Physical and photo-electrochemical properties of the hetero-junction ZnCo2O4/ZnO prepared by nitrate route: Application to photodegradation of Indigo Carmine

The semi conducting and photoelectrochemical properties of prepared ZnCo2O4 by nitrate route are investigated. The X-ray diffraction pattern of the red compound indicates a spinel type structure crystallizing in a cubic symmetry (a = 8.0989 Å). The direct optical transition of 2.10 eV, measured by diffuse reflectance spectroscopy, is assigned to unfilled Co3+: 3d orbital splitting in octahedral field (t2g → eg). The electrical conduction occurs by small lattice polaron hopping with an activation energy of 0.19 eV and a hole mobility µ300K (=3.5 × 10−4 cm2 V−1 s−1), thermally activated. ZnCo2O4 exhibits a chemical stability over the pH range (4 and 12) and a flat band potential (Efb) of 0.12 V, was deduced from the capacitance – potential (C−2 – E) plot. The positive slope characteristics p type conduction with a hole density of 1.6 × 1017 cm−3 due to mixed valences Co3+/2+. The energy of the conduction band (2.96 eV / −1.79 VSCE) makes the spinel attractive for the photo-oxidation of dyes by O2− radicals. The powder is successfully tested in the photo degradation of Indigo Carmine (10 10 mg/L) over the hetero-junction upon visible and solar lights. The best performance of the hetero- junction (ZnCo2O4 / ZnO) is obtained with (10 %/90 %) percentage with an abatement of 93 %, larger than the spinel alone (49 %) under sunlight. The kinetic follows pseudo-first order with a half-life of 23 min.