Synergistic effect of band gap and surface area on the improvement of NiTiO3 sunlight-driven photocatalysts via NiTiO3@S nanocomposites

The performance of the NiTiO3 semiconductor under sunlight was improved by its modification with sulfur (NiTiO3@S nanocomposites) through the hydrothermal-solid state method. The NiTiO3@S nanocomposites exhibited better photocatalytic activity compared to pure NiTiO3. Evaluations by DRS analysis and Tauc plots showed a mild decrement in the band gap energy from 2.49 eV to 2.25 eV by adding 5–30 wt% sulfur. The improved photocatalytic performance of NiTiO3@S relative to pure NiTiO3 could be assigned to the increase of oxygen vacancies, specific surface area, and pore diameter, as confirmed by PL and BET analyses. The presence of oxygen defect state over the valance band below the conduction band and also S species (as S+6/S+4 ions) on the NiTiO3 surface (as co-catalyst) enhanced the sunlight-driven and the photogenerated separation of electron-hole pairs, which incremented the photocatalytic efficiency of NiTiO3@S nanocomposites. Experimental Crystal-violet dye degradation tests showed almost the same results for all synthesized NiTiO3@S nanocomposites. Investigation of the effect of pH unveils better degradation in alkaline environments rather than in acidic media. The NiTiO3@S-30 nanocomposite remains stable after four consecutive cycles of the degradation process. According to Kinetics studies, both NiTiO3 and NiTiO3@S-30 can be fitted with the pseudo-first-order (p.f.o) and pseudo-second-order (p.s.o) models (R2 ≃ 1). The degradation rate constant of NiTiO3@S-30 nanocomposites (Kp.f.o = 61 · 10−4 min−1 and Kp.s.o = 66 · 10−4 min−1) is higher than that NiTiO3 (Kp.f.o = 13 · 10−4 min−1 and Kp.s.o = 8 · 10−4 min−1).