## 3 Synthesis of Transition-Metal-Sulfide-Based Nanomaterials
### SnS\({}_{x}\)  
[PERSON] et al. studied SnS\({}_{x}\) supported on carbon cloth for the electrocatalytic reduction of CO\({}_{2}\)[196]. The authors used chalcogen as a precursor for the synthesis of SnS\({}_{0.09}\) and SnS\({}_{0.55}\) supported on carbon cloth. The SnS\({}_{0.55}\) showed a 93% faradaic efficiency to produce formate at a potential of \(-\)1.1 V, with a current density of 28.4 mA/cm\({}^{2}\). [PERSON] et al. studied the formation of formate from CO\({}_{2}\) using Sn/S derived from SnS\({}_{2}\) nanosheets [197]. The authors synthesized SnS\({}_{2}\) nanosheets supported on graphene oxide, which were capable of forming formate at over-potentials of 0.23 V. However, the optimum formation of formate occurred at a current density of 13.5 mA/cm\({}^{2}\), faradaic efficiency of 85% and over-potential of 0.68 V. The examination of the catalysis showed the presence of reduced Sn in the catalyst, which was formed from the cathodic reduction, but the enhanced performance was attributed to the presence of SnS\({}_{2}\). [PERSON] et al. investigated methods to reduce sulfur dissolution and enhance the electrocatalytic reduction of CO\({}_{2}\) to formate on SnS [198]. The authors investigated In doping of the SnS structure which reduced the dissolution of the SnS. The In-SnS on carbon catalysis showed a faradaic efficiency of 97% for formate production at an over-potential of \(-\)0.6 V and a current density of 37 mV/cm\({}^{2}\).  
[PERSON] et al. investigated InO-SnS\({}_{2}\) nanosheets for the electrochemical reduction of CO\({}_{2}\) to formate [199]. The authors performed the synthesis using an InSn(OH)\({}_{6}\) precursor which was functionalized into the In-O-SnS\({}_{2}\) nanosheets. The catalysts showed a faradaic efficiency to produce formate of 89% at a current density of 22.7 mA/cm\({}^{2}\) and an over-potential of 1.0 V. The unmodified SnS\({}_{2}\) nanosheets showed a current density of 9.4 mA/cm\({}^{2}\) and a faradaic efficiency for formate production of 18%. The DFT calculations indicated a synergistic effect of the In-O interacting with the SnS\({}_{2}\) creating oxidized Sn sites next to In sites, which reduced the activation energies and speeded up the reaction [199].  
[PERSON] et al. prepared a Cu-SnS catalyst for the formation of formate from CO\({}_{2}\)[200]. The authors made a precatalyst consisting of a Cu\({}_{2}\)SnS\({}_{3}\). The as-prepared catalyst showed a faradaic efficiency of 96% at a current density of \(-\)241 mA/cm\({}^{2}\). The authors also performed DFT calculations which showed preferred formation of formate by the HCOO\({}^{*}\) intermediate on the CuSnS\({}_{x}\) catalyst. The pristine SnS\({}_{x}\) and SnS\({}_{x}\) composite materials showed a preference for the formation of formate in CO\({}_{2}\)RR. This indicates that these materials have a preference to form the HCOO\({}^{*}\) radical species over the other radical species. The studies indicate that SnS is not a great material for use in the CO\({}_{2}\)RR; however, doping SnS with a secondary compound or ion can increase the faradaic efficiency tremendously, which shows great promise in controlled CO\({}_{2}\)RR.