Phase Engineering of Transition Metal Dichalcogenides with Unprecedentedly High Phase Purity, Stability, and Scalability via Molten-Metal-Assisted Intercalation.

The crystalline phase of layered transition metal dichalcogenides (TMDs) directly determines their material property. The most thermodynamically stable phase structures in TMDs are the semiconducting 2H and metastable metallic 1T phases. To overcome the low phase purity and instability of 1T-TMDs, which limits the utilization of their intrinsic properties, various synthesis strategies for 1T-TMDs have been proposed in phase-engineering studies. Herein, a facile and scalable synthesis of 1T-phase molybdenum disulfide (MoS ) via the molten-metal-assisted intercalation (MMI) approach is introduced, which exploits the capillary action of molten potassium and the difference between the electron affinity of MoS and the ionization potential of potassium. Highly reactive molten potassium metal can readily intercalate into the MoS interlayers, inducing an efficient phase transition from the 2H to 1T crystal structure. The ionic bonding between the intercalated potassium and sulfur lowers the energy barrier of the 1T-phase transition, enhancing the phase stability of the 1T crystals. Owing to the high purity and stability of the 1T phase, the electrocatalytic performance for the hydrogen evolution reaction is significantly higher in 1T-MoS (MMI) than in 2H-MoS and even in 1T-MoS synthesized using n-butyllithium.