Controlling Zeolitic Imidazolate Framework Nano- and Microcrystal Formation: Insight into Crystal Growth by Time-Resolved In Situ Static Light Scattering

We report on a simple and straightforward method that enables the rapid room-temperature production of nanocrystals (finely tuned in size between ∼10 and 65 nm) and microcrystals (∼1 μm) of the prototypical microporous zeolitic imidazolate framework (ZIF) material ZIF-8. Control of crystal size is achieved in a novel approach by employing an excess of the bridging bidentate ligand and various simple auxiliary monodentate ligands with different chemical functionalities (carboxylate, N-heterocycle, alkylamine). The function of the monodentate ligands can be understood as a modulation of complex formation and deprotonation equilibria during crystal nucleation and growth. Using time-resolved static light scattering, the functioning of modulating ligands is monitored for the first time by in situ experiments, which offered significant insight into the crystal growth processes. Formation of nanocrystals is characterized by continuous, comparatively slow nucleation and fast crystal growth occurring on a time scale of seconds. Although nucleation and growth are not separated from each other, a significant narrowing of the particle size distribution during early stages results in rather monodisperse nanocrystals, before broadening of the particle size distribution occurs, as observed by complementary ex situ electron microscopy studies. Microcrystal growth is dominated by a particle−monomer addition mechanism, but indications for the operation of a coalescence process during early stages of growth have been also obtained. During later stages of microcrystal growth crystals change their shape from cubes to rhombic dodecahedra. The prepared phase-pure ZIF-8 nanoscale materials exhibit good thermal stability in air and large surface areas, which are comparable to those of large macrocrystals. Nanocrystal powders exhibit dual micro- and mesoporosity.