Hierarchical synthesis of corrugated photocatalytic TiO2 microsphere architectures on natural pollen surfaces

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g 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

Full

Length

Article

Hierarchical

synthesis

of

corrugated

photocatalytic

TiO2

microsphere

architectures

on

natural

pollen

surfaces

Deniz

Altunoz

Erdogan,

Emrah

Ozensoy

∗,1

DepartmentofChemistry,BilkentUniversity,06800,Ankara,Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received12October2016

Receivedinrevisedform7January2017 Accepted12January2017

Availableonline17January2017 Keywords: TiO2 Photocatalyst Ambrosiatriﬁda NO(g)oxidation RhodamineB

a

b

s

t

r

a

c

t

Biomaterialsarechallenging,yetvastlypromisingtemplatesforengineeringunusualinorganicmaterials withunprecedentedsurfaceandstructuralproperties.Inthecurrentwork,anovelbiotemplate-based photocatalyticmaterialwassynthesizedintheformofcorrugatedTiO2microspheresbyutilizingasol-gel

methodologywhereAmbrosiatriﬁda(Ab,Giantragweed)pollenwasexploitedastheinitialbiological sup-portsurface.HierarchicallysynthesizedTiO2microsphereswerestructurallycharacterizedindetailvia

SEM-EDX,Ramanspectroscopy,XRDandBETtechniquesinordertoshedlightonthesurfacechemistry, crystalstructure,chemicalcompositionandmorphologyofthesenovelmaterialarchitectures. Photo-catalyticfunctionalityofthesynthesizedmaterialswasdemonstratedbothingasphaseaswellasin liquidphase.Alongtheselines,airandwaterpuriﬁcationcapabilitiesofthesynthesizedTiO2

micro-sphereswereestablishedbyperformingphotocatalyticoxidativeNOx(g)storageandRhodamineB(aq) degradationexperiments;respectively.Thesyntheticapproachpresentedhereinoffersnew opportuni-tiestodesignandcreatesophisticatedfunctionalmaterialsthatcanbeusedinmicroreactorsystems, adsorbents,drugdeliverysystems,catalyticprocesses,andsensortechnologies.

1. Introduction

Hazardouschemicalsarisingfromcombustionoffossilfuels, suchassulphurdioxide,nitrogenoxides,mercury,aswellas indus-trialwastematerialincludingorganicdyes,andsolventsareamong theprominentcontaminantscontributingtothewater,airandsoil pollution;causingawidevarietyofseverehealthand environmen-talproblems[1–5].Heterogeneouscatalysisplays animportant roleincopingwiththeenvironmentalpollutionattheglobalscale. Oneofthemostabundantrenewableenergysourcesthatcanbe exploitedinheterogeneouscatalysisapplicationsinan environ-mentallyfriendlyandeconomicalmanneristhesolarenergy.Thus, thereexistsanimmensedemandtodevelopnovelphotocatalytic materialsusinginnovativesyntheticmethodologies[6–11].

Alargevarietyofmaterialssuchasmetaloxides,metal hydrox-ides,metalcarbides,metal nitrides,carbonallotropes and their derivativeshavebeeninvestigatedintheliteratureas photocat-alysts[12–18].Amongthem,titaniumdioxide(TiO2)hasreceived

considerableattentionsincethesuccessfulgenerationofH2from

waterviaelectrochemicalphotolysisof waterby Fujishimaand

∗ Correspondingauthor.

E-mailaddress:[email protected](E.Ozensoy). 1 _Web:_{http://www.fen.bilkent.edu.tr/∼ozensoy}

Honda[19].TiO2hasbeenthemostfrequentlyutilized

photocat-alyticmaterialduetoitsfunctionalversatilityinawiderangeof processessuchasenergystorage/conversion,photocatalytic pollu-tionabatement,andbiotechnology[20–22].

It is well known that physical and chemical properties of materialssuchasshape, texture,particlesize,porosity,specific surfacearea,crystallinity,electronicbandgap,surfacedefectsand surfacefunctionalgroupsdirectlyinfluencethephotocatalytic per-formance. Particularly, shape and surface structural properties ofphotocatalytic materialscanbecloselylinkedtothe reactiv-ity and selectivity of these systems [23–25]. One of the most efficientandsimpleapproachestopreparesophisticatedsurface structuresonmaterialsistemplating.Natural/biologicalstarting materialscanbeusedastemporalsupportsystems/sacrificial tem-platesinordertocreatewell-definedshapes,sizesandtextures onsurfaces.For instance,mesoporous hollowSnO2 microfibers

were prepared using natural kapok (Ceiba pentandra) ﬁber as a template and were found to be photocatalytically active in methylene blue dye degradation under UV irradiation [26]. In another study, freshnatural rose (Rosa hybrida L.)petals were used as a template to synthesizeTiO2 ﬂakes exhibiting higher

