purification of carbon nanotubes

However, a reproducible and reliable purificationprotocol with high selectivity, especially for SWCNTs, is still a great challenge, because the purity of CNTs depends onnot only purifica

Purification of carbon nanotubes

Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,

72 Wenhua Road, Shenyang 110016, PR China

A R T I C L E I N F O

Article history:

Received 27 June 2008

Accepted 1 September 2008

Available online 9 September 2008

A B S T R A C T

It is predicted theoretically and understood experimentally that carbon nanotubes (CNTs) possess excellent physical and chemical properties and have wide-range potential applica-tions However, only some of these properties and applications have been verified or real-ized To a great extent, this situation can be ascribed to the difficulties in getting high-purity CNTs Because as-prepared CNTs are usually accompanied by carbonaceous or metallic impurities, purification is an essential issue to be addressed Considerable pro-gress in the purification of CNTs has been made and a number of purification methods including chemical oxidation, physical separation, and combinations of chemical and physical techniques have been developed for obtaining CNTs with desired purity Here

we present an up-to-date overview on the purification of CNTs with focus on the principles, the advantages and limitations of different processes The effects of purification on the structure of CNTs are discussed, and finally the main challenges and developing trends

on this subject are considered This review aims to provide guidance and to stimulate inno-vative thoughts on the purification of CNTs

Ó2008 Elsevier Ltd All rights reserved

Contents

1 Introduction 2004

1.1 CNT synthesis techniques 2004

1.2 Impurities coexisting with CNTs 2004

1.3 Assessment of CNT purity 2005

1.4 Purpose of this review 2006

2 Purification methods 2007

2.1 Chemical oxidation 2007

2.1.1 Gas phase oxidation 2007

2.1.2 Liquid phase oxidation 2009

2.1.3 Electrochemical oxidation 2011

2.1.4 Brief summary 2011

2.2 Physical-based purification 2011

2.2.1 Filtration 2012

2.2.2 Centrifugation 2013

2.2.3 Solubilization of CNTs with functional groups 2013

2.2.4 High temperature annealing 2013

0008-6223/$ - see front matter Ó 2008 Elsevier Ltd All rights reserved

doi:10.1016/j.carbon.2008.09.009

* Corresponding author: Fax: +86 24 2390 3126

E-mail address:cheng@imr.ac.cn(H.-M Cheng)

a v a i l a b l e a t w w w s c i e n c e d i r e c t c o m

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / c a r b o n

2.2.5 Other physical techniques 2014

2.2.6 Combination of purification and separation 2014

2.2.7 Brief summary 2015

2.3 Multi-step purification 2016

2.3.1 HIDE-assisted multi-step purification 2016

2.3.2 Microfiltration in combination with oxidation 2016

2.3.3 Sonication in combination with oxidation 2017

2.3.4 High temperature annealing in combination with extraction 2018

2.3.5 Brief summary 2018

2.4 Applicability of typical purification techniques 2018

3 Challenges 2020

3.1 Synthesis methods 2020

3.2 Purification methods 2021

3.3 Purity assessment 2021

4 Concluding remarks 2021

Acknowledgements 2021

References 2021

Elemental carbon in sp2hybridization can form a variety of

amazing structures, such as graphite (3D), graphene (2D),

car-bon nanotubes (CNTs, 1D) and fullerene (0D) CNTs defined by

Iijima in 1991[1]have a unique tubular structure with

nano-meter scale dianano-meters and large length/dianano-meter ratios CNTs

may consist of one (single-walled CNTs, SWCNTs) or up to

tens and hundreds (multi-walled CNTs, MWCNTs) seamless

graphene cylinders concentrically stacked with an adjacent

layer spacing of 0.34 nm Owing to the covalent sp2bonds

formed between individual carbon atoms, CNTs are stiffer

and stronger potentially than any other known materials

Thus, CNTs have ultra-high Young’s modulus and tensile

strength, which makes them promising in serving as a

rein-forcement of composite materials with desired mechanical

properties Because of the symmetry and unique electronic

structure of graphene, the structure of a SWCNT determines

its electrical properties For a SWCNTwith a given (n, m) index

[2], when (2n + m) = 3q (q is an integer), the nanotube is

metal-lic, otherwise the nanotube is a semiconductor Not only do

these nanotubes show amazing mechanical and electronic

properties, but also possess well-defined hollow interiors

and biocompatibility with living systems As a result, CNTs

are considered to be excellent candidates for many potential

applications, including but not limited to: catalyst and

cata-lyst supports [3,4], composite materials [5,6], sensors and

actuators[7,8], field emitters[9,10], tips for scanning probe

microscopy[11,12], conductive films[13,14],

bio-nanomateri-als[15], energy storage media[16,17]and nanoelectronic

de-vices[18,19]

Nowadays, CNTs can be produced in large quantities by three

dominant techniques: chemical vapor deposition (CVD,

including high-pressure carbon monoxide (HiPco) process)

[20], arc discharge[1], and laser ablation[21] CVD involves

catalyst-assisted decomposition of hydrocarbons (commonly

benzene, ethanol, acetylene, propylene, methane, ethylene,

CO, etc.) and growth of CNTs over the catalyst (usually transi-tion metals such as Ni, Fe, Co, etc.) in a temperature range of 300–1200 °C Good alignment[22]as well as positional control

on a nanometric scale [23]can be achieved by using CVD Control over diameter, shell number, and growth rate of CNTs are also realized with this method The chief drawback of CVD is the high defect density of the obtained CNTs owing

to low synthesis temperatures, compared with arc discharge and laser ablation As a result, the tensile strength of the CNTs synthesized by CVD is only one-tenth of those made

by arc discharge[24] Typical SWCNT content in as-prepared samples by CVD is 30–50 wt%, while the content of MWCNTs is in the range of 30–99 wt% depending on their diameters The by-products are usually aromatic carbon, amorphous carbon, polyhedral carbon, metal particles, etc Arc discharge uses two electrodes (at least one electrode is made of graphite) through which a direct current (DC) is passed in a gaseous atmosphere MWCNTs can be obtained

by arc discharge without any metal catalyst, while mixed me-tal came-talysts inserted into the anode are required when syn-thesizing SWCNTs by this method In laser ablation for producing CNTs, an intense laser beam is used to ablate/ vaporize a target consisting of a mixture of graphite and me-tal came-talyst in a flow of inert gas This method favors the growth of SWCNTs with controlled diameter depending on reaction temperature[24] When using arc discharge and laser ablation for SWCNT synthesis, side products such as fuller-enes, amorphous carbon, graphite particles, and graphitic polyhedrons with enclosed metal particles are also formed The record high-purity of the SWCNTs synthesized by arc dis-charge has been reported to be 80% by volume[25]

