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Applied Catalysis B: Environmental
jo u r n al ho me p 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 c a t b
Enhanced photocatalytic activity of homoassembled ZnO
nanostructures on electrospun polymeric nanofibers: A combination of atomic layer deposition and hydrothermal growth
Fatma Kayaci
a,b, Sesha Vempati
a,∗, Cagla Ozgit-Akgun
a,b, Necmi Biyikli
a,b, Tamer Uyar
a,b,∗∗aUNAM-NationalNanotechnologyResearchCenter,BilkentUniversity,Ankara,06800,Turkey
bInstituteofMaterialsScienceandNanotechnology,BilkentUniversity,Ankara,06800,Turkey
a r t i c l e i n f o
Articlehistory:
Received10December2013
Receivedinrevisedform24February2014 Accepted4March2014
Availableonline14March2014
Keywords:
Photocatalysis ZnO
Electrospinning Atomiclayerdeposition Hydrothermal
a b s t r a c t
Wereportonthesynthesisandphotocatalyticactivity(PCA)ofelectrospunpoly(acrylonitrile)(PAN) nanofibrousmatdecoratedwithnanoneedlesofzincoxide(ZnO).Apartfromadetailedmorphologi- calandstructuralcharacterization,thePCAhasbeencarefullymonitoredandtheresultsarediscussed elaboratelywhen juxtaposedwiththe photoluminescence.Thepresenthierarchal homoassembled nanostructuresareacombinationoftwotypesofZnOwithdiverseopticalqualities,i.e.(a)controlled depositionofZnOcoatingonnanofiberswithdominantoxygenvacanciesandsignificantgrainbound- ariesbyatomiclayerdeposition(ALD),and(b)growthofsinglecrystallineZnOnanoneedleswithhigh opticalqualityontheALDseedsviahydrothermalprocess.Theneedlestructure(∼25nmindiameter withanaspectratioof∼24)alsosupportsthevectorialtransportofphoto-chargecarriers,whichiscrucial forhighcatalyticactivity.Furthermore,itisshownthatenhancedPCAisbecauseofthecatalyticactiv- ityatsurfacedefects(onALDseed),valenceband,andconductionband(ofZnOnanoneedles).PCAand durabilityofthePAN/ZnOnanofibrousmathavealsobeentestedwithaqueoussolutionofmethylene blueandtheresultsshowedalmostnodecayinthecatalyticactivityofthismaterialwhenreused.
©2014ElsevierB.V.Allrightsreserved.
1. Introduction
Photocatalysisis one ofthewidely researched[1–11]topics becauseofitsimportanceinwaterandenvironmentalpurification inthebackground ofunavoidable andever increasingindustri- alization[12].Photocatalysts(PCs) aregenerallynanostructured semiconductors which are employed either directly [1,2,5,10], dopedformat[7],defect-induced [3,4,13–15]orcombined with another material to yield a synergy effect. Such combinations exploitplasmoniceffect[6]orpropertiesofothersemiconductors [7–9,11]depictingrelativelyhigherphotocatalyticactivity(PCA) thantheirpristinecounterparts.DespitethehigherPCAthosecom- binationsalsoincreasethecomplexityoftheprocessandattheend theyshouldbecompatiblewiththeenvironment[10].Metal-doped semiconductorscanbeunstableandcorrodeafterlongtermusage
∗ Correspondingauthor.Tel.:+903122903533.
∗∗ Correspondingauthor.Tel.:+903122903571.
E-mailaddresses:svempati01@qub.ac.uk(S.Vempati), tamer@unam.bilkent.edu.tr,tameruyar@gmail.com(T.Uyar).
causingagradualdecayinthePCA[12],apartfromthedifficulties intheirpreparationandcharacterization[16].Ofcoursethisdoes notapplytonoblemetals[17]whicharestableinthecontextof photo-oxidation;ontheotherhand,ifAunanoparticlesarelarger thanacriticalsize,thentheymayactase/hrecombinationcenters whichreducethePCA[17].Inanycase,themainaimistoengineer aPCpossessingenhancedPCAaswellasstability overadecent periodoftime.
Among alist ofsemiconductors employedin photocatalysis, ZnOnanostructureshaveattractedalotofattention[1,2,4–6,15,18]
duetotheireasyprocessabilityviaavarietyofmethods[5,19–21], versatilityinnanostructuring[5,19–21],non-toxicity,abundance, low cost, etc. Having listed the properties which make the nanostructured-ZnOahighlysuitablecandidateasaPC,weshould agreewiththefactthatideallydefect-free-ZnOcanuseonlyUV region(3–4%)ofthesolarspectrumbecauseofitswiderbandgap andtherefore44–47%ofvisiblelightisleftunused.Hence,itobvi- ouslyisawisechoicetoharnessthevisibleaswellasUVregion ofsolarenergytoachievesignificantlyhigherPCA.Wealsonote that themorphology and crystal structure of ZnO at nanoscale aredetrimentalonvariouspropertiesincludingoptical[15,19–21], http://dx.doi.org/10.1016/j.apcatb.2014.03.004
0926-3373/©2014ElsevierB.V.Allrightsreserved.
photostability[22,23]and PCA[14,15,24].Inordertoinitiateor improvethelightabsorptioninthevisibleregiononecanengage thenativedefectsofZnO[13,15],whichformsub-bandgapstates [21].Forexample,oxygenvacancies(VOs)areinducedinZnOand recentlyshowntoimprovePCA[3,4,25],alongsideofothersimilar studies[13–15]. On the otherhand, VOsnot only create inter- mediatebands,butalsoactasself-dopantsandinducedbandgap reduction[25].Therefore,byconsideringthevariouspropertiesof nanostructured-ZnO,itisconvincingandlogicaltodesignasmart andefficientZnOcatalystdepictinghighPCAisoffundamentalas wellastechnologicalimportance.Herewedemonstrateanovel hybridapproach,in which wecombinechemical vapordeposi- tion(CVD)andliquidphasedeposition(LPD)techniques.Atomic layerdeposition(ALD)and hydrothermalgrowth arecombined to fabricate a hierarchy of nanostructured-ZnO onelectrospun poly(acrylonitrile)(PAN)nanofibers.TheresultingZnOnanostruc- turesdepictsynergyeffectandshowenhancedPCA.PANnanofibers arewelladoptableinwaterfiltrationwheretheiruniqueproperties suchashighsurfacearea,nanoporousstructure,lowbasisweight, easypermeability,goodstabilityandchemicalresistanceareworth mentioning[26–29].Inourpreviousstudy[2]electrospunpoly- mericnanofibersweresubjectedtovaryingALDparameterswhere wehavestudiedhow thePCAisinfluencedwhennanoparticles transformintocontinuousfilm.Wehadinferredthathighlydense nanoparticlesshowrelativelyhigherPCAduetotheincreasedsur- faceareaconsistingofoxygenvacancyandotherrelateddefects.
Webelievethathavingpolycrystallinefilmaloneisnotadequate toyieldhighPCA,henceinthisstudy,wehavegrownsinglecrys- tallineZnOnanoneedlesontheALD-seedcoating.Notethatthe singlecrystallineZnOnanoneedlescandepictthelowestpossible defectdensity.Furthermore,previousstudies[3,4,13–15,25]have introducedVOsthroughoutthecatalyst,however,incontrastwe havecombinedtwomaterialsoneofwhichisdominantinoxygen relateddefects,whiletheotherisvirtuallydefect-freesinglecrystal.
WealsoshowthatthepresentcombinationyieldsenhancedPCAin thepresenceofhighaspectratioZnOnanoneedleswithanaverage diameterandlengthof∼25nmand∼600nm,respectively.Wenote thatsurfaceareatovolumeratiocannotequivalentlyimprovethe PCA[10]whereasonedimensionalsemiconductorshavealready showntodepictvectorialtransportofphotogeneratedchargecar- riersandhelpingtoimprovethePCA[30,31].Whilemaintaining adelicatebalancebetweentheadvantages[13–15,25]andlimi- tations[10]ofthenanostructureswehaveachievedasignificant PCAwiththepresentcombination.Sincenanoneedlesweregrown onthethinfilmwhichisonthepolymericfibrousmat,theymay notbeeasilydislodgedwithoutasignificantmechanicalfatigue.
