A systemic study on Gd, Fe and N co-doped TiO2 nanomaterials for enhanced photocatalytic activity under visible light irradiation

In this study, the gadolinium (Gd), iron (Fe), and nitrogen (N) doped TiO2 nanomaterials have been synthesized by a sol-gel method with slight modification. While the effect of Gd on the crystalline morphology, specific surface area, chemical composition, and bandgap of the doped samples have been investigated by SEM, TEM, BET, XRD, UV-vis, and XPS. The bonds of Ti-O-Gd, N-Ti-O, and O-N-Ti induced by Gd and N substitution inside the TiO2 crystal lattice structure have been investigated by XPS. The UV-vis spectra showed that the absorption edges of all doped samples are significantly red-shifted, further extending towards the visible spectrum, owing to the impurity band formation of the above valence band (VB) by doping of N. With the increase in the Gd doping amount, the photocatalytic efficiency of GFNTO (tri-doped samples) followed the sequence: G(2.0)FNTO G(2.5)FNTO > G(3.0)FNTO > G(1.5)FNTO > G(3.5)FNTO > G(1.0)FNTO. The first-order reaction constant of G(2.0)FNTO for photodegradation of methylene blue (MB) is ten times higher than that of un-doped TiO2. The significant improvement in the performance of the co-doped sample is due to the finer particle size, larger specific surface area, narrowed bandgap, more defect sites, and oxygen vacancies induced by the substitution of Ti4+ by N, Fe3+, and Gd3+. The recycling experiment showed that the photocatalytic efficiency of G(2.0)FNTO can still reach 95% after its five cycles, further exhibiting the excellent stability and re-usability.