Skew trapezoidal bipyramidal distortion in MoS₆ unit stabilizing distorted phases of 1T-MoS₂ single layer

The exceptional electrocatalytic performance of the 1T phase of MoS₂ in the hydrogen evolution reaction has motivated researchers to design methods for enhancing the stability of this phase. Herein, the electronic origin of the stability of 1T-MoS₂ and its distorted phases: 1T' (zig-zag), 1T′′ (diamond chain), and 1T′′′ (triangle) was elucidated using first-principles calculations. The phase stability can be altered by the repeating MoS₆ units that play a significant role in the generation of metal clusters. A novel skew trapezoidal bipyriamidal (STB) distortion was observed in the MoS₆ unit, which increased the stability of the distorted 1T phases in MoS₂. The highly distorted STB unit was found in the stable 1T' phase with an enhanced d_yz orbital population. The short edges of the distorted STB increase the electron density fraction of the Mo atoms, promoting Mo-Mo bond formation. The 1T' phase exhibited superior stability due to stronger electron delocalization in the Mo-Mo bond compared to that in the 1T′′ and 1T′′′ phases. The nature of Mo-Mo bonding varied on different metal clusters for 1T' (σ, π, and δ), 1T′′ (π and σ), and 1T′′′ (σ) bond types. Therefore, the stability of the 1T'-MoS₂ phase depends on the extent of distortion in the MoS₆ unit, Mo-Mo bond nature, and layer thickness.