Underpotential deposition of Cu on Pt(001): Interface structure and the influence of adsorbed bromide

Using in situ x-ray diffraction, we studied the underpotential deposition (UPD) of copper onto a Pt(001) electrode both in pure perchloric acid and in the presence of bromide anions. In pure perchloric acid, the Cu is deposited in pseudomorphic p(1x1) islands. In the presence of bromide anions, the strong Pt-Br interaction significantly broadens the potential range of Cu UPD. We propose that Pr remains in the interface region throughout the UPD process, at first in a disordered Cu-Br phase and then, at more negative potential, forming a c(2x2) closed-packed monolayer on top of the completed p(1X1) Cu monolayer. The structures are compared to those found during Cu UPD onto Pt(111), and explained in terms of the metal-halide interactions and the Pt surface atomic geometry.