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Purification of carbon nanotubes
Peng-Xiang Hou, Chang Liu, Hui-Ming Cheng*
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 2008Accepted 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.
2008 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
2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2008.09.009
* Corresponding author: Fax: +86 24 2390 3126.E-mail address:[email protected](H.-M. Cheng).
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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
1. Introduction
Elemental carbon in sp2 hybridization 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 diameters and large length/diameter 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 sp2 bonds
formed between individual carbon atoms, CNTs are stiffer
and stronger potentially than any other known materials.Thus, CNTs have ultra-high Youngs 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(qis 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].
1.1. CNT synthesis techniques
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
3001200 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 3050 wt%, while the content ofMWCNTs is in the range of 3099 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 catalysts 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 catalyst in a flow of inert gas. This method favors the
growth of SWCNTs with controlled diameter depending onreaction 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].
1.2. Impurities coexisting with CNTs
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
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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.
1.3. Assessment of CNT purity
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 (UVvis-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 of
CNTs, in particular, the content of metal impurity. It is easy
and straightforward to obtain the metal impurity content
using TGAby simplyburning CNTsamples in air. A higher oxi-
dation temperature (>500 C) is always associated with purer,
less defective CNT samples. The homogeneityof CNT samples
can be evaluated by standard deviations of the oxidation tem-
perature and metal content obtained in several separate TGA
runs [29]. The real difficulty is qualitative or quantitative
assessment 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 the
relative fraction of impurities in the measured CNT sample
using the area ratio of D/G bands under fixed laser power den-
sity. In addition, the diameters and electronic structures of
CNTs can be determined by using the resonance Raman scat-
tering[30]. However, the drawback of Raman spectroscopy is
that it cannot provide direct information on the nature of
metal impurities, and it is not as effective in studying CNT
samples with a low content of amorphous carbon[31].
UVvis-NIR spectroscopy is a rapid and convenient tech-
nique to estimate the relative purity of bulk SWCNTs based
on the integrated intensity of S22 transitions compared with
that of a reference SWCNT sample [28]. It is convenient to
determine 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 their
electronic structure. For small diameter SWCNTs individually
dispersed in solution with the assistance of surfactants or
DNA molecules, the (n, m) index assignment is also possible
from UVvis-NIR spectroscopy [33,34]. The drawback of this
method is the difficulty in repeatedly preparing the standard-
ized SWCNT film or solution and controlling film thickness or
solution concentration, making it difficult for quantification
analysis. Furthermore, it is not yet possible to provide an
absolute value of the purity of SWCNTs because there is no
100% pure standard SWCNT sample or accurate extinction
coefficient for SWCNTs.
Besides the above most commonly used techniques, X-ray
photoelectron 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-quantitatively
identify 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 nanoparticles
covered by amorphous carbon layer, (c) metal nanoparticles covered by graphitic carbon multi-layer (reproduced with
permission from[27], Copyright 2004 Amercian Chemical Society).
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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 CNTsbut 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.
1.4. Purpose of this review
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 to
the issue of purification. The developed purification schemes
usually take advantage of differences in the aspect ratio and
oxidation rate between CNTs and carbonaceous impurities.
In most cases, CNT purifications involve one or more of the
following steps: gas phase oxidation, wet chemical oxida-
tion/treatment, centrifugation, filtration, and chromatogra-
phy, etc. However, a reproducible and reliable purification
protocol with high selectivity, especially for SWCNTs, is still
a great challenge, because the purity of CNTs depends on
not only purification itself, but also many other factors,
including CNT type (SWCNTs or MWCNTs), morphology and
structure (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 carbon
layers wrapping metal particles), purity assessment tech-
nique, and so on.
This article attempts to give a comprehensive survey and
analysis of the purification of CNTs. The challenges existing
in the purification methods, synthesis techniques and purity
assessments, which have to be overcome in order to enable
the 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
Technique C-Ia M-Ib F-Gc S-Dd C-Fe Advantages Limitations
EM Df D g Jh Direct observation A small amount of sample is analyzed
TGA J J Precise content of carbon and metals CNTs analyzed are completely destroyed
Raman D D J Diameter, quality and conductivity of SWCNTs Invalid for MWCNTs and metal impurities
UVvis-NIR J J Conductivity feature and content of SWCNTs Need 100% pure SWCNTs as standard
XPS J Accurate assessment of F-G on CNTs Invalid to purity assessment
EDS J Elemental contents, special for trace amounts Invalid to evaluate CNT content
a Carbonaceous impurity.
b Metal impurity.
c Functional groups.
d Structure defects.
e Conductivity feature.
f Qualitatively valid.
g Invalid.
h Valid.
