Multiwalled Carbon Nanotubes Produced byDirect-Current Arc Discharge in Foam

Yong-il Kim, Eiichi Nishikawa, and Toshihide Kioka

e-J. Surf. Sci. Nanotech. 6 (2008) 167-170

e-Journal of Surface Science and Nanotechnology

24 June 2008

Regular Paper

e-J. Surf. Sci. Nanotech. Vol. 6 (2008) 167-170

Multiwalled Carbon Nanotubes Produced by Direct-Current Arc Discharge in Foam
Yong-il Kim,∗ Eiichi Nishikawa, and Toshihide Kioka
Division of Electrical Engineering, Tokyo University of Science,
1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan(Received 21 April 2008; Accepted 5 June 2008; Published 24 June 2008)
The arc discharge method was the first method developed for producing multiwalled carbon nanotubes (MWNTs).
Generally, this method is carried out in low-pressure He gas or other neutral atmospheres, which requires the useof sealed reaction chambers and vacuum equipment. At high temperatures, the method produces well-graphitized
MWNTs. Here, we report the first successful trial of the facile synthesis of high-quality MWNTs in a high yieldusing the arc discharge method in foam. [DOI: 10.1380/ejssnt.2008.167]
Keywords: Multiwalled carbon nanotube; DC arc discharge; FoamI.

INTRODUCTION

High-quality crystalline multiwalled carbon nanotubes(MWNTs) have been generated by the arc discharge
method in gas phase; they were first prepared by Ebbesen and Ajayan in helium gas phase [1]. Generally, thearc discharge method is carried out in low-pressure Hegas or other neutral atmospheres, which requires the useof sealed reaction chambers and vacuum equipment. To
eliminate this requirement, arc discharge in liquid phasehas been investigated by some groups as an effective
method of CNT formation [2–5]. For example, Sano et al.
have described the formation of carbon nano-onions using
DC arc discharge in water [6, 7]. Moreover, Lange et al.
have generated CNTs by arc discharge in water between
pure graphite electrodes [8]. It is interesting to study thearc discharge process in liquid phase. Such a method has
the advantages of not requiring a vacuum system because
it is not carried out in gas phase, and of using simple
cost-effective approaches to CNT formation. These advantages will not only promote fundamental research but
will also foster industrial applications of CNTs.
In this study, we examine the arc discharge method in
the gas and liquid phases to determine the more costeffective means of achieving a high yield of highly crystalline CNTs. We used the foam phase for arc discharge,
which is the critical phase between gas and liquid. We
used a non-vacuum system, thereby eliminating almost
all the complex and expensive machinery usually requiredin the arc discharge method. The technique used in thisstudy facilitated the collection of fine particles including
CNT clusters with a high yield of MWNTs from the foam
surface.

II.

EXPERIMENTS

A schematic of the apparatus for arc discharge in foam
is shown in Fig. 1. The equipment is composed of a chamber, graphite electrodes as the cathode and anode, and a
DC power supply (Kenwood: PD36-20A). We used beerfoam, which is a multiphase flow of liquid and gas, as theenvironment for the gas discharge.

∗ Corresponding author: j4306702@ed.kagu.tus.ac.jp

FIG. 1: Schematic of the experimental setup for MWNT
preparation by the DC arc discharge method in foam.

It is known that foam generally has the ability to attract substances floating inside liquids. More specifically,hydrophobic substances, such as solid particles, are attracted to foam, whereas hydrophilic substances are not.
There are selective separation methods based on floatation which utilize this difference in the attractability tofoam, for example, the separation of target minerals fromunnecessary ones. Furthermore, in everyday life, the principle according to which dirt particles are attracted tofoam when doing laundry, as well as the principle where
the impurities released while boiling certain foods float
and become attached, are all related to the property offoam to “attract solid particles floating inside liquids”.
Similarly, beer froth comprises liquid membranes that
include proteins, and the structure allows liquid to becaptured between the surfaces of two membranes made ofproteins. Therefore, beer froth has the effect of attracting and agglomerating astringent impurity compounds,
and this can be felt as astringent taste. Consequently, byutilizing the agglomerating effect of beer froth as well as
the property of nanotubes, which are hydrophobic in nature, to generate attraction to the film of beer froth dueto the Coulomb interaction, we were able to investigate
both the coupling and the decoupling of nanotubes at thec 2008 The Surface Science Society of Japan (http://www.sssj.org/ejssnt)
ISSN 1348-0391 °167

Volume 6 (2008)

Kim, et al.

