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Chemical Physics Letters
j o ur na l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / c p l e t t
Breakdown of diameter selectivity in a reductive hydrogenation reaction of single-walled carbon nanotubes
Katalin Nemeth
a, Emma Jakab
b, Ferenc Borondics
a,1, Hajnalka M. Tóháti
a, Áron Pekker
a,2, Mónika Bokor
a, Tamás Verebélyi
a, Kálmán Tompa
a, Sándor Pekker
a,c, Katalin Kamarás
a,∗aInstituteforSolidStatePhysicsandOptics,WignerResearchCentreforPhysics,HungarianAcademyofSciences,P.O.Box49,H1525Budapest,Hungary
bInstituteofMaterialsandEnvironmentalChemistry,ResearchCentreforNaturalSciences,HungarianAcademyofSciences,P.O.Box286, H1519Budapest,Hungary
cÓbudaUniversity,Doberdóút6,H1034Budapest,Hungary
a r t i c l e i n f o
Articlehistory:
Received18September2014 Infinalform11November2014 Availableonline15November2014
a b s t r a c t
Reductivehydrogenationwasappliedtotwotypesofsingle-walledcarbonnanotubeswithdifferent diameter range. Alkali metal intercalation, followed by reaction with methanol, led to hydro- genatedproducts.Bothyieldandselectivityofthisreactionshowedstrongdependenceondiameter, contrary to expectation based on simple curvature effects. The observed yield, as detected by thermogravimetry–massspectrometryand1HNMR,isdrasticallyreducedinsmall-diametertubeswhere thealkalidopantdoesnotreachtheinsideofthebundles.Widerangeopticaltransmissionmeasure- mentswereemployedtodeterminetheselectivityandindicatethatbesideshigheryield,lowerdiameter selectivityoccursaboveacriticaldiameter.
©2014ElsevierB.V.Allrightsreserved.
1. Introduction
Chemicalmodificationofcarbonnanotubesurfacesbysidewall reactionsisimportantforanyapplicationwherefurtherchemical bondingorincreaseinsolubilityisdesired.Nanotubechemistry, however,constitutesmanychallengescomparedtoconventional organicchemistry methods:pristinenanotubes areinsolublein mostsolvents,andsolidsamplescontainbundlesofmanytubes, heldtogetherbyvanderWaalsforces.Toincreasecontactbetween reactantsandthetubesurface,exfoliationandsolubilizationare required.Theseuniquecharacteristicsalsomeanthatotherfactors thanthechemicalpropertieshavetobetakenintoaccountwhen predictingtheyieldofachemicalreaction.
Reductivereactions,wherethefirststepconsistsofchargingthe reactants,areespeciallywellsuitedforexfoliation.Severaltypesof modifiedBirchreductionarefoundintheliterature[1–4],using ammonia,tetrahydrofuran andethylenediamineas solvent.The
∗Correspondingauthor.
E-mailaddress:kamaras.katalin@wigner.mta.hu(K.Kamarás).
1 Presentaddress:SoleilSynchrotron,BP48,L’OrmedesMerisiers,91192Gifsur YvetteCédex,France.
2 Presentaddress:CenterforNanoscaleScienceandEngineering,Departments ofChemistryandChemical&EnvironmentalEngineering,UniversityofCalifornia, Riverside,CA92521,USA.
firststepiselectrontransferontothenanotubes,whererepulsion betweentheresultingnegativelycharged‘supermolecules’helps exfoliationandpromotesfurtherreactions.Similarprocessescan happeninalkalimetalintercalatedgraphitecompounds[5],result- inginpartiallyhydrogenatedgraphite[6].
Followinggraphiteintercalationcompounds[5],nanotubeexfo- liationpossibilitiesandprocesses[7,8],weapplieddirectreduction byintercalatingalkalimetalsintocarbonnanotubebundles.Based ontheseresultsandtherathersimilarreactivityofgraphiteandcar- bonnanotubes,weexpectedalkalimetalintercalationtoexfoliate andreducenanotubebundles.Bythismethod,thestepofcarbanion formationduringthereductivemodificationcouldbeseparatedin spaceandtimefromtheattachmentofthefunctionalgroup(unlike otherBirch-typereactions)[9].
