

I=0.5*(C(1,1)+C(1,2)+C(2,1)+C(2,2)+C(3,1)+C(3,2)+C(4,1)+C(4,2)+C(6,1)+C(6,2));

k2 = 10^(9.28-3.66*I^0.5); #Reaction rate constant for reaction 2, which varies with ionic strength,

l/mol.s

END

INITIAL

WITHINIEMDO

C(1,1)=0.04; #acidconcentration

FORa:=2TOkDO #setsallotherspeciesconcentrationtozero

C(a,1)=0;

END

#chemicalspecies(1=H+,2=H2BO3-,3=I-,4=IO3-,5=I2,6=I3-),

#setconcentrationforbuffer

#conc.mol/l

C(1,2)=0;

C(2,2)=0.09/2;

C(3,2)=0.032;

141

C(4,2)=0.006;

C(5,2)=0;

C(6,2)=0;

END

SOLUTIONPARAMETERS

ReportingInterval:=1e-3;

SCHEDULE

#OperationSchedule

Continuefor10;

142

AppendixB:Synthesisofsilvernanoparticlesinamicrofluidiccoaxialflowreactor

FigureB1:Microscopeimageshowingthefoulingofthechannelinasplitandrecombinemicromixeraftersynthesis

ofsilvernanoparticlesusingsilvernitrate,sodiumborohydrideandtrisodiumcitrate.

FigureB2:PeakabsorbanceofsilverNPssynthesizedatvarioustotalflowratesintherange1-14ml/min.Synthesis

was repeated four times for each flow rate. Concentrations of silver nitrate 0.2mM, trisodium citrate 0.2mM,

sodiumborohydride0.3mM.0.798mminnertubeI.D.

143

Figure B3: UV-vis spectra of silver NPs synthesized at various total flow rates in the range 1-14 ml/min.

Concentrationsof silvernitrate0.2mM, trisodiumcitrate0.2mM,sodiumborohydride0.3mM.0.798mm inner

tubeI.D.

FigureB4:Foulingontheinnerwalloftheinnertubewhensilvernitratesolutionflowedthroughtheinnertube

Fig.B5:Foulingontheouterwalloftheinnertubewhensilvernitratesolutionflowedthroughtheoutertube

144

AppendixC:Synthesisofsilvernanoparticlesusingamicrofluidicimpingingjet

reactor

Reproducibilityofsynthesis

The reproducibilityof the syntheses in the IJRwas investigatedby repeatingexperimentswithcitrate

and PVA. The peak absorbance obtained from UV-Vis spectroscopy was used as a measure for the

reproducibility. Figure C1 shows reproducibility for the synthesis in the 0.5mm I.D. tubing IJR using

citrate as a ligand. FigureC2 shows reproducibility for the synthesis in the0.25mmand0.5mm I.D.

tubingIJRusingPVAasaligand.

Figure C1: Peak absorbance (peak wavelength varied between 386-389 nm) obtained from UV-Vis

spectroscopyvs.WebernumberofsilverNPssynthesizedatdifferenttotalflowratesusing0.5mmI.D.

tubingIJR.Experimentsateachflowratewererepeatedthreetimes.Concentrationofsilvernitrate0.9

mM,trisodiumcitrate6mMininput1andsodiumborohydride1.8mMininput2.

145

Figure C2: Peak absorbance (peak wavelength varied between 398-401 nm) obtained from UV-Vis

spectroscopyvs.WebernumberofsilverNPssynthesizedatdifferenttotalflowratesusing: i)0.5mm

(black squares) and ii) 0.25 mm (red triangles) I.D. tubing IJR. Each flow rate was repeated twice.

Concentrationof silver nitrate 0.9mM, PVA0.02wt% in input 1 and sodiumborohydride 1.8mM in

input2.

146

ComparisonofhydrodynamicsbetweenwateronlysystemandsilverNPsynthesisusingPVAin0.25

mmIJR

FigureC3comparesthehydrodynamicsoftheimpingingjetsinthe0.25mmI.D.tubingIJRwhenwater

isusedandforthesilverNPsynthesiswhereoneofthestreamscontainsPVA.

