Nanoscale

PAPER

Publ

ishe

d on

27

Janu

ary

2014

. Dow

nloa

ded

by N

atio

nal D

ong

Hw

a U

nive

rsity

Lib

rary

on

16/0

3/20

14 0

9:32

:18.

View Article OnlineView Journal | View Issue

Department of Mechanical Engineering, KAI

305-701, Korea. E-mail: hjsung@kaist.ac.kr

Cite this: Nanoscale, 2014, 6, 2840

Received 19th November 2013Accepted 27th November 2013

DOI: 10.1039/c3nr06141h

www.rsc.org/nanoscale

2840 | Nanoscale, 2014, 6, 2840–2846

Continuous synthesis of zinc oxide nanoparticles ina microfluidic system for photovoltaic application

Hyun Wook Kang, Juyoung Leem, Sang Youl Yoon and Hyung Jin Sung*

This study describes the synthesis of zinc oxide nanoparticles (ZnO NPs) using a microfluidic system. A

continuous and efficient synthetic process was developed based on a microfluidic reactor in which was

implemented a time pulsed mixing method that had been optimized using numerical simulations and

experimental methods. Numerical simulations revealed that efficient mixing conditions could be

obtained over the frequency range 5–15 Hz. This system used ethanol solutions containing 30 mM

sodium hydroxide (NaOH) or 10 mM dehydrated zinc acetate (Zn(OAc)2) under 5 Hz pulsed conditions,

which provided the optimal mixing performance conditions. The ZnO NPs prepared using the

microfluidic synthetic system or batch-processed system were validated by several analytical methods,

including transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), X-ray

diffraction (XRD), UV/VIS NIR and zeta (z) potential analysis. Bulk-heterojunction organic photovoltaic

cells were fabricated with the synthesized ZnO NPs to investigate the practicability and compared with

batch-process synthesized ZnO NPs. The results showed that microfluidic synthesized ZnO NPs had

good preservability and stability in working solution and the synthetic microfluidic system provided a

low-cost, environmentally friendly approach to the continuous production of ZnO NPs.

Introduction

Zinc oxide nanoparticles (ZnO NPs) are widely used as n-typesemiconductor materials in the preparation of useful nanoscaledevices that depend on a direct wide band gap (3.37 eV), a highlyselective sensitivity for certain chemical species, piezoelectriccharacteristics, electrical and optical properties, and a large(60 meV) excitation binding energy.1,2 The unique collection ofproperties displayed by ZnO NPs makes the material useful intransparent conducting electrodes for photovoltaic devices,3

photonic sensors,4 and chemical sensors,5,6 and the NPs provideexcellent seed materials in ZnO nanowire synthesis.7–10 ZnO NPsmay be synthesized by a variety of methods, including copre-cipitation,11 sol–gel,12 plasma reaction,13 and solution-basedmethods.14,15 Among these methods, the solution-basedmethods are most amenable to integration with applicationsbecause they are low-temperature, rapid, and environmentallyfriendly processes. The cost-effectiveness of ZnO NP productionmay be improved by simplifying the synthetic process, as in acontinuous microuidic manufacturing system.

Microuidic devices have been developed in the context ofphysics, biology, chemistry, and engineering applications. Mostapplications rely on control over small volumes of uids on themicroliter scale. From miniaturized fuel cells to DNA and lab-on-a-chip devices, microuidic systems provide special

ST, 291 Daehak-ro, Yuseong-gu, Daejeon

functions and a high efficiency.16–19 As an example of a micro-uidic chemical synthesis application, a micro-reaction systemwas designed to enable reactions involving multiple chemicalreagents, thereby offering continuous operation, enhancingheat and mass transfer, reducing the reaction time andvolumes, and protecting the reaction from air and moisture in amicroscale device with a closed reaction environment.20–23 Insuch systems, microuidic mixing increased the rate of diffu-sion between two reagent delivery solutions. The benetsderived from rapid diffusion have motivated progress towarddevice miniaturization and mass production through the inte-gration of microscale devices into a large plant system. Micro-uidic mixing can proceed either actively or passively. Activemixing involves the application of external forces, such asacoustic,24 ultrasonic,25 electrokinetic,26 magnetic,27 orthermal28 elds, to disturb the sample reagents during a mixingprocess. In addition to these mixing enhancement methods, thetimed pulsing of reagent ows has been explored. Severalnumerical simulation and experimental studies have examinedmixing enhancement using time pulsing techniques. Glasgowand coworkers investigated the parameters associated withpulsed ow mixing using computational uid dynamics andow visualization schemes.29,30 They showed that a higherStrouhal number (ratio of the ow characteristic time scale tothe pulsing time period) and pulse volume ratio (ratio of thevolume of a pulsed uid to the volume of an inlet intersection)induced better mixing. The pulse waveform and Reynoldsnumber were found to have relatively insignicant effects on the

