ChineseJournalofCatalysis35(2014)2014–2019 

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Article 

Multiwalledcarbonnanotube/TiO2nanocompositeasahighlyactivephotocatalystforphotodegradationofReactiveBlack5dye

SharifahBeeAbdHamid*,TongLingTan,ChinWeiLai,EmyMarlinaSamsudinNanotechnology&CatalysisResearchCentre(NANOCAT),InstituteofGraduateStudiesBuilding,UniversityofMalaya,50603KualaLumpur,Malaysia

A R T I C L E I N F O  

A B S T R A C T

Articlehistory:Received11June2014Accepted20August2014Published20December2014

  A nanocomposite UV‐visible light‐responsive multiwalled carbon nanotube (MWCNT)/titaniumdioxide(TiO2)nanophotocatalystwassuccessfullysynthesizedbyamodifiedsol‐gelmethodusingtitaniumisopropoxideand functionalizedMWCNTsas thestartingprecursors.ThephotocatalyticactivityoftheTiO2andthenanohybridmaterialwasinvestigatedthroughthephotodegradationofReactive Black 5 dye under ultraviolet light irradiation. X‐ray diffraction analysis indicated thatanatasephasewasobtainedforboththepureTiO2andtheMWCNT/TiO2composite,whileRamanspectroscopyconfirmedthepresenceofMWCNTsinthecomposite.Fieldemissionscanningelec‐tronmicroscopyrevealedthatTiO2nanoparticleswithanindividualdiameterofabout10–20nmwerecoatedonthesurfaceoftheMWCNTs.Thespecificsurfaceareasofthesampleswerefoundtobe80and181m2/g for thepureTiO2andMWCNT/TiO2, respectively.Asa result,MWCNT/TiO2showed better photocatalytic performance than pure TiO2 because the high surface area ofMWCNTsenabledthemtofunctionasgoodelectronacceptorsfortheretardationofelectron‐holepairrecombination.

©2014,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences.PublishedbyElsevierB.V.Allrightsreserved.

Keywords:MultiwalledcarbonnanotubeTitaniumdioxidePhotocatalystModifiedsol‐gelReactiveBlack5Dye

 

 

1. Introduction

The quality of global water sources is affected by variouskinds of pollutants produced by industrial, agricultural, anddomestic activities. Examples of industrial effluents are syn‐thetic dyes,which are normally toxic and carcinogenic in na‐ture[1,2].Amongthenewadvancedsemiconductingmaterialsreported for use in electronic, magnetic, optical, and photo‐catalytic application, titaniumdioxide (TiO2)remains a prom‐ising photocatalyst for the degradation of organic pollutantsowing to its remarkable characteristics such as high photo‐catalytic efficiency, non‐toxicity, low cost, eco‐friendliness,photostability,andchemicalinertness[3–6].

However,thephotocatalyticefficiencyofTiO2ishinderedbyitslowsolarlightabsorptionofabout4%[7];itisonlyexcitedunderUVirradiationofwavelengthsshorterthan384nm[4].

Subsequently, the wide band‐gap energy of TiO2 (3.2 eV foranatasephaseand3.0eVforrutilephase)mayinducethefastrecombination of electron‐hole pairs, which may extensivelyrestrict the photoconversion efficiency achievable over TiO2nanoparticles[5,6].

Generally, a good photocatalystmust at least have a largespecificsurfaceareawithahighphotocatalyticefficiencyinthevisiblelightregionforittoeffectivelyadsorborganicpollutantsand conduct photogenerated electrons [8,9]. Much effort hasbeenmadetoovercomethe limitationsofTiO2photocatalyststhroughretardationofelectron‐holepairrecombination,modi‐ficationofitsband‐gapenergy,creationofactivesitesforpho‐tocatalyticreaction,andincreasingofitssurfacearea[7–10].

