Electrochemical surface science: The study of monolayers of ad-atoms and solvent molecules at charged metal interfaces

Abstract

In parallel with the spectacular advances that have been made in recent years in the field of metal/vacuum surface science, there have been corresponding interesting developments of a complementary kind in electrochemical surface science. This article describes recent advances in the latter field, and their experimental and theoretical bases. However, the molecular situation at electrode interfaces is very complex compared with that at a metal/vacuum interface, owing to adsorption and orientation of solvent molecules, and the presence of solvated ions. Hence these topics must be considered as well as the surface science of the metal electrode itself. In comparison with metal/vacuum surface science, electrochemical studies on metal interfaces can utilize the electrode potential as an additional independent variable and thus control surface charge density, coverage by Faradaically deposited ad-atoms, co-adsorption of cations or anions and solvent dipole orientation. Modulations of electrode potential in time allow the kinetic relaxation characteristics of monolayers to be followed, and the potential-modulated reflectivity and spectral absorption of species in the interphase to be studied. Electrochemically controllable sub-monolayer coverages of, e.g., metal ad-atoms have interesting catalytic effects on other continuous electrochemical reactions, such as oxygen reduction or formic acid oxidation. Experiments based on electrochemical potential pulse and sweep, and current pulse methods enable chemisorption of ad-atom monolayers to be studied in fine detail. Multiple state chemisorption in monolayers is commonly observed, even on single crystal surfaces, and fine resolution of the states can be achieved by electrochemical techniques. The kinetics of monolayer deposition and desorption can also be followed, as well as the dynamics of reconstruction processes e.g. in 0 monolayers as a function of time and potential. The electrochemical methods allow direct determination of the differential isotherm for adsorption as well as the usual integral one. The former gives details of multiple state adsorption, and provides information on interaction effects in the monolayer and kinetics of its formation. Ionicity of ad-species can also be evaluated. Recent advances in combining electrochemical systems in situ with high-vacuum LEED, Auger and ESCA apparatus are described. Since electrochemical surface science has necessarily to be able to study processes at the interfaces of metals in contact with liquid solutions, an important area of the field has been the treatment and evaluation of solvent dipole adsorption and orientation as a function of electrode potential or surface charge density.