## 1 Introduction
Its unique physical and chemical properties make it widely applicable in energy, environmental monitoring, and fire prevention. With the continuous advancement of science and technology, the application areas of Zn\({}_{2}\)SnO\({}_{4}\) will continue to expand, bringing more contributions to the development of human society [18; 22; 23; 24]. Zn\({}_{2}\)SnO\({}_{4}\) has been widely investigated as a photocatalyst, owing to its high electric conductivity, high electron mobility, stability, and charming optical properties [18; 22; 23; 24]. [PERSON] et al. composited ZTO with SnO\({}_{\text{x}}\) by a one-pot hydrothermal method [23]. The composited photocatalysts exhibit improved photocatalytic performance for the degradation of methyl orange (MO) and gaseous benzene. [PERSON] and [PERSON] synthesized highly ordered Zn\({}_{2}\)SnO\({}_{4}\) nanotube arrays by using ZnO arrays as templates [25]. These ZTO arrays exhibit enhanced photocatalytic activity. [PERSON] et al. synthesized high crystalline Zn\({}_{2}\)SnO\({}_{4}\) nanostructures using a hydrothermal method [26]. They modified the Zn\({}_{2}\)SnO\({}_{4}\) nanostructure with Pt to improve the photocatalytic performance of H\({}_{2}\) production. [PERSON] et al. synthesized Zn\({}_{2}\)SnO\({}_{4}\) with Mo doping and oxygen vacancies with high photocatalytic activity on HCHO oxidation [27]. Mo doping and the introduction of oxygen vacancies would extend the visible response, promote the separation of charge carriers, and induce new reactive sites for the ZTO-based photocatalysts. [PERSON] et al. composited ZTO with ZIF-8, which exhibits significant photocatalytic performance on Methylene Blue (MB) degradation [22]. Our previous work also reported a simple synthesis of Sn\({}^{4+}\) ions self-doped Zn\({}_{2}\)SnO\({}_{4}\) nanoplates [18]. The Sn\({}^{4+}\) ions would induce a new doping energy level below the conduction band of ZTO. The visible response is extended, and charge carriers are separated. As a result, the photocatalytic activity on the CO\({}_{2}\) reduction of Sn\({}^{4+}\) ions self-doped Zn\({}_{2}\)SnO\({}_{4}\) nanoplates is significantly improved. However, the detailed influence of the doped elements on the band structure and photocatalytic performance of ZTO-based photocatalysts still needs further investigation. In this work, Co and Ni, two iron family elements, were selected to be weaved into the lattice of Zn\({}_{2}\)SnO\({}_{4}\) to investigate the influence of foreign elements on the band structure and photocatalytic performance of Zn\({}_{2}\)SnO\({}_{4}\) samples. The photocatalytic reduction of CO\({}_{2}\) into CH\({}_{4}\) was applied to evaluate the photocatalytic performance of the as-prepared catalysts. It was revealed that the lattice Zn\({}^{2+}\) ions were replaced by the Co\({}^{3+}\) or Ni\({}^{3+}\) ions in the substitutional mode. The doped Co\({}^{3+}\) or Ni\({}^{3+}\) ions would induce a new doping energy level in the band gap of Zn\({}_{2}\)SnO\({}_{4}\). As a result, the light response is enhanced, the charge carriers are separated, and the photocatalytic activity is improved for the Ni-doped Zn\({}_{2}\)SnO\({}_{4}\) and Co-doped Zn\({}_{2}\)SnO\({}_{4}\) samples. The photocatalytic mechanism of the enhanced CO\({}_{2}\) reduction is also discussed.