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R E S E A R CH AR T I C L E

Advanced Science LettersVol 4 108ndash114 2011

Synthesis of Ag Doped Nano TiO2 as Efficient SolarPhotocatalyst for the Degradation of Endosulfan

Jesty Thomaslowast K Praveen Kumar and K R Chitra

School of Chemical Sciences Mahatma Gandhi University Kottayam Kerala State 686560 India

Silver nanoparticles doped anatase TiO2 nanocrystals were prepared via low temperature hydrothermal routeThe synthesized nanocrystals are highly efficient solar photocatalysts and showed higher photocatalytic activitythan pure nano TiO2 and commercial photocatalyst Degussa P25 under sunlight Silver nanoparticles for dop-ing were prepared by a novel single step chemical reduction and stabilization employing L-Dopa DRS studyshowed that nanosilver doping on TiO2 induces a shift of absorption edge to the visible range and there is anarrowing of the band gap The solar photocatalytic efficiency of the synthesized samples was checked by thephotocatalytic degradation of methylene blue and found that photocatalytic activity of TiO2 under solar irradia-tion was drastically increased by doping silver nanoparticles Further the synthesized solar photocatalysts wereused for the degradation of the organochlorine pesticide endosulfan and achieved near complete degradationwith the highly active calcined nanosilver doped TiO2 catalyst

Keywords Nano TiO2 Silver Nanoparticle Solar Photocatalysis Organic Pollutant DegradationEndosulfan

1 INTRODUCTIONPesticides have become ubiquitously distributed organic pollu-tants and their occurrence persistence and potential adverseeffects are a daily mass-media topic1 The presence of thesepersistent organic pollutants (POPs) in the environment is ofgrave public concern and research is intensifying in the areaof remediation of these contaminants These compounds are notsusceptible to degradation in the environment and are accumu-lated or transported to long distances and biomagnified in thefood chain Conventional biological treatment processes for theremoval of POPs are very slow or ineffective The traditionalphysico-chemical treatments such as adsorption on activated car-bon nano-filtration and ozonation are efficient in comparisonwith others but have inherent limitations in applicability effec-tiveness and cost2ndash4

Both the stereo isomers of the organochlorine pesticide endo-sulfan viz and endosulfan are resistant to direct photolysisin soil and aquatic systems but oxidized to endosulfansulfateby microorganisms which is resistant to further biodegradation5

Endosulfansulfate is equally toxic and more persistent than theparent compound Recently photocatalysis involving TiO2 isattracting interest in the degradation of endocrine disruptingchemical compounds and pesticides6 Titania aqueous disper-sions were found effective for the photocatalytic decompositionof the herbicide metolachlor with a t12 of 21 minutes withUV irradiation7 and nanosized TiO2 colloids were used for the

lowastAuthor to whom correspondence should be addressed

UV assisted degradation of the herbicide mecoprop but with areduced photocatalytic efficiency compared to Degussa P-258

Photocatalytic treatment using TiO2 combined with solar lightwas very efficient in the destruction of pesticide Diuron in the top4 cm of contaminated soils9 Microwave-assisted photocatalyticdegradation of atrazine on TiO2 nanotubes has been studied byZhanqi et al and complete degradation was obtained10 In thepresent study we have synthesised silver nano particle dopedTiO2 and was used for the degradation of the highly toxic chlori-nated hydrocarbon pesticide endosulfan (C9H6Cl6O3S) undersunlight

Although TiO2 is a good photocatalyst due to the wide bandgap (32 eV) of TiO2 an ultraviolet irradiation is necessary forphotocatalysis11 This has consequent implications for the use ofTiO2 as solar light activated catalyst because the majority ofsunlight consists of visible light and only a 3ndash5 UV light Morepractical applications can be achieved if the photocatalyticallyactive region is extended to the visible region (400ndash800 nm)12

Hence in the present work we have made an attempt to synthesizenovel TiO2 photocatalysts with reduced band gap and enhancedactivity under solar light for the degradation of endosulfan

Silver nanoparticles were synthesized by a novel strategy iein a single step from silver nitrate (AgNO3) by chemical reduc-tion technique employing a biologically active molecule L-Dopa(Scheme 1) Usually during the synthesis of silver nanoparticlesit is very difficult to control its particle size Also after its forma-tion from silver ions by reduction they need to be stabilized forfurther use But here we found that L-Dopa is an excellent reduc-ing agent and can stabilize the colloidal nanoparticles through an

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R E S E A R CH AR T I C L EAdv Sci Lett 4 108ndash114 2011

HO

HO H3N

COO

Scheme 1 Structure of L-Dopa

electrostatic interaction so that no other stabilizing agents wereused for the stabilization of silver nanoparticles

The photocatalytic activity of the synthesized samples undersunlight was studied by the degradation of methylene blue (MB)and compared with pure anatase nano TiO2 powder and com-mercial photocatalyst Degussa-P25 Further the as synthesizedhighly efficient photocatlysts were used for the degradation ofhighly persistent pesticide endosulfan We found that near com-plete degradation of endosulfan is possible by solar photocatal-ysis using the synthesized silver doped TiO2 nanoparticles

2 EXPERIMENTAL DETAILS21 MaterialsAnalytical grade titanium (IV) isopropoxide (Ti[OC3H7]4) pur-chased from Acros Organics was used for the synthesis

HO

COO

NH3

HO

HO

COO

NH3

HO

COOH3N

HOOH

HO

OOCNH3

HO

HO

OOC

NH3

OH

OH

OOC

H3N

OHAg

(d)

200 400 600 800

00

01

02

03

04

1 hour

0

Abs

orba

nce

Wavelength nm

(b)

50 nm

(c)

(a)

Fig 1 (a) Colour photograph of synthesized silver nanoparticle (b) Absorption Spectrum of silver nanoparticles recorded at different time intervals (c) TEMimage of synthesized silver nanoparticles (d) Schematic representation of L-Dopa capped silver nanoparticles

Methylene blue was obtained from Rankem Silver nitrate(AgNO3) Sodium hydroxide (NaOH) and L-Dopa were pur-chased from Aldrich chemicals endosulfan standard wasobtained from Sigma-Aldrich Chemical Co GR grade anhydroussodium sulphate dichloromethane and UniSolv grade toluenewere purchased from Ms Merck Chemicals All these chemicalswere used without further purification Deionized water was usedthrough out the experiments

22 Synthesis of Silver NanoparticlesPreparation of silver nanoparticles were carried out by mixing30 mL 670 M silver nitrate 25 L 0165 M NaOH and10 L 001 M L-DOPA The color of the solution slowly turnedto bright yellow due to the formation of silver nanoparticlesColour photograph of synthesized silver nanoparticle is givenin Figure 1(a) The formation of nanoparticles was confirmedthrough absorption spectral analysis and TEM image

23 Synthesis of Nano Silver Doped TiO2

Silver nanoparticles synthesized by the above method were addedto 70 ml of titanium (IV) isopropoxide with constant stirringto get pale yellow precipitate Distilled water was added to thisprecipitate until precipitate formation ceased The total volumeof the precursor is 70 ml and stirring was continued for 1 hrThe mixture was transferred into a stainless steel teflon linedautoclave (100 ml) and the hydrothermal reaction proceeded at

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R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

180 C for 20 hrs After hydrothermal treatment the powderswere filtered washed with water and acetone respectively anddried at 80 C This sample is designated as TLS1 To studythe effect of calcination TLS1 is calcinated at 300 C for 3 hrs(TLS2) Pure nano TiO2 powder (PT) also was synthesized usingthe above procedure without the addition of silver nanoparticles

The X-ray diffraction patterns (XRD) of the synthesized sam-ples were obtained on a Brucker D8 Advanced Diffractometerusing Cu K as radiation The diffused reflection spectra (DRS)of the samples were performed using a UV-2450 Shimadzu UV-visible spectrophotometer The photoluminescence (PL) spectralmeasurements were made with the synthesized TiO2 nanopar-ticles using Shimadzu RF-5301 spectrofluorophotometer at anexcitation wavelength of 300 nm The BET specific surface areaof the powders was measured via nitrogen adsorption using asurface area analyzer (Micromeritics Gemini USA) A Jeol JSM6500F scanning electron microscope was used for FESEM stud-ies TEM images were taken using a Jeol JEM-3010 transmissionelectron microscope

The photocatalytic activity of the synthesized samples wasassessed by photocatalytic degradation of methylene blue For atypical photocatalytic experiment 01 g of synthesized nanocrys-talline TiO2 was suspended in 100 ml of 50 ppm methyleneblue (MB) aqueous solution The resulting suspension was equi-librated by stirring in the dark for 30 min To study the pho-tocatalytic degradation under sunlight the MBndashTiO2 suspensionwas kept under sunlight at ambient temperature for 70 min from1200 noon to 110 noon The samples were withdrawn at differ-ent time intervals and centrifuged at 6000 rpm to remove theTiO2 particles The absorbance of MB was measured at 660 nmusing a UV-3101 PC UV-VIS-NIR Shimadzu scanning spec-trophotometer It was observed that no detectable degradation ofMB occurs without TiO2 or solar irradiation aloneTo study the degradation of pesticide endosulfan under solar

irradiation using the synthesized silver doped and undoped TiO2

samples an aqueous solution with a endosulfan concentration350 ppb (100 ml) was stirred with the samples PT TLS1 andTLS2 (01 g) in separate experiments The resulting suspensionwas equilibrated by stirring in the dark for 30 min and then keptunder direct sunlight from 1200 noon to 100 noon Samples werewithdrawn at regular intervals centrifuged and extracted threetimes with dichloromethane The combined organic layer is thendried over anhydrous sodium sulphate and concentrated whichis then reconstituted in toluene prior to gas chromatographicanalysis endosulfan concentrations in the photocatalytic experi-ments were determined by gas chromatograph equipped with elec-tron capture detector (GC-ECD) (Shimadzu GC 2010A) with anautosampler and autoinjector A capillary column DB-5 (30 Mtimes025 mmtimes025 m) (Agilent Technologies USA) was used forthe GC-ECD analysis as well as for the partitioning of the par-ent pesticide and other photolysis product in subsequent GC-MSanalysis Sample taken at 60 min was extracted and subjectedfor intermediate or end product analysis with GC-MS determina-tion in electrospray ionization scan mode (EI-SCAN) (ShimadzuGCMS QP2010)

3 RESULTS AND DISCUSSIONFigure 1(b) shows the absorption spectra of a solution containingAgNO3 and NaOH after the addition of L-DOPA recorded at dif-ferent time intervals The figure clearly shows the formation of a

10 20 30 40 50 60 70 80 90

Inte

nsity

(a

u)

2 Theta

PT

TLS1

TLS2

101

004200

105

211 204 215

Fig 2 XRD patterns of PT TLS1 and TLS2

new band at 408 nm which is the characteristic plasmon absorp-tion for silver nanoparticles The formation of silver nanoparticleswas confirmed by TEM image (Fig 1(c)) The results clearlyshow that L-Dopa is an excellent capping and stabilizing agentfor silver nanoparticles and the schematic representation is givenin Figure 1(d)

31 XRD AnalysisX-ray diffractograms (XRD) of silver doped (TLS1 and TLS2)and undoped samples (PT) are shown in (Fig 2) XRD patternsreveal that the samples show a high degree of crystallinity and thecrystalline phase are of typical anatase (JCPDS No 21-1272)There is slight increase in crystallinity of the sample on dopingsilver nanoparticles but no phase change is observed Hence it isclear that silver doping alters the crystallinity but not the crystalstructure After calcination at 300 C the crystallinity of the sam-ple again increased but there is no phase change in the sample(TLS2)

However there are no obvious peaks showing the presence ofsilver in the XRD patterns of TLS1 and TLS2 and is due to thelow silver content1314 The nanocrystallite size of the synthe-sized samples was estimated using Scherrerrsquos equation DXRD =09 cos where D is the crystallite size is the wavelengthof X-ray used and are full width at half maximum intensity(in radian) (FWHM) of XRD diffraction lines and half diffrac-tion angle 2 respectively The crystallite sizes of samples aregiven in Table I and is clear that doping of silver nanoparticledid not alter the crystallite size of the samples but the calcinationprocess slightly increased the crystallite sizes

Table I BET surface area particle size crystallite size and band gapof PT TLS1 and TLS2

Surface area Particle size Crystallite size BandSample (m2g) (DBET) (nm) (DXRD) (nm) gap (eV)

PT 11955 130 131 323TLS1 1139 135 137 292TLS2 9334 165 158 287

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32 Raman SpectraFigure 3 depicts the Raman spectra of the synthesized samplesThe peaks at 399 515 and 637 cmminus1 in PT TLS1 and TLS2 cor-responds to the allowed Raman active modes of anatase crystalsThe presence of silver nanoparticles on the TiO2 surface slightlyreduced the intensities of the peaks in TLS1 and TLS2 which isin good agreement with the earlier report15 But the intensities ofthe peaks of TLS2 is higher than that of TLS1 and this may bedue to the increase in crystallinity of TLS2 during calcination asis evident from XRD

33 Surface Area AnalysisBET surface areas measured for the synthesized samples aregiven in Table I and show that the surface areas of the sam-ples are high compared to earlier reports1617 Doping of silvernanoparticles does not induce appreciable change in surface areaBut there is a decrease in surface area after calcination and is dueto the increase in crystallite size during calcination The aver-age nanoparticle sizes were calculated from BET surface areausing the equation DBET = 6000S where DBET is the averagenanoparticle size (nm) is the powder density (gcm3) S is thespecific surface area (m2g) The results are shown in Table I andare comparable with the average nanocrystallite size determinedusing Scherrerrsquos equation

34 Diffuse Reflectance SpectraTo investigate optical absorption properties of the synthesizedsamples the diffuse reflectance absorption spectra (DRS) of PTTLS1 and TLS2 were examined in the range 300ndash750 nm and theresults are shown in Figure 4 It is clear from the figure that TiO2

nanocrystals without silver nanoparticles (PT) has absorption inthe UV region and is ascribed to charge transfer from valenceband (mainly formed by 2p orbitals of oxide anion) to the con-duction band (mainly formed by 3dt2g orbitals of Ti4+ cation)18

The samples with silver nanoparticles (TLS1 and TLS2) showedsignificant absorption in the visible region which is in goodagreement with the earlier reports1319 The red shifted photore-sponse of TLS1 and TLS2 as observed in the figure may leadto high photocatalytic activity under visible region which helpsin the enhancement of photocatalytic activity under sunlight The

200 300 400 500 600 700 8000

5000

10000

15000

20000

25000

30000

35000

Inte

nsity

(a

u)

Raman shift (cmndash1)

TLS1

TLS2

PT

Fig 3 Raman spectra of PT TLS1 and TLS2

300 400 500 600 700

00

02

04

06

08

10

12

14

16

Abs

orba

nce

(au

)

Wavelength (nm)

PTTLS1

TLS2

Fig 4 Diffuse reflectance absorption spectra of PT TLS1 and TLS2

absorption band around 400 nm may be attributed to the absorp-tion of the silver nanoparticles adsorbed on the surface of TiO2

particles20

The band gap energies of the synthesized samples were calcu-lated by the equation

Eg = 12398 Ref [21]

where Eg is the band gap (eV) and (nm) is the wavelengthof the absorption edges in the spectrum The band gap valuesare given in Table I and the values indicate that silver dop-ing reduced the band gap of TiO2 nanocrystals The band gapnarrowing was primarily attributed to the substitution of silvernanoparticles which introduced electron states into the band gapof TiO2 to form a new lowest unoccupied molecular orbital Theshift of absorption edge of TiO2 to visible range and narrowingof band gap increased the photocatalytic activity of TiO2 in thevisible region

