| AVS 60th International Symposium and Exhibition | |
| Nanometer-scale Science and Technology | Tuesday Sessions |
| Session NS+AS+EN+SS-TuA |
| Session: | Nanoscale Catalysis and Surface Chemistry |
| Presenter: | P. Reinke, University of Virginia |
| Authors: | J. McClimon, University of Virginia P. Reinke, University of Virginia |
| Correspondent: | Click to Email |
Tungsten carbide has garnered considerable interest for its catalytic activity and it is uniquely suited as an electrode/catalyst in microbial fuel cells. One of the bottlenecks in the use of W-carbide materials is their susceptibility to oxidation and concomitant loss in activity, albeit some surface processes benefit from the presence of surface oxide. The challenge is to establish the optimum oxide-carbide composition for a specific reaction and to stabilize this surface.
The W-carbide layers are synthesized from C60 and W, which are deposited MgO(001) by electron and beam and thermal evaporation. This affords control of film composition, and lead us to an in-depth study of W-C60 interaction, and discovery of a unique carbide nanosphere phase. The experiments are performed in UHV, and film growth and reactivity are studied with Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), which delivers conductivity maps illustrating the progression of oxidation and variation in surface electronic structure.
Our work includes study of C60 - W interaction and carbide thin film growth using deposition of C60 on W and vice versa, and co-deposition between 300-1000 K, and the study of oxidation of carbide films with a C/W ratio between 50/50 and 75/50, spanning the range from the line compound WC to carbon enriched films. The W/C 75/50 films have distinct graphite layers at the surface, while the W/C 60/40 surface presents mostly graphene patches. We will discuss the formation of carbide films, which retain some granularity defined by the C60 molecule, in terms of thermodynamic and kinetic limitations. The progression of oxidation on the bare carbide surface, and competition with graphite/graphene etching define the overall reactivity. Conductivity maps yield a nano-scale view of the oxidation reaction as a function of temperature and p(O2), which will be presented in the framework of a preliminary model.
We focus the discussion of W-C60 interaction on the two major discoveries: (1) the intercalation of W into the C60 matrix, and (2) the formation of carbide nanospheres.
The deposition of W on C60 leads to the intercalation of W, and the cessation of molecule rotation due to the formation of W-C60 complexes. The formation of exohedrally W-doped C60 is described by a competition between complex formation and W-cluster growth. The inverted experiment, where C60 is deposited on a W-surface and annealed at 600 (700)K leads to formation of carbide nanospheres, which are highly ordered with a narrow size distribution centered at 1.3 nm diameter. These nanospheres are metallic, and presumably form through the reaction between W and C60 , which acts as scaffold.