Nanoporous Carbon Produced By Ball Milling
APPLIED PHYSICS LETTERS
VOLUME 74, NUMBER 19
10 MAY 1999
Nanoporous carbon produced by ball milling
Ying Chena)
Joint Department of Engineering, Research School of Physical Sciences and Engineering,
The Australian National University, Canberra, ACT 0200, Australia
John Fitz Gerald
Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia
Lewis T. Chadderton
Atomic and Molecular Physics Laboratory, Research School of Physical Sciences and Engineering,
The Australian National University, Canberra, ACT 0200, Australia
Laurent Chaffron
Section de Recherches de Me´tullurgie Physique, CEA-Saclay F-91191 Gif sur Yvette Cedex, France
Received 11 January 1999; accepted for publication 7 March 1999
A nanoporous structure was produced in the samples of graphite after ball milling at ambient
temperature. The specific internal surface area of the micropores, as determined using the t-plot
method, is higher than the external surface area of particles and mesopores. Phase transformations
from hexagonal to turbostratic, and to amorphous structures were investigated using x-ray
diffraction analysis and transmission electron microscopy. Formation of the nanoporous structure is
associated with that of the disordered carbon. The disordered and nanoporous structure is probably
fullerene-like in nature. © 1999 American Institute of Physics. S0003-6951 99 00319-8
Phase transformations of graphite under mechanical
x-ray dispersive spectroscopy EDS in a JEOL JSM6400
treatment were investigated as early as the 1950’s.1,2 High
scanning electron microscope equipped with an Oxford ISIS
surface areas in the samples after ball milling have also been
EDXA of ATW window analysis system. The nanoporous
reported.3–5 However, nanoporous structures produced by
structure was investigated using a Gemini 2375 surface area
high-energy ball milling have not thoroughly been investi-
analyzer with the t-plot method which determines both the
gated. Recently, we found that nanosized filaments with tu-
external surface area contributed by particles and macropores
bular structures can be produced by the annealing of disor-
diameter larger than 500 Å and the internal surface area of
dered carbon6 or hexagonal boron nitride materials7,8 which
micropores which are smaller than 20 Å in diameter.12,13 The
were previously ball milled. During the thermal annealing,
measurements were conducted using nitrogen gas at liquid
nanotubes or nanocages form from the milled powders. This
nitrogen temperature. Samples were degassed at 200 °C for 1
is quite simply a solid state crystal growth process involving
h before the measurements were taken.
none of the vapor phases or chemical reactions essential in
Equal amounts
4 g
of graphite powders were ball
many other synthesis methods.9,10 To clarify the formation
milled using the Uniball mill at room temperature for various
times. Variations of the specific micropore internal surface
process of nanotubes during heat treatment, ball milled ma-
(S
terials need to be investigated from the standpoint of both
inter) , external surface ( S exter) , and the Brunaer – Emmett –
Teller BET areas (S
crystalline structure and morphologic change.
BET
Sinter Sexter) as a function of
milling time are illustrated in Fig. 1. It is found that the BET
Hexagonal graphite powder of a purity more than 99.8%
area increases to a maximum value of 589 m2 g 1 during the
was used as the starting material. Ball milling was carried
first 15 h of milling. It is important to note that more than
out at room temperature using two different kinds of ball
mills: 1 a vertical planetary ball mill Uniball mill
details
in Ref. 11 with a stainless steel container and four hardened
steel balls diameter 25.4 mm . Argon gas at 300 kPa was
used as the milling atmosphere; 2 a vibrating frame grinder
Fritsch Pulverisette 0 with a tungsten carbide WC ball
and a WC vial under vacuum (10 4 Torr . Several graphite
samples were also milled in the steel mill in a wet condition
with 10 ml of ethanol to reduce particle agglomeration. The
crystalline structure of samples was investigated by means of
both x-ray diffraction
XRD
using cobalt radiation
1.789 Å and transmission electron microscopy TEM us-
ing Philips EM430 300 kV and Hitachi 7100 100 kV
instruments. Chemical compositions were examined using
FIG. 1. Variations of the BET, internal, and external surface areas as a
function of milling time for the graphite powders milled in a steel ball mill
a Electronic mail: ying.chen@anu.edu.au
without ethanol.
0003-6951/99/74(19)/2782/3/$15.00
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© 1999 American Institute of Physics
Appl. Phys. Lett., Vol. 74, No. 19, 10 May 1999
Chen et al.
2783
FIG. 2. Variations of the BET, internal, and external surface areas as a
function of milling time for the graphite powders milled in a steel ball mill
with ethanol.
FIG. 3. X-ray diffraction patterns taken from the graphite samples milled in
a steel ball milling without ethanol for a 10 h, b 15 h, and c 50 h.
