In Situ STM Study of the Initial Stages of Anodic Oxidation of Cu(111) in the Presence of Sulfates

In situ scanning tunneling microscopy (STM) has been applied to study adsorption and oxide formation on Cu(111) in weakly alkaline, neutral, and acidic sulfate-containing solutions. In 5 × 10 − 4 M NaOH + 5 × 10 − 3 M Na 2 SO 4 ( pH ∼ 10.5 ) , a hexagonally structured OH layer is formed at potentials well below oxide formation despite the tenfold larger sulfate concentration showing a stronger interaction of OH − with the Cu surface than SO 4 2 − . In 5 × 10 − 3 M Na 2 SO 4 ( pH ∼ 7 ) , a partially structured and highly mobile adlayer is formed at potentials quite near the onset of oxide formation with coexisting ( 3 × 3 ) -ordered domains and nonordered domains. This is assigned to the competitive adsorption of water molecules with sulfate anions. In contrast, a highly ordered and stable ( 3 × 7 ) adsorbed layer is formed in 5 × 10 − 3 M H 2 SO 4 . It is assigned to the coadsorption of sulfate anions and cations ( H 3 O + or H + ) , the latter compensating the repulsive forces between sulfate anions and thus stabilizing the adlayer. The anodic Cu(I) oxide is (111)-oriented and has a faceted surface in solutions of alkaline, weakly alkaline, and neutral pH.