X-ray and Electrochemical Studies of Cu UPD on Au(111) Single-Crystal Electrodes in the Presence of Bromide

Abstract

The underpotential deposition (UPD) of copper on Au(111) in the presence of bromide anions has been investigated employing electrochemical techniques, in-situ grazing incidence X-ray diffraction (GIXD), and crystal truncation rod (CTR) measurements. At potentials positive of +0.55 V (vs Ag/AgCl), bromide adsorbs on the electrode surface forming an ordered and rotated hexagonal structure. The rotation angle of this structure with respect to the Au(01) direction and the Br−Br distance change with the applied potential. At potentials below +0.55 V, the bromide overlayer becomes disordered although its coverage remains high. The initial stages of copper deposition (at ca. +0.36 V) lower the gold's work function, giving rise to an increase in the bromide coverage and the formation of an ordered bromide layer with a coverage of 0.51. At +0.32 V there is a sharp voltammetric feature that corresponds to a phase transition to give a (4 × 4) bromide layer with a coverage of 0.563. Over the potential range from +0.32 to ca. +0.20 there are two voltammetric features associated with additional copper deposition, giving rise to a stoichiometric CuBr layer. At +0.147 V there is another sharp voltammetric feature which, again, is ascribed to a phase transition giving rise to a (1 × 1) copper layer with a bromide (4 × 4) structure adsorbed over the copper layer. At this potential, the distances between the bromide and copper layers are similar to those found in solid CuBr. At potentials negative of +0.147 V, bulk copper deposition begins.