Dynamics of Br electrosorption on single-crystal Ag(100): a computational study

We present dynamic Monte Carlo simulations of a lattice-gas model for bromi ne electrodeposition on single-crystal silver (100). This system undergoes a continuous phase transition between a disordered phase at low electrode p otentials and a commensurate c(2 x 2) phase at high potentials. The lattice -gas parameters are determined by fitting simulated equilibrium adsorption icsotherms to chronocoulometric data, and free-energy barriers for adsorpti on/desorption and lateral diffusion are estimated from ab-initio data in th e literature. Cyclic voltammograms in the quasi-static limit are obtained b y equilibrium Monte Carlo simulations, while for nonzero potential scan rat es we use dynamic Monte Carlo simulation. The butterfly shapes of the simul ated voltammograms are in good agreement with experiments. Simulated potent ial-step experiments give results for the time evolution of the Br coverage , as well as the c(2 x 2) order parameter and its correlation length. Durin g phase ordering following a positive potential step, the system obeys dyna mic scaling. The disordering following a negative potential step is well de scribed by random desorption with diffusion. Both ordering and disordering processes are strongly influenced by the ratio of the time scales for desor ption and diffusion. Our results should be testable by experiments, in part icular cyclic voltammetry and surface X-ray scattering. (C) 2000 Elsevier S cience B.V. All rights reserved.