Electrophoretic deposition of multi-walled carbon nanotubes on
porous anodic aluminum oxide using ionic liquid as a dispersing agent
F. Hekmat
a, B. Sohrabi
a,∗, M.S. Rahmanifar
b, A. Jalali
aaDepartmentofChemistry,SurfaceChemistryResearchLaboratory,IranUniversityofScienceandTechnology,P.O.Box16846-13114,Tehran,Iran
bFacultyofBasicScience,ShahedUniversity,Tehran,Iran
a r t i c l e i n f o
Articlehistory:
Received16August2014 Receivedinrevisedform 29December2014 Accepted20February2015 Availableonline28February2015
Keywords:
Vertically-alignedcarbonnanotubes Electrophoreticdeposition Anodicaluminumoxide Ionicliquid
Electrochemicaldoublelayercapacitor
a b s t r a c t
Multi-wallcarbonnanotubes(MW-CNTs)havebeenarrangedinnanochannelsofanodicaluminumoxide template(AAO)byelectrophoreticdeposition(EPD)tomakeavertically-alignedcarbonnanotube(VA- CNT)basedelectrode.WellorderedAAOtemplateswerepreparedbyatwo-stepanodizingprocessby applyingaconstantvoltageof45Vinoxalicacidsolution.ThestabilizedCNTsinawater-solubleroom temperatureionicliquid(1-methyl-3-octadecylimidazoliumbromide),weredepositedintheporesof AAOtemplateswhichwereconductivebydepositionofNinanoparticlesinthebottomofpores.In ordertoobtainidealresults,differentEPDparameters,suchasconcentrationofMWCNTsandionic liquidonstabilityofMWCNTsuspensions,depositiontimeandvoltagewhichareappliedinEPDprocess andalsooptimalconditionsforanodizingoftemplatewereinvestigated.Thecapacitiveperformanceof preparedelectrodeswasanalyzedbymeasuringthespecificcapacitancefromcyclicvoltammogramsand thecharge–dischargecurves.Amaximumvalueof50Fg−1atthescanrateof20mVs−1wasachievedfor thespecificcapacitance.
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1. Introduction
EversincetheirdiscoverybyIijimain1991[1],carbonnano- tubes(CNTs)areanovelgenerationofmaterialswhichwidelyused tomakeavarietyofadvancednanostructures,includingelectron fieldemissiondevices[2,3],storageofhydrogen[4,5],batteries[6], supercapacitors[7,8],electrochemicalsensors[9],andelectrome- chanicalactuators[10],becauseoftheiruniqueelectrical,optical, andmechanicalproperties,suchassuperiormechanicalstrength, metal-likeelectricalconductivity,andextremeaspectratios[11].
SupercapacitorsorElectrochemicaldoublelayercapacitors(EDLCs) areelectrochemicalenergystoragedevicesthatcanprovidealarge amountofelectricalenergyinashortperiodoftime[12].Because theEDLCsstorechargesintheelectricdoublelayerformedatthe electrode/electrolyteinterface,maximizingtheamountofsurface areaofelectrodesplayanimportantroleinsupercapacitors[13].
Recentresearchesdemonstratedtheimprovedratecapabilityfor alignedCNTsoverrandomlyentangledCNTs,somanyresearchers havetriedtodevelopmethodstocontrolandalignCNTsindesire direction[14–16].
∗Correspondingauthor.Tel.:+9877240540x6275.
E-mailaddress:sohrabib@iust.ac.ir(B.Sohrabi).
Twoclassesofalignmenttechniquescanbedistinguished:one attemptgrowingthetubesalongagivendirection,eitherwithin thesubstrateornormal bundles [14].Amongthemanykindof CNTsynthesismethods,chemicalvapordeposition(CVD),espe- ciallyplasmaenhancedchemicalvapordepositionwhichiscalled PECVD,iseffectiveforVA-CNTvolumeproduction[17].Inanother methodwhich is knownas post-growthmethod,a purification and aneffectivedispersionofCNTsin asuitable solventbyaid of adisparent (surfactants orpolymers) isrequired toobtaina homogenouswelldispersedCNTsuspension,whichisplayakey roleincontrollingmanipulationofCNTs[14,18].Severalinteresting approacheshavebeendevelopedtoalignCNTsaftertheirsynthe- sized.Jin etal.reportedthealignment ofCNTsbystretchingof polymer/CNTfilm[19].Bendiab andco-workersusedauniaxial pressure(∼10kbars)toalignCNTs[20].Bythedoctorbladetech- nique,theembeddedCNTsinpolymerfilm,warmedtobesoftand thenrubbedwithabladeinadesiredirectiontoinducetheelastic forcewhichcanalignCNTs[14].Bulkacousticwaves(BAWs)are usedtoalignMWCNTsinpolymercompositematerialsbyHaslam andRaeymaekers[21].Itisnoticeablethatinallofthesemethods, CNTsarehighlysurroundedbyapolymernetworkandthusCNTs willnotbeabletoexpressalloftheiruniqueproperties.During thefiltrationmethod,dispersedCNTscrossingthroughamicro- porefiltertopreparealignedCNTs[22].Alongfiberdrawingor spinningmethod,CNTsforcedtobealignedwithinthefibersdue http://dx.doi.org/10.1016/j.apsusc.2015.02.142
0169-4332/©2015ElsevierB.V.Allrightsreserved.
