Enhanced photocatalytic activity of homoassembled ZnO nanostructures on electrospun polymeric nanofibers: a combination of atomic layer deposition and hydrothermal growth

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ContentslistsavailableatScienceDirect

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,∗∗

a_{UNAM-National}_{Nanotechnology}_Research_Center,_Bilkent_University,_Ankara,_06800,_Turkey b_Institute_of_Materials_Science_and_{Nanotechnology,}_Bilkent_University,_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).Apartfromadetailed morphologi-calandstructuralcharacterization,thePCAhasbeencarefullymonitoredandtheresultsarediscussed elaboratelywhen juxtaposedwiththe photoluminescence.Thepresenthierarchal homoassembled nanostructuresareacombinationoftwotypesofZnOwithdiverseopticalqualities,i.e.(a)controlled depositionofZnOcoatingonnanofiberswithdominantoxygenvacanciesandsignificantgrain bound-ariesbyatomiclayerdeposition(ALD),and(b)growthofsinglecrystallineZnOnanoneedleswithhigh opticalqualityontheALDseedsviahydrothermalprocess.Theneedlestructure(∼25nmindiameter withanaspectratioof∼24)alsosupportsthevectorialtransportofphoto-chargecarriers,whichiscrucial forhighcatalyticactivity.Furthermore,itisshownthatenhancedPCAisbecauseofthecatalytic activ-ityatsurfacedefects(onALDseed),valenceband,andconductionband(ofZnOnanoneedles).PCAand durabilityofthePAN/ZnOnanofibrousmathavealsobeentestedwithaqueoussolutionofmethylene blueandtheresultsshowedalmostnodecayinthecatalyticactivityofthismaterialwhenreused.

1. Introduction

Photocatalysisis one ofthewidely researched[1–11]topics

becauseofitsimportanceinwaterandenvironmentalpuriﬁcation

inthebackground ofunavoidable andever increasing

industri-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.DespitethehigherPCAthose

com-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,it

obvi-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

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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 vapor

deposi-tion(CVD)andliquidphasedeposition(LPD)techniques.Atomic

layerdeposition(ALD)and hydrothermalgrowth arecombined

to fabricate a hierarchy of nanostructured-ZnO onelectrospun

poly(acrylonitrile)(PAN)nanoﬁbers.TheresultingZnO

nanostruc-turesdepictsynergyeffectandshowenhancedPCA.PANnanoﬁbers

arewelladoptableinwaterﬁltrationwheretheiruniqueproperties

suchashighsurfacearea,nanoporousstructure,lowbasisweight,

easypermeability,goodstabilityandchemicalresistanceareworth

mentioning[26–29].Inourpreviousstudy[2]electrospun

poly-mericnanoﬁbersweresubjectedtovaryingALDparameterswhere

wehavestudiedhow thePCAisinﬂuencedwhennanoparticles

transformintocontinuousﬁlm.Wehadinferredthathighlydense

nanoparticlesshowrelativelyhigherPCAduetotheincreased

sur-faceareaconsistingofoxygenvacancyandotherrelateddefects.

Webelievethathavingpolycrystallineﬁlmaloneisnotadequate

toyieldhighPCA,henceinthisstudy,wehavegrownsingle

crys-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

showntodepictvectorialtransportofphotogeneratedcharge

car-riersandhelpingtoimprovethePCA[30,31].Whilemaintaining

adelicatebalancebetweentheadvantages[13–15,25]and

limi-tations[10]ofthenanostructureswehaveachievedasigniﬁcant

PCAwiththepresentcombination.Sincenanoneedlesweregrown

onthethinﬁlmwhichisonthepolymericﬁbrousmat,theymay

notbeeasilydislodgedwithoutasigniﬁcantmechanicalfatigue.

