Preparation and photoelectrochemical characterization of the Ca2Co2O5, as novel photocatalyst for the H2 photo-production

Ca2Co2O5 was successfully synthesized from nitrate precursors. It crystallizes in the brownmillerite structure and exhibits semiconducting properties. The oxide was analyzed by X-ray diffraction (XRD), TG/DTA thermal analysis, scanning electron microscope (SEM), UV–visible spectroscopy, and electrochemical impedance spectroscopy (EIS). The XRD pattern of a powder calcined at 750 °C shows a single phase of Ca2Co2O5, in agreement with the TG/DTA plots. A direct gap of 0.72 eV was revealed from the diffuse reflectance spectroscopy. The electrical conductivity follows an exponential lawσ = σo exp{-0.10 eV/kT}. The photoelectrochemistry was undertaken and assessed for the hydrogen formation under visible light. The p-type conduction was demonstrated from the Mott–Schottky plot (C−2 vs. the applied potential); a flat band potential (Efb) of 0.07 VSCE and a hole density (ND) of 5.36 × 1019 cm−3 were determined in Na2SO4 (0.1 M) solution. Ca2Co2O5 was employed with success for H2 production under visible-light irradiation, since the conduction-band potential (-0.63 VSCE) is cathodically located with respect to the H2O/H2 couple, at neutral pH. A liberation rate of 22 µmol g−1 min−1 and a quantum yield of 0.18% were achieved under a light flux of 29 mW cm−2.