Co-adsorption of Fluoride and Hydroxide Ions on Ag(111) in Alkaline Electrolytes: Electrochemical and SHG Studies

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 cross-over 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.