New method of carbon onion growth by radio-frequency plasma-enhanced chemical vapor deposition

Article abstract of DOI:10.1016/S0009-2614(01)00085-9

Large quantities of carbon onions with high purity were synthesized by radio-frequency plasma-enhanced chemical vapor deposition. The produced carbon onions are solid, clean and can be separated easily from the catalytic particles. The formation of onions is based on the formation of many cages in successive stages from the core to the surface. Around the edge of a carbon onion, discontinuous curved lines are shown in the high-resolution TEM image, reflecting the wavy behavior of carbon onions.

Full text of DOI:10.1016/S0009-2614(01)00085-9

16 March 2001
Chemical Physics Letters 336 D2001) 201±204
www.elsevier.nl/locate/cplett
New method of carbon onion growth by radio-frequency
plasma-enhanced chemical vapor deposition
X.H. Chen a,b,*, F.M. Deng b, J.X. Wang a, H.S. Yang b, G.T. Wu b, X.B. Zhang b,
J.C. Peng a, W.Z. Li b
a
Collage of Materials Science and Engineering, Hunan University, Changsha, 410082, People's Republic of China
b
Department of Physics, Zhejiang University, Hangzhou, 310027, People's Republic of China
Received 16 June 2000; in ®nal form 5 January 2001
Abstract
Large quantities of carbon onions with high purity were synthesized by radio-frequency plasma-enhanced chemical
vapor deposition. The produced carbon onions are solid, clean and can be separated easily from the catalytic particles.
The formation of onions is based on the formation of many cages in successive stages from the core to the surface.
Around the edge of a carbon onion, discontinuous curved lines are shown in the high-resolution TEMimage, re¯ecting
the wavy behavior of carbon onions. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
by-products and are hardly separated from the
product, which brings about diculty in investi-
gation and application for carbon onions. There-
fore, searching for a new synthesis process, which
could overcome the disadvantage of these tech-
niques, is crucially important to the future appli-
cation of the carbon onions. Here we report a new
method of formation of carbon onions in mac-
roscopic quantities by radio-frequency plasma
CVD.
After the discovery of carbon onions [1], which
are made of concentric graphite spherical shells,
many e€orts have been made to create these on-
ions and understand the formation mechanism of
such materials. Carbon onions are expected to
have good prospects for some aspects of com-
posite materials, wear-resistant materials and
magnetic storage media. These carbon onions are
produced by DC arc-discharge [1]. Other ways to
form carbon onions have been reported, such as
high-energy electron irradiation of carbonaceous
materials [2], or thermal treatment of fullerene
black [3]. Onions can also be produced by shock-
wave treatment of carbon soots [4], and plasma
torch [5]. In these materials, carbon onions are
2. Experimental
The catalysts were prepared by impregnating
silica gel with a solution of 0.02 MCo ꢀNO3†2. The
solution was stirred for 24 h and then dried at
150°C for 12 h, followed by a calcining at 500°C in
¯owing N2 for 4 h. Finally, the resulting samples
were reduced at 700°C for 5 h in H2 gas environ-
ment to obtain the catalysts.
* Corresponding author. Fax: +86-731-8824525.
E-mail address: xhchen@mail.hunu.edu.cn DX.H. Chen).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 D 0 1 ) 0 0 0 8 5 - 9

