Dionysioua,∗ a Environmental Engineering and Science Program, School of Energy, Environmental, Biological, and Medical Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, U
Trang 1Miguel Pelaeza, Nicholas T Nolanb, Suresh C Pillaib, Michael K Seeryc, Polycarpos Falarasd,
Athanassios G Kontosd, Patrick S.M Dunlope, Jeremy W.J Hamiltone, J.Anthony Byrnee,
Kevin O’Sheaf, Mohammad H Entezarig, Dionysios D Dionysioua,∗
a Environmental Engineering and Science Program, School of Energy, Environmental, Biological, and Medical Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA
b Center for Research in Engineering Surface Technology (CREST), FOCAS Institute, Dublin Institute of Technology, Kevin St, Dublin 8, Ireland
c School of Chemical and Pharmaceutical Sciences, Dublin Institute of Technology, Kevin St., Dublin 8, Ireland
d Institute of Physical Chemistry, NCSR Demokritos, 15310 Aghia Paraskevi, Attiki, Greece
e Nanotechnology and Integrated BioEngineering Centre, School of Engineering, University of Ulster, Northern Ireland, BT37 0QB, United Kingdom
f Department of Chemistry and Biochemistry, Florida International University, University Park, Miami, FL 3319, USA
g Department of Chemistry, Ferdowsi University of Mashhad, Mashhad 91775, Iran
a r t i c l e i n f o
Article history:
Received 28 March 2012
Received in revised form 21 May 2012
Accepted 25 May 2012
Available online 5 June 2012
Keywords:
TiO 2
Visible
Solar
Water
Treatment
Air purification
Disinfection
Non-metal doping
Anatase
Rutile
N–TiO 2
Metal doping
Environmental application
Reactive oxygen species
Photocatalysis
Photocatalytic
EDCs
Cyanotoxins
Emerging pollutants
a b s t r a c t
FujishimaandHonda(1972)demonstratedthepotentialoftitaniumdioxide(TiO2)semiconductor mate-rialstosplitwaterintohydrogenandoxygeninaphoto-electrochemicalcell.Theirworktriggeredthe developmentofsemiconductorphotocatalysisforawiderangeofenvironmentalandenergy applica-tions.Oneofthemostsignificantscientificandcommercialadvancestodatehasbeenthedevelopment
ofvisiblelightactive(VLA)TiO2photocatalyticmaterials.Inthisreview,abackgroundonTiO2 struc-ture,propertiesandelectronicpropertiesinphotocatalysisispresented.Thedevelopmentofdifferent strategiestomodifyTiO2fortheutilizationofvisiblelight,includingnonmetaland/ormetaldoping, dyesensitizationandcouplingsemiconductorsarediscussed.Emphasisisgiventotheoriginofvisible lightabsorptionandthereactiveoxygenspeciesgenerated,deducedbyphysicochemicaland photo-electrochemicalmethods.VariousapplicationsofVLATiO2,intermsofenvironmentalremediationand
inparticularwatertreatment,disinfectionandairpurification,areillustrated.Comprehensivestudies
onthephotocatalyticdegradationofcontaminantsofemergingconcern,includingendocrinedisrupting compounds,pharmaceuticals,pesticides,cyanotoxinsandvolatileorganiccompounds,withVLATiO2
arediscussedandcomparedtoconventionalUV-activatedTiO2nanomaterials.Recentadvancesin bac-terialdisinfectionusingVLATiO2arealsoreviewed.Issuesconcerningtestprotocolsforrealvisiblelight activityandphotocatalyticefficiencieswithdifferentlightsourceshavebeenhighlighted
© 2012 Elsevier B.V All rights reserved
Contents
1 Titaniumdioxide–anintroduction 332
1.1 TiO2structuresandproperties 332
1.2 ElectronicprocessesinTiO2photocatalysis 332
1.3 Recombination 333
1.4 StrategiesforimprovingTiO2photoactivity 334
夽 All authors have contributed equally to this review.
∗ Corresponding author Tel.: +1 513 556 0724; fax: +1 513 556 2599.
E-mail address: dionysios.d.dionysiou@uc.edu (D.D Dionysiou).
0926-3373/$ – see front matter © 2012 Elsevier B.V All rights reserved.
Trang 22 Developmentofvisiblelightactive(VLA)titaniaphotocatalysts 334
2.1 Nonmetaldoping 334
2.1.1 Nitrogendoping 334
2.1.2 Othernon-metaldoping(F,C,S) 336
2.1.3 Non-metalco-doping 336
2.1.4 OxygenrichTiO2modification 336
2.2 Metaldeposition 336
2.2.1 Noblemetalandtransitionmetaldeposition 336
2.3 Dyesensitizationinphotocatalysis 337
2.4 Coupledsemiconductors 337
2.5 DefectinducedVLAphotocatalysis 339
3 Oxidationchemistry,thereactiveoxygenspeciesgeneratedandtheirsubsequentreactionpathways 339
3.1 ReactiveoxygenspeciesandreactionpathwaysinVLATiO2photocatalysis 339
3.2 Photoelectrochemicalmethodsfordeterminingvisiblelightactivity 340
4 EnvironmentalapplicationsofVLATiO2 342
4.1 WatertreatmentandairpurificationwithVLAphotocatalysis 342
4.2 WaterdisinfectionwithVLAphotocatalysis 343
5 AssessmentofVLAphotocatalystmaterials 344
5.1 Standardizationoftestmethods 344
5.2 ChallengesincommercializingVLAphotocatalysts 346
6 Conclusions 346
Acknowledgments 346
References 346
1 Titanium dioxide – an introduction
1.1 TiO2structuresandproperties
Titaniumdioxide(TiO2)existsasthreedifferentpolymorphs;
anatase,rutileandbrookite[1].Theprimarysourceandthemost
stableformofTiO2isrutile.Allthreepolymorphscanbereadily
synthesisedinthelaboratoryandtypicallythemetastableanatase
andbrookitewilltransformtothethermodynamicallystablerutile
uponcalcinationattemperaturesexceeding∼600◦C[2].Inallthree
forms,titanium(Ti4+)atomsareco-ordinatedtosixoxygen(O2 −)
atoms,forming TiO6 octahedra [3].Anatase is made upof
cor-ner(vertice)sharingoctahedrawhichform(001)planes(Fig.1a)
resultinginatetragonalstructure.In rutiletheoctahedrashare
