Unveiling the Effect of Organic Sulfur Sources on Synthesized MoS₂ Phases and Electrocatalytic Hydrogen Evolution...

Metallic-phase MoS₂ exhibits Pt-comparable electrocatalytic hydrogen evolution reaction (HER) performance in acidic conditions. However, the controllable synthesis of metallic-phase MoS₂ is quite challenging because the key factor determining the phase types of MoS₂ during synthesis is still unclear. Herein, the effect of organic sulfur sources on the formed MoS₂ phase is studied by use of thioacetamide (TAA), l-cysteine, and thiourea as sulfur sources. The TAA and l-cysteine produce metallic MoS₂, while thiourea gives rise to semiconducting MoS₂. Owing to the metallic phase and smaller size, the MoS₂ prepared with TAA and l-cysteine has a higher electrocatalytic HER activity than the MoS₂ obtained from thiourea. The HER overpotential of MoS₂ synthesized with TAA is only 210 mV for reaching the current density of 10 mA/cm², and the corresponding Tafel slope is 44 mV/decade. Further studies find that the decomposition temperature of sulfur precursors is the key factor for the formation of metallic MoS₂. Sulfur precursors with a lower decomposition temperature release sulfur ions quickly, which in turn stabilize the metallic phase and inhibit the growth of MoS₂ into large sizes. Our findings unveil the key factor for controlling the phase type of MoS₂ synthesized from organic sulfur precursors and will be very helpful for the synthesis of MoS₂ with high electrocatalytic activity.