Physical and photo-electrochemical properties of NiCr₂O₄/ZnO: application to the photo-oxidation of OII under solar light

The photoelectrochemical characterization of the spinel NiCr₂O₄ synthetized by sol gel route was investigated. The thermal analysis showed that the single phase is formed above 530 °C as confirmed by X-ray diffraction. The direct optical transition (1.79 eV), determined from the diffuse reflectance, is assigned to the lifting degeneracy of Cr³⁺: 3d³ orbital octahedrally bonded to O²⁻ ions. Field-dependent magnetization, measured at 300 K in the range (± 20 kOe), shows a maximum magnetization of ~ 0.16 emu/g with no saturation and a weak coercivity of 67 Oe. The thermal variation of the electrical conductivity indicates localized 3d-electrons and the data are fitted by a low lattice-polaron hopping with an activation energy of 0.17 eV, characteristic of a semiconducting behavior. A flat band potential (E_fb) of − 0.23 V_SCE with p-type nature was deduced from the variation of the capacitance (C⁻²) on the potential (E) of the junction NiCr₂O₄/Na₂SO₄. The p-type conduction is corroborated by the positive thermo-power (S_300K = 123 µV K⁻¹), supporting the non-degenerate semi-conductivity. The electron transfer from the conduction band (− 1.85 V_SCE) to dissolved oxygen is predicted from the potential diagram, constructed from the physic-chemical properties; this allows the oxidation of Orange II (O II) upon solar light. Similarly, ZnO enhanced the photoactivity by 68% with loading the spinel as sensitizer. O II is mineralized by O₂^•− radicals at pH ~ 7 within 1 h on the heterosystem NiCr₂O₄ (60%)/ZnO for O II concentration of 20 mg L⁻¹. The removal color obeys a pseudo first order kinetic with a rate constant of 0.023 min⁻¹ (t_1/2 = 23 min).