Synthesis of ZnOAggraphene composite and its enhanced photocatalytic efficiency

MaterialsResearchBulletin

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SynthesisofZnO/Ag/graphenecompositeanditsenhancedphotocatalyticefficiencyFangXua,b,YafeiYuanb,DapengWua,b,MeiZhaoc,ZhiyongGaoa,b,KaiJianga,b,*aEngineeringTechnologyResearchCenterofMotivePowerandKeyMaterialsofHenanProvince,Xinxiang453007,ChinaSchoolofChemistryandChemicalEngineering,HenanNormalUniversity,Xinxiang453007,ChinacQUEMINGHUScientific&TechnologicalIndustryPark,Yingkou115010,ChinabARTICLEINFOABSTRACTArticlehistory:

Received28June2012Receivedinrevisedform8February2013Accepted12February2013Availableonline1March2013

Keywords:A.CompositesB.ChemicalsynthesisC.X-raydiffractionD.CatalyticpropertiesZnO/Ag/graphenecompositewassynthesizedviaafacilelow-temperatureaqueoussolutionreaction.ThemorphologyandcrystalphaseoftheobtainedsampleswerewellcharacterizedbySEM,TEM,andXRD.TheresultsindicatedthatZnO/Agnanoparticles(NPs)areanchoredonthetwo-dimensionalgraphenesheet.ThephotocatalytictrialsconfirmedthattheZnO/Ag/graphenecompositeexhibitedimprovedphotocatalyticefficiencycomparedwiththeZnO/AgNPsandpristineZnONPs.TheincreasedphotocatalyticactivityofZnO/Ag/graphenewasattributedtotheeffectivechargeseparationandthesuppressedrecombinationofphotogeneratedelectron–holepairs.Moreover,theZnO/Ag/graphenesampleshowedexcellentstabilityafterfourphotodegradationcyclingruns.ß2013ElsevierLtd.Allrightsreserved.1.IntroductionZnOhasbeenconsideredasoneofthemostpromisingphotocatalystsbecauseofitswideband-gapenergy,physicalandchemicalstability,highoxidativecapacity,lowcost,andeaseofavailability[1–4].Therefore,ZnOhavebeenwidelyusedascandidatesindifferentterritoriessuchasopticalmaterial,sensors,solarenergyconversiondeviceandphotocatalysts[5–7].WhenZnOisirradiatedbyUVlight,theelectronsareexcitedtotheconductionband(CB),leavingholesinthevalanceband(VB),andtheas-formedelectron–holepairsareresponsibleforthephoto-catalyticactivitiesofZnO.However,thefastrecombinationofphoto-generatedelectronsandholeswilldecreasethephoto-catalyticefficiency.Moreover,theZnOnanostructurescouldbeeasilypackedintolargeaggregatesduetothehighsurfaceenergy,whichfurtherreducesitsphotocatalyticefficiency.Thus,theretardationoftheelectron–holepairrecombinationisofgreatsignificancetoimprovethephotocatalyticperformancesofZnO.Previously,manyZnObasedcompositeswereproposedtoincreasethephotocatalyticperformance[8–10].Ithasbeenreportedthattherecombinationofelectron–holepairscouldberetardedbyhybridizingZnOwithnoblemetallicparticles,suchas*Correspondingauthorat:EngineeringTechnologyResearchCenterofMotivePowerandKeyMaterialsofHenanProvince,Xinxiang453007,China.Tel.:+863733326209;fax:+863733326209.E-mailaddress:jiangkai6898@126.com(K.Jiang).Au,Ag,andPt[11–14].TheSchottkybarrieratthemetal–semiconductorinterfacecanseparatethephotoinducedelectronsandholeseffectivelybecausethenoblemetalcouldserveaselectronsinks,whichenhancedthephotocatalyticactivities.Moreover,thenoblemetalmodificationcanalsoinfluencethesurfaceproperty,especiallyonthesurfacehydroxylofthephotocatalysts[8].Itiswellknownthatgraphenesheetspossessuniquetwodimensionallayerstructureofsp2-hybridizedcarbonatoms,whichexhibitsnovelelectronicpropertyasazero-bandgapsemiconductorandishighlyelectronicallyconductivityforstoringandtransportingelectrons[15,16].Graphenebasedphotocatalystshavebeeninvestigatedintensivelybecauseoftheseuniqueproperties.Inaddition,grapheneprovidesalargescaffoldforanchoringvarioussubstancesowingtoitslargespecificsurfaceareaandtwo-dimensionalplanarconjugationstructure.Therefore,graphene-basedphotocatalystswillexhibitenhancedphotocata-lyticperformancethansingle-phasesemiconductor.Moreover,graphene-basedphotocatalystscanavoidtheaggregationofnanoparticlesanchoredongraphenesheets,whichcanenhancesurfaceareaandreactivesitesinphotocatalyticdegradationprocess.Itisworthytomentionthatitisconvenienttoseparatethegraphene-basedphotocatalystsfromwater.Therefore,thephoto-catalystscanbeusedrepeatedly.Recently,severalZnO/graphenecompositeshavebeenreportedandexploredinphotocatalysisindicatingtheelectron–holepairsseparationcouldbealsopromotedbyanchoringZnOnanoparticlesontothegraphenesheets[17–21].AlthoughtheeffectofnoblemetalandgrapheneonthephotocatalyticefficiencyofZnOhasbeendiscussedintensively,0025-5408/$–seefrontmatterß2013ElsevierLtd.Allrightsreserved.http://dx.doi.org/10.1016/j.materresbull.2013.02.034F.Xuetal./MaterialsResearchBulletin48(2013)2066–20702067

