microbowls TiO templated Thermal structure evolution and of photocatalytic activity in polymermicrosphere Applied Surface Science

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Applied Surface Science

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Thermal evolution of structure and photocatalytic activity in polymer microsphere templated TiO 2 microbowls ^夽

Deniz Altunoz Erdogan

^a

, Meryem Polat

^a

, Ruslan Garifullin

^b

, Mustafa O. Guler

^b

, Emrah Ozensoy

^a,∗,1

aDepartmentofChemistry,BilkentUniversity,06800Ankara,Turkey

bInstituteofMaterialsScienceandNanotechnology,NationalNanotechnologyResearchCenter(UNAM),BilkentUniversity,06800Ankara,Turkey

a r t i c l e i n f o

Articlehistory:

Received22January2014

Receivedinrevisedform4April2014 Accepted12April2014

Availableonline21April2014

Keywords:

TiO2

Photocatalyst

Cross-linkeddivinylbenzene NO(g)oxidation

RhodamineB

a b s t r a c t

Polystyrenecross-linkeddivinylbenzene(PS-co-DVB)microsphereswereusedasanorganictemplate inordertosynthesizephotocatalyticTiO2microspheresandmicrobowls.Photocatalyticactivityofthe microbowlsurfacesweredemonstratedbothinthegasphaseviaphotocatalyticNO(g)oxidationbyO2(g) aswellasintheliquidphaseviaRhodamineBdegradation.Thermaldegradationmechanismofthepoly- mertemplateanditsdirectinfluenceontheTiO2crystalstructure,surfacemorphology,composition, specificsurfaceareaandthegas/liquidphasephotocatalyticactivitydatawerediscussedindetail.With increasingcalcinationtemperatures,sphericalpolymertemplatefirstundergoesaglasstransition,cover- ingtheTiO2film,followedbythecompletedecompositionoftheorganictemplatetoyieldTiO2exposed microbowlstructures.TiO2microbowlsystemscalcinedat600^◦Cyieldedthehighestper-sitebasispho- tocatalyticactivity.CrystallographicandelectronicpropertiesoftheTiO2microspheresurfacesaswell astheirsurfaceareaplayacrucialroleintheirultimatephotocatalyticactivity.Itwasdemonstratedthat thepolymermicrospheretemplatedTiO2photocatalystspresentedinthecurrentworkofferapromising andaversatilesyntheticplatformforphotocatalyticDeNOxapplicationsforairpurificationtechnologies.

1. Introduction

Shape-deﬁnednanoandmicro scale titaniumdioxide(TiO₂) structuresare widelyutilized asphotocatalytic systems;where theyhaveattractedaparticularinterestinenvironmentalapplica- tions.Ithasbeenreportedthatcontrollingparticleshape,geometry, size,surfacemorphology,electronicstructure,relativeabundance ofanatase/rutile surfacedomains andthenatureof thesurface functionalgroups(suchas OH)aresomeofthekeyfactorsfor designingefﬁcientTiO₂photocatalyticarchitectures[1–5].

TiO2 materials can be produced with unique morpholo- gies, shapes and structures at the micro/nanoscale revealing extraordinaryphysical,chemical,electronicandopticalproperties, renderingthesesystemsveryversatilephotocatalysts[3].Template directedsynthesisis oneoftheapproachesforﬁne-tuningsize,

夽 ElectronicSupplementaryInformation(ESI)available:Gas-phaseandsolution- phasephotocatalyticperformenceofP25.

∗ Correspondingauthor.Tel.:+903122902121;fax:+903122664068.

E-mailaddress:ozensoy@fen.bilkent.edu.tr(E.Ozensoy).

1 Web:http://www.fen.bilkent.edu.tr/∼ozensoy.

shapeand porosityofTiO2 particles[6–8].Inparticular,utiliza- tionoforganictemplatessuchaspolymersoffersvastopportunities forcontrollingtheshapesofinorganicmaterialsatthemicrome- ter/nanometerscale.Suchstrategiescanbeexploitedtosynthesize shape-deﬁnedTiO2 materialsexhibitingnano/microspheres[9], hollowstructures[10],tubes[11],wires[3],core–shellstructures [3],andegg-yolkstructures[12].

In thecurrent report,TiO2 microbowlswere synthesizedby usingpolystyrenecrosslinkeddivinylbenzene(PS-co-DVB)microspheres.Thepolymertemplate wasremovedbycalcinationand TiO2 microbowls were produced. The effect of the calcination temperatureonthestructuralpropertiesandactivityofthepho- tocatalystswerestudiedinthegasphaseaswellasinthesolution phaseoxidationreactions.

