Three-step nucleation of metal–organic framework nanocrystals

Abstract

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.

Multistep nucleation of anisotropic molecules

“…Such clusters are referred to as pre-nucleation clusters (PNCs), although their existence is open to debate 24 , 25 . The nucleation process believed to involve PNCs evolves in order of the phase separation, condensation, and ripening, and is often referred to as multistep nucleation in a broad sense 26 – 29 .…”

Multi-Step Crystallization and Chemical Evolution of Sodium Yttrium Fluoride

“…11 Some systems have also shown a distinct intermediate step in which the DLP condenses into an amorphous phase prior to crystallization. 4 These studies, among others, 18 suggest that the two-step mechanism via a DLP can readily be accessed in a wide range of solution-based systems, because, when taken to sufficiently high supersaturation, nearly all solutions will reach their limit of stability. 19 One aspect of these two-step pathways via a DLP that remains largely unexplored is that the stoichiometry of the intermediate phase is variable and ill-defined and must evolve to generate the final stable phase.…”

“…3 Recent work has also shown a distinct intermediate step involving the formation of an amorphous aggregate. 4 Here we report a novel four-step mechanism in the aqueous synthesis of sodium yttrium fluoride involving 1) the segregation of aqueous ions into a dense liquid phase, 2) the formation of an amorphous aggregate, 3) nucleation of a cubic YF 3 phase, and 4) subsequent solid-state diffusion of sodium and fluoride ions to form a final NaYF 4 phase. The final step involves a continuous, gradual change of the solid phase's chemical stoichiometry from YF 3 toward NaYF 4 .…”

Autocatalysis and Oriented Attachment Direct the Synthesis of a Metal–Organic Framework

“… 5 , 6 However, recent research on zeolite and MOF crystallization has shown multiple deviations from classical crystallization theories. 7 , 8…”

“… 9 − 11 In fact, a three-step crystallization mechanism has been proposed recently for ZIF-8. 7 Nonclassical growth mechanisms have also been recently proposed as a deviation from the classical monomer-by-monomer based crystal growth, including for composite systems such as protein-MOF composites. 7 , 12 − 14 Nonclassical growth can occur because of the attachment of oligomers or amorphous clusters or through the oriented attachment of nanocrystals.…”