Abstract
Nanoparticles of metal–organic frameworks (nanoMOFs) boast superior properties compared to their bulk analogs, yet little is known about how common synthetic parameters dictate particle sizes. Here, we provide experimental evidence for the “seesaw” model of nanoMOF growth. Solution acidity, ligand excess, and reactant concentrations are decoupled and shown to form the key independent determinants of nanoMOF sizes, thereby validating the proposal that nanoMOFs arise from coupled equilibria involving ligand deprotonation and metal–ligand complexation. By achieving the first demonstration of a seesaw relationship between nanoMOF sizes and ligand excess, these results provide further proof of the model, as they required deliberate manipulation of relationships outlined by the model. Exploring the relative impacts of these parameters reveals that ligand excess has the greatest ability to decrease sizes, although low acidity and high concentrations can exhibit similar effects. As a complement to existing models of polymer formation and crystal growth, the seesaw model therefore offers a powerful tool for reliable control over nanoMOF sizes.
Highlights
Precise size control can yield distinct functional behavior from materials with seemingly similar compositions
We show that metal–organic frameworks (MOFs) nanoparticle sizes can be tuned through independent control over solution acidity, ligand excess, and reactant concentrations
Demonstrating that these three parameters control nanoMOF sizes supports the key claim of the model that nanoMOFs result from kinetic trapping of nanoparticles determined by competition between coupled equilibria involving metal–ligand complexation and ligand acid–base chemistry
Summary
Precise size control can yield distinct functional behavior from materials with seemingly similar compositions. Achieving control at the nanoscale, in particular, has uncovered remarkable size-dependent properties, such as the luminescence of quantum dots and the distinct catalytic activities of metal nanoparticles.[1,2] Recent reports suggest that the rich structural and compositional diversity of bulk metal–organic frameworks (MOFs) produces enhanced functional properties when realized on the nanoscale.[3] For example, advanced MOF-based gas separation technologies use nanoparticulate MOFs (nanoMOFs) dispersed into mixed matrix membranes (MMMs) to achieve enhanced efficiencies over bulk phases. MMMs that employ nanoMOFs have been shown to surpass the Robeson limit—an intrinsic trade-off between selectivity and permeability in separation membranes.[4,5] While the gas separation performance of nanoMOFs has attracted industrial interest, their improved activities as atomically de ned catalysts and drug delivery agents has opened emerging areas of research.[6,7,8] Despite advances in nanoMOF applications, accurate models are still needed to probe fundamental mechanistic details and reliably control particle sizes
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