Hydrogen evolution under visible light over LaCoO3 prepared by chemical route

Abstract

The semiconducting properties of the perovskite LaCoO3, prepared by nitrate route, are investigated for the first time by the photo-electrochemical technique. The oxide shows a direct optical transition at 1.33eV, due to Co3+: 3d orbital splitting in octahedral site and possesses a chemical stability over a fair pH range (4–14). The conductivity follows an exponential type law with a hole mobility (8.3×10−2cm2V−1s−1), thermally activated. The Mott–Schottky plot in KOH medium is characteristic of p type conduction with a flat band potential of 0 VSCE and a holes density of 1.35×1017cm−3. The electrochemical impedance spectroscopy reveals the predominance of the bulk and grains boundaries contributions with a constant phase element and a multi-relaxation type nature. As application, the hydrogen evolution upon visible light is demonstrated on the hetero-junction LaCoO3/SnO2. The best performance occurs at pH∼12.8 with an evolution rate of 0.25cm3min−1 (mg LaCoO3)−1 and a quantum yield of 0.11%. The improved activity is attributed to the wide depletion width of ∼10nm and the potential of the conduction band of LaCoO3 (−1.34VSCE), more negative than that of SnO2, the latter acts as electrons bridge for the interfacial water reduction. The relevance of 3d orbital of the performance of semi conducting photoelectrode is discussed.