This journal is © The Royal Society of Chemistry 2014

Paper Nanoscale

Publ

ishe

d on

27

Janu

ary

2014

. Dow

nloa

ded

by N

atio

nal D

ong

Hw

a U

nive

rsity

Lib

rary

on

16/0

3/20

14 0

9:32

:18.

View Article Online

mixing performance of a pulsed mixing system. Lim andcoworkers developed a pulsed ow approach using electro-osmotic ow as the external driving force and a T-shapedmicrochannel.31 They showed that mixing could be enhanced byincreasing the pulsed ow frequency up to an optimalfrequency of 4–6 Hz. Subsequent increases in the pulsed owfrequency reduced the homogeneity of mixing. Wang andcoworkers demonstrated the preparation of a magnetic particle-driven micromixer, which was also characterized by an optimalfrequency that depended on the dimensions of the micro-channel.32 In this case, the pulsed ow was driven by magneticforces acting onmagnetic particles present in the working uid,which, unfortunately, prevents the application of this approachto general chemical analysis.

Here, we report the design of a continuous ZnO NPssynthetic process using a microuidic system. The system wasoptimized for rapid synthesis through numerical simulationand experimental time pulsed mixing studies. The microuidicsynthetic ZnO NPs were compared with batch-process syntheticZnO NPs to investigate the advantages of the microuidicsynthetic system. For more applications, organic photovoltaiccells were fabricated and analyzed. The process described herepresents an environmentally friendly, fast, and low-cost massproduction system that may potentially be integrated into alarge chemical plant.

ExperimentalMicrouidic system

A microuidic system for ZnO NPs synthesis was fabricated byapplying deep reactive ion etching (DRIE) techniques to asilicon substrate. The microchannels were 200 mm wide and200 mm deep. The initial DRIE process generated the 200 mmdeep channels. The silicon substrates containing the micro-channels were then anodically bonded to a glass coverslip. Asshown in Fig. 1a, the channels formed three distinct regions toallow for preheating, mixing, and synthesis, with two inlet portsand one outlet port. A photograph of the fabricated

Fig. 1 (a) Schematic diagram of the microfluidic channel used for ZnONPs synthesis. Microfluidic channels included preheating, mixing, andsynthesis regions, with two inlets and one outlet. (b) A deep reactiveion etched microfluidic channel with an anodic bonded glass cover.

This journal is © The Royal Society of Chemistry 2014

microchannels is shown in Fig. 1b. The synthetic process pro-ceeded by injecting chemical reagents into the inlet portsdirectly connected to the preheating region. The 92.5 mm-longpreheating region functioned both to preheat and to stabilizethe reagents. The preheated reagent ows converged at theconuence of the mixing region. Aer completion of mixing,the mixed reagents owed into a 341.5 mm-long synthesisregion where the chemicals reacted under uniform heatingconditions. Aer reacting, the reaction solution was thencollected in a reservoir via the outlet port.

Chemical preparation

The chemicals were prepared according to the methoddescribed by Pacholski, with modications.15 Ethanol served asa working uid and as the solvent. The sodium hydroxide(NaOH, Sigma Aldrich) and zinc acetate dehydrate (Zn(OAc)2,Sigma Aldrich) solutions were prepared to have concentrationsof 30 mM and 10 mM, respectively. The solutions were injectedinto the microchannel through Teon microtubing using asyringe pump (Nemesys, Cetoni). The injected volume ratio ofthe two solutions was 1 : 1.924 NaOH : Zn(OAc)2. During thesynthetic process, the microchannel was heated at 60 �C (K typethermocouples) using a lm heater controlled by a proportionalintegral derivative (PID) controller. The synthesized ZnO NPswere cooled to room temperature (25 �C) and stored in a glassbottle.

