An experimental and ﬁrst-principles study of the effect of B/N doping in TiO 2 thin ﬁlms for visible light photo-catalysis

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ContentslistsavailableatSciVerseScienceDirect

Journal of Photochemistry and Photobiology A:

Chemistry

j o u r n al hom ep age : w w w. e l s e v i e r . c o m / l o c a t e / j p h o t o c h e m

An experimental and ﬁrst-principles study of the effect of B/N doping in TiO 2 thin ﬁlms for visible light photo-catalysis

Md. Nizam Uddin

^a,b,∗

, Sayed Ul Alam Shibly

^a

, Rasim Ovali

^c

, Saiful Islam

^a

, Md. Motiur Rahaman Mazumder

^a

, Md. Saidul Islam

^a

, M. Jasim Uddin

^d,e

, Oguz Gulseren

^c

, Erman Bengu

^b

aDepartmentofChemistry,ShahjalalUniversityofScienceandTechnology,Sylhet-3114,Bangladesh

bDepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey

cDepartmentofPhysics,BilkentUniversity,06800Ankara,Turkey

dHigh-PerformanceMaterialsInstitute,FloridaStateUniversity,Tallahassee,FL32310,USA

eDepartmentofChemicalEngineeringandPolymerScience,Sylhet-3114,Bangladesh

a r t i c l e i n f o

Articlehistory:

Received30July2012 Receivedinrevisedform 15December2012 Accepted30December2012 Available online 19 January 2013

Keywords:

TiO2

Photo-catalyst Thinﬁlm Doping Dipcoating

a b s t r a c t

ThinfilmsofTiO2andboron–nitrogen(B/N)co-dopedTiO2onglasssubstrateshavebeenpreparedby asimplesol–geldipcoatingroute.Titanium(IV)isopropoxide,boricacidandureahavebeenusedas titanium,boronandnitrogensources,respectively.ThefilmswerecharacterizedbyX-raydiffraction, X-rayphoto-electronspectroscopy,scanningelectronmicroscopy,RamanspectroscopyandUV–visspectroscopy.TheTiO2thinfilmswithco-dopingofdifferentB/Natomicratios(0.27–20.89)showedbetter photo-catalyticdegradationabilityofmethylenebluecomparedtothatofbare-TiO2undervisiblelight.

TheTiO2filmdopedwiththehighestatomicconcentrationofNshowedrepeatedlythebestphoto- catalyticperformance.Thehighactivityofco-dopedTiO2thinfilmstowardorganicdegradationcanbe relatedtothestrongerabsorptionobservedintheUV–visregion,redshiftinadsorptionedgesandsur- faceacidityinducedbyB/Ndoping.Furthermore,severalatomicmodelsforB/Ndopinghavebeenused toinvestigatetheeffectofdopingonelectronicstructureanddensityofstatesofTiO2throughab-initio densityfunctionaltheorycalculations.Thecomputationalstudysuggestedasignificantnarrowingofthe bandgapduetotheformationofmidgapstatesandtheshiftofFermi-levelfortheinterstitialNmodel supportingtheexperimentalresults.

1. Introduction

Theutilizationofsolarirradiationtosupplyenergyortoinitiate chemicalreactionsisalreadyawellestablishedidea[1].Anatase phaseoftitaniumdioxide(TiO₂),anon-toxicandbiocompatible wide-bandgap semiconductor, when irradiatedwitha suitable wavelengthlightisknowntofacilitatechemicalprocessesonits surfaceincludingdegradationreactions.Inaddition,TiO₂isoneof themostimportantandwidelyinvestigatedphoto-catalystmateri- als[2].Itcanbeusedindecompositionofvariousenvironmentally hazardouscompounds(organic,inorganicandbiologicalmaterials) inbothgaseousandliquidphases[3].

ItiswellknownthatTiO2hasthreepolymorphs:brookite,rutile andanatase.Mixed-phasephoto-catalystswithrutileandanatase,

∗ Correspondingauthorat:DepartmentofChemistry,ShahjalalUniversityofSci- enceandTechnology,Sylhet3114,Bangladesh.Tel.:+8801926372680;

fax:+88082171525.

E-mailaddresses:nizam3472@yahoo.com,uddinnizam@hotmail.com (Md.N.Uddin).

e.g.P25Degussa[4],phaseshavebeenreportedtoexhibitenhanced photo-activityrelativetosingle-phaseanatase.However,synthe- sisofrutile–anatasephasemixturerequiresahightemperature treatment; heatinganatase upto 600–700^◦C is needed for the transformationofanatasetorutile[5].Furthermore,therearestill problemsintheuseofTiO2forpracticalandwide-spreadphoto- catalyticapplications:

(1)Recycling of nano-particulate TiO₂ requires costly separa- tion/ﬁlteringprocesses.

(2)TiO2photo-catalysthasawidebandgap(3.2eV,foranatase), and can only be activated by UV radiation (<387nm) which constitutes only a small fraction (3–5%) of the solar spectrum. Thus, the use of visible light (400–750nm,

∼45% of thesolar spectrum) byanatase shouldbe enabled [6].

(3)TiO2 has a relatively low rate of electron transfer to oxy- genandahighrateofrecombinationwhichresultsinalow quantum yield rateand also a limited photo-oxidation rate [7].

http://dx.doi.org/10.1016/j.jphotochem.2012.12.024

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remediationoftheissueslistedabove.Someoftheeffortsarelisted asaccordingly:

(1)Nano-crystallineTiO2thinﬁlmshaveattractedagreatdealof attention[8,9]owingtotheirﬂexibilityintreatingwastes,e.g.

noseparationrequiredandreuseofphoto-catalyticproducts.