edgesat(001)planestogiveatetragonalstructure(Fig.1b),andin
brookitebothedgesandcornersaresharedtogiveanorthorhombic
structure(Fig.1c)[2,4–7]
Titaniumdioxideistypicallyann-typesemiconductordueto
oxygendeficiency[8].Thebandgapis3.2eVfor anatase,3.0eV
forrutile,and∼3.2eVforbrookite[9–11].Anataseandrutileare
themainpolymorphsandtheirkeypropertiesaresummarizedin
Table1[12,5,13].TiO2isthemostwidelyinvestigatedphotocatalyst
duetohighphoto-activity,lowcost,lowtoxicityandgoodchemical
andthermalstability[12,14,15].Inthepastfewdecadestherehave
beenseveralexcitingbreakthroughswithrespecttotitanium
diox-ide.Thefirstmajoradvancewasin1972whenFujishimaandHonda
reportedthephotoelectrochemicalsplittingofwaterusingaTiO2
anodeandaPtcounterelectrode[16].Titaniumdioxide
photocatal-ysiswasfirstusedfortheremediationofenvironmentalpollutants
in1977whenFrankandBard reportedthereductionofCN− in
water[17,18].Thisledtoadramaticincreaseintheresearchinthis
areabecauseofthepotentialforwaterandairpurificationthrough
utilizationof“free”solarenergy[12,13,19].Othersignificant
break-throughsincludedWangetal.(1997),whoreportedTiO2surfaces
withexcellentanti-foggingandself-cleaningabilities,attributedto
thesuperhydrophilicpropertiesofthephotoexcitedTiO2surfaces
[20]anduseofnanotitaniumdioxideinanefficientdyesensitized
solarcell(DSSC),reportedbyGraetzelandO’Reganin1991[21]
1.2 ElectronicprocessesinTiO2photocatalysis
Photocatalysisiswidelyusedtodescribetheprocessinwhich
theaccelerationofa reactionoccurswhenamaterial,usually a
semiconductor,interactswithlightofsufficientenergy(orofa cer-tainwavelength)toproducereactiveoxidizingspecies(ROS)which canleadtothephotocatalytictransformationofapollutant.Itmust
benotedthatduringthephotocatalyticreaction,atleasttwoevents mustoccursimultaneouslyinorderforthesuccessfulproduction
ofreactiveoxidizingspeciestooccur.Typically,thefirstinvolves theoxidation ofdissociativelyadsorbedH2Obyphotogenerated holes,thesecondinvolvesreductionofanelectronacceptor (typi-callydissolvedoxygen)byphotoexcitedelectrons;thesereactions leadtotheproductionofahydroxylandsuperoxideradicalanion, respectively[22]
Itisclearthatphotocatalysisimpliesphoton-assisted genera-tionofcatalyticallyactivespeciesratherthattheactionoflight
asacatalystinareaction[23,24].Iftheinitialphotoexcitation pro-cessoccursinanadsorbatemolecule,whichtheninteractswiththe groundstateofthecatalystsubstrate,theprocessisreferredtoasa
“catalyzedphotoreaction”,if,ontheotherhand,theinitial photoex-citationtakesplaceinthecatalystsubstrateandthephotoexcited catalysttheninteractswiththegroundstateadsorbatemolecule, theprocessisa“sensitizedphotoreaction”.Inmostcases, hetero-geneousphotocatalysisreferstosemiconductorphotocatalysisor semiconductor-sensitizedphotoreactions[22]
Inphotocatalysis,lightofenergygreaterthanthebandgapof thesemiconductor,excitesanelectronfromthevalencebandto theconductionband(seeFig.2).InthecaseofanataseTiO2,the bandgapis3.2eV,thereforeUVlight(≤387nm)isrequired.The absorptionofaphotonexcitesanelectrontotheconductionband (eCB−)generatingapositiveholeinthevalenceband(hVB+)(Eq (1.1))
ChargecarrierscanbetrappedasTi3+andO−defectsitesinthe TiO2lattice,ortheycanrecombine,dissipatingenergy[25] Alter-natively,thechargecarrierscanmigratetothecatalystsurfaceand initiateredoxreactionswithadsorbates[26].Positiveholescan oxi-dizeOH−orwateratthesurfacetoproduce•OHradicals(Eq.(1.2)) which,areextremelypowerfuloxidants(Table2).Thehydroxyl radicalscansubsequentlyoxidizeorganicspecieswith mineraliza-tionproducingmineralsalts,CO2andH2O(Eq.(1.5))[27]
Trang 3Fig 1.Crystalline structures of titanium dioxide (a) anatase, (b) rutile, (c) brookite (Reprinted with permission from Katsuhiro Nomura ( nomura-k@aist.go.jp ; http://staff.aist.go.jp/nomura-k/english/itscgallary-e.htm ) Copyright (2002)).
Electronsin the conductionband canberapidly trappedby
molecularoxygenadsorbedonthetitaniaparticle,whichisreduced
toformsuperoxideradicalanion(O2•−)(Eq.(1.4))thatmay
fur-therreactwithH+ togeneratehydroperoxylradical(•OOH)(Eq
(1.6))andfurtherelectrochemicalreductionyieldsH2O2(Eq.(1.7))
[28,29].Thesereactiveoxygenspeciesmayalsocontributetothe
oxidativepathwayssuchasthedegradationofapollutant(Eqs.(1.8)
and(1.9))[25,27,28]
1.3 Recombination
Recombinationofphotogeneratedchargecarriersisthemajor
limitationinsemiconductorphotocatalysisasitreducesthe
over-all quantum efficiency [29] When recombination occurs, the
Table 1
Physical and structural properties of anatase and rutile TiO 2
Molecular weight (g/mol) 79.88 79.88
Boiling point ( ◦ C) 2500–3000 2500–3000
Light absorption (nm) <390 <415
Crystal structure Tetragonal Tetragonal
Lattice constants ( ˚A) a=3.78 a=4.59
c = 9.52 c = 2.96
Ti O bond length ( ˚A) 1.94 (4) 1.95 (4)
1.97 (2) 1.98 (2)
dis-sipatingtheenergyaslightorheat[6,31] Recombinationmayoccureitheronthesurfaceorinthebulk and isingeneralfacilitatedbyimpurities, defects,or allfactors which introduce bulk or surface imperfections into the crystal [29,32].Serponeetal.foundthattrappingexcitedelectronsasTi3+ species occurredona time scale of∼30psand that about90%
ormoreofthephotogeneratedelectronsrecombinewithin10ns
Fig 2.Schematic of TiO 2 photocatalytic mechanism.
