First Principles Study of Nitrogen Doping at the Anatase TiO2(101) Surface

The incorporation of nitrogen impurities in the anatase TiO2(101) surface is investigated by first principles density functional theory calculations. Several substitutional and interstitial configurations and different concentrations for the N impurities in the surface and subsurface layers are considered, as well as their interactions with oxygen vacancies in the TiO2 lattice. The stability of the various investigated systems is compared on the basis of their formation energy as a function of the oxygen chemical potential, which determines whether the system is in an oxidizing or reducing environment. Under oxygen-rich conditions, N bound to a surface O is preferred, whereas, under oxygen-poor conditions, substitutional N together with oxygen vacancies is favored, as previously found for bulk TiO2. The cost of formation of a surface oxygen vacancy is almost cancelled in the presence of N impurities in subsurface layers. The incorporation of nitrogen in the lattice modifies the electronic structure by introducing localized states in the band gap, consistent with the experimentally observed absorption of N-doped anatase samples in the visible region. Also, these N impurity states are excellent traps for the Ti3+ electrons deriving from surface oxygens vacancies. STM images for the various N-doped anatase (101) surface models have been computed to provide a reference to future experimental studies of this surface.