photoreduction ultrasonic deposited spray by method: Application to HCrO photo-electrochemical Physical and characterizations of ZnO thinﬁlms Applied Surface Science

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Applied Surface Science

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Physical and photo-electrochemical characterizations of ZnO thin ﬁlms deposited by ultrasonic spray method: Application to HCrO ₄ ⁻ photoreduction

N. Zebbar

^a

, M. Trari

^b,∗

, M. Doulache

^b

, A. Boughelout

^a

, L. Chabane

^a

aDepartmentofMaterials&Compounds,FacultyofPhysics,USTHB,BP32,Algiers16111,Algeria

bLaboratoryofStorageandValorizationofRenewableEnergies,FacultyofChemistry,USTHB,BP32,Algiers16111,Algeria

a r t i c l e i n f o

Articlehistory:

Received25June2013

Receivedinrevisedform5December2013 Accepted10December2013

Available online 18 December 2013

Keywords:

ZnOthinﬁlm Ultrasonicspray Photo-electrochemical Chromate

Sunlight

a b s t r a c t

ZnOthinﬁlms,preparedbyultrasonicsprayontoglasssubstrate,crystallizeinthewurtzitestructure.

TheXRDpatternshowspreferentialorientationalongthe[002]direction.Thefilmsdepositedat350^◦C consistof60nmcrystalliteswithanaveragethicknessof∼150nmandSEMimagesshowroughsur- faceareas.Thegapincreaseswithincreasingthetemperatureofthesubstrateandavalueof3.25eVis obtainedforfilmsdepositedat350^◦C.ZnOisnominallynon-stochiometricandexhibitsn-typeconduc- tionbecauseofthenativedefectssuchasoxygenvacancies(VO)and/orinterstitialzincatom(Zni)which actasdonorshallows.Theconductivityisthermallyactivatedandobeystoanexponentialtypelawwith activationenergyof57meVandanelectronmobilityof7cm²V⁻¹s⁻¹.Thecapacitance-voltage(C⁻²V) measurementinacidelectrolyte(pH∼3)showsalinearbehaviorwithapositiveslope,characteristicof n-typeconduction.Aflatbandpotentialof−0.70VSCEandadonorsdensityof5.30×10¹⁶cm⁻³aredeter- mined.TheNyquistplotexhibitstwosemicirclesattributedtoacapacitivebehaviorwithalowdensity ofsurfacestateswithinthegapregion.Thecentreislocalizedbelowtherealaxiswithadepletionangle of16^◦ascribedtoaconstantphaseelement(CPE)duetotheroughnessofthefilm.Theenergyband diagramassessesthepotentialityofZnOfilmsforthephoto-electrochemicalconversion.Asapplication, 94%ofchromate(3.8×10⁻⁴M)isreducedafter6hundersunlight(AM1)withaquantumyieldof0.06%

andtheoxidationfollowsaﬁrstorderkinetic.

1. Introduction

Metalswhenexposedtooxygenatmosphereatmoderatetem- peraturesformthinﬁlmsuptosomehundredsofnminthickness.

Theinitialstepofoxygenchemisorptionoccursinstantaneously withadsorptionenergyclosetotheheatofformationoftheoxide.

ZnOemerged asapromising windowmaterialduetoitstrans- parencyoverthevisiblespectrum.Itcrystallizesinthewurtzite structure withabandgap(Eg)exceeding3eV[1].It is actively investigated owingtoitstechnological applicationssuchasUV lightemittingdiodes[2],solardevices[3,4],andphotocatalysis[5].

However,oneofthemajorproblemsinelaboratingthinﬁlmsis thenonreproducibilityoftheoutputparameters.ZnOﬁlmshave beengrownbydifferenttechniqueslikechemicalvapordeposition

∗Correspondingauthorat:FacultyofChemistry,TheoreticalandPhysicalChem- istry,BEZEl,AliaBP32,Algiers,Algeria.Tel.:+21321247955;

fax:+21321248008.