(2)AnyredshiftintheopticalresponseofTiO2fromtheUVband towardthevisiblespectrumwillhaveaprofoundeffectonthe photo-catalytic efficiency[10,11].Withthis view,doping of pureTiO2hasbeenundertakenbyanumberofresearchgroups [12–30]in thelast decade.Modificationby noble-transition metalshasbeenhistoricallyregardedasthefirststrategyandby nonmetalsasthesecondstrategy[12].However,thesemetal- doped TiO2 materials suffered from thermal instability and low-quantumefficiencybecauseofincreasedcarriertrapping afterdoping[13,14].

Incontrastaniondopinghasshowngreatpotentialinintroduc- ingbathochromismandintensiveeffortshavebeenundertakento synthesizeanion-dopedtitaniatowardvisible-light-activephoto- catalysts[15–23],usually byintroducing localizedstates inthe bandgap.Fittipaldietal.hascriticallydiscussedthelimitations andfuturechallenges intheuseofelectronparamagneticreso- nancetechniquein theinvestigationofaniondopedTiO2 based photocatalyst[31].Recentemphasishasbeenplacedonco-doped systems,thatis,thoseinvolvingcombinationsofcationsandanions [24]ortwoanionstogetherwithintheoxidelattice,inwhichadra- maticenhancementofphoto-catalyticbehaviorhasbeenreported [25–28].

Thus,manyresearchersstartedtoinvestigateanionicnonmetal dopantssuchasC[32–35],N[34,36–43],S[44,45],andB[46,47]for extendingthephoto-catalyticactivityintothevisible-lightregion because,therelatedimpuritystatesforsuchdopantsappearnear thevalencebandedgebutdonotactaschargecarriers[34].Gom- bacetal.havestudiedBandNco-dopedpowdered-TiO₂,where asigniﬁcantimprovementinactivityarisesmainlyfromthered shift in the absorption edge, and also B is reported to inhibit growththerebyresultinginhighsurfaceareapowders[48].Espe- cially,surface constructionof titaniaduring N-dopinghasbeen demonstratedbothexperimentallyandtheoretically[37,49].How- everN-dopingoftitaniabythermaltreatmentunderanammonia atmoshphereusuallyleadstoverylimitedvisiblelightactivitybut greatlyimpairstheUVactivity[11].Divergentresultshavebeen reportedforless-studiedsysteminvolvingB-dopedTiO₂[17,46,47].

Itisthereforehighlyimportanttodesignandconstructeffective photocatalystsurfacestructureswithsomesortsofco-dopingwith properratioofdifferentanionsaimingsynergyeffectsthatenhance theseparationandtransferofthecarrierstodevelopefﬁcientvis- iblelightphotocatalysts.However,there isstillgreat debateon thelocationof thedopants,thesynergyeffects onphotoactivi- tiesandtheimportanceoftherelativeratiosonphotoactivities [17,50].

In this work, we focused on thesynthesis of B/N co-doped anatasethinfilmsonglasssubstratesbyasimplesol–geldipcoatingroute.Itwasouraimtoexplorepossiblesynergisticadvantages arisingfromthesimultaneouspresenceofBandNasdopantsin theTiO₂structure.Asmentionedearliertherearealreadyreports onthephoto-catalyticbehaviorofboronandnitrogendopedTiO₂ nano-powders[51–54]andmesoporousforms[50].Tothebestof ourknowledge,thereisnoreportonthephoto-catalyticactivity ofboronandnitrogendopedanatasethin films.Detailedstruc- tural,compositionalandopticalcharacterizationoftheco-doped titaniathinfilmshasbeendone.Then,thesefilmswereemployed asphoto-catalystsundervisiblelightirradiationinthedegrada- tionofaqueousmethyleneblue(MB),adyeofteninvestigatedas

tivestudyofthecatalyticefﬁciencywasevaluatedasafunctionof thedopantnatureandatomicratioofthedopant.

A varietyof internal charge transfersmay takeplace inthe dopedsystems[57].Co-dopingmayalsobebeneficialtoreducethe numberofintrinsicdefectswhicharesupposedtobedetrimen- talinphotocatalyticprocessessincetheyareconsideredtofavor electron–holerecombination[57].Inordertoexplaintheimprove- mentinthephoto-catalyticactivityobservedfortheco-dopedfilms weperformedfirst-principlesplane-wavecalculations[58]based onthedensityfunctionaltheory(DFT)[59,60]onvariousatomic modelsdepictingB/Ndopinginanatasestructure.Theresultsof thesetheoreticalworkswereusedtobetterexplainthechanges inducedinthebondingbehaviorandthebandgapofanataseby doping.

2. Experimental 2.1. Materials

Titanium(IV)isopropoxide(TIP)(≥97.0%,SigmaAldrich)and triethylamine(TA)(≥99%,SigmaAldrich)wereusedasTisource andstabilizer,respectively.Microscopicsodalimeslideglasswas usedassubstrate.MBwaspurchasedfromMerck.Solutionsof1M HCland1MNaOHwereusedtoadjustthepHofthesolution.All reagentswereanalyticalgradeandusedwithoutfurtherpuriﬁca- tion.

