Ex situ scanning tunneling microscopy study of under-potential oxidation of a Ag(111) electrode in an alkaline electrolyte

A Ag(111) single crystal electrode emersed from NaF+NaOH electrolyte (pH 11) under potential control in the interval between −0.8 V and +0.2 V vs. Hg | HgO was studied by scanning tunneling microscopy (STM) in an inert atmosphere. The STM images show that the oxidation of the Ag(111) surface starts above the point of zero charge and exhibits a nucleation-growth mechanism. It starts at the steps and extends to the terraces as the electrode potential is scanned positive. Potential reversal restores the initial surface morphology. The reaction-induced features imaged in STM as dark spots are assigned as islands of chemisorbed oxygen-containing species. The irregular shape of the islands points to the diffusion of ad-species as the limiting step of the process.