Single-walled nanotubes by the pyrolysis of acetylene-organometallic mixtures

Article abstract of DOI:10.1016/S0009-2614(98)00727-1

Gas-phase pyrolysis of acetylene along with a metallocene or with a binary mixture of metallocenes in flowing Ar or Ar + H₂ at 1100°C yields single-walled carbon nanotubes. Pyrolysis of Fe(CO)₅-acetylene mixtures in Ar at 1100°C also gives single-walled nanotubes. The diameter of the nanotubes is generally around 1 nm, showing thereby that on pyrolysis under the dilute conditions employed, the organometallic precursors give rise to very fine metal particles essential for the formation of such nanotubes.

Full text of DOI:10.1016/S0009-2614(98)00727-1

21 August 1998
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Chemical Physics Letters 293 1998 47–52
Single-walled nanotubes by the pyrolysis of
acetylene-organometallic mixtures
a
b
b
a,b,1
B.C. Satishkumar , A. Govindaraj , Rahul Sen , C.N.R. Rao
a
Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India
CSIR Centre of Excellence in Chemistry, Indian Institute of Science, Bangalore 560012, India
b
Received 8 June 1998; in final form 19 June 1998
Abstract
Gas-phase pyrolysis of acetylene along with a metallocene or with a binary mixture of metallocenes in flowing Ar or Ar
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q H2 at 11008C yields single-walled carbon nanotubes. Pyrolysis of Fe CO -acetylene mixtures in Ar at 11008C also gives
5
single-walled nanotubes. The diameter of the nanotubes is generally around 1 nm, showing thereby that on pyrolysis under
the dilute conditions employed, the organometallic precursors give rise to very fine metal particles essential for the formation
of such nanotubes. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction
catalytic metal particles to be crucial for the growth
of SWNTs. Furthermore, a study of the nanotube
literature shows that large particles of metals gener-
ally form metal carbides or metal particles covered
by graphene layers. Recently, multi-walled carbon
One of the important problems in the area of
carbon nanotubes relates to the development of
methods to prepare single-walled carbon nanotubes
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SWNTs in a reproducible manner. The earliest
nanotubes MWNTs were prepared by the pyrolysis
. Ž
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M s Fe, Co or Ni in
report of the synthesis of SWNTs employed the arc
evaporation of graphite electrodes carried out in the
presence of metals such as Fe, Co or Ni 1,2 . High
yields of SWNT bundles are obtained by the arc-
evaporation of graphite electrodes filled with Y–Ni
of metallocenes, M C5H5
2
w x
admixture with benzene 7 . Our experience with the
pyrolysis of metallocene-benzene mixtures suggested
that it should be possible to synthesize SWNTs by a
careful control of the preparative conditions as well
as the relative proportions of the metal-containing
organic precursor and the hydrocarbon source. In this
communication, we report the succussful synthesis of
w
x
w x
3 . Laser vapourization of transition metal contain-
ing graphite targets is known to yield ropes of
w
x
w x
SWNTs 4,5 . Dai et al. 6 have obtained SWNTs by
the decomposition of CO at 12008C over small Mo
particles. This work suggests the small size of the
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SWNTs ;1 nm dia. by the pyrolysis of mixtures
of acetylene with metallocenes or Fe CO . The
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.
5
organometallic precursor acts as the source of small
metal particles necessary for the formation of
SWNTs.
1 Corresponding author.
0009-2614r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 98 00727-1

