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Energy & Fuels 1993, 7, 436-431
Nanotubes from Coal Louis S. K. Pang* and Michael A. Wilson' CSIRO Division of Coal and Energy Technology, P.O.Box 136, North Ryde, NSW 2113, Australia Received March 11, 1993. Revised Manuscript Received April 12, 1993
or hexagonal honeycomb structure^.^ In the micrograph Coal which has first been carbonized so that it has an shown in Figure 1 several types of nanotubes are evident electrical resistance of 1.5-2.5 ohms produces fullerenes including smail diameter (less than 5 nm) tubes (marked in much the same way as fullerenes can be generated from A), and tubes of larger diameter (marked B). The different Nevertheless the temperature needed to diameters are a result of different numbers of cylinders produce conductive coal (1200 "C) is below that of of hexagonal aromatic sheets around a central cavity. The graphitization so that graphitization is not essential for nanotubes observed in our experiments with coal are fullerene production. Yields of fullerenes c 6 0 and c70 in similar to those observed by Iijima and co-workers and coal experiments are comparable to those from graphite. Small quantities of fullerenes can also be made from coal others with graphite?l2-l5 except we have observed some by laser ablation and detected by mass s p e ~ t r o m e t r y . ~ ~ ~bent and folded tubes (Figure 1, point C). It has been suggested12J3that some nanotubes contain convex and During the arcing experiment with graphite, carbon is concave structures which originate from the incorporation vaporized from the anode and transferred to the cathode, of heptagon and pentagon rings. This could be the reason with subsequent buildup of carbon on the cathode. This for folds in tubes although the incorporation of tetrahedral transferred carbon resembles a pencil. Carbon composing carbon or even mechanical damage could also account for the pencil has been found to have a variety of molecular the folds. The angle between the folds in tubes in Figure and morphological forms."ll The most important, and novel to the arcing process, are nanotubes of graphitic 1 is about 105" which is close to the tetrahedral angle, but carbon of 4-30 nm diameter.5 Nanotubes, coined buckywe have been able to observe nanotubes that are clearly tubes by some researchers, have attracted great interest broken and others which appear to be bent due to physical since they may be raw materials for new composites, or compression. sorb materials such as lead or possibly copper, to form The material shown in Figure 1 was prepared from nanowires in which only a few atoms of metal compose the Bacchus Marsh brown coal. We have observed that brown diameter of the wire. Nanotubes should not be confused coals readily produce C6o and C70 fullerenes,16 but if the with other fibers7 which are formed in the same process, carboxylic groups are exchanged with calcium or lanthawhich we term microfibers. These have attached planes num (the latter is known to form endohedral comof graphitic layers at angles other than parallel to the ple~es),'~-'~ Cw and C7o fullerenes are not present in toluene tube main axis. extracts of the soot made from arcing electrodes made We report now that nanotubes (buckytubes) are also an from these materials. Here, we report that the pencils important component of the products from arcing coals formed on the cathode from both experiments contain and coal products such as humic acids as well as graphite. nanotubes. The reason why nanotubes but not c 6 0 or c 7 0 The nanotubes compose most of the inner soft deposits fullerenes are formed from ion-exchanged brown coals is in the cathode pencil and there seems to be about the not clear, although it is known that exchanging metal ions same proportion of nanotubes as obtained from graphite. on carboxylicunits in brown coals reduces tar yields during However, this needs to be confirmed by studies on a wide flash pyrolysis.20 It is probable that the metal ions affect variety of coals. The original cathode material does not the rate of volatilization during the carbonization process appear to be composed of nanotubes. and thus the type of material present in the electrode. Figure 1illustrates a typical electron micrograph of the The yields of soot are lower in the presence of metal, which material isolated from Bacchus Marsh brown coal humic could be due to the effect of metal on vaporization. The acid. This micrograph was obtained from material commetal may also be vaporized and reduce c60 and C ~ O posing the core of material that has built up on the cathode, formation. Whatever the explanation for the behavior of but separate from other core material composing feathers exchanged brown coals, the formation of nanotubes from coal, like formation of fullerenes from coal, offers a cheaper (1) Pang, L. S. K.; Vassallo, A. M.; Wilson, M. A. Energy Fuels 1992, and hence industrially more important route to these new 6,176179. materials than graphite. Nanotubes can be separated by (2) Pang, L. S. K.; Vassallo, A. M.; Wilson, M. A. Nature 1991,352, sonication from less valuable products,13although in any 480. (3) Greenwood, P. F.; Strachan, M. G.; El-Nakat, H. J.; Willett, G. D.; Wilson, M. A.; Attalla, M. I. Fuel 1990,69, 257-260. (4) Greenwood, P. F.; Strachan, M. G.; Willett, G. D.; Wilson, M. A. Org. Mass. Spectrosc. 1990, 25, 353-362. (5) Iijima, S. Nature 1991, 354, 5 6 5 8 . (6)Pang, L. S. K.; Wilson, M. A.; Taylor, G. H.; Fitzgerald, J.; Brunckhorst, L. Carbon 1992,30, 1130-1132. (7) Fitzgerald, J.;Taylor,G. H.;Brunckhorst, L.; Pang,L. S. K.;Wilson, M. A. Carbon 1993,31, 240-242. (8) Ugarte, D. Chem. Phys. Lett. 1992, 198, 596-602. (9) Fitzgerald, J.; Taylor, G. H. Unpublished research. (10)Bacon, R. J.Appl. Phys. 1960, 31, 283-190. (11) Baker, R. T. K. Carbon 1989,27, 313-323.
