Electrochromism in Anodically Formed Tungsten Oxide Films

By anodization of tungsten in acid solutions, oxide films were obtained Whose compositio,n depended on the working temperature. In particular, the composition.of the films was WO ~ at 25 ~ and 50 ~ but WO3 9 H20 at 70~ Studies of electrochromism in these films seem to show that the electro colora-tion occurs with different mechanisms according to the oxide chemical com-position. For WO3 films the coloration is due to a simultaneous injection of electrons and H + ions with formation of hydrogen tungsten bronzes H=WO3. For W ~ 9 H20 films the coloration could occur as a result of the injection of electrons and oxygen ion vacancies with formation of blue oxide hydrate WO~- = 9 H~O. The phenomenon of electrochromism in WO ~ films is under active investigation because of its potential use for passive alphanumeric displays. Electrochromism in WO3 films was first reported by Deb (1). A reversible blue coloration was generated at the cathodic area on applying a d-c electric field across an evaporated WO3 film. Deb has proposed (2) that the coloration occurred as a result of the formation of F or F + color centers in oxygen ion vacancies. Chang et al. (3) have attributed the blue color to the formation of reduced oxides of general formula WOs-x according to the electrochemical re-action mWO8-]- 2nil + ~ 2ne--> W,nO3m-n + nH20 [1] (blue) Other authors (4-8) have suggested the formation of hydrogen tungsten bronzes by double injection of protons and electrons WO3 +nH + + xe---> HxWO ~ (O ~ x " ~ 0.6) [2] (blue) In this hypothesis the phenomenon of coloration was explained by Faughnan et al. (8) as an intervalence transfer absorptio