Electrochemistry of Hydrous Oxide Films

Abstract

For the purpose of this review it is convenient to classify oxides into two groups: (1) compact, anhydrous oxides such as rutile, perovskite, spinel, and ilmenite in which oxygen is present only as a bridging species between two metal cations and ideal crystals constitute tightly packed giant molecules; and (2) dispersed, hydrous oxides where oxygen is present not just as a bridging species between metal ions, but also as O^-, OH, and OH₂ species, i.e., in coordinated terminal group form. In many cases the latter materials when in contact with aqueous media contain considerable quantities of loosely bound and trapped water, plus, occasionally, electrolyte species. Indeed, with highly dispersed material (dispersion here refers to the molecular level, i.e., microdispersion, not to the finely divided state where the oxide microparticles may still be compact in character, i.e., macrodispersion), the boundary between the solid and aqueous phases may be somewhat nebulous as the two phases virtually intermingle. While compact oxides are usually prepared by thermal techniques, e.g., direct combination of the elements, decomposition of an unstable salt, or dehydration of a hydrous oxide, the dispersed oxides are almost invariably prepared in an aqueous environment using, for instance, base precipitation or electrochemical techniques.