Enhanced hydrogen evolution reaction activity of hydrogen-annealed vertical MoS(2) nanosheets

Molybdenum disulfide (MoS(2)) 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(2). Here, we report an enhanced HER activity of MoS(2) by combing vertical nanosheets and H(2) annealing. Compared to horizontal MoS(2) nanosheets, pristine vertical MoS(2) nanosheets showed better HER activity due to a larger amount of edges. H(2) annealing further enhanced the HER activity of vertical MoS(2) 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 MoS(2) 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.