thesynergeticeffectofthenoblemetalandgraphenehasbeenseldomdiscussedpreviously[22].Inthiswork,ZnO/Ag/graphenecompositewassuccessfullypreparedviaalow-temperatureaqueoussolutionmethod.Thisnovelcompositeintegratestheadvantagesofitscomponents,whichleadstothesuperiorperformancesthanZnOorZnO/Agphotocatalysts.Theenhancementscouldbegenerallyattributedtothe(1)suppressedrecombinationofphotogeneratedelectron–holepairsthroughaneffectivechargetransferpathwayfromZnOtoAgandgraphene.(2)UniformdepositionofZnO/AgongraphenesheetsreducestheaggregationoftheNPsandmaximizesactivesitesand(3)thiscompositecatalystcouldbeeasilyseparatedfromthereactionsystemviacentrifugationandmaintainingagoodcyclingperformance.2.Experimental2.1.PreparationofZnO,ZnO/Ag,ZnO/Ag/graphenephotocatalystsGrapheneoxide(GO)waspreparedthroughtheHummer’smethodwithslightmodifications[23,24].Inatypicalprocedure,50mLofH2SO4wasaddedintoaflaskcontaining2ggraphiteinicebathunderstirring.Afterthat,6gpotassiumpermanganate(KMnO4)wasaddedslowlytotheabovemixture,andtheicebathwasremovedafterseveralminutes.Thecolorofthesolutionturnedtodarkgreenwhilekeepstirringfor2hat358C.Then,100mLwaterwasaddedslowlytothereactionmixtureandtheconicalflaskwasbathedinboilingwater,thereactionmixturewaskeptatthistemperatureforanother30min.Afterthat,theflaskwascooledinanicebath,and350mLdistilledwaterwasaddedtostopthereaction.20mLH2O2(30%)wasaddedtoreducetheresidualpermanganatetosolublemanganeseions.Eventually,theprecipitationwascentrifugedandwashedwith1MHClanddistilledwaterforseveraltimes,andthenwasvacuumdriedat608Cfor24h.ZnOnanoparticleswerepreparedbyadding25mLNaOHsolution(4M)into25mLof0.2MZnSO4solutionatanapproximaterateof5mL/min.Then,themixturewaskeptat608Cfor2h.TheZnO/Agcompositenanoparticleswerepreparedbyadding5mLof1MVcand3mLof1MAgNO3intothemixturesolutionofNaOHandZnSO4,andthemixturewasalsoperformedat608Cfor2h.ForthepreparationofZnO/Ag/graphene,50mgofGOwasdispersedin50mLwaterandkeptinabathtypeultrasonicatorfor30min,andthen25mLof0.2MZnSO4solution,25mLof4MNaOH,3mLof1MAgNO3and5mLof1MVcweredroppedintothemixtureatanapproximaterateof5mL/min,andthemixturewasperformedat608Cfor2h.Alltheproductswerecentrifugedandwashedbydistilledwaterforseveraltimesandweredriedinvacuumat608Cfor24h.TheZnONPs,ZnO/Ag,ZnO/Ag/graphenewereassignedasS1,S2,S3,respectively.2.2.CharacterizationThephasepurityandstructureoftheproductswerecharacterizedbyX-raydiffraction(XRD)usingaBrukerad-vance-D8XRDwithCuKaradiation(l=0.154178nm).Theacceleratingvoltagewassetat40kVwitha100mAflux.TheSEMimagesandenergydispersiveX-rayanalysisweretakenonJEOLJSM-6390LVandJED-2300,respectively.LowmagnificationTEMimageswereobtainedfromJEOLJSM-100whilethehigh-resolutionTEM(HRTEM)imagesweretakenonFEITecnaiG220.ThephotoluminescencewasperformedonJASCOFP-6500fluorophotometerunderroomtemperature.Thecyclic-voltam-metry(CV)wasmeasuredonSolartron1287electrochemicaltestsystem(SolartronAnalytical,England)withthree-electrodesystem,consistingofaAgCl/Agelectrodeasareferenceelectrode,aplatinumwireasacounterelectrodeandamodifiedAuelectrodeasworkingelectrodes.2.3.PhotocatalyticexperimentsThephotocatalyticactivitiesoftheobtainedsamplesweremeasuredbydegradationofRhBinaqueoussolutionunderUVirradiation.Inatypicalprocess,20mgofthesamplesweremixedwith50mLofRhBsolution(20mg/L)incylindricalquartzvesselsandwerestirredinthedarkfor30mintoestablishadsorption/desorptionequilibrium.Afterthat,thesuspensionwasirradiatedbya300Whigh-pressureHglampforphotodegradationunderstirring.Aquartzdouble-wall-tubeloadedwithrecyclingdistilledwaterwasplacedbetweenHglampandphotoreactionvesselstofiltratetheconcomitantheatduringirradiation.Atanintervalof10min,thesuspensionwassampledinturnandfiltratedtomeasuretheabsorptionat553nmtodeterminetheconcentrationofRhB.ThephotocatalyticperformanceisexpressedbyC0/Cvst,whereC0istheconcentrationofRhBatabsorptionequilibration.Thephotocatalyticdurabilityofthesamplewasstudiedbyrecyclingtestatthesameconditions.Thatis,thephotocatalystswereusedtodegradeRhB(20mg/L)aqueoussolutionrepeatedly.Afterthefirstcycle,thephotocatalystswerecentrifuged,washedanddried,andthesamevolumeoffreshRhBwasaddedforthesecondcycle.Theprocesswasrepeated4times.Hydroxylradicals(OH󰀂)producedbytheas-preparedphoto