Table 2
Standard electrochemical reduction potentials of common oxidants.
potential (V)
• OH (Hydroxyl radical) • OH + H + + e − → H 2 O 2.80
O 3 (Ozone) O 3 (g) + 2H + + 2e − → O 2 (g) + H 2 O 2.07
H 2 O 2 (Hydrogen peroxide) H 2 O 2 + 2H + + 2e − → 2H 2 O 1.77 HClO (Hypochlorous acid) Cl 2 (g) + 2e − → 2Cl − 1.49
Cl − (Chlorine) 2HClO + 2H + + 2e − → Cl + 2H O 1.36
Trang 4andnanosizedcrystals[40,41]haveallbeenreportedtopromote
separationoftheelectron–holepair,reducingrecombinationand
therefore improvethe photocatalytic activity.For example, the
TiO2 crystallites ofEvonik(Degussa)P25containa combination
ofanatase(∼80%)andrutile(∼20%).Theconductionband
poten-tialofrutileis morepositivethanthatofanatasewhichmeans
thattherutilephase mayactasanelectronsinkfor
photogen-eratedelectronsfromtheconductionbandoftheanatasephase
Manyresearchersattributethehighphotocatalyticactivityofthis
preparationtotheintimatecontactbetweentwophases,
enhanc-ingseparationofphotogeneratedelectronsandholes,andresulting
inreducedrecombination[42]
1.4 StrategiesforimprovingTiO2photoactivity
Variousstrategieshavebeenadoptedforimprovingthe
pho-tocatalyticefficiencyofTiO2.Theycanbesummarizedaseither
morphologicalmodifications,suchasincreasingsurfaceareaand
porosity,oraschemicalmodifications,byincorporationof
addi-tionalcomponents in theTiO2 structure Although visible light
active(VLA)TiO2 photocatalystsrequirechemicalmodifications,
whichwillbereviewedinthenextsection,theiroverallefficiencies
havebeensignificantlyenhancedbycontrollingthesemiconductor
morphology
ThemostcommonlyusedTiO2morphologyisthatof
monodis-persednanoparticleswhereinthediameteriscontrolledtogive
benefitsfromthesmallcrystallitesize(highsurfacearea,reduced
bulk recombination)withoutthe detrimentaleffects associated
withverysmallparticles(surfacerecombination,lowcrystallinity)
[43] One dimensional (1D) titania nanostructures (nanotubes,
nanorods, nanowires, nanobelts, nanoneedles) have been also
formedbyhydrothermalsynthesisbuthighemphasiswasgivenin
titaniaself-assemblednanotubularfilmsgrownbyelectrochemical
anodizationon titanium metalfoils Advantages of such
struc-turesistheirtailoredmorphology,controlledporosity,vectorial
chargetransfer[44,45]andlowrecombinationatgrainboundaries
thatresultinenhancedperformanceinphotoinducedapplications,
mainlyin photocatalysis[44,46,47] An interesting use of TiO2
nanotubesinphotocatalyticapplicationsisthegrowthof
freestand-ingflow-throughmembranes[44]
2 Development of visible light active (VLA) titania
photocatalysts
2.1 Nonmetaldoping
2.1.1 Nitrogendoping
Ultravioletlight makes up only4–5% of thesolar spectrum,
whereas approximately40% of solar photonsare in the visible
region A major drawback of pure TiO2 is the large band gap
meaningitcanonlybeactivateduponirradiationwithphotons
oflightintheUVdomain(≤387nmforanatase),limitingthe
practical efficiency for solar applications [48–50] Therefore, in
ordertoenhancethesolarefficiencyofTiO2undersolarirradiation,
itisnecessarytomodifythenanomaterialtofacilitatevisiblelight
absorption.Non-metaldopingofTiO2 hasshown greatpromise
in achieving VLA photocatalysis, with nitrogen beingthe most
promisingdopant[51,52]
NitrogencanbeeasilyintroducedintheTiO2structure,duetoits
comparableatomicsizewithoxygen,smallionizationenergyand
highstability.Itwasin1986whenSatodiscoveredthataddition
ofNH4OHinatitaniasol,followedbycalcinationofthe
precipi-tatedpowder,resultedinamaterialthatexhibitedavisiblelight
response[53,54].Lateron,Asahiandco-workersexploredforfirst
timethevisiblelightactivityofN-dopedTiO2producedbysputter depositionofTiO2underanN2/Aratmosphere,followedby anneal-ingunderN2[55].Sincethen,therehavebeenmanyreportsdealing withnitrogendopingofTiO2.Significanteffortsarebeingdevoted
toinvestigating thestructural,electronicand opticalproperties
ofN-dopedTiO2,understandingtheunderlyingmechanismsand improvingthephotocatalytic and self-cleaning efficiencyunder visibleandsolarlight[56–58].Comprehensivereviewshavebeen publishedwhichsummarizerepresentativeresultsofthesestudies [59,60].Modelpollutantsthathavebeenreportedtobeeffectively degradedbyVLAphotocatalystincludephenols,methyleneblue, methylorange(althoughdyeshavestrongabsorptioninthevisible range)andrhodamineB,aswellasseveralgaseouspollutants(e.g., volatileorganiccompounds,nitrogenoxides)
FortheefficientincorporationofnitrogenintoTiO2 eitherin thebulk or asa surfacedopant, both dry and wet preparation methods havebeen adopted.Physicaltechniquessuchas sput-tering [61–65] and ion implantation [66,67],rely onthe direct treatmentofTiO2 withenergetic nitrogenions.Gasphase reac-tionmethods[68–70],atomiclayerdeposition[71]andpulsedlaser deposition[72]havebeensuccessfullyappliedtoprepareN–TiO2,
aswell However,themost versatiletechnique for the synthe-sisofN–TiO2nanoparticlesisthesol–gelmethod,whichrequires relativelysimpleequipmentandpermitsfinecontrolofthe mate-rial’snanostructure,morphologyandporosity.SimultaneousTiO2 growthandNdopingisachievedbyhydrolysisoftitanium alkox-ideprecursorsinthepresenceofnitrogensources.Typicaltitanium salts (titanium tetrachloride) and alkoxide precursors (includ-ing titanium tetra-isopropoxide, tetrabutyl orthotitanate) have beenused.Nitrogencontainingprecursorsusedincludealiphatic amines, nitrates, ammonium salts,ammonia and urea[73–75] The synthesis root involves several steps; however, the main characteristic is that precursor hydrolysis is usually performed
at room temperature The precipitate is then dried to remove solvents, pulverized and calcined at temperaturesfrom 200to
600◦C
One promising way to increase the nitrogencontent in the TiO2latticeistocombinethetitaniumprecursorswitha nitrogen-containingligand,suchasTi4+-bipyridineorTi4+-aminecomplexes [76,77].Analternativesoftchemicalrouteisbasedontheaddition
ofureaduringthecondensationofanalkoxideacidifiedsolution, leadingtointerstitialsurfacedopingandshiftoftheabsorption edgewellintothevisiblespectralrange(from3.2to2.3eV)[78]
An innovative sol–gelrelated technique for thepreparation of efficientvisible-lightactivenanostructuredTiO2 isthe templat-ingsol–gelmethod,utilizingtitaniumprecursorscombinedwith nitrogen-containingsurfactants.Specifically,successfulsynthesis
ofvisiblelight activatedN–TiO2 hasbeenachievedbya simple sol–gelmethodemployingdodecylammoniumchloride(DDAC)as surfactant[79].TheDDACsurfactantactssimultaneouslyasapore templatingmaterialtotailor-design thestructural propertiesof TiO2(seeFig.3)aswellasanitrogendopanttoinducevisible-light photoactivityanduniquereactivityandfunctionalityfor environ-mentalapplications[80,81]
InadifferentapproachN–TiO2,wassynthesizedviatwo succes-sivesteps:synthesisofTiO2andthennitrogendopingusingvarious nitrogen-containing chemicals (e.g urea, ethylamine, NH3 or gaseousnitrogen)athightemperatures[52,82–84]orinductively coupledplasmacontaininga widerange ofnitrogenprecursors [85].Inthatcase,thenitrogenatomspredominantlyresidedonthe TiO2surface.Theoriginofthevisible-lightphotocatalyticactivity
inthesemethodsmayarisefromcondensedaromatics-triazine compoundscontainingmelemandmelonunits[73]
AlthoughmostreportsonN–TiO2concerntheanatase polymor-phicphase,visiblelightactiveN–TiO2withanatase-rutilemixed phase(Fig.4)hasalsobeenpreparedbytuningtheparametersof
Trang 5Fig 3.Templating sol–gel method utilizing nitrogen containing surfactants as both nitrogen source and pore template material (Reprinted with permission from H Choi,
M G Antoniou, M Pelaez, A A de la Cruz, J A Shoemaker, D D Dionysiou, Environ Sci Technol 41 (2007) 7530–7535 Copyright (2007) American Chemical Society).
