The Journal of
Physical Chemistry VOLUME 97, NUMBER 23, JUNE 10,1993
Q Copyright 1993 by the American Chemical Society
LETTERS High Yield Synthesis of Lanthanofullerenes via Lanthanum Carbide Shunji Bandow' Instrument center, Institute for Molecular Science, Myodaiji Okazaki 444, Japan
Hisanori Shinoharat Department of Chemistry for Materials, Mi'e University, Tsu 51 4, Japan
Yahachi Saito Department of Electric and Electronic Engineering, Mi'e University, Tsu 51 4, Japan
Masato Ohkohchi and Yoshinori Ando Department of Physics, Meijo University, Nagoya 468, Japan Received: February 18, 1993; In Final Form: April 19, 1993
A high yield synthesis for lanthanofullerenes has been studied by a newly developed fullerene arc generator equipped with an anaerobic sampling apparatus. The yield of the lanthanofullerene La@Csz has increased by a factor of 10 or more, when LaCz-enriched composite carbon rods are used for the generation of soot, instead of hydrooxidized or oxidized lanthanum composite rods. The fact is characterized by electron spin resonance and by the powder X-ray diffraction method. Furthermore, the yields of other types of lanthanofullerenes, LaCz,,, have also increased, when the LaCz-enriched composite rod is used as an arc-burning material. Introduction The first extraction of a lanthanum fullerene, La@CSZ,was achieved by Chai et a1.,1 which stimulated much research on endohedralmetallofullerenes. Since the first success of the metal encapsulation into fullerenes, the electronic properties of metallofullerenes were well studied and established to be described as M3+@ C&-.Z+ However, the yield of metallofullerenesis very low, normally far below than 1% from the original soot. It is thus extremely important to find high yield synthesis methods for these endohedral metallofullerenes. The metal encapsulation of fullerenes is normally performed by arc burning a composite rod made from a metal oxide and graphite in a depressurized He gas. Prior to arc burning, the composite rod is normally subjected to heat treatment and carbonized at a temperature of about 1000 OC in a high vacuum, To whom correspondence should be addressed. Present address: Department of Chemistry, Nagoya University, Fur* cho, Nagoya 464, Japan. f
as was described in previous studies.*-' By using such composite rods, the metallofullerenesof La,237-9 Y ,3.73 and Sc4+5v7have been successfully produced, extracted, and characterized by electron spin resonance (ESR) and mass spectrometry. In the heat treatment, it is believed that the composed metal oxide changes to the metal carbide. However, there has been no systematic study of metallofullerenegeneration which stresses the relation between the yield of the synthesis and the starting material. In this Letter, we present a systematic study of a high yield synthesis of lanthanofullerenes. The yield of lanthanofullerenes is characterized quantitatively by electron spin resonance (ESR), and the crystallographic properties of the rods for evaporation are characterized by powder X-ray diffraction. Experimental Section The lanthanum-graphite composite rods used in the present experiment were purchased from Toyo Tanso Co. Ltd., with a constituent of 8 wt % of La203. These rods were baked under the evacuationof a rotary pump at 1200 OC for 1h and evacuated
0022-365419312097-6101S04.00/0 0 1993 American Chemical Society
6102 The Journal of Physical Chemistry, Vol. 97, No. 23, 1993
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Figure 1. Configuration of the arc-burning materials. A positive and a negative electrodeare indicated by the symbols + and -, respectively.