Nanocomposites of TiO2 nanoparticles with carbonaceousmaterials have become of major interest to researchers be‐causeoftheiruniquepropertiesintermsofphysical,structural,

*Correspondingauthor.E‐mail:sharifahbee@um.edu.myDOI:10.1016/S1872‐2067(14)60210‐2|http://www.sciencedirect.com/science/journal/18722067|Chin.J.Catal.,Vol.35,No.12,December2014

SharifahBeeAbdHamidetal./ChineseJournalofCatalysis35(2014)2014–2019 2015

chemical, thermal, mechanical, electrical, and optical applica‐tion[11–13].Amongsuchcarbonaceousmaterials,multiwalledcarbon nanotubes (MWCNTs) show good intrinsic propertiesas an adsorbent and dispersing agent and may create moreactive sites for photon absorption [14,15]. In this manner,MWCNTsplayanessentialroleasagoodelectronacceptorforphotoexcitedTiO2byfacilitatingtheseparationofelectron‐holepairsandmodifying itsoptical bandgap,whichmayenhancethephotocatalyticefficiency[5,15].

Previously, MWCNTs coupled with TiO2 nanocompositeshave been prepared via several synthesis methods includingsol‐gel, hydrothermal, impregnation, chemical and physicalvapor deposition, and electrophoretic deposition [5,7,16].However, some of thesemethods are costly, time‐consuming,andrequiremultiplestepsorhighertemperatureandpressureduring the synthesis process [8,9]. Therefore, in the presentwork,asimple,cost‐effective,andhigh‐homogeneityapproachis employed to synthesizeMWCNTs/TiO2 nanocomposites bydepositingTiO2nanoparticlesonthelargesurfaceofMWCNTsusingamodifiedsol‐gelmethod.

2. Experimental

2.1. Materials

MWCNTs(>95%,10–20nmindiameter)weresuppliedbyBayerMaterial ScienceAG (Germany). Titanium isopropoxide(TTIP, 97%), concentrated nitric acid (65%), sodium do‐decylbenzesulfonate (SDS, 98%), and Reactive Black 5 (55%,molecular formula C26H21N5Na4O19S6) were purchased fromSigma‐Aldrich, USA. Absolute ethanol (>99.8%), glacial aceticacid, anddiluteammonia solution (30%)wereobtained fromFisherScientific,UK.Allchemicalswereusedasreceivedwith‐outfurtherpurification.Deionizedwaterwasusedthroughouttheexperiment.

2.2. FunctionalizationofMWCNTs

ThepureMWCNTswerefunctionalizedusingconcentratedHNO3underrefluxwithvigorousmagneticstirringat100°Cfor2 h. Under these conditions, the remaining iron particles areremoved and hydroxyl and carboxyl groups are introducedontothesurfaceoftheMWCNTs[17].About500mLofHNO3wassufficientfor10gofcarbonsample.Then,thesolutionwascooledtoroomtemperature,washedwithdistilledwater,andoven‐driedat100°C.

2.3. SynthesisofMWCNTs‐TiO2nanocomposites

Inatypicalsynthesis,MWCNTs/TiO2nanocompositeswerepreparedviaasimplemodifiedsol‐gelmethod.Anappropriateamount of functionalizedMWCNTswas added to an aqueoussolutionofSDSwithcontinualstirring for24h.ThepreparedMWCNTssolutionwasthendispersedinethanolandstirredfor30mintoachievehomogeneity(solutionA).AmixtureofTTIP,ethanol, andaceticacidwassubjected tovigorousstirring for30min to formaclear solution (solutionB).Then, solutionBwasaddeddropwiseintosolutionAundervigorousstirringat28°C for2h.Diluteammoniasolutionwas thenaddeddrop‐wiseintothemixturetohydrolyzetheTTIPandformauniformTiO2 coating on the surface of theMWCNTs.The samplewascentrifugedand rinsedwithethanol, oven‐driedat80 °C, andthencalcinedat450°C for2h.AschematicrepresentationofthepreparationoftheTiO2‐coatedMWCNTsisshowninFig.1.

