Phase Evolution of ReMoSe Alloy Nanosheets and Their Enhanced Catalytic Activity toward Hydrogen Evolution Reaction.

Two-dimensional ReSe has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), but its catalytic activity needs to be further improved. Herein, we synthesized ReMoSe alloy nanosheets with the whole range of (0-100%) using a hydrothermal reaction. The phase evolved in the order of 1T″ (triclinic) → 1T' (monoclinic) → 2H (hexagonal) upon increasing . In the nanosheets with = 10%, the substitutional Mo atoms tended to aggregate in the 1T″ ReSe phase with Se vacancies. The incorporation of the 1T' phase makes the alloy nanosheets more metallic than the end compositions. The 10% Mo substitution significantly enhanced the electrocatalytic performance toward HER (in 0.5 M HSO), with a current of 10 mA cm at an overpotential of 77 mV ( RHE) and a Tafel slope of 42 mV dec. First-principles calculations of the three phases (1T″, 2H, and 1T') predicted a phase transition of 1T″-2H at ≈ 65% as well as the production of a 1T' phase along the composition tuning, which are consistent with the experiments. At = 12.5%, two Mo atoms prefer to form a pair along the Re chains. Gibbs free energy along the reaction path indicates that the best HER performance of nanosheets with 10% Mo originates from the Mo atoms that form Mo-H when there are adjacent Se vacancies.