2. Experimental 2.1. Samplepreparation

Acustomsol–gelmethodcombinedwithapolymertemplating techniquewasusedforthesynthesisofTiO₂microbowlstructures http://dx.doi.org/10.1016/j.apsusc.2014.04.082

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Scheme1.SyntheticprotocolforPS-co-DVB-templatedTiO2microspheresandmicrobowls.

[13–15].Commerciallyavailablepolystyrenecross-linkeddivinyl benzene(PS-co-DVB)microspheres(Aldrich)withanaverageparti- clesizeofca.8␮mwereusedasthetemplatematerial.Preparation ofTiO2microspheresandmicrobowlsisshowninScheme1.First, equalmasses(i.e.1.0g)ofpolymermicrospheresandtitanium(IV) isopropoxide(TIP,97%,Aldrich)weremixedandstirredfor24h underambientconditions.Then,100mLofdeionizedwater(Milli- Q,18.2Mcm)wasaddedtothemixtureundercontinuousstirring (24h),wherehydrolysisandcondensationreactionswerecarried out.Then,microsphereswerevacuum-ﬁltered,washedwithdeion- izedwater and dried for 24hat 60^◦C inair. Later, thesample wascalcinedinair inordertoremovethepolymertemplateas wellastocrystallizetheinorganiccomponent(i.e.TiO₂).Samples werecalcinedatvarioustemperatures(200,300,400,500, 600, 700^◦C)in air for 2h(usinga heating rateof 8^◦C/min) to con- trolcrystallinity and surfacemorphology ofTiO₂ microspheres.

SynthesizedsampleswerenamedasPsTi-200,PsTi-300,PsTi-400, PsTi-500, PsTi-600, and PsTi-700 depending onthe calcination temperature.

2.2. Structuralcharacterization

Themorphologyandtheparticlesizeofthepolymertemplated TiO₂ microspheres and microbowlswere investigated by using aCarl-Zeiss Evo40environmentalscanningelectronmicroscope (SEM) equipped with a Bruker energy dispersive X-Ray (EDX) detector.Determinationofthecrystalstructureofthesynthesized materialswerecarriedoutwithaRigakuMiniflexX-raydiffractometer(XRD)equippedwithCuK␣radiationoperatedat30kV, 1.54 ˚Aand15mA.TheXRDpatternswererecordedinthe2range of10–60^◦withastepwidthof0.02s⁻¹.Ramanspectraofthesam- pleswerecollectedintherangeof200–1500cm⁻¹witharesolution of4cm⁻¹usingaHoribaJobinYvonLabRAMHR800spectrometer equippedwitha confocalRamanBX41 microscope.The Raman spectrometerwasequippedwitha Nd:YAGlaser (=532.1nm) where the laser power was 20mW. The thermal properties of the TiO₂ systems were also investigated by using thermo gravimetricanalysis(TGA).TGAmeasurementswerecarriedout between30and800^◦C(ataheatingrateof10^◦C/minandunder nitrogenflow)byusing aTA InstrumentsTGA-Q500setup.The specificsurfacearea(SSA)oftheTiO₂sampleswasdeterminedby

conventional Brunauer–Emmett–Teller (BET) N2 adsorption methodwithaMicromeriticsTristar3000surfaceareaandpore sizeanalyzer.PriortotheBETmeasurements,allofthesamples wereoutgassedinvacuumfor2hat150^◦C.

2.3. Photocatalyticperformanceanalysismeasurements

2.3.1. Gas-phasephotocatalyticoxidationperformance measurements

ReactivityoftheTiO₂ microstructureswasstudiedviaphoto- catalyticNOoxidation(NO(g)+½O₂(g)→NO₂(g)).Thegasphase photocatalyticactivityoftheTiO2microstructureswasanalyzedin acustom-madecontinuousflowreactionsystem,whichisshown inScheme2.Theexperimentalsetupwascomprisedofa high- puritygasmixturecontainingNO(g)(100ppmNO(g)inN2(g),Linde GmbH),O2(g)(99.998%,LindeGmbH)andN2(g)(99.998%,Linde GmbH) which was humidifiedwith 70%RH (relative humidity, measuredvia a Hanna HI9565 humidity analyzerat the sam- plepositioninthephotocatalyticreactor).Inatypicalgasphase photocatalytic performance analysis test, a total gas flow rate of1SLM(SLM,standardliters perminute)wasused,where the volumetric flow ratesof N2(g),O2(g)and NO(g)were settobe 0.750SLM, 0.250SLM and 0.010SLM via mass flow controllers (MFCs,MKS,1479A), respectively.Beforetheperformance tests, synthesizedTiO2 microsphere/microbowlpowder sampleswere dispersedonapoly-methylmethacrylate(PMMA)sampleholder (2×40×40mm³)andirradiatedwithUVAillumination(Sylvania UV-lamp,black-light,F8W,T5,368nm)underambientconditions for18hinordertoremovethesurfacecontaminationsandtoacti- vatethephotocatalysts.Afterthisactivationanddecontamination procedure,sampleswereinsertedintothephotocatalyticflowreac- torforperformanceanalysis.UVAilluminationsourceusedinthe performanceanalysistests(SylvaniaUV-lamp,black-light,F8W,T5, 368nm)generatedaUVAphotonfluxof7.5W/m²atthesample positionundertypicalreactionconditions.Duringtheperformance tests, reaction gases were swept over a 950mg photocatalyst sample and the concentration of NO(g), NO₂(g) and total NOx

