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 investigat ed by visible and infrared transmission spectroscopy (FTIR), scanning elect ron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The lumino us transmittance of the WO3 films was related to their state of oxidation a t each annealing temperature, both during thermal coloration and bleaching. It was found that annealing in vacuum progressively reduces the oxide to t he lower oxidation states W5+, W4+ and changes the film color. Both states contribute to the coloration of the film. However, the presence of the W4state was found to play a dominant role in deep blue coloration. Thus, a tw o step procedure model with polaron transitions from W5+ to W4+ and also fr om W6+ to W5+ would better describe the phenomenon. (C) 2001 Elsevier Scien ce B.V, All rights reserved.