purification of carbon nanotubes

CARBON

4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/carbon

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

A B S T R A C T

Article history:

It is predicted theoretically and understood experimentally that carbon nanotubes (CNTs)

Received 27 June 2008

possess excellent physical and chemical properties and have wide-range potential applica-

Accepted 1 September 2008

tions. However, only some of these properties and applications have been verified or real-

Available online 9 September 2008

ized. To a great extent, this situation can be ascribed to the difficulties in getting highpurity CNTs. Because as-prepared CNTs are usually accompanied by carbonaceous or
metallic impurities, purification is an essential issue to be addressed. Considerable progress 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 innovative thoughts on the purification of CNTs.
Ó 2008 Elsevier Ltd. All rights reserved.

Contents
1.

2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1. CNT synthesis techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2. Impurities coexisting with CNTs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.3. Assessment of CNT purity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.4. Purpose of this review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Purification methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1. Chemical oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.1. Gas phase oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2. Liquid phase oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3. Electrochemical oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.4. Brief summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2. Physical-based purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1. Filtration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2. Centrifugation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3. Solubilization of CNTs with functional groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.2.4. High temperature annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

* Corresponding author: Fax: +86 24 2390 3126.
E-mail address: (H.-M. Cheng).
0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carbon.2008.09.009

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4.

1.

CARBON

4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

2.2.5. Other physical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6. Combination of purification and separation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.7. Brief summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. Multi-step purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1. HIDE-assisted multi-step purification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2. Microfiltration in combination with oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.3. Sonication in combination with oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.4. High temperature annealing in combination with extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.5. Brief summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4. Applicability of typical purification techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Synthesis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Purification methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Purity assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction

Elemental carbon in sp2 hybridization can form a variety of
amazing structures, such as graphite (3D), graphene (2D), carbon nanotubes (CNTs, 1D) and fullerene (0D). CNTs defined by
Iijima in 1991 [1] have a unique tubular structure with nanometer 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 Young’s modulus and tensile
strength, which makes them promising in serving as a reinforcement 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 SWCNT with a given (n, m) index
[2], when (2n + m) = 3q (q is an integer), the nanotube is metallic, 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 catalyst 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-nanomaterials [15], energy storage media [16,17] and nanoelectronic devices [18,19].

1.1.

2014
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2018

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benzene, ethanol, acetylene, propylene, methane, ethylene,
CO, etc.) and growth of CNTs over the catalyst (usually transition 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 metal catalysts inserted into the anode are required when synthesizing 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 metal catalyst 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 fullerenes, amorphous carbon, graphite particles, and graphitic
polyhedrons with enclosed metal particles are also formed.
The record high-purity of the SWCNTs synthesized by arc discharge has been reported to be 80% by volume [25].

CNT synthesis techniques
1.2.

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

Impurities coexisting with CNTs

As-synthesized CNTs prepared by the above methods inevitably contain carbonaceous impurities and metal catalyst particles, and the amount of the impurities commonly increases
with the decrease of CNT diameter. Carbonaceous impurities


CARBON

4 6 ( 20 0 8 ) 2 0 0 3–20 2 5

typically include amorphous carbon, fullerenes, and carbon
nanoparticles (CNPs) (as shown in Fig. 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 polyhedrons 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 removed owing to their solubility in certain organic solvents.
Amorphous carbon is also relatively easy to eliminate because of its high density of defects, which allow it to be oxidized 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 sometimes encapsulated by carbon layers (varying from disordered
carbon layers to graphitic shells, as shown in Fig. 1b and c)
making them impervious and unable to dissolve in acids. Another problem that needs to be overcome is that carbonaceous and metal impurities have very wide particle size
distributions and different amounts of defects or curvature
depending on synthesis conditions, which makes it rather difficult to develop a unified purification method to obtain reproducibly 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. Characterization of CNT samples falls into three groups: metal catalyst,
carbonaceous impurity, and CNT structure variation (defects,
functional groups, cap opening, cutting, etc.). Their characterization mainly depends on electron microscopy (EM, including
scanning EM (SEM), and transmission EM (TEM)), thermogravimetric analysis (TGA), Raman spectroscopy and ultravioletvisible-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 algorithms to convert images into numerical data, EM cannot give
a quantitative evaluation of the purity of CNTs [28].

2005

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 TGA by simply burning CNT samples in air. A higher oxidation temperature (>500 °C) is always associated with purer,
less defective CNT samples. The homogeneity of CNT samples
can be evaluated by standard deviations of the oxidation temperature 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-destructive 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 density. In addition, the diameters and electronic structures of
CNTs can be determined by using the resonance Raman scattering [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].
UV–vis-NIR spectroscopy is a rapid and convenient technique 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 solution once the extinction coefficient of SWCNTs is known
[32]. On the other hand, SWCNTs give rise to a series of predictable electronic band transitions between van Hove singularities 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 UV–vis-NIR spectroscopy [33,34]. The drawback of this
method is the difficulty in repeatedly preparing the standardized 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 characterize functional groups on the walls of CNTs, and energy dispersive 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).


