Photocatalytic activity of non-stoichiometric ZnFe₂O₄ under visible light irradiation

Nanostructured zinc ferrites with different excess iron contents (ZnFe ₂+zO₄, where z = 0.00, 0.05, 0.10 and 0.15) have been synthesized using the sol-gel auto-combustion method. The effect of excess iron on the structural, optical and visible light photocatalytic activity of zinc ferrite samples has been investigated. X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), x-ray magnetic circular dichroism (XMCD), Brunauer-Emmett-Teller theory, scanning electron microscopy (SEM), diffuse reflectance spectroscopy (UV-Vis) and photoluminescence spectroscopy were used to characterize the synthesized non-stoichiometric ZnFe₂O₄ powders. The XRD patterns demonstrated that the samples consist of single phase spinel structure with crystallite sizes of ∼25 nm. SEM analysis indicated that the nanosized particles grow together in porous clusters with a size of several microns. The XPS and XMCD analyses revealed that the excess iron ion substitutes Zn²⁺ in tetrahedral sites and in octahedral sites Fe ²⁺ can be found in addition to Fe³⁺, which could be created to restore the overall charge balance in the crystal lattice. Stoichiometric zinc ferrite (ZnFe₂+zO₄, z = 0.00) exhibited higher photocatalytic activity (40%) than the excess iron sample (ZnFe₂+zO₄, z = 0.15, i.e. 4%) under visible light irradiation for 3 h. This was explained by the formation of Fe²⁺ in the octahedral sites of excess iron ZnFe₂O₄; these species act as recombination centres.