As-synthesized CNTs prepared by the above methods inevita-bly contain carbonaceous impurities and metal catalyst parti-cles, and the amount of the impurities commonly increases with the decrease of CNT diameter Carbonaceous impurities

typically include amorphous carbon, fullerenes, and carbon

nanoparticles (CNPs) (as shown inFig 1) Because the carbon

source in arc discharge and laser ablation comes from the

vaporization of graphite rods, some un-vaporized graphitic

particles that have fallen from the graphite rods often exist

as impurity in the final product In addition, graphitic

polyhe-drons with enclosed metal particles also coexist with CNTs

synthesized by arc discharge and laser ablation as well as

high temperature (>1000 °C) CVD Fullerenes can be easily

re-moved owing to their solubility in certain organic solvents

Amorphous carbon is also relatively easy to eliminate

be-cause of its high density of defects, which allow it to be

oxi-dized under gentle conditions The most knotty problem is

how to remove polyhedral carbons and graphitic particles

that have a similar oxidation rate to CNTs, especially

SWCNTs Metal impurities are usually residues from the

transition metal catalysts These metal particles are

some-times encapsulated by carbon layers (varying from disordered

carbon layers to graphitic shells, as shown inFig 1b and c)

making them impervious and unable to dissolve in acids

An-other problem that needs to be overcome is that

carbona-ceous and metal impurities have very wide particle size

distributions and different amounts of defects or curvature

depending on synthesis conditions, which makes it rather

dif-ficult to develop a unified purification method to obtain

repro-ducibly high-purity CNT materials To fulfill the vast potential

applications and to investigate the fundamental physical and

chemical properties of CNTs, highly efficient purification of

the as-prepared CNTs is, therefore, very important

To evaluate the purity of CNTs, the efficiency of a purification

method as well as changes in the structure of CNTs during

purification, characterization methods with rapid, convenient

and unambiguous features are urgently required

Character-ization of CNT samples falls into three groups: metal catalyst,

carbonaceous impurity, and CNT structure variation (defects,

functional groups, cap opening, cutting, etc.) Their

character-ization mainly depends on electron microscopy (EM, including

scanning EM (SEM), and transmission EM (TEM)),

thermogravi-metric analysis (TGA), Raman spectroscopy and

ultraviolet-visible-near infrared (UV–vis-NIR) spectroscopy

EM is a useful technique allowing for direct observations of

impurities, local structures as well as CNT defects Owing to

the small volume of sample analyzed and the absence of

algo-rithms to convert images into numerical data, EM cannot give

a quantitative evaluation of the purity of CNTs[28]

TGA is effective in evaluating quantitatively the quality ofCNTs, in particular, the content of metal impurity It is easyand straightforward to obtain the metal impurity contentusing TGA by simply burning CNT samples in air A higher oxi-dation temperature (>500 °C) is always associated with purer,less defective CNT samples The homogeneity of CNT samplescan be evaluated by standard deviations of the oxidation tem-perature and metal content obtained in several separate TGAruns [29] The real difficulty is qualitative or quantitativeassessment of carbonaceous impurity, which is influenced

by the amount of defects, forms of carbon, and so on.Raman spectroscopy is a fast, convenient and non-destruc-tive analysis technique To some extent, it can quantify therelative fraction of impurities in the measured CNT sampleusing the area ratio of D/G bands under fixed laser power den-sity In addition, the diameters and electronic structures ofCNTs can be determined by using the resonance Raman scat-tering[30] However, the drawback of Raman spectroscopy isthat it cannot provide direct information on the nature ofmetal impurities, and it is not as effective in studying CNTsamples with a low content of amorphous carbon[31].UV–vis-NIR spectroscopy is a rapid and convenient tech-nique to estimate the relative purity of bulk SWCNTs based

on the integrated intensity of S22transitions compared withthat of a reference SWCNT sample [28] It is convenient todetermine the concentration of SWCNTs dispersed in solu-tion once the extinction coefficient of SWCNTs is known[32] On the other hand, SWCNTs give rise to a series of pre-dictable electronic band transitions between van Hove singu-larities in the density states of nanotubes (S11, S22, and M11),therefore this technique is also used to analyze SWCNT types,i.e., metallic or semiconducting[31,33,34], according to theirelectronic structure For small diameter SWCNTs individuallydispersed in solution with the assistance of surfactants orDNA molecules, the (n, m) index assignment is also possiblefrom UV–vis-NIR spectroscopy[33,34] The drawback of thismethod is the difficulty in repeatedly preparing the standard-ized SWCNT film or solution and controlling film thickness orsolution concentration, making it difficult for quantificationanalysis Furthermore, it is not yet possible to provide anabsolute value of the purity of SWCNTs because there is no100% pure standard SWCNT sample or accurate extinctioncoefficient for SWCNTs

Besides the above most commonly used techniques, X-rayphotoelectron spectroscopy (XPS) is often used to character-ize functional groups on the walls of CNTs, and energy disper-sive spectroscopy (EDS) is also used to semi-quantitativelyidentify the metal content in CNT samples, especially for