2006

CARBON

4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

trace amounts. The major purity and quality assessment
techniques and their efficiency are summarized in Table 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 integrated 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 in Table 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 presence of impurities in CNTs, the non-uniformity in morphology 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 oxidation/treatment, centrifugation, filtration, and chromatography, 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, diameter), impurity type and their morphology (particle size, defect, curvature, the number and crystallinity of carbon
layers wrapping metal particles), purity assessment technique, 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
EM
TGA
Raman
UV–vis-NIR
XPS
EDS
a
b
c
d
e
f
g
h

Df
J
D
J



g




J


D
J



J

Jh

D






J
J



Advantages

Limitations

Direct observation
Precise content of carbon and metals
Diameter, quality and conductivity of SWCNTs
Conductivity feature and content of SWCNTs
Accurate assessment of F-G on CNTs
Elemental contents, special for trace amounts

A small amount of sample is analyzed
CNTs analyzed are completely destroyed
Invalid for MWCNTs and metal impurities
Need 100% pure SWCNTs as standard
Invalid to purity assessment
Invalid to evaluate CNT content

Carbonaceous impurity.
Metal impurity.
Functional groups.
Structure defects.
Conductivity feature.
Qualitatively valid.
Invalid.
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

Structure integrity

Pure CNTs without large defects and faults, and
no functional groups, amorphous carbon or
fullerene adhered on the tube wall

SWCNT content

The content of SWCNTs in CNTs

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
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
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 combination of both. The chemical method purifies CNTs based on
the idea of selective oxidation, wherein carbonaceous impurities are oxidized at a faster rate than CNTs, and the dissolution of metallic impurities by acids. This method can
effectively remove amorphous carbon and metal particles except for those encaged in polyhedral graphitic particles. However, 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 properties, 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 principle, 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 purification 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 amorphous 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 disadvantages 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 oxidizing carbonaceous impurities at a temperature ranging
from 225 °C to 760 °C under an oxidizing atmosphere. The

2007

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 extremely simple and successful strategy for purifying arc discharge derived MWCNTs, which are metal free and have
fewer defects on tube walls. Ebbesen et al. [40,41] first reported 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 limited
oxidation selectivity between CNTs and carbonaceous impurities. Therefore, two routes may be helpful to increase the
purification yield using this simple air oxidation. One is to ensure 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 researchers. 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 between MWCNTs and carbon impurities, the difference in oxidation rates of graphite and intercalated graphite [43–45] 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 reduces the oxidation resistance of the intercalated graphite.
Carbonaceous impurities have higher structural defect densities than CNTs, and are therefore more ready to act as reaction sites for intercalated atoms. Thus the oxidation
resistance difference between CNTs and carbonaceous impurities can be increased. As an example, Chen et al. [43] reported a combined purification process consisting of
bromination and subsequent selective oxidation with oxygen

at 530 °C for 3 days. Temperature programmed oxidation profiles of the CNT samples with and without bromine treatment
are shown in Fig. 2. It is obvious that oxidation of the brominated sample occurs more readily than that without bromination. 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 packing of the carbon, and the quality of the cathodic soot.
Although MWCNTs can be purified by a variety of gas
phase oxidation [41–45], 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 experienced by the graphene sheet of SWCNTs, and the other is


2008

CARBON

4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

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).
metal impurities catalyzing the low-temperature oxidation of
carbon. There may therefore be two ways to increase the purification 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 simultaneously protecting SWCNTs. The purification incorporates
a chlorine, water, and hydrogen chloride gas mixture to remove the impurities. A SWCNT yield of $15 wt% and a purity
of $90% indicate that the carbonaceous impurities are preferentially removed. Based on their experimental observation,
hydrogen chloride was required for selective removal of the
unwanted carbon. They proposed a mechanism for the purification. 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 hydroxyl 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 controlling 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 pressure [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 removal. They found that metal particles catalyze the oxidation
of carbons indiscriminately, destroying SWCNTs in the presence 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 procedure for purifying SWCNTs synthesized by the HiPco process.
Their method begins with cracking of the carbonaceous shells
encapsulating metal particles using wet oxygen (20% O2 in argon 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 verified 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 multistep method to remove metal catalyst and non-nanotube carbons from raw HiPco SWCNTs. Their scalable multi-step purification included two processes: oxidation and deactivation of
metal oxides. In the oxidation, metal catalysts coated by nonnanotube carbon were oxidized into oxides by O2 and 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 SWCNT oxidation. The Fe content was remarkably decreased from $30 to $1 wt% and a
yield of $25–48 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 carbonaceous impurities and opening the caps of CNTs without
vigorously introducing sidewall defects, although it cannot directly 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 particular) 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 increase the exposed surface of raw soot during thermal oxidation in air. The final purified samples had a yield of $26 wt%
and a metal impurity of $7%. Although the purity is not very
high, the technique suggests a way to purify SWCNTs on a
large scale using gas phase oxidation. This method can provide
pure 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]. For
achieving purified CNTs on a large scale, gas phase oxidation