Further the absorption spectrum of TLS2 reveals more absorp-tion in the visible region than TLS1 which indicates the existenceof surface states22 The most possible mechanism to account forthe observed absorption might involve oxygen vacancies pro-duced by thermal treatment which form localization levels withinthe band gap22

35 Morphological AnalysisFESEM and TEM images of the silver doped samples aregiven in Figure 5 FESEM (Figs 5(a and b)) In TEM images(Figs 5(c and d)) Ag nanoparticles are distinguishable and appearas dark dots on the surface of the TiO2 particles Also there is apossibility for silver to be incorporated into the interstitial posi-tions of the TiO2 semiconductor particles It is also clear thatthe crystallinity has been increased which is indicated by theincreased crystal size with well-developed faces This observa-tion is supported by the XRD spectrum (Fig 2)

36 Photoluminescence StudiesThe photoluminescence emission spectra (PL) of the synthesisedsamples were studied in the range of 200ndash600 nm to investigatethe separation efficiency of charge carriers and the results areshown in Figure 6 It is evident from the figure that the PL inten-sity of TiO2 was decreased with silver doping The lower PL

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(c)

(a) (b)

(b)

Fig 5 FESEM of TLS1 (a) TLS2 (b) and TEM images of TLS1 (c) and TLS2 (d)

intensity shows that the rate of recombination between electronand holes is low23 The electrons are excited from valance band(VB) to conduction band (CB) of TiO2 under UV irradiation andgenerate photoexcited electrons and holes Although doping withsilver narrows the band gap of TiO2 samples the recombinationof electrons and holes are relatively slow which is evident fromPL spectra

As is well known TiO2 exhibit poor photocatalytic efficiencysince the majority of photogenerated charge carriers undergorecombination In the present system silver nanoparticles act aselectron acceptors and sinks for the charge carriers which will

200 300 400 500 600 700ndash20

ndash15

ndash10

ndash05

00

05

10

15

20

25

30

Inte

nsity

(a

u)

Wavelength (cmndash1)

PT

TLS1

TLS2

Fig 6 Photoluminescence emission spectra (PL) of PT TLS1 and TLS2

enhance the efficiency of photocatalysis2324 For samples TLS1and TLS2 silver nanoparticles plays an important role in theinterfacial charge transfer and in the decrease in rate of electron-hole recombination Silver nanoparticles could act as an effec-tive electron scavenger to trap the photo induced electrons andholes of TiO2 leading to the reduction of electronndashhole recom-bination and thus improving the photocatalytic efficiency Theelectrons trapped in silver sites were subsequently transferred tothe surrounding adsorbed O2 It can be supported by PL emissionspectra which have been widely used to investigate the fate ofelectron hole pairs in semiconductor particles since PL emissionresults from the recombination of free carriers2025

37 Photocatalytic ActivityTo investigate the photocatalytic activity of synthesized TiO2

samples in solar light the degradation of MB was studiedin presence of PT TLS1 and TLS2 nanoparticles under sun-light For comparison photocatalytic studies were also performedwith commercially available photocatalyst Degussa P25 and theresults are depicted in Figure 7 The activity of different samplesin sunlight is in the order TLS2 gt TLS1 gt PT gt Degussa P25Silver doped TiO2 samples show higher activity than undopedTiO2 and Degussa P25 Diffuse reflectance spectra of silverdoped TiO2 samples showed significant absorption in the visi-ble region which enhance the photocatalytic activity in the vis-ible region The high activity of silver doped TiO2 samples ismainly attributed to the decrease in band gap (323 to 287 eV)so that visible light is enough to excite electron from valenceband to conduction band Silver nanoparticles could act as aneffective electron scavenger to trap the conduction band elec-trons of TiO2

23 In silver doped TiO2 the silver nanoparticles

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R E S E A R CH AR T I C L EAdv Sci Lett 4 108ndash114 2011

10 20 30 40 50 60 70

10

20

30

40

50

60

70

80

90

100

D

egra

datio

n of

MB

Time in min

Degussa P 25

PTTLS1

TLS2

Fig 7 Photocatalytic activity of PT TLS1 TLS2 and Degussa P25 undersunlight

play an important role in the interfacial charge transfer and inthe elimination of electron-hole recombination as is evident fromPL spectra which would be beneficial for the high photocatalyticactivity The activity of PT is less because here electrons can-not be excited from valence band to conduction band by visiblelight irradiation due to the large band gap as calculated fromDRS (323 eV) In this case an ultraviolet irradiation is requiredfor the excitation of electrons from valence band to conductionband to carry out photocatalysis which accounts for only smallfraction of the solar light Also the efficiency of the photocatal-ysis depends on the effectiveness of the photocatalytic processin transferring the photoinduced eminush+ pair from the particlevolume to the particle surface and subsequently to the surface-adsorbed species In PT there is no intermediate level to trap theelectron and the recombination possibility of electrons and holesis enhanced and the free electron or holes available for the pho-tocatalysis is less subsequently the efficiency of photocatalysisalso is less

It is well known that it is the anatase polymorph rather than thebrookite or rutile polymorph that has the highest photocatalyticactivity The most active commercially available photocatalyst(Degussa P25) has an anatase content of 75 and 25 of rutilecontent26 From the XRD pattern it is evident that the synthesizedsamples (PT TLS1 and TLS2) are purely anatase and hence itshows higher activity than Degussa P25

The synthesized silver doped samples (TLS1 and TLS2) arehighly efficient photocatalysts as is evident from the MB degra-dation studies The sample TLS2 shows higher photocatalyticactivity than the sample TLS1 (Fig 7) and is due to the increasein crystallinity of TLS2 which is clear from the XRD data How-ever no direct relation was found between the BET areas and thecatalytic activity of the catalysts as in earlier report27 The previ-ous reports show that calcination of samples and UV irradiationwas required for effective photocatalysis also the UV irradiationtime for the degradation of MB is high (60 to 120 min)2829

The studies which deals visible light photocatalysis of MB usingdoped TiO2 also required high temperature calcination (600 C)and longer irradiation time (60ndash120 min)3031

But we have used low temperature hydrothermal route to pre-pare efficient photocatalyst which is active under solar light without any high temperature treatment Also silver doping couldreplace artificial UV light irradiation by sunlight Hence it is clearthat silver doping in TiO2 can give highly efficient photocatalystswhich are active under sunlight

In addition to higher photocatalytic activity the hydrother-mally synthesized TiO2 samples are easier to separate from theaqueous media than Degussa P25 Degussa P25 forms a milkywhite turbid suspension in aqueous media Though the synthe-sized samples have equal or less particle size they do not formturbid suspension Also the synthesized catalysts settle faster andit is easier to separate from the reaction mixture by centrifuga-tion This enabled recyclability of the synthesized nanophotocat-alysts after centrifugation at the end of each photocatalytic cycleand the recycled catalyst was found efficient and stable for morethan six cycles

Degradation of the nonbiodegradable pesticide endosulfanwas also investigated in the light of the interesting photocat-alytic activity of the synthesized nanoparticles with MB Thephotocatalytic activity of PT TLS1 and TLS2 nanoparticles withthe organochlorine pesticide under sunlight was studied in sep-arate experiments Figure 8 shows the degradation kinetics of endosulfan with the nanoparticles at an initial concentration of350 ppb which is close to the maximum aqueous solubility of endosulfan Concentration of the organochlorine pesticide endosulfan was determined by GC-ECD analysis As expectedhere also the more crystalline nanoparticle sample TLS2 exhib-ited highest photocatalytic activity followed by TLS1 and PT InTLS2-Endosulfan system substantial reduction in the concentra-tion of endosulfan was observed in the initial 20 minutes at at12 of 13 minutes It was observed that more than 98 of thepesticide was decomposed in 60 minutes The GC-MS analysisin EI-SCAN mode of the sample taken at 60 minutes confirmedthe almost complete mineralization of endosulfan The envi-ronmental degradation products of endosulfan as proposed byBakouri et al32 such as the immediate hydrolysis product endo-sulfan diol the oxidation product endosulfan sulfate or the furtherdecomposition products endosulfan ether endosulfan hydroxylether or endosulfan lactone were not detected in the system Toensure degradation rather than adsorption the experiment was

0 10 20 30 40 50 6000

02

04

06

08

10

PT

TLS1

TLS2

CtC

0

Time min

Fig 8 Photocatalytic degradation of endosulfan using PT TLS1 andTLS2 under sunlight

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repeated with the same TLS2 nanoparticles (after regenerating itfrom the photocatalytic system by centrifugation) with the sameinitial concentration of endosulfan for six more cycles Sampleswere analyzed for endosulfan concentration at the end of 60 min-utes The TLS2 photocatalytic system was found stable and effi-cient after the repeated experiments with consistent endosulfandegradation capability In aqueous systems though endosulfanresist direct photolysis it is clearly observed that it undergoesnear complete photodegradation with silver doped nano TiO2

particles The heterogeneous photocatalysis with TiO2 nanopar-ticles accelerated the decomposition of otherwise recalcitrant endosulfan and silver doping shifted the photocatalysis to visibleregion to attain efficient degradation under sunlight The resultssuggest the potential of silver doped TiO2 nanomaterials in envi-ronmental pollutant remediation applications

4 CONCLUSIONSAnatase nanocrystals with enhanced photocatalytic activity undersolar irradiation have been successfully synthesized by lowtemperature hydrothermal route by silver doping Silver nanopar-ticles for doping were prepared by simultaneous chemical reduc-tion and stabilization employing L-Dopa without any additionalstabilizing agent The hydrothermally synthesized TiO2 samplesshowed higher photocatalytic activity under sunlight than thecommercially available photocatalyst Degussa P25 and couldcompletely degrade the organochlorine pesticide endosulfan bysunlight irradiation Also the synthesized catalysts settle fasterand it is easier to separate from the reaction mixture which pro-motes the reusability of these photocatalysts Decrease in bandgap in silver doped samples by the formation of an intermedi-ate level is the major reason for the high photocatalytic activ-ity of these samples under sunlight The present study suggeststhat silver doped TiO2 can be used for the development of anenvironmentally sustainable photocatalytic treatment process forthe degradation of persistent organic pollutants using sunlight inplace of an artificial light

Acknowledgments The authors wish to thank the Depart-ment of Science and Technology (DST) India for financial sup-port under woman scientist scheme (WOS-A)

References and Notes1 K B Dhanalakshmi S Anandan J Madhavan and P Maruthamuthu Solar

Energy Mat Solar Cells 92 457(2008)2 R T Meijers E J Oderwaldmuller P Nuhn and J C Kruithof Ozone-Sci

Eng 17 673 (1995)

3 E Wittmann P Cote C Medici J Leech and A G Turner Desalination119 347 (1998)

4 P Roche and M Prados Ozone-Sci Eng 17 657 (1995)5 H M Shivaramaiah and I R Kennedy J Environ Sci Health Part B 41 895

(2006)6 D Dong P Li X Li Q Zhao Y Zhang C Jia and P Li J Hazardous

Materials 174 859 (2010)7 V A Sakkas I M Arabatzis I K Konstantinou A D Dimou T A Albanis

and P Falaras Appl Catal B Environ 49 195 (2004)8 A S Topalova D V Šojica D A Molnaacuter-Gaacutebora B F Abramovica and M I

Comorb Appl Catal B Environ 54 125 (2004)9 M M Higarashi and W F Jardim Catal Today 76 201 (2002)

10 G Zhanqi Y Shaogui T Na and S Cheng J Hazard Mater 142 424(2007)

11 T Kawai H Takahashi Y Matsushima T Ogata and H Unuma Sci AdvMater 2 74 (2010)

12 Y Tokunaga H Uchiyama Y Oaki and H Imai Sci Adv Mater 2 69(2010)

13 X S Li G E Fryxell C Wang and M H Engelhard Microporous Meso-porous Mater 111 639 (2008)

14 N Ma X Fan X Quan and Y Zhang J Membr Sci 336 109(2009)

15 K Mallick M J Witcomb and M S Scurrell Appl Catal A General 259 163(2004)

16 S Je-Lueng L Chia-Hsiang C Chyow-San C Chang-Tong C Chia-Chiand C Ching-Yuan J Hazard Mater 155 164 (2008)

17 A Kubacka M Ferrer A Martiacutenez-Arias and M Fernaacutendez-Garciacutea ApplCatal B Environ 84 87 (2008)

18 N Sobana M Murugandahamm and M Swaminathan J Mol Catal AChem 258 124 (2006)

19 S Anandan P S Kumar N Pugazhenthiran J Madhavan andP Maruthamuthu Solar Energy Mat Solar Cells 92 929 (2008)

20 S V Awate R K Sahu M D Kadgaonkar R Kumar and N M GuptaCatal Today 141 144 (2009)

21 B O Regan and M Gratzel Nature 353 737 (1991)22 Z P Wang W M Cai X T Hong X L Zhao F Xu and CG Cai Appl

Catal B Environ 57 223 (2005)23 F B Li X Z Li M F Hou K W Cheah and W C H Choy Appl Catal A

General 285 181 (2005)24 B Zhu Z Sui S Wang X Chen S Zhang S Wu and W Huang Mat Res

Bull 41 1097 (2006)25 D Zhang D Yang H Zhang C Lu and L Qi Chem Mater 18 3477

(2006)26 T Ohno K Sarukawa K Tokieda and M Matsumura J Catal 203 82

(2001)27 M Hosseini S Siffert H L Tidahy R Cousin J-F Lamonier A Aboukais

A Vantomme M Roussel and B-L Su Catal Today 122 391(2007)

28 Q Xiao Z Si Z Yu and G Qiu J Alloys Compd 450 426 (2008)29 M W Yan W L Su X Zhiliang B J Xin P C Xiao and P Jie Mater Lett

60 974 (2006)30 X Qi Z Jiang X Chong S Zhichun and T Xiaoke Solar Energy 82 706

(2008)31 Q Xiao Z Si Z Yu and G Qiu Mat Sci and Eng B 137 189

(2007)32 H E Bakouri A Ouassini J M Aguado and J U Garciacutea Water Environ

Res 79 2578 (2007)

Received 30 May 2010 Accepted 6 July 2010

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R E S E A R CH AR T I C L EAdv Sci Lett 4 108ndash114 2011

HO

HO H3N

COO

Scheme 1 Structure of L-Dopa

electrostatic interaction so that no other stabilizing agents wereused for the stabilization of silver nanoparticles

The photocatalytic activity of the synthesized samples undersunlight was studied by the degradation of methylene blue (MB)and compared with pure anatase nano TiO2 powder and com-mercial photocatalyst Degussa-P25 Further the as synthesizedhighly efficient photocatlysts were used for the degradation ofhighly persistent pesticide endosulfan We found that near com-plete degradation of endosulfan is possible by solar photocatal-ysis using the synthesized silver doped TiO2 nanoparticles

2 EXPERIMENTAL DETAILS21 MaterialsAnalytical grade titanium (IV) isopropoxide (Ti[OC3H7]4) pur-chased from Acros Organics was used for the synthesis

HO

COO

NH3

HO

HO

COO

NH3

HO

COOH3N

HOOH

HO

OOCNH3

HO

HO

OOC

NH3

OH

OH

OOC

H3N

OHAg

(d)

200 400 600 800

00

01

02

03

04

1 hour

0

Abs

orba

nce

Wavelength nm

(b)

50 nm

(c)

(a)

Fig 1 (a) Colour photograph of synthesized silver nanoparticle (b) Absorption Spectrum of silver nanoparticles recorded at different time intervals (c) TEMimage of synthesized silver nanoparticles (d) Schematic representation of L-Dopa capped silver nanoparticles