Fig 1 – TEM images of (a) amorphous carbon and fullerene molecules on the surface of CNTs[26]; (b) metal nanoparticlescovered by amorphous carbon layer, (c) metal nanoparticles covered by graphitic carbon multi-layer (reproduced withpermission from[27], Copyright 2004 Amercian Chemical Society)

trace amounts The major purity and quality assessment

techniques and their efficiency are summarized inTable 1

It seems that no assessment technique mentioned above

can give a precise and comprehensive quantification of CNTs

(Table 1) Consequently, there is a need to develop an

inte-grated method by which the type, amount, and morphology

of CNT-containing materials can be accurately and precisely

quantified [35] Alternatively, a combination of different

assessment techniques may be a good choice to give a full

understanding of CNTs but this takes more time Furthermore,

a precise definition of purity should be established because

‘‘purity’’ can be different from different points of view, such

as CNT content, structure integrity, and SWCNT content From

this respect, we define the purity of CNTs as given inTable 2

Meanwhile, the major purity assessment techniques and

how to evaluate them are also briefly included

As mentioned above, a series of problems involving the

pres-ence of impurities in CNTs, the non-uniformity in

morphol-ogy and structure of both CNTs and impurities, as well as

the absence of precise characterization methods limit the

applications of CNTs Thus great attention has been paid tothe issue of purification The developed purification schemesusually take advantage of differences in the aspect ratio andoxidation rate between CNTs and carbonaceous impurities

In most cases, CNT purifications involve one or more of thefollowing steps: gas phase oxidation, wet chemical oxida-tion/treatment, centrifugation, filtration, and chromatogra-phy, etc However, a reproducible and reliable purificationprotocol with high selectivity, especially for SWCNTs, is still

a great challenge, because the purity of CNTs depends onnot only purification itself, but also many other factors,including CNT type (SWCNTs or MWCNTs), morphology andstructure (defects, whether or not they exist in bundles, diam-eter), impurity type and their morphology (particle size, de-fect, curvature, the number and crystallinity of carbonlayers wrapping metal particles), purity assessment tech-nique, and so on

This article attempts to give a comprehensive survey andanalysis of the purification of CNTs The challenges existing

in the purification methods, synthesis techniques and purityassessments, which have to be overcome in order to enablethe wide applications of CNTs, will be discussed The purity

in this article generally is referred to as CNT content in the

Table 1 – Summary of commonly used techniques for detecting the impurities in CNT samples

Raman D D J Diameter, quality and conductivity of SWCNTs Invalid for MWCNTs and metal impuritiesUV–vis-NIR J J Conductivity feature and content of SWCNTs Need 100% pure SWCNTs as standard

Table 2 – Definition of purity for CNTs from different points of view and the corresponding assessment techniques

CNT content The content of CNTs in sample containing CNTs,

carbonaceous and metallic impurities

TGAMetal content can be calculated from the ash weight aftercomplete oxidation, and carbonaceous impurity content can becalculated by corresponding peak area ratio from DTG curve.CNTs without any other carbonaceous impurity arecharacterized by one DTG peak

Structure integrity Pure CNTs without large defects and faults, and

no functional groups, amorphous carbon orfullerene adhered on the tube wall

EM in combination with XPS

EM can directly observe and qualitatively assess the amount ofdefects, amorphous carbons, fullerenes adhered on the wall ofCNTs XPS can give a quantitative characterization of type andcontent of functional groups

100% pure SWCNTs should be characterized by one G bandwith RBM and without D band

as-prepared or purified samples, and the yield means the

weight ratio of purified CNTs to that of the as-prepared CNT

sample, unless specified otherwise

2 Purification methods

Purification methods of CNTs can be basically classified into

three categories, namely chemical, physical, and a

combina-tion of both The chemical method purifies CNTs based on

the idea of selective oxidation, wherein carbonaceous

impuri-ties are oxidized at a faster rate than CNTs, and the

dissolu-tion of metallic impurities by acids This method can

effectively remove amorphous carbon and metal particles

ex-cept for those encaged in polyhedral graphitic particles

How-ever, the chemical method always influences the structure of

CNTs due to the oxidation involved The physical method

separates CNTs from impurities based on the differences in

their physical size, aspect ratio, gravity, and magnetic

proper-ties, etc In general, the physical method is used to remove

graphitic sheets, carbon nanospheres (CNSs), aggregates or

separate CNTs with different diameter/length ratios In

prin-ciple, this method does not require oxidation, and therefore

prevents CNTs from severe damage However, the physical

method is always complicated, time-consuming and less

effective The third kind of purification combines the merits

of physical and chemical purification, and we denominate it

as multi-step purification in this article This method can lead

to high yield and high-quality CNT products Owing to the

diversity of the as-prepared CNT samples, such as CNT type,

CNT morphology and structure, as well as impurity type and

morphology, it needs a skillful combination of different

puri-fication techniques to obtain CNTs with desired purity

The carbonaceous impurities co-existing with as-synthesized

CNTs are mainly amorphous carbon and CNPs Compared

with CNTs, these impurities usually have higher oxidation

activity The high oxidative activity demonstrated by

amor-phous carbon is due to the presence of more dangling bonds

and structural defects which tend to be easily oxidized;

meanwhile the high reactivity of the CNPs can be attributed

to their large curvature and pentagonal carbon rings[36,37]

Therefore, chemical oxidation purification is based on the

idea of selective oxidation etching, wherein carbonaceous

impurities are oxidized at a faster rate than CNTs In general,

chemical oxidation includes gas phase oxidation (using air,

O2, Cl2, H2O, etc.), liquid phase oxidation (acid treatment

and refluxing, etc.), and electrochemical oxidation The

disad-vantages of this method are that it often opens the end of

CNTs, cuts CNTs, damages surface structure and introduces

oxygenated functional groups (–OH, –C@O, and –COOH) on

CNTs As a result, the purified CNTs in turn can serve as

chemical reactors or a starting point for subsequent nanotube

surface chemistry[38,39]

2.1.1 Gas phase oxidation

In gas phase oxidative purification, CNTs are purified by

oxi-dizing carbonaceous impurities at a temperature ranging

from 225 °C to 760 °C under an oxidizing atmosphere The

commonly used oxidants for gas phase oxidation include air[40–46], a mixture of Cl2, H2O, and HCl[47], a mixture of Ar,