(a) Normal arc-burningconnection between the composite rod (rod A/or B) and a pure carbon rod. (b) Schematic illustration of the carbonaceous deposit produced during the evaporation of the composite rod. (c) Configuration of the carbonaceousdeposit. further by a diffusion pump a t 7 X le5Torr for another hour. Moreover, the temperature was elevated to 2000 "C under the atmospheric pressure of an Ar stream (2 L/min) for 5 h. After these heat treatments, one of the two composite rods was put into a sample bottle which was filled with Ar gas (rod A, an anaerobically stocked rod) and the other was kept in air (rod B, an aerobically stocked rod). The arc burning of the composite rods in the dc mode was performed under controlled conditions: We have applied a potential of ca. 30 V with a current density of 2.3 A/mm2 in a He gas flow of 43 Torr.L/s at a pressure of 100 Torr. The arc burning of the composite rods was conducted by the following two types of experiments: One is that the pure carbon rod sets as a negative end and the composite one sets as a positive end, as shown in Figure la. In this configuration, the positive end of the composite rod mainly evaporates to generate soot. During the arc burning, a carbonaceousdeposit is simultaneouslyproduced on the top of the negative carbon rod (see Figure lb). The other type of evaporation is to utilize this deposited material: The electrode with the carbonaceous deposit is used as a positive end, while the pure carbon electrode is used as a negative end, as indicated in Figure IC. The evaporation conditions were the same as the former case. The sootgenerated was collected anaerobically by the so-called "gas flow cold trap" methodlo by using toluene as a solvent. The sample solution was collected by a luar lock type syringe and filtered with a disposable syringe filter unit (0.5 pm) to remove insoluble soot (the extract was ca. 3 wt % from the original soot). The sample was then put in an ESR tube and degassed by the freeze and thaw technique in vacuo. In each sample preparation for ESR measurements, great care was taken to keep the concentration of fullerenes (mainly Cm and (270) constant to compare with the yield of metallofullerenes: The concentration of Cm was quantitatively measured after the ESR measurement by an analytical type HPLC (high-pressure liquid chromatograph, Hewlett Packard HP1090) with a detective wavelength of 320 nm and was found to be in the order of l O I 7 ~ m for - all ~ samples. All the sample handling processes were conducted under anaerobic conditions. Since theline widthsof theoctet ESR due to lanthanofullerenes attain their minimum values a t the temperature of 220 K,9 ESR spectra weretakenat this temperature for all samples. Thespectra were recorded by a Bruker ESP300E equipped with a liquid nitrogen flow cryostat (ER4111VT) using an X-band frequency of 9.43 GHz with a field modulation frequency of 25 kHz. The powder X-ray diffraction was taken by a MAC Science vertical type powder X-ray diffractometer (MXPVA) with an X-ray source of Cu Ka.
Figure 2. ESR spectra of anaerobically sampled lanthanofullerenes in toluene. ESR spectra were taken at 220 K. Sptctrum (a) is from the sample produced by the composite rod B (an aerobically stocked rod), (b) from the carbonaceous deposited material of rod B, (c) from the composite rod A (an anaerobically stocked rod), and (d) from the carbonaceousdeposited material of rod A. One ESR octet is revealed in (a), while five ESR octets are revealed in each spectrum (b) to (d).
Spectra (b) to(d) givenearlythesameESRintensitiesandeachcomponent ratio (see ref 9 for the explanation of each component). The arrows indicate the position of the g value of the main octet, 2.0008.
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29 ( d e g ) Figure 3. Powder X-ray diffraction patterns of the starting materials for arc burning. The diffraction (a) is for a pure graphitic carbon rod, (b) for the compositerod B (an aerobically stocked rod, kept 10 days in air), (c) for the composite rod A (an anaerobically stocked rod), (d) for the carbonaceousdeposited material of the composite rod A after 2 months left in air (a depositedoneof the compositerod B gives a similardiffraction pattern), and (e) for the carbonaceous deposited material of a pure graphitic carbon. Half-crossed square, half-closed square, solid circle, and open circle indicate the diffraction peaks from graphite, La(OH)3, LaC2, and the carbonaceous deposit of a pure graphitic carbon, respectively. The Miller indices of graphite, La(OH),, LaC2, and the carbonaceous deposit of a pure graphitic carbon are indicated in the diffractions (a), (b), (c), and (e), resptctively. The X-ray source is Cu KCY.
