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 x 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-vacancyrelated 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.