Electrochemical quartz crystal microgravimetry of gold in perchloric and sulfuric acid solutions

Formation of oxidized surface films at a gold electrode in sulfuric acid medium during anodization was studied using the electrochemical quartz crystal microbalance (EQCM) technique. The oxidation process consists of two single-electron steps. Forming a surface OH species from adsorbed water is the first step. Generating a surface O species from the OH species is the second step. The second step is accompanied by a concerted place exchange reaction involving the surface O species and gold atoms below the surface, and rehydration of the gold surface. The reduction of the surface retraces the oxidation processes. During the faradaic processes, sulfate species are adsorbing on (or desorbing from) the gold surface, but produce no detectible mass change. They have C_3v symmetry on the surface in the relevant potential region and compete for five surface gold atoms with five adsorbed water, OH and/or O moieties. In the double-layer region, mass changes are caused by the adsorption/desorption of sulfate species. Changing frequency-potential responses accompany cyclic potential scans because of roughening of the gold electrode's surface caused by sulfate-induced gold dissolution and redeposition processes.