Photocatalytic-Water-Splitting-Recent-Progress-and

PublishedonWebDate:August26,2010r2010AmericanChemicalSociety2655DOI:10.1021/jz1007966|J.Phys.Chem.Lett.2010,1,2655–2661PERSPECTIVEpubs.acs.org/JPCLPhotocatalyticWaterSplitting:RecentProgressandFutureChallengesKazuhikoMaedaandKazunariDomen*DepartmentofChemicalSystemEngineering,TheUniversityofTokyo,7-3-1Hongo,Bunkyo-ku,Tokyo113-8656,JapanABSTRACTWatersplittingtoformhydrogenandoxygenusingsolarenergyinthepresenceofsemiconductorphotocatalystshaslongbeenstudiedasapotentialmeansofclean,large-scalefuelproduction.Ingeneral,overallwatersplittingcanbeachievedwhenaphotocatalystismodifiedwithasuitablecocatalyst.Itisthereforeimportanttodevelopbothphotocatalystsandcocatalysts.Inthepastfiveyears,therehasbeensignificantprogressinwatersplittingphotocatalysis,especi-allyinthedevelopmentofcocatalystsandrelatedphysicalandmaterialschem-istry.Thisworkdescribesthestateoftheartandfuturechallengesinphotocatalyticwatersplitting,withafocusontherecentprogressofourownresearch.Photocatalyticoverallwatersplittingtoformhydrogenandoxygenhasattractedconsiderableattentionasapotentialmeansofrenewableenergyproductionwithnorelianceonfossilfuelsandnocarbondioxideemission.1-5AsillustratedinFigure1,currentsuccessfulphotocatalyticsystemsforoverallwatersplittingcanbedividedintotwoprimaryapproaches.OneapproachistosplitwaterintoH2andO2usingasinglevisible-light-responsivephotocatalystwithasufficientpotentialtoachieveoverallwatersplitting.Inthissystem,thephotocatalystshouldhaveasuitablethermo-dynamicpotentialforwatersplitting,asufficientlynarrowbandgaptoharvestvisiblephotons,andstabilityagainstphotocorrosion.Becauseofthesestringentrequirements,thenumberofreliable,reproduciblephotocatalystssuitableforone-stepwatersplittingislimited.6,7Theotherapproachistoapplyatwo-stepexcitationmechanismusingtwodifferentphotocatalysts.8ThiswasinspiredbynaturalphotosynthesisingreenplantsandiscalledtheZ-scheme.TheadvantagesofaZ-schemewatersplittingsystemarethatawiderrangeofvisiblelightisavailablebecauseachangeinGibbsfreeenergyrequiredtodriveeachphotocatalystcanbereducedascomparedtotheone-stepwatersplittingsystemandthattheseparationofevolvedH2andO2ispossible.Itisalsopossibletouseasemiconductorthathaseitherawaterreductionoroxidationpotentialforonesideofthesystem.Forexample,somemetaloxides(e.g.,WO3andBiVO4)functionasagoodO2evolutionphotocatalystinatwo-stepwatersplittingsystemusingaproperredoxmediator,althoughtheyareunabletoreducewater.9,10Successfuloverallwatersplittingviatwo-stepphotoexcitationbyvisiblelightusingseveralcombinationsofphotocatalystsandelectronrelayshasbeenreported.9-16However,challengesremaininthepromotionofelectrontransferbetweentwosemiconductorsandinthesuppressionofbackwardreactionsinvolvingshuttleredoxmediators.AswecanexpectfromthereactionschemeshowninFigure1,photocatalyticactivityforoverallwatersplittingisstronglydependentonthephysicochemicalpropertiesofaphotocatalyst,thenatureoftheactivesites(so-calledcocatalyst),andthereactionconditions.2,4Inthepastdecade(inparticular,from2000to2005),anumberofmaterialshavebeenreportedasvisible-light-drivenphotocatalystscapableofproducingbothhydrogenandoxygenundervisiblelight.1-4,17Somehavesuccessfullyachievedoverallwatersplittingwithoutanysacrificialreagents.Inthepast5years,signifi-cantprogresshasbeenmadeoncocatalystdevelopmentandtheelucidationofreactionmechanisms.ThisPerspec-tivehighlightssomeimportantaspectsofrecentwatersplittingresearch.NanoscaleDesignofHydrogenEvolutionSites.Althoughsomeofthephotocatalystsdevelopedtodate(e.g.,layeredcompoundsandtantalates)candecomposewaterwithoutacocatalyst,4mostrequiretheloadingofasuitablecocatalyst(suchasNiOxandRuO2)toobtainahighactivityandreason-ablereactionrates.Itisbelievedthatthecocatalystsprovidereactionsitesanddecreasetheactivationenergyforgasevolution.NoblemetalssuchasPtandRhareexcellentpromotersforH2evolutionbutcanalsocatalyzeabackwardreaction,formingwaterfromH2andO2,limitingtheirusefulnessascocatalystsforphotocatalyticoverallwaterLoadingnanoparticulatecocata-lystsontoaphotocatalystsignificantlyimprovesthewater-splittingrate.ReceivedDate:June9,2010AcceptedDate:August20,2010r2010AmericanChemicalSociety2656DOI:10.1021/jz1007966|J.Phys.Chem.Lett.2010,1,2655–2661PERSPECTIVEpubs.acs.org/JPCLsplitting.Toavoidthebackwardreaction,transition-metaloxidesthatdonotexhibitactivityforwaterformationfromH2andO2areusuallyappliedascocatalystsforoverallwatersplitting.However,nococatalystmoreeffectivethanNiOxorRuO2wasfound,untilrecently.OurreportonananoparticulaterhodiumandchromiummixedoxidewithasolidsolutionofGaNandZnO(representedGaN:ZnOhereafter),publishedin2006,18high-lightedtheimportantroleofcocatalystsinphotocatalyticwatersplittingandstimulatedadditionalresearchonsuchcocatalysts.19-21Aftertheinitialstudy,weattemptedtodevelopanewcocatalystusingnanotechnology.22-25Core/shell-structurednanoparticles(withanoblemetalormetaloxidecoreandachromia(Cr2O3)shell)werepresentedasanewtypeofcocatalystforphotocatalyticoverallwatersplitting.22AsschematicallyillustratedinFigure2A,core/shellnanoparticlescanbe