Template Assisted Synthesis of Photocatalytic Titanium Dioxide Nanotubes by Hot Filament Chemical Vapor Deposition Method

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

jo u r n al h om ep a 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

Template

assisted

synthesis

of

photocatalytic

titanium

dioxide

nanotubes

by

hot

ﬁlament

chemical

vapor

deposition

method

Mustafa

Karaman

a,b,∗

_,

_Fatma

_Sarıipek

a,b

_,

_Özcan

_Köysüren

a

_,

_H.

_Bekir

_Yıldız

c

a_Department_of_Chemical_Engineering,_Selcuk_University,_Turkey

b_Advanced_Technology_Research_and_Application_Center,_Selcuk_University,_Turkey

c_Department_of_Chemistry,_{Karamano˘glu}_Mehmet_Bey_University,_Turkey

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received22April2013

Receivedinrevisedform4July2013

Accepted12July2013

Available online 20 July 2013 Keywords:

Chemicalvapordeposition

Titaniumdioxide

Photocatalysis

a

b

s

t

r

a

c

t

Titaniumdioxidethinfilmsweredepositedconformallyoverelectrospunpolymethylmethacrylate (PMMA)fibersbyhotfilamentchemicalvapordepositionmethod.Depositionrateswereobservedto beveryhightoallowforrapidcoatings.Thermalannealingofasdepositedmaterialsleadstheclean decompositionofthepolymericinnerlayerandformationofrandomlydistributedanataseTiO2

nano-tubes.NanotubularTiO2 structurewasclearlyidentiﬁedbySEMandthatstructureisidealforgood

photocatalyticactivitybecauseofitshighsurfaceareaperunitvolumeratio.FTIRandXPSresultsshow theformationofstoichiometricTiO2,andthecrystallineformoftheﬁnalnanotubeswasfoundtobe

anatase(101)afterXRDanalysis.HighphotocatalyticactivityofTiO2nanotubesunderUVirradiation

wasobservedwithanapparentrateconstantof0.74h−1_for_methyl_orange_{decomposition.}

1. Introduction

Titaniumdioxide(TiO2)presentsanumberofattractive

proper-tiessuchashighrefractiveindex,highdielectricconstant,chemical stabilityandsemiconductorproperties.Ithasfoundapplications inmanyareasincludingcatalysisandphotocatalysis,asgas sen-sors,inelectricalandopticalapplications[1].Itisawell-known photocatalyticmaterialwhichcaneffectivelyconvertsolarenergy intousefulchemicalenergy,whichcanbeutilizedtodecompose harmfulmaterialsinairorinwater[2–4].Beingchemicallyinert and stable,TiO2 hasfoundmany applicationsasphocatalyst in

antibacterialandself-cleaningcoatings[5–7].Dependingonthe productionmethodandconditions,TiO2 canbeamorphous,orit

mayacceptoneofthethreecrystallineformsofanatase,rutileor brookite.Thenanostructureandcrystallinestatesarevery impor-tantregardingtospeciﬁcapplications.WhileamorphousTiO2 is

usedforopticalcoatings,anataseisusedfordye-sensitizedsolar cellsmainlyduetoitshigherbandgap[8,9].Inliterature,solution basedmethodstosynthesizethinﬁlmsoftitaniumdioxidefrom metallo-organicprecursorsincludesol–gelprocessingand layer-by-layerdepositiontechniques[3,10–12].TiO2nanotubearraysof

variousporesizes,lengths,andwallthicknessescanbegrownby anodizationoftitaniuminﬂuoride-basedbaths[1,13].Apartfrom

∗ Correspondingauthorat:DepartmentofChemicalEngineering,Selcuk

Univer-sity,42031Konya,Turkey.Tel.:+903322231972;fax:+903322410635.

