Hydrogen production under visible illumination on the spinel NiMn2O4 prepared by sol gel

The spinel NiMn2O4 prepared by sol-gel is applied for the hydrogen liberation upon visible light. The oxide crystallizes in a cubic symmetry with a crystallite dimension of 63 nm, smaller than that obtained by transmission electron microscopy (TEM; 470 nm), indicating crystallites agglomeration. p-type conductivity is demonstrated by the positive thermo power (+130 µV at 300 K), with a hole mobility of 2.09 × 10−4 cm2 V−1 s−1 and an activation energies of 70 meV. The optical gap (1.13 eV), attributed to 3d orbital splitting, is determined from the diffuse reflectance and is appropriately matched to the sun spectrum. The physical and chemical characterizations are correlated to show the feasibility of NiMn2O4 for the hydrogen formation. The capacitance measurement gives a flat band potential of −0.20 VSCE and a hole density of 0.29 × 1016 cm−3. The potential of the conduction band (−1.26 VSCE) is below that of H2O/H2 level (∼−0.40 VSCE), allowing H2-liberation upon visible irradiation. The NiMn2O4 mass, pH and concentration of the hole scavenger S2O32− are optimized. Under optimized conditions, the H2volume liberation reaches 77 µmol with rate of 0.024 µmol mg−1 min−1.