Hydrogen evolution under visible light over the solid solution NiFe2−xMnxO4 prepared by sol gel

• The work deals with the system NiFe2−xMnxO4 (0 ≤ x ≤ 2) for the hydrogen production.
• The solid solution NiFe2−xMnxO4 is prepared by sol gel.
• The system NiFe1.6Mn0.4O4/S2O32− shows a volume V(H2) of 124 μmol after 20 min.
• The energy diagram shows the optimal feasibility of hydrogen evolution at pH ∼9.4.

The continuous solid solution NiFe2−xMnxO4 (0 ≤ x ≤ 2) prepared by sol–gel is successfully tested for the hydrogen production under visible light. All spinels crystallize in a cubic structure with a particle size ranging from 30 to 63 nm. They exhibit p-type conductivity with an average thermopower of ∼135 μV K−1, a hole mobility of ∼9 × 10−5 cm2 V−1 s−1 and an activation energies in the range (0.12–0.33 eV). The optical transition of the most active photocatalyst NiFe1.6Mn0.4O4 (1.53 eV) is well matched to the solar spectrum. The chemical and physical properties are correlated to assess the feasibility of the spinels for the hydrogen liberation. The conduction band (−1.54 VSCE) is located below the H2O/H2 potential (∼−0.8 VSCE), allowing H2-evolution under visible light. The spinel dose, pH and S2O32− concentration are optimized. Under the ideal conditions, the volume of H2 liberation reaches 124 μmol at saturation after 20 min of irradiation.