CARBON

4 6 ( 20 0 8 ) 2 0 0 3–20 2 5

2009

need to be modified in the following ways: one is to look for a
simple approach and non-toxic reagents to remove metal particles 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 centrifugation, 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 simultaneously removes both amorphous carbon and metal catalyst
was developed. Oxidative ions and acid ions dissolved in solution can evenly attack the network of raw samples, and therefore 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], H2O2 or a mixture of H2O2 and 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 functional 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 selectively remove amorphous carbon. In addition, it is inexpensive and nontoxic, capable of removing metal catalysts and
no secondary impurities are introduced.
Dujardin et al. [58] reported a one-step method using concentric nitric acid to purify SWCNTs synthesized by laser ablation. Briefly, as-synthesized SWCNTs were sonicated in
concentrated nitric acid for a few minutes followed by refluxing under magnetic stirring 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 purification is that the permeation rate during filtration was very
low because SWCNTs packed together and the filter membrane 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 HNO3 for 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 nitric acid treatment on the purification of SWCNTs is confirmed, the relationship between purification yield and purity
with systematic and quantitative measurements was not reported before Hu et al.’s work [60]. They established a systematic 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 nitric 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 percent of each component calculated from TGA and NIR spectra
is plotted in Fig. 3. It is clear that the purity and the yield of
SWCNTs with nitric acid treatment depend on the concentration of the nitric acid and the time of reflux. The treatments 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).

3 M HNO3 for 12 h and 7 M HNO3 for 6 h were the most efficient.
Nitric acid treatment destroys SWCNTs, leading to the production of carbonaceous impurities. Nevertheless, with the ability
to dissolve the metal catalyst, intercalate SWCNT bundles, attack amorphous carbon, and break large carbon particles, the
nitric acid treatment can be a viable first step for SWCNT purification. The key to achieving high-purity SWCNTs is a subsequent 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 attack the carbon surface. The disadvantage of H2O2 is also obvious. It cannot remove metal particles. Therefore, it is usually used together with HCl. HCl is a
widespread chemical that can be easily converted into a harmless salt. Therefore, purifying CNTs using H2O2 followed by HCl
treatment to remove metal particles has also been intensely
investigated. Macro-scale purification, including a first refluxing 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 Wang’s work [63].
Wang et al. [63] tried to explain the above question. They
combined two known reactions (oxidation of amorphous carbon with H2O2 and removal of metal particles with HCl) into a
single pot, which simplified the process. Surprisingly, the
product yield and purity were improved. Typically, carboncoated iron impurities were simply dissolved by reacting with
an aqueous mixture of H2O2 and HCl at 40–70 °C for 4–8 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 purification can be summarized as following. First, Fe particles act as
a catalyst by Fenton chemistry [70], producing hydroxyl radicals (ÅOH), a more powerful oxidant than H2O2. Second, HCl
dissolves the iron nanoparticles upon their exposure. The exposed iron releases ferrous ions as a result of dissolution of


2010

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4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

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 effect (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 H2SO4 can rapidly absorb microwave energy and dissolve metals efficiently without damaging the tube wall structure in a short time.
As discussed above, HNO3, H2O2, as well as microwave-assisted inorganic acid treatments can effectively remove metal

particles, but they are not so effective in removing carbona-

Fig. 4 – Scheme of localized catalytic reaction of H2O2 with
carbon-coated iron nanoparticles (not drawn to scale,
reprinted with permission from [63], Copyright 2007
Amercian Chemical Society).

ceous particles owing to the relative mildness in their oxidation. In order to get rid of carbonaceous impurities, liquid
oxidants with stronger oxidation activity were also investigated. 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 carboxylic acid groups at the open ends. Wiltshire et al. [65] reported 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 in Fig. 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 condition 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/HNO3 mixture (3:1).
Colomer et al. [68] reported an effective method for removing amorphous carbon by refluxing as-prepared MWCNTs in
acidified KMnO4 at low-temperature (80 °C). 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. KMnO4 in alkali solution is a much more moderate

Fig. 5 – TEM images of purified SWCNTs: (a) sonication in 6 M HNO3 for 4 h, (b) refluxing in 6 M HNO3 at 120 °C for 4 h,
(c) refluxing in concentrated H2SO4/HNO3 mixture (3:1) at 120 °C for 2 h, (d) refluxing in concentrated H2SO4/HNO3 mixture
at 120 °C for 4 h (reprinted with permission from [66], Copyright 2004 Institute of Physics Publishing).


CARBON

4 6 ( 20 0 8 ) 2 0 0 3–20 2 5

oxidant than in acidic solution. The solution cannot effectively 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 MnO2 generated during the oxidation.
It is desirable to remove carbonaceous impurities by converting them into soluble or volatile products, and from this point
of view, KMnO4 seems 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 preferentially 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 mechanically reinforced composites, in use as scanning probe microscopy 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 biotechnology is based on the premise that short CNTs are dispersible in water [81,82]. The main problem of this liquid
oxidation strategy is the damage to CNTs, the inability to remove 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, carbon materials with fewer defects usually show a lower corrosion rate under electrochemical oxidation. Therefore, it is
reasonable to deduce that CNTs with fewer defects should
show higher electrochemical oxidation resistance than carbon impurities with more defects.
Fang et al. [27] investigated the electrochemical cyclic voltammetric (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 nanoparticles, hence facilitating the elimination of the metal impurities
by subsequent HCl washing. The redox peaks from the electrochemical 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 oxidation 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.