Addingalkalimetalstocarbonnanotubesinexcess,overtime asaturationconcentration(aboutKC9 forpotassium)isreached [10].Astablephase(withcompositionKC27determinedbyXPS)is formedinafewminutesat180◦C.StructuralcharacterizationofK- andCs-dopedSWNTsbyX-raydiffractionandelectronmicroscopy provedthatthealkaliionsareattachedtothetubesurfaces,increas- ingthebundlesize,insteadofbeingencapsulatedintothetubes [11].Sidewallfunctionalizationofthenanotubescanproceedfrom thedopedphasebyelectrophilicaddition.
Thegeneral selectivityof sidewallreactionsin nanotubes is believedtobedeterminedprincipallybystructuralstraincaused http://dx.doi.org/10.1016/j.cplett.2014.11.019
0009-2614/©2014ElsevierB.V.Allrightsreserved.
by-orbitalmisalignmentonthecurvedsurface [12],resulting in higher reactivity ofsmaller diametertubes. Considering one nanotube,thesefeaturesdeterminediameterselectivitythermo- dynamically.Takingintoaccount,however,thatrealisticnanotube samplesconsistofbundles,thereareotherimportantphenomena thatmustbeconsidered,suchaskinetics,stericeffectsandener- geticsofallprocessesandintermediateproductsinareaction.
Theeffect of bundling isan unusualand exciting featureof nanotubepropertiesandreactions,andhasbeensubjectofmany previousinvestigations.Oneespeciallyrelevanttoreductivereac- tionsisthecarefulanddetailedstudybyKukoveczetal.[13]on thefirststepofthesereactions,thedopingofbundlednanotubes, using resonant Ramanspectroscopy and conductivitymeasure- ments.TheyfoundthatinHiPcotubesaminimumindopinglevel appearsataround1.1nm.Theyexplained theirfindingsbytak- ingintoaccountthemorphologyof thebundles, andcalculated therequiredrelativelatticeexpansionasafunctionofthesizeof theintertubechannelsinthebundles,whichisproportionaltothe tubediameter.Accordingtotheircalculations,above1.3nmtube diameter,thereisnoneedforlatticeexpansionforincorporatingK+ ions.Atlowdiameters,however,anothereffecthastobetakeninto account,theincreasingstackinginteractioninthebundle,inversely proportionaltotubediameter[14].Thislattereffectcompeteswith theincreasingreactivityathighercurvature.
Inthisletter,wedescribetheresultsofintercalationofsingle- walledcarbonnanotubes(SWNTs)byalkalimetals,followedby electrophilicadditionofprotonstoyieldhydrogenatedproducts.
Weusedtwotypesofcommercialnanotubesamplesasstarting materials,withdifferentdiameterrange,aboveandbelow1.2nm.
Asdopantionsweusedpotassiumandrubidium,similarinchem- icalpropertiesbutdifferinginsize,inordertoreachtwoextreme combinations(potassiumwiththelargerdiametertubes,rubid- iumwiththesmallerones).The productswere investigatedby thermogravimetry–mass spectrometry (TG–MS), 1H NMR spec- troscopy,Ramanspectroscopyandwiderangeopticaltransmission spectroscopytoobtaindetailed informationaboutthereactivity throughthe composition,thermal and opticalproperties ofthe samples,withspecialattentiontodiameterselectivity.Wefind thataboveacriticalintertubechannel/ionicradiusratio,theyield increasesandbecomesindependentofdiameter.
2. Experimental
Twotypesofcommerciallyavailablesingle-walledcarbonnano- tubes, differing in average diameter, were used: P2 by Carbon Solutions,Inc.andCoMoCatCGbySouthWestNanoTechnologies.
