Phase Evolution of Re1–xMoxSe₂ Alloy Nanosheets and Their Enhanced Catalytic Activity toward Hydrogen Evolution Reaction

Two-dimensional ReSe₂ has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), but its catalytic activity needs to be further improved. Herein, we synthesized Re1-xMoxSe₂ alloy nanosheets with the whole range of x (0-100%) using a hydrothermal reaction. The phase evolved in the order of 1T″ (triclinic) → 1T' (monoclinic) → 2H (hexagonal) upon increasing x. In the nanosheets with x = 10%, the substitutional Mo atoms tended to aggregate in the 1T″ ReSe₂ 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 H₂SO₄), with a current of 10 mA cm⁻² at an overpotential of 77 mV (vs RHE) and a Tafel slope of 42 mV dec⁻¹. First-principles calculations of the three phases (1T″, 2H, and 1T') predicted a phase transition of 1T″-2H at x ≈ 65% as well as the production of a 1T' phase along the composition tuning, which are consistent with the experiments. At x = 12.5%, two Mo atoms prefer to form a pair along the Re₄ 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.