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Jun.2010ExpandingIndustrializationofPhotocatalystsTaketoshiMurakamiSeniorResearcherattheKanagawaAcademyofScienceDirectorofthePhotocatalysisMuseumAkiraFujishimaPresidentofTokyoUniversityofScienceChairmanoftheKanagawaAcademyofScienceandTechnologyPhotocatalysts,typifiedbytitaniumoxide,promotereactionsbyabsorbinglight,andtheyareeffectiveatdeodorizationandpreventingcontamination,inadditiontobeingantibacterialandsterilizing.Industrializationismovingforwardrapidlyinavarietyoffields.◆IntroductionThetwenty-firstcenturyisbeingcalledtheenvironmentcentury.Therehasbeenanintensificationofglobalwarmingaswellasairandwaterpollutionassociatedwithindustrialization;thisisdueinalargeparttotherapideconomicgrowthofdevelopingcountriessuchasChina,firstandforemost,andthenIndiaandBrazil.Inaddition,anewstrainofinfluenzahasspreadthroughouttheworld,becomingamajorsocialissue.Photocatalysttechnologyissaidtobeanenvironmentalcleanuptechnology.Usingthenaturalenergyofsunlight,itcontributestothecleaningofairandwater,anditisattractingattentionasanapproachtoenvironmentalcleanupbyfightingbacteriaandprovidingsterilizationtocombatinfluenzaandotheroutbreaksofillness.Herewewilldescribetheprinciplesofphotocatalysttechnology,itseffectsandtheprogressthathasbeenmadeinindustrializingphotocatalystproducts.Wewillalsocommentonthedirectioninwhichthingsarelikelytomoveinthefuture.●Photocatalysts1.DefinitionofaPhotocatalystPhotocatalystsaredefinedassubstancesthatpromotereactionsbyabsorbinglightwithoutbeingalteredthemselves,beforeorafterthereaction.Wewilltakealookatthephotocatalyticfunctionsoftitaniumoxide,whichistypicalofthemainstreamphotocatalystscurrentlyinuse.Whentitaniumoxideisexposedtoalightsourcesuchassunlightorfluorescentlight,itexhibitsbothoxidativedecompositionandsuperhydrophilicproperties,whichcanbeeffectivelyusedfordeodorization,fightingbacteria,preventingcontamination,antifogging,andotherapplications.2.FunctionsandPrinciplesofPhotocatalystsOxidativedecompositionmeansthatiftitaniumoxideisexposedtolight,organicsubstancesonitssurfacewillbebrokendownbyoxidation,eventuallyformingcarbondioxideandwater.Thishasapplicationsinfieldssuchasdeodorization,eliminationofvolatileorganiccompounds(VOCs),eliminationofsoiling,antibacterialactionandsterilization.Titaniumoxideissaidtobeanopticalsemiconductor,anditchangesfromaninsulatortoaconductorbyabsorbingtheenergyoflight.Thisoccursbecauseelectronsareexcitedfromthevalencebandtotheconductionband,causingtheformationofholesinthevalenceband.Theexcitedelectronsreactwithoxygenintheatmosphereformingsuperoxideanions(•O2-),andtheholesreactwithmoistureintheatmosphereproducinghydroxylradicals(•OH).Theseactiveoxygenspeciesareextremelyreactive,andoxidizeanddecomposeorganicsubstances(Figure1).Figure1PrinciplesofoxidativedecompositionofphotocatalystsWhentitaniumoxideisexposedtolight,itssurfaceexhibitssuperhydrophilicity,withwatercontactanglesof5°orless.Althoughtheunderlyingmechanismisagaintheformationofelectron-holepairsunderlightirradiation,thesuperhydrophilicityisdirectlytheresultoftheformationofoxygenvacanciesbytheinteractionofholeswithoxygenatomsinthecrystallinelattice.Suchdefectsformahydrophilicdomain,andsincewateriseasilyadsorbedthere,generalsuperhydrophilicityresults.Figure2Self-cleaningeffectThesesuperhydrophilicpropertiesgiverisetoanantifoggingeffect.Inaddition,whensurfacescoatedwithatitaniumoxidephotocatalystareexposedtocontaminantssuchasoil,thecombinationofsuperhydrophilicityandoxidativedecompositionleadstothecontaminationbeingwashedawaybyrainwater,asshowninFigure2.Thus,theoriginalcleanstateofthesurfacecanbemaintained,andthisisreferredtoasaself-cleaningeffect.●HistoricalbackgroundofphotocatalystsThediscoveryofthesephotocatalyticpropertiesoftitaniumoxidebeganin1967whenoneoftheauthors,AkiraFujishima,thenagraduatestudentatTokyoUniversity,noticedthedecompositionofwaterintohydrogenandoxygeninthepresenceoflightirradiation.Takingthediscoverersnames,thiseffectwascalledtheHonda-Fujishimaeffect,anditreceivedworldwideattentionwithanannouncementinNaturein1972.Thisdiscoveryledtoactiveresearchanddevelopmentrelatedtophotocatalysts.Thiswasaroundthetimeoftheinternationaloilcrisis,andtherewasagreatdealofinterestinhydrogen-basedsolarenergyproductionusingphotocatalystsasanalternativetopetroleum.However,thiswasfoundtobefarfrompracticalbecausetheefficiencywassolow.Therefore,Fujishimaandotherschangeddirectionsandfocusedontheoxidativedecompositioncapabilitiesoftitaniumoxidephotocatalysts,concentratingtheirmainresearcheffortsintothedevelopmentofapplicationsbasedonthesecapabilities.Onexamplewasdeodorizationandantibacterialproductsforbathrooms,andin1990jointresearchwasinitiatedwithTOTOLtd.regardingtitaniumoxidecoatings.Itwasfoundthattilescoatedwiththephotocatalystdidnotsoileasily,andtheantibacterialanddeodorizingeffectswereconfirmed.In1994,aphotocatalystantibacterial,stainresistanttilewasdevelopedandmadepractical.Inaddition,duringthejointdevelopmentperiod,thephoto-excitedsuperhydrophilicityofphotocatalystswasdiscovered.Severalcompaniestriedtodeveloppr