Unveiling the structure-mechanical strength relationship of single crystal mixed-linker ZIF-8 and ZIF-67 via nanoindentation.

Zeolitic imidazolate frameworks (ZIFs) have garnered considerable attention in recent years due to their facile synthesis, versatile chemical properties, large pore volumes, and chemical stability. Researchers have developed organic glasses/membranes based on ZIFs for various applications, including gas storage and molecular separation. Probing the structural-mechanical property relationship of ZIFs can provide the foundation for designing high-performance ZIF-based materials with controllable physical structures and mechanical responses. Therefore, we aim to define the factors governing mechanical performance through systematic structural variations and comprehensive mechanical characterization. We synthesized micron-sized single crystals of ZIF-8 and ZIF-67, which have identical crystal structures but different metal-linker connectivity, and studied their mechanical properties using single-particle nanoindentation. We found that ZIF-67 exhibited a higher Young's modulus and hardness when compared with ZIF-8, which was attributed to the stronger Co-N bond in ZIF-67. Moreover, we controlled the linker content (20% imidazole and 80% 2-methylimidazole), as confirmed by NMR for both ZIF-8 and ZIF-67 and by single-crystal X-ray diffraction (SCXRD) for ZIF-67. The resulting mixed-linker ZIFs showed decreased crystallinity compared with the parent ZIFs and exhibited missing-linker defects, as determined by thermogravimetric analysis (TGA). As a result, the mixed-linker ZIFs displayed lower Young's modulus and hardness. Our approach offers significant insight into the design and fabrication of mechanically robust ZIFs/MOFs, paving the way for future research.