2H-2M Phase Control of WSe₂ Nanosheets by Se Enrichment Toward Enhanced Electrocatalytic Hydrogen Evolution Reaction

The phase control of transition metal dichalcogenides (TMDs) is an intriguing approach for tuning the electronic structure toward extensive applications. In this study, WSe₂ nanosheets synthesized via a colloidal reaction exhibit a phase conversion from semiconducting 2H to metallic 2M 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 are used to identify the stacking sequence of the 2M 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 2M phase. The 2M phase WSe₂ 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 2M phase WSe₂ enhanced the HER catalytic activity, which is 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.