In-depth first-principle study on novel MoS2 polymorphs

MoTe2 Polymorphs: A DFT Approach to Structural, Electronic, Mechanical and Vibrational Properties

Molybdenum ditelluride (MoTe2), a key member of the transition metal dichalcogenides (TMDCs) family, holds significant potential for applications in electronics, energy storage, and catalysis. Despite its importance, the range of MoTe2 structural forms that has been explored is still limited. The primary aim of this research is to identify new stable MoTe2 polymorphs that may exist under zero-temperature and zero-pressure conditions. This study offers an in-depth analysis of 11 different structural variations (polymorphs) of MoTe2 using advanced computational methods based on density functional theory (DFT). By employing the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, accurate calculations of electronic properties, such as band structure, are achieved. Bonding analysis, including charge density and electron localization, reveals consistent covalent interactions across the hexagonal and trigonal forms of MoTe2. The study also assesses the mechanical stability of these polymorphs using elastic constants, identifying both stable and metastable forms. Additionally, phonon and thermal properties, including heat capacity and entropy, are calculated for all dynamically stable polymorphs. Raman and infrared spectra provide insights into their distinct vibrational modes. These findings help distinguish structural attributes relevant to layer-specific applications. This comprehensive investigation of MoTe2 polymorphs uncovers new stable structures and provides crucial insights for their potential use in technological applications.

Effects of Unit Cell Size on Thermal Conductivity in Two Different Polymorphs of Niobium Diselenide

2D materials possess weak inter-layer van der Waals bonding, allowing them to exist as different polymorphs depending on the stacking sequence of the layers. Herein, the thermal conductivities of the 2H-NbSe2 and 2H-3R-NbSe2 polymorphs by conducting experimental measurements and theoretical analysis are comparatively studied. Owing to its 1.8 times larger unit cell, 2H-3R-NbSe2 has a considerably greater number of optical phonon branches than does 2H-NbSe2, suggesting that 2H-3R-NbSe2 absorbs thermal energy rather than transporting it. In addition, scattering is more likely to occur in 2H-3R-NbSe2 because a far greater number of states satisfy the selection rule. As a result of these, the 2H-3R-NbSe2 has considerably lower thermal conductivity than that of the 2H-NbSe2. The results highlight how the size of the unit cell affects the thermal conductivities of polymorphs.

A first-principles study of interfacial vacancies in the β-CsPbI3/1T-MoS2 heterostructure towards photocatalytic applications

Halide perovskite (HP) composites with transition metal dichalcogenides (TMDs) have attracted attention as promising photocatalysts for hydrogen production through solar-driven water splitting but their working mechanism is yet unclear. Here, we propose novel heterostructures composed of all-inorganic HP β-CsPbI3 and metallic TMD 1T-MoS2 and investigate the influence of interfacial vacancies on their interfacial properties using first-principles calculations. Using CsPbI3(001)/MoS2(001) interface slab models with a minimal lattice mismatch, we calculate the interface formation and interlayer binding energies, finding that the PbI2-terminated interfaces have better stability and stronger binding strength than the CsI-terminated ones and iodine vacancy enhances the binding properties. Our calculations demonstrate that photo-generated electrons are transferred from CsPbI3 to MoS2, inducing a dipole moment at the interface that prevents recombination of electrons and holes, and this desirable process for the hydrogen evolution reaction (HER) is enhanced by forming an I vacancy. Through analysis of the electronic density of states, we reveal that the I vacancy reduces the band gap of CsPbI3 by down-shifting its conduction band minimum level and forming a shallow defect state, being favourable for enhancing the HER performance on the MoS2 surface. This work highlights a way to design advanced photocatalysts based on HP/TMD composites for hydrogen production using solar energy.

Impact of Atomic Defects on Water Contact Angle of 2D Molybdenum Disulfide Surfaces

Interfacial dynamics within nanofluidic systems are crucial for applications like water desalination and osmotic energy harvesting. Understanding these dynamics can inform the rational optimization of two-dimensional (2D) materials and devices for such applications. This study explores the wetting behavior of realistic 2D MoS2 surfaces incorporating vacancies and atomic steps, known as atomic defects. We employ a combined density functional theory (DFT) and molecular dynamics (MD) computational approach to elucidate the influence of atomic defects on the MoS2-water interface. DFT calculations are utilized to determine the charge distribution within MoS2. Subsequently, free energy calculations are obtained through MD simulations of the MoS2-water interface. Our findings underscore the importance of incorporating atomic defects into MoS2 surfaces for accurate water contact angle (WCA) predictions in nanofluidic simulations, particularly when using Abal et al. force field parameters. However, the force field developed by Liu et al. yielded more accurate results for pristine MoS2 surfaces. While these parameters provide reliable outcomes for pristine MoS2 surfaces, their application to surfaces with defects may lead to underestimation of WCA. This highlights the critical need for realistic surface representations in nanofluidic modeling to accurately capture the complex interactions between water and MoS2 materials.

