Thermal Stability of Tungsten Oxide Clusters

Thermal reduction of tungsten oxide nanoclusters supported on thin silicon and aluminum oxide films and ranging in size from 2 to 8 nm was studied by in situ X-ray photoelectron spectroscopy. We observed that with a decrease in size oxide clusters demonstrate enhanced resistance to the oxide reduction when heated in vacuum. The effect is presumed to be related to the point defects that serve as nucleation sites in the oxide reduction process. It is suggested that due to the increased Laplace pressure small clusters contain lower defect density thus making them less susceptible to the thermal reduction. Preamorphization of the oxide particles by low-energy ion bombardment demonstrated a lowering of oxide decomposition temperature thus supporting the inference on the defect stimulated oxide reduction. These results provide a novel model system to understand the reduction behavior of bulk tungsten oxides and might be of crucial relevance for applications where the use of clusters is of special importance.