Thismakesthemeasiertohandleandrecycleinaqueousenviron- mentunlikethecasewithnanosizedparticles[6,14,15,25,32].Of courseonecanemploytheexpensiveindiumtinoxide/fluorinetin oxidesubstratebasedcatalysts[10,33].Asanadditionaladvantage, nanoneedlesareboundtothesurfaceoftheZnOfilmonthepoly- mericfiberandtheyarewellseparatedfromeachotherduringand aftertheprocessincontrasttonanoparticle-basedcatalystswitha significantdrawbackofagglomeration.
2. Experimental 2.1. Materials
PAN(Mw:∼150,000)waspurchased fromScientificPolymer Products,Inc.N,N-dimethylformamide(DMF,Pestanal,Riedel)was usedasasolvent.ALD ofZnOwasperformedusingdiethylzinc (DEZn,Sigma–Aldrich)and HPLCgradewater (H2O) asthezinc precursorandoxidant,respectively.Forhydrothermalprocesszinc acetatedihydrate(ZAD,≥98%,Sigma–Aldrich)andhexamethylene tetramine(HMTA,≥99%,AlfaAesar)wereused.Methyleneblue
(MB,Sigma–Aldrich,certifiedbytheBiologicalStainCommission) wasusedasamodelorganicdyetotestPCAofthePANnanofibers andPAN/ZnOnanofibrousmats.Allmaterialswereusedwithout anypurification.De-ionized(DI)waterisobtainedfromMillipore Milli-Qsystem.
2.2. ElectrospinningofPANnanofibers
Inbrief,wehaveoptimizedthePANconcentration(12%(w/v) inDMF)toyielduniformandbead-freenanofibers.Priortoelec- trospinning,PANsolutionwasstirredfor3hatroomtemperature (RT)toobtainhomogeneousandclearsolution.Well-stirredsolu- tionwastakenina5mLsyringefittedwithametallicneedleof
∼0.8mmofinnerdiameter.Thesyringewasfixedhorizontallyon thesyringepump(KDScientific,KDS101)withafeedratesetto 1mL/h.Ahighvoltageof15kVisapplied(Matsusada,AUSeries) betweenthesyringeneedleandastationarycylindricalmetalcol- lector(wrappedwithacleanaluminumfoil)locatedat12cmfrom theendofthetip.Theelectrospinningprocesswascarriedoutat
∼25◦Cand22%relativehumidityinanenclosedchamber.
2.3. PreparationofZnOseedstructurebyALD
ZnO deposition on electrospun PAN nanofibers was carried outat∼200◦CinaSavannahS100ALDreactor(CambridgeNan- otech Inc.). N2 was used as a carrier gas at a flow rate of
∼20sccm.400cycleswereappliedviaexposuremode(atrade- mark of Ultratech/CambridgeNanotech Inc.) in which dynamic vacuum was switched to static vacuum before each precursor pulse.This is achieved by closing the valve betweenthe reac- tion chamber and the pump. After a predetermined exposure time,thevacuumwasswitchedbacktodynamicmodeforpurg- ing excess precursor molecules and gaseous byproducts. One ALD cycle consists of the following steps: valve OFF/N2 flow setto 10sccm/H2O pulse (0.015s)/exposure (10s)/valveON/N2 purge(20sccm,10s)/valveOFF/N2flowsetto10sccm/DEZnpulse (0.015s)/exposure(10s)/valveON/N2purge(20sccm,10s).
2.4. GrowthofZnOnanoneedlesbyhydrothermalmethod
ZnOcoatedPANnanofibers(PAN/ZnOseed)wereusedasaseed substrateforthegrowthofZnOnanoneedles.∼3.6mgofPAN/ZnO seednanofibrousmatwasimmersedinto∼33mLaqueoussolution ofequimolarZAD,HMTA(0.02M)andmildlystirredovernightat RT.Thissolutionisthenheatedto90◦Candkeptfor5h. When thecruciblecooleddowntoRT,thenanofibrousmatwasthor- oughlyrinsedwithDIwatertoremoveanyresidualsaltsanddried invacuumovenat∼40◦Cfor12h.
2.5. Characterizationtechniques
Themorphologyofthesampleswasstudiedusingascanning electronmicroscope(SEM,FEI–Quanta200FEG)withanominal 5nm of Au/Pd sputter coating. These images are used to esti- matetheaveragefiberdiameter(AFD).Fortransmissionelectron microscopy(TEM)imaging,samplesweresonicatedinethanolfor 5minandthedispersioniscollectedonholeycarboncoatedTEM grid.TEM(FEI–TecnaiG2F30)andelementalanalysis(energydis- persiveX-rayspectroscopy,EDX)wasperformedonthePAN/ZnO seed nanofibers. Selected area electron diffraction (SAED) pat- ternsofthePAN/ZnOseednanofiberswerealsoobtained.X-ray diffraction(XRD)patternsfromthepristinePAN,PAN/ZnOseed andPAN/ZnOneedlesampleswerecollected(2=10◦–100◦)using PANalyticalX’PertProMPDX-rayDiffractometerusingCuK␣radi- ation(=1.5418 ˚A).Forsurfaceanalysis,samplesweresubjected toX-rayphotoelectronspectroscopy(Thermoscientific,k-Alpha)
underAlK␣(h=1486.6eV)linewithachargeneutralizer.Pass energy, stepsize and spotsizewere 30eV,0.1eV and 400m, respectively.Peak deconvolution wasperformedwithAvantage softwarewhere thenumber ofpeaks waschosenbased onthe physicsofthematerialwhilethespectrallocationandfullwidth athalfmaximum(FWHM)wereallowedtovary.Photolumines- cence(PL)measurementswereperformedusingHoribaScientific FL-1057TCSPCatanexcitationwavelengthof360nm.
2.6. Photocatalyticactivityofthenanofibers
ThePCAsofthePANnanofibers,PAN/ZnOseedandPAN/ZnO needlesampleswereanalyzedthroughphotoinduceddegrada- tionofMBinaqueousmedium(18.8M).Thenanofibrousmats (weight: 3.6mg) were immersed in quartz cuvettes containing theMBsolution.ThecuvetteswereexposedtoUVlight(300W, Osram,Ultra-Vitalux,sunlightsimulation)placedatadistanceof
∼15cm.Dyeconcentrationsinthecuvettesweremeasuredusinga UV–Vis-NIRspectrophotometer(VarianCary5000)atregulartime intervals.Thenanofibrousmatswerepushedtothebottomofthe cuvettesduringtheUV–Visspectroscopy.TheweightofPAN/ZnO seedsamplebeforeandaftertheneedlegrowthwas∼3.6mgand
∼3.9mgrespectivelywhichisequivalenttoanincreaseof∼8wt%.
ThentheweightofPAN/ZnOneedlesamplewascorrectedtoequate PAN/ZnOseedsample(3.6mg).Hencethe3.6mgofPAN/ZnOnee- dlewasfoundtocontain3.32mgofseedand0.28mgofneedles.
Therateofdyedegradationwasquantifiedviafirstorderexponen- tialfit(y=y0+Ae-x/t)foreachdataset.Thisfitwasperformedunder automatedroutinewithOrigin6.1,wherealltheparametersareset asfreeuntilconvergence.WehavealsorepeatedthePCAexperi- menttwice(i.e.2ndand3rdcycles)forPAN/ZnOneedlesample (∼3.3mg)todeterminethereusabilityversusperformance.
3. Resultsanddiscussion
ZnO nanoneedles were hydrothermally grown on the ZnO seed-coatedpolymericnanofiberswhichwerefabricatedthrough combiningelectrospinning and ALD.Theprocess for fabricating thehierarchicalpolymer/ZnOnanofiberisillustratedinFig.1and variousstepsareannotatedontheimage.
