Metal-organic frameworks (MOFs) are among the most attractive porous materials known today. Their miniaturization to the nanoscale--into nanoMOFs--is expected to serve myriad applications from drug delivery to membranes, to open up novel avenues to more traditional storage and catalysis applications, and to enable the creation of sophisticated superstructures. Here, we report the use of spray-drying as a versatile methodology to assemble nanoMOFs, yielding spherical hollow superstructures with diameters smaller than 5 µm. This strategy conceptually mimics the emulsions used by chemists to confine the synthesis of materials, but does not require secondary immiscible solvents or surfactants. We demonstrate that the resulting spherical, hollow superstructures can be processed into stable colloids, whose disassembly by sonication affords discrete, homogeneous nanoMOFs. This spray-drying strategy enables the construction of multicomponent MOF superstructures, and the encapsulation of guest species within these superstructures. We anticipate that this will provide new routes to capsules, reactors and composite materials.

Self-assembly of particles into long-range, three-dimensional, ordered superstructures is crucial for the design of a variety of materials, including plasmonic sensing materials, energy or gas storage systems, catalysts and photonic crystals. Here, we have combined experimental and simulation data to show that truncated rhombic dodecahedral particles of the metal-organic framework (MOF) ZIF-8 can self-assemble into millimetre-sized superstructures with an underlying three-dimensional rhombohedral lattice that behave as photonic crystals. Those superstructures feature a photonic bandgap that can be tuned by controlling the size of the ZIF-8 particles and is also responsive to the adsorption of guest substances in the micropores of the ZIF-8 particles. In addition, superstructures with different lattices can also be assembled by tuning the truncation of ZIF-8 particles, or by using octahedral UiO-66 MOF particles instead. These well-ordered, sub-micrometre-sized superstructures might ultimately facilitate the design of three-dimensional photonic materials for applications in sensing.

Controlling the shape of metal-organic framework (MOF) crystals is important for understanding their crystallization and useful for myriad applications. However, despite the many advances in shaping of inorganic nanoparticles, post-synthetic shape control of MOFs and, in general, molecular crystals remains embryonic. Herein we report using a simple wet-chemistry process at room temperature to control the anisotropic etching of colloidal ZIF-8 and ZIF-67 crystals. Our work enables uniform reshaping of these porous materials into unprecedented morphologies, including cubic and tetrahedral crystals, and even hollow boxes, via acid-base reaction and subsequent sequestration of leached metal ions. Etching tests on these ZIFs reveal that etching occurs preferentially in the crystallographic directions richer in metal-ligand bonds; that, among these directions, the etching rate tends to be faster on the crystal surfaces of higher dimensionality; and that the etching can be modulated by adjusting the pH of the etchant solution. KeywordsMetal-Organic Frameworks; Zeolitic-Imidazolate Frameworks; anisotropic etching; hollow particles; porosity Chemical etching is an ancient fabrication method that was used by metal and glass craftsmen to obtain sophisticated surface designs. With the advent of controlling the etching orientation at the microscale and nanoscale, anisotropic wet-chemical etching has become highly useful for shaping many materials for diverse applications.[1,2] For example, the anisotropic wet-chemical etching of single-crystal silicon in the presence of a base is essential in microelectronics manufacturing.[1] Anisotropic etching can also be applied to preparation of metal nanocrystals from oxidative species and coordination ligands, for which it enables unprecedented morphologies and complexities, and unique physical properties.[2] Here, we introduce the concept of anisotropic wet-chemical etching for metal-organic frameworks (MOFs)-specifically, for the zeolitic-imidazolate framework (ZIF) subfamily. Figure S1).[9] Recently, several studies have shown that the crystal growth of ZIF-8 starts with formation of cubes exposing six {100} facets, which gradually evolve into truncated rhombic dodecahedra exposing six {100} and twelve {110} facets, and finally, into the thermodynamically more stable rhombic dodecahedra, in which only the twelve {110} facets are exposed ( Figure 1a). [10,11] Figure 1a illustrates the different exposed crystallographic planes for the cubes and for the truncated and non-truncated rhombic dodecahedra. As shown in this figure, each crystal shape has different exposed crystallographic planes, and each crystallographic plane, a distinct chemical composition. In ZIF-8, {100} and {211} planes contain several Zn-2-MIM linkages, whereas the {110} and {111} planes do not contain any of these linkages ( Figure 1b).By exploiting the aforementioned differences, we devised a method for anisotropic wet etching of colloidal crystals of isostructural ZIF-8 and ZIF-67. Our approach combines ...

As a pioneering MOF constituent of colloidal science, this tutorial summarizes the advances in the synthesis of colloidal ZIF-8.

While bulk-sized metal–organic frameworks (MOFs) face limits to their utilization in various research fields such as energy storage applications, nanoarchitectonics is believed to be a possible solution.