## 3 Results and Discussion  
### Morphology, Microstructure, and Composition Characterizations  
The morphologies of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanostructures were characterized by SEM, as shown in Figure 2. When using ethanol and deionized water (volume ratio = 3:1) as reaction solvent, the resultant product was composed of aggregates of nanoparticles with about 30-40 nm in diameter (Figure 2a). When using deionized water, the resultant product was composed of nanorods with about 30-40 nm in width and 90-120 nm in length (Figure 2b). When using TEG and deionized water (volume ratio = 3:1), the resultant product consisted of many nanowires with about 20-30 nm in width and 200-300 nm in length (Figure 2c). When using glycerol and deionized water (volume ratio = 3:1), some microspheres with about 0.3 \(\upmu\)m in diameters are observed, which are composed of small nanoparticles (Figure 2d). Therefore, in our synthesis method, NiCo\({}_{2}\)O\({}_{4}\) nanostructures with different morphologies can easily obtained by adjusting the type of reaction solvents. Figure 2: Scanning electron microscopy (SEM) images (secondary electron imaging mode) of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanostructures obtained by different reaction solvents: (**a**) Ethanol and deionized H\({}_{2}\)O (3:1); (**b**) deionized H\({}_{2}\)O; (**c**) tetraethylene glycol (TEG) and deionized H\({}_{2}\)O (3:1); and (**d**) glycerol and deionized H\({}_{2}\)O (3:1). The crystalline structure and phase composition of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanostructures were analyzed by XRD (Cu K\(\alpha\), \(\lambda\) = 0.15406 nm). Figure 3a shows that all samples exhibited similar diffraction peaks, which matched well with the standard diffraction data (PDF. No. 73-1702) for spinel structured NiCo\({}_{2}\)O\({}_{4}\) phase [13]. All diffraction peaks correspond to the (220), (311), (333), (400), (422), (511), and (440) crystal planes of spinel structure NiCo\({}_{2}\)O\({}_{4}\). The lattice parameters (a) of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanoparticles, nanorods, nanowires, and microspheres were calculated by the XRD data, which are 0.8113, 0.8116, 0.8112, and 0.8111 nm, respectively. All calculated lattice parameters are closed to the lattice parameter a = 0.8114 nm in the standard data (PDF. No. 73-1702) for spinel structured NiCo\({}_{2}\)O\({}_{4}\) phase. No other diffraction peaks of impurities phase were observed. It indicated that pure phase of spinel structured NiCo\({}_{2}\)O\({}_{4}\) are obtained by our synthesis method. The typical spinel structure of NiCo\({}_{2}\)O\({}_{4}\) is illustrated in Figure 3b. To further investigate the microstructural information of as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires sample, TEM, high-resolution TEM (HRTEM), and selected area electron (SAED) patterns were measured and the results are shown in Figure 4. Figure 4a shows that as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires sample was composed of nanoparticles of 20-30 nm in diameter, which were connected to one another to form nanowire-like structure. The SAED pattern indicated good polycrystallinity of the sample (inset in Figure 4b). Figure 4c is the HRTEM image of enlarged part marked by red square in Figure 4b. It reveals distinct lattice lines with spacings of 0.286 and 0.244 nm according to the HRTEM measurement and subsequent calculation by the software of Digital Micrograph (Gatan Inc., Pleasanton, CA) applied in the HRTEM, which are corresponding to the (220) and (311) planes of spinel structure NiCo\({}_{2}\)O\({}_{4}\), respectively [14]. The corresponding elemental mapping (Figure 4d) shows evenly distributed elements of Ni, Co, and O in the single nanowire, which supports the XRD results that the as-prepared NiCo\({}_{2}\)O\({}_{4}\) nanowires are multi-component metal oxide, instead of metal oxide composites. The corresponding EDS spectrum shows that the atomic ratio of Ni:Co for the single NiCo\({}_{2}\)O\({}_{4}\) nanowire is approximately 1:2 (Figure 4e).