Letters to the Editor / Carbon 39 (2001) 287 – 324
311
Synthesis of diamond from carbon nanotubes under high
pressure and high temperature
a,
b
c
b
c
Limin Cao *, Cunxiao Gao , Haiping Sun , Guangtian Zou , Ze Zhang ,
a
a
a
a
a
Xiangyi Zhang , Meng He , Ming Zhang , Yancun Li , Jun Zhang , Daoyang Dai a ,
Liling Sun a , Wenkui Wang a
a
Institute of Physics, Center for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100080, PR China
b
State Key Laboratory for Superhard Materials, Jilin University, Changchun 130023, PR China
c
Beijing Laboratory of Electron Microscopy, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PR China
Received 26 July 2000; accepted 12 September 2000
Keywords: A. Carbon nanotubes, Carbon onions, Diamond; B. High pressure
The investigation on elemental carbon has long been of
considerable interest because of its great importance in
both science and technology. One of the most outstanding
achievements which occurred in carbon science was the
synthesis of diamond under high-pressure–high-temperature (HPHT) conditions [1,2]. Now, man-made diamond is
commercially available and plays an indispensable role in
modern industry for abrasives, tool coatings, microelectronics, optics and other applications. Another important
advance in carbon science was the discovery of fullerenes
and carbon nanotubes [3,4]. These novel carbon phases
have gained high visibility because they have the potential
to exhibit unique structural variety and extraordinary
optical, mechanical, and electronic properties [5]. Recently, a number of studies relating to the behaviors of
fullerenes under high pressure have been reported which
were focused on probing the cage structure stability and
looking for new novel structures [6–8]. Moreover, it has
been demonstrated that fullerenes can convert to diamond
by applying high pressure at either room temperature [9] or
high temperature [10]. However, only a few works have
been so far focused on the structural stability and phase
transformation of carbon nanotubes at high pressure
[11,12]. As we know, the transformation of graphite or
graphite-like materials to diamond is of great technological
importance and therefore remains an exciting field in both
experimental and theoretical studies. Therefore, a detailed
study of the behaviors of carbon nanotubes at high
pressure is very necessary for further understanding their
structures and properties and comparing with that of
graphite.
In the present work we report the conversion of carbon
*Corresponding author. Tel.: 186-10-8264-9531; fax: 18610-8264-9147.
E-mail address: [email protected] (L. Cao).
nanotubes to diamond at 4.5 GPa and 13008C using a
six-anvil high pressure apparatus with the existence of
NiMnCo catalyst. The detailed characterization conducted
shows that carbon nanotubes transform to quasi-spherical
onion-like structures first and then to diamond crystals. It
is different from the phase transformation behavior of
graphite to diamond under high pressure and high temperature conditions.
For the experiments presented here, multiwalled carbon
nanotubes with diameters of 20–50 nm produced by
catalytic chemical vapor deposition (CCVD) were used as
starting material (Fig. 1). A 63600 ton six-anvil high
pressure apparatus with an electric current heating device
was employed for the high pressure experiments. In each
experiment, about 50 mg of carbon nanotubes placed
between two NiMnCo alloy flakes were put into the high
pressure cavity and a cubic body with a cylinder sample
chamber of pyrophyllite was used as pressure seal and
transmitting medium. Before and after high pressure runs,
the samples were monitored by transmission electron
microscopy (TEM; H8100), scanning electron microscopy
(SEM; S-4200) and Raman spectroscopy (T64000). Highresolution transmission electron microscopy (HRTEM)
investigations were carried out using a JEOL2010 microscope operating at 200 kV.
Fig. 1 shows the representative morphology of carbon
nanotubes in the starting material. The carbon nanotubes
exhibit typical concentric graphitic shell structure with a
hollow core. The diameters of most carbon nanotubes in
our sample are around 30–40 nm uniformly. No other
forms of carbon can be detected in the sample by TEM
study.