Inordertoreachtwoextremesofthedopantradiustodiameter ratiorD/dNT,P2tubesweredopedwithpotassiumandCoMoCat tubeswithrubidium.Parametersofthereactantsaresummarized inTable1.Calculatedintertubechannelradiifromthemeandiam- etervaluesinTable1,accordingtoRef.[13],are0.15nmforP2and 0.096nmforCoMoCat.Comparingthesevalueswiththedopant radii,theK/P2combinationrepresentsacasewherethechannels caneasilyaccomodatethedopantions,whereasforRb/CoMoCat theionicradiusofthedopantbyfarexceedstheintertubechannel radius.ThisdifferenceisexpressedintherD/dNTratio,giveninthe lastcolumnofTable1.
Fig.1.Directreductionofsingle-walledcarbonnanotubesbyintercalatingpotas- sium.
The hydrogenationmethod, which wasinspired bygraphite intercalationcompounds,isdescribedbelowandshowninFigure1.
InthecaseofP2,about100mgofas-receivedSWNTwasfirst annealedindynamicvacuum(at10−6mbar)at250◦Cfor12h,fol- lowedbytransferintoanargondrybox.Potassiumwasaddedina glassvial,keepingtheSWNT-Kmolarratio4:1.Theglassvialwas sealedonavacuumline.Annealingat200◦Cfor12hwasenough forpotassiumtointercalatethenanotubes.Intercalationwasindi- catedbythecopper/goldcolorofthesample.Subsequently,the samplewasputintoaSchlenk-typeflaskwithafunnel.40mlanhy- droustoluene(Sigma–Aldrich,cryo-distilledfromNa–Kalloy)was addedtotheflaskand20mlintothefunnel.Toluenewasusedas atotallyaproticsolventtoavoidsidereactionswithanyotherH source,andtocontrolalkalimetalatomstointercalatethenano- tubesandnottodissipateinthesolvent.Sonicationwasappliedfor 15mintoenhancetheintercalationprocess.Next,5mlmethanol (Sigma–Aldrich,usedasreceived)wasfilledcarefullyintothefun- nel.Methanolwasaddeddropwiseintotheflaskduringsonication.
Sonicationwascontinuedfortwomorehours,andthemixturewas leftovernight.ThesamplewasfilteredonaMilliporenylonmem- branefilter(0.1mporesize),washedwithethanol,1:3HCl:H2O, distilledwater,ethanolandacetone.Finallyitwasdriedindynamic vacuumat200◦Cfor12h.Theproductobtainedthiswaywastrans- ferredbackintothedrybox.Thewholeprocessdescribedabove, exceptpriorannealing,wasrepeatedtwomoretimesinorderto investigatewhetheritispossibletoimprovethedegreeofhydro- genationbyapplyingsuccessivesteps.ForCoMoCat,rubidiumwas usedinsteadofpotassiumasintercalatingagent,inordertoachieve largerbundleexpansion[15].Themainproductsofthesereactions are hydrogenatednanotubes,but there areside reactions, such ashydrogenevolution,whenattachmentofHtothenanotubeis kineticallyhindered,orwhentheunreactedalkalimetalreduces methanoldirectly.
Reference samples were made of pristine P2 and CoMoCat by performingthesame stepsaswiththe hydrogenationreac- tions(initialannealing,annealinginsealedglasstube,additionof methanol,washing,annealingindynamicvacuum),exceptaddition ofalkalimetal.
Thermogravimetry–mass spectrometry (TG–MS) measure- mentswereusedtodeterminethecompositionofthesamples, particularlytheHcontent.Themainpurposeofthemeasurement isthequantitativedeterminationofthedecompositionproducts asa functionof temperature.Masschangewithtemperatureis directlymeasuredbyaPerkin-ElmerTGS-2thermobalanceanda HIDENHAL2/301PICquadrupolemassspectrometer.2–4mgsam- pleinaPtvesselwasheatedupto800◦Cwith20◦C/minratein Aratmosphere.Aportionofthevolatileproductswasintroduced intothemassspectrometer(operatedat70eVinelectronimpact ionizationmode)throughaheatedglass-linedsteelcapillary.Ion intensities were normalized to 38Ar isotope of the carrier gas toeliminateerrorsresultingfromtheshiftinMSintensities.To
Table1
Specificparametersofnanotubereactions.