tothecombinationofshearforcesandtheliquidcrystallinephase [21,23].Lungmuir–Blodgett(LB)techniqueisgentlydonebysink- ingatemplateintoaCNTssuspensionandpullingitoutslowlyto produceathinandhomogenouslayerofCNTsontemplate[14].
Successofthesemethodstightlydependsonprecisioninspeed, directionandintensityofappliedmechanicalforce.Amongmany methodshave beenreportedforcontrollingalignment ofCNTs, applyinganexternal magneticor/andelectricfieldwerewidely usedforvariousapplications.Unlikeothermethods,thesetwolast mentionedmethodscausedminimumstructuraldamagewithout considerablechangesincharacteristicpropertiesofCNTs.Byusing apowerfulmagneticfield(>7T)awelldispersedCNTsuspensionis developonsubstrate.Despitetheefficiencyofthismethod,itsdis- advantageiscostsandpreparationofastrongmagneticfield[14].
ThelastmethodwhichiscalledEPD,allowscontrollingofthickness andshapeofdepositedfilmbyusinganelectricfield[17].However, thereareavarietyofpost-growthmethodstoalignCNTs,butthe successofEPDistothefactthatitisfast,costeffectivetechnique usuallyrequiringsimpleequipment[24].
BecauseofthestrongvanderWaalsinteractionbetweentubes, CNTshave extremely poorsolubilityinmostof solvents and it makestheircharacterizationandprocessingdifficult,whichisthe majorlimitationtotheirpractical applications[25].Specifically, CNTsexistasheavilyentangledaggregatesofbundles[26,27]con- taininghundredsofcloselypackedCNTs[26].
Recentlyroomtemperatureionicliquids(RT-ILs),withunique physicochemicalpropertiessuchaswideliquidtemperaturerange, highthermalstability,negligiblevaporpressure,non-flammability, increasedelectrochemicalwindow,andrelativelyhighioniccon- ductivity have beenwidely appliedas excellent green reaction media[28,29].ModificationofCNTswithILsisexpectedtoimprove theircompatibilityandstability[30].NowadaysdispersingCNTs withILsseemstobecomeaninterestingtopicandattractedmuch attention,sosomestudieshavebeendoneinthisfield.Imidazolium basedILs,suchas1-butyl-3-methylimidazoliumtetrafluoroborate triggeredsinglewalledcarbonnanotubes(SWCNTs)toformgel called“Buckygel”[29].Kocharovaetal.demonstratedthatCNTs couldbestablydispersed inwater withaidofsmallamountof 1-dodecyl-3-methylimidazolium bromide [31]. Crescenzo and co-workersindicatedthat1-hexadecyl-3-vinyl-imidazoliumbro- mideisagoodagenttodispersingSWCNTs[31].Liuetal.found thationic-liquid-typeGeminiimidazoliumsurfactant([Cn-C4-Cn
im] Br2, n=12, 14) can effectively disperse MWCNTs even at verylowconcentration[32].Gaoandco-workersdiscussedthat electrostatic repulsion, hydrophobic interactions, cation–, and n–interactionsplayedimportantrolesindispersionofMWCNTs byPoly(1-glycidyl-3-methylimidazoliumchloride)(PGMIC)[33].
Lu et al. reported that MWCNTs can be effectively dispersed in aqueous solution by alkyl-triphenyl phosphonium bromide (CnTPB,n=12and14)[25].Despitemanyresearcheswerecarried out,therearestillsignificantchallengestoobtainwell-separated CNTswithaidofroomtemperatureionicliquidstoachievedense arrayof vertically aligned carbon nanotubes (VA-CNTs)in post growthaligningmethods.Inthiswork,weusedasmallamountof 1-methyl-3-octadecylimidazoliumbromidetodisperseMWCNTs inaqueoussolution.Asalreadymentioned,thehomogeneousdis- tributionofCNTswithinasuspensionplaysakeyroleininflicting manyimportantpropertiesduringthepost-growthmethodsfor aligningCNTs.Inrecentyears,varioustechniquesweresuggested toobtainaquantitativeassessmentforexfoliatedCNTsconcen- trationinsuspensions.Incomparewithcommonmethodssuch asRamanspectroscopyandatomicforcemicroscopy(AFM)which possessagoodabilityondeterminationofdispersionqualityand exfoliationstate,butthesecannotgiveanyquantitativeinforma- tiononexfoliatedCNTconcentration[34].TheUV–visapproachis asimpleandeffectivemethodforcalculatingtheconcentrationof
surfactant-dispersedCNTsinsolution[34–37].Inthisstudy,we followedamethodproposedbyLiuetal.[35]forquantifyingdis- persionofCNTswhichisbasedonasimpleultracentrifugeprocess.
