Removal of Oxidative Debris from Chemically Functionalized ... No. 1 2018 JAN/Vol_11_No... · simplified scheme shows the steps of MWCNTs functionalization involving nitric acid reflux,

InternationalJournalofNanoelectronicsandMaterialsVolume11,No.1,Jan2018[43‐48]

RemovalofOxidativeDebrisfromChemicallyFunctionalized

Multi‐walledCarbonNanotube(MWCNT)

MohdHamzahHarun1*,NikGhazaliNikSalleh1,MohdSofianAlias1,MahathirMohamed1,MohdFaizalAbdulRahman1,MohdYusofHamzah1,KhairilNurKamalUmar1and

NorfazlinayatiOthman2*

1MalaysianNuclearAgency,Bangi,43000,Kajang,Selangor,MALAYSIA2FacultyofScience,UniversitiPutraMalaysia,43400,Serdang,Selangor,MALAYSIA

Received18May2017;Revised24July2017;Accepted16Aug2017

ABSTRACTOxidizedmulti‐walled carbonnanotubes (MWCNTs)werepreparedby functionalizingMWCNTs with nitric acid (70%). In order to remove the oxidative debris, in whichpartially attached onto the outer layer of MWCNTs side wall, the functionalizedMWCNTs(f‐MWCNTs)byrefluxmethodwerecarriedoutwithNaOH(1M)andfollowedbyHCl(1M)wash.Thepresenceofthecarboxyl(–COOH)groupthatcovalentlyattachedonto the f‐MWCNTs sidewall is confirmedwith acid‐base titration (Boehm titration).The transmission electron microscopy images show the comparison of pristineMWCNTs, f‐MWCNTsandbase‐acidwashof f‐MWCNTs,whereas energydispersiveX‐ray analysis confirms the removal of sulphur, a common catalyst material typicallyemployed in theproductionofcarbonnanotubes (CNTs).TheUV‐VisiblespectroscopyshowsthedispersibilityofpureandfunctionalizedMWCNTsinwater(H2O). Keywords:Oxidativefragments/debris,MWCNTs,chemicalfunctionalization.

1.INTRODUCTION Multi‐walledcarbonnanotubes(MWCNTs)canbeconsideredasanarrayofgraphenesheetsrolledupintoaseriesofhollowcylindersarrangedcoaxiallywithregularincreasingdiameter[1]. Due to their extraordinary mechanical, chemical and electronic properties, researchersactivelycarryoutstudiesandinvestigationinordertounderstandtheirpotential.Asgenerallyknown,pristineCNTs(Figure1)areimpureandsufferpoordispersibilityandlowsolubility.Therefore,reliablepurificationprocessmustbeconductedbeforeMWCNTscanbeemployedfor potential commercial utilization. The functionalization ofMWCNTs is a key approach inexploiting their potentials which helps improves the solubility, processability anddispersibility ofMWCNTs. A commonmethod to produce the functionalizedMWCNTs is bytreatingpureMWCNTswithstrongacidviarefluxprocess.Nitricacid(highconcentration)ismostlyusedinthisstudy.BytreatingMWCNTswithnitricacid(HNO3),theMWCNTs‐COOHisformedinwhichcarboxylicgroupattachedonthesidewallofMWCNTs.Despitethat,recentstudies show that the majority of the –COOH functionality created when refluxing pureMWCNTs with nitric acid is present on carboxylated carbonaceous fragment (CCF) i.e.moleculardebris rather than–COOHcovalently attachedon the sidewall ofMWCNTs [2‐5].

*Correspondingauthorinformation:MohdHamzahB.Harun,NikGhazaliB.NikSalleh,MohdSofianB.Alias,MahathirB.Mohamed,MohdFaizalB.AbdulRahman,MohdYusofB.Hamzah,KhairilNurKamalUmar,andNorfazlinayatiBt.Othman,MalaysianNuclearAgency,Bangi,43000,Kajang,Selangor,MALAYSIA,FacultyofScience,UniversitiPutraMalaysia,43400,Serdang,Selangor,MALAYSIA.Emailaddress:[email protected]

MohdHamzahHarun,etal./RemovalofOxidativeDebrisfromChemicallyFunctionalized…

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Therefore, this work focused on the removal of this oxidative debris and furthercharacterization.

Figure1.Simplediagramofpristinecarbonnanotubesandderivatives

2. MATERIALSANDMETHODMWCNTsthatiscommerciallyavailableviachemicalvapordeposition(CVD)grownMWCNTs(SigmaAldrich)wererefluxedinnitricacid(70%,30ml)for6hoursat100oC.Aftercooldowntoroomtemperature,theywerevacuum‐filteredthrougha0.22 mmilliporeofpolycarbonatemembraneandwashedwithdeionizedwateruntilthefiltrateachievedneutralpH.Then,thefunctionalized MWCNTs (f‐MWCNTs) were dried overnight in a vacuum oven at 105oC. Inordertoremoveanyf‐MWCNTsoxidativefragments,f‐MWCNTswererefluxedagainin1.0MNaOHfor1hour,washedandfiltereduntilaneutralpHwasreachedandthenrefluxedin1.0MHCltoregeneratetheacidicsites(referFigure2).

