Potentiodynamic desorption spectra of metallic monolayers of Cu, Bi, Pb, Tl, and Sb adsorbed at (111), (100), and (110) planes of gold electrodes

Abstract

The formation of metallic adsorption layers was studied in solutions of Cu 2+, Bi 3+, Pb 2+, Tl + and Sb 3+ at (111), (100) and (110) planes of gold single crystal electrodes. Potentiodynamic desorption spectra were recorded with a sweep rate of 20 mV s −1 for all systems. Characteristic peak structures were obtained which depend strongly on the nature of the adsorbate as well as on the substrate orientation. The half width of the peaks indicates attraction and repulsion respectively for various systems. In most systems more than one peak was observed. This is explained by the formation of various ordered structures. At low coverages peak charge data obtained by integration of current/time curves yield surface concentrations which fit those of ordered structures well, e.g. c(2×2) on (100) or p( 3 × 3 ) R 30° on (111). The adsorption behaviour of the (110) plane is similar in all systems because atomic chains seem to be generally stable. Near the equilibrium potential of the correspondent metal electrode, ϵ r = 0, a “mono-molecular” adsorption layer was found for Cu 2+, Pb 2+ and Bi 3+. In the case of the small copper atom, a 1:1 adsorption was found for all planes. Larger atoms like bismuth and lead form epitactic layers at low coverages; at high coverages they form close-packed monolayers with surface concentrations independent of the substrate structure but decreasing with increasing adsorbate radius. The coulometric data for antimony and thallium are not so conclusive. Measurements with various sweep rates show that the adsorption reaction is a slow potential dependent process in various systems. The underpotential/work function correlation of Kolb, Gerischer and Przasnyski is discussed with respect to these experiments. It follows that this concept developed for polycrystalline electrodes is qualitatively valid for (110), but not clearly so for (100) and (111).