Methylene blue was obtained from Rankem Silver nitrate(AgNO3) Sodium hydroxide (NaOH) and L-Dopa were pur-chased from Aldrich chemicals endosulfan standard wasobtained from Sigma-Aldrich Chemical Co GR grade anhydroussodium sulphate dichloromethane and UniSolv grade toluenewere purchased from Ms Merck Chemicals All these chemicalswere used without further purification Deionized water was usedthrough out the experiments

22 Synthesis of Silver NanoparticlesPreparation of silver nanoparticles were carried out by mixing30 mL 670 M silver nitrate 25 L 0165 M NaOH and10 L 001 M L-DOPA The color of the solution slowly turnedto bright yellow due to the formation of silver nanoparticlesColour photograph of synthesized silver nanoparticle is givenin Figure 1(a) The formation of nanoparticles was confirmedthrough absorption spectral analysis and TEM image

23 Synthesis of Nano Silver Doped TiO2

Silver nanoparticles synthesized by the above method were addedto 70 ml of titanium (IV) isopropoxide with constant stirringto get pale yellow precipitate Distilled water was added to thisprecipitate until precipitate formation ceased The total volumeof the precursor is 70 ml and stirring was continued for 1 hrThe mixture was transferred into a stainless steel teflon linedautoclave (100 ml) and the hydrothermal reaction proceeded at

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180 C for 20 hrs After hydrothermal treatment the powderswere filtered washed with water and acetone respectively anddried at 80 C This sample is designated as TLS1 To studythe effect of calcination TLS1 is calcinated at 300 C for 3 hrs(TLS2) Pure nano TiO2 powder (PT) also was synthesized usingthe above procedure without the addition of silver nanoparticles

The X-ray diffraction patterns (XRD) of the synthesized sam-ples were obtained on a Brucker D8 Advanced Diffractometerusing Cu K as radiation The diffused reflection spectra (DRS)of the samples were performed using a UV-2450 Shimadzu UV-visible spectrophotometer The photoluminescence (PL) spectralmeasurements were made with the synthesized TiO2 nanopar-ticles using Shimadzu RF-5301 spectrofluorophotometer at anexcitation wavelength of 300 nm The BET specific surface areaof the powders was measured via nitrogen adsorption using asurface area analyzer (Micromeritics Gemini USA) A Jeol JSM6500F scanning electron microscope was used for FESEM stud-ies TEM images were taken using a Jeol JEM-3010 transmissionelectron microscope

The photocatalytic activity of the synthesized samples wasassessed by photocatalytic degradation of methylene blue For atypical photocatalytic experiment 01 g of synthesized nanocrys-talline TiO2 was suspended in 100 ml of 50 ppm methyleneblue (MB) aqueous solution The resulting suspension was equi-librated by stirring in the dark for 30 min To study the pho-tocatalytic degradation under sunlight the MBndashTiO2 suspensionwas kept under sunlight at ambient temperature for 70 min from1200 noon to 110 noon The samples were withdrawn at differ-ent time intervals and centrifuged at 6000 rpm to remove theTiO2 particles The absorbance of MB was measured at 660 nmusing a UV-3101 PC UV-VIS-NIR Shimadzu scanning spec-trophotometer It was observed that no detectable degradation ofMB occurs without TiO2 or solar irradiation aloneTo study the degradation of pesticide endosulfan under solar

irradiation using the synthesized silver doped and undoped TiO2

samples an aqueous solution with a endosulfan concentration350 ppb (100 ml) was stirred with the samples PT TLS1 andTLS2 (01 g) in separate experiments The resulting suspensionwas equilibrated by stirring in the dark for 30 min and then keptunder direct sunlight from 1200 noon to 100 noon Samples werewithdrawn at regular intervals centrifuged and extracted threetimes with dichloromethane The combined organic layer is thendried over anhydrous sodium sulphate and concentrated whichis then reconstituted in toluene prior to gas chromatographicanalysis endosulfan concentrations in the photocatalytic experi-ments were determined by gas chromatograph equipped with elec-tron capture detector (GC-ECD) (Shimadzu GC 2010A) with anautosampler and autoinjector A capillary column DB-5 (30 Mtimes025 mmtimes025 m) (Agilent Technologies USA) was used forthe GC-ECD analysis as well as for the partitioning of the par-ent pesticide and other photolysis product in subsequent GC-MSanalysis Sample taken at 60 min was extracted and subjectedfor intermediate or end product analysis with GC-MS determina-tion in electrospray ionization scan mode (EI-SCAN) (ShimadzuGCMS QP2010)

3 RESULTS AND DISCUSSIONFigure 1(b) shows the absorption spectra of a solution containingAgNO3 and NaOH after the addition of L-DOPA recorded at dif-ferent time intervals The figure clearly shows the formation of a

10 20 30 40 50 60 70 80 90

Inte

nsity

(a

u)

2 Theta

PT

TLS1

TLS2

101

004200

105

211 204 215

Fig 2 XRD patterns of PT TLS1 and TLS2

new band at 408 nm which is the characteristic plasmon absorp-tion for silver nanoparticles The formation of silver nanoparticleswas confirmed by TEM image (Fig 1(c)) The results clearlyshow that L-Dopa is an excellent capping and stabilizing agentfor silver nanoparticles and the schematic representation is givenin Figure 1(d)

31 XRD AnalysisX-ray diffractograms (XRD) of silver doped (TLS1 and TLS2)and undoped samples (PT) are shown in (Fig 2) XRD patternsreveal that the samples show a high degree of crystallinity and thecrystalline phase are of typical anatase (JCPDS No 21-1272)There is slight increase in crystallinity of the sample on dopingsilver nanoparticles but no phase change is observed Hence it isclear that silver doping alters the crystallinity but not the crystalstructure After calcination at 300 C the crystallinity of the sam-ple again increased but there is no phase change in the sample(TLS2)

However there are no obvious peaks showing the presence ofsilver in the XRD patterns of TLS1 and TLS2 and is due to thelow silver content1314 The nanocrystallite size of the synthe-sized samples was estimated using Scherrerrsquos equation DXRD =09 cos where D is the crystallite size is the wavelengthof X-ray used and are full width at half maximum intensity(in radian) (FWHM) of XRD diffraction lines and half diffrac-tion angle 2 respectively The crystallite sizes of samples aregiven in Table I and is clear that doping of silver nanoparticledid not alter the crystallite size of the samples but the calcinationprocess slightly increased the crystallite sizes

Table I BET surface area particle size crystallite size and band gapof PT TLS1 and TLS2

Surface area Particle size Crystallite size BandSample (m2g) (DBET) (nm) (DXRD) (nm) gap (eV)

PT 11955 130 131 323TLS1 1139 135 137 292TLS2 9334 165 158 287

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32 Raman SpectraFigure 3 depicts the Raman spectra of the synthesized samplesThe peaks at 399 515 and 637 cmminus1 in PT TLS1 and TLS2 cor-responds to the allowed Raman active modes of anatase crystalsThe presence of silver nanoparticles on the TiO2 surface slightlyreduced the intensities of the peaks in TLS1 and TLS2 which isin good agreement with the earlier report15 But the intensities ofthe peaks of TLS2 is higher than that of TLS1 and this may bedue to the increase in crystallinity of TLS2 during calcination asis evident from XRD

33 Surface Area AnalysisBET surface areas measured for the synthesized samples aregiven in Table I and show that the surface areas of the sam-ples are high compared to earlier reports1617 Doping of silvernanoparticles does not induce appreciable change in surface areaBut there is a decrease in surface area after calcination and is dueto the increase in crystallite size during calcination The aver-age nanoparticle sizes were calculated from BET surface areausing the equation DBET = 6000S where DBET is the averagenanoparticle size (nm) is the powder density (gcm3) S is thespecific surface area (m2g) The results are shown in Table I andare comparable with the average nanocrystallite size determinedusing Scherrerrsquos equation

34 Diffuse Reflectance SpectraTo investigate optical absorption properties of the synthesizedsamples the diffuse reflectance absorption spectra (DRS) of PTTLS1 and TLS2 were examined in the range 300ndash750 nm and theresults are shown in Figure 4 It is clear from the figure that TiO2

nanocrystals without silver nanoparticles (PT) has absorption inthe UV region and is ascribed to charge transfer from valenceband (mainly formed by 2p orbitals of oxide anion) to the con-duction band (mainly formed by 3dt2g orbitals of Ti4+ cation)18

The samples with silver nanoparticles (TLS1 and TLS2) showedsignificant absorption in the visible region which is in goodagreement with the earlier reports1319 The red shifted photore-sponse of TLS1 and TLS2 as observed in the figure may leadto high photocatalytic activity under visible region which helpsin the enhancement of photocatalytic activity under sunlight The

200 300 400 500 600 700 8000

5000

10000

15000

20000

25000

30000

35000

Inte

nsity

(a

u)

Raman shift (cmndash1)

TLS1

TLS2

PT

Fig 3 Raman spectra of PT TLS1 and TLS2

300 400 500 600 700

00

02

04

06

08

10

12

14

16

Abs

orba

nce

(au

)

Wavelength (nm)

PTTLS1

TLS2

Fig 4 Diffuse reflectance absorption spectra of PT TLS1 and TLS2

absorption band around 400 nm may be attributed to the absorp-tion of the silver nanoparticles adsorbed on the surface of TiO2

particles20

The band gap energies of the synthesized samples were calcu-lated by the equation

Eg = 12398 Ref [21]

where Eg is the band gap (eV) and (nm) is the wavelengthof the absorption edges in the spectrum The band gap valuesare given in Table I and the values indicate that silver dop-ing reduced the band gap of TiO2 nanocrystals The band gapnarrowing was primarily attributed to the substitution of silvernanoparticles which introduced electron states into the band gapof TiO2 to form a new lowest unoccupied molecular orbital Theshift of absorption edge of TiO2 to visible range and narrowingof band gap increased the photocatalytic activity of TiO2 in thevisible region

Further the absorption spectrum of TLS2 reveals more absorp-tion in the visible region than TLS1 which indicates the existenceof surface states22 The most possible mechanism to account forthe observed absorption might involve oxygen vacancies pro-duced by thermal treatment which form localization levels withinthe band gap22

35 Morphological AnalysisFESEM and TEM images of the silver doped samples aregiven in Figure 5 FESEM (Figs 5(a and b)) In TEM images(Figs 5(c and d)) Ag nanoparticles are distinguishable and appearas dark dots on the surface of the TiO2 particles Also there is apossibility for silver to be incorporated into the interstitial posi-tions of the TiO2 semiconductor particles It is also clear thatthe crystallinity has been increased which is indicated by theincreased crystal size with well-developed faces This observa-tion is supported by the XRD spectrum (Fig 2)

36 Photoluminescence StudiesThe photoluminescence emission spectra (PL) of the synthesisedsamples were studied in the range of 200ndash600 nm to investigatethe separation efficiency of charge carriers and the results areshown in Figure 6 It is evident from the figure that the PL inten-sity of TiO2 was decreased with silver doping The lower PL

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(c)

(a) (b)

(b)

Fig 5 FESEM of TLS1 (a) TLS2 (b) and TEM images of TLS1 (c) and TLS2 (d)

intensity shows that the rate of recombination between electronand holes is low23 The electrons are excited from valance band(VB) to conduction band (CB) of TiO2 under UV irradiation andgenerate photoexcited electrons and holes Although doping withsilver narrows the band gap of TiO2 samples the recombinationof electrons and holes are relatively slow which is evident fromPL spectra

As is well known TiO2 exhibit poor photocatalytic efficiencysince the majority of photogenerated charge carriers undergorecombination In the present system silver nanoparticles act aselectron acceptors and sinks for the charge carriers which will

200 300 400 500 600 700ndash20

ndash15

ndash10

ndash05

00

05

10

15

20

25

30

Inte

nsity

(a

u)

Wavelength (cmndash1)

PT

TLS1

TLS2

Fig 6 Photoluminescence emission spectra (PL) of PT TLS1 and TLS2

enhance the efficiency of photocatalysis2324 For samples TLS1and TLS2 silver nanoparticles plays an important role in theinterfacial charge transfer and in the decrease in rate of electron-hole recombination Silver nanoparticles could act as an effec-tive electron scavenger to trap the photo induced electrons andholes of TiO2 leading to the reduction of electronndashhole recom-bination and thus improving the photocatalytic efficiency Theelectrons trapped in silver sites were subsequently transferred tothe surrounding adsorbed O2 It can be supported by PL emissionspectra which have been widely used to investigate the fate ofelectron hole pairs in semiconductor particles since PL emissionresults from the recombination of free carriers2025

37 Photocatalytic ActivityTo investigate the photocatalytic activity of synthesized TiO2

samples in solar light the degradation of MB was studiedin presence of PT TLS1 and TLS2 nanoparticles under sun-light For comparison photocatalytic studies were also performedwith commercially available photocatalyst Degussa P25 and theresults are depicted in Figure 7 The activity of different samplesin sunlight is in the order TLS2 gt TLS1 gt PT gt Degussa P25Silver doped TiO2 samples show higher activity than undopedTiO2 and Degussa P25 Diffuse reflectance spectra of silverdoped TiO2 samples showed significant absorption in the visi-ble region which enhance the photocatalytic activity in the vis-ible region The high activity of silver doped TiO2 samples ismainly attributed to the decrease in band gap (323 to 287 eV)so that visible light is enough to excite electron from valenceband to conduction band Silver nanoparticles could act as aneffective electron scavenger to trap the conduction band elec-trons of TiO2

23 In silver doped TiO2 the silver nanoparticles

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R E S E A R CH AR T I C L EAdv Sci Lett 4 108ndash114 2011

10 20 30 40 50 60 70

10

20

30

40

50

60

70

80

90

100

D

egra

datio

n of

MB

Time in min

Degussa P 25

PTTLS1

TLS2

Fig 7 Photocatalytic activity of PT TLS1 TLS2 and Degussa P25 undersunlight

play an important role in the interfacial charge transfer and inthe elimination of electron-hole recombination as is evident fromPL spectra which would be beneficial for the high photocatalyticactivity The activity of PT is less because here electrons can-not be excited from valence band to conduction band by visiblelight irradiation due to the large band gap as calculated fromDRS (323 eV) In this case an ultraviolet irradiation is requiredfor the excitation of electrons from valence band to conductionband to carry out photocatalysis which accounts for only smallfraction of the solar light Also the efficiency of the photocatal-ysis depends on the effectiveness of the photocatalytic processin transferring the photoinduced eminush+ pair from the particlevolume to the particle surface and subsequently to the surface-adsorbed species In PT there is no intermediate level to trap theelectron and the recombination possibility of electrons and holesis enhanced and the free electron or holes available for the pho-tocatalysis is less subsequently the efficiency of photocatalysisalso is less

It is well known that it is the anatase polymorph rather than thebrookite or rutile polymorph that has the highest photocatalyticactivity The most active commercially available photocatalyst(Degussa P25) has an anatase content of 75 and 25 of rutilecontent26 From the XRD pattern it is evident that the synthesizedsamples (PT TLS1 and TLS2) are purely anatase and hence itshows higher activity than Degussa P25

The synthesized silver doped samples (TLS1 and TLS2) arehighly efficient photocatalysts as is evident from the MB degra-dation studies The sample TLS2 shows higher photocatalyticactivity than the sample TLS1 (Fig 7) and is due to the increasein crystallinity of TLS2 which is clear from the XRD data How-ever no direct relation was found between the BET areas and thecatalytic activity of the catalysts as in earlier report27 The previ-ous reports show that calcination of samples and UV irradiationwas required for effective photocatalysis also the UV irradiationtime for the degradation of MB is high (60 to 120 min)2829

