Effect of sulfuric acid concentration on the structure of sulfate adlayer on Au(1 1 1) electrode

The adsorption of sulfate on Au(1 1 1) single-crystal electrode was examined in sulfuric acid solutions of various concentrations (0.5–7.5 M) by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The well-known sulfate adlayer with (√3 × √7) unit cell was found in 0.5 M sulfuric acid by in situ STM. In 2.5 and 5.0 M sulfuric acid, a new order–disorder phase transition, from the (√3 × √7) adlayer to a disordered structure, was found to take place at ca. 1.1 V vs. Ag∣AgCl∣KCl(sat.). In 7.5 M sulfuric acid, no current spikes were seen in the CV profile, and in situ STM showed that the sulfate adlayer was disordered.