Structural Disorder of ZnFe2O4 Spinel controlling Oxygen Vacancy and Photocatalytic Activity

Abstract:

Spinel ferrite minerals widely distributes in supergene environments of solid planets. With two kinds of cations over tetrahedral and octahedral sites, the relationship between their complicated cation configuration and versatile physicochemical properties has not been fully resolved yet. In this study, spinel ferrite samples (Zn_1-xFe_x)[Zn_xFe_2-x]O₄ (x=0, 0.03, 0.06, 0.15, 0.20) were prepared at selected annealing temperature. With increasing temperature up to 1200ºC, ZnFe₂O₄ becomes disordered and its lattice parameter (a=b=c) descends from 8.4407 to 8.4320 Å with the increase of inversion coefficient (x) from 0 to 0.20. Cation-oxygen polyhedrons ZnO₆ and FeO₄ appear since 600ºC, which bring about three new Raman bands at 299-317 (F_2g mode), 482-484 (F_2g mode) and 690-696 cm^-1 (A_1g mode). Moreover, the amount of oxygen vacancy and adsorbed hydroxyl species on ZnFe₂O₄ surface show linear increase with increasing x (R²=0.943 and 0.997, respectively).

The disordered structure and oxygen vacancy both enhance the optical absorption of ZnFe₂O₄, which exhibits red-shift absorption edges in tens of nanometers and additional impurity band in 700-900 nm. Both the spectroscopic measurements and density functional theory calculations clearly indicate the valence band edge keeps unchanged while the conduction band edge gradually goes lower as ZnFe₂O₄ turns disordered. In addition, the bandgap of ZnFe₂O₄ is first proved to linearly decrease from 2.08 to 1.95 eV (R²=0.971) when x increases from 0 to 0.20, suggesting its controllability and dependency on structural disorder. Furthermore, two impurity levels in oxygen-defective ZnFe₂O₄ cause dual fluorescence signals at 670 nm and 740-850 nm. Compared with normal ZnFe₂O₄, the disordered ZnFe₂O₄ with narrower bandgap, more oxygen vacancies and more adsorbed hydroxyl species is able to produce more hydroxyl radical (•OH) under simulated solar light.

Now that natural spinel-type minerals always occur with structural disorder and lattice defects, their capability of converting solar to chemical energy form may be underestimated before. The structural disordering degree of spinel-type minerals controls the concentration of oxygen vacancy, configuration of electronic structure and thus the photocatalytic activity, which endow the natural spinel ferrite minerals with surprising chemical reactivity on the surface of solid planets.