E-mailaddress:karamanm@selcuk.edu.tr(M.Karaman).

thesewettechniques,chemicalvapordepositionprocessallows titaniumdioxidecoatingsinadryatmosphere.IntheCVDprocess oftitaniumdioxide,titaniumalkoxidesandtitaniumtetrachloride arethemostwidelyusedprecursors,inwhichtitaniumalkoxides havebeenpreferredmorebecauseoftheirlowvaporpressuresand highreactivitiesatlowtemperatures[14–16].Itisalsonot conve-nienttousetitaniumtetrachloridewhenchlorinecontaminationis notdesires,especiallyforelectronicapplications.Deposition tem-peraturegreatlydeterminesthecrystalstructureofTiO2thinﬁlms,

whicharegenerallyamorphousfordepositiontemperaturesbelow 350◦C,abovewhichanataseisformed.Themoststablecrystalline phaserutileisformedabove800◦C.ThehotfilamentCVDtechnique fallswithinthebroaderclassofthermal(orclassical)CVD tech-nique.UnliketheclassicalthermalCVDmethod,wherethewhole reactorbodyandhencethesubstratesareheatedinafurnace, resis-tivelyheatedfilamentsabovethesubstrateprovidetheenergyfor reactioninHFCVD[16,17].Thereforethesubstratetobecoated remainsfreefromthehightemperatures,plasmaorlightsources, whichcanalterthechemicaland/orphysicalnatureofthe frag-ilesubstrates,suchaspolymers.Also,theconformalnatureofthe HFCVDprocessallowsuniformcoatingsaroundsubstrateshaving complicatedgeometriessuchasfibers[18].

For best photocatalyticactivity, a highsurface area, anatase crystallinestructureofTiO2 isthemostdesiredstructure[7,19].

Electrospunﬁbermatswhich havehighsurfaceareatovolume ratioswereconsideredtobesuitablesubstratematerialstodeposit photocatalytic TiO2 thin ﬁlms. Electrospinningis a unique

pro-cessthatcreatesﬁberswithmicrometerandnanometerdiameters

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believedtoproduceTiO2 crystalswithhighersurfaceareathan

classicalmethods,andthereforeresultantmaterialsareexpected toshow improvedcatalytic activities. Thephotocatalytic activ-ities ofthe ﬁnal TiO2 nanotubes have alsobeen evaluated and

reported.

2. Experimental

2.1. Electrospinningofpolymethylmethacrylatenanoﬁbers

Laboratoryscaleelectrospinningunit(NE-100,Inovenso)was usedtopreparePMMA nanoﬁbersonaluminum foilsubstrates. PMMA(MW=1,20,000,AlfaAesar)wasdissolvedinacetonetogive a2wt.%solution,whichwasthenfedintoasyringe.Thepolymer blendsolutionwasfedthroughthesyringetipwithaﬂowrateof 0.1ml/husingafeedpump.Anelectricpotentialdifferenceof30kV wasappliedbetweenthesyringetipandcollector,onwhichsilicon substratewasplaced.Thedistancebetweenthecollectorandthe tipwas10cm.

2.2. DepositionofTiO2thinﬁlmsonPMMAnanoﬁbersbyhot

ﬁlamentchemicalvapordeposition

TiO2thinﬁlmsweredepositedonelectrospunPMMAﬁbermats

byhotfilament chemicalvapordepositionmethodina custom buildvacuumreactor.Inthisreactor,thereactantgaseswere ther-mallyactivatedbyheatingatungstenfilamentarrayresistively.The filamentarray,whichconsistsof15paralleltungstenfilamentsof 15cmlength,ismountedabovethewatercirculatedcoolingplate onwhichthesubstrateswereplaced.Theclearancebetweenthe wirearrayandthecoolingplatewas30mm.Alldepositionswere carriedoutatasubstratetemperatureof30◦C.Titanium(IV)tetra isopropoxide(TTIP)(99.999%,Aldrich)wasfedtothereactoras 0.5sccmthroughatemperaturecontrolledbubblerat50◦C,using 50sccmO2(99.999%)asthecarriergasintoa20Pareactorpressure

andaﬁlamenttemperatureof500◦C.Thereactorwasequippedby alaserinterferometrysystemforrealtimemonitoringofthe depo-sitionthickness.Theinterferometrysystemconsistsofa633nm He–Nelasersourceand a laser powermeter.Reactor was cov-eredbyaquartzplateatthetop,allowingvisualinspectionand laserinterferometry.Duringthedepositions,siliconwaferswere alsoplacednexttothePMMAmatsubstrates,forcharacterization purposes,andforinterferometricthicknessdeterminations. Depo-sitiontimewasvariedbetween1and10mintoobtaincoatings withdesiredthicknesses.