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X.H. Chen et al. / Chemical Physics Letters 336 12001) 201±204
A ceramic boat containing 25 mg of catalyst
was placed in the reaction chamber of the radio-
frequency plasma CVD. A mixture of CH4 and H2
DH2 : CH4 ˆ 20 : 1) was introduced into the
chamber at a ¯ow rate of 850 ml/min at 573 K. The
pressure was kept at 1 Torr and the voltage output
was 35±50 V for 15 h. The surface of the catalysts
was deposited with black material. The specimens
were dispersed in alcohol by ultrasound and then
dropped on the carbon grids for TEMand
HRTEMobservation.
3. Results and discussion
Fig. 1 is a typical TEMimage of the high-per-
centage carbon spheres on the catalysts. The
product contains no carbon tubes, only carbon
spheres, which are solid, smooth and clean. The
carbon spheres have an average diameter ranging
from a few to several tens of nanometers, roughly
the same as the diameter of multiwall tubes. Fig. 2
gives an HRTEMimage of spherical particles,
where the concentric graphitic layers are resolved
clearly, related to the so-called carbon onion.
From Fig. 2 we can also see the broken curved
lines around the edge of the particle. This may
mean that the carbon sheets are wavy, which is
di€erent from the continuous lines observed in
carbon onions obtained by the electron irradiation
of carbonaceous materials [2]. The interplanar
Fig. 2. HRTEMimage of a typical onion formed on catalyst.
The carbon onion contains several cores in its center.
distance is modulated by the wavy structure of the
carbon sheets. Otherwise, carbon spheres con-
taining several cores in their centers, as shown in
Fig. 2, were found fairly frequently.
The radio-frequency plasma-enhanced CVD
process is similar to the classical pyrolytic CVD
process for the growth of carbon ®bers or carbon
nanotubes, but the key di€erence is the gas de-
composition principle. The pyrolytic CVD tech-
niques use conventional Fe, Co, Ni and their
alloys as catalysts. Up to now, these methods have
not produced any carbon onions. It may be spec-
ulated that the radio-frequency plasma-enhanced
CVD process is suitable for the formation of
pentagonal carbon rings, which is indispensable
for curvature of the graphite sheet.
A striking feature is that the carbon onions
growing in our conditions do not encapsulate the
catalysts, as opposed to data reported earlier on
®lament or tube formation on transition metals
[6,7]. Generally, the catalyst particle plays the
important role of carbon transporter. The hydro-
carbon is adsorbed and decomposed on the front
of the metal particle, and then carbon layers form
on the back of the metal particle. As a result, the
Fig. 1. A TEMimage of synthesized carbon onions showing
the monodispersed size distribution. The product contain no
carbon tubes, only carbon onions.

X.H. Chen et al. / Chemical Physics Letters 336 12001) 201±204
203
catalyst is encapsulated by carbon layers. How-
ever, in our case, the carbon onions contain no
catalysts. This indicates a di€erence in growth
mechanism compared with the one previously
postulated.
carbon cage gives broken curved lines in the im-
age.
It is quite noteworthy that carbon onions are
obtained here without the help of high energy, as
reported in the literature [1±3]. For the condition
of arc-discharge, the formation of curvature of
graphite sheets happened during the cooling of
quasi-liquid carbon. In the case of high-tempera-
ture-treated or electron-irradiated amorphous
carbon particles, the multiple-shell spheres were
formed based on irradiation-stimulated graphiti-
zation. Such graphitization initiates at the external
surface of the particle and progresses toward its
center. In the case of radio-frequency plasma-en-
hanced CVD with low temperature, it is obvious
that the formation process of carbon onions is
di€erent. The following growth processes are in
agreement with our ®ndings DFig. 3).
The 3D nucleus is assumed to consist of a
fullerene `dome' formed by one cap of a C60 or C70
molecule at the surface of the catalyst. Further
growth of this dome by the addition of hexagons
and pentagons would lead to the formation of a
®rst cage. This ®rst cage is the substrate on which
further layers may nucleate and grow into a sec-
ond, third, etc. spherical cage. With the increase of
cage surface area, the new secondary surface may
involve pentagons, hexagons and heptagons which
deposit with various degrees of epitaxial coher-
ence, resulting in formation of wavy carbon sheets.
Such a process would explain the unclosed-shell-
like morphology observed.
In order to describe the formation mechanism
of the carbon onions, a possible picture is given.
Initially, the hydrocarbon is adsorbed at the sur-
faces of the cobalt catalyst particles. In the
meantime, the process of glow discharge leads to
CH4 decomposition, resulting in the formation of
carbon rings, which tend to nucleate on the sur-
faces of the catalysts. In addition, in the presence
of atomic hydrogen in the plasma, free carbon
valences on the ends of carbon layers can be sta-
bilized by the formation of C±H bonds. In fact,
similar behavior of Co±silica catalysts has been
reported during the growth process of carbon na-
notubes [8]. It is not clear what e€ect SiO2 has on
the growth of carbon nanotubes or carbon onions.
It seems that the unique catalytic behavior of Co±
silica needs further study.
It is interesting to note that there were many
discontinuous curved lines in the image DFig. 2),
which is similar to those produced by Kang et al.
[9]. This type of microtexture of carbon onions is
assumed to be associated with the combination of
the various kinds of carbon rings formed during
the growth of carbon onions. It is well known that
carbon atoms can form three di€erent types of
graphitic carbon rings. The ¯at graphite layer is
composed of hexagonal carbon rings. The pen-
tagonal carbon ring causes the hexagonal network
to curve inward, forming a surface with positive
curvature. The heptagonal carbon ring forces the
hexagonal network to curve outward, forming a
negative curvature. The coexistence of pentagonal
and heptagonal atom rings is the basis for the
closure and bending of carbon tubes. Thus, a pure
hexagonal network cannot form a perfectly closed
cage. Pentagonal carbon rings are essential for
forming closed carbon cage structures, such as C60.
If the carbon cage is only composed of pentagonal
and hexagonal carbon rings, the perfect carbon
cage will be obtained, and it gives continuous lines
in the image. However, if the heptagonal carbon
rings are also introduced in the hexagons network,
the carbon sheet will be wavy. In this case, the
Fig. 3. Successive stages in the formation of onions: Da) the
nucleus is a dome formed by one cap of a C60 or C70 molecule;
Db) the nucleus grow into a cage by the addition of hexagons
and pentagons; Dc) a second cage is nucleated on the ®rst cage as
substrate; Dd) a new secondary surface grows into the second
cage and encapsulate the ®rst cage; De) a large spherical particle
containing many closed-shell cages.

