英文-无机纳米材料光解水ppt课件

FrankE.OsterlohUniversityofCalifornia,DavisDepartmentofChemistry,Inorganicnanostructuresforphotoelectrochemicalandphotocatalyticwatersplitting,Contents,Conclusionandoutlook,4,ThesolarenergyreceivedontheEarthssurfacemeetscurrentandfuturehumanenergydemand.,carbonfreeenergytechnology,1Introduction,photovoltaiccellsThesolarenergycanbeconvertedintoelectricitywithupto43.5%efficiency.Thesecellsaremoreexpensivethanvirtuallyfreecoal.Electricityisdifficulttostoreandtodistributeoverlongdistances.,1Introduction,Convertingthephotochemicalenergydirectlyintofuel:H2O1/2O2(g)+H2(g);G=+237kJ/molSeveralwaystofacilitatereaction:PhotovoltaiccellpluselectrolyzerThermochemicalmethods,etc.Themosteconomicalones:Photoelectrosyntheticcells(PECs)Suspendedphotocatalysts,ThebestperformingphotoelectrochemicaldevicesknowntodayareTandemcells.acombinationoftwoormoresemiconductorsconnectedinseriesSemiconductorswithsmallerbandgapscanbeused,canabsorbagreaterfractionofthesolarspectrum,Photoelectrochemicaldevices,Photoelectrochemicaldevices,Efficienciesbetween12.4%andover18%havebeenachieved,i.e.abouthalfofthetheoreticalefficiencylimitforthesedevices.Buttheperformancecannotbesustained.Thephotoelectrodematerialsarenotstableunderoperatingconditionsandundergophotocorrosion.SimilarstabilityproblemsexistformostothervisiblelightabsorbingIIVI,IIIV,andgroup14elementsemiconductors.,Photoelectrochemicaldevices,threemainstrategies,1,Coatconventionalphotovoltaiccellswithcocatalystsforwatersplittingorwithprotectinglayerstoinhibitphotocorrosion,2,Developmentofnewmetaloxidematerialsthatcombinesuitablepropertiesforphotoelectrochemicalwatersplitting,3,Exploitscalinglawsandspecificeffectsatthenanoscaletoenhancetheefficiencyofexistingsemiconductorsandmetaloxides,Briefhistoryofnanoscalephotoelectrochemistryandphotocatalysis,Modernnanoscience:onlyabout40yearsold,beganin1974withDinglesdiscoveryofquantumsizeeffectsinthinfilmsPhotoelectrochemistry:startedin1955withBrattainselectrochemicalstudiesongermaniumelectrolytejunctionsWaterphotoelectrolysisatilluminatedTiO2electrodes:in1971byFujishimaandHondasuspendedsemiconductorparticles:Barddemonstratedthephotocatalyticeffectsphotochemicaldiodes:ArthurNozikformulatedtheconcept,PhotoelectrochemicalDiode,Theonlyfourknownnanoscalephotocatalystsforoverallwatersplitting,Nanostructuredmaterials,Electrodefunctions:waterreductionandoxidation,lightabsorption,chargeseparation,chargetransportAdjustingthenanoscalemorphologyofthephotoelectrodecanimprovesomeoftheseelectrodefunctions,butatthesameitdiminishesothers.Therefore,asystematicoptimizationofphotoelectrodesrequiresawholesystemapproachthatconsidersbothpositiveandnegativeeffectsofnanoscaling.,ShortenedcarriercollectionpathwaysImprovedlightdistributionQuantumsizeconfinementPotentialdeterminingions(PDI)Surfacearea-enhancedchargetransferMultipleexcitongeneration,Advantages,1Shortenedcarriercollectionpathways,ThefreediffusionlengthLdependsonthecarrierdiffusionconstantDandthecarrierlifetimetandadimensionalityfactor.L2=qDtForintrinsicsemiconductors,usuallyLeLh.Foroptimumcollectionofbothcarriertypesatthebackcontact,thesemiconductorfilmthicknessdhastobeinthesamerangeasLeandLh.,d:filmorparticlethickness;Le:electrondiffusionlength;Lh:holediffusionlength.Suspendedparticlesrequirebothelectronsandholestobecollectedatthescelectrolyteinterface,2Improvedlightdistribution,Theabilitytoabsorblight:LambertBeerlawThewavelength-dependentabsorptioncoefficientThelightpenetrationdepth-1referstothedistanceafterwhichthelightintensityisreducedto1/eoftheoriginalvalue.Surface-structuringonthemicro-ornanoscalecanincreasethedegreeofhorizontallightdistributionvialightscattering.Lightscatteringismaximalinparticlesuspensions.,Toensure90%absorptionoftheincidentlight,thefilmthicknessmustbe2.3timesthevalueof-1,3Quantumsizeconfinement,Thequantumsizeeffectdependsalsoonthematerialandthenanocrystalshape.Withincreasingthebandgap,theconductionbandedgeshiftstomorereducingandthevalencebandtomoreoxidizingpotentials.FromMarcusGerischertheoryitisexpectedthatthisincreaseinthermodynamicdrivingforceincreasestheratesforinterfacialchargetransferandwaterelectrolysis.Indeed,thiscanbeexperimentallyobservedforchargetransferacrosssolidsolidinterfaces.,Quantumsizeeffectinnanocrystals,4Potentialdeterminingions(PDI),Theeffectofpotentialdeterminingionsontheinterfacialenergeticsiswellknown.Duetothesmallthicknessofnanostructures,externalelectricfieldscanreachintothenanomaterialinteriorandmodifythelocalenergeticstructure.Thus,thebandedgepotentialsofnanomaterials,andresultingfunctions,erfacialchargetransfercanbecontrolledwithPDIs.,EffectofPDIsonnanocrystalenergetics,5Surfacearea-enhancedchargetransfer,Thelargerspecificsurfaceareaofnanomaterialspromoteschargetransferacrossthematerialinterfaces(solidsolidandsolidliquid),allowingwaterredoxreactionstooccuratrelativelylowcurrentdensitiesand,correspondingly,lowoverpotentials.Increasesofsurfaceareareducetheneedforhighlyactive,andoftenexpensivecocatalysts,basedonIr,RhorPt.,6Multipleexcitongeneration,Thealteredelectronicstructureofstronglysize-confinednanocrystalsgivesrisetomultipleexcitongeneration(MEG),i.e.theformationofseveral(n)electronholepairsafterabsorptionofonephotonwithanenergyntimesofthebandgapofthedot.,Theeffecthasnotyetbeenappliedtowaterphotoelectrolysis.FutureMEGenhancedwatersplittingdeviceswilllikelybeTandemormulti-junctiondevices,becausetheindividualquantumdotscannotproduceasufficientpotentialforoverallwatersplitting.Thisisbecauseforefficientsolarenergyconversion,thebandgapsoftherelevantdotsneedtobeafractionoftheenergyofvisiblelightphotons(E=1.553.1eV).,Whilenanostructuringcanimprovethelightharvesting,chargetransport,kinetic,andenergeticparametersofphotoelectrosyntheticcells,italsohassignificantdisadvantagesthatcanleadtoreducedpowerconversionefficiencyandlowerdurabilityofdevices.,IncreasedsurfacerecombinationReducedspacechargelayerthicknessLowerabsorbedphotonfluxSlowinterparticlechargetransport,Disadvantages,1Increasedsurfacerecombination,Photogeneratedchargecarriersrecombinethroughradiativeornonradiativeprocessesinthebulkphaseofthesemiconductor,inthedepletionregion,oratdefectsatthesurface.,Innanostructuredsemiconductors,surfaceandinterfacialrecombinationratesareenhancedduetotheirlargerspecificinterfacialareas.Itmightbepossibletocontroltheselossesbyreducingsurfacedefectsthroughsurfacetreatments.Forexample,formacroscopicsiliconfilms,treatmentwithHFcansuppresssurfacerecombination.Analogoustreatmentswillhavetobedevelopedfornanomaterials.,2Reducedspacechargelayerthickness,Innanomaterials,carrierseparationismoredifficultthaninthebulk,becauseataveragedopingconcentrations(n0=1017cm-