Photoinduced Formation of Defects and Nitrogen Stabilization of Color Centers in N-Doped Titanium Dioxide

Second-generation photocatalysts based on the metal oxide TiO 2 and doped with various anions (e.g., N, S, and C) and cations have recently been the object of intense scrutiny as a result of the red-shift of the absorption edge of TiO 2 to longer wavelengths, thereby increasing the photocatalytic efficacy based on total UV and visible light absorbed relative to pristine nondoped TiO 2 which can only absorb UV radiation. This article examines the optical behavior (diffuse reflectance spectroscopy) of a nitrogen-doped TiO 2 specimen and explores the photoinduced formation of defects when the N-doped specimen is subjected to oxidative (O 2 ) and reductive (H 2 ) stresses relative to vacuum. The resulting absorption spectrum in the visible spectral region (400 nm < λ < 900 nm) of the N-doped TiO 2 consists of overlapping single absorption bands, each one of which reflects absorption by the constituent color centers. Kinetics of formation and accumulation of these color centers (Ti 3+ centers) have been assessed. The electron nature of the color centers has been unraveled by the effect(s) that hydrogen and oxygen have on the photocoloration of TiO 2 under UV radiation and by the photobleaching of the photoinduced defect states by red light (λ > 610 nm). A model is described that pertains to the stabilization of such color centers by the azide anions through a defect charge compensation effect. Different mechanisms prevail for the physical relaxation of the electronic subsystem and for the chemical pathways when the N-doped metal oxide is subjected to UV-light or to visible-light irradiation.