C A R B O N 5 0 ( 2 0 1 2 ) 4 1 8 4 – 4 1 9 1

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Room-temperature synthesis of single-wall carbon nanotubesby an electrochemical process

Ahmed Shawky, Satoshi Yasuda, Kei Murakoshi *

Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Hokkaido, Japan

A R T I C L E I N F O

Article history:

Received 23 February 2012

Accepted 27 April 2012

Available online 11 May 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.04.068

* Corresponding author: Fax: +81 11 706 4810E-mail address: kei@sci.hokudai.ac.jp (K.

A B S T R A C T

Single-wall carbon nanotubes (SWCNTs) were produced by an electrochemical route by

applying a small negative potential to a solution of acetic acid over a Au surface supporting

Ni nanocatalysts. Ni nanocatalysts were grown electrochemically on Au surface and their

particle sizes were controlled by deposition time. Raman spectroscopy and scanning probe

microscopy observations of the catalyst and as-deposited samples and revealed that the

catalyst structure strongly affects the SWCNT diameter distribution. The deposited carbon

structure depended on the catalyst particle size and structure. Raman spectra confirmed

the existence of selectively grown semiconducting SWCNTs with very narrow diameter

distribution.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Single-walled carbon nanotubes (SWCNTs) have attracted a

lot of interest since the landmark paper by Iijima and Ichih-

ashi [1]. The unique properties of SWCNTs makes them suit-

able for many applications including optoelectronic [2,3] and

catalytic [4] applications as well as in nanodevices such as

sensors [5], memory elements, and field-effect transistors

[6–8]. Over the past decade and a half, the synthesis of

SWCNTs has been greatly improved, resulting in orders of

magnitude increases in nanotube lengths and yields [9,10].

Several research works have been performed seeking the

selective growth of metallic or semiconducting SWCNTs with

narrower chirality distribution in order to obtain unique prop-

erties for direct use in applications [7,8,11–14]. However, there

is still relatively feeble control of chirality and diameter distri-

bution of such nanotubes which mainly determine its elec-

tronic properties [15]. One reason why it is difficult to

control SWCNTs is ascribed to the high temperature growth

condition. Conventional techniques for synthesizing

SWCNTs, such as arc discharge, laser ablation, and chemical

vapor deposition (CVD), are performing high temperatures to

er Ltd. All rights reserved

.Murakoshi).

activate the catalyst and decompose carbon feedstock. How-

ever, such high temperature generates large thermal fluctua-

tions at the nanocatalyst, which are expected to deform and

aggregate particles. These dynamic changes to the catalyst

during SWCNT growth are thought to induce particle aggrega-

tion on the substrates by Ostwald ripening and altering the

interaction between the catalyst and graphitic lattices, result-

ing in the formation of SWCNTs with wide chirality distribu-

tions [7,16,17]. This significant problem points out the

extreme challenging to achieve diameter selectivity [18,19]

or to develop a technique for production of uniform SWCNTs

[13,16,20–22].

As to one of the solutions to develop selective SWCNT syn-

thesis, an electrochemical approach would be considered.

The thermal fluctuations should be reduced by this method

because synthesis proceeds in the liquid phase and room

temperature conditions. Consequently, it is expected to be a

possible method for the selective growth of SWCNTs with

well-specified structures. Carbon deposition was first per-

formed by electrolysis using methanol and carbon dioxide

as carbon sources [23,24]. A mixture of diamond-like carbon

(DLC) and amorphous carbon has been produced from the

.

Fig. 1 – (a) Schematic representation of experimental setup

used for catalyst/carbon deposition and (b) represented bias/

current waveforms generated for deposition.

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 8 4 – 4 1 9 1 4185

reaction of electrochemically produced hydrocarbon radicals

[25]. It has been reported that nanofibers and multi-walled

carbon nanotubes (MWCNT) were investigated [26–29]. How-

ever, SWCNTs have not been yet produced.

Carbon deposition by electrochemical process mostly per-

formed at high applied potentials (orders of kilovolt) to

decompose carbon sources by electrostatic force. Conse-

quently, considerable amounts of radicals are expected to be

generated on the substrate surface. These radicals induce

undesirable side reactions on nanotube structures that are

expected to inhibit efficient formation. In addition, numerous

CVD studies have revealed that using appropriate metal cata-

lysts smaller than 2 nm results in preferential growth of

SWCNTs rather than MWCNTs [30]. Thus, it is very important

to control the size of metal catalysts on electrodes. However,

there have been no experimental studies of electrochemical

deposition of carbon over size-controlled metal catalysts.