2.4. Characterization

X‐raydiffraction(XRD)wasusedtostudythecrystalstruc‐tureanddegreeofcrystallinityofMWCNTs/TiO2.TheXRDwasperformed at 40 kV and 30mA at a scanning rate of 0.01°/sfrom10° to 80°with CuKα radiation (λ = 0.15406 nm) on aBruker AXSD8Advance diffractometer. Themorphology andparticlesizeweredeterminedbyfieldemissionscanningelec‐tronmicroscopy(FESEM)usingaQuantaFEG450operatedat5.00kVandhighvacuumwithamagnificationof100000.De‐terminationoftheBrunauer‐Emmett‐Teller(BET)specificsur‐facearea,SBET,ofthesampleswasperformedonaMicromerit‐ics ASAP 2010 sorptometer with N2 adsorption‐desorptionisothermmeasurementsat–196°C.Therequired1.0gofsam‐ple was degassed at 200 °C. Raman spectroscopy (RenishawinVia)with a 514.5nmAr+ laser as an excitation sourcewasusedtoconfirmthegraphiticstructureofthecarbonbondandanalyzedefects.

2.5. PhotocatalyticactivityofMWCNTs/TiO2nanocomposites

ThephotocatalyticdegradationofReactiveBlack5overtheobtainednanocompositeswasevaluatedunderultraviolet(UV)lightirradiationusingaUVlamp(96W).Typically,0.05goftheMWCNTs/TiO2photocatalystwasaddedto100mLofReactiveBlack 5 solution (15 mg/L). Initially, the mixture was ultra‐sonicatedinawaterbathfor10mintoensuregooddispersionofthephotocatalyst.Thesuspensionwasthenstirredindark‐

Oxidation

TiO2-coated MWCNTs

Conc. HNO3

Washed, dried and calcined at 450 oC

in air

Hydrolyzed TiO2 precursorPristine MWCNTs Functionalized MWCNTs

COOH

COOHCOOHCOOH

COOH

COOH

COOHCOOH COOH

Ti

OH

OH

O

O+

Fig.1.SchematicrepresentationofthepreparationofTiO2‐coatedMWCNTs.

2016 SharifahBeeAbdHamidetal./ChineseJournalofCatalysis35(2014)2014–2019

nessfor30mintoreachadsorptionequilibriumbeforetheUVlampwasswitchedontoinitializethephotodegradationoftheReactiveBlack5.Partofthereactionsolutionwascollectedatcertain time intervals and centrifuged to separate the photo‐catalyst, and the concentration of Reactive Black 5 in the su‐pernatantwasthendeterminedbyUV‐visspectrophotometry.

3. Resultsanddiscussion

3.1. Morphologyandelementalanalysisresults

The surface morphologies of the functionalized MWCNTs,pure TiO2 nanoparticles, and MWCNTs/TiO2 nanocompositeswereexaminedusingFESEM.FromFig.2(a),itcanbeobservedthat the functionalizedMWCNTswere intertwinedwith eachother. The individualMWCNTs exhibited a tubular‐like struc‐turewithanaveragediameterof20nm[18].Thesol‐gelsyn‐thesizedpureTiO2nanoparticleshadanalmostsphericalshapewithanindividualdiameterof10–20nm(Fig.2(b)).FromFig.2(c), the TiO2 nanoparticles were found to be homogenouslydeposited on the MWCNTs but showed some agglomerationalongtheMWCNTs[19,20].

HRTEM imagesof thepureTiO2 (Fig.3(a)) revealedanal‐most spherical shape with an average particle size of 10–20nm. The distance between the adjacent lattice fringes wasmeasuredandfoundtomatchthed‐spacingoftheTiO2latticeplaneintheXRDresults.TheTiO2nanoparticlesshowedalat‐ticespacingofd=0.35nm,correspondingtothe(101)planeofanataseTiO2,inboththepureTiO2andMWCNTs/TiO2compo‐sites[21,22].

Quantitative elemental analysis of the functionalizedMWCNTs, TiO2, and MWCNTs/TiO2nanocomposites was car‐ried out by FESEM‐EDX, and the percentage of each elementdetected ispresented inTable1.For functionalizedMWCNTs,

onlythepresenceofCandOelementsandnoothertraceim‐puritieswerefound.ThehighestmasspercentofTiandOwasobserved for the pure TiO2 nanoparticles. For theMWCNTs/TiO2nanocomposites,TiandOwerepresentas themajorelementswithaminoramountofCelement[11,20].

