Brownmillerite Ca₂Fe₂O₅: structural, optical and photoelectrochemical properties for efficient visible-light-driven...

The main objective of this study was to investigate the structural, optical, electrochemical, and photocatalytic properties of brownmillerite Ca₂Fe₂O₅ synthesized via the nitrate route. Comprehensive structural, morphological, and surface characterization experiments were performed using X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX), and X-ray photoelectron spectroscopy (XPS). The results confirmed the formation of phase-pure orthorhombic brownmillerite with a homogeneous elemental distribution and expected surface chemical states (Ca²⁺, Fe³⁺, and lattice oxygen). The material exhibited a relatively low specific surface area (∼8 m² g^-1). Diffuse reflectance spectroscopy revealed a direct optical band gap (E _g) of 1.92 eV, attributed to the d-d transition of the Fe³⁺: 3d orbital in 6-fold coordination, enabling efficient utilization of visible solar radiation, determined by diffuse reflectance spectroscopy. The capacitance-potential (C ^-2 vs. potential) characteristic in a Na₂SO₄ (0.1 M) solution revealed p-type conduction, with a flat band potential (E _fb) of 0.21 V. The current density-potential (J-E) profile demonstrates electrochemical oxygen intercalation. The semicircle in the Electrochemical Impedance Spectroscopy is ascribed to the bulk contribution. Electrical conductivity measurements revealed a thermally activated conduction mechanism governed by small-polaron hopping, with an activation energy (E _a) of 0.31 eV, indicating favorable charge transport within the brownmillerite structure. The conduction band potential, being more negative than the O₂/O₂˙^- redox level, supports the feasibility of photoinduced water reduction. Owing to its high chemical stability over a wide pH range (5-12), Ca₂Fe₂O₅ demonstrated efficient photocatalytic activity for the degradation of methylene blue under visible light, highlighting its potential for environmental remediation and sustainable photocatalytic applications.