## 3 Results and Discussion
### Characteriziaton of Phase Structure and Microstructure
The clear lattice fringes observed in HRTEM images (Figure 3f) indicate the good crystallinity of the NiO microsphere sample. Figure 3g shows that there are many aggregations of sub-microspheres with a size of 100 nm in the NiO sub-microspheres. The high-magnification TEM image in Figure 3h indicates that the sub-microspheres are composed of many grains of 10-20 nm in size and a large number of well-distributed pores are observed. The SAED pattern is also a polycrystalline ring (inset in Figure 3h). The HRTEM image (Figure 3i) shows clear lattice fringes with a spacing of 0.244 nm, which can be attributed to the (111) crystal plane of the NiO phase. The corresponding energy dispersive spectroscopy (EDS) spectra (Figure S1) indicates that the atomic ratio of Ni and O in the samples are close to 1:1. Figure 3: TEM and HRTEM images of as-prepared NiO nanostructures: **(a)** low-magnification TEM image, **(b)** high-magnification TEM image, and **(c)** HRTEM image of NiO nanosheet; **(d)** low-magnification TEM image, **(e)** high-magnification TEM image, and **(f)** HRTEM image of NiO microspheres; **(g)** low-magnification TEM image, **(h)** high-magnification TEM image, and **(i)** HRTEM image of NiO sub-microsphere. The insets in **(b)**, **(e)**, and **(h)** show the corresponding selected area electron diffraction (SAED) patterns. To study the element composition and valence state of NiO sub-microsphere sample, XPS technique was performed (Figure 4). From the survey spectrum (Figure 4a), it can be seen that the binding energy peaks of Ni and O atoms exist in the spectrum. Figure 4b shows the Ni 2p binding energy spectrum. There are eight fitted binding energy peaks in the range of 850-885 eV. Among them, the two peaks located at 853.4 and 855.3 eV correspond to the characteristic peaks of Ni 2p 3/2, and the two peaks around 870.7 and 872.7 eV correspond to the Ni 2p 1/2 peak. The doublet peaks of 853.4 and 870.7 eV are related with Ni\({}^{2+}\) component, and the peaks of 855.3 and 872.7 eV are ascribed to Ni\({}^{3+}\) component [25]. The remaining four characteristic peaks at 860.6, 864.7, 877.9, and 880.9 eV are satellite peaks. Figure 4c shows the O1s binding energy spectrum. It can be fitted to three characteristic peaks of 529.2, 530.9, and 532.1 eV, which correspond to Ni-O bond, oxygen in OH\({}^{-}\) groups, and adsorbed water on the surface of the sample, respectively. These results indicate the formation of NiO phase, which is consistent with XRD results. Since porosities were observed in the NiO samples by TEM observation, the surface area and pore structure of the NiO samples were further investigated and the results are shown in Figure 5. The N\({}_{2}\) adsorption-desorption isotherm curve (Figure 5a) of the NiO nanosheet sample shows a typical IV type isotherm with a H2 hysteresis loop [26], which indicates the existence of a mesoporous structure in the sample. The BET surface area and pore volume are 40.5 m\({}^{2}\)/g and 0.11 cm\({}^{3}\)/g, respectively. The NiO microsphere sample exhibits a similar isotherm curve (Figure 5b), with the BET surface area of 19.8 m\({}^{2}\)/g and pore volume of 0.06 cm\({}^{3}\)/g. Figure 5c shows N\({}_{2}\) adsorption-desorption isotherm curve of the NiO sub-microsphere sample. It can be seen that the enclosure area increases obviously though the same IV type isotherm with a H2 hysteresis loop is observed. The BET surface area and pore volume are 89.5 m\({}^{2}\)/g and 0.58 cm\({}^{3}\)/g, respectively. Figure 5d shows the pore size distribution derived from the adsorption branches of the isotherm data in Figure 5a-c of the NiO samples. A peak centered at  
Figure 4: X-ray photoelectron spectroscopy (XPS) spectrum of as-prepared NiO sub-microsphere: (**a**) survey spectrum; (**b**) Ni 2p spectrum; and (**c**) O 1s spectrum. about 3 nm was observed in the NiO nanosheet and microsphere samples. Comparatively, three peaks located at 9, 15, and 30 nm are observed in the NiO sub-microsphere sample, as well as the apparently increased peak intensity.