## 3 Results and Discussion
### Morphologies of Nanofibers before and after Calcination
41-1142), respectively [36]. As observed in Figure 5c, all diffraction peaks are well indexed to pure Co\({}_{3}\)O\({}_{4}\) with the structure of JCPDS card no. 65-3103, where the diffraction peaks at 20 values of 18.9\({}^{\circ}\), 31.3\({}^{\circ}\), 36.9\({}^{\circ}\), 38.6\({}^{\circ}\), 44.9\({}^{\circ}\), 59.4\({}^{\circ}\) and 65.3\({}^{\circ}\) are ascribed to the reflections of (111), (220), (311), (222), (400), (511) and (440) planes of the Co\({}_{3}\)O\({}_{4}\), respectively [40]. Figure 5d,e indicate the phase of NiO and ZnO. The formation of NiO is revealed by the diffraction peaks at 20 values of 37.3\({}^{\circ}\), 43.3\({}^{\circ}\), 62.9\({}^{\circ}\), 75.3\({}^{\circ}\) and 79.2\({}^{\circ}\) corresponding to (111), (200), (220), (311) and (222) crystal planes of NiO, respectively [41], while the diffraction peaks at 20 values of 31.9\({}^{\circ}\), 34.5\({}^{\circ}\), 36.4\({}^{\circ}\), 47.7\({}^{\circ}\), 56.7\({}^{\circ}\), 63.1\({}^{\circ}\), 66.2\({}^{\circ}\), 68.0\({}^{\circ}\), 69.2\({}^{\circ}\), 72.6\({}^{\circ}\) and 77.3\({}^{\circ}\) correspond to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) crystal planes of ZnO, respectively [42]. In addition, to verify the XRD results, the HRTEM diagrams of the nanofibers are shown in Figure 3a'-e' respectively. As presented in Figure 3a', the interplanar distances between lattice fringes of 0.350 nm and 0.327 nm correspond to the anatase phase (101) and the tetragonal rutile phase (110) in TiO\({}_{2}\), respectively. The lattice fringes with a spacing of 0.276 nm can be clearly observed in Figure 3b', which is in agreement with the spacing of (222) planes of Mn\({}_{2}\)O\({}_{3}\). Figure 3c' shows clear lattice fringes of 0.279 nm corresponding to the crystallographic (111) plane of Co\({}_{3}\)O\({}_{4}\), which indicates that the sample was structurally uniform and well crystallized. As shown in Figure 3d',e', the interplanar distances between lattice fringes of 0.240 and 0.279 nm correspond to the facet (101) of NiO and the facet (110) of ZnO, respectively. The results of XRD and HRTEM analysis suggest that the calcined nanofibers are composed of metal oxide nanocrystals. Figure 5: X-ray diffraction (XRD) spectra of a: TiO\({}_{2}\) nanofiber, b: Mn\({}_{2}\)O\({}_{3}\) nanofiber, c: Co\({}_{3}\)O\({}_{4}\) nanofiber, d: NiO nanofiber, e: ZnO nanofiber. N\({}_{2}\) Adsorption-Desorption Isotherms and Pore Size Distribution of TiO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), NiO and ZnO Nanofibers  
To characterize these porous nanofibers further, the N\({}_{2}\) adsorption-desorption isotherms and the pore diameter distributions of all the samples are shown in Figure 6, and a summary of the specific surface area, pore volume and average pore diameter data is shown in Table 2. According to the classification of international union of pure and applied chemistry (IUPAC), all the samples exhibit characteristic type IV isotherms with H3-type hysteresis loops, indicating the presence of uniform microporous structure. The pore size distributions of TiO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), NiO and ZnO nanofibers are 3.823, 3.820, 3.834, 3.816 and 3.407, respectively, which is consistent with the pore size distributions of the corresponding samples obtained from the desorption data of isotherms. The pore volume of the TiO\({}_{2}\), Mn\({}_{2}\)O\({}_{3}\), Co\({}_{3}\)O\({}_{4}\), NiO and ZnO nanofibers are 0.079, 0.072, 0.082, 0.328 and 0.081 m\({}^{3}\)/g, respectively, and the specific areas are 121.445, 18.600, 52.717, 393.645 and 57.219 m\({}^{2}\)/g, respectively. Obviously, NiO nanofibers have the largest specific surface area compared with other nanofibers, which agrees with the result of the TEM. The specific surface area ascribes to the porous structure and the small particle size [43; 44; 45], and the smaller the particle size, the larger specific surface area.