FIG. 2: DC arc discharge in foam: (a) Arc discharge in foam;(b) Surface of the foam after DC arc discharge.

same time.
We used a beer dispenser (Fukushima Industries Corporation: FTN-38S1) as the beer foam supply, and thepressure of the carbon dioxide gas in the dispenser was
set to 2.4 kg/cm2 . Generally, foam can be prepared byusing surfactants, which are primarily proteins in the caseof beer. Moreover, because the foaming ingredients create
viscous membranes, it is possible to obtain a highly dense
foam phase that entraps the carbon dioxide gas from thebeer. In addition, there is not enough surface tension in
the foam to break the membranes, so direct current arc
plasma inside the foam can be entrapped for comparatively long periods of time. The average free path of anarc discharge in foam is longer in comparison with thatin water, and it is possible to achieve good arc discharge,
as in gas phase.
Arc discharge was conducted as follows. Both graphite
electrodes were dipped in an open container filled withhigh-density foam from a beer dispenser. They were momentarily brought into contact, and arc discharge was initiated as shown in Fig. 2(a). Discharge voltage and current were optimized and maintained at 20 V and 10 A,respectively. The synthesis time was 2 min. Note thatduring continuous operation large quantities of fine particles adhered onto the surface of the foam.
Figure 2(b) shows the foam surface after the arc discharge. The products of the arc discharge in the foam
were separated into fine particles trapped on the foam
surface and fine particles at the bottom of the beaker. We
placed some of the fine particles on a microscope specimen
grid and examined them under a high-resolution transmission electron microscopy (HR-TEM) system operating at168

FIG. 3: (a)-(c) TEM images of fine particles at the bottom ofthe beaker. (b) and (c) are high-magnification views of (a).

200 kV (HR-TEM model H9000; Hitachi Co., Ltd.).

III.

RESULTS AND DISCUSSION

Figures 3(a)-(c) show HRTEM images of the fine particles at the bottom of the beaker. Figures 3(b) and 3(c)are enlarged views of Fig. 3(a). These micrographs show alarge cluster of 2 µm diameter and consisting of numerous
MWNTs, as shown in Fig. 3(c). From the fine particles
at the bottom of the beaker, clusters of approximately
1-10 µm diameter and a small cluster of 1 µm diameter
were observed.
Figures 4(a)-(c) show the fine particles on the foam surface. Figures 4(b) and 4(c) are enlarged views of Fig. 4(a).
The cluster from the foam surface was different from thecluster at the bottom of the beaker. Many small clusters were observed from the sample of the foam surface.

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FIG. 5: Raman spectra of CNT cluster produced by arc discharge in foam.

FIG. 4: TEM images of fine particles on the foam surface:(a)-(c) low-magnification views; (e) high-magnification viewof (d).

