Paper EN-ThP1
Tungsten Carbide: Synthesis and Reactivity with Oxygen on the Nanoscale

Thursday, November 1, 2012, 6:00 pm, Room Central Hall

Transition metal carbides have shown considerable promise as catalysts, which is often attributed the similarities in electronic structure between noble metals and transition metal carbides. The addition of tungsten carbide to microbial fuel cells, and their use as an anode material has proven to be quite advantageous. However, the bottleneck in the use of tungsten carbide lies in the loss of function due to partial or complete oxidation of the surface. We study the reactivity of tungsten, and carbon-rich tungsten carbide clusters with oxygen to investigate the progression of oxidation. We suggest that carbon-rich carbide surfaces are less susceptible to oxidation and can be regenerated by annealing.

The tungsten and tungsten carbide clusters are made by the co-deposition of W and C₆₀ which enables us to fine-tune composition in a wide range. The morphology and electronic structure of the surface is probed with STM in-situ, which is supplemented by chemical analysis with XPS, and structural analysis with TEM. The progression of oxidation is observed with bandgap maps with spatial resolution in the nm-range. The experiments were performed on graphite substrates, where the metal clusters remain highly mobile and do not react in the temperature regime of our work (< 650 K).

We begin with a discussion of the synthesis of clusters with compositions ranging from pure W-clusters, to carbon-rich surfaces. The cluster sizes are between 5-30 nm, and the carbon is introduced either by co-deposition of W and C₆₀ or by the deposition of fullerenes on pure W-clusters (and vice-versa). The C₆₀ aggregates to large islands and a reaction with the W-clusters is only initiated by annealing, and leads to carbon-terminated metallic clusters. The motion of fullerene molecules is reflected in the sawtooth signature of tip induced displacement, which is also a probe for the surface chemistry. The co-deposition of W and C₆₀ (and W deposition on C₆₀) leads to the formation of spherical structures whose granularity reflects the dimensions of the C₆₀ cage. We assume that the C₆₀ cage reacts with surface W but the low temperature prevents collapse. The oxidation of tungsten clusters has been observed as a function of oxygen partial pressure, and shows the progression of the reaction as a function of cluster size and surface morphology. We will present a complete set of bandgap maps, which are recorded during the oxidation at room temperature and elevated temperature for the different carbide structures. A model describing the oxidation as a function of carbide structure and composition will be presented.