Photovoltaic device fabrication

The bulk-heterojunction organic photovoltaic cells were fabri-cated by depositing the following thin layers onto the indiumtin oxide coated glass (15 U sq�1): 80 nm of ZnO NPs (spincoated with 1000 rpm for 30 s aer drying at 150 �C for 10 min),400 nm of P3HT:PCBM, 1 mm of PEDOT:PSS, and 1 mm of Ag(printed by screen printer). The fabricated cell area was 1 cm�1.

Measurements

The transmission electron microscopy (TEM) images and energydispersive spectrometry (EDS) data were acquired using anFE-TEM (Tecnai F20, Philips). X-ray diffraction (XRD) measure-ments were collected using a thin lm X-ray diffractometer(D/MAX-RC, Rigaku) by q–2q scanning. Zeta (z) potentialmeasurements were performed using a zeta analyzer (ELS-22,Otsukael) based on Smoluchowski's theory. Specular trans-mission spectra were obtained using a UV/VIS NIR spectropho-tometer (V-570, Jasco). The current density–voltage curves wereobtained using a solar simulator (IVT Solar) under 100mW cm�2

of AM 1.5 global solar illumination with a calibrated light sourceby a standard Si photodiode.

Numerical simulations

The computational domain of the simulation is shown in Fig. 2.The T-shape microchannel indicates the conuence at themixing region. The system coordinates were positioned suchthat the origin was located at the center of the two inletbranches, the streamwise direction of the outlet branch was

Nanoscale, 2014, 6, 2840–2846 | 2841

Fig. 2 Computational domain of the numerical simulation. The x-coordinate corresponds to the streamwise direction of the outletbranch, the y-coordinate corresponds to the direction perpendicularto the outlet branch, and the z-coordinate corresponds to the lateraldirection of the microchannel.

Nanoscale Paper

Publ

ishe

d on

27

Janu

ary

2014

. Dow

nloa

ded

by N

atio

nal D

ong

Hw

a U

nive

rsity

Lib

rary

on

16/0

3/20

14 0

9:32

:18.

View Article Online

oriented along the X-axis, the direction perpendicular to theoutlet branch was oriented along the Y-axis, and the lateraldirection of the channel was oriented along the Z-axis. They-directional distance between the two inlets was 3 mm, and thex-directional distance from the conuence and the outlet was7 mm. The width and height of the microchannel were each200 mm. The ow rates of the NaOH and Zn(OAc)2 solutionswere 0.855 mL min�1 and 1.645 mL min�1, respectively. Aerconuence, the total ow rate was 2.5 mL min�1, with a meanvelocity of 10.42 mm s�1. This ow rate was determined bycalculating the maximum theoretical ow rate under anexpected total pressure drop (DPtotal) of less than 20 kPa for astable process. The total pressure drop was calculated from thesum of the pressure drops in the rst preheating region DPph1,second preheating region DPph2, mixing region DPmixing, andsynthesis region DPsynthesis as follows,

DPtotal ¼ DPph1 + DPph2 + DPmixing + DPsynthesis. (1)

Each pressure drop was calculated using the pressure dropequation described by Bahrami,33

DP ¼ 16p2m�uIPLA�3 (2)

where m is the dynamic viscosity (0.604 � 10�3 kg m�1 s), �u isthe mean velocity (m s�1), L is the length of the streamwisedirection (m), A is the cross-sectional area (m2), and IP is thepolar moment of inertia (m4),

IP ¼ 0.25pwh(w2 + h2), (3)

where w and h are the dimensions of the channel width andheight (m), respectively. The ow rate at each inlet could beexpressed in the form of a sine function, 0.855(1 + sin( f � 2pt +p)) mL min�1 and 1.645(1 + sin( f� 2pt)) mL min�1 for the NaOHand Zn(OAc)2 solutions, respectively (t: time, f: frequency). Thephase difference between the pulse injection proles at the twoinlets was explored as a function of frequency. The diffusionconstant (D) was D ¼ 10�9 m2 s�1, which is typical of ion diffu-sion in aqueous solutions. The time step in the numericalsimulation was set to 50 steps per pulse cycle. Data were recor-ded every 10 steps and analyzed according to the mass fractionof the selected regions to evaluate the effectiveness of mixing.

