In Situ Scanning Tunneling Microscopy of Au(111) in Acidic and Alkaline Potassium Cyanide

Abstract

Linear sweep voltammetry has been used to examine Au(111) electrodes in 50 μM KCN solutions with pH's varying between 1 and 11. In situ scanning tunneling microscopy (STM) was employed to probe the adsorption of cyanide species and the dissolution/deposition processes of gold in pH 11, 0.1 M KF, and 0.1 M HClO 4 solutions containing 50 μM KCN. Dissolution of gold commenced at a potential as negative as −0.8 V (vs SCE) in alkaline KCN solutions. Both in situ STM and voltammetry indicate that dissolution and deposition of gold were electrochemically reversible. STM atomic resolution yielded a reconstructed Au(111) surface rather than the cyanide adlattices, while etching proceeded at −0.8 V in an alkaline KCN solution. The dissolution of gold at −0.8 V in pH 11 KCN solution was highly selective and anisotropic, manifesting through step retreat in directions perpendicular to the herringbone features of the reconstructed Au(111) surface, or the [11̄0] direction. Nearly all gold atoms on step ledges resided at unfaulted face-center cubic (fcc) sites. In contrast, a number of cyanide adlattices were observed on Au(111) in acidic KCN, including a (6 × 6) structure on the reconstructed Au(111) at 0.1 V (vs SCE), followed by a phase transition to give (√7 × √7) R 19.1° and (√7 × √21) oblique structures on the unreconstructed Au(111) within the potential region of 0.2 and 0.3 V. Two intertwined ordered structures, (3 × 2√3) rect and (2 × 2), appeared at potentials more positive than 0.3 V, before dissolution of gold became apparent at 0.4 V.