E-mailaddress:solarchemistry@gmail.com(M.Trari).

[6],pulsedlaserdeposition[7],sol–gel[8],magnetronsputtering [9],spraypyrolysis[10,11],andultrasonicspray[12,13].Thelast methodis lowcost andpermitstheﬁlmstobedeposited with variousgeometriesonanysizeandshapeofsubstrate.However, comparedtotheenormousstudy,relativelylittleworkwasdone onthephoto-electrochemical(PEC)characterizationofthinﬁlms.

Withtheaimofpreparinginexpensivedevicesforthesolarenergy conversion,thepresentworkisdevotedtoZnOthinﬁlmsgrown byultrasonicspray.Thehomogeneityandstoichiometryaredif- ﬁculttoachieve and this turns toadvantagesince it offersthe opportunitytocharacterizetheoxidephotoelectrochemically.The electrochemicalimpedancespectroscopy(EIS)isalsostudied.

Ontheotherhand,thesolarenergymeetsagrowingdemand andthephotocatalysisisactivelydeployedtomanagetheenvi- ronmentalprotection[14].Theecologicalaspectthatwedevelop istheremovalofheavymetals,recognizedasaworldwidepol- lution problem[15].HexavalentstateCr(VI)ishighly toxicand asapplication, wedemonstratethefeasibilityof ZnOthin ﬁlms forthechromatereductionundersolarirradiationtolessharm- ful forms. The potential sources of chromium include tannery, 0169-4332/$–seefrontmatter© 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.apsusc.2013.12.059

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dyesbatteriesandsteelalloyswhoseefﬂuentsaredischargedin theaquaticmediumwithnorestrictioncausinganenvironmental damage.

2. Experimental

Thesemiconductormustadhereonthesubstrateandtheultra- sonicsprayisaneconomictechniquewithhighgrowthrateand goodareauniformity.ZnOfilmsaredepositedunderatmospheric conditionsandthesprayedsolutionispreparedbydissolving0.1M ofZn(CH₃CO₂H)₂,H₂Oinmethanol.Theglasssubstratesaretreated bythestandardcleaningprocedure[16].Thesolutionissprayedfor 5minatthesubstratetemperature(Ts)range(300–400^◦C).Thefor- mationofthephaseisconfirmedbyX-raydiffractionusingBrukers D8AdvancediffractometerwithCuK_˛radiation(=1.5403 ˚A)over the2range(25–75^◦C).Themorphologyofthefilmsisanalyzed byscanningelectronmicroscopy(SEM,InstrumentJSM-6360).The FTIRspectrumisplottedwithaThermo-Nicolet,Nexusspectrom- eterwitharesolution of4cm⁻¹.Theopticaltransmittancedata arerecordedwithadoublebeamspectrophotometer(3101PCShi- madzu)whichgoesfrom200to800nm.Theabsorptioncoefficient (˛)iswavelengthdependent:˛=(1/d)log(1/T)whereTisthetrans- mittanceanddthefilmthickness.Thegapenergy(E_g)iscalculated fromtheTaucrelation:(˛h)²=C(h−Eg)whereCisaconstantand (h)thephotonenergy.

Electricalcontactonthefilmismadebyvacuumevaporationof Alascoplanarcontactofknownsize;theohmicityofthecontact ischeckedbythecurrent–voltageplot.Thecurrent-temperature measurementsareperformedovertherange(30–120^◦C).Theelec- triccurrentthroughthefilmismeasuredundervacuumwitha Keithley617picoammeter bothinthedarkandunderUV light (100mWcm⁻²).Thetemperaturedependenceoftheconductivity isderivedfromthecurrent-temperaturemeasurementtakinginto accountthegeometricalsizeofthecontactandthefilmthickness.