2842 | Nanoscale, 2014, 6, 2840–2846

Results and discussionNumerical simulations

Effective mixing conditions were identied in the numericalsimulations by measuring the degree of mixing based on themass fractions of the synthetic solutions. The degree of mixingwas dened as

Degree of mixing ¼ 1�"Xn

i¼1

ðfi � fÞ2ðyi � ymeanÞ=n#0:5

f�1;

(4)

where n is the number of cells in a selected region, yi is thevelocity of the ith cell, ymean is the mean velocity in the selectedregion, fi is the mass fraction of the ith cell, and f is the massfraction assuming equal mixing between the two mixed solu-tions. Due to differences in the ow rates of each solution, fwasdened as

f ¼ qNaOH

.�qNaOH þ qZnðOAcÞ2

�for NaOH solution

f ¼ qZnðOAcÞ2

.�qNaOH þ qZnðOAcÞ2

�for ZnðOAcÞ2 solution

(5)

where �qNaOH is the mean ow rate of the NaOH solution and�qZn(OAc)2 is the ow rate of the Zn(OAc)2 solution. The mixingefficiency was quantied based on the degree of mixing over therange [0 to 1], where a value of 0 indicated no mixing and 1indicated full mixing. To avoid round-off errors from thenumerical simulation, full mixing was dened as a degree ofmixing greater than 0.995. In addition to the degree of mixing,the full mixing time (tf) and length (‘f) were calculated to form abasis for effective mixing comparisons. Full mixing was denedas the point aer which the degree of mixing always exceeded0.995. The full mixing time was dened as the time required forthe uid initially at the conuence (t¼ 0) to reach the rst pointat which full mixing was observed. The full mixing length wasdened as the distance from the conuence point to the fullmixing point.

Fig. 3 shows the degree of mixing as a function of time forvarious mixing frequencies (1, 3, and 5 Hz) at a cross section ofX ¼ 7.0 mm. From 0 to 0.4 s, no mixing was observed becausethe synthetic solutions had not yet owed. The degree of mixingincreased signicantly aer 0.4 s. The initial rate of mixing wasmost rapid in the system that underwent 1 Hz pulsed mixing;however, the degree of mixing in this system did not reach thefull mixing region and uctuated over the range 0.6–0.9. Thesystem that underwent 3 Hz pulsed mixing tended to uctuateover the range 0.9–0.99. On the other hand, the system thatunderwent 5 Hz pulsed mixing showed a stable increase in thedegree of mixing up to the full mixing region. Full mixing wasobtained at 1.4 s, and no signicant uctuations were observedthereaer. The effects of pulsed ow are apparent from thespatial proles of the mass fractions for various pulsefrequencies, as shown in Fig. 4. The contours of the Zn(OAc)2mass fraction are shown at several perpendicular cross sectionscorresponding to Z ¼ 100 mm, X ¼ 0, 1.5, 3.0, 5.0, and 7.0 mm.As shown in Fig. 4a and b, the large plug stream prevented

This journal is © The Royal Society of Chemistry 2014

Fig. 3 The degree of mixing depended on the mixing time and mixingfrequency (1, 3, and 5 Hz). The full mixing region indicates a 0.995degree of mixing.

Fig. 4 Numerical simulations of the pulse mixing system. (a) 1 Hz, (b) 3Hz, and (c) 5 Hz. The contours indicate the mass fraction of theZn(OAc)2 solution.

Fig. 5 Productivity factor as a function of the mixing frequency. Themixing enhancement region indicates regions in which mixing wasmore efficient under pulse mixing conditions than under continuousmixing conditions.

Paper Nanoscale

Publ

ishe

d on

27

Janu

ary

2014

. Dow

nloa

ded

by N

atio

nal D

ong

Hw

a U

nive

rsity

Lib

rary

on

16/0

3/20

14 0

9:32

:18.

View Article Online

effective mixing between two solutions. Cross-sectional views ofthe streamwise mass fraction distribution show that effectivemixing was not achieved in these cases, even up to the end ofthe mixing region. By contrast, the sample that underwent 5 Hzpulsed ow mixing displayed effective mixing at the end of themixing region, as shown in Fig. 4c. The small plug stream

This journal is © The Royal Society of Chemistry 2014

facilitated efficient mixing at the interface of the mixing solu-tions. In this case, full mixing was observed prior to the end ofthe mixing channel.

