Harnessing the synergistic advantages of functional modified (defect generated, Ni and S co-doped) TiO₂/NiS S-scheme photocatalyst and halloysite nanotube support for efficient photodegradation of several volatile organics

Efficient heterojunction (HJ) photocatalysts require precise energy-level alignment, rapid charge transfer, and stable interfaces. In this work, commercial P25-TiO₂ (P-T) was modified through defect generation, Ni/S co-doping, and coupling with nickel sulfide (NiS), followed by immobilization on halloysite nanotubes (HNTs) to form P-T(d, Ni,S)/NiS@HNT S-scheme heterojunctions. A simple three-step process involving chemical reduction, calcination, and impregnation was employed. Comprehensive characterization using ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), electrochemical impedance spectroscopy (EIS), and X-ray photoelectron spectroscopy (XPS) confirmed structural and electronic modifications. The optimized photocatalyst achieved complete (100 %) degradation of various volatile organic compounds (VOCs)—including propanol, toluene, trichloroethylene, and benzaldehyde—surpassing the performance of T(d,Ni,S)/NiS and T(d) systems. The enhanced activity arises from efficient charge separation and transfer through the S-scheme mechanism, as evidenced by Urbach tail analysis, flat-band potential shifts, and favorable band alignment. The prolonged charge-carrier lifetime of P-T(d,Ni,S)/NiS (∼14.65 ns) compared to P-T(d) (4.02 ns) further validates the S-Scheme mechanism. The HNT support enhanced photocatalytic efficiency through its tubular morphology, oppositely charged surfaces, and high surface area, ensuring uniform dispersion and strong interfacial coupling of active sites.