Formation of well-ordered surface compounds by coadsorption of thallium and bromide on the Au(111) electrode surface

The structures of the adlayers formed on Au(111) in acid solution containing thallium and bromide have been investigated by using in situ surface X-ray diffraction. At the most positive potentials, where Br forms close-packed rotated-hexagonal monolayers, the surface is free of Tl. The Br adlayer disorders at the potential where thallium starts to coadsorb. With further decreasing potential, two well-ordered commensurate and one weakly-ordered incommensurate phases appear over a 0.7 V potential region prior to the formation of the close-packed rotated-hexagonal Tl monolayer, The structure factor analysis for the commensurate phases shows the formation of TlBr2 and TlBr surface compounds in the 3-(root 13 x root 13) and 2-(3 x root 5) phases, respectively. In addition, an adlayer-induced subsurface relaxation has been observed. Under the 2TlBr-(3 x root 3) adlayer, where bromide ions are at the bridge sites and thallium ions are situated near the hollow sites, the top layer of Au atoms displaces laterally along the root 3 direction. This results in more uniform spacings between T1 and the three neighboring gold atoms, as well as an increase of the Br-Au separation. (C) 1998 Elsevier Science B.V. All rights reserved.