Green-synthesized Fe-doped TiO₂/ZnO heterojunction nanophotocatalyst for efficient visible-light-driven gas-phase...

In this study, a green-synthesized Fe-doped TiO₂/ZnO heterojunction nanophotocatalyst was successfully fabricated via a combined sol-gel/hydrothermal method using pomegranate peel extract as a natural reducing and capping agent. The optimized catalyst (TiO₂/ZnO = 70 : 30 w/w, 3 mol% Fe, 50 mL extract) exhibited a mesoporous structure with a high surface area (85 m² g^-1), reduced crystallite size (∼18 nm), and enhanced visible-light absorption due to Fe³⁺-induced defect states and oxygen vacancies. Photocatalytic performance was evaluated for gas-phase toluene degradation in a continuous-flow quartz reactor under visible LED irradiation (420-700 nm, 30 mW cm^-2). The optimized sample (TZFA1) achieved 84% removal efficiency at 50 ppm within 120 min, following pseudo-first-order kinetics (k _app ≈ 0.014 min^-1, R ² > 0.98). Reactive oxygen species trapping tests showed that O₂ ^-˙ plays the dominant role in toluene degradation, with additional contributions from ˙OH and h⁺, supporting the proposed photocatalytic mechanism. The enhanced performance is attributed to the synergistic effects of efficient charge separation via type-II TiO₂/ZnO heterojunction, Fe³⁺/Fe²⁺ redox cycling, and improved surface properties induced by green modification. Systematic optimization of compositional and operational parameters confirmed the robustness of the system under varying conditions. The catalyst also demonstrated acceptable reusability over five cycles with moderate activity loss. These findings highlight a sustainable and efficient platform for practical VOC removal in air purification applications.