MXenes, an emerging two-dimensional (2D) transition metal carbides, nitrides and carbonitrides, have exhibited great potential as electrocatalysts for hydrogen evolution reaction (HER) due to the excellent characters, including excellent structural and chemical stability, superior electrical conductivity, and large active surface area. In this comprehensive study, firstly, the preparation advances of MXenes are systematically summarized. Then, the representative applications of MXenes-based HER electrocatalysts are introduced, from experimental and theoretical aspects. Thirdly, the strategies for improving HER catalytic activity of MXenes are demonstrated, such as optimizing active sites by termination modification and metal-atom doping, increasing active sites by fabricating various nanostructures. Finally, the existing challenges and new opportunities for MXenes-based electrocatalysts are also elucidated. This paper provides reference for the future development of new and efficient MXenes-based electrocatalysts for hydrogen production through water-splitting technology.

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The controlled anisotropic growth of two-dimensional (2D) materials along a specific crystal axis is very crucial to explore the axis-dependent performance, which derives from the difference of atom arrangement in different crystal axes, while the relevant research is still insufficient. Here, we demonstrated a systematic investigation on the controllable anisotropic growth of 2D SnSe along different axis directions from both the experimental and theoretical perspectives. The anisotropy growth of 2D SnSe was attributed to the difference in atomic structures and bonding features between the zigzag edge (along the b-axis) and armchair edge (along the c-axis), as well as the chemical inertness of the base plane in the a-axis. Scanning transmission electron microscopy (STEM) images further verified that the long side of rectangular 2D SnSe is an armchair edge and the short side is a zigzag edge. The growth rates along the b-and c-axes could be effectively adjusted by hydrogen, attributed to the difference in desorption energy between hydrogen and different edge structures. The higher hydrogen concentration led to a shorter b-axis and a longer c-axis. The morphologies of 2D SnSe could be regulated in the range from square nanosheets to linear nanowires. In terms of the a-axis, 2D SnSe was precisely limited to a minimum value of 7.42 nm via a one-step rapid cooling method and could be further thinned to a bilayer using a two-step method with an additional annealing etching step. This study has demonstrated a facile strategy toward the structure-dependent controllable growth of 2D anisotropic materials, which shed new light on the orientation judgment and axisdependent mechanism of the crystal axis of anisotropic materials.

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