Essays

A review on the visible light active titanium dioxide photocatalysts for environmental applications

Description
Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for
Categories
Published
of 19
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  AppliedCatalysisB:Environmental 125 (2012) 331–349 ContentslistsavailableatSciVerseScienceDirect Applied   CatalysisB:   Environmental  journalhomepage:www.elsevier.com/locate/apcatb Review A   review   on   the   visible   light   active   titanium   dioxide   photocatalysts   forenvironmental   applications  Miguel   Pelaez a ,   Nicholas   T.   Nolan b ,   Suresh   C.   Pillai b ,   Michael   K.   Seery c ,   Polycarpos   Falaras d ,Athanassios   G.   Kontos d ,   Patrick   S.M.   Dunlop e ,    Jeremy   W.J.   Hamilton e ,    J.Anthony   Byrne e ,Kevin   O’Shea f  , Mohammad   H.   Entezari g , Dionysios   D.   Dionysiou a , ∗ a EnvironmentalEngineeringandScienceProgram,SchoolofEnergy,Environmental,Biological,andMedicalEngineering,UniversityofCincinnati,Cincinnati,OH45221-0012,USA b CenterforResearchinEngineeringSurfaceTechnology(CREST),FOCASInstitute,DublinInstituteofTechnology,KevinSt,Dublin8,Ireland c SchoolofChemicalandPharmaceuticalSciences,DublinInstituteofTechnology,KevinSt.,Dublin8,Ireland d InstituteofPhysicalChemistry,NCSRDemokritos,15310AghiaParaskevi,Attiki,Greece e NanotechnologyandIntegratedBioEngineeringCentre,SchoolofEngineering,UniversityofUlster,NorthernIreland,BT370QB,UnitedKingdom f  DepartmentofChemistryandBiochemistry,FloridaInternationalUniversity,UniversityPark,Miami,FL3319,USA g DepartmentofChemistry,FerdowsiUniversityofMashhad,Mashhad91775,Iran a   r   t   i   c   l   e   i   n   f   o  Articlehistory: Received28March2012Receivedinrevisedform21May   2012Accepted25May   2012 Available online xxx Keywords: TiO 2 VisibleSolarWaterTreatmentAirpurificationDisinfectionNon-metaldopingAnataseRutileN–TiO 2 MetaldopingEnvironmentalapplicationReactiveoxygenspeciesPhotocatalysisPhotocatalyticEDCsCyanotoxinsEmergingpollutants a   b   s   t   r   a   c   t Fujishima   and   Honda   (1972)   demonstrated   the   potential   of    titanium   dioxide   (TiO 2 )   semiconductor   mate-rials   to   split   water   into   hydrogen   and   oxygen   in   aphoto-electrochemical   cell.   Their   work   triggered   thedevelopment   of    semiconductor   photocatalysis   for   a   wide   range   of    environmental   and   energy   applica-tions.One   of    the   most   significant   scientific   and   commercial   advances   to   date   has   been   the   developmentof    visible   light   active   (VLA)   TiO 2  photocatalytic   materials.   Inthis   review,   abackground   onTiO 2  struc-ture,   properties   and   electronic   properties   inphotocatalysis   ispresented.   The   development   of    differentstrategies   to   modify   TiO 2  for   the   utilization   of    visible   light,   including   non   metal   and/or   metal   doping,dye   sensitization   and   coupling   semiconductors   are   discussed.   