Table 2 Definition of purity for CNTs from different points of view and the corresponding assessment techniques
Purity definition Assessment technique and methods
CNT content The content of CNTs in sample containing CNTs,
carbonaceous and metallic impurities
TGA
Metal content can be calculated from the ash weight after
complete oxidation, and carbonaceous impurity content can be
calculated by corresponding peak area ratio from DTG curve.
CNTs without any other carbonaceous impurity are
characterized by one DTG peak
Structure integrity Pure CNTs without large defects and faults, and
no functional groups, amorphous carbon or
fullerene adhered on the tube wall
EM in combination with XPS
EM can directly observe and qualitatively assess the amount of
defects, amorphous carbons, fullerenes adhered on the wall of
CNTs. XPS can give a quantitative characterization of type and
content of functional groups
SWCNT conten t The content of SWCNTs in CNTs Raman spectroscopy
100% pure SWCNTs should be characterized by one G band
with RBM and without D band
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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.
2.1. Chemical oxidation
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
[4046], a mixture of Cl2, H2O, and HCl[47], a mixture of Ar,
O2, and H2O [4850], 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 have
fewer 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 750C for
30 min. However, only a limited amount of pure MWCNTs
(12 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 limited
oxidation selectivity between CNTs and carbonaceous impu-
rities. Therefore, two routes may be helpful to increase the
purification 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 oxidation
resistance 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 the
purification yield to 35 wt% by rotating the quartz tube in
which the sample was placed, in order to evenly expose the
CNTs and carbonaceous impurities to air at 760 C for 40 min.
To increase the difference in oxidation resistance to air be-
tween MWCNTs and carbon impurities, the difference in oxi-
dation rates of graphite and intercalated graphite[4345]was
taken into account. Graphite intercalation compounds are
formed by the insertion of atomic or molecular layers of other
chemical species between graphite layers. This interaction
causes 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 oxidation
resistance difference between CNTs and carbonaceous impu-
rities can be increased. As an example, Chen et al. [43] re-
ported a combined purification process consisting of
bromination and subsequent selective oxidation with oxygen
at 530 C for 3 days. Temperature programmed oxidation pro-
files of the CNT samples with and without bromine treatment
are 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 open
were enriched in the purified sample, and the yield obtained
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 pack-
ing of the carbon, and the quality of the cathodic soot.
Although MWCNTs can be purified by a variety of gas
phase oxidation [4145], attempts to use similar procedures
for SWCNTs result in nanotubes etching away. For example,
using the bromine and oxygen system, the yield was
3 wt% [47] for SWCNT purification, which implies that a
large fraction of SWCNTs are consumed in the process. This
large difference between MWCNTs and SWCNTs results from
two factors. One is the larger amount of curvature experi-
enced by the graphene sheet of SWCNTs, and the other is
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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 SWCNTyield of 15 wt% and a purity
of90% 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 2050 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 much
lower density (the densities of Fe and Fe2O3 are 7.86 and
5.18 g/cm3, respectively). Such significant expansion broke
the 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 shells
encapsulating 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 dissolve
the iron particles. After filtering and drying, the oxidation
and acid extraction cycle was repeated once more at 325 C,
followed by an oxidative baking at 425 C. Finally, 99.9% pure
SWCNTs (with respect to metal content) with a yield of
30 wt% were obtained. The validity of this method was veri-
fied by another group[50]. However, owing to the complicated
purification steps, it is hard to purify SWCNTs in a large scale.
Xu et al. [51] developed a controlled and scalable multi-
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 of
metal oxides. In the oxidation, metal catalysts coated by non-
nanotube carbon were oxidized into oxides by O2and exposed
by using a multi-step temperature increase program. In the
deactivation step, the exposed metal oxides were deactivated
by conversion to metal fluorides through reacting with
C2H2F4, SF6, or other fluorine-containing gases to avoid the
catalytic effect of iron oxide on SWCNToxidation. The Fe con-
tent was remarkably decreased from 30 to 1 wt% and a
yield of2548 wt% was achieved. However, the shortcoming
of this method is that it is limited to HiPco SWCNTs, in which
the dominant impurity is metal catalyst. Furthermore, the
toxicity of the reagents used in this method and the resulting
gases are undesirable features.
Gas phase oxidation is a simple method for removing car-
bonaceous impurities and opening the caps of CNTs without
vigorously 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 derived
MWCNTs, which contains no metal catalyst. For purifying
SWCNTs or MWCNTs (synthesized by other techniques), acid
treatment 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 homogeneously
contacting the whole sample. In order to obtain high-purity
CNTs, the amount of sample to be purified each time is quite
limited (tens to a hundred milligrams). Therefore, methods
that can cause the oxidant gas to homogeneously contact
CNT samples are urgently required to obtain high-purity CNTs
on a large scale. Recently, Tan et al.[55]mixed raw SWCNTs
with 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 of7%. Although the purity is not very
high, the technique suggests a way to purify SWCNTs on a
large scale using gasphase oxidation. This method canprovide
pure and opened CNTs without heavily damaging tube walls,
which is a good choice for theapplication of open-ended CNTs
as nano-size reaction tubes or chemical reactors[56,57]. For
achieving 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).
