Crystal size-dependent framework flexibility of a prototypical metal organic framework is related to metal content: zeolitic imidazolate framework-7

Crystal size engineering is an emerging strategy to modulate mechanical and gas adsorption properties of metal-organic frameworks (MOFs). Fundamental principles on how the properties of these materials change with size remain to be understood and explored. Here, supermicron-, micro-, and nano-sized single crystals of a prototypical MOF zeolitic imidazolate framework-7 (ZIF-7) were generated using a solvothermal method. Atomic force microscopy (AFM) imaging revealed that nano- and micro-sized crystals exhibited rounded and prism-like morphologies, respectively. AFM nanoindentation was used to assess the stiffness (Young's modulus) of the rounded and prism-like crystals as a function of crystal size. We demonstrate that the framework flexibility increases (Young's modulus decreases) with crystal size reduction for both morphologies, which can be attributed to a larger number of point defects (missing metal nodes and/or missing linkers) for smaller crystals. Remarkably, scanning electron microscopy (SEM) energy dispersive X-ray spectroscopy measurements on individual prism-like micro-sized crystals of various sizes revealed a decreasing trend in the Zn/C ratio with crystal size reduction. Collectively, the size-dependent SEM and AFM characterization studies suggest that smaller crystals have lower relative metal content via a larger number of missing metal node defects. Our findings highlight how the mechanical properties of MOFs can vary significantly as a function of crystal size likely due to a variable and size-dependent number of missing metal node defects. Such size-dependent behavior especially towards the nanoscale is thus important to consider for the rational design of various functional crystalline materials.