The structure of adsorbed bromide concurrent with the underpotential deposition (UPD) of Cu on Pt(111)

The adsorption of bromide anions concurrent with the underpotential deposition (UPD) of Cu on Pt(111) was examined utilizing ex-situ LEED and in-situ surface X-ray scattering for structure determination, in combination with coverage determination by rotating ring disk flux measurements with the Pt(111) single crystal as the disk electrode. The results show definitively that Cu UPD on Pt(111) in the presence of bromide is a two-step process, with the total amount of Cu deposited at underpotentials ∼ 0.95 ± 5% ML (1 ML=1 adatom per Pt atom). The results also indicate that the surface coverage by adsorbed bromide undergoes only a small (<0.05 ML) change upon the deposition of Cu, even up to a nominal monolayer. We propose a model wherein the first stage of deposition occurs by displacement of the close-packed hexagonal layer of Br adatoms by Cu adatoms through a "turn-over" process, in which Cu is sandwiched between the Pt surface and the Br overlayer, leading to the formation of an ordered Pt(111)-Cu-Br bilayer intermediate phase which closely resembles the (111) planes of the Cu(I)Br crystal. The coverage of both Cu and Br in this intermediate phase is ∼0.5 ML. The second stage is the filling-in of the Cu plane of the bilayer to form a pseudomorphic (1×1) Cu monolayer and a disordered Br adlayer with a coverage of ∼0.4 ML. The same mechanism is suggested for Cu UPD on Pt(111) in solutions containing chloride anions.