FigureC3:Highspeedcameraimagesofhydrodynamicsinthe0.25mmI.D.tubingIJRforasystemusingonlywater

and under silver NP synthesis conditions of concentration of silver nitrate 0.9 mM, PVA 0.02 wt%, sodium

borohydride1.8mM.Totalflowrateswere:A:18ml/min(We:32),B:22ml/min(We:48),C:26ml/min(We:68),

D:30ml/min(We:90),E:34ml/min(We:115),F:38ml/min(We:144)andG:42ml/min(We:176).

147

TEMimageofsilverNPssynthesizedusinganincreasedPVAconcentration

FigureC4showsthesilverNPsobtainedwhenthePVAconcentrationwasdoubled.

FigureC4:TEMimageandparticlesizedistributionofsilverNPssynthesizedusingthe0.5mmI.D.tubingIJRata

flow rateof72ml/min (We:65).Concentrationof silvernitrate0.9mM,PVA0.04wt%, sodiumborohydride1.8

mM.

148

AppendixD:Effectofhydrodynamicsandmixingconditionsonthecontinuous

synthesisofgoldandsilvernanoparticlesinacoaxialflowdevice

GoldNPsynthesisrepeatability

FigureD1:Averagediameterof goldNPs synthesizedusing theCFRwithaCFI residence loopat flow

ratesof0.5,1and3ml/minobtainedusingDCSanalysis.Concentrationoftetrachloroauricacid,0.557

mM; concentration of trisodium citrate, 0.09M; volumetric flow rate ratio, 32.3: 1 (Qout:Qin, HAuCl4:

Na3citrate);molarflowrateratio,1:5(HAuCl4:Na3citrate);temperature,80°C.

149

Figure D2: Average diameter of gold NPs synthesized using the CFR with a CFI residence loop at

temperatures between 60-100°C obtained using DCS analysis. Concentration of tetrachloroauric acid,

0.557 mM; concentration of trisodium citrate, 0.09 M; volumetric flow rate ratio, 32.3: 1 (Qout:Qin,

HAuCl4:Na3citrate);molarflowrateratio,1:5(HAuCl4:Na3citrate);flowrate,1ml/min.

150

VisualizationofflowintheCFRusingturbulentconditions

Figure D3: Flow visualization in the CFR with an inner tube I.D. of 0.798 mm at different Reynolds

numbersa)5.1ml/min(Reinnertube,133,Remainchannel,54),b)10.1ml/min(Reinnertube,265,

Remainchannel,107)andc)15.1ml/min (Re inner tube,397,Remainchannel,160).BasicBluedye

was pumped through the inner tube between 5 and 15ml/min andwaterwas pumped through the

outertubeatafixedflowrateof0.1ml/min.1indicateswhendyeisinitiallypumpedthroughtheinner

tubewithincreasingnumbersgoinguptoshowtheCFRatsteadystate.

151

SilverNPsynthesisrepeatabilitywithinnertubeI.D.variation

FigureD4: Peak absorbance obtained fromUV-Vis spectra vs. inner tube internal diameter of repeat

experiments for silver NPs synthesized using the CFR with inner tube internal diameters ranging

between 0.142 and 0.798 mm. Concentration of silver nitrate, 0.1 mM; concentration of sodium


NaBH4:AgNO3);molarflowrateratio,1:3:5(AgNO3:NaBH4:Na3citrate);flowrate,1ml/min,temperature

22-24°C.

152

Listofpublications

Thefollowinglistarepublicationswhichresultedfromtheworkcontainedinthisthesis:

1. R.Baber,L.Mazzei,N.T.K.ThanhandA.Gavriilidis,RSCAdvances,2015,5,95585-95591.

2. R.Baber,L.Mazzei,N.T.ThanhandA.Gavriilidis,JournalofFlowChemistry,2016,1-11.

153

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