Therepresentative SEM imagesofPAN nanofibersare given in Fig. 2(a1 and 2). The nanofiber morphology was optimized against several PAN concentrations (results not shown here).
About 12% (w/v) was found to be the optimum for the cho- senparameters yielding bead-free morphology withan AFD of
∼655±135nm.Inelectrospinning,itisverytypicaltoobtainfibers inarangeofdiametersasreportedbyus[34,35]andmanyoth- ers[36,37].Acloseinspectionofthemorphologyrevealsatexture likestructure,which issometimesobservedfor certainelectro- spunpolymericnanofibersduetothetypeofsolventused[36–38].
Thesenanofiberswereemployedforthesecondstepofseeding withALD[39–41]byapplying400cyclesat200◦Cusingexposure mode(Section2.3).AftertheALDprocess,wehaverecordedthe SEMimageswhichareshowninFig.2(b1and2)wheretheAFD is∼715±125nm.ThismeasurementsuggestedanincreaseinAFD becauseofALDcoating.Thefiberstructurewasnotdestroyeddur- ingtheALDprocesswhereawelldefinedandstablefiberstructure suggeststhesuitabilityofthechosenparameters.Itisimportant topointouttheneedofcompatibilitybetweentheprecursorand polymerastheformercandegradethelatterbychemicallyreact- ingwithit;seethecasewithALDprocessingofAl2O3 [42].On theotherhand,inthecaseofpoly(propylene)fibersAl2O3base layerisemployedtodepositZnO,wheretheformerprotectsthe diffusion of DEZn into the polymer [43]. Despite these limita- tions,ALDcoatingcanyieldcoral[44],core–shell[45]likecomplex nanostructures. Such structures are potentialfor photocatalytic
applications[46].Inthepresentcasethemorphologicalchanges aresimilartoourearlierobservation[1,2].Itisclearfromtheimage (Fig.2(b2))thatthesurfaceroughnessisincreasedafterALDpro- cess,whichismostprobablyduetothegrainystructureofZnO[1,2].
Inourearlierinvestigation[2],wehaveshownthegrainformation underALDfordifferentstagesofprocessingcycles.Thecloselyand uniformlypackedgrainsactedastheseedlayerforthesubsequent growthofnanoneedlesofZnOinhydrothermalprocess.Notethat thisgrainystructureisnotundesired,ontheotherhandithelpsto enhancethePCA,wherewecanexpecttheformationofdepletion layerwithinthegrainboundaries[47,48].Such depletionlayers areextremelyhelpfulandwewilladdresstheminthecontextof PCAlatterinthisarticle.Subsequently,thehydrothermalmethod wasemployedtogrowZnOnanoneedlesontheZnOseed-coated polymericnanofibers.Fig.2(c1and 2)shows therepresentative SEMimagesoftheresultingnanoneedleassemblies(PAN/ZnOnee- dle).ItcanbeseenthatnanoneedlescoverthesurfaceoftheZnO seed-coatedPANnanofibers.Thenanoneedleswerestraightandno branchingwasobserved.Branchinggenerallyoccursbecauseofthe irregularityintheseedwhereitcanpromotethegrowthofmore thanoneneedle.Notably,inthepresentcontexttheseedsgrown throughALD-processwereuniformanddidnotinitiateorsupport multi-needlegrowth.SEMimagesofPAN/ZnOneedlesatdifferent magnificationsaregiveninFig.S1ofSupportingInformation.By analyzingtheSEMimageswehaveestimatedtheaveragediam- eterandlengthofthenanoneedlestobe∼25nmand ∼600nm, respectively(Fig.2(c2)).Detaileddiscussiononthemechanismof thegrowthofZnOnanoneedlescanbefoundintheliterature[15].
SeedingofZnOwithALDprocessandsubsequenthydrothermally grownnanorodsof∼50nmdiameterwithalengthof∼0.5–1m canbeseenintheliterature[49,50].
ThemorphologiesofthePAN/ZnOseednanofiberswerefur- therinvestigatedbyTEMandshowninFig.3(a1).Theconformal coatingofZnOcanbeevidencedfromtheimagewithauniform thickness(∼50nm)inspiteoftherelativelylargesurfaceareaofthe nanofibers.Notablythissupportstheearlierargumentonunifor- mityofthegrainswhicharenotfavorableformulti-needlegrowth.
ALDiswellsuitableforthehighsurfaceareasubstratessuchasa non-wovennanofibersmatasshownbyusearlier[1,2].Growthof ZnOonthePANnanofiberswascalculatedtobe∼1.25 ˚A/cycleinthe presentALDconditions.Inourpreviousstudy[1]weobservedthat ALDofZnOwith0.015spulsesand10spurgesunderthedynamic vacuumconditionsresultsinuniformcoatingsonlyaftera cer- tainnumber ofALDcycles.In contrast,herewehaveemployed exposuremode(seeSection2.3)whichalsoresultedinacontinu- ousanduniformZnOcoatingwithouttheneedofhighnumberof ALDcycles.Exposuremodekeepstheprecursormoleculesinside thereactionchamberfor acertainperiod oftime whichallows themtodiffuseintothesubstrate.Thelocalcrystalstructureof theALD-ZnOisinvestigatedthroughSAEDpattern,andshownin Fig.3(a2).ThepatternrevealsthepolycrystallinenatureofZnO seed.Moreover,thebrightspotsonthepolycrystallinediffraction ringsindicatethepresenceofwellcrystallinegrains[2].Various diffractedplanesareannotatedontheimageandareconsistent withtheliterature[20,21].EDXanalysis(Fig.S2ofSupportingInfor- mation,leftpanel)onthePAN/ZnOseednanofibershasshownzinc, oxygen,carbon,nitrogenandcopper(fromTEMgrid)elements.Zn andOoriginatefromZnOseed,whereasCandNareduetothe polymericcorestructureofPAN.Alsothequantification(Fig.S2of SupportingInformation,rightpanel)ofZnandOatomicpercent- agessuggeststhatthematerialisnominallyoxygenrich,within thedetectionlimitofEDX.Thisisbecauseoftheveryhighsurface areayieldingdefectivesites(oxygenvacancies)wheremolecular oxygencanbeadsorbed.Itisnotablethatoxygenvacanciesare typicalforZnOinotherprocessingtechniquesaswell,whichwere determinedthroughanindirectmethod[19–21,48].Wewillsee
Fig.1.Schematicrepresentationsof(a)electrospinningofPANsolution,(b)ALDofZnOseedontoPANnanofiber,and(c)fabricationprocessforhierarchicalPAN/ZnOneedle nanofiber.
thattheoxygendeficiencyisconsistentwiththePLofZnO.Fur- thermore,HRTEMimagedemonstratedasinglecrystallinenature ofZnOnanoneedles(Fig.3(b1)).Thelatticespacingwasmeasured tobe∼0.525nmcorrespondingtothec-axisofZnO,whichisthe preferentialgrowthdirectionofthenanoneedles.Itisimportant todeterminethegrowthdirectionwherethepolarplanesofZnO haveshowntodepictrelativelyhigherPCA[13].ThefastFourier transform(FFT)imageisshowninFig.3(b2)alsodemonstratesthe singlephase[32]structure.
TheXRDpatternsofpristinePAN,PAN/ZnOseedandPAN/ZnO needlenanofibersareshowninFig.4.TheXRDpatternofpurePAN nanofibersshowsapeakat∼16.93◦correspondingtoorthorhom- bicPAN (110) reflection[51] withan FWHM of∼2.282◦.Also abroad andless intensepeak-like structurecanbeseen inthe range of 20–30◦ which corresponds to the (002) reflection of PAN [52]. Afterthe ALD process the(110) planehasshown a significantreduction in the FWHM (to ∼0.761◦)and the broad peak(2=20◦–30◦ indicatedwith‘*’ontheimage)hasstabilized at ∼29.69◦ and became more sharp (FWHM ∼0.776◦). This is becauseofthereorganizationofthepolymericchainsat∼200◦C (ALDprocessingtemperature)equivalenttotypicalannealing.Fur- thermore,sincethenanoneedlesweregrownatslightlyelevated temperature (∼90◦C) for substantial period of time, there is a nominalincreaseintheFWHMofthePANdiffractionpeaks((110) and(002))becauseoftheincompatibleprocessingtemperature.
