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.

Controlling the shape of metal-organic framework (MOF) crystals is important for understanding their crystallization and useful for myriad applications.H owever,d espite the many advances in shaping of inorganic nanoparticles,postsynthetic shape control of MOFs and, in general, molecular crystals remains embryonic.H erein, we report using as imple 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,i ncluding cubic and tetrahedral crystals,and even hollow boxes,b yanacid-base reaction and subsequent sequestration of leached metal ions.E tching tests on these ZIFs reveal that etching occurs preferentially in the crystallographic directions richer in metal-ligand bonds;t hat, along these directions,the etching rate tends to be faster on the crystal surfaces of higher dimensionality;a nd that the etching can be modulated by adjusting the pH of the etchant solution.Chemical etching is an ancient fabrication method that was used by metal and glass craftsmen to obtain sophisticated surface designs.W ith the advent of controlling the etching orientation at the microscale and nanoscale,anisotropic wetchemical etching has become highly useful for shaping many materials for diverse applications. [1,2] Fore xample,t he anisotropic wet-chemical etching of single-crystal silicon in the presence of ab ase is essential in microelectronics manufacturing. [1] Anisotropic etching can also be applied to preparation of metal nanocrystals from oxidative species and coordination ligands,f or which it enables unprecedented morphologies and complexities,a nd unique physical properties. [2] Herein, we introduce the concept of anisotropic wetchemical etching for metal-organic frameworks (MOFs) and, in particular,f or the zeolitic-imidazolate framework (ZIF) subfamily.MOFs (and by extension, ZIFs) are an emerging class of porous materials that show extremely large surface areas (S BET )a nd potential for myriad applications,i ncluding gas sorption and separation, catalysis,s ensing,a nd biomedicine, among others. [3,4] MOFs are built up from metal ions/clusters connected through organic linkers.T heir exposed crystal facets,e dges,a nd vertices can exhibit different chemical compositions.W ehypothesized that agents capable of breaking the coordination bonds between the metal ions/clusters and the organic linkers could be exploited to preferentially etch specific external crystal surfaces (with more density of coordination bonds) over others.W ee nvisioned that such control would enable us to post-synthetically tailor the shape of MOF crystals.T od ate,p ost-synthetic random etching of MOF crystals using H + ,N a + ,a nd quinone has already been demonstrated. [5,6] Inspired by similar results with zeolites,this strategy has enabled researchers to prepare hierarchical MOF crystals and/or create macropores on the MOF crystal surfaces. [7,8] However, to date,n oo ne has demonstrated the ab...

The physical and even chemical properties of crystals often differ with crystal orientation, [1] due to the distinct atomic interactions and bond distances along the crystal directions, which can strongly affect the electronic, mechanical, and/or magnetic characteristics. Accordingly, the integration of crystals into devices requires control of their surface orientation. [2] Mesoscale self-assembly of particles into supercrystals is important for the design of functional materials such as photonic and plasmonic crystals. However, while much progress has been made in self-assembling supercrystals adopting diverse lattices and using different types of particles, controlling their growth orientation on surfaces has received limited success. Most of the latter orientation control has been achieved via templating methods in which lithographic processes are used to form a patterned surface that acts as a template for particle assembly. Herein, a template-free method to self-assemble (111)-, (100)-, and (110)-oriented face-centered cubic supercrystals of the metal-organic framework ZIF-8 particles by adjusting the amount of surfactant (cetyltrimethylammonium bromide) used is described. It is shown that these supercrystals behave as photonic crystals whose properties depend on their growth orientation. This control on the orientation of the supercrystals dictates the orientation of the composing porous particles that might ultimately facilitate pore orientation on surfaces for designing membranes and sensors. Mesoscale AssemblyFor instance, the importance of controlled growth of oriented crystalline (111)-silicon and (0001)-ZnO nanowires, [3,6] (001)-YBCO superconductors, [4] and phosphorene semiconductors [5] on surfaces in electronic, photovoltaic, and photonic devices has been described. To date, crystal orientation is controlled chiefly via direct growth methods, including vapor/ liquid/solid, [7] oxide-assisted, [8] and template-based growth methods. [9] Controlled orientation of crystals on surfaces can also improve the performance of porous materials integrated into devices or membranes. For example, Tsapatsis and coworkers demonstrated that zeolite ZSM-5 membranes in the (010) orientation perform better at separation of xylene isomers than those in other orientations do. They attributed this advantage to the larger, straighter pores accessible along the b-axis throughout the membrane thickness, compared to the narrower, sinusoidal pores along the a-axis. [10] Likewise, MFI-type zeolite membranes in the (010) orientation showed better separation performance and mass transfer than those in other orientations did. [11] Similar trends are expected for metal-organic frameworks (MOFs), an emerging class of porous materials that can be synthesized in various shapes and pore sizes and that show extremely large surface areas and tailored internal surfaces. [12,13] Preliminary advances in controlling the orientation of MOF crystal growth on surfaces have been reported by Biemmi et al., [14] for HKUST-1, and by Zacher et a...