Imaging Aspect Ratio-Dependent Anisotropic Conversion of Bismuth-Based Metal-Organic Frameworks into Bismuth Oxides

Metal-organic frameworks (MOFs) have emerged as promising templates and precursors that can be converted to a series of functional materials. Probing the dynamic conversion processes within individual MOF crystal particles in real-time is key to understanding the structure-activity relationship, but this remains challenging, particularly for anisotropic MOF crystals. Here, using dark-field optical microscopy, we visually in situ image and quantify the dynamic oxidation conversion of a single bismuth-based metal-organic framework (Bi-MOF) into bismuth oxides by NaClO. The reactivity of single rod-shaped Bi-MOFs is anisotropic, and its two ends have higher activity than those in the middle regions, which depend on the aspect ratio. Through analysis of the channel directions and theoretical diffusion coefficients, this anisotropic behavior may be attributed to the inherently low diffusion barrier of the NaClO oxidant due to the larger accessible rectangular channels at both ends along the c-axis. Our findings unveil comprehensive kinetic information about the MOF conversion reaction at the subparticle level, favoring the design of future MOF-derived materials.