Downsizing metal–organic frameworks by bottom-up and top-down methods

Downsizing metal–organic framework (MOF) crystals into the nanoregime offers a promising approach to further benefit from their inherent versatile pore structures and surface reactivity. In this article, downsizing is referred to as the deliberate production of typical large MOF crystals into their nanosized versions. Here, we discuss various strategies towards the formation of crystals below 100 nm and their impact on the nano-MOF crystal properties. Strategies include an adjustment of the synthesis parameters (e.g., time, temperature, and heating rate), surface modification, ligand modulation, control of solvation during crystal growth and physical grinding methods. These approaches, which are categorized into bottom-up and top-down methods, are also critically discussed and linked to the kinetics of MOF formation as well as to the homogeneity of their size distribution and crystallinity. This collection of downsizing routes allows one to tailor features of MOFs, such as the morphology, size distribution, and pore accessibility, for a particular application. This review provides an outlook on the enhanced performance of downsized MOFs along with their potential use for both existing and novel applications in a variety of disciplines, such as medical, energy, and agricultural research. Methods for enhancing the properties of porous materials known as metal–organic frameworks (MOFs) by making the crystals smaller have been reviewed by scientists in Australia and the Philippines. MOF crystals have an open atomic structure which includes large voids. MOFs are highly crystalline materials, typically generated in powder form, useful for applications such as hydrogen storage and carbon capture. Reducing the crystal sizes to nanometer scales significantly enhances the material’s physical and chemical properties. Ken Usman, Ludovic Dumee and Joselito Razal from Deakin University, Geelong, Australia, and co-workers have reviewed the latest methods for synthesizing MOF crystals smaller than a hundred nanometers. Synthesis strategies include altering a wide range of parameters such as time, temperature and heating rate. The authors show how these different approaches allow the properties of nano-sized MOF, including morphology and size distribution, to be controlled to suit a specific application. Metal organic frameworks are typically synthesized at the macroscale, into powders, films or as coatings generated across appropriate supporting materials. The downsizing of metal–organic frameworks offers opportunities to not only benefit from their properties at the nanoscale but also to enhance surface interactions and reactivities. The potential and challenges with current downsizing techniques are discussed in this review in light of materials properties and application performance.