2M Phase Engineering of W_1-xMo_xS₂ Alloy Nanosheets for Enhanced Catalytic Hydrogen Evolution Reaction

The phase engineering of two-dimensional transition-metal dichalcogenides (TMDs) remains difficult yet crucial for both fundamental studies and practical applications. In this work, WS₂-MoS₂ (W_{1- x}Mo_xS₂) alloy nanosheets with tunable crystal structures (2H, 2 M, and 1T') were prepared via a colloidal synthesis conducted at 220-300°C. Interestingly, 2 M → 2H phase transition temperature was higher for WS₂ than for MoS₂ (e.g., 300°C vs. 260°C), indicating that WS₂ possesses a more stable metastable phase. W-rich alloying was found to stabilize further the metastable structure of 2 M phase. First-principles calculations consistently revealed that alloying, NH₄ ⁺ intercalation, and sulfur/metal vacancies all contribute to enhancing the stability of the metastable phases. In acidic media, the W-rich alloy nanosheets exhibited improved hydrogen evolution reaction (HER) activity, which is likely associated with their 2 M phase. The calculations also suggested that the 2 M phase alloys effectively activate the basal S atoms toward HER, offering valuable insight into how alloying modulates the catalytic properties of TMD-based electrocatalysts.