Here, we present an electrochemical method for producing

SWCNTs. Ni nanoparticles with sizes controlled to 2 nm were

prepared on Au surfaces by electrochemical deposition and

subsequent reduction of acetic acid solution as carbon source.

A moderate deposition potential was used for carbon deposi-

tion. By combining both processes, SWCNTs were success-

fully synthesized for the first time by an electrochemical

process at room temperature. Furthermore, Raman spectros-

copy analysis suggested the production of pure semiconduct-

ing SWCNTs with narrow diameter distribution. This room-

temperature synthesis method in the liquid phase is expected

to be a key technique for realizing industrial-scale production

of SWCNTs and electronic nanodevices.

2. Experimental

Fig. 1(a) schematically depicts the experimental setup. All

chemicals were purchased from WAKO chemicals, Japan

without further purifications. A homemade three-electrode

cell was used with a Pt sheet as the counter electrode and a

Ag/AgCl in a saturated KCl solution as the reference electrode.

Au beads with atomically flat (111) facets were prepared by

flame annealing and were used as the working electrode.

Such (111) surfaces enable detailed and precise characteriza-

tion of deposited structures. Both the catalyst and carbon

were deposited at controlled electrochemical potentials using

a Hokuto Denko (HSV-100) potentiostat with waveform shown

in Fig. 1(b). Ar or N2 gases were bubbled from the inlet to re-

move the oxygen contents from the electrolytes. Ni was elec-

trochemically deposited at �1.2 V for the polarization times of

5, 10, and 100 ms from a modified Watt’s electrolyte contain

0.01 M NiSO4 + 0.01 M H3BO3 + 10�4 M H2SO4. After depositing

the Ni nanoparticles, carbon was deposited on the Au beads

at �1.0 V for 30 min from an aqueous solution contain a mix-

ture of 1% (v/v) CH3COOH as the carbon source and 0.1 M

Na2SO4 as the supporting electrolyte (final concentration of

0.17 M; pH 2.9). Before characterization, the as-deposited

samples were rinsed with distilled water and dried with Ar

gas. Raman spectra measurements using different excitation

lasers with wavelengths of 785, 632.8, and 532 nm were per-

formed in air using RENISHAW Raman spectrometer and

homemade one to characterize the deposited carbon materi-

als. Tapping atomic force microscope (AFM) and scanning

tunneling microscope (STM) (Nanoscope IIIa, Veeco) were ob-

tained in air to analyze the deposited structures.

3. Results and discussion

3.1. Ni catalyst deposition

We used Ni as a metal catalyst because it is a very effective

metal for growing carbon nanotubes and graphene by CVD

[9,18]. Ni nanoparticles were deposited on an Au surface

and their size controllability was investigated. The cyclic vol-

tammogram (CV) of Au beads immersed in the solution con-

tain Ni ions revealed that Ni starts to be deposited on Au

beads at a negative bias of more than �0.65 V [31]. According

to previous work, STM measurements revealed the formation

of Ni particles on Au (111) for overpotential deposition of Ni

at �1.2 V, and the particles sizes of 10–20 nm were grown after

just 50 s deposition time [32]. From the consideration of these

results, we have shortened the deposition time to produce

smaller Ni particles. When the deposition time reduced to

the order of milliseconds, we revealed that a smaller, repro-

ducible and well-controlled Ni nanoparticle could be pre-

pared. STM analysis was used to characterize the as-

deposited Ni structures in detail. Fig. 2(a–c) show STM images

and corresponding particle height histograms (below each

image) for Ni nanoparticles formed at deposition times of 5,

10, and 100 ms, respectively. As seen from this figure, Ni

nanoparticles with heights of approximately 0.7 and 0.5 nm

were grown over Au (111) surface for the deposition times

Fig. 2 – 3D STM images of Ni nanoparticles deposited on Au (111) facets at �1.2 V for deposition times of (a) 5, (b) 10, and (c)

100 ms and corresponding particle height histograms for deposited Ni nanoparticles (below each image), (d–f) are higher

magnified images of Ni nanoparticles deposited over Au (111) surface for deposition times of (d) 5, (e) 10, and (f) 100 ms. The

insets show cross-sectional analysis of selected particles. They indicate that increasing the deposition time can affect the

particle size and change the structures produced from protrusions in (d) to small islands in (e) to larger islands in (f).