(g)speciesinthephotocatalyticreactorwerequantitativelymea- sured online with a Horiba APNA-370 chemiluminiscence NO_x analyzer.

Scheme2.Gas-phasephotocatalyticperformanceanalysissetup.

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Gasphasephotocatalyticactivitymeasurementsarereportedin termsofpercentphotonicefﬁciencies(%)asdescribedinEqs.(1) and(2).

%= nNO_x

nphoton×100 (1)

wherenNO_xcorrespondstoeitherthedecreaseinthetotalnumber ofmolesofallgaseousNOxspeciesorthenumberofmolesofNO2(g) generatedina60min(i.e.3600s)photocatalyticperformancetest.

Ontheotherhand,n_photoncorrespondstothetotalnumberofmoles ofincidentUVAphotonsimpingingonthecatalystsurfacein3600s, whichcanbecalculatedthroughEq(2)as:

nphoton=(ISt)

(Nhc) (2)

whereIrepresentsthephotonpowerdensity oftheUVAlamp, experimentallymeasuredatthesamplepositioninthephotocat- alyticreactor(typically,7.5W/m²),istherepresentativeemission wavelengthoftheUVAlamp(i.e.368nm),Sisthesurfaceareaof thephotocatalystsampleholderinthereactorthatisexposedto theUVAirradiation(i.e.4cm×4cm=16cm²);tisthedurationof theperformancetest(i.e.3600s),NistheAvogadro’snumber,his Planck’sconstantandcisthespeedoflight.

2.3.2. Liquid-phasephotocatalyticoxidationperformance measurements

Liquid-phase photocatalytic oxidation activity of the TiO₂ microstructureswasdemonstratedbyphotodegradation[16–18].

Oxidative degradation of Sulforhodamine B (RhB, 95%, Sigma) underUVA irradiation (SylvaniaUV-lamp, F8W,T5, Black-light, 8W,368nm)wasconductedinabatch-modephotocatalyticreac- torofdimensions45×23×28cm³.AnaqueousRhBsolutionat concentrationof1mg/Land 30mgof TiO2 microstructureswas addedintothereactorandstirredcontinuouslyatastirringrateof 100rpm.Then,thephotocatalyticdegradationprocesswasstud- iedbymeasuringthechange inthe dyeconcentrationwithan UV–visspectrophotometer(Carry300,Agilent).Attenuationofthe majorabsorptionbandofRhB(564nm)associatedwiththeS₀→S₁ absorption[19]wasrecordedevery30minuntilthetestsolution becamevisuallytransparent.BeforetheUV–visabsorptionmea- surements,testsolutionswerecentrifugedandtheabsorbanceof theﬁltratewasrecorded.Byusingacalibrationcurve(R²=9994)of thedyesolution,thepercentdecolorizationefﬁciency(D_ef)ofthe systematanirradiationtimet(min)wascalculatedasdescribedin Eq.(3)[20].

D_ef(%)=(C0−Ct)

C0 ×100 (3)

InEq.(3),C₀andCtrepresenttheconcentrationofthetestsolu- tionbeforeandafterirradiationattimet,respectively.AplotofC₀/C versusirradiationtime(t)determinesthedecolorizationdegreeof thetestsolution.

3. Resultsanddiscussion

3.1. Structuralcharacterizationofpolymer-templatedTiO₂ microstructures

TheSEM images in Fig.1a–d illustratethe morphology and theparticlesizeoftheTiO2coatedPS-co-DVBmicrospheres.The particlesizevariationinthemicrostructuresstemsfromthecor- respondingsizedistributioninthenascentcommercialPS-co-DVB material.SEMimagesinFig.1a–dandthecorrespondingEDXmea- surements(Fig.1e)oftheTiO₂-coatedmicrospheresrevealedthat thesurfaceofthepolymermicrosphereswascoatedwithathin layerofTiO2andadditionalTiO2wasalsofurtherdeposited.