Emphasis   is   given   to   the   srcin   of    visiblelight   absorption   and   the   reactive   oxygen   species   generated,   deduced   byphysicochemical   and   photo-electrochemical   methods.   Various   applications   of    VLA   TiO 2 ,   interms   of    environmental   remediation   andinparticular   water   treatment,   disinfection   andair   purification,   areillustrated.   Comprehensive   studiesonthe   photocatalytic   degradation   of    contaminants   of    emerging   concern,   including   endocrine   disruptingcompounds,   pharmaceuticals,   pesticides,   cyanotoxins   and   volatile   organic   compounds,   with   VLA   TiO 2 are   discussed   and   compared   to   conventional   UV-activated   TiO 2  nanomaterials.   Recent   advances   in   bac-terialdisinfection   using   VLA   TiO 2  are   also   reviewed.   Issues   concerning   test   protocols   for   real   visible   lightactivity   and   photocatalytic   efficiencies   with   different   light   sources   have   been   highlighted. © 2012 Elsevier B.V. All rights reserved. Contents 1.Titanium   dioxide   –   an   introduction   ...   ...   .   ...   .   ...   .   ...   ....   ...   .   ...   ....   .....   ....   ..   ...   ...   .   ....   .   ...   .   ..   ...   ..   ...   ....   ...   .   ...   .   .   ..   .   .   .   .   .   .   ...   .......   ...   ...   ...... 332 1.1.   TiO 2  structures   and   properties   ...   .   ...   ....   ...   .   ...   ...   .   ...   ...   .   ...   .   .......   ...   ...   ....   ...   ....   ...   ...   .   ...   .   ...   ...   ..   .   .   ...   .   ....   .   ....   .   .   .   ..   .   .   ...   ......   .   ... 332 1.2.Electronic   processes   in   TiO 2  photocatalysis.   .   ....   ...   .   .......   ...   .   ...   ...   ...   .   ...   ....   ...   ....   ...   .   ...   .   ...   ...   ...   .   ...   .   ...   .   ..   .   .   ...   .   .   .....   ....   .   .   .   .   .   .   .   .   . 332 1.3.   Recombination   .....   ...   ...   .   ...   .   ...   ....   ..   .   .   ..   .   .   ..   .   .   ..   .   .   ..   .   ..   .   .   ..   .   .   ...   ...   ..   .   ..   .   .   ..   .   ...   ..   .   ...   .   ...   .   .   ...   .   ...   ....   ..   .   ...   .   ....   .   .....   .   ..   ..   .   ..   .   .   .   .   .   .   .   .   .   . 333 1.4.   Strategies   for   improving   TiO 2  photoactivity   ..   .   ..   .   ...   .   ..   .   .   .   ..   .   .   .   ...   .   ..   .   ...   ......   .   ...   .   ...   .   .   ..   ..   .   ....   ...   .   .   ..   .   .   ..   .   ..   .   ..   .   .   .   .   .   .   .   .......   ..   .   .   ...   ..   .   . 334  Allauthorshavecontributedequallytothisreview. ∗ Correspondingauthor.Tel.:+15135560724;fax:+15135562599. E-mailaddress: dionysios.d.dionysiou@uc.edu(D.D.Dionysiou).0926-3373/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcatb.2012.05.036  M.Pelaezetal./AppliedCatalysisB:Environmental 125 (2012) 331–349 333 Fig.1. Crystallinestructuresoftitaniumdioxide(a)anatase,(b)rutile,(c)brookite(ReprintedwithpermissionfromKatsuhiroNomura(nomura-k@aist.go.jp;http://staff.aist.go.jp/nomura-k/english/itscgallary-e.htm)   Copyright(2002)). O 2 + e CB − → O 2 •− (1.4) • OH + pollutant →→→ H 2 O + CO 2  (1.5)O 2 •− + H + →  • OOH(1.6) • OOH +  • OOH → H 2 O 2 + O 2  (1.7)O 2 •− + pollutant →→→ CO 2 + H 2 O(1.8) • OOH + pollutant → CO 2 + H 2 O(1.9)Electronsintheconductionbandcanberapidlytrappedbymolecularoxygenadsorbedonthetitaniaparticle,whichisreducedtoformsuperoxideradicalanion(O 2 •− )(Eq.