Improved visible light photocatalytic activity of fluorine and nitrogen co-doped TiO₂ with tunable nanoparticle size

Graphical abstract: - Highlights: • The particle sizes of the prepared samples were tunable. • The economic titanium sulfate (Ti(SO₄)₂) was used as precursor. • Co-doped catalyst exhibited high photocatalytic activity. • The synergistic effect of the two nonmetal elements was investigated. • Potential degradation mechanism was proposed. - Abstract: Fluorine and nitrogen co-doped TiO₂ (F-N-TiO₂) photocatalysts with enhanced photocatalytic activities were facilely synthesized by a simple one-step hydrothermal method using Ti(SO₄)₂ as an economical precursor, and hydrofluoric acid and ammonia as F and N source, respectively. The structure, morphology, and optical properties of produced nanoparticles were characterized by X-ray diffraction (XRD), N₂ adsorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectra (FT-IR) methods. The synergistic effects of F and N doping were systematically examined by changing the molar ratio of F/N. Compared with the un-doped F or N mono-doped TiO₂, the co-doped samples exhibited significantly improved photocatalytic performance due to their synergistic effects under visible light. It was shown that F dopant promoted the crystal growth and crystallinity of samples, while N dopant hindered it to some extent, which resulted in the tunable particle size of obtained F-N-TiO₂ materials. The effects of F and N dopants on the enhanced photocatalytic activity of modified TiO₂ materials were also discussed. The degradation rate of methylene blue (MB) was achieved at 97.31% after 5 h reaction under visible light over the optimized sample of FN_3.5T. The materials also showed excellent stability according to the recycling tests of the photodegradation of MB