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

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.