Blowing molybdenum sulfide bubbles

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ResearchBlogging.orgThere has been a veritable hype around fullerenes and carbon nanotubes in recent years, so this modification of carbon has extensively researched. What’s a little less known, is that there are other, very similar structures, made of inorganic building blocks, usually transition metal chalcogenides. There is, however, a difference: In most of the inorganic fullerens (IF) there is no preferred minimal structure like C60.

The one notable exception appears to be MoS2, which forms regular octahedral of discrete sizes, which are about 4 times larger than C60. Calculations show that these octahedra are onion-like, made up of several layers and contain between 1000 and 80000 atoms. As with normal carbon fullerenes, there are several ways to obtain such structures, but their growth mechanism is largely unknown.

Well, what do you do if you don’t know the mechanism of a reaction? You try to catch the intermediates somehow. That’s what researchers from Mainz did with the formation of the MoS2-octahedra. It turns out that the shrink-wrap model of fullerene formation applies to this material as well: In the CVD chamber, large sheets form first, which later on contract into the well-known stable forms. To catch the intermediates, their formation needs to be accelerated, which was why iodine was used to increase solid-state diffusion. The result was rather interesting, because the iodine didn’t do just that.

The first step, generating precursor particles by CVD with molybdenum hexacarbonyl, iodine and hydrogen sulfide, yielded normal-looking amorphous particles with diameters between 15 to 20 nanometers. However, the annealing step at 850 degrees showed that there was more to the particles that showed in the TEM images. Precursor particles prepared at 350 and 450 degrees apparently contained a considerable amount of iodine. At temperatures higher than that the iodine gets away during CVD and the final result looks very much like the inorganic fullerenes that form when the reaction is performed without iodine.

The iodine has a rather neat effect on the annealing process. At the required temperature of about 850 degrees it changes into gas phase, while trapped in the molybdenum siulfide shell like in a balloon. The authors of the paper calculate that under annealing conditions the pressure within the particles rises to over 1000 Bar. The pressure literally blows a growing bubble. The outer shell, consisting of 5-6 layers of molybdenum sulfide sheets, constantly reorganized via surface diffusion. Out of this came thin-walled bubbles with about 120 nanometers in diameter, as seen in the images.


Molydenum sulfide bubbles created by pressure expansion. Left: Bubbles from precursors generated by CVD at 350 degrees. Right: Bubbles within bubbles, from precursors generated by CVD at 450 degrees. Source: Yella, A., Panthöfer, M., Kappl, M., & Tremel, W. (2010). Snapshots of the Formation of Inorganic MoS

Onion-Type Fullerenes: A “Shrinking Giant Bubble” Pathway
Angewandte Chemie International Edition DOI: 10.1002/anie.200902481

There are some more goodies in the paper (like nanopods, and trying to burst a bubble with an AFM cantilever tip), but I think I’ll leave it at that. It’s a rather neat article that’s fun to read, so if you find the time, have a look at it.

Yella, A., Panthöfer, M., Kappl, M., & Tremel, W. (2010). Snapshots of the Formation of Inorganic MoS

Onion-Type Fullerenes: A “Shrinking Giant Bubble” Pathway
Angewandte Chemie International Edition DOI: 10.1002/anie.200902481

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One Response to “Blowing molybdenum sulfide bubbles”

  1. Children’s Learning Toys | babyguga Says:

    […] Blowing molybdenum sulfide bubbles « […]

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