new energy sources, such as hydrogen, has attracted much attention in recent years. [1][2][3] Electrochemical water splitting technology is a highly efficient and promising technology for green hydrogen production. The entire water-splitting process consists of a cathodic hydrogen evolution reaction (HER) and an anodic oxygen evolution reaction (OER). [4,5] The electrocatalyst is a key part in the process of driving the reaction and reducing power consumption. Conventional electrocatalysts are mainly based on rare precious metal nanomaterials, such as Pt, Ru, Rh, Ir, Pd and RuO 2 , IrO 2 , etc. They are considered to be the most efficient HER and OER electrocatalysts. [6][7][8] However, their high price and low abundance have led to limited large-scale applications in industry. Therefore, it is necessary to develop economical, stable, and highly active nonprecious metal catalysts as an alternative. 2D transition metal dichalcogenides (2D TMDs) materials based on earth-abundant elements have triggered a surge of research due to their unique optical and electrical properties. [9] Among them, MoS 2 , a typical representative of TMDs, has a hexagonal crystal system with a 2D structure, a narrow and flexible band gap, a peelable layered structure, a tunable active site, and a transformable phase structure (2H-MoS 2 to 1T/1T′-MoS 2 ), which are effective ways to improve its catalytic performance. [10][11][12][13][14][15] In theory, MoS 2 has a hydrogen adsorption-free energy close to that of the precious metal Pt. Being a non-precious metal material with both costeffectiveness and high efficiency, it is affirmed by researchers as a promising application in the field of hydrogen energy. [16,17] In the same way that the catalytic activity of most inorganic solid catalysts depends on the number and stable mass of active reaction sites, the catalytic activity of MoS 2 is limited by the sparse number of surface sites exposed by the layered structure, while the bulk material is relatively inert. [18] All possible catalytically active reaction centers of MoS 2 have been investigated as promising low-cost catalysts for applications. They are mainly the grain boundary sites located at the basal plane, the generation of sulfur vacancy defect sites, and the structurally undercoordinated molybdenum atomic sites at the edge plane. [19][20][21] It is well known that the (002) crystal plane is the main exposed basal plane of 2D MoS 2 . It has a centrosymmetric Mo-2S structure. Next is the (100) edge plane, which has high surface energy as well as a non-centrosymmetric Mo-2S structure with 2D molybdenum disulfide (MoS 2 ) is developed as a potential alternative non-precious metal electrocatalyst for energy conversion. It is well known that 2D MoS 2 has three main phases 2H, 1T, and 1T′. However, the most stable 2H-phase shows poor electrocatalysis in its basal plane, compared with its edge sites. In this work, a facile one-step hydrothermal-driven in situ porousizing of MoS 2 into self-supporting nano islands to maximally expose the e...
