films Characterization of MWCNT-TiO QDs and TiO QDs inself-assembled Optik

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Optik

jou rn a l h om ep ag e :w w w . e l s e v i e r . d e / i j l e o

Original

research

article

Characterization

of

MWCNT-TiO

2

QDs

and

TiO

2

QDs

in

self-assembled

films

Ümit

Özlem

Akkaya

Arıer

_,

_Bengü

_Özu˘gur

_Uysal

a_{DepartmentofPhysics,MimarSinanFineArtsUniversity,Bomonti,Istanbul34349,Turkey}

b_{FacultyofEngineeringandNaturalSciences,KadirHasUniversity,CibaliCampus,Fatih,Istanbul34083,Turkey}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received22November2016

Receivedinrevisedform26April2017

Accepted28April2017 Keywords: MWCNT-TiO2 QDscomposites Films Growthkinetics

a

b

s

t

r

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t

Inthisstudy,thesolutionwhichincludesTiO2quantumdots(QDs)wasmixedwiththe

multi-wallcarbonnanotubes(MWCNTs)toprepareMWCNT-TiO2QDscompositefilms.

TheeffectofmicrostructuresonthestructuralandopticalpropertiesofMWCNT-TiO2QDs

compositefilmswasevaluated.TheactivationenergyforcrystallitegrowthofTiO2QDs

whichareproducedinbrookitephaseswascalculatedas20.3kJ/mol.Thepropertiesof MWCNT-TiO2QDscompositefilmswerecharacterizedbyX-raydiffraction(XRD),

scan-ningelectronmicroscopy(SEM),atomicforcemicroscopy(AFM)andultraviolet–visible absorptionspectroscopy(UV–vis).

©2017ElsevierGmbH.Allrightsreserved.

1. Introduction

Single-walledandmulti-walledcarbonnanotubes(CNTs)haveinterestingphysicalandtechnologicalproperties.CNTs

havelargesurfaceareas,highstrengthandgoodelectricalconductivities[1,3].MWCNT-TiO2 compositestructureshave

anumberofapplicationsincludingphotocatalyticandhydrophilicactivities,solarcells,lithiumionbatteries,biosensors

andelectrochemicalcapacitorduetotheirmechanical,chemicalandelectricalproperties[2–12].MWCNT-TiO2composites

showhigherphotocatalyticactivitythanpureTiO2structures[2,13].MWCNT-TiO2structureshavebeenpreparedwith

differentmethodssuchassol-geldeposition,chemicalvapordeposition(CVD),hydrothermaldeposition,and

electrospin-ningdepositionetc.[1–3,13].MWCNT-TiO2QDscompositescanbeproducedaspowder,films,etc.[4–6].CombiningTiO2

filmswithCNTsimprovesaspectssuchasmechanicalandoptoelectronicefficiency[14].TiO2quantumdotshavedifferent

propertiesbecauseofquantumconfinementeffects,smallsize,andhighsurfacearea.Iftheparticlesizeiscomparabletothe

Bohrexcitonradiusofthematerial,onecaninvestigatethatthismaterialwhichcontainQDsdisplayquantumconfinement

effectsconsiderably.Onlyifradiusofparticlesissmallerthanexcitonradius,particlesexhibitstrongconfinement.While

excitonBohrradiusofhydrogen:0.53Å,theexcitonradiiofTiO2nano-particlesarebetween7.5and19Å[15].Quantum

sizeeffectoccurredinTiO2particleswhichhaveradiusintherangeof1–10nm[16,17].QuantumsizeeffectofQDsmeans

controllingthebandgap,thustuningthediscretenessoftheenergylevelsdependingonthesize.BrookitephaseofTiO2is

difficulttoproducepurelyandthereisnoanystudyaboutbrookitephaseofMWCNT-TiO2QDscompositefilms[16].The

effectofprecursorratiosonthefilmisherepresentedwhereTiO2quantumdotsareincorporatedinthefilmstructurein

additionwithMWCNTinordertoformthedispersedmediaofTiO2QDsinMWCNT-TiO2films.

∗ Correspondingauthor.

E-mailaddress:oarier@gmail.com(Ü.Ö.A.Arıer).

http://dx.doi.org/10.1016/j.ijleo.2017.04.101

Ü.Ö.A.Arıer,B.Ö.Uysal/Optik140(2017)1032–1037 1033

Table1

SizecalculationsofQDsfromXRDandUV–vismeasurementsbyusingScherrerandBrusequationsfordifferentAcid:TiO2ratios.

Data Acid:TiO2ratio XrdgraphScherrer’sequ.