4186 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 8 4 – 4 1 9 1

of 5 and 10 ms, respectively, whereas the 100 ms deposition

produces a larger particles and islands of approximately

0.4 nm in heights. Fig. 2(d�f) displays a magnified STM images

for Ni nanoparticles with individual height profile insets. It is

clear that the structure of Ni nanoparticle differed greatly.

The height profiles revealed that the particles formed for a

deposition time of 5 ms consisted of a single protrusion,

whereas the particles formed for deposition times of 10 and

100 ms are of island structure. Moreover, the Ni nanoparticles

formed after a deposition time of 10 ms are almost three

times larger than those deposited at 5 ms. The island size in-

creased to be approximately 10 nm for the deposition time of

100 ms. These results signify that the Ni catalyst particle size

and structure are strongly depended on deposition time and it

could be controlled prior to carbon deposition.

3.2. Carbon deposition over Ni catalysts

Carbon electrodeposition was performed over Au beads cata-

lyzed with Ni nanoparticles. Acetic acid has been used previ-

ously as the carbon source for DLC synthesis [33]. Thus, its

polar nature could be also suitable as source for carbon depo-

sition. We first investigated the reduction of acetic acid via the

CV measurements. Fig. 3 shows CVs of bare Au beads in

H2SO4 aqueous solution in the absence of acetic acid (blue

curve), and 0.1 M Na2SO4 aqueous solution containing acetic

acid (red curve). To estimate the effect of hydrogen evolution,

the pH of H2SO4 aqueous solution in the absence of acetic acid

was adjusted to be 2.9 by adding 1 M NaOH solution. At the

negative potential scanning, current density at H2SO4 solution

without acetic acid increased due to hydrogen evolution and

reaches its maximum at �0.76 V to be �1.63 mA cm�2. On

the other hand, in the solution containing acetic acid, the cur-

rent density increased significantly reaching �7.1 mA cm�2 at

�0.7 V in spite of the comparable pH of H2SO4 solution with-

out acetic acid. These observations suggest the increment in

the current density originates from polarization and reduc-

tion of acetic acid molecules on the Au surface. We also per-

formed CV for Au beads with Ni nanocatalysts. However

there was no significant increments in the reduction curve,

Fig. 3 – Cyclic voltammograms of Au beads in solution of

H2SO4 (pH 2.9) without acetic acid and Na2SO4 (pH 2.9) with

acetic acid (blue colored). The black colored arrow at �1.0 V

shows the increase of the cathodic current due to addition of

acetic acid; scan rate: 50 mV/s. (For interpretation of the

references to colour in this figure legend, the reader is

referred to the web version of this article.)

Fig. 4 – Raman spectra of as-deposited carbon structures

over different sized Ni nanocatalysts (colored) compared to

reference carbon materials (dark gray). The Raman spectra

denoted for reference carbons are (a) HOPG, (b) commercial

SWCNT sample, (c) DLC film and for as-deposited samples

are (d) over bare Au surface and over Au supporting Ni

nanoparticles deposited for (e) 5, (f) 10, and (g) 100 ms.

Samples are in coincidence to the referenced carbon

materials. (For interpretation of the references to colour in

this figure legend, the reader is referred to the web version

of this article.)

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 8 4 – 4 1 9 1 4187

suggesting a small amount of catalyst are grown on Au sur-

face. In this study, carbon deposition was performed at

�1.0 V for 30 min.

The electrodeposited carbon structures were analyzed by

Raman spectroscopy which is a powerful tool for characteriz-

ing the structures of carbon materials. In particular, it can be

used to evaluate the chirality, type (i.e., semiconducting or

metallic), and defects in SWCNTs [34–43].

Although the SWCNTyield was quite low, we carefully per-

formed Raman measurements for different samples by

observing many sites on Au electrode to characterize via Ra-

man measurements of SWCNT.

Fig. 4 shows Raman spectra for the as-deposited samples

grown over Au (111) surface supporting different-sized Ni

nanocatalysts (denoted as d, e, f and g for 0, 5, 10 and

100 ms Ni deposition times, respectively). For comparison, Ra-

man spectra of highly ordered pyrolytic graphite (HOPG),

commercial SWCNTs grown by CVD, and DLC spectra [44]

are denoted as a, b and c, respectively in the same figure.

