Non-Monotonic Variation of Potential-Dependent Surface Diffusion at Electrochemical Interfaces in the Presence of Coadsorbates

The influence of coadsorbed ions on adsorbate diffusion, an inherent effect at solid-liquid interfaces, was studied for adsorbed sulfur on Ag(100) electrodes in the presence of bromide or iodide. Quantitative in situ high-speed scanning tunnelling microscopy (video-STM) measurements were performed both in the potential regime of the c(2×2) halide adlayer at its saturation coverage and in the regime of a disordered adlayer where the halide coverage increases with potential. These studies reveal a surprising non-monotonic potential dependence of Sad diffusion with an initial increase with halide coverage, followed by a decrease upon halide adlayer ordering into the c(2×2) structure. Density functional theory (DFT) and Monte Carlo (MC) simulations only qualitatively reproduce the rise in Sad mobility with halide coverage, suggesting that many-adsorbate interactions and the presence of the electrolyte need to be considered.