Influence of Pr3+ doping on the synthesis of colloidal sols and nanoparticulate TiO2 xerogels and their photocatalytic activity

Newtonian nanoparticulate TiO2 sols without and with Pr3+ doping (0.5, 1, 2, 3, 5, and 10 mol%) were synthesized by the aqueous colloidal sol-gel method, next elucidating that the Pr3+ doping increasingly retards their peptization while makes them more viscous at high doping contents. Subsequently, undoped and Pr3+-doped TiO2 xerogels were obtained by drying the colloidal sols, demonstrating that they crystallize as tiny nanocrystals of anatase with some brookite, with the Pr3+ cations in substitutional solid solution within the TiO2 crystal lattice. It was also shown that moderate calcination (i.e., at 400-500 °C) causes only a slight nanocrystal growth, and a minor transformation to rutile in the xerogels with lowest Pr3+ doping proportions (i.e., <1 mol%). Finally, a preliminary study of the photocatalytic activity under ultraviolet-visible light irradiation of the undoped and Pr3+-doped TiO2 xerogels calcined at 400 °C for 10 min was performed. The xerogel doped with 0.5 mol% Pr3+ and calcined at 400 °C ranks first and that those with Pr3+ doping proportion above 1 mol% behave worse than the undoped one. Also, calcination at 500 °C was found to worsen the photocatalytic activity. The photocatalysis observations were rationalized on the basis of the dependence of the polymorph composition, crystal size, bandgap, and specific surface area as a function of the dopant content.