Article

Separation of surfactant functionalized single-walled carbon nanotubes via free solution electrophoresis method

Central European Journal of Physics (Impact Factor: 1.09). 04/2011; 9(2):325-329. DOI: 10.2478/s11534-010-0083-z

ABSTRACT

This work presents the application of the free solution electrophoresis method (FSE) in the metallic / semiconductive (M/S)
separation process of the surfactant functionalized single-walled carbon nanotubes (SWCNTs). The SWCNTs synthesized via laser
ablation were purified through high vacuum annealing and subsequent refluxing processes in aqua regia solution. The purified
and annealed material was divided into six batches. First three batches were dispersed in anionic surfactants: sodium dodecyl
sulfate (SDS), sodium cholate (SC) and sodium deoxycholate (DOC). The next three batches were dispersed in cationic surfactants:
cetrimonium bromide (CTAB), benzalkonium chloride (BKC) and cetylpyridinium chloride (CPC). All the prepared SWCNTs samples
were subjected to FSE separation process. The fractionated samples were recovered from control and electrode areas and annealed
in order to remove the adsorbed surfactants on carbon nanotubes (CNTs) surface. The changes of the van Hove singularities
(vHS) present in SWCNTs spectra were investigated via UV-Vis-NIR optical absorption spectroscopy (OAS).

