Large-scale and low-cost synthesis of single-walled carbon nanotubes by the catalytic pyrolysis of hydrocarbons

Article abstract of DOI:10.1063/1.121624

Rope-like bundles of single-walled carbon nanotubes (SWNTs) similar to those obtained by laser vaporization and electric-arc techniques were synthesized on a relatively large scale and at low cost by the catalytic decomposition of hydrocarbons at a temperature of about 1200°C using an improved floating catalyst method. The SWNTs thus obtained have larger diameters and are self-organized into ropes. The addition of thiophene was found to be effective in promoting the growth of SWNTs and in increasing the yield of either SWNTs or multiwalled carbon nanotubes under different growth conditions.

Full text of DOI:10.1063/1.121624

Large-scale and low-cost synthesis of single-walled carbon nanotubes by
the catalytic pyrolysis of hydrocarbons
H. M. Cheng, F. Li, G. Su, H. Y. Pan, L. L. He et al.
Citation: Appl. Phys. Lett. 72, 3282 (1998); doi: 10.1063/1.121624
View online: http://dx.doi.org/10.1063/1.121624
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APPLIED PHYSICS LETTERS
VOLUME 72, NUMBER 25
22 JUNE 1998
Large-scale and low-cost synthesis of single-walled carbon nanotubes
by the catalytic pyrolysis of hydrocarbons
H. M. Chenga)
Institute of Metal Research, Academia Sinica, Shenyang 110015, People’s Republic
of China and Department of Physics, Massachusetts Institute of Technology, Cambridge,
Massachusetts 02139
F. Li and G. Su
Institute of Metal Research, Academia Sinica, Shenyang 110015, People’s Republic of China
H. Y. Pan and L. L. He
Laboratory for Atomic Imaging of Solids, Academia Sinica, Shenyang 110015, People’s Republic of China
X. Sun and M. S. Dresselhaus
Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
͑
Received 19 January 1998; accepted for publication 20 April 1998͒
Rope-like bundles of single-walled carbon nanotubes ͑SWNTs͒ similar to those obtained by laser
vaporization and electric-arc techniques were synthesized on a relatively large scale and at low cost
by the catalytic decomposition of hydrocarbons at a temperature of about 1200 °C using an
improved floating catalyst method. The SWNTs thus obtained have larger diameters and are
self-organized into ropes. The addition of thiophene was found to be effective in promoting the
growth of SWNTs and in increasing the yield of either SWNTs or multiwalled carbon nanotubes
under different growth conditions. © 1998 American Institute of Physics.
͓
S0003-6951͑98͒01125-5͔
A great amount of research on carbon nanotubes has
been carried out since the first pioneering report of the
catalytic particles dispersed on the support, which was not
easy to control precisely, was found to be important in con-
trolling the shape of the nanotubes that are produced.
We report in this letter a novel synthesis method for
SWNTs using an improved floating catalyst approach in
which benzene was catalytically pyrolyzed at 1100–1200 °C.
In contrast to the electric-arc and laser vaporization methods,
this method allows lower growth temperature and easier con-
trol over the growth conditions, permits semicontinuous or
continuous preparation, and produces SWNTs at high purity,
large quantity and at relatively low cost.
