Central European Journal of Chemistry Gas-phase synthesis of inorganic fullerene-like structures and inorganic nanotubes * E-mail: reshef.tenne@weizmann.ac.il Received 14 March 2008; Accepted 17 June 2008 Abstract: Following the discovery of fullerenes (C 60 ) and carbon nanotubes, it was shown that nanoparticles of inorganic layered compounds, like WS 2 and MoS 2 , are unstable in the planar form and they form closed cage structures with polyhedral or nanotubular shapes. Although initially the method of synthesis for the formation of such closed caged structures and nanotubes involved starting from the respective oxides, it is now well established that the gas-phase synthetic route (using metal chlorides, carbonyls etc) provides an alternative which is suitable for the synthesis of very many closed caged structures and nanotubes hitherto unknown. Various issues with this method of synthesis, including its fundamentals, mechanism, and the properties of the inorganic fullerene-like structures produced are reviewed, together with some possible applications. © Versita Warsaw and Springer-Verlag Berlin Heidelberg. Keywords: Inorganic fullerenes (IF) • Inorganic nanotubes (INT) • Gas-phase synthesis • Electron microscopy • Doping • Tribology Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel F. L. Deepak, Reshef Tenne * Review Article 1. Introduction The formation of closed-cage structures, namely the fullerenes (C 60 ) and nanotubes, which was known previously for carbon, is no longer unique and exclusive to this element alone [1,2]. Such inorganic counterparts as the inorganic fullerene-like nanoparticles (IF) and inorganic nanotubes (INTs), discovered in 1992, have elicited considerable interest ever since in this emerging feld [3-5]. The reason for the formation of such closed- cage structures akin to carbon is intriguing. In the case of the layered compounds of MS 2 (M = Mo, W) (Fig. 1b) which were the frst examples of the closed-cage inorganic fullerene-like nanoparticles and inorganic nanotubes, the molecular sheet is made of a layer of M atoms sandwiched between two outer sulfur layers. Each M atom binds to six sulfur atoms to form a trigonal biprism. In analogy to graphite (Fig. 1a), weak van der Waals forces are responsible for the stacking of the S-M-S layers together. Like graphite, such compounds are highly anisotropic with respect to many of their physical and chemical properties. The basal (Van der Waals) surfaces of the crystal, which are perpendicular to the c axis, consist of sulfur atoms that form bonds to three underlying W/Mo atoms; these sulfur atoms are chemically quite inert. However, rim W/Mo and S atoms, which are abundant in the nano-regime, are only four- and twofold bonded, respectively, making these nanostructures unstable in the planar form. Therefore, by folding the molecular sheet and stitching the rim atoms together, seamless and stable nano-tubular (one dimensional) and spherical (zero-dimensional) structures with all W/Mo and S atoms being six- and threefold-bonded, respectively, are obtained [6,7]. Initially only the transition metal chalcogenides of WS 2 and MoS 2 were known in the form of closed cage structures and nanotubes. However, over the last decade this family has been expanded considerably and it now encompasses a large number of other compounds like oxides, nitrides, chlorides, sulphides and selenides. For example they now include metal chalcogenides: IF- W(Mo)Se 2 nanoparticles and nanotubes, IF-In 2 S 3 and IF-VS 2 nanoparticles, IF-NbS 2 nanoparticles, NbS 2 and TaS 2 nanotubes, IF-TiS 2 nanoparticles and nanotubes, ZrS 2 , and HfS 2 nanotubes, IF-ReS 2 nanoparticles and nanotubes, Bi 2 S 3 nanotubes; for transition-metal halides: IF-NiCl 2 nanoparticles and nanotubes, IF-CdCl 2 Cent. Eur. J. Chem. • 6(3) • 2008 • 373–389 DOI: 10.2478/s11532-008-0043-2 373