Ab-initio study of the adsorption of Cu on Pt(100)

Electrochemical and gas-phase deposition experiments show a conflicting dep endence of the adsorption geometries on the adsorption conditions for Cu on Pt(100). To clarify the experimental findings, the adsorption was investig ated by an ab-initio method. According to the calculated results, (1 x 1) a dsorption is favoured over the c(2 x 2) adsorption for the deposition from the gas phase as well as from electrolyte solution. It seems that, in the e lectrochemical experiments, adsorbate-size effects are responsible for the observed c(2 x 2) geometry. The calculated adsorption energy of 0.25 eV/Cu for c(2 x 2) coverage compares well with the experimentally measured underp otential. The atomic positions were relaxed for the topmost three layers. I t turns out that the relaxation energy of -0.45 eV gives a very important c ontribution to the adsorption energy of -0.25 eV of the system Cu c(2 x 2)/ Pt(100). Upon Cu adsorption the work function of Pt is decreased already fo r half a monolayer from 6.23 to 5.14 eV, corresponding to the work function of clean Cu(100). Furthermore, the differences of the surface energies of the clean surfaces of Cu (0.78 eV atom(-1)) and Pt (1.20 eV atom(-1)) indic ate wetting of the Pt surface by Cu.