Revisiting Microstructure, Facet Exposure, and Lattice Distortion in Bismuth Oxyhalide (BiOX, X = Cl, Br, I) Nanomaterials for Catalysis

We present a comprehensive structural study of BiOCl, BiOBr, and BiOI nanomaterials, focusing on the impact of varying the synthesis pH and halide on the structural properties of BiOX catalysts. By employing a combination of advanced structural analysis techniques, including Rietveld refinement of powder X-ray diffraction (PXRD) data, X-ray pair distribution function (PDF) analysis, and X-ray absorption spectroscopy (XAS), notable structural insights into crystallite dimensions, microstrain, and lattice distortion were elucidated. Most notably, all BiOX materials comprised anisotropic, platelet-shaped crystallites with dominant {001} facets when using a microwave-assisted synthesis protocol, irrespective of the synthesis pH or halide. While large cylindrical crystalline platelets formed in acidic conditions, significantly smaller crystalline regions were formed under neutral-mildly alkaline conditions, with dimensions in the layer stacking direction (i.e., along the crystallographic c axis) becoming ultrathin (approximately 4 nm for BiOBr synthesized at pH 9.0), corresponding to only 4 bound unit cells. As the crystalline regions become smaller, the microstrain increases and the crystal lattice experiences increased distortion, expanding in the c direction and contracting in the a = b directions. Given the widespread application of BiOX nanomaterials within photo- and electro-catalysis, the practical significance of the structural characteristics on properties relevant to catalytic performance is discussed throughout. Namely, the crystallite dimensions and facet exposure have important implications for the available surface area for catalytic application, while the tunable microstrain, unit cell distortion, and ultrathin morphology might influence the electronic band structure, optical properties, and charge carrier dynamics. This study provides valuable insight into tuning the properties of BiOX nanocatalysts for their intended application, as well as demonstrating the merit of performing detailed structural analysis on BiOX nanomaterials using X-ray probes.