Effect of valence band energy on the photocatalytic performance of N-doped TiO2 for the production of O2 via the oxidation of water by visible light

• N-doped TiO2 were used for production of O2 via the oxidation of water by visible light.
• The migration of h+ to the surface leads to the evolution of O2 and the formation of OH radicals.
• Eg for TiO2−xNx is optimized to 2.8 eV for the evolution of O2 under the irradiation by visible light.

Five types of N-doped TiO2 (TiO2−xNx, x = 0, 0.005, 0.015, 0.03, and 0.06) nanoparticles were prepared and evaluated for use as a catalyst for production of O2 via the oxidation of water by irradiation with visible light. The resulting materials were characterized using ultraviolet diffuse reflectance spectroscopy (UV-DRS), electrochemical impedance spectroscopy (EIS), and photoluminescence (PL) techniques. The results indicate that a change in band gap energy (Eg) as a function of N-doping level in the materials resulted in a change in the valence band (VB) level, but not the conduction band (CB) level. The very small upward shift in VB enhanced the photocatalytic formation of OH radicals produced by irradiation with visible light. In addition, the small amount N-dopant (up to 3 at%) incorporated into the samples accelerated the evolution of O2, rather than the formation of OH species. The incorporation of larger amounts of N-dopant in the TiO2−xNx reduced the rates of both O2 evolution and OH formation, due to the small energy level of VB and the short life time of the photo-generated charges. The performance of TiO1.97N0.03 nanoparticles with an Eg value of 2.8 eV was outstanding for the evolution of O2 from the water under the experimental conditions used.

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