(1.4))thatmay   fur-therreactwithH + togeneratehydroperoxylradical( • OOH)(Eq.(1.6))andfurtherelectrochemicalreductionyieldsH 2 O 2  (Eq.(1.7))[28,29].Thesereactiveoxygenspeciesmay   alsocontributetotheoxidativepathwayssuchasthedegradationofapollutant(Eqs.(1.8)and(1.9))[25,27,28]. 1.3.Recombination Recombinationofphotogeneratedchargecarriersisthemajorlimitationinsemiconductorphotocatalysisasitreducestheover-allquantumefficiency[29].Whenrecombinationoccurs,the  Table1 PhysicalandstructuralpropertiesofanataseandrutileTiO 2 .PropertyAnataseRutileMolecularweight(g/mol)79.8879.88Meltingpoint( ◦ C)18251825Boilingpoint( ◦ C)2500–30002500–3000Lightabsorption(nm)<390<415Mohr’sHardness5.56.5–7.0Refractiveindex2.552.75Dielectricconstant31114Crystalstructure TetragonalTetragonalLatticeconstants( ˚A)  a =3.78 a =4.59 c    =9.52 c  =2.96Density(g/cm 3 )3.794.13Ti   Obondlength( ˚A) 1.94(4)1.95(4)1.97(2) 1.98(2) excitedelectronrevertstothevalencebandwithoutreactingwithadsorbedspecies(Eq.(1.2))[30]non-radiativelyorradiatively,dis- sipatingtheenergyaslightorheat[6,31].Recombinationmay   occureitheronthesurfaceorinthebulkandisingeneralfacilitatedbyimpurities,defects,orallfactorswhichintroducebulkorsurfaceimperfectionsintothecrystal[29,32].Serponeetal.foundthattrappingexcitedelectronsasTi 3+ speciesoccurredonatimescaleof  ∼ 30psandthatabout90%ormoreofthephotogeneratedelectronsrecombinewithin10ns Fig.2. SchematicofTiO 2  photocatalyticmechanism.  Table2 Standardelectrochemicalreductionpotentialsofcommonoxidants.OxidantHalf-cellreactionOxidationpotential(V) • OH(Hydroxylradical)  • OH+H + +e − → H 2 O2.80O 3  (Ozone)O 3  (g)+2H + +2e − → O 2  (g)+H 2 O2.07H 2 O 2  (Hydrogenperoxide)H 2 O 2  +2H + +2e − → 2H 2 O1.77HClO(Hypochlorousacid)Cl 2  (g)+2e − → 2Cl − 1.49Cl − (Chlorine)2HClO+2H + +2e − → Cl 2  +2H 2 O1.36  334  M.Pelaezetal./AppliedCatalysisB:Environmental 125 (2012) 331–349 [33].Dopingwithions[34–36],   heterojunctioncoupling[37–39]andnanosizedcrystals[40,41]haveallbeenreportedtopromote separationoftheelectron–holepair,reducingrecombinationandthereforeimprovethephotocatalyticactivity.Forexample,theTiO 2  crystallitesofEvonik(Degussa)P25containacombinationofanatase( ∼ 80%)andrutile( ∼ 20%).Theconductionbandpoten-tialofrutileismorepositivethanthatofanatasewhichmeansthattherutilephasemay   actasanelectronsinkforphotogen-eratedelectronsfromtheconductionbandoftheanatasephase.Manyresearchersattributethehighphotocatalyticactivityofthispreparationtotheintimatecontactbetweentwophases,enhanc-ingseparationofphotogeneratedelectronsandholes,andresultinginreducedrecombination[42]. 