The Raman spectra of the carbon materials generally

showed characteristic features of the bands, namely radial

breathing mode (RBM) at 100–300 cm�1, D-band (1300–

1350 cm�1), and G-band (1500–1600 cm�1) as shown in Fig. 4.

The RBM originates from vibrations of tube structures and

its frequency is inversely proportional to the tube diameter.

The RBM can be observed only SWCNTs that are resonant

with the laser excitation energy. It can be used to specify

the chirality of SWCNTs. The D-band is due to disorder-in-

duced C–C bonds and it appears when there are disordered

structures or defects. The G-band originates from the vibra-

tion of sp2 graphitic carbon structures and is visible in the

spectra of the SWCNTs and HOPG. As can be seen from

Fig. 4, the Raman spectrum of carbon deposited without Ni

nanocatalysts (Fig. 4(a)) did not exhibit any RBM and G-band

features. Rather, it has a broad D-band centered at

1360 cm�1, indicating the formation of an amorphous carbon

phase. By contrast, the Raman spectra of carbon deposited

over Ni nanocatalysts differed greatly. For carbon deposited

over 5 ms Ni nanocatalysts, sharp RBM and G-band peaks

were clearly observed at 224 and 1593 cm�1, respectively

(Fig. 4(e)). These observed Raman features strongly indicate

the deposition of SWCNTs over the Ni nanocatalysts. Further-

more, the observed single RBM peak corresponds to an indi-

vidual SWCNT, suggesting that the synthesized SWCNTs

have a very narrow diameter distribution. From the inversely

proportional relationship between the RBM frequency and the

SWCNT diameter, the produced SWNTs are assigned to (10,5)

semiconducting SWCNTs with a diameter of 1.03 nm [41–43].

Fortunately, The Raman feature of SWCNTs was also ob-

served for the sample deposited over of 10 ms Ni nanocatalyst

(Fig. 4(f)). Though multiple RBM peaks appeared at 219, 229,

and 238 cm�1, which correspond to semiconducting SWCNTs

with chiral indices of (9,7), (8,7), and (12,1), respectively

[41,42]. The broad D-band at 1317 cm�1 indicates that the

SWCNTs contain disordered structures. Observation of multi-

ple RBMs strongly designates that the larger catalyst size pro-

duce SWCNTs with different chiralities. On the other hand,

carbon deposited over the 100 ms Ni islands had a different

Raman spectrum from the SWCNT spectra (Fig. 4(g)), indicat-

4188 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 8 4 – 4 1 9 1

ing that another phase had formed. No RBM peaks were ob-

served, while there were several peaks at 1287, 1353, 1433,

and 1515 cm�1 and a G-band peak at 1600 cm�1. The origin

of the Raman peak at 1515 cm�1 is related to amorphous car-

bon phase [44]. The peaks at 1287, 1353, and 1433 cm�1 may

arise from stretching vibrations of alkyl groups and different

carbon phases, that the produced structures are composed

of graphitic and hydrogenated carbon structures mixed with

a DLC phase [40,44].

The above results imply the effect of the Ni particle size on

the structure of deposited carbons. Based on STM analysis, it

was realized that the Ni initially forms protrusion-like struc-

tures that become islands by increasing deposition time. The

100 ms deposition time produced island-like structures that

were approximately 10 nm in size. We suppose, at this large

island, there is no longer ability for the SWCNT growth due

to lower catalytic activity of these Ni islands, causing the for-

Fig. 5 – (a) STM image for as-deposited sample on bare Au

(111) surface. (b) AFM image for as-deposited sample on Au

(111)/Ni nanoparticle surface. Deposition potential was

�1.0 V for 30 min. Images are clearly showing the formation

of tube structures rather than layers in case of using Ni

(indicated by arrows) as a catalyst. Insets are higher

magnified images.

mation of graphitic and hydrogenated carbons rather than

SWCNTs.

Morphological study by using STM and AFM were also ap-

proved the formation of nanotubes in case of using Ni nano-

catalyst rather than carbon layers on the bare Au (111)

surface. Fig. 5 showed the morphology of the as-deposited

carbon structures over the Au (111) surface with and without

Ni nanocatalyst. As we can see in Fig 5a, the STM image

showed amorphous layers with approximate thickness of

0.32 nm were grown from the Au (111) terrace and nanoparti-

cles on the facet surface can be easily observed in magnified

image. On the other hand, AFM images in Fig.5 indicated

tubular structures of cross section 0.8–1.6 nm grown from

the Ni nanocatalyst particles deposited over Au (111) surface.

