Effect of the tungsten oxidation states in the thermal coloration and bleaching of amorphous WO3 films

The structural, optical and electronic changes caused by thermal annealing in vacuum and in oxygen atmosphere on evaporated WO3 films, were investigated by visible and infrared transmission spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The luminous transmittance of the WO3 films was related to their state of oxidation at each annealing temperature, both during thermal coloration and bleaching. It was found that annealing in vacuum progressively reduces the oxide to the lower oxidation states W5+, W4+ and changes the film color. Both states contribute to the coloration of the film. However, the presence of the W4+ state was found to play a dominant role in deep blue coloration. Thus, a two step procedure model with polaron transitions from W5+ to W4+ and also from W6+ to W5+ would better describe the phenomenon. (C) 2001 Elsevier Science B.V, All rights reserved.