2.2. PreparationofTiO₂ﬁlm

TIP (1.5ml) was added to anhydrous (Anh.) ethanol (10ml) under vigorous stirring conditions and then TA (0.35ml) was addedas a stabilizerof thesolutionand stirredat 200rpm for 2–3min under N2 environment (solution-A). A secondsolution wasprepared separatelyby mixing hydrochloricacid(0.92ml), water(0.15ml)andAnh.ethanol(10ml)usingamagneticstirrer at200rpm(solution-B).Thetwosolutionswerethenmixeddrop wiseandstirredvigorouslyfor60minunderN₂environment.The formedTiO₂solwastransparent,quitestableandhighlysensitive totheamountofTAandwater.Thenthesolwasagedfor24hand servedforﬁlmpreparation.Thetransparentsolwasstableforthree weeks.

TiO2 thinﬁlmswerepreparedbyadip-coatingmethod.Prior tothecoating process,soda-lime–silicaglasssubstrates(micro- scopeslides)withdimensionof 10mm×60mm×1.5mmwere grinded by a commercial bench grinder (model: ST-150), then cleanedinpotassiumdichromateanddichloromethanesolution.

Finally those abrasive substrateswere rinsed withalcohol and deionizedwaterandthendriedat100^◦Cinamicrooven.TheTiO₂ gelﬁlmwasobtainedbydippingthesubstrateintheprecursor solutionbathandpulledupwardswithaspeedabout4cm/min.

Thesubstratescoatedwithgelwerepretreatedinroomtemper- atureandthenannealedfor20minusingamicroovenat200^◦C and finally thefilm wasvapor treated in the vapor of boiling waterfor30storemovethelooselybondedparticles.Thecoat- ingprocesswasrepeatedfivetimesforthickfilmpreparationand finallyannealedat500^◦Cfor2husingamufflefurnace(JSMF-30T, Korea).

B/NdopedTiO₂ wassynthesizedbyasimilarmethodwhere appropriateamountsofboricacid(H3BO3)(solution-C)andurea (solution-D)weredissolvedseparatelyinAnh.ethanol(5ml)and rapidly added to the mixture of solution-A and solution-B. A schematicﬂow chartof thepreparationof dopedTiO2 ﬁlmsby sol–geldipcoatingisshowninFig.1.

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Fig.1.SchematicﬂowchartofthepreparationofdopedTiO2thinﬁlmonglasssubstratebysol–geldip-coatingprocess.

2.3. Filmcharacterization

Thesurfacemorphologyand thethicknessofthefilmswere investigated by scanning electron microscopy (Carl-Zeiss EVO 40 usingLaB6 filament).TheRamanspectrafromtheannealed sampleswererecordedwithaHoriva(Jobin–YvonMicroRaman) spectrometer.XRDpatternswererecordedbyRigaku(MiniFlesx) usingCuK␣radiation(=1.5406 ˚A).TheScherrer equationwas appliedtotheanatase(101)diffractionpeaktocalculatetheaver- agecrystallinesizes.XPSinaSPECSPHOIBOS100hemispherical electrostaticenergyanalyzerwasusedforthecheckingofatomic percentage of the dopant and others species of thefilms. The absorptionspectraofthebandwidthofthedopedandbare(un- doped)-TiO2filmsrangingfrom300to800nmwereinvestigated byUV–visspectroscopy(ThermoScientific:Evolution160).

2.4. Photo-catalyticstudies

Photo-chemicaldegradationwascarriedout inanopenvisi- blelightchamberasshowninFig.2.Anouterwaterpumpwas usedtocirculateconstanttemperaturewaterthroughthesystem continuouslytokeepthetemperatureconstantduringthedegra- dationstudy.Thatchamberconsistedoftwomagneticstirrers,two coolingfansanda200Wtungstenlamp.Degradationwascarried outundervisiblelight.50ml1×10⁻⁵MMBsolutionwastaken andtwocatalystﬁlmswereusedfordegradationstudy.Thesur- faceareaofeachﬁlmwas6cm².Changeintheconcentrationof MBsolutionduringphoto-catalyticdegradationatdifferenttime intervalswasmonitoredbyUV–visspectroscopy(Simadju1800).

Theabsorptionspectrawererecordedandrateofdecolorizationof

MBwasobservedintermsofchangeintheintensityatmaxofthe dye.Thedecolorizationefﬁciency(%)hasbeencalculatedas:efﬁ- ciency(%)=((A₀−At)/A₀)×100,whereA₀isthelightabsorbanceof MBbeforethetreatmentandAtisthatofaftertreatmentattimet.

Beforetakingthesamplesundervisiblelightirradiation,thesolu- tionofMBwastreatedwithcoatedﬁlmsfor30mininthedarkto getadsorptionanddesorptionequilibrium.

2.5. Computationaldetails

Wehaveperformedtheﬁrstprinciplesplane-wavecalculations [58] basedonDFT [59,60]using theprojector-augmented-wave (PAW)potentials[61,62]implementedinViennaab-initiosimu- lationpackage(VASP)program[61–65].Theexchange-correlation potential was expressed in terms of the generalized gradient approximation(GGA)(Perdew–Wang91type)[66].Aplane-wave cut-offenergyof500eVwasusedinallcalculationstoachievethe desiredaccuracy.Thevariable-cellstructuraloptimizationwasper- formedfordopedmodelsinsuchawaythattheexternalpressure waslessthan1.0Kbarandmaximumforcemagnituderemained oneachatomwassetatmostto0.05eV/Å.Weused2×2×2cell forwhichthepureanatasecellcontains48ionsintotal.7× 7×7 Monkhorst–Pack(MP)[67]meshwasusedfork-pointsamplingin theBrillouinzone.ThepartialoccupancyaroundtheFermilevel wastreatedbyGaussiansmearingwithasmearingparameterof 0.05eV.Spinpolarizationwasincludedandthetotalenergywas minimizedupto10⁻⁵eVaccuracyforallcalculations.Duringthe optimizations no symmetry constraintswere imposed in order toavoidentrapmentinlocalminimumconﬁgurationsfordoped models.

