Observations of Isolated Mobile Au-Br and Au-S Surface Complexes on Au(100) Electrodes

Wet-chemical processing of metals, for example, galvanic deposition, etching, and nanoparticle synthesis, usually requires complexing agents. In particular, for noble metal processing, anionic complexing species, such as halides or sulfide are indispensable. While it is known that these species strongly adsorb on metal surfaces and affect metal nucleation and growth, the detailed role of these anions in the underlying atomistic processes is less clear. Often, it is assumed that surface complexes are involved, but experimental evidence for the latter is still lacking. Here, we present direct in situ video-rate scanning tunneling microscopy observations of gold-bromide and gold-sulfur surface complexes on Au(100) electrodes. Based on the intramolecular resolution images obtained in these studies, these species can be assigned to a dimeric planar Au2Br6${\rm Au}_2{\rm Br}_6$ and a linear Au2S2${\rm Au}_2{\rm S}_2$ complex. Once formed, the surfaces complexes are stable even at rather negative potentials and diffuse as molecular species on the Au surface.