## 3 Results and Discussion  
### Morphologies of Nanofibers before and after Calcination  
SEM measurements were performed to study morphology of the nanofibers before and after calcination. Figure 2a-e represents Ti(BuO)\({}_{4}\)/PVP, Mn(Ac)\({}_{2}\)/PVP, Co(Ac)\({}_{2}\)/PVP, Ni(Ac)\({}_{2}\)/PVP and Zn(Ac)\({}_{2}\)/PVP blended nanofibers with fiber diameters in the range of 300-700, 200-600, 300-600, 800-1000 and 350-600 nm, respectively. It can be seen that the nanofibers exhibit uniform diameter and continuous long nanofiber smooth morphology. Furthermore, due to unsteady bending of the spinning jet, the nanofibers are arranged in random orientation [33]. Therefore, the nanofibers with a three-dimensional random orientation network structure are finally formed. Figure 2a'-e' represents TiO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), NiO and ZnO nanofibers calcined at 500 \({}^{\circ}\)C, respectively. It can be seen that the calcined nanofibers still remain the three-dimensional scaffold, and consist of well-twisted continuous nanofiber structures. Due to the oxidative decomposition of organometallic salt and removal of PVP, the nanofibers show a shape of shrinkage. The diameters of the nanofibers decreased to 200-600 nm for TiO\({}_{2}\), 190-260 nm for Mn\({}_{2}\)O\({}_{3}\), 150-370 nm for Co\({}_{3}\)O\({}_{4}\), 350-550 nm for NiO and 240-500 nm for ZnO nanofibers. In detail, all five obtained nanofibers are composed of neat nanoparticles with a necklace-like arrangement, and exhibit high spatial orientation and necklace-like porous structure. TiO\({}_{2}\) and ZnO nanofibers are curled irregularly. Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\) and NiO nanofibers show straight geometry with granular rough microstructure. In addition, Figure 3a-e exhibits the TEM of TiO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), NiO and ZnO nanofibers, respectively, from which the nanoparticles and the porous surface structures can be clearly reflected. Figure 2: SEM (scanning electron microscope) images of the electrospun nanofiber (**a**) Ti(BuO)\({}_{4}\)/PVP, (**b**) Mn(Ac)\({}_{2}\)/PVP, (**c**) Co(Ac)\({}_{2}\)/PVP, (**d**) Ni(Ac)\({}_{2}\)/PVP, (**e**) Zn(Ac)\({}_{2}\)/PVP and the nanofibers calcined at 500 \({}^{\circ}\)C (**a**) TiO\({}_{2}\) nanofiber, (**b**’) Mn\({}_{2}\)O\({}_{3}\) nanofiber, (**c**’) Co\({}_{3}\)O\({}_{4}\) nanofiber, (**d**’) NiO nanofiber, (**e**’) ZnO nanofiber. Figure 3: TEM and high resolution transmission electron microscopy (HRTEM) images of (**a**,**a**) TiO\({}_{2}\) nanofiber, (**b**,**b**) Mn\({}_{2}\)O\({}_{3}\) nanofiber, (**c**,**c**) Co\({}_{3}\)O\({}_{4}\) nanofiber, (**d**,**d**) NiO nanofiber, (**e**,**e**) ZnO nanofiber. The grains are densely packed along the nanofiber length. Specifically, it can be seen that the TiO\({}_{2}\) and NiO nanoparticles have a uniform and compact arrangement with an average diameter of 10 and 20 nm, respectively. While, the sizes of Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\) and ZnO nanoparticles are relatively large, with sizes of 80-100, 60-80, 50-75 nm, respectively. In addition, the edges of Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\) and ZnO nanofibers are rough, and the arrangement of Mn\({}_{2}\)O\({}_{3}\) particles is relatively dispersed. It is obvious that the crystallinity of TiO\({}_{2}\) and NiO nanofibers is different from others. After careful consideration, we think the possible reasons are as follows. Due to the different solubility of these five metal salts in PVP solution, only the electrospun precursors of TiO\({}_{2}\) and NiO nanofibers are clear and transparent, while the other three precursors are homogeneous emulsions. In the calcination process, when the temperature rises above the decomposition point of organometallic salts, a small amount of metastable nucleations appears in the amorphous region, and the nanocrystal grains with smaller size are formed as the surrounding atoms are adsorbed to the crystal nucleus.