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 dyz 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.