photocatalyticactivitythanthecommercialDegussaP25 photocat-alyst[27].Also,cerium-dopedTiO2mesoporousnanoﬁberswere

preparedby a single-potsynthesis methodusing collagenﬁber

http://dx.doi.org/10.1016/j.apsusc.2017.01.107

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160 D.A.Erdogan,E.Ozensoy/AppliedSurfaceScience403(2017)159–167

biotemplates[28].A variety of synthetic techniqueshave been developedtodepositthedesiredphotocatalyticmaterialonthe sur-faceofbio-templatesincludingsputtering,sol-gel,electrochemical deposition,andchemicalvapourdepositionapproaches.Usingsuch syntheticmethods,micro/nanostructuressuchaswires,tubes,rods orspherescanbefabricatedpreservingtheoriginalshapeandsize oftheinitialnaturaltemplate.Pollengrainsattractattentionas ver-satilebiotemplatesduetotheiruniqueandsophisticatedsurface structuresatthemicro/nanoscale[7,29–33].

Thus,inthepresentstudy,asimplebiotemplateassisted sol-gelrouteispresentedinordertosynthesizeTiO2 photocatalytic

microsphereswithuniquesurfacemorphologies,whereAmbrosia triﬁda (Ab, Giant ragweed) pollen is used as the starting bio-substrate. Ab is selected as a biotemplate due to its unusual micron-sizedsurfacemorphologyexhibitingconicalnano-spikes. Upondetailed structural characterizationof this novelmaterial platform,photocatalyticfunctionalityofthesehierarchicalsystems underultraviolet-A(UVA)irradiationisalsodemonstratedattwo differentinterfacesnamely,RhodamineB(RhB)photodegradation attheliquid/solidinterfaceaswellasthephotocatalyticoxidative storageofNOx(g)atthegas/solidinterface;illustratingthecatalytic versatilityofthisnewfamilyofmaterials.

2. Experimental

2.1. Materials

Ambrosiatriﬁda(Ab,Giantragweed)pollenswereobtainedfrom Bonapola.s.Company(CzechRepublic).Titanium(IV)isopropoxide (TIP, 97%), ethanol (_≥99.8%), and Rhodamine B (RhB, dye con-tent ∼95%)were purchased fromSigma-Aldrich (Germany).All chemicalswere of analytical grade and used as received with-out any further treatment. Milli-Q ultra-pure deionized water (18.2Mcm)wasalsousedasasolvent.

2.2. SynthesisofbiotemplatedTiO2microspheres

BiotemplatedTiO2microsphereswerepreparedusingamethod

analogousto theone described in one of ourprevious reports

[7].Brieﬂy,Ab pollenswerewashedwithanhydrousethanolto removesurfaceimpuritiesandsubsequentlydriedunderambient conditionsfor48h. Then,titanium(IV)isopropoxide(TIP,4mL) precursorwasmixedwithethanol(2mL)foraperiodof10min atroomtemperature.100mg cleanAb pollen(i.e.,biotemplate) wasaddedtothepreparedprecursorsolutionandtheslurrywas stirredvigorouslyfor30min.Afterdepositingprecursorsolution ontheoutersurface(i.e.exine)ofthebiotemplate,themixture wasﬁlteredtoremovetheexcessdecantate.Coatedsamplewas agedfor60minunderambientconditionsinordertoallowforthe hydrolysisandpolycondensationreactionstoproceed,formingan amorphousTiO2shellonthebiotemplatesurface.Then,calcination

stepswereexecutedinamufﬂefurnaceatvarioustemperatures varyingwithin400◦C–900◦C(for2.5hpercalcinationstep)inair, wherethesacriﬁcialbiotemplatewaseliminatedandthe crystal-lizationandorderingoftheTiO2overlayerwereachieved.Products obtainedattheendofthesynthesisprotocolarenamedasAbTi-X, whereXindicatesthecalcinationtemperature.

2.3. Characterization

Surfacestructureandmorphologyofthesampleswere investi-gatedviaaCarl-ZeissEvo40scanningelectronmicroscope(SEM) withanacceleratingvoltagevaryingwithin5–10kV.For elemen-talanalysis,energydispersiveX-ray(EDX)analysisofthepowder samplesdispersedonanelectricallyconductivecarbonﬁlmwas performedusinganacceleratingvoltageof10kV.

Crystallographicchangesonthesamplesaftercalcinationwere determined via XRD measurements performed using a Rigaku (Japan)X-raydiffractometerequippedwithaMiniﬂexgoniometer andamonochromatedhigh-intensityCuK␣radiation(␭=1.5405Å, 30kV,15mA)source.XRDdatawerecollectedbyscanningthe2␪ rangewithin10–60◦ usingastepsizeof0.02◦s−1.Identiﬁcation oftheunknownphasesin thepowder XRDdataweremadeby utilizingPowderDiffractionFile(PDF)databasemaintainedbythe InternationalCentreforDiffractionData(ICDD).

