Metal-organic frameworks (MOFs) are known for their versatile combination of inorganic building units and organic linkers, which offers immerse opportunities in a wide range of applications. Such applications are often designed by considering the inherent properties of MOFs, which are governed by the crystal structures. Therefore, development of new structural characterization techniques parallels discovery of new materials. Although single crystal X-ray diffraction (SCXRD) is the routine method for structure determination, the acquisition of adequate data quality from weakly scattering nanocrystals remains a challenge. While powder X-ray diffraction (PXRD) is more suitable for handling small crystals, structure determination of MOFs can be perticularly challenging due to severe peak overlap. Three-dimensional electron diffraction (3DED) techniques have been developed for structural analysis of 'intractable' crystals that are too small for SCXRD. Compared to X-ray, electrons generate much higher signal-to-noise ratios, even when the volume of the crystals are 6 or 7 orders of magnitude smaller. I will discuss applying 3DED in revealing the unique properties of MOFs for photo- and electrocatalysis. I will first present it on investigating heteroatom distribution in photoactive MOFs, PCN-415 and PCN-416, where the discovery of the [Ti8Zr2O12(COO)16] cluster is key to its photocatalytic activity1. Further example will be given on the study of the electrocatalyst PCN-226. Based on Zr-oxo chains, PCN-226 was found not only have a novel 3D net, but also have an ideal spacing between active-sites for oxygen reduction reaction (ORR)2. We therefore presents a new design strategy on using Zr-chains to promote high stability and, more importantly, tailor the immobilization and packing of redox active-sites at a density that can improve the reaction kinetics. I believe 3DED as a significant development for the community of MOFs, as it allows to obtain accurate atomic information from nanocrystals, and thus show the poential to avoid the slow and arduous process of crystal growth.

Structure and property analysis of metal-organic framework nanocrystals by three-dimensional electron diffraction

ACS Spring Meeting 2021

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