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

Summary

Host-guest interactions govern the chemistry of a broad range of functional materials, but direct imaging using conventional transmission electron microscopy has not been possible. This problem is exacerbated in metal-organic framework materials, which are easily damaged by the electron beam. Here, we use cryogenic electron microscopy (cryo-EM) to stabilize the host-guest structure and resolve the atomic surface of zeolitic imidazolate framework (ZIF-8) and its interaction with guest CO₂ molecules. We image step-edge sites on the ZIF-8 surface that provide insight to its growth behavior. Furthermore, we observe two distinct binding sites for CO₂ within the ZIF-8 pore, which are predicted by density functional theory to be energetically favorable. This CO₂ insertion induces an apparent ∼3% lattice expansion along the <002> and <011> directions of the ZIF-8 unit cell. The ability to stabilize and preserve host-guest chemistry opens a rich materials space for scientific exploration and discovery using cryo-EM.