Thismakesthemeasiertohandleandrecycleinaqueous

environ-mentunlikethecasewithnanosizedparticles[6,14,15,25,32].Of

courseonecanemploytheexpensiveindiumtinoxide/ﬂuorinetin

oxidesubstratebasedcatalysts[10,33].Asanadditionaladvantage,

nanoneedlesareboundtothesurfaceoftheZnOﬁlmonthe

poly-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.Priorto

elec-trospinning,PANsolutionwasstirredfor3hatroomtemperature

(RT)toobtainhomogeneousandclearsolution.Well-stirred

solu-tionwastakenina5mLsyringeﬁttedwithametallicneedleof

∼0.8mmofinnerdiameter.Thesyringewasﬁxedhorizontallyon

thesyringepump(KDScientiﬁc,KDS101)withafeedratesetto

1mL/h.Ahighvoltageof15kVisapplied(Matsusada,AUSeries)

betweenthesyringeneedleandastationarycylindricalmetal

col-lector(wrappedwithacleanaluminumfoil)locatedat12cmfrom

theendofthetip.Theelectrospinningprocesswascarriedoutat

∼25◦_C_and_22%_relative_humidity_in_an_enclosed_chamber.

2.3. PreparationofZnOseedstructurebyALD

ZnO deposition on electrospun PAN nanoﬁbers was carried

outat∼200◦_C_in_a_Savannah_S100_ALD_reactor_(Cambridge

Nan-otech Inc.). N2 was used as a carrier gas at a ﬂow rate of

∼20sccm.400cycleswereappliedviaexposuremode(a

trade-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,thevacuumwasswitchedbacktodynamicmodefor

purg-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,thenanoﬁbrousmatwas

thor-oughlyrinsedwithDIwatertoremoveanyresidualsaltsanddried

invacuumovenat∼40◦_C_for₁₂_h.

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(energy

dis-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

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underAlK␣(h=1486.6eV)linewithachargeneutralizer.Pass

energy, stepsize and spotsizewere 30eV,0.1eV and 400␮m,

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

needlesampleswereanalyzedthroughphotoinduced

degrada-tionofMBinaqueousmedium(18.8␮M).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/ZnO

nee-dlewasfoundtocontain3.32mgofseedand0.28mgofneedles.

Therateofdyedegradationwasquantiﬁedviaﬁrstorder

exponen-tialﬁt(y=y0+Ae-x/t)foreachdataset.Thisﬁtwasperformedunder

automatedroutinewithOrigin6.1,wherealltheparametersareset

asfreeuntilconvergence.WehavealsorepeatedthePCA

experi-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]andmany

oth-ers[36,37].Acloseinspectionofthemorphologyrevealsatexture

likestructure,which issometimesobservedfor certain

electro-spunpolymericnanoﬁbersduetothetypeofsolventused[36–38].

Thesenanoﬁberswereemployedforthesecondstepofseeding

withALD[39–41]byapplying400cyclesat200◦Cusingexposure

mode(Section2.3).AftertheALDprocess,wehaverecordedthe

SEMimageswhichareshowninFig.2(b1and2)wheretheAFD

is∼715±125nm.ThismeasurementsuggestedanincreaseinAFD

becauseofALDcoating.Theﬁberstructurewasnotdestroyed

dur-ingtheALDprocesswhereawelldeﬁnedandstableﬁberstructure

suggeststhesuitabilityofthechosenparameters.Itisimportant

topointouttheneedofcompatibilitybetweentheprecursorand

polymerastheformercandegradethelatterbychemically

react-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))thatthesurfaceroughnessisincreasedafterALD

pro-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/ZnO

nee-dle).ItcanbeseenthatnanoneedlescoverthesurfaceoftheZnO

seed-coatedPANnanoﬁbers.Thenanoneedleswerestraightandno

branchingwasobserved.Branchinggenerallyoccursbecauseofthe

irregularityintheseedwhereitcanpromotethegrowthofmore

thanoneneedle.Notably,inthepresentcontexttheseedsgrown

throughALD-processwereuniformanddidnotinitiateorsupport

multi-needlegrowth.SEMimagesofPAN/ZnOneedlesatdifferent

magniﬁcationsaregiveninFig.S1ofSupportingInformation.By

analyzingtheSEMimageswehaveestimatedtheaverage

diam-eterandlengthofthenanoneedlestobe∼25nmand ∼600nm,

respectively(Fig.2(c2)).Detaileddiscussiononthemechanismof

thegrowthofZnOnanoneedlescanbefoundintheliterature[15].