thesol–gelsynthesis.Suchheterojunctionphotocatalystsseemto
effectivelytransferphoto-excitedelectronsfromtheconduction
bandof anatasetothatofrutile,favoring electron–hole
separa-tionandenhancingthevisiblelightphotocatalyticactivity.[86,87]
Etacherietal.havesuccessfullydevelopednitrogendoped
anatase-rutileheterojunctionswhichwerefoundtobeninetimesmore
photocatalyticallyactiveatwavelengthshigherthan450nm(blue
filter)incomparisonwithEvonikP25
Mostoftheabovemethodshavealsobeensuccessfullyapplied
for the doping of 1D titania nanostructures with nitrogen In
this way,N-doped anatasetitania nanobeltswere preparedvia
hydrothermalprocessingandsubsequentheattreatmentinNH3
[88].Similarpost-treatmentwasemployedfordopinganodized
titaniananotubes[89],whilehighenergyion implantationwas
foundto bemore efficient in introducingN atoms in theTiO2
lattice[90].Nitrogenlocalizedstateshave alsobeenintroduced
into highly ordered TiO2 nanotubes via nitrogen plasma [91]
Visiblelight-activeN–TiO2 nanoarray filmshave alsobeen
pre-pared on sacrificial anodized alumina liquid phase deposition
withureamixedwith(NH4)2TiF6aqueoussolution[92].Recently,
surfaceN-doping ontitaniananowires,theirlateraldimensions
reaching the atomic scale, was achieved by the introduction
of amines duringthe condensation stageof the titania
precur-sor[93].Otherapproaches forpreparingdopedTiO2 nanotubes
includeemploymentofnitrogensourcesintheelectrolyte solu-tionsofelectrochemicalanodization[94]orintheinitialsolution
ofhydrothermalgrowth[95,96] Manyresults,uptonow,describenitrogendopingas substitu-tionalelementontheoxygenlatticesitesoratinterstitiallattice sites.ThetwositescanbeinprinciplediscriminatedbyX-ray pho-toelectronspectroscopy(XPS)relyingonthedistinctN1sbinding energiesat396and400eV,respectively[51,69,97–99].XPSpeak assignmentforN-dopedvisiblelightactivatedtitaniaisstillunder debate[57,100].ManyresearchersreportedthatN1speaksaround
397eVarerepresentativeofsubstitutionalnitrogen[57,100,101] whilepeaksatbindingenergies>400eVareassignedtoNO(401eV)
orNO2 (406eV)indicatinginterstitialnitrogen[101].DiValentin
etal [57]employeddensity functionaltheory(DFT)to demon-strateinterstitialnitrogenascharacterNOwithinanataseTiO2
Itwasalsofoundthatthereisnosignificantshiftintheconduction
orvalencebandsof theTiO2.Theenergybondingstates associ-atedbelowthevalencebandandanti-bondingstatespresentabove thevalenceband.Theanti-bonding*N Oorbitalsbetweenthe TiO2 valencebandand conductionbandisbelieved tofacilitate visiblelightabsorptionbyactingasasteppingstoneforexcited electronsbetweenconductionandvalencebands.Nspecies dif-ferentfromthephotoactiveonesinNdopedTiO2caninterferein spectroscopicmeasurementssincetheyhavepeaksaround400eV
Fig 4. Electron transfer mechanism in N-doped anatase rutile heterojunction (Reprinted with permission from V Etacheri, M K Seery, S J Hinder, S C Pillai, Chem Mater.
