Fig. 5.

Typical atomic-level STEM images and DFT calculations of defect-riched IFONFs-45. a HAADF-STEM image of IFONFs-45 taken from [1–10] direction of FeF₂. The enlarged views of exact objects in the dashed-box depict detailed distribution of Fe atoms. b BF-STEM image of IFONFs-45. The FeF₂ tetragonal lattice consisting of Fe (purple dots) and alternating F (turquoise dots) atoms is readily distinguished in left side of dashed-box. The neighboring columns depict faint O signals (magenta dots) at the right of interface, as Fe and F atoms are brighter than the substituted O atoms in bright-field mode. Atomic structure of FeF₂ (110) surface, with stoichiometric termination (c), and adsorbed with O, Fe, and H (d). Fe: purple; F: turquoise; O: magenta; H: white. Both top and side views are revealed. e Free energy of H adsorption on different sites. “per” means perfect (stoichiometric) surface, “v” represents vacancies, “ad” for adatoms, “sub” for F substituted by O, and “sub-ad-Fe” for Fe adatom on O that substituted F. Scale bar, 1 nm