  Host-guest interactions govern the chemistry of a broad range of functional
materials. However, the weak bonding between host and guest prevents
atomic-resolution studies of their structure and chemistry using transmission
electron microscopy (TEM). This problem is exacerbated in metal-organic
frameworks (MOF), in which the host framework is easily damaged by the electron
beam. Here, we use cryogenic-electron microscopy (cryo-EM) to simultaneously
address these two challenges and resolve the atomic surface structure of
zeolitic imidazolate framework (ZIF-8) and its interaction with guest CO2
molecules. We image atomic step-edge sites on the ZIF-8 surface that provides
possible insight to its growth behavior. Furthermore, we observe two distinct
binding sites for CO2 within the ZIF-8 pore, which are both predicted by
density functional theory (DFT) 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 MOFs and preserve
their host-guest chemistry opens a rich materials space for scientific
exploration and discovery using cryo-EM.
