Effect of silica capping on the oxidation of Fe${}_3$O${}_4$ nanoparticles in dispersion revealed by x-ray absorption spectroscopy

Fe${}_3$O${}_4$ nanoparticles have been investigated as they are biocompatible and their surface can be functionalized. We synthesized iron oxide nanoparticles using a water-in-oil microemulsion method. Bare and silica-coated iron oxide nanoparticles of a core size of 6 nm dispersed in ethanol have been investigated by means of x-ray absorption spectroscopy (XAS). Due to a dedicated experimental setup the particles can be measured directly in dispersion. XAS allows us to disentangle the contributions of the Fe${}^{2+}$ and Fe${}^{3+}$ ions and therefore to estimate the amount of Fe${}_3$O${}_4$ in the particles. In case of the silica coated particles a high amount of magnetite was obtained. In contrast, the bare nanoparticles showed indications of a further oxidation into $\gamma$-Fe${}_2$O${}_3$ even in dispersion.

Figure 1

Unit cell of ideal Fe${}_3$O${}_4$ (magnetite). Fe${}^{3+}$ ions are equally distributed on tetrahedral (red spheres) and octahedral (blue spheres) lattice sites. The remaining octahedral sites are occupied by Fe${}^{2+}$ ions. The gray spheres represent the oxygen anions.

Figure 2

TEM image and related size distribution of bare iron oxide nanoparticles.

Figure 3

TEM image and related size distribution of silica-coated iron oxide nanoparticles.

Figure 4

Low-temperature (4.2 K) Mössbauer spectrum of 6 nm nanoparticles with organic ligands. The experimental spectrum (dots) has been fitted with 3 sextets accounting for the contributions of Fe${}^{3+}$ and Fe${}^{2+}$ ions in tetrahedral (A sites: Fe${}^{3+}$ $T_d$) and octahedral (B sites: Fe${}^{3+}$ $O_h$ and Fe${}^{2+}$ $O_h$) environment in an external magnetic field of 5 T. Data taken from Ref. 27.

Figure 5

Normalized XAS at the Fe $L_{3,2}$ edges of bare and silica-coated iron oxide nanoparticles dispersed in ethanol.

Figure 6

(a) Calculated XAS at the Fe $L_3$ edge for Fe${}_3$O${}_4$ revealing the contributions from the different surroundings of the Fe${}^{2+}$ and Fe${}^{3+}$ ions. (b) Normalized experimental XAS at the Fe $L_3$ edge.

Figure 7

Normalized XAS at the Fe $L_3$ edge of bare and silica-coated iron oxide nanoparticles dispersed in ethanol. The spectrum of iron oxide nanoparticles, where the organic ligands are partially washed out, is plotted for comparison (green line).

Figure 8

Calculated x-ray absorption spectra for different degrees of inversion and oxidation (violet spectrum). The spectra are an average over the contributions from the inequivalent sites and are normalized at the $L_3$-edge maximum.