Results and Discussion
Figure 2 shows the ESR spectra for the samples extracted anaerobically by toluene from the soots produced from different starting materials. The ESR intensities for the samples from arc-burning soots of rod A and of the carbonaceous deposits are about 10-20 times greater than that produced by rod B. The concentration of La@Cgz for each sample was estimated by comparing theESRintensityofLa@CCg2octetswiththat ofDPPH (l,l-diphenyl-2-picrylhydrazyl)and was found to be in the order of I O l 4 ~ m for - (a) ~ and of ~ m for - (b), ~ (c), and (d). Since
The Journal of Physical Chemistry, Vol. 97, No. 23, 1993 6103
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26 ( d e g ) Figure 4. Expanded form of X-ray diffraction. Solid, dash-dotted, and dotted lines indicate the positions of the diffraction from graphite (002), LaCz(lOl),andthecarbonaceousdepit(002) ofapuregraphiticcarbon, respectively. Symbols (a) to (e) and marks have the same meaning as in Figure 3.
the concentration of Cm for each sample is on the order of 1017 cm-3 as was noticed in the previous section, the yields of La@Cs2 normalized with Cm concentration are roughly estimated to be 0.1% for rod B and 1% for both rod A and carbonaceousdeposits. Furthermore, the five types of ESR octets related to different lanthanofullerenes reported previously9 are observed for rod A and the deposited ones. For CS2 extraction, the ESR intensities of the main octet observed for rod A and the deposited ones are also an order of magnitude stronger than that for rod B, which are identical to toluene extraction. The powder X-ray diffraction patterns for each of the starting materials are shown in Figure 3; (a) is for the pure graphitic carbon rod, (b) for composite rod B, (c) for composite rod A, (d) for the carbonaceous deposited material of the composite rods (rods A and B give nearly the same X-ray diffraction pattern), and (e) for the deposit produced from a pure graphitic carbon rod. It is obvious that rod A and the carbonaceous deposited materials include a considerableamount of LaC2. On the other hand, rod B does not give a strong X-ray diffraction due to LaC2; Le., most of the diffraction peaks related to La are due to La(OH),. Figure 4 is an expanded form of the diffraction peaks for the spacings (002) of a pure graphitic carbon and for (101) of Lac,; each spacing is very close, and it is very difficult to separate them from each other. However, it is seen that at least two diffraction peaks are superimposed in diffraction (d): One is from LaCz (101), and theother is from thecarbonaceousdeposit (002) which presumably stems from what are called carbon nanotubesl1-l3 and nanocapsules (nanometer scale graphitic particles).14.15 It should be noted that t h e (002) spacing of the carbonaceous deposit (0.343 nm) is wider by 1.8% than that of a pure graphitic carbon (0.337 nm); the values of each spacing are from Figure 4, e and a, respectively. Combining the results of ESR with those of powder X-ray diffraction, the yields of lanthanofullerenes are about 10 times higher for samples produced from LaC2-enriched materials (rod A and the carbonaceous deposits) than from the normal composite rod (rod B). Furthermore, it should be noticed that the yield of lanthanofullerenes has not changedbetween the samples produced
from composite rod A and from the carbonaceous deposits. As a matter of fact, very recent HRTEM (high-resolution transmission electron microscope) studies of carbonaceous deposits reveal the presence of Lac2 nanocrystals encapsulated by nanocapsules.16 This fact seems to indicate that the efficient production of lanthanofullerenesis closely related to the presence of Lac2 encapsulated by nanocapsules. However, since the powder X-ray diffraction from rod A (Figure 4c) provides no evidence of the presence of carbon nanotubes and nanocapsules (only indicatingthe presence of a pure graphitic carbon and Lac2 from Figure 3c), the presence of LaCz-encapsulated nanocapsules enhances the yield of lanthanofullerenes but is not a necessary condition for the high yield synthesis of the metallofullerenes. Furthermore, it has been confirmed by a recent arc-burning experiment using starting material with 9 wt % of LaCz (Rare Metallic, 99.9%) mixed anaerobically with the carbon powder that the yield of lanthanofullerenes is almost the same as that from rod A and the carbonaceous deposits (ca. 3 wt % of extract from the original soot and 1% of La@Cs2 normalized with Cm concentration). The present study actually demonstrates that the LaCz-enriched carbon compound is the most probable candidate for the starting material needed for high yield synthesis of various lanthanofullerenes. It is concluded that the high yield synthesis of lanthanofullerenes is achieved by using LaC2-enriched composite rods. The presence of LaCz in a supersaturated carbon ~apor17-~1 in arc burning may play an important role in generating the lanthanofullerenes.
