Effect of Trace Amounts of Chloride on Roughening of Au(111) Single-Crystal Electrode Surface in Sulfuric Acid Solution during Oxidation-Reduction Cycles.

This study investigates the impact of varying trace-level chloride ion concentrations on the roughening of a Au(111) electrode during oxidation-reduction cycles (ORCs) in 0.1 M sulfuric acid by in situ scanning tunneling microscopy (STM). At the higher chloride concentration (50 μM), rapid dissolution of Au atoms and step line recession are observed in the recorded in situ STM images. The high surface mobility of Au atoms resulted in a lack of detectable vacancy islands in the images with minimal changes in cyclic voltammograms (CVs) and the complete absence of nano-island formation, which is observed in pure sulfuric acid. At moderate concentration (10 μM), the dissolution rate decreased substantially, so the initial step lines are still distinguishable after the 200 ORCs. The lower surface mobility leads to the formation of vacancy islands in the terraces, and these newly formed step sites give rise to additional peaks in the CVs. At the lowest concentration (1 μM), nano-island formation is observed. However, inhomogeneous chloride adsorption (showing as darker areas in the EC-STM images) on the sample at high enough anodic potential (0.9 V) led to previously unreported behavior, showing very inhomogeneous roughening, with parts on the surface showing reduced Au atom mobility and minimal changes even after 200 ORCs.