Defect-rich exfoliated MoSe2 nanosheets by supercritical fluid process as an attractive catalyst for hydrogen evolution in water

Single- to few-layers MoSe2 nanosheets were obtained from bulk MoSe2 by a supercritical fluid exfoliation. High angle annular dark field (HAADF) imaging showed the presence of a great variety of vacancies and edge defects within the exfoliated MoSe2 nanosheets. Thanks to these defects, the exfoliated MoSe2 nanosheets showed attractive catalytic performance and robustness for the hydrogen evolution reaction in water. The best electrode made of these nanosheets required 300 mV overpotential to generate a catalytic current of 10 mA/cm(2) and showed a Tafel slop of 90 mV/s. The catalytic performance was further enhanced by twice when the MoSe2 nanosheets catalyst was conditioned at a mild oxidative potential, e.g. + 0.73V vs. RHE. The mild oxidation treatment was proposed to create novel Se-vacancy and See-vacancy while an oxidation treatment at higher potential, e.g. + 1.23 V vs. RHE, completely oxidized MoSe2 into inactive MoO3. As a consequence, the later caused a rapid degradation of catalytic performance.