Fig.1.(a–d)SEMimagesand(e)arepresentativeEDXspectrumofTiO2-coated PS-co-DVBmicrospheresbeforecalcination.

UponcalcinationoftheTiO₂ coatedPS-co-DVBmicrospheres between200and700^◦C,signiﬁcantmorphologicalchangeswere observed. The microspheres were converted into microbowls (Fig.2).Thisobservationwasalsoaccompaniedbyaconsiderable weightloss,which willbediscussedfurtherinthetext(Fig.3).

Fig.2showstheSEMimagesandthecorrespondingEDXspectrum ofthepolymer-templatedTiO2microbowls,whichwerecalcined at600^◦Catambientconditionsfor2h.Duetodecompositionof thepolymertemplateandtheassociatedformationofHxCy(g)and HxCyOz(g),pressureaccumulationinsidethemicrosphereleadsto theruptureofthesphericalmorphologyduringtheevolutionof theentrappedgas.Theresultingopenmicrobowlstructuresare shownintheinsetofFig.2b.Theinteriorcavitiesofthemicrobowls haveanaveragediameterof8␮mwithanaveragewallthickness of600nm.TheEDX spectrumof themicrobowls(Fig.2b)indicates TiO2/TiOx content witha relativelyminor contributionof carbon-basedspecies.Ontheotherhand,EDXspectrumofthesame samplesbeforethecalcinationrevealedexcessiveCsignal(Fig.1e).

EvolutionofHxCy(g)andHxCyOz(g)andtheanticipatedweight loss of the sample upon the decomposition/degradationof the polymertemplatebelow600^◦Cisinperfectagreementwiththe TGAresultsshowninFig.3,whichshowasharpgravimetricloss

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Fig.2. (a)SEMimage,(b)EDXspectrumofPS-co-DVBtemplatedTiO2microbowls aftercalcinationat600^◦Cfor2h(insetshowsthedetailedmorphologyofthe microbowlsinSEM).

within400–500^◦C.TheTGAcurveofTiO₂-coatedPS-co-DVBmicro- spheres(Fig.3)exhibitsa2.7wt%lossinthetemperaturerangeof 30–250^◦Cduetotheevaporationofwaterandothervolatileorgan- ics.TiO₂ revealsa negligiblegravimetric losswithin30–800^◦C, whilepure/uncoatedpolystyreneundergoesalmost100wt%loss within 300–500^◦C due to decomposition/degradation [21–23].

Fig.3.TGAmeasurementforPS-co-DVBtemplatedTiO2microspheres.

Thus, TGAdata in Fig. 3, suggest that after the 71wt% loss at T>400^◦C,alargeportionoftheremainingsample,whichcorre- spondsto29%oftheoriginalsampleweight,isduetotheinorganic content(i.e.TiO₂).

In order toinvestigatethe inﬂuenceof thecalcination temperature on the photocatalyst structure and thephotocatalytic activity,sampleswerecalcinedatdifferenttemperatureswithin 200–700^◦C.Corresponding XRD patternsand Ramanspectraof thesesamplesarepresentedinFig.4.Calcinationat200and300^◦C leadstotheformationofanamorphousTiO₂/TiOxstructure,which startstocrystallizeintoaratherdisorderedanatasephaseat400^◦C withasmallaverageparticlesize,evidentfromthecorresponding broadanataseXRDdiffractionsignals(ICDDCardNo:21-1272)in Fig.4aandthecharacteristicallyintenseanataseRamanscattering observedat144cm⁻¹ [24–26].At500^◦C,awell-orderedanatase phasewithalargeraverageparticlesizeisformedascanbeseen fromthesharpandintenseanatasesignalsinbothXRD(Fig.4a) andRaman(Fig.4b)results.Atthistemperature,rutilephasealso appearsasasecondaryphaseinbothXRDresultsshowninFig.4a (ICDDcardno:04-0551)aswellasintheRamandatainFig.4b.For- mationoftherutilephaseleadstotheevolutionoftypicalRaman scatteringfeaturesat236,447,612,826cm⁻¹[24–26].Rutilephase becomesmorecrystallineandabundantathighercalcinationtem- peratures.Uponcalcinationat700^◦C,rutilebecomesthedominant phase,althoughanatasephasecanstillbedetectedasasecondary phase(Fig.4aandb).

