Surface models for Fe single atoms on SiO₂. (a) Schematic of Fe⁰ grafting over the SiO₂ ; shown is a zoomed-out (left) and a zoomed-in (right) image for the optimised geometry of the Fe⁰@SiO₂ system. (b) Schematic of the Fe²⁺ grafting over the SiO₂; shown is a zoomed-out (left) and a zoomed-in (right) image for the optimised geometry of the Fe²⁺@SiO₂ system. The relevant distances are depicted in angstroms (Å). Colour-coding: white, H atoms; grey, C atoms; red, O atoms; beige, Si atoms; and orange, Fe atoms.