Elaboration, physical and photo-electrochemical properties of NiCr2O4. Application to hydrogen production under visible irradiation

Abstract

The physical and photoelectrochemical characterization of NiCr2O4, prepared by sol gel route, were investigated to be applied for the H2 production. The thermal gravimetry (TG) indicates that the single phase is formed above 530 °C as confirmed by X-ray diffraction (XRD). The Nano powder crystallizes in a tetragonal structure with lattice constants: a = 8.3276 Å and c = 8.5542 Å and a particle size of 63 nm, smaller than that obtained by Transmission Electronic Microscopy (TEM) analysis; the latter gives sizes between 80 and 150 nm, indicating crystallites agglomeration. The variation of the dielectric constant (ε) with temperature gives a relative value of 26 at 10 kHz. A direct optical transition at 1.79 eV is determined from the diffuse reflectance spectroscopy assigned to Cr3+ octahedrally coordinated. The thermal variation of the conductivity shows that 3d-electrons are localized and the data are modelled by a lattice-polaron hopping with an activation energy of 0.17 eV. The dependence of the interfacial capacitance on the potential (C−2 - E) indicates p-type behavior with a flat band potential (Efb) of −0.23 VSCE and holes density (NA) of 5.88 × 1016 cm−3. The potential of the conduction band (−1.85 VSCE) is below the H2O/H2 level (∼-1.2 VSCE), allowing a spontaneous H2-release under visible light. The O2 evolution occurs at high over-voltage as shown from the intensity-potential (J-E) characteristic in Na2SO4 solution (0.1 M) and a hole scavenger was used to preclude the photo corrosion. The NiCr2O4 mass, pH and the hole scavenger (S2O32− and NO2−) were optimized. The H2 volume reached 65 μmol with an evolution rate of 8.6 μmol g−1 min−1, liberated under optimized conditions {1.2 g catalyst L−1, pH ∼9 with thiosulfate S2O32− [10−3 M]}.