Fullerene production in alternative atmospheres

393

Letters to the Editor

Fullerene

production

in alternative

atmospheres

(Received 27 October 1992; accepted in revisedform 21 December 1992)

Key Words - Fullerenes, arcing, cyclopentadiene, hydrogen

The electrical arcing of graphite [l-3] or coal rods [4,5] has been shown to be a successful method for the production of fullerenes [6,7]. In conventional fullerene production. an inert atmosphere of helium or argon is used, and fullerenes are produced in about 10% yield by toluene extraction. However, it is clear that C60 is trapped in the toluene insoluble soot [8-lo] and that this soot contains a wide range of other unusual carbon forms [ll-141. In laser ablation mass spectrometry studies, we have observed that C6n and other fullerenes can be generated in other atmospheres such as hydrogen, methane and benzene I1 51. Hence it seemed worthwhile to undertake some preliminary arcing experiments in alternative atmospheres. In this work, we chose two experiments, one in an atmosphere of methane to provide a potential hydrogen source and another in an atmosphere saturated with cyclopentadiene monomer vapour prepared by distilling the dimer. The latter was chosen since to form C60 twelve pentagon faces are needed. If the ratio of pentagons to hexagons is increased, then larger yields of C&-Jmay form. Thus the addition of a source of five membered rings is of interest. Experiments were carried out using the experimental conditions outlined in Table 1. The product was Soxhlet extracted with toluene and examined by laser desorption mass spectrometry, infra-red

spectrometry and solution nuclear magnetic resonance spectroscopy and gas chromatography mass spectrometry. Methods of analysis are given in detail elsewhere [5]. Infrared spectra of the two extracts are shown in Figure 1. C60 has i&a-red absorptions [6,7] at 527, 576,1182,1428 cm-l. The spectrum produced from the toluene extractable material from the cyclopentadiene monomer experiment shows strong absorptions at 527 and 576 cm-1 (Figure la) showing the presence of Q-J. However, C&I appears not to be present in the extract generated from arcing in methane (Figure lb). Both spectra contain absorption from aliphatic and aromatic materials. The aryl-H absorption at 3031 cm-1 is present, as are absorptions at 2951,2922 and 2848 cm-l characteristic of aliphatic CHa structures. It is clear that in the cyclopentadiene experiments, some carbonyl material is present, possibly generated by oxidation in the presence of water or air introduced dunng addition of the cyclopentadiene. Additional information is available from the lH and 13C NMR spectra (Figure 2). Cm is identified by one line at 143 ppm in the 13C NMR spectrum. This is not present in the spectrum of the extract from the methane experiment, but is present in the extract from the cyclopentadiene experiment. The 1H spectra (not shown) indicate a range of other products present best

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Figure 1. FTIR spectra of toluene extract from a) Cp/He experiment b) CI-I@Ie experiment. Besides the absorptions from f&o, absorptions from C60_cyclope.ntacliene complex are present. Cjo is known to react with cyclopentadiene19.

Letters to the Editor

394

Table 1 Reaction Conditions Used During Electtical Arcing of Graphite

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a initial partial pressure of methane b Cp = cyclopentadiene monomer

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Figure 2. 1% NMR spectrum of extract from a) CH&Ie experiment (in CDC13) b) Cp/He experiment (in C&j). identified by gas chromato~aphy mass sp~~ornet~ (gclms) . Typical gc/ms spectra are shown in Figure 3. Both experiments produced a wide range of polycyclic hydrocarbons, but in general larger aromatic clusters were observed in the methane experiment. Both these experiments indicate that hydrogen can be captured from cyclopentadiene or methane, so that Ceo production is reduced by quenching reactive radicals, and instead of

ring closure to form f&o or extended graphitic sheet formation, small ring hydrocarbons are produced. The capture of hydrogen as polycyclic hydrocarbons is strong evidence for a mechanism of competitive ring closure for fulierene production [6,13,16- 181. These products however, represent only a small part of the product since yietds are small (Table 1). Most of the product is soot. Examination of the soot by solid state t3C NMR proved unsuccessful since it was

395

Letters to the Editor

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Figure 3. GUMS spectra of toluene extract from (a) CIVHe experiment (b) C@Ie experiment Infra-red spectroscopy was also paramagnetic. Instead, the extracted soots were uniformative. examined by laser desorption/ablation, Fourier transform mass spectrometry. Details of the method are described elsewhere 116,17-J.

