Morphological aspects of sulfate-induced reconstruction of Cu(111) in sulfuric acid solution: in situ STM study

Using in situ STM the atomic structure and the morphology of a Cu(I 1 1) surface exposed to a dilute sulfuric acid solution have been studied depending on the applied electrode potential. At anodic potentials near the onset of copper dissolution the electrode surface is reconstructed (expanded) caused by the specific adsorption of sulfate anions. The extent of the surface reconstruction strongly depends on the sulfate adsorption rate. Fast sulfate adsorption results in a mainly disordered sulfate adlayer on an unreconstructed copper surface. Conversely, slow sulfate adsorption produces a mainly reconstructed copper surface with a highly ordered sulfate/water coadsorption layer. This adsorbate structure shows an additional long-range Moire modulation, due to a misfit between the first reconstructed and the second unreconstructed copper layer. This is verified by spectroscopy-like STM experiments, which allow the imaging not only of the adsorbate overlayer, but also of the underlying reconstructed substrate. This type of adsorbate-induced reconstruction is characterized by an expansion of the topmost copper layer. The kinetically slow process of reconstruction can be easily followed by dynamic STM measurements revealing a mass transport out of the topmost copper layer during the slow sulfate adlayer formation. Characteristically, new copper islands nucleate and grow, while the sulfate Moire adlayer expands over the electrode surface. At cathodic electrode potentials the desorption of the sulfate adlayer is accompanied by the lifting of the surface reconstruction and the massive formation of surface defects, such as small pits and vacancy islands. A continuous cycling of the electrode potential leads to an enormous roughening of the surface morphology. (C) 2001 Elsevier Science Ltd. All rights reserved.