Among different bismuth oxyhalides (BiOX; X=F, Cl, Br, I), the moderate bandgap of ~2.8 eV of BiOBr makes it a suitable visible-light-driven photocatalyst. However, a comparison of its different morphologies, e.g., plate-like and hierarchical structures, is not yet reported, leaving behind fundamental questions as to which one is better for photocatalytic water treatment and why? These questions were addressed in this study by conducting a vis-à-vis comparison of photocatalytic performance of the two structures in degradation of pharmaceuticals and personal care products (PPCPs) — an emerging class of ubiquitous water pollutants. The photocatalytic degradation by the hierarchical structure was three times faster than its plate-like counterpart. Contrary to the widespread notions...[ Read more ]

Among different bismuth oxyhalides (BiOX; X=F, Cl, Br, I), the moderate bandgap of ~2.8 eV of BiOBr makes it a suitable visible-light-driven photocatalyst. However, a comparison of its different morphologies, e.g., plate-like and hierarchical structures, is not yet reported, leaving behind fundamental questions as to which one is better for photocatalytic water treatment and why? These questions were addressed in this study by conducting a vis-à-vis comparison of photocatalytic performance of the two structures in degradation of pharmaceuticals and personal care products (PPCPs) — an emerging class of ubiquitous water pollutants. The photocatalytic degradation by the hierarchical structure was three times faster than its plate-like counterpart. Contrary to the widespread notions in the literature that a hierarchical photocatalyst performs better than its plate-like counterpart due to its better light harvestability and/or adsorption capability, charge carrier separation was found to be the mechanistic feature predominantly affecting photocatalytic performance. Subsequently, material properties of the two structures were comparatively examined using material characterization and photoelectrochemical techniques. Compared to other material properties, crystal disorders were found to have a profound effect on photocatalytic activity. Examination of property-performance relationships, evaluated for a solid solution system, BiOBr_xI_1–x (0 ≤ x ≤ 1), verified that charge carrier separation is a predominant mechanistic feature, and crystal disorders constitute one of the most influential material properties governing photocatalytic activity. Further, a magnetically separable BiOBr/Fe₃O₄@SiO₂ photocatalyst was synthesized exhibiting not only fast PPCP degradation kinetics but also mineralization capability. Conduction band electron-mediated reactions and direct-hole oxidation comprised major degradation routes. The recycled BiOBr/Fe₃O₄@SiO₂ maintained about 80% of its activity over five cycles. The typically co-present wastewater constituents, including NOM and anions, inhibited photocatalytic performance to varying extents in real sewage. The results of prototype photocatalytic experiments were comparable to those of batch experiments, and a magnetic separation efficiency of ~99% was achievable in 5 min.