J. Phys. Chem. 1995,99, 16076-16079
16076
Single-Layered Carbon Nanotubes Synthesized by Catalytic Assistance of Rare-Earths in a Carbon Arc Yahachi Saito,* Kenichiro Kawabata, and Mitsumasa Okuda Department of Electrical and Electronic Engineering, Mie University, Tsu 514, Japan Received: June 6, 1995; In Final Form: August 9, 1995@
Formation of single-layered (SL) carbon tubes by the arc-evaporation method has been studied using a series of rare-earth elements (Sc, Y, and La through Lu, excluding Pm) as catalysts. Electron microscopy of soot materials revealed that Y, La, Ce, R,Gd, Tb, Dy, Ho, Er, and Lu (including Gd, Y,La and Nd, which are already reported to be active catalysts) assisted the formation of SL tubes. The tube length ranged from 10 to 100 nm depending on the catalytic activity of the elements, while diameters were 1.8-2.1 nm regardless of the elements. Y, La, and Ce, which were the three rare-earth elements with the lowest vapor pressures, showed the highest activity among the elements investigated. The relation between the manifestation of the catalytic activity and the volatility of the elements was discussed.
Introduction Evaporation of carbon together with metals by arc discharge produces a wide range of novel carbon-based materials, e.g., fullerenes containing metal atoms (metallofullerenes),’-3filled nanocapsules,4-6 and single-layered (SL) SL tubes were serendipitously discovered in the course of the synthesis experiments of nanowpsules containing magnetic materials such as Fe,’ C O , and ~ Ni9 in 1993. Subsequently, it was revealed that Gd,I0 Y,ll La,’2 and NdI3 also catalyze the formation of SL tubes. The growth morphology of the tubes synthesized with rare-earths resembled a “sea urchin”,I0i.e., radially growing from a core particle. For Ni, similar growth morphology has been observed.I4 With regard to the growth mechanism of SL tubes, it has been proposed that tubes nucleate on the surface of a carbon-saturated metal particle, and then the nuclei radially grow by absorbing carbon on their roots via bulk diffusion of carbon through the base particle or via surface diffusion on the particle.l 4 3 l 5 In the present study, we carried out arc-evaporation experiments synthesizing SL tubes using a series of rare-earth elements and systematically investigated their catalytic activity. In addition to Gd, Y , La, and Nd, which were already reported’O-I2 to assist the formation of SL tubes, we found Ce, PI, Tb, Dy, Ho, Er, and Lu as new members exhibiting catalytic activity with varying activities. The growth morphology of SL tubes was similar to that already reported;l0-I2tubes protrude radially from a core particle.
Experimental Section The method employed here to produce SL tubes was the same as that previously used for the production of filled nanocaps u l e ~as~well , ~ as SL tube^:^*'^ a dc (direct current) arc discharge between a graphite cathode and a metal-loaded graphite anode in an atmosphere of helium. The graphite rods containing rareearth elements were prepared as follows: graphite rods (purity 99.998%), 50 mm long by 6 mm diameter, were drilled out to 30 mm depth by 3.2 mm diameter and filled with only metaloxide powder. Metal-oxides and their purity used are listed in Table 1. The cathode was a pure graphite rod (99.998%) with diameter of 10 mm. After evacuating the arc-discharge chamber
* To whom correspondence should be addressed. ‘Abstract published in Advance ACS Absrracfs, September 15, 1995. 0022-365419512099-16076$09.00/0
TABLE 1: Catalytic Activities of Rare-Earth Elements of Formation of Single-Layered (SL) Tubes (Elements That Form Filled Nanocapsules and Metallofullerenes Are Also Indicated) rare-earth element
oxide powder“
activity of SL tube formation
nanocapsulesb
metallofullerenes‘
no high high high low low no no intermediate intermediate low intermediate intermediate no no intermediate Metal-oxide powder packed in graphite rods. Punty of oxide powders was 99.9% except for La203 (99.99%) and Ce02(99.99%). See ref 6. See ref 18. XRD suggested the formation of encapsulated TmC2, although nanocapsules were not observed by
down to less than Torr, helium gas (purity 99.99%) was introduced to 100,500, or 1500 Torr, and the dc arc (70 A, 25 V) was fired to evaporate the metal-packed graphite anode. Sootlike materials were formed and deposited on the inner walls of the reaction chamber and on the cathode surface. The sootlike materials were collected from the inner walls of the arc chamber and from the surface of the cathode; we call the former soot “chamber soot” and the latter “cathode soot” hereafter. The collected soot were examined using a Philips EM400 transmission electron microscope (TEM) operating at 120 kV and a Hitachi H800 at 200 kV. Samples of TEM were prepared by sonicating the soot materials in acetone and putting a drop of the suspension onto a copper grid covered with a perforated carbon film.