2011

They recently reported an ultra-fast and complete opening
and purification of MWCNTs through electrochemical oxidation in acid solution. The vertically aligned MWCNT (with herringbone structure) arrays investigated were grown on a
carbon microfiber network through DC plasma-enhanced
CVD. Electrochemical oxidation for tip opening and purification of MWCNT arrays was performed in an aqueous solution
of 57% H2SO4 at room temperature. SEM and TEM images before 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 performed 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 selectively opening and purifying vertically aligned CNT arrays.
The desired vertical orientation can be maintained and facilitates the use of CNT arrays as fuel cell electrodes, sensor
platforms, nanoreactors, field emitter components, and other

applications. However, little polyhedral carbon, graphite particles, 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 amorphous carbon, polyhedral carbon, and metal impurities at
the expense of losing a considerable amount of CNTs or
destroying CNT structures. Gas phase purification is characterized by opening the caps of CNTs without greatly increasing sidewall defects or functional groups. Liquid phase
oxidation introduces functional groups and defects preferentially on CNT side walls, and may cut CNTs into shorter ones
with different lengths. The electrochemical oxidation is suitable for purifying CNT arrays without destroying their alignment. 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 applications of CNTs in some fields, for example, electronic devices.

2.2.

Physical-based purification

A big problem in chemical purification is that it always destroys 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


2012

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4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

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).

treatment are highly desirable. The morphology and physical
properties of CNTs, such as aspect ratio, physical size, solubility, gravity, and magnetism are different from impurities.
These differences enable one to separate CNTs from impurities by adopting some physical techniques. Therefore, physical-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 requires 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 particles and polyaromatic carbons or fullerenes. Small size particles 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 removed 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 technique is that CNTs or large particles deposited on the filter often block the filter, making the filtering prohibitively slow and

inefficient. Therefore, a stable suspension of CNTs and a technique 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 sonication to purify MWCNTs. The as-prepared MWCNTs were dispersed 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 enough to allow the sonication of the colloidal suspension to
extract larger particles. In order to enhance the separation yield, successive filtrations were carried out until the desired 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 30–70 wt%.
Bandow et al. [86] developed a purification process (shown
in Fig. 7) consisting of filtration and microfiltration under an
overpressure ($2 atm) of N2 to separate CNSs, metal nanoparticles, 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 CS2 extracts were separated from the as-prepared lasersynthesized SWCNTs.
A major advantage of filtration is that it is driven by pure
physicochemical interactions of carbon products with amphiphilic molecules and the filter membrane, leaving the nanotubes undamaged. However, this procedure relies on the
quality of raw samples and is time-consuming. In addition,


CARBON

4 6 ( 20 0 8 ) 2 0 0 3–20 2 5

As-prepared carbonaceous sample in CS2

(SWCNT, CNS, C60, C70, polyaromatic carbons)

Solids caught
on filter

Extract

Sonication in aqueous
solutions (0.1% surfactant)

Evaporate CS2

Microfiltration under
over pressure (~ 2 atm)

Fullerenes (C60, C70),
polyaromatic carbons

Liquid
Evaporate and
collect solids

CNSs

Solids caught
on filter

Purified SWCNTs

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).

amorphous and spherical carbon particles stuck on the tube
walls cannot be effectively removed.

2.2.2.

bon and CNPs is shown in Fig. 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.

Filtration

Centrifugation

Centrifugation is based on the effect of gravity on particles
(including macromolecules) in suspension because two particles of different masses settle in a tube at different rates in response 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 stabilities resulted from the different (negative) surface charges
introduced by acid treatment [87,88]. Low-speed centrifugation (2000g) is effective in removing amorphous carbon and
leaving SWCNTs and CNPs in the sediment. High-speed centrifugation (20000g) works well in settling CNPs, while leaving
SWCNTs suspended in aqueous media. The effectiveness of
centrifugation in separating SWCNTs from amorphous car-

2013

Solubilization of CNTs with functional groups

The principle of this purification step is to solubilize CNTs by
introducing functional groups onto their surface. These soluble nanotubes allow for the application of other techniques
such as filtration or chromatography as a means of tube purification. To regain reasonable quantities of un-functionalized
but purified nanotubes, the functional groups should be removed 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-phenylene-co-2,5-dioctoxy-p-phenylenevinylene (PmPV)) in toluene. 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 filtration, giving CNTs with a purity of 91% (estimated from electron 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 polymethylmethacrylate (PMMA) with an ultrasonic cleaner. The mixture was
homogenized through an ultrasonic-homogenizer and filtered. 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 (À78 °C) were also reported to solubilize
CNTs 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 deionized water at 0 °C (in an ice-water bath) for 30 min with
0.5 mg/mL total cellular RNA. The solution was then centrifuged 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 suggested that RNA/DNA was more efficient in solubilizing CNTs
than SDS as there is a large surface area of phosphate backbone which interacts with water, while similarly there are
many bases to bind the CNTs.
The advantage of this process is that it can always preserve 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).