However the relative intensity of this peak was considerably subduedbecauseoftheZnOnanoneedles.
MovingontothepeakscorrespondingtotheZnO,wehaveanno- tatedthereflectionsontheimageforthesamplesPAN/ZnOseed, andPAN/ZnOneedle(Fig.4a).PAN/ZnOseedandneedlesamples
exhibiteddiffractionpeaksofhexagonalwurtzitestructureofZnO (ICDD01-074-9940)revealing thesuccessful depositionof ZnO seedas wellasnanoneedles onelectrospunPAN nanofibersby ALDandhydrothermaltechnique,respectively.TheXRDpatterns ofPAN/ZnOseedandPAN/ZnOneedlematchwiththereference patternintermsofpeakpositions.Alsothesepeakpositionsmatch withtheliterature,whenZnOispreparedthroughdifferentmeth- ods[19–21,48].However,acloseobservationof(100),(002)and (101)reflections(Fig.4b)revealvitalinformation.Ifwecompare theFWHMvaluesofthesepeaksacrossseedandneedlesamples, wecanseethattheformerislesscrystallinethanthelatter.There isalsoashiftinthepositionofthepeakstowardshigher2value uponneedleformation.ForthediffractionpatternofPAN/ZnOnee- dle,wecanseeashoulder-likestructure(denotedwithsinFig.4b) whichcorrespondstothePAN/ZnOseed,whileamoreintensepeak correspondingtothehighlycrystallineZnOnanoneedle.Peakshift isgenerallyassociatedwiththeresidualstress(defect-induced)in thematerial.Thestressmightbeoriginatedfromtheoxygenvacan- ciesinthelattice[25]orthesubstrate(PANnanofibers)[53].Since thepeakshiftisnoticedfor(100),(002)and(101)reflectionswe canexpectthatthesampleisundercompressivestraininthesaid crystaldirections[53].Notably,theXRDpatternofPAN/ZnOseed isconsistentwithSAEDpattern.Furthermore,theintensityratios of(002)polarplaneto(100)nonpolarplaneisestimatedandit turnsouttobethecasethatseed(∼0.78)samplehaslargefraction ofpolarplanesthanneedle(∼0.65)sample,wherelargerfraction indicatespossiblyhigherPCA[15].Inthiscontext,itisnotablethat theXRDisastatisticalaverageofplanesfromsurfaceaswellassub- surfaceregions.Hence,thereisapossibilitythatthepolarplanes maybeexposedtothesurfacetoenhancethePCA.
Fig.2.RepresentativeSEMimagesof(a1and2)pristinePAN,(b1and2)PAN/ZnOseed,and(c1and2)PAN/ZnOneedlenanofibersatdifferentmagnifications.
The ionic state of oxygen generally determines the optical emission properties in visible region [19–21] (associated with oxygenrelated defects)andhencethephotocatalyticproperties [3,4,13–15].TheO1sXPSspectrumcanbedeconvolutedintotwo peaksasshowninFig.5,withthepeakpositionsannotated on theimage.Thepeak at∼530.5eVcorrespondstotheoxygenin ZnO,whichisnominallyatthesamespectralpositionforboththe samples.Theotherpeakseenat∼531.8eVand∼532.2eVforseed andneedlesamplerespectivelycorrespondstothechemisorbed oxygenoftwodifferentchemicalorigins.Thepeaksat531eVand 531.5eVareattributedtoO-xions(O-andO-2ions)intheoxygen deficientregions,whilepeaksat532.3eVand532.7eVaregener- allyascribedtothepresenceofoxygenrelatedspeciessuchas–OH, –CO,adsorbedH2OorO2onthesurfaceofZnO[25,54,55].Dur- ingthehydrothermalgrowthoftheneedles,grainboundariesand oxygendeficientregionsareexposedtohydroxylions.Theseions perhapsoccupysomeoftheoxygendeficientregionsasreflected withapeakat∼532.2eV.Apartfromthedifferenceinthespectral
location,thenumberdensityofsuchoccupanciesisseenin the areaofthepeakwhereforPAN/ZnOseedthearearatiois(∼49%) significantlyhigherthanneedlecase(∼22%).Itisnotablethatthe signalfromPAN/ZnOseedsamplecanbeattributedtothesample directlywithoutanyambiguity, however,forthePAN/ZnOnee- dlecase,itcanbeanintegralspectrumofseedaswellasneedle.
Despitethelatterambiguity,theabovegiveninterpretationisstill wellapplicableandwewillseethatitisinlinewithopticaland PCAmeasurements.Finally,alargeoxygen-deficientstateofthe surfacelayer[25,54]canbeseenforPAN/ZnOseed,whileincon- trast,PAN/ZnOneedlesamplehasshownsignificantlylessoxygen vacancies.NeedlesresultedinamorestableZnOenrichmenton PANnanofiberswhencomparedtoPAN/ZnOseed[54].
ThevalencebandspectraforPAN/ZnOseedaswellneedleis showninFig.S3ofSupportingInformation,wheretheintensity axisisnormalizedagainstthemaximumcountsandplottedwith referencetothebindingenergyineV.Inzincoxide,theconduction band(CB)andthevalenceband(VB)areformedfromO1sandZn2p
Fig.3. Representative(a1)TEMimageand(a2)SAEDpatternofPAN/ZnOseednanofibers;(b1)HRTEMimageand(b2)FFTimageofZnOneedle.
orbitals,respectively.Asawhole,both thesampleshaveshown thedensityofstateswhicharetypicaltozincoxide[25].Alsothe featuresandtheirspectrallocationforboththesamplesareexactly retraced(Fig.S3ofSupportingInformation).Thisisincontrasttoan earlierobservation[25]inwhichVOshaveshowntoinduceaband gapnarrowingbyexpandingtheminimumofCB.However,here theVOsdidnotinduceanysuchtailingofCBthoughevidencedin O1sXPSanalysis.Theenergeticlocationofoxygenvacancydefects withinthebandgapwillbediscussedinthecontextofPL.
Asmentionedearlier,thesurfacedefectsplayacrucialrolein determiningthePCA; we caninfer theinformation aboutsuch defectsthroughPLspectroscopy.ThePLspectraofPAN/ZnOseed andPAN/ZnOneedlenanofibersshowninFig.6awereobtainedat RT.Asshowninearlierinvestigations[19,21,48],thevisibleemis- sionfromZnOcanbedecomposed(fittingsnotshown)intovarious plausibletransitionswhichwillbediscussedaswegoalong.Itis knownthatthetypicalexcitionemissionbandliesintheUVregion forZnO, whilethedefect related emissionin thevisibleregion [19–21,47,48,56].Basedontheliteraturethepossibletransitions andthecorrespondingemissionwavelengthsareschematizedin Fig.6b,whicharecross-annotatedonFig.6awitharrowsonthe wavelengthaxis.
