IN-SITU ATOMIC-STRUCTURE OF UNDERPOTENTIALLY DEPOSITED MONOLAYERS OF PB AND TL ON AU(111) AND AG(111) - A SURFACE X-RAY-SCATTERING STUDY

We describe in-situ surface X-ray scattering measurements of the electrochemical underpotential deposition (UPD) of Tl and Pb monolayers on Au(111) and Ag(111) with the aim of understanding the factors that influence the atomic structure and compressibility of these systems. We find that the UPD deposits form incommensurate, hexagonal monolayers that are compressed compared to the bulk metals and rotated from the substrate [011 $$($) over bar] direction by several degrees. These structures are similar to those found for the vapor-deposited analog and for the closest-packed planes of the bulk phases. In these systems (as for other incommensurate UPD systems), the in-plane spacing between adatoms decreases with decreasing electrode potential, and from such data the monolayer compressibility (kappa(2D)) is calculated. Values of kappa(2D) are tabulated for these four systems, for Bi on Au(111) and Ag(111), and for a simple model of kappa(2D); we discuss the trends evident in this table. To understand the differences in the voltammograms of Pb/Ag(111) in acetate and perchlorate, the monolayer structures of these systems were determined. The same compressed, incommensurate, hexagonal monolayer was found, and at any given potential, the near-neighbor spacings were nearly identical. Our results show that for these incommensurate UPD systems, as for Bi on Ag(111) and Au(111), (i) the adatom-adatom interaction is the primary force determining the monolayer structure, (ii) although the adatom-substrate interaction is strong and results in the formation of the monolayer, it only influences the monolayer structure slightly, and (iii) the presence of the large concentration of water molecules or adsorbing anions in contact with the monolayer does:not affect its structure. For UPD Pg/Ag(111) our results show that kinetics and not thermodynamics causes some of the observed differences in the voltammograms.