Results Catalytic activities of rare-earth elements to produce SL tubes are summarized in Table 1. La, Ce, and Y produced abundant SL tubes in both the chamber soot and the cathode soot under 0 1995 American Chemical Society
Single-Layered Carbon Nanotu bes
J. Phys. Chem., Vol. 99, No. 43, 1995 16077
Figure 1. TEM image of single-layered (SL) carbon tubes collected from chamber soot produced by coevaporation of carbon and Ce.
He gas of 500 Torr. The fraction of SL tubes grown from La, Ce, and Y was estimated to be roughly 10% by volume from visual inspection of TEM images. Pr, Nd, and Dy showed weak activities for the formation SL tubes, the fraction being on the order of 0.1% or less. Pr and Nd produced SL tubes only in the chamber soot but not in the cathode soot, and Dy showed activity only at the highest He pressure (1500 Torr) in the present experiment, although SL tubes were observed in both the chamber and the cathode soot. Gd, Tb, Ho, Er, and Lu showed intermediate activities. The remaining rare-earth elements including Sc did not produced any SL tubes. Figure 1 shows a typical TEM image of a colony of SL tubes synthesized with Ce. This sample was produced under 500 Torr He and collected from the chamber soot. SL tubes with diameter of 1.8-2.1 nm and length of 80- 100 nm grow radially, showing “starfish”-like morphology. The tips of the tubes are capped, and no evidence of the presence of metal atoms or clusters was observed. The overall morphology of the colonies of SL tubes resembles the “sea urchin” like morphology revealed for Gd,lo Y,ll and La.12 However, core (carbide) particles emitting SL tubes, which were commonly observed for Gd, Y, and La, were rarely observed in the Ce case; no distinct dark contrast representing the presence of core particles was found in the center of “starfishes”, as displayed in Figure 1 for the most of the “starfishes” produced from Ce. Small CeC2 particles were of course observed in the chamber soot, but most of them were buried in amorphous carbon globules.
Figure 2. Tip of a curved single-layer tube with a shape of a “boot”. Arrows indicate the locations where a pentagon and a hexagon are introduced .
Deviations from straight tubes, induced by placing pentagons and heptagons into hexagon networks, have been observed not only for multilayered nanotubes16 but also for SL tubes.17 Iijima” showed several examples of such defected tubes: a 180”-foldedSL tube (with “candy cane” shape) and a SL tube with an indentation in the middle of the tube. In the present SL tubes produced from Ce, curved tips looking like “boots” as shown in Figure 2 were often observed. A pentagon and heptagon pair is presumably placed in the curved portion, indicated by a pair of arrows in the figure. The tips of the tubes were closed and empty like those for straight SL tubes. On the surface of the SL tubes often observed were hollow carbon spheres (carbon bubbles) with outer diameters ranging
Saito et al.
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I
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Figure 3. Hollow carbon bubbles (gigantic fullerenes) on the surface of single-layer tubes.
from 0.8 to 1.2 nm, as shown in Figure 3. Under an assumption that the carbon bubbles have fullerene structures (Le., consisting of 12 pentagons and arbitrary numbers of hexagons), a bubble indicated by an arrow A in Figure 3 corresponds to a gigantic fullerene consisting of about 170 carbon atoms. The presence of gigantic fullerenes on the surface of SL tubes was already reported for Ni-catalyzed tubes.9 The gigantic fullerenes seem to be liable to form together with SL tubes. SL tubes synthesized with Gd, Tb, Ho, Er, and Lu were short (10-30 nm) compared to those produced from Y,La, and Ce, although the diameter of the tubes was the same. Figure 4 shows a TEM picture of short SL tubes produced from Ho.Core particles were distinctly observed for Gd, Tb,Ho,Er, and Lu, as exemplified in Figure 4. The particles were dicarbides (RC2) and were covered with graphitic layers, as has been observed for SL tubes produced from Y by Zhou et al." Pr, Nd, and Dy showed the lowest activity and produced only a small amount of SL tubes. The lengths of the tubes produced from Pr and Nd (-80 nm) were the same as those produced from Y, La, and Ce, while the tube length for Dy (-20 nm) was short, like for Gd, Tb, Ho, Er, and Lu.
