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#
x-ray dispersive spectroscopy ~EDS! in a JEOL ~JSM6400! scanning electron microscope equipped with an Oxford ISIS EDXA of ATW window analysis system. The nanoporous structure was investigated using a Gemini 2375 surface area analyzer with the t-plot method which determines both the external surface area contributed by particles and macropores ~diameter larger than 500 Å! and the internal surface area of micropores which are smaller than 20 Å in diameter.12,13 The measurements were conducted using nitrogen gas at liquid nitrogen temperature. Samples were degassed at 200 °C for 1 h before the measurements were taken. Equal amounts ~4 g! of graphite powders were ball milled using the Uniball mill at room temperature for various times. Variations of the specific micropore internal surface (S inter), external surface (S exter), and the Brunaer–Emmett– Teller ~BET! areas (S BET5S inter1S exter) as a function of milling time are illustrated in Fig. 1. It is found that the BET area increases to a maximum value of 589 m2 g21 during the first 15 h of milling. It is important to note that more than
Phase transformations of graphite under mechanical treatment were investigated as early as the 1950’s.1,2 High surface areas in the samples after ball milling have also been reported.3–5 However, nanoporous structures produced by high-energy ball milling have not thoroughly been investigated. Recently, we found that nanosized filaments with tubular structures can be produced by the annealing of disordered carbon6 or hexagonal boron nitride materials7,8 which were previously ball milled. During the thermal annealing, nanotubes or nanocages form from the milled powders. This is quite simply a solid state crystal growth process involving none of the vapor phases or chemical reactions essential in many other synthesis methods.9,10 To clarify the formation process of nanotubes during heat treatment, ball milled materials need to be investigated from the standpoint of both crystalline structure and morphologic change. Hexagonal graphite powder of a purity more than 99.8% was used as the starting material. Ball milling was carried 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 (1024 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 ~l 51.789 Å! and transmission electron microscopy ~TEM! using 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 without ethanol.
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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.
50% of this surface area is found to come from the nanosized pores with the specific internal area of 335 m2 g21, while the external surface area is 254 m2 g21. This reveals that a large number of micropores was created in the milled graphite sample by ball milling. During further milling the external surface decreases to about 102 m2 g21 after 50 h and afterwards remains constant up to 150 h. This is a typical surface area change during milling for nonporous materials. The increase in the external surface area during the first period of milling simply is a result of particle fracturing induced by ball impacts, and the later reduction in surface area is linked to agglomeration effects. The external surface area becomes constant when the particle fracturing is in balance with the formation of agglomerates. In contrast, the micropore area is apparently maintained at a high level during the extended milling. The nanoporous structure was also found in samples milled for 150 h in the WC mill with a micropore area of 284 m2 g21 in an external surface area of 269 m2 g21. Therefore, nanoporous carbon powders were produced using two different ball mills and was clearly unaffected by milling contaminants. Figure 2 shows different changes in the surface areas of the graphite samples during ball milling in the presence of liquid. The BET surface area increases slowly to about 102 m2 g21 during 100 h of milling. The internal surface area is very low ~only 21 m2 g21!, being a maximum at 20% of the total surface area. Hence, milling in a wet condition increases only the external surface as a result of fracturing of large particles. Three typical XRD patterns taken from the graphite samples milled in the steel mill without ethanol for 10, 15, and 50 h are shown in Fig. 3. The hexagonal structure of graphite is still the dominant phase in the sample after milling for 10 h, but this phase is no longer present in the sample after milling for 15 h, having been replaced by a turbostratic structure with lattice spacings of d(002)53.4660.05 Å and d(100)52.1060.05 Å. The asymmetric shape of the ~002! peak is probably due to the possible presence of an amorphous carbon phase. Dominant amorphous phase is found in the sample after milling for 50 h ~TEM reveals in fact that carbon nanocrystallites can still be found in the amorphous phase!. Similar structures were produced in the samples after further milling up to 150 h. Longer milling times lead to a high level of iron contamination. The Fe content in the sample after milling for 150 h is found to be about 3.5 wt %
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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.
