Probing defects and disorders in nanoporous materials by electron crystallography

Nanoporous materials including zeolites and metal-organic frameworks (MOFs) have well-defined pores and channels in molecular dimensions.They have a wide range of applications from adsorption, separation to catalysis.The properties of a nanoporous material depend both on its crystal structure and on the defects and disorders in the crystal.It is therefore important to elucidate the periodic atomic structure as well as the defects.Point defects caused by vacancies or incorporation of additional atoms and extended defects (1 D and 2 D) appear frequently in nanoporous materials [1].They often impact the performance of the materials.High-resolution (scanning) transmission electron microscopy (HRTEM/HRSTEM) has unique advantages in observing defects and disorders [2].However, it is very challenging to obtain high atomic resolution images from porous materials because of electron beam damage.Three-dimensional electron diffraction (3D ED) requires, on the other hand, two orders of magnitude lower electron dose than HRTEM/HRSTEM, which is more useful for studying beam-sensitive samples [3][4].3D ED has been shown to be powerful for determination of atomic structures from nano-and micron-sized crystals that are too small to be studied by single crystal X-ray diffraction [5].In this talk, I will give several examples on how various imaging and 3D ED techniques can be used for ab initio structure determination of novel nanoporous materials, and for studying defects and crystal intergrowth.Using HRTEM imaging, it was possible to observe atomic surfaces and crystal interfaces in zeolites [6][7], as well as missing linkers/clusters in MOFs.Using 3D ED, we can identify missing clusters (0 D), chain disorders (1 D) and layer stacking faults (2 D) [8][9].With the sub-Ångström resolution of 3D ED data, it was possible to resolve and refine disordered atomic sites that are only 0.4 Å apart, from which the nature of the disorders in zeolites could be elucidated.Finally I will show how 3D ED can be used to visualize the structural transformation of a series of zeolites obtained by defect engineering.