1.4.StrategiesforimprovingTiO  2  photoactivity Variousstrategieshavebeenadoptedforimprovingthepho-tocatalyticefficiencyofTiO 2 .Theycanbesummarizedaseithermorphologicalmodifications,suchasincreasingsurfaceareaandporosity,oraschemicalmodifications,byincorporationofaddi-tionalcomponentsintheTiO 2  structure.Althoughvisiblelightactive(VLA)TiO 2  photocatalystsrequirechemicalmodifications,whichwillbereviewedinthenextsection,theiroverallefficiencieshavebeensignificantlyenhancedbycontrollingthesemiconductormorphology.ThemostcommonlyusedTiO 2  morphologyisthatofmonodis-persednanoparticleswhereinthediameteriscontrolledtogivebenefitsfromthesmallcrystallitesize(highsurfacearea,reducedbulkrecombination)withoutthedetrimentaleffectsassociatedwithverysmallparticles(surfacerecombination,lowcrystallinity)[43].Onedimensional(1D)titaniananostructures(nanotubes,nanorods,nanowires,nanobelts,nanoneedles)havebeenalsoformedbyhydrothermalsynthesisbuthighemphasiswasgivenintitaniaself-assemblednanotubularfilmsgrownbyelectrochemicalanodizationontitaniummetalfoils.Advantagesofsuchstruc-turesistheirtailoredmorphology,controlledporosity,vectorialchargetransfer[44,45]andlowrecombinationatgrainboundaries thatresultinenhancedperformanceinphotoinducedapplications,mainlyinphotocatalysis[44,46,47].   AninterestinguseofTiO 2 nanotubesinphotocatalyticapplicationsisthegrowthoffreestand-ingflow-throughmembranes[44]. 2.Developmentofvisiblelightactive(VLA)titaniaphotocatalysts  2.1.Nonmetaldoping  2.1.1.Nitrogendoping  Ultravioletlightmakesuponly4–5%ofthesolarspectrum,whereasapproximately40%ofsolarphotonsareinthevisibleregion.AmajordrawbackofpureTiO 2  isthelargebandgapmeaningitcanonlybeactivateduponirradiationwithphotonsoflightintheUVdomain(  ≤ 387nmforanatase),limitingthepracticalefficiencyforsolarapplications[48–50].   Therefore,inordertoenhancethesolarefficiencyofTiO 2  undersolarirradiation,itisnecessarytomodifythenanomaterialtofacilitatevisiblelightabsorption.Non-metaldopingofTiO 2  hasshowngreatpromiseinachievingVLAphotocatalysis,withnitrogenbeingthemostpromisingdopant[51,52].NitrogencanbeeasilyintroducedintheTiO 2 structure,duetoitscomparableatomicsizewithoxygen,smallionizationenergyandhighstability.Itwasin1986whenSatodiscoveredthatadditionofNH 4 OHinatitaniasol,followedbycalcinationoftheprecipi-tatedpowder,resultedinamaterialthatexhibitedavisiblelightresponse[53,54].Lateron,Asahiandco-workersexploredforfirst timethevisiblelightactivityofN-dopedTiO 2  producedbysputterdepositionofTiO 2  underanN 2 /Aratmosphere,followedbyanneal-ingunderN 2  [55].Sincethen,therehavebeenmanyreportsdealingwithnitrogendopingofTiO 2 .Significanteffortsarebeingdevotedtoinvestigatingthestructural,electronicandopticalpropertiesofN-dopedTiO 2 ,understandingtheunderlyingmechanismsandimprovingthephotocatalyticandself-cleaningefficiencyundervisibleandsolarlight[56–58].   Comprehensivereviewshavebeenpublishedwhichsummarizerepresentativeresultsofthesestudies[59,60].   