AfterdispersingCNTsbythislongchainIL,welldispersedCNTs arealignedondesiredtemplatebyapplyingelectricfield.EPDis achievedviathemotionofchargedCNTsdispersedinIL,toward aconductingelectrode underanappliedelectricfield. Thefinal goalisproductionofafilmwheremostCNTsarealignedtocertain direction[14].
ElectrophoreticmotionofchargeddispersedCNTsduringEPD resultsintheaccumulationofCNTs,whichareencapsulatewithin anILmicelleandformationofhomogenousandrigiddepositatthe relevantelectrode[17,24].Inthisstudyweusedaperiodicporous anodicaluminumoxide(AAO)electrode,whichisbeingconduc- tivesubstratebyelectrochemicaldepositionofNinanoparticlesas catalystinthebottomofthenanoholesofAAOtemplatetoalign individualdispersedCNTs.ThestructureofAAOtemplatecanbe describedasclose-packedhexagonalarraysofparallelcylindrical nanoporousperpendiculartothesurfaceonthetopoftheunder- lyingAlsubstrate[38].
WereporthereaVA-CNTbasedelectrodepreparedbytheEPD technique,whichhastheadvantagesoffastandsimpleformation.
Theelectrodesbuiltfromsuchmethodshaveexhibitedacceptable electrochemicalresults,whichindicatethatthispost-growthtech- niqueisaneffectivemethodinfabricatingCNTelectrodesforEDLCs andothersimilardevices.
2. Experimental 2.1. Chemicals
The ionic liquid was 1-methyl-3-octadecylimidazolium bro- mide([C18C1im]Br)(>98%),whichhasbeenpurchasedfromKimia Exir(Iran)andusedasreceived.TheMWCNTs(length0.1–10m, outer mean diameter10–15nm, mean number of walls 5–15), weresuppliedbyArkemaCo.Ltd.(France).High-purity(99.9995%) 0.3mm thickness, 30mm width aluminum foil was purchased fromMerckKGaA(Germany).Othersolventsandreagents,except chromicanhydrate,obtainedfromMerck(Germany)andtheyused asreceived.ChromicanhydratewaspurchasedfromKANTOChem- icalCo.,Inc.(Japan).Finally,doublyionizedwaterwasobtained fromanOESwaterpurificationsystem(USA).
2.2. Equipments
Carbonnanotubesweredispersedstablyinwaterinpresence ofsmallamountoflongchainILbyusinga UP400Sultrasound processor fromHielscherUltrasoundGmbH (Teltow,Germany).
DispersedMWCNTswerecharacterizedbyUV–visonacomputer- controlledspectrophotometer (UVmini1240,Shimadzu,Japan).
Thewavelength range from200to 800nm wasusedto exam- inetheabsorbanceofMWCNTsuspensions.Beforemeasurements, each samplewasdilutedby di-ionized watertotheconcentra- tionofone-fifteenthofitsinitialconcentration,andsimultaneously thecorrespondingILaqueoussolutionwithoutCNTswasusedas thebulk.TheinteractionsbetweenMWCNTsandILswerestud- iedbyInfraredspectrometer(8400S,Shimadzu,Japan).Thezeta potentialmeasurementsweredonebyusingaMalvernZetaSizer 3000HAS(Malverninstruments,UK),Becauseof themostnec- essaryprerequisiteforsuccessfulEPDisdependsonpreparation ofastabledispersionofCNTs,whichhavea highzetapotential [39].
DCpowersuppliers(MP6003,Megatek, Germany)wereused in order toapply electric field in preparation of AAO template andalsotodepositionofdispersedCNTsduringtheEPDprocess.