O2, and H2O [48–50], a mixture of O2, SF6 and C2H2F4 [51],

H2S and O2[52], and steam[53].High temperature oxidation in air is found to be an extre-mely simple and successful strategy for purifying arc dis-charge derived MWCNTs, which are metal free and havefewer defects on tube walls Ebbesen et al [40,41] first re-ported a gas phase purification to open and purify MWCNTs

by oxidizing the as-prepared sample in air at 750 °C for

30 min However, only a limited amount of pure MWCNTs(1–2 wt%) remained after the above purification, which can

be ascribed mainly to two reasons One is uneven exposure

of CNTs to air during oxidation, and the other is the limitedoxidation selectivity between CNTs and carbonaceous impu-rities Therefore, two routes may be helpful to increase thepurification yield using this simple air oxidation One is to en-sure that the as-synthesized CNT samples are evenly exposed

to air, and the other is to enhance the difference in oxidationresistance to air between CNTs and carbonaceous particles.The above suggestions have been verified by some research-ers As an example, Park and coworkers [42]increased thepurification yield to 35 wt% by rotating the quartz tube inwhich the sample was placed, in order to evenly expose theCNTs and carbonaceous impurities to air at 760 °C for 40 min

To increase the difference in oxidation resistance to air tween MWCNTs and carbon impurities, the difference in oxi-dation rates of graphite and intercalated graphite[43–45]wastaken into account Graphite intercalation compounds areformed by the insertion of atomic or molecular layers of otherchemical species between graphite layers This interactioncauses an expansion of carbon interlayer spacing, which re-duces the oxidation resistance of the intercalated graphite.Carbonaceous impurities have higher structural defect densi-ties than CNTs, and are therefore more ready to act as reac-tion sites for intercalated atoms Thus the oxidationresistance difference between CNTs and carbonaceous impu-rities can be increased As an example, Chen et al [43]re-ported a combined purification process consisting ofbromination and subsequent selective oxidation with oxygen

be-at 530 °C for 3 days Temperbe-ature programmed oxidbe-ation files of the CNT samples with and without bromine treatmentare shown inFig 2 It is obvious that oxidation of the bromi-nated sample occurs more readily than that without bromin-ation TEM studies showed that CNTs with both ends openwere enriched in the purified sample, and the yield obtained

pro-by the above process varied from 10 to 20 wt% with respect

to the weight of the original carbon sample Furthermore,they found that the yield depended crucially on the flow rate

of oxidant, the amount of initial sample, the manner of ing of the carbon, and the quality of the cathodic soot.Although MWCNTs can be purified by a variety of gasphase oxidation [41–45], attempts to use similar proceduresfor SWCNTs result in nanotubes etching away For example,using the bromine and oxygen system, the yield was

pack-3 wt%[47] for SWCNT purification, which implies that alarge fraction of SWCNTs are consumed in the process Thislarge difference between MWCNTs and SWCNTs results fromtwo factors One is the larger amount of curvature experi-enced by the graphene sheet of SWCNTs, and the other is

metal impurities catalyzing the low-temperature oxidation of

carbon There may therefore be two ways to increase the

puri-fication yield of SWCNTs using gas phase oxidation One is to

select oxidants that can selectively oxidize carbonaceous

impurities by a unique selective carbon surface chemistry

while leaving SWCNTs intact The other is to remove metal

particles before gas phase oxidation Some positive results

have been obtained following the above suggestions

Zimmerman et al [47] first reported suitable conditions

allowing for the removal of amorphous or spherical carbon

particles, with or without metal catalyst inside, while

simul-taneously protecting SWCNTs The purification incorporates

a chlorine, water, and hydrogen chloride gas mixture to

re-move the impurities A SWCNT yield of 15 wt% and a purity

of 90% indicate that the carbonaceous impurities are

prefer-entially removed Based on their experimental observation,

hydrogen chloride was required for selective removal of the

unwanted carbon They proposed a mechanism for the

puri-fication Chlorine gas mixture interacted with the nanotube

cap and formed a hydroxy-chloride-functionalized nanotube

cap Hydrogen chloride in the gas phase purification mixture

protected the caps that are more reactive, by preventing

hy-droxyl groups from deprotonating The disadvantage of this

method is that only small quantities (5 mg) of SWCNTs were

purified each time Furthermore, the reagents and produced

gases are toxic and explosive, which limits its practical use

At the same time, some other oxidants that can selectively

oxidize carbonaceous impurities were also reported For

example, hydrogen sulfide was reported to play a role in

enhancing the removal of carbon particles as well as

control-ling the oxidation rate of carbon A purity of 95% SWCNTs

with a yield of 20–50 wt% depending on the purity of raw

material was reported[52] In addition, steam at 1 atm

pres-sure [53], local microwave heating in air [46], air oxidation

and acid washing followed by hydrogen treatment[54]were

also reported to work well to improve the purification yield

It was Chiang et al.[48,49]who clearly elucidate the role of

metals in oxidizing carbons and the need for their prior

re-moval They found that metal particles catalyze the oxidation

of carbons indiscriminately, destroying SWCNTs in the

pres-ence of oxygen and other oxidizing gases Encapsulated metal

particles can be exposed using wet Ar/O2(or wet air) oxidation

at 225 °C for 18 h This exposure was attributed to the

expan-sion of the particles because oxidation products have a muchlower density (the densities of Fe and Fe2O3 are 7.86 and5.18 g/cm3, respectively) Such significant expansion brokethe carbon shells, and the particles were exposed as a result.Based on the above results, they proposed a multi-stage proce-dure for purifying SWCNTs synthesized by the HiPco process.Their method begins with cracking of the carbonaceous shellsencapsulating metal particles using wet oxygen (20% O2in ar-gon passed through a water-filled bubbler) at 225 °C, followed

by stirring in concentrated hydrochloric acid (HCl) to dissolvethe iron particles After filtering and drying, the oxidationand acid extraction cycle was repeated once more at 325 °C,followed by an oxidative baking at 425 °C Finally, 99.9% pureSWCNTs (with respect to metal content) with a yield of