Westartwiththepeakscorrespondingtotheinterbandtran- sition(excitonicrecombination)whichistheleastcontroversial emission.Asexpected,relativelybettercrystallinePAN/ZnOneedle hasshownaclearpeakat∼3.25eV,whileincontrast,onlyasig- natureofsuchemissionisnoticedforPAN/ZnOseedsample.This emissionpeakisconsistentwiththeliteratureintermsofspectral location[21],yieldingabandgapof3.31eV[21]whenanexcition bindingenergyof60meVisassumed.Therelativeintensityofthis
interbandtransitionisenhancedupon needleformationonALD seedlayer,whichsuggestsanimprovementintheoveralloptical qualityofthematerial.Intheliterature[18,64,65],wecanseenee- dle/rodlikestructure,however,thesamplesdepictedbroadnear UVemissionandalmostnegligiblevisibleemission.However,in contrast,wehavecomparativelysharpUVemissionandsignifi- cantvisibleemission,whereweareaimedtoharnessthedefect relatedPCA.NotethattheemissionfromPAN/ZnOneedleisthe integralresponseoftwocomponents,oneofwhichisfromthenee- dleitself,whiletheotherisfromPAN/ZnOseed.AsPAN/ZnOseed didnotshowanyclearexcitionemission,thepeakseeninneedle samplearisesfromtheZnO-nanoneedle.InthecaseofPAN/ZnO seedbecauseofthelargegrainboundariesfromthefilmlikestruc- tureonthecylindricalperipheral,asignificantamountofsurface recombinationtakesplacegivingthepredominantvisibleemis- sion.VariousplausibleemissionsareschematizedinFig.6band shownwithnumerals(1)though(7)wheretheenergeticlocations ofthedefectshavebeenobtainedfromthecorrespondingrefer- ences;(1)[57],(2)[58],(3)[59,60],(4)[61],(5)[47,48],(6)[62]and (7)[63].Violetemissionscenteredatabout410nmarebroader, andnotasprominentasgreenemissioncenteredaround520nm, whichwereinterpretedtoberelatedtothedefectssuchaszinc interstitials(Zni)andVOs,respectively.Violetemissioncanresult fromanintegralresponseofthreetransitions[57–60]asdenoted onFig.6bwithAthroughE.Inthevisibleregionofthespectrum, boththesampleshaveexhibitedabroademissionwhichisagain anintegralresponseofthedefectsoftwodifferentorigins.Also,a slightthoughnoticeableblueshiftcanbenoticedinthecenterof thepeak(peakpositionsareannotatedontheimage)forPAN/ZnO needlefromitsseedcounterpart.Althoughthevariationisnominal
Fig.4. (a)XRDpatternsofnanofibersofPAN,PAN/ZnOseedandPAN/ZnOneedle,and(b)magnifiedXRDpatternsintherangeof31–37.5◦.
(∼0.02eV),whenitcomestothedensityofthedefects,itplaysa crucialroleindeterminingthePCAofthematerial.Theopticalqual- ityofthesemiconductorcanbeestimatedbytakingtheintensity ratiosofUVtovisibleemission[19–21].Itisworthnotingthatthe ratiooftheintensityofbandtobandtransition(∼381nm)tothe intensityofthedefectlevelemission(∼520nm)istentimeshigher forthePAN/ZnOneedlethanPAN/ZnOseed.Thishighratioindi- cateshigheropticalqualityofthePAN/ZnOneedlesample.Unlike thevioletemission,greenemissionisslightlycomplex[47,48].In thebulkgrainregion(BGR)singlypositivelychargedVOcaptures anelectronfromCBandformsaneutralVO(i.e.VO+→VO*).Inthe depletionregion(DR)ifthesinglypositivelychargedVOcaptures aholefromtheVB,itformsdoublypositiveVO(i.e.VO+→VO++).
Hence,thegreenemissionisacombinationoftransitionsfromVO* totheVBandCBtoVO++emittingFandGwavelengths,respec- tively (Fig. 6b)[47,48]. Also, relatively lowerintense interband emissionsuggeststhatthephoto-generatedelectronsandholesare capturedbyVO+emittingphotonsinthevisibleregionofthespec- trum.ThisinterpretationwillbeemployedtoexplainthePCAofthe samples.
WehavecomparativelyinvestigatedthePCAofPANnanofibers, PAN/ZnOseedandPAN/ZnOneedlebyanalyzingthetimedepen- dentdecompositionofMBinaqueousmediumunderillumination.
ToevaluatethedegradationrateofMB,itscharacteristicabsorption
peak(∼665nm)ismonitoredagainstUV-exposuretime.Therate ofdegradationisdefinedasC/CowhereCoandCrepresenttheini- tialconcentrationofMBbeforeandafterirradiationatagiventime respectively.ThepristinePANnanofibersareporoustoadsorb(not degrade)thedyeuptoanoticeablelevel(resultsnotshown)until equilibriumbetweenadsorptionanddesorptionisattained.Hence wehavetakenthesurfaceadsorptionasreferenceandanalyzed thePANnanofiberseffectondyedegradation,wherenoeffectis seen(Fig.7a).Thisisconsistentwiththeliterature[66].On the otherhand,itis notablethatALD unveilsconformalcoatingon electrospunnanofibersandhencetheexposureofPANnanofibers directlytothedyecanbeveryunlikely.InthecaseofPAN/ZnO seedandneedlecaseswehaveobservedanonlinearbehavior,and theelectrontransferbetweendonorstatesandthedyegoverns thedegradationratio[67,68].Hencewehaveaddressedthenature ofdegradationinthecontextofeachsampleindependently.When thecatalystsareimmersedintheMBsolution,thePCAwithrespect toUVirradiationtimeisdepictedinFig.7aalongwiththepris- tineMBsolutionwhichwassubjectedtothesameUVtreatment.
AccordingtotheLangmuir–Hinshelwoodmodeltheexponential relationshipof(C/Co)againsttimeindicatesthatMBdegradation followspseudo-first-orderkinetics.Wehaveperformedexponen- tialfittotheeachdatasetandthedecayconstantsaregivenonthe figure.
Fig.5.Peakdeconvolutionofcore-levelXPSspectraofO1sfromPAN/ZnOseedandPAN/ZnOneedlesamples.Thespectrallocationsofthepeaksareannotatedontheimage.
Inthecase ofUV exposuretopristinesolutionthedatahas showndecayconstantof∼157min.Notablythoughtothenaked eye,thedegradationoftheMB(withoutnanofibers)isnotclearly observeduponexposuretoUVradiationfor210min(seeFig.S4 ofSupportingInformationfordigitalphotographs).ForPAN/ZnO seed,∼47%ofMBdecomposedinnominal60minyieldingadegra- dationrateof∼113min.Animprovementof∼28%isnoticedwhen
comparedtothedegradationrateinthecaseofnocatalyst.Even- tually,thebluesolutionwasalmostdecolorizedafter∼210minof UVirradiation(Fig.S4ofSupportingInformation).Inthecaseof PAN/ZnOneedle,thedecompositionofMBwas∼93%in∼60min.
Interestingly,in thecase ofPAN/ZnO needles,at a degradation rateof∼15minhasshownimprovementof∼91%and∼87%for no catalyst and PAN/ZnO seed samples, respectively. PCA was
Fig.6.(a)PLspectraofPAN/ZnOseedandneedlecounterpart,and(b)depictsvariouscrystaldefectsandpossibletransitions[21].Theenergeticlocationofeachdefectlevel (denotedbynumerals)isobtainedfromthecorrespondingreferences(1)[57],(2)[58],(3)[59,60],(4)[61],(5)[47,48],(6)[62]and(7)[63].Thealphabetsstandforemission energiesinnanometer,whereA=395,B=437,C=405,D=440,E=455,F=∼500,andG=564.VZnislocated0.30eVabovetheVB,whileZniisat0.22eVbelowtheCB.Inthe bulkgrainregion(BGR)andinthedepletionregion(DR)VO+→VO*andVO+→VO++processestakeplace,respectively.
