Anisotropic Roughening of a Au(111) Single-Crystal Electrode Surface in HClO₄ Solution during Oxidation–Reduction Cycles

This study investigates the inhomogeneous roughening of a Au(111) single-crystal electrode surface during oxidation–reduction cycles (ORCs) in a 0.1 M perchloric acid (HClO₄) solution using electrochemical scanning tunneling microscopy (EC-STM). The results reveal that, even in ultrapure HClO₄, the presence of minor impurities can lead to three distinguishable surface evolutions, on one and the same crystal: surface roughening by the formation of adatom and vacancy islands, gold dissolution resulting in vacancy island formation (in conjunction with step-line recession), and the surface remaining intact even during oxidation–reduction cycling. The impact of trace impurities, specifically sulfate (SO₄^2–) and chloride (Cl^–), on the surface structure development is investigated by adding 10 μM of H₂SO₄ and HCl to the HClO₄ solution. Our results reveal that sulfate significantly promotes uniform roughening, while chloride accelerates gold dissolution and step-line recession. These findings highlight the critical role of minor impurities in altering the electrochemical behavior of gold surfaces and how sensitive the local evolution of the surface structure is to these effects.