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B.C. Satishkumar et al.rChemical Physics Letters 293 1998 47–52
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2. Experimental
end of the second furnace marked 2 in Fig. 1 a .
While, MWNTs were present at the inlet end, marked
In order to prepare SWNTs by metallocene pyrol-
1.
Ž .
Ž
.
5
ysis, the set-up shown in Fig. 1 a was employed.
Fe CO was pyrolyized along with acetylene by
Ž .
Ž
The pyrolysis set-up consists of stainless steel gas
flow lines and a two-stage furnace system fitted with
using the set-up shown in Fig. 1 b . Acetylene flow
rate s 50 sccm was bubbled through a known
Ž . Ž .
quantity of Fe CO 1 ml and the mixture carried
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by argon gas flow rate s 1000 sccm into the
furnace, maintained at 11008C. The argon gas acts as
a dilution agent. SWNTs were found at the outlet
.
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a quartz tube 10 mm i.d . The flow rate of the gases
is controlled using UNIT mass flow controllers. A
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known quantity ;100 mg of the metallocene or a
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mixture of metallocenes of 99% purity was taken
in a quartz boat and placed inside the first furnace.
The metallocene was sublimed by raising the tem-
perature of this furnace to 3508C at a heating rate of
1008Crmin. Argon and acetylene were passed
through the quartz tube at flow rates of 1000 sccm
and 50 sccm respectively. The metallocene vapour
was carried by this gas mixture into the second
furnace maintained at 11008C. The pyrolysis yielded
copious quantities of carbon deposits inside the sec-
ond furnace. SWNTs were mainly found at the outlet
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end of the furnace marked 2 in the Fig. 1 b , MWNTs
being present at the inlet end, marked 1. For the
purpose of comparison, we also synthesized SWNTs
by the arc-evaporation of graphite electrodes filled
with Y2O3 and Ni powder 1 at.% Y and 4.2 at.%
Ni in a He atmosphere of 660 Torr. The graphite
rods 6 cm long, 6 mm dia. with a 4 mm dia. hole
drilled in the centre filled with Y2O3, Ni and graphite
powder were treated with hydrogen at 7008C for 3h,
prior to arcing 3,8 .
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w
x
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Ž .
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Fig. 1. Pyrolysis apparatus employed for the synthesis of SWNTs by pyrolysis of a metallocenes and b Fe CO along with acetylene.

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B.C. Satishkumar et al.rChemical Physics Letters 293 1998 47–52
49
The carbon products were sonicated in carbon
tetrachloride and loaded onto holey carbon copper
rise to web-like deposits on the chamber walls near
the cathode. The web-like deposit consists of SWNT
bundles. The high resolution transmission electron
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grids. Transmission electron microscopy TEM was
performed with a JEOL JEM-3010 microscope oper-
ating at 300 kV. The microscope is equipped with a
Gatan fiber optically coupled TV system, model
622SC for high resolution imaging. Scanning elec-
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.
microscopic image HREM of SWNTs so prepared
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is shown in Fig. 2 a . The image shows bundles
consisting of 10–20 SWNTs forming some kind of
high-way junctions. The diameter of the SWNTs is
in the range of 1–2 nm and the length extended upto
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.
tron microscopy SEM was performed on the as-
prepared samples using a LEICA S440i instrument.
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Ž .
several microns. Fig. 2 b and c show the TEM
images of isolated SWNTs obtained by the pyrolysis
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of nickelocene and acetylene flow rate s 50 sccm
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3. Results and discussion
at 11008C in Ar flow rate s 1000 sccm . The
diameter of SWNT in Fig. 2 b is 1.4 nm. It is
Ž .
The arc evaporation of graphite rods filled with
noteworthy that pyrolysis of nickelocene in admix-
ture with benzene under similar conditions primarily
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Ni and Y2O3 in a He atmosphere 660 Torr gives
Ž .
Fig. 2. High resolution transmission electron microscopic images of a SWNTs obtained by arcing graphite electrodes filled with Ni and
Ž
. Ž .
Ž .
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Y2O3 in a He atmosphere 660 Torr , b and c SWNTs obtained by the pyrolysis of nickelocene and C2 H2 flow rate s 50 sccm at
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.
Ž .
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11008C in a flow of Ar 1000 sccm and d SWNT obtained by the pyrolysis nickelocene and C2 H2 flow rate s 25 sccm at 11008C in a
flow of Ar flow rate s 1000 sccm and H2 flow rate s 25 sccm .
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B.C. Satishkumar et al.rChemical Physics Letters 293 1998 47–52
yields MWNTs. Acetylene appears to be a better
carbon source for the preparation of SWNTs, since it
contains a smaller number of carbon atoms; it seems
plausible that benzene favours the formation of
MWNTs since it is more carbon-rich. The bottom
Ž .
portion of the SWNT in Fig. 2 b shows an amor-
phous carbon coating around the tube. Indeed, a
majority of the SWNTs show the presence of such
amorphous carbon coating. This can be avoided by
Fig. 4. HREM images of SWNTs obtained by the pyrolysis of
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.
C2 H2 flow rate s 50 sccm with a cobaltocene-ferrocene, b
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c ferrocene-nickelocene mixtures,
nickelocene-cobaltocene and
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.
Ž
at 11008C in a flow of Ar flow rate s 975 sccm and H2 flow
.
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rate s 25 sccm . A small bundle of SWNTs is also shown in a .
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reducing the proportion of the hydrocarbon C2 H2
and mixing hydrogen into the Ar stream. The SWNTs
obtained with Ar–H2 as the flow gas show less
amorphous carbon deposition around the tubes, as
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revealed by the HREM image in Fig. 2 d . Pyrolysis
of cobaltocene and acetylene under similar condi-
tions also gives isolated SWNTs. The yield of
SWNTs is however considerably better with cobal-
tocene compared to nickelocene. The SEM image in
Ž .
Ž .
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c HREM images of SWNTs
Fig. 3.
a
SEM image,
b
and
obtained by the pyrolysis of a mixture of cobaltocene and C2 H2
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flow rate s 50 sccm at 11008C in a flow of Ar flow rate s
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Fig. 3 a demonstrates the high yield of SWNTs,
.
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975 sccm and H2 flow rate s 25 sccm . The image in c
present along with other carbonaceous deposits. In
shows a thick coating of amorphous carbon on the SWNT.