(12) Iijima, S.;Ichihashi, T.; Ando, Y. Nature 1992, 356, 776-779. (13) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220-222. (14) Lenosky, T.; Gonze, X.; Teter, M. Nature 1992,355,333-334. (15) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333-334. (16) Pang, L. S.K. Fuel Process. Technol., in press. (17) Johnson, R. D.; de Vries, M. S.; Salem, J. R.; Bethune, D. S.; Yannoni, C. S. Nature 1992, 355, 239-240. (18) Smalley, R. E. Acc. Chem. Res. 1992,25,9&105. (19) Johnson, R. D.; Bethune, D. S.; Yannoni, C. S. Acc. Chem. Res. 1992,25, 169-175. (20) Tyler, R.; Schafer, H. N. S. Fuel 1990,59, 487-494.
0887-0624/93/2507-0436$04.00/00 1993 American Chemical Society
Communications
Energy & Fuels, Vol. 7, No. 3,1993 437
Figure 1. Electron micrograph of core cathode material from Bacchus Marsh brown coal 200 OOOX magnification: A, B, nanotubes of different diameters (buckytubes); C, folded nanotube.
industrial process other methods such as densityseparation say, of metal-filled tubes, may be preferable. Experimental Section Brown coals from Loy Yang, Bacchus Marsh, Yallourn and Morwell mines, Latrobe Valley, Victoria, Australia were dried a t 40 OC for 4 days. When dry, the coals were crushed to pass a 212-pm screen before use. Acid or Cation Exchange of Coal and Humic Acid. Loy Yang coal was ion exchanged using similar procedures described previously.21*nBriefly, an acid-exchanged coal was first prepared by stirring the coal (25 g) in excess HCl(1 L, 0.1 M) for 20 h. The coal was then filtered and washed with distilled water (about 2 L) until neutral. The calcium-exchanged coal was prepared by stirring the acid form of coal in 600 mL 0.2 M CaC12 solution for 20 h. The exchanged coal was filtered, washed with distilled water, and dried in a vacuum oven a t 80 "C for 20 h. Humic acid was prepared according to a published method.= Briefly, Bacchus Marsh coal (20g) was stirred in a 2 L 0.5 M NaOH solution at 60 "C for 20 h. After digestion, the mixture was centrifuged (10min a t 2500 rpm) to precipitate the insoluble humin fraction. The supernatant was acidified with dilute HCl to precipitate the humic acid. This suspension was then centrifuged and the precipitate washed with distilled water (3 X 500 mL) until neutral and dried in a vacuum oven a t 80 "C for 20 h. The humic acid yield was 30% based on dry coal. (21) Schafer, H.N.S. Fuel 1970,49, 197-213. (22) Schafer, H.N. S . Fuel 1972,51, 4-9. (23) Pang, L.S. K.,Vassallo, A. M.; Wilson, M . A. Org. Ceochem. 1990, 16,853-864.
The humic acid was exchanged with lanthanum by stirring the humic acid with a La(NO& solution (0.2 m) filtered and washed to neutral using a similar procedures as for the Loy Yang coal. The La(N03)3solution was prepared by neutralizing dilute HN03 with La203 (99.99%, Alfa Chemical Co.). Preparationof Conductive Material. Each material (about 15 g) was mixed with finely ground pitch (Kopper's) as a binder as described previously.'S2 The coal-pitch mixtures were packed into a Swageloktype stainless steel tube of size 12 mm i.d. by 152 mm length and sealed. The system was heated to 500 "C for 20 h to obtain a rod-shaped electrode. Further carbonization at 1200 OC for 5 h in an argon flow resulted in the formation of a conductive rod. Fullerene Production. Fullerenes were produced by the vaporization of the conductive rods by an electrical arc in helium as described previously.1*2Direct current was used throughout the present experiments. Graphite was used as the cathode electrode and the coal rods were used as the anode electrode. Only the coal rods were vaporized in these experiments. The chamber was evacuated and purged with helium and the process repeated 5 times to totally eliminate oxygen. The chamber was finally filled to 250 Torr with helium. A Lincoln dc arc welder capable of delivering up to lo00 A a t 65 V was used to supply the power for the experiment. Currents of 40-100 A dc a t 30 V were applied. The char vaporized in the electric plasma arc condensed as soot on the condenser and other parts of the chamber. The soot collected was Soxhlet extracted with toluene to isolate the fullerenes. The chamber was dismantled to isolate the carbon pencils which were sectioned for microscopicanalysis.
