Synthesis of visible light-driven Eu, N co-doped TiO2 and the mechanism of the degradation of salicylic acid

Europium and nitrogen co-doped TiO₂ was successfully synthesized by the precipitation–peptization method. The structure and properties of the catalysts were characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and UV–vis diffuse reflectance spectra. The photocatalytic efficiency was evaluated by monitoring the photocatalytic degradation of salicylic acid under visible light irradiation. It was verified that TiO₂ co-doped with nitrogen and 1% europium showed the highest photocatalytic activity. The adsorption isotherms were obtained by measuring the salicylic acid concentration before and after the dark adsorption at different original solution concentrations. The results illustrated that the doping of Eu was beneficial to the adsorption of salicylic acid. The probable degradation mechanism of salicylic acid was examined by the addition of NaF, Na₂S₂O₃, and K₂S₂O₈ as the probe molecules. It was verified that salicylic acid was first adsorbed on the surface of the catalysts, followed by the degradation by the photogenerated holes (h _vb ⁺ ).