Observation of gold electrode surface response to the adsorption and oxidation of thiocyanate in acidic electrolyte with broadband sum-frequency generation spectroscopy

• Phase change of the S-bounded adsorbed thiocyanate around 0.3 V on Au electrode surface.
• Oxidation and adsorption of the products occurs simultaneously at surface potentials higher than 0.7 V.
• SFG signal from the on Au electrode surface strongly affected by the adsorption of polarized ions and compounds.

The adsorption and oxidation of thiocyanate (SCN−) on polycrystalline Au electrode surface in acidic electrolyte (10 mM NaSCN/0.1 M HClO4) have been investigated with broadband sum-frequency generation spectroscopy (BB-SFG). Potential-dependent behaviors of both the surface adsorbed species and the Au nonresonant (NR) electrode surface have been observed through BB-SFG band. A phase change of the S-bounded SCN− on the Au electrode surface occurs around 0.3 V in both the anodic (increase of surface potential) and cathodic (decrease of surface potential) sweeps. At surface potentials >0.7 V, oxidation and adsorption of the products (AuCN and Au(CN)2−) have been observed. Possible oxidation/reduction pathways have been discussed. The Stark slopes of the adsorbed SCN− SFG bands are significantly different in the anodic scan (before oxidation) from that in the cathodic scan (after the oxidation). The Au surface NR SFG band in the cathodic scan is much stronger than that in the anodic scan at the double layer surface potentials (−0.2 V to 0.5 V). The potential dependent Au surface NR BB-SFG band clearly shows that the adsorption/desorption and oxidation/reduction of surface species will affect the surface electronic structure.