6. Monte Carlo Simulations of the Underpotential Deposition of Metal Layers on Metallic Substrates: Phase Transitions and Critical Phenomena

The underpotential deposition (UPD) of metal submonolayers and monolayers on metal substrates for the systems Ag/Au(100), Au/Ag(100), Ag/Pt(100), Pt/Ag(100), Au/Pt(100), Pt/Au(100), Au/Pd(100), and Pd/Au(100) is studied by means of lattice Monte Carlo simulations. Interaction energies among different metal atoms are evaluated by using the embedded-atom method. A wide variety of physical situations are found and discussed, including systems exhibiting the sequential adsorption of atoms on kink and step sites, prior to the completion of the monolayer. On the other hand, for other systems, we observe the formation of 2D alloys between substrate and adsorbate atoms, and our predictions are compared with available experimental data. The adsorption isotherms determined for most of the systems studied exhibit sharp transitions in the coverage when the chemical potential is finely tuned. In particular, on the basis of the fact that the UPD of Ag atoms on the Au(100) surface exhibits a sharp first-order phase transition, at a well-defined value of the (coexistence) chemical potential, we also performed extensive simulations aimed at investigating the hysteretic dynamic behavior of the system close to coexistence upon the application of a periodic potential signal.