30 wt% were obtained The validity of this method was fied by another group[50] However, owing to the complicatedpurification steps, it is hard to purify SWCNTs in a large scale

veri-Xu et al [51]developed a controlled and scalable step method to remove metal catalyst and non-nanotube car-bons from raw HiPco SWCNTs Their scalable multi-step puri-fication included two processes: oxidation and deactivation ofmetal oxides In the oxidation, metal catalysts coated by non-nanotube carbon were oxidized into oxides by O2and exposed

multi-by using a multi-step temperature increase program In thedeactivation step, the exposed metal oxides were deactivated

by conversion to metal fluorides through reacting with

C2H2F4, SF6, or other fluorine-containing gases to avoid thecatalytic effect of iron oxide on SWCNT oxidation The Fe con-tent was remarkably decreased from 30 to 1 wt% and ayield of 25–48 wt% was achieved However, the shortcoming

of this method is that it is limited to HiPco SWCNTs, in whichthe dominant impurity is metal catalyst Furthermore, thetoxicity of the reagents used in this method and the resultinggases are undesirable features

Gas phase oxidation is a simple method for removing bonaceous impurities and opening the caps of CNTs withoutvigorously introducing sidewall defects, although it cannot di-rectly get rid of metal catalyst and large graphite particles.Thus it is a good choice to purify arc discharge derivedMWCNTs, which contains no metal catalyst For purifyingSWCNTs or MWCNTs (synthesized by other techniques), acidtreatment to remove the metal catalyst is always necessary.Another point worth noting is that CNTs (SWCNTs in particu-lar) in agglomerates prevent oxidant gas from homogeneouslycontacting the whole sample In order to obtain high-purityCNTs, the amount of sample to be purified each time is quitelimited (tens to a hundred milligrams) Therefore, methodsthat can cause the oxidant gas to homogeneously contactCNT samples are urgently required to obtain high-purity CNTs

car-on a large scale Recently, Tan et al.[55]mixed raw SWCNTswith zirconia beads to enhance air flow uniformity and in-crease the exposed surface of raw soot during thermal oxida-tion in air The final purified samples had a yield of 26 wt%and a metal impurity of 7% Although the purity is not veryhigh, the technique suggests a way to purify SWCNTs on alarge scale using gas phase oxidation This method can providepure and opened CNTs without heavily damaging tube walls,which is a good choice for the application of open-ended CNTs

as nano-size reaction tubes or chemical reactors [56,57] Forachieving purified CNTs on a large scale, gas phase oxidation

Fig 2 – Temperature programmed oxidation profiles of the

cathodic soot before (CS) and after (BS) bromination

(reprinted with permission from[43], Copyright 1996 Wiley–

VCH Verlag GmbH & Co KGaA)

need to be modified in the following ways: one is to look for a

simple approach and non-toxic reagents to remove metal

par-ticles encapsulated by carbon layers; the other is to look for a

way that can make oxidant gas homogeneously contact the

as-prepared CNTs In addition, the gas phase oxidation can

be combined with other techniques, such as filtration or

cen-trifugation, to further enhance the purification efficiency

2.1.2 Liquid phase oxidation

Although the merits of gas phase oxidation are obvious, it has

a drawback that metal particles cannot be directly removed,

and further acid treatment is needed In order to overcome

this limitation, liquid phase purification that always

simulta-neously removes both amorphous carbon and metal catalyst

was developed Oxidative ions and acid ions dissolved in

solu-tion can evenly attack the network of raw samples, and

there-fore selection of oxidant type and precise control of treatment

condition can produce high-purity CNTs in a high yield The

commonly used oxidants for liquid phase oxidation include

HNO3[58–60], H2O2or a mixture of H2O2and HCl[61–63], a

mixture of H2SO4, HNO3, KMnO4 and NaOH [64–67], and

KMnO4[67–69] The shortcomings of this method are that it

causes reaction products on the surface of CNTs, adds

func-tional groups, and destroys CNT structures (including cutting

and opening CNTs)

Nitric acid is the most commonly used reagent for SWCNT

purification for its mild oxidation ability, which can

selec-tively remove amorphous carbon In addition, it is

inexpen-sive and nontoxic, capable of removing metal catalysts and

no secondary impurities are introduced

Dujardin et al.[58]reported a one-step method using

con-centric nitric acid to purify SWCNTs synthesized by laser

abla-tion Briefly, as-synthesized SWCNTs were sonicated in

concentrated nitric acid for a few minutes followed by

reflux-ing under magnetic stirrreflux-ing at 120–130 °C for 4 h The yield

reached 30–50 wt% of the raw sample and the metal amount

was decreased to 1 wt% One problem in the above

purifica-tion is that the permeapurifica-tion rate during filtrapurifica-tion was very

low because SWCNTs packed together and the filter

mem-brane was blocked This makes it difficult to purify CNTs on

a large scale, and some small carbonaceous impurity particles

cannot permeate the filter To solve this problem, Rinzler et al

[59] adopted hollow-fiber cross-flow filtration (CFF) to filtrate

SWCNTs that had been refluxed in 2.6 M HNO3for 45 h Highly

pure SWCNTs with a yield of 10–20 wt% were obtained with

this readily scalable method, which opens up a way to purify

SWCNTs on a large scale Even though the effectiveness of

ni-tric acid treatment on the purification of SWCNTs is

con-firmed, the relationship between purification yield and purity

with systematic and quantitative measurements was not

re-ported before Hu et al.’s work[60] They established a

system-atic and quantitative relationship between yield and purity by

using solution phase NIR spectroscopy In their experiments,

1 g of the as-prepared SWCNT sample was refluxed in 3 M

ni-tric acid for 12, 24 and 48 h, in 7 M nini-tric acid for 6 and 12 h,

and in concentrated nitric acid for 6 and 12 h The weight

per-cent of each component calculated from TGA and NIR spectra

is plotted inFig 3 It is clear that the purity and the yield of

SWCNTs with nitric acid treatment depend on the

concentra-tion of the nitric acid and the time of reflux The treatments of

3 M HNO3for 12 h and 7 M HNO3for 6 h were the most efficient.Nitric acid treatment destroys SWCNTs, leading to the produc-tion of carbonaceous impurities Nevertheless, with the ability

