Bulk production of quasi-aligned carbon nanotube bundles by the catalytic chemical vapour deposition (CCVD) method

Article abstract of DOI:10.1016/S0009-2614(99)00202-X

The large-scale production of quasi-aligned carbon nanotube bundles is reported. The method includes catalytic decomposition of acetylene over well-dispersed metal particles embedded in commercially available zeolite at a lower temperature (700°C). In-depth studies of this nanotube through scanning electron microscopy and transmission electron microscopy reveal their homogeneity as well as perfect graphitization, along with inner (2.5-4 nm) and outer diameters (10-12 nm) of the tubes. Details of the optimum conditions for producing these nanotubes are also described.

Full text of DOI:10.1016/S0009-2614(99)00202-X

2 April 1999
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Chemical Physics Letters 303 1999 117–124
Bulk production of quasi-aligned carbon nanotube bundles by the
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catalytic chemical vapour deposition CCVD method
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Kingsuk Mukhopadhyay , Akira Koshio , Toshiki Sugai , Nobuo Tanaka ,
Hisanori Shinohara a,), Zoltan Konya , Janos B. Nagy
Faculty of Science, Department of Chemistry, Nagoya UniÕersity, Nagoya 464-8602, Japan
Faculty of Engineering, Department of Applied Physics, Nagoya UniÕersity, Nagoya 464-8603, Japan
Laboratoire de Resonance Magnetique Nucleaire, FUNDP, 61 rue de Bruxelles, B-5000 Namur, Belgium
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Received 29 December 1998; in final form 29 January 1999
Abstract
The large-scale production of quasi-aligned carbon nanotube bundles is reported. The method includes catalytic
decomposition of acetylene over well-dispersed metal particles embedded in commercially available zeolite at a lower
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temperature 7008C . In-depth studies of this nanotube through scanning electron microscopy and transmission electron
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microscopy reveal their homogeneity as well as perfect graphitization, along with inner 2.5–4 nm and outer diameters
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10–12 nm of the tubes. Details of the optimum conditions for producing these nanotubes are also described. q 1999
Elsevier Science B.V. All rights reserved.
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1. Introduction
lene 8 . Of these production procedures reported,
only the CCVD method leads to a large-scale yield
of carbon nanotubes. The lower reaction temperature
coupled to the low cost of production, as compared
with others, is of fundamental importance for the
CCVD method. However, the separation of carbon
nanotubes from the support, as well as from the
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Since the discovery of carbon nanotubes 1 , vari-
ous synthetic procedures of the nanotubes have been
reported, such as carbon arc discharge in the pres-
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ence of a metal catalyst 1,2 , laser vaporization of a
graphite electrode in the presence of metal catalysts
3 , pyrolysis of hydrocarbon vapour over a transition
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amorphous carbon, has been difficult 9 .
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metal incorporated silicarzeolite support catalytic
It is well known that on proper alignment these
carbon nanotubes act as good electron field emitters
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chemical vapour deposition, CCVD technique 4,5 ,
pyrolysis of metallocenes e.g., ferrocene, cobal-
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10 . Hence, attention has been drawn to align them.
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tocene and nickelocene 6 , chemical vapour deposi-
tion on silicon wafers containing lithographically
deposited metal salts and alumina 7 and laser-as-
sisted production of carbon nanotubes from acety-
Recently, some reports concerning the alignment of
nanotubes have attracted much interest. Terrones et
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al. 11 have reported the formation of aligned nan-
otubes by the pyrolysis of a triazine compound over
well-aligned laser-deposited Co particles. Li et al.
)
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12 have reported the synthesis of aligned nanotubes
by the pyrolysis of acetylene over Fe-incorporated
Corresponding author. Fax: q81 52 789 2962; e-mail:
nori@chem2.chem.nagoya-u.ac.jp
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 99 00202-X