KeywordsSWCNTs–surfactants–separation–electrophoresis

Full-text

Available from: Ryszard Jozef Kalenczuk, Nov 25, 2015
Cent. Eur. J. Phys. • 9(2) • 2011 • 325-329
DOI: 10.2478/s11534-010-0083-z
Central European Journal of Physics
Separation of surfactant functionalized single-walled
carbon nanotubes via free solution electrophoresis
method
Research Article
Blazej Scheibe
1
, Mark H. Rümmeli
2
, Ewa Borowiak-Palen
1
, Ryszard J. Kalenczuk
1
1 West Pomeranian University of Technology, Institute of Chemical and Environment Engineering,
ul. Pułaskiego 10, 70-322 Szczecin, Poland
2 Leibniz Institute for Solid State and Materials Research Dresden,
Helmholtzstr. 20, 01069 Dresden, Germany
Received 31 July 2010; accepted 2 September 2010
Abstract: This work presents the application of the free solution electrophoresis method (FSE) in the metallic /
semiconductive (M/S) separation process of the surfactant functionalized single-walled carbon nanotubes
(SWCNTs). The SWCNTs synthesized via laser ablation were purified through high vacuum annealing
and subsequent refluxing processes in aqua regia solution. The purified and annealed material was di-
vided into six batches. First three batches were dispersed in anionic surfactants: sodium dodecyl sulfate
(SDS), sodium cholate (SC) and sodium deoxycholate (DOC). The next three batches were dispersed in
cationic surfactants: cetrimonium bromide (CTAB), benzalkonium chloride (BKC) and cetylpyridinium chlo-
ride (CPC). All the prepared SWCNTs samples were subjected to FSE separation process. The fractionated
samples were recovered from control and electrode areas and annealed in order to remove the adsorbed
surfactants on carbon nanotubes (CNTs) surface. The changes of the van Hove singularities (vHS) present
in SWCNTs spectra were investigated via UV-Vis-NIR optical absorption spectroscopy (OAS).
PACS (2008): 61.48.De, 78.67.Bf, 78.67.-n, 64.75.Jk
Keywords: SWCNTs • surfactants • separation • electrophoresis
© Versita Sp. z o.o.
1. Introduction
Single-walled carbonnanotubes(SWCNTs),due totheir
prominentelectronicproperties,chemicalandthermalsta-
bility, high tensile strength, and ultra-light weight are
recognizedasamaterialofthefutureinmanydifferent
E-mail:eborowiak@zut.edu.pl (Corresponding author)
nanoelectronicbranches.Thereisastrongneedforprepa-
rationofcarbonnanotubes(CNTs)withhomogenouselec-
tronicproperties. However,currentlysynthesizedSWC-
NTscontainapproximately30%metallicand70%semicon-
ductingtubes[1,2]. ThedifferenceinSWCNTsconduc-
tivity is aserious problemin theelectronic industry that
islimitingtheirfurtherapplications. Forthisreason,a
lotoftheresearchgroupsaretryingtosolvethisprob-
lemand toobtainelectronically homogenousfractions of
SWCNTs. So far many different separation techniques
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Separation of surfactant functionalized single-walled carbon nanotubes via free solution electrophoresis method
werefoundandtested: selectivedispersion[3],selective
functionalization [47], selective destruction [8], as well
as densitygradientultracentrifugation[9, 10],chromatog-
raphy[11], electrophoresis techniques [1215], andoth-
ers[16]. Thisworkpresentstheresultsofthemetallic/
semiconductive(M/S)separationofthesingle-walledcar-
bonnanotubesinafreesolutionelectrophoresissystem
(FSE).Fractionationviaelectrophoresisisbasedonmi-
gration of themolecules in theelectric fieldaccording to
charge,mass,oranisoelectricpoint. Thepristinenan-
otubes, after purificationand high-temperature annealing
arechemicallyinert.Theelectricchargeneededforsepa-
rationwasaddedviaphysical adsorptionoftheanionicor
cationic surfactantonto CNTssurface.Asknown fromthe
stateof theart,the non-covalentfunctionalizationofthe
nanotubes canbe a selectiveprocess, dueto the applica-
tion of thespecific surface agents [15]. This is causedby
the difference inSWCNTs’ electronic properties, which is
relatedtoadifferentforceandrateofthefunctionalization
byaspecificsurfaceagent.Threeanionicsurfactantswere
selected for the experiment. Sodium dodecyl sulphate
(SDS) is an anionic surfactant commonly used for the
electrophoresis separation process of biomolecules and
foundapplicationinnanotubesseparation[15]. Sodium
cholate (SC)and sodiumdeoxycholate (DOC)are anionic
bile-acidsaltsfeaturinggreatdispersionpotentialused
inCNTs’separationprocesses[1517]. Ascounterparts
threecationicsurfactantsweretestedfortheseparation
process. Cetrimoniumbromide(CTAB)isusedmainlyin
enzymeelectrophoresis,wherecatalyticactivityhastore-
mainafter theseparation process andinthe dispersionof
nanotubes[17,18]. Cetylpyridiniumchloride(CPC)was