The studies which deals visible light photocatalysis of MB usingdoped TiO2 also required high temperature calcination (600 C)and longer irradiation time (60ndash120 min)3031

But we have used low temperature hydrothermal route to pre-pare efficient photocatalyst which is active under solar light without any high temperature treatment Also silver doping couldreplace artificial UV light irradiation by sunlight Hence it is clearthat silver doping in TiO2 can give highly efficient photocatalystswhich are active under sunlight

In addition to higher photocatalytic activity the hydrother-mally synthesized TiO2 samples are easier to separate from theaqueous media than Degussa P25 Degussa P25 forms a milkywhite turbid suspension in aqueous media Though the synthe-sized samples have equal or less particle size they do not formturbid suspension Also the synthesized catalysts settle faster andit is easier to separate from the reaction mixture by centrifuga-tion This enabled recyclability of the synthesized nanophotocat-alysts after centrifugation at the end of each photocatalytic cycleand the recycled catalyst was found efficient and stable for morethan six cycles

Degradation of the nonbiodegradable pesticide endosulfanwas also investigated in the light of the interesting photocat-alytic activity of the synthesized nanoparticles with MB Thephotocatalytic activity of PT TLS1 and TLS2 nanoparticles withthe organochlorine pesticide under sunlight was studied in sep-arate experiments Figure 8 shows the degradation kinetics of endosulfan with the nanoparticles at an initial concentration of350 ppb which is close to the maximum aqueous solubility of endosulfan Concentration of the organochlorine pesticide endosulfan was determined by GC-ECD analysis As expectedhere also the more crystalline nanoparticle sample TLS2 exhib-ited highest photocatalytic activity followed by TLS1 and PT InTLS2-Endosulfan system substantial reduction in the concentra-tion of endosulfan was observed in the initial 20 minutes at at12 of 13 minutes It was observed that more than 98 of thepesticide was decomposed in 60 minutes The GC-MS analysisin EI-SCAN mode of the sample taken at 60 minutes confirmedthe almost complete mineralization of endosulfan The envi-ronmental degradation products of endosulfan as proposed byBakouri et al32 such as the immediate hydrolysis product endo-sulfan diol the oxidation product endosulfan sulfate or the furtherdecomposition products endosulfan ether endosulfan hydroxylether or endosulfan lactone were not detected in the system Toensure degradation rather than adsorption the experiment was

0 10 20 30 40 50 6000

02

04

06

08

10

PT

TLS1

TLS2

CtC

0

Time min

Fig 8 Photocatalytic degradation of endosulfan using PT TLS1 andTLS2 under sunlight

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repeated with the same TLS2 nanoparticles (after regenerating itfrom the photocatalytic system by centrifugation) with the sameinitial concentration of endosulfan for six more cycles Sampleswere analyzed for endosulfan concentration at the end of 60 min-utes The TLS2 photocatalytic system was found stable and effi-cient after the repeated experiments with consistent endosulfandegradation capability In aqueous systems though endosulfanresist direct photolysis it is clearly observed that it undergoesnear complete photodegradation with silver doped nano TiO2

particles The heterogeneous photocatalysis with TiO2 nanopar-ticles accelerated the decomposition of otherwise recalcitrant endosulfan and silver doping shifted the photocatalysis to visibleregion to attain efficient degradation under sunlight The resultssuggest the potential of silver doped TiO2 nanomaterials in envi-ronmental pollutant remediation applications

4 CONCLUSIONSAnatase nanocrystals with enhanced photocatalytic activity undersolar irradiation have been successfully synthesized by lowtemperature hydrothermal route by silver doping Silver nanopar-ticles for doping were prepared by simultaneous chemical reduc-tion and stabilization employing L-Dopa without any additionalstabilizing agent The hydrothermally synthesized TiO2 samplesshowed higher photocatalytic activity under sunlight than thecommercially available photocatalyst Degussa P25 and couldcompletely degrade the organochlorine pesticide endosulfan bysunlight irradiation Also the synthesized catalysts settle fasterand it is easier to separate from the reaction mixture which pro-motes the reusability of these photocatalysts Decrease in bandgap in silver doped samples by the formation of an intermedi-ate level is the major reason for the high photocatalytic activ-ity of these samples under sunlight The present study suggeststhat silver doped TiO2 can be used for the development of anenvironmentally sustainable photocatalytic treatment process forthe degradation of persistent organic pollutants using sunlight inplace of an artificial light

Acknowledgments The authors wish to thank the Depart-ment of Science and Technology (DST) India for financial sup-port under woman scientist scheme (WOS-A)

References and Notes1 K B Dhanalakshmi S Anandan J Madhavan and P Maruthamuthu Solar

Energy Mat Solar Cells 92 457(2008)2 R T Meijers E J Oderwaldmuller P Nuhn and J C Kruithof Ozone-Sci

Eng 17 673 (1995)

3 E Wittmann P Cote C Medici J Leech and A G Turner Desalination119 347 (1998)

4 P Roche and M Prados Ozone-Sci Eng 17 657 (1995)5 H M Shivaramaiah and I R Kennedy J Environ Sci Health Part B 41 895

(2006)6 D Dong P Li X Li Q Zhao Y Zhang C Jia and P Li J Hazardous

Materials 174 859 (2010)7 V A Sakkas I M Arabatzis I K Konstantinou A D Dimou T A Albanis

and P Falaras Appl Catal B Environ 49 195 (2004)8 A S Topalova D V Šojica D A Molnaacuter-Gaacutebora B F Abramovica and M I

Comorb Appl Catal B Environ 54 125 (2004)9 M M Higarashi and W F Jardim Catal Today 76 201 (2002)

10 G Zhanqi Y Shaogui T Na and S Cheng J Hazard Mater 142 424(2007)

11 T Kawai H Takahashi Y Matsushima T Ogata and H Unuma Sci AdvMater 2 74 (2010)

12 Y Tokunaga H Uchiyama Y Oaki and H Imai Sci Adv Mater 2 69(2010)

13 X S Li G E Fryxell C Wang and M H Engelhard Microporous Meso-porous Mater 111 639 (2008)

14 N Ma X Fan X Quan and Y Zhang J Membr Sci 336 109(2009)

15 K Mallick M J Witcomb and M S Scurrell Appl Catal A General 259 163(2004)

16 S Je-Lueng L Chia-Hsiang C Chyow-San C Chang-Tong C Chia-Chiand C Ching-Yuan J Hazard Mater 155 164 (2008)

17 A Kubacka M Ferrer A Martiacutenez-Arias and M Fernaacutendez-Garciacutea ApplCatal B Environ 84 87 (2008)

18 N Sobana M Murugandahamm and M Swaminathan J Mol Catal AChem 258 124 (2006)

19 S Anandan P S Kumar N Pugazhenthiran J Madhavan andP Maruthamuthu Solar Energy Mat Solar Cells 92 929 (2008)

20 S V Awate R K Sahu M D Kadgaonkar R Kumar and N M GuptaCatal Today 141 144 (2009)

21 B O Regan and M Gratzel Nature 353 737 (1991)22 Z P Wang W M Cai X T Hong X L Zhao F Xu and CG Cai Appl

Catal B Environ 57 223 (2005)23 F B Li X Z Li M F Hou K W Cheah and W C H Choy Appl Catal A

General 285 181 (2005)24 B Zhu Z Sui S Wang X Chen S Zhang S Wu and W Huang Mat Res

Bull 41 1097 (2006)25 D Zhang D Yang H Zhang C Lu and L Qi Chem Mater 18 3477

(2006)26 T Ohno K Sarukawa K Tokieda and M Matsumura J Catal 203 82

(2001)27 M Hosseini S Siffert H L Tidahy R Cousin J-F Lamonier A Aboukais

A Vantomme M Roussel and B-L Su Catal Today 122 391(2007)

28 Q Xiao Z Si Z Yu and G Qiu J Alloys Compd 450 426 (2008)29 M W Yan W L Su X Zhiliang B J Xin P C Xiao and P Jie Mater Lett

60 974 (2006)30 X Qi Z Jiang X Chong S Zhichun and T Xiaoke Solar Energy 82 706

(2008)31 Q Xiao Z Si Z Yu and G Qiu Mat Sci and Eng B 137 189

(2007)32 H E Bakouri A Ouassini J M Aguado and J U Garciacutea Water Environ

Res 79 2578 (2007)

Received 30 May 2010 Accepted 6 July 2010

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180 C for 20 hrs After hydrothermal treatment the powderswere filtered washed with water and acetone respectively anddried at 80 C This sample is designated as TLS1 To studythe effect of calcination TLS1 is calcinated at 300 C for 3 hrs(TLS2) Pure nano TiO2 powder (PT) also was synthesized usingthe above procedure without the addition of silver nanoparticles

The X-ray diffraction patterns (XRD) of the synthesized sam-ples were obtained on a Brucker D8 Advanced Diffractometerusing Cu K as radiation The diffused reflection spectra (DRS)of the samples were performed using a UV-2450 Shimadzu UV-visible spectrophotometer The photoluminescence (PL) spectralmeasurements were made with the synthesized TiO2 nanopar-ticles using Shimadzu RF-5301 spectrofluorophotometer at anexcitation wavelength of 300 nm The BET specific surface areaof the powders was measured via nitrogen adsorption using asurface area analyzer (Micromeritics Gemini USA) A Jeol JSM6500F scanning electron microscope was used for FESEM stud-ies TEM images were taken using a Jeol JEM-3010 transmissionelectron microscope

The photocatalytic activity of the synthesized samples wasassessed by photocatalytic degradation of methylene blue For atypical photocatalytic experiment 01 g of synthesized nanocrys-talline TiO2 was suspended in 100 ml of 50 ppm methyleneblue (MB) aqueous solution The resulting suspension was equi-librated by stirring in the dark for 30 min To study the pho-tocatalytic degradation under sunlight the MBndashTiO2 suspensionwas kept under sunlight at ambient temperature for 70 min from1200 noon to 110 noon The samples were withdrawn at differ-ent time intervals and centrifuged at 6000 rpm to remove theTiO2 particles The absorbance of MB was measured at 660 nmusing a UV-3101 PC UV-VIS-NIR Shimadzu scanning spec-trophotometer It was observed that no detectable degradation ofMB occurs without TiO2 or solar irradiation aloneTo study the degradation of pesticide endosulfan under solar

irradiation using the synthesized silver doped and undoped TiO2

samples an aqueous solution with a endosulfan concentration350 ppb (100 ml) was stirred with the samples PT TLS1 andTLS2 (01 g) in separate experiments The resulting suspensionwas equilibrated by stirring in the dark for 30 min and then keptunder direct sunlight from 1200 noon to 100 noon Samples werewithdrawn at regular intervals centrifuged and extracted threetimes with dichloromethane The combined organic layer is thendried over anhydrous sodium sulphate and concentrated whichis then reconstituted in toluene prior to gas chromatographicanalysis endosulfan concentrations in the photocatalytic experi-ments were determined by gas chromatograph equipped with elec-tron capture detector (GC-ECD) (Shimadzu GC 2010A) with anautosampler and autoinjector A capillary column DB-5 (30 Mtimes025 mmtimes025 m) (Agilent Technologies USA) was used forthe GC-ECD analysis as well as for the partitioning of the par-ent pesticide and other photolysis product in subsequent GC-MSanalysis Sample taken at 60 min was extracted and subjectedfor intermediate or end product analysis with GC-MS determina-tion in electrospray ionization scan mode (EI-SCAN) (ShimadzuGCMS QP2010)

3 RESULTS AND DISCUSSIONFigure 1(b) shows the absorption spectra of a solution containingAgNO3 and NaOH after the addition of L-DOPA recorded at dif-ferent time intervals The figure clearly shows the formation of a

10 20 30 40 50 60 70 80 90

Inte

nsity

(a

u)

2 Theta

PT

TLS1

TLS2

101

004200

105

211 204 215

Fig 2 XRD patterns of PT TLS1 and TLS2

new band at 408 nm which is the characteristic plasmon absorp-tion for silver nanoparticles The formation of silver nanoparticleswas confirmed by TEM image (Fig 1(c)) The results clearlyshow that L-Dopa is an excellent capping and stabilizing agentfor silver nanoparticles and the schematic representation is givenin Figure 1(d)

31 XRD AnalysisX-ray diffractograms (XRD) of silver doped (TLS1 and TLS2)and undoped samples (PT) are shown in (Fig 2) XRD patternsreveal that the samples show a high degree of crystallinity and thecrystalline phase are of typical anatase (JCPDS No 21-1272)There is slight increase in crystallinity of the sample on dopingsilver nanoparticles but no phase change is observed Hence it isclear that silver doping alters the crystallinity but not the crystalstructure After calcination at 300 C the crystallinity of the sam-ple again increased but there is no phase change in the sample(TLS2)

However there are no obvious peaks showing the presence ofsilver in the XRD patterns of TLS1 and TLS2 and is due to thelow silver content1314 The nanocrystallite size of the synthe-sized samples was estimated using Scherrerrsquos equation DXRD =09 cos where D is the crystallite size is the wavelengthof X-ray used and are full width at half maximum intensity(in radian) (FWHM) of XRD diffraction lines and half diffrac-tion angle 2 respectively The crystallite sizes of samples aregiven in Table I and is clear that doping of silver nanoparticledid not alter the crystallite size of the samples but the calcinationprocess slightly increased the crystallite sizes

Table I BET surface area particle size crystallite size and band gapof PT TLS1 and TLS2

Surface area Particle size Crystallite size BandSample (m2g) (DBET) (nm) (DXRD) (nm) gap (eV)

PT 11955 130 131 323TLS1 1139 135 137 292TLS2 9334 165 158 287

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32 Raman SpectraFigure 3 depicts the Raman spectra of the synthesized samplesThe peaks at 399 515 and 637 cmminus1 in PT TLS1 and TLS2 cor-responds to the allowed Raman active modes of anatase crystalsThe presence of silver nanoparticles on the TiO2 surface slightlyreduced the intensities of the peaks in TLS1 and TLS2 which isin good agreement with the earlier report15 But the intensities ofthe peaks of TLS2 is higher than that of TLS1 and this may bedue to the increase in crystallinity of TLS2 during calcination asis evident from XRD

33 Surface Area AnalysisBET surface areas measured for the synthesized samples aregiven in Table I and show that the surface areas of the sam-ples are high compared to earlier reports1617 Doping of silvernanoparticles does not induce appreciable change in surface areaBut there is a decrease in surface area after calcination and is dueto the increase in crystallite size during calcination The aver-age nanoparticle sizes were calculated from BET surface areausing the equation DBET = 6000S where DBET is the averagenanoparticle size (nm) is the powder density (gcm3) S is thespecific surface area (m2g) The results are shown in Table I andare comparable with the average nanocrystallite size determinedusing Scherrerrsquos equation

34 Diffuse Reflectance SpectraTo investigate optical absorption properties of the synthesizedsamples the diffuse reflectance absorption spectra (DRS) of PTTLS1 and TLS2 were examined in the range 300ndash750 nm and theresults are shown in Figure 4 It is clear from the figure that TiO2

nanocrystals without silver nanoparticles (PT) has absorption inthe UV region and is ascribed to charge transfer from valenceband (mainly formed by 2p orbitals of oxide anion) to the con-duction band (mainly formed by 3dt2g orbitals of Ti4+ cation)18

The samples with silver nanoparticles (TLS1 and TLS2) showedsignificant absorption in the visible region which is in goodagreement with the earlier reports1319 The red shifted photore-sponse of TLS1 and TLS2 as observed in the figure may leadto high photocatalytic activity under visible region which helpsin the enhancement of photocatalytic activity under sunlight The