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B.C. Satishkumar et al.rChemical Physics Letters 293 1998 47–52
51
Ž .
Ž .
Fig. 3 b and c , we show the HREM images of
SWNTs, with diameters of 1 nm and 1.5 nm respec-
tively. Some of the nanotubes have an excessive
coating of amorphous carbon as indicated by Fig.
lene. This is because, binary metal alloys have been
reported to give good yields of SWNTs by arc
w x
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vapourization 9 . In Fig. 4 a we show a HREM
image of an isolated SWNT obtained on pyrolysis of
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3 c , where the coating is ;7 nm thick.
acetylene flow rate s 50 sccm with a cobal-
Having succeeded in synthesizing SWNTs by the
pyrolysis of metallocenes in admixture with acety-
lene, we carried out the pyrolysis of binary mixtures
tocene-ferrocene mixture at 11008C in a flow of Ar
flow rate s 975 sccm and H2 flow rate s 25
sccm . Fig. 4 b and c show the HREM images of a
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1:1 by weight of metallocenes along with acety-
SWNT from the pyrolysis of acetylene with a nicke-
locene-cobaltocene mixture and
a ferrocene-
nickelocene mixture respectively, obtained under
similar conditions. We have not found major differ-
ences in the yields of SWNTs by using binary
mixtures of metallocenes instead of a single metal-
locene. However, the nanotubes are somewhat
cleaner with the use of the mixture. Occasionally
SWNT bundles are obtained when binary mixtures
Ž
of metallocenes are used as metal precursors see
Ž ..
inset of Fig. 4 a .
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.
We have employed Fe CO as a source of the
5
metal instead of metallocenes and carried out the
pyrolysis of acetylene in mixture with the carbonyl
at 11008C. Interestingly, SWNTs were obtained by
this method as well. The flow rate of acetylene was
maintained at 50 sccm and the Ar flow rate at 1000
Ž .
sccm. The TEM image in Fig. 5 a shows the pres-
ence of a few SWNTs along with metal encapsulated
Ž .
Ž .
graphite nanoparticles. The images in
b and c
show the single-walled tubes with diameters in the
0.9–1.5 nm range. SWNT bundles are more fre-
Ž
.
5
quently found when Fe CO
is used as the
organometallic precursor.
4. Conclusions
In conclusion, the present study shows that the
pyrolysis of dilute acetylene–metallocene mixtures
yields isolated SWNTs. The use of hydrogen in the
flow gas along with Ar minimizes the amorphous
carbon coating on the nanotubes. Binary metallocene
mixtures also yield isolated SWNTs and occasional
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.
bundles on pyrolysis with acetylene. Fe CO is a
5
good metal precursor to obtain SWNTs by acetylene
pyrolysis. It is noteworthy that the diameters of the
SWNTs obtained in the present study are all around
1 nm. Thus, the study confirms that on pyrolysis, the
organometallic precursor molecules provide very
Fig. 5. TEM images of SWNTs obtained by the pyrolysis of
Ž
.
Ž
.
Ž
Fe CO and C2 H2 flow rate s 50 sccm at 11008C in Ar flow
5
.
rate s 1000 sccm flow.

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B.C. Satishkumar et al.rChemical Physics Letters 293 1998 47–52
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C. Journet, W.K. Maser, P. Bernier, A. Loiseau, M. Lamy de
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SWNTs.
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Nature 388 1997 756.
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A. Thess, R. Lee, P. Nikolaev, H. Dai, P. Petit, J. Robert, C.
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Acknowledgements
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273 1996 483.
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T. Guo, P. Nikolaev, A. Thess, R.E. Smalley, Chem. Phys.
The authors thank the Indo-French Centre for
promotion of Advanced Scientific Research, New
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H. Dai, A.G. Rinzler, P. Nikolaev, A. Thess, D.T. Colbert,
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