N doping of TiO ₂(110) Photoemission and density-functional studies

The electronic properties of N-doped rutile TiO ₂(110) have been investigated using synchrotron-based photoemission and density-functional calculations. The doping via N ₂ ⁺ ion bombardment leads to the implantation of N atoms (∼5% saturation concentration) that coexist with O vacancies. Ti 2p core level spectra show the formation of Ti ³⁺ and a second partially reduced Ti species with oxidation states between +4 and +3. The valence region of the TiO _2-xN _y(110) systems exhibits a broad peak for Ti ³⁺ near the Fermi level and N-induced features above the O 2p valence band that shift the edge up by ∼0.5 eV. The magnitude of this shift is consistent with the "redshift" observed in the ultraviolet spectrum of N-doped TiO ₂. The experimental and theoretical results show the existence of attractive interactions between the dopant and O vacancies. First, the presence of N embedded in the surface layer reduces the formation energy of O vacancies. Second, the existence of O vacancies stabilizes the N impurities with respect to N ₂(g) formation. When oxygen vacancies and N impurities are together there is an electron transfer from the higher energy 3d band of Ti ³⁺ to the lower energy 2p band of the N ^2- impurities.