Origin of Photoactivity of Nitrogen-Doped Titanium Dioxide under Visible Light

Abstract

Nitrogen-doped titanium dioxide (N-TiO2), a photocatalytic material active in visible light, has been investigated by a combined experimental and theoretical approach. The material contains single-atom nitrogen impurities that form either diamagnetic (Nb-) or paramagnetic (Nb*) bulk centers. Both types of Nb centers give rise to localized states in the band gap of the oxide. The relative abundance of these species depends on the oxidation state of the solid, as, upon reduction, electron transfer from Ti3+ ions to Nb* results in the formation of Ti4+ and Nb-. EPR spectra measured under irradiation show that Nb centers are responsible for visible light absorption with promotion of electrons from the band gap localized states to the conduction band or to surface-adsorbed electron scavengers. These results provide a characterization of the electronic states associated with N impurities in TiO2 and, for the first time, a picture of the processes occurring in the solid under irradiation with visible light.

“Black” TiO2 Nanotubes Formed by High-Energy Proton Implantation Show Noble-Metal-co-Catalyst Free Photocatalytic H2-Evolution

“…This feature becomes even more visible under illumination. This Ti 3+ center can be described by orthorhombic g values [1.990 1.929 1.909], with a considerable distribution in the g strain contribution which is significantly different from the typically observed Ti 3+ centers found in the reduced titania [36,37].Overall the PL and EPR findings strongly support that a main electronic effect of p + -implantation is the creation of paramagnetically active recombination states that are close to the band gap of the anatase. Such features were also observed for high pressure hydrogenated anatase, where they have been identified as a key indicator for establishing intrinsic H 2 evolution ability in TiO 2 [20,21].…”

“…This feature becomes even more visible under illumination. This Ti 3+ center can be described by orthorhombic g values [1.990 1.929 1.909], with a considerable distribution in the g strain contribution which is significantly different from the typically observed Ti 3+ centers found in the reduced titania [36,37].…”

Influence of the chemical synthesis on the physicochemical properties of N-TiO2 nanoparticles

“…1) and can be described in terms of interstitial nitrogen into the TiO 2 lattice. This species is the photoactive centre as proved on the basis of irradiation experiments showing that its EPR spectral intensity varies when irradiating the sample at a wavelength corresponding to the maximum of the optical absorption in the visible region [8].…”

“…These centres (labelled as N i O • ) are paramagnetic and were identified via Electron Paramagnetic Resonance (EPR) spectroscopy [7][8][9].…”

Unraveling the Cooperative Mechanism of Visible-Light Absorption in Bulk N,Nb Codoped TiO2 Powders of Nanomaterials

“…[8][9] Actually, these factors control the material microstructure, that in turn determines the concentration and nature of both extrinsic and intrinsic point defects. As for the latter, it is known that oxygen vacancies, VO •• , whose formation is favored by N doping 10 , might reduce neighboring Ti ions and determine the formation of new gap states just below the conduction band (CB). It is believed that such defect-induced states could play a crucial role in enhancing the visible-light activity of N-doped TiO2.…”