Before annealing carbon nanotubes under high pressure,
we pressed the carbon nanotubes up to a hydrostatic
pressure of 6 GPa for 4 h at room temperature. After
removal from the high pressure cell, some of the samples
were dispersed in alcohol, placed on a holey carbon
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312
Letters to the Editor / Carbon 39 (2001) 287 – 324
Fig. 1. Transmission electron microscopy (TEM) micrograph of
carbon nanotubes in the starting material.
microgrid and then observed directly with TEM. No
structural changes were found and all the nanotubes kept
their hollow one-dimensional cylinders as the original.
According to previous reports, after the graphitization at
28008C under ambient pressure as well as argon protection
or high vacuum, the structural characteristics of carbon
nanotubes may be not only kept but also improved [13].
Monthioux also predicted that the structure of nanotubes
may not be lost under 30008C [14]. All of these prove that
carbon nanotubes are stable under a pressure below 6.0
GPa or a temperature below 28008C, respectively.
When the samples were annealed at 13008C for 12 min
under 4.5 GPa, we found some diamond crystals grown on
the NiMnCo flake. Fig. 2 shows a characteristic SEM
image of the diamond crystals which are 0.1–0.5 mm in
size and with the shapes of cubes, cubo-octahedra or
octahedra. Energy-dispersive X-ray analysis (EDX) with
these crystals showed the carbon K a line as the only
Fig. 2. Scanning electron microscopy (SEM) image of diamond
crystals grown on the NiMnCo catalyst flake.
Fig. 3. Raman spectrum of the diamond crystals measured at
room temperature.
relevant peak in the spectrum, indicating that these crystals
are made of pure carbon.
Raman spectroscopy was used to further characterize
these carbon crystals. We used a Jobin-Yvon T64000
micro-Raman system with the 514.5 nm line of an argonion laser for excitation at an output power of 200 mW. The
measurement was performed at room temperature. The
Raman spectrum, shown in Fig. 3, revealed the characteristic peak centered at 1332 cm 21 , which corresponds to
cubic diamond. This analysis further demonstrates that
these crystals are diamond rather than other carbon structures.
In order to investigate the growth mechanism of
diamond from carbon nanotubes and compare with that of
graphite, carbon nanotubes and graphite flakes were placed
at the symmetric position of temperature and pressure
between two NiMnCo catalyst flakes in the high pressure
cell, and were subjected to high pressure and high temperature treatments. At 4.5 GPa and 13008C, no conversion of
graphite to diamond was observed. This suggests that the
formation of diamond is favored if carbon nanotubes are
used as starting material and there must be some differences in the process of transformation to diamond between
graphite and carbon nanotubes.
For further study, small pieces taken off from the
produced carbon nanotubes treated at 13008C and 4.5 GPa
were crushed into small chips and characterized by highresolution transmission electron microscopy. Fig. 4a shows
a characteristic TEM image of the products. It can be seen
that the hollow long cylindrical structures of the carbon
nanotubes disappear. Instead, some quasi-spherical carbon
particles can be observed to form. High-resolution TEM
image of the quasi-spherical carbon particle (Fig. 4b)
indicates the formation of an onion-like graphitic structure
which is composed of concentrically nested graphitic
Letters to the Editor / Carbon 39 (2001) 287 – 324
Fig. 4. (a) Representative TEM image of the quasi-spherical
carbon particles. (b) High-resolution transmission electron microscopy (HRTEM) image showing the nested onion-like graphitic
structure.
shells. Zhang et al. reported a similar transformation of
carbon nanotubes to quasi-spherical onion-like graphitic
particles under high pressure and high temperature [11].
And the beautiful transmission electron microscopy studies
done by Ugarte [15,16] and Banhart et al. [17–20] have
revealed that spherical carbon onions were generated under
intense electron or ion irradiation of carbon nanotubes and
other carbonaceous precursors. We think that the high
pressure and high temperature take a similar role in the
transformation of carbon nanotubes to onion-like structures
in our experiments. Under high pressure and high temperature, the tubular structures collapse and graphitic layers of
the nanotubes break. These broken graphitic shells will
curl up and close into spheroidal networks to eliminate
their dangling bonds at the edges and reach the lowest
energy level. Thus, quasi-spherical onion-like graphitic
particles develop by successive curvature and eventually
313
closure of the graphene planes. The high curvature of the
nested graphitic shells and cross-links between the layers
in the onion-like structures formed lead to an increased
fraction of sp 3 bonds, which facilitates the formation of
diamond.
