Photocatalytic degradation of Rhodamine B dye over oxygen-rich bismuth oxychloride Bi24O31Cl10 photocatalyst under UV and Visible light irradiation: Pathways and mechanism

Photocatalytic removal of organic pollutants from wastewater has recently garnered significant attention due to its environmental and ecological significance. In this study, a bismuth-rich bismuth oxychloride (Bi₂₄O₃₁Cl₁₀) was synthesized using a single-step solid-state reaction and applied as a photocatalyst for the degradation of rhodamine B in aqueous solution. The photocatalyst was prepared through an eco-friendly solid-state method by mixing bismuth oxide (Bi₂O₃) and bismuth oxychloride (BiOCl), followed by direct annealing at 600 °C. The synthesized material was characterized through various techniques, including X-ray diffraction (XRD), Fourier-transform infrared (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, energy dispersive X-ray spectroscopy (EDS), diffuse reflectance, UV–Visible, and photoluminescence (PL) spectroscopies. The electronic structure of the Bi₂₄O₃₁Cl₁₀ bulk material was analyzed using Density Functional Theory (DFT).

Rietveld refinement confirmed the formation of a pure monoclinic Bi₂₄O₃₁Cl₁₀ phase with the space group P2/c, and the XRD patterns indicated well-crystallized material. SEM revealed micron-sized crystallites, while BET surface area analysis showed a value of 22.546 m²/g, suggesting that a larger surface area could enhance photocatalytic performance. The band gap of the material was determined to be 2.88 eV, with an absorption edge at 430 nm, indicating a promising response to visible light.

The photocatalytic activity of Bi₂₄O₃₁Cl₁₀ was demonstrated by the degradation of rhodamine B (RhB), with complete degradation achieved in 90 min under UV light. Under visible light, 98 % degradation efficiency was reached after 180 min. Kinetic studies showed that the degradation followed a pseudo-first-order model. Optimal conditions for maximum degradation were found at a solution pH of 5, a catalyst concentration of 1 g/L, and a dye concentration of 5 mg/L. Remarkably, the photocatalyst exhibited excellent reusability, maintaining high efficiency over five cycles, with only a slight decrease from 100 % to 90 %.

Trapping experiments identified that reactive species such as superoxide radicals (∙O₂⁻) and hydroxyl radicals (∙OH) played key roles in the photocatalytic process. The possible reaction mechanism was proposed, and degradation products were monitored using liquid chromatography-mass spectrometry (LC-MS), allowing for the elucidation of the degradation pathways of RhB. Additionally, the photocatalyst's effectiveness was tested on Methyl Orange (MO) and Methylene Blue (MB), achieving nearly 100 % degradation for MO and 41 % for MB under UV light. This research highlights the significant potential of Bi₂₄O₃₁Cl₁₀ as a photocatalyst for dye degradation in wastewater treatment.