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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 [5860], H2O2 or a mixture of H2O2 and HCl [6163], a
mixture of H2SO4, HNO3, KMnO4 and NaOH [6467], and
KMnO4 [6769]. 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 stirring at 120130 C for 4 h. The yield
reached 3050 wt% of the raw sample and the metal amount
was decreased to 1 wt%. One problem in the above purifica-
tion is that the permeation rate during filtration 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 1020 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 nitric 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 HNO3 for 12 hand7 M HNO3 for6 hwere 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, at-
tack amorphous carbon, and break large carbon particles, the
nitric 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 that
remain in the sample after nitric acid treatment. In this case,
a preferred step is hollow-fiber CFF[59].
Hydrogen peroxide (H2O2) is also a mild, inexpensive and
green oxidant, which can attackthe carbonsurface.The disad-
vantage of H2O2is also obvious. It cannot remove metal parti-
cles. Therefore, it is usually used together with HCl. HCl is a
widespread chemical that can be easilyconverted into a harm-
less salt. Therefore,purifying CNTs using H2O2 followedby HCl
treatment to remove metal particles has also been intensely
investigated. 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 results
showed that the size of Fe particles has a great influence on
the oxidation of amorphous carbon. However, this was still a
question about the effect of Fe before Wangs work[63].
Wang et al.[63]tried to explain the above question. They
combined two known reactions (oxidation of amorphous car-
bon with H2O2and removal of metal particles with HCl) into a
single pot, which simplified the process. Surprisingly, the
product yield and purity were improved. Typically, carbon-
coated iron impurities were simply dissolved by reacting with
an aqueous mixture of H2O2 and HCl at 4070 C for 48 h.
With this treatment, the purification yield was significantly
increased 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 radi-
cals (OH), a more powerful oxidant than H2O2. Second, HCl
dissolves the iron nanoparticles upon their exposure. The ex-
posed iron releases ferrous ions as a result of dissolution of
Fig. 3 Mass balance of the normalized weight percentage of
all components including SWCNTs, metal, carbonaceous
impurities, and weight loss of the SWCNT samples
(reprinted with permission from[60], Copyright 2003
Amercian Chemical Society).
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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 [7176]. 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 oxida-
tion. In order to get rid of carbonaceous impurities, liquid
oxidants with stronger oxidation activity were also investi-
gated. These oxidants are predominantly mixture of acids
and KMnO4.
Liu et al. [64]use a mixture of concentrated H2SO4/HNO3(3:1 by volume) to cut highly tangled long ropes of SWCNTs
into 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 continuous
diameter-selective process, eliminating SWCNTs with smaller
diameter by oxidizing the sidewalls. Li and coworkers [66]
investigated the purification effectiveness of concentrated
H2SO4/HNO3 (3:1) treatment and compared this with 6 M
HNO3 treatment. Typical TEM images of purified SWCNTs
after different treatment conditions are shown inFig. 5, from
which 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 diameter
nanotubes for a 3 h reflux process using a concentrated
H2SO4/HNO3mixture (3:1).
Colomer et al.[68]reported an effective method for remov-
ing amorphous carbon by refluxing as-prepared MWCNTs in
acidified KMnO4 at low-temperature (80C). According to
them, amorphous carbon was completely removed at the cost
of more than 60% carbon loss. TEM observation of the purified
CNTs indicated that all amorphous carbon aggregates were
removed and the CNT caps were opened. Hernadi et al. [69]
verified the above conclusion. They obtained MWCNTs with
oxygen functional groups which were free from amorphous
carbon by KMnO4 oxidation. Zhang et al. [67] investigated
the effect of KMnO4 in alkali solution on the purification of
SWCNTs. KMnO4in alkali solution is a much more moderate
Fig. 5 TEM images of purified SWCNTs: (a) sonication in 6 M HNO 3for 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).
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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 [7780]. 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/sp2 carbon ratio
from C1s XPS 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 purification 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 opening
and purification of MWCNTs through electrochemical oxida-
tion in acid solution. The vertically aligned MWCNT (with her-
ringbone structure) arrays investigated were grown on a
carbon microfiber network through DC plasma-enhanced
CVD. 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 be-
fore and after purification (Fig. 6) indicated that the CNT tips
were opened, and entrapped metals were removed during the
electrochemical oxidation. The results of inductively coupled
plasma-mass spectrometry indicate that 98.8% of the Ni was
removed after the electrochemical oxidation in acid. The
authors also investigated a series of electrolyte solutions for
electrochemical opening of CNT tips at room temperature.