The above observations indeed suggest the formation of

SWCNT depend on the presence of Ni nanocatalyst. These re-

sults are supporting the Raman spectra showing SWCNT for-

mation in Au/Ni system (see Supplementary data).

To confirm the growth of uniform SWCNTs, Raman spectra

were obtained using different laser excitation wavelengths

because the RBM in the Raman spectra of SWCNTs depends

strongly on the laser excitation wavelength [37,39]. When

the laser excitation energy is equal to the band-gap energy

of a SWCNT, strong optical absorption increases the RBM sig-

nal intensity; this is known as the resonant Raman process.

When there is no matching, the RBM signal disappears due

to the off-resonance Raman process. The Raman spectra at

different excitation wavelengths of 785, 632.8 and 532 nm

for as-deposited sample grown over Ni protrusion is shown

in Fig. 6. The obtained Raman spectra with 785 nm excitation

has a single RBM peak at 229 cm�1 assigned to (8,7) SWCNT.

D-band and G-band features were also observed at 1295 and

1593 cm�1, respectively. However the RBM feature was not ob-

served for excitation wavelengths of 632.8 and 532 nm, indi-

cating the off-resonance Raman process. According to

Kataura et al. [41], the excitation wavelengths of 632.8 and

532 nm are off resonance for (8,7) semiconducting SWCNTs

and it does not show a RBM signal. This agrees with the pres-

ent experimental results. Thus, we conclude that SWCNTs

Fig. 6 – Raman spectra of selected as-deposited SWCNTs on

Au (111)/Ni protrusion surface at different excitation

wavelengths of (a) 785 nm (1.58 eV), (b) 632.8 nm (1.96 eV)

and (c) 532 nm (2.33 eV). Resonance with the 1.58 eV laser

indicates semiconducting SWCNTs with a narrow chirality

distribution (8,7).

C A R B O N 5 0 ( 2 0 1 2 ) 4 1 8 4 – 4 1 9 1 4189

are indeed synthesized by an electrochemical process at room

temperature. Generally, when the laser excitation wavelength

changed, another SWCNTwith a different chirality can be ob-

served for the SWCNTs samples with wide-diameter distribu-

tion. However, the fact that other RBM frequencies were not

observed at other excitation wavelengths specifies the selec-

tive production of (8,7) semiconducting SWCNTs. These Ra-

man spectra analyses demonstrate that appropriately sized

Ni nanoparticles catalyze SWCNT growth under moderate

electrochemical reduction. Particularly, the Ni nanocatalyst

size and shape significantly affect the carbon structural phase

produced and either SWCNTs or other carbon structures can

be grown. Surprisingly, the excitation wavelength depen-

dence strongly suggests highly selective growth of semicon-

ducting SWNTs.

3.3. Growth mechanism of SWCNTs

We considered the growth mechanism of electrodeposited

SWCNTs. Tremendous studies have investigated for cathodic

reduction of carboxylic acid. This reaction produces a carbox-

ylate anion, an aldehyde, an alcohol, and a hydrocarbon

through one-, two-, four-, and six-electron reductions, respec-

tively. Acetic acid can exhibit these reaction processes as

follows:

