Simulation of S Tracer Diffusion Processes at the Ag(100) and Au(100) Surfaces in the Presence of Br Coadsorbates Based on DFT
Surface diffusion governs adsorbate self-assembly, catalysis, and metal deposition. Experiments using electrochemical video-STM have shown that S tracer diffusion in halide adlayers on coinage metals displays potential-dependent activation energies, attributed to the interaction of the adsorbates' dipole moments with the double layer field. Unexpectedly, in the disordered Br phase on Ag(100), S adatom hopping accelerates with increasing potential and Br coverage. This work investigates S/Br/Ag(100) in vacuum using periodic density functional theory (DFT) simulations. Br–Br and S–Br interactions are dominated by dipole–dipole forces, with additional elastic and oscillatory contributions. A simple polarizability model reproduces adatom depolarization. Monte Carlo simulations based on DFT interactions reproduce the experimentally observed rise in S nearest-neighbor hopping only partially; many-body effects increase the rate, yet it still falls far short of experimental values. A complementary S–Ag vacancy ingress–egress mechanism is proposed to account for longer jumps. For the ordered c(2×2)-Br phase on Ag(100), DFT suggests Ag-vacancy-assisted subsurface S diffusion paths, but estimated subsurface occupation probabilities are too low to match experiment. A related study of S in the c(√2×2√2)R45° Br adlayer on Au(100) finds strong binding between S and adlayer vacancies, yet a Br-vacancy-mediated diffusion path is identified whose energy barrier remains too low to explain observed mobility. Overall, discrepancies between theory and experiment indicate that realistic modeling must include electrolyte effects absent from vacuum DFT.
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