Polytypic Phase Transition of Nb_{1- x}V_xSe₂ via Colloidal Synthesis and Their Catalytic Activity toward Hydrogen Evolution Reaction

Polytypes of two-dimensional transition metal dichalcogenide can extend the architecture and application of nanostructures. Herein, Nb_1-xV_xSe₂ alloy nanosheets in the full composition range (x) were synthesized by a colloidal reaction. At x = 0.1-0.3, a phase transition occurred from various hexagonal (three 2H and one 4H types) phase NbSe₂ to an atomically homogeneous 1T phase VSe₂. Density functional theory calculations also revealed a polytypic phase transition at x = 0.3, which was shifted close to 0 in the presence of Se vacancies. Furthermore, the calculations validate favorable formation of Se vacancies at the phase transition. The sample at x = 0.3 exhibited enhanced electrocatalytic activity toward the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄. The Gibbs free energy indicates that the catalytic HER performance is correlated with the active Se vacancy sites of polytypic structures.