Graphitic hollow carbon calabashes

Article abstract of DOI:10.1016/S0009-2614(98)00428-X

Graphitic carbon calabashes, a new structure of carbon, have been found among the solid carbon spheres synthesized by a mixed-valent oxide-catalytic carbonization (MVOCC) process. The calabashes are as large as 0.8 μm and are constructed by graphitic carbon shells of thicknesses ~10 nm. The calabash can be hollow or filled with a carbon core. Oxygen gas has been found to be trapped inside the calabashes, suggesting they could be used for gas/energy storage.

Full text of DOI:10.1016/S0009-2614(98)00428-X

5
June 1998
Chemical Physics Letters 289 Ž1998. 189–192
Graphitic hollow carbon calabashes
)
Z.L. Wang , J.S. Yin
School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA
Received 10 February 1998; in final form 7 April 1998
Abstract
Graphitic carbon calabashes, a new structure of carbon, have been found among the solid carbon spheres synthesized by a
mixed-valent oxide-catalytic carbonization ŽMVOCC. process. The calabashes are as large as 0.8 mm and are constructed by
graphitic carbon shells of thicknesses ;10 nm. The calabash can be hollow or filled with a carbon core. Oxygen gas has
been found to be trapped inside the calabashes, suggesting they could be used for gasrenergy storage. q 1998 Published by
Elsevier Science B.V. All rights reserved.
Carbon is probably the most versatile element
because it can form various structures. The success-
ful growth of diamond films, fullerene molecule C60
and its family w1x, carbon nano-tubes w2x, and carbon
onion w3x has excited many researchers and raised
The MnO catalysis was placed at the end of the gas
inlet. As the temperature increased at a rate of
2
98Crmin, decomposition of CH occurred when the
4
chamber temperature reached 900–10508C; carbon
solid spheres and calabashes were formed on the
wall of the quartz tube. Although carbon spheres
were the most dominant product, carbon calabashes
were found using transmission electron microscopy
ŽTEM..
many speculations about their potential applications
w4–14x. The variety of structures produced by carbon
1
2
are determined by its unique hybridization sp , sp
3
and sp bonding. One of the most striking features
of carbon is that it can form pentagonal and heptago-
nal carbon-rings, and the combination of the two
basic structural units with the hexagonal carbon-rings
can form a variety of geometrical configurations. In
this Letter, we report the finding of hollow graphitic
carbon calabashes in the carbon products synthesized
by a mixed-valent oxide-catalytic carbonization
ŽMVOCC. process.
Fig. 1a is a bright-field TEM image of the carbon
calabashes with spherical and dumbbell shapes. The
carbon calabash has the size of ;0.8 mm, which is
much larger than any of the fullerenes known. A
careful examination of the image shows that the wall
of the calabash is fairly uniform in thickness Žsee
arrowheads 1 and 3.. The calabash can be an hollow
enclosed shell or filled with a carbon core, and in
some cases the separation between the carbon shell
and its core is almost equal distance Žsee arrowhead
3.. The carbon calabashes and spheres stand by
The MVOCC process uses natural gas Žmethane.
as the source of carbon for synthesis of carbon
products w7–14x. Natural gas flowed through a quartz
tube placed inside a horizontal cylindrical furnace.
themselves without any contact or encapsulation with
the MnO2 catalyst. Chemical microanalysis using
energy-dispersive X-ray spectroscopy ŽEDS. showed
)
Corresponding author. E-mail: zhong.wang@mse.gatech.edu
no Mn present in the carbon structures. The shell
0
009-2614r98r$19.00 q 1998 Published by Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž98.00428-X

1
90
Z.L. Wang, J.S. YinrChemical Physics Letters 289 (1998) 189–192
ness is ;10 nm. A sharp drop in the image contrast
right inside the inner wall of the shell indicates that
the calabash is hollow. Electron diffraction pattern
shows that, if the electron beam is positioned at the
center of a calabash ŽFig. 2b., two broad rings
corresponding to graphite Ž101. and Ž112. are ob-
served; the absence of the Ž002. reflection indicates
Fig. 1. Ža. Bright-field and Žb. dark-field TEM images of carbon
calabashes found in carbon spheres synthesized by MVOCC.
Arrowheads 1, 2, 3 and 4 indicate a spherical calabash, dumbbell
calabash, dumbbell calabash containing a solid core and solid
carbon spheres, respectively. The images were recorded at 200 kV
using a Hitachi HF-2000 TEM equipped with a field emission
source. The dark-field image was recorded by selecting the Ž101.
and Ž112. reflection rings.
structure of the carbon calabash is also clearly re-
vealed by the dark-field TEM image ŽFig. 1b. since
the image contrast is directly related to the projected
density of carbon and from which the hollow shell
structure is apparent. The core structure of the cal-
abashes and the solid carbon spheres are clearly
revealed. The solid spheres are composed of graphitic
flakes and their growth model has been proposed
w15–18x.
Fig. 2. Ža. High-resolution TEM image recorded from a carbon
calabash, showing a hollow shell structure. Electron diffraction
patterns recorded from a hollow spherical carbon shell when the
electron probe was positioned at Žb. the center and Žc. the edge of
the calabash. The Ž002. reflection was over exposured in Žc..
The graphitic shell of the calabashes is best seen
in the high-resolution TEM image ŽFig. 2a.. The
calabash shell is rather uniform and the shell thick-