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K. Mukhopadhyay et al.rChemical Physics Letters 303 1999 117–124
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mesoporous silica. In the former report, the tempera-
ture of the laser deposition system as well as of the
furnace is quite high and in both cases the formed
nanotubes contain 40–60 graphitic layers, having
outer diameters of 30–50 nm and an inner diameter
has a length of 1.5 m with outer 8 cm and inner
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7.5 cm diameters.
The catalyst was prepared by dissolving cobalt
acetate 2.5 wt% and vanadium acetate 2.5 wt% in
25 ml of water and then mixing them with a com-
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of ;4 nm. Rao et al. 13 have reported the forma-
tion of aligned nanotube bundles from ferrocene
pyrolysis but in this case also the furnace tempera-
ture is high and tubes are ;30–40 nm in diameter.
mercially available zeolite support Y-type zeolite
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from Zeolyst International . The mixture was vigor-
ously stirred in order to obtain the resulting mixture
of semisolid form. This semisolid mixture was then
dried overnight at 1408C which was then ground into
a fine powder and used in the production process.
The so-prepared catalyst ;1.5 g was sprayed
over the quartz boat and was placed in the central
position of the furnace. It was then activated by
passing nitrogen gas 99.99% through the chamber
for 15 min at 120 mlrmin flow rate. The tempera-
ture of the reaction was fixed at 7008C. It was
followed by the flow of acetylene 99.6% at the rate
of 15 mlrmin for 1 h through the reaction chamber
while nitrogen gas flow was maintained at 120
mlrmin flow rate. After the scheduled time, the
furnace was cooled to room temperature by the
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Ren et al. 14 have synthesized well-aligned carbon
nanotubes by plasma-enhanced hot filament chemi-
cal vapour deposition on nickel-coated glass at 6668C
but the synthesized nanotubes have many structural
defects and have outer diameters in the range of
30–50 nm.
Here, we report the bulk synthesis of thin, quasi-
aligned nanotubes over well-dispersed metal parti-
cles on a commercially available zeolite support by
the pyrolysis of acetylene at much lower temperature
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7008C . We also present the effects of reaction
parameters on the quality and quantity of nanotubes.
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passage of nitrogen gas 120 mlrmin . The sample
was then collected as a black powder from the quartz
boat. In this way, we can synthesize nanotubes in
2. Experimental
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The experimental procedure is illustrated schemat-
ically in Fig. 1. It involves a horizontal furnace with
three thermocouples to regulate and measure the
temperatures of the furnace precisely. The length of
the furnace is 1 m and has an inner diameter of 10
cm. The quartz tube used to carry out the deposition
gram quantities ;500 mg of carbon nanotubes per
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hour .
The resulting carbon nanotube samples were char-
acterized by scanning electron microscopy SEM:
Hitachi S-900 and transmission electron microscopy
TEM: Hitachi H-800 and JEOL JEM 2010 . Details
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Fig. 1. A schematic representation of nanotube production unit: A the furnace; B a quartz tube with gas inlet and outlet system; C a
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quartz boat for carrying the catalyst during the synthesis; and D the thermocouples.