200 300 400 500 600 700 8000

5000

10000

15000

20000

25000

30000

35000

Inte

nsity

(a

u)

Raman shift (cmndash1)

TLS1

TLS2

PT

Fig 3 Raman spectra of PT TLS1 and TLS2

300 400 500 600 700

00

02

04

06

08

10

12

14

16

Abs

orba

nce

(au

)

Wavelength (nm)

PTTLS1

TLS2

Fig 4 Diffuse reflectance absorption spectra of PT TLS1 and TLS2

absorption band around 400 nm may be attributed to the absorp-tion of the silver nanoparticles adsorbed on the surface of TiO2

particles20

The band gap energies of the synthesized samples were calcu-lated by the equation

Eg = 12398 Ref [21]

where Eg is the band gap (eV) and (nm) is the wavelengthof the absorption edges in the spectrum The band gap valuesare given in Table I and the values indicate that silver dop-ing reduced the band gap of TiO2 nanocrystals The band gapnarrowing was primarily attributed to the substitution of silvernanoparticles which introduced electron states into the band gapof TiO2 to form a new lowest unoccupied molecular orbital Theshift of absorption edge of TiO2 to visible range and narrowingof band gap increased the photocatalytic activity of TiO2 in thevisible region

Further the absorption spectrum of TLS2 reveals more absorp-tion in the visible region than TLS1 which indicates the existenceof surface states22 The most possible mechanism to account forthe observed absorption might involve oxygen vacancies pro-duced by thermal treatment which form localization levels withinthe band gap22

35 Morphological AnalysisFESEM and TEM images of the silver doped samples aregiven in Figure 5 FESEM (Figs 5(a and b)) In TEM images(Figs 5(c and d)) Ag nanoparticles are distinguishable and appearas dark dots on the surface of the TiO2 particles Also there is apossibility for silver to be incorporated into the interstitial posi-tions of the TiO2 semiconductor particles It is also clear thatthe crystallinity has been increased which is indicated by theincreased crystal size with well-developed faces This observa-tion is supported by the XRD spectrum (Fig 2)

36 Photoluminescence StudiesThe photoluminescence emission spectra (PL) of the synthesisedsamples were studied in the range of 200ndash600 nm to investigatethe separation efficiency of charge carriers and the results areshown in Figure 6 It is evident from the figure that the PL inten-sity of TiO2 was decreased with silver doping The lower PL

111

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R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

(c)

(a) (b)

(b)

Fig 5 FESEM of TLS1 (a) TLS2 (b) and TEM images of TLS1 (c) and TLS2 (d)

intensity shows that the rate of recombination between electronand holes is low23 The electrons are excited from valance band(VB) to conduction band (CB) of TiO2 under UV irradiation andgenerate photoexcited electrons and holes Although doping withsilver narrows the band gap of TiO2 samples the recombinationof electrons and holes are relatively slow which is evident fromPL spectra

As is well known TiO2 exhibit poor photocatalytic efficiencysince the majority of photogenerated charge carriers undergorecombination In the present system silver nanoparticles act aselectron acceptors and sinks for the charge carriers which will

200 300 400 500 600 700ndash20

ndash15

ndash10

ndash05

00

05

10

15

20

25

30

Inte

nsity

(a

u)

Wavelength (cmndash1)

PT

TLS1

TLS2

Fig 6 Photoluminescence emission spectra (PL) of PT TLS1 and TLS2

enhance the efficiency of photocatalysis2324 For samples TLS1and TLS2 silver nanoparticles plays an important role in theinterfacial charge transfer and in the decrease in rate of electron-hole recombination Silver nanoparticles could act as an effec-tive electron scavenger to trap the photo induced electrons andholes of TiO2 leading to the reduction of electronndashhole recom-bination and thus improving the photocatalytic efficiency Theelectrons trapped in silver sites were subsequently transferred tothe surrounding adsorbed O2 It can be supported by PL emissionspectra which have been widely used to investigate the fate ofelectron hole pairs in semiconductor particles since PL emissionresults from the recombination of free carriers2025

37 Photocatalytic ActivityTo investigate the photocatalytic activity of synthesized TiO2

samples in solar light the degradation of MB was studiedin presence of PT TLS1 and TLS2 nanoparticles under sun-light For comparison photocatalytic studies were also performedwith commercially available photocatalyst Degussa P25 and theresults are depicted in Figure 7 The activity of different samplesin sunlight is in the order TLS2 gt TLS1 gt PT gt Degussa P25Silver doped TiO2 samples show higher activity than undopedTiO2 and Degussa P25 Diffuse reflectance spectra of silverdoped TiO2 samples showed significant absorption in the visi-ble region which enhance the photocatalytic activity in the vis-ible region The high activity of silver doped TiO2 samples ismainly attributed to the decrease in band gap (323 to 287 eV)so that visible light is enough to excite electron from valenceband to conduction band Silver nanoparticles could act as aneffective electron scavenger to trap the conduction band elec-trons of TiO2

23 In silver doped TiO2 the silver nanoparticles

112

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R E S E A R CH AR T I C L EAdv Sci Lett 4 108ndash114 2011

10 20 30 40 50 60 70

10

20

30

40

50

60

70

80

90

100

D

egra

datio

n of

MB

Time in min

Degussa P 25

PTTLS1

TLS2

Fig 7 Photocatalytic activity of PT TLS1 TLS2 and Degussa P25 undersunlight

play an important role in the interfacial charge transfer and inthe elimination of electron-hole recombination as is evident fromPL spectra which would be beneficial for the high photocatalyticactivity The activity of PT is less because here electrons can-not be excited from valence band to conduction band by visiblelight irradiation due to the large band gap as calculated fromDRS (323 eV) In this case an ultraviolet irradiation is requiredfor the excitation of electrons from valence band to conductionband to carry out photocatalysis which accounts for only smallfraction of the solar light Also the efficiency of the photocatal-ysis depends on the effectiveness of the photocatalytic processin transferring the photoinduced eminush+ pair from the particlevolume to the particle surface and subsequently to the surface-adsorbed species In PT there is no intermediate level to trap theelectron and the recombination possibility of electrons and holesis enhanced and the free electron or holes available for the pho-tocatalysis is less subsequently the efficiency of photocatalysisalso is less

It is well known that it is the anatase polymorph rather than thebrookite or rutile polymorph that has the highest photocatalyticactivity The most active commercially available photocatalyst(Degussa P25) has an anatase content of 75 and 25 of rutilecontent26 From the XRD pattern it is evident that the synthesizedsamples (PT TLS1 and TLS2) are purely anatase and hence itshows higher activity than Degussa P25

The synthesized silver doped samples (TLS1 and TLS2) arehighly efficient photocatalysts as is evident from the MB degra-dation studies The sample TLS2 shows higher photocatalyticactivity than the sample TLS1 (Fig 7) and is due to the increasein crystallinity of TLS2 which is clear from the XRD data How-ever no direct relation was found between the BET areas and thecatalytic activity of the catalysts as in earlier report27 The previ-ous reports show that calcination of samples and UV irradiationwas required for effective photocatalysis also the UV irradiationtime for the degradation of MB is high (60 to 120 min)2829

The studies which deals visible light photocatalysis of MB usingdoped TiO2 also required high temperature calcination (600 C)and longer irradiation time (60ndash120 min)3031

But we have used low temperature hydrothermal route to pre-pare efficient photocatalyst which is active under solar light without any high temperature treatment Also silver doping couldreplace artificial UV light irradiation by sunlight Hence it is clearthat silver doping in TiO2 can give highly efficient photocatalystswhich are active under sunlight

In addition to higher photocatalytic activity the hydrother-mally synthesized TiO2 samples are easier to separate from theaqueous media than Degussa P25 Degussa P25 forms a milkywhite turbid suspension in aqueous media Though the synthe-sized samples have equal or less particle size they do not formturbid suspension Also the synthesized catalysts settle faster andit is easier to separate from the reaction mixture by centrifuga-tion This enabled recyclability of the synthesized nanophotocat-alysts after centrifugation at the end of each photocatalytic cycleand the recycled catalyst was found efficient and stable for morethan six cycles

Degradation of the nonbiodegradable pesticide endosulfanwas also investigated in the light of the interesting photocat-alytic activity of the synthesized nanoparticles with MB Thephotocatalytic activity of PT TLS1 and TLS2 nanoparticles withthe organochlorine pesticide under sunlight was studied in sep-arate experiments Figure 8 shows the degradation kinetics of endosulfan with the nanoparticles at an initial concentration of350 ppb which is close to the maximum aqueous solubility of endosulfan Concentration of the organochlorine pesticide endosulfan was determined by GC-ECD analysis As expectedhere also the more crystalline nanoparticle sample TLS2 exhib-ited highest photocatalytic activity followed by TLS1 and PT InTLS2-Endosulfan system substantial reduction in the concentra-tion of endosulfan was observed in the initial 20 minutes at at12 of 13 minutes It was observed that more than 98 of thepesticide was decomposed in 60 minutes The GC-MS analysisin EI-SCAN mode of the sample taken at 60 minutes confirmedthe almost complete mineralization of endosulfan The envi-ronmental degradation products of endosulfan as proposed byBakouri et al32 such as the immediate hydrolysis product endo-sulfan diol the oxidation product endosulfan sulfate or the furtherdecomposition products endosulfan ether endosulfan hydroxylether or endosulfan lactone were not detected in the system Toensure degradation rather than adsorption the experiment was

0 10 20 30 40 50 6000

02

04

06

08

10

PT

TLS1

TLS2

CtC

0

Time min

Fig 8 Photocatalytic degradation of endosulfan using PT TLS1 andTLS2 under sunlight

113

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R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

repeated with the same TLS2 nanoparticles (after regenerating itfrom the photocatalytic system by centrifugation) with the sameinitial concentration of endosulfan for six more cycles Sampleswere analyzed for endosulfan concentration at the end of 60 min-utes The TLS2 photocatalytic system was found stable and effi-cient after the repeated experiments with consistent endosulfandegradation capability In aqueous systems though endosulfanresist direct photolysis it is clearly observed that it undergoesnear complete photodegradation with silver doped nano TiO2

particles The heterogeneous photocatalysis with TiO2 nanopar-ticles accelerated the decomposition of otherwise recalcitrant endosulfan and silver doping shifted the photocatalysis to visibleregion to attain efficient degradation under sunlight The resultssuggest the potential of silver doped TiO2 nanomaterials in envi-ronmental pollutant remediation applications

4 CONCLUSIONSAnatase nanocrystals with enhanced photocatalytic activity undersolar irradiation have been successfully synthesized by lowtemperature hydrothermal route by silver doping Silver nanopar-ticles for doping were prepared by simultaneous chemical reduc-tion and stabilization employing L-Dopa without any additionalstabilizing agent The hydrothermally synthesized TiO2 samplesshowed higher photocatalytic activity under sunlight than thecommercially available photocatalyst Degussa P25 and couldcompletely degrade the organochlorine pesticide endosulfan bysunlight irradiation Also the synthesized catalysts settle fasterand it is easier to separate from the reaction mixture which pro-motes the reusability of these photocatalysts Decrease in bandgap in silver doped samples by the formation of an intermedi-ate level is the major reason for the high photocatalytic activ-ity of these samples under sunlight The present study suggeststhat silver doped TiO2 can be used for the development of anenvironmentally sustainable photocatalytic treatment process forthe degradation of persistent organic pollutants using sunlight inplace of an artificial light

Acknowledgments The authors wish to thank the Depart-ment of Science and Technology (DST) India for financial sup-port under woman scientist scheme (WOS-A)

References and Notes1 K B Dhanalakshmi S Anandan J Madhavan and P Maruthamuthu Solar

Energy Mat Solar Cells 92 457(2008)2 R T Meijers E J Oderwaldmuller P Nuhn and J C Kruithof Ozone-Sci

Eng 17 673 (1995)

3 E Wittmann P Cote C Medici J Leech and A G Turner Desalination119 347 (1998)

4 P Roche and M Prados Ozone-Sci Eng 17 657 (1995)5 H M Shivaramaiah and I R Kennedy J Environ Sci Health Part B 41 895

(2006)6 D Dong P Li X Li Q Zhao Y Zhang C Jia and P Li J Hazardous

Materials 174 859 (2010)7 V A Sakkas I M Arabatzis I K Konstantinou A D Dimou T A Albanis

and P Falaras Appl Catal B Environ 49 195 (2004)8 A S Topalova D V Šojica D A Molnaacuter-Gaacutebora B F Abramovica and M I

Comorb Appl Catal B Environ 54 125 (2004)9 M M Higarashi and W F Jardim Catal Today 76 201 (2002)

10 G Zhanqi Y Shaogui T Na and S Cheng J Hazard Mater 142 424(2007)

11 T Kawai H Takahashi Y Matsushima T Ogata and H Unuma Sci AdvMater 2 74 (2010)

12 Y Tokunaga H Uchiyama Y Oaki and H Imai Sci Adv Mater 2 69(2010)

13 X S Li G E Fryxell C Wang and M H Engelhard Microporous Meso-porous Mater 111 639 (2008)

14 N Ma X Fan X Quan and Y Zhang J Membr Sci 336 109(2009)

15 K Mallick M J Witcomb and M S Scurrell Appl Catal A General 259 163(2004)

16 S Je-Lueng L Chia-Hsiang C Chyow-San C Chang-Tong C Chia-Chiand C Ching-Yuan J Hazard Mater 155 164 (2008)

17 A Kubacka M Ferrer A Martiacutenez-Arias and M Fernaacutendez-Garciacutea ApplCatal B Environ 84 87 (2008)

18 N Sobana M Murugandahamm and M Swaminathan J Mol Catal AChem 258 124 (2006)

19 S Anandan P S Kumar N Pugazhenthiran J Madhavan andP Maruthamuthu Solar Energy Mat Solar Cells 92 929 (2008)

20 S V Awate R K Sahu M D Kadgaonkar R Kumar and N M GuptaCatal Today 141 144 (2009)

21 B O Regan and M Gratzel Nature 353 737 (1991)22 Z P Wang W M Cai X T Hong X L Zhao F Xu and CG Cai Appl

Catal B Environ 57 223 (2005)23 F B Li X Z Li M F Hou K W Cheah and W C H Choy Appl Catal A

General 285 181 (2005)24 B Zhu Z Sui S Wang X Chen S Zhang S Wu and W Huang Mat Res

Bull 41 1097 (2006)25 D Zhang D Yang H Zhang C Lu and L Qi Chem Mater 18 3477

(2006)26 T Ohno K Sarukawa K Tokieda and M Matsumura J Catal 203 82

(2001)27 M Hosseini S Siffert H L Tidahy R Cousin J-F Lamonier A Aboukais

A Vantomme M Roussel and B-L Su Catal Today 122 391(2007)

28 Q Xiao Z Si Z Yu and G Qiu J Alloys Compd 450 426 (2008)29 M W Yan W L Su X Zhiliang B J Xin P C Xiao and P Jie Mater Lett

60 974 (2006)30 X Qi Z Jiang X Chong S Zhichun and T Xiaoke Solar Energy 82 706

(2008)31 Q Xiao Z Si Z Yu and G Qiu Mat Sci and Eng B 137 189

(2007)32 H E Bakouri A Ouassini J M Aguado and J U Garciacutea Water Environ

Res 79 2578 (2007)

