## 3 Results and Discussion  
### Physicochemical Properties of the NFs  
Morphological and textural properties of the electrospun NFs were investigated by SEM and TEM analyses (Figure 1). In both types of produced NFs (Fe\({}_{2}\)O\({}_{3}\) NFs and Fe\({}_{2}\)O\({}_{3}\):Ge NFs), no beads were observed. Nonetheless, the results revealed that the addition of GeIPO to the FeAc\({}_{2}\)-PAN-DMF precursor solution translated into evident differences in the morphology and texture of the calcined NFs. SEM images showed that the NFs prepared without adding GeIPO had rough and porous surface (Figure 1a,b). Their diameters, as calculated by the image analysis software, varied mainly between 120 and 500 nm, although largely thicker NFs (>0.8 \(\upmu\)m) were also present. The center of the diameter distribution was located at 230 nm. Differently, the surface of the germanium-containing NFs was smoother (Figure 1c,d) and their size varied more uniformly in the 65-1180 nm range. The center of the distribution moved at higher values (360 nm). Many factors, such as feeding rate of the solution, applied voltage, collection distance and ambient humidity, affect the morphology and diameters of the fibers. In the present study, feeding rate of the solution, applied voltage and collection distance were kept constant. In addition, the relative air humidity (40%) was the same in the two syntheses. Nonetheless, the well-known extreme moisture sensitivity of the germanium isopropoxide and miscibility of water with DMF might have played a role in giving rise to a more uniform fiber diameter distribution since humidity causes changes in the nanofibers diameter by controlling the solidification process of the charged jet. The TEM analysis shed light on the different texture of the fibers (Figure 1e-j). Although both types of NFs exhibited a hierarchical structure, in the absence of germanium (Figure 1e-g), the NFs consisted of interconnected rounded and smooth crystalline grains, about 65 nm in size [39]. The resulting coral-like architecture seems to be peculiar to electrospun iron oxide NFs, as it has been frequently observed regardless of the precursors utilized to prepare the spinnable solution and the heat treatment conditions [46; 47; 48; 49; 50; 51; 52; 53]. Previous studies on NFs prepared under the same conditions [42] had revealed the single-crystal or quasi-single-crystal nature of the oxide grains, as well as the hollow section of the fibers that is due to the large temperature gradient along the radial direction experienced by the FeAc\({}_{2}\)-PAN NFs upon calcination [54]. Different from the pure iron oxide NFs, the  
Figure 1: Morphology of the electrospun NFs, as resulting from (**a–d**) SEM and (**e–j**) TEM analyses. The shown SEM images refer to (**a,b**) Fe\({}_{2}\)O\({}_{3}\) NFs and (**c,d**) Fe\({}_{2}\)O\({}_{3}\):Ge NFs. The shown TEM images refer to (**e–g**) Fe\({}_{2}\)O\({}_{3}\) NFs and (**h–j**) Fe\({}_{2}\)O\({}_{3}\):Ge NFs. germanium-containing ones consisted of finer and elongated nanostructures that developed mainly along the longitudinal axis of the fiber (Figure 1h-j). A hierarchical structure with similar characteristics had been previously reported for electrospun Si-doped iron oxide NFs [36]. Indeed, both silicon and germanium are believed to drive the growth along preferential crystallographic directions to a limited extent [41; 55; 56]. The spatial distribution of the NF components was investigated via TEM/EDX (Figure S3). The elemental maps revealed a homogeneous distribution of iron, oxygen and germanium within the NFs, with no organic residuals from the pristine NFs. The oxide phase in the NFs and its crystallinity were investigated by X-ray and neutron diffraction and MRS analyses. Figure S4 compares the diffraction patterns of the NFs produced from the FeAc\({}_{2}\)-PAN-DMF and GeIPO-FeAc\({}_{2}\)-PAN-DMF solutions.