In-situ STM investigation of adsorbate structures on Cu(111) in sulfuric acid electrolyte

Abstract

The process of sulfate adsorption on Cu(111) from a sulfuric acid electrolyte (5 mM H 2SO 4) and the resulting surface structures have been investigated by in-situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). Using the so-called potentiodynamic STM scan mode, it was possible to correlate the anodic peak and the cathodic peak in the CV with the adsorption and desorption of a sulfate adlayer on Cu(111). At cathodic potentials, the bare copper surface could be imaged with an atomic resolution. After adsorption at anodic potentials, the sulfate forms an ordered superstructure with a short range periodicity (molecular pattern) and a superimposed long-range periodicity (Moiré pattern). The Moiré pattern occurs in three anisotropic rotational domains. High-resolution STM images manifest a coadsorption of H 2O molecules and sulfate anions. Closely packed rows of sulfate ions are separated by zigzag chains of water molecules. The short-range lattice vectors are very similar to those found for sulfate adlayers on other f.c.c.(111) surfaces (Au, Pt, Rh). For the first time, the adlayer formation process of an anionic adsorbate on a single crystal electrode was observed directly. If the potential scan is stopped between the desorption peak and the adsorption peak in the anodic run of the CV, the slow formation of the Moiré structure at a constant potential could be followed. At an early stage of the adlayer formation, different domains of metastable striped patterns occur that are successively replaced by the growing Moiré structure. Its formation starts locally at upper step edges and spreads from the step edges over the terraces. Even in the submonolayer regime, the Moiré structure occurs, which can be seen as a clear evidence for a strong attractive interaction within the adlayer. During the adlayer formation process, a mass transport out of the top copper layer also takes place, resulting in a drastic change of the surface topography due to the formation of characteristically shaped islands and step edges. A structure model is proposed that consists of a ( 3 × 7 ) SO 2− 4 structure on a reconstructed first Cu layer. The mismatch of the latter with the (111) structure of the second Cu layer is thus suggested to be the origin of the observed Moiré pattern. The proposed reconstruction mechanism is consistent with the slow Moiré formation kinetics as well as the observed island growth structures.