## 3 Results and Discussion
### Physicochemical Properties of the NFs
In the spectrum of Fe\({}_{2}\)O\({}_{3}\):Ge NFs (Figure 2), the peaks of both \(\alpha\)- and \(\gamma\)-phase of the iron oxide were clearly visible, in full agreement with the evidences of XRD analysis. On the contrary, no obvious peak ascribable to germanium dioxide was detected. Spectra recorded at different random locations were different from each other (Figure S5), indicating that the incorporation of germanium led to the spatially non-uniform formation of \(\alpha\)- and \(\gamma\)-polymorphs of the oxide. Figure 3a displays the X-ray photoelectron spectra of Fe 2\(p\) core level in the electrospun NFs. The spectrum of Fe2O3 NFs consisted of two spin-orbit components at binding energies of 710.5 eV (2\(p_{3/2}\)) and 724.0 eV (2\(p_{1/2}\)) and of two Fe3+ shake-up satellites at 718.3 and 732.1 eV, as typical of hematite [63,75]. In agreement with the literature [36,41,56], the incorporation of germanium in the iron oxide lattice did not caused appreciable variations in the spectral profile, but a slight shift (\(\sim\)0.4 eV) of spin-orbit doublets and shake-up satellites towards higher binding energies, for which both co-formation of maghemite [76] and incorporation of germanium [41] could have been responsible. In the Fe2O3:Ge NFs, the binding energy position of the Ge 2\(p_{3/2}\) (1220 eV, inset to Figure 3a) and Ge 3\(d\) (32 eV, not shown) core levels evidenced the presence of Ge ions with +4 oxidation state [77, 78, 79]. O 1s core level of the two samples exhibited evident differences both in terms of binding energy and shape (Figure 3b), hinting at different surrounding cations. Two components at 530.5 (O1) and 532.7 eV (O1)were singled out in the spectrum of Fe2O3 NFs (Figure S6a); they were ascribed to O2 - ions of the surface lattice oxygen in \(\alpha\)-Fe2O3 matrix and to surface hydroxyl (Fe-OH) groups, respectively [80]. The spectrum of Fe2O3:Ge NFs exhibited larger broadening (Figure 3b), mirroring the presence of more kinds of oxygen species [81] and clearly pointing out the incorporation of Ge ions into the \(\gamma\)-Fe2O3 lattice (i.e., solid solutions), as well as the co-presence of segregate phases of \(\gamma\)-Fe2O3 and Ge2O3 oxides [81,82]. In maghemite, surface lattice oxygen has been reported to contribute to the O1s intensity at lower binding energy compared with hematite [83,84], whereas the O1s peak of germanium dioxide has been detected at 531.8 eV [77]. Thus, in agreement with the results of XRD and MRS analyses, not only Fe-O species in \(\alpha\)-Fe2O3 nanocrystals, but also surface lattice oxygen in \(\gamma\)-Fe2O3 and Ge2O2 phases were identified in the O1s spectrum of Fe2O3:Ge NFs, as well as surface hydroxyl groups (Figure 6b). The different  
Figure 3: X-ray photoelectron spectra of (**a**) Fe 2p and (**b**) O 1s core levels. Inset to plot (**a**): Ge 2p core level in Fe2O3:Ge NFs. Figure 2: Micro-Raman spectra of the electrospun NFs, as resulting from the average on different random locations in each specimen. oxygen species were quantified assuming that the contributions at lower bindings energies (530.1-531.6 eV), O\({}_{\text{Ia}}\) and O\({}_{\text{Ib}}\), originate from Fe-O and Ge-O lattice oxygen, respectively, whereas the signal at higher binding energy (532.7 eV), O\({}_{\text{II}}\), is related to either the presence of molecular oxygen weakly bounded or oxygen from adsorbed species (e.g., hydroxyl species) [81]. Accordingly, the concentrations of the three species were estimated to be 57.6% (O\({}_{\text{Ia}}\)), 31.4% (O\({}_{\text{Ib}}\)) and 11.0% (O\({}_{\text{II}}\)). Therefore, considering that the fraction of oxygen relating to the isolated GeO\({}_{2}\) phase (OI\({}_{\text{Ib}}\)) is 31.4%, being the atomic surface abundance of oxygen 68.8%, that of germanium 10.9% (Table S1) and the stoichiometric ratio between oxygen and germanium of 2:1, it followed that almost all of the Ge ions on the surface of the Fe\({}_{2}\)O\({}_{3}\):Ge NFs should be involved in the formation of the oxide phase (Table S2).