A modified sol-gel synthesis to yield a stable Fe3+/ZnO photocatalyst: Degradation of water pollutants and mechanistic insights under UV and visible light

Abstract We synthesized pristine and Fe3+ doped ZnO photocatalysts with a modified sol-gel method, which included a peptization step. X-ray diffraction, scanning and transmission microscopy identified the crystalline nature of pure and doped ZnO having wurtzite (hexagonal) structure. UV–vis diffuse spectroscopy detected a red shift towards longer wavelengths upon incorporation of Fe3+ in ZnO, revealing that the Fe3+ ions replaced some of the crystal lattice Zn2+ ions. X-ray photoelectron spectroscopy demonstrated the coexistence of Fe2+ and Fe4+ ions in addition to the presence of Zn vacancies. Fe doping also reduces the dissolution of ZnO in acidic or alkaline solution, as well as its photocorrosion through self-oxidation. Moreover, the nano-oxides were stable at a pH of 7.5. The photocatalytic activity was tested to degrade methylene blue (MB, 12 mg/L, reaction volume of 2 L)) under UV and 4-chlorophenol (4-CP, 0.6 mg/L, reaction volume of 0.8 L) under Vis light. Fe0.8-ZnO had the highest activity, degrading 93 % MB (UV, 2 h) and 73 % 4-CP (Vis, 1 h), respectively. The increased activity ascribes to higher BET surface area (136 m2 g−1 vs 42 m2 g−1 for the control ZnO), surface hydroxyl groups, increased photons absorption at longer wavelength and decreased electron-hole recombination. Hydroxyl radicals (OH ) and superoxide anions (O2 -) were the key species responsible for the pollutant decomposition as the scavenging tests demonstrated. In the light of the observed characterization data and ROS (reactive oxidizing species) experiments, we propose a photocatalytic mechanism.