## 2 Materials and Methods
### Characterization  
After un-controlled cooling to RT, the as-calcined samples (Figure S2) were analyzed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), powder neutron diffraction (PND), micro-Raman spectroscopy (MRS), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS).  
SEM images were acquired on a Zeiss microscope (15 kV), equipped with software for the automated measurement of the NF diameters from their SEM images. A Philips CM 200 microscope (200 kV), equipped with energy-dispersive X-ray (EDX) spectrometer, was utilized for the TEM analysis. XRD patterns were recorded with a Bruker D2 diffractometer using Ni \(\beta\)-filtered Cu-K\({}_{\alpha}\) radiation source (\(\lambda\) = 0.1541 nm). Raman scattering was measured in air at RT by means of a NTEGRA--Spectra SPM NT-MDT confocal microscope coupled to a solid-state laser operating at 633 nm. A variable ND filter set the laser power at 250 \(\upmu\)W at the sample surface to prevent local heating. The scattered light from the sample, collected by means of a 100X Mitutoyo objective, was detected by a cooled ANDOR iDus CCD Camera. Spectra from several random positions on each specimen were recorded to investigate its spatial homogeneity. The collected spectra were averaged to have a reliable picture of the entire sample. XPS analysis was conducted with a Physical Electronics GMBH PHI 5800-01 spectrometer, equipped with a monochromatic Al-K\({}_{\alpha}\) source (1486.6 eV) with a power beam of 300 W. The C 1\(s\) peak at 284.5 eV was assumed as reference for binding energies. Further details on the instrumentation and experimental data processing can be found elsewhere [36; 37; 39; 42].  
The XAS spectra were collected at the Fe and Ge K-edges (7112 and 11,103 eV, respectively) at the XAFS beamline [43] operating at the Elettra synchrotron radiation facility in Trieste, Italy. The spectra were recorded at RT in transmission mode; for the measurements, a proper amount of sample (as to give a unit jump in the absorption coefficient) was mixed with cellulose and pressed into a pellet. Energy calibration was performed measuring the absorption spectrum of either metallic Fe pellet and Ge foil, placed in a second experimental chamber after the sample and after the II ionization chamber. The ring current and the energy were 200 mA and 2.4 GeV, respectively. A Si(111) double-crystal monochromator was used, ensuring high-order harmonic rejection by detuning of the second crystal. A water-cooled Pt-coated silicon mirror was used to obtain vertical collimation of the beam. The X-ray signal extraction and analysis, including the linear combination fit, were performed by means of Athena, belonging to the set of interactive programs IFEFFIT [44; 45]. For the X-ray absorption near edge structure (XANES) analysis, the raw spectra were first background subtracted using a straight line and normalized to unit absorption above 800 eV, where the extended X-ray absorption fine structure (EXAFS) oscillations are no more visible. The EXAFS data analysis at the Ge K-edge was performed employing the Excurve code using a k2 weighing scheme and isolating the x(k) signal given by the first shell through Fourier filtering, using a Gaussian window between 0.7 and 2.4 A.  
A PND pattern of the Fe\({}_{2}\)O\({}_{3}\):Ge NFs was collected at RT in a vanadium sample holder on the high intensity D20 diffractometer at the Institut Laue-Langevin (ILL), Grenoble, France, with the use of 1.54 A wavelength, in the 0-150\({}^{\circ}\) angular range, using the high-resolution option with a takeoff angle of 90\({}^{\circ}\). Data were analyzed using the FullProf Suite to perform LeBail fitting and traditional Rietveld refinement.