Second Harmonic Generation Studies of Anionic Chemisorption at Polycrystalline Platinum Electrodes

Nonresonant optical second harmonic generation (SHG) is used to monitor the adsorption of bisulfate, chloride, bromide, and iodide anions at polycrystalline platinum electrodes in aqueous solutions. Different polarization combinations of the fundamental and second harmonic beams are used to probe changes in the surface nonlinear susceptibility tensor elements caused by adsorption, reaction, and the electrode potential. The relative surface coverages of chloride and bromide as a function of solution concentration at a fixed potential are calculated from the changes observed in the nonresonant SHG signal. The reversibility of the chloride chemisorption is demonstrated by additional SHG measurements. Unlike the chloride and bromide chemisorption, iodide solutions lead to a monolayer of zero-valent iodine that is irreversibly adsorbed onto the electrode surface over a wide potential range. For partial iodine monolayers, the relative surface coverage of iodine can be determined from the SHG signal. The adsorption of various iodine species at platinum electrodes during the oxidation and reduction of solution iodide is studied in acidic and basic media. In basic solutions, the adsorption of atomic hydrogen in the presence of iodide is confirmed.