N Doping of Rutile TiO₂ (110) Surface. A Theoretical DFT Study

A realistic model, consisting of six-layer slabs, and density-functional calculations were used to perform a detailed analysis of the structural and electronic properties of N doped TiO 2 (110). All the positions examined for adsorption of atomic N are unstable regarding the formation and further escape of N 2 (g). The adsorption of atomic N could take place only when having isolated adatoms. In this case, N prefers to bond to O centers located either on the surface or in the interstitial channels of the oxide lattice. These N adatoms probably give rise to the peak seen at ∼400 eV in N 1s XPS spectra. The coexistence of N with O vacancies and surface reconstructions are explained in terms of cooperative behaviors and the special electronic structure of the TiN x O 2-2 x (110) system. Here, electrons move from the O vacancies to implanted N to fill up its electronic shell. Such electron transfer yields the normal oxidation state of nitrogen, N 3-, and explains a number of things: the easiness to form O vacancies when implanted N is present, the easiness to implant N when O vacancies are present, and the difficulty for the implanted N to escape. It is not likely that N-doping will improve the photocatalytic behavior of TiO 2- x (110) surfaces. For these compounds the band gap will be always equal or larger than that of pure stoichiometric TiO 2 .