Surface Diffusion on Ag(100) and Cu(100) Electrodes in the Presence of Low Coverage Halides

Abstract

Atomic-scale processes at the electrochemical interface are of great importance to not only the fundamental research of electrochemistry, but also the newly developing technologies including energy conversion and bottom-up nanofabrication. In particular, the surface-active additive, such as halides, can strongly affect these processes, hence change the micro or macro behaviors of the studied system. For example, adding halides to the electrolyte is a feasible way to change the shape of the synthesized nanoparticles [1]. Besides, the modification of catalytic activity and selectivity by halide-containing electrolyte in CO2 electrochemical reduction on Cu surface has also been reported [2]. Therefore, a detailed and quantitative study of the electrochemical interface processes in the presence of co-adsorbed halides species may pave the way for more efficient design of electrochemistry-related procedures. Surface diffusion of adsorbates on the electrode is one of these processes, playing an important role e.g. in electrocatalysis, and electrochemical metal deposition or dissolution. Video-rate scanning tunneling microscope (Video-STM) in an electrochemical environment enables the recording of real-time videos of atomic-scale motion on electrode surfaces, which makes the observation and quantification of surface diffusion of individual adsorbed atoms and molecules possible [3].Halides form strongly adsorbed c(2x2) structures on various metal surfaces at the positive end of the double layer range. Our previous studies revealed a significant potential dependence of the surface diffusion of sulfur on Cu(100) and Ag(100) electrodes in the presence of close-packed c(2x2) halide adlayers [3,4]. In this range, sulfur atoms diffuse by hopping between neighboring sites of the c(2x2) lattice. The mobility of the atoms exponentially decreases with increasing potential, following a linear relationship between the potential and the diffusion barrier. More specifically, for a given system, the change of the diffusion barrier with the potential can be attributed to the change in the surface dipole moment along the diffusion path. Further studies demonstrated that the potential-dependence is independent of the diffusing species, which means that the co-adsorbed halides may dominate the change of the dipole moment.Here, instead of the c(2x2) potential range, the surface diffusion is studied in the negative potential range of Br-covered Ag(100), where Br forms a highly mobile lattice gas on the surface and the coverage increases with potential. Sulfur diffusion via hopping between neighboring sites of the (1x1) Ag substrate lattice was observed. Surprisingly, the diffusion rates increase with increasing potential, suggesting a different diffusion mechanism in the presence of low coverage halides. However, there is a wide potential gap between (1x1) and c(2x2) range, where quantitative Video-STM studies are difficult due to the high mobility of sulfur atoms at room temperature. Therefore, we also collected diffusion data at low temperature, which show the same potential dependence as the room temperature data, apart from a reduced mobility. These experiments allowed to show that the crossover between increasing and decreasing diffusion rates with potential indeed occurs at the phase transition between the disordered and the c(2x2) ordered Br adlayer, resulting in a maximum diffusion rate near the onset of Br adlayer ordering.In addition, the diffusion of CH3Sad on c(2x2)-Brad covered Cu(100) was studied to test the hypothesis that the inverted potential dependence (the diffusion rates increase with increasing potential in the ordered coverage range) of Sad diffusion on this surface is attributed to a Br-induced exchange diffusion [5]. The methyl group should inhibit the exchange of sulfur with a Cu surface atom and thus suppress an exchange mechanism for steric reasons. We therefore expect that the diffusion of CH3Sad on c(2x2)-Brad covered Cu(100) shows the same potential dependence as the diffusion of Sad and CH3Sad on c(2x2)-Clad covered Cu(100) [6-7], that is, a decrease of the diffusion rate with increasing potential.We acknowledge financial support by the Deutsche Forschungsgemeinschaft via project no. 504552981.