Electrochemical formation of Se atomic layers on Au(1 1 1) surfaces: the role of adsorbed selenate and selenite

We report an investigation of the electrodeposition of Se atomic layers on Au(1 1 1) surfaces from aqueous SeO2 solutions. The behavior of this system is complex, and characterized by two major surface-limited reduction waves in the potential window positive of the onset of Se bulk deposition. These two waves occur at potentials negative of the known E° for the Se(IV)/Se(0) couple due to the large overpotential for Se deposition on Au surfaces, and therefore cannot be considered true underpotential deposition. On the basis of potential switching and concentration dependence experiments, the first surface wave is unambiguously assigned as arising from the two-electron reduction of an adsorbed selenate species to adsorbed selenite. In some earlier reports, this feature has been assigned as corresponding to the four-electron reduction of selenite to elemental selenium. The second major surface-limited wave corresponds to the four-electron reduction of adsorbed selenite to elemental selenium. Coulometric measurements indicate that the selenium coverage corresponds to approximately 0.41 monolayers, in good agreement with previous scanned probe microscopy investigations which indicate that Se forms a (√3×√3)R30° adlayer on Au(1 1 1) surfaces.