Determining factors in the growth of MOF single crystals unveiled by in situ interface imaging

We trace the evolution of crystal interface during the solvothermal synthesis of metal-organic frameworks (MOFs) in flow cell by in situ dark-field microscope. Precise measurement of the growth rate at specific crystal facets allows for the extraction of the key determining factors for the crystal growth of five typical MOFs, i.e., Cu-MOF-74 (1D), Co-ZIF, Cu-MOF-2-BDC, Cu-MOF-2-NDC (2D), and HKUST-1 (3D). Rather than strongly correlating to the corresponding chemical formulas, the compositional organic linker and metal ion play vital but separate roles for different MOFs, giving insight into the crystal growth mechanism. Moreover, upon interruption, retreat and recovery of the crystal interface are observed, unveiling the influence from interface to MOF crystal growth. This also leads to the discovery of a transition layer that mediates the linkage of molecular building blocks to form MOF single crystals at the interface.