Preferential arc-discharge production of higher fullerenes

8 December 1995

ELSEVIER

CHEMICAL PHYSICS LETTERS Chemical Physics Letters 246 (1995) 571-576

Preferential arc-discharge production of higher fullerenes Takumi Kimura a, Toshiki Sugai a, His od Shinohara a, Takashi Goto b, Kazuyuki Tohji h Isao Matsuoka b a Department of Chemistry, Nagoya University, Nagoya 464, Japan " Department of Resources Engineering, Tohoka University, Sendal 980, Japan

Received 26 August 1995; in final form 28 September 1995

Abstract

A new method for efficiently generating higher fullerenes is reported. By utilizing the arc-discharge of boron-doped graphite (bulk boronized graphite) electrodes, we have. found that the yield of higher fullerenes (C76-C96) in toluene extracts becomes 35-40 wt% with an optimum boron doping, which is more than twice as large as that obtained in the conventional arc-discharge of 100% graphite rods. A typical absolute yield of the higher fullerenes in soot is 5 - 6 wt%. The results suggest that some boron-carbon binary clusters play an important role in an early stage of the formation of highe~ fullerenes.

1. Introduction Just after the discovery of the macroscopic arcdischarge synthesis of C6o and C7o [1], it was found that there also exists in soot, in some quantity, the so-called higher fullerenes (C,; n > 70) [2]. In fact, it has been known [3-5] that by using multiple solvent extraction a series of higher fullerenes up to C5oo is actually observed in raw soot produced by the arc-discharge of graphite rods. Recent progress in the chromatographic separation of fullerenes, on the other hand, provides us with the complete isolation not only of various sizes of higher fullerenes [6] but also fullerene-related novel materials such as endohedral metallofullerenes [7,8]. So far m3C NMR spectroscopy [9-11] and UHV scanning tunnelling microscopy (STM) [ 12-14] have proved to be useful experimental techniques for structural analyses of several higher fullerenes. However, due to the scarcity of higher fullerenes in soot

prepared by the arc-discharge method, only a few isomers of the higher fullerenes have been studied and analyzed by such techniques. For example, a typical yield of higher fullerenes in raw soot is in the range 1-3 wt%. It has been found [15] that the production of higher fullerenes depends on the inert gas used in the arc-discharge. Under these circumstances, a high-yield synthesis of higher fullerenes is needed for a further structural and electronic study of these materials. In the present study, we have found that under optimum conditions the soot produced by the arcdischarge of bulk boronized graphites (boron-doped graphite) contains, typically, 5-6 wt% of higher fullerenes, which is more than twice as abundant as that observed in normal soot. The enhancement of higher fullerenes is sensitive to the doping ratio of boron. The production of some structural isomers of higher fullerenes such as CTs-C2v, is particularly enhanced, which can be interpreted within the frame-

0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All tights reserved SSDI 0009-2614(95)01174-9

572

T. Kimura et a l . / Chemical Physics Letters 246 (1995) 571-576

work of the catalytic effects of boron-carbon binary clusters at an early stage of fullerene formation.

matogram was monitored by a UV detector (A = 312 nm). The size distribution of higher fullerenes was determined by the integrated intensities of the HPLC peaks.

