Capture--Store--and-Discharge.-Shuttling-Photogene

TAKAIANDKAMATVOL.5’NO.9’7369–7376’2011
SilverInterfaceAzusaTakai†andPrashantV.Kamat*RadiationLaboratory,DepartmentsofChemistryandBiochemistryandChemical&BiomolecularEngineering,UniversityofNotreDame,NotreDame,Indiana46556,UnitedStates.†Presentaddress:DepartmentofAppliedChemistry,FacultyofScienceandEngineering,WasedaUniversity,3-4-1Ohkubo,Shinjuku-ku,Tokyo169-8555,Japan.Semiconductor
metalnanocompositesareeffectiveinfacilitatingphotocata-lyticprocesses.1
9Thecouplingofsemiconductorandmetalnanoparticlesprovidesauniquepathwayforthedis-chargeofelectronsattheelectrolyteinterface.10
14Onesuchapplicationofthisprocessisinthephotocatalyticproductionofhydrogen.15,16Manyrecenteffortsofphotocatalyticsplittingofwaterrequirein-clusionofametalcocatalystsuchasPt.17,18ThenoblemetalssuchasAuandAgpossesselectronstoragepropertieswhichinturnfacil-itatesimprovedchargeseparationinsemicon-ductor
metalcompositesystems.19,20Recentstudieshavealsofocusedonsurfaceplasmonexcitationofmetalnanoparticlesandim-provementinsolarcellandphotocatalyticperformanceofsemiconductor
metalnano-composites.21
27Althoughevidenceispre-sentedinthesestudiesforobservingelectroninjectionfrommetalnanoparticlesintosemi-conductorand/orplasmonresonancein-ducedfieldeffects,oftenthesereportedenhancementsaresmallcomparedtothebulksemiconductorexcitationeffects.Spectroscopicstudiesindicatethattheelectrontransferfromasemiconductortoametalnanoparticleisanultrafastpro-cess.28,29Forexample,theelectrontransferfromCdSeintoPtoccurswitharatecon-stantof1.22109s
1.28WhilemetalssuchasPtandPdprovideanohmiccontact,metalssuchasAgandAuexhibitcapacitiveproperties.Thesecoinagemetalswhentheyaresmall(2
5nmindiameter)arecapableofstoringelectrons.30,31Suchelectronsto-ragecausesashiftintheFermilevel.Forexample,earlierstudieshaveshownthattheshiftinFermilevelcouldbeashighas0.1Vperstoredelectron.32
35Theelec-tronstoragepropertycanbebeneficiallyusedtoimprovethephotoelectrochemicalperformanceofnanostructuredsemicon-ductorfilms.Inpreviousstudies,wehaveshownthatAunanoparticlesdepositedonTiO2candirectlyparticipateinFermilevelequilibrationunderUVirradiation,therebyshiftingtheFermilevelofthecompositetomorenegativepotentials.36
41ThestoredelectronscanbereadilyestimatedbytitratingwithaknownredoxcouplesuchasthionineorC60.SmallersizeparticleswerefoundtoinducemaximumshiftintheFermilevelofthesemiconductor
metalcomposite.Anotherconvenientwaytoprobetheelectronstorageinmetalnanoparticlesisbymonitoringitsplasmonfrequency.Theadditionofelectronstosilverandgoldnanoparticlesornanorodscausesablueshiftintheabsorptionspectrumduetotheincreasingsurfaceplasmonfrequencyoftheelectrongas.41,42Thecorrelationbe-tweenthenumberofstoredelectronsandtheshiftinplasmonfrequencywascom-paredinthesestudies.Wehavenowutilizedthispropertytoestablishthemechanismwithwhichelectronchargeanddischargecyclesoperateinasemiconductor-assisted*[e-mail]21,2011andacceptedAugust5,2011.Publishedonline10.1021/nn202294bABSTRACTUVirradiationofTiO2nanoparticlesinthepresenceofAgþionsresultsinthequantitativereductionanddepositionofsilveronitssurface.ContinuedUVirradiationfollowingthedepositionofAgontheTiO2surfacecausesablueshiftinthesurfaceplasmonpeakfrom430to415nmastheseparticlesbecomechargedwithexcesselectrons.UnderUVirradiation,boththecharginganddischargingofelectronsoccuratdifferentrates,thusallowingthesystemtoattainasteadystate.UponstoppingtheUVirradiation,afractionoftheseelectronsremainstored.TheelectronstorageisdependentontheamountofAgdepositedonTiO2nanoparticleswithmaximumcapacityseenat8.6μMofAginasuspensioncontaining5.8mMofTiO2.Suchelectroncharginganddischargingprocessesinsemiconductor
metalcompositesneedtobetakenintoaccountwhileevaluatingtheplasmonresonanceinducedeffectsinphotocatalysisandphotoelectrochemistry.KEYWORDS:semiconductor
metalcomposite.photocatalysts.TiO2.silvernanoparticles.plasmonresonance.solarenergyconversion.electronstorageARTICLETAKAIANDKAMATVOL.5’NO.9’7369–7376’2011þIonsattheTiO2Interface.ThereductionofmetalionsontheUV-irradiatedTiO2surfacehasbeenthetopicofextensiveinvestigationsinceitsfirstreportbyBardandco-workers.43Whereassuchmetaldepositionisbeneficialinboostingthephotocatalyticreductionprocessesbycapturingtheelectronsfromexcitedsemiconductornanoparticles,thepresenceofexcesselectronsalsoshiftstheapparentFermileveltomorenegativepotentials.2,3,39Inaddition,thesemi-conductor/metalinterfaceisdynamicasreductionofmetalionsandoxidationofmetalcontinuestopro-ceedduringUVirradiationofTiO2.44InordertoobtainaquantitativemeasureoftheelectrontransfertoAgþions,wefirstirradiatedtheTiO2sampletotrapelectronsandthentitratedtrapped(orstored)elec-tronswithexternallyaddedAgþsolution(previously