ThemorphologyoftheresultantAAOtemplatesbeforeandafter
Fig.1.SchematicproceduresforproductionofILstabilized-MWCNTsuspension:(a)randomlyentangledCNTs,(b)1-methyl-3-octadecylimidazoliumbromide,and(c)well dispersedMWCNTsinaqueoussolutionby[C18C1im]Br.
depositionofCNTshasbeeninvestigatedbyusingofascanning electron microscopy(TESCAN, VEGA,Czech Republic) and their EDXanalysiswereobtainedwithascanningelectronmicroscope (FE-SEM/EDX) (TESCAN,MiraIILMU,CzechRepublic). Different electrochemicaltechniqueswereusedtocharacterizethecapac- itiveperformanceofaspreparedelectrodes.Cyclicvoltammetry (CV)andcharge–dischargegalvanostaticexperimentswerecon- ductedElectroanalyzersystemSAMA500(Iran)and Kimiastat (Iran),respectively.Alloftheelectrochemicaltestswerecarriedout in1.0MNa2SO4aqueoussolutionatroomtemperatureinathree electrodesconfigurationusingasaturatedAg/AgClasthereference electrodeandagraphitecounterelectrode.
2.3. PreparationofCNTsuspensionforEPD
Thesuspensions ofMWCNTswere preparedbyadding 4mg MWCNTsinto10mlof1-methyl-3-octadecylimidazoliumbromide aqueoussolutions,andthenthesolutionsweresonicatedfor10min byprobesonicatorwithapowerdensityof80Wcm−2.Inorder tosedimentationoflargetubebundles,thesuspensionofMWC- NTswasstoredatroomtemperaturefor24h,Afteroneday,the resultingsuspensionwerecentrifugedfor10minwith2000rpm, inordertoseparatetheprecipitationfromthebulksolutions.Fig.1 schematicallyillustratesthedispersingprocessofCNTsbyusing smallamountof[C18C1im]Brasdisparent.Toevaluatethequan- titative characterization of CNTs dispersions, thesuspension of MWCNTsin[C18C1im]Braqueoussolution(C0)waspreparedwith themethodwhichwasmentionedabove.SincetheBeer-Lambert lawisnotobeyedinthestrongabsorptionwhichisattributedtothe highanalyteconcentration[34],aseriesofdilutedsampleswith concentrationsofC0/2,C0/4, C0/8, C0/16,C0/32,andC0/64were prepared.Preparedsampleswerecentrifugedforvariedperiodof time(1,2,5,10,30,60min)with14,000rpm[35],thenaUV–vis basedmethodwereusedforthesemiquantitativecharacterization ofpreparedCNTsuspensions.
2.4. PreparationofAAOtemplate
TheAAOtemplateisfabricatedbyusingatwo-stepanodization processasshowninFig.2.First,ahigh-purityaluminumsheetwith dimensionsof30mm×30mm×0.3mmwasdegreasedinacetone ultrasoundandrinsedinanethanolsolution,followedbyannealing at500◦Cfor5h[40].Beforeanodizing,theannealedsamplewas electropolishedina1:4volumemixtureofperchloricacid(60wt%) andethanol(96wt%)at3◦C,andconstantDCvoltageof20Vfor 1minwereappliedtoobtainmirrorfinish.Thetwo-stepanodiza- tionwaschosentoprepareanorderedporousAAOtemplate.The firstanodizingprocesswascarriedoutunderaconstantvoltageof
45Vdcina0.3Moxalicacidsolutionat5◦Cfor20h.TheAAOfilm waschemicallyetchedinamixtureofchromicacid(1.8wt%)and phosphoricacid(6wt%)at75◦Cfor3h,thenthesecondanodizing processwasdonefor1h,underthesameconditionsasthefirst anodizationprocessmentionedabove,whichresultedinforma- tionofahighlyorderedporousAAOtemplatewithaporedepthof about2m[41,42].Inordertofacilitatetheuniformelectrodepo- sitionofNinanoparticlesascatalyst,thevoltagewasdroppedfrom 45to14Vattherateof0.5Vmin−1,whichwasdoneimmediately attheendofthesecondanodization.Becausethethicknessofthe barrierlayerisproportionaltoappliedvoltage,thebarrierlayer wasalmostcompletelyremovedwhenthevoltagereached14V [43,44].
Aftereliminationofthebarrierlayer,theremainingAlsubstrate wasremovedinasaturatedcoppersulphate(CuSO4)aqueoussolu- tioninchloridricacid(HCl)atroomtemperature[45].Finally,the resultingAAOtemplatewasimmersedina1.0Maqueousphos- phoricacidsolutionatroomtemperaturefor40mintoeliminate thebarrierlayeronthebottom side,andsimultaneouslywiden thepores[40].Onesideof AAOmembrane wassputtered with a layerof Auasa workelectrode. The Nicatalyst waselectro- chemicallydepositedattheporebottomoftheAAOtemplatein athree-electrodeelectrochemicalsystemfromanaqueoussolu- tionof0.32MNiSO4·6H2O,0.08MNiCl2·7H2O,and0.28MH3BO3, whichiscalledWattbath[45].Inordertohavethebetterresults inelectrodepositionofNicatalystsinthebottomofpores,thepH valueoftheelectrolytewasadjustedtoabout2.0with0.1MH2SO4 solution[45].TheelectrochemicaldepositionofNicatalystwasper- formedbyusingAAOtemplate,whichsputteredwithalayerofAu asworkingelectrode,platinumascounterelectrode,andAg/AgClas referenceelectrode,andallofthepotentialsrefertothereference electrode.