3.2. PhasestructureandBETsurfaceareaanalysisresults

The XRD patterns of MWCNTs, TiO2 nanoparticles, andMWCNTs/TiO2 nanocomposites are shown in Fig. 4. For theMWCNTssample,thediffractionpeaksobservedat2θ=25.9°and43.2°correspondwiththe(002)and(100)diffractionsofthe hexagonal graphite structure [23]. The sharp crystallinepeaksat2θ=25.2°,37.8°,47.9°,54.0°,and62.6°observedforthe nanoparticles were assigned to the (101), (004), (200),(105), (211), and (204) planes, respectively, of pure anatasephase tetragonal TiO2 (JCPDS 21‐1272) [24,25]. In theTiO2/MWCNTscomposites,thepeaksofanataseTiO2werestillobservableafterincorporationwiththeMWCNTs.Thegraphite

     Fig.2.FESEMimagesof(a)MWCNTs,(b)TiO2,and(c)MWCNTs/TiO2nanocomposites.

Fig.3.HRTEMimagesof(a)TiO2and(b)MWCNTs/TiO2.

Table1EDX elemental analysis of MWCNTs, TiO2, and MWCNT/TiO2 nano‐composites.

SampleMasspercent(wt%)

C O TiMWCNTs 94.4 5.6 —TiO2 — 36.7 63.3MWCNT/TiO2 12.1 38 49.9

10 20 30 40 50 60 70 80

Inte

nsity

(a.

u.)

2/( o )

(1)

(2)

(3)

(101)

(002)(100)

(004) (200)

(105)(211)

(204)

Fig.4.XRDpatternsof(1)MWCNTs,(2)TiO2,and(3)MWCNTs/TiO2.

SharifahBeeAbdHamidetal./ChineseJournalofCatalysis35(2014)2014–2019 2017

peak at 2θ = 25.9° could not be seen in the pattern of theMWCNTs/TiO2 nanocomposites owing to overlapping by thestronganataseTiO2peakat2θ=25.2°.Thehigherintensityofthe TiO2 peaks caused by the higher crystallinity of the TiO2nanoparticlescomparedwiththatoftheMWCNTsshieldedthepeaks of the MWCNTs [11,14]. However, the presence ofMWCNTs in the compositeswas further confirmedbyRamananalysisasdiscussedlater.

Table2presents thecrystallite size,BETsurfacearea,andpore volume of the MWCNTs, TiO2 nanoparticles, andMWCNTs/TiO2nanocomposites.Acorrelationbetweencrystal‐lizesizeandsurfaceareawasobserved;thesmallerthecrystal‐lizesize, the largerthesurfacearea.Thecrystallitesizeofthesampleswascalculatedbythewell‐knownScherrer’sequation:D =kλ/(βcosθ),whereD is the crystallite particle size,k is aconstantof0.9,λisthewavelength(nm)oftheX‐raysused,βistheFWHM,andθistheBraggangle[26].

The nitrogen sorptionmeasurements revealed the surfacearea and porosity of the MWCNTs, TiO2 nanoparticles, andMWCNTs/TiO2nanocomposites. As shown in Table 2, the in‐corporation of TiO2 nanoparticles on the surface of theMWCNTscausedanincreaseintheSBETandtheporevolumescomparedwiththoseofthepureTiO2nanoparticles,whichmaylead to an increase in pollutant adsorption capability. Thus,MWCNTsmayactasbothasupportandstrongadsorbent forenhancingthephotocatalyticperformanceofTiO2materialsbyenhancingtheirsurfaceproperties[11,13].

3.3. Ramanspectroscopicanalysisresults

TheRamanspectraofthefunctionalizedMWCNTs,TiO2,andMWCNTs/TiO2 nanocomposites are shown in Fig. 5. For the

functionalized MWCNTs, two typical peaks were observed ataround1314.95and1593.54cm−1.Thebandat1314.95cm−1istheD‐band,whichisattributedtosp3defectsincarbon[27,28],while the band at 1593.54 cm−1 is the G‐band, which corre‐spondstosp2carbondefects.TheRamanspectrumoftheTiO2nanoparticlesshowsfourmainpeaks(141.85,396.53,517.12,and 641.86 cm−1) assignable to the Eg(1), B1g(1), A1g and Eg(2)modes, respectively, of anatase. Although the XRDanalysis oftheMWCNTs/TiO2nanocompositesonlyreflectedthepresenceofpureanataseTiO2,theRamanresultsconfirmedthepresenceof MWCNTs in the composite. While both themain peaks ofanatase and MWCNTs appeared in the Raman spectrum, thepeakswere slightly broadened because of interfacial integra‐tionbetweentheTiO2andMWCNTs[13,29].

