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Usually, it is not easy to distinguish γ-Fe2O3 from Fe3O4 in XRD patterns because of their similar structures. Therefore, another characterization method, X-ray photoelectron spectrum, was also used to differentiate the structures of iron oxides as presented in the Fe 2p core-level XPS spectra of γ-Fe2O3, Fe3O4 and α-Fe2O3 (Fig. 2). Both spectra of γ-Fe2O3 and α-Fe2O3 show very similar patterns with main peak of Fe 2p3/2 around 710.8 eV, which is attributed to the Fe3+ component in γ-Fe2O3 and α-Fe2O3. As we know, the XPS Fe 2p core-level spectra of γ-Fe2O3 and α-Fe2O3 are almost identical with each other despite their large differences in crystal structures [21]. On the other hand, the binding energy of Fe 2p3/2 of the Fe2+ component in Fe3O4 is around 708.5 eV [22]. Therefore, the two samples can be assigned to γ-Fe2O3 or α-Fe2O3. As observed in their spectra, the Fe 2p3/2 and 2p1/2 main peak of two samples is accompanied by a satellite structure around 718.9 eV. Just as proven in other work [21], the XPS Fe 2p spectrum of γ-Fe2O3 possesses smaller satellite intensity as compared with that of α-Fe2O3 due to the larger Fe 3d to O 2p hybridization in γ-Fe2O3. The Fe 2p3/2 peak in the spectrum of Fe3O4 sample is much wider than those in γ-Fe2O3 and α-Fe2O3, which is considered to be composed of two peaks in the positions of 708.6 and 710.8 eV. The binding energy of 708.6 eV is attributed to Fe3O4, while 710.8 eV can be assigned to γ-Fe2O3. This is because Fe3O4 nanoparticles are quite sensitive to oxygen even at low temperature, resulting in the oxidation of the surface of Fe3O4 nanoparticles to γ-Fe2O3 during the drying and preparation of XPS samples. |
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