Chemistry for Everyone edited by
Products of Chemistry
George B. Kauffman California State University Fresno, CA 93740
Inorganic Fullerenes, Onions, and Tubes Andrew P. E. York† The Wolfson Catalysis Centre, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, United Kingdom;
[email protected] The discovery of the soccer-ball shaped Buckminsterfullerene in 1985 was an exciting and unexpected event, especially considering carbon is one of the two nonmetallic elements that was known even to the ancients (brimstone or sulfur being the other one). Less surprising was the award in 1996 of the Nobel Prize to the researchers who made the remarkable find, Harry Kroto, Richard Smalley, and Robert Curl. The new form of carbon prompted a huge research effort worldwide. In electron microscopy studies at the NEC Corporation in Tsukuba, Japan, Sumio Iijima noticed that some samples of fullerene-rich carbon also contained long carbon tubes with nanometer diameters. The fullerene family was further expanded to include larger carbon balls, such as C70, which has a rugby-ball shaped structure, C84, C270, and so forth, and fullerenes nested within fullerenes dubbed carbon onions. Many applications have been proposed for these new carbon materials. Buckminsterfullerene may be a good lubricant, owing to its ball structure, or a new superconductor. The nanotubes may be useful as catalyst supports, scanning probe microscopy tips, gas storage media, or electronic displays. But while some scientists were hypothesizing about uses for the carbon structures, others were dreaming of fullerenes and fullerene-like materials composed of elements other than carbon. The synthesis of these inorganic fullerenes involves a great deal of interdisciplinary research between physicists, materials scientists, engineers, and chemists from many fields. The novelty and potential importance of the fullerene and inorganic fullerene-like materials makes them ideal candidates for study in inorganic, solid state, or materials chemistry courses, and their structural and bonding characteristics are interesting subjects in introductory chemistry courses. Molybdenum Fullerenes One group working on noncarbon structures analogous to fullerenes and led by Reshef Tenne, working at the Weizmann Institute in Rehovot, Israel, suggested that molybdenum sulfide (MoS2) should be able to form closed-cage structures like those formed by carbon. This is not such an unreasonable idea when the structures of these two apparently very different materials are considered. Both graphitic carbon and molybdenum sulfide are layered structures. Graphite is made up of layers of carbon sheets, resembling chicken wire, stacked in a staggered form one layer upon the other, with strong bonds between carbon atoms within each † Current address: Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading, RG4 9NH, United Kingdom
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layer and weaker van der Waals interactions holding the layers together. Molybdenum sulfide has a similar structure, with each stacking layer consisting of sulfur–molybdenum–sulfur groups (Figure 1). The weak interlayer interactions imbue these materials with excellent lubricating properties, and molybdenum sulfide is used as a dry lubricant in high temperature scenarios where hydrocarbons would burn. The slippery nature of graphite is known to those who have rubbed some between their fingers. Its appearance highlights another similarity with molybdenum sulfide, namely, that it is also black. Indeed, molybdenum sulfide and graphite were originally thought to be the same material, and also confused with black lead until, in 1778, the Swedish chemist Karl Wilhelm Scheele experimented with a black powder and found that it contained molybdenum. In fact, it was the similarity of molybdenum sulfide to black lead that gave molybdenum its name; the word molybdae, meaning leadlike, was applied to these materials by Dioscurides, a Greek pharmacist, and used by the Roman scholar Pliny the Elder (1). Then in 1923 Linus Pauling, in his first research paper, showed that molybdenum sulfide has a layered structure, analogous to that of graphite (2). Tenne’s group has been able to extend the similarities between graphitic carbon and molybdenum sulfide to include formation of fullerene-like structures (3, 4). The researchers have also synthesized closed cages of tungsten sulfide, molybdenum selenide, and tungsten selenide, all of which have layered structures. The group has been able to produce macroscopic amounts of these new inorganic fullerene-like materials by reacting metal oxides, such as molybdenum oxide or tungsten oxide, with hydrogen sulfide. This slowly reduces the oxide, but instead of forming metal it gives metal sulfide. In this way the sulfur replaces the oxygen in the solid, and a growth front of sulfide slowly eats away at the oxide precursor until after a few hours the reaction is complete. At this point the oxide structure has been totally consumed and a layered fullerene-like sulfide material with an empty core is produced. By stopping the reaction at an earlier stage it is also possible to produce beautiful structures consisting of reduced metal oxide protected by a sulfide skin (Figure 2A and B). Seen under a suitable transmission electron microscope, which permits atomic level resolution, the inorganic fullerenelike materials are strikingly similar to the nested carbon fullerenes and carbon nanotubes. Furthermore, it is thought that the metal sulfides and selenides are able to form fullerenelike structures by incorporating non-six-membered rings into their structures, as is the case for carbon fullerenes (Figure 1). It is impossible to form Buckminsterfullerene without including 12 five-membered rings. When the curvature of a
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Figure 1. Representations of (top) the bulk structures of graphitic carbon, boron nitride, and molybdenum sulfide and (bottom) how the incorporation of pentagonal defects (carbon), rhombohedral defects (boron nitride), and trigonal defects (molybdenum sulfide) can induce curvature into these structures. Incorporation of the right number of defects in the right places can lead to the formation of closed structures such as fullerene molecules (e.g., C60), or nanotubes and nanoparticles.