3.4. Opticalproperties

The optical properties of the TiO2 and MWCNTs/TiO2nanocomposites were determined using UV‐vis spectroscopy.As shown inFig.6(a), theabsorptionedgesofMWCNTs/TiO2wereredshiftedtowardsahigherwavelength,whichsuggest‐edanarrowingofthebandgapcausedbytheintroductionofMWCNTs. In addition, a typical absorption with an intensetransitionintheUVregionwasobservedinthespectraofbothsamples,assignabletotheintrinsicband‐gapabsorptionofTiO2

Table2Phase identification, crystallite size, specific surface area, and porevolumeofMWCNTs,TiO2nanoparticles,andMWCNTs/TiO2nanocom‐posites.

Sample PhaseCrystallitesize(nm)in(101)plane

SBET(m2/g)

Porevolume(cm3/g)

MWCNTs — — 231 0.1715TiO2 Anatase 12.9 80 0.0708

MWCNTs/TiO2 Anatase 8.9 181 0.1421

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Inte

nsi

ty (

a.u

.)

Raman shift (cm-1)

D band

G band

(a)

(b)

(c)

 Fig.5.Ramanspectraof(a)MWCNTs,(b)TiO2,and(c)MWCNTs/TiO2nanocomposites.

2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0200 300 400 500 600 700 800

3.02 eV

[F(R

)h]

0.5 (

a.u.

)

h (eV)

3.23 eV

(1)

(2)

(b)

(2)

Inte

nsity

(a.

u.)

Wavelength (nm)

(1)

(a)

 Fig.6.(a)UV‐Visdiffusereflectancespectraand(b)variationof(αhν)2versusphotonenergy(hν)of(1)MWCNTs/TiO2and(2)TiO2.

2018 SharifahBeeAbdHamidetal./ChineseJournalofCatalysis35(2014)2014–2019

forelectrontransitionfromthevalencebandintotheconduc‐tionband(O2p→Ti3d)[21,30].

TheopticalbandgapoftheTiO2andMWCNTs/TiO2nano‐composites was determined using the Kubelka‐Munk expres‐sionviathetransformationoftheirdiffusereflectancespectra(Fig. 6(b)) according to the equation [F(R)hν]1/2 =A(hν −Eg),where F(R), h, Eg, ν, and A are the Kubelka‐Munk function,Planck’s constant, the band gap, and a constant, respectively.TheestimatedbandgapsofTiO2andMWCNTs/TiO2were3.23and 3.02 eV, respectively, which are in accordance with thequalitativeobservationofablueshiftintheabsorptionedgeofthe TiO2 nanoparticles compared with that of theMWCNTs/TiO2nanocomposite[23,31].

3.5. Thermalbehavior

Thethermalstabilityandoxidativemass lossconditionsofMWCNTs and MWCNTs/TiO2 were determined via thermo‐gravimetricanalysis(TGA)(Fig.7).Thecombustionpointofthefunctionalized MWCNTs was found to be located at approxi‐mately 654 °C owing to the highest rate ofmass loss, 86.8%(Fig.7(a)).FortheMWCNTs/TiO2composites,theinitialmasslosswascausedbytheevaporationofwaterataround100°Cand was followed by the thermal decomposition of theMWCNTsat637°C.Thetotalmasslosswasdeterminedtobe25.8%, indicating that TiO2 became themajor component re‐mainingafterthecombustionoftheMWCNTs[7,11].