In our experiment, an improved floating catalyst method
was employed in which a vapor-phase catalyst was flowed
into a horizontal reactor to achieve semicontinuous growth
of SWNTs. The process diagram is given in Fig. 1. Benzene
was used as the carbon source, hydrogen as the carrier gas,
and ferrocene as the catalyst precursor. To enhance the
growth of SWNTs, a sulfur-containing additive ͑thiophene͒
was employed. It is known that ferrocene begins to vaporize
at about 185 °C and decomposes above 400 °C. During
preparation, ferrocene was vaporized and carried into the re-
1
,2
discovery of multiwalled carbon nanotubes ͑MWNTs͒ by
3
Iijima. Among them, the research on single-walled carbon
nanotubes ͑SWNTs͒ is particularly active and fruitful be-
cause SWNTs represent the ultimate form of carbon nano-
tubes, and because of their especially interesting properties,
both predicted theoretically and demonstrated experimen-
tally. Initially, about 1–2% yields of SWNTs were found in
1
993 in soot generated in an arc struck between graphite
4
5
6
electrodes containing a few at % Fe, Co, and Ni. Since
then, the preparation of high-quality SWNTs on a large scale
has been the objective of many research endeavors.7
Among the reported techniques, the pulsed laser vaporization
–11
9
of a heated carbon target containing a Ni/Co catalyst and
more recently, an electric-arc technique using a Ni/Y
1
0
catalyst have produced SWNTs at high yield. However,
these methods may not be the best methods for obtaining a
continuous process for carbon nanotube production on a
commercial scale.12 From a standpoint of applications, the
emphasis will be on high-purity, high-yield, and low-cost
nanotube growth.
Since 3d transition metals seem to be important for the
synthesis of SWNTs, as is also the situation for the growth of
vapor-grown carbon fibers, it is therefore likely that SWNTs
can be synthesized through the catalytic pyrolysis of hydro-
carbons. Fonseca et al. reported the synthesis of MWNTs13
and bundles of aligned and straight 1 nm diameter
1
4
nanotubes over supported catalysts by the decomposition
of hydrocarbons. However, the amount and dimension of the
FIG. 1. Schematic diagram of the apparatus used for the synthesis of our
SWNTs.
a͒Electronic mail: cheng@imr.ac.cn
0003-6951/98/72(25)/3282/3/$15.00
3282
© 1998 American Institute of Physics
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Appl. Phys. Lett., Vol. 72, No. 25, 22 June 1998
Cheng et al.
3283
action tube with a mixture of benzene and thiophene vapor,
and hydrogen gas. The vaporized ferrocene was firstly re-
duced by hydrogen to form atomic iron, allowing atomic iron
to agglomerate into iron clusters appropriate for the growth
of SWNTs. Although the preparation conditions were not
well optimized, we obtained quantities of SWNTs with a
Ϫ7
ferrocene vaporization rate of 1.5–4.5ϫ10 mol/s, and a
H gas flow rates of 70–90 and 150–225 ml/min, respec-
2
tively, directly through the oven and through the benzene
that was kept at room temperatures (18–25 °C) and con-
tained 0.5–5 wt % of thiophene. The reaction temperature
was maintained at 1100–1200 °C, and synthesis times were
1
to 30 min ͑but they can be much longer͒. The grown nano-
tubes were transported out of the reaction zone by the flow-
ing gases and were collected on a graphite plate or on the
thermocouple-protecting tube and the tube wall. Because the
ferrocene flows through the reaction region continuously,
SWNTs can be grown continuously and the production of
SWNTs can be quite large, thus making full use of the ad-
vantages of the floating catalyst method. The morphology
and structure of the SWNTs that were obtained were ob-
served in their as-prepared state and after dispersion in alco-
hol by ultrasound, using scanning electron microscopy
͑SEM͒, transmission electron microscopy ͑TEM͒, and high-
resolution TEM ͑HRTEM͒. Resonant Raman scattering mea-
surements were also carried out to characterize our SWNTs.
