PHOTOCURRENT IMAGING OF THE LAYERS FORMED DURING THE ELECTROOXIDATION OF GOLD

A scanning laser electrochemical microscope was used to study localized photocurrents during the electrooxidation of gold. Images obtained solely with the supporting electrolyte suggested that the oxide layer is virtually homogenous. However in the presence of ferrocyanide a spectacular contrast was observed. A number of possible sources of image contrast are discussed and in part differentiated. In the presence of ferrocyanide the main mechanism for photoinduced current variations at a gold electrode is the effect of the small temperature change induced by the illumination on the rate of electrooxidation of Fe(II). The origin of the chemical heterogeneity thereby revealed was explored by examining the influence of electrode potential and pretreatment and is ascribed to a chemical species formed below regular monolayer oxide formation. Significantly this ''incipient oxide'' was found to be exceptionally stable, affecting image contrast up to oxygen evolution, and only being removed by prolonged evolution of hydrogen. Intense illumination resulted in a photoinduced aging or reconstruction of the oxide layer.