5
Figure 4.
TEM image of single-layer carbon tubes produced from
Ho. Group A
Group B
Discussion It is known that metallofullerenes and filled nanocapsules have been produced using coevaporation of carbon and rareearth metals. Rare-earth elements which have been revealed so far to form metallofullerenes'*and nanocapsules6are shown in Table 1, together with the present results on SL tube formation. With the exception of Sc and Tm, the elements which assisted to form SL tubes also form metallofullerenes and filled nanocapsules. The importance of vapor pressure of metal elements and carbon in the formation of nanocapsules and metallofullereneshas been pointed out;6nonvolatile metals (group A: Sc, Y,La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu) were encapsulated for both metallofullerenesand nanocapsules, while volatile metals (group B: Sm, Eu, Tm, and Yb) were not. This finding brought out an argument that the vapor pressure of a metal is an important factor for the encapsulation of the metal. Figure 5 shows vapor pressure curves19 against temperature for the rare-earth metals and carbons. From the viewpoint of formation kinetics, to form precursor clusters (transient clusters comprising carbon and metal atoms) of filled nanocapsules or metallofullerenes, metal and carbon have to condense simultaneouslyin a spatial region within an arc-reactor vessel; that is, the two regions where metal and carbon condense have to overlap with each other spatially and chronologically. The coincidence of the elements which assisted the formation of SL tubes with those which formed filled nanocapsules and
1000
1500
2Ooo
2500
3000
350(
Temperature [ K ] Figure 5. Vapor pressure curves of rare-earth metals and carbon drawn from the data of Honig.I9 Rare-earth elements are distinguished by their vapor pressures. Sm, Eu, Tm, and Yb (group B) are volatile, while Sc, Y,La, Ce, FV, Nd, Gd, Tb, Dy, Ho, Er, and Lu (group A) are nonvolatile.
metallofullerenes suggests that the volatility of metals is an important factors in manifesting catalytic activity for the formation SL tubes. Incidentally, Fe, Co, and Ni, which are already known to assist the SL tube formation, exhibit vapor pressure curves categorized into group A (nonvolatile metals). Sc did not show any catalytic activity for the formation of SL tubes, although it forms metallofullerenes as well as filled nanocapsules. The reason is not yet clear, but it may stem from the fact that Sc lacks 4f electrons and has different chemical
Single-Layered Carbon Nanotubes properties from the main series of lanthanides (for example, Sc3C4 is encapsulated in nanocapsules for Sc, while RC2, for the other rare-earths, and the oxidation state of Sc in sC@c82 is $2, while that of La in La@Cg;?is +3).20 Tm also did not show any catalytic activity for the SL tube formation, although it is reported to form a trace amount of nanocapsulesS6 Since this element is classified into group B (volatile metals), neither metallofullerenes nor filled nanocapsules are expected to form. In fact, formation of metallofullerenes filled with a Tm atom has not been reported as yet, and the nanocapsules filled with TmC2 were not observed by TEM owing to the scarcity of the nanocapsules. Therefore, it is natural to say that Tm shows very low or practically no activity for formation of SL tubes due to high volatility. Y, La, and Ce produced the most abundant and the longest SL tubes among the rare-earths. Vapor pressures of these three elements are the lowest among the rare-earths and closest to the vapor pressure of carbon (see Figure 5). This finding confirms that the vapor pressure is a crucial factor for the formation of SL tubes by using arc-evaporation of carbon and metal catalysts.