2. Experimental

The details of the large-scale dc arc-discharge apparatus with an anaerobic soot sampling mechanism for fullerene production have been described previously [16,17], The samples of higher-fullerene rich soot were produced by utilizing the arc-discharge of boron-doped graphite rods. We used the so-called bulk boronized graphite (BBG) composite rods (200 × 6 × 6 ram; Toyo Tanso Co. Ltd.), with several mixing ratios of boron/carbon, as the positive electrodes, The rods used in the present study are GB-101, GB-103 and GB-110 with a total boron concentration of I, 3 and 11 wt%, respectively. Pressed composites were baked around 2000°C in a vacuum to remove impurities prior to the arc-discharge. For comparison, non-boronized (pure) graphite (GB-100, Toyo Tanso Co. Ltd.), which has essentially the same matrix structure as that of BBG, was used for producing the soot of pure carbon. Detailed characteristics of BBG can be fotmd elsewhere [18]. Direct current arc-discharge (100 A, 20 V) of the composite rods was performed under a flow of He (40 Tort). The soot produced was collected under totally anaerobic conditions to avoid unnecessary contamination from air [16,17] and was Soxhlet extracted by carbon disulfide for 10 h or refluxed with pyridine, Tile extracts were mass-analyzed by laser-desorption time-of-flight mass spectrometry (LD-TOF-MS). The samples coated on a quartz plate were mounted in the ion acceleration region of a reflectron type TOF mass spectrometer. Desorption/ionization was done by the third harmonic of a Nd:YAG laser (Spectra Physics GCR150) at 355 nm. The pulsed acceleration voltage was typically in the range 7001000 V with 5-10 p.s duration• To quantitatively analyze the size distribution of fullerenes in the extracts, we also used preparative high-performance liquid chromatography (HPLC; Japan Analytical Industry LC-908-C60) with a Cosmosil Buckyprep column (20-250 ram; Nacalai Tesque) and with 100% toluene eluent. The chro-

3. Results and discussion

3.1. Preferential production of higher fullerenes via boron doping Figs. la and lb show LD-TOF mass spectra of extracts of the soot prepared by the arc-discharge of pure graphite and bulk boronized carbon (BBG 1%) composite rods, respectively, where the two mass spectra were measured under the same experimental conditions. Fig. la is a typical mass spectrum of fullerenes in CS2 extracts of the soot produced by the arc-discharge of pure graphite rods, which conI

I

I

I

I

I

I

I

I

I

(a) Pure graphite

e-

< e-

C60

C70

C7s

(b) Boron 1%

c C96

L,I , , 150 160 170

, I , I I 180 190 200 210 220 230 240

Time of Flight //zs Fig. I. Laser desorption (355 nm) TOF mass spectra of carbon disulfide extracts of the soot prepared by the arc-discharge of (a) a pure graphite rod and (b) bulk boronized graphite i%.

573

T. Kimura et al./ Chemical Physics Letters 246 (1995) 571-576

sists primariiy of C6o and C70 with a small amount of higher f~,|lerenes. Fig. lb, in contrast, shows a totally different trend; the mass spectrum is composed of enhanced peaks due to the higher fulle;enes (up to at least ca. Ci5 o) and of much reduced peaks of C6o and C7o relative to that in Fig. la. It is known [3,4] that the various sizes of fullerenes are not uniformly distributed in extracts. In fact, the size distribution of the fullerenes observed in the present LD-TOF mass spectra was somewhat sensitive to laser-desorption spots on the samples. However, in qualitative terms, the results shown in Fig. 1 are regarded as typical size distributions of fullerenes. In addition to the general feature of the preferential production of the higher fullerenes via the arcdischarge of bulk boronized graphites, one of the salient features in the present results is that the CTa fullerene is particularly enhanced (Fig. l b). To quantitatively analyze the enhancement of CTS, we have performed HPLC (high-performance liquid chromatography) analyses of the corresponding CS 2 extracts. Fig. 2 shows HPLC chromatograms of the extracts from the soot of pure graphite, bulk boronized graphite (BBG 1%) and boronized graphite (BBG 3%) with toluene eluent in the fullerene size range C~6-Cs6. As observed in the mass spectrum (Fig. Ib), the peak due to C78 is especially enhanced also in the HPLC chromatograms for BBG 1 and 3% samples r¢!ative to that for the pure graphite. Furthermore, the HPLC chromatogram reveals that one of the Cva isomers is particularly enhanced with boron doping in the arc-discharge. It is well known [9,10] that there are three solvent-extractable C:a isomers (D 3, C2v and C2,,), which are actually produced by the arc-discharge, out of the five isolated pentagon rule (IPR) sati.~fying isomers of C~s (D 3, C2~, C2~,, D3h and Dab,) [19]. The present HPLC chromatograms reveal that out of the three extractable isomers of C7s the amount of the C~s-C2,, isomer is particularly increased whereas the intensities of the other two isomers remain almost the same. A similar result was reported by Wakabayashi et al. [20] using a high-temperature laser-vaporization of graphite rods. According to their results, the production of the C~s-C2~, isomers is clearly affected by the buffer He pressure and also by the temperature around the laser-vaporization spot; the production rate increases drastically as the He

