Highly Efficient Low-Temperature N-Doped TiO₂ Catalysts for Visible Light Photocatalytic Applications

In this paper, TiO₂ prepared with an aqueous sol-gel synthesis by peptization process is doped with nitrogen precursor to extend its activity towards the visible region. Three N-precursors are used: urea, ethylenediamine and triethylamine. Different molar N/Ti ratios are tested and the synthesis is adapted for each dopant. For urea- and trimethylamine-doped samples, anatase-brookite TiO₂ nanoparticles of 6-8 nm are formed, with a specific surface area between 200 and 275 m²·g^-1. In ethylenediamine-doped samples, the formation of rutile phase is observed, and TiO₂ nanoparticles of 6-8 nm with a specific surface area between 185 and 240 m²·g^-1 are obtained. X-ray photoelectron spectroscopy (XPS) and diffuse reflectance measurements show the incorporation of nitrogen in TiO₂ materials through Ti-O-N bonds allowing light absorption in the visible region. Photocatalytic tests on the remediation of water polluted with p-nitrophenol show a marked improvement for all doped catalysts under visible light. The optimum doping, taking into account cost, activity and ease of synthesis, is up-scaled to a volume of 5 L and compared to commercial Degussa P25 material. This up-scaled sample shows similar properties compared to the lab-scale sample, i.e., a photoactivity 4 times higher than commercial P25.