Aurivillius type BiCrVOx -Catalyzed Photocatalytic degradation of methylene blue in aqueous media under visible- light irradiation

Abstract

This work reports on a newly developed Aurivillius-type BiCrVO_x series (Bi₂Cr_xV1-_xO_5.5-x, 0.05 ≤ x ≤ 0.20), focusing on its phase stability and photocatalytic behavior under visible light irradiation. The samples were synthesized via an ethylene glycol–citrate sol–gel method, followed by microwave-assisted calcination. X-ray diffraction (XRD) and differential thermal analysis (DTA) indicated the stabilization of three crystalline phases, α-monoclinic, β-orthorhombic, and γ-tetragonal, at room temperature. XRD refinement revealed that an increase in chromium content led to lattice expansion owing to the substitution of smaller V⁵⁺ ions (0.54 Å) with larger Cr³⁺ ions (0.755 Å). The direct band gap energy (Eg), determined by UV–Vis spectroscopy, achieved values ranging from 1.64 to 2.68 eV. The photocatalytic performance was assessed through the degradation of methylene blue (MB) under visible- light irradiation with λ > 400 nm. The highest performance was observed for the γ-tetragonal phase at x = 0.15, with an apparent first-order rate constant (k_app) of 13.22 × 10^-3 min^-1. The enhanced activity is attributed to band gap narrowing caused by disordered oxygen vacancies at equatorial sites. In contrast, a decline in performance at x = 0.20 is likely due to vacancy ordering associated with apical oxygen positions. Overall, the results demonstrate that BiCrVO_x is an effective photocatalyst for the removal of recalcitrant organic pollutants from industrial wastewater. 68