ModelpollutantsthathavebeenreportedtobeeffectivelydegradedbyVLAphotocatalystincludephenols,methyleneblue,methylorange(althoughdyeshavestrongabsorptioninthevisiblerange)andrhodamineB,aswellasseveralgaseouspollutants(e.g.,volatileorganiccompounds,nitrogenoxides).FortheefficientincorporationofnitrogenintoTiO 2  eitherinthebulkorasasurfacedopant,bothdryandwetpreparationmethodshavebeenadopted.Physicaltechniquessuchassput-tering[61–65]andionimplantation[66,67],relyonthedirect treatmentofTiO 2  withenergeticnitrogenions.Gasphasereac-tionmethods[68–70],   atomiclayerdeposition[71]andpulsedlaser deposition[72]havebeensuccessfullyappliedtoprepareN–TiO 2 ,aswell.However,themostversatiletechniqueforthesynthe-sisofN–TiO 2  nanoparticlesisthesol–gelmethod,whichrequiresrelativelysimpleequipmentandpermitsfinecontrolofthemate-rial’snanostructure,morphologyandporosity.SimultaneousTiO 2 growthandNdopingisachievedbyhydrolysisoftitaniumalkox-ideprecursorsinthepresenceofnitrogensources.Typicaltitaniumsalts(titaniumtetrachloride)andalkoxideprecursors(includ-ingtitaniumtetra-isopropoxide,tetrabutylorthotitanate)havebeenused.Nitrogencontainingprecursorsusedincludealiphaticamines,nitrates,ammoniumsalts,ammoniaandurea[73–75].Thesynthesisrootinvolvesseveralsteps;however,themaincharacteristicisthatprecursorhydrolysisisusuallyperformedatroomtemperature.Theprecipitateisthendriedtoremovesolvents,pulverizedandcalcinedattemperaturesfrom200to600 ◦ C.OnepromisingwaytoincreasethenitrogencontentintheTiO 2  latticeistocombinethetitaniumprecursorswithanitrogen-containingligand,suchasTi 4+ -bipyridineorTi 4+ -aminecomplexes[76,77].Analternativesoftchemicalrouteisbasedontheadditionofureaduringthecondensationofanalkoxideacidifiedsolution,leadingtointerstitialsurfacedopingandshiftoftheabsorptionedgewellintothevisiblespectralrange(from3.2to2.3eV)[78].Aninnovativesol–gelrelatedtechniqueforthepreparationof efficientvisible-lightactivenanostructuredTiO 2  isthetemplat-ingsol–gelmethod,utilizingtitaniumprecursorscombinedwithnitrogen-containingsurfactants.Specifically,successfulsynthesisofvisiblelightactivatedN–TiO 2  hasbeenachievedbyasimplesol–gelmethodemployingdodecylammoniumchloride(DDAC)assurfactant[79].TheDDACsurfactantactssimultaneouslyasapore templatingmaterialtotailor-designthestructuralpropertiesof TiO 2  (seeFig.3)aswellasanitrogendopanttoinducevisible-light photoactivityanduniquereactivityandfunctionalityforenviron-mentalapplications[80,81].InadifferentapproachN–TiO 2 ,wassynthesizedviatwosucces-sivesteps:synthesisofTiO 2  andthennitrogendopingusingvariousnitrogen-containingchemicals(e.g.urea,ethylamine,NH 3  orgaseousnitrogen)athightemperatures[52,82–84]orinductively coupledplasmacontainingawiderangeofnitrogenprecursors[85].Inthatcase,thenitrogenatomspredominantlyresidedontheTiO 2  surface.Theoriginofthevisible-lightphotocatalyticactivityinthesemethodsmay   arisefromcondensedaromatics-triazinecompoundscontainingmelemandmelonunits[73].AlthoughmostreportsonN–TiO 2  concerntheanatasepolymor-phicphase,visiblelightactiveN–TiO 2  withanatase-rutilemixedphase(Fig.4)hasalsobeenpreparedbytuningtheparametersof   M.Pelaezetal./AppliedCatalysisB:Environmental 125 (2012) 331–349 335 Fig.3. Templatingsol–gelmethodutilizingnitrogencontainingsurfactantsasbothnitrogensourceandporetemplatematerial.(ReprintedwithpermissionfromH.Choi,M.   