SeedingofZnOwithALDprocessandsubsequenthydrothermally

grownnanorodsof∼50nmdiameterwithalengthof∼0.5–1␮m

canbeseenintheliterature[49,50].

ThemorphologiesofthePAN/ZnOseednanoﬁberswere

fur-therinvestigatedbyTEMandshowninFig.3(a1).Theconformal

coatingofZnOcanbeevidencedfromtheimagewithauniform

thickness(_∼50nm)inspiteoftherelativelylargesurfaceareaofthe

nanoﬁbers.Notablythissupportstheearlierargumenton

unifor-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)whichalsoresultedina

continu-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.S2ofSupporting

Infor-mation,leftpanel)onthePAN/ZnOseednanoﬁbershasshownzinc,

oxygen,carbon,nitrogenandcopper(fromTEMgrid)elements.Zn

andOoriginatefromZnOseed,whereasCandNareduetothe

polymericcorestructureofPAN.Alsothequantiﬁcation(Fig.S2of

SupportingInformation,rightpanel)ofZnandOatomic

percent-agessuggeststhatthematerialisnominallyoxygenrich,within

thedetectionlimitofEDX.Thisisbecauseoftheveryhighsurface

areayieldingdefectivesites(oxygenvacancies)wheremolecular

oxygencanbeadsorbed.Itisnotablethatoxygenvacanciesare

typicalforZnOinotherprocessingtechniquesaswell,whichwere

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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◦_{corresponding}_to

orthorhom-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,wehave

anno-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,whenZnOispreparedthroughdifferent

meth-ods[19–21,48].However,acloseobservationof(100),(002)and

(101)reﬂections(Fig.4b)revealvitalinformation.Ifwecompare

theFWHMvaluesofthesepeaksacrossseedandneedlesamples,

wecanseethattheformerislesscrystallinethanthelatter.There

isalsoashiftinthepositionofthepeakstowardshigher2value

uponneedleformation.ForthediffractionpatternofPAN/ZnO

nee-dle,wecanseeashoulder-likestructure(denotedwithsinFig.4b)

whichcorrespondstothePAN/ZnOseed,whileamoreintensepeak

correspondingtothehighlycrystallineZnOnanoneedle.Peakshift

isgenerallyassociatedwiththeresidualstress(defect-induced)in

thematerial.Thestressmightbeoriginatedfromtheoxygen

vacan-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

theXRDisastatisticalaverageofplanesfromsurfaceaswellas

sub-surfaceregions.Hence,thereisapossibilitythatthepolarplanes

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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-x_ions_(O-_and_O-2_ions)_in_the_oxygen

deﬁcientregions,whilepeaksat532.3eVand532.7eVare

gener-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/ZnO

nee-dlecase,itcanbeanintegralspectrumofseedaswellasneedle.

Despitethelatterambiguity,theabovegiveninterpretationisstill

wellapplicableandwewillseethatitisinlinewithopticaland

PCAmeasurements.Finally,alargeoxygen-deﬁcientstateofthe

surfacelayer[25,54]canbeseenforPAN/ZnOseed,whilein

con-trast,PAN/ZnOneedlesamplehasshownsigniﬁcantlylessoxygen

vacancies.NeedlesresultedinamorestableZnOenrichmenton

PANnanoﬁberswhencomparedtoPAN/ZnOseed[54].

ThevalencebandspectraforPAN/ZnOseedaswellneedleis

showninFig.S3ofSupportingInformation,wheretheintensity

axisisnormalizedagainstthemaximumcountsandplottedwith

referencetothebindingenergyineV.Inzincoxide,theconduction

(6)

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],thevisible

emis-sionfromZnOcanbedecomposed(ﬁttingsnotshown)intovarious

plausibletransitionswhichwillbediscussedaswegoalong.Itis

knownthatthetypicalexcitionemissionbandliesintheUVregion

forZnO, whilethedefect related emissionin thevisibleregion

[19–21,47,48,56].Basedontheliteraturethepossibletransitions

andthecorrespondingemissionwavelengthsareschematizedin

Fig.6b,whicharecross-annotatedonFig.6awitharrowsonthe

wavelengthaxis.

