Synthesis of carbon nanotubes from bulk polymer

Article abstract of DOI:10.1063/1.117949

Carbon nanotubes have been synthesized by heat treating the polymer at 400°C in air which was obtained by polyesterification between citric acid and ethylene glycol. Transmission electron micrographs and an electron diffraction pattern showed the formatio

Full text of DOI:10.1063/1.117949

Synthesis of carbon nanotubes from bulk polymer
WooSeok Cho, Etsuo Hamada, Yukihito Kondo, and Kunio Takayanagi
Citation: Applied Physics Letters 69, 278 (1996); doi: 10.1063/1.117949
View online: http://dx.doi.org/10.1063/1.117949
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Synthesis of carbon nanotubes from bulk polymer
Woo-Seok Cho, Etsuo Hamada, Yukihito Kondo, and Kunio Takayanagi
Takayanagi Particle Surface Project, ERATO, Research Development Corporation of Japan, 3-1-2
Musashino, Akishima, Tokyo 196, Japan
͑
Received 8 April 1996; accepted for publication 3 May 1996͒
Carbon nanotubes have been synthesized by heat treating the polymer at 400 °C in air which was
obtained by polyesterification between citric acid and ethylene glycol. Transmission electron
micrographs and an electron diffraction pattern showed the formation of carbon nanotubes. The
diameter of the tubes ranged from 5 to 20 nm, whereas the lengths were less than 1 ␮m. © 1996
American Institute of Physics. ͓S0003-6951͑96͒04528-7͔
1
The recent discovery of carbon nanotubes has stimu-
lated intensive scientific investigations aiming at elucidating
precursors was heat treated at 400 °C for 8 h in air on an
Al O boat followed by natural furnace cooling to room tem-
perature. The samples were dispersed in ethanol, placed on a
microgrid and observed using a transmission electron micro-
scope ͑JEM-2010F, JEOL͒ at 200 kV.
2
3
2
–6
7–9
the formation mechanism,
applications
properties,
and potential
of the new material. Clearly, experimental
1
0–14
tests for understanding of the unique properties, as well as
the technological applications, demand a large quantity of
the material. By far the most successful way to date for pro-
ducing carbon nanotubes is the arc-discharge method.11–13
The following typical experimental conditions must be con-
trolled in the arc discharges to produce nanotubes efficiently;
the current density, the interelectrode distance, and the pres-
sure of inert gas filled in the discharge chamber. However, it
is relatively expensive, and only a small quantity is formed,
because the carbon-arc experiments are done with the low
yield in a closed reaction vessel through which an inert gas
flows at a controlled pressure. Carbon nanotubes have also
The general morphology of the samples obtained by heat
6
been prepared by the plasma decomposition of benzene in a
chamber filled with a gas mixture of Ar/He at a pressure of
5
0 Torr or the high-temperature ͑Ͼ600 °C͒ catalytic
2
,15
decomposition
of organic vapors such as acetylene or
benzene on a metallic catalyst such as Co, Fe, or Ni in N2
and H gases. The experimental complexity and high capital
2
equipment required for the processes make carbon nanotubes
expensive. If carbon nanotubes are ever to achieve wide-
spread application, some simple and inexpensive fabrication
route must be found. One possibility is to introduce a chemi-
cal route using polymers, which mainly consist of carbon.
The essential point is that the bonds between the carbon and
other elements can be removed by simple thermal treatment.
We have attempted to heat-treat the polymer obtained by
polyesterification between citric acid and ethylene glycol.
Here we report that carbon nanotubes can be formed by heat
treatment of polymer in air.
Carbon nanotubes were synthesized as follows. One
mole of anhydrous citric acid (HOOCCH C͑OH͒
2
͑
COOH͒CH COOH͒ was first dissolved into 4 mole of eth-
2
ylene glycol (HOCH CH OH). The mixture was stirred for
2
2
2
h at 50 °C until it became transparent. The pale yellow
solution thus prepared was heated at 135 °C for 5 h to pro-
mote polymerization and remove excess solvents. With con-
tinued heating at 135 °C, the solution became more viscous,
and gelation occurred. The resulting gel was a transparent
brown resin. Charring the resin at 300 °C for 2 h in an elec-
tric furnace resulted in a black solid mass, which was lightly
ground into a powder with a Teflon rod. The powder thus
obtained is referred to as the ‘‘precursor’’ hereinafter. The
FIG. 1. ͑a͒ Transmission electron micrograph showing bundles of carbon
nanotubes. ͑b͒ Transmission electron micrograph showing an enlarged car-
bon nanotube.
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37.149.200.5 On: Sat, 22 Nov 2014 21:39:33
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1

synthesized in spite of the existence of other competing re-
actions. The decomposition of polymer might generate dan-
gling bonds of carbon at the temperature, then they will be
reconstructed. Thus, nanotube growth seems to take place as
a nonequilibrium reaction during the processes.
The ability to simply produce carbon nanotubes is of
importance in materials science and applications. Further-
more, it is noteworthy that carbon nanotubes can be formed
by the heat treatment of the polymer in air. Thus, such a
simple method makes the overall process more feasible and
enables us to mass-produce nanotubes. We confirmed that
carbon nanotubes could also form by heat treatment of the
polymer even in a vacuum chamber. Therefore, quantity and
quality of nanotubes would be controlled by the vacuum
state or inert gases under atmospheric pressure. The fact that
the nanotubes can be formed by the mild heat treatment
without electrons, plasmas, ions, metallic catalyst, etc. will
provide a powerful clue to elucidate the formation mecha-
nism and possibilities of the synthesis of new type fullerenes
as well as those1
6,17
already known. In summary, the novel
method is easily accessible to other researchers with an in-
terest in the new materials, and should offer exciting oppor-
tunities for fundamental research and potential applications
in fullerenes science.
FIG. 2. Electron diffraction pattern taken from a multishell nanotube about
0 nm in diameter.
1
The authors would like to thank Dr. N. Minami in their
laboratory for helpful discussions.
treatment of the precursor at 400 °C for 8 h is shown in Fig.
. The tube-like samples had the structures reported by
1
1
Iijima: multiwalled cylindrical nanotubes. The diameter of
the tubes in our samples ranged from 5 to 20 nm, whereas
the lengths were less than 1 ␮m. The spacing between tube
walls was about 0.34 nm. This well agrees with the wall
separation of conventional multiwalled nanotubes. Figure 2
shows an electron diffraction pattern taken from a multishell
tubule. The ring-like patterns originated from h k 0 spots,
and the spots indicated by the arrows corresponded to 0 0 2n,
as was found previously for conventional carbon nanotubes.1
The tip shapes were similar to the tips of the tubes formed by
arc-discharge method, as shown in Fig. 1͑b͒. These results
indicate that carbon nanotubes form by the present method.
Besides the tubes, we also observed small carbon graphitic
particles and amorphous sheet-like carbons.
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17
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