After HFCVD deposition of TiO2 on PMMA nanoﬁbers, TiO2

nanotubesweredevelopedfromthePMMA–TiO2nanocomposite

ﬁbersbythermal degradationofthepolymericcores.The ther-maltreatments were carried out at 600◦C for 4h in a tubular furnace.

Fig.1. FTIRspectraofTTIPprecursor,as-depositedTiO2ﬁlm,andpost-annealed

TiO2ﬁlm.

2.3. Characterization

Real timethickness controlsofthe depositionsonreference siliconwafersweremadeusingthelaserinterferometrysystem. Thethicknessandrefractiveindicesoftheﬁlmswerealso deter-minedbyusinganellipsometer(Woollam M-2000)atanangle of70◦ andwithinaspectralrangeof315–720nm.Fourier Trans-formInfrared(FTIR)measurementsweredoneonaNicolet380 spectrometerintransmissionmode.Thechemicalbondingstates wereanalyzedusingX-rayphotoelectronspectrometer(XPS)on aCratosAxisUltraspectrometerusingamonochromatizedAlK␣ source.Morphologyoftheﬁlmswasstudiedbyscanningelectron microscopy(Jeol).ThecrystalstructuresofTiO2nanotubesbefore

andaftertheheattreatmentwereanalyzedbyusingX-ray diffrac-tion(XRD)method(Rigaku).Toobservethephotocatalyticactivity ofthepreparedTiO2nanotubes,20mgTiO2samplewasplacedina

quartzcellcontaining50mlaqueousmethylorangesolutionwith aconcentrationof10mg/l.TheconcentrationofTiO2 nanotubes

inthecellwasirradiatedby4×6W,365nmUVlampsin aUV curer(ChematKW-4AC).Duringirradiation,thesolutionwas bub-bledwithair.Thechangeinthemethylorangeconcentrationwas observedbyaUV–visiblespectrophotometer(ShimadzuUV-1800).

3. Resultsanddiscussions

3.1. HFCVDofTiO2thinﬁlms

Theinsitumeasurementofthedepositionthicknessbylaser interferometryindicatesadepositionrateof120nm/minonthe referencesiliconwafer.Thechemicalstructureofthedeposited TiO2 thin ﬁlmswasanalyzedbyFTIRin thewavenumberrange

between 400and 4000cm−1. Fig. 1 shows theFTIR spectra of as-deposited and post annealed films together with the spec-trumoftheTTIPprecursor, which wasdrop castontoa silicon waferforcharacterizationpurpose.Allofthepeaksrelatedwith TTIPprecursor were observedtodisappear in theas deposited films.TheresultedFTIRspectrumoftheasdepositedfilmshows abroadabsorbancepeakbetweenthewavenumbersof400and 800cm−1,whichisrelatedwiththeTi Obond.Thecauseofthe peak broadening is attributed tothe amorphous nature of the filmduetothecarbonincorporationintotheTi Onetwork.The

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Fig.2. HighresolutionC1s,Ti2pand01sXPSscansoftheTiO2ﬁlmdepositedfromHFCVD.

featuresobservedbetween1400and1600cm−1wererelatedwith carbonaceousmaterials.Thepresenceofcarbonrelatedgroupsin TiO2 ﬁlms,especiallytheonesdepositedatlowtemperaturesby

variousmeansiscommonespecially whenthestartingmaterial isTTIP[20].Tobetterunderstandthechemicalnatureoftheas depositedﬁlms,XPSanalysiswascarriedout.XPSsurveyscan indi-catesC1s,O1s,andTi2ppeakswithatomicpercentagesof40.92,42.5,

and14.18,respectively.InFig.2,theTi2p,and01shighresolution

spectraobtainedfromtheXPSscanarepresented.The deconvo-lutionofTi2pspectrumshowshighintensitypeakscenteredatthe

bindingenergyvalues of 459.2and464.9eV,whichcorrespond

toTi2p3/2 andTi2p1/2states.TheO1sspectrumhasamajorpeak

centeredat530.8eV,indicatingTi Obond,andashoulderonthe lefthandsidecenteredat532.3eVcorrespondingtothehydroxyl species.Hydroxylbondsareattributedtowater, whichisa by-productoftheTTIPaccordingtothefollowingreaction:TTIP+O2

