www.elsevier.com/locate/diamond

Diamond & Related Materi

Spectroscopic study during single-wall carbon nanotubes production by

Ar, H2, and H2–Ar DC arc discharge

Y. Guoa,b, T. Okazakib,*, T. Kadoyab, T. Suzukib, Y. Andob

aSchool of Information Science and Engineering, Shenyang University of Technology, Tiexiqu, Shenyang 110023, PR ChinabDepartment of Materials Science and Engineering, Meijo University, Tempaku-ku, Nagoya 468-8502, Japan

Available online 8 December 2004

Abstract

A new method to produce a macroscopic oriented web (30 cm in length) of single-wall carbon nanotubes (SWNTs) has been developed in

our laboratory. In order to understand the growth mechanism of SWNTs, the optical emission spectra during SWNTs production in pure Ar,

H2 gas, and H2–Ar mixture gas were investigated. In pure Ar gas, Fe spectra are strongly appeared, in which SWNTs could not be formed,

but in pure H2 gas, Fe spectra almost disappeared in which small amount of SWNTs were formed. In the case of H2–Ar gas, Fe and C2

species were commonly identified, in which SWNTs were highly produced. H2–Ar gas provides the optimum condition for high production

and high quality of SWNTs. Spectroscopic study during carbon nanotubes production by DC arc discharge provides the useful method to

under the growth mechanism of nanotubes.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Nanotubes; Optical emission; DC arc discharge; Catalytic processes

1. Introduction

Carbon arc discharge is widely used for producing

spheroidal fullerenes and carbon nanotubes in various

gases, such as He [1–6], Ar [3], CH4 [3,4], H2 [7,8], and

Ar+CH4 [9]. It is known that C60 is effectively produced in

He gas [1] but multiwalled carbon nanotubes (MWNTs) are

productive in CH4 or H2 gas [4,8]. The single-wall carbon

nanotubes (SWNTs) were also produced with the help of

catalysts, Fe, Ni–Co, Co–Y, Ni–Y, and S, in Ar–CH4 [9], He

[6], H2 [10], and H2–Ar [11]. The effect of spatial

distributions of plasma temperature and density of carbon

species in DC arc discharge for multiwalled carbon nano-

tubes (MWNTs) were also investigated [12]. It is reported

that, when the temperature at the axial center of the cathode

was high or the potential drop at the cathode sheath was

high, MWNTs were highly productive. It is clear that

chemical species in the evaporation source and surrounding

space strongly affect the production of fullerenes and

0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.diamond.2004.10.041

* Corresponding author. Tel.: +81 528382410; fax: +81 528321170.

E-mail address: tokazaki@ccmfs.meijo-u.ac.jp (T. Okazaki).

nanotubes. In our previous paper [13] and other reports

[14–17] on MWNTs or spheroidal fullerenes production by

optical emission study, it was concluded that H atoms

hamper the formation of spheroidal fullerenes but were

conductive for the preparation of high-quality MWNTs. In

the previous paper [11], we reported that the SWNTs of high

crystallinity and purity higher than 70 at.% were formed.

The present study is the report of optical emission spectra

during production of these SWNTs, spectroscopically. For

the SWNTs production in H2 and H2–Ar gases, Fe, C2, CH,

and H species play a significant role.

2. Experimental

The experimental apparatus is shown in Fig. 1 and briefly

described here as reported in detail in the previous paper for

producing SWNTs [11], and for optical measurement [13].

Two carbon rods were installed vertically in the center of a

3�104 cm3-volume working chamber with a gap of 2 mm.

A carbon rod, 10 mm in diameter, laid at upper side was

used as a cathode and another one, 6 mm in diameter with

als 14 (2005) 887–890

Fig. 1. DC arc discharge apparatus.

Fig. 3. Optical photograph of the SWNTs grown in the space around the

upper carbon rod in H2–Ar gas, taken 60 sec after ignition. The arrow in the

photograph shows the as-grown SWNTs web.

Y. Guo et al. / Diamond & Related Materials 14 (2005) 887–890888

Fe catalyst, laid at lower side served as an anode. Carbon

rods with 0.5, 1, 2, and 5 at.% Fe were supplied by Toyo

Carbon. After the chamber was evacuated by an oil

diffusion pump and a rotary pump to a pressure below

1.5�10�5 Torr, desired ambient gas was introduced. The

Fig. 2. Emission spectra at the M position for three evaporation times, (a) 2 sec, (b

are existing in this range.

required pressure was fixed to 200 Torr for Ar and H2, and

for Ar–H2 mixture, 100 Torr each. Arc current was

generated by commercially used welder device (Fancy

201FP, Daiden). The emitted light was guided to a 320 or

1000 mm monochromator (Jobin Yvon, HR-320 or THR-

1000, respectively) equipped with a 1200 or 1800 line/mm

ruled grating (FWHM, 0.06 or 0.013 nm, respectively). The

diffracted light was recorded by a CCD (800�2000

elements) camera operated with a computer system (Jobin

Yvon, Spectra One). A conventional digital camera (3.3

mega pixels) can take as-grown SWNTs web.

