## 3 Results and Discussion
### Photocatalytic Activities of Metal Oxide Nanofibers  
Finally, in order to verify the excellent properties of the prepared nanocrystalline metal oxide nanofibers, the photocatalytic experiments were carried out. Figure 8a-e presents the absorption spectra of MB aqueous solution in the presence of the ZnO, TiO\({}_{2}\), NiO, Mn\({}_{2}\)O\({}_{3}\) and Co\({}_{9}\)O\({}_{4}\) nanofibers under exposure to UV light for different durations. In Figure 8a-c, with the increase of the irradiation time, the absorption peak corresponding to MB at 664 nm diminishes gradually, testifying the degradation of MB. It indicates that MB can be degraded by ZnO, TiO\({}_{2}\) and NiO nanofibers, respectively. However, the absorption peak of MB remained relatively unchanged in Figure 8d,e, indicating that Mn\({}_{2}\)O\({}_{3}\) and Co\({}_{9}\)O\({}_{4}\) could not induce the degradation of MB. As shown in Figure 8f, under dark conditions, MB dye was decolorized about 3.7%, 5.5% and 4.5% in the presence of ZnO, TiO\({}_{2}\), NiO nanofibers due to adsorption of the dye. After irradiation with UV light for 150 min, 97.6% of MB was decomposed by using the ZnO nanofiber as the photocatalyst, while at the same time TiO\({}_{2}\) and NiO nanofibers revealed 93.8% and 86.7% degradation. Table 4 shows a summary of TiO\({}_{2}\), ZnO and NiO nanofibers employed in the photocatalysis of a wide range of dyes and pollutants. The results indicate that the prepared TiO\({}_{2}\), ZnO and NiO nanofibers in the paper have good photocatalytic properties, especially NiO nanofibers. Figure 8: Adsorption spectra of methylene blue solution under UV light in the presence of (**a**) TiO\({}_{2}\) nanofiber, (**b**) Mn\({}_{2}\)O\({}_{3}\) nanofiber, (**c**) Co\({}_{9}\)O\({}_{4}\) nanofiber, (**d**) NiO nanofiber, (**e**) ZnO nanofiber, and (**f**) photocatalytic MB dye degradation activity. In order to further investigate the photocatalytic activities of these samples, the photocatalytic degradation kinetics were analyzed. From Figure 9a, the samples exhibit a good linear relationship between ln(C/C\({}_{0}\)) and irradiation time \(t\). The reaction rate constants (slope of the fitting line) of ZnO, TiO\({}_{2}\) and NiO nanofibers are 0.02447, 0.01874 and 0.01269 min\({}^{-1}\), respectively. In addition, the recycle ability of ZnO, TiO\({}_{2}\) and NiO nanofibers were also studied (Figure 9b), after the third recycle, the photocatalytic efficiency remains 95.9%, 87.1% and 65.4%, respectively, which indicates that ZnO and TiO\({}_{2}\) nanofibers have an excellent catalytic cycling ability. Many researchers have explained the mechanism of photolysis [55; 56; 57; 58]: when the ZnO, TiO\({}_{2}\) and NiO are excited by UV light with energy higher than or equal to their band gaps, electrons (e\({}^{-}\)) in the valence band (VB) can be excited to the conduction band (CB), leaving corresponding holes (h\({}^{+}\)) in the VB. Photogenerated electrons (e\({}^{-}\)) on the surface of nanoparticles are easily captured by electronic acceptors such as oxygen dissolved in water to form superoxide radicals (\(\cdot\)O\({}_{2}\)\({}^{-}\)). At the same time, photo-induced holes (h\({}^{+}\)) can oxidize the organic chemicals adsorbed on the surface of metal oxides or be trapped by electronic donors like OH\({}^{-}\) and H\({}_{2}\)O to produce \(\cdot\)OH. The oxidation ability of \(\cdot\)OH and \(\cdot\)O\({}_{2}\)\({}^{-}\) are very strong, which can make the combination bond of various organic compounds break, so that MB can be oxidized to obtain CO\({}_{2}\) and H\({}_{2}\)O.