to dissolve the metal catalyst, intercalate SWCNT bundles, tack amorphous carbon, and break large carbon particles, thenitric acid treatment can be a viable first step for SWCNT puri-fication The key to achieving high-purity SWCNTs is a subse-quent process for removing the carbonaceous impurities thatremain in the sample after nitric acid treatment In this case,

at-a preferred step is hollow-fiber CFF[59].Hydrogen peroxide (H2O2) is also a mild, inexpensive andgreen oxidant, which can attack the carbon surface The disad-vantage of H2O2is also obvious It cannot remove metal parti-cles Therefore, it is usually used together with HCl HCl is awidespread chemical that can be easily converted into a harm-less salt Therefore, purifying CNTs using H2O2followed by HCltreatment to remove metal particles has also been intenselyinvestigated Macro-scale purification, including a first reflux-ing treatment in H2O2 solution and then rinsing with HCl,was reported by Zhao et al.[61,62] Their experimental resultsshowed that the size of Fe particles has a great influence onthe oxidation of amorphous carbon However, this was still aquestion about the effect of Fe before Wang’s work[63].Wang et al.[63]tried to explain the above question Theycombined two known reactions (oxidation of amorphous car-bon with H2O2and removal of metal particles with HCl) into asingle pot, which simplified the process Surprisingly, theproduct yield and purity were improved Typically, carbon-coated iron impurities were simply dissolved by reacting with

an aqueous mixture of H2O2and HCl at 40–70 °C for 4–8 h.With this treatment, the purification yield was significantlyincreased to 50 wt% and the purity was up to 96 wt%.According to Wang, the effect of this process on the purifica-tion can be summarized as following First, Fe particles act as

a catalyst by Fenton chemistry[70], producing hydroxyl cals (OH), a more powerful oxidant than H2O2 Second, HCldissolves the iron nanoparticles upon their exposure The ex-posed iron releases ferrous ions as a result of dissolution of

radi-Fig 3 – Mass balance of the normalized weight percentage ofall components including SWCNTs, metal, carbonaceousimpurities, and weight loss of the SWCNT samples(reprinted with permission from[60], Copyright 2003Amercian Chemical Society)

the Fe particles in the acid solution The ferrous ions quickly

diffuse into the acid solution, thereby eliminating iron and

iron hydroxide precipitation and their unwanted catalytic

ef-fect (Fig 4) Therefore, by confining the catalytic effect to the

vicinity of the carbon-coated iron nanoparticles, both a high

selectivity in removing iron particles and carbonaceous shells

and a low consumption of SWCNTs are accomplished

At almost the same time, microwave-assisted inorganic

acid treatment for the effective removal of metal particles

was reported [71–76] The principle of this method is that

inorganic acids such as HNO3, HCl, and H2SO4can rapidly

ab-sorb microwave energy and dissolve metals efficiently

with-out damaging the tube wall structure in a short time

As discussed above, HNO3, H2O2, as well as

microwave-as-sisted inorganic acid treatments can effectively remove metal

particles, but they are not so effective in removing

carbona-ceous particles owing to the relative mildness in their tion In order to get rid of carbonaceous impurities, liquidoxidants with stronger oxidation activity were also investi-gated These oxidants are predominantly mixture of acidsand KMnO4

oxida-Liu et al.[64]use a mixture of concentrated H2SO4/HNO3(3:1 by volume) to cut highly tangled long ropes of SWCNTsinto short, open-ended pipes, and thus produced many car-boxylic acid groups at the open ends Wiltshire et al.[65]re-ported that liquid phase oxidation could be a continuousdiameter-selective process, eliminating SWCNTs with smallerdiameter by oxidizing the sidewalls Li and coworkers [66]investigated the purification effectiveness of concentrated

H2SO4/HNO3 (3:1) treatment and compared this with 6 MHNO3 treatment Typical TEM images of purified SWCNTsafter different treatment conditions are shown inFig 5, fromwhich it can be concluded that concentrated H2SO4/HNO3(3:1) is more effective than nitric acid in removing impurities.Furthermore, it was reported that the best purification condi-tion could reach 98% purity of SWCNTs with a yield of 40 wt%within 2 h, without decreasing the number of small diameternanotubes for a 3 h reflux process using a concentrated

H2SO4/HNO3mixture (3:1)

Colomer et al.[68]reported an effective method for ing amorphous carbon by refluxing as-prepared MWCNTs inacidified KMnO4 at low-temperature (80 °C) According tothem, amorphous carbon was completely removed at the cost

remov-of more than 60% carbon loss TEM observation remov-of the purifiedCNTs indicated that all amorphous carbon aggregates wereremoved and the CNT caps were opened Hernadi et al.[69]verified the above conclusion They obtained MWCNTs withoxygen functional groups which were free from amorphouscarbon by KMnO4 oxidation Zhang et al [67] investigatedthe effect of KMnO4in alkali solution on the purification ofSWCNTs KMnO4in alkali solution is a much more moderate

Fig 5 – TEM images of purified SWCNTs: (a) sonication in 6 M HNO3for 4 h, (b) refluxing in 6 M HNO3at 120 °C for 4 h,(c) refluxing in concentrated H2SO4/HNO3mixture (3:1) at 120 °C for 2 h, (d) refluxing in concentrated H2SO4/HNO3mixture

at 120 °C for 4 h (reprinted with permission from[66], Copyright 2004 Institute of Physics Publishing)