2014

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4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

is of fundamental importance for the use of nanotubes as biosensors [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]. However, 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-materials, 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 metals 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 temperatures higher than their evaporation point. Therefore, it is expected 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 particles. The effectiveness of removing metal particles in
MWCNTs using high temperature annealing was also verified
by a few reports [99–101]. Their results suggest that high-purity (99.9%) CNTs with respect to metal particles can be obtained 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 [98–105] 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. Therefore, this method can be used to remove residual metal particles 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 particles entrapped by carbon layers without changing the inherent properties of the CNTs.

Kang and Park [107] demonstrated magnetophoretic purification of SWCNTs (produced by HiPco process) from superparamagnetic 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 microfluidic and a magnetic field-induced saw-tooth nickel microstructure, 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 asymmetrically generated perpendicular to the streamline. SWCNTs
with a purity of $98–99% with respect to metal content were
obtained. However, some SWCNTs containing iron particles
were also removed, which decreased the yield of this method.
Sc-CO2 has 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-CO2 as a solvent to clean metal impurities from asgrown SWCNTs produced by the HiPco process. The first step
of this method is a pretreatment procedure using bulk electrolysis with ethylenediaminetetraacetic acid. The second
step is an in situ chelation/(Sc-CO2) extraction to remove metal 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 catalysts 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 nanoparticles (zirconium oxide, diamond, ammonium chloride, or calcium 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 materials such as catalyst, amorphous carbon, and carbon-coated
nanoparticles, the sorting of SWCNTs according to their
length becomes particularly important in light of their potential 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 nondestructive for the purification and length-dependent separation 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 differences in partitioning behavior between a flowing mobile
phase and a stationary phase to separate the components


CARBON

4 6 ( 20 0 8 ) 2 0 0 3–20 2 5

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 different total transit times through the column depending on
their size and shape. Chromatography was first used to separate CNTs from carbonaceous impurities [114–117]. 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 electric field between an electrode and a charged body. Electrophoresis can be achieved regardless of the electric field’s
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 polarized. The result may produce a torque, but not a net translational 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 electric 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 lengthbased 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

2015

CNTs, not only by differences in length [112] but also by differences 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 advantage of the difference in the relative dielectric constants of
two species with respect to the solvent, resulting in an opposite 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 chromatography is that FFF separation is (ideally) induced only by physical
interactions with an external field rather than physicochemical 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 analytical, nanotechnology and nanoelectronics fields [123,124].
Therefore, they are not merely purification methods. A common feature of these techniques is that they require high dispersibility of isolated CNTs in solution. However, the CNT
surface is hydrophobic, and the existing state of CNTs, especially 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 chromatography, electrophoresis, and FFF can separate CNTs according 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 SiO2
substrates (reprinted with permission from [111], Copyright 2005 Amercian Chemical Society).


2016

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4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

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 samples 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, except for the case where a small amount of CNTs with a particular structure or property are required.

2.3.

Multi-step purification

As discussed above, gas phase oxidation is effective in removing 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 oxidation with strong oxidants is effective in removing carbonaceous impurities and metal particles simultaneously, whereas
purified CNTs are always cut, opened and damaged. Physical
purifications are effective in partly removing isolated carbonaceous or metallic impurities, while amorphous and spherical 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 purification methods are reported. For example, it is difficult to remove carbonaceous impurities adhering to the sidewalls of
CNTs for both chemical and physical purifications. To solve
this problem, hydrothermally initiated dynamic extraction
(HIDE) [44,125–127] or sonication [128–133] was adopted in
many chemical purification procedures. Graphite particles
existing in CNTs synthesized by arc discharge or laser ablation 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 oxygen and other oxidizing gases. To overcome this problem,
purification combining gas phase oxidation and acid treatment were widely investigated [136–145]. In fact, several techniques such as oxidation, sonication, HIDE, or filtration are
simultaneously adopted in one purification procedure to obtain high-purity CNTs with high yield.

2.3.1.

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 in Fig. 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 removed 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 process 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 multistep 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 1–2
days at room temperature to remove metal particles. The purity 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 filtration alone, or combining it with chemical-based or annealingassisted 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-assisted multi-step purification

HIDE provides comminution on a microscopic scale as a result of collision between soot particles and water molecules
during thermal treatment [146]. Thus during HIDE, water molecules break the network between SWCNTs, amorphous carbons 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 layers that wrap the metal particles. Consequently, incorporating HIDE in the purification procedure exposes the metal

HIDE treatment
for 12 h

Purified
SWCNT

Removing
exposed metal
6 M HCl
treatment

Filtration
and dry
Removing
Soxhlet extraction fullerenes
in CS2

Removing
amorphous carbon
Oxidation at 470ºC

for 20 min

Fig. 10 – A diagram showing the purification of SWCNTs
with a multi-step process incorporating HIDE treatment.