They concluded that if electrochemical oxidation was per-
formed in neutral or basic aqueous solutions, no significant
tip opening was observed. If aqueous solutions of common
strong or medium strength acids (5% H2SO4, 5% HNO3, or
25% HNO3+ 25% H2SO4, 5% H3PO4 and 5% CH3COOH) were
used, not only can the amorphous carbon be readily etched
but also the metal catalyst can be dissolved.
Superior to the gas phase oxidation and wet oxidation, the
optimum time and degree of electrochemical oxidation for
CNT purification can be easily determined. This method can
get 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, sensor
platforms, nanoreactors, field emitter components, and other
applications. However, little polyhedral carbon, graphite par-
ticles, and metal particles enwrapped by carbon layers with
fewer defects can be removed by the CV oxidation. Moreover,
the purity of the obtained sample greatly depends on the
starting materials, and the amount of sample purified for
each batch is too small to make the method practical.
2.1.4. Brief summary
Chemical-based purification can effectively remove amor-
phous carbon, polyhedral carbon, and metal impurities at
the expense of losing a considerable amount of CNTs or
destroying CNT structures. Gas phase purification is charac-
terized by opening the caps of CNTs without greatly increas-
ing sidewall defects or functional groups. Liquid phase
oxidation introduces functional groups and defects preferen-
tially on CNT side walls, and may cut CNTs into shorter ones
with different lengths. The electrochemical oxidation is suit-
able for purifying CNT arrays without destroying their align-
ment. These features allow chemical purification adopted by
researchers to fulfill different requirements. The most serious
problem of this technique is that the structure of CNTs may
be destroyed by the reactants, and hence limits the applica-
tions of CNTs in some fields, for example, electronic devices.
2.2. Physical-based purification
A big problem in chemical purification is that it always de-
stroys the structure of CNTs or changes their natural surface
properties. To elucidate the inherent physical and chemical
properties of CNTs, purifications that do not involve oxidative
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treatment are highly desirable. The morphology and physical
properties of CNTs, such as aspect ratio, physical size, solubil-
ity, gravity, 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 par-
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 tech-
nique preventing them from deposition and aggregation are
very important during filtration. Thus, surfactants are widely
used to make a stable CNT suspension, and ultrasonication is
usually 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 a
stabilized colloidal suspension was formed. The suspension
was filtered using a filtration apparatus with a funnel large en-
ough to allow the sonication of the colloidal suspension to
extract 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 above
procedure to purify SWCNTs and obtained SWCNT material
with a purity of more than 90% (estimated by EM) and a yield
of 3070 wt%.
Bandow et al.[86]developed a purification process (shown
inFig. 7) consisting of filtration and microfiltration under an
overpressure (2 atm) of N2to separate CNSs, metal nanopar-
ticles, polyaromatic carbons and fullerenes from SWCNTs.
The microfiltration was repeated three times to minimize
the amount of residual CNSs and metal particles. Using this
technique, 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 pure
physicochemical interactions of carbon products with amphi-
philic molecules and the filter membrane, leaving the nano-
tubes undamaged. However, this procedure relies on the
quality 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).
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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, which
introduces functional groups on their surface.
2.2.3. Solubilization of CNTs with functional groups
The principle of this purification step is to solubilize CNTs by
introducing functional groups onto their surface. These solu-
ble nanotubes allow for the application of other techniques
such as filtration or chromatography as a means of tube puri-
fication. To regain reasonable quantities of un-functionalized
but 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 soot
using 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 nanotubes
indefinitely whilst the accompanying graphitic particles settle
out. 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 was
homogenized through an ultrasonic-homogenizer and fil-
tered. The MCB was removed by evaporation at 150 C, and
PMMA was removed by burning it off in 200-Torr oxygen gas
at 350 C. At the same time, azomethine ylides and solution
phase ozonolysis (78 C) were also reported to solubilize
CNTs via 1,3 dipolar cycloaddition[9294].
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 with
0.5 mg/mL total cellular RNA. The solution was then centri-
fuged to pellet the insoluble particles. RNA-wrapped CNTs
were treated with enzyme ribonuclease to remove the RNA
and thereby precipitate the CNTs. Jeynes et al. [95]also sug-
gested that RNA/DNA was more efficient in solubilizing CNTs
than SDS as there is a large surface area of phosphate back-
bone which interacts with water, while similarly there are
many bases to bind the CNTs.
The advantage of this process is that it can always pre-
serve the surface electronic structure of CNTs. This property
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).