3.6. Photocatalyticactivity

TheschematicmechanismofthephotocatalyticdegradationofReactiveBlack5over theMWCNTs/TiO2nanocomposite is

shown in Fig. 8. When the TiO2 photocatalyst is illuminatedwithphotonshavingenergygreaterthantheband‐gapenergy,thephotonswillexciteelectronsfromthevalencebandoftheTiO2 into the conductionband.Theholes in thevalencebandwillmeetwithOH−andproducehydroxylradicals.Therefore,thetargetedpollutantsadsorbedonthesurfaceofthecatalystwill be oxidized by •OH. However, the excited electron‐holepairsmay instead recombine [6].TheMWCNTsact asa goodelectron acceptor and thereby facilitate the separation of theelectron‐hole pairs to prevent their recombination [15]. Themore electrons that are transferred from the TiO2 to theMWCNTs, the better the retardation of electron‐hole pair re‐combination[32,33].

ThephotocatalyticactivityofpureTiO2andMWCNTs/TiO2wasstudiedusingthedegradationofReactiveBlack5.InFig.9,C is the concentrationofReactiveBlack5afterdifferent lightirradiationtime,andC0 istheinitialconcentrationofReactiveBlack5before light irradiation.ReactiveBlack5,whichhasamajorabsorptionpeakat595nm,isoneofthemostcommonorganicpollutantsinindustrialwastewater.ThefastestrateofReactiveBlack5degradationwasobtainedduringthefirst15min of irradiation, and the rate gradually decreased as timeincreased. The hybridMWCNTs/TiO2exhibited better perfor‐mance for the photodegradationofReactiveBlack5 than thepureTiO2.ThismaybecausedbythehighsurfaceareaoftheMWCNTs,whichconsistofmultiplelayersofgraphitethatcan

e-

TiO2

O2

O2.-

MWCNTs

H2O

HO.

hv e-

CB

VB

 Fig.8.PhotocatalyticmechanismofReactiveBlack5degradationovertheMWCNTs/TiO2nanocomposite.

0 10 20 30 40 50 600.0

0.2

0.4

0.6

0.8

1.0

C/C

0

Time (min)

(1)

(2)

Fig.9.PhotocatalyticdegradationofReactiveBlack5over(1)TiO2and(2)MWCNTs/TiO2nanocompositeunderUVlight.

0

20

40

60

80

100

Tp = 637 oC

Mas

s (%

)

(a)

Tp = 654 oC

86.8%

Residue 12.3%

0 200 400 600 800 100070

75

80

85

90

95

100

Mas

s (%

)

Temperature (oC)

(b)

25.8%

Residue 72.0%

Fig.7.TGAcurvesof(a)MWCNTsand(b)MWCNTs/TiO2nanocompo‐site.

SharifahBeeAbdHamidetal./ChineseJournalofCatalysis35(2014)2014–2019 2019

facilitate the separationof electron‐holepairsatTiO2‐CNT in‐terfaces[26,6].

4. Conclusions

A simple sol‐gel technique was used to synthesizeMWCNTs/TiO2 nanocomposites. A variety of techniques(FESEM, HRTEM, XRD, BET, Raman, TG‐MS) were used tocharacterize the obtained materials, and the TiO2/MWCNTsnanocatalystwasfoundtoexhibitgoodphysical,chemical,andoptical properties that enhanced their activity for the photo‐degradationofReactiveBlack5underUVlight.

Acknowledgments

The authors would like to thank University of Malaya forsponsoringthisworkunderUniversityMalayaResearchGrant(UMRGRP022‐2012A)andBrightSparksProgram.

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GraphicalAbstract

Chin.J.Catal.,2014,35:2014–2019 doi:10.1016/S1872‐2067(14)60210‐2

Multiwalledcarbonnanotube/TiO2nanocompositeasahighlyactivephotocatalystforphotodegradationofReactiveBlack5dye

SharifahBeeAbdHamid*,TongLingTan,ChinWeiLai,EmyMarlinaSamsudinUniversityofMalaya,Malaysia

Thispaperdecribesthefeasibilitystudyofamodifiedsol‐gelmethodforfabricationofTiO2nanoparticlesonthesurfaceofMWCNTs.Thehybridmaterial showedahigherphotocatalytic activity thanpureTiO2 for thephotodegradationofReactiveBlack5dye in aqueoussolutionunderUVlightirradiation.