CARBON

2017

4 6 ( 20 0 8 ) 2 0 0 3–20 2 5

Cathode deposit
(850 mg)

Sediment
(320 mg)
Remove small graphite

Grinding in water

Ultrasonic
irradiation

Centrifugation

Ultrasonic
irradiation

Add
surfactant

Dispersed
material, 68 mg

37 μm filtration

HIDE
treatment

Oxidation, 700 ºC
for 15 min

Filtration
(390 mg)

Purified
MWCNTs, 16 mg

Remove large graphite
Cake
(460 mg)

Fig. 11 – A diagram showing the purification of MWCNTs with a multi-step process incorporating HIDE and wet grinding
treatment (modified with permission from [127], Copyright 2001 Amercian Chemical Society).

treated samples. By combining chemical and magnetic purification, 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 oxidants. Therefore, effective removal of metal particles is desirable 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 solvent molecules are able to interact with CNTs and hence lead
to solubilization, which can improve purification effectiveness when some other steps are followed.
We [128] developed an effective multi-step purification approach to purify SWCNTs by combining sonication with oxidation in air. The biggest problem in purifying SWCNTs
synthesized by arc discharge is how to remove graphite particles produced from the graphite rod. Ultrasonication in ethanol 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 organic impurities after sonication for about 5 min. This procedure 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 Raman spectra verified the effectiveness of the above purification 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 synthesized by hydrogen arc discharge, in which SWCNTs exist as
ropes with fewer defects.

Fig. 12 – TEM images of (a) the as-prepared SWCNTs, and (b)
the purified SWCNTs [128].

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 HNO3 to remove metal particles, ultrasonication in acid solution (H2SO4/HNO3, H2SO4/H2O2) to cut and
polish the SWCNTs, and heat treatment in an NH3 atmosphere to remove carbon impurities and heal structural defects. Recently, they [133] further improved their method
mainly by replacing the above acid solution with an
(NH4)2S2O8/H2SO4 solution. In addition, ultrasonication time

2018

CARBON

4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

As-prepared
SWCNTs
Fullerenes
and soluble
impurities

Sohxlet extraction
with toluene

Amorphous
carbons

Sonication in
H2O2 solution

Metallic
particles
Graphite and
protected metallic
impurities

Extracted
material

Sonication in
HNO3 /HF/ SDS

Separation in
SDS solution

High purity
SWCNTs
Fig. 13 – A flowchart illustrating the multi-step purification
of SWCNTs. The removed species are listed to the left of the
chart (modified after [130]).

was extended to 4–30 h to control the length of the SWCNTs.
The final SWCNT product had a metal content lower than
1 wt%, a length distribution between 1 and 2 lm, and very
few defects.

2.3.4. High temperature annealing in combination with
extraction
As discussed above, high temperature annealing is an effective method to remove metal particles, while it makes the removal of carbonaceous impurities much more difficult in the
following chemical treatments. To solve this problem, Zhang
et al. [150] proposed a method involving high temperature
annealing followed by a dispersant extraction treatment
using a polymer. Briefly, the MWCNT samples with micrometer lengths and diameters of 20–50 nm were annealed at
2600 °C for 60 min to remove metal catalyst. The graphitized
MWCNT product was sonicated in a solution of dispersing
agent 710 (basic urethane copolymer) to remove CNPs. Dispersing agents for carbon black have a two-component structure: one is specific anchoring groups which can be strongly
adsorbed on the particle surface; the other is polymeric
chains which dangle into the solvent and repel other polymers. Considering that CNPs and MWCNTs present different
dimensionalities, the interaction potential between two parallel, infinitely long, and perfect MWCNTs as well as two CNPs

was calculated and compared on the basis of the continuum
Lennard-Jones model (Fig. 14). It was found that the attractive
potential between two CNPs was relatively low and of short
range. Therefore, the polymer chain length and surface coverage for the dispersion of CNPs was lower than that needed for
the dispersion of MWCNTs. On the basis of this theoretical

Fig. 14 – Potential energy (per atom) of MWCNTs and CNPs
(reproduced with permission from [150], Copyright 2006
Amercian Chemical Society).
prediction, CNPs and MWCNTs were effectively separated
from each other by choosing an appropriate polymer and suitable concentration (for instance, 0.04–0.1 wt% for the dispersing agent 710). Thus an effective and nondestructive method
for the purification of MWCNTs with high yield ($90 wt%) was
developed. Furthermore, the high temperature annealing step
in this purification can improve the structures of CNTs, which
will be of great importance for the exploitation of their excellent mechanical and electrical properties in various
applications.

2.3.5.

Brief summary

Based on the above description of multi-step purification, it
can be concluded that the processes can be effective in
removing both carbonaceous and metal impurities. These
methods have been rapidly developed and widely used in recent years. Most importantly, one can selectively open tips,
cut CNTs, add functional groups on sidewalls, separate CNTs
according to length or conductivity, or maintain an undamaged CNT structure by skillfully combining different techniques. The key point of this general method is how to
combine different methods according to one’s requirement
and the quality of the raw CNTs. Although considerable progress has been made, some merits of physico-chemical techniques have not been fully used and combined. For example,
some physical methods capable of removing metal particles

(magnetophoretic, Sc-CO2 extraction, etc.) are rarely reported
to be combined with gas oxidation. This combination may
greatly improve gas phase purification yield owing to the early
elimination of metal particles, which can catalyze the CNT
oxidation [48,49].