ExtractSolids caught
on filter
As-prepared carbonaceous sample in CS2
(SWCNT, CNS, C60, C70, polyaromatic carbons)
Evaporate CS2
Fullerenes (C60, C70),
polyaromatic carbons
Evaporate and
collect solids
Sonication in aqueous
solutions (0.1% surfactant)
CNSs
Filtration
Purified SWCNTs
Microfiltration under
over pressure (~ 2 atm)
Liquid
Solids caught
on filter
Fig. 7 A diagram illustrating the technique used forseparating coexisting CNSs, metal nanoparticles, and
polyaromatic carbons or fullerenes from the
laser-synthesized SWCNTs (modified with permission
from[86], Copyright 1997 Amercian Chemical Society).
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is of fundamental importance for the use of nanotubes as bio-
sensors[97]. On the other hand, the capability of dispersing
CNTs in solution is a very important step for using CNTs as
vectors to deliver therapeutics (drug, nucleic acid) [97]. How-
ever, the effectiveness of this technique is not high for CNT
samples containing a large amount of impurities or bundled
SWCNTs.
2.2.4. High temperature annealing
For some applications of CNTs, such as their use as bio-mate-
rials, complete removal of metal particles is of particular
importance. However, it is very difficult to achieve this by acid
washing because most of the metal particles are enwrapped
by carbon layers. The physical properties of carbon and met-
als are different at high temperature (>1400 C) under inert
atmosphere or high vacuum. It is well known that, graphite
is stable even at 3000 C, while metal evaporates at tempera-
tures higher than their evaporation point. Therefore, it is ex-
pected that high temperature annealing can effectively
remove metal particles.
Lambert et al.[98]first attempted to remove metal catalyst
particles from SWCNTs by heating the material above the
evaporation temperature of the metal. The results showed
that this might be a good way to eliminate the catalyst parti-
cles. The effectiveness of removing metal particles in
MWCNTs using high temperature annealing was also verified
by a few reports[99101]. Their results suggest that high-pur-
ity (99.9%) CNTs with respect to metal particles can be ob-
tained by a high temperature (P1800 C) annealing
treatment. Further research indicated that high temperature
annealing (>1400 C) can change the structure of CNTs, such
as removing structural defects [102], enlarging diameter
[103], transforming SWCNTs to MWCNTs [104] or MWCNTs
to double-walled CNTs (DWCNTs) [105] at appropriate
temperatures.
In brief, high temperature annealing of CNTs is one of the
most efficient methods for the removal of metal particles at
the tips or in the hollow core of CNTs [98105] and also for
structural evolvement from disordered to straight, crystalline
layers [106]. High temperature annealing not only increases
the mechanical strength and thermal stability of CNTs but
also affects their electronic transport property. The drawback
of this method is that carbonaceous impurities still exist and
become more difficult to remove after graphitization. There-
fore, this method can be used to remove residual metal parti-
cles of purified CNTs obtained by other techniques for
achieving metal free CNTs. It can also be used to remove
the metal particles in as-prepared CNT samples that contain
a small amount of carbonaceous impurities or in the case
where the existence of carbon impurities is not of much
concern.
2.2.5. Other physical techniques
Some other physical methods were also explored to remove
metal particles, including a magnetophoretic technique
[107], supercritical fluid carbon dioxide (Sc-CO2) extraction
[108], and a mechanically ejecting technique [109]. These
techniques are reported to be effective in removing metal par-
ticles entrapped by carbon layers without changing the inher-
ent properties of the CNTs.
Kang and Park[107]demonstrated magnetophoretic purifi-
cation of SWCNTs (produced by HiPco process) from super-
paramagnetic iron-catalyst impurities in a microfluidic
device. The flow of a fluid through a microfluidic channel is
completely laminar and no turbulence occurs due to the
small dimension of the microchannels. By employing micro-
fluidic and a magnetic field-induced saw-tooth nickel micro-
structure, a highly enhanced magnetic force in adjoining
microchannels was exploited. The iron impurities of SWCNTs
were attracted towards areas of higher magnetic-flux density
in the microchannels where magnetic field was asymmetri-
cally generated perpendicular to the streamline. SWCNTs
with a purity of 9899% with respect to metal content were
obtained. However, some SWCNTs containing iron particles
were also removed, which decreased the yield of this method.
Sc-CO2has both gas-like and liquid-like properties; thus it
can penetrate into small pores like a gas and dissolve organic
substances like a liquid. Based on the above mechanism,
Wang and coworkers[108]developed a two-step purification
using Sc-CO2as a solvent to clean metal impurities from as-
grown SWCNTs produced by the HiPco process. The first step
of this method is a pretreatment procedure using bulk elec-
trolysis with ethylenediaminetetraacetic acid. The second
step is an in situ chelation/(Sc-CO2) extraction to remove me-
tal particles. Over 98% of the iron impurity (measured by EDS)
in the as-grown SWCNTs were removed using this two-step
extraction.
Thien-Nga et al.[109]developed a mechanical purification
to remove ferromagnetic particles from their graphitic shells.
