Carbon nanotubes produced by tungsten-based catalyst using vapor phase deposition method

Article abstract of DOI:10.1016/S0009-2614(02)00982-X

We have demonstrated that W-based catalysts can produce carbon nanotubes (CNTs) effectively. Well-aligned, high-purity CNTs were synthesized using the catalytic reaction of C₂H₂ and W(CO)₆ mixtures. The CNTs had a multiwalled structure with a hollow inside. The graphite sheets of CNTs were highly crystalline but the outmost graphite sheets were defective.

Full text of DOI:10.1016/S0009-2614(02)00982-X

6 August 2002
Chemical Physics Letters 361 (2002) 469–472
www.elsevier.com/locate/cplett
Carbon nanotubes produced by tungsten-based catalyst
using vapor phase deposition method
a,
a
b
c
Cheol Jin Lee *, Seung Chul Lyu , Hyoun-Woo Kim , Jong Wan Park ,
d
Hyun Min Jung , Jaiwook Park
d
a
Department of Nanotechnology, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Republic of Korea
b
School of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea
School of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea
Department of Chemistry, Pohang University of Science and Technology, Pohang 440-746, Republic of Korea
c
d
Received 25 April 2002; in final form 10 June 2002
Abstract
We have demonstrated that W-based catalysts can produce carbon nanotubes (CNTs) effectively. Well-aligned, high-
purity CNTs were synthesized using the catalytic reaction of C2H2 and WðCOÞ mixtures. The CNTs had a multiwalled
6
structure with a hollow inside. The graphite sheets of CNTs were highly crystalline but the outmost graphite sheets were
defective. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
solution in the metal, thus the chemical nature as
well as the shape and size of the catalyst also af-
fects the CNT growth and morphology [9]. With
regard to the species of metal catalyst, iron (Fe)
[10,11], cobalt (Co) [12], nickel (Ni) [13,14], or
their bimetallic systems [15,16] have been mostly
utilized for CNT growth, and additionally mo-
lybdenum (Mo) [17], rhodium (Rh) [18], Rh–Pt
[19], palladium (Pd) [20], gold (Au) [21], silver (Ag)
[22] and copper (Cu) [23] have been studied as a
candidate material. Although synthesis of CNTs
with the mixture of cobalt and tungsten (W) cat-
alysts using arc discharge has been reported [24],
there is no report on the tungsten-catalyzed syn-
thesis of CNTs using chemical vapor deposition or
vapor phase deposition method until now.
Since the first observation of carbon nanotubes
(CNTs) [1], CNTs have been of great interests
because of their novel physical and chemical
properties, suggesting that they have numerous
potential applications such as electron field emitter
[2,3], scanning microscopy tips [4], hydrogen
storage materials [5,6], and molecular electronic
devices [7,8].
In preparation of CNTs using catalytic meth-
ods, the initial stage of growth involves the de-
composition of carbon-containing gas on the
surface of the metal catalyst and subsequent dis-
* Corresponding author. Fax: +82-2-2290-0768.
E-mail address: cjlee@hanyang.ac.kr (C. Jin Lee).
In this work, we report the production of CNTs
from tungsten-based catalyst using vapor phase
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)00982-X

