Materials Research Lecture

In addition to carbon nanotubes, non-carbon nanostructures have attracted much attention recently. In particular, inorganic nanotubes and fullerenes have become a key topic in nanoscale research. Chalcogenide nanostructures are of interest for a variety of applications ranging from nanotribology, nanoelectronics or battery materials to the active use in heterogeneous catalysis.

Similar as their carbon congeners chalcogenide nanoparticles are not equilibrium but high-temperature and low-pressure phases in that they are obtained by quenching hot gas phase species. Various approaches to chalcogenide nanoparticles have been established so far, among them oxide to sulfide conversion, MOCVD reactions, or from the elements using metal droplets. Except for the oxide to sulfide conversion, which has been scrutinized, the growth mechanism of such nanostructures is still subject to discussion.

We have used a novel approach to unravel the formation mechanism of MoS2 nanotubes and fullerenes by taking TEM snapshots of reaction intermediates that were obtained by annealing amorphous Mo-S nanoparticle precursors captured from MOCVD reactions. This approach allows a trapping of reaction intermediates by thermal quenching and a control of the reaction process and monitoring the particle growth by the choice of the experimental conditions (precursor etc.). Thermal decomposition of the precursors leads in the first step to the formation of giant fullerenes with diameters > 150 nm. In the next step fullerene particles segregate within these giant fullerenes, and a shrink-down process leads to the formation of tubular MoS2 structures containing fullerene-particles in a peapod-type fashion. In the final steps nested fullerene particles are formed. The resulting chalcogenide nanostructures can reversibly be functionalized to make them dispersible in solvents. Graphene-type chalcogenides can also be obtained via sulfidization of oxide precursors. The resulting graphene-type WxNb1-xS2 2D-nanosheets have lateral diameters of about 30 nm and exhibit good electrochemical performance for lithium-ion batteries and lubrication.