In situ STM study of the initial stages of oxidation of Cu(111) in aqueous solution

In situ scanning tunneling microscopy measurements of Cu(111) in 0.1 M NaOH in the underpotential range of Cu2O formation are reported. Adsorption and desorption processes are observed at the reversible potential of −0.675±0.02 V/SHE. These processes are initiated preferentially at the step edges on the upper terrace side. The adsorption decreases the mobility of the Cu atoms along the step edges. It also induces the lateral growth of the terraces, which is thought to result from the accumulation, at the step edges, of Cu atoms ejected from the adlayer superstructure due to reconstruction of the outermost Cu plane of the substrate. The same mechanism leads, in the last stages of the adsorption process, to the formation of Cu adislands on the terraces when the adlayer grow in adsorbate-free areas surrounded by the superstructure. The adlayer forms an ordered superstructure with an hexagonal lattice having a unit vector of 0.6±0.02 nm and one adspecies per unit cell (coverage of ca. 0.2 ML). The lattice parameters suggest that the outermost Cu plane is reconstructed according to the Cu plane in Cu2O(111), i.e., with CuCu distances of ca. 0.3 nm, with the adlayer forming a (2×2) lattice. The comparison between the STM data and the charge transfer measurements indicates that the adspecies are hydroxide or hydroxyl groups rather than adsorbed oxygen.