Fig.7.(a)DegradationrateofMBinaqueousenvironmenttestedforpristine,inthepresenceofPANnanofibers,PAN/ZnOseedandPAN/ZnOneedle(1stcycle)cases,(b) plausiblemechanismofphotocatalysisinvolvingoxygenvacancies,where(i)and(ii)standforprocessesacceptor→acceptor–anddonor→donor+respectively,and(c)PCA ofPAN/ZnOneedlenanofibersfor1st,2ndand3rdcycles.
relativelyhigherforthePAN/ZnOneedlethanPAN/ZnOseed,which isbecauseofnotonlyrelativelyhighersurfaceareabut alsoits highercrystalqualityoftheneedle-morphology.AspointedinSec- tion2.6,ZnO-seedcontentinPAN/ZnOneedlesampleislessthan PAN/ZnOseed,wheretheneedlescompensatetheremainderof theweight.Althoughtheneedlesareabout0.02mginPAN/ZnO needletheyshowsignificanteffectonPCA.Asanasidetheimprove- mentinthesurfaceareaisabout30times,where∼1200–1500 needlesareapproximatedonfiber(∼800nmand715nmoflength anddiameterrespectively).Inourpreviousstudy[2]highdensity nanoparticleshaveshown∼1.2timeshigherPCAthannanocoat- ing case. It needs tobe emphasizedthat within this study we haveachievedanimprovementofdyedegradationrateofnearly8 timesforneedlecasewhencomparedtoseedcase.Inthefollow- ing,weestablishtheargumentforPCAandlattercorrelatewith eachofthesamples.Undersuitableilluminationelectronscanbe excitedfromtheVBtoreachtheCB,leavingbehindholesintheVB [21,48].Iftheseseparatedchargescanmigratetothesurfaceofthe semiconductorbeforetheyrecombine,thentheyhaveachanceto participateintheredoxreactions[69].Formationofhydroxylrad- ical(˙OH)is thekey forthePCA,inwhich holes[70] aswellas electrons(whichmaybecapturedbymolecularoxygenforming superoxideanions[71],˙O2-)areinvolvedatVBandCB,respec- tively.Becauseofthepresenceofhighlyoxidativeholeaswellas
˙OHradicalstheorganicdyecanbedecomposedeitherpartiallyor completely.Wehaveshownthepossiblemechanism[10,70,71]in Fig.S5ofSupportingInformation.Intheliterature[10,72],itisdis- cussedthatPCAtakesplaceattheVBandthedefectstate(formed eitherbydoping[72]orintrinsic[10]e.g.VOs),wherethelatter capturesafreeelectronfromtheCB.However,underillumination, O2cancaptureanelectronfromCBpromotingthePCA.Thebasis forthisargumentistheinterbandtransitionseeninthePLspec- trumfromPAN/ZnOneedle samplewhichsuggestsapossibility ofphoto-electrons recombiningwithholesin CB,bypassingthe defectstate.Hence,atagiventime,underilluminationelectrons arepopulatedinCBtobecapturedbyO2.Ontheotherhand,the photo-electronscanalsobecapturedbyO2atVOsproducingsuper- oxideradicalanions.ItisalsoshownearlierthattheVOscanactas activesitesforPCAinZnOnanostructures[13,25,73].SincetheVOs arelocatedonthesurface(interfacesofthegrainsanddepletion regions)[47,48]theydirectlyinvolveinPCA[74].Notably,VOis treatedaselectronacceptors[74]bycapturinganelectronfromCB [21,47,48]andhencetherecombinationprocessisdelayed[10].In thePCAatheterojunction(e.g.ZnO/ZnSe[32],ZnO/Cu2O[33])(i) acceptor→acceptor– and(ii)donor→donor+ processesoccurat
CBofZnOandVBofZnSe(orCu2O),respectively,wherethecharge migrationacrosstheheterojunctiondelaystherecombinationpro- cess.InthecontextofPt–ZnOnanocomposite[74],awelldefined emissionfrominterbandtransitioninPLisnotseenbecauseofthe lowrecombinationrateofe/hpairswhichisinducedbyPt.
Inthebackgroundoftheabovediscussion,forahypothetical caseofvirtuallydefectfreenanoneedle(i.e.highopticalquality, Fig.7b,needleonly),PCAisbecauseof(i)and(ii)processestaking placeatCBandVBofZnOnanoneedlerespectively.Inthecaseof PAN/ZnOseed,thereisjustasignatureofinterbandtransitioninPL, hencethePCAthattakesplaceatCBandVBisnotdominant,which isdenotedwith(i)*and(ii)*,respectively(Fig.7b,seed).Thedefect siteVO+islocatedinthebulkofthegrain[19,21,47,48](Fig.6b) andhenceitisnotaccessibleforPCA,unlessthecapturedelectron migratestothesurface.Thismaybeaveryunlikelycaseasthese statesarehighlylocalized.Furthermore,thePCAassociatedwith VO+ isrelativelyweakandisdenotedwith(i)**.Incontrast,the defectsiteVO++,whichislocatedinthedepletionregion(e.g.grain boundaries[19,21,47,48],Fig.6b),iswellaccessibleforPCAandis denotedwith(ii).Inprinciple,thepresentALDgrownZnOfilmis evidencedtobegrainywithlargeportionsofgrainboundaries.For PAN/ZnOneedlecase,thePCAisanintegraleffectofVOs((ii),from PAN/ZnOseedsample)aswellasthecatalysistakingplaceatCB(i) andVB(ii)ofneedle(Fig.7b,seed/needle).Thecombinedeffectof alltheseprocessesyieldedsignificantlyhigherPCA.Wehavealso seenthattheseedsamplehaslargefractionofpolarplanesthan needlesample(analysisfromXRD),henceitisexpected[15]that seedsampleshouldhaveshownbetterPCA.Althoughitappearsto benotthecasehere,acarefulunderstandingofthebothmaterials revealsthatthepresentresultsareinlinewithRef.[15].Itiswell agreedthatthesamplewithlargerfractionofpolarplanesyield higherPCA(owingtotheirVOs)whatweseeisasynergyeffectof theneedleandtheseed,hencetheseresultsarenotincontrastto anearlierobservation[15].
The structuraldurability of thePAN/ZnO seedand PAN/ZnO needle nanofibers was also examined through SEM after the photocatalysis(Fig.S6ofSupportingInformation).Wenotethat the stability as well as durability plays a vital role because of their potential application in water purification of the organic pollutants. Asoutlinedin theintroduction,we characterize the materialintermsoftheircatalyticefficiencyanddurabilitywith reference to recycling. We have repeated the PCA experiment twiceforthePAN/ZnOneedle(Fig.7c).Thereisaslightdecreasein theefficiencyofPCAfrom1stcycletothefollowingcycles,where the1stcyclehasshown∼93%in∼60minofUVirradiation(Fig.7c).
Thedeteriorationcouldhaveoccurredfromvariousfactors.Firstly, asmallquantity(∼0.3mg)ofnanofibrousmatwasusedforSEM analysisafterthe1stcycle,leavingbehindlessamountofcatalytic materialforthe2ndand3rdcycles.Secondly,byconsideringthe SEM imagesof thelatter cycles(after 1st cycle, Fig.S6b; after 3rdcycle,Fig.S6cofSupportingInformation),itisclearthatthe densityofnanoneedles isdecreased toa certaindegree.Thisis becauseofthemechanicalfatiguewhileinsertingthenanofibrous matthroughatinyholeofthecuvetteandUV–Visspectroscopy.
Ifthenanofibrousmathasbeenhandledcarefullythenwebelieve thattheperformanceofcatalystafterthe2ndcyclewillbeasgood asoratleastcomparablewiththatofafreshsample.
4. Conclusions
HerewehavereportedtheresultsofaninvestigationonZnO- basedphotocatalystsynthesizedonelectrospunPANnanofibers.
ThiscatalystharnessesPCAatthreedifferentenergeticlocations withintheband gapof ZnO, namely, oxygenvacancy sites,VB and CB. In order toachieve this, morphologically well defined PAN nanofibersareproduced via electronspinning,followed by ALDtodepositZnOin awellcontrolledmanneryieldinga thin andconformalcoatingonthenanofibers.Thelaststepconsistsof hydrothermalgrowthofZnOsinglecrystalneedlelikestructures ontheALDseedcoating.Thepresentinvestigationalsore-iterates theflexibilityofvarioustechniquesandacombinationofALDand hydrothermalgrowth.TheALDparametersareoptimizedinsuch awaythattheseedsdonotinitiatemulti-needlegrowthwhichin turnimprovesthesubsequentprocessingofhydrothermalgrowth asinthepresentcaseorothermethodssuchassol–gel.Thestruc- turalinvestigation(XRD)revealedthestressrelatedinformationof thewurtzitestructuredPAN/ZnOseedaswellasPAN/ZnOneedle.