Received 30 May 2010 Accepted 6 July 2010

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32 Raman SpectraFigure 3 depicts the Raman spectra of the synthesized samplesThe peaks at 399 515 and 637 cmminus1 in PT TLS1 and TLS2 cor-responds to the allowed Raman active modes of anatase crystalsThe presence of silver nanoparticles on the TiO2 surface slightlyreduced the intensities of the peaks in TLS1 and TLS2 which isin good agreement with the earlier report15 But the intensities ofthe peaks of TLS2 is higher than that of TLS1 and this may bedue to the increase in crystallinity of TLS2 during calcination asis evident from XRD

33 Surface Area AnalysisBET surface areas measured for the synthesized samples aregiven in Table I and show that the surface areas of the sam-ples are high compared to earlier reports1617 Doping of silvernanoparticles does not induce appreciable change in surface areaBut there is a decrease in surface area after calcination and is dueto the increase in crystallite size during calcination The aver-age nanoparticle sizes were calculated from BET surface areausing the equation DBET = 6000S where DBET is the averagenanoparticle size (nm) is the powder density (gcm3) S is thespecific surface area (m2g) The results are shown in Table I andare comparable with the average nanocrystallite size determinedusing Scherrerrsquos equation

34 Diffuse Reflectance SpectraTo investigate optical absorption properties of the synthesizedsamples the diffuse reflectance absorption spectra (DRS) of PTTLS1 and TLS2 were examined in the range 300ndash750 nm and theresults are shown in Figure 4 It is clear from the figure that TiO2

nanocrystals without silver nanoparticles (PT) has absorption inthe UV region and is ascribed to charge transfer from valenceband (mainly formed by 2p orbitals of oxide anion) to the con-duction band (mainly formed by 3dt2g orbitals of Ti4+ cation)18

The samples with silver nanoparticles (TLS1 and TLS2) showedsignificant absorption in the visible region which is in goodagreement with the earlier reports1319 The red shifted photore-sponse of TLS1 and TLS2 as observed in the figure may leadto high photocatalytic activity under visible region which helpsin the enhancement of photocatalytic activity under sunlight The

200 300 400 500 600 700 8000

5000

10000

15000

20000

25000

30000

35000

Inte

nsity

(a

u)

Raman shift (cmndash1)

TLS1

TLS2

PT

Fig 3 Raman spectra of PT TLS1 and TLS2

300 400 500 600 700

00

02

04

06

08

10

12

14

16

Abs

orba

nce

(au

)

Wavelength (nm)

PTTLS1

TLS2

Fig 4 Diffuse reflectance absorption spectra of PT TLS1 and TLS2

absorption band around 400 nm may be attributed to the absorp-tion of the silver nanoparticles adsorbed on the surface of TiO2

particles20

The band gap energies of the synthesized samples were calcu-lated by the equation

Eg = 12398 Ref [21]

where Eg is the band gap (eV) and (nm) is the wavelengthof the absorption edges in the spectrum The band gap valuesare given in Table I and the values indicate that silver dop-ing reduced the band gap of TiO2 nanocrystals The band gapnarrowing was primarily attributed to the substitution of silvernanoparticles which introduced electron states into the band gapof TiO2 to form a new lowest unoccupied molecular orbital Theshift of absorption edge of TiO2 to visible range and narrowingof band gap increased the photocatalytic activity of TiO2 in thevisible region

Further the absorption spectrum of TLS2 reveals more absorp-tion in the visible region than TLS1 which indicates the existenceof surface states22 The most possible mechanism to account forthe observed absorption might involve oxygen vacancies pro-duced by thermal treatment which form localization levels withinthe band gap22

35 Morphological AnalysisFESEM and TEM images of the silver doped samples aregiven in Figure 5 FESEM (Figs 5(a and b)) In TEM images(Figs 5(c and d)) Ag nanoparticles are distinguishable and appearas dark dots on the surface of the TiO2 particles Also there is apossibility for silver to be incorporated into the interstitial posi-tions of the TiO2 semiconductor particles It is also clear thatthe crystallinity has been increased which is indicated by theincreased crystal size with well-developed faces This observa-tion is supported by the XRD spectrum (Fig 2)

36 Photoluminescence StudiesThe photoluminescence emission spectra (PL) of the synthesisedsamples were studied in the range of 200ndash600 nm to investigatethe separation efficiency of charge carriers and the results areshown in Figure 6 It is evident from the figure that the PL inten-sity of TiO2 was decreased with silver doping The lower PL

111

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R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

(c)

(a) (b)

(b)

Fig 5 FESEM of TLS1 (a) TLS2 (b) and TEM images of TLS1 (c) and TLS2 (d)

intensity shows that the rate of recombination between electronand holes is low23 The electrons are excited from valance band(VB) to conduction band (CB) of TiO2 under UV irradiation andgenerate photoexcited electrons and holes Although doping withsilver narrows the band gap of TiO2 samples the recombinationof electrons and holes are relatively slow which is evident fromPL spectra

As is well known TiO2 exhibit poor photocatalytic efficiencysince the majority of photogenerated charge carriers undergorecombination In the present system silver nanoparticles act aselectron acceptors and sinks for the charge carriers which will

200 300 400 500 600 700ndash20

ndash15

ndash10

ndash05

00

05

10

15

20

25

30

Inte

nsity

(a

u)

Wavelength (cmndash1)

PT

TLS1

TLS2

Fig 6 Photoluminescence emission spectra (PL) of PT TLS1 and TLS2

enhance the efficiency of photocatalysis2324 For samples TLS1and TLS2 silver nanoparticles plays an important role in theinterfacial charge transfer and in the decrease in rate of electron-hole recombination Silver nanoparticles could act as an effec-tive electron scavenger to trap the photo induced electrons andholes of TiO2 leading to the reduction of electronndashhole recom-bination and thus improving the photocatalytic efficiency Theelectrons trapped in silver sites were subsequently transferred tothe surrounding adsorbed O2 It can be supported by PL emissionspectra which have been widely used to investigate the fate ofelectron hole pairs in semiconductor particles since PL emissionresults from the recombination of free carriers2025

37 Photocatalytic ActivityTo investigate the photocatalytic activity of synthesized TiO2

samples in solar light the degradation of MB was studiedin presence of PT TLS1 and TLS2 nanoparticles under sun-light For comparison photocatalytic studies were also performedwith commercially available photocatalyst Degussa P25 and theresults are depicted in Figure 7 The activity of different samplesin sunlight is in the order TLS2 gt TLS1 gt PT gt Degussa P25Silver doped TiO2 samples show higher activity than undopedTiO2 and Degussa P25 Diffuse reflectance spectra of silverdoped TiO2 samples showed significant absorption in the visi-ble region which enhance the photocatalytic activity in the vis-ible region The high activity of silver doped TiO2 samples ismainly attributed to the decrease in band gap (323 to 287 eV)so that visible light is enough to excite electron from valenceband to conduction band Silver nanoparticles could act as aneffective electron scavenger to trap the conduction band elec-trons of TiO2

23 In silver doped TiO2 the silver nanoparticles

112

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R E S E A R CH AR T I C L EAdv Sci Lett 4 108ndash114 2011

10 20 30 40 50 60 70

10

20

30

40

50

60

70

80

90

100

D

egra

datio

n of

MB

Time in min

Degussa P 25

PTTLS1

TLS2

Fig 7 Photocatalytic activity of PT TLS1 TLS2 and Degussa P25 undersunlight

play an important role in the interfacial charge transfer and inthe elimination of electron-hole recombination as is evident fromPL spectra which would be beneficial for the high photocatalyticactivity The activity of PT is less because here electrons can-not be excited from valence band to conduction band by visiblelight irradiation due to the large band gap as calculated fromDRS (323 eV) In this case an ultraviolet irradiation is requiredfor the excitation of electrons from valence band to conductionband to carry out photocatalysis which accounts for only smallfraction of the solar light Also the efficiency of the photocatal-ysis depends on the effectiveness of the photocatalytic processin transferring the photoinduced eminush+ pair from the particlevolume to the particle surface and subsequently to the surface-adsorbed species In PT there is no intermediate level to trap theelectron and the recombination possibility of electrons and holesis enhanced and the free electron or holes available for the pho-tocatalysis is less subsequently the efficiency of photocatalysisalso is less

It is well known that it is the anatase polymorph rather than thebrookite or rutile polymorph that has the highest photocatalyticactivity The most active commercially available photocatalyst(Degussa P25) has an anatase content of 75 and 25 of rutilecontent26 From the XRD pattern it is evident that the synthesizedsamples (PT TLS1 and TLS2) are purely anatase and hence itshows higher activity than Degussa P25

The synthesized silver doped samples (TLS1 and TLS2) arehighly efficient photocatalysts as is evident from the MB degra-dation studies The sample TLS2 shows higher photocatalyticactivity than the sample TLS1 (Fig 7) and is due to the increasein crystallinity of TLS2 which is clear from the XRD data How-ever no direct relation was found between the BET areas and thecatalytic activity of the catalysts as in earlier report27 The previ-ous reports show that calcination of samples and UV irradiationwas required for effective photocatalysis also the UV irradiationtime for the degradation of MB is high (60 to 120 min)2829

The studies which deals visible light photocatalysis of MB usingdoped TiO2 also required high temperature calcination (600 C)and longer irradiation time (60ndash120 min)3031

But we have used low temperature hydrothermal route to pre-pare efficient photocatalyst which is active under solar light without any high temperature treatment Also silver doping couldreplace artificial UV light irradiation by sunlight Hence it is clearthat silver doping in TiO2 can give highly efficient photocatalystswhich are active under sunlight

In addition to higher photocatalytic activity the hydrother-mally synthesized TiO2 samples are easier to separate from theaqueous media than Degussa P25 Degussa P25 forms a milkywhite turbid suspension in aqueous media Though the synthe-sized samples have equal or less particle size they do not formturbid suspension Also the synthesized catalysts settle faster andit is easier to separate from the reaction mixture by centrifuga-tion This enabled recyclability of the synthesized nanophotocat-alysts after centrifugation at the end of each photocatalytic cycleand the recycled catalyst was found efficient and stable for morethan six cycles

Degradation of the nonbiodegradable pesticide endosulfanwas also investigated in the light of the interesting photocat-alytic activity of the synthesized nanoparticles with MB Thephotocatalytic activity of PT TLS1 and TLS2 nanoparticles withthe organochlorine pesticide under sunlight was studied in sep-arate experiments Figure 8 shows the degradation kinetics of endosulfan with the nanoparticles at an initial concentration of350 ppb which is close to the maximum aqueous solubility of endosulfan Concentration of the organochlorine pesticide endosulfan was determined by GC-ECD analysis As expectedhere also the more crystalline nanoparticle sample TLS2 exhib-ited highest photocatalytic activity followed by TLS1 and PT InTLS2-Endosulfan system substantial reduction in the concentra-tion of endosulfan was observed in the initial 20 minutes at at12 of 13 minutes It was observed that more than 98 of thepesticide was decomposed in 60 minutes The GC-MS analysisin EI-SCAN mode of the sample taken at 60 minutes confirmedthe almost complete mineralization of endosulfan The envi-ronmental degradation products of endosulfan as proposed byBakouri et al32 such as the immediate hydrolysis product endo-sulfan diol the oxidation product endosulfan sulfate or the furtherdecomposition products endosulfan ether endosulfan hydroxylether or endosulfan lactone were not detected in the system Toensure degradation rather than adsorption the experiment was

0 10 20 30 40 50 6000

02

04

06

08

10

PT

TLS1

TLS2

CtC

0

Time min

Fig 8 Photocatalytic degradation of endosulfan using PT TLS1 andTLS2 under sunlight

113

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R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

repeated with the same TLS2 nanoparticles (after regenerating itfrom the photocatalytic system by centrifugation) with the sameinitial concentration of endosulfan for six more cycles Sampleswere analyzed for endosulfan concentration at the end of 60 min-utes The TLS2 photocatalytic system was found stable and effi-cient after the repeated experiments with consistent endosulfandegradation capability In aqueous systems though endosulfanresist direct photolysis it is clearly observed that it undergoesnear complete photodegradation with silver doped nano TiO2

particles The heterogeneous photocatalysis with TiO2 nanopar-ticles accelerated the decomposition of otherwise recalcitrant endosulfan and silver doping shifted the photocatalysis to visibleregion to attain efficient degradation under sunlight The resultssuggest the potential of silver doped TiO2 nanomaterials in envi-ronmental pollutant remediation applications

4 CONCLUSIONSAnatase nanocrystals with enhanced photocatalytic activity undersolar irradiation have been successfully synthesized by lowtemperature hydrothermal route by silver doping Silver nanopar-ticles for doping were prepared by simultaneous chemical reduc-tion and stabilization employing L-Dopa without any additionalstabilizing agent The hydrothermally synthesized TiO2 samplesshowed higher photocatalytic activity under sunlight than thecommercially available photocatalyst Degussa P25 and couldcompletely degrade the organochlorine pesticide endosulfan bysunlight irradiation Also the synthesized catalysts settle fasterand it is easier to separate from the reaction mixture which pro-motes the reusability of these photocatalysts Decrease in bandgap in silver doped samples by the formation of an intermedi-ate level is the major reason for the high photocatalytic activ-ity of these samples under sunlight The present study suggeststhat silver doped TiO2 can be used for the development of anenvironmentally sustainable photocatalytic treatment process forthe degradation of persistent organic pollutants using sunlight inplace of an artificial light

Acknowledgments The authors wish to thank the Depart-ment of Science and Technology (DST) India for financial sup-port under woman scientist scheme (WOS-A)

References and Notes1 K B Dhanalakshmi S Anandan J Madhavan and P Maruthamuthu Solar

Energy Mat Solar Cells 92 457(2008)2 R T Meijers E J Oderwaldmuller P Nuhn and J C Kruithof Ozone-Sci

Eng 17 673 (1995)

3 E Wittmann P Cote C Medici J Leech and A G Turner Desalination119 347 (1998)

4 P Roche and M Prados Ozone-Sci Eng 17 657 (1995)5 H M Shivaramaiah and I R Kennedy J Environ Sci Health Part B 41 895

(2006)6 D Dong P Li X Li Q Zhao Y Zhang C Jia and P Li J Hazardous

Materials 174 859 (2010)7 V A Sakkas I M Arabatzis I K Konstantinou A D Dimou T A Albanis

and P Falaras Appl Catal B Environ 49 195 (2004)8 A S Topalova D V Šojica D A Molnaacuter-Gaacutebora B F Abramovica and M I

Comorb Appl Catal B Environ 54 125 (2004)9 M M Higarashi and W F Jardim Catal Today 76 201 (2002)

10 G Zhanqi Y Shaogui T Na and S Cheng J Hazard Mater 142 424(2007)

11 T Kawai H Takahashi Y Matsushima T Ogata and H Unuma Sci AdvMater 2 74 (2010)

12 Y Tokunaga H Uchiyama Y Oaki and H Imai Sci Adv Mater 2 69(2010)

13 X S Li G E Fryxell C Wang and M H Engelhard Microporous Meso-porous Mater 111 639 (2008)

14 N Ma X Fan X Quan and Y Zhang J Membr Sci 336 109(2009)

15 K Mallick M J Witcomb and M S Scurrell Appl Catal A General 259 163(2004)

16 S Je-Lueng L Chia-Hsiang C Chyow-San C Chang-Tong C Chia-Chiand C Ching-Yuan J Hazard Mater 155 164 (2008)

17 A Kubacka M Ferrer A Martiacutenez-Arias and M Fernaacutendez-Garciacutea ApplCatal B Environ 84 87 (2008)

18 N Sobana M Murugandahamm and M Swaminathan J Mol Catal AChem 258 124 (2006)

19 S Anandan P S Kumar N Pugazhenthiran J Madhavan andP Maruthamuthu Solar Energy Mat Solar Cells 92 929 (2008)

20 S V Awate R K Sahu M D Kadgaonkar R Kumar and N M GuptaCatal Today 141 144 (2009)

21 B O Regan and M Gratzel Nature 353 737 (1991)22 Z P Wang W M Cai X T Hong X L Zhao F Xu and CG Cai Appl

Catal B Environ 57 223 (2005)23 F B Li X Z Li M F Hou K W Cheah and W C H Choy Appl Catal A