RamanexperimentswerecarriedoutusingaLabRAMHR800 spectrometer(HoribaJobinYvon,Japan)equippedwithaNd:YAG laser(␭=532.1nm,20mW)andanintegratedconfocalOlympus BX41microscope.Thesystemwascalibratedusingthereference Si Ramanshiftat 520.7cm−1 byadjusting the zero-order posi-tionofthegrating.Powdersamplewasevenlyspreadonasingle crystalSiwaferandRamanspectrawererecordedintherangeof 100–1500cm−1withaspectralresolutionof4cm−1atroom tem-perature.

TheBrunauer-Emmett-Teller(BET)SSAmeasurementsofthe synthesized catalystsweredetermined by nitrogen adsorption-desorptionisothermsusingaMicromeriticsTristar 3000surface areaandporesizeanalyser.PriortoSSAanalysis,allsampleswere outgassedinvacuumfor2hat150◦C.

2.4. Liquidphasephotocatalyticactivitytestsforthedegradation ofRhB(aq)

Photocatalyticfunctionality of thebiotemplatedTiO2

micro-spheres in liquid phase was demonstrated via RhB (aq) dye degradationunderUVA irradiationat roomtemperature.RhB is afrequentlyusedmodelpollutantfortesting thephotocatalytic activityofnovelmaterialsinwater.RhBdegradationexperiments wereperformedinaphotocatalyticreactorequippedwithan8W SylvaniaUVA-lamp(F8W,T5,Black-light,368nm).Acoolingfan wasalsoinstalledinsidethereactorfortemperatureregulation. Ini-tially,a48mLaqueoussolutionofRhB(10mgL−1)wasprepared indarkand25mgofbiotemplatedTiO2microsphereswere

ultra-sonicallydispersedinthissolutiontoformasuspension.Then,the samplecontainerwasplacedataspeciﬁedpositioninsidethe pho-tocatalyticreactor,wherethedistancebetweenthelightsourceand thesuspensionwasﬁxedat13cm.PriortoUVAlightirradiation, thesuspensionwasmagneticallystirredinsidethereactorunder darkconditions for 30minin order toestablish an adsorption-desorptionequilibriumbetweenthephotocatalystandRhB(aq). BeforetheUVAlightexposure,a3mLaliquotwasextractedfrom thesuspensionunderdarkconditionsandtheconcentrationofthis startingsolutionwasdesignatedasC0. Then,identicalamounts

ofsampleswereobtainedduringtheUVAlight irradiationafter certaintimeintervalswhoseconcentrationsweredenotedasCt. Afterremovingthephotocatalystfromtheextractedsamplesvia centrifugation,RhBconcentrationoftheextractedsolutionswere determinedusingaUV–visspectrophotometer(Carry300,Agilent) withthehelpofacalibrationcurveutilizingtheRhBcharacteristic maximalabsorptionbandatca.553nm.Thetypicalphotonpower density(irradiance)duringtheexperimentswas7.4Wm−2which wasmeasuredbyaphotoradiometer(DeltaOhm,HD2302.0,Italy) equippedwithaUVAprobe(DeltaOhm,LP471UVA).The photocat-alyticdyedegradationefﬁciency(D_eff)ofthephotocatalystswas calculatedaccordingtofollowingequation;

Deff(%)= (C0_C−Ct)

0 ×

100 (1)

where,C0istheinitialRhBconcentrationandCtistheRhB concen-trationatagiventimet.

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Fig1. (a)EDXspectrumofthebare(uncoated)Abpollenobtainedfromtheblue-colouredcircularregionin(b).(b)SEMimageoftheuncoatedAbpollen.(c)Schematic describingthevariousbio-structuralsectionsoftheAbpollen.(d)EDXspectraobtainedaftercalcinationoftheuncoatedAbpollensat800◦_C._Red_and_black_spectra_were obtainedfromthecircularregionswiththecorrespondingcolourspresentedin(e).(e)SEMimageoftheuncoatedAbpollensaftercalcinationat800◦_C._(For_{interpretation} ofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

2.5. Gasphasephotocatalyticactivitytestsfortheremovalof gaseousnitricoxide

Inordertodemonstratethefunctionalversatilityofthe syn-thesizedbiotemplatedTiO2microspheres,inadditiontotheliquid

phase photocatalytic tests, obtained materials were also used inphotocatalyticoxidative storageofNO atthesolid/gas inter-face. Photocatalytic removal of NO(g) over biotemplated TiO2

microsphereswasperformedatroomtemperatureina custom-madecontinuousphotocatalyticﬂowreactorwhichwasdesigned consideringtheISO22197-1:2007standard[8–10,34].This pho-tocatalyticreactionsystemwascomposedofa gassupply unit, aﬂat-bedphotoreactorchamberhousingthesample,aUVA illu-minationsourceand a chemiluminescentNOxanalyser (Horiba APNA-370) for continuous inline monitoring of the NO, NO2

andtotalNOxconcentrations[8–10].Inthegassupplyunit,NO (100ppm NO in N2 balance, Linde GmbH) was mixed withO2

