PHOTOCURRENT IMAGING OF THE LAYERS FORMED DURING THE ELECTROOXIDATIONOF GOLD

A scanning laser electrochemical microscope was used to study localize d photocurrents during the electrooxidation of gold. Images obtained s olely with the supporting electrolyte suggested that the oxide layer i s virtually homogenous. However in the presence of ferrocyanide a spec tacular contrast was observed. A number of possible sources of image c ontrast are discussed and in part differentiated. In the presence of f errocyanide 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 or igin of the chemical heterogeneity thereby revealed was explored by ex amining the influence of electrode potential and pretreatment and is a scribed to a chemical species formed below regular monolayer oxide for mation. Significantly this ''incipient oxide'' was found to be excepti onally stable, affecting image contrast up to oxygen evolution, and on ly being removed by prolonged evolution of hydrogen. Intense illuminat ion resulted in a photoinduced aging or reconstruction of the oxide la yer.