The structure of bromide and chloride adlayers on Au(100) electrodes: an in situ STM study

Bromide adsorption on Au(100) electrodes was investigated using in situ scanning tunneling microscopy (STM). In perfect agreement with published surface X-ray scattering data, two quasi-hexagonal structures were found: a c(2×22)R45° commensurate structure at potentials between the first and second spike in the cycling voltammogram of Au(100) in Br− containing solutions and a uniaxially incommensurate c(2×p)R45° structure (22≥p≥2.5), at potentials positive of the second spike. The incommensurate character of the latter bromide structure provokes the appearance of stripes which run parallel to one of the 2 directions of the substrate. The electrocompression of this adlattice is reflected in the change of shape and corrugation length of the stripes upon changing the potential. In the potential region of the ordered structures, the Au(100) surface is covered by gold islands which are nearly hexagonal in shape and by steps forming angles of ca. 120°, indicating that, in the presence of the ordered Br− adlayer, the gold steps run parallel to the close-packed rows of the bromide structure. Chloride adsorption on Au(100) electrodes was also investigated by in situ STM. A uniaxially incommensurate structure was found at potentials positive of the spike in the cyclic voltammogram of Au(100) in Cl− containing solutions. It shows a quasi-hexagonal geometry and can be described as a c(2×p)R45° superstructure (22≥p≥2.3). As in the case of the uniaxially incommensurate bromide superstructure, stripes can be seen in the STM images of the chloride adlattice, the corrugation length of which depends on the electrode potential.