2M Phase Engineering of W _1‐x Mo _x S ₂ Alloy Nanosheets for Enhanced Catalytic Hydrogen Evolution Reaction

ABSTRACT The phase engineering of two‐dimensional transition‐metal dichalcogenides (TMDs) remains difficult yet crucial for both fundamental studies and practical applications. In this work, WS ₂ ‐MoS ₂ (W _{1‐ x} Mo _x S ₂ ) 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 WS ₂ than for MoS ₂ (e.g., 300°C vs. 260°C), indicating that WS ₂ 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, NH ₄ ⁺ 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.