Bundles of aligned carbon nanotubes obtained by the pyrolysis of ferrocene-hydrocarbon mixtures: Role of the metal nanoparticles produced in situ

Article abstract of DOI:10.1016/S0009-2614(99)00521-7

Aligned nanotube bundles have been produced by the pyrolysis of ferrocene along with methane, acetylene or butane. Ferrocene-acetylene mixtures are found to be ideal for the production of compact aligned nanotube bundles. The nanotube bundles are associat

Full text of DOI:10.1016/S0009-2614(99)00521-7

2
July 1999
Chemical Physics Letters 307 Ž1999. 158–162
www.elsevier.nlrlocatercplett
Bundles of aligned carbon nanotubes obtained by the pyrolysis of
ferrocene–hydrocarbon mixtures: role of the metal nanoparticles
produced in situ
a
a
a,b,)
B.C. Satishkumar , A. Govindaraj , C.N.R. Rao
a
Solid State and Structural Chemistry Unit and CSIR Centre of Excellence in Chemistry, Indian Institute of Science, Bangalore 560 012,
India
b
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for AdÕanced Scientific Research, Bangalore 560 064, India
Received 1 March 1999
Abstract
Aligned nanotube bundles have been produced by the pyrolysis of ferrocene along with methane, acetylene or butane.
Ferrocene–acetylene mixtures are found to be ideal for the production of compact aligned nanotube bundles. The nanotube
bundles are associated with iron nanoparticles of diameters in the range 2–13 nm. These nanoparticles are ferromagnetic,
showing low saturation magnetization compared to bulk iron. The ferromagnetism of the transition metal nanoparticles is
likely to be responsible for the alignment of the nanotubes. q 1999 Elsevier Science B.V. All rights reserved.
1
. Introduction
lyst substrates. Ren et al. w8x have employed plasma-
enhanced chemical vapor deposition and synthesized
aligned nanotubes on nickel-coated glass, using
acetylene and ammonia mixtures. The mechanism of
growth of nanotubes by this method and the exact
role of the metal particles, however, are not clear,
although a nucleation process involving the metal
particles is considered to be an important step. More
recently, Fan et al. w9x have obtained aligned nan-
otubes by employing chemical vapor deposition on
porous silicon and plain silicon substrates patterned
with Fe films. Here again, the exact role of the Fe
particles has not been unravelled. The role of the
transition metal particles assumes importance in view
of the report of Pan et al. w10x that aligned nanotubes
Carbon nanotubes are potential candidates for use
in many functional devices such as field emitters
w1,2x and also for use in nanoelectronics w3x. Of
particular relevance to this area is the synthesis of
aligned nanotube bundles. Aligned nanotube bundles
have been obtained by using chemical vapor deposi-
tion over transition metal catalysts embedded in the
pores of mesoporous silica or the channels of alu-
mina membranes w4,5x. Terrones et al. w6,7x have
prepared aligned nanotubes over laser patterned cata-
)
can be obtained by the pyrolysis of acetylene over
ironrsilica catalyst surfaces. In the light of our
Corresponding author. Fax: q91 80 846 2766; e-mail:
cnrrao@jncasr.ac.in
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž99.00521-7

B.C. Satishkumar et al.rChemical Physics Letters 307 (1999) 158–162
159
earlier work on the synthesis of carbon nanotubes by
the pyrolysis of mixtures of organometallic precur-
sors and hydrocarbons w11x, we expected that the
small transition metal particles produced in situ in
the pyrolysis may not only nucleate the formation of
carbon nanotubes but also align them. We have
examined this aspect by carrying out the pyrolysis of
ferrocene along with additional hydrocarbon sources,
in a suitably designed apparatus.
duced into the pyrolysis zone of the second furnace
soon after the Arqferrocene stream was let in. The
main variables in the experiment were the amount of
ferrocene and partial pressure of the hydrocarbon.
Large quantities of deposits accumulated at the inlet
end of the second furnace. These deposits were
removed and examined by electron microscopy.
Scanning electron microscope ŽSEM. images were
recorded on the as-deposited samples using a LEICA
S440i instrument, to check the nature of alignment
and propensity of aligned nanotube bundles. Trans-
mission electron microscope ŽTEM. images were
2
. Experimental
obtained with a JEOL JEM-3010 microscope operat-
ing at 300 kV.
In order to prepare aligned nanotube bundles, the
pyrolysis apparatus shown in Fig. 1 was employed.
The apparatus consists of stainless steel gas flow
lines and a two-stage furnace system fitted with a
quartz tube Žinner diameter, 10 mm.. The flow rate
3
. Results and discussion
of gases was controlled using UNIT mass flow con-
trollers. A known quantity Ž;100 mg. of the fer-
rocene Žpresublimed, 99% pure. was taken in a
In Fig. 2a,b,c we show the SEM images of aligned
nanotubes obtained by the pyrolysis of ferrocene
with methane, acetylene and butane respectively Žat
quartz boat and placed inside the quartz tube. The
part of the quartz tube containing the ferrocene was
in the first furnace and the temperature was raised to
an Ar flow of 950 sccm and hydrocarbon flow of 50
sccm, sccmsstandard cubic centimeter per minute.
at 1373 K. The SEM images of larger bundles of the
aligned nanotubes are shown in Fig. 3. The average
length of the nanotubes was generally around 60 mm
with methane and acetylene. We see from the SEM
image in Fig. 2a that, although the nanotubes were
aligned in the case of methane, the packing density
6
23 K at a fast heating rate Ž1008rmin.. Argon gas
was passed through the quartz tube at the desired
flow rate and the ferrocene vapour was carried by
the Ar gas into the second furnace, maintained at
1
373 K. Methane, acetylene or n-butane was intro-
Fig. 1. The pyrolysis apparatus employed for the synthesis of aligned nanotube bundles by the pyrolysis of ferrocene–hydrocarbon mixtures
at 1373 K.