Conclusion The activity of rare-earths to assist the formation of SL tubes was investigated. Among the series of rare-earths elements investigated (Sc, Y, and Ln = La through Lu except for Pm), La, Ce, and Y were the most active; abundant colonies of SL tubes growing radially were observed in both the chamber and the cathode soot. The length and diameter of tubes were typically 80 and 2 nm, respectively. Gd, Tb, Ho, Er, and Lu showed intermediate activity next to the above three elements. Pr, Nd, and Dy showed weak activity, and SL tubes were observed only in the chamber soot for Pt and Nd. Dy produced SL tubes only at the highest pressure of He gas (1500 Torr) within the present experimental condition. The remaining rareearth elements including Sc did not produced any SL tubes. Except for Sc, correlation between the activity for SL tube formation and the vapor pressure of the elements was found; nonvolatile metals (Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu) showed the catalytic activity, while volatile metals (Sm, Eu, Tm, and Yb) did not. The three elements with the lowest
J. Phys. Chem., Vol. 99, No. 43, 1995 16079 vapor pressures, La, Ce, and Y, produced SL tubes most abundantly.
Acknowledgment. The authors express thanks to the Electron Microscope Center, National Institute for Physiological Science, for the use of the transmission electron microscope. This work was supported by the Ministry of Education, Science and Culture of Japan (Grants-in-Aid for Scientific Research on Priority Area and for New Program). References and Notes (1) Chai, Y.; Guo, T.; Jin, C.; Haufler, R. E.;Chibante, L. P. F.; Fure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. J. Phys. Chem. 1991, 95, 7564. (2) Shinohara, H.; Sato, H.; Ohkohchi, M.; Ando, Y.; Kodama, T.; Shida, T.; Kato, T.; Saito, Y. Nature 1992, 357, 52. (3) Bethune, D. S.; Johnson, R. D.; Salem, J. R.; de Vries, M. S.; Yannoni, C. S. Nature 1993, 366, 123. (4) Ruoff, R. S.; Lorents, D. C.; Chan, B.; Malhotra, R.; Subramoney, S. Science 1993, 259, 346. c5) Tomita, M.; Saito, Y.; Hayashi, T. Jpn. J . Appl. Phys. 1993, 32, L280. (6) Saito, Y.; Okuda, M.; Yoshikawa, T.; Kasuya, A,; Nishina, Y. J. Phys. Chem. 1994, 98, 6696. (7) Iijima, S.; Ichihashi, T. Nature 1993, 363, 603. (8) Bethune, D. S.; Kiang, C. H.; de Vries, M. S.; Gorman, G.; Savoy, R.; Vazquez, J.; Beyers, R. Nature 1993, 363, 605. (9) Saito, Y.; Yoshikawa, T.; Okuda, M.; Fujimoto, M.; Sumiyama, K.; Suzuki, K.; Kasuya, A.; Nishina, Y. J. Phys. Chem. Solids 1993, 54, 1849. (10) Subramoney, S.; Ruoff, R. S.; Lorents, D. C.; Malhotra, R. Nature 1993, 366, 637. (11) Zhou, D.; Seraphin, S.; Wang, S. Appl. Phys. Lett. 1994,65, 1593. (12) Saito, Y. Fullerenes: Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; Electrochemical Society: Pennington, 1994, p 1419. (13) Kiang, C.-H.; Goddard, W. A., 111: Beyers R.; Bethune, D. S. Carbon 1995, 33, 903. (14) Saito, Y.; Okuda, M.; Fujimoto, N.; Yoshikawa, T.; Tomita, M.; Hayashi, T. Jpn. J . Appl. Phys. 1994, 33, L526. (15) Saito, Y.; Okuda, M.; Tomita, M.; Hayashi, T. Chem. Phys. Leu. 1995, 236, 419. (16) Iijima, S.; Ichihashi, T.; Ando, Y. Nature 1992, 356, 776. (17) Iijima, S. MRS Bull. 1994, 19, 43. (18) Moro, L.; Ruoff, R. S.; Becker, C. H.; Lorents, D. C.; Malhotra, R. J . Phys. Chem. 1993, 97, 6801. (19) Honig, R. E. RCA Rev. 1962, 23, 567. (20) Nagase, S.; Kobayashi, K. Chem. Phys. Lett. 1993, 214, 57. JP95 15578
