## 3 Synthesis of Transition-Metal-Sulfide-Based Nanomaterials
### Hydrothermal Synthesis  
Similar to the low-temperature aqueous TMS nanomaterial synthesis, a reaction for TM formation is performed using a water-soluble sulfide source, such as Na\({}_{2}\)S, (NH\({}_{4}\))\({}_{2}\)S, or NaHS in aqueous solution. The mixture is then transferred to an autoclave and heated to a specific temperature. In these reactions, there is control over the synthesized nanomaterials. Different TMS and TMS-composite nanomaterials have been synthesized using hydrothermal techniques [112, 113, 114, 115, 116, 117]. During the hydrothermal synthesis, the crystallite size, structure, and morphology are controlled by both the temperature and the starting materials, as well as the ratios of the materials in the solution mixture.  
[PERSON] et al. synthesized MoS\({}_{2}\) nanowires using MoO\({}_{2}\) and Na\({}_{2}\)S as the starting materials in an aqueous solution of HCl at 260 \({}^{\circ}\)C [112]. [PERSON] et al. have used hydrothermal synthesis techniques to produce MoS\({}_{2}\) nanosheets [113]. The authors used a combination of NaMoO\({}_{4}\) with thioacetamide and a small amount of silicontungstic acid (2.8 mM). The mixture was held at a constant 220 \({}^{\circ}\)C for 24 h. The results also showed that amorphous nanospheres and nanoflowers were formed at temperatures between 120-150 \({}^{\circ}\)C. [PERSON] et al. synthesized MoS\({}_{2}\) using hydrothermal synthesis to generate different morphologies [114]. The results of the study showed that depending on the sulfur and Mo sources and a reductant (either citric acid or ascorbic acid), the morphology and crystallinity of the MoS\({}_{2}\) could be controlled while the electrochemical properties could be altered.  
[PERSON] et al. used hydrothermal synthesis at a temperature of 200 \({}^{\circ}\)C for 24 h to generate PbS, CdS, ZnS, CoS, and Cu\({}_{2}\)S nanoparticles using 1-butyl-3-methylimidazole thiocyanate as the sulfur source [115]. The authors used the chloride of the respective metals as the metal ion source for the sulfide. The results showed the formation of dendrites by PbS and CdS while the other metals formed spherical particles. [PERSON] and [PERSON] used hydrothermal synthesis to make NiS\({}_{2}\), CoS\({}_{2}\), FeS\({}_{2}\), NiSe2, MoS\({}_{2}\) and MoSe\({}_{2}\) at the nanoscale size [116]. The authors used Na\({}_{2}\)S\({}_{2}\)O\({}_{3}\) with the chloride salts of Ni\({}^{2+}\), Co\({}^{2+}\), and Fe\({}^{2+}\) metal ions over a temperature range between 140 and 150 \({}^{\circ}\)C for 12 h, which resulted in the pyrite type structure [116]. NiSe\({}_{2}\) was synthesized using NiCl\({}_{2}\) and Na\({}_{2}\)SeSO\({}_{3}\), which resulted in the formation of NiSe\({}_{2}\) nanocrystals. The synthesis of MoS\({}_{2}\) and MoSe\({}_{2}\) was successful with Na\({}_{2}\)MoO\({}_{4}\) and Na\({}_{2}\)S\({}_{2}\)O\({}_{3}\) or Na\({}_{2}\)SeSO\({}_{3}\) which resulted in the formation of a hexagonal layered structure after annealing in an inert atmosphere at 350 \({}^{\circ}\)C. [PERSON] et al. synthesized NiMnS at different ratios using hydrothermal synthesis for use as electrodes in energy storage applications [117]. The authors used metal chlorides in the presence of sodium sulfide at a reaction temperature of 170 \({}^{\circ}\)C for 1 day which resulted in the formation of mixed metal TMS with high specific capacity and rate performance. The carbon hybrid delivered an energy density of 46 Wh/kg.