Nanotube Diameterrange Meandiameter Dopant Ionicradius rD/dNT
nm nm nm
P2 1.2–1.7 1.60 K 0.138 0.086
CoMoCat 0.57–1.17 0.90 Rb 0.152 0.169
measurehydrogen(m/z2),a calibrationwithTiH2 isnecessary.
Duringthemeasurements,signalsof16ionswerefollowed.
Wideline1HNMR measurementsanddataacquisitionwere accomplishedbya BrukerAVANCE IIINMR spectrometeratthe frequency of 82.4MHz witha stability better than ±10−6. The inhomogeneity of the magnetic field was 2ppm. Free induc- tiondecays(FIDs)weremeasuredatroomtemperature.Known amounts (weight) of the nanotubes (typically 7–15mg) and adamantane(99+%,Sigma–Aldrich)wereputin Tefloncapsules.
TheFIDmeasuredontheemptycapsulewassubtractedfromthe FIDoftheactualcapsuledsampletocorrectforbackground.The amplitudeoftheFIDatzerotimeisproportionaltothenumber of1Hnucleiinthesample[16].Thefirst9–10softheFIDwas lostinthedeadtimeofthespectrometer.TheobservedFIDswere extrapolatedbacktozerotime by fittingGaussian functionsto obtainitszero-timeamplitude.TheFIDofadamantanewasused forcalibrationincalculatingthehydrogenconcentrations.There wereresidualmagneticcatalystparticlesinthesamples(typically 2–5w/w%),buttheydidnotdisturbthemeasurementsignificantly [3].
RamanspectraweretakenbyaRenishaw1000Bspectrometer using785nmexcitationwavelength,with4cm−1spectralresolu- tion.Thelaserpowerwaskeptsufficientlylowinordertoexclude heatdamage.
Widerangeopticaltransmissionmeasurementswerecarried outonself-supportingthinfilmsmadeofthesamples[17].Trans- missiondatabetween 25 and 52500cm−1 wererecorded, by a BrukerIFS 66v/s FT-IR instrument in the far and mid-infrared region,aBrukerTensor37inthenearinfrared,andaJascov550 spectrometer in the visible and UV. Optical conductivity data wereobtainedbyperformingKramers–Kronig(KK)transformation onthetransmissiondata(atomicforcemicroscopy wasusedto determinesamplethicknessforKKcalculations)[18].Opticalcon- ductivityspectrawerefittedbytheDrude–Lorentzmodel.After subtractingthebackgroundandallotherpeaksrelatedtotransi- tionsbetweendifferentVanHovesingularities,weobtainedthe contributionofthespecificpeaks[19].
3. Resultsanddiscussion
Ramanspectrawererecordedinordertodetectchangesinthe D/Gmodeintensityratio.TheincreaseinD/Gratioindicatesthe presenceofmoredefectsinthenanotubesidewall.Thisischarac- teristicoffunctionalizednanotubes,inwhichthesp3carbonatoms inthesidewallsactasdefects[20,21].Ramanspectraofbothsam- plesareshowninFigure2.EachspectrumisnormalizedtotheG bandtobecomparable.TheD/Gratioincreasesuponfunctional- ization,butthechangecannotbeusedforquantitativeestimation ofthesidegroupcontent,becauseofthecomplicatedoriginofthe Dband[22].Todeterminethehydrogencontentmoreprecisely, TG–MSandNMRspectrometrywereemployed.
ResultsoftheTG–MSmeasurementsareshowninFigure3and Table2.IncaseoftheCoMoCattubestheH-contentwassosmall thatthehydrogenevolutionpeakfadedintothebackground,there- foretheexact temperatureof hydrogenevolution couldnotbe determined.