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K. Mukhopadhyay et al.rChemical Physics Letters 303 1999 117–124
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Table 1
Effect of reaction temperature, flow rate of the gases, support and metal particles on the qualityrquantity of nanotubes
Support
Metals used
Reaction time
Temp.
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8C
Gas flow rate:
N2 flow;
Comment
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min
C2 H2 flow
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mlrmin
Y-type zeolite
Y-type zeolite
Y-type zeolite
Y-type zeolite
Y-type zeolite
Y-type zeolite
Co–V
Co–V
Co–V
Co–V
Co–V
Co–V
60
60
30
45
60
60
700
600
700
700
700
700
120; 15
120; 15
120; 15
120; 15
200; 25
85; 25
high density and quasi aligned
less dense but quasi aligned
less dense but quasi aligned
less dense but quasi aligned
quasi aligned but not homogeneous in nature
high amount of amorphous carbon
Y-type zeolite
Y-type zeolite
Co–Fe
Co–Fe
60
60
600
700
120; 15
120; 15
high density and quasi aligned
quasi aligned but high amount of amorphous
carbon and also slightly thick nanotubes
Y-type zeolite
Y-type zeolite
Y-type zeolite
Y-type zeolite
Y-type zeolite
Co–Ni
Co–Pt
Co–Y
Co–Cu
Co–Sn
60
60
60
60
60
700
700
700
700
700
120; 15
120; 15
120; 15
120; 15
120; 15
less dense and not aligned
less dense and not aligned
less dense and not aligned
not active, almost no nanotubes
not at all active
Commercial
Co–V
60
700
120; 15
high density and quasi aligned
grade alumina
Commercial
grade silica
Co–V
60
700
120; 15
high density but not aligned
of the experimental results, along with the optimiza-
layers on both sides. The as-prepared carbon nan-
otubes have an inner diameter of ;3.5 nm and an
external diameter of ;12 nm. It should be noted
that during TEM, as well as electron-dispersive X-ray
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tion of experimental parameters such as metal parti-
cles, flow rate of the reaction gas and temperature of
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the furnace , are summarized in Table 1.
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EDAX analyses of the tubes, we have found no
trace of the metal particles, either at the base or at
the tip of the tubes, which might be closely related to
the growth process of the nanotubes by the present
3. Results and discussion
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CCVD method 4 .
3.1. SEMrTEM obserÕations
Fig. 2a–d shows the SEM topographs of as-
synthesized carbon nanotube samples. It clearly illus-
trates the purity and homogeneity of the tubes, where
graphitic particles and nano-capsules are completely
absent. Fig. 3 is a low-resolution TEM image of the
nanotubes. It shows that the tubes are thin and
quasi-aligned. It also depicts the parallel alignment
of the tubes on the catalyst support. Fig. 4 shows a
3.2. Optimization of the production yield of the
nanotubes
The synthesis of nanotubes by the above-men-
tioned procedure involves the following parameters:
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1 the temperature of the furnace; 2 the flow rate
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of the gases; 3 the reaction time; 4 the kind of the
embedded metal particles; and 5 the composition of
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high-resolution TEM image HREM of the carbon
nanotubes. It is clear from these images that the
tubes are perfectly graphitized, having ;10 graphitic
the support. In the following, we present the effect of
each parameter which is summarized in Table 1.

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K. Mukhopadhyay et al.rChemical Physics Letters 303 1999 117–124
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Fig. 2. SEM images of the sample: a and
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b
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show the overall morphology of the sample while c and d
reveal one of the nanotube
bundles. These images show the morphology of as-prepared samples which clearly reveal the high purity and homogeneity of the sample.

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K. Mukhopadhyay et al.rChemical Physics Letters 303 1999 117–124
121
Fig. 3. Low-resolution TEM image. It clearly shows that the bundles are quasi-aligned.
Fig. 4. An HREM image of a typical carbon nanotube. The tube has 10 layers on both the sides and is highly graphitized.