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Fig.2. Experimentalsetupforphoto-catalyticreactions.

3. Resultsanddiscussion 3.1. Morphologyandstructure

Inordertoinvestigatethemorphologyoftheobtainedsam- ples,acomparisonbetweentheSEMimagesofthe500^◦Cfor2h annealedsamplesweremade;bare-TiO2 anddifferentco-doped TiO2 filmsisillustratedinFig.3(a)–(d).Topandbottomimages ofthosefiguresshowedlowandhighmagnificationoftheimages, respectively.Fig.3(e)showsatypicalcrosssectionviewtakenfrom sample‘b’.AsestimatedfromtheFig.3(e),thethicknessofthe filmswasapproximately4␮m.Significantcrackingwasobserved onthesurfaceofallfilms.Thefilmsmorphologyarecomparableto thoseofFe³⁺andW⁶⁺dopedTiO2filmspreparedonceramicand Tisubstrate,respectivelybyYaoetal.[68].

Fig.4showstheRamanspectraofthestandardTiO2(anatase) andthesampleswithdifferentB/Nratios.Comparisonwiththe RamanspectrumofpureanataseTiO₂(curved),whichshowsband at143,196,396,515,and638cm⁻¹,demonstratesthatthesurface layerismainlyconstitutedbyanatase.Conversely,Ramanspec- trumofthesampleswithN andBdopingshowastrongsharp bandat140.7cm⁻¹ (Eg),threemid-intensity bandsat393(B1g), 513.5(A1gandB1g)and634.8cm⁻¹(Eg),andaveryweakbandat

∼200cm⁻¹(Eg).ThosebandshavebeenascribedforanataseTiO₂ [69–71].TheintensitiesofvariousTiO₂featureshavebeenclearly decreasedwithB/NdopingwithrespecttothatofbareTiO2[69].

SincechangeintheparticlesizeswithrespecttoB/Ncontentisnot sosigniﬁcantforoursamples(havebeenexplainedinXRDpart) comparetothoseshowninRef.[69],thereisnotveryclearpicture ofintensitychangesofdifferentanatasefeatureswithindifferent

Fig.3. SEMimagesoftheB/Nco-dopedandbare-TiO2ﬁlmsobtainedonglasssubstratesandannealedat500^◦Cfor2hwhereatomicratioofeachdopingisgivenatthe bottomofeachpair.

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0 100 200 300 400 500 600 700 800 900 B/N: 0.27 (a)

B/N: 3.83 (b) B/N: 20.89 (c) Bare TiO2 (d)

Eg

Raman Intensity (a.u.)

634.8 Eg 513.5

A1g

B1g 393 B1g

Raman shift (cm^-1)

140.7 Eg

Fig.4.RamanspectraofdifferentTiO2thinﬁlms.Themodesofsymmetryofthe anataseareindicated.

B/Ndopingfilms.ItisusefultounderlinethattheRamanpeaksof theB/NdopedTiO2arebroaderthanthoseofthepureanatase.In particular,particleshavingsmalldimensionandlowcrystallinity arecharacterizedbyRamanpeakbroadening.Theshifts(5cm⁻¹) intheRamanbandscomparetothereferences[69]mightbedue totheNdopinganddifferenceinparticlesizephononconfinement andoxygendeficiency.

XRDpatternsofdopedand bare-TiO₂ thin ﬁlmsannealedat 500^◦C for 2h are shown in Fig. 5. It is identiﬁed that all the diffraction peaks can beindexed tothe anatase phase of TiO₂ (101),(004),(200),(105),(211),(204),(116)and(215)planes [JCPDS:00-002-0406] and noother phase can be detected.No B-and N-derived peaksdue tootheroxides andnitrides have alsobeendetectedinallthepatterns,indicatingthatBandNas dopantin TiO2 exhibitnotendencytosegregateand/orprecip- itate indifferent phases duringthe syntheticprocess [72].The Band N in thematrix are assumed to beeither interstitial or systematicallysubstituteTiorOwithoutchangingthehostTiO2

matrix.ItcanbefoundthattheXRDpeakpositionsofdopedsam- plesareingoodagreementwiththosereferenceanatasephaseof TiO₂[48,JCPDS:00-002-0406].

Fig.5. XRDpatternsofthethinﬁlmsofco-dopedTiO2:(a)B/N;0.27,(b)B/N;3.83, (c)B/N;20.89and(d)bare-TiO2.

600 550 500 450 400 350 300 250 200 150 100 50 0

N 1s Si 2s Si 2pAr Ti 3pTi 3s

O 1s

Ti 2s Ti 2p B 1sC 1s

B/N: 20.89 (c) Bare-TiO₂ (d)

Intensity (a.u)

Binding Energy (eV)

Fig.6. TypicalXPSsurveyscansofbare-TiO2andB/N:20.89dopedTiO2ﬁlms.