Thedegrees of hydrogenation measuredbyNMR follow the sametrendasTG–MS:thevalueobtainedforCoMoCatissmaller thanthatforP2.Bothmethodsprovedthepresenceofhydrogen, althoughtheyhavenotyetbeensystematicallycompared.Since NMRmeasuresthetotalhydrogencontentinthesample,wecon- sider theTG–MS methodas yielding more precisequantitative resultsofhydrogenwithchemicalbondingtothenanotubewalls.
ThehydrogencontentoftheproductsformedfromP2isbetween2 and4at%.Incontrast,insmallerdiameterCoMoCatsamples,where
Fig.2.Ramanspectraofthetwosamples(excitationwavelength:785nm)before functionalization(reference)andafterthreefunctionalizationsteps.Redframe:D modes;blueframe:Gmodes.EachspectrumisnormalizedtotheGmode.Increasing D/Gratioindicatessidewallhydrogenation.(Forinterpretationofthereferencesto colorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)
Rbwasusedasintercalatingalkalimetal,thedegreeofhydrogena- tionisverysmall.Thisfactisingoodagreementwithprevious studies[23,24].
Fromspecial features intheoptical spectra,conclusionscan bedrawnaboutthereactivityofthestartingmaterial[21,25,26].
Theenergyof theinterbandtransitions(orlikewise, transitions betweenexcitoniclevelsclosetothebandedge)scaleswiththe inversediameteroftherespectivetubes.Thewidthofthepeaks intheopticalspectrareflectsmainlythediameterdistributionof peaksinthesample,and thereforethechangein shapeupon a chemicalreactionreflectsthechangeindistributionbetweenthe startingmaterialandtheproduct.Themostpreciseprocedureto extractthesechangesfromsolid-statespectraistocomparethe opticalconductivity.Thisquantityisadditivewhenseveralinde- pendentprocessesareinvolved(likelightabsorptionbydifferent nanotubes)anditscalculationtakesintoaccountthereflectanceat theinterfaces,whichcanheavilyinfluencetheopticaldensitycal- culatedfromtransmittance[18].Themodelweused[19]involvesa reliablebackgroundcorrectionandresultsincurvesreflectingone typeoftransitionacrossvariousdiametersamples.Theadvantage ofusingopticalspectratodeterminechangesindiameterdistribu- tionoverRamanspectroscopyisthatthereisnoresonanceprocess whichpreferscertainnanotubesoverotherswithselectiveincrease inthescatteringintensity.
Figure 4 compares the change in optical conductivity upon hydrogenationinthespectralregionsconnectedtothefirst(S11) and second(S22)transitions betweensemiconducting nanotube energylevels.Thecurvesshownarethoseoftheproductswith thehighesthydrogencontent,obtainedafterthreestepsofhydro- genation. Intensity loss due to hydrogenation is independent of frequency in P2, and increases withincreasing frequency in
Table2
Hcontentandthermalstabilityofthesamples.NMRdataarerelatedtothesampleswiththehighestdegreeofhydrogenationmeasuredbyTG–MS.Hcontentdetermined byNMRarecorrectedwiththeHcontentofthereferencesamplestocorrectwiththeHcontentofotherHcontainingmaterials(toluene,etc.).
Sample Tmax[H] H/100C(TG-MS) [H](NMR) H/100C(NMR)
◦C Samplenumber mmol/g
1 2 3
P2 350 2.17 2.41 3.61 4.63±0.27 5.6
CoMoCat – <1 <1 <1 2.39±0.15 2.9
CoMoCat.Thischangeindicatesthatthereactivityisdetermined bythepenetrationofthedopantintothebundles:inP2,theinten- sitydecreasesindependentlyoffrequency,whereasinCoMoCatthe inversediameterdependencereturns.Inthelattercase,theeffect ofthebundlesseemstodisappear,sincethealkaliatomswillcover onlythebundlesurface.Thiseffectresultsinmuchloweryieldbut
‘normal’diameterselectivity,determinedbythecurvatureofthe nanotubes.
