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

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.