small section of graphite sheet reaches a critical point the dangling bonds at the periphery can react with an opposite edge, and a stable ball or tube is formed. Similarly very small sections of molybdenum sulfide sheet are unstable, but this time the curvature is induced by three- and four-membered groups, leading to closed cages and tubes. Some of the molybdenum sulfide tubes have even been shown to exhibit a helical structure, chirality being another property they can share with carbon nanotubes (5). Unfortunately, the inorganic fullerenes are not perfectly analogous to the carbon fullerenes, and the Israel group have been careful to call their materials fullerene-like. The first obvious difference is shown when the tubes are examined. While carbon nanotubes form with one seated inside the next, like a Russian doll, the sulfiding reaction mechanism for inorganic sulfide fullerene formation leads to a structure that more closely resembles a rolled up scroll. Secondly, carbon is able to form discrete cage sizes, favoring structures such as C60 and C70, whereas the inorganic fullerene-like materials discussed so far are much less uniform and selective in their choice of size, shape, and structure. One of the main reasons for this difference is that each MoS2 layer is three-dimensional whereas graphite can be thought of as a flat two-dimensional system within each layer (see Figure 1). Therefore, it is much easier for the graphite sheets to curve, roll, or wrap themselves into spheres and tubes, than is the case for MoS2 sheets. However, this point has also now been addressed. A research group working at the National Renewable Energy Laboratory in Colorado, United States, used laser ablation of molybdenum sulfide targets to synthesize inorganic 674
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fullerenes, a method employed by carbon nanotube researchers. Amid the jumble of nanotubes and nested fullerene-like materials, similar to those seen by the Israeli group and in carbon samples, and among unexciting amorphous molybdenum sulfide, small discretely sized nano-octahedral cages of molybdenum sulfide were seen, possibly the first example of “true” inorganic fullerenes (6). Similarly ellipsoidal structures, containing pentagonal faces and with a remarkably close structural appearance to the cage carbon fullerenes (i.e., C60, C70, C84, etc.), have been synthesized based on indium. The In74 cages were discovered using single crystal X-ray crystallography and are contained within a crystal array, also made up of In–Ni species and sodium counter ions. These fullerene arrays are an example of a Zintl phase (7). Nickel Chloride Fullerenes After their initial successes with the metal sulfides, Tenne and his group wanted to investigate whether other layered materials could also form fullerene-like structures. They turned their attention to nickel chloride (NiCl2), another layered material that can also form closed cages resembling fullerenes and nanotubes (8). Under normal circumstances nickel chloride is nonmagnetic, as the dipoles in each layer cancel out. However, on a nanoscale it is possible to produce materials with an odd number of layers, thereby inducing magnetic properties. Tenne and his team produced cages with one to four layers, and tubes with 20 or more layers of nickel chloride. Those with one and three layers may have potential as tiny magnets. Since they are smaller than the mag-
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Figure 2. Transmission electron micrograph showing: (A) Formation of tungsten sulfide (which has the same structure as molybdenum sulfide) around a tungsten oxide particle. The number in curly brackets, {100}, indicates a specific crystallographic plane within the oxide material. (B) Enlargement of a portion of the micrograph showing the interface between the sulfide (right) and the oxide (left). (C) A structural representation of the micrograph. The dark lines in the image B and C correspond to W layers as these are the most electron dense. The diagonal line in B, marked {103}, corresponds to shear planes in the tungsten oxide caused by partial reduction of the oxide. In C this is represented by corner-sharing squares (unreduced octahedra) and darker edge-sharing squares (reduced octahedra).
