WATER-INTERFACE STRUCTURES ON Cu(111) ELEC-TRODES. I. K. Robinson, Y. S. Chu and A. A. Gewirth, University of Illinois, Urbana IL 61801, USA

We have investigated the structure of Cu(111)/water interfaces using in-situ X-ray reflectivity and crystal-truncation-rod (CTR) diffraction. A thin-layer geometry and synchrotron radiation were used to obtain monolayer sensitivity. Careful electrochemical and contamination control were required to stabilize the interface structure during the time needed to complete the measurements.

We will first present the results of an experiment in which a monolayer of Pb was introduced on the electrode by underpotential deposition. This monolayer was found to be hexagonal, aligned with the substrate, but incommensurate. The Pb in-plane lattice parameter was found to change with potential, showing electro-compression. However the numerical value of the compressibility was different both from values

observed on Ag(111) or Au(111) and from the value expected for an ideal 2D metal. The distance of the monolayer from the substrate was found to have little potential dependence.

Our second experiment looked at X-ray reflectivity data for the clean Cu(111) electrode in 0.1M perchloric acid at different potentials within its range of stability (between dissolution and hydrogen-evolution potentials). Substantially different curves were observed at -0.60V vs Ag/AgCl (within a narrow region close to the potential for hydrogen evolution), and at -0.05V vs Ag/AgCl, characteristic of most of the potential range. The change was well correlated with the cyclic voltammetry, which displayed a well-defined cathodic peak near the hydrogen evolution potential too. The difference in the reflectivity could be explained by fitting of these curves to simple layered models of the Cu/water interface, leading to the conclusion that oxygen becomes specifically bound to the electrode over most of the potential range. Since our experiments cannot detect the presence of H, it seems plausible that the bound species is OH, rather than just O.