We observed large quantities of web-like, silver-black,
very light and very thin, half-transparent, but free-standing
materials in the products. A characteristic SEM image of the
products ͓Fig. 2͑a͔͒ shows a large amount of entangled car-
bon nanotube filaments, homogeneously distributed over
large areas ͑one piece of the thin, web-like product can have
an area of about 10 mmϫ50 mm͒. Typical TEM images of
the as-prepared product ͓Figs. 2͑b͒ and 2͑c͔͒ show that it
consists primarily of many rope-like bundles of SWNTs, in
which the SWNTs are aligned and maintain constant rope
diameters, ranging from 10 to 40 nm, over the entire length,
which is very similar to those grown by other methods.9
Figure 2͑b͒ is particularly interesting, when viewed as a
three-dimensional image, from which we can see that the
bundles have a roughly round rope-like shape with some
small bundles or single nanotubes split off from the larger
ropes. This splitting may occur during the collection of the
products from the furnace or during preparation of the TEM
specimen. The number of SWNTs within a rope-like bundle
is typically above 10. However, we could not identify any
rope ends from the microscopic observation ͑the length of
our SWNT ropes can be several tens of micrometers based
on the SEM observation͒. An HRTEM image of the products
FIG. 2. Images of our SWNTs obtained from the catalytic pyrolysis of
hydrocarbons. ͑a͒ SEM image showing a high density of entangled ropes;
͑
b͒ TEM image showing SWNT ropes; ͑c͒ TEM image showing SWNT
bundles ͑inset is an HRTEM image for a 2-nm diam SWNT͒.
,10
with a circular cross section,16 thus indicating a larger pro-
jected diameter under TEM observation. Due to the low
preparation temperature and sulfer addition that increased the
growth rate and yield, some defects are observed in our
SWNTs. It was found that, within the range of 0.5 to 5 wt %
of thiophene addition, the lower the addition, the purer but
fewer were the products of SWNTs that were obtained. Fig-
͓see inset of Fig. 2͑c͔͒ shows that the tube diameter is about
2
nm with a hollow core of 1.4 nm. The large hollow core of
these SWNTs may be very useful for the newly discovered
application of SWNTs as a hydrogen storage material.15
Figure 3͑a͒ shows that a carbon nanotube with a diam-
eter of about 5 nm was bent like ‘‘z’’ but still was not bro-
ken, indicating that the tube is very flexible. This tube is
likely single-walled with a large inside hollow core, because
MWNTs are less flexible and more prone to buckling. This
nanotube may have an elliptical or dog-bone cross section
based on a deformation of the normally cylindrical SWNT
FIG. 3. TEM images. ͑a͒ Showing a 5 nm projected diameter carbon nano-
tube that is bent like a letter ‘‘z’’ but still not broken; ͑b͒ Showing SWNT
bundles with some wavy parts.
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3284
Appl. Phys. Lett., Vol. 72, No. 25, 22 June 1998
Cheng et al.
ure 3͑b͒ shows wavy regions in part of the SWNT bundles,
and we also can see that some amorphous carbon is attached
on the surface of some SWNTs ͓see inset of Fig. 2͑c͔͒, as is
commonly found on vapor-grown MWNTs prepared from
hydrocarbons.
The spectra from resonant Raman scattering measure-
ments show characteristic features associated with the
SWNT radial breathing mode and with the C–C stretching
In conclusion, our improved floating catalyst method is
able to synthesize SWNTs on a large scale using hydrocar-
bons. The equipment needed is only a simple furnace, the
process can be easily modified for continuous production,
and the raw materials are cheap and easy to obtain. There-
fore, it is a very promising method to produce SWNTs on a
large scale at low cost.
1
7
Ϫ1
The IMR group acknowledges NSFC Grant No.
motions. Typical peaks at 158, 1570 and 1591 cm were
observed with a laser excitation wavelength of 647.1 nm,
which indicates that our nanotubes are SWNTs, similar to
5
9672024. The work at MIT was supported in part by NSF
DMR 95-10093. The authors thank Professor G. Dresselhaus
for illuminating discussions, and to Miss S. D. A. Brown,
Professor M. A. Pimenta and Dr. A. Marucci for help with
Raman measurements and for valuable discussions.
17
those obtained by laser vaporization and electric-arc
techniques. Further work on Raman measurements of our
SWNTs is in progress.
1
0
One important innovation in our method worthy of men-
tion is its capability to grow both SWNTs and MWNTs, and
their absolute and relative yields were greatly influenced by
S addition ͑i.e., a growth promotor͒ under our preparation
conditions. With the addition of different amounts of
thiophene, either SWNTs ͑0.5–5 wt % addition͒ or MWNTs
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14
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