## 3 Synthesis of Transition-Metal-Sulfide-Based Nanomaterials
### Computational Studies Using MoS\({}_{2}\)  
The results from simulations have also been reported on for the reduction of CO\({}_{2}\) using different metal sulfides with the aim to provide more insights into its mechanisms, efficacy, and efficiency. [PERSON] et al. studied the mechanism of reduction of CO\({}_{2}\) to CO using MoS\({}_{2}\) monolayers as catalysts [157]. The simulation results were performed using DFT calculations. The results indicated that the Mo exposed on the edges of the MoS\({}_{2}\) were the adsorption sites for the CO\({}_{2}\) and the O=C=O bonds which were reconfigured during the adsorption process between two Mo centers. In addition, the first H\({}^{+}\) reduction reaction occurred through a different pathway which pushed product selectivity towards the formation of CO. [PERSON] et al. on the other hand, investigated the effect of modification of the MoS\({}_{2}\) edge structure by the addition of transition metals on electrocatalytic CO\({}_{2}\) reduced to CO using DFT calculations [158]. The focus of the study was on the insertion of V, HF, and Zr into edge sites of the MoS\({}_{2}\). The simulation or (modeling) results indicated that the activity did not depend on dopant concentration. However, locating the dopant close to the Mo active site was extremely important for desorption. Similarly, [PERSON] et al., using DFT calculations, investigated the reduction reaction of CO\({}_{2}\) on MoS\({}_{2}\) monolayers [159]. In that work, the authors investigated the formation of 2H, 1T, and 1T\({}^{\prime}\) phases in MoS\({}_{2}\) supported on Ag, Au, and Cu. The results indicated that the MoS\({}_{2}\) basal planes were relatively unaffected by the supports, but the Mo active sites were poisoned by the support. Additionally, the support effects were important and needed to be considered.  
[PERSON] et al. used computational methods to investigate single-atom catalysts based on first-row transition elements on MoS\({}_{2}\) for the electrocatalytic reduction of CO\({}_{2}\) to methanol [160]. The results showed that the Ni-MoS\({}_{2}\) combination was the most stable catalyst of the series using electronic structure studies. [PERSON] et al. used DFT computational methods to investigate single-atom-supported catalysts on MoS\({}_{2}\) for the electrocatalytic reduction of CO\({}_{2}\)[161]. The authors investigated Pd, Co, No, and Pt metals supported on MoS\({}_{2}\), which were shown to be efficient in the electrocatalytic reduction of CO\({}_{2}\). The Fe, Co, Ni, and Pt on MoS\({}_{2}\) produced primarily CH\({}_{4}\) and had a low limiting potential. In addition, the formation of HCOO\({}^{*}\) was determined to be one of the key reactive descriptors for the reaction. The authors also noted that the MoS\({}_{2}\), WS\({}_{2}\), and WSe\({}_{2}\) systems with single-atom catalysts showed promise. [PERSON] et al. on the other hand used DFT calculations to explore the effects of edge-exposed MoS\({}_{2}\) hybridized with N-doped carbon [162]. The results in that study showed that a high number of exposed edge sites on the MoS\({}_{2}\) provided a high number of active sites, which resulted in lowering the onset potential. In addition, the results showed a 93% faradaic efficiency at an over-potential of 0.59 V. CO was the major product formed in the reaction [162].  
The modeling results discussed above are of great interest, as a new mechanism for CO\({}_{2}\) reduction was explored, which is difficult to be studied experimentally. These computational studies really supplemented and helped explain the reaction mechanisms. However, more work needs to be done on other layered TMS. For example, TiS\({}_{2}\) has been also used for CO\({}_{2}\) reduction and more importantly, both modeling and experimental results were reported on its effect on the electrocatalysis reduction of CO\({}_{2}\).