2M Phase Stability of WSe2-MoSe2 Alloy Nanosheets via Colloidal Reaction and Their Se-Rich Model Calculations

Abstract

The phase-controlled synthesis of two-dimensional transition-metal dichalcogenides (TMDs) is a compelling area of interest for both basic research and potential applications. Herein, we successfully synthesized 2M phase WSe₂-MoSe₂ (=W_1-xMo_xSe₂) alloy nanosheets with a full composition tuning using a colloidal reaction. As x increased, the 2M-2H phase transition temperature (T_2M) decreased, indicating the higher stability of the 2M phase at lower x. A Se-rich growth condition stabilized the 2M phase at all x. The more significant defects of the 2M phase at lower x or lower reaction temperature suggest that the Se-rich defects stabilize the 2M phase. First-principles calculations consistently showed that the 2M phase formed more favorably at lower x, especially in the presence of Se-rich defects (e.g., metal vacancies, Se intercalation). The density of states and Gibbs free energy for H adsorption indicated that the Se-rich defects in the metallic 2M phase effectively activate the Se atoms for hydrogen evolution reaction (HER). The calculations supported experimental data, providing insights into controlling the crystal phase in TMD catalysts.