Abstract
Porous nanocrystals of metal-organic frameworks (MOFs) offer greater bioavailability, higher surface-to-volume ratios, superior control over MOF membrane fabrication, and enhanced guest-sorption kinetics compared to analogous bulk phases, but reliable synthesis of uniformly sized particles remains an outstanding challenge. Here, we identify the smallest and most probable sizes of known MOF nanocrystals and present an exhaustive comparative summary of nano- versus bulk-MOF syntheses. Based on critical analysis of reported size data and experimental conditions, an alternate to the LaMer model is proposed that describes nanocrystal formation as a kinetic competition between acid-base and metal-ligand reactivity. Particle growth terminates when ligands outcompete metal-ion diffusion, thereby arresting polymerization to produce kinetically trapped particle sizes. This model reconciles disparate trends in the literature and postulates that minimum particle sizes can be achieved by minimizing the relative ratios of metal-to-linker local concentrations. By identifying conditions that disfavor small nanocrystal sizes, this model also provides routes towards macroscopic MOF single crystals. A universal "seesaw" relationship between nanocrystal sizes and the concentrations of acidic surface-capping ligands provides a roadmap for achieving precise synthetic control. Best practices in synthesis, characterization, and data presentation are recommended for future investigations so that MOF nanocrystals may achieve their full potential as advanced nanomaterials.
Highlights
Nanocrystals are distinguished from their bulk counterparts by the extreme size-dependence of their functional properties
Factors controlling metal–organic frameworks (MOFs) nanocrystal sizes We propose that the kinetic trapping of MOF nanocrystals of particular sizes depends on the competition between four chemical equilibria (Scheme 1): (1) linker deprotonation; (2) modulator deprotonation; (3) linker complexation, and (4) termination
The metal–ligand chemistry outlined in eqn (1)– (4) provides a framework for understanding trends in reported nano-MOF sizes
Summary
Nanocrystals are distinguished from their bulk counterparts by the extreme size-dependence of their functional properties. Either the differing reactant concentrations (2.34 mM versus 0.17 mM), solvent conditions (DMF/H2O/EtOH mixture versus butanol), or solvothermal versus microwave synthetic routes involved distinct processes that produced stark size differences In response to such cases, we focus our mechanistic analysis on reports that employed “coordination modulators”—typically monotopic acid ligands—as these represent the bulk of literature examples, small particles of MOFs have been generated by many other techniques, such as preparation via microemulsions,[32,33] dual injection,[34] and metal–organic gels.[35]. We support this kinetic model with literature examples that illustrate the role performed by each parameter and apply this insight to rationalizing previously unexplained phenomena
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