The influence of halides on the initial selective dissolution of Cu3Au (1 1 1)

We report an electrochemical in situ X-ray diffraction (XRD) and ex situ atomic force microscopy (AFM) study on the influence of the halide additives bromide and iodide in comparison to chloride in 0.1 M H2SO4 solution on the selective dissolution of Cu3Au (1 1 1). Our results suggest that the addition of these halides have a pronounced effect on the dealloying process and lead to peculiar changes in the resulting surface morphologies. For an additive of bromide we observed (similar to an earlier study with chloride) a sequence of specific Au surface layers with an initial ultra-thin stacking-inverted Au-rich layer, subsequent pure Au islands, and finally substrate oriented nanometer-scale ligaments (nanoporosity). These structures occur with a negative shift in the transformation potentials compared to pure sulfuric acid solution. With an additive of iodide no stable passive Au film is formed and a porous surface was observed at relatively short times and low potentials. A precipitation of copper iodide (CuI) visible in the ex situ AFM images leads to the formation of surface porosity with bimodal length scales.