Underpotentially deposited thallium on silver (111) by in situ surface x-ray scattering

Using in situ surface x-ray scattering, we have investigated the atomic structure, the stability, and the dependence of the structure on electrode potential for electrochemically deposited Tl monolayers and bilayers on Ag(111). The layers were formed by underpotential deposition (UPD) at electrode potentials positive of the reversible potential for bulk Tl deposition. At potentials between -475 and -680 mV (versus Ag/AgCl), the Tl deposit forms an incommensurate, hexagonal two-dimensional (2D) monolayer that is compressed relative to bulk Tl by 1.4–3.0 % and rotated from the Ag [011¯] direction by Ω=4°–5°. The structure of the monolayer does not change over at least 24 h (the longest we waited). From diffraction scans of the Tl Bragg rods, we find that the in-plane and vertical root-mean-square displacement amplitudes are 0.36±0.1 and 0.46±0.1 Å, respectively.

The monolayer structure is the same as that of vapor-deposited Tl/Ag(111), and this shows that the interaction between the solvent molecules and the Tl adatoms does not influence the monolayer structure. Since the monolayer has a structure that is about the same as the closest-packed planes of bulk Tl, we deduce that the adatom-adatom interactions are the most important structure-determining forces. The compression of the monolayer (compared to bulk Tl) is explained in terms of effective-medium theory. With decreasing electrode potential, the in-plane spacing between Tl adatoms decreases and this permits a calculation of the 2D compressibility. This decreases with atomic spacing, but has an average value ${\mathrm{\kappa }}_{2\mathrm{D}}$=1.54±0.10 A${\mathrm{̊}}^2$/eV, which is similar to previously measured compressibilities of UPD monolayers and is in reasonable agreement with theoretical estimates. The rotation angle Ω depends on electrode potential and adatom spacing, but irreversibly decreases with potential cycling (which is possibly due to the adsorption of trace impurities). Despite this irreversibility, the dependence of Ω on adatom spacing qualitatively agrees with theory. We have investigated the structure of monolayers where the deposition potential is reached in either an anodic or cathodic scan and found these structures to be identical. This shows that the monolayer is in thermodynamic equilibrium and that the finite width and offset of the peaks in the deposition curves are due to kinetics, the influence of adsorbed anions, and/or substrate heterogeneity. At potentials between the monolayer region and bulk deposition, Tl forms a bilayer and this also has a hexagonal structure that is incommensurate with the Ag(111) substrate. In the bilayer, the compression is 1.0% (compared to bulk Tl) and the rotation from the Ag [011¯] direction is 3.9°; these are both less than in the monolayer.