**2.2 Solvothermal synthesis of TMCs**

Wet chemical synthesis, a bottom-up process, is broadly used to synthesize TMC nanosheets with tunable thickness and size [24–26]. The desired nanosheets form directly in solution or onto substrate in high yield. The synthesis of TMCs via wet chemical method usually relies on the chemical reaction of metal salts and sulfur/selenide-based materials as precursors. The solvothermal method is one of the most typical routes of wet chemical method, in which the reaction is conducted in a sealed autoclave at suitable temperature (**Figure 4**) [27]. As a representative example, Xie et al. [28] synthesized oxygen incorporated MoS2 nanosheets from the

precursors of (NH4)6Mo7O244H2O and thiourea through a solvothermal method. By tuning the synthetic temperatures, the disordered structure and intrinsic conductivity of MoS2 could be controlled to a moderate degree. Thus, the optimal

overpotential as low as 120 mV. It is well known that the materials with different crystal orientations have the anisotropic properties. This phenomenon appears in TMCs depending on stacking sequence of the chalcogen and transition-metal coordination. The commonly found polymorphs in TMC materials are the so-defined 1 T, 2H, and 3R, where the number and letter, respectively, indicate the layer number in the unit cell and the type of symmetry with T, H, and R representing for tetragonal, hexagonal, and rhombohedral, respectively (**Figure 5**). For example, Zhang et al. synthesized metallic CoS2 nanopyramid array on carbon fiber paper by an one-step solvothermal synthesis [29]. In brief, the carbon fiber papers were transferred to the precursor solution containing CoCl26H2O and CS(NH2)2, which was maintained at 180°C for 12 hours. The unique 3D nanostructure and intrinsic metallic properties of the CoS2 by this method contribute to the ultrahigh activity

catalyst for electrocatalytic hydrogen evolution reaction exhibits onset

*Different metal coordination and stacking sequence in TMC unit cells [10].*

*Transition Metal Chalcogenides for the Electrocatalysis of Water*

*DOI: http://dx.doi.org/10.5772/intechopen.92045*

**3. Hydrogen evolution reaction (HER) based on TMC electrodes**

In order to improve HER performance, three main factors including the number of active sites, intrinsic catalytic activity, and the conductivity of TMCs play crucial roles. In 2005, Hinnemann et al. suggested the active sites for HER only exist in the edges of TMCs by density functional theory (DFT) calculation [30]. Thus, many research works are dedicated to enrich the defect sites and/or active sites by nanostructural engineering [31–33]. Zhang et al. synthesized the edge-rich 3D MoS2 coupling with conductive polymer polyaniline (PANI) as catalyst (**Figure 6**) [34].

toward hydrogen evolution reaction.

**Figure 5.**

**81**

**Figure 3.**

*The general introduction to the CVD synthesis, CVD setup, and growth mechanism of MoS2 layer [23].*

*The synthesis process of MoS2 nanosheets via a simple hydrothermal method [27].*

*Transition Metal Chalcogenides for the Electrocatalysis of Water DOI: http://dx.doi.org/10.5772/intechopen.92045*

of TMCs based on VIII B group metals such as cobalt [20] and nickel [21] are also

Wet chemical synthesis, a bottom-up process, is broadly used to synthesize TMC nanosheets with tunable thickness and size [24–26]. The desired nanosheets form directly in solution or onto substrate in high yield. The synthesis of TMCs via wet

chemical method usually relies on the chemical reaction of metal salts and sulfur/selenide-based materials as precursors. The solvothermal method is one of the most typical routes of wet chemical method, in which the reaction is conducted in a sealed autoclave at suitable temperature (**Figure 4**) [27]. As a representative example, Xie et al. [28] synthesized oxygen incorporated MoS2 nanosheets from the

*The general introduction to the CVD synthesis, CVD setup, and growth mechanism of MoS2 layer [23].*

*The synthesis process of MoS2 nanosheets via a simple hydrothermal method [27].*

widely studied in recent years.

*Advanced Functional Materials*

**Figure 3.**

**Figure 4.**

**80**

**2.2 Solvothermal synthesis of TMCs**

**Figure 5.** *Different metal coordination and stacking sequence in TMC unit cells [10].*

precursors of (NH4)6Mo7O244H2O and thiourea through a solvothermal method. By tuning the synthetic temperatures, the disordered structure and intrinsic conductivity of MoS2 could be controlled to a moderate degree. Thus, the optimal catalyst for electrocatalytic hydrogen evolution reaction exhibits onset overpotential as low as 120 mV. It is well known that the materials with different crystal orientations have the anisotropic properties. This phenomenon appears in TMCs depending on stacking sequence of the chalcogen and transition-metal coordination. The commonly found polymorphs in TMC materials are the so-defined 1 T, 2H, and 3R, where the number and letter, respectively, indicate the layer number in the unit cell and the type of symmetry with T, H, and R representing for tetragonal, hexagonal, and rhombohedral, respectively (**Figure 5**). For example, Zhang et al. synthesized metallic CoS2 nanopyramid array on carbon fiber paper by an one-step solvothermal synthesis [29]. In brief, the carbon fiber papers were transferred to the precursor solution containing CoCl26H2O and CS(NH2)2, which was maintained at 180°C for 12 hours. The unique 3D nanostructure and intrinsic metallic properties of the CoS2 by this method contribute to the ultrahigh activity toward hydrogen evolution reaction.
