Structural disorder controlled oxygen vacancy and photocatalytic activity of spinel-type minerals: A case study of ZnFe2O4

Spinel ferrite minerals widely distribute in supergene environments. This study mainly focus on investigating the relationship between the crystal chemistry (structural disordering, oxygen vacancy) and semiconducting property (optical absorbance, band structure, photocatalytic activity) of spinel ferrites (Zn1-xFex)[ZnxFe2-x]O-4 (x = 0, 0.03, 0.06, 0.15, 0.20). With the increase of inversion coefficient (x) from 0 to 0.20, ZnFe2O4 becomes disordered and its lattice parameter descends from 8.4407 to 8.4320 angstrom. New polyhedrons ZnO6 and FeO4 appear since x = 0.03, which induce three new Raman bands at 299-317 (F-2g((2))), 482-484 (F-2g((3))) and 690-696 cm(-1)(A(1g)), respectively. The amount of oxygen vacancy and adsorbed hydroxyl species on ZnFe2O4 surface show linear increase with increasing x (R-2 = 0.943 and 0.997, respectively), as indicated by electron paramagnetic resonance and X-ray photoelectron spectroscopy. Both the spectroscopic analysis and density functional theory calculations suggest the valence band edge of ZnFe2O4 keeps unchanged as the structure goes disorderly, while its conduction band edge gradually goes lower. Therefore, the bandgap linearly decreases from 2.08 to 1.95 eV (R-2 = 0.971) when x increases from 0 to 0.20. Furthermore, two impurity levels are produced in oxygen-defective ZnFe2O4 and cause dual fluorescence signals at 670 and 740-850 nm. Compared with normal ZnFe2O4, the disordered ZnFe2O4 produces more hydroxyl radical (center dot OH) under the same solar irradiation conditions. In summary, the structural disordering degree of spinel ferrites controls the concentration of oxygen vacancy, configuration of electronic structure and thus the photocatalytic activity, which endow natural bulk spinel ferrite minerals with surprising chemical reactivity on the surface of solid planets.