50% of this surface area is found to come from the nanosized
pores with the specific internal area of 335 m2 g 1, while the
using EDS. Similar carbon structure was produced by ball
external surface area is 254 m2 g 1. This reveals that a large
milling with the presence of a higher Fe content of 10
number of micropores was created in the milled graphite
at. %.14 In the case of milling in WC mill, the turbostratic
sample by ball milling. During further milling the external
structure is found in the sample milled for 50 h and the
surface decreases to about 102 m2 g 1 after 50 h and after-
amorphous phase is dominant in the samples after 200 h of
wards remains constant up to 150 h. This is a typical surface
milling. Again, contaminant WC is found in the sample, but
area change during milling for nonporous materials. The in-
the milling contaminations seem to have no effect on the
crease in the external surface area during the first period of
structural changes of the graphite during ball milling. In con-
milling simply is a result of particle fracturing induced by
trast, in the case of milling with ethanol, the hexagonal
ball impacts, and the later reduction in surface area is linked
graphite structure remains stable even after 200 h of milling,
to agglomeration effects. The external surface area becomes
suggesting that no disordered phases were produced during
constant when the particle fracturing is in balance with the
wet milling.
formation of agglomerates. In contrast, the micropore area is
The above results reveal that formation of the nanopo-
apparently maintained at a high level during the extended
rous structure is probably linked with the formation of the
milling. The nanoporous structure was also found in samples
disordered structure such as the amorphous phase. Mi-
milled for 150 h in the WC mill with a micropore area of 284
cropores presumably formed during agglomeration of fine
m2 g 1 in an external surface area of 269 m2 g 1. Therefore,
carbon particles during welding under the ball impacts. The
nanoporous carbon powders were produced using two differ-
nanoporous structure also could be created by severe plastic
ent ball mills and was clearly unaffected by milling contami-
deformation of the 002 planes.15 In the case of wet milling
nants.
the low internal surface area is due both to much reduced
Figure 2 shows different changes in the surface areas of
plastic deformation and agglomeration of particles.
the graphite samples during ball milling in the presence of
TEM confirms the formation of the turbostratic and
liquid. The BET surface area increases slowly to about 102
amorphous structures in the sample after ball milling for 15 h
m2 g 1 during 100 h of milling. The internal surface area is
in the steel ball mill. As shown in Fig. 4, an agglomerate of
very low only 21 m2 g 1 , being a maximum at 20% of the
size about 100 nm is seen. Nanosized narrow ribbons width
total surface area. Hence, milling in a wet condition in-
of about 5 nm with parallel fringes corresponding to 002
creases only the external surface as a result of fracturing of
basal planes identified by microdiffraction are characteris-
large particles.
tic of the turbostratic structure. Holes with different sizes and
Three typical XRD patterns taken from the graphite
curved surfaces can also be observed in the agglomerate,
samples milled in the steel mill without ethanol for 10, 15,
revealing the porous structure, although the micropores can-
and 50 h are shown in Fig. 3. The hexagonal structure of
not be observed directly in the TEM micrographs. For
graphite is still the dominant phase in the sample after mill-
samples milled for 50 h or longer, more amorphous-like
ing for 10 h, but this phase is no longer present in the sample
structure and fewer nanosized ribbons were observed. The
after milling for 15 h, having been replaced by a turbostratic
mixture of amorphous and nanocrystalline phases in the ball
structure with lattice spacings of d(002)
3.46
0.05 Å and
milled samples revealed by TEM is in agreement with those
d(100)
2.10
0.05 Å. The asymmetric shape of the 002
reported by Tan et al.16 and by Shen et al.17 using high reso-
peak is probably due to the possible presence of an amor-
lution TEM. The formation of a turbostratic structure sug-
phous carbon phase. Dominant amorphous phase is found in
gests fracturing of the hexagonal structure of graphite into
the sample after milling for 50 h TEM reveals in fact that
small basal planes during the first period of milling. Further
carbon nanocrystallites can still be found in the amorphous
milling leads to broken-up graphene layers which eventually
phase . Similar structures were produced in the samples after
transform to the amorphous structure. The high number of
further milling up to 150 h. Longer milling times lead to a
micropores in the milled samples suggests a high number of
high level of iron contamination. The Fe content in the
Gaussian curved layers possibly due to the presence of pen-
sample after milling for 150 h is found to be about 3.5 wt %
tagons, as is the well-known case for fullerenes. This disor-
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Appl. Phys. Lett., Vol. 74, No. 19, 10 May 1999
Chen et al.
pacts. Milling contaminations have no effect on the forma-
tion of this nanoporous structure.
The authors thank T. Hwang for help in some of the
surface area measurements, M. Marsh for some ball milling
experiments, and Drs. S. Stowe and F. Brink from the EMU
of the Australian National University for assistance with
TEM.
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11
FIG. 4. TEM micrograph taken from the graphite sample after milling for
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13
dered nanoporous structure is probably responsible for pro-
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16
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