(a)

(b)

Figure2.Schemaofthe(a)FunctionalizationstepsofMWCNTsbychemicalmethod,and(b)A

simplifiedschemeshowsthestepsofMWCNTsfunctionalizationinvolvingnitricacidreflux,NaOHwashandHClwash


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2.1 The–COOHgroupdetermination

Atypicalacid‐baseorBoehmtitration[6]wasusedtodeterminedistributionofthefunctionalgroups namely –COOH group: 20 ml of 0.01 M base solution NaHCO3 in 0.1 M NaCl waspipettedslowlyintoavialcontainingf‐MWCNTs.Thevialwassealedandplacedinashakingincubator(25oC)at150rpmfor24hours.Sampleswerethenfilteredand5mLaliquotswereremoved and titrated with 0.01 M HCl in 0.1 M NaCl solution. A control sample, withoutMWCNTs was also titrated under the same condition. Titration steps were carried out intriplicateandtypicalpHmeter(MettlerToledo)wasusedtomeasurethepHofthetitrant.2.2TEMandEDXmeasurement

Pristine CNTs, f‐MWCNTs and base and acid wash f‐MWCNTs were sonicated withtetrahydrofuran (THF) prior to TEMmeasurement and allowing a drop to dry onto a holeycarbonfilm.ImageofthesamplesweretakenusingTEM(JEOL,JEM2100)operatedat100kV.EDXanalysiswascarriedoutbyusingEDXanalyzer(FEIQuanta)andisoperatedat20kV.

3. RESULTSANDDISCUSSION3.1 RemovalofPartiallyOxidizedDebris

The–COOHgroupisintroducedontothesidewallofMWCNTsbyrefluxingwithnitricacid.Duetothepresenceofoxidizedfragments,thebasewashingwascarriedoutbyusingNaOH.Thebasewashingconvertsanyacidicgrouppresentswithinthesampletotheirconjugatebaseandhence solubilizes any partially oxidized fragments that remain as contaminants [2‐4]. Theremovalofthecontaminantsisconfirmedclearlyfromtheorange‐redcolorobtainedfromtheleachate of the sample indicating the removal of the oxidized fragment/debris as shown inFigure 3. Addition of HCl for reprotonation changes the leachate color to orange yellowishcolor.

Figure3.Watersolubleleachatefroma)OxidizedMWCNTs,b)Afterbasewash,andc)AfteracidificationwithHCl.

3.2 Boehmtitration

IntheBoehmtitrationofoxidizedMWCNT,itcanbeseeninFigure4,thedistinctivepeakforNaHCO3‐MWCNTsismoreacidicthanblankNaHCO3whichsuggestedthepresenceof–COOHgrouponMWCNTssidewall.AsfortitrationusingNaHCO3asbase,theearlyreductionfrom0to1mlHClcanbeignoredassupportedbyliterature[1].Itiscalculatedthattheconcentrationof–COOHgroupattachedonMWCNTssidewallis0.011meq/g.


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(a) (b)

Figure4.Acidbasetitration(Boehmtitration)for(a)blankNaHCO3and(b)NaHCO3containingMWCNTs.

3.3TEMimagingFigure5showstheTEMimageforas‐producedMWCNT,functionalizedMWCNTandacid‐basewashMWCNTs. For pristineMWCNTs, it is clearly seen that it is highly amorphous, whichmight comprise of the clump containing catalyst particles and amorphous carbon [3]. Afterrefluxing f‐MWCNTwith nitric acid, the clump reduces in consistency as depicted in Figure5(b). It appears that functionalization caused erosion on the outermost layer of MWCNTs.Therefore, it is expected that prolonging the reflux time could destroy the upper layer ofMWCNTs. After acid and basewashing, area of clean surface is obtained and the degree ofamorphousphaseisseentodecreaseasshowninFigure5(c)[3].

(a) (b) (c)

Figure5.TEMimage(scale,10nm)ofa)pristineMWCNT,b)acidfunctionalizedMWCNTandc)base‐

acidwashMWCNT.Further investigationbyEDX compares the individual composition forpristineMWCNTandbase‐acidwashMWCNT.ThefindingfromtheEDXinvestigationshowninFigure6(a)confirmsthepresenceofsulphur(commonlyusedintheproductionofCNTsascatalystmaterial[7])inpristineMWCNT.Asreportedintheliterature,sulphurplaysanimportantroleininducingthegrowthofCNT,thusincreasethediameteroftube.This isthereasonforthepresenceoftheapparentclumpthatcanbeseen inTEMimageofpristineCNTs.Ontheotherhand,nosuchobservationfromtheEDXspectrumofbase‐acidwashoffunctionalizedMWCNTasdepictedintheFigure6(b)anditiswithintheagreementwiththeas‐obtainedTEMimageinwhichcleansurfaceisobservedforbase‐acidwashfunctionalizedMWCNT.

