Formation and Diffusion of Subsurface Adsorbates at Electrodes.

We report direct observation of the formation of a subsurface species at metal electrodes in liquid electrolytes and of its migration within the solid's surface layer, below a chemisorbed electrochemical double layer. Using in situ video-rate scanning tunneling microscopy, we find for adsorbed sulfide on bromide-covered Ag(100) electrodes reversible transitions between adsorption sites on top of the surface and within a vacancy in the first Ag layer. In the latter state, the sulfide surface diffusion can be enhanced by orders of magnitude, which we attribute to vacancy-mediated diffusion underneath the bromide adlayer. The high dynamics within the surface layer, indicated by these observations, may open up alternative pathways in electrocatalytic reactions and growth processes.

Coadsorbate-Induced Reversal of Solid-Liquid Interface Dynamics.

Coadsorbed anions are well-known to influence surface reactivity and dynamics at solid-liquid interfaces. Here we demonstrate that the chemical nature of these spectator species can entirely determine the microscopic dynamic behavior. Quantitative in situ video-STM data on the surface diffusion of adsorbed sulfur atoms on Cu(100) electrodes in aqueous solution covered by bromide and chloride spectators, respectively, reveal in both cases a strong exponential potential dependence, but with opposite sign. This reversal is highly surprising in view of the isostructural adsorbate arrangement in the two systems. Detailed DFT studies suggest an anion-induced difference in the sulfur diffusion mechanism, specifically an exchange diffusion on the Br-covered surface. Experimental evidence for the latter is provided by the observation of Cu vacancy formation in the Br system, which can be rationalized by a side reaction of the sulfur exchange diffusion.

Structural Reorganization of Pt(111) Electrodes by Electrochemical Oxidation and Reduction.

The surface restructuring of Pt(111) electrodes upon electrochemical oxidation/reduction in 0.1 M HClO4 was studied by in situ grazing-incidence small-angle X-ray scattering and complementary scanning tunneling microscopy measurements. These methods allow quantitative determination of the formation and structural evolution of nanoscale Pt islands during potential cycles into the oxidation region. A characteristic ripening behavior is observed, where these islands become more prominent and homogeneous in size with increasing number of cycles. Their characteristic lateral dimensions primarily depend on the upper potential limit of the cycle and only slightly increase with cycle number. The structural evolution of the Pt surface morphology strongly resembles that found in studies of Pt(111) homoepitaxial growth and ion erosion in ultrahigh vacuum. It can be fully explained by a microscopic model based on the known surface dynamic behavior under vacuum conditions, indicating that the same dynamics also describe the structural evolution of Pt in the electrochemical environment.

Potential-Dependent Adlayer Structure and Dynamics at the Ionic Liquid/Au(111) Interface: A Molecular-Scale In Situ Video-STM Study.

Room-temperature ionic liquids are of great current interest for electrochemical applications in material and energy science. Essential for understanding the electrochemical reactivity of these systems are detailed data on the structure and dynamics of the interfaces between these compounds and metal electrodes, which distinctly differ from those in traditional electrolytes. In situ studies are presented of Au(111) electrodes in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSA]) by high-speed scanning tunneling microscopy (video-STM). [BMP][TFSA] is one of the best-understood air and water stable ionic liquids. The measurements provide direct insights into the potential-dependent molecular arrangement and surface dynamics of adsorbed [BMP](+) cations in the innermost layer on the negatively charged Au electrode surface. In particular, two distinct subsequent transitions in the adlayer structure and lateral mobility are observed with decreasing potential.