ThecrystalsizesofallthesamplesareestimatedusingtheScher- rerequation:

B(2)= K

Lcos

whereListhefullwidthathalfmaxima(FWHM)ofthediffrac- tionpeakofanatase,K=0.89istheshapefactor,isthediffraction angleandistheX-raywavelengthcorrespondingtotheCuK␣ irradiation.Consideringpeakof(101)planesinaccount,theaver- agecrystallinesizesofthesample‘a’,‘b’,‘c’and‘d’are13.43,11.67, 12.99and13.43nm,respectively.Itsuggestedthatthechangein particlesizewithrespecttoB/Ncontentisnotsigniﬁcant.Inaddi- tion,thereisnochangeinthedspacingvalue;3.5 ˚A,whichimplies that B/N modiﬁcationin co-doping samplesdo not change the averageunitcelldimension.Foroursamplestheaveragelattice parametersareasfollows:a=3.78 ˚A,c=9.44 ˚Aandunitcellvol- ume=134.52 ˚A³.

3.2. XPSstudy

XPSstudiesusingmono-chromatedAlK␣X-raysourcewere performedtochecktheatomicpercentageofdifferentdopingele- mentswithintheannealedfilmsandbondingenvironmentamong Ti,Oanddopant.Atomicpercentagesofdifferentelementswere calculatedfromtheXPSnarrowscanpeakintensityconsidering therelativesensitivityfactorofeachelement.Theatomicratiosof borontonitrogen(B/N)fordifferentsamplesareasfollows;0.27(a), 3.83(b)and20.89(c).Fig.6showsthetypicalXPSspectrumofthe B/N:20.89co-dopedTiO₂film.Thespectrumfrombare-TiO₂sam- pleisalsoshowninthatfigureforcomparison.XPSpeaksshowed thatthedopedsamplescontainedTi,O,C,SielementswithBand Nasdopant.Thepresenceofcarboncouldbeascribedtotheresid- ualcarbonfromtheprecursor’ssolution.Fig.7(i)showstheB1s XPSspectrumforthesample‘c’whichcontainshighestatomicper- centageofB.Itshowssinglepeakcenteredatbindingenergy(BE) of192.1eV.ReferringtothestandardBEofB1sinB₂O₃(193.1eV, B Obond)[73]andTiB2(187.5eV,B Tibond)[74],itisspeculated thatboronatommightprobablybeincorporatedintoTiO₂matrix andformTi O BorTi B OorO Ti Bbond[52,75].Hencepeak at192.1eVhavebeenassignedtoTi O BorTi B OorO Ti B bond.Fig.7(ii)showstheN1sXPSspectrumforthesample‘a’which containshighestatomicpercentageofN.Consideringsignificant asymmetry,N1speakhasbeendeconvolutedwithintwopeaksat

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196 195 194 193 192 191 190 189 192.1

B/N : 20.89 (c)

(i)

B1s

Intensity (a.u.)

Binding Energy (eV)

405 404 403 402 401 400 399 398 397 396 395 B/N: 0.27 (a)

(ii )

400.0 398.5 N1s

Intensity (a.u.)

Binding Energy (eV)

Fig.7.XPSnarrowscanofB1sandN1sofdoped-TiO2thinﬁlmswithB/N:20.89 and0.27,respectively.

398.5and400.0eV.Thepeakat398.5eVmaybebecauseofanionic NincorporatedinTiO₂inO Ti Nlinkages[22,76],whichmight beresponsibleforvisiblelightphoto-catalysis.Thehigherbind- ingenergypeakat400.0eVareattributedtooxidizednitrogenin theformofTi O NorTi N Olinkages[22,76].Theabsenceofa peakatornear396eVfortheN1scorelevelimpliedthattheTiN phaseorchemisorbednitrogenisnotformedinthenanomaterials [77].

3.3. UV–visabsorptionspectra

Fig.8comparestheUV–visabsorptionspectraofthinﬁlmsof bare-andB/Nco-dopedTiO2 withdifferentatomicratios.After B/Nco-dopingtotheTiO₂,theabsorptionsofcatalystsincreased signiﬁcantlyintherangeofwavelengthsfrom400to800nm.This clearred-shiftinUV–visspectrarevealsthevisiblelightabsorp- tionwithB/Nco-dopedTiO₂[52,78–81].Thered-shiftwithinthe absorptionedgesfollowsanincreasingorderas(a)>(b)>(c)(d).

ItimpliesthatasatomicpercentageofNishigherinB/Nco-doping case,theredshiftintheabsorptionedgeishigherwhichisingood agreementwiththeresultinreference[52].Morevertheredshift observedintheco-dopingcasesaresigniﬁcantlyhighercompare tothatofbare-TiO2case.

300 350 400 450 500 550 600 650 700 750 800

Absorbance (a.u.)

(nm)

B/N:0.27 (a) B/N:3.83 (b) B/N:20.89 (c) Bare TiO

2 (d)

Fig.8. UV–visabsorptionspectraofco-dopedandbare-TiO2thinﬁlms.

