Unveiling the Origin of the High Catalytic Activity of Ultrathin 1T/2H MoSe2 Nanosheets for the Hydrogen Evolution Reaction: A Combined Experimental and Theoretical Study

2 D transition metal dichalcogenide materials with layered nanostructures and specific phases usually exhibit excellent catalytic activities for the hydrogen evolution reaction (HER). A facile solvothermal process was used to prepare ultrathin noble-metal-free 2 D biphasic MoSe₂ nanosheets composed of a metastable metallic 1T phase and a semiconducting 2H phase. High metallic 1T phase content and few-layer-thick MoSe₂ nanosheets were obtained by tuning the amount of NaBH₄ used in the reaction. The optimal integration of a metallic 1T phase and an environmentally stable 2H phase in MoSe₂ electrocatalysts provides abundant active sites and good conductivity beneficial for the HER. The combination of experimental results and DFT calculations implies that the electrocatalytic activity for the HER on the MoSe₂ surface goes through a collaborative Heyrovsky and Volmer reaction process. The theoretical studies suggest that the presence of 1T-MoSe₂ could reduce the band energy relative to 2H-MoSe₂ and, consequently, accelerate the sluggish HER kinetics of 2H-MoSe₂ . This work provides valuable and novel insights into the understanding of the structure-activity relationships in 2 D transition metal dichalcogenide electrocatalysts.