Synthesis of Fe-doped ZnO by supercritical antisolvent precipitation for the degradation of azo dyes under visible light

Abstract

Supercritical antisolvent precipitation was used to obtain ZnO (undoped) and Fe-doped ZnO photocatalysts at different Fe/Zn molar ratio percentages (0.57, 0.75 and 0.84 mol%). Wide-angle X-ray diffraction showed the hexagonal wurtzite as the main crystalline phase of the cauliflower-like nanoparticle aggregations (observed from Field Emission Scanning Electron Microscopy) with a mean diameter of 54.5 nm for the pure ZnO nanoparticles and a higher mean diameter (in the range 109.1–121.5 nm) for Fe-doped ZnO nanoparticles. XRF confirmed the approximate Fe/Zn ratios (0.55, 0.77, 0.82 mol%). The band gap energy, derived from UV − Vis diffuse reflectance measurements decreased upon Fe doping with values ranging from 3.22 (ZnO) to 2.83 eV (0.57 mol%Fe). The photocatalytic activity results showed that the undoped ZnO photocatalyst completely removes color from an aqueous solution containing Acid Orange 7 dye (AO7) after 30 min of UV light irradiation. Whereas Fe-doped ZnO photocatalyst with 0.75 mol% Fe content (0.75Fe) exhibited the highest discoloration efficiency (52 %) and total organic carbon (TOC) removal efficiency (∼36 %) of AO7 after 180 min under visible light. Moreover, the main reactive oxygen species involved in the AO7 degradation were assessed with the optimized photocatalyst (0.75Fe) in presence of scavenger molecules. The experimental results revealed that positive holes and superoxide are responsible for target dye degradation under visible light.