Experiments in our laboratory [9,10] and elsewhere [81 have shown that in conventional fullerene production experiments Ca is entrapped in the toluene extracted soot and can be removed bv ovridine or detected by laser ablation at 2kWcm-2. The experiments

396

Lettersto the Editor

presented here showed that C&o and C70 could be observed at this low laser power or slightly higher laser power (Figure 4). This suggests Ceo and C7u are entrapped or some material is present in the toluene extracted soot which is readily converted to Qn and C70. We have shown that laser desorption of C!tjoand C70 occurs between 2 and 90kWcm-l without any decomposition [161. However, in separate studies of coal liquefaction residues which do not contain Gju and C7o but contain large amounts of polycyclic hydr~ar~n, we were able to detect Go and C7u at 5SkWcm-2 laser power. Thus, caution should be made in definitively concluding from laser desorption/ablation work alone that residues contain entrapped C!a and C7u. Pyridine extraction did not yield material in which C&o and C7u could be observed by i&a-red spectroscopy. Moreover at around 10kWcm3, the residues appear to begin yielding higher fullerenes as well as C&Jand C7o which are the products of thermal decomposition/ desorption of other components of the soot. These results strongly suggest that the soot from these experiments consists of material which readily forms C& and Cn, but is not purely graphitic since at these laser powers graphite is stable [6,151. It is clear that these soot residues do not resemble conventional soot, and represent a new class of heterogeneous carbon materials important in their own right.

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CSfRODivisionofCoal and Energy Technology P-0. Box 136 North Ryde NSW 2113 A USTRALIA School of Chemistry Universi~ of NSW P.O. Box 1 Kensington NSW 2033 AUSTRALIA

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REFERENCES 1. 2. 3.

4. 5. 6. 7. 8.

9.

R. Taylor, J.P. Hare, A.K. Abdul-Sada and H.W. Kroto, J. Chem. Sot. Chem. Commun., 1423 (1990). W. Kratschmer, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature, 347,354 (1990). RF+. Haufler, J. Conceicao, L.P.F. Chibante, Y. Chai, N.E. Byrne, S. Flanagan, M.M. Haley, SC. O’Brien, C. Fan, Z. Xiao, W.E. Billups, M.A. Ciufolini, R.H. Hauge, J.L. Margrave, L.J. Wilson, R.F. Curl and R.E. Smalley, J. Phys. Chem., 94, 8434 (1990). L.S.K. Pang, A.M. Vassallo and M.A. Wilson, Nature, 352, 480 (1991). L.S.K. Pang, A.M. Vassallo and M.A. Wilson, Energy and Fuels, 6, 176 (1992). M.A. Wilson, L.S.K. Pang, G.D. Willett, K.J. Fisher and LG. Dance, Carbon, 30,675 (1992). H.W. Kroto, A.W. Allaf and S.F. Balm, Chem. Rev., 91, 1213 (1991). D.H. Parker, P. Wurz, K. Chatterjee, K.R. Lykke, J.E. Hunt, M.J. Pellin, J.C. Hemminger, D.M. Gruen and L.M. Stock, J. Amer. Chem. SOC., 113, 7499 (1991). F. Hopwood, K.J. Fisher, LG. Dance, G.D. Willett, M.A. Wilson, L.S.K. Pang and J.V. Hanna, Preprints Amer. Chem. Sot. Div. Fuel, 37(2). 568 (1992).

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m/r Figure 4. Laser de~~ti~ab~tion spectra of toluene extracted soot from a) CH&Ie experiment b) Cp/He experiment. The irradiation power is shown.

