Phase transformation from α-Bi2O3 needles to petal-shaped β-Bi2O3 nanoparticles via ball milling for boosted photocatalytic activity of rhodamine B dye

Abstract

Tetragonal bismuth oxide (β-Bi₂O₃) is regarded as a promising heterogeneous photocatalyst for the degradation of organic dye molecules in polluted water, owing to its high surface area, wide absorption band edge, effective exciton separation, and abundant oxygen vacancies. Here, we propose a phase transformation from needle-shaped monoclinic (α) Bi₂O₃ to petal-shaped β-Bi₂O₃ nanoparticles through a ball milling process. The α-Bi₂O₃ and β-Bi₂O₃ materials were extensively characterized through various techniques to investigate their structural, morphological, surface chemicals, and optical properties. X-ray powder diffraction (XRPD), high-resolution transmission electron microscopy (HR-TEM), and selected-area electron diffraction (SAED) patterns reveal that the ball-milled α-Bi₂O₃ has a polycrystalline structure and identify the (201) and (101) planes, which correspond to the β-Bi₂O₃ phase. The Tauc plot yields band gaps of 2.9 eV for α-Bi₂O₃ and 2.5 eV for β-Bi₂O₃. Furthermore, β-Bi₂O₃ exhibited a broad absorption band from UV to 540 nm. This finding suggests that β-Bi₂O₃ has an enhanced ability to absorb UV–visible light, thereby improving photon utilization efficiency. The petal-shaped β-Bi₂O₃ nanoparticles demonstrated improved UV–visible light absorption, enhanced charge carrier separation, and superior photocatalytic properties. The petal-shaped β-Bi₂O₃ nanoparticles exhibited excellent photocatalytic activity for the degradation of Rhodamine B in aqueous phase under UV–visible light and solar simulator irradiation. Notably, the β-Bi₂O₃ system achieved 97 % degradation in 75 min under UV–visible light and 91 % degradation in 150 min under the solar simulator. Both catalytic reactions followed pseudo-first-order kinetics, with kinetic constants (k) registering at 8.7 ± 0.6 × 10⁻³ and 48.1 ± 4.7 × 10⁻³ min⁻¹ for Rhodamine B (RhB) under UV–vis light. Additionally, by expanding the number of reactive sites and oxygen vacancies, the catalyst's large surface area boosted electron-hole separation and its photocatalytic performance.