Title: Polytypic Phase Transition of Nb1-xVxSe2 via Colloidal Synthesis and Their Catalytic Activity toward Hydrogen Evolution Reaction

Authors: In Hye Kwak, Ik Seon Kwon, Getasew Mulualem Zewdie, Tekalign Terfa Debela, Seung Jae Lee, Ju Yeon Kim, Seung Jo Yoo, Jin-Gyu Kim, Jeunghee Park*, and Hong Seok Kang*

DOI: 10.1021/acsnano.1c10301

Abstract: Polytypes of two-dimensional transition metal dichalcogenide can extend the architecture and application of nanostructures. Herein, Nb1-xVxSe2 alloy nanosheets in the full composition range (x) were synthesized by a colloidal reaction. At x = 0.1-0.3, a novel 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.