Soft X-ray Spectroscopy Study of the Electronic Structure of Oxidized and Partially Oxidized Magnetite Nanoparticles

Abstract

The crystal structure of magnetite nanoparticles may be transformed to maghemite by complete oxidation, but under many relevant conditions the oxidation is partial, creating a mixed valence material with structural and electronic properties that are poorly characterized. We used X-ray diffraction, Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and soft X-ray absorption and emission spectroscopy to characterize the products of oxidizing uncoated and oleic-acidcoated magnetite nanoparticles in air. The oxidization of uncoated magnetite nanoparticles creates a material that is structurally and electronically indistinguishable from maghemite. By contrast, while oxidized oleic-acid-coated nanoparticles are also structurally indistinguishable from maghemite, Fe Ledge spectroscopy revealed the presence of interior reduced iron sites even after a 2-year period. We used X-ray emission spectroscopy at the O K-edge to study the valence bands (VB) of the iron oxide nanoparticles, using resonant excitation to remove the contributions from oxygen atoms in the ligands and from low-energy excitations that obscured the VB edge. The bonding in all nanoparticles was typical of maghemite, with no detectable VB states introduced by the long-lived, reduced-iron sites in the oleic-acid-coated sample. However, O K-edge absorption spectroscopy observed a ~0.2 eV shift in the position of the lowest unoccupied states in the coated sample, indicating an increase in the semiconductor band gap relative to bulk stoichiometric maghemite that was also observed by optical absorption spectroscopy. The results show that the ferrous iron sites within ferric iron oxide nanoparticles coated by an organic ligand can persist under ambient conditions with no evidence of a distinct interior phase, and exert an effect on the global electronic and optical properties of the material.This phenomenon resembles the band gap enlargement caused by electron accumulation in the conduction band of TiO 2 .

An X-ray magnetic circular dichroism (XMCD) study of Fe ordering in a synthetic MgAl2O4-Fe3O4(spinel-magnetite) solid-solution series: Implications for magnetic properties and cation site ordering

“…For the two most Mg-rich synthetic samples (SP3 and SP5) and the natural magnesian spinel (Cressey et al, 1993) the Fe L 2,3 -edge peaks have a larger energy separation (1.64-1.71 eV; 1.68 ± 0.03 eV) than for the Fe-rich samples. This separation is similar to that (~ 1.6 eV) between reduced (lower energy peak) and oxidised Fe in an L 2,3 spectroscopy study of zoned oxidised magnetites (Gilbert et al, 2010) and in reduced and oxidised versions of organic dinuclear Fe-oxo compounds (Peng et al, 1995) . Sample SP9 shows intermediate properties with a peak separation of ~1.45 eV.…”

Fe-incorporated TiO2 nanotube arrays: Electronic structure and magnetic response

“…Fe L 3,2 -edge NEXAFS: The Fe L 3,2 -edge is reported to be very sensitive to the Fe oxidation state [34] [31]. The absorption spectra of the Fe L 3,2 -edge for the Fe-incorporated nanotubes in the as-synthesized and annealed states are shown in Figure 8(a), compared to the those of the reference Fe-based compounds.…”

Synthesis, Functionalized and Characterization of Magnetic Fe3O4 Nanoparticles Protected with MCM-41 Mesoporous

“…Magnetic nanoparticles and their functionalized, attract plenty attention through their markable usages in different field such as biotechnology [1], metal ion extraction [2], carriers for targeted drug delivery [3], optical imaging [4], electronic [5], magneto resistance [6] and catalyst [7]. In which magnetic nanoparticles are present, specially for Fe3O4 nanoparticles have individual, features including superparamagnetism, high surface area, large surface-tovolume ratio, facile separation by external magnetic fields [3,[8][9][10].…”