In situ infrared spectroscopic study of the electrochromic reactions of tungsten trioxide films

Thin W03 films are transparent in the oxidized state and colored in the reduced state. These changes in optical properties are associated with compositional variations of the material. Changes in vibrational intensities of W=O, W—O, and W—O—H bonds in the electrochromic W03 film were detected by an in situ FTIR technique at various stages of reduction (coloration). The absorbance due to O-H stretching and bending vibrations was found to increase during the electrochemical reduction of the film, indicating the incorporation of water into the film along with the formation of HxW03 bronze during coloration. The absorbance due to W=0 vibration decreased while that due to W—O vibration increased during reduction. These observations suggest that during the coloration process W=0 bonds break and new W—O bonds form in the film, and thus, provide direct evidence for the electrochromic reaction, 02W=O + xH+ + xe <-» 02W—O—Hr.