## 3 Results and Discussion
### Morphology, Microstructure, and Composition Characterizations
To investigate the chemical composition and element valence of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires sample, the XPS analysis was performed and the results are shown in Figure 5. The survey spectrum showed the presence of the core-levels peaks of Ni 2p, Co 2p, and O 1s (Figure 5a). The Ni 2p spectrum (Figure 5b) exhibits several strong peaks, which are fitted to the 2p\({}_{3/2}\) and 2p\({}_{1/2}\) peaks and the satellite peaks. The peaks at 854.3 and 871.4 eV are associated with Ni\({}^{2+}\) component and the peaks located at 856.1 and 873.2 eV are related with Ni\({}^{3+}\) component in NiCo\({}_{2}\)O\({}_{4}\)[15]. The peaks at 861.1 and 879.4 eV are the satellite peaks. The Co 2p spectrum (Figure 5c) consists of two spin-orbit doublets and two satellite peaks. The doublet peaks at 779.7 and 794.5 eV are characteristic of Co\({}^{3+}\) component. The doublet peaks at 781.7 and 796.0 eV are characteristic of Co\({}^{2+}\) component. The satellite peak at 788.4 and 803.7 eV are associated with Co\({}^{3+}\) component and Co\({}^{2+}\)/ Co\({}^{3+}\), respectively [16]. The O 1s spectrum (Figure 5d) can be fitted to four  
Figure 3: (a) X-ray diffraction (XRD) (Cu K\(\alpha\), \(\lambda\) = 0.15406 nm) patterns of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanostructures; (b) structural illustration of spinel NiCo\({}_{2}\)O\({}_{4}\). peaks at 529.5, 530.9, 531.9, and 533.3 eV, which correspond to metal-oxygen bonds, oxygen in hydroxyl groups, oxygen defects, and surface adsorbed water, respectively [17]. These results confirmed the formation of spinel NiCo\({}_{2}\)O\({}_{4}\) phase, consistent with the XRD and TEM results. Since the gases produced during AP thermal decomposition mainly react on the surface of additives, and the gases are diffused through open pores rather than closed pores, the properties of catalytic additives depend on the surface area and open porosities. The surface areas and open porosities of the samples were studied by the surface area and porosity analyzer, and the results are illustrated in Figure 6. The surface areas were calculated by BET method, which are 34.3, 39.3, 43.7, and 30.1 m\({}^{2}\)/g for as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanoparticles, nanorods, nanowires, and microspheres, respectively. The absorption-desorption isothermals curves of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanostructures are shown in Figure 6a. All samples show a typical IV type isotherm with a H2 hysteresis loop according to IUPAC [18], indicating the mesoporous characteristic of the samples. Figure 6b shows the pore size distribution curves of the samples determined from the desorption branches. All samples exhibited mesoscale pores range from 2-20 nm, with a peak centered around 2.5 nm. Therefore, all as-prepared NiCo\({}_{2}\)O\({}_{4}\) samples possess mesoporous structure. Figure 4: (**a**) Low-magnification transmission electron microscopy (TEM) image; (**b**) high-magnification TEM image, (**c**) high-resolution TEM (HRTEM) image of enlarged part marked by red square in (**b**), (**d**) elemental mapping, and (**e**) energy dispersive spectrometer (EDS) spectrum of single NiCo\({}_{2}\)O\({}_{4}\) nanowire.