Sequential oxidation and reduction of tungsten/tungsten oxide

The reduction of oxide films on the surface of tungsten was studied. The oxide films were created by briefly exposing smooth W foils to oxygen plasma generated using a microwave discharge at 500 W and an oxygen pressure of 40 Pa. Oxidation resulted in the formation of a WO3 film with a thickness of approximately 1700 nm. The oxidised samples were then exposed to hydrogen plasma that was generated using the same conditions as the oxygen plasma and simultaneously heated by concentrated solar radiation. Several peaks were observed in the plot of the recorded sample temperature versus hydrogen plasma treatment time, indicating surface reactions. The treated samples were characterised using scanning electron microscopy, Auger electron spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction. The first step in the reduction process was the transformation of WO3 to W18O49 which was followed by the rapid and complete reduction of the entire oxide film. The surface morphology changed upon reduction of the oxide film, and nanostructured tungsten remained on the surface, particularly after three sequential oxidation/reduction cycles. The reduction by hydrogen plasma was accomplished in a few seconds at approximately 1100 K, indicating the interaction of reactive hydrogen species, such as hydrogen atoms, with the W18O49 compound with a rich morphology. Treatment in equilibrium hydrogen at the same pressure without igniting plasma required a considerably higher reduction temperature of approximately 1400 K and an order of magnitude longer reduction time.