## 3 Results and Discussion
### Physicochemical Properties of the NFs
Thus, summarizing, the XRD, XPS, and MRS analyses indicated that Fe\({}_{2}\)O\({}_{3}\) NFs can be considered as a single-phase compound; differently, the Fe\({}_{2}\)O\({}_{3}\):Ge NFs are more complex, both in terms of phase composition and compositional homogeneity, being composed by at least two crystalline phases, and an amorphous component. To further explore the compositional and structural aspects of Fe\({}_{2}\)O\({}_{3}\):Ge NFs, X-ray absorption spectroscopy and neutron diffraction analysis were carried out. XAS measurements were acquired on Fe\({}_{2}\)O\({}_{3}\):Ge NFs at the Fe and Ge K-edges, aiming at selectively obtaining electronic and structural information on both atomic species. The XANES region gives direct indication of the oxidation state; moreover, its fingerprinting properties allow discerning between different species by rapid comparison with the spectra of reference materials. For this purpose, Figure 4a compares the XANES spectrum of the sample at the Fe K-edge with those of hematite (\(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)) and maghemite (\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\)) [85]. The energy position of the rising edge is indicative of the oxidation state of the sample: in this case, the edge energy of Fe\({}_{2}\)O\({}_{3}\):Ge NFs, marked as a black dotted line, was the same as in \(\alpha\)- and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), indicating that all iron is present as Fe(III), in agreement with results of XPS analysis. All the spectral features in the spectrum were attributable either to maghemite or hematite; for a quantification of the effective composition, a linear combination fit was performed (see Figure S7). The best fit, obtained with 77 (1)% maghemite and 23 (1)% hematite, showed a very good agreement with the experimental signal. However, the presence of a small amount of iron-based oxyhydroxide species, especially \(\gamma\)-FeOOH (lepidocrocite) and ferrihydrite, which present some spectral features in the same energy region as maghemite [86; 87], could not be excluded. Figure 4b compares the spectrum of Fe\({}_{2}\)O\({}_{3}\):Ge NFs at the Ge K-edge with the reference spectrum of GeO\({}_{2}\) in the quartz-like structure. The edge energy position was coincident in the two spectra, indicating that Ge has oxidation state 4+, in full agreement with the XPS analysis. All the spectral features were visibly broadened in Fe\({}_{2}\)O\({}_{3}\):Ge NFs, as expected  
Figure 4: (a) XANES spectra at the Fe K-edge of Fe\({}_{2}\)O\({}_{3}\):Ge NFs, hematite and maghemite (these last spectra taken from ref [85]. The spectra are shifted along the \(y\)-axis for better comparison. The absorption edge energy, common to all the three spectra, is indicated as a black dotted line. (b) XANES spectra at the Ge K-edge of Fe\({}_{2}\)O\({}_{3}\):Ge NFs and of quartz-like GeO\({}_{2}\). The spectra are shifted along the \(y\)-axis for better comparison. for nanosized and/or amorphous samples, and displayed a striking similarity. At the Ge K-edge, the main absorption peak (white line, WL) at 11,112 eV is due to the transition from filled 1\(s\) orbitals to empty 4\(p\) orbitals. The WL of the spectrum of Fe\({}_{2}\)O\({}_{3}\):Ge NFs, as well as for quartz-like GeO\({}_{2}\), was unique and intense, as expected in case of tetrahedral coordination. Indeed, when Ge is in an octahedral environment (e.g., in rutile-like GeO\({}_{2}\)), the WL is broadened and split into three distinct features [88; 89]. In order to gain further evidence on the coordination around Ge, the EXAFS signal and the corresponding Fourier transform (FT) were extracted (Figure S8a,b). The FT showed an intense peak around 1.7 A, due to the first Ge-O distance, while all the other contributions above 3 A became negligible, confirming the amorphousness of the Ge local environment. The \(x(k)\) signal resulting from the first shell was isolated through a Fourier-filtering process and fitted, allowing to selectively consider the contribution of the first neighboring atoms around the photoabsorber.