1
60
B.C. Satishkumar et al.rChemical Physics Letters 307 (1999) 158–162
study appear to have a greater packing density than
those obtained with Fersilica catalysts by Pan et al.
w10x.
When butane was used as the additional hydrocar-
bon source, there was a greater proportion of extra-
Fig. 2. SEM images Žviewed perpendicular to the nanotube axis.
of aligned nanotubes obtained from ferrocene–hydrocarbon mix-
tures: Ža. methane, Žb. acetylene and Žc. butane Žhydrocarbon at
5
0 sccm. in an Ar flow of 950 sccm.
was not high. With acetylene, however, more com-
pact nanotube bundles were obtained as seen from
Fig. 2b. A negligible proportion of graphite-covered
metal nanoparticles was present along with the nan-
otubes with methane and acetylene as the additional
hydrocarbon sources. Furthermore, the aligned nan-
otube bundles obtained with acetylene in the present
Fig. 3. SEM images showing the bundles of aligned nanotubes
obtained by pyrolysing ferrocene along with Ža. methane, Žb.
acetylene and Žc. butane.

B.C. Satishkumar et al.rChemical Physics Letters 307 (1999) 158–162
161
Fig. 4. TEM image of a part of an aligned nanotube bundle Žcorresponding to Fig. 2b. obtained from the pyrolysis of an acetylene–ferrocene
mixture, showing Fe nanoparticles along with the nanotubes.
neous carbon deposits, mainly consisting of iron
particles encapsulated inside graphite sheaths. The
average length of the nanotubes was also shorter
Ž;35 mm.. It therefore appears that the ferrocene–
acetylene mixture is ideal for preparing compact
bundles of aligned carbon nanotubes. The advantage
of the present method is that the large aligned nan-
otube bundles are produced in one step, without any
prior preparation of the substrates.
In order to study the nature of the metal nanopar-
ticles involved in the formation of the nanotube
bundles, we analyzed their size distribution. For this
purpose, the iron metal particles associated with the
aligned nanotube bundles obtained by the pyrolysis
of ferrocene–acetylene mixtures Žcorresponding to
Fig. 2b. were examined by TEM. A TEM image of a
part of an aligned nanotube bundle is shown in Fig.
4
. The image shows the presence of Fe nanoparticles
along with the nanotubes. The size distribution of
these iron nanoparticles is shown in Fig. 5a. The
particle diameters are in the range of 2–13 nm. For
the purpose of comparison, we analysed the size
distribution of graphite-coated iron nanoparticles ob-
tained by the pyrolysis of excess ferrocene Ž500 mg.
with acetylene Ž100 sccm.. The particle size distribu-
Fig. 5. Histogram showing the size distribution of Fe nanoparti-
cles present Ža. in aligned nanotube bundles obtained from the
pyrolysis of the ferrocene–acetylene mixture Žcorresponding to
Fig. 2b. and Žb. in large graphite covered nanoparticles obtained
tion is shown in Fig. 5b. We see that the particle
as a major product in the pyrolysis of ferrocene–acetylene mix-
diameters are in the range of 20–40 nm, consider-
ture, containing an excess of the former.

1
62
B.C. Satishkumar et al.rChemical Physics Letters 307 (1999) 158–162
ably larger than those in the case of the nanoparticles
associated with the formation of nanotube bundles.
Magnetization measurements were carried out on
the aligned nanotube bundles from acetylene–ferro-
cene mixtures Žcorresponding to Fig. 2b., to examine
play an important role in the process of alignment
because of their magnetic nature.
Acknowledgements
the magnetic nature of the iron nanoparticles. A
M–H hysteresis loop with a saturation magnetization
of around 20 emurg was obtained. Considering that
the average particle diameter is small Ž2–13 nm., we
One of the authors ŽBCS. acknowledges CSIR,
New Delhi, for a senior research fellowship.
would expect such a reduction in the value of satura-
tion magnetization, compared to the bulk value Ž222
emurg.. It is noteworthy that the iron nanoparticles
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