Trang 6thatNphotoactivespeciescorrespondingtointerstitialnitrogen
withbindingenergyinthe400–401eVregion,preparedfromglove
dischargeinmolecularnitrogeninthepresenceofpureanatase,
havebeenprovidedbyNapolietal.[102].Moreover,Livraghietal
showedthat, bycoupling XPSand solid stateNMR, the 400eV
peakfromammoniumionsreducesitsintensityuponwashingthe
solid[103].ComparedwiththeUVactivityofundopedTiO2,the
visiblelightactivityof N–TiO2 is ratherlow.Thereisalsosome
conflictintheliteratureconcerningthepreferredNsites,
substitu-tionalorinterstitial,whichinducethehighestphotocatalyticaction
[69,83,99,104].Independentlyoftheoriginofvisiblelight
absorp-tioninsubstitutionalorinterstitialnitrogendiscreteenergystates,
thelowphotocatalyticefficiencyismainlyattributedtothelimited
photo-excitationofelectronsinsuchnarrowstates,theverylow
mobilityofthecorrespondingphoto-generatedholes[105]andthe
concomitantincreaseoftherecombinationrateduetothecreation
ofoxygenvacanciesbydoping[106]
2.1.2 Othernon-metaldoping(F,C,S)
FluorinedopingdoesnotshifttheTiO2 bandgap;howeverit
improvesthesurfaceacidityandcausesformationofreducedTi3+
ionsduetothechargecompensationbetweenF−andTi4+
. Thus, chargeseparationispromotedandtheefficiencyofphotoinduced
processesis improved[107] Insertionoffluorine intotheTiO2
crystal latticehasalsobeen reportedto elevate theanataseto
rutilephasetransformationtemperature.Padmanabhanetal
suc-cessfullymodifiedtitaniumisopropoxidewithtrifluoroaceticacid
carryingoutasol–gelsynthesis.Theresultingmaterialprovedtobe
morephotocatalyticallyactivethanEvonikP25whilealsoretaining
anataseattemperaturesofupto900◦C[108]
Carbon,phosphorousandsulphurasdopantshavealsoshown
positiveresultsforvisiblelightactivityinTiO2 [48,49].The
non-metaldopantseffectivelynarrowthebandgapofTiO2 (<3.2eV)
[50,109,110].Thechangeoflatticeparameters,andthepresence
oftrapstateswithintheconductionandvalencebandsfrom
elec-tronicperturbations,givesrisetobandgapnarrowing[111].Not
onlydoesthisallowforvisiblelightabsorptionbutthepresence
oftrapsiteswithintheTiO2bandsincreasesthelifetimeof
photo-generatedchargecarriers
SuccessfulinsertionofsulfurintotheTiO2 latticeisfarmore
difficulttoachievethan nitrogen,duetoitslargerionicradius
Insertionofcationicsulfur(S6+)ischemicallyfavourableoverthe
ionicform(S2−)lattice.Cationic(sulfur)andanionic(nitrogen)
co-dopedwithTiO2 hasalsobeensynthesisedfromasinglesource,
ammoniumsulfate,usingasimplesol–geltechnique[112].Periyat
etal.successfully developedS-dopedTiO2 throughmodification
oftitaniumisopropoxidewithsulphuricacid.Theyfoundthat
for-mationoftitanyloxysulfateresultsintheretentionofanataseat
increasedtemperatures(≥800◦C)andthatthepresenceofsulfur
causesincreasedvisiblelightphotocatalyticactivityofthe
synthe-sisedmaterials.[113].Recently,visiblelight-activatedsulfurdoped
TiO2 films were successfully synthesized using a novelsol–gel
methodbasedontheself-assemblytechniquewithanonionic
sur-factanttocontrolnanostructureandH2SO4asaninorganicsulfur
source[114].Sulfurspeciesdistributeduniformlythroughoutthe
filmswereidentifiedbothasS2−ionsrelatedtoanionic
substitu-tionaldopingofTiO2aswellasS6+/S4+cations,attributedmainlyto
thepresenceofsurfacesulfategroups.AstrongEPRsignal,whose
intensitycorrelatedwiththesulfurcontentandmostimportantly
wasmarkedly enhanced under visible lightirradiation, implied
formationoflocalizedenergystatesintheTiO2 bandgapdueto
aniondopingand/oroxygenvacancies.Calcinationat350◦C for
2hprovidedsulfurdopedTiO2filmswiththehighestsulfur
con-tentandBETsurfacearea,smallcrystallitesize,highporosity,and
largeporevolumetogetherwithverysmoothanduniformsurface
ThecorrespondingmesoporousS–TiO2filmwasthemosteffective photocatalystforthedegradationofmicrocystin-LR(MC-LR)under visiblelightirradiation
2.1.3 Non-metalco-doping N–Fco-dopedTiO2hasbeenexploredinvisiblelight photocatal-ysis[115,116]duetothesimilarstructuralpreferencesofthetwo dopants.Inaddition,thecombinedstructureretainstheadvantages
ofN-dopinginhighvisiblelightresponseandtheF-doping signif-icantroleinchargeseparation.Furthermore,synergeticeffectsof theco-dopinghavebeenfound.Infact,surfacefluorinationinhibits phase transformation fromanatasetorutile and removalof N-dopantsduringannealing[117].Inaddition,itreducestheenergy costofdopingandalsotheamountofoxygendefectsinthe lat-tice,asa consequenceofthechargecompensationbetweenthe nitrogen(p-dopant)andthefluorine(n-dopant)impurities[118] Theseeffectsstabilizethecompositesystemandeffectivelyreduce theconcomitantelectron–holerecombination thathampersthe photocatalyticperformanceofsinglydopedN–TiO2
ThesynergisticapproachoftheN–Fdopinghasbeenfurther exploitedemployingamodifiedsol–geltechniquebasedona nitro-gen precursor and a Zonyl FS-300nonionic fluorosurfactant as bothfluorinesourceandporetemplatematerialtotailor-design the structural properties of TiO2 [119] The obtained materials areactiveundervisiblelightilluminationandhavebeenusedfor thephotocatalyticdegradationofavarietyofpollutantsinwater Veryrecently,theseN–Fdopedtitaniamaterialsweresuccessfully immobilizedonglasssubstratesemployingthedip-coatingmethod withsubsequentdryingunderinfraredlamp,followedby calcina-tionat400◦C.Thenanostructuredtitaniadopedthinfilmspreserve theirvisible light induced catalytic activity[120].Furthermore, comparativeEPRmeasurementsbetweentheco-dopedand refer-encesamplesidentifieddistinctNspinspeciesinNF–TiO2,witha highsensitivitytovisiblelightirradiation.Theabundanceofthese paramagneticcentersverifiestheformationoflocalizedintra-gap statesinTiO2andimpliessynergisticeffectsbetweenfluorineand nitrogendopants[120]
Significantimprovement of thevisible-light photoactivity of N–Fco-dopedtitaniafilmshasbeenobservedbyemployingan inverseopalgrowthmethod,usinga silicacolloidalcrystalasa templateforliquidphasedepositionofNF–TiO2.Inthisway, hierar-chicalmeso-macroporousstructuresarepreparedwhichpromote efficientandstablephotocatalysisviatunedmorphologyand pho-tonmultiplescatteringeffects[121]
2.1.4 OxygenrichTiO2modification Followinganotherapproach, recentlythe visiblelight active photocatalytic properties have been achieved by the in situ generation of oxygen through the thermal decomposition of peroxo-titania complex[122].IncreasedTi O Tibond strength and upward shiftingof the valence band(VB) maximum were responsiblefor thevisible lightactivity.Theupwardshiftingof theVBmaximumforoxygenrichtitaniaisidentifiedasanother crucialreasonresponsibleforefficientvisiblelightabsorption Typ-icalbandgapstructuresofcontrolandoxygenrichtitaniasamples obtainedarerepresentedinFig.5
2.2 Metaldeposition 2.2.1 Noblemetalandtransitionmetaldeposition ModificationsofTiO2withtransitionmetalssuchasCr,Co,Vand
FehaveextendedthespectralresponseofTiO2wellintothe vis-ibleregionalsoimprovingphotocatalyticactivity[107,123–128] However,transitionmetals mayalsoactasrecombinationsites for thephotoinducedcharge carriers thus, loweringthe quan-tumefficiency.Transitionmetals havealsobeenfoundtocause
Trang 7Fig 5. Mechanism of band gap narrowing by oxygen excess Number 2 and 16 in H 2 O 2 –TiO 2 was used to identified two different modified titania samples (Reprinted with permission from V Etacheri, M K Seery, S J Hinder,S C Pillai, Adv Funct Mater 21 (2011) 3744–3752 Copyright (2011) Wiley VCH).