(99.998%,LindeGmbH) andN2 (99.998%, LindeGmbH)atroom

temperature.Thetotal gasflow rateinthereactorwaskeptat ca.1.0SLM(standardlitersperminute)viamassflowcontrollers (MFCs, MKS,1479A)byadjusting theflowrateof eachgas(i.e. N2=0.75 SLM,O2=0.25 SLM, and NO=0.01 SLM).The gas

mix-turewasalsopassedthroughawaterbubblerbeforethereactor forhumidiﬁcationandtherelativehumidity(RH)ofthegas mix-turewasmeasuredviaaHannaHI9565humidityanalyzeratthe samplepositionin thephotocatalytic reactoratroom tempera-ture.RHwasdetectedtobetypicallyca.70%atroomtemperature duringthemeasurements.Synthesizedphotocatalystpowder sam-ples(250mg)weregentlypressedonapoly-methylmethacrylate (PMMA)sample holder (2×20×20mm3₎ _to_produce _a _smooth

surface. Inorder toactivatethephotocatalysts and remove the initialsurfacecontaminants,beforethegasphasephotocatalytic activitymeasurements,sampleswereexposedtoUVAirradiation underambientconditionsfor18h.Then,thesamplewasplaced intothephotocatalyticreactor,whereaUVAlamp(Sylvania UV-lamp,black-light,F8W,T5,368nm)wasplacedabovethereactor. Next,thegasmixturewasfedtothephotocatalyticreactor,where thegasfeedsweptthesurfaceofthepowderphotocatalystsample. Afterestablishingtheadsorption-desorptionequilibriuminsidethe photocatalyticreactor,UVAilluminationsourcewasactivatedto initiatethephotocatalyticreaction.Controlexperimentscarried out intheabsence ofa photocatalyst(i.e.in theempty reactor undertheUVAillumination)revealednocatalyticconversion. Pho-tocatalyticconversion efﬁciencyfor NOand photocatalyticNO2

productionefﬁciency(␨%)overTiO2microsphereswerecalculated

asfollows:

%=nNOxornNO2

nphoton ×100 (2)

where, nNOx is thedecreaseinthetotal numberof molesof all

gaseous NOx species and nNO2 is the number of moles of NO2

generatedin 60min(i.e.over thecourseofafullphotocatalytic NOxremovalexperiment).Inthisequation,nphotoncorrespondsto

thetotalnumberofmolesofincidentUVAphotonsimpingingon thecatalystsurfaceduringthe60mintime interval.nphoton was

calculatedby usingthephoton powerdensityoftheUVAlamp (I=7.4Wm−2), representativeemission wavelengthof the UVA lamp(␭=368nm),surfaceareaofthesampleholderthatisexposed to theUVA irradiation (S=2cm×2cm=4cm2_), _duration _of _the

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Fig.2.SchematicillustrationofoneofthepossiblesyntheticroutesleadingtotheformationofbiotemplatedTiO2microspheres:(i)coatedAbpollen,(ii)biotemplatedTiO2 microspheresaftertheremovaloftheAbpollenbycalcination.

photocatalytictest(t=3600s),Avogadro’snumber(N_A),Planck’s constant(h),andthespeedoflight(c)asshowninEq.(3)below:

nphoton=_NISt

AhC (3)

3. Resultsanddiscussion

3.1. Structureandmorphology

Surface elemental composition and the morphology of the uncoatedAbpollengrainswereinvestigatedusingSEMandEDX techniques(Fig.1aandb).Macroscopicstructuralcomponentsof theAbpollenswerealsoschematicallydescribedinFig.1c.Ascan beseeninFig.1c,Abpollensarecomposedoftwonestedlayers coveringthelivingmatterandprotectingitagainsttheexternal physicaland chemical adverseeffects [35,36].The robustouter surfacecalledexine(Fig.1c)iscomposedofahighlycrosslinked organicsubstancethatcanincludefattyacids,phenylpropanoids, andphenolicsporopollenins.Theinnerlayerofthepollen(Fig.1c)is calledtheintineandisprimarilycomposedofcellulosicmaterials andpolysaccharides[35,36].Inordertostudythemorphological andstructuralalterationsoccurringuponcalcination,uncoatedAb pollensampleswereinvestigatedcomparativelybySEMandEDX analysisbeforeandaftercalcination(Fig.1a–e).Fig.1bshowsthat, uncoatedAbpollenshaveasphericalshapewithanaveragepollen sizeof23.5±1.5␮mdecoratedwithconicalnano-spikes/thorns. Afterthecalcinationoftheuncoatedpollensat 800◦C, obvious structuraland morphological changes wereobserved signifying visible geometric deformation (Fig. 1e). According to the EDX spectragiveninFig.1a,whileuncoatedmicrosphereshavea car-bonaceousoutermostlayerexhibitingmainlyCandOsignalsbefore calcination,uponcalcinationat800◦C,pollensseemtolosetheir structuralintegrityanddeformfromtheiroriginalshapes, reveal-ingavarietyofEDXsignalscorrespondingtoelementssuchC,O, Mg,P, S, K,and Ca(Figs.1dand e). It islikely that duringthe calcinationprocess,outerexinelayerof theuncoatedpollensis partiallydestroyedandthebiologicalmaterialinsidetheintine cap-sule,whichmayinvolvevariousmineralsforvitality,diffusetothe