3.2. Photocatalyticactivityofthepolymer-templatedTiO2

microspheresandmicrobowls

3.2.1. Gas-phasephotocatalyticoxidationperformance

ThephotocatalyticNO(g)oxidationwithO₂(g)wasusedasa modelreaction[27–32].Fig.5illustratesatypicalgasphasepho- tocatalyticperformanceanalysistestinwhichthephotocatalyst sampleisexposedtoafeedgasmixturecontaining1ppmNO(g) aswellasacertaincompositionofN₂(g)andO₂(g)witha70%RH (seeSection2fordetails).Fig.5showsthetime-dependentpro- ﬁlesforthetotalNOxconcentration(i.e.sumoftheconcentrations ofalloftheNOxspeciesexistinginthereactor,i.e.bluecurvein Fig.5)aswellas separateNO(g)(blackcurve) andNO₂(g)(red curve)concentrationsinthephotocatalyticreactormeasuredby thechemiluminiscenceNOxanalyzer.AsshowninFig.5,during theinitial15minoftheperformancetest,gasmixturecontaining 1ppmNO(g)isfedtothephotocatalystwhileUVAlampisinoff positionandthereactoriskeptindarkinordertopreventany exposuretosunlight.Thisleadstoaminortransientfallinthetotal NOx(g)andNO(g)concentrations,whichisassociatedwiththedilu- tionofthegasinthereactorpipelineandthethermaladsorption ofNOx speciesonthegaslines,reactorwallsaswellasonthe photocatalystsurface.Asthesystemiskeptindarkunderthese conditions,nophotocatalyticactivityisobservedduringthisini- tialstage,whichisevidentbythelackofanyNO₂(g)production.

Aftertheinitialtransientperiod,reactorwallsandthephotocata- lystsurfacearesaturatedwithNOx,afterwhichNOx(g)andNO(g) tracesquicklyreturntotheoriginalinletconcentrationvalueof 1ppm.

Next,UVAlampisturnedonandthephotocatalyticreaction isstarted. UponUVA radiation,asharpanda permanentfallin the NO(g) and total NOx(g) concentrations along witha quick risein NO2(g) signal,wereobserved.This iscaused byconver- sionofNO(g)intoNO₂(g)viaphotocatalyticoxidation.Inaddition, generated NO₂(g) can alsoadsorb onthe photocatalystsurface intheformofchemisorbedNO2,nitric/nitrousacid,nitritesand nitrates [24–26,33] and stored in the solid state, leading to a furtherdecreaseintheNO(g)signal.Furthermore,directphotocat- alyticdecompositionandphoto-reductionofNO(g)formingN2(g)

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100 200 300 400 500 600 700 800 A A A

PsTi-200 PsTi-300 PsTi-400

PsTi-500 PsTi-700

Raman Intensity (a.u.)

Raman Shift (cm-1) x20

R A

R R

A A

PsTi-600 A

R A R

A

10 000

10 20 30 40 50 60

Intensity (a.u)

2θ(deg) PsTi-700

PsTi-600

PsTi-500

PsTi-400 PsTi-300 PsTi-200

1000 R

R R AR R R A

A

AA R A

RR A R R A

R AA A

RR A R R A

A A A A

(a) (b)

Raman Intensity (a.u.)

Raman Shi (cm-1)

Fig.4. (a)XRDpatterns,(b)RamanspectraofPS-co-DVBtemplatedTiO2microspheres/microbowlsuponcalcinationat200^◦C,300^◦C,400^◦C,500^◦C,600^◦C,and700^◦Cfor 2hunderambientconditions(insethighlightsthedetailedRamanfeaturesofPsTi-600andPsTi-700samples).A:anatase,R:rutile.

and/orN2O(g)cannotberuledout[34].ThetotalNOxconcentra- tion(blue)curve(whichismostlycomprisedofthesumofNO(g) andNO₂(g)signals)inFig.5staysalwaysbelow1ppmduringthe UVA-activatedregime,illustratingthecontinuousphotocatalytic activity.

Gas-phasephotocatalyticperformancetestssimilartotheone giveninFig.5werealsoperformedonotherPS-co-DVBtemplated TiO₂microsphere/microbowlphotocatalysts,whichwerecalcined atvarioustemperaturesbetween200and700^◦C.Percentphotonic efficiencyvaluesderivedfromsuchexperimentsareshowninFig.6, wherebluebarsrepresentthe%photonicefficiencyoftotalNO_x(g) decrease,whileredbarscorrespondtothe%photonicefficiencyof NO2(g)production.

Fig.6showsthatPsTi-200samplerevealsbothconsiderableNOx

storage(bluebar)and NO₂(g) production(redbar) capabilities.

