An optical study of hydrogen insertion in the anodic oxide of molybdenum

• Published in: Journal of the Electrochemical Society 1987-07, Vol.134 (7), p.1734-1740
• Main Authors: DE SMET, D. J, ORD, J. L
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Physical properties of thin films, nonelectronic; Physics; Surfaces and interfaces; thin films and whiskers (structure and nonelectronic properties)
• ISSN: 0013-4651; 1945-7111
• DOI: 10.1149/1.2100747

Ellipsometry is used to study the coloring and bleaching processes which occur when films of molybdenum oxide are electrochemically reduced and then subsequently reoxidized. The oxide films are grown by anodic oxidation in acetic acid electrolyte, and the cycles used to study the electrochromic conversion processes are applied at thicknesses ranging from 40-180 nm. The optical data show that the cathodic cycle involves two stages. In the first stage, the optical absorption increases and the oxide film is reduced electrochemically to a film with a maximum hydrogen-to-oxygen ratio of 0.88. In the second stage, the optical absorption decreases, and there is no further change in the oxidation state of the film. The electrochemical reduction of the oxide film is found to be a rate sensitive process. At low current density, the end point of stage one is reached at a H-to-O ratio of 0.50, and the film is more highly absorbing optically than films reduced at higher current density. The optical data show that reduction of the oxide is a heterogeneous process in which a phase boundary sweeps inward across the film from the electrolyte interface. Reoxidation returns oxide film to its initial state, but the process shows a high degree of electrochemical irreversibility, and its features are sensitive to the rate and duration of the preceding cathodic cycle. The results are interpreted in terms of a model in which the reduced film has two different sites for H insertion, and a mechanism in which H moves freely through the reduced film, but requires a field on the order of the anodizing field to move through the oxide film. 19 ref.--AA