G.Antoniou,M.   Pelaez,A.A.delaCruz,J.A.Shoemaker,D.D.Dionysiou, Environ.Sci.Technol. 41(2007)7530–7535.Copyright(2007)AmericanChemicalSociety). thesol–gelsynthesis.Suchheterojunctionphotocatalystsseemtoeffectivelytransferphoto-excitedelectronsfromtheconductionbandofanatasetothatofrutile,favoringelectron–holesepara-tionandenhancingthevisiblelightphotocatalyticactivity.[86,87].Etacherietal.havesuccessfullydevelopednitrogendopedanatase-rutileheterojunctionswhichwerefoundtobeninetimesmorephotocatalyticallyactiveatwavelengthshigherthan450nm(bluefilter)incomparisonwithEvonikP25.Mostoftheabovemethodshavealsobeensuccessfullyappliedforthedopingof1Dtitaniananostructureswithnitrogen.Inthisway,N-dopedanatasetitaniananobeltswerepreparedviahydrothermalprocessingandsubsequentheattreatmentinNH 3 [88].Similarpost-treatmentwasemployedfordopinganodizedtitaniananotubes[89],whilehighenergyionimplantationwas foundtobemoreefficientinintroducingNatomsintheTiO 2 lattice[90].Nitrogenlocalizedstateshavealsobeenintroduced intohighlyorderedTiO 2  nanotubesvianitrogenplasma[91].Visiblelight-activeN–TiO 2  nanoarrayfilmshavealsobeenpre-paredonsacrificialanodizedaluminaliquidphasedepositionwithureamixedwith(NH 4 ) 2 TiF 6  aqueoussolution[92].Recently, surfaceN-dopingontitaniananowires,theirlateraldimensionsreachingtheatomicscale,wasachievedbytheintroductionofaminesduringthecondensationstageofthetitaniaprecur-sor[93].OtherapproachesforpreparingdopedTiO 2  nanotubesincludeemploymentofnitrogensourcesintheelectrolytesolu-tionsofelectrochemicalanodization[94]orintheinitialsolution ofhydrothermalgrowth[95,96].Manyresults,uptonow,describenitrogendopingassubstitu-tionalelementontheoxygenlatticesitesoratinterstitiallatticesites.Thetwo   sitescanbeinprinciplediscriminatedbyX-raypho-toelectronspectroscopy(XPS)relyingonthedistinctN1sbindingenergiesat396and400eV,respectively[51,69,97–99].XPSpeak assignmentforN-dopedvisiblelightactivatedtitaniaisstillunderdebate[57,100].ManyresearchersreportedthatN1speaksaround 397eVarerepresentativeofsubstitutionalnitrogen[57,100,101]whilepeaksatbindingenergies>400eVareassignedtoNO(401eV)orNO 2  (406eV)indicatinginterstitialnitrogen[101].DiValentin etal.[57]employeddensityfunctionaltheory(DFT)todemon-strateinterstitialnitrogenas  characterNOwithinanataseTiO 2 .ItwasalsofoundthatthereisnosignificantshiftintheconductionorvalencebandsoftheTiO 2 .Theenergybondingstatesassoci-atedbelowthevalencebandandanti-bondingstatespresentabovethevalenceband.Theanti-bonding  *NOorbitalsbetweentheTiO 2  valencebandandconductionbandisbelievedtofacilitatevisiblelightabsorptionbyactingasasteppingstoneforexcitedelectronsbetweenconductionandvalencebands.Nspeciesdif-ferentfromthephotoactiveonesinNdopedTiO 2  caninterfereinspectroscopicmeasurementssincetheyhavepeaksaround400eV. Fig.4. ElectrontransfermechanisminN-dopedanataserutileheterojunction.(ReprintedwithpermissionfromV.Etacheri,M.   K.Seery,S.J.Hinder,S.C.Pillai, Chem.Mater. 22   (2010)3843–3853.Copyright(2010)AmericanChemicalSociety).
Search
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks