Surface electron accumulation and enhanced hydrogen evolution reaction in MoSe₂ basal planes

The spontaneous formation of surface electron accumulation (SEA) was observed in synthesized molybdenum diselenide (MoSe₂) layered crystals with two-hexagonal (2 H) structure. An anomalously high electron concentration at the surface up to 10¹⁹ cm⁻³ is several orders of magnitude higher than that (3.6 × 10¹² cm⁻³) of the inner bulk. The SEA is found to be generated easily by mechanical exfoliation and room temperature deselenization. Se-vacancies have been confirmed to be the major source resulting in SEA and n-type conductivity, and also the active sites for electrochemical catalysis in MoSe₂. Noted that the SEA conjugated with the Se-vacancy-related surface defects enhances the electrochemical hydrogen evolution reaction (HER) activity substantially. The optimized HER efficiency with an overpotential at 0.17 V and Tafel slope at 60 mV/dec of the basal plane of 2 H-MoSe₂ was achieved by the nitrogen plasma treatment, which has outperformed several nanostructures, thin films, and hybrid counterparts. This study reveals the intriguing surface-dominant electronic property and its effect on the HER enhancement of the basal plane, which is crucial for development of a stable, low-cost and highly efficient catalyst using 2 H-MoSe₂.