## 3 Synthesis of Transition-Metal-Sulfide-Based Nanomaterials
### [PERSON] et al. investigated the reduction of CO\({}_{2}\) in an ionic liquid using AgS [201]. The study utilized a cell with a Nafion membrane with aqueous solution voltages ranging from \(-\)0.144 to approximately \(-\)1.156 V (RHE), and in the ionic liquid the potential ranged from \(-\)0.136 to \(-\)1.164 V with a scan rate of 50 mV s\({}^{-1}\). The authors noted that the formation of CO, H\({}_{2}\), and CH\({}_{4}\) were the only reactions with a combined faradaic efficiency of 100%. However, the CO formation had a maximum efficiency of 92% around \(-\)0.864 V. In the presence of KHCO\({}_{3}\) the efficiency was lower. [PERSON] et al. investigated the interaction of CO\({}_{2}\) with Ag\({}_{2}\)S/Ag for the electrocatalytic formation of CO [202]. The study showed that the hybrid AgS\({}^{2}\)/Ag heterogenous catalyst achieved a large current density of approximately 422 mA/cm\({}^{2}\) at an over-potential of \(-\)0.70 V. However, a steady current density of 244 mA/cm\({}^{2}\) was achieved at an over-potential of \(-\)0.49 V, which gave a 99% faradaic efficiency. [PERSON] et al. on the other hand investigated Ag\({}_{2}\)S on Au as an heterogenous catalyst for CO\({}_{2}\) reduction to CO [203]. The authors observed a faradaic efficiency of 94.5% at an over-potential of \(-\)0.8 V. In addition, the catalyst showed a current density of 9.2 mA/cm\({}^{2}\) with stability over 30 h. The authors performed DFT calculations, which indicated that the surface was favorable to the formation of COOH\({}^{*}\) radical species.  
[PERSON] et al. generated Ag\({}_{2}\)S electrocatalysts for the reduction of CO\({}_{2}\) in organic media [204]. The authors synthesized the Ag\({}_{2}\)S through the electro-oxidation of Ag in an aqueous solution on a Ag foil. In that work, the Ag\({}_{2}\)S/Ag catalyst was considered as an effective electrocatalyst in propylene carbonate/tetrabutylammonium perchlorate [204]. The reaction showed a faradaic efficiency of 92% over 4 h of reaction at a current density of 9.85 mA/cm\({}^{2}\) and an over-potential of \(-\)2.3 V (compared to Fe/Fe+). [PERSON] et al. investigated the in situ growth of Ag/AgCl\({}_{2}\) nanowire clusters in a H\({}_{2}\)S plasma for the formation of CO from CO\({}_{2}\)[205]. The authors used a H\({}_{2}\)S plasma to treat a Ag foil, which generated Ag/AgCl\({}_{2}\)S nanowire clusters. The nanocluster wires showed faradaic efficiencies of 83% at an over-potential of \(-\)0.4 V and a current density of 3 mA/cm\({}^{2}\). The highest faradaic efficiencies were observed with larger nanowires with diameters of 100 nm and lengths of microns. [PERSON] et al. investigated Ag\({}_{2}\)S on reduced graphene oxides for the electrocatalytic reduction of CO\({}_{2}\) to CO [206]. The authors used a hydrothermal method to produce the Ag\({}_{2}\)S on S/N-doped rGO. The Ag\({}_{2}\)S on S/N-doped rGO showed a selectivity of 87.4% towards producing CO from CO\({}_{2}\). In addition, the catalyst showed stability over 40 h at a current density of 70 uA/cm\({}^{2}\).  
[PERSON] et al. investigated the electroreduction of CO\({}_{2}\) using Ag\({}_{2}\)S on CdS nanorods [207]. The authors studied the formation of CO, which was very low on CdS. However, the application of Ag\({}_{2}\)S nanodots promoted the formation of CO at 5 wt% loading at a current density of 10.6 mA/cm\({}^{2}\) where 95% faradaic efficiency was achieved, which was approximately 93% enhancement over the pristine CdS.  
The use of silver sulfide as a CO\({}_{2}\)RR catalyst has typically shown the production of CO in the reaction. The generation of CO seems independent of the doping material or the support used for the Ag\({}_{2}\)S. This is a very interesting result as it shows controlled electrocatalysis in the CO\({}_{2}\)RR reaction towards a single product.