Westartwiththepeakscorrespondingtotheinterband

tran-sition(excitonicrecombination)whichistheleastcontroversial

emission.Asexpected,relativelybettercrystallinePAN/ZnOneedle

hasshownaclearpeakat∼3.25eV,whileincontrast,onlya

sig-natureofsuchemissionisnoticedforPAN/ZnOseedsample.This

emissionpeakisconsistentwiththeliteratureintermsofspectral

location[21],yieldingabandgapof3.31eV[21]whenanexcition

bindingenergyof60meVisassumed.Therelativeintensityofthis

interbandtransitionisenhancedupon needleformationonALD

seedlayer,whichsuggestsanimprovementintheoveralloptical

qualityofthematerial.Intheliterature[18,64,65],wecansee

nee-dle/rodlikestructure,however,thesamplesdepictedbroadnear

UVemissionandalmostnegligiblevisibleemission.However,in

contrast,wehavecomparativelysharpUVemissionand

signiﬁ-cantvisibleemission,whereweareaimedtoharnessthedefect

relatedPCA.NotethattheemissionfromPAN/ZnOneedleisthe

integralresponseoftwocomponents,oneofwhichisfromthe

nee-dleitself,whiletheotherisfromPAN/ZnOseed.AsPAN/ZnOseed

didnotshowanyclearexcitionemission,thepeakseeninneedle

samplearisesfromtheZnO-nanoneedle.InthecaseofPAN/ZnO

seedbecauseofthelargegrainboundariesfromtheﬁlmlike

struc-tureonthecylindricalperipheral,asigniﬁcantamountofsurface

recombinationtakesplacegivingthepredominantvisible

emis-sion.VariousplausibleemissionsareschematizedinFig.6band

shownwithnumerals(1)though(7)wheretheenergeticlocations

ofthedefectshavebeenobtainedfromthecorresponding

refer-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

(7)

Fig.4. (a)XRDpatternsofnanoﬁbersofPAN,PAN/ZnOseedandPAN/ZnOneedle,and(b)magniﬁedXRDpatternsintherangeof31–37.5◦_.

(∼0.02eV),whenitcomestothedensityofthedefects,itplaysa

crucialroleindeterminingthePCAofthematerial.Theoptical

qual-ityofthesemiconductorcanbeestimatedbytakingtheintensity

ratiosofUVtovisibleemission[19–21].Itisworthnotingthatthe

ratiooftheintensityofbandtobandtransition(∼381nm)tothe

intensityofthedefectlevelemission(∼520nm)istentimeshigher

forthePAN/ZnOneedlethanPAN/ZnOseed.Thishighratio

indi-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+emittingphotonsinthevisibleregionofthe

spec-trum.ThisinterpretationwillbeemployedtoexplainthePCAofthe

samples.

WehavecomparativelyinvestigatedthePCAofPANnanoﬁbers,

PAN/ZnOseedandPAN/ZnOneedlebyanalyzingthetime

depen-dentdecompositionofMBinaqueousmediumunderillumination.

ToevaluatethedegradationrateofMB,itscharacteristicabsorption

peak(∼665nm)ismonitoredagainstUV-exposuretime.Therate

ofdegradationisdeﬁnedasC/CowhereCoandCrepresentthe

ini-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.7aalongwiththe

pris-tineMBsolutionwhichwassubjectedtothesameUVtreatment.

AccordingtotheLangmuir–Hinshelwoodmodeltheexponential

relationshipof(C/Co)againsttimeindicatesthatMBdegradation

followspseudo-ﬁrst-orderkinetics.Wehaveperformed

exponen-tialﬁttotheeachdatasetandthedecayconstantsaregivenonthe

(8)

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%ofMBdecomposedinnominal60minyieldinga

degra-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

(9)

Fig.7.(a)DegradationrateofMBinaqueousenvironmenttestedforpristine,inthepresenceofPANnanoﬁbers,PAN/ZnOseedandPAN/ZnOneedle(1stcycle)cases,(b) plausiblemechanismofphotocatalysisinvolvingoxygenvacancies,where(i)and(ii)standforprocessesacceptor→acceptor–_and_donor_→_donor+_{respectively,}_and_(c)_PCA

ofPAN/ZnOneedlenanoﬁbersfor1st,2ndand3rdcycles.

