Photoinduced formation of defects and nitrogen stabilization of color centers in n-doped titanium dioxide

Second-generation photocatalysts based on the metal oxide TiO2 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 TiO2 to longer wavelengths, thereby increasing the photocatalytic efficacy based on total UV and visible light absorbed relative to pristine nondoped TiO2 which can only absorb UV radiation. This article examines the optical behavior (diffuse reflectance spectroscopy) of a nitrogen-doped TiO2 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 < I < 900 nm) of the N-doped TiO2 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 (Ti3+ 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 TiO2 under UV radiation and by the photobleaching of the photoinduced defect states by red light ( lambda > 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.