Evidence for Fe ²⁺ in wurtzite coordination: Iron doping stabilizes ZnO nanoparticles

First-principles calculations are used to investigate the structural and electronic properties of Fe-doped ZnO nanoparticles. Based on extensive validation studies surveying various density functionals, the hybrid functional PBE0 is employed to calculate the structures, formation energies, and electronic properties of Fe in ZnO with Fe concentrations of 6.25, 12.5, and 18.75 at%. Substitution of Zn by Fe, zinc vacancies, and interstitial oxygen defects is studied. High-resolution inner-shell electron energy loss spectroscopy measurements and X-ray absorption near-edge structure calculations of Fe and O atoms are performed. The results show that Fe-doped ZnO nanoparticles are structurally and energetically more stable than the isolated FeO (rocksalt) and ZnO (wurtzite) phases. The Fe dopants distribute homogeneously in ZnO nanoparticles and do not significantly alter the host ZnO lattice parameters. Simulations of the absorption spectra demonstrate that Fe ²⁺ dominates in the Fe-doped ZnO nanoparticles reported recently, whereas Fe ³⁺ is present only as a trace. Flame spray pyrolysis allows the easy doping of ZnO nanoparticles (NPs) with other metals. Iron-doped ZnO is less toxic and less soluble than pure NPs. ZnO can be doped with up to 10 at% Fe without significant changes in the lattice. Fe ²⁺ distributes homogeneously inside the NPs and substitutes Zn ²⁺ ions at its lattice sites.