Bismuth oxyiodides: photocatalytic performance, by-products, and degradation pathways

Abstract The increasing global demand for environmental remediation strategies has led to significant interest in the development of efficient photocatalysts. Semiconductor photocatalysts, such as titanium dioxide (TiO₂ P25), have been extensively studied for addressing challenges such as water purification and air decontamination. However, TiO₂ P25’s wide band gap restricts its efficacy under visible light, which limits its practical use in real-life applications. Bismuth oxyiodides have emerged as highly promising alternatives due to their narrow band gaps and visible-light responsiveness. In this study, BiOI, Bi5O7I, and BiOI/Bi5O7I have been synthesized by pH-dependent co-precipitation and hydrothermal methods and evaluated their photocatalytic performance for phenol degradation and nitrogen oxides (NOx) oxidation. Under visible light irradiation, BiOI-co pH 10 and BiOI/Bi5O7I-co pH 12 demonstrated promising phenol degradation rates (≈51%) compared to the TiO2 P25 benchmark (≈ 11%). In terms of mineralization efficiency, as measured by the total organic carbon (TOC)/phenol ratio (0.6–0.7), Bi5O7I-UV, BiOI/Bi5O7I-VIS, and TiO2 P25-UV showed similar capabilities. Only under UV light irradiation did TiO2 P25 (phenol removal≈100%; NO removal≈86%) surpass the bismuth oxyiodides. Despite showing minimal production of aromatic by-products (e.g., hydroquinone, benzoquinone, and catechol) during phenol degradation, the bismuth oxyiodides exhibited higher NO2 production compared to TiO2 P25 during NOx oxidation. One possible explanation for this phenomenon may be attributed to different ROS-mediated mechanisms present in TiO2 P25 and bismuth oxyiodide compounds. However, the possibility of significant adsorption of intermediates in solution onto bismuth oxyiodide materials cannot be neglected. Quencher experiments, electron paramagnetic resonance (EPR), and terephthalic acid-fluorescence probe method revealed that hydroxyl radicals (HO·) are not the major oxidant specie in in bismuth oxyiodide-mediated photocatalysis. Using evidence from EPR spectroscopy, a photodegradation pathway, involving singlet oxygen (1O2), was proposed. These findings provide valuable insights into the photocatalytic behavior of bismuth oxyiodides and highlights the importance of understanding the mechanisms to optimize their use for environmental applications.