General 285 181 (2005)24 B Zhu Z Sui S Wang X Chen S Zhang S Wu and W Huang Mat Res

Bull 41 1097 (2006)25 D Zhang D Yang H Zhang C Lu and L Qi Chem Mater 18 3477

(2006)26 T Ohno K Sarukawa K Tokieda and M Matsumura J Catal 203 82

(2001)27 M Hosseini S Siffert H L Tidahy R Cousin J-F Lamonier A Aboukais

A Vantomme M Roussel and B-L Su Catal Today 122 391(2007)

28 Q Xiao Z Si Z Yu and G Qiu J Alloys Compd 450 426 (2008)29 M W Yan W L Su X Zhiliang B J Xin P C Xiao and P Jie Mater Lett

60 974 (2006)30 X Qi Z Jiang X Chong S Zhichun and T Xiaoke Solar Energy 82 706

(2008)31 Q Xiao Z Si Z Yu and G Qiu Mat Sci and Eng B 137 189

(2007)32 H E Bakouri A Ouassini J M Aguado and J U Garciacutea Water Environ

Res 79 2578 (2007)

Received 30 May 2010 Accepted 6 July 2010

114

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R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

(c)

(a) (b)

(b)

Fig 5 FESEM of TLS1 (a) TLS2 (b) and TEM images of TLS1 (c) and TLS2 (d)

intensity shows that the rate of recombination between electronand holes is low23 The electrons are excited from valance band(VB) to conduction band (CB) of TiO2 under UV irradiation andgenerate photoexcited electrons and holes Although doping withsilver narrows the band gap of TiO2 samples the recombinationof electrons and holes are relatively slow which is evident fromPL spectra

As is well known TiO2 exhibit poor photocatalytic efficiencysince the majority of photogenerated charge carriers undergorecombination In the present system silver nanoparticles act aselectron acceptors and sinks for the charge carriers which will

200 300 400 500 600 700ndash20

ndash15

ndash10

ndash05

00

05

10

15

20

25

30

Inte

nsity

(a

u)

Wavelength (cmndash1)

PT

TLS1

TLS2

Fig 6 Photoluminescence emission spectra (PL) of PT TLS1 and TLS2

enhance the efficiency of photocatalysis2324 For samples TLS1and TLS2 silver nanoparticles plays an important role in theinterfacial charge transfer and in the decrease in rate of electron-hole recombination Silver nanoparticles could act as an effec-tive electron scavenger to trap the photo induced electrons andholes of TiO2 leading to the reduction of electronndashhole recom-bination and thus improving the photocatalytic efficiency Theelectrons trapped in silver sites were subsequently transferred tothe surrounding adsorbed O2 It can be supported by PL emissionspectra which have been widely used to investigate the fate ofelectron hole pairs in semiconductor particles since PL emissionresults from the recombination of free carriers2025

37 Photocatalytic ActivityTo investigate the photocatalytic activity of synthesized TiO2

samples in solar light the degradation of MB was studiedin presence of PT TLS1 and TLS2 nanoparticles under sun-light For comparison photocatalytic studies were also performedwith commercially available photocatalyst Degussa P25 and theresults are depicted in Figure 7 The activity of different samplesin sunlight is in the order TLS2 gt TLS1 gt PT gt Degussa P25Silver doped TiO2 samples show higher activity than undopedTiO2 and Degussa P25 Diffuse reflectance spectra of silverdoped TiO2 samples showed significant absorption in the visi-ble region which enhance the photocatalytic activity in the vis-ible region The high activity of silver doped TiO2 samples ismainly attributed to the decrease in band gap (323 to 287 eV)so that visible light is enough to excite electron from valenceband to conduction band Silver nanoparticles could act as aneffective electron scavenger to trap the conduction band elec-trons of TiO2

23 In silver doped TiO2 the silver nanoparticles

112

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R E S E A R CH AR T I C L EAdv Sci Lett 4 108ndash114 2011

10 20 30 40 50 60 70

10

20

30

40

50

60

70

80

90

100

D

egra

datio

n of

MB

Time in min

Degussa P 25

PTTLS1

TLS2

Fig 7 Photocatalytic activity of PT TLS1 TLS2 and Degussa P25 undersunlight

play an important role in the interfacial charge transfer and inthe elimination of electron-hole recombination as is evident fromPL spectra which would be beneficial for the high photocatalyticactivity The activity of PT is less because here electrons can-not be excited from valence band to conduction band by visiblelight irradiation due to the large band gap as calculated fromDRS (323 eV) In this case an ultraviolet irradiation is requiredfor the excitation of electrons from valence band to conductionband to carry out photocatalysis which accounts for only smallfraction of the solar light Also the efficiency of the photocatal-ysis depends on the effectiveness of the photocatalytic processin transferring the photoinduced eminush+ pair from the particlevolume to the particle surface and subsequently to the surface-adsorbed species In PT there is no intermediate level to trap theelectron and the recombination possibility of electrons and holesis enhanced and the free electron or holes available for the pho-tocatalysis is less subsequently the efficiency of photocatalysisalso is less

It is well known that it is the anatase polymorph rather than thebrookite or rutile polymorph that has the highest photocatalyticactivity The most active commercially available photocatalyst(Degussa P25) has an anatase content of 75 and 25 of rutilecontent26 From the XRD pattern it is evident that the synthesizedsamples (PT TLS1 and TLS2) are purely anatase and hence itshows higher activity than Degussa P25

The synthesized silver doped samples (TLS1 and TLS2) arehighly efficient photocatalysts as is evident from the MB degra-dation studies The sample TLS2 shows higher photocatalyticactivity than the sample TLS1 (Fig 7) and is due to the increasein crystallinity of TLS2 which is clear from the XRD data How-ever no direct relation was found between the BET areas and thecatalytic activity of the catalysts as in earlier report27 The previ-ous reports show that calcination of samples and UV irradiationwas required for effective photocatalysis also the UV irradiationtime for the degradation of MB is high (60 to 120 min)2829

The studies which deals visible light photocatalysis of MB usingdoped TiO2 also required high temperature calcination (600 C)and longer irradiation time (60ndash120 min)3031

But we have used low temperature hydrothermal route to pre-pare efficient photocatalyst which is active under solar light without any high temperature treatment Also silver doping couldreplace artificial UV light irradiation by sunlight Hence it is clearthat silver doping in TiO2 can give highly efficient photocatalystswhich are active under sunlight

In addition to higher photocatalytic activity the hydrother-mally synthesized TiO2 samples are easier to separate from theaqueous media than Degussa P25 Degussa P25 forms a milkywhite turbid suspension in aqueous media Though the synthe-sized samples have equal or less particle size they do not formturbid suspension Also the synthesized catalysts settle faster andit is easier to separate from the reaction mixture by centrifuga-tion This enabled recyclability of the synthesized nanophotocat-alysts after centrifugation at the end of each photocatalytic cycleand the recycled catalyst was found efficient and stable for morethan six cycles

Degradation of the nonbiodegradable pesticide endosulfanwas also investigated in the light of the interesting photocat-alytic activity of the synthesized nanoparticles with MB Thephotocatalytic activity of PT TLS1 and TLS2 nanoparticles withthe organochlorine pesticide under sunlight was studied in sep-arate experiments Figure 8 shows the degradation kinetics of endosulfan with the nanoparticles at an initial concentration of350 ppb which is close to the maximum aqueous solubility of endosulfan Concentration of the organochlorine pesticide endosulfan was determined by GC-ECD analysis As expectedhere also the more crystalline nanoparticle sample TLS2 exhib-ited highest photocatalytic activity followed by TLS1 and PT InTLS2-Endosulfan system substantial reduction in the concentra-tion of endosulfan was observed in the initial 20 minutes at at12 of 13 minutes It was observed that more than 98 of thepesticide was decomposed in 60 minutes The GC-MS analysisin EI-SCAN mode of the sample taken at 60 minutes confirmedthe almost complete mineralization of endosulfan The envi-ronmental degradation products of endosulfan as proposed byBakouri et al32 such as the immediate hydrolysis product endo-sulfan diol the oxidation product endosulfan sulfate or the furtherdecomposition products endosulfan ether endosulfan hydroxylether or endosulfan lactone were not detected in the system Toensure degradation rather than adsorption the experiment was

0 10 20 30 40 50 6000

02

04

06

08

10

PT

TLS1

TLS2

CtC

0

Time min

Fig 8 Photocatalytic degradation of endosulfan using PT TLS1 andTLS2 under sunlight

113

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R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

repeated with the same TLS2 nanoparticles (after regenerating itfrom the photocatalytic system by centrifugation) with the sameinitial concentration of endosulfan for six more cycles Sampleswere analyzed for endosulfan concentration at the end of 60 min-utes The TLS2 photocatalytic system was found stable and effi-cient after the repeated experiments with consistent endosulfandegradation capability In aqueous systems though endosulfanresist direct photolysis it is clearly observed that it undergoesnear complete photodegradation with silver doped nano TiO2

particles The heterogeneous photocatalysis with TiO2 nanopar-ticles accelerated the decomposition of otherwise recalcitrant endosulfan and silver doping shifted the photocatalysis to visibleregion to attain efficient degradation under sunlight The resultssuggest the potential of silver doped TiO2 nanomaterials in envi-ronmental pollutant remediation applications

4 CONCLUSIONSAnatase nanocrystals with enhanced photocatalytic activity undersolar irradiation have been successfully synthesized by lowtemperature hydrothermal route by silver doping Silver nanopar-ticles for doping were prepared by simultaneous chemical reduc-tion and stabilization employing L-Dopa without any additionalstabilizing agent The hydrothermally synthesized TiO2 samplesshowed higher photocatalytic activity under sunlight than thecommercially available photocatalyst Degussa P25 and couldcompletely degrade the organochlorine pesticide endosulfan bysunlight irradiation Also the synthesized catalysts settle fasterand it is easier to separate from the reaction mixture which pro-motes the reusability of these photocatalysts Decrease in bandgap in silver doped samples by the formation of an intermedi-ate level is the major reason for the high photocatalytic activ-ity of these samples under sunlight The present study suggeststhat silver doped TiO2 can be used for the development of anenvironmentally sustainable photocatalytic treatment process forthe degradation of persistent organic pollutants using sunlight inplace of an artificial light

Acknowledgments The authors wish to thank the Depart-ment of Science and Technology (DST) India for financial sup-port under woman scientist scheme (WOS-A)

References and Notes1 K B Dhanalakshmi S Anandan J Madhavan and P Maruthamuthu Solar

Energy Mat Solar Cells 92 457(2008)2 R T Meijers E J Oderwaldmuller P Nuhn and J C Kruithof Ozone-Sci

Eng 17 673 (1995)

3 E Wittmann P Cote C Medici J Leech and A G Turner Desalination119 347 (1998)

4 P Roche and M Prados Ozone-Sci Eng 17 657 (1995)5 H M Shivaramaiah and I R Kennedy J Environ Sci Health Part B 41 895

(2006)6 D Dong P Li X Li Q Zhao Y Zhang C Jia and P Li J Hazardous

Materials 174 859 (2010)7 V A Sakkas I M Arabatzis I K Konstantinou A D Dimou T A Albanis

and P Falaras Appl Catal B Environ 49 195 (2004)8 A S Topalova D V Šojica D A Molnaacuter-Gaacutebora B F Abramovica and M I

Comorb Appl Catal B Environ 54 125 (2004)9 M M Higarashi and W F Jardim Catal Today 76 201 (2002)

10 G Zhanqi Y Shaogui T Na and S Cheng J Hazard Mater 142 424(2007)

11 T Kawai H Takahashi Y Matsushima T Ogata and H Unuma Sci AdvMater 2 74 (2010)

12 Y Tokunaga H Uchiyama Y Oaki and H Imai Sci Adv Mater 2 69(2010)

13 X S Li G E Fryxell C Wang and M H Engelhard Microporous Meso-porous Mater 111 639 (2008)

14 N Ma X Fan X Quan and Y Zhang J Membr Sci 336 109(2009)

15 K Mallick M J Witcomb and M S Scurrell Appl Catal A General 259 163(2004)

16 S Je-Lueng L Chia-Hsiang C Chyow-San C Chang-Tong C Chia-Chiand C Ching-Yuan J Hazard Mater 155 164 (2008)

17 A Kubacka M Ferrer A Martiacutenez-Arias and M Fernaacutendez-Garciacutea ApplCatal B Environ 84 87 (2008)

18 N Sobana M Murugandahamm and M Swaminathan J Mol Catal AChem 258 124 (2006)

19 S Anandan P S Kumar N Pugazhenthiran J Madhavan andP Maruthamuthu Solar Energy Mat Solar Cells 92 929 (2008)

20 S V Awate R K Sahu M D Kadgaonkar R Kumar and N M GuptaCatal Today 141 144 (2009)

21 B O Regan and M Gratzel Nature 353 737 (1991)22 Z P Wang W M Cai X T Hong X L Zhao F Xu and CG Cai Appl

Catal B Environ 57 223 (2005)23 F B Li X Z Li M F Hou K W Cheah and W C H Choy Appl Catal A

General 285 181 (2005)24 B Zhu Z Sui S Wang X Chen S Zhang S Wu and W Huang Mat Res

Bull 41 1097 (2006)25 D Zhang D Yang H Zhang C Lu and L Qi Chem Mater 18 3477

(2006)26 T Ohno K Sarukawa K Tokieda and M Matsumura J Catal 203 82

(2001)27 M Hosseini S Siffert H L Tidahy R Cousin J-F Lamonier A Aboukais

A Vantomme M Roussel and B-L Su Catal Today 122 391(2007)

28 Q Xiao Z Si Z Yu and G Qiu J Alloys Compd 450 426 (2008)29 M W Yan W L Su X Zhiliang B J Xin P C Xiao and P Jie Mater Lett

60 974 (2006)30 X Qi Z Jiang X Chong S Zhichun and T Xiaoke Solar Energy 82 706

(2008)31 Q Xiao Z Si Z Yu and G Qiu Mat Sci and Eng B 137 189

(2007)32 H E Bakouri A Ouassini J M Aguado and J U Garciacutea Water Environ

Res 79 2578 (2007)

Received 30 May 2010 Accepted 6 July 2010

114

Delivered by Ingenta toGuest User

IP 122172114214Sun 12 Dec 2010 235621

R E S E A R CH AR T I C L EAdv Sci Lett 4 108ndash114 2011

10 20 30 40 50 60 70

10

20

30

40

50

60

70

80

90

100

D

egra

datio

n of

MB

Time in min

Degussa P 25

PTTLS1

TLS2

Fig 7 Photocatalytic activity of PT TLS1 TLS2 and Degussa P25 undersunlight

play an important role in the interfacial charge transfer and inthe elimination of electron-hole recombination as is evident fromPL spectra which would be beneficial for the high photocatalyticactivity The activity of PT is less because here electrons can-not be excited from valence band to conduction band by visiblelight irradiation due to the large band gap as calculated fromDRS (323 eV) In this case an ultraviolet irradiation is requiredfor the excitation of electrons from valence band to conductionband to carry out photocatalysis which accounts for only smallfraction of the solar light Also the efficiency of the photocatal-ysis depends on the effectiveness of the photocatalytic processin transferring the photoinduced eminush+ pair from the particlevolume to the particle surface and subsequently to the surface-adsorbed species In PT there is no intermediate level to trap theelectron and the recombination possibility of electrons and holesis enhanced and the free electron or holes available for the pho-tocatalysis is less subsequently the efficiency of photocatalysisalso is less

