## 3 Synthesis of Transition-Metal-Sulfide-Based Nanomaterials
### FeS\({}_{x}\)  
[PERSON] et al. studied the use of FeS surfaces for the electrochemical hydrogen evolution and CO\({}_{2}\) reduction reactions [181]. The authors investigated a greigite (Fe\({}_{3}\)S\({}_{4}\))-modified electrode, which was oxidized in an HER reaction to form a 60/40% mixture of greigite and goethite (FeOOH). The results in that study showed that the HER reaction was slowed dramatically in saturated CO\({}_{2}\) solution under reducing conditions. In addition, the binding indicated that the HCO\({}_{3}\)\({}^{-}\) was surface-bound to the FeS [181]. [PERSON] et al. investigated the application of bio-inspired CO\({}_{2}\) conversion using FeS catalysts [182]. The study was focused on the application of Fe\({}_{3}\)S\({}_{4}\) (greigite). In that work, the reduction of CO\({}_{2}\) was performed in a saturated solution at pHs of 4.5, 6.5, and 10.5, which showed that the formic acid was the major product at all pHs studied, with methanol, acetic acid, and pyruvate acid as the minor products. The authors used DFT calculations to study the CO\({}_{2}\) reduction, which indicated that the predominance of the formation of formic acid was due to the solution species of either CO\({}_{2}\) (neutral molecule), HCO\({}_{3}\)\({}^{-}\) at intermediate pHs, or CO\({}_{3}\)\({}^{2-}\) at high pHs. The molecule interacted with either the 001 or the 111 surface of the Fe\({}_{3}\)S\({}_{4}\) particles [182]. [PERSON] et al. investigated the reduction of CO\({}_{2}\) on pyrite as a mechanism of abiogenic formation of organic molecules [183]. The authors used pyrite as a rotating disc electrode to form formate at high pressure (50 atm). As the potential applied to the electrode increased, the concentration of formate formed in solution increased, while a maximum current efficiency of 0.12% was observed at \(-\)1 V. The authors proposed that the reaction occurred at deep ocean vents (black smokers) on the ocean floors. [PERSON] et al. developed a fast microwave process to synthesize Ni-substituted Fe\({}_{3}\)S\({}_{4}\) nanosheets for CO\({}_{2}\) reduction to CO [184]. The authors showed a faradaic efficiency for the NiFe\({}_{3}\)S\({}_{4}\) nanomaterial of 6% at a potential of \(-\)0.7 V. However, at lower potentials, below \(-\)0.8 V, the HER reaction became predominate. [PERSON] et al. studied the effect of S/Se exchange on Fe/Ni pendalite for the electrocatalytic reduction of CO\({}_{2}\) to form CO [185]. The results in that study showed that the introduction of higher amounts of Se increased the formation of CO. Using galvanostatic conditions in acetonitrile, Fe\({}_{4.5}\)Ni\({}_{4.5}\)S\({}_{4}\)Se\({}_{4}\) fardic efficiencies of approximately 100% were achieved at current densities of 50 mA/cm\({}^{2}\) and 100 mA/cm\({}^{2}\). [PERSON] and [PERSON] investigated the reduction of CO\({}_{2}\) to C\({}_{1}\) and C\({}_{2}\) products over sulfur deficient mackinawite (FeS) using DFT calculations [186]. The results showed that the adsorption energy for CO\({}_{2}\) onto pristine FeS was \(-\)0.17 eV, and on sulfur deficient surfaces \(-\)1.62 eV, which showed preferential binding and reduction of S-deficient surfaces. The favored product was methane in potential-limiting \"HCO \(\rightarrow\) CH\({}_{2}\)O (\(\Delta\)G = 1.27 eV) and OHCH\({}_{2}\)O\({}^{*}\)\(\rightarrow\) CH\({}_{2}\)O (\(\Delta\)G = 0.78 eV) reverse water-gas shift and formate pathways, respectively. For the formation of the C\({}_{2}\) species, a high concentration of CO\({}^{*}\) on the surface was required to yield ethane. The preliminary studies reported on the use Fe\({}_{y}\)S\({}_{x}\) nanomaterials in CO\({}_{2}\)RR have been used in an attempt to explain the origin of early life on earth. However, the application of Fe\({}_{y}\)S\({}_{x}\) nanomaterials in electrocatalysis has shown great promise in developing value-added products from the CO\({}_{2}\)RR.