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Abstract

Hydroxide (OH–) adsorption at Ag(111)/alkaline electrolyte interfaces is the initial step leading to the formation of surface oxide. We investigate how OH– anion adsorption is influenced by co-adsorption of supporting electrolyte anions (F–). Pure NaF and NaOH as well as mixed electrolytes ranging from pH=5.8 to 14 are investigated. The rotational anisotropy of the second harmonic generation (SHG) signal is measured to distinguish adsorptive processes from structural changes on the Ag surface. Our results clearly show a crossover from F–-dominated to OH–-dominated adsorption, while both species remain charged upon adsorption. At more positive potentials, but below the reversible potential of bulk oxide growth, OH is discharged leading to sub-monolayer oxide build-up, whereas in acidic electrolyte, Ag is dissolved. At intermediate pH, the formation of a structured OH–/F– co-adsorbed layer is proposed.