## 3 Synthesis of Transition-Metal-Sulfide-Based Nanomaterials
### CuS\({}_{x}\)-Based Nanomaterials  
[PERSON] et al. investigated the application of CuS\({}_{x}\) in the electrochemical reduction of CO2 to formate [168]. The authors used industrial CO2, which contained H2S and performed an in situ formation of CuS\({}_{x}\) from a Cu foil. The study showed that the average size of the CuS\({}_{x}\) nanoparticles was 133.2 nm and the nanoparticles had a faradaic efficiency of up to 72% at \(-\)6 V and showed stability for up to 72 h of operation. [PERSON] et al. investigated the deposition of CuS on Cu foils for the electrocatalytic reduction of CO2 to formate [169]. In that study, CuS was deposited on Cu foil using electrodeposition to prepare a nanowire of the CuS. The products of the reduction included hydrogen at low potential, then CO and formic acid at intermediate over-potentials, and CH4 at a high over-potential. The faradaic efficiency of the CuS on Cu foils was higher than of the foil alone. [PERSON] et al. reported results on the investigation of copper sulfide compound properties in the electrochemical reduction of CO2 [170]. The material was synthesized through electrochemical deposition of CuS. The results of the study showed that, as particle size increased, better faradaic efficiencies were observed, up to approximately 70%. The results showed low production of CO, with formic acid formation preferred at intermediate current densities of between \(-\)1 and \(-\)10 mA/cm\({}^{2}\). [PERSON] et al. investigated Cu2S nanoparticles on carbon for the electrochemical reduction of CO2 and the formation of formate [171]. The authors prepared the CuS and Cu2S nanoparticles through liquid phase sulfidation of CuO on a carbon substrate. The results showed that with less than 4% of the carbon substrate covered, the reaction was selective towards the formation of formate at low current densities [171]. In general, the Cu2S showed higher faradaic efficiencies over the CuS and the CuO nanoparticles at current densities of \(-\)1.5, \(-\)3, and \(-\)4.5 mA/cm\({}^{2}\). [PERSON] et al. used FTIR to investigate the formation of formate by the deposition of copper sulfide on nitride graphene [172]. The synthesized CuS on nitride graphene (C3N4) showed high faradaic efficiencies of 82% at an over-potential of 57 mV for the formate formation. The FTIR results showed that one of the oxygen atoms in the CO2 was bound to the Cu2S/C3N4 to form OCHO' species. The authors also used DFT calculations which showed that the sulfur promoted the activation of the interfacial water formation H species. [PERSON] et al. investigated the use of CoS\({}_{2}\) as a template for the generation of Cu2S as a CO2 reduction catalyst for the generation of formate [173]. The authors used electrochemically driven cation exchange to replace the Co to generate the Cu2S while maintaining the morphology of the CoS\({}_{2}\) nanoparticles. The copper-based catalysts showed a faradaic efficiency of 87% for the generation of formate. [PERSON] et al. studied the selective electrochemical reduction of CO2 to formate on CuS\({}_{x}\) catalysts [174]. The authors prepared electrochemically deposited CuS\({}_{x}\) catalysts that could reduce CO2 to produce formate. The faradaic efficiency of the nanomaterial was 75% with a current density of \(-\)9.0 mA/cm\({}^{2}\) at an over-potential of \(-\)0.9 V. The \begin{table}
\begin{tabular}{c c c} \hline
**CO2 RR Product** & **Material** & **Reference** \\ \hline CO, CH3 CH2 CH2 OH & MoS2 & [145; 146] \\ CO & MoS2-C3N4 & [147] \\ CO & MoS2 exfoliated-flurosilane capped & [148] \\ CH3 CH2 OH, CH3 OH, CO & TiO2-MoS2 & [149; 150; 151] \\ CH3 OH & Co–MOS2 & [152] \\ CH3 CH2 OH, CH3 OH & Cu–C3N4-MoS2 & [153] \\ CO & Nb52–MoS2/ionic liquid & [154] \\ CH3 OH & CuO–ZnO–MoS2 & [155] \\ CO, CH4, C2H4 & Cu–MoS2 & [156] \\ CO & ITS2 & [163; 164] \\ CO & WSe2 & [165] \\ \hline \end{tabular}
\end{table} 
Table 1: Summary of products in the CO2 RR using the various TMS-layered nanomaterials and their associated composite nanomaterials.