The optimal frequency range for achieving effective mixingwas identied based on a productivity factor (h), dened as theratio of the full mixing times (at X¼ 7.0 mm) and lengths underpulsed or continuous ow mixing (no pulse),

h = (tf‘f/tclc) � 1 (6)

where tc and ‘c are the full mixing time and length undercontinuous ow mixing, respectively. Numerical simulationswere used to measure the full continuous ow mixing time andlength, found to be tc ¼ 1.62 sec and ‘c ¼ 6.75 mm, respectively.Fig. 5 plots the productivity factor as a function of the mixingfrequency. Mixing was not enhanced at frequencies below 4 Hz(the poor mixing region). Efficient mixing was obtained in thefrequency range 4–15 Hz (the mixing-enhanced region). Afrequency of 5 Hz yielded the best mixing enhancement.Productivity factors at frequencies beyond 15 Hz asymptoticallyapproached the continuous ow limit value. The validationbetween simulation and experiment should be made. However,it is difficult in the present system to visualize the mixingphenomena due to the opaque silicon based microuidicchannel and the transparent synthetic solutions (ethanol).

Synthesis of ZnO NPs

ZnO NPs were synthesized in a batch process for comparisonpurposes. Sodium hydroxide (NaOH) and zinc acetate dehydrate(Zn(OAc)2) were dissolved in ethanol (C2H5OH) to prepare30 mM and 10 mM solutions, respectively. The solutions wereheated in an Erlenmeyer ask immersed in a silicon oil bathmaintained at 60 �C. Subsequently, 25 mL of the Zn(OAc)2solution were poured into an Erlenmeyer ask and incubatedfor 30 min to ensure uniform heating conditions and

Nanoscale, 2014, 6, 2840–2846 | 2843

Nanoscale Paper

Publ

ishe

d on

27

Janu

ary

2014

. Dow

nloa

ded

by N

atio

nal D

ong

Hw

a U

nive

rsity

Lib

rary

on

16/0

3/20

14 0

9:32

:18.

View Article Online

stabilization. Finally, 13 mL of the NaOH solution were slowlyadded to the Zn(OAc)2 solution, and the reaction was stirred for2 hours to ensure full reaction. The synthesized ZnO NPs werecooled to room temperature and stored in a glass bottle. Fig. 6ashows a TEM image of the NPs produced from the batchprocess. ZnO NPs were 3–5 nm in diameter and were crystallinein structure, as shown in the inset of Fig. 6a. Red ellipsesindicate the single ZnO NPs.

ZnO NP synthesis was conducted in a microuidic systemaccording to the optimal system parameters identied in thenumerical simulations described above. A 5 Hz pulse frequencywas applied using the pulse injection mode of a syringe pump.The mean ow rates were 0.855 mL min�1 for the NaOH and1.645 mL min�1 for the Zn(OAc)2 solutions. The pulsed owcould be described by the expressions: 0.855(1 + sin(10pt + p))and 1.645(1 + sin(10pt)) mL min�1 for the NaOH and Zn(OAc)2solutions, respectively. The Reynolds number was 2.72 and theStrouhal number was 0.1. As an initial condition, the micro-channel was lled with pure ethanol and preheated to 60 �Cusing a thin lm heater. The 30mMNaOH and 10mM Zn(OAc)2solutions were injected from separate syringes and delivered tothe inlet ports via Teon microtubes. The injected solutionswere heated in the 92.5 mm preheating region. The NaOH andZn(OAc)2 solutions remained in the preheating region for 26 sand 13.5 s, respectively, and were fully heated to 60 �C. Thesolutions subsequently reached the conuence point markingthe start of the mixing region. A uniform temperature acrossboth solutions avoided abnormal ZnO NP synthetic conditions.In the mixing region, the injected solutions converged andmixed under the differential pulse phase conditions. Fullmixing between the two solutions was achieved in the mixingregion. The mixed solution subsequently entered the synthetic

Fig. 6 Transmission electron microscopy (TEM) images of (a) thebatch process-synthesized zinc oxide nanoparticles (ZnO NPs) and (b)the microfluidic system-synthesized ZnO NPs under 5 Hz pulse mix-ing. The insets indicate a magnified view (individual ZnO NPs arehighlighted in the red ellipses). Electron dispersive spectrometry (EDS)data for (c) the batch process-synthesized ZnO NPs and (d) themicrofluidic system-synthesized ZnO NPs.