v = 5

dpH

/dV

pH

Volume of HCl (ml)

blank NaHCO3


47

(a) (b)

Figure6.EDXspectrumfor(a)pristineMWCNTand(b)base‐acidwashoffunctionalizedMWCNT.3.4 UVVisibleSpectroscopyThetypicalabsorptionspectraforthedispersionofpureandfunctionalizedMWCNTsinH2Oare indicated inFigure7.Asseenfromthegraph, theabsorption intensity for functionalizedMWCNT is higher thanpureMWCNTs.This indicateshigher dispersibility for functionalizedMWCNTas compared topureMWCNTs.A shoulder at around360nm in the functionalizedMWCNTspectrumismightbeattributedtonto *transitionofC=ObondinMWCNTslattice[8].ThisshoulderisnotclearlyapparentforpureMWCNTssuggestingthatitispureMWCNTssince it contain no functional group in its lattice. The dispersion in distilledwater for bothsamples is also indicated in the inset in Figure 7. (p‐C for pristine MWCNTs and f‐C forfunctionalizedMWCNTs).

Figure7.UV‐VisibleSpectroscopyforpureandfunctionalizedMWCNTinH2O.Insetphotois

functionalizedandpureMWCNTsinH2O.

0

1

2

3

280 380 480 580 680 780 880

Absorbancec (%

)

Wavelength, nm

MWCNT

f‐MWCNT


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4. CONCLUSION

AcidfunctionalizationMWCNTsusingnitricacid(HNO3)createsasignificantlevelofoxidationfragmentwhichisdifficulttoberemovedbywaterwashing.Inordertoobtainhighpurityoff‐MWCNTs,thisdebriscanbeeffectivelyremovedbyfacileandmild,base‐washingprocedure.Thewashingstep is facile since it canbeundergoneat roomtemperaturebymeansofbasedilutionfollowedbyacidwashing.ThistreatmentconvertstheweakeracidicgroupstotheirconjugatesaltsthusincreasingthewatersolubilityofimpuritiesandtheMWCNTs.Subsequenttreatmentwithdiluteacidreprotonates the functionalgroupremainedon theMWCNTssidewallthusincreasethedispersibilityinwaterhenceimproveitsprocessability.Thispurificationmethodisfacile,cost‐effectiveandscalable.Inaddition,BoehmtitrationresultsrevealthatitisthebesttechniquetodetermineandquantifythefunctionalgroupattachedontotheMWCNTssidewall.On the other hand, theUVVisible spectroscopy shows that betterdispersibility isobserved as for the functionalized MWCNTs compared to the pristine MWCNTs. The as‐preparedfunctionalizedMWCNTsisbelievedtohasmanypotentialapplicationsincludedrugdelivery,functionalcoatings(hydrophobicorconductive),andreinforcedfillers.ACKNOWLEDGEMENT

Authorswould like to thankUniversityofBrighton for theirkindness inproviding technicalguidance for thisproject.Authorsalsowould like toexpressdeepestgratitude toMr.KhirulHafizandMrs.ZaitonfortheirassistanceonTEMandEDXmeasurementsrespectively.REFERENCES[1] Wang,Z.,Shirley,M.D.,Meikle,S.T.,Whitby,R.L.D.,Mikhalovsky,S.V.(2009),The

surfaceacidityofacidoxidizedmulti‐walledcarbonnanotubesandtheinfluenceofin‐situgeneratedfulvicacidsontheirstabilityinaqueousdispersion.Carbon47,73‐79.

[2] Salzmann,C.G.,Llewellyn,S.A.,Tobias,G.,Ward,M.A.H.,Huh,Y.,Green,M.L.H.(2007),Theroleofcarboxylatedcarbonaceousfragmentsinthefunctionalizationandspectroscopyofasingle‐walledcarbonnanotubematerial.Adv.Mater.19,883‐887.

[3] Fogden,S.,Verdejo,R.,Cottam,B.,Shaffer,M.(2008),Purificationofsinglewalledcarbonnanotubes:Theproblemwithoxidationdebris.Chemi.Phys.Lett.460,162‐167.

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[5] Alias,S.H.,Buang,N.A.,MohdYusof,A.,Ibrahim,M.L.(2014),Aspirinadsorptiononmultiwalledcarbonnanotubesanditsreleasecharacteristicsinsimulatedbodyfluid.Int.J.NanoelectronicsandMaterials7,35‐43.

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Removal of Oxidative Debris from Chemically Functionalized ... No. 1 2018 JAN/Vol_11_No... · simplified scheme shows the steps of MWCNTs functionalization involving nitric acid reflux,