3. Results and discussion

We obtained the SWNTs web using a rod with Fe

containing 0.5 to 5 at.% and at arc current 30 to 70 A.

) 5 sec, and (c) 180 sec after ignition. Intense spectra labeled CH, Fe and C2

Fig. 4. The intensity ratio for Fe and C2 spectra indicating the ratio of

species density in the space. The density ratio does not change so much

through the evaporation except the beginning at the M position. The

evaporation is carried out in Ar–H2 gas.

Y. Guo et al. / Diamond & Related Materials 14 (2005) 887–890 889

Evaporation rate of carbon is 6 mg/sec at arc current 50 A.

In these conditions, when we carried out with 1 Fe at.%

carbon rod and at arc current 50 A, SWNTs were most

productive. Therefore, results with this condition are shown

in the present paper.

Visually, we can recognize that large amounts of carbon

vapor evolve spherically from the source. Thereafter,

convection flow of smoke occurred in the upward

direction. Evaporation continued stably from several

seconds to several minutes. Emission spectra were taken

through the evaporation from just after 2 to 180 sec and at

two different positions: (i) center of the gap (M position)

and (ii) 6 mm horizontally away from the center (L

Fig. 5. Emission spectra observed in the range of 535–600 nm in Ar 200 Torr

interpretation of the references to colour in this figure legend, the reader is referr

position) as shown in Fig. 1. Fig. 2 shows time-dependent

spectra evaporated in the range of 428.5–442 nm, in which

three kinds of species, CH, Fe, and C2, are observed. In

the initial stage (just 2 sec after ignition) as shown in Fig.

2a, very strong CH and C2 spectra are identified. After

several seconds, CH spectra disappeared as shown in Fig.

2b and c because of presumed breakdown into C and H

atoms. Eventually, a SWNT web is formed around the

anode and on the ceiling as shown in Fig. 3. As intense C2

and Fe spectra are observed, these species are widely

spread in the space surrounding the upper carbon rod. The

intensity of emitting light shows the density of ingredients.

It is difficult to estimate the distribution rate in the

growing space. However, it is confirmed that the stable

evaporation is going on, in which the ratio for Fe/C2 is

estimated by comparing with the emission intensity of Fe

432.5765 nm spectrum and C2 436.4929 nm band origin

as shown in Fig. 4. From the beginning to about 180 sec,

the ratio at L position does not change so much as shown

in Fig. 5. But at the M position, Fe/C2 ratio is high at just

after ignition and gradually decreases from 9.2 to 2.3 after

several 10 sec. The interelectrode gap is kept 2 mm

throughout the evaporation by manual driving. In the case

of 1 at.% Fe anode in H2–Ar mixture at arc current 50 A,

productivity is the highest, but in pure H2, it is small, and

in pure Ar, there is no production. Fig. 5 shows the spectra

in the range 535–600 nm taken in three different gases at

the L position. In the case of Ar, Fe spectra are

predominantly observed but C2 spectra are very weak.

Judging from the smoke, visually, evaporation is very

small. On the contrary, in the case of H2 gas, strong C2

spectra are observed but Fe spectra are weak or not

, H2 200 Torr, and H2–Ar 100 Torr, each, and at arc current 50 A. (For

ed to the web version of this article).

Y. Guo et al. / Diamond & Related Materials 14 (2005) 887–890890

observed. Here, carbon evaporation is very fast so that the

density of carbon species is high but that of Fe is small,

hence the SWNT productivity is low. In our another study,

the observations of TEM showed that no SWNTs grew in

Ar arc plasma, whereas a few SWNTs grew in H2 arc

plasma. In the case of H2–Ar, however, as seen in Fig. 5,

Fe and C2 species are observed. In this condition,

productivity is at the highest and it is the optimum

condition. Hence, we can assume that carbon and

decomposed carbon from CH form SWNTs with the help

of floating Fe catalyst in the space. H2–Ar gas that used 1

at.% Fe anode provides the optimum condition of

distribution of Fe and C and/or Cn molecule, and temper-

ature to condensate these species.

4. Conclusions

We have presented the first optical emission spectro-

scopic investigation of SWNTs production in pure Ar,

H2, and H2–Ar mixture gas. In the case of Ar, Fe

spectra are predominantly observed but C2 spectra are

very weak and no SWNTs were produced. In the case of

H2 gas, strong C2 spectra are observed but Fe spectra

are very weak or not observed and small amount of

SWNTs were formed. On the contrary, in the case of

H2–Ar gas, emission spectra for Fe atoms and C2 dimers

were commonly observed in the evaporation source and

its surrounding space. The productivity was the highest

in this condition. Therefore, H2–Ar gas provides the best

condition, density balance of Fe and C or Cn, and

growing temperature for forming SWNTs in the space.

This study shows that spectroscopic measurement is one

of the useful methods to understand the SWNTs growth

mechanism.

Acknowledgements

The authors thank Mr. S. Inoue and Dr. X. Zhao in our

laboratory for their help and valuable discussion through the

study. This work was supported by the DAIKO FOUNDA-

TION, No. 10083.

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