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SLIMMING DOWN INORGANIC TUBES
Synthesis of the narrowest single-walled MoS2 nanotubes seems to require C60
Buckminsterfullerene has many reasons for being in the scientific spotlight, but now comes a new one: It has been found to be essential for the growth of the narrowest molybdenum disulfide nanotubes ever reported.
Earlier reports have described MoS2 nanotubes with diameters as small as 15 nm. But the tubes reported last week by Maja Remkar and Ales Mrzel at Jozef Stefan Institute in Ljubljana, Slovenia, and coworkers are considerably narrower--they are all just under 1 nm across [Science, 292, 479 (2001)]. Moreover, they are single-walled and produced in yields of up to 15%, which is relatively high for nanotubes.
A model of an MoS2 nanotube, seen from the side and in cross section (Mo = red; S = yellow).
According to physicist Dragan Mihailovic, a senior scientist on the team, the work demonstrates that subnanometer-diameter nanotubes made of a metal disulfide can be synthesized efficiently. And this suggests that large-scale production of uniform inorganic nanotubes may be feasible--an important consideration if the tubes are found to be commercially useful.
Other forms of MoS2 are already used in a number of applications, including catalysis, high-energy-density lithium batteries, and solid lubricants. Mihailovic suggests that the properties of these new MoS2 nanotubes could prove to be a boon to some of these applications.
The nanotubes are grown from MoS2 powder in an evacuated silica ampoule that is placed in a high-temperature gradient. A small amount of C60 is added as a growth promoter. Iodine is used as a transport agent to shuttle atoms of molybdenum and sulfur from the MoS2 powder at the higher temperature end of the ampoule to the nanotubes growing at the lower temperature end. The detailed growth mechanism, including C60's exact role, isn't clear at present. "What is clear is that the synthesis doesn't work without the C60," Mihailovic tells C&EN.
The nanotubes produced via this process are up to hundreds of micrometers long. They assume a number of different forms, ranging from twisted chiral bundles containing hundreds of thousands of tubes to regularly shaped "furry" forms, according to the paper.The bundles, which contain iodine between the individual nanotubes, can be readily disassembled into individual nanotubes.
Reshef Tenne, a professor in the department of materials and interfaces at Weizmann Institute of Science, Rehovot, Israel, believes the work is remarkable for a number of reasons: This is the first report describing single-walled nanotubes of an inorganic compound that have been prepared in large amounts and with precise control of the tube diameter, he points out. "The fact that MoS2 can fold into such a small-diameter (0.96-nm) nanotube is also remarkable," he says. Such nanotubes possibly could be used to separate gas mixtures such as H2/O2--the hydrogen would pass through, leaving the larger oxygen molecules behind.
It should also be possible to tune the optical properties of these nanotubes all the way from the ultraviolet-visible to the infrared using the tube's diameter and chirality, Tenne suggests.
"I think this is one of the most exciting papers in the field of nanomaterials in recent years," he remarks.
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Copyright © 2001 American Chemical Society