Elaboration and characterization of the brownmillerite Ca2Fe2O5 synthesized by citrate sol-gel method. Application for photocatalytic H2-Production under visible light over Ca2Fe2O5/ZnO hetero-system

Abstract

The present work is devoted to the synthesis of the ferrite Ca2Fe2O5 as photocatalyst crystallizing in the brownmillerite structure. The ternary oxide is prepared by sol-gel auto combustion and characterized by physical and electrochemical methods. The thermal analysis (TG/DSC) shows that, the formation of the brownmillerite is observed above 660 °C. The X-ray diffraction and BET analysis show respectively a single phase with an active surface area of ∼6 m2 g−1. The SEM micrographs exhibit an inhomogeneous structure formed by agglomeration of irregular shaped grains, confirmed by the laser granulometry analysis. The forbidden band (∼2.3 eV) determined from the diffuse reflectance, permits to explore ∼ 30% of the sun spectrum into chemical energy. The p-type comportment of Ca2Fe2O5 is demonstrated by the capacitance-potential (C−2 - E) graph with a flat band potential (Efb = 0.93 VSCE), due to oxygen over-stoichiometry. The negative potential of the conduction band (−1.06 VSCE) predicts the feasibility of the H2 generation. Indeed, Ca2Fe2O5 is chemical stable in a wide pH domain and is positively experimented as photocatalyst for the H2-production under visible light. The best performance is obtained in alkaline medium (NaOH, 0.1 M) with a mean evolution rate of 18 μmol g−1 min−1. However, Ca2Fe2O5 coupled to ZnO sol-gel (ZnO-SG) improves the catalytic performance. The H2 evolution rate over (Ca2Fe2O5/ZnO-SG) reached 24 μmol g−1 min−1 after 60 min. It has also been shown that ZnO–P, prepared by precipitation, is more efficient than that synthetized by sol-gel method (ZnO-SG) and TiO2–P25.