RELATING THE IN-SITU, EX-SITU, AND NON-SITU ENVIRONMENTS IN SURFACE ELECTROCHEMISTRY

The relationship between ultrahigh vacuum surface science and electrochemistry is examined by comparison of so-called non-situ and ex-situ experiments performed in the vacuum environment with in-situ electrochemical experiments. Preadsorbed ClO4, on Ag(110) may be hydrated by post-adsorbed water and lifted off the surface as a hydrated complex at 170 K. This result directly illustrates the electrochemical concepts of anion desorption and nonspecific adsorption and can be explained by analogy to electrochemistry. For ex-situ studies three models exist to describe the key step of removing the electrode from the electrolyte (emersion): ideal, superequivalent, and dynamic. Ideal emersion obtains upon satisfying the criteria of (1) a 1:1 relationship of emersed work function with emersion potential and (2) zero charge transfer upon emersion. These criteria can be tested by Kelvin probe measurements of the work function in vacuum and re-immersion charge transient measurements in the electrochemical cell, respectively. Emersion of Pt(111) from 0.1M HClO4, + 1mM Cu2+ exhibits ideal emersion at potentials greater than 0.7V(RHE) and superequivalent emersion, so called because superequivalent adsorption of ClO4-, and Cl- establishes a constant work function, at emersion potentials less than 0.6V(RHE). The Pb/Pt(111) system exhibits dynamic emersion behavior, characterized by a surface redox reaction between Pb-0 and Pb2+ that discharges the double layer after emersion. Theoretical relationships among the non-situ, ex-situ, and in-situ methodologies are also briefly reviewed.