Effect of anion adsorption on early stages of copper electrocrystallization at Au(111) surface

The monolayer and submonolayer deposition of copper on Au(111) electrode surface in the presence of chloride and sulfate ions was studied by in situ X-ray absorption and electrochemical techniques, The anions coadsorb with the deposited copper adatoms and have a strong influence on the structure of these mixed overlayers. Copper deposited in the presence of chloride forms a bilayer in which copper atoms are sandwiched between the gold substrate and the top layer of chloride ions. The bilayer is well ordered and has a (5 x 5) long range structure, The copper atoms are packed in registry with the top layer of chloride ions. In contrast, copper adatoms deposited in the presence of sulfate ions are packed in registry with respect to the Au(lll) substrate. The coadsorbed copper and sulfate form a highly corrugated overlayer. The copper adatoms assume a honeycomb (root 3 x root 3) structure with the center of the honeycomb occupied by sulfate. The sulfate ion adsorbs with three of its four oxygens directed towards the hexagon of copper adatoms. The bond angle between the copper adatom and the oxygen of the sulfate ion is approximately equal to 45 degrees. Our data indicate that, in contrary to the literature reports, the (root 3 x root 3) structure observed on STM and AFM images corresponds to the corrugation of adsorbed sulfate ions rather than copper adatoms.