Cryo-EM Structures of Atomic Surfaces and Host-Guest Chemistry in Metal-Organic Frameworks

Abstract

Metal-organic frameworks (MOFs) are a large class of highly porous materials whose chemistry and structure can be tuned for potential applications in gas storage, separations, and catalysis. Interactions between the host framework and guest molecule are central to these applications, but the local atomic structure and chemistry of such interactions is poorly understood. Using cryogenic electron microscopy, we reveal the structural changes of MOFs during guest intercalation and spatially resolve their binding sites within the framework.

Imaging Beam‐Sensitive Materials by Electron Microscopy

“…Similar idea can be extended to high‐fidelity 3D reconstruction by low‐dose orientation sampling. Moreover, as described in Section 3.3, the D c values can also be enhanced by diverse specimen treatment methods, including specimen coating or freezing 49,57. These cutting‐edge dose‐efficient EM technologies and methodologies have led to groundbreaking scientific discoveries associated with diverse beam‐sensitive materials, as reviewed in Section 4.…”

“…Generally, TEM mode has a uniform and continuous illumination onto the specimen while STEM mode has a much more intensive instantaneous illumination together with an accumulated dose 10 4 –10 5 times higher 75. Although advanced electron detection technology, such as direct‐detection and pixelated cameras, has drastically brought down the typical dose used for imaging several orders of magnitude for both imaging modes,34,35,49,135 their distinct beam conditions are suited for imaging beam‐sensitive materials with different dose‐rate effects. For materials with a direct dose‐rate effect (e.g., damage dominant by heating or charging57), which have elevated damage per unit dose with increased dose rate, TEM mode usually leads to less structural damage.…”

“…Using cryo‐EM at liquid‐nitrogen temperature, the pores of MOF‐5 can be clearly resolved, as shown in Figure 8c–g 213. Cui and co‐workers used the cryoholder to image ZIF‐8 nanocrystals and observed an elevated critical dose compared with conventional imaging conditions 49. The liquid nitrogen temperature also stabilizes the ZIF‐8‐CO 2 heterostructure and allows the identification of two distinct CO 2 binding sites and considerable lattice expansions within the framework of ZIF‐8 (Figure 8h–j).…”

“…Accordingly, the high‐frequency diffraction peak at ≈5 nm −1 starts to fade when the cumulative dose reaches about 17 e Å −2 34. In contrast, ZIF‐8(Zn) composed of 4‐coordinated Zn ions with quite ionic ZnN bonds has its diffraction peaks at a much lower spatial frequency faded under a similar dose of ≈25 e Å −2 and completely loses its crystallinity at only ≈75 e Å −2 35,49. The chemical bonding also determines the critical doses of organic compounds.…”

“…For example, MOFs comprise a large family of porous crystalline materials featuring highly designable and flexible pore architectures, framework topologies and structural functionalities, and they hold great promise for a wide range of applications, such as gas separation and storage, sensing, and catalysis 46–48. However, an unambiguous structure determination of MOFs through TEM imaging is unfortunately very challenging because most MOFs typically collapse after exposure to only a few electrons Å –2 34,35,49. As another example, graphene, well known for its 2D structure, unprecedented mechanical strength and high charge carrier mobility, is susceptible to degradation for electron beam energies exceeding 80 keV 45.…”