2844 | Nanoscale, 2014, 6, 2840–2846

region to synthesize the ZnO NPs. The solution remained in thesynthesis region for 35 s and was uniformly heated to 60 �C. TheZnO NPs synthesized in ethanol were collected in an enclosedreservoir via Teon microtubing connected to the outlet port.Fig. 6b shows a TEM image of ZnO NPs. The ZnO NPs were 3–5nm in diameter, crystalline in structure, and morphologicallyindistinguishable from the NPs prepared in the batch syntheticprocess. The EDS results (Fig. 6c and d) showed the presence ofZnK (elemental zinc) and OK (elemental oxygen) peaks for NPssynthesized by either method, indicating a zinc and oxygenelemental composition without other chemical components.The peaks corresponding to C and Cu were introduced from theTEM grid. The NP compositions were compared quantitativelybased on the XRD data. Fig. 7 shows the XRD proles for theZnO NPs synthesized via batch or microuidic systems. TheXRD data were collected along the q–2q angles and the powderdiffraction le (PDF) was analyzed. The peaks of both sampleswere coincident at 31.75, 34.20, 36.20, 46.63, and 56.58. ThePDF revealed the presence of crystalline ZnO, indicating thatthe ZnO NPs synthesized using a pulse ow mixing microuidicsystem were indistinguishable from those synthesized using abatch synthetic process.

The measured properties of microuidic/batch synthesizedZnO NPs showed the same results in the structure morphologyand chemical formation. However, the preservability and dis-persibility of synthesized ZnO NPs in ethanol solution showeddifferent results depending on their synthetic conditions. InFig. 8a and b, the transmittance results of ZnO NPs dispersedethanol solution are presented according to the storage time

Fig. 7 X-ray diffraction (XRD) data for the (a) batch process-synthe-sized ZnO NPs and (b) the microfluidic system-synthesized ZnO NPs.

This journal is © The Royal Society of Chemistry 2014

Paper Nanoscale

Publ

ishe

d on

27

Janu

ary

2014

. Dow

nloa

ded

by N

atio

nal D

ong

Hw

a U

nive

rsity

Lib

rary

on

16/0

3/20

14 0

9:32

:18.

View Article Online

and synthetic methods. The transmittance of microuidicsynthesized ZnO NPs in ethanol solution showed 90.26% and85.74% in the range of visible wavelength (390–700 nm) forstorage times of 24 and 330 hours respectively. On the otherhand, the transmittance of batch process-synthesized ZnO NPsethanol solution signicantly decreased from 89.82% for astorage time of 24 hours to 52.23% for 330 hours storage withthe samemeasurement conditions. In addition, the visible colorof batch process-synthesized ZnO NPs ethanol solution slightlychanged from transparent to white opaque, while the micro-uidic synthetic ZnO NPs ethanol solution remained trans-parent up to a storage time of 330 hours. These effects arosefrom the heat conditions of the synthetic process which affectedthe electrical stabilization of synthesized ZnO NPs. As describedbefore, the microuidic system provides rapid heat and masstransfer conditions during the synthetic process that contributeto reduce the reaction time and maintain the stability ofsynthesized ZnO NPs. In contrast, the batch process-syntheticapproach needs more time than the microuidic system toensure the full reaction of ZnO NPs because of the low heattransfer and mixing efficiency. By these effects, the earliersynthesized ZnO NPs were held at high temperature until n-ishing the synthesis and cooling to room temperature, and theelectrical stability of ZnO NPs became worse as they tend toeasily coagulate with other ZnO NPs. For more quantitativeanalysis, the zeta (z) potential value was measured as presentedin the tables in Fig. 8a and b. The results show that the

Fig. 8 Transmittance and zeta potential values of ZnO NPs ethanolsolutions depending on the storage time after synthesis (24 or 330hours): (a) microfluidic synthesis, and (b) batch-process synthesis. (c)Current density versus voltage results for fabricated organic photo-voltaic cells with synthesized ZnO NPs.

