## 3 Results and Discussion  
To investigate the crystal structure of the ZTO-based samples, the XRD patterns of ZTO, Co-ZTO, and Ni-ZTO are shown in Figure 1A. All three samples show strong and sharp XRD peaks at 20 = 17.7\({}^{\circ}\), 29.1\({}^{\circ}\), 34.3\({}^{\circ}\), 35.9\({}^{\circ}\), 41.7\({}^{\circ}\), 51.7\({}^{\circ}\), 55.1\({}^{\circ}\), and 66.4\({}^{\circ}\), corresponding to the (111), (220), (311), (222), (400), (422), (511), and (440) planes of Zn\({}_{2}\)SnO\({}_{4}\) (JCPDS. No. 24-1470). There is no other diffraction peak related to SnO\({}_{2}\), ZnO, NiO, or CoO observed in Figure 1A. Figure 1B shows the enlargement of the (311) plane for ZTO, Ni-ZTO, and Co-ZTO around 34.3\({}^{\circ}\). Compared with the pure ZTO samples, the peak position of the (311) plane for Co-ZTO and Ni-ZTO shifted to a larger diffraction angle. In addition, the lattice parameters and cell volume also decreased for Co-ZTO and Ni-ZTO, in comparison with the pure ZTO samples. This implies that the introduced Co and Ni atoms were weaved into the lattice of Zn\({}_{2}\)SnO\({}_{4}\). It is known that the ionic radius of Zn\({}^{2+}\) ions and Sn\({}^{4+}\) ions is 74 pm and 71 pm, respectively. As iron family elements, the valence of Co and Ni atoms usually exist as +2 or +3 in metal oxide. The ionic radius of Co\({}^{2+}\) and Ni\({}^{2+}\) is 74 pm and 72 pm. The ionic radius of Co\({}^{3+}\) and Ni\({}^{3+}\) is 63 pm and 62 pm. According to the doping theory for metal oxide, it is difficult for the Ni and Co atoms to be incorporated into the Zn\({}_{2}\)SnO\({}_{4}\)'s lattice in the interstitial mode, as the ionic radius of Ni and Co is close to the ionic radius of Zn and Sn. Thus, it becomes reasonable to deduce that the Co and Ni ions are doped in the Zn\({}_{2}\)SnO\({}_{4}\)'s lattice in the substitutional mode. Moreover, if the lattice's Zn\({}^{2+}\) or Sn\({}^{4+}\) ions are replaced by Co\({}^{2+}\) or Ni\({}^{2+}\) ions, the lattice parameters and cell volume of Zn\({}_{2}\)SnO\({}_{4}\) should remain unchanged, owing to the similar ionic radius. If the lattice's Zn\({}^{2+}\) or Sn\({}^{4+}\) ions are replaced by Co\({}^{3+}\) or Ni\({}^{3+}\) ions, the lattice parameters and cell volume of Zn\({}_{2}\)SnO\({}_{4}\) should be reduced, as the ionic radius of the Co\({}^{3+}\) ions (63 pm) and Ni\({}^{3+}\) ions (62 pm) is much smaller than that of the Zn\({}^{2+}\) ions (74 pm) and Sn\({}^{4+}\) ions (71 pm). According to experimental data in Figure 1B and Table 1, it is reasonable to deduce that the Co\({}^{3+}\) and Ni\({}^{3+}\) ions are doped in the Zn\({}_{2}\)SnO\({}_{4}\)'s lattice in the substitutional mode. To further investigate the chemical states of the Co\({}^{3+}\) and Ni\({}^{3+}\) ions in Zn\({}_{2}\)SnO\({}_{4}\), the XPS characterization is applied in the following section. As the existing states of the Co and Ni ions in Zn\({}_{2}\)SnO\({}_{4}\) are closely related to the valence of the Co and Ni ions, the XPS Survey, Co 2p of the Co-ZTO sample, Ni 2p of the Ni-ZTO sample, O 1s, Zn 2p, and Sn 3d spectra for the ZTO, Co-ZTO, and Ni-ZTO samples are plotted in Figure 2. All the ZTO-based photocatalysts exhibit strong XPS peaks related to the C 1s, O 1s, Sn 3d, and Zn 2p. As shown in Figure 2B, a weak XPS peak centered at about 779.2 eV is observed for the Co-ZTO samples. This weak Co 2p peak can be ascribed to the Co\({}^{3+}\) ions [29]. As shown in Figure 2C, the weak XPS Ni 2p peak at about 855.6 eV can be attributed to the Ni\({}^{3+}\) ions [30]. Hence, it can be confirmed by the XPS that the introduced Co and Ni atoms exist as +3 valence in this work. Moreover, as the ionic radius of the Co\({}^{3+}\) ions (63 pm) and Ni\({}^{3+}\) ions (62 pm) is much smaller than that of Zn\({}^{2+}\) ions (74 pm) and Sn\({}^{4+}\) ions (71 pm), the lattice's Zn\({}^{2+}\) or Sn\({}^{4+}\) ions are replaced by Co\({}^{3+}\) or Ni\({}^{3+}\) ions in the substitutional mode, leading to the reduced parameters and cell volume.