Density Functional Theory Study of Edge-Induced Atomic-Scale Structural Phase Transitions of MoS₂ Nanocrystals: Implications for a High-Performance Catalyst

Molybdenum disulfide (MoS2) has attracted much attention as a material to replace the noble-metal-based hydrogen evolution reaction catalyst. Polymorphism is an important factor in improving the catalytic performance of transition-metal dichalcogenides (TMDs) including MoS2. Several methods have been proposed to synthesize the 1T/1T′ phase with high catalytic efficiency, and a gas–solid reaction has recently been proposed as one of the alternative methods. However, the atomic-scale reaction mechanism between gas molecules and MoS2 has not been clarified. Here, we report a detailed atomic-scale mechanism of structural phase transition of MoS2 nanocrystals under reaction with CO gas molecules using density functional theory calculations. We confirm that the evaporation of S atoms at the edge is much faster than the evaporation at the basal plane of MoS2 nanocrystals. It is found that the S evaporation at the edge induces the structural change from 2H to 1T/1T′ in the basal plane of nanocrystals. The structural change is also attributed to the chain reaction due to the sequential migration of S atoms to the octahedral sites, which is energetically favorable. The present results provide a guideline for the gas–solid reaction-based phase control of TMDs including MoS2 to synthesize a high-performance catalyst.