→ TiO2+4C3H6+2H2O

During the deposition by HFCVD, water and carbon related productsaretrappedandadsorbedonlowtemperaturesubstrates togetherwithTiO2.Waterrelatedpeaksaround3000–3600cm−1

were diminished in the FTIR spectrum after high temperature annealingoftheas-depositedﬁlms.TheFTIRandXPSresultsshow thatstoichiometricTiO2ﬁlms(withrespecttoTi/Oatomicratio)

canbedepositedbyHFCVDonsubstratesatroomtemperature, withtrappedwaterandcarbonrelatedmaterialsintheﬁlms.Post heattreatmentoftheﬁlmscanbedonetoremoveordecreasethe amountofsuchimpurities.

Theopticalconstantsandthicknessesoftheﬁlmsonsilicon sub-stratesweremeasuredbyspectrophotometricellipsometry.Fig.3

showstheresultsfortheHFCVDgrownTiO2ﬁlmsbeforeandafter

heattreatment.Asdepositedfilmhasathicknessof125nmwith refractiveindexof1.81at500nm.Thislowrefractiveindexvalue canbeattributedtothecarbonrelatedimpuritiesandwaterinthe as-depositedfilms.Afterannealingat600◦Cfor4h,thefilmshowed

ahigherrefractiveindexvalueof2.11,accompaniedbyadecrease ofthicknessto74nm.Thedecreaseinthethicknessmaybearesult ofwaterremovalfromthefilmstructureandreorganizationoffilm structureinamoredenselypackedcrystalformofanatase,which isverifiedafterXRDstudies(asdiscussedlater).

3.2. DepositionofTiO2thinﬁlmsaroundelectrospunPMMAﬁber

mats

SEMimageoftheelectrodepositedPMMAmatontungstenfoil isgiveninFig.4a.Thisimageillustratesthepresenceofpolymer nanofibersonthesubstrate.Thediametervaluesofindividualfibers rangefrom230nmto250nm.Rarebeadstructures,characteristic oflowsolutionviscosity,appearontheSEMimageofthe electro-spinnedpolymersample.Concentrationofthesolutionprepared forelectrospinningisquitedilute,resultinglowsolution viscos-ity.Atalowviscosity,itiscommontofindbeadsalongthefibers depositedonthecollectionplate.

After electrospinning process, the substrate was placed in HFCVDsystemandTiO2wascoatedonthenano-ﬁbrousstructures.

Fig.4bshowstheSEMimagesoftheasdepositedTiO2coatings.Itis

clearfromtheﬁgurethatTiO2wasconformallycoatedaroundthe

individualfibers.Thediametervaluesofthecoatedfibersrange from280nmand 300nm,which indicateacoatingthicknessof approximately25nm,whichislowerthanthatontheflatsubstrate (125nm).Thesurface areaof flatsubstrateismuch lowerthan thatofporouselectrospunfibers.Thisisbelievedtobethemain reasonforthecoatingthicknessdifference.Also,thethermal insu-lationdifferencehaslargeeffectondepositionratesbetweentwo kindsofsubstrates,andthatmaybethesecondreasonforthelarge thicknessdifference.Heattreatmentoftheproducedpolymer-TiO2

nanocompositematerialat600◦Cresultsintheremovalofthe poly-mercorelayer.TheresultingTiO2nanotubestructureisclearlyseen

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Fig.3.RefractiveindexvaluesofHFCVDdepositedTiO2ﬁlmsbeforeandafterheattreatmentat600◦C.