3.4. Evaluationofphoto-catalyticactivity

Thephoto-catalyticactivityofbare-andB/Nco-dopedTiO₂has beenexaminedbyphoto-catalyticdegradationofMB.Fig.9shows thepercentageofdegradationefficiencyforthesamefilmsin(i) cyclenumber1(1^◦ cycle),(ii)cyclenumber2(2^◦cycle)and(iii) cyclenumber3(3^◦cycle).Dataclearlyshowsthatthedegradation efficiencyincreasescontinuouslyduringtheprocessofdegradation forallcycles.Howevertherateofdegradationisclearlydiffered frombare-toco-dopedTiO2films.ThedopedTiO2filmswithB/N atomicratios:0.27,3.83and20.89showedhigherdegradationper- formance,upto70%,whereasbare-TiO₂filmsshowedthatof30%.

Thephoto-catalyticperformancewithintheco-dopedfilmswere increasedwithNcontentsanddecreasedwithBcontents.However allco-dopedfilmsshowedsignificantlybetteractivityundervisible rangelightcomparetobare-TiO₂films.For1^◦cycle,degradation performanceforco-dopedfilmsandbare-TiO2filmsare51–63%

and28%,respectively.

One of the main problems in the useof TiO₂-based photo- catalystsisthedeactivation;mostlyattributedtoblockageofthe activesitesonthesurface,lossofcoatingmaterialfromthesurface duetoerosionoretc[82,83].Toevaluatethepossibilityofcatalyst recoveryand re-use,successivephoto-catalytic MBdegradation wasperformedforallofthedopedandbare-films.Aftereachcycle, filmswereannealedfor1hat500^◦Candthenre-used.Fig.9(ii) showsthatsuccessiveutilizationfor2^◦cycleinducedevenbetter performanceforallthefilms(a),(b),(c)and(d).For2^◦and3^◦cycles theefficiencyincreasedto62–69%forco-dopedfilmswhereasthat increasedto32–34%forbare-films.Theincreasingofefficiencyfor 2^◦and3^◦cyclesmightbe(i)duetothesurfacechemistrychanging ateachtimeheattreatmentaftereachdegradationstudyand/or(ii) thenitrogenconcentrationwithinthestructuremightbeincreased fortheMBsolutionadsorptiononthefilm.Itisstillingreatdebate ofthesynergyeffectsonphotoactivitiesandtheimportanceofthe relativeratiosonphotoactivities[17,50].Furtherworkisongoing tounderstandthisbehavior.

3.5. Theoreticalresults

WehavealsoperformedDFTcalculationsforbulkanatase,single dopedB-toO-andN-toO-andco-dopedBN-orNB-toTiO-cases whereatitaniumatomwasreplacedwithBorNandanoxygen atomwasreplacedbyNorB,respectively.Wehaveinvestigated varioussingledopedcases(e.g.BandNsubstitutiononTisiteand

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0 40 80 120 160 200 240 0

10 20 30 40 50 60 70

(i)

B/N : 0.27 (a)

B/N : 3.83 (b) B/N : 20.89 (c)

Bare TiO2 (d)

% of effeciency

Irradiation time (min)

0 40 80 120 160 200 240

0 10 20 30 40 50 60 70

(ii)

B/N : 0.27 (a)

B/N : 3.83 (b) B/N : 20.89 (c)

Bare TiO2 (d)

% of efficiency

0 40 80 120 160 200 240

0 10 20 30 40 50 60

70

(iii)

B/N : 0.27 (a)

B/N : 3.83 (b) B/N : 20.89 (c)

Bare-TiO₂ (d)

% of Efficiency

Fig.9.Percentageofdegradationefﬁciencyofﬁlms(i)incyclenumber1(1^◦cycle),(ii)incyclenumber2(2^◦cycle)and(iii)incyclenumber3(3^◦cycle).

onOsite,andinterstitialofBandN)andco-dopedcases(e.g.sub- stitutionofBandNondifferentsites,andfordifferentseparations betweenBandN).Wearepresentingthemostrelevantcasesfor theexperimentalsituations.Thelatticeparametersofbulkanatase werefoundtobe3.81 ˚Aand9.72 ˚A(aandc),andc/aratiowas2.55 whichareclosetoourexperimentalvalues(a=3.78 ˚A,c=9.44 ˚A andc/a=2.50).Contrarytotheearlierreportclaiminglargevol- umeexpansion,whichhassigniﬁcanteffectontheelectronicband structures,upondoping[84],thevolumechangeislessthan1.5%

forallofthedopedstructuresinourcalculations.

Fig.10displaysthestructuralmodelsandthetotal(DOS)and partialdensityofstates(PDOS)ofpureTiO₂,singledopedB-orN- toO-site andinterstitialN.Thebandgapofanataseis2.04eV, lowerthan experimentalvalue; 3.2eV [85] butconsistent with theprevioustheoreticalstudies[39,86]underestimationofband gapisawell-knowndeﬁciencyofDFTcalculations.Acorrection canbemadeusinghybridfunctional, self-interactioncorrection or‘DFT+U’methods,butinthisstudyourprimarypurposeisto understandthechangesinducedbythedopantsontheelectronic structuresofbulkanatase,nottheexactvalueofthebandgap.PDOS analysisshowsthatthetopmostvalencebandsareformedmainly fromO2pstateswhiletheconductionbandsminimumareformed fromTi3dstates.ForthecaseofBsubstitutingOatom(Fig.10(b)), thehybridizationofB2p,Ti3dandO2porbitalsgivesrisetogap statesappearingat0.27eVbelowtheconductionbandforB-doped O-model.Inthisstructure,Batommakesastrongbondingwithone oftheneighborOatomswithbondlengthas1.36 ˚A.B Tibonddis- tanceis2.11 ˚AwhichisevenlargerthanTi Obonddistanceofpure anatase(1.95 ˚A).Thesystematicinvestigationofnon-metaldoping