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X.H. Chen et al. / Chemical Physics Letters 336 12001) 201±204
4. Conclusion
Carbon onions with closed-shell structure may
exhibit unique physical and mechanical properties.
We have established a processing route to syn-
thesize carbon onions in macroscopic quantities by
using radio-frequency plasma-enhanced CVD. The
product is pure and not encapsulated on the sur-
faces of the catalysts. The signi®cance of the pre-
sent results is that the method of radio-frequency
plasma-enhanced CVD has overcome the short-
comings of other techniques. The method de-
scribed here might be used to prepare a wide
variety of carbon onions on a large scale.
Fig. 4. Two-dimensional structural models of carbon species
produced by the combination of pentagonal D5), hexagonal D6)
and heptagonal D7) carbon rings
From the discussion above, we can conclude
that the ®nal spherical structure corresponds to the
formation of a multiple-shell sphere with a very
small central cage. A considerable size D50 nm in
diameter and ꢁ70 shells for the particle shown in
Fig. 2) will give us information about the distor-
tion present in the closed graphite sheets. Fol-
lowing Euler's theorem, any perfect closed shell
contains exactly 12 pentagonal rings in the hex-
agonal network, although the number of hexagons
is arbitrary; this is the case in the spherical cage
DFig. 3a). A large spherical particle cannot be
formed if all of the arriving species are pure pen-
tagons because a pentagonal carbon ring will lead
to the growth of large faceted polyhedrons, such as
the rosette, hollow-shell particles found in carbon
black [10]. Therefore, the pentagons need to be
paired with the heptagons in order to nucleate a
non-faceted graphitic layer that follows the
smooth topology of the inner layers. If heptagonal
carbon rings are introduced, the closure requires
that the di€erence in number between pentagons
Dn5) and heptagons Dn7) should satisfy [11]
n5 À n7 ˆ 12. Two adjacent paired pentagons and
heptagons are unstable and tend to recombine to
form two hexagonal carbon rings. However, when
the two paired pentagons and heptagons are sep-
arately by a hexagonal network, various structural
con®gurations can be formed, and the structures
are expected to be stable, as shown in Fig. 4. The
inward and outward corners represent pentagonal
and heptagonal carbon rings, respectively. The
straight line represent hexagonal carbon rings.
The corners in these models are responsible for
the wavy behavior of the carbon sheets.
Acknowledgements
The authors acknowledge ®nancial support
from the Natural Science Foundation of China
DGrant No. 59972031).
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