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K. Mukhopadhyay et al.rChemical Physics Letters 303 1999 117–124
The TEM and SEM studies have revealed that at
slightly lower temperatures than the optimized
and were able to produce almost same kind of
quasi-aligned carbon nanotube bundles. Right now,
we are studying the activity of these two catalyst
supports by SEM and TEM and hence a comparative
report on the activity of these two catalysts will be
reported elsewhere.
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7008C , i.e. between 500 and 6008C, the number of
nanotubes formed was fewer, and below 5008C no
nanotubes were formed at all. At temperatures higher
than the optimized temperature, the amount of amor-
phous carbon increased. However, we have noted
that this optimized temperature may vary with the
different metal catalysts used in the synthesis pro-
cess. For example, with Zeolite Y plus Co and V, the
optimum temperature was 7008C, whereas the opti-
mum temperature was found to be 6008C for Zeolite
3.3. The nature of the support and a probable growth
mechanism of the nanotubes
The support obviously plays a pivotal role in the
synthesis of the present carbon nanotubes. The pore
size of the support is a key factor in shaping the
inner diameter of the tubes. We have used zeolites
with a definite pore size. We are currently using
various types of zeolites having smaller pore size, in
an attempt to obtain single-walled nanotubes. Silica
and alumina may also be used as a support, to obtain
the same kind of quasi-aligned carbon nanotube bun-
dles but they have their own drawbacks. We have
observed that in the case of silica, the removal of
amorphous carbon from the sample is extremely
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Y plus Co and Fe cf. Table 1 .
Acetylene and nitrogen gases were used in this
synthetic procedure. The rates at which they were
passed through the reaction chamber during the ex-
periment were 15 and 120 mlrmin for acetylene and
nitrogen gases, respectively. It should be noted that
while doing the deposition, nitrogen and acetylene
gases flow at the same time. We have concluded that
these flow rates should be maintained during the
reaction to obtain the maximum yield. Any variation
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of these flow rates, viz.: 1 acetylene 25 mlrmin
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. Ž .
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.
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and nitrogen 85 mlrmin ; 2 acetylene 25 mlrmin
difficult since Si and C are in the same group, so the
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.
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and nitrogen 200 mlrmin ; and 3 acetylene 15
resemblance of their reactivity might play a role in
.
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mlrmin and nitrogen 85 mlrmin , has shown that
the yield of the nanotube decreased and the quality
of the tubes also affected. It should be mentioned
here that the optimized flow rates were irrespective
of the catalyst used.
this case . The removal of alumina from the nan-
otube sample is almost impossible because under the
experimental temperature the structure of alumina
changes. It becomes sparingly soluble in acid and
alkali solutions and hence it is quite difficult to
isolate pure nanotubes from the solutions.
In our synthetic process, the reaction time was
maintained at 60 min. Any change of this reaction
time resulted in lower yields of the nanotubes, as
revealed by SEM and TEM observations. For exam-
ple, on reducing the reaction time to 30 or 45 min,
the yield was decreased. In addition, on increasing
the time, for instance to 90 min, amorphous carbon
was concomitantly produced. Hence, we think that
the optimum reaction time for the maximum produc-
tion of the tubes is ;60 min in the current experi-
mental arrangements.
The formation mechanism, as well as such an
alignment of the present carbon nanotubes, is most
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interesting. The HREM image Fig. 4 shows that
there are 10 concentric graphitic layers with the
regular circular shaped inner pore. The central ques-
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tions to be answered are: 1 why are the tubes so
selective about the outer and inner diameter, in
Ž .
particular? and 2 what is the driving force behind
the formation of 10–12 graphitic layers? In an effort
to elucidate the growth mechanism of these nan-
otubes, we have noticed that to get the maximum
activity of the catalyst, during the synthesis of aligned
carbon nanotubes, the metal particles should be on
the surface of zeolite and not inside the pore of
It is known that a zeolitersilica support contain-
ing Co particles could produce non-aligned carbon
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nanotubes 4 . Therefore, we have examined the
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yield of the tubes on binary metallic particles zeolite
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plus Co–M where M is transition metal atoms .
zeolite 4 . This largely depends on the mixing time
Almost all the transition metals were used. Of all the
combinations, Co–V and Co–Fe gave the best yield
of metalrmetals salts with the zeolite powder at the
preparatory phase of the catalyst itself. The optimum