thermalinstabilitytotheanatasephaseofTiO2[29].Kangargues
thatdespitethefactthatadecreaseinbandgapenergyhasbeen
achievedbymany groups throughmetaldoping,photocatalytic
activityhasnot beenremarkablyenhanced because themetals
introducedwerenotincorporatedintotheTiO2framework.In
addi-tion,metals remaining ontheTiO2 surface block reactionsites
[129].Morikawaetal.showedthatdopingTiO2withCrwasfound
toreducephotocatalyticactivitybutCrandVionimplantedTiO2
showedhigherphotocatalyticperformancesthanbareTiO2didfor
thedecompositionofNO undersolarirradiation[130] Another
techniqueinvolvesmodifyingTiO2withtransitionmetalssuchas
Fe,Cu,Co, Ni,Cr, V, Mn,Mo,Nb, W,Ru, Ptand Au[131–140]
Theincorporationoftransitionmetalsinthetitaniacrystallattice
mayresultintheformationofnewenergylevelsbetweenVBand
CB,inducingashiftof lightabsorptiontowardsthevisiblelight
region.Photocatalyticactivityusuallydependsonthenatureand
theamountofdopingagent.Possiblelimitationsare
photocorro-sionandpromotedchargerecombinationatmetalsites[132]
DepositionofnoblemetalslikeAg,Au,PtandPdonthe
sur-faceofTiO2 enhancesthephotocatalyticefficiencyundervisible
lightbyactingasanelectrontrap,promotinginterfacialcharge
transferandthereforedelayingrecombinationoftheelectron–hole
pair[131,141–144].Hwangetal.showedthatplatinumdeposits
onTiO2trapphoto-generatedelectrons,andsubsequentlyincrease
thephoto-inducedelectrontransferrateattheinterface.Seeryetal
showedenhanced visible lightphotocatalysiswithAg modified
TiO2[145].WhileGunawanetal.demonstratedthereversible
pho-toswitchingofnanosilveronTiO2wherereducedsilveronaTiO2
supportexposedtovisiblelight(>450nm)resultedinexcitation
andreverseelectronflowfromsilvertotheTiO2support,oxidising
silver(Ag0→Ag+)intheprocess[146].Thevisiblelight
respon-sivenessofTiO2wasaccreditedtothesurfaceplasmonresonance
ofsilvernanoparticles(Fig.6)[146,147]
2.3 Dyesensitizationinphotocatalysis
Dyephotosensitizationhasbeenreportedbydifferentgroups
andtobeoneofthemosteffectivewaystoextendthe
photore-sponseofTiO2intothevisibleregion[148–151].Indeedthesetypes
ofreactionsareexploitedinthewellknowndyesensitizedsolar
cells[21].Themechanismofthedyesensitizedphoto-degradation
ofpollutantsisbasedontheabsorptionofvisiblelightforexciting
anelectronfromthehighestoccupiedmolecularorbital(HOMO)
tothelowestunoccupiedmolecularorbital(LUMO)ofadye.The
exciteddye moleculesubsequently transferselectrons intothe
conductionbandofTiO2,whilethedyeitself isconvertedtoits
cationicradical.TheTiO2 actsonlyasamediatorfortransferring
electronsfromthesensitizertothesubstrateontheTiO2surface
as electron acceptors, and the valence band of TiO2 remains unaffected.Inthisprocess,theLUMOofthedyemoleculesshould
bemorenegativethantheconductionbandofTiO2.Theinjected electronshopover quicklytothe surfaceof titaniawhere they are scavenged bymolecularoxygen toformsuperoxide radical
O2•−andhydrogenperoxideradical•OOH.Thesereactivespecies canalsodisproportionatetogivehydroxylradical[152–154].In addition tothementioned species,singlet oxygenmayalso be formedundercertainexperimentalconditions Oxygenhastwo singletexcitedstatesabovethetripletgroundones.Suchrelatively longliveoxygenspeciesmaybeproduced byquenchingof the excitedstate ofthephotosensitizer byoxygen The subsequent radicalchainreactionscanleadtothedegradationofthedye[154] Knowledgeof interfacialelectrontransfer between semicon-ductorandmolecularadsorbatesisof fundamentalinterestand essentialforapplicationsofthesematerials[155–158].Ultrafast electroninjectionhasbeenreportedformanydye-sensitizedTiO2 systems.Thisinjectiondepends onthenatureofthesensitizer, thesemiconductor,andtheirinteraction.Asburyetal.observed verydifferentelectroninjectiontimesfromfemtotopicosecondby changingthesemiconductorunderthesameconditions[156] 2.4 Coupledsemiconductors
Manyeffortshavebeenmadeinthesynthesisofdifferent cou-pledsemiconductorssuchasZnO/TiO2[159],CdS/TiO2[160],and
Bi2S3/TiO2 [161] Thesynthesized couplessignificantlyenhance thephotocatalyticefficiencybydecreasingtherecombinationrate
Fig 6. Mechanism for light absorption of silver supported in TiO 2 (Adapted with permission from N T Nolan, M K Seery, S J Hinder, L F Healy, S C Pillai J Phys Chem C 114 (2010), 13026–13034 Copyright (2010) American Chemical Society).
Trang 8Fig 7.TEM and mechanistic image of the interface between CdS nanowires and TiO 2 nanoparticles TiO 2 provide sites for collecting the photoelectrons generated from CdS nanowires, enabling thereby an efficient electron–hole separation (Reprinted with permission from S.J Jum, G.K Hyun, A.J Upendra, W.J Ji, S.L Jae, Int J Hydrogen Energy,
33 (2008) 5975 Copyright (2008) Elsevier).