Acknowledgements The authors express their gratitude to Dr. Hiroshi Kitagawa for his stimulated discussions throughout this work. S.B. and H.S.are indebted to Japanese Ministry of Education,Scienceand Culture (Grant-in- Aid for New Program No. 03NW301) for the support of the present study. H.S.and Y.S. also thank the Japanese Ministry of Education, Scienceand Culture (Grant-in-Aid for General Scientific Research No. 04650047 and for Scientific Research on Priority Area No. 04230102) for the partial support of the present study. References and Notes (1) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.; Chibante, L. P. F.; Fure, J.; Wang, L.; Smalley, R. E. J . Phys. Chem. 1991, 95, 7564. (2) Johnson, R. D.; de Vries, M. S.; Salem, J.; Bethune, D. S.;Yannoni, C. S. Nature 1992. 355. 239. (3) Shinohara;H.; Sato, H.; Saito, Y.; Ohkohchi, M.; Ando, Y. J. Phys. Chem. 96. -. ... 1992. -. .-,. ., 3511. . . (4) Shinohara,H.;Sato,H.;Ohkohchi, M.;Ando, Y.;Komada,T.;Shida, T.; Kato, T.; Saito, Y. Nature 1992, 357, 52. (5) Yannoni,C.S.;Hoikina,M.;deVries,M. S.; Beyhune,D.S.;Salem, J. R.; Crowder, M.S.; Johnson, R. D. Science 1992, 256, 52. (6) Weaver, J. H.; Chai, Y.; Kroll, G. H.; Jin, C.; Ohno, T. R.; Haufler, R. E.; Guo, T.; Alford, J. M.; Conceiao, J.; Chibante, L. P. F.; Jain, A,; Palmer, G.; Smalley, R. E. Chem. Phys. Letr. 1992, 190, 460. (7) Suzuki, S.;Kawata, S.; Shromaru, H.; Yamauchi, K.; Kikuchi, K.; Kato, T.; Achiba, Y. J. Phys. Chem. 1992, 96, 7161. (8) Hoinkis, M.; Yannoni, C. S.; Bethune, D. S.; Salem, J. R.; Johnson, R. D.; Crowder, M. S.; de Vries, M. S. Chem. Phys. Lett. 1992, 198,461. (9) Bandow, S.; Kitagawa, H.; Mitani, T.; Inokuchi, H.;Saito, Y.; Yamaguchi, H.; Hayashi, N.; Sato, H.; Shinohara, H. J. Phys. Chem. 1992, 96, 9609. (10)Kimura, K.; Bandow, S . Bull. Chem. Soc. Jpn. 1983,56, 3578. (1 1) Iijima, S.Nature 1991,354, 56. (12) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220. (13) Saito, Y.; Yoshikawa,T.; Bandow, S.; Tomita, M.; Hayashi,T. Phys. Rev. .. B. in o r a . ~
(14j Sako,Y.;Yoshikawa,T.;Inagaki,M.;Tomita,M.;Hayashi,T. Chem. Phys. Lett. 1993, 204, 277. (15) Ruoff, R. S.; Lorents, D. C.; Chan, B.; Malhotra, R.; Subramoney, S. Science, in press. (16) Tomita, M.; Saito, Y.; Hayashi, T. Jpn. J . Appl. Phys. 1993, 32, T
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(17) Chelikowsky, J. R. Phys. Rev. Lett. 1991, 67, 2970. (18) Ebbesen, T.W.; Tabuchi, J.; Tanigaki, K. Chem. Phys. Lett. 1992, 191, 336. (19) Wang,C. Z.;Xu, C.H.;Chan,C. T.;Ho,K.M.J. Phys. Chem. 1992, 96, 3563. (20) Ford, I. J.; Clement, C. F. Phys. Rev. Left. 1992, 69, 387. (21) Jing, X. D.;Chelikowsky, J. R. Phys. Rev. B 1992, 46, 15503.
