2H-2 M Phase Control of WSe 2 Nanosheets by Se Enrichment Toward Enhanced Electrocatalytic Hydrogen Evolution Reaction.

The phase control of transition metal dichalcogenide (TMD) is an intriguing approach for tuning the electronic structure towards extensive applications. In this study, WSe 2 nanosheets synthesized via a colloidal reaction exhibited a phase conversion from semiconducting 2H to metallic 2 M under Se-rich growth conditions (i.e., increasing the concentration of Se precursor or lowering the growth temperature). High-resolution scanning transmission electron microscopy images were used to identify the stacking sequence of the 2 M phase, which is distinctive from that of the 1T' phase. First-principles calculations employing various Se-rich models (intercalation and substitution) indicated that Se enrichment induces conversion to the 2 M phase. The 2 M phase WSe 2 nanosheets with the Se excess exhibited enhanced electrocatalytic performance in the hydrogen evolution reaction (HER). In situ X-ray absorption fine structure studies suggested that the excess Se atoms in the 2 M phase WSe 2 enhanced the HER catalytic activity, which was supported by the Gibbs free energy (ΔG H* ) of H adsorption and the Fermi abundance function. These results provide an appealing strategy for phase control of TMD catalysts. This article is protected by copyright. All rights reserved.