Metal-metal interfacial bonding: monolayer c(2 × 2) Cu on a Pt(001) surface

First steps in understanding from first principles the underpotential electrodeposition of a metal (Cu) onto the surface of another metal (Pt) are reported. Results of local density full potential linearized augmented plane wave calculations for a slab consisting of five layers of Pt(001) with a Cu monolayer of c(2 × 2) geometry on top of each surface are presented. Minimization of the total energy as a function of the distance between the uppermost Pt layer and the Cu layer yielded a shrinkage of the CuPt nearest-neighbor distance by 13%. This result illustrates the very strong bonding between the Cu adsorbate and the Pt(001) substrate in agreement with measurements of electrochemical underpotentials. A comparison of the density of states (DOS) of the overlayer system with the DOS of an isolated slab of Pt and the DOS of a single Cu monolayer shows that energetically the Cu d-states coincide with the Pt surface states. The analysis of the bonding charge density shows a strong build up of charge in-between the CuPt neighbors, a smoothening of the corrugation, and also an increase of the Pt interlayer bonding. The nature of the chemical bonding between adsorbate and substrate atoms is examined with a discussion of the charge distribution of several special occupied electronic states.