2.4.

Applicability of typical purification techniques

A high yield of high-purity CNTs can hardly be obtained using
a physical or chemical method alone because of specific limitations, while high-purity and high yield CNTs can be obtained using multi-step purification by using the advantages
of the both processes. Furthermore, it is possible to control
the morphology and structure of CNTs by selecting suitable
methods. To enable one to fully understand the characteristics of each method and make a wise combination based on


Table 3 – Applicability of some typical purification technologies
Technologies

Characteristics

Yield (wt%)

Applicability
Carbonaceous impurity
Cama/Cnpb

Chemical method

Gas phase

Liquid phase

Electrochemical

Filtration
Centrifugation
Solubilization with functional groups

Multi-step method

a
b
c
d
e
f
g
h
i
j
k
l
m
n

HIDE, wet grinding, filtration, oxidation, sonication,
centrifugation
Filtration/magnetic filtration, oxidation, annealing

Sonication in H2O2, HNO3 /HF/ SDS, filtration
High temperature annealing, extraction

Adding functional groups,
cutting or opening CNTs

Suitable for aligned CNTs
Non-destructive
Non-destructive, small
amount
Improve crystallinity
High selectivity to metal
Separate CNTs by length or
conductivity
High-purity, low yield
High-purity respect to metal
High-purity, little damage
Metal free, improving
crystallinity

Csole

Mexpf Mpolg Mcnth

$2–35
$15
$30
25–48
$30–50
10–75

30–75
10–60
$80

Ji
J
J
J
J
J
J
J*
J*

j





J*



J*k
J*
J*
J*
J*
J*

J*
J*












Jml
Jm
Jm
J
J
J
J
J
Jm

J*mm
J*m
J*m
J
J*
J*

J
J*


J*m
J*m
J*m
J*
J*
J*
J*
J*
J/J*

[41–45]
[47]
[48,49]
[51]
[58,59]
[61–63]
[64–69]
[71–76]
[27,83]

$30–84
$10–40
$17–50

J*
J*

J*

J*
J*
J*





J
J
J

J*
J*


J*
J*
J*





[84–86]
[87,88]
[89–92]

$70–90
$10–/n
/



J



J









J
J
J*

J
J
J*

J
J



[98–102]
[107–109]
[110–113]

$2%

J

J

J*

J

J

J*

J*

[127]

$9–20%
$25
$90%

J
J

J*


J*
J*

J
J*
J

J
J
J

J
J
J

J
J*
J

J*
J*
J

[135]
[130]
[150]

4 6 ( 20 0 8 ) 2 0 0 3–20 2 5

High temperature annealing
Other techniques to remove metal particles
Chromatography, electrophoresis, FFF

Opening CNT caps, low yield,
less damage to tube wall

Cadd

CARBON

Physical method

Air
Cl2, H2O, HCl
H2O, Ar, O2
O2, C2H2F4, SF6
HNO3
H2O2, HCl
Mixture of acid or KMnO4
Microwave in inorganic acid
Alkali or acid solution

Cgc

Ref.

Metal impurity

Amorphous carbon.
CNP.
Graphite particles.
Carbon impurity adhering to CNT walls.
Soluble carbon in some organic solutions (CS2, toluene).
Exposed metal.
Metal wrapped by polyhedral carbon.
Metal encapsulated in CNTs.
Effective.
Not effective.
Partly removed.
Further HCl treatment.
Further HCl treatment and partly removed.
No report.

2019


2020

CARBON

4 6 ( 2 0 0 8 ) 2 0 0 3 –2 0 2 5

one’s need and the quality of the as-prepared CNTs, it is
essential to give a clear image of the basic characteristics of
different techniques. Based on up-to-date knowledge, we
summarize the applicability of representative methods in Table 3. In general, chemical oxidation is effective in eliminating
amorphous carbon, adding functional structures, and cutting

and/or opening CNTs; inorganic acid treatment can effectively remove exposed metal particles. Physical treatment is
capable of removing metal particles, sorting CNTs by length
or conductivity, and, to some extent, separating isolated carbonaceous particles by size.
It is well known that the content, morphology and structure of impurities in as-prepared CNT samples strongly depend on their synthesis methods and conditions. Therefore,
there is no normalized method for routinely creating highquality CNTs. Here we summarize a flow chart showing representative procedures based on the function and characteristics of the purifications (Fig. 15). It should be a reference for
obtaining purified CNTs according to different requirements.

3.

Challenges

Obviously, notable progress has been made on the purification
of CNTs in recent years, and small amounts of high-purity
CNTs can be readily obtained in spite of the disadvantages

of high cost and long time involved. However, there are still
many technical barriers to be overcome when cost, reproducibility, environment compatibility, and scaling-up are taken
into account. Considering that the purification of CNTs is a
complex issue involving the quality of the raw CNT sample,
purification and purity assessment, we point out the following three main challenges: synthesis methods, purification
and purity assessment.

3.1.