The electrochemical characterization is done in a one com- partmentcell.TheauxiliaryelectrodeisalargeareaPtelectrode (Tacussel,1cm²)andthepotentials aregivenwithrespecttoa saturatedcalomelelectrode(SCE).Thesolutioninthereference electrodeis changedwhennecessarytopreventcontamination.

Theintensity-potentialJ(E)curvesareplottedwithaPGZ301poten- tiostat(Radiometeranalytical)inKOHsolution(10⁻²M,pH∼12) atascanningrateof10mVs⁻¹.Theﬁlmsareimmersedovernight inthesolutionpriorthemeasurement.TheMott–Schottkycharac- teristicsarerecordedatafrequencyof1kHz.Theelectrochemical impedancespectroscopy(EIS)iscarriedoutusingsmallamplitude wavesignals.Aresponseanalyzergenerates10mVperturbingsig- nalsoverthefrequencyrange(1mHz–100kHz)andsubjectsthe ﬁlmtovariouspotentials.

Thephotocatalytic tests aredone underdirect sunlight; the fluxintensitymeasuredwithdigitalfluxmeter,fluctuatesbetween 105and 93mWcm⁻² overtheday.Foreach experiment,20mL ofsolution(HCrO4−,10mgL⁻¹)areused.Thechromateanalysis isperformedwithaUV–visspectrophotometerShimadzu1800 (max=348nm)using1cmquartzcell.Thekineticofthechromate reductionisperformedunderartificiallightusingUVlamp(Osram Hg100,0.6-1A);thealiquotsareperiodicallyremovedforthetitra- tion.Thesolutionsarefreshlypreparedfromanalyticalsubstances intwicedistilledwater(∼1Mcm).

3. Resultsanddiscussion

Forphotovoltaicdevices andPECcells, itwouldbeadvanta- geousforeconomicreasonstousethinﬁlmswithpolycrystalline naturewhere thetransportpropertiesbehaverather differently fromthebulkmaterial.Tohaveinsightsaboutthemorphology,

Fig.1. SEMimageoftheﬁlmdepositedat400^◦C.

wehavereportedinFig.1theSEMimageofthefilmdepositedat 350^◦Cwhichshowsaroughandratherporoussurfaceandadheres tightlyonthesubstrate.ThepurityischeckedbyXRD;ZnOhas onestablephase andcrystallizesinthewurtzitestructure with spacegroup(P63mc).Thepatterns(Fig.2)clearlyshowthediffrac- tionpeaksofthehexagonalphasewithapreferentialorientation alongc-axis.Moreover,oneobservesanarrowingofthepeakswith increasingthetemperatureT_Sandthereforethefilmbecomeswell crystallized.ThelatticeconstantsatTS=350^◦C:a=0.32288nm,and c=0.51635nm(c/a=1.599)areinagreementwiththeJCPDSCard N^◦36–1451.Inthehcpwurtzite,eachionistetrahedrallycoordi- natedbyoppositeions;thestructureisunderstoodintermofions sizesofinvolvedionswithsmallradiiratio(Zn²⁺/O²⁻=0.41)and intermediatecovalency,duetothemoderatedifferenceofelectro negativities.Thecrystallitesizeisdeterminedfromthefullwidthat halfmaximum(=0.94/ˇcos),ˇ(radian)beingthebroadeningof theintensepeak(002).Asexpected,thecrystallitesizeincreases from39nm(T_S=300^◦C)to66nm(T_S=400^◦C),withanimprove- mentofthecrystallinity.Fig.3showstheFTIRspectraofthefilm depositedontosilicon.Thepeakat411cm⁻¹ correspondstothe stretchingvibrationofZn Obond,assigned totetrahedralZn²⁺

environment[16,17].Theadditionalpeaksareattributedtothesil- iconsupport.Thefilmsdepositedintherange(300–400^◦C)exhibit ahightransparency(∼85%)overthevisiblerange.InFig.4,we havereportedthevariationoftheabsorptioncoefficient(˛)versus thephotonenergy(h)thecoefficient˛iswavelengthdependent andaverages10⁴cm⁻¹at400nm,suchhighvalueinspayedfilms

(002) (103)

Intensity [a.u ]

400°C

350°C

(102) (112)

(110)

(101)

(100)

300°C

20 30 40 50 60 70 80 90

2θ [degrees]

Fig.2.XDRpatternsofZnOelaboratedbyultrasonicspayatdifferenttemperatures.