Fig 4 – Scheme of localized catalytic reaction of H2O2with

carbon-coated iron nanoparticles (not drawn to scale,

reprinted with permission from[63], Copyright 2007

Amercian Chemical Society)

oxidant than in acidic solution The solution cannot

effec-tively open the tube, while it is strong enough to attack the

nanotube walls and generate abundant functional groups

The problem of this process is that additional steps are

needed to remove the MnO2generated during the oxidation

It is desirable to remove carbonaceous impurities by

convert-ing them into soluble or volatile products, and from this point

of view, KMnO4seems to be a less suitable oxidation agent

Liquid phase oxidation is a continuous process that can

eliminate impurities on a large scale, and it is hoped that it

can be widely used for industrial application in the future

This method often leads to surface modification that

prefer-entially takes place on CNT sidewalls, which increases the

chemical activity and the solubility of CNTs in most organic

and aqueous solvents This surface modification effect shows

great potential for improving their physical and chemical

properties for specific applications, e.g., in making

mechani-cally reinforced composites, in use as scanning probe

micros-copy tips with tailored chemical sensitivity, and in producing

nanotube derivatives with altered electronic structures and

properties[77–80] Furthermore, CNTs can be cut into short

fragments decorated with oxygen functional groups under

suitable treatment conditions, which greatly increases their

dispersibility and facilitates their practical applications For

example, the application of CNTs in the field of emerging

bio-technology is based on the premise that short CNTs are

dis-persible in water [81,82] The main problem of this liquid

oxidation strategy is the damage to CNTs, the inability to

re-move large graphite particles, and the loss of a large amount

of SWCNTs with small diameter It is very difficult to obtain

purified SWCNTs with high-purity and high yield without

damage by simply using liquid phase oxidation

2.1.3 Electrochemical oxidation

As with liquid phase oxidation and gas phase oxidation,

car-bon materials with fewer defects usually show a lower

corro-sion rate under electrochemical oxidation Therefore, it is

reasonable to deduce that CNTs with fewer defects should

show higher electrochemical oxidation resistance than

car-bon impurities with more defects

Fang et al.[27]investigated the electrochemical cyclic

vol-tammetric (CV) oxidation behavior of an arc discharge derived

SWCNT sample in KOH solution Amorphous carbon in the

as-grown SWCNT sample was effectively removed by the CV

oxidation, as confirmed by analyzing the sp3/sp2carbon ratio

from C1sXPS spectra and TEM observations The removal of

amorphous carbon led to the exposure of metal

nanoparti-cles, hence facilitating the elimination of the metal impurities

by subsequent HCl washing The redox peaks from the

elec-trochemical redox reactions of Fe and Ni impurities can be

considered as an indication of the extent of removal of the

amorphous carbon, and the optimum electrochemical

oxida-tion time for the purificaoxida-tion of the as-grown SWCNT sample

can be determined in real time during the CV oxidation

treatment

The above electrochemical oxidation was performed in

KOH solution, which needs further acid treatment to remove

metal particles This makes the purification complex If the

solution is acidic, the post-treatment should be omitted,

which makes the purification easier Ye et al.[83]verified this

They recently reported an ultra-fast and complete openingand purification of MWCNTs through electrochemical oxida-tion in acid solution The vertically aligned MWCNT (with her-ringbone structure) arrays investigated were grown on acarbon microfiber network through DC plasma-enhancedCVD Electrochemical oxidation for tip opening and purifica-tion of MWCNT arrays was performed in an aqueous solution

of 57% H2SO4at room temperature SEM and TEM images fore and after purification (Fig 6) indicated that the CNT tipswere opened, and entrapped metals were removed during theelectrochemical oxidation The results of inductively coupledplasma-mass spectrometry indicate that 98.8% of the Ni wasremoved after the electrochemical oxidation in acid Theauthors also investigated a series of electrolyte solutions forelectrochemical opening of CNT tips at room temperature.They concluded that if electrochemical oxidation was per-formed in neutral or basic aqueous solutions, no significanttip opening was observed If aqueous solutions of commonstrong or medium strength acids (5% H2SO4, 5% HNO3, or25% HNO3+ 25% H2SO4, 5% H3PO4 and 5% CH3COOH) wereused, not only can the amorphous carbon be readily etchedbut also the metal catalyst can be dissolved

be-Superior to the gas phase oxidation and wet oxidation, theoptimum time and degree of electrochemical oxidation forCNT purification can be easily determined This method canget rid of impurities to some extent, particularly for selec-tively opening and purifying vertically aligned CNT arrays.The desired vertical orientation can be maintained and facil-itates the use of CNT arrays as fuel cell electrodes, sensorplatforms, nanoreactors, field emitter components, and otherapplications However, little polyhedral carbon, graphite par-ticles, and metal particles enwrapped by carbon layers withfewer defects can be removed by the CV oxidation Moreover,the purity of the obtained sample greatly depends on thestarting materials, and the amount of sample purified foreach batch is too small to make the method practical

Chemical-based purification can effectively remove phous carbon, polyhedral carbon, and metal impurities atthe expense of losing a considerable amount of CNTs ordestroying CNT structures Gas phase purification is charac-terized by opening the caps of CNTs without greatly increas-ing sidewall defects or functional groups Liquid phaseoxidation introduces functional groups and defects preferen-tially on CNT side walls, and may cut CNTs into shorter oneswith different lengths The electrochemical oxidation is suit-able for purifying CNT arrays without destroying their align-ment These features allow chemical purification adopted byresearchers to fulfill different requirements The most seriousproblem of this technique is that the structure of CNTs may

amor-be destroyed by the reactants, and hence limits the tions of CNTs in some fields, for example, electronic devices

applica-2.2 Physical-based purification

A big problem in chemical purification is that it always stroys the structure of CNTs or changes their natural surfaceproperties To elucidate the inherent physical and chemicalproperties of CNTs, purifications that do not involve oxidative