0 20 40 60 80

0.0 0.2 0.4 0.6 0.8 1.0

Concentration (ppm)

Time(min)

Thermal NOx adsorpon

Light-on Light-oﬀ

NOx (g)

NO_(g)

NO2(g)

Fig.5.Typicaltime-dependentconcentrationproﬁlesfortotalNOx(g),NO(g)and NO2(g)overPS-co-DVBtemplatedTiO2microbowlphotocatalyst(PsTi-600)during gas-phasephotocatalyticNOoxidationactivitytests.(Forinterpretationoftheref- erencestocolorinthisﬁgurelegend,thereaderisreferredtothewebversionof thisarticle.)

Ontheotherhand,uponincreasingthecalcinationtemperature to300^◦C,bothNOxstorageandNO₂(g)productionperformances wereobservedtodeclinedrastically.Ontheotherhand,aftercalci- nationat400^◦C,NOxstoragecapabilityisrecoveredwhileNO2(g) productionisstillnoticeablysuppressed.Above500^◦C,although NOxstorage capacitydecreasestoacertainextent,NO₂(g)pro- ductioncapabilityisfullyregained,reachingitshighestvalueat 600^◦C.Increasingthecalcinationtemperatureto700^◦Cleadstoa decreaseintheNOxstorageandNO₂(g)productionperformances simultaneously.

Interestinggas-phasephotocatalyticperformancetrendsgiven inFig.6canbeelucidatedbyusingthestructuralpropertiesofthe polymer-templatedTiO2microstructuresshowninScheme3.The crosslinkedpolystyrenesystemshavetypicalglasstransitiontem- peratures(Tg)within100–150^◦C,abovewhichthesolidpolymer tendstoswitchtoamobilemolten/glassystate[23].Ascanbeseen fromthespeciﬁcsurfacearea(SSA) resultsshownin Scheme3, PsTi-200samplehasamoderatelyhighSSA(86m²/g)suggesting that the mobilized PS-co-DVB microsphere template starts to segregateontheverytopsurface,onlypartiallycovering/blocking

Fig.6.ComparisonofthephotonicefﬁcienciesofTiO2microspheres/microbowls.

(Forinterpretationofthereferencestocolorinthisﬁgurelegend,thereaderis referredtothewebversionofthisarticle.)

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Scheme3.Temperature-inducedstructuralevolutionofTiO2microspheres/microbowls.

the amorphous TiO2/TiOx coating on the microsphere system.

Thus, at this calcination temperature, TiO₂/TiO_x coating is still partiallyaccessibleforgasphasephotocatalyticNOxstorageand NO2(g)production(Fig.6).

However,uponcalcinationat300^◦C,theSSAwasobservedto decreasebyabout50%,whichisaccompaniedbyatotallossofpho- tocatalyticNOxstorageandNO2(g)productionactivities(Fig.6).

Apparently,calcinationat300^◦Cleadstothesegregationofthe mobilized PS-co-DVB microsphere template onto the TiO₂/TiOx

coating(Scheme3).Hence,accesstothephotocatalyticactivesites toNO(g)iscompletelyblockedandthephotocatalyticactivityis entirelylost.

Increasingthecalcinationtemperatureto400^◦Cshowsaunique switchinthephotocatalyticactivity.Thisistheborderlinetem- perature, where the PS-co-DVB template starts to decompose leadingtotheruptureofthemicrospheresandformationofthe microbowls.Formationofmicrobowlsandeliminationofthecar- bonaceous/polymericﬁlmat400^◦Cisalsofullyconsistentwiththe drasticincreaseintheSSAofthesystemto159m²/g(Scheme3).

The increase in the SSA is alsoaccompanied by the formation of a cavity inside the microspheres due to the degradation of thePS-co-DVBtemplate,generatingadditionaladsorptionsites.At thistemperature,Ti-coatingrevealsmostlyanamorphous/porous nature, which also exhibits poorly crystalline anatase domains (Fig.4).Thus,duetothedecomposition/removalofthepolymer template,mostofthephotocatalyticactivesitesontheamorphous Ti-coatingbecomereadilyaccessibleandphotocatalyticNOoxida- tioncanbeperformedefﬁcientlywhichisevidentbytherecovery ofthephotocatalyticNOxstorage(bluebarforPsTi-400inFig.6).

AlthoughPsTi-400samplecanefﬁcientlyperformphotocatalytic NO_xstorage,yetitgeneratesarelativelysmallamountofNO₂(g).

ThiscouldbeduetothelargeSSAofthePsTi-400sample with alargenumberofadsorptionsitesthatcanimmediatelycapture

NO2(g)intheformofnitritesandnitratesontheTiO2surfaceand preventNO₂(g)slipintothegasphase.

