Enhanced photocatalytic reduction of Cr(VI) by the novel hetero-system BaFe2O4/SnO2

Abstract

In this study, BaFe2O4 nanoparticles with a crystallite size of 86 nm were produced using the sol–gel method and applied in the photoreduction of chromate. The spinel phase was identified by X-ray diffraction and scanning electron microscopy-energy-dispersive X-ray analysis. The structure was refined in orthorhombic symmetry by Rietveld analysis. A forbidden band (Eg) of 2.03 eV was extracted from the diffuse reflectance data and the optical transition was directly permitted. The thermal dependence of the electrical conductivity (σ) followed an exponential law with an activation energy of 0.23 eV. In addition, photo-electrochemical characterization of the spinel was conducted to establish the energy diagram for the hetero-system BaFe2O4/SnO2/HCrO4− solution in order to assess the interfacial reaction mechanism. Similar to most spinels, BaFe2O4 exhibits p-type behavior with a flat band potential (Efb) of 0.52 VSCE. Therefore, the potential of the conduction band (ECB) is sufficiently negative to be involved in an electronic injection with SnO2. The photoluminescence spectrum obtained for the rutile contained an intense peak at 590 nm and its intensity was substantially reduced in the hetero-system. Electrochemical impedance spectroscopy detected a depressed arc assigned to a constant phase element and its resistance (1.64 kΩ cm2) decreased to 1.05 kΩ cm2 under visible light, thereby confirming the semiconductor behavior. The reduction of Cr(VI) was successfully achieved with the BaFe2O4/SnO2 hetero-system under visible light as an application. Photocorrosion was prevented by using oxalate (C2O42−) as a scavenger to captures the holes and improve the photo-activity. The quasi-total chromate reduction had a reduction efficiency of 99% with a concentration of 30 mg L−1 under a light flux of 16 mW cm−2. The kinetics followed a first-order model with a photocatalytic half-life of 38 min and the results were fitted in an appropriate manner to the Langmuir–Hinshelwood model. Moreover, BaFe2O4/SnO2 exhibited photocatalytic efficacy for five successive runs and the final reduction efficiency was still 92%.