Enhanced hydrogen evolution reaction activity of hydrogen-annealed vertical MoS nanosheets.

Molybdenum disulfide (MoS) is a promising electrocatalyst for hydrogen evolution reaction (HER), but only edges and S-vacancies are catalytic active sites for the HER. Therefore, it is crucial to increase edge sites and S-vacancies for enhancing the HER activity of MoS. Here, we report an enhanced HER activity of MoS by combing vertical nanosheets and H annealing. Compared to horizontal MoS nanosheets, pristine vertical MoS nanosheets showed better HER activity due to a larger amount of edges. H annealing further enhanced the HER activity of vertical MoS nanosheets remarkably. Scanning electron microscopy (SEM), X-ray photoelectron spectra (XPS) and electrochemical impedance spectroscopy (EIS) were used to elucidate the enhanced HER activity by H annealing. SEM images showed that H annealing roughened the MoS edges, leading to more edge sites. XPS data revealed the smaller S : Mo ratio after H annealing, meaning more S-vacancies. Meanwhile, EIS measurements showed that charge transfer was accelerated by H annealing. These findings elaborated the H annealing induced enhancement of the HER activity, which were further confirmed by the subsequent re-sulfurization experiment.