Elementary steps of electrochemical oxidation of single-crystal planes of Au Part II. A chemical and structural basis of oxidation of the (111) plane

A detailed experimental and mechanistic analysis of the elementary surface-chemical steps involved in the beginning stages of electrooxidation of Au is given with respect to the processes that occur on the (111) plane. At Au, specific adsorption of anions, even oxyanions such as ClO−4 and SO2−4 or HSO−4, plays a major role in the initial stages of oxidation of the metal. In the case of the (111) plane investigated here, the symmetry of the surface lattice has the strongest effect on the specific adsorption of the above anions compared with behaviour at other planes. This is connected with the matching symmetry of the tetrahedral ions with the trigonal symmetry of an unreconstructed (111) plane. The results indicate that the hydrated anions, which are found to be partially or fully discharged on the metal surface, form overlay-lattice arrays. This network of hydrated anions blocks the surface, preventing the formation of sublattices of OH discharged on the free surface of the metal but allows partial discharge of water molecules of the hydration shells of the ions in two energy states in a fast upd reaction. This provides the path for the initial stages of 2-dimensional oxidation of the surface. The resulting network of anion-OH(1−γ)− complexes stabilizes the surface, preventing the turn-over process which is possible only after, or coupled with, the desorption of the anions in an anion replacement, MOH turn-over process. This is possible when a potential is reached at which partly discharged OH(1−γ)− in complexes with anions can become fully discharged, forming free MOH's which are no longer part of the complex. In this way, the network is destroyed and the stabilization of the anions by the H-bonds of hydration H2O or partially discharged OH no longer remains, allowing the anions to be desorbed and processes of phase-oxide development to begin.