Combined Scanning Tunneling Microscopy and Synchrotron X-Ray Photoemission Spectroscopy Results on the Oxidative CuI Film Formation on Cu(111)

Abstract

The initial stage of oxidative CuI film formation on Cu(111) has been studied in an electrochemical environment by means of cyclic voltammetry (CV), in situ scanning tunneling microscopy (STM) and ex situ synchrotron X-ray photoemission spectroscopy (SXPS). Cyclic voltammetric studies indicate a significant acceleration of copper oxidation in the presence of iodide. The reason for that is the iodide-mediated stabilization of cuprous species resulting in a downward shift of the onset potential for copper oxidation. Reactive sites for the copper oxidation followed by iodide complexation are exclusively defects such as substrate step edges. It is the surface-confined supersaturation of mobile CuI species that leads to the two-dimensional (2D) CuI film formation via nucleation and growth of a Cu/I bilayer on top of the preadsorbed iodide phase. In an advanced stage of copper oxidation, however, terraces are directly transformed into the 2D CuI film at the reactive boundary between metallic copper and the growing 2D CuI film. Structurally, this 2D CuI film is closely related to the (111) plane of crystalline CuIbulk (zinc blende type). There is no significant passivation of the copper surface against the oxidative dissolution reaction in the presence of the 2D CuI film. Copper dissolution in the presence of the 2D CuI film proceeds also via an inverse step flow mechanism involving the concerted receding of four atomic layers. A model of this process will be discussed on the basis of STM results. The transition from 2D to a three-dimensional (3D) CuI growth mode is observed for an advanced stage of copper oxidation.