foundtoimproveelectricconductivityofCNTs,decrease
naphthalenesorption,andwasalsousedforthedisper-
sionprocess[17,19,20].Benzalkoniumchloride wasused
fordispersionduringcarbonnanotubespurificationpro-
cess[21,22]. Noneofthespecifiedcationicsurfactants
were used forthe electrophoretical, single-walled carbon
nanotubes M/Sseparationprocess.The yieldof theFSE
separationwasestimatedfromvanHovesingularity(vHS)
peak areas in the UV-Vis-NIR spectra.
2. Experimental
Hydrochloric acid 35%-38%p.a., nitric acid 65% p.a., boric
acid 99% p.a., and acetone 99.5% p.a. were purchased
from Chempur. Tris(hydroxymethyl)aminomethane 99%
was purchased from Poch. Tricine 99%, sodium dode-
cylsulfate(SDS)>98.5%,sodiumcholatehydrate(SC)
fromoxorsheepbile>99%,hexadecyltrimethylammonium
bromide (CTAB) >99%, and hexadecylpyridinium chlo-
ridemonohydrate(CPC)99.0 102.0% werepurchased
fromSigma. Sodiumdeoxycholate(DOC)>97%andL-
arginine>98%wereobtainedfromSigma-Aldrich.Benza-
lkoniumchloridewaspurchasedfromAldrich.Allthesolu-
tionswerebasedonROH
2
O(typeIIwaterobtainedinre-
versed osmosis process with conductance 0.056 µS cm
1
).
Single-walledcarbonnanotubessynthesizedvialaserab-
lationtechnique (Pt/Rh/Recatalysts)underwent theini-
tialannealing processat600°Cinvacuumcondition(10
5
mbar) for4hours. Next,the annealedmaterial wassub-
jected to a three-fold refluxing process in diluted (4 x
ROH
2
O)aquaregia(HCl:HNO
3
=3:1)solutionfor8
hoursat 175°C.Inorderto removethefunctionalgroups
introducedduringtherefluxingprocess,thepurifiedma-
terial was annealedat 1100°C in vacuum (10
5
mbar) for
1 hour.Reference SWCNTs material was divided into six
batches.Each batch was dispersedwith assistance of an
ultrasonicbathandhorn(12hourstotal),in5mlofthe
1%selectedsurfactantsolution: SDS,SC,DOC,CTAB,
BKC, or CPC.Forthe electrophoretic separation process
twobufferswereprepared:0.1Mtris-borate(pH8.25)and
0.1Mtricine-arginine(pH8.25). Thesebuffersolutions
werechosenasanelectrophoreticalmediaintheM/Ssep-
arationprocesses forSWCNTsfunctionalizedbyanionic
andcationicsurfactants. Theelectrophoresis processes
proceededfor24hoursat300V.Anelectrophoreticalsys-
temwasbuiltfromcustom-madeglasschambersconnected
withaglasstube(diameter9mm,length120mm),withan
inletport placedatthe centreofthe glasstube andtwo
platinumelectrodesplacedineachchamber. Twofrac-
tionswerecollectedaftereachseparationprocess. The
migratingfractionwascollectedfromtheelectrodearea
(anodeorcathode dependingonthesurfactant),andthe
controlfractionwascollectedfromtheinletport. Next,
theobtainedsampleswerefilteredthroughthepolycar-
bonatefilter (Whatmanpore size0.2 µm) andrinsed thor-
oughly withROH
2
O andacetone.Subsequently,in order
toremoveadsorbedsurfactantsfromthecarbonnanotubes
surface,allthesampleswereannealedat600°Cinvac-
uum(10
5
mbar)for1hour. Afterthat, inordertoob-
servethechangesinvanHovesingularitiesbeforeand
after the separation process, the optical absorption spec-
tra (OAS) inthe range UV-Vis-NIRwere analyzed using
Jasco V 570.
3. Results and discussion
TheUV-Vis-NIRspectrumofsingle-walledcarbonnan-
otubes presents three peaks, known as the van Hove sin-
gularities. Theappearanceofthesepeaksisrelatedto
electronic properties of SWCNTs.Two of them
E
S
11
, E
S
22
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Blazej Scheibe, Mark H. Rümmeli, Ewa Borowiak-Palen, Ryszard J.
Kalenczuk
correspondto the optical transition betweendensityof
states (DOS)in semiconductingCNTs, andthe thirdone
E
M
11
correspondstothemetallicones[1]. Additionally,
the energy position of theSWCNTs OAS spectrum is in-
versely proportional tothe nanotubes diameter.Figure 1
presents a typical UV-Vis-NIR spectrum of the SWC-
NTsinrelation todensity ofstates. The insetequations
present the relation between energyand SWCNTs diam-
eter, where, a
0
- is the C-C distance in graphite (1.42 Å),
γ
0
-isthetightbindingoverlapintegral(3eV)[23,24],
and d - is the diameter of SWCNT.
Figure 1. Typical UV-Vis-NIR spectrum of SWCNTs in relation to the density of states.
Figure2 presents UV-Vis-NIR spectra of the reference
sample, as well as the migrating fractions and control
fractionsoftheSWCNTssamples(functionalizedbyan-
ionicandcationicsurfactants)aftertheseparationpro-
cess. Fromthisfigure,noticethesignificantred-shiftof
the E
S
22
and E
M
11
peaks in eachcontrol fractionin compar-
isonto thecorrespondingpeaks inthereference sample.
Moreover,inall ofthe controlfractions (except theDOC
functionalizedsample)theintensityoftheE
M
11
ishighly
decreased, which indicates low content of the metallic
SWCNTs. TheE
M
11
peakpositionoftheall-migrating
fractions(excepttheDOCfunctionalizedsample)isthe