Z.L. Wang, J.S. YinrChemical Physics Letters 289 (1998) 189–192
191
that the local Ž001. graphite plane is nearly perpen-
dicular to the electron beam. The continuous Ž101.
and Ž112. rings mean the random twist among Ž001.
graphitic layers forming the shell. When the electron
beam is positioned at the edge of the calabash ŽFig.
EDS spectrum acquired from the edge of a solid
carbon sphere ŽFig. 3c. shows a negligible oxygen
peak, indicating that the oxygen adsorbed on the
carbon surface, if any, is vanishingly small. Quanti-
tative analysis of Fig. 3a gives the atom ratio of
wn rn x sŽ2.3"0.3.%, where the oxygen signal
2
c., the pattern shows an arc-shape of the Ž002. and
O
C Ža.
Ž004. reflections due to the curvature of the graphitic
is contributed by the adsorbed oxygen and the oxy-
gen gas enclosed inside the shell. In the case of Fig.
3b, wn rn x sŽ0.5"0.3.%, and the oxygen sig-
layers, thus, the Ž001. plane is nearly parallel to the
electron beam, in agreement with the result provided
by the TEM image. The elliptical shape of the Ž101.
O
C Žb.
nal is dominated by the adsorbed oxygen on the
shell. With consideration the large size of the cal-
abash in comparison to the beam size, the volume
density of oxygen rO enclosed inside the shell is
determined by
reflection is due to the spherical shape of the graphitic
layers, and it indicates that the size of the graphitic
layer is large w19x. This elliptical shape is a distinct
difference from the diffraction pattern recorded from
a solid carbon sphere composed of small-size
graphitic flakes w15–18x.
ro yri
ri
rO f
Ä
wn rn x y2wn rn xŽb.4r ,
O
C
O
C
C
1
Ž .
Ža.
From the TEM images presented above it can be
concluded that the carbon calabashes are either hol-
low or containing a carbon core. It is our interest to
find the type of gas enclosed inside the calabashes.
To probe this phenomenon from a single carbon
calabash, EDS in TEM was employed. The electron
beam was focused to a size of 30 nm, so that the
chemical composition of the carbon calabash, as
shown in the inset of Fig. 3, can be detected near its
center and at its edge. In reference to the carbon
K line, the oxygen K line in the EDS spectrum
acquired from the center of the calabash shell ŽFig.
where r and r are the outer and inner radii of the
o
i
shell, respectively, and rC is the atom volume den-
sity of graphite. From the kinetics of gases, the
partial pressure of oxygen is given by
rO
PO s
RT ,
2
Ž .
NA
where NA is the Avogadro’s number, R the gas
constant and T temperature. For Ts300 K and Žr
o
5
y r .rr s 0.04, Eq. Ž2. yields P f 2.4 =10
i
i
O
2
Nrm . This means that the oxygen partial pressure
inside the calabash is ;2.4 atmospheric pressure.
This is a reasonable number with consideration of
our experimental condition.
3
a. is apparently stronger than that in the spectrum
acquired from the edge of the shell ŽFig. 3b.. The
The gases trapped inside the hollow carbon cal-
abashes can be oxygen, hydrogen andror CH . If
4
the trapped oxygen were in solid state and were
distributed uniformly on the surface of the inner
shell, there would be no difference between the EDS
spectra acquired when the electron probe was posi-
tioned at the center and at the edge of the shell. The
oxygen could be provided by the MnOx catalysis
during the carbonization process.
From the literature, a curling graphitic carbon
particle is believed to be nucleated from a pentago-
nal atom ring, and its growth gives a quasi-icosa-
Fig. 3. Energy-dispersive X-ray spectra acquired from Ža. the
center of a carbon calabash, Žb. the edge of the calabash, and Žc.
the edge of a solid carbon sphere. The Cu L line came from the
copper grid used in TEM analysis. The relative intensity of the Cu
L line with respect to that of the carbon K line depends on the
specimen thickness, but the relative magnitude of O K line to C
K line is the intrinsic property of the local region.
hedral spiral shell carbon particle 20,21 . A pentago-
nal carbon ring produces an inward surface with
positive curvature, resulting in spiral growth. This
has been proposed as the nucleation of fullerenes and
solid carbon spheres w15–18,20,21x. The structure of
w
x

1
92
Z.L. Wang, J.S. YinrChemical Physics Letters 289 (1998) 189–192
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the carbon calabashes reported here is, however,
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The most striking feature of the carbon calabashes is
the large-size, hollow-shell graphitic structure. The
growth of a large hollow calabash may require the
formation of a large graphitic spherical layer at least
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Acknowledgements
Thanks NSF ŽDMR-9632823 and CHE-9727633.
for financial support and Dr. Z.C. Kang for discus-
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