The as-synthesized Fe3O4 nanoparticle assemblies can be transformed
easily into either ç-Fe2O3 or R-Fe nanoparticle assemblies
by annealing at different atmospheres. Figure 2E is the XRD pattern
of a red-brown nanoparticle assembly obtained from O2 oxidation
of a black 16-nm Fe3O4 particle assembly at 250 °C. Compared to
Figure 2D, the large-angle peaks in Figure 2E shift slightly to higher
angles, and additional small peaks appear at lower angles. All peak
positions and relative intensities match well with those of commercial
ç-Fe2O3 powder materials (Aldrich catalog No. 48,066-5),
indicating that the oxidation of Fe3O4 under O2 leads to ç-Fe2O3.
Accordingly, due to the oxidation, the saturation magnetization (ós)
of the assembly is reduced from 82 emu/g to 70 emu/g (300 K).17
After annealing under Ar + 5% H2 at 400 °C, the Fe3O4
nanoparticle assembly is reduced to bcc-Fe nanoparticle assembly,
as confirmed by XRD shown in Figure 2F. The reduction leads to
the drastic increase in ós to 186 emu/g, a value close to 210 emu/g
of bulk Fe. Magnetic measurements show that all as-synthesized
Fe3O4 nanoparticles are superparamagnetic at room temperature.17