ofthephotogeneratedelectron–holepairsandpresentpotential
applicationsinwatersplitting,organicdecomposition,and
photo-voltaicdevices[162–164].Thesecompositeswerealsoconsidered
aspromisingmaterialstodevelopahighefficiencyphotocatalyst
activatedwithvisiblelight.Theycanalsocompensatethe
disad-vantagesoftheindividualcomponents,andinduceasynergistic
effectsuchasanefficientchargeseparationandimprovementof
photostability [158,159] Therefore, visible light-drivencoupled
photocatalyststhatcandecomposeorganicmaterialareofgreat
interest[163,166,167]
Analysisofthemicrostructureandphase compositionofthe
coupledsemiconductorofBiFeO3/TiO2revealedthatacore-shell
structure was formed [168] This couple resulted in extended
photo-absorptionbandsintothevisiblewhichwasdependenton
theBiFeO3 content.Thiscouplewasreportedtobemore
effec-tivefor thephotocatalytic degradationof congo red dyeunder
visiblelightirradiation,ascomparedtopureBiFeO3andTiO2
pow-ders.SensitizingTiO2nanotubearrayswithZnFe2O4wasfoundto
enhancephotoinducedchargeseparationandtoextendthe
pho-toresponsefromtheUVtothevisibleregion,too[169]
Upuntilnow,themaineffortshavebeendevotedtothe
synthe-sisofvariouscore-shellnanocrystals.Theprevalentviewpointis
thatitrequiresalatticematchingbetweenshellsandcore
materi-alstoachieveabetterpassivationandminimizestructuraldefects
[164–173].Inaddition,thecouplingofalargebandgap
semicon-ductorwitha smallerone,which can beactivatedwithvisible
light,isofgreatinterestforthedegradationoforganicpollutants
usingsolar radiation.Blocking trapstates by coatingthe
parti-cleswiththin layersofa widebandgapmaterial canleadtoa
drasticenhancementofthephotostability[174–176].Forinstance,
CdSisafascinatingmaterialwithidealbandgapenergyforsolar
andvisiblelight applications(2.4eV) However,CdSis proneto
photo-anodiccorrosion in aqueousenvironments Toovercome
thisstabilityproblemandimprovethephotoactivity,CdShasbeen
combinedwithawidebandgapsemiconductor,suchasZnOand
TiO2[163,177],andthiscouplinggivesimprovedchargeseparation
ofphotogeneratedelectronsandholes(seeFig.7)
Inadditiontotheflatbandpotentialofthecomponents,the
photocatalyticperformanceofthecoupledsemiconductorsisalso
relatedtothegeometryoftheparticles,thecontactsurfacebetween
particles,andtheparticlesize[178,179].Theseparametersstrongly dependonthemannerwithwhichthecouplesareprepared Var-iouscore/shelltype nanocrystalshave beenextensivelystudied usingdifferentmethods.Synthesismethodsnormallyrequirehigh temperatures,longtimes,strictinertatmosphereprotectionand complexmultistepreactionprocess
Byapplyingultrasoundunderspecificconditions,thereisthe possibilityofsynthesizingnano-compositesinashorttime,under mildconditions,inair,andwithoutcalcination[160].Forexample, TiO2-coatednanoparticleswitha core-shellstructurehavebeen preparedwithultrasoundtreatment.TheTiO2wasfoundtobe uni-formlycoatedonthesurfaceofCdSandthisledtoanenlargement
ofthenanoparticles.Intheabsenceofultrasound,theformation
oflargeirregularaggregateswasobserved.TheUV–visabsorbance spectraofthepureandcompositesemiconductorsareshownin Fig.8[160].TheabsorptionbandofCdSnanoparticleswasfound
ataround450–470nmincomparisonwiththebulkcrystallineCdS whichappearedatabout515nm(Eg=2.4eV)[180].Inthecaseof
Fig 8. The UV–vis absorbance spectra of pure and composite semiconductors (Reprinted with permission from N Ghows, M.H Entezari, Ultrason Sonochem., 18
Trang 9Fig 9.Proposed mechanism that shows the interaction of one species from the core with one species from the shell for the removal of RB5 by nanocomposite CdS/TiO 2 (Reprinted with permission from Ref [245] Copyright (2011) Elsevier).
TiO2,theonsetabsorptionfornanoparticlespreparedunder
ultra-soundwasabout360nm,whileforthebulkitwasabout385nm
(Eg=3.2eV)[181].ItisfoundthatmodificationofTiO2 withCdS
particlesextends theopticalabsorptionspectrum intothe
visi-bleregionincomparison withthatofpureTiO2.Increasingthe
amountofTiO2ledtoafurtherred-shiftoftheabsorptionbandin
compositephotocatalysts.Theredshiftofspectraaretypical
char-acteristicsofcore-shellnano-crystals,originatingfromtheefficient
diminishingofthesurfacedefectsofcorenano-crystalsafter
cap-pingthemwithhigherbandgapshells[173].Thisisinagreement
withthepreviousreportbyKischetal.thatthebandgapofCdS
employedincompositephotocatalystsisshiftedbyanelectronic
semiconductor-supportinteraction[182,183]
ThesynthesizedCdS/TiO2nano-compositesystemwasapplied
fortheremovalofReactiveBlack5inaqueoussolution,under
dif-ferentconditions,andemployingvisibleandsolarlightirradiation
Themechanismforthedegradationthatisproposedisbasedon
thereactionsinFig.9[245].In semiconductorcore-shell
struc-tureselectronicinteractionsthatoccurattheheterojunctioncan
trapphoto-generatedelectronsattheinterfaceandimprovethe
efficiencyofthephotocatalyticactivity.Thephoto-generated
elec-tronsandholesinduceredoxreactionsaccordingtotherelative
potentialsoftheconductionandvalencebandsofthetwo
semicon-ductors.Suchcore-shellnano-compositesmaybringnewinsights
into thedesign of highly efficient photocatalysts and potential
applicationsintechnology
2.5 DefectinducedVLAphotocatalysis
VLAtitania canalsobe formedby introducingcolorcenters
insidethematerial[44,56].Thisdefectinduceddopingcanbe
pro-ducedeither byheat treatmentof TiO2 in vacuumorinertgas
environmentsor byintercalation of smallcations(H+,Li+, etc.)
intothelattice.In somecases,O2 is releasedfromthematerial
and Ti3+ centers are formed Veryrecently, hydrogenation has
beendemonstratedasaveryeffectiveroutetoengineerthe
sur-faceofanataseTiO2nanoparticleswithanamorphouslayerwhich,
insteadofinducingdetrimentalrecombinationeffects,resultedin
themarkedextensionoftheopticalabsorptiontotheinfraredrange
andremarkableenhancementofsolar-drivenphotocatalytic
activ-ity[184]
3 Oxidation chemistry, the reactive oxygen species generated and their subsequent reaction pathways
3.1 ReactiveoxygenspeciesandreactionpathwaysinVLATiO2 photocatalysis
Asamodel,thereactionpathwaysofvisiblelight-induced pho-tocatalytic degradation of acidorange 7 (AO7) in thepresence
of TiO2 has been investigated [185], monitoringthe formation andthefateofintermediatesandfinalproductsinsolutionand
onthephotocatalystsurfaceasafunctionofirradiationtime.It wasobservedthattheintensityofthechromophorebandofAO7 reducedexponentiallywithtimeanddisappearedafterabout60h Theintensitiesoftheabsorbancepeaksrelatedtothenaphthalene andbenzeneringsinAO7decreasedwithaslowerratecompared
tothatofdecolorizationofthesolutionduringthefirst60h.After completedecolorization,theabsorbanceduetothenaphthalene andbenzeneringsremainedconstant.Thisobservationconfirmed thatintheabsenceofcoloredcompoundsonthephotocatalyst sur-face,visiblelightcannoteffectivelydegradefragmentscontaining thebenzeneandnaphthaleneringsproducedbythecleavageof thedyemolecule.ItshouldbenotedthatAO7solutionwasstable undervisiblelightwithoutTiO2,andthattheTiO2suspensionwas unabletoinitiatethedyedegradationinthedark.Bothvisiblelight andTiO2particleswereindispensableforthedegradationofAO7
inaqueoussolution.DuringtheirradiationofAO7-TiO2suspension withvisiblelightdifferentintermediatessuchascompounds con-taininganaphthalenering,phthalicderivatives,aromaticacids,and aliphaticacidswereidentified.Inaddition,theevolutionof inor-ganicionssuchassulfate,nitrate,nitrite,andammoniumionswere monitoredduringtheirradiationbyvisiblelight
By using appropriate quenchers, the formation of oxidative speciessuchassingletoxygen,superoxide,andhydroperoxide rad-icalsandtheirroleinthedegradationofthedyemoleculesduring illuminationwasstudied[185].Itwasobservedthatinthe pres-enceof1,4-benzoquinone(BQ),whichisasuperoxidequencher and agood electronacceptor[123],bothphotodegradationand formationofhydrogenperoxidewerecompletelysuppressed.This indicatesthatthesuperoxideradicalisanactiveoxidative interme-diate.Additionofsodiumazide,whichisasingletoxygenquencher [186]andmayalsointeractwithhydroxylradical[187],initiallydid
Trang 10Fig 10.%IPCE as a function of wavelength for the photooxidation of water on TiO 2 (red triangles) and WO 3 (blue squares) (Adapted with permission from J.W J Hamilton, J.
