## 3 Results and Discussion  
### Characteriziaton of Phase Structure and Microstructure  
The phase structure and morphology of the as-prepared precursors were characterized by XRD and SEM, respectively, as shown in Figure 1. Figure 1a shows the XRD results of as-prepared precursors. When the ratio of Ni\({}^{2+}\)/HMT is 2:1 and 1:2, all the diffraction peaks of the precursor can be indexed as Ni\({}_{3}\)(NO\({}_{3}\))\({}_{2}\)(OH)\({}_{4}\) phase by JCPDS (Joint Committee on Powder Diffraction Standards) card (No. 22-0752) [22]. When the ratio of Ni\({}^{2+}\)/HMT is 1:3, the diffraction peaks of the precursor can be indexed to Ni(HCO\({}_{3}\))\({}_{2}\) phase (JCPDS card No. 15-0782) [23]. Figure 1b-d show the SEM images of the as-prepared precursors. It can be seen that when the ratio of Ni\({}^{2+}\)/HMT is 2:1, the product consists of many nanosheets with an average width of about 1 \(\upmu\)m; when the ratio of Ni\({}^{2+}\)/HMT is 1:2, the product is composed of microspheres with a size of 3-5 \(\upmu\)m; when the ratio of Ni\({}^{2+}\)/HMT is 1:3, the product aggregates 100-200 nm of sub-microspheres. In short, the precursors of Ni\({}_{3}\)(NO\({}_{3}\))\({}_{2}\)(OH)\({}_{4}\) nanosheets, Ni\({}_{3}\)(NO\({}_{3}\))\({}_{2}\)(OH)\({}_{4}\) microspheres, and Ni(HCO\({}_{3}\))\({}_{2}\) sub-microspheres are obtained by adjusting the ratio of Ni\({}^{2+}\)/HMT from 2:1 to 1:2 or 1:3 by our synthesis method. Figure 1: (**a**) XRD (Cu K\(\alpha\), \(\lambda\) = 0.15406 nm) patterns and SEM (secondary electron imaging mode) images of as-prepared precursor obtained by different ratios of Ni\({}^{2+}\)/HMT (hexamethylenetetramine): (**b**) 2:1; (**c**) 1:2; and (**d**) 1:3. The precursors were calcined at 400 \({}^{\circ}\)C for 2 h to obtain the NiO samples. Figure 2 shows the XRD and SEM images of the calcined samples. It can be seen from Figure 2a that the calcined products were well indexed to the NiO phase (JCPDS card No. 47-1049) [24], and no impurity phases such as Ni and Ni(OH)\({}_{2}\) were found. It indicates that the precursor was transformed into pure NiO phase after calcination. Figure 2b-d show SEM images of as-prepared NiO samples. By comparing the morphology of precursors in Figure 1b-d, it is obvious that the morphologies of NiO samples are basically the same as the overall morphologies of the corresponding precursors, which indicates the good structure stability of the materials during calcination. Therefore, NiO nanosheets, microspheres, and sub-microspheres were successfully prepared by calcining precursors. In order to further study the morphology and microstructure of the NiO samples, TEM, high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) patterns were performed and the results are shown in Figure 3. It can be seen from Figure 3a that the NiO nanosheet sample was composed of an aggregation of nanosheets, in accordance with the SEM observation (Figure 2b). The nanosheets are composed of many grains with diameters of 30-50 nm, and a large number of holes are also observed (Figure 3b). The SAED pattern exhibits a typical polycrystalline ring, which indicates that the material has good crystallinity (inset in Figure 3b). The HRTEM image (Figure 3c) demonstrates clear crystal lattice fringes with lattice spacing of 0.208 nm, corresponding to the (200) crystal plane of the NiO phase. Figure 3d indicates that the NiO microsphere sample was composed of about 2 \(\upmu\)m microspheres and some fragmented aggregates with the size of 500 nm which could be caused by the ultrasonic vibration during the TEM sample preparation process. Figure 3e shows that the microsphere  
Figure 2: (a) XRD (Cu K\(\alpha\), \(\lambda\) = 0.15406 nm) patterns and SEM images (secondary electron imaging mode) of as-prepared (**b**) nickel oxide (NiO) nanosheet; (c) NiO microsphere; and (d) NiO sub-microsphere. consists of rod-like particles with a diameter of 20 nm and a length of 100 nm. The SAED pattern also indicates a typical polycrystalline ring (inset in Figure 3e).