ofspeciesofB,C,NandFforsubstitutionalandinterstitialcases hasbeencarriedbyValentinandPacchioni[57].Thegapstatesare expectedforsubstitutionalnon-metaldopingandthepositionsof thesestatesaremainlydependonthenucleareffectivechargeof thedopant.Forlighterelements,the2pstatesinthegapappear higherinthegapthatisclosetotheconductionbandminimum (CBM).OurresultsforBsubstitutiontoOcaseareingoodagree- mentwiththisobservation.However,thestates inthegapalso containTi3dstates.ThetypicalpositionofTi³⁺statesisjustbelow theCBM.Thissituationcanalsobeaddressedtolowelectroneg- ativityofBatomcomparedtoOandlesschargetransferfromTi toB.We canconcludethatsubstitutionof BtoOsite produces statesinthegap,whichprovidestheabsorptionofvisiblelight,as wellasTi³⁺ionswhichactlikeelectron–holerecombinationcen- tersandthereforediminishthephotocatalyticactivity.Moreover, ahighfrequencyelectronparamagneticresonance(HF-EPR)mea- surementsonBdopedanatase[51]observedsomespindensity onB,TiandOatomswhichmightbeexplainedastheformation ofsubstitutionofBtoOsite.ThevalenceelectronsfromBmight betransferredtolatticeTiions(Ti⁴⁺)producingTi³⁺ionsanditis well-knownthatthepresenceofTi³⁺ionsproducemidgapstates [85].

ForthecaseofNsubstitutingOatom(Fig.10(c)),N2pdefect statesarelocatedatthetopofthevalancebandmaximum.The positionofthesestatesmightberelatedwiththeeffectivenuclear chargeofNatom[57],thatistheN2pstatesappearlowcompared tothatofB2pintheforbiddenregionduetohigheffectivenuclear chargeofNatomcomparedtoBatom.TheN2pstatesarelocalized atthedefectsiteandthepositionofN2pstates.Alongwiththis,

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Fig.10.Relaxedstructuresandpartialdensityofstates(PDOS)of(a)pureanatase,(b)B-dopedO-,(c)N-dopedO-and(d)Ninterstitialmodels.Energiesareshiftedsuch thatFermilevelsarematchedwithzeroofenergy.Thebondlengthsbetweendopedatomsandﬁrstneighborsareindicated.Solidlines,grayshaded,greenshadedandblue shadedareasrepresentthetotalDOS,PDOSofTi3d,O2pandN2pstates,respectively.(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderisreferred tothewebversionofthearticle.)

theEPRmeasurementsalsoindicatethelocalizationofunpaired electrononNdopant[37].Thevisiblelightabsorptionmightbe enhancedbythesubstitutionofNatomtoOsiteduelocalizedstates inthegapregion.Duetothesedefectstates,bandgapisreduced to1.66eVand1.87eVfordownandupspinstates,respectively.

Nitrogenatomhasﬁveelectronsinthevalanceshellthereforethe structureisparamagneticandanacceptorstateappearsjustabove theFermilevel.

FortheinterstitialNmodel(Fig.10(d)),therewillbeastrong N Obondingwithabondlengthof1.34 ˚A.Sincetheelectronega- tivityofOishigher,NtendstodonateelectronstoOandN O Ti conﬁgurationisformedatthedefectregion.Theseexcesselectrons behaveinthesamemannerofB-toO-model(Fig.10(b))result- ingtheformationofmidgapstates[37].However,inthiscase,in

additiontotheun-occupiedstateformedattheedgeofconduction band,extramidgapstatesappearbelowtheFermilevel.

Inordertounderstandtheeffectofco-dopinginanatasewesub- stitutedBandNatomsforvariouspositionsofTiandOsites.Fig.11 displaystherelaxedstructuresandPDOSforBN-andNB-doping toTiO-positionsaswellasthebondlengthstonearestatomsof dopedspeciesisindicated.ForBN-dopedtoTiO-model,Batom transfersitselectronstoOatomsandreachesclosed-shellconﬁgu- rationwhichpreventsBNbonding.TheinteractionofNatomwith itssurroundingatomsproduces midgapbandswhichbehaveas acceptorstatesandalocalizednitrogenstateappearsatthetop ofthevalenceband.Thus,N Ti Otypeofhybridizationproduces midgapstates.PDOSanalysisindicatesthatthereisnoboroncon- tributiontoDOSaroundFermilevelbutbandgapofthiscaseis

Fig.11.Co-dopingofanatase:(a)BatomreplacedwithTiandNatomreplacedwithOand(b)NatomreplacedwithTi,andBatomreplacedwithO.Solidlines,grayshaded, greenshadedandblueshadedareasrepresentthetotalDOS,PDOSofTi3d,O2pandN2pstates,respectively.(Forinterpretationofthereferencestocolorinthisﬁgurelegend, thereaderisreferredtothewebversionofthearticle.)

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-4.7 -4.0 -3.0 -2.0 -1.0 0.0 0

1 2 3 4 5 6 7

Ti-Rich O-Rich

N doped O B doped O N Interstitial BN doped TiO NB doped TiO

Formation Energy (eV)

O

Fig.12.Formationenergyasafunctionofchemicalpotentialofoxygenforthe dopedstructures.

foundtobe1.76eVforthespinupstatesand1.2eVforspindown states.Comparedtosingle-dopedmodels,thisisarelativelylarger narrowinginthebandgap.Thus,co-dopingofanatasewithB/Ncan beexpectedtoimprovephoto-degradationwithrespecttosingle- dopedcasesviabandgapnarrowing.