Synthesis of sp 3 -bonded cubic diamond of high quality
at high pressures and high temperatures with transition
metal catalysts from sp 2 -bonded graphite-like carbon precursors is of great technical importance and considerable
commercial interest. However, the mechanism by which
transition metals catalyze the transformation from graphitic
materials to diamond (and vice versa) is still obscure [21].
Diamond synthesis is sensitive to the nature of the starting
carbon materials. In our experiments, no conversion to
diamond was observed from graphite in the presence of
NiMnCo catalyst or carbon nanotubes in the absence of
NiMnCo catalyst at 4.5 GPa and 13008C. Therefore,
NiMnCo catalyst plays the key role for the transformation
of carbon nanotubes to diamond. High-resolution transmission electron microscopy studies demonstrated that carbon
nanotubes transformed into quasi-spherical onion-like particles first under high pressure and high temperature. We
think that the diamond crystals could be nucleated from
these onion-like particles with the aid of NiMnCo catalyst.
In this process, the atoms of the metal catalyst diffuse into
the onion-like graphitic particles, promote the deformation
of graphene layers of the quasi-spherical shells, which are
then converted into diamond layers. The increased fraction
of sp 3 bonds induced by the high curvature of the graphitic
layers and cross-links between the graphitic shells might
make the diamond formation favored from the quasispherical graphitic structures. Detailed studies of the
transformation mechanism of nanotube-onion and oniondiamond are in progress.
In summary, we have synthesized diamond using carbon
nanotubes as starting material at 4.5 GPa and 13008C with
the existence of NiMnCo catalyst. Under the same condition, no diamond crystals were obtained when graphite
was used as the carbon source. The lower pressure and
temperature in the current work suggests that carbon
nanotubes can be used as another carbon source for the
synthesis of high-quality diamond. It is expected that the
pressure and temperature will be further reduced through
the optimization of catalyst and technological process,
which are of great importance from both basic and applied
viewpoints.
Acknowledgements
This work was supported by the National Natural
Science Foundation of China. The authors wish to thank
Dr. Y.W. Pan for Raman scattering analysis, Dr. H.Y. Chen
for TEM, Professor C.Y. Wang for SEM measurements and
for valuable discussions.
Letters to the Editor / Carbon 39 (2001) 287 – 324
314
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Mesostructure of polymer / carbon black composites observed by
conductive probe atomic force microscopy
J. Ravier a , *, F. Houze´ b , F. Carmona a , O. Schneegans b , H. Saadaoui a
a
b
Centre de Recherche Paul Pascal, Av. du Dr. Schweitzer, 33600 Pessac, France
´
Laboratoire de Genie
Electrique de Paris, Plateau de Moulon, 91192 Gif-Sur-Yvette Cedex, France
Received 28 July 2000; accepted 13 September 2000
Keywords: A. Carbon black, Carbon composites; C. Atomic force microscopy, Image analysis; D. Microstructure
The electric conductivity of random composites made of
an insulating polymer matrix filled with conductive carbon
black particles undergoes an insulator–conductor transition
when the carbon black concentration increases. This
transition is usually described by the percolation theory
which predicts the emergence of an infinite cluster of
connected particles [1]. The electrical transport properties
*Corresponding author. Fax: 133-5-5684-5600.
E-mail address: [email protected] (J. Ravier).
of such disordered conductor–insulator composites are
closely tied to their mesostructure, i.e. to the distribution of
the carbon black aggregates in the matrix at every scale
larger than the particle size. A better description of this
mesostructure will help us understand the origin of their
macroscopic properties, as some of them have important
practical applications [2].
Previous experimental access to the microscopic structure was made using neutrons and X-ray diffraction [3] or
TEM ImagE ANalysis and reconstruction using mathe-
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