This journal is © The Royal Society of Chemistry 2014

measured z potential values were 31.3 eV (24 h) to 10.3 eV (330h) and 28.7 eV (24 h) to 2.39 eV (330 h) for microuidic andbatch processed syntheses respectively. A low z potential valueindicates high occulation tendency in a colloid.34,35 From thispoint of view, the microuidic synthesized ZnO NPs have morestable electrical and storage properties than batch process-synthesized ZnO NPs; this shows the advantages of the micro-uidic synthetic system and their high potential for practicalapplications. To measure the performance difference ofsynthesized ZnO NPs in electrical device applications, bulk-heterojunction organic photovoltaic (OPV) cells with an inver-ted conguration were tested with the 330 hours stored ZnONPs as an electron transporting layer. The OPV cells werefabricated using the following built-up sequence of thin layerson the indium tin oxide (ITO) coated glass (15 U sq�1): 80 nm ofZnO NPs, 400 nm of P3HT:PCBM, 1 mm of PEDOT:PSS (holetransporting), and 1 mm of Ag. In Fig. 8c, the current density–voltage characteristics of the devices under 100 mW cm�2 of AM1.5 illumination are presented. The power conversion efficiency(PCE) of the OPV with microuidic synthesized ZnO NPs was1.67%, higher than the PCE of the OPV with batch process-synthesized ZnO NPs (0.09%). In addition to the PCE, the opencircuit voltage (VOC), short circuit current (JSC), and ll factor(FF) values also show that the microuidic synthetic ZnO NPsbased OPV has higher performance than the batch process-synthesized ZnO NPs based OPV. These results show that themicrouidic system has advantages to maintain the character-istics of synthesized ZnO NPs and its applications, and guar-antees the good preservability and performance. We anticipatethat a microuidic system and ow control technique will beapplicable to more functional nanomaterials synthesis andpresent economic and environmental benets through massproduction plant systems with closed reaction conditions.

Conclusions

We successfully prepared a microuidic system for use in ZnONP synthesis. The degree of mixing enhancement throughapplication of time pulsing was investigated using numericalsimulations. Efficient mixing was observed over the range 5–15Hz. A pulse frequency of 5 Hz was predicted to display the bestmixing enhancement. A microuidic channel was designed toallow for the mixing and reaction of two components. Themicrochannels included regions for preheating, mixing, andsynthesis, where uniform heating at 60 �C was applied using athin lm heater. Ethanol solutions of 30 mM sodium hydroxideand 10 mM dehydrate zinc acetate were prepared for thesynthesis of the ZnONPs. The solutions were pulse owed with a180� phase difference to achieve a uniform ow rate at the outlet.The ow rates were selected based on the system efficiency andthe pressure drop across the microuidic channel. The meanow rates were 0.855 mLmin�1 and 1.645 mLmin�1 for the NaOHand Zn(OAc)2 solutions, respectively. The synthetic results wereanalyzed by TEM, EDS, and XRD. ZnO NPs were synthesized by abatch process for comparison purposes. The ZnO NPs synthe-sized in the microuidic system were 3–5 nm in diameter,crystalline in structure, and indistinguishable from the

Nanoscale, 2014, 6, 2840–2846 | 2845

Nanoscale Paper

Publ

ishe

d on

27

Janu

ary

2014

. Dow

nloa

ded

by N

atio

nal D

ong

Hw

a U

nive

rsity

Lib

rary

on

16/0

3/20

14 0

9:32

:18.

View Article Online

batch-synthesized ZnO NPs. However, the measurements oftransparency and zeta potential showed that the preservabilityand dispersibility in solution of synthesized ZnO NPs had betterperformance than batch-synthesized ZnO NPs. In terms of theperformance in a practical application, the fabricated OPV cellswith microuidic synthesized ZnO NPs showed better resultsthan batch-synthesized ZnO NPs based OPV. These results wereinduced by the rapid heat and mass transfer effects of themicrouidic system with reaction conditions closed to theenvironment. This useful ZnO NP synthetic microuidic systemis low-cost and environmentally benign, offering a powerfulmeans for efficient mass production.

Acknowledgements

This work was supported by the Creative Research Initiatives(no. 2012-0000246) program of the National Research Founda-tion of Korea.

References

1 Z. L. Wang, J. Phys.: Condens. Matter, 2004, 16, R829.2 Z. Fan and J. G. Lu, J. Nanosci. Nanotechnol., 2005, 5, 1561.3 F. S. F. Morgenstern, D. Kabra, S. Massip, T. J. K. Brenner,P. E. Lyons, J. N. Coleman and R. H. Friend, Appl. Phys.Lett., 2011, 99, 183307.

4 Y. Jin, J. Wang, B. Sun, J. C. Blakesley and N. C. Greenham,Nano Lett., 2008, 8, 1649.

5 J. H. Jun, J. Yun, K. Cho, I. S. Hwang, J. H. Lee and S. Kim,Sens. Actuators, B, 2009, 140, 412.

6 N. Han, X. Wu, L. Chai, H. Liu and Y. Chen, Sens. Actuators, B,2010, 150, 230.

7 S. H. Ko, D. Lee, H. W. Kang, K. H. Nam, J. Yeo, S. J. Hong,C. P. Grigoropoulos and H. J. Sung, Nano Lett., 2011, 11, 666.

