Adsorbate-induced etching of Au(111) surfaces: A combined in-situ infrared spectroscopy and scanning tunneling microscopy study

In this paper the initial stages of the etching of Au(lll) electrode surfaces in the presence of adsorbed tetramethylthiourea (TMTU) are investigated using the complementary methods of in-situ IR spectroscopy and STM. Scanning tunneling microscopy (STM)has been used in-situ to examine changes in surface topography associated with the surface etching. STM images show that the etching process proceeds from step edges and eventually leads to the exposure of monatomic steps which are orientated at 60 degrees/120 degrees with respect to each other. This indicates that the more lowly co-ordinated kink atoms are more readily etched than ''straight'' steps, leading to an appearance of anisotropic etching of steps. Iri-situ IR spectroscopy confirms that etching leads to the formation of gold-TMTU complexes, which are soluble in the electrolyte solution. It is also apparent from these spectroscopic data that no significant TMTU decomposition occurs during the anodic surface etching. The potential dependence of the IR band intensity of the Au-TMTU complex indicates the important role which adsorbed TMTU plays in the etching process. Upon subsequently lowering the electrode potential a slow growth of the step edges is seen by in-situ STM. Deductions concerning the growth could be made by following the development of the surface topography with in-situ STM. Growth preceded two-dimensionally from substrate step edges and gradually gave rise to semicircular step edges which advanced slowly across the surface. This can be interpreted as the nucleation and growth of gold islands by electrodeposition of the TMTU-complexed gold.