A Byrne, P S M Dunlop, N M D Brown, International Journal of Photoenergy (2008) Article ID 185479 Copyright (2008) Hindawi Publishing Corporation) (For interpretation
of the references to color in this figure legend, the reader is referred to the web version of the article.)
notsignificantlyaffectthedegradationofAO7buttheinhibition
becameimportantafter40min,indicatingthedelayedformation
ofsinglet oxygenandpossiblyhydroxyl radicalspecies
Forma-tionofhydrogenperoxidewasalsosuppressedin thepresence
ofthisinhibitor.Similarresultswereobtainedbyadditionof
1,4-diazabicyclo[2,2,2]-octane(DABCO)[188],whichisalsoasinglet
oxygenquencher.Theimportantpointoftheworkin[185]isthat
whencompletedecolorizationofthesolutionwasachieved,the
for-mationofactiveoxidationspeciesandhydrogenperoxidestopped,
theoxidation reactionsceased and theconcentrations of
inter-mediatesremainedconstant.Thisisbecauseonlyinthepresence
ofvisiblelightabsorbingcompounds,theformationofoxidizing
specieswaspossible
Inavisiblelight/sensitizer/TiO2 system,oxygenis
indispens-ableinorder togenerateactiveoxygenradicals [189].The role
ofdissolvedoxygenandactivespeciesgeneratedinthe
photocat-alyticdegradationofphenolwasinvestigatedbyusingapolymer
sensitizedTiO2undervisiblelight[190].Theexperimentalresults
showedthatthephotocatalyticdegradationofphenolwasalmost
stopped under nitrogen atmosphere Therefore, oxygenis very
importantinphotocatalyticreactionsinducedbyvisiblelightand
itactsasanefficientelectronscavenger.Inthissystem,the
degra-dationofphenolgraduallydecreasedbyincreasingsodiumazide
concentration.Thisindicatedthatsingletoxygenwasgenerated
undervisiblelight irradiation.Singlet oxygencandegrade
phe-noldirectly toabout40%which is due toitshighenergy level
(22.5kcalmol−1).Inaddition,singletoxygencanbemeasuredby
phosphorescenceinnearIRasadirectmethodofdetection.There
isarangeofdifferentfluorescenceorspin-trapprobesforindirect
measurements of singlet oxygen and/or superoxide The
spin-trap2,2,6,6-tetramethyl-4-piperidone-N-oxide(TEMP)isgenerally
usedasaprobeforsingletoxygeninEPRstudies.Thereactions
ofTEMPwithsingletoxygenyieldsastableradicaladduct[191]
Anotherusefulspintrapsystemisthe5,5dimethylpyrrolineloxide
(DMPO)[192–194].Monitoringintermediate5,5
dimethylpyrro-lineloxide(DMPO)-OH• radicalsformedinthesuspensionduring
illumination[190]isdonebyitscharacteristic1:2:2:1quartetEPR
spectrumandprovidesevidenceofhydroxylradicalsinthe
sys-tem.Inaddition,somealcoholsarecommonlyusedasdiagnostic
toolsfor hydroxylradical mediatedmechanisms [195,196].The
degradationofphenolbyaddingi-PrOHorMeOHwasdecreased
byabout60%whichindicatedthatbothofthemseriously inhib-itedthephotocatalyticdegradationofphenol[190].Thisconfirmed thathydroxylradicalswerethepredominantactivespeciesinthis system,butdidnotprobethemechanismofhydroxylradical for-mation
3.2 Photoelectrochemicalmethodsfordeterminingvisiblelight activity
Ifthephotocatalyticmaterialis immobilizedontoan electri-callyconductingsupportingsubstrate,onecanusethiselectrode
inaphotoelectrochemicalcelltomeasurepropertiesincludingthe bandgapenergy,flatbandpotential,dopantdensity,kineticsof holeand electrontransfer,and theenergiesof dopantlevels.If oneexaminesthecurrent-potentialresponseunder potentiomet-riccontrol,forann-typesemiconductore.g.TiO2,inthedarkno significantanodic(positive)currentisobservedbecausethereare essentiallynoholesinthevalenceband.Whenirradiatedwithlight equaltothebandgapenergy,electronsarepromotedtothe con-ductionband,leavingpositiveholesinthevalenceband,andan increaseisobservedintheanodiccurrentatpotentialsmore pos-itivethattheflatbandpotentialEfb.Thedifferencebetweenthe currentobservedinthelightandthatinthedarkiscalledthe pho-tocurrent(Jph)anditisameasureofthehole-transferrateatthe SC-electrolyteinterface.Attheflatbandpotential,nonetcurrent
isobservedasallchargecarriersrecombine.Forap-type semicon-ductor,thesituationisreversedandanincreaseincathodiccurrent
isobservedunderbandgapirradiationforpotentialsmorenegative thanEfb.Ifamonochromatorisusedalongwithapolychromatic source,e.g.xenon,toirradiatetheelectrodeonecandeterminethe spectralphotocurrentresponseandtheincidentphotontocurrent conversionefficiency(IPCE)
IPCE= Jph I0F whereJphisthephotocurrentdensity(Acm−2),I0istheincident light flux (moles of photonss−1cm−2)and F is Faraday’s con-stant(Cmol−1).Forann-typesemiconductor,thisisthequantum efficiencyforhole-transfertotheelectrolyte.Themaximum wave-lengthatwhichphotocurrentisobservedwillcorrelatetotheband gapenergyforthematerial.Therefore,thevisiblelightactivitycan