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1200 1100 1000 900 800 700 600 500 400 ZnO ZnO

Absorbance [a.u]

Wavenumber [cm^-1] deposited at 400°C at 350°C at 300°C

Fig.3. FTIRspectraofZnOﬁlmdepositedat300–400^◦C.

3.10 3.15 3.20 3.25 3.30 3.35

10⁴

10⁵ deposited at 400°C at 350°C at 300°C

hν [eV]

α [cm-1 ]

Fig.4.Theabsorptioncoefﬁcient(˛)asafunctiontheenergyphotonoftheﬁlms depositedatdifferenttemperatures.

isduetothefactthattheincidentlightisscatteredinalldirec- tions,thusthepathtraveledbyincidentlightisextendedleading toahighrugositybecauseoftheroughsurfaceoftheﬁlm.This lightscatteringinalargeangleisinterestingforthesolarenergy conversionsinceitoffersalargeactivesurfaceareawheremore photonscontributetothephotoactivity.Theopticalgapincreases onlyslightlyfrom3.25to3.28eV(Fig.5)whenthesubstratetem- perature increasesfrom300 to400^◦C becauseofthe bandtail energy(Urbachtail),duetotheimprovedﬁlmcrystallinity.The smallchangeofthegapresultsfromtwocompetingmechanisms:

awideningduetotheBurstein–Mosseffect,enhancedwhenthe freeelectronconcentrationincreases,andanarrowingattributed toelectron-electronandelectron-ionscattering.

2.6 2.8 3.0 3.2 3.4

0.0 5.0x10⁹ 1.0x10¹⁰ 1.5x10¹⁰ 2.0x10¹⁰

(αhν)2 [eV2 .cm-2 ]

hν [eV] 300°C 350°C 400°C

Fig. 5.The direct optical transitions of ZnO ﬁlms deposited at different temperatures.

2.6 2.8 3.0

10^-3 10⁷

Light

Dark E_a=0.057 eV

Conductivity[Ω cm]-1

1000/T [K^-1]

Fig.6. Theplotsoflogconductivityvs.1000/Tinthedarkandunderillumination ZnOﬁlmdepositedat350^◦C.

Thesimultaneousoccurrenceofopticaltransparencyinthevis- ibleregionandelectronicconductivityrequirethegenerationof electronsdegeneracy.Theoxygendeﬁciencyextendsthespectral photoresponsetowardlongerwavelengthsandproducesexcess chargecarrierswhichshouldyieldn-typeconductivity(seephoto- electrochemistrybelow).So,thequestionthatcanbesettledisthe following:istheconductivity()increasesbydeviationfromthe stochiometry.Toanswerthisquestion,wehaveplottedthetem- peraturedependenceof(Fig.6):

=oexp

−Ea

kT

(1) Whereoisthepre-exponentialterm;theroomtemperature conductivity300Kis0.066(cm)⁻¹.Thethermaldependence(T) followsanexponentiallaw,indicatingnondegeneratesemicon- ducting behavior withan activationenergy(Ea)of 57meV.The conductiontakesplacebetweenmixedzincvalencesintetrahedra sharingcommoncorners.Thedonorshallowsarepartiallyionized atroomtemperature,givingrisetoasmallpolaronhopping,based onelectron-latticeinteraction.Thefreeelectronsmainlyarisefrom theoxygenionization:

Oo↔ 0.5O2+Vo••+2e⁻ (2)

andareassociatedtotheformationofvacanciesandzincinter- sticesZn_i,thespeciesarewrittenaccordingtotheKroger–Vink notation.Thelocalizedenergylevelsleadtoweakelectronmobility (e):

