Synthesis and Oxidation of non-Stoichiometric Tungsten Carbide Studied by Scanning Tunneling Microscopy/Spectroscopy

Tungsten Carbide (WC) is a promising catalyst material for applications where cost is paramount, such as microbial fuel cells. Unfortunately, oxidation depresses performance which is a critical problem with respect to long term performance. In this work, non-stoichiometric carbon-rich WC is investigated to infer whether an excess of carbon near the surface can reduce or be utilized to repair oxidation of the active catalytic surface. The carbides are synthesized via a physical vapor deposition of metallic tungsten and C60, which allows fine control over the composition of the resulting film. The films are studied predominantly via Scanning Tunneling Microscopy/Spectroscopy (STM/STS).
Prior to synthesizing the carbide, a number of RT depositions are performed onto graphite substrates to understand the interactions between the C60 and W prior to thermally-induced destruction of the C60 cage and formation of the carbides. These experiments showed a surprising lack of interaction between small W clusters and adjacent C60 molecules. This result contrasts with literature of analogous experiments showing significant charge transfer from W to C60 and strong interaction as a result. For W deposited on top of C60 layers, it is found that isolated W atoms diffuse easily into the interstices of the C60 matrix and that for high coverages, interactions between the W and C60 induce a significant reduction in the C60 bandgap. These interactions can also stabilize very small islands of C60 on graphite that otherwise require hundreds of molecules before a stationary C60 island can form.
Intermediate annealing at 400-500C of C60 on epitaxial W/MgO(100) produces novel nanostructures with a very similar size and shape to C60 but with metallic conductivity, a unique atomic-scale stripe pattern on their surface, occasional mobility under the influence of the STM tip, and show signs of evolving towards WO3 under oxidation. These structures increase in diameter and slowly dissolve into the W surface under increasing annealing temperatures.
Carbide thin films are synthesized by codeposition onto MgO substrates held above 600˚C. These thin films are determined to be predominantly metastable, cubic WC1-x phase. At a C/W ratio of 60/40 and above, carbon is observed to surface segregate and form graphite on the surface of the film. At a ratio of 60/40 the surface is heavily populated with graphene, rather than graphite.
Oxidation of the films at elevated temperatures in UHV shows that morphological changes, even at atomic resolution, were absent. STS shows that signs of oxidation overspread the surface immediately but that WO3 nucleates well-defined islands which grow in size with harsher oxidation conditions. For the graphite covered surfaces, graphite appears to protect the underlying carbide from oxidation, and between 300 and 400˚C, O2 begins to etch the graphite. The underlying carbide appears to oxidize similarly to films without a graphitized surface. Overall, the WC films oxidize much less rapidly than W clusters on graphite. Attempts to regenerate the oxidized carbide surface via annealing met with mixed results. Carbon diffuses easily through the oxide film and graphitizes the surface. There are signs that the WO3 layer might decrease in thickness.