## 3 Results and Discussion
As shown in Figure 6B,C, the valence bands of Co-ZTO and Ni-ZTO are mainly composed of the O 2p states as well as the Sn and Zn states. The conduction bands of Co-ZTO and Ni-ZTO are mainly contributed by the Zn and Sn states, with a small amount of the O 2p states. Moreover, compared with pure ZTO samples, new doping energy levels occurred in the band gaps of Co-ZTO and Ni-ZTO, respectively. The introduced doping energy levels are mainly contributed by the Co/Ni and O elements, suggesting the occurred energy levels originated from the doped Co and Ni ions. Hence, it can be concluded that the introduced Co\({}^{3+}\) and Ni\({}^{3+}\) ions in the substitutional mode would induce new doping energy levels above the valence band of ZTO. To explore the influence of the doped Co and Ni ions on the band structure and light absorption capacity of the Zn\({}_{2}\)SnO\({}_{4}\) samples, the UV-vis diffuse reflectance absorption spectra of the pure ZTO, Co-ZTO, and Ni-ZTO samples are shown in Figure 7A. The pure ZTO samples show strong absorption in the UV region, which can be attributed to the electrons' transition from the valence band to the conduction band of ZTO. For the Co-ZTO and Ni-ZTO samples, no blue shift or red shift is observed for the absorption edge of the spectra. This suggests that the doped Co\({}^{3+}\) and Ni\({}^{3+}\) ions did not change the band gap of the Zn\({}_{2}\)SnO\({}_{4}\) microspheres. Moreover, it is found that the absorption spectra from the UV to the visible region are enhanced, which is caused by the doped Co\({}^{3+}\) and Ni\({}^{3+}\) ions in the substitutional mode. According to the DFT calculation results, the extended absorption can be ascribed to the electrons' transition from the energy levels of doped Co\({}^{3+}\) and Ni\({}^{3+}\) ions in the substitutional mode to the conduction band of ZTO. In addition,  
Figure 6: Calculated density of states (DOS) for the (**A**) ZTO, (**B**) Co-ZTO, and (**C**) Ni-ZTO samples. the Tauc plots of ZTO, Co-ZTO, and Ni-ZTO are plotted in Figure 7B. The band gap for ZTO, Co-ZTO, and Ni-ZTO is estimated to be 3.30 eV. As the doped Co and Ni ions did not change the morphology and nanostructure of the ZTO microspheres, the band gap of Co-ZTO and Ni-ZTO remained unchanged, in comparison with the pure ZTO samples. Therefore, according to the UV-vis absorption spectra, it can be confirmed that new doping energy levels originated from the doped Co\({}^{3+}\) and Ni\({}^{3+}\) ions that occurred in the band gap of ZTO. The efficient separation and transfer of photogenerated charge carriers is beneficial for enhancing photocatalytic performance. Figure 7C illustrates the photocurrent response versus time for the ZTO-based samples, which was analyzed to evaluate the separation efficiency of the charge carriers. Notably, the photocurrent response, as observed in Figure 7C, exhibited a remarkable improvement for Co-ZTO and Ni-ZTO when compared to pure ZTO, during five on/off repetition cycles of the light. This observation strongly indicates an enhanced separation efficiency of the charge carriers in the Co-ZTO and Ni-ZTO samples. The photocatalytic reduction of CO\({}_{2}\) into CH\({}_{4}\) is performed to investigate the effect of doped Co\({}^{3+}\) and Ni\({}^{3+}\) ions on ZTO samples, shown in Figure 8. For the pure ZTO samples, only a small amount of CH\({}_{4}\) (1.02 \(\times\) 10\({}^{-7}\) mol) is yielded after 8 h irradiation. For the Co-ZTO samples, more CH\({}_{4}\) is produced under irradiation. There is about 1.78 \(\times\) 10\({}^{-7}\) mol of CH\({}_{4}\) detected for 8 hours' irradiation. The Ni-ZTO samples exhibit the highest photocatalytic activity and about 1.92 \(\times\) 10\({}^{-7}\) mol of CH\({}_{4}\) is produced, which is almost twice that for pure ZTO. Hence, doping Co or Ni ions into the lattice of ZTO is an effective method to improve the photocatalytic performance of ZTO-based photocatalysts.