Defects in ZIF-8 crystallization and their impact on mechanical properties

Abstract: The growth process of metal-organic frameworks (MOFs) defines their properties for functional applications. However, it is very plausible that defects may occur during the crystallization of even seemingly perfect MOFs, such as ZIF-8, and yet, direct probing of such structural defects has been challenging due to the lack of techniques to locally examine individual nanocrystals. Now, we directly study local defects - such as missing linkers or metal vacancies - in ZIF-8 nano- and microcrystals with near-field infrared nanospectroscopy combined with density function theory calculations. We have tracked the chemical changes during crystallization and found that structural defects like zinc-rich regions gradually disappear with the ripening of the crystals, while missing linker defects prevail. The resulting open metal sites reduce the Young's modulus, as measured with tip force microscopy and supported by theoretical modelling, but also open the door for defect engineering to tune the adsorption and catalytic performance of ZIF-8.