Controllable Synthesis of Nanosized Amorphous MoS_x Using Temporally Shaped Femtosecond Laser for Highly Efficient Electrochemical Hydrogen Production

Amorphous molybdenum sulfide (a-MoS_x) is regarded as a promising electrocatalyst for hydrogen evolution reaction (HER) due to its disorder structures with a significant number of defect-rich active sites. Here, a green, one-step, and controllable method is developed to photoregulate the chemical reactions and synthesize nanosized a-MoS_x by temporally shaped femtosecond laser ablation of ammonium tetrathiomolybdate aqueous solution. By adjusting the laser energy and pulse delay to control photoinduced and/or photothermal-induced reduction/oxidation, the S to Mo ratio x can be modulated from 1.53 to 3.07 and the ratio of the Mo^V defect species, bridging S₂ ²⁻, and terminal S₂ ²⁻ ligands can be controlled. The optimized a-MoS_x catalysts (x = 2.73) exhibit high catalytic activity with a low Tafel slope of 40 mV dec⁻¹, high double-layer capacitance of 74.47 mF cm⁻², and large current density of 516 mA cm⁻² at an overpotential of 250 mV. The high catalytic activity can be mainly attributed to Mo^V defect species and bridging S₂ ²⁻ ligands, or most likely dominated by the Mo^V defect species. This study not only provides an alternatively controllable method to prepare a-MoS_x as efficient HER catalysts but also contributes to the understanding of the origin of its catalytic activity.