Fig.6showsthatasthecalcinationtemperatureisincreased from400^◦Cto500^◦C,thephotocatalyticNOx storagedecreases signiﬁcantlyincontrasttothenoticeableincreaseintheNO₂(g) production.Within400–500^◦C,PsTisamplesundergoasubstantial crystallographictransformation(Fig.4),whereporousandamor- phous TiO2 domains crystalize into ordered anatase and rutile domainsresultinginasigniﬁcantlossintheSSA.Alongtheselines, PsTi-500samplehasaSSAof13.9m²/g(Scheme3).Thus,thepho- tocatalyticNOxstoragecapacityfallsinlinewiththecorresponding theSSAloss,suggestingthatNO₂(g)generatedviaphoto-oxidation readilyslipsintothegasphase.However,thisdoesnotmeanthat thephotocatalyticactivitydecreasesuponincreasingthetempera- turefrom400^◦Cto500^◦C.BycomparingthecombinedNOxstorage andNO₂formationresults(i.e.sumoftheredandbluebarsinFig.6) for400^◦Cand500^◦CalongwiththecorrespondingSSAvaluessug- geststhatPsTi-500samplehasaconsiderablyhigherper-sitebasis photocatalyticactivitywithrespecttoPsTi-400.

Fig. 6 indicates that the optimum gas-phase photocatalytic activityisreachedforthePsTi-600sample,whichrevealsalower anatase/rutileratio(Fig.4aandScheme3)estimatedbyXRDresults byusingtheapproachdevelopedbySpurrandMyers[35].Onthe otherhand,asthecalcinationtemperatureisincreasedto700^◦C, concomitanttothefurtherdecreaseintheanatase/rutileratio,pho- tocatalyticactivitystartstodecrease.Thus,itisapparentthatrather thanthesoleSSAvalues,crystallographicandelectronicproperties oftheTiO₂ microspheres/microbowlsplayamajorroleindeter- miningtheirultimategas-phasephotocatalyticactivities.

3.2.2. Solution-phasephotocatalyticoxidationperformance PhotocatalyticactivityofTiO₂microstructurecalcinedatdiffer- enttemperatureswasalsostudiedbyconventionalsolutionphase

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Fig.7. Time-dependentUV–VisabsorptionspectrashowingUVA-inducedpho- tocatalyticdegradation of RhB in thepresence ofPS-co-DVB templated TiO2

microbowlscalcinedat600^◦Cfor2h.

photocatalyticoxidationofRhB.Atypicalseriesoftime-dependent UV–visabsorptionspectraobtainedduringtheUVAirradiationis presentedinFig.7.ThisseriesofspectracorrespondstothePsTi- 600samplewhichiscomprisedofTiO₂microbowls(Fig.2).During thephotocatalyticreaction,thecharacteristicRhBabsorptionband locatedat564nm graduallydecreasesindicating photocatalytic degradation/oxidationofRhB.After330minofUVA irradiation, thedyesolutionbecomesvisiblycolorlessandthe564nmsignal vanishesalmostcompletely.

Time-dependent decolorization efﬁciency results for the remainingsamplesaresummarizedinFig.8a.Thesolutionphase photocatalyticoxidationexperimentscouldnotberealizedforthe PsTi-200andPsTi-300samplesduetolow densityofthecorre- spondingsolidphotocatalysts(originatingfromtheirhighpolymer content),whichresultsintheﬂoatingofthemicrospheresonthe

Fig.8.(a)Liquid-phasephotocatalyticreactivity ofPS-co-DVBtemplated TiO2

microspheres/microbowlsinRhodamineBphotodegradationviaUVAirradiation, (b)photocatalyst-containing1mg/LRhBsolutionsafter18hUVAirradiation.

aqueousmediumpreventingtheirefﬁcientmixingandhomoge- nousUVAexposure.Fig.8ashowsthatRhBconcentrationinthe solutiondecreasesmonotonicallywithincreasingirradiationtime whichis alsoillustratedinFig.8b(forphotocatalyst-containing 1mg/LRhBsolutionsafter18hUVAirradiation).Controlexperi- mentsperformedbyexposing1mg/LRhBsolutiontoUVAinthe absenceofaphotocatalyst(datanotshown)didnotleadtoany decolorizationundertypicalreactionconditions.Theliquid-phase photocatalyticactivityofthesynthesizedTiO₂structuresexhibits astrongdependenceonthecalcinationtemperature.Fig.8aclearly indicatesthatPsTi-600samplewhichhasamicrobowlstructure (Fig.2)andexhibitspredominantlyanatasephase(inadditionto rutileasasecondaryphase)revealsthehighestliquid-phasephotocatalyticactivity.ThePsTi-400sampleissigniﬁcantlylessactive thanalloftheanalyzedsamples(Fig.8),andiscomprisedofapoorly crystallineanatasephase(Fig.4).Thissuggeststhatsolution-phase photocatalyticactivityrequiresformationoforderedanatase/rutile crystallographicphases.Ontheotherhand,Fig.8aalsoshowsthat thesolution-phasephotocatalyticactivitytendstodecreaseatele- vatedcalcinationtemperaturessuchas700^◦C,suggestingthata rutile-dominantTiO2microbowlstructureisnotfavorable.