Fig.3.SEMimageandthecorrespondingEDXspectrumoftheAbbiotemplateafter titanium(IV)isopropoxide(TIP)depositionat25◦_C_(AbTi-25).

surfaceatelevatedtemperatures,leadingtothedetectionofthe EDXelementalsignalsforC,O,Mg,P,S,K,andCa(Figs.1dande).

Inthecurrentwork,Abpollenswereselectedasabiotemplate todirecttheformationofbiomorphicTiO2microspheresusingthe

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sol-gelprocess.Fig.2providesoneofthepossiblereaction path-waysforthesol-gelsyntheticrouteusedherein.Asillustratedin

Fig.2,after thealcoholysisreaction,metalalkoxidespecies are expected tobind tothe naturally functionalizedsurface of the pollentemplate throughcondensationandhydrolysis reactions. Extent of themetalalkoxide depositionand thecorresponding thicknessoftheultimateTiO2 overlayerwerecontrolledbythe

composition/concentrationoftheprecursorsolutionaswellasthe durationofthealcoholysisreaction.AftertheformationoftheTiO2

overlayer,calcinationprocessleadstotheformationofa biomor-phicTiO2surfacepreservingmostoftheoriginalshape,size,and

morphologyoftheAbbiotemplate.

Fig. 3 shows the SEM image and the corresponding EDX spectrum of the TiOx deposited Ab pollens after hydrolysis andpolycondensationreactionsatroomtemperature(i.e.before calcination). SEM image reveals the formation of a homoge-neous/continuous TiOx overlayer preserving the characteristic microstructureofthenascentpollensurface.Thisisalsoevident bytheEDXspectruminFig.3,indicatingastrongTisignal over-whelmingthatoftheotherpre-existingelementsonthesurface suchasCa,S,K,andP.

Calcination was employed in order to convert amorphous TiOx coating onthe Ab pollensinto crystallineTiO2 overlayers

(Fig.4).Low-magniﬁcationSEMimage(Fig.4a)shows thatTiO2

microspheresare relativelywell-dispersed rather than severely aggregated.Whilethecalcinationprocessinducesthe crystalliza-tionoftheTiOxoverlayertoTiO2,italsoleadstomorphological

modiﬁcationsatthenanometerscaleresultingintheformationofa spongy/porousandacorrugatednetworkonthesurface(Fig.4b–d andf).ComparisonofthebareAb(Fig.1b)orcoatedAbpollens beforecalcination(AbTi-25,Fig.3),withtheonesobtainedafter cal-cination(e.g.600◦Cand800◦C)suggestsdeformationofthesharp conicalspikes(Fig.4b–dandf),inadditiontotheshrinkingofthe pollenstoasmalleraveragediameterofca.13␮m.

Aftercalcination(Fig.4e),P,K,andCasignalsoriginatingfrom thebiotemplatebecomesdiscernibleontheAbTi-600andAbTi-800 surfaces.ItcanbeseeninFigs.4band4ethatanaperturewith anapproximatediameterof2.5␮mexistsintheAbpollen struc-ture,fromwhichthepollentubeextendsatgerminationtofertilize theovum.Thus,itisfeasiblethatduringthecalcinationprocess,the interiorpartofthepollen(i.e.intineandotherbiologicalliving mat-terdepictedinFig.1c)diffusesoutboundthroughthisapertureand spilledoverontheTiO2surfaceatelevatedtemperatures.

Fig.5aandbillustratethephasechangesoccurringontheAbTi materialsasa function ofcalcinationtemperature viaXRDand Ramanspectroscopy,respectively.Itisapparentthatthediffraction signalsinFig.5a intensifyand sharpenwithincreasing calcina-tiontemperaturessuggestingorderingandcrystallizationofthe TiO2 overlayer on the AbTi surface. XRD patterns of the AbTi

microspheresrevealpredominantlyanatasephase atcalcination temperatures≤600◦_C;_while _two_different_ordered_TiO₂ _phases

namely,anatase(ICDDNo.00-021-1272)andrutile(ICDDNo. 00-021-1276)arevisibleforthecalcinationtemperaturesabove600◦C (Fig.5a).Also,Fig.6depictsrelativemassfractionofanataseand rutilephasesforvarioussamplescalculatedusingtheXRDdatavia SpurrandMyersapproach[37].Itisclearthattheanatasetorutile phasetransitionontheAbsurfacestartstooccurpredominantlyat T≥800◦C.RutilemassfractionincreasesdrasticallyforT≥800◦C, whilefortheAbTi-900sample,anataseandrutilephasesreveal almostequalmassfractions.

