Charge Mediated Semiconducting-to-Metallic Phase Transition in Molybdenum Disulfide Monolayer and Hydrogen Evolution Reaction in New 1T′ Phase

The phase transition of single layer molybdenum disulfide (MoS₂) from semiconducting 2H to metallic 1T and then to 1T′ phases, and the effect of the phase transition on hydrogen evolution reaction (HER) are investigated within this work by density functional theory. Experimentally, 2H-MoS₂ has been widely used as an excellent electrode for HER and can get charged easily. Here we find that the negative charge has a significant impact on the structural phase transition in a MoS₂ monolayer. The thermodynamic stability of 1T-MoS₂ increases with the negative charge state, comparing with the 2H-MoS₂ structure before phase transition and the kinetic energy barrier for a phase transition from 2H to 1T decreases from 1.59 to 0.27 eV when 4e^– are injected per MoS₂ unit. Additionally, 1T phase is found to transform into the distorted structure (1T′ phase) spontaneously. On their activity toward hydrogen evolution reaction, 1T′-MoS₂ structure shows comparable hydrogen evolution reaction activity to the 2H-MoS₂ structure. If the charge transfer kinetics is taken into account, the catalytic activity of 1T′-MoS₂ is superior to that of 2H-MoS₂. Our finding provides a possible novel method for phase transition of MoS₂ and enriches understanding of the catalytic properties of MoS₂ for HER

Versatile Single-Layer Sodium Phosphidostannate(II): Strain-Tunable Electronic Structure, Excellent Mechanical Flexibility, and an Ideal Gap for Photovoltaics

Density functional theory (DFT) calculations were performed to study the structural, mechanical, electrical, optical properties, and strain effects in single-layer sodium phosphidostannate­(II) (NaSnP). We find the exfoliation of single-layer NaSnP from bulk form is highly feasible because the cleavage energy is comparable to graphite and MoS₂. In addition, the breaking strain of the NaSnP monolayer is comparable to other widely studied 2D materials, indicating excellent mechanical flexibility of 2D NaSnP. Using the hybrid functional method, the calculated band gap of single-layer NaSnP is close to the ideal band gap of solar cell materials (1.5 eV), demonstrating great potential in future photovoltaic application. Furthermore, strain effect study shows that a moderate compression (2%) can trigger indirect-to-direct gap transition, which would enhance the ability of light absorption for the NaSnP monolayer. With sufficient compression (8%), the single-layer NaSnP can be tuned from semiconductor to metal, suggesting great applications in nanoelectronic devices based on strain engineering techniques

Zero-Dimensional Interstitial Electron-Induced Spin–Orbit Coupling Dirac States in Sandwich Electride

The development of inorganic electrides offers new possibilities for studying topological states due to the nonnuclear-binding properties displayed by interstitial electrons. Herein, a sandwich electride 2[CaCl]+:2e− is designed, featuring a tetragonal lattice structure, including two atomic lattice layers and one interstitial electron layer. The interstitial electrons form nonsymmorphic-symmetry-protected Dirac points (DPs) at the X and M points, which are robust against the spin–orbit coupling effect. DPs exhibit an approximately elliptical shape, characterized by a relatively high anisotropy, resulting from the interplay between the electron and atomic layers. In addition, 2[CaCl]+:2e− possesses a lower work function (WF) (3.43 eV), endowing it with robust electron-supplying characteristics. Due to the low WF and interstitial electrons, 2[CaCl]+:2e− loaded Ru shows outstanding catalytic performance for N2 cleavage. A potential research platform for exploring the formation of topological states and promoting nitrogen cracking in electrides is provided

Predicting Single-Layer Technetium Dichalcogenides (TcX₂, X = S, Se) with Promising Applications in Photovoltaics and Photocatalysis

One of the least known compounds among transition metal dichalcogenides (TMDCs) is the layered triclinic technetium dichalcogenides (TcX₂, X = S, Se). In this work, we systematically study the structural, mechanical, electronic, and optical properties of TcS₂ and TcSe₂ monolayers based on density functional theory (DFT). We find that TcS₂ and TcSe₂ can be easily exfoliated in a monolayer form because their formation and cleavage energy are analogous to those of other experimentally realized TMDCs monolayer. By using a hybrid DFT functional, the TcS₂ and TcSe₂ monolayers are calculated to be indirect semiconductors with band gaps of 1.91 and 1.69 eV, respectively. However, bilayer TcS₂ exhibits direct-bandgap character, and both TcS₂ and TcSe₂ monolayers can be tuned from semiconductor to metal under effective tensile/compressive strains. Calculations of visible light absorption indicate that 2D TcS₂ and TcSe₂ generally possess better capability of harvesting sunlight compared to single-layer MoS₂ and ReSe₂, implying their potential as excellent light-absorbers. Most interestingly, we have discovered that the TcSe₂ monolayer is an excellent photocatalyst for splitting water into hydrogen due to the perfect fit of band edge positions with respect to the water reduction and oxidation potentials. Our predictions expand the two-dimensional (2D) family of TMDCs, and the remarkable electronic/optical properties of monolayer TcS₂ and TcSe₂ will place them among the most promising 2D TMDCs for renewable energy application in the future