## 3 Results and Discussion
### Possible Mechanism Analysis of Metal Oxide Nanofibers  
In order to explore the formation mechanism of nanocrystalline metal oxide nanofibers, TGA experiments of each nanofiber were evaluated in air. As shown in Figure 7, the five nanofibers are roughly divided into three stages during the calcination process from 35 to 500 \({}^{\circ}\)C. The first stage is at about 35-180 \({}^{\circ}\)C, and in this stage, the weight loss is relatively small (<10%), mainly caused by the residual organic reagents in the nanofiber and the evaporation of water. The second stage is at approximately 150-350 \({}^{\circ}\)C, which is caused by the decomposition of acetate and the side chain of PVP. The third stage is at 350-500 \({}^{\circ}\)C, which is mainly caused by the decomposition of residual PVP and crystal formation [46]. The specific data are shown in Table 3. On the basis of the aforementioned experimental analysis, a possible formation mechanism of the nanocrystalline metal oxide nanofibers \begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Nanofibers** & **TiO\({}_{2}\)** & **Mn\({}_{2}\)O\({}_{3}\)** & **Co\({}_{3}\)O\({}_{4}\)** & **NiO** & **ZnO** \\ \hline Specific area (m\({}^{2}\)/g) & 121.445 & 18.600 & 52.717 & 393.645 & 57.219 \\ Pore volume (m\({}^{3}\)/g) & 0.079 & 0.072 & 0.082 & 0.328 & 0.081 \\ Pore diameter (nm) & 3.823 & 3.820 & 3.834 & 3.816 & 3.407 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Specific area distribution, pore volume distribution and pore diameter distribution of the metal oxides nanofibers. Figure 6: (a) N\({}_{2}\) adsorption-desorption isotherms and (b) pore diameter distribution of electrospun nanofibers calcined at 500 \({}^{\circ}\)C.  
can be explained as follows. During the calcination process, the PVP in the nanofiber will undergo intermolecular and intramolecular thermal decomposition and be accompanied by nucleation and growth of grains, which will result in a corresponding change in surface morphology. Specifically, when the temperature rises above the decomposition point of organometallic salts, a small amount of metastable nucleations appears in the amorphous region due to the low concentration of metal ions in the nanofibers, and the nanocrystal grains with smaller size are formed as the surrounding atoms are adsorbed to the crystal nucleus. At this time, the PVP molecules become flexible but do not decompose, so the small nanocrystal grains will move gently with the PVP molecules. As the probability of the movement toward each orientation is equivalent, the particles are uniformly dispersed in the PVP nanofibers. Then, when the temperature continues to increase, the PVP molecules move dramatically and begin to decompose, which is the main reason for the bending of the calcined nanofibers. At this moment, the newly formed nanocrystal grains grow larger, and have a better crystallinity. Furthermore, due to the limitation of the one-dimensional morphology of nanofibers, the annexation and growth of the grains can be limited in the axial direction of the nanofibers that retain the morphology of the original shape. Finally, when the PVP molecules are completely decomposed, the grains become coarser and connected with each other in the process of heat preservation, and the nanocrystalline metal oxide nanofibers with porous structure are finally formed. It is worth noting that according to the analysis of the above process, temperature plays a crucial role. As shown in Figure 2a',e', the obtained TiO\({}_{2}\) and ZnO nanofibers are curled irregularly and are uneven in thickness, which may be due to the dramatic movement, collapse and adhesion of PVP caused by the excessive temperature rise. In addition, the appearance of two crystal forms of TiO\({}_{2}\) is also related to temperature. TiO\({}_{2}\) is mainly composed of the anatase phase at a relatively low temperature stage. When the temperature is elevated, the anatase phase is unstable and then gradually converted into the rutile phase [47].