Thecrystallinestructureofas-depositedandheattreatedTiO2 nanotubeswerestudiedbyXRD.Fortheas-depositedtitaniaﬁlms onthe electrospunﬁbers there is no signof diffraction peaks, whichindicatethatas-depositedtitanialayerisamorphous.The diffractogram of the nanotubes treated at 600◦C is shown in

Fig.5.Themostintensediffractionpeakobservedfromthe(101) reﬂection at 2 position of 25.2 is characteristic peak fort the anatase structure. Other reﬂections at 2 angular positions of 36.81,37.73, 38.51,48, 53.77,and 54.93are all characteristics ofanatasephase and there isnoindicationof othercrystalline phases.Thus,XRDanalysisshowsthatacalcinationtemperatureof

600◦Cissufﬁcientforconversionofamorphousas-deposited tita-niaﬁlmsintonanotubeshavingpureanatasecrystallinestructure. Thesetubularanatasestructures,coveringwholesurfaceofthe sub-stratematerial,arethoughttobeidealstructuresforphotocatalytic activity.

3.3. PhotocatalyticactivityofTiO2nanotubes

Photodegradation of organic compounds in the presence of UV-illuminatedTiO2 in aqueousenvironments is a well-known

phenomenon.ToprovethephotocatalyticefﬁciencyoftheTiO2

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Fig.5.XRDpatternofpost-annealedTiO2nanotubes.

nanotubestructures,substratescontainingTiO2 nanotubeswere

suspended in methyl orange, and theywere treated under UV lighttostartthedegradationreactions.AsTiO2isirradiatedbyUV

light(=365nm),rapiddecolorizationofthemethylorange solu-tionwasobservedbyUVvisiblemeasurements.Thehighintensity absorbancebandofmethylorangewasobservedat464nm.The decompositionkineticscanbewellexplainedbypseudoﬁrstorder kineticsaccordingtothefollowingrelations[21]:

C=C0exp(−kt) (1)

ln

C0

C

=kt (2)

Therewasalinearrelationshipbetweentheabsorbance(A)and concentration(C)ofmethylorangeat464nmaccordingtothe cal-ibrationexperiments.HenceconcentrationtermsinEq.(2)canbe replacedbytheabsorbanceterms.

ln

_A 0 A

=kt

wheretisthetimeandkistheapparentrateconstantwhich canbedeterminedfromtheslopeoftheln(A0/A)vs.tgraph.Fig.6

presentsthechangeintheabsorbanceofmethylorangesolutionat 460nmduringirradiationbyUVlight.Withoutlightillumination, thecontentofmethylorangedoesnotchangewithtime.Methyl orangesolutioncontainingamorphousTiO2onPMMAﬁberscauses

aslowdecolorization withanapparentrateconstantofaround 0.18h−1(R2₌_0.97)._On_the_other_hand,_the_solution_containing_TiO

2

Fig.6.TimechangeofUVabsorbanceofmethylorangesolutionat460nmunder

irradiationof365nmUVlight.

nanotubesshowsarapiddecolorization.Transformationof amor-phousTiO2containingﬁbersintocrystallineTiO2nanotubescauses

around4-foldincreaseupto0.74h−1(R2₌_0.98)_in_the_apparent_rate

constant.

4. Conclusions

TiO2 thin ﬁlmswere successfully deposited by hot ﬁlament

chemical vapor deposition method. HFCVD method was capa-bleofdepositingTiO2conformallyaroundveryfragilepolymeric

nanofibers at room temperature. The size and shapes of the nanofibersdoesnotchangeduringthedeposition,duetothelackof thermalorplasmaeffectsonthesubstrates.Postheattreatmentof thefilmsat600◦Ctotallyremovesthepolymericcorelayer, leav-ingananotubularTiO2 havingacrystal formofanatase.Ahigh

photocatalyticactivityofTiO2nanotubeswasobservedunderUV

irradiation.Thehighphotocatalyticactivitywasattributedtothe convenientchemicalstructureandthehighsurfacetovolumeratio oftheasproducedTiO2nanotubes.

Acknowledgements

This research was supported in part by the Scientiﬁc and TechnologicalResearchCouncilofTurkey(TUBITAK)(ProjectNo. 110M088) and Scientiﬁc Research Project of Selcuk University (BAP-09401061).

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