(a) Puregraphite A

~/

/~Czv.

~

=£

C

.Q

I'i

~

i/IJl I\,

¢-

ou~

(b)Boron1%

/\

e-

c,? c,.

"-'c?-

............. "c,o

"

fi (c) Boron 3% L

i <,fi i

/ !,

\

,'~.

I

I

I

I

25

30

35

40

RetentionTime/ min Fig. 2. HPLC chromalograms for the higher fullerenes in the size range C76 -C8~', (a) pure graphite, (b) bulk boronized graphite 1% and (c) bulk boronized graphite 3%. The chromatograms are normalized to C~t,; the absolute peak intensities in (b) and (c) are much higher than that in (a).

pressure and the temperature increase, while those of the other two isomers remain relatively constant. In the present study, boron doping during arc-discharge provides a similar effect to those of He pressure and temperature on the production of C7a-C2v,. The relative HPLC intensities of the higher fullerenes, from C76 to C96, of the two extracts (pure graphite and BBG 1%) are schematically presented in Fig. 3. Since, in general, the solubility of fullerenes in toluene decreases as the size of the fullerenes increases, the relative abundance of the fullerenes higher than Ca4 should be much larger than that

T. Kimura et aL / Chemical Physics Letters 246 (1995) 571-575

574

[]

76

78

80

82

84

86

88

Pure graphite

90

92

94

I

96

Cn Fig, 3, Schematic presentation of the relative abundance of higher fullerenes (C76-C96) as determined by the corresponding HPLC chromatogram areas for pure graphite and boron I% samples using the same amount of extract.

presented in Fig. 3. Namely, the HPLC results on the size distribution of the higher fullerenes are qualitatively in good agreement with those of the massspectral measurements (Fig. 1). HPLC analysis indicates that the amount of C~ in the BBG 1% sample decreases to about 80% and the production of the higher fullerenes (C76-C96) increases to about three times that of the pure graphite sample. It may also be that the discharge temperature increases on boron doping, so that the production of the isomer increases. The enhancement of the higher fullerenes and the reduction of C60 was found to be a universal tendency on boron doping in arc-discharge; the production of C60 competes with that of the higher fullercnes and they may share the same carbon clusters as important components at an early stage of their growth [21].

3.2. Absolute yields of higher fullerenes To study further the enhancement of the higher fullerenes on boron doping and the effectiveness of the present method for the synthesis of higher fullerenes, we have examined their absolute yields. To obtain the absolute yield, we first determined the absolute yield of the total fullerenes in soot by actually weighing the extract/soot ratio. The absolute yields of higher fullerenes (C76-C96) were then

deteflnined by measuring the corresponding HPLC chromatogram area and by the ratio of (C~6C96)//(C60-C96 ). Table 1 presents the yield of total fullerenes and that of higher fullerenes. It was observed that the sum of the areas for 100% graphite and BBG 1% are almost the same. This indicates that a reasonable estimate of the absolute yield of higher fullerenes can be obtained as given in Table 1. The best yield of the higher fullerenes (5.6%) was obtained by using a BBG 1% composite rod although in general the total yield of fullerenes decreases as the boron/carbon ratio in the rods increases. The relative yield of the higher fullerenes in the toluene extracts increased to 38% by using a 1% boron-doped rod. Furthermore, the absolute yield of the higher fullerenes with BBG 1% is more than twice as large as that with a pure graphite rod. This shows that the present boron doping technique is a useful method for the efficient production of higher fuUerenes. 3.3. Boron doping and the formation of higher ful!erenes

