In-situ STM investigation of Tl and Pb underpotential deposition on chemically polished Ag(111) electrodes

Abstract

In-situ STM was used to investigate the adlayer formation and structure of Tl adsorbates at chemically polished Ag(111) electrodes in perchlorate electrolyte, and to study the effect of extended polarization of incomplete Tl and Pb adlayers at high initial adsorbate coverage. In the Tl+/Ag(111) system, the full first monolayer adsorbate has a hexagonally close packed structure with interatomic distances that are compressed in comparison with the bulk Tl phase. The subsequently formed second monolayer has also a hexagonally close packed (hcp) structure, but shows more pronounced disordered domains. Incomplete first Tl monolayers of high coverage have the same hcp structure as a complete monolayer, but do not extend to the very boundary of the substrate terrace. Extended polarization of such monolayers leads to a conversion of the original hcp layer into a coverage with [√3 × √3]R30 °—symmetry which is interpreted as a mixed coverage, where every third Ag atom is replaced by a Tl atom. Similar phenomena are observed during long-time polarization of an incompletely formed Pb-coverage. The results are discussed with regard to the role of the atomic steps and reveal the importance of surface singularities for the understanding of the adsorption behaviour at real electrodes.