relativelyhigherforthePAN/ZnOneedlethanPAN/ZnOseed,which

isbecauseofnotonlyrelativelyhighersurfaceareabut alsoits

highercrystalqualityoftheneedle-morphology.Aspointedin

Sec-tion2.6,ZnO-seedcontentinPAN/ZnOneedlesampleislessthan

PAN/ZnOseed,wheretheneedlescompensatetheremainderof

theweight.Althoughtheneedlesareabout0.02mginPAN/ZnO

needletheyshowsigniﬁcanteffectonPCA.Asanasidethe

improve-mentinthesurfaceareaisabout30times,where∼1200–1500

needlesareapproximatedonﬁber(∼800nmand715nmoflength

anddiameterrespectively).Inourpreviousstudy[2]highdensity

nanoparticleshaveshown∼1.2timeshigherPCAthan

nanocoat-ing case. It needs tobe emphasizedthat within this study we

haveachievedanimprovementofdyedegradationrateofnearly8

timesforneedlecasewhencomparedtoseedcase.Inthe

follow-ing,weestablishtheargumentforPCAandlattercorrelatewith

eachofthesamples.Undersuitableilluminationelectronscanbe

excitedfromtheVBtoreachtheCB,leavingbehindholesintheVB

[21,48].Iftheseseparatedchargescanmigratetothesurfaceofthe

semiconductorbeforetheyrecombine,thentheyhaveachanceto

participateintheredoxreactions[69].Formationofhydroxyl

rad-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],itis

dis-cussedthatPCAtakesplaceattheVBandthedefectstate(formed

eitherbydoping[72]orintrinsic[10]e.g.VOs),wherethelatter

capturesafreeelectronfromtheCB.However,underillumination,

O2cancaptureanelectronfromCBpromotingthePCA.Thebasis

forthisargumentistheinterbandtransitionseeninthePL

spec-trumfromPAN/ZnOneedle samplewhichsuggestsapossibility

ofphoto-electrons recombiningwithholesin CB,bypassingthe

defectstate.Hence,atagiventime,underilluminationelectrons

arepopulatedinCBtobecapturedbyO2.Ontheotherhand,the

photo-electronscanalsobecapturedbyO2atVOsproducing

super-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+ _processes_occur_at

CBofZnOandVBofZnSe(orCu2O),respectively,wherethecharge

migrationacrosstheheterojunctiondelaystherecombination

pro-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

(10)

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

Herewehavereportedtheresultsofaninvestigationon

ZnO-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.The

struc-turalinvestigation(XRD)revealedthestressrelatedinformationof

thewurtzitestructuredPAN/ZnOseedaswellasPAN/ZnOneedle.

Thestressinthematerialmighthavebeenoriginatedduetothe

polymericnanoﬁbroussubstrateandtheassociatedhighsurface

area.Investigationonlocalcrystalstructure(TEM)alsosupported

thewurtzitestructureandhintedoxygendeﬁciencyinALD-ZnO.

However,asexpectedhydrothermallygrownZnOhappenedtobe

insinglecrystallinestateandnomultiplephaseswereobservedin

theFFTimage.Theoriginofthedefectandtheoxygendeﬁciency

canbeidentiﬁedwithXPSratherprecisely,wherewehavenoticed

thatPAN/ZnOseedsampleconsistsofO-x_type_ions,_while_PAN/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/ZnOneedlesof

rel-ativelybetteropticalquality.Wenotetheconsistencybetweenthe

PLandXPSmeasurements.Basedontheliterature,various

emis-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 synthesizedonﬂexible

poly-mericnanoﬁbers,themembranecanbehandledrathereasily(Fig.

S7ofSupportingInformation).Finallyitisconvincingthatthese

ZnOnanostructuresarewellsuitedandpotentialcandidatesfor

wastewatertreatmentwithsolarenergywheretheirperformance,

structuralstabilityandreusabilityareworthmentioning.

Acknowledgements

S.V.thanksTheScientiﬁc&TechnologicalResearchCouncilof

Turkey(TUBITAK)(TUBITAK-BIDEB2216,ResearchFellowship

Pro-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.

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