2.5. ElectrophoreticdepositionofCNTs
AsitshowninFig.3theEPDwasdoneinatwo-electrodesystem, whereresultingconductiveAAOtemplatebydepositionofNicata- lyst,withexposedareaof1cm×1cmwereusedascathode,andNi paperelectrodewithanexposedareaof2cm×2cmwereusedas anode.Thesetwoelectrodeswereusedinanelectrophoreticdepo- sitioncellwithanelectrodegapof7mm.Carewastakenthatthese twoelectrodeswereparalleltoeachother.TheDCelectrophoretic depositionofVA-CNTswasconductedat100Vfor1min.Aftercom- pletingthedeposition,theAAOmembranewasdriedunderroom temperatureindesiccatorsfor24h.Thenpreparedelectrodeplaced atroomtemperatureinNaOHetchingsolutiontoremovingtheAAO templatepartiallyandtheresultingtemplateimmersedindilute waterimmediately.
Fig.2.SchemeofAAOprocess:(a)highpurityaluminumsheet,(b)electropolishedaluminumsheet,(c)Firstanodizedaluminumsheet,(d)chemicaletchedaluminalayer, (e)preparedAAOafteratwo-stepanodizationprocess,(f)AAOtemplateafterremovingexcessaluminumlayer,(g)AAOtemplatewithdepositedAulayer,and(h)resulting conductiveAAOtemplatebyelectrochemicaldepositionofNiparticles.
3. Resultsanddiscussion
3.1. DispersionofMWCNTsby1-methyl-3-octadecylimidazolium bromideIonicLiquid
Fig.4ashowstheMWCNTsinaqueoussolutionbeforeadding 1-methyl-3-octadecylimidazolium bromide ([C18C1im] Br). We noticed that even after 1hr of ultrasound, MWCNTs hardly agglomerate in aqueous solution. In contrast, after adding 1- methyl-3-octadecylimidazoliumbromideintothesameaqueous solution, a black homogenous dispersion was obtained, which couldkeepstableformorethan10months.Becauseaddingsmall amountof1-methyl-3-octadecylimidazoliumbromideintoaque- oussolution(only0.4mMinFig.4b)todispersetheMWCNTs,we canclaimthat1-methyl-3-octadecyleimidazoliumbromideisan excellentdispersantfordispersingCNTs.
In this work, we studied the effect of 1-methyl-3- octadecylimidazolium bromide concentration on dispersion
ofMWCNTsin aqueoussolution.UV–visisusuallyusedtoesti- mate thestability of dispersedCNTs, It hasbeen reportedthat there is almost no absorption bandin UV–vis regionfor bun- dled CNTs, however individual CNTs are active in this region and strongabsorptioncan beobserved[32].Fig.5ashows that 1-methyl-3-octadecylimidazolium bromide aqueous solutions displayonlynegligibleabsorptionsinthewavelengthrangefrom 200to800nm,sotheeffectofILsontheabsorptionsofMWCNT suspensionsinUV–visregioncanbeignored[32].Fig.5bshows thattheMWCNTsuspensionshavestrongUV–visadsorption,and themaximumabsorbanceappearedaround260nm,whichisin agreementwithpreviousreports[32].
On theotherhand,becauseof theUV–visregionis scarcely activatedbybundledCNTs,whileitcanbeactivatedbyindividual CNTs,sowithincreasingofindividualCNTsconcentrationinaque- oussuspension,itcanbeexpectedthattheabsorbanceofCNTsat thesamewavelengthbeingincreased[33].Accordingtotheseevi- dences,wecanjudgeabouttheamountofindividuallydispersed
Fig.3.SchematicdiagramofanEPDcellforelectrophoreticdepositionofwelldispersedCNTsonthecathode(aspreparedAAOtemplate).
CNTsbasedontheintensityofUV–visabsorptionpeaks,andwe candeterminetheabilityofdifferentconcentrationsof1-methyl- 3-octadecylimidazoliumbromidetodisperseMWCNTsbyUV–vis spectra.
Fig. 6a shows the absorbance of different concentrations of MWCNTsuspensions,andinFig.6bshownthatthereisnolinear relationshipbetweentheabsorbanceintensityofMWCNTsandIL concentration.