=(qNDe) (3) Themobility,deﬁnedasthemeandriftvelocityinanelectric ﬁeldofunitforce,isquotedas7cm²V⁻¹s⁻¹.Itisgovernedbythe scatteringmechanism,duetotheobstructionofO²⁻ionstothe hopping process.Under illumination,theconductivityincreases byfourordersofmagnitudewhiletheactivationenergyremains nearlyunchangedandthisindicatesaconstantmobility.

ZnO is applied in the environmental protection [18] and the chemical stability is a crucial requirement for long term applications.ZnO isstable over afair pHrange (2–12)but dis- solves in strong alkaline and acid solutions. The dark current (j_d∼25␮Acm⁻²) in the J(E) characteristic (Fig. 7) plotted in acidicmedium(pH∼3)isconsistentwithagoodelectrochemical stabilitywherethesumofH₂andO₂over-voltagesaverages1.2V.

Thepeakat−0.65VisduetoZn²⁺reductionandisclosetothe standardpotentialofZn²⁺/Zncouple[19].Thisrequiresthatthe freepotentialmustbelesscathodicthan−0.65V,otherwiseZnO corrodeselectrochemicallyinsolution.Thecurrentshootsupdras- ticallybelow−1Vandisaccompaniedbyhydrogenevolution.The positionofthebandsiscrucialinphotocatalysis.Whendrawing

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Fig.7.TheJ(E)characteristicofZnOinacidicsolution(H2SO4pH∼3).Thearrow indicatesthezoomofreductionzone.

thecapacitanceC⁻² versustheappliedpotential,thelinearpart obeystotheMott–Schottkyrelation:

C⁻²=

₂

eεεoND

_V₋_V

fb−kT e

(4) Thesymbolshavetheirusualmeanings.Theplotislinearaslong asthespacechargeregioniselectrons-depleted.Theextrapolated plot(Fig.8)convergestotheﬂatbandpotentialV_fb(−0.70V)while theelectrondensity(ND=5.30×10¹⁶cm⁻³)isdeterminedfromthe slope.Thepositiveslopeprovidesunambiguousevidenceofn-type behaviorwhichtakesitsoriginfromoxygenoffstochiometry.The potentialV_fboutlinestheenergeticpositionoftheconductionband (ZnO-CB)withrespecttovacuumFig.9:

ECB=4.75+eVfb+Ea (5) TheE_CB value(−0.76V/3.99eV)indicatesthattheconduction bandislargelyderivedfromZn²⁺:4sorbitalwithasmalladmix- tureofoxygencharacterwhilethevalenceband(2.49V/7.24eV) consists mainly of O²⁻: 2p orbital and the light absorption is attributedtothechargetransferO²⁻:2p→Zn²⁺:4s.ZnOisapplied fortheenvironmentalprotectionandthechromatereductionis useda reactiontest.Itiswellestablishednowthatchromateis highlytoxicandcomesfromvariousindustriesliketanneryand batteries.So, its eliminationis of great interestand thephoto- catalysisundersolarinsulation(AM1)issuitableforitsreduction tolessharmful oxidation states.The combinationof thephysi- calandphoto-electrochemicalcharacterizationspermitstodraw theenergy banddiagramof thejunctionZnO/HCrO₄⁻ solution.

ThepotentialV_fbisnegativeenoughtogiverisetoalargeband bendingattheinterfacefortheseparationof(e⁻/h⁺/pairs.Asappli- cation,ZnOissuccessfullyusedforthereductionHCrO₄⁻intoCr³⁺.

