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 particleÀmonomer 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.

Prenucleation clusters: in situ synchrotron X-ray scattering with a one-second time resolution revealed the occurrence of nano-sized clusters during the nucleation and early growth of nanocrystals of a zeolitic imidazolate framework (ZIF). The complex crystallization process exhibits similarities with crystallization processes of zeolites from solution. Hmim= 2-methylimidazole.

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MOF-5 nanoparticles were prepared by mixing a solution of [Zn4O(C6H5COO)6] with a solution of benzene-1,4-dicarboxylic acid in DMF at ambient conditions. The former species mimics as a secondary building unit (SBU), and the latter acts as linker. Mixing of the two solutions induced the formation of MOF-5 nanoparticles in dilute suspension. The applied conditions were identified as suitable for a closer investigation of the particle formation process by combined light and small angle neutron scattering (SANS). Scattering analysis revealed a significant impact of the molar ratio of the two components in the reaction mixture. Excessive use of the building unit slowed down the process. A similar effect was observed upon addition of 4n-decylbenzoic acid, which is supposed to act as a modulator. The formation mechanism leads to initial intermediates, which turn into cubelike nanoparticles with a diameter of about 60–80 nm. This initial stage is followed by an extended formation period, where nucleation proceeds over hours, leading to an increasing number of nanoparticles with the same final size of 60–80 nm.

The present work is a comparative study on the scattering behaviour of hollow and filled rectangular parallelepipeds. Comparison is based on the model formfactors of rectangular parallelepipeds, which cover the entire regime of variable wall thickness. The entire regime of wall thicknesses has been made available by the present work, which completed the set of formulas by calculating the formfactor for the limit of hollow parallelepipeds with infinitely thin walls, which was still lacking. The formfactors are expressed as a function of the momentum transfer q. Discrimination between massive and hollow structures by means of the q-dependent scattering data SF(q) gets possible once the Guinier radius and the particle volume can be established at the lower limit of q or a power law of SF(q) ∼ q α can be extracted towards large q. Whereas the former requires extrapolation of the scattering data to q = 0 with high accuracy, the latter needs experiments over a very broad q-regime. If experimental data is restricted to a q-regime which includes only the first peak in a Kratky representation of q 2 SF(q) vs. q without giving way to an accurate extrapolation to q = 0 nor to a clear power law in q, discrimination between a hollow and a massive structure gets extremely difficult. Yet, the effect of anisometry is striking and enables extraction of a crude guess of the degree of anisometry already from the first Kratky peak. This could be achieved by introducing a dimensionless parameter established from the width and location of the first Kratky peak.