Asitcanbeexpected,thebestconcentrationof1-methyl-3- octadecylimidazoliumbromidetodisperseCNTsisaroundcritical micelle concentration (CMC), so we investigated dispersion of CNTsbyusing[C18C1im]BraroundCMCofionicliquid,andthe curvesinFig.6,suggestthatthebestconcentrationof1-methyl-3- octadecylimidazoliumbromidetodisperse4mgofMWCNTswas 0.4mM.
Fig.4.AqueousMWCNTdispersions:(a)withoutIL,imagedafter1hr(b)with 0.4mMof1-methyl-3-octadecylimidazoliumbromide,imagedafter10months.
Semi-quantitativecharacterizationofCNTdispersionwasdone byusingasimpleultracentrifugemethodforevaluatingthesta- bilityofCNTsuspensions[35].Fig.7ashowstheUV–visspectra of diluted 1-methyl-3-octadecylimidazolium bromidestabilized MWCNTdispersions (C0/16,C0/32,and C0/64)which werecen- trifugedfordifferentperiods(0,1,2,5,10,30,60min).Asitcan beseeninFig.7a,theabsorbanceofsuspensionwhichreferstothe individualCNTsweredecreasedbydecreasingtheinitialconcen- trationandalsodecreasedwithincreasingthecentrifugationtime [35].Absorbanceattwoarbitrarywavelength(260and600nm) wasplottedversustheircorrespondingconcentrationvalues(the insetofFig.7a),whichfollowingtheLambert–Beer’slaw[35,46].
Theabsorbancediagramsoftheseriesdiluted[C18C1im]Br- stabilizedMWCNTsuspensions centrifugedfordifferentperiods oftime at=600nm,A(Cd,t),againsttheirabsorbancewithout centrifugation,A(Cd,t=0),atthesamewavelengtharelinear(the insetofFig.7b).Liuetal.derivealogicalrelationshipbetweenthe absorbanceA(Cd,t)andA(Cd,t=0)asfollows:
A(Cd,t)=F(t)A(Cd,t=0) (1)
Fig. 5. UV–vis spectra of MWCNT suspensions (a) 0.4mM of 1-methyl-3- octadecylimidazoliumbromideaqueoussolutionand(b)0.004gMWCNTsdispersed in10mlof0.4mMof1-methyl-3-octadecylimidazoliumbromideaqueoussolution.
Fig.6.(a)UV–visspectraofaqueousMWCNTsuspensionwithdifferentconcentrationof1-methyl-3-octadecylimidazoliumbromide,(b)absorptionofMWCNTsdispersions at260nmwithincreasingILconcentration(0.004gMWCNTsdispersedin10mlofdifferentconcentrationof[C18C1im]Brinaqueoussolutions).
where F(t)issedimentation factor, whichcan beused tochar- acterizedthesedimentationbehaviorofagivendispersion[35].
Fig.7billustratedthevariationofsedimentationfunctionwithcen- trifugationtime.AsitcanbeseeninFig.7b,thesedimentation functionisdecreasedbyraiseofthecentrifugationtime.Whena shorttimeultrasonication(10min)wereusedforpreparingawell dispersedCNTssuspension,itclearlyindicatedthatcannotdam- agethestructure(shorteningorcutting)ofCNTs[35,46],whichis stronglydesiredinpreparationofwelldispersedCNTsforusingin supercapacitors.
The zeta potential measurement is a common means to evaluatethestabilityof colloidalparticles inaqueoussolutions, thisimportantparametercanvisually reflectthesurfacecharge and themagnitude of electrostatic interactions [39]. Whenthe concentration of 1-methyl-3-octadecyleimidazolium bromide is only 0.4mM, zeta-potential for the surface of MWCNTs is
about 60mV, which can be attributed to the tight adsorption of 1-methyl-3-octadecylimidazoliumbromideon thesurface of MWCNTs.Thelargeaverageamountofzetapotentialfordispersed CNTs in prepared suspension (60mV), trustworthy confirmed theirstabilityagainstaging[25].Basedontheseresultsweclaim thatthestabilityofthesuspensionwasachievedbyelectrostatic repulsion.Thereforelonghydrophobicchainand imidazolering headgroupmake[C18C1im]Brasasuitabledispersingagent.