3)spacechargelayersarenoteffectiveonthenanoscale.Forsphericalnanoparticles,thespacechargelayerthicknesscannotexceedtheradiusoftheparticle,whichrestrainsthepossiblebarrierheightattheinterface.,Spacechargelayersinlargeandsmallparticles,Theabsenceofaninternalmechanismforchargeseparationalsoincreasese/hrecombination.Thus,additionalenergyisnecessarytoseparatethecharges.Thisisanalogoustoorganicphotovoltaiccells,whereahigherexcitonbindingenergyresultsfromthelowerdielectricconstantoforganicpolymers.,3Lowerabsorbedphotonflux,Asecondconsequenceoftheincreasedjunctionareainnanostructuredphotoelectrochemicalcellsislowerabsorbedflux(iflightscatteringisneglected).,AccordingtotheShockleydiodeequation:J0,reversesaturationcurrentofdiode;Jphot,photocurrent=photonfluxtimesxirradiatedarea;VOC,opencircuitvoltage.SinceJphotJ0,thevoltagedecreasesby0.059VforeverydecadicdecreaseofJphot(i.e.decadicincreaseofsurfaceroughness).Thisreducesthethermodynamicdrivingforceavailableforwaterelectrolysis.,4Slowinterparticlechargetransport,Innanocrystallinefilms,chargecarriersmovebydiffusioninsteadofdrift.Asaresult,chargetransportismuchslowerthaninthebulk,increasingthechancesforrecombinationandbackreactions.Theproblemofchargetransportovermacroscopicdistancecanbeentirelyavoidedwithnanoscalesuspendedcatalysts.,Ifthenanoparticlesarenotfusedtogether,additionalbarriersarisefrominterparticlechargetransport,whichoccursbythermallyactivatedhoppingandbyelectrontunneling.,Equivalentcircuitdiagram,Anusefulwaytoillustratetheeffectsofnanostructuringonphotoelectrosyntheticdevices.,Equivalentcircuitdiagram,Forananostructuredphotoelectrochemicalcell,thedecreaseinRSHcorrespondstotheleakagecurrentsJetandJT.IfthiscurrentexceedsthegenerationcurrentIL,waterelectrolysiswillcometoastop.ThedecreaseofRSduetothereductioninelectronholetransportresistancescanleadtoimprovedperformanceofananostructureddevice.Asthesemiconductorparticlesapproachthenanoscale,thereductionofthespacechargelayereffectivenessdiminishestherectifyingproperties.ThisalsocorrespondstoreductionofRSH.,Nanostructuredphotoelectrodematerials,Metaloxidesclosedshellsystems:TiO2andtitanates,ZnO,Niobates,WO3,Cu2O,BiVO4,etc.Metaloxideswithpartiallyfilledd-orbitals:Fe2O3,IrO2,MnO2,VO2,etc.Nitridesandsulfides:Metalnitrides,Metalchalcogenides.Maingroupelement:SiliconNanostructuredcocatalystsandplasmonicnanostructures:Carbonnitride,nanoparticlesascocatalysts,Pt/Pd/Rh/Ru,IrO2,Co-PiandCo-oxides,metalsulfides,Nanoparticlesforplasmoniceffects.,TiO2andtitanates,TypesofNanostructuredTiO2photoanodes,TiO2andtitanates,Underequalexperimentalconditions,thephotocurrentdensityfromwateroxidationincreasesingoingfromparticlestorodsandtobranchedrods,mainlyduetoimprovedelectrontransporttotheback-contactandimprovedholetransporttothesolidliquidinterface.,ZnO,ZnOislimitedbyalargebandgapthatpreventsvisiblelightabsorption,andbychemicalinstabilityunderbothanodicandcathodicbias.ZnOnanorodarrayafterelectrodepositionofCoPicatalyst,improvesthephotocurrentonsetpotentialby0.23V.,dxsystems,Metaloxideswithpartiallyfilledd-shellstypicallyexhibitlowe/hlifetimes.Inaddition,thefirstrowtransitionmetaloxidesalsosufferfromlowe/hmobilitytheyareclassifiedasMottinsulators.Intheory,bothproblemscanbeaddressedthroughnanoscalingandsodxsystemshavebeenintensivelystudiedasphotoelectrodes.ThebestknownmetaloxidehereisFe2O3,butreportsonimprovedphotocatalyticpropertieswithnano-MnO2andnano-IrO2haveappearedaswell.,Metalnitrides,Becausetheelectronegativityofnitrogenislowerthanthatofoxygen,metalnitridesarepronetoanodicphotocorrosion.However,theyhaveconsiderablepotentialasphotocathodes.W2Nnanowirearrays:FormedbynitridationofWO3Supportanodicphotocurrentsof0.2mAcm-2Butperformancerapidlydecaysduetoformationoftheoxides.ImprovedstabilitywasfoundforthemixedphaseW2NWO3,Metalchalcogenides,Metalchalcogenides,especiallythoseofthegroup12metals:quantumconfinementeffectsBecauseofphotocorrosion,metalchalcogenidenanocrystalsaremainlyusedforvisiblelightsensitizationofmetaloxidephotoelectrochemicalcells,andforbasicsciencestudiesonphotocatalyticwaterreduction,interfacialchargetransportandquantumconfinement.,Maingroupelementsandcompounds,Silicon:suitablebandgapandhighconductionbandedgeTocompensateforthelowerminoritycarrierdiffusionlengthofreagentgradesilicon,theelectrodesurfaceareaneedstobeincreasedaccordingly.Thisalsoallowsforthepossibilityoflessexpensivewaterreductioncocatalystwithlowerperatomactivity.Toincreasee/hseparationatthesiliconelectrolyteinterface,an+pjunctionwasadded,andPtnanoparticleswerechemicallydepositedattheSitipstoreducethekineticbarrierforprotonreduction.,Fabrication,morphology,andlightscatteringpropertiesofSiradialmicrowirearrayphotocathodes.,Carbonnitride,C3N4:consistsofgraphite-likelayersinwhichsomeofthecarbonatomsarereplacedbynitrogenatoms.,TEMimagesofporous(A)andbulkC3N4(B).,Carbonnitride,Thebandgapandphotocatalyticwatersplittingpropertiescanbeadjustedbycopolymerizationwithbarbituricacid,aminoborane,sulfur,orotherheteroatomsources.Basedonsurfacephotovoltagemeasurements,andphotoelectrochemistry,g-C3N4isann-typesemiconductorthatfunctionsasaweakphotoanodematerial.Thelowphotocurrentssuggestfaste/hrecombinationandslowchargetransport.Therefore,nanostructuringcanbeusefulforincreasingthespecificsurfaceofthecompoundanditsactivity.,Nanoparticlesascocatalysts,Watersplittingcocatalysts:Towardsmoreabundantandlessexpensiveelementsthataremoresuitableforlargescaleoperation.LowertheactivationbarrierforwaterredoxreactionsAssistine/hseparation.Pt/Pd/Rh/Ru,IrO2,3dmetaloxides,Co-PiandCo-oxides,Metalsulfides,Nanoparticlesascocatalysts,Aproblemofcocatalystsemiconductorinterfacesisthattheygeneraterecombinationsitesforchargecarries,therebydeterioratingphotoelectrochemicalperformance.However,thisnegativeeffectisgenerallyovercompensatedbyimprovedwaterredoxkinetics.,Nanoparticlesforplasmoniceffects,Nanoparticles:surfaceplasmonresonancetoboostvisiblelightabsorptionofphotoelectrodes.Fromincreasedelectricalfieldstrengthneartheparticle,andduetoenhancedlightscatteringthroughoutthephotoelectrode.Surfaceplasmonresonancescanimpactlightabsorptionandphotocurrentgenerationbyphotoelectrodes,butthatinsulatingcoatingsneedtobeemployedtosuppressenhancedchargetrapping,transfer,andrecombinationatthemetalsemiconductorinterface.,Conclusionandoutlook,Thenanoscalingapproach：improvetheperformanceofmetaloxidephotoanodematerialswithlowcarriermobilityandwithshortexcitedstatelifetimes.Forelectrocatalysts：increasetheelectroactivesurfaceareaandallowformoreefficientmaterialsuse.Forselectedmetalchalcogenides：thequantumsizeeffecthasbeenusefulforcontrollinginterfacialchargetransport,andinpromotingphotocatalyticprotonreduction.,Con