These small clusters were typically 500-1000 nm in diameter. Compared with that at the bottom of the beaker, thecluster on the foam surface showed high-purity MWNTs
with a small percentage of amorphous carbon or onions,as shown in Figs. 4(b) and 4(c).
The TEM images provided valuable information on thequality of the nanotubes produced. On the other hand,the HR-TEM images revealed that such nanotubes wereMWNTs of 25 nm diameter and tended to be very straightand clean, as shown in Figs. 4(d) and 4(e).
Table I shows the percentages of fine particles in theclusters collected at the bottom of the beaker and thesurface of the foam. We took 10 samples each from thebottom of the beaker and on the foam surface, and divided
them into MWNTs, onions and amorphous carbon by
TEM observation. In particular, we have defined clusters
with an MWNT ratio of 50 % or more as CNT clusters,
and the first column of Table I provides the CNT cluster
ratio at the two sampling sites. In the second column ofTable I, we have defined spherical graphite as “Onion”
and shapeless amorphous carbon as “Amorphous”, where
both are non-CNT materials, and have presented the component ratios of the materials included in the CNT clusters.
As seen from the table, most of the CNT clusters wereincluded in the foam. We observed more than 60 % CNTclusters from the foam, and less than 20 % CNT clusters
at the bottom of the beaker. In addition, CNT clustersof small diameter (< 1 µm) were often found on the foamsurface, whereas CNT clusters of large diameter (> 1 µm)
were found at the bottom of the beaker with small CNT
clusters of more than 1 µm diameter.
In addition, Table I shows the material ratio of the CNTclusters. CNT clusters containing high-purity MWNTs
were those containing MWNTs at greater than 60 %; however, these CNT clusters contained onions and amorphous
carbon from 5 % to 40 %. In particular, CNT clusters
with many MWNTs but with few of onions or amorphous
carbon, and CNT clusters with a purity of MWNT exceeding 90 % were often found.
From the TEM observation, CNT clusters of small diameter were often found on the foam surface, whereas
CNT clusters of large diameter were found at the bottomof the beaker. Therefore, it is considered that CNT clusters separated by size or mass into the foam surface andthe bottom of the beaker.
Figure 5 shows the Raman scattering spectra of theCNT cluster in the regions of 50-1800 cm−1 . The excitation wavelength of the Raman spectrum device (JASCO
Corporation: NRS-3000) used green laser of 532.03 nm.
A strong peak at 1587 cm−1 and a weak peak at
1367 cm−1 can be observed, which were assigned to theD-band and G-band signals, respectively. The signal intensity ratio of the G-band to the D-band (G/D) is high,indicating that high-quality MWNTs contain few impurities. Low-frequency peaks (at -220 cm−1 ) can also be

TABLE I: Percentages of production collected from the foam.
CNT clusters
Material ratio of CNT clusters
Form surface 20-40 %MWNT 60-95 %; Others
Bottom of beaker 5-20 %(onion, amorphous) 5-40 %http://www.sssj.org/ejssnt (J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/)

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Kim, et al.

observed, which are assigned to RBM vibrations. The
tube diameter d (in nm) can be estimated using the relation d=223.75/w [9], where w (in cm−1 ) denotes the RBM
vibrational wave number observed. The RBM peaks at
72, 86, 107 and 126 cm−1 correspond to MWNTs withdiameters of 3.1, 2.6, 2.1 and 1.7 nm, respectively.
IV.

CONCLUSION

We reported the characteristics of MWNTs produced
by arc discharge in foam as determined from the TEM
images and Raman spectra of the nanotubes. From theTEM observation, CNT clusters containing high-purity
MWNTs were often found. These clusters also contained
small percentages of onions and amorphous carbon, but
a large percentage MWNTs at more than 90 %. CNT[1] T. W. Ebbesen and P. M. Ajayan, Nature 358, 220 (1992).
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clusters with a large percentage of high-quality MWNTs
were easily collected on the foam surface. We observed
more than 40 % CNT clusters on the foam surface andless than 20 % CNT clusters at the bottom of the beaker.
From these results, it is considered that CNT clusters separate by size or mass into the foam surface and the bottomof the beaker. The yield of MWNTs by the arc synthesis
method is highest in foam, and might be increased withfurther research.
Acknowledgments

The authors thank Dr. A. Suzuki of RIKEN for fruitful
discussion about the Raman spectra. This work was partially supported by the Center for Drug Delivery Research(DDS), Tokyo University of Science.

[6] N. Sano, H. Wang, M. Chhowalla, I. Alexandrou, and G.
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