Facile fabrication of novel magnetic 3-D ZnFe₂O₄/ZnO aerogel based heterojunction for photoreduction of Cr(Ⅵ) under visible light: Controlled synthesis, facial change distribution, and DFT study

A novel magnetic type Ⅱ ZnFe₂O₄/ZnO aerogel (ZZA) heterojunction photocatalyst is firstly developed for photocatalytic reduction of Cr(Ⅵ) to Cr (III). The optimal ZZA-400 shows typical “pearl-like” nanostructures, with particle diameters at around 20–30 nm. It possesses a large BET specific surface area of 127.5 m²/g and an appropriate bandgap of 1.9 eV. It exhibits an efficiency of 87.0 % for photocatalytic Cr(VI) reduction within 120 min under extremely high Cr(VI) concentration (100 mg/L), with the rate constant up to 7.08 × 10⁻³ min⁻¹. Density functional theory (DFT) calculations show that the incorporation of ZnFe₂O₄ (ZFO) introduces an impurity energy level in the gap region of ZnO (ZO), which contributes to the optical absorption in the visible light region. It is energetically favorable for the electrons' migration from ZnO to ZnFe₂O₄ due to the lower Fermi energy level of ZnFe₂O₄. However, the electrons migrate from ZnFe₂O₄ to the conduction band of ZnO with the assistance of the built-in electric field under visible light illumination, therefore, the separation of electron-hole pairs can be greatly promoted. The combination of formed [rad]O₂^–, the electrons in the conduction band of ZnO, and the reductive [rad]CO₂^– radical together enhance the photocatalytic property of the aerogel heterojunction.