Concurrent Vacancy and Adatom Defects of MoNbSe Alloy Nanosheets Enhance Electrochemical Performance of Hydrogen Evolution Reaction.

Earth-abundant transition metal dichalcogenide nanosheets have emerged as an excellent catalyst for electrochemical water splitting to generate H. Alloying the nanosheets with heteroatoms is a promising strategy to enhance their catalytic performance. Herein, we synthesized hexagonal (2H) phase MoNbSe nanosheets over the whole composition range using a solvothermal reaction. Alloying results in a variety of atomic-scale crystal defects such as Se vacancies, metal vacancies, and adatoms. The defect content is maximized when approaches 0.5. Detailed structure analysis revealed that the NbSe bonding structures in the alloy phase are more disordered than the MoSe ones. Compared to MoSe and NbSe, MoNbSe exhibits much higher electrocatalytic performance for hydrogen evolution reaction. First-principles calculation was performed for the formation energy in the models for vacancies and adatoms, supporting that the alloy phase has more defects than either NbSe or MoSe. The calculation predicted that the separated NbSe domain at = 0.5 favors the concurrent formation of Nb/Se vacancies and adatoms in a highly cooperative way. Moreover, the Gibbs free energy along the reaction path suggests that the enhanced HER performance of alloy nanosheets originates from the higher concentration of defects that favor H atom adsorption.