Averagecrystallitesizesoftheanataseandrutilephaseswere alsocalculatedusingtheanatase(101)andrutile(110)diffraction signalsviaScherrerequation[38,39](Fig.6).Theseresultssuggest thatanataseandrutiledomains havesimilaraveragecrystallite sizes(ca.20–30nm)forAbTi-700andAbTi-800sampleswhilethey drasticallydivergefromeachotherfortheAbTi-900sample,where

rutilecrystallitesizesurpassesthatoftheanatase(ca.47nmfor anataseandca. 134nm forrutile).Fig.6indicatesthat increas-ingcalcinationtemperaturesresultsinamonotonicincreaseinthe crystallitesizesofanataseandrutiledomainsduetosintering.

Ramanspectroscopic measurementswerealso performedin ordertoconﬁrmthestructuralpropertiesofthebiotemplatedTiO2

microspheres.Fig.5bdisplaystheRamanspectraofthesamples preparedby calcinationof thecoatedAbTi samplesat different temperatures.CharacteristicanataseRamanscatteringfeaturesat 147cm−1(Eg),397cm−1(B1g),515cm−1(A1g),and641cm−1(Eg) areobservedforallsamplesexcepttheAbTi-400sample.Forthe AbTi-900sample, additionalRamanpeaks at445cm−1 (Eg)and 612cm−1 (A1g)arevisiblewhichcanbeattributedtotherutile phase.In goodaccordancewiththecurrent XRDmeasurements (Fig.5a),RamandatainFig.5balsoindicatethattherutilecontent ofthesamplesincreaseswithincreasingcalcinationtemperatures. Inadditiontothesesignals,someoftheRamanspectrainFig.5balso includesanadditionalfeatureat484cm−1(labelled“withthe sym-bol“”inFig.5b)whichcantentativelybeattributedtocomplex temporalspeciesgeneratedduringthecalcinationofthe biopoly-mermatrixoftheunderlyingAmbrosiatemplate[7].

3.2. PhotocatalyticactivityofthebiotemplatedTiO2microspheres

Fig.7apresentsthephotocatalyticRhB(aq)degradationstudies performedunderUVAirradiationatroomtemperaturebyusing biotemplatedTiO2microspherescalcinedatvarioustemperatures.

Fig.7bshowsatypicalseriesoftime-dependentUV–vis absorp-tionspectraoftheRhB(q)containingtheAbTi-800sampleobtained duringtheUVAirradiation.ItisapparentthatthecharacteristicRhB absorptionbandat553nmgraduallydecreaseswhilethe photocat-alyticdyedegradationreactionproceeds.After120minUVAlight exposure,colororiginatingfromRhBdyeisvirtuallydisappears evi-dentbythevanishingabsorptionsignalat553nm.Notethatthe photocatalyticRhB(aq)degradationperformanceoftheAbTi-400 sampleisnotreportedinFig.7.Thisisduetothefactthatsuchlow calcinationtemperaturesdonotallowthecompleteremovalofthe biotemplatewhichinturn,leadstotheformationofgrainswithlow materialdensitythatcanﬂoatonthetopoftheRhB(aq)solution preventingtheirhomogenousmixinganduniformirradiation.

Asastandardcontrolexperiment,measureddecreaseinRhB concentrationoftheRhB(aq)solutionunderUVAirradiationinthe absenceofacatalystwasalsomonitoredinordertoinvestigatethe non-catalyticselfphotodegradationoftheRhBdye(Fig.7a).Ascan beseeninFig.7a,within500–800◦C,increasingcalcination tem-peratureleadstoamonotonicenhancementinthephotocatalytic RhB(aq)degradation.However,calcinationathighertemperatures suchas900◦CresultsinanattenuationofthephotocatalyticRhB (aq)decompositionperformance.Basedonthestructural charac-terizationdataprovidedinFig.6andtheliquidphasephotocatalytic activitydatagiveninFig.7,itcanberealisedthattheoptimum photocatalyst samplefor RhB (aq) degradation (i.e.AbTi-800)is comprisedofbothanataseandrutiledomainswithaspeciﬁc sur-faceareaofca.7–8m2_/g._It_is_also_apparent_that_the_monotonic

increaseinthephotocatalyticRhB(aq)decompositionperformance within500–800◦Cisconcomitanttotheincreaseinthecrystallinity aswellastheaveragesizeoftheanatasedomains,wherethelatter convergestoca.30nmfortheAbTi-800sample.Fig.6alsosuggests thattheAbTi-800sampleiscomprisedof94.9%anataseand5.1% rutilebymass.Ontheotherhand,atelevatedcalcination tempera-turessuchas900◦C,relativerutilemassfractionincreasestovalues abovetheoptimalvalue,leadingtoattenuationinthe photocat-alyticperformance(Figs.6and7).Itisapparentthattheoptimum bio-templatedphotocatalyststudiedinthecurrentworkforthe RhB (aq) degradation processes possesses co-existing anatase and rutile domains functioning in a synergistic manner with