sameas inthereference sample. Additionally,this peak
exhibits differentintensity foreach analyzedsample. In-
terestingly, theE
S
22
peaksofthe allmigratingfractions
(exceptDOCandBKCfunctionalizedsamples)partially
overlapwithE
S
22
ofthereferencesample,duetotheen-
ergyloss;neverthelessnoblue-shiftwasdetected. The
red-shiftofE
S
22
andE
M
11
peaksofthecontrolareasandthe
energylossatE
S
22
ofthemigratingfractionscanleadtothe
conclusionthattheseparation ofSWCNTswassuccess-
fullyperformed. IncaseofDOCfunctionalizedsamples,
the red-shiftof the E
S
22
and E
M
11
peaks occurin migrating
andcontrolfractions. InFigure2conecanalsonotice
theoverlappingofthepeakareasandsimilarintensity
of thepeaks indicating thatno M/Sseparation occurred.
Additionally, theposition of thepeaks in OAS spectra of
SWCNTsisdiametersensitive.Theirpositioncanbealso
influenced by electron doping. However, this effect isex-
cludedhereduetohigh-temperature annealingofeach
collectedfraction,hencerecoveringthepropertiesofthe
pristine SWCNTs.Therefore, it is clearthat almost every
surfactantisdiametersensitive, which means thatthey
functionalize tubes with the selected diameter.In case of
thered-shiftofvHSpositionsofthetubescollectedinthe
controlarea(non-migratingfraction),thefunctionalization
and migration of the tubes with narrower diameter would
beexpected.Thelargerdiametertubesstayinthecontrol
area.
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Separation of surfactant functionalized single-walled carbon nanotubes via free solution electrophoresis method
Figure 2. UV-Vis-NIR spectra of SWCNTs functionalized with surfactants: a) SDS, b) SC, c) DOC, d) CTAB, e) CPC and f) BKC after FSE
separation processes.
Toestimatetheyieldoftheseparationprocessbycom-
parison of the E
S
22
and E
M
11
peak areas inthe UV-Vis-NIR
spectraofthereferencesampleandthefractionscollected
aftertheseparationprocess,thefollowingequationwas
applied [1]:
M =
1
1+
1.2·
E
S
22
E
M
11
!
· 100%.
Table1summarizesmetallicityofallsamplesobtainedaf-
terFSEseparationprocesses.ThemetallicSWCNTscon-
tent ofthe reference sampleagrees withcurrent thestate
oftheart[1]. The datapresentedinTable1leadtothe
conclusion thatthe mostselective surfactantis SC.Only
8%ofthemetallictubes wereleftinthecontrolfraction,
whichmeansthat92%ofSWCNTsaresemiconducting.On
theotherhandthemostselectivesurfactanttowardmetal-
lic tubes is CTAB. The metallicity content is increased in
comparison to the reference sample.
Accordingtotheliterature, duringseparationprocesses
SDSisselectivetowardsmetallicSWCNTs[15,16].How-
ever, FSEseparationprocessshowsa three-fold decrease
ofthemetallicitycontentatthecontrol fraction,incom-
parisontothereferencesampleandno increaseofthe
Table 1. Content of the metallic SWCNTs after FSE separation pro-
cesses.
Reference 32% Metallicity content %
Surfactant SDS SC DOC CTAB CPC BKC
Migrating +/- 31 21 24 35 21 30
Control 10 8 25 18 13 15
metallicityin themigratingfraction. Itleads tothecon-
clusionthatSDSappliedintheFSEseparationsystemis
semiselectivetowards semiconducting SWCNTs. The se-
lectedanionic bile-acidsaltsdifferonlybyoneadditional
hydroxylgroup. ContrarytoselectiveSC,theDOChas
greatpotentialtoformastablesuspensionofSWCNTs
[17],but noselectivity inM/S separation viaFSE process
isobserved. TheCPCandBKCusedintheFSEsepa-
rationprocesshave somedegreeof selectivity; howeverit
was found insufficient.
4. Conclusion
The freesolutionseparation methodisa goodtechnique
forM/Sseparationof carbonnanotubes.This methodcan
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Blazej Scheibe, Mark H. Rümmeli, Ewa Borowiak-Palen, Ryszard J.
Kalenczuk
be widely applied. However, its effectiveness depends on
the selective functionalization of the carbon nanotubes.
Amongtheanionicsurfactants, sodiumcholateisfound
to be most selective towards semiconductive nanotube
species.In the caseof the cationic surfactants,the cetri-
monium bromide showsgood selective properties towards
metallic SWCNTs. In order to achieve highly efficient
separationtoward metallicCNTs, themultistep procedure
shouldbeapplied. Theexclusionofthesemiconducting
nanotubes via sodium cholate in FSE process and ap-
plicationofthe cetrimoniumbromideinsubsequentFSE
separationofthepreviouslycollectedmigratingfraction
couldleadtoahighly-metallicenrichedSWCNTssam-
ple. This processseems to be also diameter selective.
However, thisstatementshouldbeconfirmedbythe study
ofmorphology ofthecollected fraction,e.g.,bymeans of
high resolution transmission electron microscopy.
Acknowledg ements
This work was sponsored bythe Polish Research Project
in years 2009-2012.
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