The basic principle of the method is like a snooker game,
where the energy of elastic impact between encapsulated cat-
alysts and small hard inorganic particles is used to eject metal
kernels and trap them by a strong magnet. Typically, SWCNTs
were first dispersed in various solvents. Insoluble nanoparti-
cles (zirconium oxide, diamond, ammonium chloride, or cal-
cium carbonate) in the given medium were then added to
the suspension. This slurry was sonicated typically for 24 h.
This process enables the production of laboratory quantities
of SWCNTs containing no magnetic impurities.
2.2.6. Combination of purification and separation
Following the purification of CNTs to remove foreign materi-
als such as catalyst, amorphous carbon, and carbon-coated
nanoparticles, the sorting of SWCNTs according to their
length becomes particularly important in light of their poten-
tial applications. Various techniques have been employed to
purify CNTs and simultaneously sort them by length [110
113]. Chromatography is useful for the length fractionation
of shortened CNTs less than 300 nm in length [110,111]. For
longer CNTs, techniques such as capillary electrophoresis
(CE)[112]and field-flow fractionation (FFF) appear to be more
applicable [113]. These techniques are simple and non-
destructive for the purification and length-dependent separa-
tion of CNTs. These techniques require CNTs to be purified
are individually dispersed. Therefore, complex procedures
are needed to obtain this kind of highly dispersible CNT
solution.
Chromatography is a separation method that relies on dif-
ferences in partitioning behavior between a flowing mobile
phase and a stationary phase to separate the components
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in a mixture. Materials that are smaller than the pore size can
enter the pores and therefore have a longer path and longer
transit time, while larger materials that cannot enter the
pores are eluted first. Different molecules, therefore, have dif-
ferent total transit times through the column depending on
their size and shape. Chromatography was first used to sepa-
rate CNTs from carbonaceous impurities[114117]. After this,
chromatography is usually used to separate CNTs by length.
For example, purification and length separation of oxidatively
shortened SWCNTs were achieved by this technique in an
alkalescent water solution [110]and SWCNTs with different
lengths were separated. Huang and coworkers [111] used
chromatography to purify DNA-wrapped CNTs and sort them
into fractions of uniform length. As observed by atomic force
microscopy (AFM), the length variation was typically within
10% or less for each of the measured fractions (Fig. 9).
Electrophoresis is caused by electrostatic forces, which are
generated by applying an alternating current (AC) or DC elec-
tric field between an electrode and a charged body. Electro-
phoresis can be achieved regardless of the electric fields
uniformity. A charged particle is pulled along the field lines
toward the electrode carrying an opposite charge to that of
the particle. In the same field, a neutral body is merely polar-
ized. The result may produce a torque, but not a net transla-
tional force, without which the body as a whole will not move
towards either electrode. Electrophoresis can be appreciable
even when the free charge per unit weight of the particle is
quite small. Therefore, it is possible to purify CNTs in an elec-
tric field by using the motion difference between CNTs and
carbon impurities [118]. This motion depends not only on
the intrinsic electric properties but also on the diameter and
length of CNTs. Therefore, this technique can also be used
to separate CNTs with length, diameter, and conductivity by
refining experimental conditions. Yamamoto et al.[119]first
reported a purification and orientation method based on AC
electrophoresis in isopropyl alcohol. To increase separation
rate and effectiveness, Doorn et al.[112]adopted CE to purify
and separate CNTs by size. The CE was performed in narrow
tubes (in the order of lm) and resulted in rapid separation
based on charge- and size-dependent mobility of solution
phase species under the influence of an applied electric field.
And AFM observations on fractions demonstrated a length-
based separation mechanism that leads to elution of short
tubes first, followed progressively by longer tubes. Further
work [120] indicated that CE has the potential to separate
CNTs, not only by differences in length[112]but also by differ-
ences in size or other geometric factors, such as diameter or
cross section.
At the same time, AC dielectrophoresis was reported to be
capable of separating metallic SWCNTs from semiconducting
SWCNTs in SDS suspension[121]. This method takes advan-
tage of the difference in the relative dielectric constants of
two species with respect to the solvent, resulting in an oppo-
site movement of metallic and semiconducting tubes along
the electric field gradient. Metallic tubes are attracted toward
a microelectrode array, leaving semiconducting tubes in the
solvent. An enrichment of metallic tubes up to 80% was
achieved by a comparative Raman spectroscopy study on
the dielectrophoretically deposited tubes and a reference
sample.
In addition, FFF was also developed to separate CNTs by
length [113,122]. FFF is a chromatography-like separation
and sizing technique based on elution through a thin empty
channel. The main difference between FFF and chromatogra-
phy is that FFF separation is (ideally) induced only by physical
interactions with an external field rather than physicochem-
ical interactions with a stationary phase. Compared with
chromatography and CE, FFF can separate CNTs by length
over a larger range and in larger quantities.