-catalystsweremeasuredbyfluorescencemethodwithterephtha-licacid(TA)usedasprobemolecule.Inbrief,10mgofeachZnO/Ag/graphenewasmixedwith40mLofTAsolution(5Â10À4M),andthemixturewasdissolvedintoNaOH(2Â10À3M)aqueoussolution.Then,theresultingsuspensionwasexposedtoUVlight.Atintervalsof10min,1.2mLofthesuspensionwascollectedandcentrifugedtomeasurethemaximumfluorescenceemissionintensityat425nmwithanexcitationwavelengthof315nm.3.ResultsanddiscussionTheXRDpatternsofthepristineZnONPs,ZnO/AgandZnO/Ag/graphenewereshowninFig.1.AllthediffractionpeaksofZnONPsareconsistentwiththehexagonalphasewurtziteZnO(JCPDSNo.36-1451).FortheXRDpatternofZnO/Agcomposite,thediffractionpeakslocatedat38.178,44.288,and64.438couldbeindexedtothecubicphasesilver(JCPDSNo.04-0783),suggestingtheAgparticlesweremodifiedontheZnOsurface.ThediffractionpeaksofFig.1.XRDpatternsofZnONPs,Ag/ZnOandZnO/Ag/graphene.2068F.Xuetal./MaterialsResearchBulletin48(2013)2066–2070Fig.2.(a)SEMimageofZnOnanoparticles;(b)SEMimageofGO;(c)SEMimageofZnO/Ag/graphene;(d)TEMimageofZnO/Ag/graphene;(e)EDSspectrumofZnO/Ag/graphene.graphenecanhardlybedetectedintheXRDpatternofZnO/Ag/graphene,whichindicatesthattheinsertionofnanoparticlesbetweentheadjacentgraphenesheetscausedthedisorderoflayerspacing.TheSEMandTEMimagesoftheobtainedproductsaredepictedinFig.2.TheSEMimageshowstheas-preparedZnONPshasauniformdiameterof$200nm(Fig.2a).TheSEMimageofGOindicatestheproducthastwo-dimensionalsheetsstructurewithsilkveilwaves(Fig.2b).AsshowninFig.2c,theZnO/AgNPsdispersedhomogeneouslyonthegraphenesheets,whichcouldbefurtherconfirmedbythecorrespondingTEMimageofthecomposite(Fig.2d)ConsideringtheoxygencontaininggroupslikecarboxylinGOserveasanchoringsitestodirectcoatingprocess,theZnO/AgNPswerehomogeneouslyformedonthegraphenesheetsandfreefromaggregation.SeveralZnO/AgNPswerefounddetachedfromthegraphenesheet,whichmayprobablyresultfromtheultrasonictreatmentinpreparingtheTEMsamples.ThewrinklesintheZnO/Ag/graphenecompositealsorevealthetypicalfeatureofgraphenesheet.ThechemicalcompositionofZnO/Ag/graphenewasanalyzedbyanenergydispersivespectrometer(EDS)spectrumonanSEM(Fig.2e).TheresultshowsthepeaksofC,O,ZnandAg,confirmingthesampleisofhighpurity.Photoluminescence(PL)canprovideinformationabouttheseparationandrecombinationofphotoinducedelectronsandholes.Theelectronintheexcitedstatewouldundergorecombi-nationwiththeholesbeforediffusedtothesurfaceofZnO,andthisirradiativereleaseofenergyresultedinfluorescence.BasedonthefluorescencecharacteristicsofZnO,theinteractionbetweenZnO,Ag,andgraphenesheetscanbemonitoredbymeansofmeasuringthefluorescencedecayofthesamples[25].ThePLspectraofZnONPs,ZnO/AgandZnO/Ag/graphenewererecordedatroomtemperaturewithexcitationwavelengthof325nm,anddisplayedinFig.3a.ItcanbeseenthatZnONPsshowstrongfluorescenceemissionat387nm,465nmand540nm.Thepeakat387nmcanbeascribedtotherecombinationofthechargecarriersnearthebandedgesofZnO.While,theblueandgreenemissionscenteredataround460nmand540nmaregenerallybelievedcomingfromthesurfacedefectsofZnO.ItisevidentthatthefluorescenceemissionintensityofAg/ZnOdecreasesdramatically.Moreover,thePLintensityofAg/ZnO/grapheneistheweakestamongallthethreesamples,indicatingthefluorescenceofthecompositeisquenchedmoreefficientlythanthatofAg/ZnObytheincorpo-rationofgraphene.ThisphenomenonsuggeststheefficientinteractionbetweenZnO,Agandgraphene,indicatingtheincorporationofAgandgrapheneenhancestheseparationofphotoinducedelectronsandholes.TheworkfunctionofZnOandAgisabout5.2eVand4.26eV.Thus,theFermienergylevelofZnOislowerthanthatofAg,whichfavorsthetransferofelectronsfromAgtoZnOtoreachequilibriumandformthenewFermienergy[5].AsshowninFig.3b,undertheUVlightirradiation,theelectronsinCBofZnOtendtobetransferredtoAg,andholesareremainedinVBofZnO.Therefore,therecombinationofelectronsandholescanbeeffectivelydelayed.GrapheneisazerobandgapsemiconductorandhasaworkfunctionofÀ4.42eV.ThephotoinducedelectronscanalsobetransferredfromZnOtographene,leavingholesinVBofZnO.Therefore,thephotoinducedelectronsandholescanalsobeFig.3.(a)RoomtemperaturephotoluminescencespectraofS1,S2,andS3;(b)SchematicdiagramofthechargetransferbetweenZnO,AgandgrapheneunderUVlightirradiation.F.Xuetal./MaterialsResearchBulletin48(2013)2066–20702069

