In-situ surface x-ray scattering of metal monolayers adsorbed at solid-liquid interfaces

Abstract

This paper discusses experiments experiments on electrochemically deposited monolayers of Pb, Tl, and Bi on Ag and Au (111) electrodes. Tl and Pb form 2-D, incommensurate hexagonal solids that are compressed relative to bulk and rotated by 4 - 5 ° with respect to the substrate. As the applied electrode potential decreases, the in-plane atomic spacing also decreases, since the chemical potential of the monolayer increases. From these data, the 2-D compressibility of the monolayer can be calculated. We find that the compressibility is only slightly dependent on substrate, being smaller on Ag(111) than on Au(111). For Tl/Ag(111), the intensity of the Ag surface diffraction changes when the monolayer is adsorbed. This results from a substrate- induced modulation of the atomic positions in the incommensurate monolayer and we have quantified this modulation. Bi/Ag(111) forms an unusual structure: a rectangular lattice that is uniaxially commensurate with the hexagonal surface. This unusual structure reflects the tendency toward covalent bonding found in Bi and a fortuitous match between the atomic spacings of the Ag substrate and the close packed planes of bulk Bi. In contrast to Tl and Pb where the compressibility is isotropic, Bi/Ag(111) compresses anisotropically and this maintains the uniaxially commensurate structure. Our results show that for these metal monolayer systems the adatom-adatom interactions determine the atomic structure of the monolayer and the adatom-substrate interactions only weakly affect this structure. Furthermore, the structure is not influenced by the presence of the large concentration of adsorbed water molecules or anions.