With a BBG 11% composite rod, only a few fullerenes were extracted from the soot and their absolute yield was substantially lower than that of BBG 3%. The laser-vaporization TOF mass spectrometry of a BBG 11% rod (Fig. 4) in the gas phase, on the other hand, indicated that the formation of various sizes of boron-carbon clusters C.B,n (58 ~
Table I Absolute yields of higher fullerenes a (C 76 -C96) for pure graphite and bulk boronized graphite Ratio of boron in rods (wt%)

Absolute yield Ratio of higher of total fullerenes in fullerenes extract (%) b in soot (wt%)

Absolute yield of higher fullerenes in soot (wt%)

0 I 3

18.7 14.7 4.7

2.7 5.6 1.5

14.9 38.2 31.3

Q Typical values, which may vary by 10%-20% depending on the arc-discharge conditions. b (C76_C96)/(Coo_C96) as determined by the areas of the HPLC chromatogram.

T. Kimura et aL / Chemical Physics Letters 246 (1995) 571-576

B.Cs.-.

~s9

l

Css

575

Japanese Ministry of Education, Science and Culture for Grants-in-Aid for Scientific Research on Priority Areas (No. 05233108) and General Scientific Research (No. 06403006).

C6o

BnC6o-n /

References

i

.

137

.

.

.

,

138

.

.

.

.

t

139

.

.

.

.

I

.

.

.

.

140

i

141

.

.

.

.

I

.

.

.

.

142

Time of F l i g h t / a s

Fig. 4. Laser-vaporization (355 nm) cluster-beam TOF mass spectrum of a boron-doped composite rod (bulk boronized graphite 11%) around the cluster size of 60. Various types of boron-carbon binary clusters (C,B,,,; n + m = 58, 60, 62) are predominant. Only a small amount of pure carbon clusters was seen (see text).

of pure carbon clusters is small in the cluster size range of C60. These results are similar to those reported by Guo et al. [22]. Although we have looked for boron-substituted heterofullerenes in the CS 2 extracts up to C96 by high-resolution mass spectrometry, no trace of the heterofullerenes was observed. However, in a recent report by Muhr et al. [23] mono-boron heterofullerenes of the type C,_ ]B were generated by the arc-discharge of composite rods and were solvent extracted only under completely anaerobic conditions. Namely, the boron-substituted heterofullerenes are extremely air-sensitive and are readily subject to degradation under ambient conditions. At present, we presume that the observed decrease in the total yield of fullerenes on boron doping is due to an increase in the formation of boron-substituted heterofullerenes of the CnB,,, most of which have degraded under aerobic conditions. The observed enhancement of the production in the higher fuilerenes by a certain boron doping level (1% in the present study) suggests that boron-carbon binary clusters play some catalytic roles at an early stage of the fullerene formation.

Acknowledgements The authors thank Toyo Tanso Co. Ltd. for supplying bulk boronized graphites. HS thanks the

