Research Activities

Inorganic Fullerene-like Nanostructures and Inorganic Nanotubes

In 1992 (Ref. No.91) we showed that nanoparticles of the layered compound WS₂ are unstable in the platelet form and they spontaneously form closed cage structures akin to carbon fullerenes and carbon nanotubes. This instability was attributed to the highly reactive dangling bonds of both sulfur and tungsten atoms, which appear at the periphery of the nanoparticles. We therefore proposed that this instability is generic to layered compounds and consequently named this nanostructures, inorganic fullerene-like (IF) structures and inorganic nanotubes (INT). Studying the growth mechanism of IF and INT of WS₂ (MoS₂) in great detail, we were able to scale-up their synthesis using the fluidized bed reactor.

This work was followed by a series of works in which fullerene-like nanoparticles and nanotubes of various layered compounds, like MoS₂, and the respective selenides have been synthesized by our group. More recently, various IF and INT from metal dihalide compounds, like NiCl₂, with layered structure were synthesized.

Electron micrograph taken by the late Dr. Lev Margulis

Transmission electron microscopy of a WS2 nanotube Image courtesy of J.L. Hutchison, Oxford University

Inorganic nanotubes and fullerene-like structures of various materials have been synthasized, such as NiCl₂ nanotubes, and fullerene-like Cs₂O.

TEM of NiCl2 nanotube

TEM image of fullerene-like Cs2O nanoparticle

Production of large amounts of multiwall WS₂ nanotubes


Courtesy of Dr. A. Zak and Dr. M. Genut, "NanoMaterials", Ltd.

Technological applications for some of these IF materials, have been pursued. In collaboration with the group of Lev Rapoport, from the Holon Institute of Technology, we have demonstrated that IF-WS₂ and IF-MoS₂ can serve as superior solid lubricants. In particular, self-lubrication has been demonstrated for various matrixes impregnated with these nanoparticles, with numerous potential applications. At left is "NanoLub", produced by NanoMaterials, an oil additive based on IF-WS₂ nanoparticles. Other proposed applications include ultra high strength nanocomposites; tips for scanning probe microscopy and more. Other groups have studied enticing potential applications of IF nanoparticles and INT in (electro)chemical hydrogen storage; rechargeable lithium batteries; catalysis; photocatalysis, etc.

One of the most challenging questions regards the structure and properties of the smallest IF nanoparticles. As early as 1993 (Ref. No. 97), we have proposed that rectangles and triangles may be formed in the apex. This idea was verified by Heben et.al., (Nature, 397, 114 (1999)), who used laser ablation of MoS₂ targets to obtain MoS₂ nanooctahedra, having six rectangles in the corners. Jointly with Prof. G. Seifert (TU Dresden) (Ref. No. 218) we have studied the stability of the MoS₂ nanooctahedra using a combination of ab-initio and experimental techniques. 3-5 wall octahedra were found to be indeed the smallest (3-8 nm; 10³-10⁵ atoms) stable hollow-closed nanoparticles and can therefore be considered the “true” MoS₂ fullerenes.

TEM imaging of a 3-layered nanooctahedron in various tilting angles (left) The corresponding images of the atomic model for an (MoS2)784@(MoS2)1296@(MoS2)1936 octahedral fullerene (right)

DFTB optimized structure of a (MoS2)576 fullerene degraded to Mo576S1140 (left) and a Mo576S1140 nanooctahedron after an MD experiment at 300 K (right) Courtesy of Prof. G. Seifert

Not less important are the questions of how unique are the properties of these nanoparticles in comparison to the macroscopic materials. For example, using density functional theory, Seifert et.al., (Phys. Rev. Lett. 85, 146, (2000)), have shown that the bandgap of MoS₂ and WS₂ nanotubes is reduced with shrinking nanotube diameter. This result is counterintuitive in comparison to semiconductor quantum dots, in which the bandgap increases with shrinking diameter. Indeed using optical measurements (Ref. 122), and more recently scanning probe microscopy (Ref. 169) of IF-WS₂, we have reached similar conclusions. A series of careful mechanical measurements of individual WS₂ nanotubes were carried out and compared with ab-initio calculations (see for example Ref. 200). The high strength of the nanotubes (16 GPa) and their elongation (12%) are compatible with their structural perfectness. This series of studies are indicative of their numerous applications, e.g. in ultra high strength nanocomposites.

View these relevant movies:

• Compression test of an individual WS₂ nanotube

• Tensile test of an individual nanotube

• DFTB-MD calculation of the tensile test of a MoS₂ nanotube (Courtesy of Prof. G. Seifert, TU Dresden)

In 2002 “Yeda” the R&D arm of the Weizmann Institute has decided to license the extensive knowhow and intellectual property which was accumulated in our group over the years, to ApNano Materials. After extensive R&D work, the company has launched during 2008 its first commercial products, i.e. solid lubricant additives for the automotive and machine industry. It is likely that products based on self-lubricating coatings, which are in great demand by the medical, aerospace, electronic and many other industries, will be commercialized within the next few years. Ultrastrong nanocomposites impregnated with WS₂ nanotubes are likely to be commercialized in the foreseeable future as well.

Succsseful scale-up efforts in ApNano Materials (NanoMaterials, Ltd.) have recently led to the synthesis of macroscopic amounts of WS₂ nanotubes.

SEM image of WS₂ nanotuves prepared in "NanoMaterials"

Numerous synthetic challenges remain to be solved, whereupon interesting questions regarding the chemistry and physical behavior of these nanostructures can be addressed and new applications can be found, particularly in the field of nanotechnology.