Structure and dynamics of oxy-overlayer on Cu(1 1 1) electrode surfaces in alkaline aqueous solution revealed by electrochemical STM and quartz crystal microbalance measurement

The redox reactivity of copper in aqueous solution has attracted much attention due to practical applications such as catalysis, electrodeposition, microelectronic device fabrication. In this work, the structure and dynamic behavior of Cu(1 1 1) electrodes were investigated in 0.01 M NaOH aqueous solution in the potential range where copper electrode shows redox reactivity for aldehyde oxidation by using electrochemical scanning tunneling microscopy (EC-STM), cyclic voltammetry, and electrochemical quartz crystal microbalance (EQCM). EC-STM observations revealed that the Cu(1 1 1) electrode surface was covered with an oxy-overlayer (layer of adsorbates containing oxygen), which formed a terrace-step structure with a monoatomic step height. Two kinds of ordered structures on the terrace were observed at the potential negative-side of the cathodic peak at −0.85 V vs. Ag/AgCl: hexagonal lattice with a unit vector of 0.70 ± 0.03 nm (structure A), and rhombic lattice with a unit vector of 1.40 ± 0.03 nm (structure B). Species with these structures seem responsible for aldehyde oxidation, based on the good agreement between the cyclic voltammogram (CV) for these species and that previously reported for Cu electrodes active for aldehyde oxidation. EC-STM revealed the deposition and dissolution of the oxy-overlayer on Cu(1 1 1) electrode surfaces. Combining the EQCM measurement with EC-STM images suggests that a cluster of water molecules had adsorbed on a species that had ordered structure A at the potential of −0.65 V in an anodic scan. This hydration probably transforms the species into a soluble species, such as Cu(OH)2−, which induces the partial dissolution of a terrace from a step edge. In addition, dependence of the position of the anodic peak at −0.65 V on a cathode limit was observed for the first time. EC-STM observation revealed that this electrochemical behavior of the anodic peak correlated with the surface structure of the oxy-overlayer, namely, correlated with the ratio of the surface areas covered by ordered structure A and B.