-0.8 -0.4 0.0 0.4

0 75 150 225

V_fb= -0.70 V C-2 1012 [ F-2 cm4 ]

Potential [V]

Fig.8. TheMott–SchottkycharacteristicofZnOelaboratedat350^◦Cinacidicsolu- tion(H2SO4,pH∼3).

e^-

h⁺

H2O O2

HCrO4-

Cr³⁺

H2O H2

Eg= 3.25 eV hν

e^-

-4.74 V

h⁺

Vacuum

~ -1 V

Potential (VSCE) 2.44 V CB

VB

-0.75 V

~ 0 V ~ 1.5 V

n-ZnO Solution

Fig.9. TheenergydiagramofthejunctionZnO/HCrO4−solutionatpH∼2.5.

ThespecieHCrO₄⁻ predominatesatlow pHs becauseofit acid- ityconstantpKa (Cr₂O₄²⁻+H₂O→HCrO₄⁻+OH⁻,pKa=1.6)[19].

The potential of the couple HCrO4−/Cr³⁺ is found to be 0.56V undertheoperatingconditions whiletheopencircuitpotential (OCP=−0.125V)islesscathodicthanthepotentialV_fb(−0.70V), andthisafurtheradvantagesincethechromatereductiondoesnot requireanexternalbias.Inaddition,thepotentialofphotoelec- tronsinZnO-CBismorenegativethantheHCrO₄⁻/Cr³⁺leveland shouldreducespontaneouslyHCrO4−intoCr³⁺.ZnO-CBvarieswith pH(−0.06VpH⁻¹)butlessthanthecoupleHCrO₄⁻/Cr³⁺(−0.14V pH⁻¹).So, we have exploitedthis property tohave an optimal bandbendingatpH∼3wherethechargecarriersareseparatedby theelectricalﬁelddevelopedatthejunctionZnO/HCrO4−solution.

BecauseofthewidegapofZnO,theholesarescavengedbywater resultinginprolongedlifetimeofcarriers.Indeed,thepotentialof O2/H2OcoupleliesbelowZnO-VBandthereactionsoccurringat theinterfacearethefollowing:

ZnO+h→h⁺_VB+e⁻_CB (6)

HCrO₄⁻+7H₃O⁺+3e⁻→Cr³⁺+11H₂O (7)

H₂O+2h⁺→ 0.5O₂+2H⁺ (8)

Thesurfaceadsorptionhasadirecteffectonthephotoactivity viatheincreasingnumberofphotocatalyticsites.Theattachmentof chromateonthecatalystpowdermakeseasiertheelectrontransfer.

ThesurfaceofZnOispositivelychargedforpH<pHzpc,andnega- tivelychargedforpH>6.3,pHpzzpisthepointofzerozetapotential

0 0 6 0

0 4 0.0

0.1 0.2 0.3

absorbance [a.u]

λ [nm]

initial solution

8 h illumination (pH=2.5) 8 h illumination (pH=7)

Fig.10.UV–VisiblespectraofchromatesolutionsatpH2.5and7overillumination time.

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Fig.11.(a)TheNyquistplotofZnO-HCrO4−solutionpH∼3.(b)ThecorrespondingBodrepresentation.

(=7.74)¹.Fig.10givestheUV–VisiblespectraatpH∼3and7of chromatesolutionsafter4hillumination.Thedecayofabsorbance (max=350nm)isduetoHCrO4−lightinducedreductionsinceno changeinabsorbancehasbeenobservedinthedarkorinabsenceof ZnO.Thereductionrateaverages94%atpH∼3againstonly28%in neutralsolution(pH∼7).AtransferofthreeelectronsperHCrO4−

moleculeisrequired(reaction7)andthequantumyield( )isgiven by:

=3

numberofHCrO⁻₄convertedmol s⁻¹ photonsﬂux s⁻¹

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Avalueof0.06%isobtainedundertheworkingconditions;the lowquantumyieldisduetothesmallpartofUVradiationinthe sunspectrumabsorbedbyZnO(<380nm).Theregressionofthe photoactivityevidencedbythebendingoverthecurveisdueto thesaturationof photocatalyticsitesbythehydroxideCr(OH)₃, suchhypothesisiscorroboratedbytheabsenceofCr³⁺peakinthe spectrum,duetotheadsorptionofCr(OH)₃becauseofthelowsolu- bilityproduct(Ks=5.4×10⁻³¹).Thehalflife(t_1/2),thetimeneeded toreducehalfofHCrO4−presentinitially,isfoundtobeconcen- trationdependent,indicatingaﬁrstorderkinetic.Indeed,wehave foundalinearrelationbetween(lnCt)andirradiationtime.