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K. Mukhopadhyay et al.rChemical Physics Letters 303 1999 117–124
123
mixing time should thus be maintained to obtain the
most activated catalyst. Experimentally we have con-
firmed that the diameter of the nanotubes becomes
larger as the reaction time increases. The pore size of
the zeolite also plays a significant role for the deter-
mination of the inner diameter of the aligned carbon
nanotubes. For example, the zeolite we used has the
pore size of 2.5 nm and the inner diameter of aligned
carbon nanotubes is almost of the same size.
smaller diameter nanotube in the near future by
using much smaller pore sized catalyst and metal
particles. The different parameters guiding the reac-
tion process have also been discussed with regard to
optimizing the quality and quantity of the sample.
Detailed studies of the electrical conductivity, mag-
netic properties, elastic character, electron energy
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loss spectroscopy EELS as well as field emission
studies of these bundles and the atomic resolution of
these nanotube bundles using STM are underway in
our group and hence will be reported in subsequent
publications.
The inner diameter of carbon nanotubes, there-
fore, varies with the size of metal particles, mixing
time and the pore size of the zeolite used. As the
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gases here acetylene and nitrogen are allowed to
pass through the reaction chamber, the reaction pro-
ceeds from the reaction center towards the periphery.
This leads to the formation of graphitic layers of
generally 8–10 fold, depending on the extent of the
reaction from the center to the periphery. In this
regard, we think a hypothesis on capillary action and
Acknowledgements
KM thanks the Advance Process Research for the
Future Programme of JSPS for a fellowship. A part
of this study was financially supported by the JSPS
Future Programme ‘‘New Carbon Materials Pro-
cesses’’.
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a rolling effect of the metal particles 2,15,16 , which
tries to explain the formation of these graphitic rings
as well as the formation of nanotubes, seem to be
somewhat clumsy. As already mentioned above, the
circumstantial evidence from TEM images as well as
the EDX analysis does not clarify that the metal
particles, which, according to the hypothesis, should
have been present at the tip or at the base of the
tubes, are absent or rarely present. We therefore
believe that the building up process of these carbon
nanotubes has a different formation mechanism.
The zeolite used here, acting as a support in this
reaction process, has regular pores. The metal parti-
cles are embedded within these pores and position
themselves in a regular array, similar to the case of
regular arrays of Co particles as reported elsewhere
References
w x
Ž
.
Ž
.
1
w x
2
S. Iijima, Nature London 354 1991 56.
C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M. Lamy de
la Chapelle, S. Lefrant, P. Deniard, R. Lee, J.E. Fischer,
Ž
.
Ž
.
Nature London 388 1997 756.
w x
3
A. Thess, R. Lee, P. Nikolaev, P. Dai, P. Petit, J. Robert, C.
Xu, Y.H. Lee, S.G. Kim, A.G. Rinzler, D.T. Colbert, G.E.
Scuseria, D. Tomanek, J.E. Fisher, R.E. Smalley, Science
Ž
.
273 1996 483.
w x
4
K. Mukhopadhyay, A. Koshio, N. Tanaka, H. Shinohara,
Jpn. J. Appl. Phys. 37 1998 L1257.
Ž
.
w x
5
C.E. Snyder, W.H. Mandeville, H.G. Tennent, L.K. Trues-
dale, J.J. Barber, Int. Pat. WO 9r07163, 1989.
R. Sen, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 267
w
x
11 . The metal salts used in the present case have
extremely low melting points, and hence at the reac-
w x
6
Ž
.
1997 276.
Ž
.
w x
tion temperature 7008C they are in a highly reac-
7
J. Kong, H.T. Soh, A.M. Cassell, C.F. Quate, H. Dai, Nature
Ž
.
Ž
.
London 395 1998 878.
Ž
tive vapour phase. As the gases nitrogen and acety-
lene are passed through the reaction chamber, car-
w x
8
H. Kanzow, A. Schmalz, A. Ding, Chem. Phys. Lett. 295
.
Ž
.
1998 525.
bon nanotubes align themselves in the direction of
the gas flow. Presumably, the highly reactive metal
salts in the gaseous state flow down along with the
gases, leaving almost empty tipped nanotubes.
Here, we have presented a simple and novel way
to synthesize aligned nanotube bundles at lower tem-
peratures, in bulk quantities and at the same time we
have also tried to develop the idea of obtaining
w x
9
E.F. Kukovitskii, L.A. Chernozatonkii, S.G. Lvov, N.N.
Ž
.
Melnik, Chem. Phys. Lett. 266 1997 323.
w
w
x
10 Y. Saito, K. Hamaguchi, K. Hata, K. Uchida, Y. Tasaka, F.
Ž
.
Ikazaki, M. Yumura, A. Kasuya, Y. Nishina, Nature London
Ž
.
389 1997 554.
x
11 M. Terrones, N. Grobert, J. Olivares, J.P. Zhang, H. Ter-
rones, K. Kordatos, W.K. Hsu, J.P. Hare, P.D. Townsend, K.
Prassides, A.K. Cheetham, H.W. Kroto, D.R.M. Walton,
Ž
.
Ž
.
Nature London 388 1997 52.

(
)
124
K. Mukhopadhyay et al.rChemical Physics Letters 303 1999 117–124
w
w
w
x
w
x
15 L.C. Qin, D. Zhou, A.R. Krauss, D.M. Gruen, D.M. Grow-
12 W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y.
Ž
.
Ž .
ing, Appl. Phys. Lett. 72 1998 3437.
Zhou, R.A. Zhao, G. Wang, Science 274 1996 1701.
x
w
x
13 C.N.R. Rao, R. Sen, B.C. Satishkumar, A. Govindaraj, J.
16 H. Kanzow, A. Schmalz, A. Ding, Chem. Phys. Lett. 295
Ž
.
Ž
.
Chem. Soc., Chem. Commun. 15 1998 1525.
1998 525.
x
14 Z.F. Ren, Z.P. Huang, J.W. Xu, J.H. Wang, P. Bush, M.P.
Ž
.
Siegal, P.N. Provencio, Science 282 1998 1105.