470
C. Jin Lee et al. / Chemical Physics Letters 361 (2002) 469–472
3. Results and discussion
deposition method, moreover investigate the
structure and crystallinity of CNTs. For large-
scale production of CNTs, several research groups
have employed catalytic pyrolysis of hydrocarbons
[25–31]. We have synthesized CNTs by the cata-
lytic reaction of tungsten hexacarbonyl (WðCOÞ6)
and C2H2 mixtures using hot-wall type furnace.
Fig. 1a shows SEMimages of deposit on the
inner surface of the quartz tube, revealing that well
aligned carbon deposit was densely compacted and
uniformly grown. The average length of deposits is
about 30 lm as shown in Fig. 1b and the diameter
of deposits is in the range 20–40 nm as shown in
Fig. 1d. By adapting our method, the deposits are
homogeneously produced along the total heating
zone of the quartz tube.
2. Experimental
We employed Raman spectroscopy analysis to
investigate the elements and structure of deposits.
Fig. 2 shows a Raman spectrum of the deposits,
indicating the graphite structure. The G band at
1584 cmꢁ1 with a small bump at 1621 cmꢁ1 re-
veals that deposits are multiwalled CNTs. The D
band at 1337 cmꢁ1 indicates the existence of de-
fects or carbonaceous particles on the surface
of CNTs. Since the CNTs synthesized using
W-based catalyst have a little carbonaceous par-
ticles on the surface as shown in Fig. 1, it is
suggested that the D band mainly indicates the
defective feature of graphite sheets. In Raman
spectra, we notice that the relative intensity of G
band to D band is close to unity, indicating good
crystallinity of graphite sheets. Therefore, Raman
analysis confirms that the CNTs have a high-
crystalline multiwalled structure and some defects
on the wall surface.
A representative TEMimage of the synthe-
sized CNTs is shown in Fig. 3a. The CNTs have
a multiwalled structure with a hollow inside. In
our experiment, W catalyst particles are largely
existed on the one side of CNTs, but sometimes
they are appeared in CNTs inside hollow. There
were many reports for the synthesis of CNTs
using various catalysts instead of Fe, Ni, and Co
catalysts [17–23]. It reveals that many metal
particles can contribute to CNT growth as an
effective catalyst. In our previous results, we
suggested that a nanometer-sized catalyst particle
is inevitably necessary to synthesize CNT using
the CVD method [32]. We could find that there
was no CNT growth if the size of catalyst particle
(Fe, Co, Ni) is larger than about 300 hundreds
nanometer. Our result shows that W particles can
also perform catalytic role for the CNT growth
The vapor phase deposition system consists of a
two-stage furnace with an inner diameter of 20 mm
and a length of 300 mm. The first heating zone was
maintained at a low-temperature of 180 °C for
sublimation of catalyst and the second heating
zone was maintained at a high-temperature of 950
°C for CNT growth. A known quantity (ꢀ50 mg)
of the WðCOÞ powder (99.9+%, Sigma-Aldrich)
6
was loaded on the quartz boat located at the first
heating zone and was sublimated inside the first
heating zone. The WðCOÞ6 vapor was carried by
Ar gas with the flow rate of 1500 standard cubic
centimeters per minute (sccm) and was introduced
into the reactor. C2H2 gas with the flow rate of 30
sccm was used as a carbon source. To synthesize
CNTs, C2H2 and Ar gases were supplied into the
reactor when the temperature of the first heating
zone and the second heating zone are arrived at
180 and 950 °C, respectively. After the reaction,
the reactor was cooled down to room temperature
in flowing 500 sccm of Ar gas. Large quantities of
black-colored deposits were homogeneously pro-
duced inside the second heating zone.
The deposits were scraped off the wall of the
quartz tube and examined by electron microscopy.
A scanning electron microscopy (SEM) (Hitachi
S-4700) was used to measure the length and di-
ameter of CNTs. A transmission electron micro-
scopy (TEM) (JEOL, JEM-3011, 300 kV) was used
to analyze the structure and crystallinity of CNTs.
A Raman spectrometer (Renishaw micro-Raman
2000) was used to evaluate the overall crystallinity
of CNTs. The 632.8 nm line of a He–Ne laser was
used for excitation. Since TEMdoes not provide
the overall information on the crystallinity of en-
tire specimen, we attempted to overcome this
shortcoming by using a Raman spectroscopy.

C. Jin Lee et al. / Chemical Physics Letters 361 (2002) 469–472
471
Fig. 1. SEMmicrographs of deposits produced by W-based catalyst at 950 °C. (a) Large quantities of carpet-like deposits were scraped
off the inner wall of the quartz tube. (b) Magnified view of (a), revealing that the deposits have a length about 30 lm. (c) Magnified
view of (b). (d) High-resolution SEMimage of deposits, showing the average diameter of about 20–40 nm.
graphite sheets have straight fringes, indicating a
good crystallinity but the outmost graphite sheets
have a defective crystalline structure. HRTEM
images demonstrate that highly crystalline graph-
ite sheets are generated by the introduction of the
W-based catalyst.
In summary, we have demonstrated that
W-based catalysts can be used to synthesize CNTs
using the vapor phase deposition method effec-
tively. Well-aligned, high-purity CNTs are syn-
thesized by the catalytic reaction of C2H2 and
WðCOÞ6 mixtures at 950 °C. The CNTs are ho-
mogeneously produced along the total heating
zone of the quartz tube. The Raman spectra and
TEMimages indicate that the CNTs have multi-
Fig. 2. Raman spectra of CNTs, showing multiwalled structure
walled structure and most graphite sheets of CNTs
of CNTs.
have a good crystallinity but the outmost graphite
sheets show a defective structure.
effectively when their size is limited to several tens
nanometer.
Fig. 3b shows
a
high-resolution TEM
Acknowledgements
(HRTEM) image, showing the wall structure of a
CNT. The fringes on each side of the tube repre-
sent an individual cylindrical graphite sheet. Most
This work was supported by Center for Nano-
tubes and Nanostructured Composites.

472
C. Jin Lee et al. / Chemical Physics Letters 361 (2002) 469–472
Fig. 3. (a) TEMimage of CNTs, showing a multiwalled structure with a hollow inside. (b) HRTEMimage of a CNT, showing high-
crystallinity of graphite sheets.
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