Synthesis methods

Carbonaceous and metallic impurities are produced as byproducts during the synthesis of CNTs. If pure CNTs without
any impurity were directly synthesized, the purification
would be obviated. A decade ago, Kyotani et al. [151] synthesized MWCNTs by a template CVD using an anodic aluminum
oxide film as template. The resulting MWCNTs are free of carbonaceous and metal impurities. Furthermore, the length,

diameter and wall thickness of the CNTs can be uniformly
controlled by easily controlling the structure of the template.
Recently, Li et al. [152] reported that pure SWCNTs with a purity of $96% were produced by CVD using ethanol as carbon
source. The only impurity was iron nanoparticles that were
exposed on the surface and could be easily removed by acid.
Batch-scale preparation of ultralong SWCNT arrays with high-

As-prepared CNT sample
CS2 extraction to remove fullerene or aromatic carbon
Purity evaluation
Low purity
High purity

Physical separation method

Removing metal impurity
Wrapped by carbon layers

Exposed metal

Improve crystallinity

Maintain CNT structure

High temperature annealing

Other physical techniques

HCl treatment

Removing carbonaceous impurity
Removing carbonaceous impurity
Polymer extraction
Adding functional groups
or cut CNTs
Liquid phase oxidation

Maintain structure

Physical method

Opening caps

Gas phase oxidation

Separation
Conductivity

Length < 300 nm

Length > 300 nm

Dielectrophoresis

Chromatography

Electrophoresis or FFF

Fig. 15 – A flow chart showing the representative procedures based on the function and characteristics of the purifications.

CARBON

4 6 ( 20 0 8 ) 2 0 0 3–20 2 5

quality was achieved by ultralow flow rate CVD [153]. At the
same time, cloning growth from an existing nanotube segment was also reported [154,155], which ties synthesis and
selectivity in chirality into one single step. Despite the progress achieved in directly synthesizing high-purity CNTs,
the following challenging issues in synthesis still remain:
(a) Large-scale synthesis of high-purity CNTs with controllability in diameter, length, wall thickness, chirality and
conductivity.
(b) Synthesis of isolated CNTs with desired structures.
(c) Growth of CNTs at preset positions for device
applications.

3.2.

Purification methods

As pointed out above, most of the as-prepared CNTs contain a
certain amount of impurity. To obtain high-quality CNTs,
purification is inevitable. The demand for obtaining uniform,
high-purity CNTs with low cost and high efficiency on a large
scale requires the following major challenges to be overcome.
(a) How to efficiently combine different purification techniques according to the requirements of various applications and the characteristics of the raw CNT samples.
(b) How to solve the problems brought about by the scaling-up of CNT purification, such as the uniformity of
the CNTs and the homogeneous contact between
impurities and oxidants.
(c) How to develop novel techniques that can effectively
achieve purification, structure tailoring, and structure/

property sorting of CNTs.

3.3.

the purification strategy must vary according to the features
of the raw CNTs, and the specific requirement and target
applications of the purified CNTs. Although considerable progress has been achieved, it is still difficult to find a simple
method that can purify CNTs effectively. Multi-step methods
that combine the advantages of different purifications provide a solution for getting CNTs with higher purity.
With regard to the different types of CNTs, the purification
of MWCNTs is relatively easy and effective, whereas the purification of SWCNTs and DWCNTs always results in low yield
and defect formation. Because of this, more efforts aimed at
selectively sorting SWCNTs by length, diameter, transport
properties, and even chirality, are required. Therefore, challenges still remain in the purification of CNTs. High-purity,
high yield, low cost, and environmental friendliness are
essential. Establishing a CNT purity assessment standard is
necessary for evaluating and improving the validity of purification. Without question, the exploitation of efficient CNT
purification techniques that can produce high-quality and
homogeneous CNTs will greatly facilitate fundamental research and practical applications of CNTs.

Acknowledgements
We acknowledge financial support from MOST (2006
CB932703), NSFC (90606008, 50672103), and Chinese Academy
of Sciences. The authors appreciate very much Prof. Peter
Thrower’s advice and comments. We thank Dr. Ren WC and
Liu BL for useful discussion on Raman and UV–vis-NIR characterization methods.

R E F E R E N C E S

Purity assessment

An important issue on the purification of CNTs is purity evaluation. Although several techniques have been used for evaluating the purity of CNTs, a standard and well-recognized
purity assessment protocol has not been established. This
leads to uncertainty in the purity of the products obtained
as well as the efficiency of purification processes. Therefore,
the following challenges also remain for purity assessment:
(a) To establish a standard characterization protocol to
evaluate and compare different CNT samples.
(b) To establish a standard that can fully describe the characteristics of the purified CNTs, such as CNT content,
amounts of different impurities, defects, etc.
(c) To establish a standard that can give an overall evaluation of the efficiency purification, according to the CNT
quality, yield, cost, environment compatibility, etc.

4.

2021

Concluding remarks

Techniques for the purification of CNTs have been reviewed,
with an emphasis on their purification principles. Since the
quality and accompanying impurities of as-prepared CNTs
strongly depend on the synthesis method and experimental
conditions, it is hard to propose a universal method. In fact,

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