Surface Alloying During Pb Underpotential Deposition on Au(111)

The surface alloying during Pb underpotential deposition (UPD) on Au(111) films was studied using electrochemical techniques. This UPD system has been known for its unusual stress behavior associated with surface alloying during epitaxial monolayer formation. The characteristic cyclic voltammetry of Pb UPD on Au(111) exhibits an anodic peak at the most positive potentials that does not have a symmetric cathodic counterpart. The peak can be associated with the surface structural changes due to the Pb dealloying from the top substrate layer. Two electrochemical approaches were used to study the surface transformations: i) extended polarization (up to 60 min) at high Pb coverage of 0.85 ML, and ii) repeated cycling 1150 times between the potentials corresponding to 0.25 ML and 1 ML Pb coverages. In both approaches, it was observed that with the increased time of polarization or number of potential cycles, the prominent UPD peaks gradually reduced in magnitude, became broader and lost their original double-peaks structure. At the same time, the dealloying (the most anodic) peak shifted positive about 0.1 V and increased in magnitude. Quantitative analysis of the changes estimated the coverage of Pb alloying with a surface of 0.28-0.30 ML.