Synthesis and characterization of N-doped TiO2 photocatalysts with tunable response to solar radiation

Abstract

• N-doped TiO 2 photocatalysts have been synthesized by annealing TiO 2 powder under flowing NH 3 . • Absorption at λ > 400 nm increases monotonically with increase of synthesis temperature. • The decrease of the effective band gap is due to the creation of localized states above the VB of TiO 2 . • The electronic structure of TiO 2 can be tailored to absorb light in the visible and NIR regions. Modification of the electronic structure of wide band gap semiconductors by anion doping is an effective strategy for the development of photocatalytic materials operating under solar light irradiation. In the present work, nitrogen-doped TiO 2 photocatalysts of variable dopant content were synthesized by annealing a sol–gel derived TiO 2 powder under flowing ammonia at temperatures in the range of 450–800 °C, and their physicochemical and optical properties were compared to those of undoped TiO 2 samples calcined in air. Results show that materials synthesized at T = 450–600 °C contain relatively small amounts of dopant atoms and their colour varies from pale yellow to dark green due to the creation of localized states above the valence band of TiO 2 and the formation of oxygen vacancies. Treatment with NH 3 at T > 600 °C results in phase transformation of anatase to rutile, in a significant decrease of the specific surface area and in formation of TiN at the surface of the TiO 2 particles. The resulting dark grey ( T = 700 °C) and black ( T = 800 °C) materials display strong absorption in both the visible and NIR regions, originating from partial reduction of TiO 2 and formation of Ti 3+ defect states. The present synthesis method enables tailoring of the electronic structure of the semiconductor and could be used for the development of solar light-responsive photocatalysts for photo(electro)chemical applications.