Physical and photoelectrochemical properties of the spinel ZnCr^sub 2^O^sub 4^ prepared by sol gel: Application to Orange II degradation under solar light

The physical and photoelectrochemical properties of the spinel ZnCr2O4 prepared by Sol Gel method are investigated. The thermal analysis indicates that the cubic spinel is formed beyond 500 °C with a lattice constant of 0.8328(3) nm. The Rietveld texture analysis indicates the absence of inter-granular residual stresses. The attenuated transmission reflectance confirmed the phase formation and forbidden band is found to be 1.85 eV. The valence and conduction bands have respectively Cr3+-t2g Cr3+-eg character. The magnetic curves M(H) of ZnCr2O4 exhibit a low hysteresis and the magnetization rises as the applied field up to 19 kOe with no saturation. The electrical properties of ZnCr2O4 indicate semiconducting behavior with activation energy of 43 meV while the thermo-power characterizes hole doped spinel. The capacitance measurement (C−2 - E) in Na2SO4 solution (0.1 M) shows p type conduction with a flat band potential (Efb) of −0.061 VSCE and a hole density (NA) of 2.7 × 1018 cm-3. The semicircle in the Nyquist plot is assigned to the bulk contribution (7700 kΩ cm2) with a depletion angle of 4° due to a constant phase element. As application, orange II (OII) is successfully oxidized on the spinel under solar and artificial lights. The conduction band is cathodic enough to generate radicals O2 which mineralizes OII. The best activity occurs at pH ∼7 under solar light (44%) and the process follows a first order kinetic with a half-life of 170 min. As expected, the COD removal efficiency increases, with irradiation time and reaches 20% after 1 h under solar light.