Fig.4.TheCVcurvesforS1,S2andS3modifiedgoldelectrodeinsaturatedaqueoussolutionofKCl.separatedbecauseoftheincorporationofgraphenesheets.Conclusively,thephotoinducedelectronsandholescanbeeffectivelyseparatedduetotheexistenceofAgandgraphene.Thecyclic-voltammetry(CV)curveswerealsoutilizedtoevaluatetheinteractionsbetweenZnONPs,ZnO/AgNPsandtheZnO/Ag/graphenecomposites.Fig.4showsCVcurvesoftheelectrodesmodifiedbythethreesamplesatthepotentialfrom0to0.5Vwithscanrateof50mVsÀ1.TheshapesoftheCVloopsarequasi-rectangularalongthecurrent–potentialaxiswithoutobvi-ousredoxpeaks.ThecurrentresponseofZnO/AgcompositeislittlehigherthanthatofZnONPs,andZnO/Ag/graphenecompositeshowsmuchhighercurrentresponse.Therefore,itcanbeconcludedthatZnO/Ag/grapheneproducesahigheraveragecurrentatvariousdifferentpotentialscomparedtoZnONPsandZnO/Ag,indicatingtheincorporationofAgandgraphenecanimprovethepolarityoftheZnONPs,whichfavoritesthechargeseparationofthecomposite[26,27].ThephotocatalyticdegradationofRhBunderUVlightirradia-tionisusedtoevaluatethephotocatalyticactivitiesofZnO,ZnO/AgandZnO/Ag/graphene.AsshowninFig.5a,RhBiscompletelydegradedwithin90mininthepresenceofZnONPs.And,thecompletedegradationofRhBisabout50minasS3wasusedasphotocatalyst,whichiscomparabletoP25.Thedegradationrateorderofthesamplesis:ZnO[1]X.W.Duan,G.Z.Wang,H.Q.Wang,Y.Q.Wang,C.Shen,W.P.Cai,CrystEngComm12

