## 3 Results and Discussion
### Morphologies of Nanofibers before and after Calcination
However, the particles of Mn(Ac)\({}_{2}\) and Co(Ac)\({}_{2}\), Zn(Ac)\({}_{2}\) are relatively large, the sizes of Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\) and ZnO nanocrystal grains are much larger than that of TiO\({}_{2}\) and NiO during nucleation, and have a relatively loose arrangement. Therefore, the crystallinity of TiO\({}_{2}\) and NiO nanofibers is different from the others. FTIR Characterization of TiO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), NiO and ZnO Nanofibers  
In order to determine the composition and structural changes of nanofibers before and after calcination, FTIR was carried out. It can be seen from Figure 4a that there is a weak broad peak at about 3447 cm\({}^{-1}\), which is the -OH stretching vibration peak of the water molecule adsorbed on the surface of PVP nanofibers. The peaks at 2951, 1422 and 1286 cm\({}^{-1}\) correspond to the -CH\({}_{2}\) asymmetric addition. There is a strong absorption peak at 1661 cm\({}^{-1}\), which is attributed to the CH\({}_{2}\) stretching vibration in PVP [34]. In addition, the broad hump around 1000 cm\({}^{-1}\) is probably due to the uneven baseline, which may be caused by poor particle size and poor transparency of the sample. From Figure 4b, we can see that the infrared characteristic peaks of PVP disappear after calcination at 500 \({}^{\circ}\)C, indicating that the PVP is completely oxidized and decomposed. There is a wide peak at 532 cm\({}^{-1}\) in TiO\({}_{2}\) nanofibers, which is caused by the stretching vibration of Ti-O-Ti. It has been proven that TiO\({}_{2}\) nanocrystals are formed [35]. The peaks at 670 and 578 cm\({}^{-1}\) in the infrared peak of Mn\({}_{2}\)O\({}_{3}\) nanofibers are caused by the longitudinal optical (LO) mode and the transverse optical (TO) mode in the tensile vibration (Mn-O) of Mn\({}_{2}\)O\({}_{3}\), respectively, while the peak at 529 cm\({}^{-1}\) is caused by the bending vibration (Mn-O) of Mn\({}_{2}\)O\({}_{3}\)[36]. Co\({}_{3}\)O\({}_{4}\) nanofibers exhibit strong absorption peaks at 666 and 573 cm\({}^{-1}\), which is related to the tensile vibration of the Co-O band [37]. The single peak at 466 cm\({}^{-1}\) in NiO nanofibers and 437 cm\({}^{-1}\) in ZnO nanofibers can be assigned to Ni-O and Zn-O vibrational peaks respectively [38; 39]. In brief, it can be seen that nanocrystalline metal oxide nanofibers are formed by calcination. Figure 4: FTIR spectra of (**a**) the as-spun nanofibers and (**b**) electrospun nanofibers calcined at 500 \({}^{\circ}\)C.  
XRD Characterization of TiO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), NiO and ZnO Nanofibers  
For determining the crystalline phase of the calcined nanofibers, the samples were characterized by XRD. Figure 5a-e represents TiO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), NiO and ZnO nanofibers, respectively. It indicates that diffraction peaks appearing at 20 of 25.3\({}^{\circ}\), 37.9\({}^{\circ}\), 48.1\({}^{\circ}\) and 62.9\({}^{\circ}\) in the XRD pattern are well attributed to the anatase phase with a tetragonal structure of TiO\({}_{2}\) (JCPDS card no. 21-1272), corresponding to its (101), (004), (200) and (204) facets, respectively. In addition, the peaks at 20 of 27.4\({}^{\circ}\), 36.1\({}^{\circ}\), 41.2\({}^{\circ}\), 44.0\({}^{\circ}\), 54.4\({}^{\circ}\), 56.6\({}^{\circ}\), 64.0\({}^{\circ}\) and 69.0\({}^{\circ}\) in the XRD pattern belong to the tetragonal rutile phase of TiO\({}_{2}\). This reveals that there were two kinds of crystal forms in the obtained TiO\({}_{2}\) nanofibers. As shown in Figure 5b, the formation of Mn\({}_{2}\)O\({}_{3}\) is revealed by the diffraction peaks at 20 values of 23.2\({}^{\circ}\), 33.0\({}^{\circ}\), 38.3\({}^{\circ}\), 45.4\({}^{\circ}\), 49.4\({}^{\circ}\), 55.2\({}^{\circ}\), 60.7\({}^{\circ}\), 64.1\({}^{\circ}\), 65.9\({}^{\circ}\) and 67.6\({}^{\circ}\) corresponding to the (211), (222), (400), (332), (431), (440), (611), (541), (622) and (631) crystal planes of Mn\({}_{2}\)O\({}_{3}\) (JCPDS card no.