Sulfate structures on copper deposits on Au(111): In situ STM investigations

Abstract

Motivated by some incomprehensible aspects of the electro-deposition of copper on a Au(111) surface from copper sulfate containing sulfuric acid solution published in the literature, we have revisited this system using cyclic voltammetry (CV) and scanning tunneling microscopy (STM). Specifically, we have examined the different stages of Cu deposition associated with 2/3 ML, 1 ML, supposedly 5/3 ML and multilayer coverages, and present a consistent structure model for the coadsorbed sulfate overlayers on the copper deposits of varying thickness. At a Cu coverage of 2/3 ML the coadsorbed sulfate anions exhibit the well known (3×3)R30° structure. In the regime where the full pseudomorphic Cu monolayer is supposed to exist first the formation of a disordered sulfate layer is found, which only at more negative potentials transforms into a (3×7)R19.1° sulfate structure (on the first completed Cu monolayer). In view of the (3×7)R19.1° structures found on both the Au(111) and the bulk Cu(111) surface a (3×7)R19.1° structure on a pseudomorphic Cu monolayer on Au(111) appears much more likely than a (3×3)R30° structure as claimed in the literature. Moreover, this (3×7)R19.1° structure on 1ML Cu/Au(111) could only be found at rather negative potentials and, in particular, low Cu concentrations in the solution. Further deposition of Cu then leads again to the formation of a (3×3)R30° structure on a 2/3 filled and slightly contracted second Cu layer, i.e. at a local copper coverage of 5/3 ML. Consistent with this contraction the STM images indicate a weak Moiré superstructure of the second layer. The formation of this (3×3)R30° structure is again rationalized by the adsorption of sulfate anions in atomic vacancies within the second Cu layer, similar to what was previously established for a copper coverage of 2/3 ML on Au(111). Cu islands, which exhibit multilayer thickness show the characteristic sulfate induced Moiré structure similar, but not identical, to that known from bulk Cu(111).