(2010)2821.

[2]T.J.Sun,J.S.Qiu,C.H.Liang,J.Phys.Chem.C112(2008)715.

[3]H.Q.Wang,G.H.Li,L.C.Jia,G.Z.Wang,C.J.Tang,J.Phys.Chem.C112(2008)11738.[4]Y.H.Zhen,C.Q.Chen,Y.Y.Zhan,X.Y.Lin,Q.Zheng,K.M.Wei,J.F.Zhu,Y.J.Zhu,Inorg.

Chem.46(2007)6675.

[5]D.P.Wu,Z.Y.Bai,K.Jiang,Mater.Lett.63(2009)1057.

[6]D.P.Wu,Z.Y.Gao,F.Xu,J.L.Chang,W.G.Tao,J.J.He,S.Y.Gao,K.Jiang,CrystEng-Comm15(2013)1210.

[7]D.P.Wu,Z.Y.Gao,F.Xu,Z.P.Shi,W.G.Tao,K.Jiang,2012,CrystEngComm14(2012)

7934.

[8]S.Y.Gao,X.X.Jia,S.X.Yang,Z.D.Li,K.Jiang,J.SolidStateChem.184(2011)764.[9]D.P.Wu,Y.Jiang,Y.F.Yuan,J.S.Wu,K.Jiang,J.Nanopart.Res.13(2011)2875.[10]F.Xu,Y.F.Yuan,D.P.Han,H.J.Wu,Z.Y.Gao,K.Jiang,CrystEngComm14(2012)

3615.