Itisworthmentioningthatthesolution-phasephotocatalytic reactivitytrendspresentedinFig.8acannotbeexplainedsolely basedontheSSAvaluesof thesynthesizedmaterials.Although PsTi-400 sample reveals a signiﬁcantly higher SSA than all of the other synthesized materials, it has a considerably lower liquid-phasephotocatalyticactivity(Fig.8).Inotherwords,crys- tallographic and the electronic properties of the TiO2-coated Ps-co-DVBmicrospheres/microbowlsseemtoplayamajorrolein theirliquid-phasephotocatalyticreactivity.

It isworth mentioningthat wehave alsoperformedsimilar liquid-phaseand gas-phase photocatalyticactivitytestsusing a benchmarkphotocatalyst(P25)(Figs.S1andS2,ESI†).Weobserved thattotalphotocatalyticactivityforP25inbothliquidandgasphase experimentswereabouttwotimeshigherthanthat ofthebest Ps-co-DVBtemplatedTiO₂ microsphere/microbowlphotocatalyst (PsTi-600).TheSSAofP25isabout50m²/g,whichisaboutmore than5timesgreaterthanthatofPsTi-600.Thus,per-sitebasispho- tocatalyticactivityofPsTi-600isstill 2.5timeshigherthanthat ofP25.Thissuggeststhatbyoptimizingthepolymermicrosphere templatingstrategy(forinstancebyusingpolymernanospheres withsmalleraverageparticlesizesandthushigherSSA),advanced photocatalyticsystemscanbedesigned,whichrevealhigherphoto- catalyticperformancebothintermsoftotalphotocatalyticactivity aswellasper-site-basisphotocatalyticactivity.Inaddition,further improvements in the photocatalytic performance of PS-co-DVB templatedTiO2microsphere/microbowlphotocatalystscanalsobe achievedbyincorporatingplasmonicmetalnanoparticlestothese systems[36].Suchexperimentaleffortsarecurrentlyunderwayin ourresearchgroup[37].

4. Conclusions

In this work, Ps-co-DVB microsphere templated TiO₂ photocatalysts were synthesized via sol–gel method. Inﬂuence of thecalcinationtemperatureonthestructuralpropertiesandthe photocatalytic activity of these systems under UVA excitation wereinvestigatedbothin thegasphase (bystudyingphotocat- alyticNO(g)oxidationbyO2(g))aswellasinthesolutionphase (by monitoring Rhodamine B photocatalytic degradation). The polymermicrosphereswerefoundtobecoveredwithathinﬁlmof TiO2/TiOxaswellasTiO2/TiOxnanoparticles.Photocatalyticactivity carriedoutinthesolutionphaseandinthegasphaseshowedthat the photocatalyst calcined at 600^◦C exhibiting a microbowl structure, yielded the highest per-site-basis photocatalytic

(8)

activity which is even greater than that of the commercial benchmarkP25.Ourﬁndings indicatethatnotonlythespeciﬁc surfaceareabutalsothecrystallographicandelectronicproperties oftheTiO₂microstructuresplayamajorroleindeterminingtheir ultimate photocatalytic activities. This suggests that polymer- templated TiO₂ microstructures offer a promising versatile syntheticplatformforphotocatalyticDeNOxapplications,which canbefurtherimprovedbyusingpolymernanospheretemplates withhigherSSAorbyadditionalfunctionalizationwithtransition metalnanoparticlesand/orplasmoniccomponents.

Acknowledgments

AuthorsgratefullyacknowledgeAssociateProf.Dönüs¸Tuncel for fruitful discussions,and Zafer Sayfor performing BET measurements.E.O.alsoacknowledgesfinancialsupportfromTurkish AcademyofSciencesthroughthe“TUBA-GEBIPOutstandingYoung Scientist Prize” and from Fevzi Akkaya Science Fund (FABED) throughEserTümenScientificAchievementAwardaswellasthe Scientific and Technical Research Council of Turkey (TUBITAK) (ProjectCode:109M713).

AppendixA. Supplementarydata

Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10.1016/j.

apsusc.2014.04.082.

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