using EDS. Similar carbon structure was produced by ball milling with the presence of a higher Fe content of 10 at. %.14 In the case of milling in WC mill, the turbostratic structure is found in the sample milled for 50 h and the amorphous phase is dominant in the samples after 200 h of milling. Again, contaminant WC is found in the sample, but the milling contaminations seem to have no effect on the structural changes of the graphite during ball milling. In contrast, in the case of milling with ethanol, the hexagonal graphite structure remains stable even after 200 h of milling, suggesting that no disordered phases were produced during wet milling. The above results reveal that formation of the nanoporous structure is probably linked with the formation of the disordered structure such as the amorphous phase. Micropores presumably formed during agglomeration of fine carbon particles during welding under the ball impacts. The nanoporous structure also could be created by severe plastic deformation of the ~002! planes.15 In the case of wet milling the low internal surface area is due both to much reduced plastic deformation and agglomeration of particles. TEM confirms the formation of the turbostratic and amorphous structures in the sample after ball milling for 15 h in the steel ball mill. As shown in Fig. 4, an agglomerate of size about 100 nm is seen. Nanosized narrow ribbons ~width of about 5 nm! with parallel fringes corresponding to ~002! basal planes ~identified by microdiffraction! are characteristic of the turbostratic structure. Holes with different sizes and curved surfaces can also be observed in the agglomerate, revealing the porous structure, although the micropores cannot be observed directly in the TEM micrographs. For samples milled for 50 h or longer, more amorphous-like structure and fewer nanosized ribbons were observed. The mixture of amorphous and nanocrystalline phases in the ball milled samples revealed by TEM is in agreement with those reported by Tan et al.16 and by Shen et al.17 using high resolution TEM. The formation of a turbostratic structure suggests fracturing of the hexagonal structure of graphite into small basal planes during the first period of milling. Further milling leads to broken-up graphene layers which eventually transform to the amorphous structure. The high number of micropores in the milled samples suggests a high number of Gaussian curved layers possibly due to the presence of pentagons, as is the well-known case for fullerenes. This disor-
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pacts. Milling contaminations have no effect on the formation 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. G. E. Bacon, Acta Crystallogr. 3, 320 ~1950!. G. E. Bacon, Acta Crystallogr. 5, 392 ~1952!. 3 H. Hermann, Th. Schubert, W. Gruner, and N. Mattern, Nanostruct. Mater. 8, 215 ~1997!. 4 F. Disma, L. Aymard, L. Dupont, and J.-M. Tarascon, J. Electrochem. Soc. 143, 3959 ~1996!. 5 C. S. Wang, G. T. Wu, and W. Z. Li, J. Power Sources 76, 1 ~1998!. 6 Y. Chen, J. Fitz Gerald, and L. T. Chadderton ~unpublished!. 7 Y. Chen, J. Fitz Gerald, J. S. Williams, and S. Bulcock, Chem. Phys. Lett. 299, 260 ~1999!. 8 Y. Chen, L. T. Chadderton, J. Fitz Gerald, and J. S. Williams, Appl. Phys. Lett. ~to be published!. 9 T. W. Ebbesen, Annu. Rev. Mater. Sci. 24, 235 ~1994!. 10 C. Journet and P. Bernier, Appl. Phys. A: Mater. Sci. Process. 67, 1 ~1998!. 11 Y. Chen, T. Halstead, and J. S. Williams, Mater. Sci. Eng., A 206, 24 ~1996!. 12 G. Halsey, J. Chem. Phys. 16, 931 ~1948!. 13 W. D. Harkins and G. J. Jura, J. Chem. Phys. 11, 431 ~1943!. 14 T. Tanaka, M. Motoyama, K. N. Ishihara, and P. H. Shingu, Mater. Trans., JIM 36, 276 ~1995!. 15 A. Oberlin, Chemistry and Physics of Carbon ~Marcel Dekker, New York, 1989!, p. 94. 16 J. Tang, W. Zhaoi, L. Li, A. U. Falster, W. B. Simmons, W. L. Zhou, Y. Ikuhara, and J. H. Zhang, J. Mater. Res. 11, 733 ~1996!. 17 T. D. Shen, W. D. Ge, K. Y. Wang, M. X. Quan, J. T. Wang, W. D. Wei, and C. C. Koch, Nanostruct. Mater. 7, 393 ~1996!. 1 2
FIG. 4. TEM micrograph taken from the graphite sample after milling for 15 h in a steel mill without ethanol, arrows showing the nanosized ribbons.
dered nanoporous structure is probably responsible for providing the precursors to homogeneous nucleation for the growth of carbon nanotubes during subsequent anneal. In summary, a disordered and nanoporous carbon powder was produced by the ball milling of graphite powder at room temperature. Micropores are apparently formed though agglomeration of nanosized, disordered layers under ball im-
