2D Metallic Transitional Metal Dichalcogenides for Electrochemical Hydrogen Evolution

2D metallic transition metal dichalcogenides (MTMDCs) have become the focus of intense research in many fields due to their novel physical and chemical properties (e.g., charge-density wave order, unconventional superconductivity, and magnetism), as well as their extraordinary performance in advanced nanoelectronics and energy-related fields. Herein, the preparation methods of 2D MTMDCs, such as chemical exfoliation via alkali metal intercalation, colloidal synthesis, and chemical vapor deposition (CVD), and their applications in electrocatalytic hydrogen evolution reactions (HERs) are comprehensively discussed. In particular, the first part focuses on various synthetic routes of 2D MTMDC nanosheets with different thicknesses and domain sizes, as well as the crucial factors in the corresponding synthetic process. In addition, the different growth mechanisms via the CVD processes caused by common insulating substrates (e.g., SiO2/Si, mica), unique conducing substrates (e.g., Au foil), and novel porous substrates (e.g., nanoporous gold) are also compared. In the second part, the excellent electrocatalytic performances of 2D MTMDC nanosheets in HER are also demonstrated by combining density functional theory (DFT) calculations and experimental results. Finally, challenges regarding the preparation of MTMDC nanosheets and probable routes for further improving the HER performance are also highlighted, and the future research directions are also proposed.