## 3 Results and Discussion
In Figure 2D, the lattice's O 1s peak is at about 530.1 eV, and the surface's O 1s peak is at about 531.5 eV for all three samples. In Figure 2E, the Zn 2p\({}_{3/2}\) is at 1020.8 eV for ZTO and 1020.9 eV for Co-ZTO and Ni-ZTO. In Figure 2F, the Sn 3d\({}_{5/2}\) and Sn 3d\({}_{3/2}\) are at 486.2 eV and 494.7 eV for all ZTO-based samples. To investigate the morphology of Co-doped Zn\({}_{2}\)SnO\({}_{4}\) and Ni-doped Zn\({}_{2}\)SnO\({}_{4}\), the TEM and HR-TEM images of Co-ZTO and Ni-ZTO are shown in Figure 3. As shown in Figure 3A,B, both the Ni-ZTO and Co-ZTO samples consist of microspheres with an average diameter of 300 nm\(\sim\)500 nm. Moreover, these microspheres are constructed by a series of nanoplates, which are intercrossed with each other by self-assembly. These microsphere structures may leave many pores on the surface, increase the specific surface area, and improve the stability of photocatalysts, eventually benefiting the photocatalytic performance. Moreover, as shown in Figure 3C, the fringe lattice spacing for Co-ZTO is estimated to be 0.488 nm, corresponding to the (111) plane of Zn\({}_{2}\)SnO\({}_{4}\). As shown in Figure 3D, the lattice spacing for Ni-ZTO is estimated to be 0.260 nm, corresponding to the \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{4}{c}{**Lattice Parameters**} & \multirow{2}{*}{**Cell Volume (Å\({}^{3}\))**} \\ \cline{2-2} \cline{4-5}  & **a** & **b** & **c** \\ \hline ZTO & 8.6711 & 8.6711 & 8.6711 & 651.97 \\ Co-ZTO & 8.6546 & 8.6546 & 8.6546 & 648.24 \\ Ni-ZTO & 8.6414 & 8.6414 & 8.6414 & 645.29 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Lattice parameters and cell volume of pure ZTO, Co-ZTO, and Ni-ZTO samples. Figure 1: **(A) XRD patterns of ZTO, Ni-ZTO, and Co-ZTO. (B) Enlarged XRD peaks of the (311) plane for ZTO, Ni-ZTO, and Co-ZTO. **  
(311) plane of Zn\({}_{2}\)SnO\({}_{4}\). It should be noted that the lattice spacing for Co-ZTO and Ni-ZTO in the HR-TEM images is smaller than that of the standard Zn\({}_{2}\)SnO\({}_{4}\) cards. This may be caused by the substitutionally doped Co\({}^{3+}\) and Ni\({}^{3+}\) ions in the Zn\({}_{2}\)SnO\({}_{4}\)'s lattice. Thus, it can be concluded from the TEM images that both the Co-ZTO and Ni-ZTO samples are composed of microspheres, which consist of nanoplates intercrossed with each other. To better investigate the morphology of the as-prepared Co-ZTO and Ni-ZTO samples, the EDS elemental mapping of the Zn, Sn, O, Co and Zn, Sn, O, Ni for Co-ZTO, and Ni-ZTO samples are shown in Figures 4 and 5, respectively. It is clear in Figures 4 and 5 that the ZTO-based samples mainly consist of Zn, Sn, and O elements. In addition, a small amount of Ni and Co elements are randomly dispersed and did not enrich in some points. This also suggests the existence of Co\({}^{3+}\) and Ni\({}^{3+}\) ions doped in the lattice of Zn\({}_{2}\)SnO\({}_{4}\) in the substitutional mode. Figure 3: TEM image of (**A**) Co-ZTO and (**B**) Ni-ZTO; HR-TEM image of (**C**) Co-ZTO and (**D**) Ni-ZTO. Figure 2: XPS (**A**) Survey, (**B**) Co 2p of the Co-ZTO sample, (**C**) Ni 2p of the Ni-ZTO sample, (**D**) O 1s, (**E**) Zn 2p, and (**F**) Sn 3d spectra for ZTO, Co-ZTO, and Ni-ZTO samples. Figure 4: EDS elemental mapping of the Co-ZTO samples. Figure 5: EDS elemental mapping of the Ni-ZTO samples. To obtain physical insight into the band structure of Co\({}^{3+}\)-doped Zn\({}_{2}\)SnO\({}_{4}\) and Ni\({}^{3+}\)-doped Zn\({}_{2}\)SnO\({}_{4}\), a theoretical calculation based on the density functional theory calculation (DFT) is carried out, and the corresponding density of the states for ZTO, Co-ZTO, and Ni-ZTO is shown in Figure 6. Theoretical calculations were performed using the Vienna ab initio software package (VASP.5.4.4). As shown in Figure 6A, the band gap of pure ZTO is estimated to be about 0.93 eV, which is much smaller than the experimental data. This is because the DFT calculation usually underestimates the band gap of the semiconductors [31; 32; 33].