Amorphous molybdenum sulfide monolayer nanosheets for highly efficient electrocatalytic hydrogen evolution

Monolayer amorphous molybdenum sulfide (ML-a-MoSx) nanosheets were synthesized via a space-confined strategy, and their electrocatalytic activity was evaluated towards the hydrogen evolution reaction. The so-obtained ML-a-MoSx, with a quite high monolayer ratio of ~ 93%, shows a polymeric chain or network structure consisting of trinuclear [Mo3] clusters, with abundant Mo defects. ML-a-MoSx exhibits much higher activity in comparison with bulk a-MoSx (B-a-MoSx), and is superior among the Mo-S electrocatalysts reported to date. In particular, ML-a-MoSx can deliver a high current density of 400 mA cm−2 at an overpotential of 230 mV vs. RHE, which is similar to the state-of-the-art catalyst commercial 20% Pt/C. The Tafel slope of ML-a-MoSx remains almost unchanged with an increase in overpotential, very conducive to practical applications. The higher electrocatalytic activity of ML-a-MoSx than B-a-MoSx results from its larger specific surface area, higher active site density, stronger conductivity, and faster turnover frequency. The active sites of ML-a-MoSx are identified as Mo defects. This is the first report on the fabrication and catalytic activity of ML-a-MoSx. This work provides an effective route for large-scale fabrication of ML-a-MoSx and contributes to the understanding of the origin of its excellent electrocatalytic activity.