Thestressinthematerialmighthavebeenoriginatedduetothe polymericnanofibroussubstrateandtheassociatedhighsurface area.Investigationonlocalcrystalstructure(TEM)alsosupported thewurtzitestructureandhintedoxygendeficiencyinALD-ZnO.
However,asexpectedhydrothermallygrownZnOhappenedtobe insinglecrystallinestateandnomultiplephaseswereobservedin theFFTimage.Theoriginofthedefectandtheoxygendeficiency canbeidentifiedwithXPSratherprecisely,wherewehavenoticed thatPAN/ZnOseedsampleconsistsofO-xtypeions,whilePAN/ZnO needlesampleconsistsof–OH,–CO,H2O,orO2adsorbedatthe defect site. The former furthersupports theexistence of grain boundaries in the PAN/ZnO seed and less defective PAN/ZnO needle.BeingverycrucialforPCA,theresultsfromPLsuggestedan oxygendeficientPAN/ZnOseedwhilethePAN/ZnOneedlesofrel- ativelybetteropticalquality.Wenotetheconsistencybetweenthe PLandXPSmeasurements.Basedontheliterature,variousemis- sionbandshavebeenascribedtotheirplausibleorigin.Wehave suggestedamechanismfortheimprovedPCAofPAN/ZnOneedle sample,whencomparedwithPAN/ZnOseed.Wehaveinterpreted the PCA in conjunction with PL, where we point out the fact thatoxygenvacancycapturesaholefromtheCBandhencethe recombinationprocessisdelayed.Alsothiscapturedholecantake partinPCAasitislocatedwithinthegrainboundaryregion.The improvementisattributedtothecollectiveeffectwhichenabled theactiveparticipationofdefectstateandthecatalysistakingplace atCBaswellasVB.Ifphotocatalysisconsistsofonlydefectrelated activity,orthattakesplaceatCBandVBisnotsufficienttoachieve higherPCA.Ontheotherhand,thediscussiononPCAassumesthat thesurfacedefectsonnanoneedlesarenegligibleatanacceptable levelbygivenitscrystallinity,andtherelativeintensityofvisible emissionhasinfactsubduedwhencomparedtotheUVemission.
Furthermore,thesamplesaresubjectedtorecyclingandnominally thePAN/ZnOneedledepictedacomparableperformancewiththe
freshsample.Since thecatalystis synthesizedonflexiblepoly- mericnanofibers,themembranecanbehandledrathereasily(Fig.
S7ofSupportingInformation).Finallyitisconvincingthatthese ZnOnanostructuresarewellsuitedandpotentialcandidatesfor wastewatertreatmentwithsolarenergywheretheirperformance, structuralstabilityandreusabilityareworthmentioning.
Acknowledgements
S.V.thanksTheScientific&TechnologicalResearchCouncilof Turkey(TUBITAK)(TUBITAK-BIDEB2216,ResearchFellowshipPro- grammeforForeignCitizens)forpostdoctoralfellowship.F.K.and C.O.-A.thanksTUBITAK-BIDEBforaPhDscholarship.N.B.thanks EU FP7-Marie Curie-IRG for funding NEMSmart (PIRG05-GA- 2009-249196).T.U.thanksEU FP7-MarieCurie-IRG(NANOWEB, PIRG06-GA-2009-256428)andTheTurkishAcademyofSciences –OutstandingYoungScientistsAwardProgram(TUBA-GEBIP)for funding. Authorsthank M.Gulerfor technicalsupport for TEM analysis.
AppendixA. Supplementarydata
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/
j.apcatb.2014.03.004.
References
[1]F.Kayaci,C.Ozgit-Akgun,I.Donmez,N.Biyikli,T.Uyar,ACSAppl.Mater.Inter- faces4(2012)6185–6194.
[2]F.Kayaci,C.Ozgit-Akgun,N.Biyikli,T.Uyar,RSCAdv.3(2013)6817–6820.
[3]Z.Pei,L.Ding,J.Hu,S.Weng,Z.Zheng,M.Huang,P.Liu,Appl.Catal.B.143–143 (2013)736–743.
[4]T.-T.Chen,I.-C.Chang,M.-H.Yang,H.-T.Chiu,C.-Y.Lee,Appl.Catal.B.142–143 (2013)442–449.
[5]H.U.Lee,S.Y.Park,S.C.Lee,J.H.Seo,B.Son,H.Kim,H.J.Yun,G.W.Lee,S.M.Lee, B.Nam,J.W.Lee,Y.S.Huh,C.Jeon,H.J.Kim,J.Lee,Appl.Catal.B.144(2014) 83–89.
[6]Y.Zuo,Y.Qin,C.Jin,Y.Li,D.Shi,Q.Wu,J.Yang,Nanoscale5(2013)4388–4394.
[7]M.Pelaez,N.T.Nolan,S.C.Pillai,M.K.Seery,P.Falaras,A.G.Kontos,P.S.M.Dun- lop,J.W.J.Hamilton,J.A.Byrne,K.O’Shea,M.H.Entezari,D.D.Dionysiou,Appl.
Catal.B.125(2012)331–349.
[8]S.Hotchandani,P.V.Kamat,J.Phys.Chem.96(1992)6834–6839.
[9]N.Chouhan,C.L.Yeh,S.-F.Hu,R.-S.Liu,W.-S.Chang,K.-H.Chen,Chem.Commun.
47(2011)3493–3495.
[10]F.Xu,Y.Shen,L.Sun,H.Zeng,Y.Lu,Nanoscale3(2011)5020–5025.
[11]D.-X.Xu,Z.-W.Lian,M.-L.Fu,B.Yuan,J.-W.Shi,H.-J.Cui,Appl.Catal.B.142–143 (2013)377–386.
[12]M.R.Hoffmann,S.T.Martin,W.Choi,D.Bahnemann,Chem.Rev.95(1995) 69–96.
[13]J.Wang,P.Liu,X.Fu,Z.Li,W.Han,X.Wang,Langmuir25(2009)1218–1223.
[14]Y.Zheng,C.Chen,Y.Zhan,X.Lin,Q.Zheng,K.Wei,J.Zhu,Y.Zhu,Inorg.Chem.
46(2007)6675–6682.
[15]G.R.Li,T.Hu,G.L.Pan,T.Y.Yan,X.P.Gao,H.Y.Zhu,J.Phys.Chem.C112(2008) 11859–11864.
[16]X.Liu,L.Pan,T.Lv,Z.Sun,C.Sun,RSCAdv.2(2012)3823–3827.
[17]V.Subramanian,E.E.Wolf,P.V.Kamat,Langmuir19(2003)469–474.
[18]T.J. Athauda, U. Butt, R.R. Ozer, RSC Adv. (2013), http://dx.doi.org/
10.1039/c1033ra43672a.
[19]S.Vempati,A.Shetty,P.Dawson,K.Nanda,S.B.Krupanidhi,J.Cryst.Growth343 (2012)7–12.
[20]S.Vempati,A.Shetty,P.Dawson,K.K.Nanda,S.B.Krupanidhi,ThinSolidFilms 524(2012)137–143.
[21]S.Vempati,J.Mitra,P.Dawson,NanoscaleRes.Lett.7(2012)470.
[22]D.Chu,Y.Masuda,T.Ohji,K.Kato,Langmuir26(2010)2811–2815.
[23]N.Kislov,J.Lahiri,H.Verma,D.Y.Goswami,E.Stefanakos,M.Batzill,Langmuir 25(2009)3310–3315.
[24]T.J.Sun,J.S.Qiu,C.H.Liang,J.Phys.Chem.C112(2008)715–721.
