Green synthesis of Zn-doped TiO2 nanomaterials for photocatalytic degradation of crystal violet and methylene blue dyes under sunlight

In this study, anatase titanium dioxide (TiO₂) and 7 mol% zinc-doped TiO₂ nanoparticles (Zn-TiO₂ NPs) were synthesized using a low-cost green method based on Salvadora persica leaf extract. The NPs were characterized using Fourier transform infrared spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface area, photoluminescence, energy dispersive X-ray spectrometry (EDS), ultraviolet–visible spectroscopy, and thermogravimetric analysis. The XRD patterns indicated a pure anatase structure with crystallite size of 18.80 and 8.85 nm, TEM suggested a wide distribution of particle sizes with spherical and faceted shapes, EDS confirmed the target elemental composition of the catalyst, and BET revealed type IV isotherms with meso- and macro-pores structures, and specific surface area of 23.540 and 34.449 m²/g, respectively. The incorporation of Zn into the TiO₂ structure has reduced crystal growth, thereby decreasing both the bandgap of TiO₂ (from 3.20 to 3.14 eV) and the electron–hole recombination rate. The photocatalytic activity was tested under sunlight against nitrogenous crystal violet and methylene blue dyes. Importantly, both TiO₂ and Zn-TiO₂ have a high potential for dye degradation. The experimental data demonstrated performance enhancement of the photocatalyst after doping. In all cases, degradation data was better fit the pseudo-first-order kinetic model with a higher reaction rate observed for the doped catalyst and against methylene blue. More interestingly, even after four cycles, the catalyst’s stability and activity remained unchanged, supporting their applicability as efficient photocatalysts against organic pollutants.