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

Abstract

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 C 60 , 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 C 60 and W prior to thermallyinduced destruction of the C 60 cage and formation of the carbides. These experiments showed a surprising lack of interaction between small W clusters and adjacent C 60 molecules. This result contrasts with literature of analogous experiments showing significant charge transfer from W to C 60 and strong interaction as a result. For W deposited on top of C 60 layers, it is found that isolated W atoms diffuse easily into the interstices of the C 60 matrix and that for high coverages, interactions between the W and C 60 induce a significant reduction in the C 60 bandgap. These interactions can also stabilize very small islands of C 60 on graphite that otherwise require hundreds of molecules before a stationary C 60 island can form.Intermediate annealing at 400-500C of C 60 on epitaxial W/MgO(100) produces novel nanostructures with a very similar size and shape to C 60 but with metallic conductivity, a unique atomic-scale stripe pattern on their surface, occasional mobility under the influence of the STM iv tip, and show signs of evolving towards WO 3 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 WC 1-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 WO 3 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, O 2 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 cluste...

Electronic Structure and Band Gap of Fullerenes on Tungsten Surfaces: Transition from a Semiconductor to a Metal Triggered by Annealing

“…W thin films were deposited on a MgO(100) substrate followed by deposition of the C 60 layer. The as-received MgO (CrysTec) was (1) etched for 30 s in 14.6 M H 3 PO 4 and rinsed with deionized water and acetone and (2) annealed 10 h in air at 1273 K in a box furnace, and (3) the calcium (Ca) spires from impurity segregation , were removed in a second etching step followed by a 5 h annealing in air . All MgO surfaces were checked with atomic force microscopy prior to use.…”