In Situ Video-STM Studies of Adsorbate Dynamics at Electrochemical Interfaces

The dynamic behavior of individual adsorbates at electrochemical interfaces was studied directly by in situ high-speed scanning tunneling microscopy, using sulfur adsorbed on Cu(100) electrodes in 0.01 M HCl solution as an example. By dosing from diluted Na2S solutions S-ad coverages of a few percent can be prepared, with the sulfur adsorbates occupying positions within the c(2x2) lattice of coadsorbed chloride. S-ad tracer diffusion occurs via hopping between neighboring c(2x2) lattice sites at considerably higher rates than those of sulfur on Cu(100) under UHV conditions, indicating a pronounced influence of the electrochemical environment on the adsorbate surface dynamics. The diffusion barrier linearly increases by 0.5 eV per V with potential and is strongly affected by neighboring S-ad and surface defects. The S-ad-S-ad interactions extend over approximate to 7 angstrom. They are repulsive between nearest-neighbor and attractive between next-nearest-neighbor sites, respectively, and result in significantly reduced diffusion barriers. S-ad on the upper terrace side of steps are transiently trapped and exhibit lower diffusion rates, leading to the formation of small metastable p(2x2) domains. Attractive interactions between S-ad and domain boundaries in the c(2x2) adlayer result in boundary pinning as well as transient trapping and enhanced diffusion of S-ad along the boundary.