Over 100,000 types of MOFs, a class of crystalline materials built up from inorganic nodes and organic multidentate ligands, have been reported in recent years. Their immense versatility, arising from the many permutations of metal-ligand combinations affords them great potential for many applications, such as for electronic or optical devices, fluorescence-based sensing, CO2 capture, and increasingly, for energy applications such as fuel cell membranes. Nevertheless, in spite of all their promise, there are very few commercialized MOF products hitting the market. A major reason for this is the glaring technological gap in our ability to control and process MOF materials.
The need to control orientation: The majority of MOFs possess very distinct physicochemical properties (and hence functionality) along different crystallographic directions due to their anisotropic lattices. Despite the dependence of MOF performance on the crystallographic direction, methods to control the orientation of MOF materials are extremely limited. As MOFs are generally synthesized as bulk powders, it is tempting to apply well-established processing methods to MOF materials, such as pressurized compaction or polymer blending and extrusion to form desired structures, but these do not address the problem of orientational control. Furthermore, directional functionality is not unique to MOFs, but is common to many classes of materials, ranging from biological composites like wood to inorganic materials like zeolites. The challenge of effectively exploiting such functional anisotropy arises when the materials of interest are loose nano or microcrystals, which are difficult to physically manipulate into user-directed orientations and positions.
Current approaches to achieve orientational control of MOFs focus on directional growth of MOFs on suitable substrates by carefully tailoring growth conditions. However, the laborious and time-consuming aspects of this approach have hindered its widespread adoption. Furthermore, such methods afford MOF crystals with a static orientation, preventing MOF reorientation under changing conditions and do not address the major question of how to manipulate and orientate free-standing MOF crystals. Therefore, the ability to precisely and dynamically control the orientation and motion of free-standing MOF crystals would represent a major step-change in our ability to manipulate MOF materials, which would be highly desirable for their integrated processing and for their incorporation into MOF devices.
The aim of DYNAMOF is to establish a toolbox of ground-breaking methods to control the orientation and alignment of free-standing and supported MOF crystals, in a manner that can be integrated into other processing techniques, thereby allowing major advancements in the performance of MOF materials, composites and devices.
Especially exciting is that MOFs are an ideal platform for understanding colloidal interactions and developing techniques for their manipulation, as they allow us unprecedented fine-tuning of particle size, shape, composition, surface functionality, density, zeta potential and dielectric constants and so forth, which are important parameters in colloidal assembly. The term ‘colloidal’ as used in this proposal describes particles of sizes ranging from one nanometre to several microns. The lessons learned herein will be important for controlling other colloidal materials such as covalent organic frameworks, hydrogen bonded frameworks, zeolites or carbon nanotubes, with applicability across different materials classes.
Furthermore, the control of materials across multiple length scales remains a significant scientific challenge. Although scientists are skilled at controlling molecular and macroscopic materials, as these clearly fit into traditional key disciplines of chemistry and materials science/engineering, many advancements remain to be made for mesoscale structural control and manipulation of materials, which falls into a highly interdisciplinary area, requiring insights from physics, chemistry, and engineering. Through this work, we aim to advance our ability to control nano and microscale materials, thereby helping to fill this key area.
