INSITU X-RAY ABSORPTION-SPECTROSCOPY STUDY OF UNDERPOTENTIAL DEPOSITION OF COPPER ON PLATINUM (100)

X-Ray absorption spectroscopy (XAS) spectra have been recorded for the electrochemical adsorption of Cu on Pt(100) in the fluorescence detection mode at grazing incidence, with polarization parallel to the surface. During data collection, the Pt(100) surface was under potentiostatic control in a 10(-4) M Cu2+ electrolytic solution, in order to maintain a monolayer coverage. The near-edge structure of the copper K-edge resembles that of Cu+. The Fourier transform of the EXAFS spectra shows a first peak at 0.15 nm, and a broad unresolved peak with maximum at 0.30 and a shoulder at 0.37 nm. The first single peak is associated with oxygen neighbours at 0.197 nm. The unresolved peak is built up from contributions of copper first and second neighbours. The distance of first copper neighbours is 0.309 nm, and that of second copper neighbours is 0.393 nm, equal to the substrate lattice parameter (0.3929 nm). This suggests that a first copper adlayer of half monolayer density is in registry with the platinum unreconstructed (100) surface net. A second copper adlayer of the same density lies over the first adlayer, at a distance of 0. 1 35 nm. The copper atoms in this second layer are in the fourfold geometry with respect to the underlying copper half-monolayer. These layers form a c(2 x 2) periodicity on the Pt (100) substrate. The copper-oxygen distance is about half the second copper-copper distance. The sharing of oxygen between the two copper adlayers is unclear.