Fe₂O₃ nanoparticle structures investigated by X-ray absorption near-edge structure, surface modifications, and model calculations

The structures of Fe₂O₃ nanoparticles with different sizes were investigated using Fe K-edge X-ray absorption near-edge structure (XANES) and the FEFF calculations, as well as surface modification with enediol ligands. The studies not only revealed the existence of under-coordinated Fe sites in the nanoparticles but also confirmed that these under-coordinated sites were located on the surface. Upon binding of enediol ligands, surface sites were restructured to octahedral sites. In particular, the nature of the surface defects and their correlation with the unique properties of the nanoparticles were discussed. Model calculations were conducted for Fe_mO_n (m ≥ 1, n ≥ 4) clusters of various sizes centered at Fe sites with octahedral (O_h), distorted octahedral (C_3v) and tetrahedral (T_d) coordination geometry using FEFF8.10 programs. The main features of the calculated spectra agree with the experimental results and were correlated to the density of states, the Fe coordination geometry, and the long-range order of the lattice.