de-treatment are highly desirable The morphology and physical

properties of CNTs, such as aspect ratio, physical size,

solubil-ity, gravsolubil-ity, and magnetism are different from impurities

These differences enable one to separate CNTs from

impuri-ties by adopting some physical techniques Therefore,

physi-cal-based methods including filtration, chromatography,

centrifugation, electrophoresis, and high temperature (1400–

2800 °C) annealing, have been extensively investigated The

most striking feature of these methods is a non-destructive

and non-oxidizing treatment Another feature is that the

purifications are mostly performed in solution, which

re-quires the as-prepared samples to have a good dispersibility

in the solutions To meet this requirement, surfactants and/

or sonication are often used

2.2.1 Filtration

Separation by filtration is based on the differences in physical

size, aspect ratio, and solubility of SWCNTs, CNSs, metal

ticles and polyaromatic carbons or fullerenes Small size

par-ticles or soluble objects in solution can be filtered out, and

SWCNTs with large aspect ratio will remain Polyaromatic

carbons or fullerenes are soluble in some organic solvents,

such as CS2, toluene, etc The impurities can be easily

re-moved by immersing the as-prepared sample in these organic

solutions followed by filtering The impurity particles smaller

than that of the filter holes flow out with the solution during

filtration, while large impurity particles and small ones

adhering to the CNT walls remain One problem of this

tech-nique is that CNTs or large particles deposited on the filter

of-ten block the filter, making the filtering prohibitively slow and

inefficient Therefore, a stable suspension of CNTs and a nique preventing them from deposition and aggregation arevery important during filtration Thus, surfactants are widelyused to make a stable CNT suspension, and ultrasonication isusually adopted to prevent the filter from being blocked.Bonard et al.[84]first applied filtration assisted with sonica-tion to purify MWCNTs The as-prepared MWCNTs were dis-persed in water with sodium dodecyl sulfate (SDS), and astabilized colloidal suspension was formed The suspensionwas filtered using a filtration apparatus with a funnel large en-ough to allow the sonication of the colloidal suspension toextract larger particles In order to enhance the separa-tion yield, successive filtrations were carried out until the de-sired purity is reached Shelimov et al [85]used the aboveprocedure to purify SWCNTs and obtained SWCNT materialwith a purity of more than 90% (estimated by EM) and a yield

tech-of 30–70 wt%

Bandow et al.[86]developed a purification process (shown

inFig 7) consisting of filtration and microfiltration under anoverpressure (2 atm) of N2to separate CNSs, metal nanopar-ticles, polyaromatic carbons and fullerenes from SWCNTs.The microfiltration was repeated three times to minimizethe amount of residual CNSs and metal particles Using thistechnique, 84, 10, and 6 wt% of purified SWCNTs, CNSs,and CS2extracts were separated from the as-prepared laser-synthesized SWCNTs

A major advantage of filtration is that it is driven by purephysicochemical interactions of carbon products with amphi-philic molecules and the filter membrane, leaving the nano-tubes undamaged However, this procedure relies on thequality of raw samples and is time-consuming In addition,Fig 6 – SEM (a, b) and TEM (c, d) images of MWCNT arrays: (a, c) as-grown, (b, d) purified (reprinted with permission from[83], Copyright 2006 Amercian Chemical Society)

amorphous and spherical carbon particles stuck on the tube

walls cannot be effectively removed

2.2.2 Centrifugation

Centrifugation is based on the effect of gravity on particles

(including macromolecules) in suspension because two

parti-cles of different masses settle in a tube at different rates in

re-sponse to gravity On the other hand, centrifugation can also

separate amorphous carbon and CNPs based on the different

stabilities in dispersions consisting of amorphous carbon,

SWCNTs, and CNPs in aqueous media The different

stabili-ties resulted from the different (negative) surface charges

introduced by acid treatment[87,88] Low-speed

centrifuga-tion (2000g) is effective in removing amorphous carbon and

leaving SWCNTs and CNPs in the sediment High-speed

cen-trifugation (20000g) works well in settling CNPs, while leaving

SWCNTs suspended in aqueous media The effectiveness of

centrifugation in separating SWCNTs from amorphous

car-bon and CNPs is shown inFig 8 The drawback of this process

is that CNTs need to be first treated with nitric acid, whichintroduces functional groups on their surface

2.2.3 Solubilization of CNTs with functional groupsThe principle of this purification step is to solubilize CNTs byintroducing functional groups onto their surface These solu-ble nanotubes allow for the application of other techniquessuch as filtration or chromatography as a means of tube puri-fication To regain reasonable quantities of un-functionalizedbut purified nanotubes, the functional groups should be re-moved by thermal treatment or other techniques

Coleman et al [89,90] described a one-step, high yield,nondestructive purification for MWCNTs containing sootusing a conjugated organic polymer host (poly(m-pheny-lene-co-2,5-dioctoxy-p-phenylenevinylene (PmPV)) in tolu-ene PmPV is shown to be capable of suspending nanotubesindefinitely whilst the accompanying graphitic particles settleout Finally the host polymer was removed by Buchner filtra-tion, giving CNTs with a purity of 91% (estimated from elec-tron paramagnetic resonance) In this case, a yield of 17 wt%pristine nanotubes was reclaimed from the soot

Yudasaka et al.[91]mixed as-grown SWCNTs with a 2%monochlorobenzene (MCB) solution of polymethylmethacry-late (PMMA) with an ultrasonic cleaner The mixture washomogenized through an ultrasonic-homogenizer and fil-tered The MCB was removed by evaporation at 150 °C, andPMMA was removed by burning it off in 200-Torr oxygen gas

at 350 °C At the same time, azomethine ylides and solutionphase ozonolysis ( 78 °C) were also reported to solubilizeCNTs via 1,3 dipolar cycloaddition[92–94]

Recently, Jeynes et al.[95]and Sanchez-Pomales et al.[96]reported a method for purifying CNTs using RNA and DNA.Briefly, arc discharge derived CNTs were sonicated in deion-ized water at 0 °C (in an ice-water bath) for 30 min with0.5 mg/mL total cellular RNA The solution was then centri-fuged to pellet the insoluble particles RNA-wrapped CNTswere treated with enzyme ribonuclease to remove the RNAand thereby precipitate the CNTs Jeynes et al.[95]also sug-gested that RNA/DNA was more efficient in solubilizing CNTsthan SDS as there is a large surface area of phosphate back-bone which interacts with water, while similarly there aremany bases to bind the CNTs

The advantage of this process is that it can always serve the surface electronic structure of CNTs This property

pre-Fig 8 – TEM images of (a) SWCNT-COOH material showing embedded catalyst particles, (b) purified SWCNT-COOH fraction,and (c) carbon particle fraction (reprinted with permission from[88], Copyright 2006 Amercian Chemical Society)

Extract Solids caught

Fig 7 – A diagram illustrating the technique used for

separating coexisting CNSs, metal nanoparticles, and

polyaromatic carbons or fullerenes from the

laser-synthesized SWCNTs (modified with permission

from[86], Copyright 1997 Amercian Chemical Society)