netic domains in current magnetic storage media, it is suggested they may be able to increase the storage density of hard disks. Boron Nitride Fullerenes Another material expected to have potential as an inorganic fullerene precursor is boron nitride, which appears to be very different from graphite because of its white color. However, like graphite, boron nitride is a slippery material, because it is also made up of weakly interacting stacked layers. In boron nitride the layers consist of hexagons in which B and N alternate. The most common phase of boron nitride has a similar structure to graphite (Figure 1) and is isoelectronic with graphite. The boron and nitrogen contribute three and five electrons, respectively, to the covalent bonding while, within graphite, the carbon atoms each contribute four. Fullerenic boron nitride structures have been known for some time. For example, in 1995 boron nitride nanotubes were synthesized using the Krätschmer–Huffman arc method (9), employed for carbon nanotube synthesis. Boron nitride onions and fullerene-like materials have been observed using electron microscopy. However, the boron nitride materials differ from the carbon fullerenes in that they do not typically have closed ends, with fullerene moieties forming a cap, but are more frequently closed by metal particles. This difference arises because boron nitride is an insulator, which should give these nanotubes very different properties from the carbon fullerenes and necessitates the use of a metal electrode, usually tungsten, to enable the arc discharge process. www.JCE.DivCHED.org
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More recently, researchers at Northwestern University successfully grew boron nitride nanotubes on a hot tungsten surface under high vacuum (10). In earlier work it was possible that the nanotubes were created owing to structural defects introduced by interaction with oxygen or water vapor in the atmosphere. This work demonstrated that boron nitride is certainly able to form fullerenic materials. The product contained a mixture of fullerenes, onions, and nanotubes, and this time the nanotubes were capped by fullerenic boron nitride structures. Simulations suggest that the boron nitride nanotubes are composed of hexagonal rings, just like their carbon cousins, but the defects and curvature are most likely the result of four- or eight-membered rings and not pentagons. Five-membered rings, which would have to involve B⫺B or N⫺N bonding, are unfavorable in boron nitride, but four- and eight-membered rings allow the boron nitride structure to curve while still satisfying the electronic requirements of boron and nitrogen. Another method used for the synthesis of boron nitride nanotubes is known as chemical vapor deposition (CVD). In CVD, a reactant containing boron and nitrogen, for example borazine (B3N3H6 ), is passed over a catalyst, such as nickel or nickel boride, at very high temperature (>1000 ⬚C); the boron nitride nanotubes then grow from the catalyst (11). These nanotubes appear to be very similar to those synthesized by arc discharge. Indeed, carbon nanotubes can also be synthesized using the CVD method and a carbon containing precursor such as a hydrocarbon or ferrocene, (C5H5)2Fe. Finally, it is interesting to note that carbon nanotubes themselves can be used as a template for growing noncarbon nanotubes: this technique is known as carbon nanotube template (CNT) synthesis. For example, in the case of boron nitride nanotube synthesis, the carbon nanotubes were reacted with boron oxide and nitrogen at elevated temperature (12, 13). Niobium sulfide nanotubes have been produced by essentially the same method. Niobium chloride and carbon nanotubes were mixed together and heated to high temperature (>1000 ⬚C) in air. Then hydrogen sulfide gas was passed over the resulting solid, leading to a carbon template sheathed in niobium sulfide nanotubes (14). Tungsten sulfide nanotubes have also been produced by the CNT method. Applications Research into the possible applications of inorganic fullerenes is still in its infancy, but many uses have been proposed. For example, boron nitride nanotubes are expected to be semiconductors, while the high stability of the metal sulfides may make them candidates for scanning probe microscope tips, or they could be used as tools in the electronics industry for probing silicon chips for flaws. Perhaps the most promising studies on inorganic fullerene applications have concentrated on the tribological, or lubricating, properties of the metal sulfide fullerenes. Tenne and co-workers compared the lubrication properties of the tungsten sulfide fullerenes with standard layered sulfides. They found that under certain conditions, particularly at lower forces and loadings, the fullerene-like materials were superior (15). The fullerenic sulfides are able to act as nanoball bearings, rolling instead of sliding to reduce friction. In addition, the fullerenic sulfides are very elastic and chemi-
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cally inert because of the absence of reactive dangling bonds. This could result in longer-lived lubricants that would need changing less often. Therefore, these new materials could be the basis for future environmentally friendly oils, used in engines and transmission systems. Unfortunately, initial testing at higher forces showed that the metal sulfide ball-bearings underwent plastic deformation, and were even broken or popped like a balloon. Poor high-load durability has been addressed by incorporating a hard material at the center of the metal sulfide fullerenes, namely tungsten carbide (16), which is one of the hardest materials known, approaching diamond. The surface of tungsten carbide is usually covered with a skin of oxide, and this can react with hydrogen sulfide to form a fullerenelike structure, as was the case for molybdenum oxide. The fullerene structure surrounds tungsten carbide (see Figure 3), leading to very robust and rigid nano-ball bearings. These may be useful as lubricants under higher loadings, avoiding the particle deformation or “popping” observed with the noncarbide fullerene-like materials.