QDsize(nm)

Brusequ.QDssize(nm)

a 0.05 2.99 2.18

b 0.1 3.58 3.81

c 0.2 4.22 4.13

d 0.4 5.52 5.15

2. Experimental

2.1. Preparationofsolutionsandcoatings

TiO2QDsandMWCNT-TiO2QDscompositethinfilmswerepreparedusingsol-gelmethod.TiO2solutionwasprepared

bydissolving1.2mltitaniumisopropoxideTi(OCH(CH3)2)4 inthesolutionwhichcontains11mlethanol,0.12mlacetic

acid(AcAc)and0.038mlwater.Firstly,titaniumisopropoxide,ethanolandwaterconcentrationswereheldfixed,andthen

acid:TiO2volumeratioswerechangedto0.4,0.2,0.1,0.05.

Secondly,TiO2solution(acid:TiO2volumeratios:0.05)andMWCNT(outerdiameter20–40nm,cheaptubes-commercial)

wereusedtopreparethefourdifferentsolutionsforMWCNT:TiO2ratio:0.005,0.00375,0.0025and0.00125.Thesolutions

weremixedusingamagneticstirrerfor3handthentheyweredepositedoncorning2947glasssubstratesbyaspin-coating

techniquewithaspeedof3000rpm.Eightdifferentsol-coatedglasssubstrateswereannealedat470◦C.Theprocesswas

repeatedat420,520,and570◦CheattreatmenttemperaturesforTiO2QDscontainedfilmwithvolumeratioofacid:TiO2=0.2.

2.2. Characterizationofcoatings

TiO2QDsandMWCNT-TiO2 QDscompositefilmsdepositedonglasssubstrateswerecharacterizedbyX-ray

diffrac-tometer(XRD,PhilipsPW-1800,Cu-K␣radiation).Ascanningelectronmicroscopy(SEM-S-3100H,HitachiLtd.)andatomic

forcemicroscope(AFM,ShimadzuscanningprobemicroscopeSPM-9500J3)wereusedtoinvestigatethesurface

morphol-ogyofthesefilms.UV–visspectroscopicanalysisoffilmswasperformedusingUV–visibleabsorptionspectrophotometer

(Perkin–ElmerLambda900withLabsphereintegratingsoftware).

3. Resultsanddiscussion

3.1. XRDanalysis

XRDwasemployedtostudythecrystalstructureofthepreparedfilms.TiO2QDsfilmswhichwereproducedwithvarious

acid:TiO2ratiosandMWCNT-TiO2QDscompositefilmsfordifferentMWCNT:TiO2ratioswereanalyzedinFig.1.

ThediffractionpatternsofthefilmsdemonstratethatbothTiO2 QDsandMWCNT-TiO2 QDsfilmsincludedbrookite

(orthorhombic)phaseof(211)(JCPDScardNo.75-1582)[17].

ThecrystalsizesofTiO2QDsfordifferentacid:TiO2ratiosandTiO2QDsinMWCNT-TiO2QDscompositefilmarecalculated

withScherrer’sformulaaccordingtotheXRDresults.TheyaredisplayedinTable1.TiO2QDscrystalsizesweredetermined

between2.99and5.52nmfordifferentacid:TiO2 ratiosandthesevalueswerefoundneartheTiO2 excitonBohrradius.

TheresultsindicatethatadecreaseintheratioleadstotheincreaseintheTiO2QDssize.Thatistosay,thesizesofTiO2

QDsdecreasedwiththeincreaseinacid:TiO2ratiosduetotheacceleratedreactionwiththeincreaseintheamountofacid.

Furthermore,theincreaseintheamountofTiO2causestheagglomeration,sothecrystalsizeofTiO2QDsinMWCNT-TiO2

QDscompositefilmsincreaseswiththisincrement.

TheactivationenergyofTiO2QDsfilmswascalculatedbyArrheniusequation.TheaverageQDssize(d)canbedetermined

usingEq.(1):aistheintercept,Ristheuniversalgasconstant,Tisthetemperature(Kelvin),andEistheactivationenergy.

lnd= −E/RT+lna (1)

Fig.2representsaplotoflogarithmoftheparticlesizeversusthetemperature(1000/T).Theactivationenergyforthe

TiO2QDsgrowthwasdeterminedusingtheslopeofthelineinFig.2andEq.(1)as20.3kJ/mol.Theactivationenergyof

therarebrookitephasehasbeenfoundlessthanthevaluesofotherphasesintheliteraturebecauseofthedifficultiesin

obtainingbrookitecrystalphase.Inthiscase,thetotalsurfaceenergyincreasesandthatleadstogrowthofQDsrequireless

energy.

TEMimagesofTiO2QDsareshowninFig.3.Theparticlessizewasapproximatelydeterminedas5nmforacid:TiO2ratio

of0.05.TEMimagesofthefilmsaredifficulttomeasurebecausethefilmsareverythin.