CH3COOHþHþ þ e� ! CH3CO�2 þ 1=2H2 ð1Þ

CH3COOHþ 2Hþ þ 2e� ! CH3CHOþH2O ð2Þ

CH3COOHþ 4Hþ þ 4e� ! CH3CH2OHþH2O ð3Þ

CH3COOHþ 6Hþ þ 6e� ! CH3CH3 þ 2H2O ð4Þ

During cathodic deposition on Au/Ni system, hydrogen is

evolved at biases of more than �0.7 V in aqueous solution of

acetic acid, suggesting one electron reduction of protons

according to Eq. (1). Further reduction will occur and the final

reduction process from ethanol to ethane (Eq. (4)) will pro-

duce ethane radicals. We conjecture that CH3 and C2H5 radi-

cals produce carbon materials by partial dehydrogenation

and recombination of each radical. The above results indicate

that these radicals adsorbed on the edges of the Au terraces

and that they formed the carbon structures by dehydrogena-

Fig. 7 – Proposed mechanism for SWCNT growth on Au s

tion (Fig. 4(d)). Although the sp2 hybridization mechanism is

still unclear, the presence of Ni nanocatalysts is considered

to accelerate the growth of sp2 carbons due to their perfect

matching with the graphene lattice [17,45–47]. In fact, SWCNT

vibrational spectra were observed at the substrate with Ni

nanocatalyst (Fig. 4(e and f)). The SWCNT growth observed

in our results may be based on the root growth mechanism

of carbon atoms from Ni nanocatalyst [17]. During the initial

growth stage, the acetate ion adsorbed on the catalyst sur-

face, and then hydrocarbon radicals (C2H5� and CH3

�) diffused

over it and dehydrogenated by subsequent applied potential

forming carbonaceous structures which may tend to form

small fullerene-like hemispheres that encapsulate the Ni

nanocatalyst particles. Additional carbon atoms produced

from radical dehydrogenation tended to diffuse toward the

sides of the catalyst, leading to SWCNT formation. The pro-

posed growth mechanism can be seen schematically in

Fig. 7. During the growth process, the moderate applied po-

tential suppresses the generation of large amounts of radicals

so that the undesired side reaction between the radicals and

the produced SWCNT is effectively inhibited. Eventually,

SWCNT formation is considered to be promoted. Further-

more, room-temperature growth stabilizes the catalyst parti-

cle structure so that there is little probability of deformation

occurring. We suppose the selective SWCNT growth is due

to the reduction of thermal fluctuations during synthesis pro-

cess. As the SWCNT diameter is directly correlated with the

catalyst particle size and structure [7,18], small structural

changes in the Ni nanoparticles will affect the diameter and

chirality of SWNTs. Hence, we believe that the proposed

method is a promising approach for achieving chiral selective

growth of SWCNTs.

It should be noticed that the SWCNT yield is still very low.

We suppose that such low yield come from difficulty in the

formation of carbon–carbon bond. To produce carbon nano-

tubes, effective formation of carbon–carbon bonds via the

reductive reaction of carbon source to generate hydrocarbon

radicals. However, most of reactions may produce aldehyde

by two-electron reduction, and only a few reactions may con-

tribute to the formation of the carbon nanotube. Efficient gen-

eration of hydrocarbon radical should be the next challenges

for high-yield synthesis of SWCNT.

upporting Ni nanocatalyst by electrochemical process.

4190 C A R B O N 5 0 ( 2 0 1 2 ) 4 1 8 4 – 4 1 9 1

4. Summary

We have synthesized SWCNTs and other carbon materials by

an electrochemical method at room temperature. We found

that using size-controlled nanocatalysts and applying a mod-

erate potential are key aspects in the synthesis of SWCNTs by

our approach. In this process, size-controlled Ni nanocata-

lysts were initially electrodeposited on a Au (111) surface,

and then carbon was deposited on the Au/Ni system by the

electroreduction of acetic acid solution.

Raman spectroscopy and SPM results indicated clear sig-

natures for SWCNTs formation in presence of Ni catalyst

rather than amorphous carbon layers on bare Au surface.

RBM, D-band and G-band were surprisingly appeared for

SWCNT existence, chirality assignment based on the RBM fre-

quency revealed the existence of semiconducting SWCNTs

with a narrow diameter distribution. Raman spectra obtained

using different laser excitation wavelengths evidently show

the production of pure semiconducting SWCNTs. This

room-temperature process appears promising technique for

the future production of various electronic devices, particu-

larly flexible SWCNT-based devices on plastic substrates. It

should be mentioned that the SWCNT yield is currently very

low for industrial applications. However, it seems possible

to increase the yield and the quality of produced SWCNTs

through further work.

Acknowledgements

We thank Associate Prof. K. Ikeda and J. Sato of Hokkaido Uni-

versity for help with some Raman spectra, AFM measure-

ments and useful discussions. This work was partly

supported by the GCOE project ‘‘Catalysis as the Basis for

the Innovation in Materials Science’’ at Hokkaido University.

We also acknowledge partial financial support from the Min-

istry of Education, Culture, Sports, Science, and Technology

(MEXT) of Japan through Grants-in-Aid (Nos. 19049003 and

22016001).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at http://dx.doi.org/10.1016/j.carbon.

2012.04.068.

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