Inthefirstapproximation,intercalationshouldproceedaccord- ingtothefirsttransitionenergy(S11insemiconductingnanotubes).
Sincethisquantityscalesinverselywithdiameter,thefirstreac- tionstepwouldbemoreprobableforhigherdiametertubes.These twooppositetrends(thatoftheionicintercalationstepandthe chemicalreaction,describedabove)determinetherateofthereac- tionandthereforeitisnotsurprisingthatitisnotmonotonous withdiameter.Anaddedcomplicationarisesbecausetheinterca- lationhappensinsidebundles,introducingadditionalfactors.Since intercalationintobundlesformedbylargerdiametertubesisless hindered,thesebundleswillbemorereactive,bothbytheirlarger
Fig.3.TypicalTG–MScurvesofthesamples.(a)P2,(b)CoMoCat.Blacksolidlinesin bothgraphsrepresentmasslossanditsderivative,respectively.Notationsforgraph (a):(redcircles)m/z2hydrogen;(bluetriangles)m/z16methane;(greendiamonds) m/z31methanol;(magentasquares)m/z92toluene.Forgraph(b):(redcircles)m/z 2hydrogen;(bluetriangles)m/z28CO;(greendiamonds)m/z44CO2;(magenta squares)m/z92toluene,wheremistheatomicmassandztheatomicnumber.(For interpretationofthereferencestocolorinthisfigurelegend,thereaderisreferred tothewebversionofthearticle.)
Fig.4.Theeffectofhydrogenationonthefirst(S11)andsecond(S22)semiconducting interbandtransitionsintheopticalspectra.Inbothcases,thereferencesampleand theproductwiththehighesthydrogencontentareshown.
electron density and better availability for reactant molecules [13].Abovethediameterlimitestimated(1.3–1.5nm),wherethe intertubechannelsarewiderthantheionicradius,potassiuminter- calationbecomesunhinderedandbecauseofhighavailabilityfor reactantmolecules(loosenedbundles)verylittlediameterselec- tivity can be detected.This is thecase of theK/P2 sample. As thechannelsbecomenarrowercomparedtotheintercalantion, intercalationbecomesmoreandmorehindereduntilitwillbeener- geticallytotallyunfavorable.Inthiscase(Rb/CoMoCat),electron transferispossibleonlyatthesurfaceofthebundles,whichcauses adrasticloweringinavailabilityoftubes[23].Belowthetransi- torydiameterrangethediameterselectivitywillbedeterminedby thechemicalreaction,favoringhighercurvature,asobservedinthe caseofCoMoCat.
Wedidnotaddressthequestionofselectivitybetweenmetallic andsemiconductingtubes;asinthefirstintercalationstepallthe tubesacquiremetalliccharacterduetodopingwithchargecarriers fromthealkalimetal[27],wedonotconsiderthiseffectsignificant.
4. Conclusion
Wehavesynthesizedhydrogenatedsingle-walledcarbonnano- tubesbyalkalimetalintercalationandsubsequentreactionwith methanol.Byusingtwosampleswithwidelydifferentdiameter distributions (P2 and CoMoCat), we could investigatediameter selectivity.Wehaverevealedtheroleofalkalimetalintercalation indiameterselectivityofsimilarreductivereactions.Theresults areinagreementwithpreviousstudiesonalkalimetalintercalated nanotubes. We demonstrated that factorsother thanstructural strainmustbeconsideredinreactionsthataredriveninnottotally homogeneousmediasuchasbundlednanotubes.Forpracticalpur- posesofnanotubechemistry,ourresultsexplainwhyananotube
samplewithlowaveragediametercaninfactproduceloweryield inchemical reactionsthan nanotubes abovea critical diameter limit.
Acknowledgment
Thisresearchwasfunded bytheHungarian NationalScience Fund(OTKA)undergrantNo.105691.
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