Incontrastwiththepreviousco-dopingcase,fortheNB-doped TiO-model,thereisabondingbetweenBandNatomswithbond- lengthof1.617 ˚A.SimilartotheB-toO-case,theFermilevelis alsopinnedforthismodeltothebottomoftheconductionband causingaconductorlikebandstructure,whichobviouslydoesnot contributeinthephoto-catalyticactivityofTiO2.

In summary, band gap narrowing leads to the red shift of the absorbance spectra to the visible light region. Meanwhile, the appearance of midgap states could be responsible for the enhancementofthephoto-catalyticactivityeitherbyimproving theabsorbance oravoiding the recombinationof electron–hole pairs[87].

XPS results of the doped films suggested the formation of N Ti OconfigurationupontheexaminationoftheN1sandTi2p spectra(Section3.2).AsshowninFig.10(c)and(d),N Ti Ocon- figurationcanbeformedinbothN-toO-andNinterstitialcases.

Therefore,in ordertounderstandtheenergetically morestable structures,we havecompared theformation energies ofdoped modelsusingthefollowingformalism:

Eform=E

TiO₂+B N

−E(TiO2)−kB−lN+mO+nTi

whereE(TiO2+B/N)and E(TiO2)arethetotalenergies ofdoped structureand pureanatase,respectively and’sarethechemi- calpotentialsofcorrespondingspecies.ChemicalpotentialsofN andBarecalculatedfromgaseousphaseofN₂and␣-boronphase, respectively.Oissetasthechemicalpotentialofthegasstateof O₂forthecaseofOrichenvironment.Ontheotherhand,Tiisset frommetallicTiphaseforthecaseofOpoor(Tirich)environment.

FurthermoreweimposedtheconstraintasTi+2O=TiO2 in ordertoensuretheequilibriumofbulkanatasephase.Integersm andnarethenumbersofOandTivacancies,respectivelywhilek andlarethenumbersofdopedBandNatoms(ifany),respectively.

Fig.12displaystheformationenergyofthedifferentdopedmodels asafunctionofthechemicalpotentialofoxygenwithrespecttothe valueofgasphase(O).Variationofoxygenchemicalpotential indicatestherelativeabundanceofoxygeninthesynthesismedium withintwolimitingcases;O₂-richandO₂-poorenvironments.Two limitsintheﬁgurerepresenttheenvironments;O=0eVforthe

O₂-richandO=−4.7eVfortheO₂-poororTi-rich.Asdepictedin Fig.12,N-dopedO-caseisenergeticallythemostfavorabledoping modelintheOpoorenvironment,whileBN-dopedTiO-isthestable oneintheOrichenvironment.Thereisacrossoverbetweenthese twomodelsatO=−2.3eV.HoweverNinterstitialcasemight bestabilizedinOrichenvironmentwithinthesingledopedmod- els.ThismightbethereasonfortheformationofN O Tior/and N Ti Obondinginthecaseofourexperiment.

4. Conclusions

Anexperimentalandtheoreticalstudyhasbeenperformedto realizethesynergisticeffectofnonmetaldopinginTiO₂forphoto- catalytic degradation.Co-doped TiO₂ thinfilmswithboronand nitrogenhavebeensuccessfullysynthesizedbysimplesol–geldip coatingmethod.TheB/Nco-dopedTiO₂filmsdemonstratedupto 40%higherphoto-catalyticactivitiesthanbare-TiO₂ filmsunder visiblelightirradiation.Theabsorptionedgesforthedopedfilms werefoundtobeshiftedtowardthevisibleregion,whiletheover- allabsorptionremarkablyincreasedfordopedfilms.Thefilmwith theB/Natomicratioof0.27displayedthehighestdegradationrate amongalldopedfilms.Thedopedfilmsretainedtheirsuperiorcat- alyticactivityforextended periods.Computationalstudieswere conducted onseveralatomic models describing various doping schemes.TheresultsshowedthatdopingwithBand/orNinduced (a)bandgapnarrowing (redshiftof theabsorbance spectrato thevisiblelightregion)and(b)formationofmidgapstatesespe- ciallyincaseofNinterstitialmodel.Theseresultsalsosupported theobservedsynergisticeffectsofB/Ndopingforhigherphoto- degradationactivity.Thesecomputationalfindingssupportedour experimentaldatabyindicatingthepossibleroutesthat canbe responsiblefortheimprovementofthephoto-catalyticactivityin TiO₂ duetoBand Ndoping.Itis revealedthatB/NdopedTiO₂ filmscouldbeapotentialcandidateforscalingupforindustrial applications.

Acknowledgments

WeacknowledgethesupportsfromtheDepartmentofChem- istryofShahjalalUniversityofScienceandTechnology,Bangladesh andTÜB˙ITAK, TheScientiﬁcandTechnologicalResearchCouncil ofTurkey(Grantno:TBAG110T394).Computingresourcesused inthisworkwereprovidedbytheNationalCenterforHighPer- formance Computing of Turkey (UYBHM) under grant number 10362008.OGacknowledgesthesupportofTheTurkishAcademy ofSciences,TÜBA.PartialfundingforM.N.Uddinisprovidedbythe TurkishUndersecretariatforDefenseIndustries.

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