These techniques can separate CNTs according to their
size or electronic properties, which represents an important
improvement in size and conductivity selectivity. This will
promote the development and application of CNTs in the ana-
lytical, nanotechnology and nanoelectronics fields [123,124].
Therefore, they are not merely purification methods. A com-
mon feature of these techniques is that they require high dis-
persibility of isolated CNTs in solution. However, the CNT
surface is hydrophobic, and the existing state of CNTs, espe-
cially for SWCNTs, is interconnected or in a thick bundle. In
order to obtain high-purity CNTs, pre-treatment is required
to obtain isolated CNTs having high dispersibility in solution.
2.2.7. Brief summary
Physical-based purification can maintain the intrinsic
structure of CNTs, which is desirable for elucidating their
properties. Furthermore, some techniques such as chroma-
tography, electrophoresis, and FFF can separate CNTs accord-
ing to their differences in length or conductivity in addition to
their purification function, which is a key step for using CNTs
in devices such as nano- and micro-electronics. However,
Fig. 9 AFM images of three representative chromatography fractions of SWCNTs deposited onto alkyl silane-coated SiO 2substrates (reprinted with permission from[111], Copyright 2005 Amercian Chemical Society).
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there are still some problems in these techniques that need to
be solved. One is that these methods are not very effective in
removing impurities. Another is that they require CNT sam-
ples be highly dispersible. Therefore, the as-prepared sample
is always first dispersed in solution by adding surfactants or
treated by a chemical process to cut and/or add functional
groups before purification. The third problem is the limited
amount of sample that can be purified each time. Based on
the above facts, physical methods are more suitable for use
as an assistant step combined with chemical purification, ex-
cept for the case where a small amount of CNTs with a partic-
ular structure or property are required.
2.3. Multi-step purification
As discussed above, gas phase oxidation is effective in remov-
ing amorphous carbon and polyhedral carbon at the cost of
losing some CNTs but fails to remove a significant fraction
of graphite particles and metal impurities. Liquid phase oxi-
dation with strong oxidants is effective in removing carbona-
ceous impurities and metal particles simultaneously, whereas
purified CNTs are always cut, opened and damaged. Physical
purifications are effective in partly removing isolated carbo-
naceous or metallic impurities, while amorphous and spher-
ical particles stuck to sidewalls or metal particles
encapsulated in CNTs remain. In order to achieve desirable
CNT purity with high yield, combinations of chemical and
physical purifications are being intensely investigated.
According to different needs, various kinds of multi-step puri-
fication methods are reported. For example, it is difficult to re-
move carbonaceous impurities adhering to the sidewalls of
CNTs for both chemical and physical purifications. To solve
this problem, hydrothermally initiated dynamic extraction
(HIDE) [44,125127] or sonication [128133] was adopted in
many chemical purification procedures. Graphite particles
existing in CNTs synthesized by arc discharge or laser abla-
tion are hard to remove by chemical oxidation, so filtration
is adopted in some purification[134,135]. It is clear that metal
particles catalyze the low-temperature oxidation of carbons
indiscriminately, destroying SWCNTs in the presence of oxy-
gen and other oxidizing gases. To overcome this problem,
purification combining gas phase oxidation and acid treat-
ment were widely investigated[136145]. In fact, several tech-
niques such as oxidation, sonication, HIDE, or filtration are
simultaneously adopted in one purification procedure to ob-
tain high-purity CNTs with high yield.
2.3.1. HIDE-assisted multi-step purification
HIDE provides comminution on a microscopic scale as a re-
sult of collision between soot particles and water molecules
during thermal treatment[146]. Thus during HIDE, water mol-
ecules break the network between SWCNTs, amorphous car-
bons and metal particles, and also attack the graphitic layers
encapsulating metal particles. As a result, almost all graphitic
nanoparticles and CNSs are washed out from the soot. The
graphitic sheets of the CNSs, for the most part, have defects
and dislocations, in contrast to SWCNTs [147]. It is believed
that the reaction of H2O with carbon breaks the graphitic lay-
ers that wrap the metal particles. Consequently, incorporat-
ing HIDE in the purification procedure exposes the metal
particles and this allows them to be dissolved by hydrochloric
acid in the final step of the treatment [125].
Tohji et al. [125,126]first reported a multi-step method to
purify SWCNTs by combining HIDE with other processes as
illustrated inFig. 10. SWCNTs with a purity of 95 wt% and a
yield of about 2 wt% were obtained by this process.
Graphite fragments from the graphite rod cannot be re-
moved by oxidation. To solve this problem, the as-prepared
MWCNTs were treated by a multi-step process (shown in
Fig. 11) combining wet grinding, HIDE, oxidation and other
techniques [127]. TEM observations indicated that this pro-
cess was effective in removing graphite and carbonaceous
particles and opening CNT caps. However, the yield was only
2 wt% due to the high oxidation temperature (700 C).
