Oxidation of the Nb(1 1 0) surface by ab initio calculations

We report ab initio (FPLAPW) results referring to the early stages of oxygen deposition on the Nb(1 1 0) surface. We considered the cases of half and full monolayer depostion and we evaluated the structural and electronic properties of the resulting overlayers. From the calculated electronic strctures, it came out that the oxygen atoms form stronger bonds with the Nb surface atoms in the [0 0 1] than in the [ 1   1 ¯   0 ] direction, a fact that implies also shorter bond lengths in the former direction and different structural relaxations in the neighbouring Nb atoms. In addition, we found that the Nb–O bonding, which is attributed to d–p hybridization of double bond-like character, results in the introduction of new electronic states that lay well below the lowest states of the clean Nb face, while it also contributes into the increase of states around the Fermi level. Interestingly, the Nb–O bonding type along the preponderated direction yields free oxygen pz orbitals that can be regarded as dangling-like bonds. These results elucidate the early stages of Nb oxidation and can be used to enlighten the mechanisms of surface reactions, catalytic activity and oxides growth mechanisms.