Hydrogen production under visible illumination on NiFe_0.8Mn_1.2O₄ synthesized by sol–gel processing

The spinel NiFe_0.8Mn_1.2O₄ synthesized by sol–gel processing was successfully exploited for the hydrogen evolution reaction (HER) under visible light. XRD studies indicated that the oxide crystallizes in a spinel structure with a crystallite size of 51 nm, smaller than that observed by transmission electron microscopy (TEM, 465 nm), indicating an agglomeration process. p-type conduction is indicated by a positive thermoelectric power (S_300K = +134 µV K⁻¹), with an activation energy of 290 meV. The direct band gap (E_g = 1.10 eV), arising from the lifting of degeneracy of the 3d orbital in (Fe, Mn)O₆ octahedra, was obtained from the diffuse reflectance, ideal for the exploitation of the visible solar spectrum. The correlated physical and electrochemical properties predict the feasibility of NiFe_0.8Mn_1.2O₄ for HER from an energy diagram. The “Capacitance –Potential” characteristics yield a hole concentration (N_A) of 2.8 × 10¹⁵cm^-3 and a flat band potential (E_fb) of −30 mV_SCE. The cathodic conduction band (−0.84 V_SCE) is ideally positioned relative to the H₂O/H₂ couple (~ −0.48 V_SCE), facilitating HER upon visible light irradiation. The optimization of NiFe_0.8Mn_1.2O₄ dose, pH and S₂O₃^2- concentration has been undertaken, giving a maximum HER volume of 73 µmol with an evolution rate of 8.1 µmol g⁻¹ min⁻¹.