Electron Microscopy Reveals Inhomogeneous Adsorption of Iodine and Concurrent Defect Formation in a Metal–Organic Framework

Adsorption behaviors are typically examined through adsorption isotherms, which measure the average adsorption amount as a function of partial pressure or time. However, this method is incapable of identifying inhomogeneities across the adsorbent, which may occur in the presence of strong intermolecular interactions of the adsorbate. In this study, we visualize the adsorption of molecular iodine (I2) in the metal-organic framework material MFM-300(Sc) using high-resolution scanning transmission electron microscopy (STEM). Our observations demonstrate that, counterintuitively, I2 adsorption in MFM-300(Sc) occurs in an inhomogeneous manner, regardless of the I2 uptake level. Even at adsorption saturation, corresponding to an average of 23 iodine atoms per unit cell, MOF channels with significantly varying iodine contents─from nearly empty to densely filled─coexist. Image simulations suggest that the most densely packed I2 may locally form the previously proposed triple-helix structure, corresponding to up to 142 iodine atoms per unit cell. Furthermore, STEM imaging reveals that I2 adsorption can induce the formation of structural defects, such as edge dislocations and stacking faults, within the MOF framework. These defects persist even after the complete removal of I2 molecules. Additionally, we have developed a surfactant-capping strategy to minimize the release of adsorbed I2 from MFM-300(Sc) and validated its effectiveness using STEM imaging.