An Optical Study of Hydrogen Insertion in the Anodic Oxide of Tungsten

Ellipsometry is used to study electrochromic processes in anodic oxide films on tungsten electrodes. The oxide films are grown by galvanostatic anodization in 0.1 M H 2 SO 4 , and electrochromic cycles are applied at 5V intervals beginning at 10V. The anodic oxide is transparent and has n = 2.188 in its initial (bleached) state. After a cathodic cycle designed to give a 1 / 15 hydrogen to oxygen ratio (assuming 100% efficiency) the film is optically absorbing (colored) and has an extinction coefficient k = 0.176 . For anodizing potentials below 50V the optical cycling data exhibit little hysteresis and fit a model in which the hydrogen concentration builds up uniformly in the film. We conclude, therefore, that hydrogen moves freely through tungsten oxide even at a low concentration gradient. At higher anodizing potentials the optical cycling data exhibit progressively more hysteresis and fit neither a uniform buildup nor a stratified layer model of the conversion process. We attribute this behavior to inhomogeneity introduced by the transport of hydrogen along the pores which act as breakdown sites at higher potential. There is evidence that a substantial fraction of the hydrogen inserted during a coloring cycle is still present in the film when the field is reestablished at the end of the bleaching cycle. Since this is true for both short and extended cycles, the field reestablishment cannot be a consequence of a conductor‐insulator transition at a specific bulk hydrogen concentration in the film.