Thick oxide growth on gold in base

Abstract

The formation of thick oxide films (ca. 100 monolayer equivalents at 2.3 V) on a gold anode was observed under steady-state polarization conditions in base. As reported by other authors for acid, the layer produced was of duplex character with a compact, largely anhydrous inner film at the metal surface, and a much thicker, porous, highly hydrated outer film at the oxide-solution interface. These deposits were readily distinguished, and quantified coulometrically, due to differences in reduction potential under cathodic sweep conditions. The onset of thick film growth occurred in a potential region where linear Tafel behaviour is observed for oxygen gas evolution on gold, and changes in oxygen coordination of cations in the outer region of the compact layer under these conditions is assumed to enhance the conversion of material in the outer region of the latter to the more hydrated form. As compared with acid, oxide growth was much slower (and reached a limiting value) in base, the outer film was reduced less readily and inhibition of oxygen gas evolution in the thick film region (at ca. 2.35 V) was observed. The results were accounted for by assuming increasing hydroxide ion coordination by cations in the hydrous layer with increasing pH — the more highly coordinated species being thermodynamically more stable but kinetically less capable of rearranging to form a crystalline product. The relationship between the hydrous film on gold and those produced on a wide range of other transition metals is briefly outlined.