EIS isperturbation techniqueof thedynamic ofthe electro- chemicalprocesswherethebulk,grainboundariesanddiffusion contributionsare quantifiedbytheresponsesof ACsignalsub- jectedtovariablefrequencies.Theplotofimaginaryimpedance versustherealimpedance(Fig.11)showstwodepressedsemicir- cles,thefirstonewitharesistance(R₂=1.89kcm²)isassigned tothechargetransferinconformitywiththemoderateconduc- tivity.Thesecondsemicircle(R3=7.12kcm²)isattributedtothe grainboundaries.Theslightoffsetneartheorigin(R₁=0.4kcm²) isduetotheionicelectrolytebecauseofthehighmobilityofpro- ton(350⁻¹cm²mol⁻¹).Thesemicirclesarecenteredbelowthe abscissa axis due to a constant phase element (CPE). CPErep- resents thedeviationfrom anideal capacitorand is definedas Z_CPE=[C(jω)ⁿ]⁻¹whereωistheangularfrequencyandnthehomo- geneityfactor(−1≤n≤1).CPEtakesitsoriginfromtheroughness of theelectrode aswellas defectstates withinthe gapregion.

Thefactorn(=0.82)isreadilyobtainedfromtherelation{=/2 (1−n)}.Let’srecallthatnvalueclosetounityindicatesacapacitive behavior. The absence of straight line at low frequencies indi- catesthatthechromatereductionisunderkineticcontrolandthe

1Thepointofzerocharge(pzc)isobtainedfromtheequilibriumofZnOpowder suspensionindistilledwater.

electrontransferattheinterfaceistherate-limitingstep.Themin- imumangularfrequency(ωmin=2␲f_min)isusedtodeterminethe lifetimeofelectrons(23ms=ωmin−1).Theexperimentaldataare ﬁttedbytheZviewsoftwarefortheequivalentelectricalcircuit (Fig.11a,Inset).FutureinvestigationsfocusonthegrowthofZnO ﬁlmsontosiliconsinglecrystalanditsapplicationfortheenviron- mentalprotection.Theworkispresentlyunderwayandtheresults willbereportedsubsequently.

4. Conclusion

ZnOthinfilmsaredepositedonglasssubstratesbyultrasonic spray. The technique is low cost technique and both the size and the stoichiometryof thefilms areaffected by thethermal treatment.TheXRDanalysisrevealsthehexagonalwurtzitestruc- turewithpreferential orientationalongthecaxis.ZnO exhibits ahightransparencyoverthevisibleregionanddisplaysasemi- conductingbehavior,dominatedbythermallyactivatedhopping ofsmallpolarons.Anexcellentrectificationisobservedinacidic mediumandZnObehavesaschemicaldiode.Theoffstochiometry isconfirmedbythecapacitancemeasurements.Thephysicaland photo-electrochemicalcharacterizationspermittobuildtheenergy diagramwhichpredictsthechromatereductionintotrivalentstate underUVlight.95%ofCr(VI)arereducedafter6hofexpositionto solarillumination.Theoxidationfollowsafirstorderkinetic.The impedancedataindicatenondiffusioncontrolledprocess.

Acknowledgments

The authors would like to thank Dr M. Khitous for the spectrophotometricmeasurements.Theworkwasﬁnanciallysup- portedbyboththedepartmentsofPhysicsandChemistry(Algiers).

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