8 H. W. Kang, J. Yeo, J. O. Hwang, S. Hong, P. Lee, S. Y. Han,J. H. Lee, Y. S. Rho, S. O. Kim, S. H. Ko and H. J. Sung,J. Phys. Chem. C, 2011, 115, 11435.

9 S. H. Ko, D. Lee, N. Hotz, J. Yeo, S. Hong, K. H. Nam andC. P. Grigoropoulos, Langmuir, 2012, 28, 4787.

10 H. W. Kang, J. Leem, S. H. Ko, S. Y. Yoon and H. J. Sung,J. Mater. Chem. C, 2013, 1, 268.

11 B. L. Cushing, V. L. Kolesnichenko and C. J. O'Connor,Chem. Rev., 2004, 104, 3893.

12 M. Niederberger and G. Garnweitner, Chem.–Eur. J., 2006, 12,7282.

2846 | Nanoscale, 2014, 6, 2840–2846

13 H. F. Lin, S. C. Liao and S. W. Hung, J. Photochem. Photobiol.,A, 2005, 174, 82.

14 E. A. Meulenkamp, J. Phys. Chem. B, 1998, 102, 5566.15 C. Pacholski, A. Kornowski and H. Weller, Angew. Chem., Int.

Ed., 2002, 41, 1188.16 S. M. Mitrovski, L. C. C. Elliott and R. G. Nuzzo, Langmuir,

2004, 20, 6974.17 S. M. Mitrovski and R. G. Nuzzo, Lab Chip, 2006, 6,

353.18 Y. K. Cho, J. G. Lee, J. M. Park, B. S. Lee, Y. Lee and C. Ko, Lab

Chip, 2007, 7, 565.19 R. S. Jayashree, M. Mitchell, D. Natarajan, L. J. Markoski and

P. J. A. Kenis, Langmuir, 2007, 23, 6871.20 H. R. Sahoo, J. G. Kralj and K. F. Jensen, Angew. Chem., 2007,

119, 5806.21 A. N. Hellman, K. R. Rau, H. H. Yoon, S. Bae, J. F. Palmer,

K. S. Phillips, N. L. Allbritton and V. Venugopalan, Anal.Chem., 2007, 79, 4484.

22 V. Kumar, M. Paraschivoiu and K. D. P. Nigam, Chem. Eng.Sci., 2011, 66, 1329.

23 W.Wang, S. Zhao, T. Shao, Y. Jin and Y. Cheng, Chem. Eng. J.,2012, 192, 252.

24 R. H. Liu, R. Lenigk, R. L. Druyor-Sanchez, J. Yang andP. Grodzinski, Anal. Chem., 2003, 75, 1911.

25 G. G. Yaralioglu, I. O. Wygant, T. C. Marentis and B. T. Khuri-Yakub, Anal. Chem., 2004, 76, 3694.

26 C. Y. Lee, G. B. Lee, L. M. Fu, K. H. Lee and R. J. Yang,J. Micromech. Microeng., 2004, 14, 1390.

27 C. Y. Wen, C. P. Yeh, C. H. Tsai and L. M. Fu, Electrophoresis,2009, 30, 4179.

28 B. Xu, T. N. Wong, N. T. Nguyen, Z. Che and J. C. K. Chai,Biomicrouidics, 2010, 4, 044102.

29 I. Glasgow and N. Aubry, Lab Chip, 2003, 3, 114.30 I. Glasgow, S. Lieber and N. Aubry, Anal. Chem., 2004, 76,

4825.31 C. Y. Lim, Y. C. Lam and C. Yang, Biomicrouidics, 2010, 4,

014101.32 Y. Wang, J. Zhe, B. T. F. Chung and P. Dutta, Microuid.

Nanouid., 2007, 4, 375.33 M. Bahrami, M. M. Yovanovich and J. R. Culham, Int. J. Heat

Mass Transfer, 2007, 50, 2492.34 S. Liufu, H. Xiao and Y. Li, Powder Technol., 2004,

145, 20.35 D. Hanaor, M. Michelazzi, C. Leonelli and C. C. Sorrell,

J. Eur. Ceram. Soc., 2012, 32, 235.

This journal is © The Royal Society of Chemistry 2014