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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]

[Display omitted] • Novel Janus Monolayer 1T'-MoSF is found through theoretical calculations. • Janus Monolayer 1T'-MoSF has high energetic, mechanical, dynamical, and thermodynamic stability. • Janus Monolayer 1T'-MoSF exhibits significant anisotropy, and superior mechanical flexibility. Janus monolayers of transition-metal dichalcogenides (TMDs) have garnered significant attention in various scientific fields due to their exceptional physical properties. Among the known Janus monolayer TMDs, MoSX holds particular significance, with X typically representing chalcogen atoms. The successful synthesis of MoSH has pushed the boundaries of X beyond the conventional chalcogens. Nevertheless, it remains an open question whether other elements can also yield stable Janus monolayer MoSX. Herein, we employ theoretical methods to undertake a comprehensive evaluation of the stability of Janus monolayer MoSX (X = Li, Be, B, C, N, F, Na, Mg, Al, Si, P, Cl). The analysis of stability encompasses various aspects, including energetic, mechanical, dynamical, and thermodynamic features. Interestingly, our investigations have revealed the existence of a hitherto undiscovered stable 1T'-MoSF structure. Remarkably, this structure exhibits significant anisotropy, providing superior mechanical flexibility due to its low Young's modulus. In addition, despite the dynamical instability of 2H/1T-MoSN and the mechanical instability of 2H/1T-MoSC/Mg, they could potentially exist under specific conditions. This theoretical study not only provides insights into the structural characteristics and stability of novel Janus monolayer MoSX, but also extends the Janus monolayer MoSX family, which would be valuable to future applications of Janus TMDs. [ABSTRACT FROM AUTHOR]

• Higher oxidization degree of GO membrane favors water permeation. • Optimal microstructures for highly separating water from ethanol were uncovered. • With low O/C ratio, diffusion effect governs water permeation behavior. • With high O/C ratio, preferential adsorption governs water permeation process. Molecular dynamics (MD) simulations were employed to investigate water-ethanol separation through monolayer graphene oxide (GO) membranes with different pore sizes and O/C ratios. The ultrahigh water flux and infinite water separation factors were achieved. The separation properties under 50/50 w/w water-ethanol mixtures reveal that higher O/C ratios favor water selectivity, and water flux is enhanced with O/C ratio and pore size. With the help of highest oxidization, water largely sorption amount governs permeation process. On the contrary, with low oxidization degree of GO membrane, water diffusion effect becomes dominant factor of permeation. We screen an optimal microstructure of GO membrane which equipped with an adequately sized pore (D = 2.4 Å) and a highest O/C ratio (R = 0.49). This optimal one D2.4Å_R0.49 achieves the highest water flux and fully ethanol rejection in the mimicked experimental system. This simulation study elucidates the role of O/C ratio and pore diameter in water-ethanol separation through porous GO membranes on the microscopic level and uncovers the governing effects for water permeation and also suggests a potential candidate as a water-ethanol separation membrane. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pristine C 3 N monolayer shows stronger O 2 physisorption than pristine graphene. • The most preferable O 2 dissociation is an indirect two-step path with chemisorbed O 2. • Two dangling C-O bonds on the para-carbon site is the most stable oxidized structure. • Pristine C 3 N monolayer is more susceptible to oxidation than pristine graphene. • Electronic properties can be altered significantly due to the presence of two chemisorbed O atoms. The oxygen dissociation and the oxidized structure on the pristine C 3 N monolayer in exposure to air are the inevitably critical issues for the C 3 N engineering and surface functionalization yet have not been revealed in detail. Using the first-principles calculations, we have systematically investigated the possible O 2 adsorption sites, various O 2 dissociation pathways and the oxidized structures. It is demonstrated that the pristine C 3 N monolayer shows more O 2 physisorption sites and exhibits stronger O 2 adsorption than the pristine graphene. Among various dissociation pathways, the most preferable one is a two-step process involving an intermediate state with the chemisorbed O 2 and the barrier is lower than that on the pristine graphene, indicating that the pristine C 3 N monolayer is more susceptible to oxidation than the pristine graphene. Furthermore, we found that the most stable oxidized structure is not produced by the most preferable dissociation pathway but generated from a direct dissociation process. These results can be generalized into a wide range of temperatures and pressures using ab initio atomistic thermodynamics. Our findings deepen the understanding of the chemical stability of 2D crystalline carbon nitrides under ambient conditions, and could provide insights into the tailoring of the surface chemical structures via doping and oxidation. [ABSTRACT FROM AUTHOR]