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Fig.4.SEMimagesofthebiotemplatedTiO2microspherescalcinedfor2.5hinairat(a)800◦C(lowmagniﬁcationimage,AbTi-800)(b)600◦C(AbTi-600),and(c,d,and f)800◦_C_(AbTi-800)._Image_(d)_also_emphasizes_the_SEM_image_showing_the_detailed_morphology_of_the_AbTi-800_pollen_surface_exhibiting_a_porous_and_a_corrugated_TiO₂ overlayerstructure.(e)EDXspectraobtainedfromthecircledregionslabelledas1,2,3in(f).

Fig.5.(a)XRDpatternsand(b)theRamanspectraofthebiotemplatedTiO2microspherescalcinedat400,500,600,700,800,and900◦Cfor2.5hinairaftercoating.“A”and “R”letterscorrespondtoanataseandrutilephases;respectively(seetextfordetails).

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Fig.6.VariationoftheanataseandrutileaveragecrystallitesizesandmassfractionsonthecoatedAbTisamplesasafunctionofcalcinationtemperature.

Fig.7. (a)PhotocatalyticRhB(aq)degradationperformanceofbiotemplatedTiO2microspheresunderUVAilluminationatroomtemperature.Measurementlabelledasthe “RhBSolution”wasperformedintheabsenceofaphotocatalystunderUVAirradiation.(b)Time-dependentUV–visabsorptionspectraoftheAbTi-800sampleduringthe photocatalyticRhB(aq)degradationprocess.

particularcrystallitesizesandauniquemassfraction.This obser-vationisinperfectagreementwithformerphotocatalyticstudies onotherTiO2-basedsystemsintheliterature[40–42].Itshould

benotedthatthephotocatalyticactivityofAbTisystemsare typ-icallylowerthanthatofaconventionalbenchmarkcatalystsuch asDegussaP25revealing%photonicefﬁcienciesof0.45and0.11 forNO2(g)productionandNOxstorage;respectively.Thiscanbe

attributedtothehigherSSAofP25(ca.50m2_/g).

Afterhavingdemonstratedthephotocatalyticwater puriﬁca-tioncapabilitiesoftheAb-tempaltedTiO2microspheresunderUVA

irradiation,weperformedfurtherstudiesinordertoestablishthe photocatalyticactivityofthisnewfamilyofmaterialsin

photocat-alyticairpuriﬁcationapplications.Alongtheselines,photocatalytic NO(g)oxidationandstorageexperimentswerecarriedoutusinga custom-madephotocatalyticﬂow reactorunderUVA irradiation

[8–10].Fig.8presentsresultsofthesegasphasephotocatalytic activitytests.Theresultingtypicaltime-dependentconcentration proﬁlesforthephotocatalyticNOoxidativestorageexperimentis alsoshownintheinsetofFig.8.InthehistogramsofFig.8,per centphotonicefﬁciencyvaluesfortotalNOxremoval(bluebars) andNO2production(redbars)areshown.Itisworthmentioning

thatanidealphotocatalystforgasphaseDeNOxapplicationsshould exhibita highNOx(g)storage/removalefﬁciencyaswellaslow NO2(g)generation/releasecharacteristics.Photocatalyticoxidative

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Fig.8. PhotocatalyticNO(g)oxidationandstorageperformanceresultsobtainedviaUVAirradiationatroomtemperatureforbiotemplatedTiO2microspheresinitially calcinedatvarioustemperatures(insetshowsthetypicaltime-dependentconcentrationproﬁlesfortotalNOx(g),NO(g),andNO2(g)overAbTi-600.(Forinterpretationofthe referencestocolourintext,thereaderisreferredtothewebversionofthisarticle.)

storageofNO(g)includesoxidationsteps[11,34,43,44]involving theformationofNO2(g),wheretheeventualstorageofNOxspecies

onthecatalystsurfacemayoccurintheformofchemisorbedNO, NO2/NO2−,N2O,andNO3−.Thus,maximizingtheoxidativeNOx

storageatthesolidstate,whilesimultaneouslyminimizingthegas phasereleaseoftoxicNO2(g)requiresoptimizationofthechemical,

electronicandsurfacestructureofthephotocatalysts.