SEMandAFMimagesofTiO2QDsandMWCNT-TiO2QDscompositefilmsareshowninFig.4andFig.5.Fig.4indicate

thatTiO2QDswerelocatedontheMWCNTs.SurfaceroughnessvaluesofthefilmsweredeterminedtobeRms:5.07;5.65;

Fig.1.X-raydiffractionpatternsof(I)TiO2QDsfilmsfordifferentacid:TiO2ratios:(a)0.05,(b)0.1,(c)0.2,(d)0.4;(II)MWCNT-TiO2QDscompositesfilms

fordifferentMWCNT:TiO2ratios:(a)0.00125,(b)0.0025,(c)0.00375,(d)0.005,(III)TiO2QDsfilmsfordifferentheattreatmenttemperatures:(a)570,(b)

520,(c)470,(d)420◦_{Cforacid:TiO2volumeratios:0.05.}

Ü.Ö.A.Arıer,B.Ö.Uysal/Optik140(2017)1032–1037 1035

Fig.3.TEMimagesofTiO2QDswithdifferentacid:TiO2ratios:(a)0.1,(b)0.05.

Fig.4. SEMimagesofTiO2withdifferentacid:TiO2ratios:(a)0.1,(b)0.05,MWCNT-TiO2QDscompositefilmswithdifferentMWCNT:TiO2ratios:(c)0.005,

(d)0.0025.

to0.4,0.2,0.1,0.05,theroughnessvalueswereevaluatedtobeRms:1.9;2.2;3.5;4.51.Theroughnessincreasedwiththe

decreasingacid:TiO2andMWCNT:TiO2ratios.

AbsorptiongraphsofTiO2QDsandMWCNT-TiO2QDscompositefilmsarerepresentedinFig.6.WhenMWCNT’samount

washeldfixed,absorptionedgeshiftstoalongerwavelengthbyincreasingtheTiO2ratios.

ThesizesofTiO2QDscanbecalculatedusingtheabsorptionshiftandeffectivemassmodel(particleinaboxproblem)

Fig.5.AFMimagesofTiO2withdifferentacid:TiO2ratios:(a)0.1,(b)0.05,MWCNT-TiO2QDscompositefilmswithdifferentMWCNT:TiO2ratios:(c)0.005,

(d)0.0025.

Fig.6. UV–visspectraof(a)TiO2QDsfilmsfordifferentacid:TiO2ratios:0.4;0.2;0.1;0.05,(b)MWCNT-TiO2QDscompositefilmsfordifferentMWCNT:TiO2

ratios:0.005;0.00375;0.0025;0.00125.

(r)ofQDsusingEq.(2),wherehisPlanck’sconstant,_␮effectivemass,_␧isthedielectricconstant,Eg=Eg-Ebulkistheband

gapshift.EffectivemassforTiO2istakenas1.63mo,whichisexperimentallyestimatedbyKormannetal.[19,20].

Emin=Eg+h−−− 2 ␲2 2r2 − 1.8e2 4␲





IfradiusofQDsissmallerthanexcitonradius,thefirsttermisdominantasquantumconfinementintheequation,and

thelasttermisbecauseoftheCoulombinteractionbetweentheelectronandtheholeinEq.(1).

XrdanduvresultsofTiO2QDs’sizeswerecompared,anditwasfoundthatthesizesincreasedwithacid:TiO2ratiosin

Table1.TheUV–visabsorptionspectrarepresentthatabsorptionedgeshiftstoalongerwavelengthwiththeincreasing

Ü.Ö.A.Arıer,B.Ö.Uysal/Optik140(2017)1032–1037 1037

4. Conclusions

TiO2QDsandMWCNT-TiO2QDscompositefilmsweredepositedbysol-gelspincoatingtechnique.QDssizesofthefilms

dependontheprecursorratiossuchasacid:TiO2andMWCNT:TiO2ratios.TiO2QDsandMWCNT-TiO2QDscompositefilms

wereobtainedasbrookiteformandsingleoriented(211).TiO2QDsdispersedeasilyinMWCNT-TiO2QDscompositefilms.

ItisfoundthatsizeofTiO2QDscanbecontrolledbychangingtheacid:TiO2andMWCNT:TiO2ratios.Thesizesofparticles

weredeterminedwithdifferentwaysinTable1.Thefilmsexhibitabsorptioninthelongerwavelengthregionwiththe

incrementQDssize.Withtheoptimizationoftheprocessparameters,untoxicTiO2filmsincludingQDsinpreferredsizes

andfeaturescanbeusedinopticandelectronicindustry.

Acknowledgment

TheResearchFundofMimarSinanFineArtsUniversity(BAPProjectNo:201206)hassupportedthisresearch.

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