[25]S.A.Ansari,M.M.Khan,S.Kalathil,A.N.Khan,J.Lee,M.H.Cho,Nanoscale5 (2013)9238–9246.
[26]N.Scharnagl,H.Buschatz,Desalination139(2001)191–198.
[27]S.Yang,Z.Liu,J.Membr.Sci.222(2003)87–98.
[28]L.Zhang,J.Luo,T.J.Menkhaus,H.Varadaraju,Y.Sun,H.Fong,J.Membr.Sci.369 (2011)499–505.
[29]Y.Mei,C.Yao,K.Fan,X.Li,J.Membr.Sci.417(2012)20–27.
[30]X.Zhang,V.Thavasi,S.G.Mhaisalkar,S.Ramakrishna,Nanoscale4(2012) 1707–1716.
[31]H.Tong,S.Ouyang,Y.Bi,N.Umezawa,M.Oshikiri,J.Ye,Adv.Mater.24(2012) 229–251.
[32]S.Cho,J.-W.Jang,J.S.Lee,K.-H.Lee,Nanoscale4(2012)2066–2071.
[33]T.Jiang,T.Xie,L.Chen,Z.Fu,D.Wang,Nanoscale5(2013)2938–2944.
[34]T.Uyar,R.Havelund,J.Hacaloglu,X.Zhou,F.Besenbacher,P.Kingshott,Nano- technology20(2009)125605.
[35]S.Vempati,J.B.Veluru,R.G.Karunakaran,D.Raghavachari,T.S.Natarajan,J.
Appl.Phys.110(2011)113718.
[36]S.Ramakrishna,K.Fujihara,W.Teo,T.Lim,Z.Ma,AnIntroductiontoElec- trospinningandNanofibers,WorldScientificPublishingCompany,Singapore, 2005.
[37]J.H.Wendorff,S.Agarwal,A.Greiner,Electrospinning:Materials,Processing, andApplications,Wiley-VCH,Germany,2012.
[38]J.V. Nygaard,T.Uyar, M.Chen,P.Cloetens,P.Kingshott,F. Besenbacher, Nanoscale3(2011)3594–3597.
[39]S.M.George,Chem.Rev.110(2009)111–131.
[40]M.Leskelä,M.Ritala,Angew.Chem.Int.Ed.42(2003)5548–5554.
[41]C.Detavernier,J.Dendooven,S.P.Sree,K.F.Ludwig,J.A.Martens,Chem.Soc.
Rev.40(2011)5242–5253.
[42]C.J.Oldham,B.Gong,J.C.Spagnola,J.S.Jur,K.J.Senecal,T.A.Godfrey,G.N.Par- sons,J.Electrochem.Soc.158(2011)D549–D556.
[43]W.J.Sweet,J.S.Jur,G.N.Parsons,J.Appl.Phys.113(2013)194303.
[44]P.Heikkilä,T.Hirvikorpi,H.Hilden,J.Sievänen,L.Hyvärinen,A.Harlin,M.Vähä- Nissi,J.Mater.Sci.47(2012)3607–3612.
[45]E.Santala,M.Kemmel,M.Leskela,M.Ritala,Nanotechnology20(2009)035602.
[46]I.M.Szilagyi,E.Santala,M.Heikkila,V.Pore,M.Kemmel,T.Nikitin,G.Teucher,T.
Firkala,L.Khriachtchev,M.Rsanen,M.Ritala,M.Leskala,Chem.Vap.Deposition 19(2013)149–155.
[47]J.D.Ye,S.L.Gu,F.Qin,S.M.Zhu,S.M.Liu,X.Zhou,W.Liu,L.Q.Hu,R.Zhang,Y.
Shi,Y.D.Zheng,Appl.Phys.A:Mater.Sci.Process.81(2005)759–762.
[48]S.Vempati,S.Chirakkara,J.Mitra,P.Dawson,K.K.Nanda,S.B.Krupanidhi,Appl.
Phys.Lett.100(2012)162104.
[49]B.Gong,Q.Peng,J.-S.Na,G.N.Parsons,Appl.Catal.A407(2011)211–216.
[50]A.Sugunan,V.K.Guduru,A.Uheida,M.S.Toprak,M.Muhammed,J.Am.Ceram.
Soc.93(2010)3740–3744.
[51]P.Liu,Y.Zhu,J.Ma,S.Yang,J.Gong,J.Xu,Colloid.Surf.A436(2013)489–494.
[52]Z.Zhang,L.Zhang,S.Wang,W.Chen,Y.Lei,Polymer42(2001)8315–8318.
[53]B.D.Cullity,S.R.Stock,ElementsofX-rayDiffraction,3rded.,PrenticeHall, 2001.
[54]M.Chen,X.Wang,Y.Yu,Z.Pei,X.Bai,C.Sun,R.Huang,L.Wen,Appl.Surf.Sci.
158(2000)134–140.
[55]A.St˘anoiu,C.E.Simion,S.Som˘acescu,Sens.ActuatorsB:Chem.186(2013) 687–694.
[56]A.Djuriˇsi ´c,W.C.Choy,V.A.L.Roy,Y.H.Leung,C.Y.Kwong,K.W.Cheah,T.Gundu Rao,W.K.Chan,H.FeiLui,C.Surya,Adv.Funct.Mater.14(2004)856–864.
[57]C.H.Ahn,Y.Y.Kim,D.C.Kim,S.K.Mohanta,H.K.Cho,J.Appl.Phys.105(2009) 013502.
[58]E.G.Bylander,J.Appl.Phys.49(1978)1188.
[59]B.Lin,Z.Fu,Y.Jia,Appl.Phys.Lett.79(2001)943.
[60]P.S.Xu,Y.M.Sun,C.S.Shi,F.Q.Xu,H.B.Pan,Nucl.Instrum.Meth.B199(2003) 286–290.
[61]H.Zeng,G.Duan,Y.Li,S.Yang,X.Xu,W.Cai,Adv.Funct.Mater.20(2010)561.
[62]K.Vanheusden,W.L.Warren,C.H.Seager,D.R.Tallant,J.A.Voigt,B.E.Gnade,J.
Appl.Phys.79(1996)7983.
[63]A.V.Dijken,E.A.Meulenkamp,D.Vanmaekelbergh,A.Meijerink,J.Lumin.90 (2000)123–128.
[64]T.J.Athauda,P.Hari,R.R.Ozer,ACSAppl.Mater.Interfaces5(2013)6237–6246.
[65]T.J.Athauda,R.R.Ozer,Cryst.GrowthDesign13(2013)2680–2686.
[66]C.Prahsarn,W.Klinsukhon,N.Roungpaisan,Mater.Lett.65(2011)2498–2501.
[67]S.Baruah,S.S.Sinha,B.Ghosh,S.K.Pal,A.K.Raychaudhuri,J.Dutta,J.Appl.Phys.
105(2009)074308.
[68]S.S.Warule,N.S.Chaudhari,B.B.Kale,M.A.More,Cryst.Eng.Commun.11(2009) 2776–2783.
[69]N.Daneshvar,D.Salari,A.R.Khataee,J.Photochem.Photobiol.A162(2004) 317–322.
[70]R.W.Matthews,J.Catal.97(1986)565–568.
[71]I.Izumi,W.W.Dunn,K.O.Wilbourn,F.R.F.Fan,A.J.Bard,J.Phys.Chem.84(1980) 3207–3210.
[72]Y.Yang,Y.Li,L.Zhu,H.He,L.Hu,J.Huang,F.Hu,B.Hec,Z.Ye,Nanoscale5 (2013)10461–10471.
[73]J.Wang,Z.Wang,B.Huang,Y.Ma,Y.Liu,X.Qin,X.Zhang,Y.Dai,ACSAppl.
Mater.Interfaces4(2012)4024–4030.
[74]C.Yu,K.Yang,Y.Xie,Q.Fan,J.C.Yu,Q.Shu,C.Wang,Nanoscale5(2013) 2142–2151.