[11]W.W.Lu,S.Y.Gao,J.J.Wang,J.Phys.Chem.C112(2008)16792.

[12]S.Pyne,G.P.Sahoo,D.K.Bhui,H.Bar,P.Sarker,S.Samanta,A.Maity,A.Misra,

Spectrochim.ActaA93(2012)100.

[13]N.Udawatte,M.Lee,J.H.Kim,D.Lee,ACSAppl.Mater.Interfaces3(2011)4531.[14]N.Morales-Flores,U.Pal,E.SanchezMora,Appl.Catal.,A394(2011)269.[15]M.J.Allen,V.C.Tung,R.B.Kaner,Chem.Rev.110(2010)132.

[16]C.N.R.Rao,A.K.Sood,K.S.Subrahmanyam,A.Govindaraj,Angew.Chem.Int.Ed.48

(2009)7752.

[17]X.J.Liu,L.K.Pan,Q.F.Zhao,G.Zhu,T.Q.Chen,T.Lu,Z.Sun,C.Q.Sun,Chem.Eng.J.

183(2012)238.

[18]D.Y.Fu,G.Y.Han,Y.Z.Chang,J.H.Dong,Mater.Chem.Phys.132(2012)673.[19]T.Lv,L.K.Pan,X.J.Liu,T.Lu,G.Zhu,Z.Sun,J.AlloysCompd.509(2011)10086.[20]J.L.Wu,X.P.Shen,L.Jiang,K.Wang,K.M.Chen,Appl.Surf.Sci.256(2010)2826.[21]O.Akhavan,ACSNano4(2010)4174.

[22]D.H.Yoo,T.V.Cuong,V.H.Luan,N.T.Khoa,E.J.Kim,S.H.Hur,S.H.Hahn,J.Phys.

Chem.C116(2012)7180.

[23]W.S.Hummers,R.E.Offeman,J.Am.Chem.Soc.80(1958)1339.[24]C.Nethravathi,M.Rajamathi,Carbon46(2008)1994.

[25]Z.Y.Gao,N.Liu,D.P.Wu,W.G.Tao,F.Xu,K.Jiang,Appl.Surf.Sci.258(2012)2473.[26]Y.P.Zhang,H.B.Li,L.K.Pan,T.Lu,Z.Sun,J.Electroanal.Chem.634(2009)68.[27]T.Lu,Y.P.Zhang,H.B.Li,L.K.Pan,Y.L.Li,Z.Sun,Electrochim.Acta55(2010)

4170.

[28]Z.G.Xiong,L.L.Zhang,J.Z.Ma,X.S.Zhao,Chem.Commun.46(2010)6099.[29]D.Wang,Y.Duan,Q.Luo,X.Li,L.Bao,Desalination270(2011)174.

5.ConclusionsInsummary,ZnO/Ag/graphenecompositewaspreparedviaalow-temperatureaqueoussolutionmethodusingVcasreductant.TheSEMandTEMimagesshowthattheZnO/Agnanoparticleswerehomogeneouslyanchoredonthegraphenesheets.MoreovertheinteractionoftheZnOwiththeAgandthegraphenesheetwasfurtherinvestigatedbythePLandtheCVmeasurements.Theas-preparedZnO/Ag/graphenecompositeexhibitsenhancedphoto-catalyticbehaviorsindegradingRhBcomparedwiththeZnONPsandZnO/AgNPs,whichcanbeattributedtoenhancedelectron–holesseparationatthehetero-interface.Moreover,ZnO/Ag/graphenecompositeshowsasuperiorstabilityaccordingtothecyclingtests.Thedegradationprocessofthephotocatalystwasalsoinvestigatedintermsofthegenerationofhydroxylradicalsandthedeterminationofcarriertype.Ourmethodpavesawaytodesignandpreparemulti-componentgraphene-basedphoto-catalystswithhighphotocatalyticefficiency.Acknowledgement

ThisworkwassupportedbytheNationalNaturalScienceFoundationofChina(Nos.61176004and61204078)andScienceandTechnologyBreakthroughProjectofHenanProvince(Nos.122102210561,13A150517).