[1] W. Kr'itschmer, L.D. Lamb, K. Fostiropoulos and D.R. Huffman, Nature 347 (1990) 354 [2] H. Ajie, M.M. Alvarez, SJ. Anz, R.D. Beck, F. Diederich, K. Fostiropoulos, D.R. Huffman, W. Kf;itschmer, Y. Rubin, K.E, Schriver, D. Sensharma and R.L. Whetten, J. Phys. Chem. 94 (1990) 8630. [3] H. Shinohara, H. Sato, Y. Saito, M. Takayama, A. lzuoka and T. Sugawara, J. Phys. Chem. 95 (1991) 8449. [4] H. Shinohara, H. Sato, Y. Saito, A. Izuoka, T. Sugawara, H. Ito, T. Sakurai and T. Matsuo, Rapid Commun. Mass Spcctrom. 6 (1992) 413. [5] D.H, Parker, P. Wurz, K. ChaUerjee, K.R. Lykke, J.E. Hunt, M.J. Pellin, J.C. Hemminger. D.M. Gruen and L.M. Stock, J. Am. Chem. Soc. 113 (1991) 7,-'1-99. [6] K. Kikuchi, N. Nakahara, T. W&~.abayashi, M. Honda, H. Matsumiya, T. Moriwaki, S. Suzuki, H. Shiromaru, K. Saito, K. Yamauti, I. lkemoto and Y. Achiba, Chem. Phys. LeUcrs 188 (1992) 177. [7] H. Shinohara, H. Yamaguchi, N. Hayashi, H. Sato, M. Ohkohchi, Y. Ando and Y. Saito, L Phys. Chem. 97 (1993) 4259. [8] K. Kikuchi, S. Suzuki, Y. Nakao, N, Nakahara, T. Wakabayashi, H. Shiromaru, I. Saito, I. Ikemoto and Y. Achiba, Chem. Phys. Letters 67 (1993) 216. [9] R. Ettle, l. Chao, F. Diederich and R.L. Whetten, Nature 353 (1991) 149. [10] F. Diederich, R.L. Wbettcn, C. Thilgen, R. Ettle, 1. Chao and M.M. Alvarez, Science 254 (1991) 1768. [11] K. Kikochi, N. Nakahara, T. Wakabayashi, S. Suzuki, H Shinomaru, Y. Miyake, K. Saito, I. Ikemoto, M. Kainosho and Y. Achiba, Nature 357 (1992) 142. [12] T. Hashizume, X.D. Wang, Y. Nishina, H. Shinohara, Y. Saito and T. Sakurai, Japan. J. Appi. Phys. 32 (1993) L132. [13] X.D. Wang, T. Hashizume, H. Shinohara, Y. Saito, Y. Nishina and T. Sakurai, Phys. Rev. B 47 (1993) 15923. [14] H. Shinohara, N. Hayashi, H. Sato, Y. Saito, X.D. Wang, T. Hashizume and T. Sakurai, J. Phys. Chem. 97 (1993) 13438. [15] Y. Saito, M.lnagaki, H. Shinohara, H. Nagashima, M. Ohkohchi and Y. Ando, Chem. Phys. Letters 200 (1992) 643 [16] H. Shinohara, M. Inakuma, N. Hayashi, H. Sato, Y. Saito, T. Kato and S. Bandow, J. Phys. Chem. 98 (1994) 8597. [17] H. Shinohara, M. Takata, M. Sakata, T. Hashizume and T. Sakurai, in: Cluster-assembled solids, ed. K. Sattler (Transtech Publications, 1995), in press. [18] Y. Hirooka, R.W. Conn, MJ. Khandagle, (3. Chevalier, T. Sogabe, T. Matsuda, H. Ogura, H. Toyoda and It. Sugai, Fusion Technol. 19 (1991) 2059.

576

T. Kimura et al./ Chemical Physics Letters 246 (1995) 571-576

[19] P.W. Fowler and D.E. Manolopoulos, An arias of fullerenes (Clarendon Press, Oxford, 1995) p. 254. [20] T. Wakabayashi, K. Kikuchi, S. Suzuki, H. Shiromam and Y. Achiba, J. Phys. Chem. 98 (1994) 3090. [21] G. yon Hclden, N.G. Gotts and M.T. Bowers, Nature 363 (1993) 60.

[22] T. Guo, C. Jin and R.E. Smalley, J. Phys. Chem. 95 (1991) 4948 [23] HJ. Muhr, R. Nesper, F. Banhart and M, Zwanger, Abstract, International Winterschool on Elecu'onic Properties of Novel Materials: FuUerides and Fulleroids, March 1995, Tyrol.