To increase the purification yield, we developed a multi-
step method combining sonication, HIDE, bromination, gas
phase oxidation and acid treatment [44]. It was found that
bromination can increase the purification yield from 25 to
50 wt%. The effect of bromination on the purification of
CNTs was also verified by Fan et al. [148]. The problem with
this technique is that it is not suitable for purifying SWCNTs.
On the other hand, the onset burning temperature of
MWCNTs was decreased after purification, suggesting that
defects or functional groups were introduced.
2.3.2. Microfiltration in combination with oxidation
Bandow et al. [134] purified SWCNTs synthesized by laser
ablation by combining microfiltration [86] with oxidation in
air. In a typical procedure, as-prepared soot containing
SWCNTs was first purified using microfiltration to remove
large CNSs. The obtained SWCNTs were then oxidized in air
at 450 C for 20 min to remove CNSs adhering to the SWCNT
walls, followed by soaking in concentrated HCl (36%) for 12
days at room temperature to remove metal particles. The pur-
ity of the SWCNTs after purification was greater than 90%.
To remove metal particles before oxidation, Kim and Luzzi
[135]developed magnetic filtration carried out in a magnetic
field. They investigated the efficiency of using magnetic filtra-
tion alone, or combining it with chemical-based or annealing-
assisted oxidation treatments. Using magnetic filtration
alone, the catalyst content was reduced from 11.7 to
3.7 wt%, much better than obtained in oxidation or chemical
As-produced
SWCNTs
HIDE treatment
for 12 h
Filtration
and dry
Soxhlet extraction
in CS2
Oxidation at 470C
for 20 min
Removing
fullerenes
Removing
amorphous carbon
Removing
exposed metal
6 M HCl
treatment
Purified
SWCNT
Fig. 10 A diagram showing the purification of SWCNTs
with a multi-step process incorporating HIDE treatment.
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treated samples. By combining chemical and magnetic purifi-
cation, the metal catalyst content was reduced to 0.3 wt%.
These results allowed the authors to conclude that magnetic
filtration is effective in removing metal catalysts, producing
CNTs with high-quality and yield. It is well known that metal
particles can catalyze carbon oxidation in the presence of oxi-
dants. Therefore, effective removal of metal particles is desir-
able for the following purification of CNTs using chemical
oxidation. Thus magnetic filtration combined with chemical
purification opens a new way to obtain purified CNTs with
high yield.
2.3.3. Sonication in combination with oxidation
Sonication is identified as one of the effective processes to get
rid of the amorphous impurities adhering to the walls of
CNTs using suitable solvents[149]. During sonication, the sol-
vent molecules are able to interact with CNTs and hence lead
to solubilization, which can improve purification effective-
ness when some other steps are followed.
We[128]developed an effective multi-step purification ap-
proach to purify SWCNTs by combining sonication with oxi-
dation in air. The biggest problem in purifying SWCNTs
synthesized by arc discharge is how to remove graphite parti-
cles produced from the graphite rod. Ultrasonication in etha-
nol was adopted to first remove the graphite particles.
Because the SWCNTs used were rope-like, we decanted the
alcohol solution containing graphite particles and other or-
ganic impurities after sonication for about 5 min. This proce-
dure was repeated five times. The graphite-free material was
oxidized and then soaked in HCl to remove amorphous
carbon and metal particles. TEM observations (Fig. 12) and Ra-
man spectra verified the effectiveness of the above purifica-
tion procedure. With this procedure, a 41 wt% yield of
SWCNTs with a purity of about 96% was achieved. However,
the above procedure is only applicable to SWCNTs synthe-
sized by hydrogen arc discharge, in which SWCNTs exist as
ropes with fewer defects.
We [129] also developed a multi-step method to purify
SWCNTs synthesized by CVD, which includes acid washing,
ultrasonication and freezing treatments in liquid nitrogen.
After purification, SWCNTs with a purity of 95% (estimated
from EDS and SEM) and a yield of 40 wt% were obtained and
the procedure did not destroy the SWCNT bundles.
Montoro and Rosolen[130]reported a four-step method to
purify SWCNTs synthesized by arc discharge as shown in
Fig. 13. This new procedure is efficient and appropriate for
obtaining highly pure SWCNTs with minimum damage to
the CNT walls and minimum modification in the CNT length.
Wang et al.[132]developed a three-step method to purify
and cut SWCNTs synthesized by CVD. This method included
refluxing in 2.6 M HNO3to remove metal particles, ultrasoni-
cation in acid solution (H2SO4/HNO3, H2SO4/H2O2) to cut and
polish the SWCNTs, and heat treatment in an NH3 atmo-
sphere to remove carbon impurities and heal structural de-
fects. Recently, they [133] further impro
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