UPD of Cu on Pt(100): effects of anions on adsorption isotherms and interface structures

The results presented in this study show definitely that copper deposition in perchloric acid is a very slow, kinetically controlled process. In nearl y halide free supporting electrolytes du appeared to be deposited at underp otential in metallic islands (or 'patches') having the platinum lattice con stant. The presence of Br- anions significantly enhances the kinetics of th e Cu UPD on the Pt(100) electrode. The total amount of Cu deposited at unde rpotential is ca. 1 ML (1 ML = 1 Cu adatom per Pt atom). The results obtain ed from the surface X-ray diffraction measurements showed the formation of an ordered structure of Pr into a c(2 x 2) unit cell on the top of a p(1 x 1) pseudomorphic layer of Cu. In a solution free of Cu no ordered structure of Pr has been found on Pt(100). RRDE and SXS results indicate beyond any doubt that the surface coverage by Br undergoes only negligible changes upo n the deposition of Cu even up to a nominal monolayer. We propose, therefor e, that the mechanism of Cu UPD occurs by displacement of bromine adatoms f rom the Pt surface by Cu adatoms through a 'turn-over' process in which Cu is sandwiched between the Pt surface and the Brad overlayer, i.e. a Pt(100) -Cu-Br bi-layer structure. (C) 1998 Elsevier Science Ltd. All rights reserv ed.