Figure 3. Transmission electron micrograph showing: (A) Encapsulation of one of the hardest known materials, tungsten carbide, by one of the best known lubricants, tungsten sulfide. (B) Microstructure at the interface (enlargement obtained from the indicated region in A) revealing the dense carbide structure in the middle left (WC) and the layered WS2 structure (2H) at the top. The latter “grows into” the former.
Summary Inorganic fullerenes are new and exciting materials that should be the subject of study at undergraduate and postgraduate levels. They make an ideal subject for literature studies, both the fullerenes themselves and the inorganic cage materials. Furthermore, if students construct the cages and tubes using molecular modeling kits or computer simulations, this is a field of chemistry that can excite interest in the study of bonding and molecular structure. Acknowledgment I would like to thank Jeremy Sloan of University of Oxford, United Kingdom, for helpful discussions and supplying the figures. Literature Cited 1. Pliny the Elder, Historia Naturalis; 77 B.C.E. 2. Dickinson, R. G.; Pauling, L. J. Am. Chem. Soc. 1923, 45, 1466–1471. 3. Tenne, R.; Margulis, L.; Genut, M.; Hodes, G. Nature 1992, 360, 444. 4. Tenne, R.; Homyonfer, M.; Feldman, Y. Chem. Mater. 1998, 10, 3225–3238.
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5. Margulis, L.; Dluzewski, P.; Feldman, Y.; Tenne, R. J. Microscopy 1996, 181, 68–71. 6. Parilla, P. A.; Dillon, A. C.; Jones, K. M.; Riker, G.; Schulz, D. L.; Ginley, D. S.; Heben, M. J. Nature 1999, 397, 114. 7. Sevov, S. C.; Corbett, J. D. Science 1993, 262, 880–883. 8. Rosenfeld Hacohen, Y.; Grunbaum, E.; Tenne, R.; Sloan, J.; Hutchison, J. L. Nature 1998, 395, 336–337. 9. Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966– 967. 10. Bengu, E.; Marks, L. D. Phys. Rev. Lett. 2001, 86, 2385–2387. 11. Lourie, O. R.; Jones, C. R.; Bartlett, B. M.; Gibbons, P. C.; Ruoff, R. S.; Buhro, W. E. Chem. Mater. 2000, 12, 1808–1810. 12. Han, W.; Bando, Y.; Kurashima, K.; Sato, T. Appl. Phys. Lett. 1998, 73, 3085–3087. 13. Chen, Y.; Conway, M.; Williams, J. S.; Zou, J. J. Mater. Res. 2002, 17, 1896–1899. 14. Zhu, Y.-Q.; Hsu, W.-K.; Kroto, H. W.; Walton, D. R. M. Chem. Commun. 2001, 2184–2185. 15. Rapoport, L.; Feldman, Y.; Homyonfer, M.; Cohen, H.; Sloan, J.; Hutchison, J. L.; Tenne, R. Wear 1999, 229, 975–982. 16. Rothschild, A.; Sloan, J.; York, A. P. E.; Green, M. L. H.; Hutchison, J. L.; Tenne, R. Chem. Commun. 1999, 363–364.
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