Inthispaper,theinteractionbetweenMWCNTsand1-methyl- 3-octadecylimidazolium bromide were studied with the FT-IR spectra.TheFT-IRspectraof[C18C1im]Br,whichwerecomparedto [C18C1im]Br–stabilizedMWCNTsdispersionareshowninFig.8a and b,respectively. Thepeak shifts definedthecharge transfer between[C18C1im]BrandMWCNTs[25,33].Thebandsin2916and 2851cm−1areattributedtoasymmetricandsymmetricvibration ofmethylenegroups(CH2)nof[C18C1im]Br,respectively.Because
Fig.7.(a)UV–visspectraofdiluted[C18C1im]BrstabilizedMWCNTsuspension(C0/16,C0/32,andC0/64)andcorrespondingcentrifugedsupernatants.Theinsetshows thevariationoftheabsorbanceat260and600nmvs.theCNTconcentrationoftheseriesdispersionswithoutcentrifugation,(b)sedimentationfunctionof1-metyl-3- octadecylimidazoliumbromidestabilizedCNTssuspensions.TheinsetrepresentstheplotofA(Cd,t)vs.A(Cd,t=0)at600nm.
Fig.8.FT-IRspectraof(a)1-methyl-3-octadecylimidazoliumbromide,comparing with(b)[C18C1im]Br-stabilizedMWCNTdispersion.
ofthehydrophobicinteractionbetweenMWCNTsandhydrocarbon chainsof1-methyl-3-octadecylimidazoliumbromide,theup-shift ofbandscanbeseeninthespectraofMWCNTsandILmixture [33].ThebendingvibrationofN-CinILat1571.88showsanup shiftinmixture.Allthepeakshiftsdemonstratedthat1-methyl-3- octadecylimidazoliumbromidecantightlyadsorbonthesurfaceof MWCNTs.
AccordingtotheUV–vis,FT-IR,andzeta-potentialresults,we canclaimthatwhen1-methyl-3-octadecylimidazoliumbromide
andcross-sectionalviewsofAAOtemplateaftertwo-stepanodiz- ingandporeswideningtreatment.Wecanseethestraightparallel pores,perpendiculartotheAAOtemplatesurface.
ForbothSEMimagesinFig.9,theEDXspectraconfinedtheexist- enceofAluminum,oxygen,andAu(asconductinglayersputtered ontheonesideoftheAAOtemplates)inthetemplate,thatreferred toformationaperiodichexagonalclose-packedmembranewhich iscalledAAOtemplate(Fig.9c).
Fig.10aandbillustratestheSEMimagesofthesurfaceandcross- sectionalviewsoftheAAOtemplateafterelectrodepositionofNiin thebottomofpores.TheEDXdataanalysisofcanddareaswhich remarkedinFig.10bwereshowninFig.10candd,respectively.
TheEDXresultsaretrustworthyevidencethatasmallamountof Ninanoparticleswereuniformlyelectrodepositedatthebottomof theporesintheAAOtemplate,whichcouldplayanimportantrole indepositionofCNTsinthenanochannelsofAAOtemplate.
Fig.11aillustratesthearrangementofwelldispersedCNTson theAAOtemplateafterapplyinganexternalfield.Asitcanbeseen, theporousstructureofAAOtemplateandthesuperiorabilityof [C18C1im]BrindispersingCNTsresultedinauniquestructureof CNTbasedelectrode(withoutanybundlesofCNTsontheupper
Fig.9.SEMimagesof(a)plane,(b)cross-sectionalviewofporestructuresofanodizedaluminumoxide(AAO)template.InsetshowstheEDXspectrumobtainedfromregion whichremarkedwithc,and(c)isitsEDXanalysis.
Fig.10. SEMimagesof(a)surfacemorphology,(b)cross-sectionalimagesofporestructuresofAAOtemplate,afterNinanoparticlesdeposition,respectively.Insetshows theEDXspectrumobtainedfromregionswhichremarkedwithcandd,(c)and(d)aretheEDXquantitativeresultsofremarkedregionsinFig.10b.
surfaceofthetemplate)whichcangranteeagoodwetabilityof electrodewhenitusedinEDLCs.Fig.11arepresentsthecrosssec- tionalviewofalignedCNTswhichfabricatedinthenanoporesof AAOtemplate.Incomparewiththecrosssectionalviewofpre- paredAAOtemplatebeforedepositionofCNTs(seeFig.9c),itcan clearlyseenthatthenanochannelsofAAOtemplatehavebeenfilled withCNTsduringtheEPDprocess.Theinsetisahighmagnifica- tionimageofCNTswhichhavebeendepositedintheporesofAAO template.Fig.11billustratesEDXanalysisoftheregionwhichwas shownintheinsetofFig.11a.Itisevidentthatthematerialconsists ofC,Al,O,Au,andNielements.ThepresenceofAuandNielements isattributedtotheAAOtemplatecoatedwithanAulayerandbeing conductivewithNinanoparticleswhichhavebeendepositedinthe bottomofAAOpores.Accordingtotheresults,theEPDofwelldis- persedCNTsontheAAO nanoporesfilter,leadstoalignment of CNTsintheporesofAAOtemplate.