Letters to the Editor

10. F. Hopwood, K.J. Fisher, I.G. Dance. G.D. Willett, M.A. Wilson, L.S.K. Pang and J.V. Hanna, Org. Mass Specrrom., 27. 1006 (1992). 11. T.W. Ebbesen and P.M. Ajayan, Nature. 358.220 (1992). 12. S. Iijima, Nature, 354, 56 (1991). 13. L.S.K. Pang, M.A. Wilson, G.H. Taylor, J. Fitzgerald and L. Brunckhorst, Carbon, 30, 1130 (1992). 14. J. Fitzgerald, G.H. Taylor, L. Brunckhorst, L.S.K. Pang, M.A. Wilson,Carbon,31, 240 (1993). 15. I.G. Dance, K.J. Fisher, G.D. Willett and M.A.Wilson, J. Phys. C/rem., 95, 8425 (1991).

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16. P.F. Greenwood, I.G. Dance, K.J. Fisher, G.D. Willett, L.S.K. Pang and M.A. Wilson, Org. Mars. Spectrom., 26, 920 (1991). 17. R.E. Smalley, Act. Chem. Res., 25,98 (1992). 18. T.M. Chang, A. Naim, S.N. Ahmed, G. Goodloe, P.B. Sherlin, J. Amer. Chem. Sot., 114. 7603 (1992). 19. F. Wudl, A. Hirsch, K.C. Khemani, T. Suzuki, P.M. Allemand, A. Koch, H. Eckert, G. Srdanov and H.M. Webb, ACS - Symposium Series 481, ~161-177, Amer. Chem. Sot., Washington, 1992.

Radiation damage of C60 CryStalS (Received 4 January 1993; accepted 15 January 1993)

Key Words - Radiation damage, fullerene, HRTEM imaging

Attendant with the intense interest in the fullerenes is an increased awareness of the utility of the transmission electron microscope (TEM) as an analytical instrument, especially for studying crystalline forms. However, radiation damage is a prominent feature of fullerene crystals when studied using high-voltage electron beams [ 1.21. In addition to the frustration that is inevitable when trying to capture the fascinating features the eye sees momentarily on the TEM viewing screen, there is also the possibility that the fullerenes could change character when subjected to this high-energy environment. Such a suggestion was made as a sort of “throw-away” comment sneculating about the origin of the elongated cigar-like “bucky-b&” reported by Wang and Buseck f31. Since these were the lrueest fullerenes reported to date, there was considerable iserest whether they formed during the original fullerene synthesis or within the TEM. Clearly, the study of the effects of radiation damage on fullerenes could be significant. An understanding of the radiation damage depends on a knowledge of the electron dose received by the sample. The stacking relations in fullerene crystals are well viewed by imaging along . Therefore we studied radiation damage in crystals having this orientation. The following images were all obtained with an accelerating potential of 200 keV. Incident beam currents were measured at the image plane using an electrometer, and the current density at the specimen was determined by the relation

approximately 2.3 x 1025 e/m2 for a Cm crystal in the Figures 2a, b, and c show the same crystal at three stages during the process of radiation damage. A photograph taken as soon as possible after locating the crystal shows well-defined Cjo spheres in a the recognizable close-packed configuration; corresponding selected-area electron diffraction (SAED) pattern contains sharp diffraction spots (Fi 2a). After the specimen received a dose of 2.3 x 109i5 e/m2, the orientation.

&p&men = (magnification)2. @image where 0 = current density. The dose to amorphization was calculated by multiplying the current density by the total exposure time. The image in Figure la was obtained after a dose of less than 1 x 1022 e/m2, whereas the one in Figure lb was taken just after damage was noticeable. We estimate the dose to amorphization as approximately 1 x 1023 e/m2. We also used a TEM having a 400 keV accelerating potential, and in this case damage was delayed slightly. The comparable dose to the initiation of noticeable amorphization in this instance is

Fig. 1 (a) and (b) [l 111 TEM image of a @I crystal supported on a holey carbon film; the crystal in (a) is minimally damaged, as can be seen along its edge, and that in (b) is extensively damaged.