Oxygen adsorption on Zr(0 0 0 1) surfaces: Density functional calculations and a multiple-layer adsorption model

The adsorption of oxygen atoms on the Zr(0 0 0 1) surface is investigated by the use of ab initio total energy density functional methods within the generalized gradient approximation. Considering the repulsive interactions between the adsorbed oxygen atoms, a multiple-layer adsorption model (MLAM), in which the adsorbed oxygen atoms take the sites in different adsorption layers, was used. Our calculated results with a p(2 × 2) unit cell show that the surface face-centered cubic (SFCC) sites are the most favorable sites at the oxygen coverage of 0.25 monolayer (ML). However, the system with one oxygen adatom taking the SFCC sites and the other one taking the octahedral sites between second and third Zr layers (Octa(2,3)) is the most stable configuration at the coverage of 0.50 ML. As the oxygen coverage is increased to 1.0 ML, each of the oxygen atoms prefers taking the SFCC and octahedral sites such as Octa(2,3), Octa(4,5) and Octa(6,7) in the alternate layers, respectively. Our calculated results of the work functions with the MLAM successfully explain the interesting and unusual experimental work function changes upon oxygen adsorption. It should be noticed that the MLAM must be taken into account for oxygen adsorption on the Zr(0 0 0 1) surface.