Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) 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 in turn 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 most practiced and routine method for structure determination, the acquisition of adequate data quality from weakly scattering nano- and submicro-sized crystals remains a challenge for this technique. While powder X-ray diffraction (PXRD) is more suitable technique for handling small crystals, structure determination of MOFs in particular can be challenging due to severe peak overlap as a consequence of large unit cell parameters, complexity of the structures themselves, as well as phase mixtures.
Three-dimensional electron diffraction (3DED) techniques have shown to be powerful for structural determination of 'intractable' crystals that are too small for SCXRD analysis. These techniques benefit from the strong Coulomb interaction between electrons and matter. 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 a photoactive MOF1. Further example will be given by applying 3DED on the study of the electrocatalyst PCN-226, where the spacing between active-sites are found crucial for its activity2. Last, I will demonstrate 3DED as a high throughput single crystal approach that can accelerate the discovery of new materials, especially in a phase mixture of nanocrystals, which makes the structure determination inaccessible to other characterization techniques. I believe 3DED as a significant development for the community of MOFs, where it allows to obtain accurate atomic information from nanocrystals, and can thus avoid the slow and arduous process of crystal growth.
(1) Yuan, S.; Qin, J.-S.; Xu, H.-Q.; Su, J.; Rossi, D.; Chen, Y.; Zhang, L.; Lollar, C.; Wang, Q.; Jiang, H.-L.; Son, D. H.; Xu, H.; Huang, Z.; Zou, X.; Zhou, H.-C. ACS Cent. Sci. 2018, 4 (1), 105–111.
(2) Cichocka, M. O.; Liang, Z.; Feng, D.; Back, S.; Siahrostami, S.; Wang, X.; Samperisi, L.; Sun, Y.; Xu, H.; Hedin, N.; Zheng, H.; Zou, X.; Zhou, H.-C.; Huang, Z. J. Am. Chem. Soc. 2020, 142, 15386−15395.