Video-STM-Untersuchungen der Diffusion von Adsorbaten in in elektrochemischer Umgebung
The surface diffusion of adsorbates at the electrochemical interface between electrolytes and electrodes plays an important role in many phase transfer processes, e.g. the formation of ordered adsorbate layers, galvanic deposition and electrocatalytic reactions. It is well-known that transportation processes at electrochemical interfaces depend on the electrode potential. The present work gives a detailed view on the elementary process of the tracer diffusion of individual adsorbates on single-crystal electrode surfaces. The diffusion of isolated sulfide adsorbates in the c(2x2)-Cl adsorbate lattice of a Cu(100) electrode in 0.1 mM HCl was investigated as a function of temperature and electrode potential using an in-situ Video-STM. The statistical analysis of the Video-STM sequences of isolated sulphur atoms and its dimers shows that the jump rates Gamma exhibit an exponential dependence on the electrode potential phi at constant temperature. Gamma proportional exp(-alpha x phi/kB x T) with alpha = 0.5 · eV/V Further temperature-dependent investigations at constant electrode potentials show that the thermally activated adsorbates jump to next neighbouring place, with a potential-indepent attempted frequency. In total, the diffusion barrier Ed can be represented as sum of a constant term E0 (with respect to Reference electrode) and a linear term alpha · phi dependent on the potential alpha, so that Ed = E0 + alpha x phi. This can be explained on the basis of a simple model, in which this effect is caused by the change of the dipole moment of the adsorbate during diffusion and a contribution of the co-adsorbates.