Basedonthemechanismofgrowingnano-particlesinthepores ofAAOtemplate,wesuggestthatthejunctionbetweentheelec- trodesurfaceandthebottomedgeofthetemplateporeservesasa
preferentialsiteforthedepositionofindividualCNTs,becausethe innerwallsofthenanochannelshavesurfaceadsorptionenergy [47].
Basedonourexperimentalresults,structureofdepositedfilm tightlydependsondepositiondurationandappliedvoltage,sowe studiedtheeffectoftheseparametersondepositionof[C18C1im]
Br-stabilizedMWCNTdispersion,whichareresultedthatthebest timeandvoltageare1minand100V,respectively.
3.3. Electrochemicaltests
The cyclic voltammograms (CVs) obtained from electrodes which prepared by electrophoretic deposition of MWCNT- [C18C1im]BronporousAAOtemplatein1.0MNa2SO4atdifferent scanratesareshown inFig.12a.TheCVcurvespresentregular shapesatrelativelyhighscanrate(100mVs−1),indicatingwell- developedcapacitiveproperties.Theelectrodewhichpreparedby EPDexhibitsalmostmirrorvoltagramswithrespecttothezero- current line, except for the small peaks at +0.41 and −0.24V,
Fig.11. (a)SEMmicrographsofCNTsdepositedinananodicaluminumoxidetemplatebyEPDfromsuspensionof0.004gCNTsdispersedin0.4mM[C18C1im]Br,at 100Vcm−1for1min.Inset:CrosssectionalviewofdepositedCNTsinAAOnanochannels(b)theEDXspectrumobtainedfromregionwhichshownintheinsetofImage11a, and(c)correspondingEDXresults.
attributabletotheredoxreactionscausedbythedispersantagents whichwereadsorbedonthesurfaceofMWCNTs[48].Atthescan rateof20mVs−1,capacitanceoftheVA-CNTelectrodewasfound tobe51Fgr−1.AsillustratedinFig.12b,thecapacitanceofresultant electrodedecreaseduponthescanratesincreased.Toinvestigate theperformanceof preparedVA-CNT electrodes,potential-time profilesofindividualelectrodesvs.Ag/AgClattheconstantcurrent densityof1.25Agr−1andcutoffvoltagesforcharging-discharging of−1.2to0.8Vin1.0MNa2SO4solution,werepresented.
AsshowninFig.13a,thechronopotentiogramoftheprepared VA-CNTelectrodesindicatesagoodlinearvariation ofpotential versustime,which istypicalcharacteristicofanidealcapacitor [49].AccordingtoFig.13b,thespecificcapacitanceofresultantelec- trodesslightlydecreasesfrom40to27Fgr−1,withtheincreaseof dischargecurrentdensityfrom1.25to2.5Agr−1.Sothereisdrop ofthespecificcapacitancewithincreasingthedischargecurrent density.Fig.13cillustratesthecyclicdurabilityofpreparedVA- CNTelectrodesin 1000cyclesata dischargecurrent densityof
Fig.12.(a)CVsobtainedfromaVA-CNTelectrodein1.0MNa2SO4atthescanrateincreasingfrom10to100mVs−1,and(b)showsthevariationofcapacitancewithincreasing thescanrate.
Fig.13.(a)Constantcurrentcharge/dischargecurvesforVA-CNTelectrodemeasuredat1.25Agr−1in1.0MNa2SO4solution,between−1200and400mV,and(b)Theratioof changingspecificcapacitancevaluesofpreparedelectrodeasafunctionofdischargecurrentdensitiesand(c)Capacitanceretentionratioofresultantelectrodeasafunction ofcyclenumber.
1.25Agr−1,thiscurvesclearlyshownthattheresultingelectrodes retainedacapacitiveover70%ofthefirstcycle[50–52].
4. Conclusion
We have developed an easy method to fabricate well dispersed CNTs with aid of small amount of 1-methyl-3- octadecylimidazoliumbromideby EPDinAAOsubstrate, where synthesizedbytwo-stepanodizationofAluminumat5◦C.TheAAO templateswerebecameconductivebyelectrochemicaldeposition ofNinanoparticlesinthebottomofpores.Thismethodenablesus toobtainverticallyalignedCNTs.Weexpectthatthismethodcan bedirectlyappliedtofabricationofaVACNTbasedelectrodeforthe CNT-basedEDLCs.EtchingofthetemplategivesusCNTswithvery similardimensions.WehavedevelopedanewgenerationofVA- CNTwith50Fgr−1specificcapacitance.Itseemstobeveryuseful toamodelstudyformanyCNT-basedapplications.
Acknowledgements
WearegratefulforfinancialsupportfromIranNationalScience Foundation(INSF),Iran(GrantNo.93/36575).
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