Nitrogen-Doped Titanium Dioxide Active in Photocatalytic Reactions with Visible Light: A Multi-Technique Characterization of Differently Prepared Materials

Nitrogen-doped TiO2 materials were successfully prepared following three different preparation routes (sol−gel, mechanochemistry, and oxidation of TiN) and characterized by X-ray diffraction, electron microscopy, and various spectroscopic techniques. All samples absorb visible light, and the one obtained via sol−gel, showing the anatase structure, is the most active in the decomposition of organic compounds under visible light. Various nitrogen-containing species have been observed in the materials, whose presence and abundances depends on the preparative route. Ammonium NH4 + ions are residual of the synthesis using ammonium salts (sol−gel, mechanochemistry) and are quite easily eliminated, as shown by the parallel behavior of both NMR and XPS spectra. Cyanide CN− ions form at high temperature in parallel with the phase transition of the solid to rutile. Molecular nitric oxide forms in the case of materials exhibiting close porosity. The already reported bulk radical species, Nb •, is the only paramagnetic center observed in all types of samples, and is responsible for the visible light sensitization of TiO2. A mechanism for the formation of such a species in chemically prepared N-doped TiO2 materials is for the first time proposed based on the reduction of Nitric Oxide (NO) at oxygen vacancies