Effect of the tungsten and molybdenum oxidation states on the thermal coloration of amorphous WO3 and MoO3 films

Transmission and X-ray photoelectron spectroscopy (XPS) was used to investigate the optical and electronic changes caused by thermal annealing in UHV of amorphous WO3 and MoO3 thin films suitable for electrochromic applications. The core level XPS measurements showed that annealing in UHV progressively reduces the oxides to the lower oxidation states and causes changes in their color. Their luminous transmittance was related to their state of oxidation at each annealing temperature during thermal coloration. Only the O2p peak characterizes the valence band spectra of the oxides before coloration. After annealing, the 5d and 6s states of tungsten and 4d, 5s of molybdenum are partially filled by electrons released due to the reduction of the oxides. The MoO3 film exhibits deep blue coloration below 400°C, associated to a continuous distribution between Mo6+ and Mo5+ states. For the WO3 film the W4+ state, which appears above 400°C, plays the dominant role in deep coloration. At lower temperatures distinct W5+ states are formed with a limited contribution to coloration.