Enhanced hydrogen evolution reaction activity of hydrogen-annealed vertical MoS2 nanosheets

Molybdenum disulfide (MoS2) 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 MoS2. Here, we report an enhanced HER activity of MoS2 by combing vertical nanosheets and H-2 annealing. Compared to horizontal MoS2 nanosheets, pristine vertical MoS2 nanosheets showed better HER activity due to a larger amount of edges. H-2 annealing further enhanced the HER activity of vertical MoS2 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-2 annealing. SEM images showed that H-2 annealing roughened the MoS2 edges, leading to more edge sites. XPS data revealed the smaller S:Mo ratio after H-2 annealing, meaning more S-vacancies. Meanwhile, EIS measurements showed that charge transfer was accelerated by H-2 annealing. These findings elaborated the H-2 annealing induced enhancement of the HER activity, which were further confirmed by the subsequent re-sulfurization experiment.