Three-step nucleation of metal–organic framework nanocrystals

Significance Nucleation is ubiquitous in many natural and industrial processes, yet our understanding of how a nucleus first emerges is limited. According to classical nucleation theory, a nucleus from which crystals grow forms in a single, spontaneous step. In this work, we combine ultralow-dose in situ liquid-phase and cryogenic transmission electron microscopy to show that metal–organic frameworks (MOFs) nucleate through a different, nonclassical pathway with three distinct steps. These steps are phase separation, condensation, and crystallization. Our observations of intermediate steps during the crystallization of MOFs provide important insight for controlling the nucleation and growth of both crystalline and amorphous materials. Metal–organic frameworks (MOFs) are crystalline nanoporous materials with great potential for a wide range of industrial applications. Understanding the nucleation and early growth stages of these materials from a solution is critical for their design and synthesis. Despite their importance, the pathways through which MOFs nucleate are largely unknown. Using a combination of in situ liquid-phase and cryogenic transmission electron microscopy, we show that zeolitic imidazolate framework-8 MOF nanocrystals nucleate from precursor solution via three distinct steps: 1) liquid–liquid phase separation into solute-rich and solute-poor regions, followed by 2) direct condensation of the solute-rich region into an amorphous aggregate and 3) crystallization of the aggregate into a MOF. The three-step pathway for MOF nucleation shown here cannot be accounted for by conventional nucleation models and provides direct evidence for the nonclassical nucleation pathways in open-framework materials, suggesting that a solute-rich phase is a common precursor for crystallization from a solution.