Alongtheselines,photocatalyticDeNOxperformanceofthe syn-thesizedAbTi photocatalysts were investigated asa function of thecalcinationtemperatureusedinthesyntheticprotocol,inan attempttomonitorthestructure-functionalityrelationships. Pho-tocatalyticactivitydatapresentedinFig.8canbeanalysedinthe lightofthesearguments(Figs.1–6).Itisapparentthatunlikethe liquidphaseRhB(aq)degradationresultsgiveninFig.7,suggesting AbTi-800astheoptimumcatalystintheliquidphase,Fig.8shows thatAbTi-800haslimitedphotocatalyticNOxabatementcapability ingasphase.Thisobservationmaysuggestrelativelydifferent reac-tionmechanismsandinvolvementofdissimilaractivesitesforthe photocatalyticliquidphasewaterpuriﬁcationprocessesas com-paredtothegasphasephotocatalyticDeNOxprocessesoccurring onthesamecatalystsurface.

The highest total photocatalytic gas phase activity can be assignedtotheAbTi-600catalystgivenin Fig.8 duetothefact thatthiscatalyst revealsmaximumNOxremoval efﬁciencyand maximum photocatalyticoxidation of NO(g) toNO2(g). On the

otherhand,AbTi-600shouldnotbeidentiﬁedasthe photocata-lystofchoiceduetoitshighNO2(g)releasetotheatmosphere.

Comparisonof theAbTi-600catalyst withAbTi-500 revealsthat theAbTi-500hasacomparableNOxstorageefﬁciencytothatof theAbTi-600catalyst,whileexhibitingmuchlowerNO2(g)release.

Hence,AbTi-500canbeconsideredasthepreferablecatalyst in theseriesforgasphasephotocatalyticDeNOx applications.It is likelythatthegasphasephotocatalyticoxidationofNO(g)requires thepresenceoforderedanatasedomains,whilepreventionofthe NO2(g)sliptotheatmosphererequiresaporous/highsurfacearea

catalystthatcanoptimizecapture/adsorption/solidstatestorage ofthegeneratedNO2(g).Thisisconsistentwiththeobservation

thattheAbTi-400catalystobtainedaftera low-temperature cal-cinationstephaslimitedNOxremovalefﬁciencyaswellaslow NO2(g)production,duetothelackoforderedanatasedomainsand

presenceofsmallanataseparticlesanddisordered(amorphous) domains.Inotherwords,themainreasonforthepoorperformance ofAbTi-400seemstobeitslimitedphotocatalyticoxidation capa-bilityratherthanitslackofsurfaceareaforNOxstorage.Incontrast, forthecatalystscalcinedatT≥700◦C,themaincatalytic disadvan-tagecouldbeshrinkingofthepollensathightemperatures(i.e. decreaseintheavailablesurfacesitesforadsorptionandstorage ofoxidizedNOxspecies)anddecreaseinthenumberofexposed activesites,whichinturnhinderthestorageofphotocatalytically producedNO2species,resultingindetrimentalNO2releasetothe

atmosphere.Comparisonoftheliquidphasephotocatalytic activ-ityofAbTisystemswiththatofabenchmarkcatalyst(i.e.Degussa P25)revealsthatthelattersystemhasahigherphotocatalytic activ-ity,where100%decolourizationefﬁciencycanbereachedafterca. 70min.Thisobservationcanbeassociatedwiththehighersurface areaofthelattersystem.

4. Conclusions

A novel biotemplate-based photocatalytic material platform wassynthesizedbyutilizingAmbrosiatriﬁda(Ab,Giantragweed) pollenastheinitialbiologicalsupportsurface.Structural charac-terizationofthesynthesizedbiotemplatedTiO2microsphereswas

performedusingSEM-EDX,Ramanspectroscopy,and XRD tech-niques.Photocatalyticfunctionality ofthesynthesizedmaterials wasdemonstrated both in gasphase (via photocatalytic oxida-tiveNOxstorage)as wellasin liquid phase (viaphotocatalytic Rhodamine B(aq)degradation)asa function ofthecalcination temperatureusedinthesyntheticprotocol.Optimumcatalystfor RhB(aq)photocatalyticdegradationintheliquidphasewasfound tobeAbTi-800,whiletheoptimumcatalystforgasphase photocat-alyticoxidativeNOxstoragewasAbTi-500;emphasizingdifferent structural/functionalrequirementsfordifferentcatalyticreactions occurringonthesamecatalyticsurface.Thesyntheticapproach presentedhereinoffersnewopportunitiesforobtainingadvanced functionalmaterialswhichcanhavepotentialprospective applica-tionsinmicroreactorsystems,adsorbents,drugdeliverysystems, catalyticprocesses,andsensortechnologies.

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Acknowledgments

EOacknowledgesﬁnancialsupportfrom“TheScienceAcademy” (Turkey) through “Young Scientists Award Program (BAGEP)”. AuthorsalsoacknowledgethescientiﬁccollaborationwithTARLA projectfoundedbytheMinistryofDevelopmentofTurkeyunder grantnoDPT2006K-120470.

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