It is well known that it is the anatase polymorph rather than thebrookite or rutile polymorph that has the highest photocatalyticactivity The most active commercially available photocatalyst(Degussa P25) has an anatase content of 75 and 25 of rutilecontent26 From the XRD pattern it is evident that the synthesizedsamples (PT TLS1 and TLS2) are purely anatase and hence itshows higher activity than Degussa P25

The synthesized silver doped samples (TLS1 and TLS2) arehighly efficient photocatalysts as is evident from the MB degra-dation studies The sample TLS2 shows higher photocatalyticactivity than the sample TLS1 (Fig 7) and is due to the increasein crystallinity of TLS2 which is clear from the XRD data How-ever no direct relation was found between the BET areas and thecatalytic activity of the catalysts as in earlier report27 The previ-ous reports show that calcination of samples and UV irradiationwas required for effective photocatalysis also the UV irradiationtime for the degradation of MB is high (60 to 120 min)2829

The studies which deals visible light photocatalysis of MB usingdoped TiO2 also required high temperature calcination (600 C)and longer irradiation time (60ndash120 min)3031

But we have used low temperature hydrothermal route to pre-pare efficient photocatalyst which is active under solar light without any high temperature treatment Also silver doping couldreplace artificial UV light irradiation by sunlight Hence it is clearthat silver doping in TiO2 can give highly efficient photocatalystswhich are active under sunlight

In addition to higher photocatalytic activity the hydrother-mally synthesized TiO2 samples are easier to separate from theaqueous media than Degussa P25 Degussa P25 forms a milkywhite turbid suspension in aqueous media Though the synthe-sized samples have equal or less particle size they do not formturbid suspension Also the synthesized catalysts settle faster andit is easier to separate from the reaction mixture by centrifuga-tion This enabled recyclability of the synthesized nanophotocat-alysts after centrifugation at the end of each photocatalytic cycleand the recycled catalyst was found efficient and stable for morethan six cycles

Degradation of the nonbiodegradable pesticide endosulfanwas also investigated in the light of the interesting photocat-alytic activity of the synthesized nanoparticles with MB Thephotocatalytic activity of PT TLS1 and TLS2 nanoparticles withthe organochlorine pesticide under sunlight was studied in sep-arate experiments Figure 8 shows the degradation kinetics of endosulfan with the nanoparticles at an initial concentration of350 ppb which is close to the maximum aqueous solubility of endosulfan Concentration of the organochlorine pesticide endosulfan was determined by GC-ECD analysis As expectedhere also the more crystalline nanoparticle sample TLS2 exhib-ited highest photocatalytic activity followed by TLS1 and PT InTLS2-Endosulfan system substantial reduction in the concentra-tion of endosulfan was observed in the initial 20 minutes at at12 of 13 minutes It was observed that more than 98 of thepesticide was decomposed in 60 minutes The GC-MS analysisin EI-SCAN mode of the sample taken at 60 minutes confirmedthe almost complete mineralization of endosulfan The envi-ronmental degradation products of endosulfan as proposed byBakouri et al32 such as the immediate hydrolysis product endo-sulfan diol the oxidation product endosulfan sulfate or the furtherdecomposition products endosulfan ether endosulfan hydroxylether or endosulfan lactone were not detected in the system Toensure degradation rather than adsorption the experiment was

0 10 20 30 40 50 6000

02

04

06

08

10

PT

TLS1

TLS2

CtC

0

Time min

Fig 8 Photocatalytic degradation of endosulfan using PT TLS1 andTLS2 under sunlight

113

Delivered by Ingenta toGuest User

IP 122172114214Sun 12 Dec 2010 235621

R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

repeated with the same TLS2 nanoparticles (after regenerating itfrom the photocatalytic system by centrifugation) with the sameinitial concentration of endosulfan for six more cycles Sampleswere analyzed for endosulfan concentration at the end of 60 min-utes The TLS2 photocatalytic system was found stable and effi-cient after the repeated experiments with consistent endosulfandegradation capability In aqueous systems though endosulfanresist direct photolysis it is clearly observed that it undergoesnear complete photodegradation with silver doped nano TiO2

particles The heterogeneous photocatalysis with TiO2 nanopar-ticles accelerated the decomposition of otherwise recalcitrant endosulfan and silver doping shifted the photocatalysis to visibleregion to attain efficient degradation under sunlight The resultssuggest the potential of silver doped TiO2 nanomaterials in envi-ronmental pollutant remediation applications

4 CONCLUSIONSAnatase nanocrystals with enhanced photocatalytic activity undersolar irradiation have been successfully synthesized by lowtemperature hydrothermal route by silver doping Silver nanopar-ticles for doping were prepared by simultaneous chemical reduc-tion and stabilization employing L-Dopa without any additionalstabilizing agent The hydrothermally synthesized TiO2 samplesshowed higher photocatalytic activity under sunlight than thecommercially available photocatalyst Degussa P25 and couldcompletely degrade the organochlorine pesticide endosulfan bysunlight irradiation Also the synthesized catalysts settle fasterand it is easier to separate from the reaction mixture which pro-motes the reusability of these photocatalysts Decrease in bandgap in silver doped samples by the formation of an intermedi-ate level is the major reason for the high photocatalytic activ-ity of these samples under sunlight The present study suggeststhat silver doped TiO2 can be used for the development of anenvironmentally sustainable photocatalytic treatment process forthe degradation of persistent organic pollutants using sunlight inplace of an artificial light

Acknowledgments The authors wish to thank the Depart-ment of Science and Technology (DST) India for financial sup-port under woman scientist scheme (WOS-A)

References and Notes1 K B Dhanalakshmi S Anandan J Madhavan and P Maruthamuthu Solar

Energy Mat Solar Cells 92 457(2008)2 R T Meijers E J Oderwaldmuller P Nuhn and J C Kruithof Ozone-Sci

Eng 17 673 (1995)

3 E Wittmann P Cote C Medici J Leech and A G Turner Desalination119 347 (1998)

4 P Roche and M Prados Ozone-Sci Eng 17 657 (1995)5 H M Shivaramaiah and I R Kennedy J Environ Sci Health Part B 41 895

(2006)6 D Dong P Li X Li Q Zhao Y Zhang C Jia and P Li J Hazardous

Materials 174 859 (2010)7 V A Sakkas I M Arabatzis I K Konstantinou A D Dimou T A Albanis

and P Falaras Appl Catal B Environ 49 195 (2004)8 A S Topalova D V Šojica D A Molnaacuter-Gaacutebora B F Abramovica and M I

Comorb Appl Catal B Environ 54 125 (2004)9 M M Higarashi and W F Jardim Catal Today 76 201 (2002)

10 G Zhanqi Y Shaogui T Na and S Cheng J Hazard Mater 142 424(2007)

11 T Kawai H Takahashi Y Matsushima T Ogata and H Unuma Sci AdvMater 2 74 (2010)

12 Y Tokunaga H Uchiyama Y Oaki and H Imai Sci Adv Mater 2 69(2010)

13 X S Li G E Fryxell C Wang and M H Engelhard Microporous Meso-porous Mater 111 639 (2008)

14 N Ma X Fan X Quan and Y Zhang J Membr Sci 336 109(2009)

15 K Mallick M J Witcomb and M S Scurrell Appl Catal A General 259 163(2004)

16 S Je-Lueng L Chia-Hsiang C Chyow-San C Chang-Tong C Chia-Chiand C Ching-Yuan J Hazard Mater 155 164 (2008)

17 A Kubacka M Ferrer A Martiacutenez-Arias and M Fernaacutendez-Garciacutea ApplCatal B Environ 84 87 (2008)

18 N Sobana M Murugandahamm and M Swaminathan J Mol Catal AChem 258 124 (2006)

19 S Anandan P S Kumar N Pugazhenthiran J Madhavan andP Maruthamuthu Solar Energy Mat Solar Cells 92 929 (2008)

20 S V Awate R K Sahu M D Kadgaonkar R Kumar and N M GuptaCatal Today 141 144 (2009)

21 B O Regan and M Gratzel Nature 353 737 (1991)22 Z P Wang W M Cai X T Hong X L Zhao F Xu and CG Cai Appl

Catal B Environ 57 223 (2005)23 F B Li X Z Li M F Hou K W Cheah and W C H Choy Appl Catal A

General 285 181 (2005)24 B Zhu Z Sui S Wang X Chen S Zhang S Wu and W Huang Mat Res

Bull 41 1097 (2006)25 D Zhang D Yang H Zhang C Lu and L Qi Chem Mater 18 3477

(2006)26 T Ohno K Sarukawa K Tokieda and M Matsumura J Catal 203 82

(2001)27 M Hosseini S Siffert H L Tidahy R Cousin J-F Lamonier A Aboukais

A Vantomme M Roussel and B-L Su Catal Today 122 391(2007)

28 Q Xiao Z Si Z Yu and G Qiu J Alloys Compd 450 426 (2008)29 M W Yan W L Su X Zhiliang B J Xin P C Xiao and P Jie Mater Lett

60 974 (2006)30 X Qi Z Jiang X Chong S Zhichun and T Xiaoke Solar Energy 82 706

(2008)31 Q Xiao Z Si Z Yu and G Qiu Mat Sci and Eng B 137 189

(2007)32 H E Bakouri A Ouassini J M Aguado and J U Garciacutea Water Environ

Res 79 2578 (2007)

Received 30 May 2010 Accepted 6 July 2010

114

Delivered by Ingenta toGuest User

IP 122172114214Sun 12 Dec 2010 235621

R E S E A R CH AR T I C L E Adv Sci Lett 4 108ndash114 2011

repeated with the same TLS2 nanoparticles (after regenerating itfrom the photocatalytic system by centrifugation) with the sameinitial concentration of endosulfan for six more cycles Sampleswere analyzed for endosulfan concentration at the end of 60 min-utes The TLS2 photocatalytic system was found stable and effi-cient after the repeated experiments with consistent endosulfandegradation capability In aqueous systems though endosulfanresist direct photolysis it is clearly observed that it undergoesnear complete photodegradation with silver doped nano TiO2

particles The heterogeneous photocatalysis with TiO2 nanopar-ticles accelerated the decomposition of otherwise recalcitrant endosulfan and silver doping shifted the photocatalysis to visibleregion to attain efficient degradation under sunlight The resultssuggest the potential of silver doped TiO2 nanomaterials in envi-ronmental pollutant remediation applications

4 CONCLUSIONSAnatase nanocrystals with enhanced photocatalytic activity undersolar irradiation have been successfully synthesized by lowtemperature hydrothermal route by silver doping Silver nanopar-ticles for doping were prepared by simultaneous chemical reduc-tion and stabilization employing L-Dopa without any additionalstabilizing agent The hydrothermally synthesized TiO2 samplesshowed higher photocatalytic activity under sunlight than thecommercially available photocatalyst Degussa P25 and couldcompletely degrade the organochlorine pesticide endosulfan bysunlight irradiation Also the synthesized catalysts settle fasterand it is easier to separate from the reaction mixture which pro-motes the reusability of these photocatalysts Decrease in bandgap in silver doped samples by the formation of an intermedi-ate level is the major reason for the high photocatalytic activ-ity of these samples under sunlight The present study suggeststhat silver doped TiO2 can be used for the development of anenvironmentally sustainable photocatalytic treatment process forthe degradation of persistent organic pollutants using sunlight inplace of an artificial light

Acknowledgments The authors wish to thank the Depart-ment of Science and Technology (DST) India for financial sup-port under woman scientist scheme (WOS-A)

References and Notes1 K B Dhanalakshmi S Anandan J Madhavan and P Maruthamuthu Solar

Energy Mat Solar Cells 92 457(2008)2 R T Meijers E J Oderwaldmuller P Nuhn and J C Kruithof Ozone-Sci

Eng 17 673 (1995)

3 E Wittmann P Cote C Medici J Leech and A G Turner Desalination119 347 (1998)

4 P Roche and M Prados Ozone-Sci Eng 17 657 (1995)5 H M Shivaramaiah and I R Kennedy J Environ Sci Health Part B 41 895

(2006)6 D Dong P Li X Li Q Zhao Y Zhang C Jia and P Li J Hazardous

Materials 174 859 (2010)7 V A Sakkas I M Arabatzis I K Konstantinou A D Dimou T A Albanis

and P Falaras Appl Catal B Environ 49 195 (2004)8 A S Topalova D V Šojica D A Molnaacuter-Gaacutebora B F Abramovica and M I

Comorb Appl Catal B Environ 54 125 (2004)9 M M Higarashi and W F Jardim Catal Today 76 201 (2002)

10 G Zhanqi Y Shaogui T Na and S Cheng J Hazard Mater 142 424(2007)

11 T Kawai H Takahashi Y Matsushima T Ogata and H Unuma Sci AdvMater 2 74 (2010)

12 Y Tokunaga H Uchiyama Y Oaki and H Imai Sci Adv Mater 2 69(2010)

13 X S Li G E Fryxell C Wang and M H Engelhard Microporous Meso-porous Mater 111 639 (2008)

14 N Ma X Fan X Quan and Y Zhang J Membr Sci 336 109(2009)

15 K Mallick M J Witcomb and M S Scurrell Appl Catal A General 259 163(2004)

16 S Je-Lueng L Chia-Hsiang C Chyow-San C Chang-Tong C Chia-Chiand C Ching-Yuan J Hazard Mater 155 164 (2008)

17 A Kubacka M Ferrer A Martiacutenez-Arias and M Fernaacutendez-Garciacutea ApplCatal B Environ 84 87 (2008)

18 N Sobana M Murugandahamm and M Swaminathan J Mol Catal AChem 258 124 (2006)

19 S Anandan P S Kumar N Pugazhenthiran J Madhavan andP Maruthamuthu Solar Energy Mat Solar Cells 92 929 (2008)

20 S V Awate R K Sahu M D Kadgaonkar R Kumar and N M GuptaCatal Today 141 144 (2009)

21 B O Regan and M Gratzel Nature 353 737 (1991)22 Z P Wang W M Cai X T Hong X L Zhao F Xu and CG Cai Appl

Catal B Environ 57 223 (2005)23 F B Li X Z Li M F Hou K W Cheah and W C H Choy Appl Catal A

General 285 181 (2005)24 B Zhu Z Sui S Wang X Chen S Zhang S Wu and W Huang Mat Res

Bull 41 1097 (2006)25 D Zhang D Yang H Zhang C Lu and L Qi Chem Mater 18 3477

(2006)26 T Ohno K Sarukawa K Tokieda and M Matsumura J Catal 203 82

(2001)27 M Hosseini S Siffert H L Tidahy R Cousin J-F Lamonier A Aboukais

A Vantomme M Roussel and B-L Su Catal Today 122 391(2007)

28 Q Xiao Z Si Z Yu and G Qiu J Alloys Compd 450 426 (2008)29 M W Yan W L Su X Zhiliang B J Xin P C Xiao and P Jie Mater Lett

60 974 (2006)30 X Qi Z Jiang X Chong S Zhichun and T Xiaoke Solar Energy 82 706

(2008)31 Q Xiao Z Si Z Yu and G Qiu Mat Sci and Eng B 137 189

(2007)32 H E Bakouri A Ouassini J M Aguado and J U Garciacutea Water Environ

Res 79 2578 (2007)

Received 30 May 2010 Accepted 6 July 2010

114