In-situ X-ray diffraction and STM studies of bromide adsorption on Au(111) electrodes

The structure of bromide adlayers at the Au(111)-aqueous solution interface has been studied by in-situ surface X-ray scattering (SXS) and scanning tunneling microscopy (STM). Both techniques show the existence of a hexagonal close-packed adlayer phase above a critical potential and are in good quantitative agreement on the adlayer structural parameters. The bromide-bromide spacing changes continuously between 4.24 Angstrom at the critical potential and 4.03 Angstrom at a potential 300 mV more positive. The adlayer is rotated relative to the substrate by an angle dependent on potential and bromide concentration. The potential-dependent adlayer density corresponding to these structural results agrees well with Br surface excess densities from published electrochemical measurements. At very positive potentials a bromide-induced step-flow etching of the Au substrate is observed. The results are used to compare the different techniques and to discuss the adlayer structure, the phase behavior, and the halide-gold chemical interaction.