Preparation and characterization of the brownmillerite Sr2Co2O5 as novel photocatalyst in the hydrogen generation

Sr2Co2O5 is a semiconductor belonging to the brownmillerite family; it is prepared by nitrate route and the photo-electrochemical properties are assessed for the first time for the photocatalytic hydrogen production. Thermal analysis indicates the formation of the semiconductor phase at 750 °C. An optical transition at 1.10 eV, directly allowed is obtained from the diffuse reflectance spectrum, due to the internal Co3+: d-d transition in octahedral coordination. A flat band potential of 0.037 VSCE is determined in KOH solution (0.1 M) from the Mott-Schottky characteristic and the results are relevant for the water reduction. The conduction band of Sr2Co2O5 (−0.85 VSCE), deriving from Co3+: 3d orbital is more cathodic than the potential of H2O/H2 couple and hydrogen is successfully evolved under visible light. A rate evolution of 68 µmol (g catalyst)−1 min−1 at pH ∼ 12 and a light-to-chemical energy efficiency of 0.82% are determined.