THE EFFECT OF HALIDES ON THE STRUCTURE OF COPPER UNDERPOTENTIAL-DEPOSITED ONTO PT(111) - A LOW-ENERGY ELECTRON-DIFFRACTION AND X-RAY PHOTOELECTRON-SPECTROSCOPY STUDY

The influence of chloride and bromide ions on the underpotential deposition of copper onto Pt(111) in sulphuric acid solutions was studied ex situ by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). It is demonstrated that the growth behaviour of the copper monolayer is drastically altered when Cl or Br- is added to the electrolyte. The halides form densely packed, incommensurate (4 x 4) and (7 x 7) structures respectively, on the full copper monolayer (which itself is in registry with the substrate) before they arrange themselves in a more open structure at more negative potentials. Furthermore, it is shown that both halides have a marked influence on the cyclic voltammogram of underpotential deposited copper, which renders a reliable evaluation of the copper coverage from charge measurements impossible. The correct isotherm for copper on Pt(111) in the presence of Cl- is evaluated from XPS data.