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MoSH is a representative example of a Janus two-dimensional monolayered system consisting of a molybdenum atomic layer sandwiched between sulfur and hydrogen atomic layers. Extensive experimental and theoretical efforts have suggested the great promise of the MoSH material, but the validity of the MoSH model (with a Mo–S–H ratio of 1:1:1) remains uncertain. While various experiments have established the Mo-to-S ratio to be close to 1:1, the hydrogen content has remained elusive even with the use of state-of-the-art characterization techniques due to its lightweight nature. In this study, we present a theoretical investigation aiming to determine the positions and ratios of the hydrogen atoms on MoSHx (with x ranging from 0 to 3) as well as assess their structural stability. By evaluating the energetic, mechanical, and thermodynamic properties, we confirm the existence of stable MoSHx structures within a critical range of hydrogen atom ratios, specifically 0.5 ≤ x ≤ 2.0. Additionally, as the hydrogen atom ratio increases, we observe a transition in the preferred adsorption sites of hydrogen atoms from the center of the hexagonal ring composed of molybdenum and sulfur atoms to the upper region of the molybdenum atoms. This study offers critical insight into the structural characteristics and stability of Janus monolayer MoSHx, contributing to the advancement and application of Janus MoSHx in various fields. [ABSTRACT FROM AUTHOR]

Structural phase transition materials have attracted extensive attention in various fields due to their tunable physical properties. Transition metal dichalcogenide (TMD) nanomaterial is a critical representation. However, the phase transition of Janus MoSH, a sub-member of TMD nanomaterial, remains poorly understood. In this study, we employed a theoretical approach to investigate the phase transition of MoSH monolayer. Our results reveal temperature-dependent structural phase transitions of MoSH from 2H to 1T′. As the temperature reaches a critical value, a spontaneous structural phase transition occurs from 2H to 1T′ phase. Remarkably, our study identifies the 1T′ phase of MoSH as the most stable phase (including energetic, mechanical, dynamic, and thermodynamic stability) at most temperature ranges, and we provide insight into the reaction pathway of MoSH from 2H to 1T′ phase. Moreover, the structural phase transition of MoSH with hydrogen vacancies is also temperature-dependent, with the critical temperature decreasing with the incremental number of vacancies. Overall, our findings not only reveal the temperature-dependent structural phase transition of MoSH but also confirm the stable structure of MoSH in 1T′ phase at room temperature, which is significant for potential applications of 2D MoSH nanomaterial. [ABSTRACT FROM AUTHOR]