Adsorption and dissociation of hydrogen molecules over S-vacancies in a Nb-doped MoS2monolayer

Motivated by the recent interest in the hydrogen energy, we have carried out a complete study of the catalytic activity of a defective molybdenum disulfide monolayer (MoS2) by means of density functional theory (DFT) calculations. The MoS2monolayer is characterized by a nonreactive basal plane. In principle, its catalytic activity is concentrated at the edges, but an alternative way to increase such activity is obtained by creating active sites where the molecules can dissociate. These defects can be easily produced experimentally by different techniques. In our study, we have performed an atomic, energetic and electronic analysis of a hydrogen molecule adsorbed on a MoS2monolayer. In a first step, we have found that the H2molecule remains physisorbed over both doped-free and Nb-doped MoS2monolayers, showing that the Nb atom does not increase the poor reactivity of the clean MoS2layer. Interestingly, our energetic results suggest that the vacancies will prefer to be formed close to the Nb atoms in the doped monolayer, but the small energy difference would allow the formation in non-doped like sites. Theoretically, we found out the conditions for the molecular dissociation on a S vacancy. In both cases, with and without Nb, the molecule should rotate from the original perpendicular position to an almost parallel orientation jumping an energetic barrier. After that, the atoms are separated binding to the Mo atoms around the missing S atom. Ourab initiomolecular dynamics simulations show that for low pressure conditions (using one single molecule in the system) the H2prefers to desorb from the vacancy, while for larger pressures (when additional H2molecules are added to the system) the molecule is finally dissociated on the vacancy. Our long simulations confirm the great stability of the structure with the two H atoms binding to the Mo atoms close to the vacancy. Finally, the inclusion of a third (or a fourth) H atom in the vacancy leads to the formation and desorption of a H2molecule, leaving one (or two) atoms in the vacancy.

Doped MoS2 Polymorph for an Improved Hydrogen Evolution Reaction

Green hydrogen produced from solar energy could be one of the solutions to the growing energy shortage as non-renewable energy sources are phased out. However, the current catalyst materials used for photocatalytic water splitting (PWS) cannot compete with other renewable technologies when it comes to efficiency and production cost. Transition-metal dichalcogenides, such as molybdenum disulfides (MoS₂), have previously proven to have electronic and optical properties that could tackle these challenges. In this work, optical properties, the d-band center, and Gibbs free energy are calculated for seven MoS₂ polymorphs using first-principles calculations and density functional theory (DFT) to show that they could be suitable as photocatalysts for PWS. Out of the seven, the two polymorphs 3H_a and 2R₁ were shown to have d-band center values closest to the optimal value, while the Gibbs free energy for all seven polymorphs was within 5% of each other. In a previous study, we found that 3H_b had the highest electron mobility among all seven polymorphs and an optimal bandgap for photocatalytic reactions. The 3H_b polymorphs were therefore selected for further study. An in-depth analysis of the enhancement of the electronic properties and the Gibbs free energy through substitutional doping with Al, Co, N, and Ni was carried out. For the very first time, substitutional doping of MoS₂ was attempted. We found that replacing one Mo atom with Al, Co, I, N, and Ni lowered the Gibbs free energy by a factor of 10, which would increase the hydrogen evolution reaction of the catalyst. Our study further shows that 3H_b with one S atom replaced with Al, Co, I, N, or Ni is dynamically and mechanically stable, while for 3H_b, replacing one Mo atom with Al and Ni makes the structure stable. Based on the low Gibbs free energy, stability, and electronic bandgap 3H_b, MoS₂ doped with Al for one Mo atom emerges as a promising candidate for photocatalytic water splitting.

First-Principles Insights into the Relative Stability, Physical Properties, and Chemical Properties of MoSe2

A fascinating transition-metal dichalcogenide (TMDC) compound, MoSe₂, has attracted a lot of interest in electrochemical, photocatalytic, and optoelectronic systems. However, detailed studies on the structural stability of the various MoSe₂ polymorphs are still lacking. For the first time, the relative stability of 11 different MoSe₂ polymorphs (1H, 2H, 3H_a, 3H_b, 2T, 4T, 2R₁, 1T₁, 1T₂, 3T, and 2R₂) is proposed, and a detailed analysis of these polymorphs is carried out by employing the first-principles calculations based on density functional theory (DFT). We computed the physical properties of the polymorphs such as band structure, phonon, and elastic constants to examine the viability for real-world applications. The electronic properties of the involved polymorphs were calculated by employing the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06). The energy band gap of the polymorphs (1H, 2H, 3H_a, 3H_b, 2T, 4T, and 2R₁) is in the range of 1.6-1.8 eV, coinciding with the experimental value for the polymorph 2H. The covalent bonding nature of MoSe₂ is analyzed from the charge density, charge transfer, and electron localization function. Among the 11 polymorphs, 1H, 2H, 2T, and 3H_b polymorphs are predicted as stable polymorphs based on the calculation of the mechanical and dynamical properties. Even though the 4T and 3H_a polymorphs' phonons are stable, they are mechanically unstable; hence, they are considered to be under a metastable condition. Additionally, we computed the direction-dependent elastic moduli and isotropic factors for both mechanically and dynamically stable polymorphs. Stable polymorphs are analyzed spectroscopically using IR and Raman spectra. The thermal stability of the polymorphs is also studied.

Structural, Electronic Properties, and Relative Stability Studies of Low-Energy Indium Oxide Polytypes Using First-Principles Calculations

Materials made of indium oxide (In₂O₃) are now being used as a potential component of the next generation of computers and communication devices. Density functional theory is used to analyze the physical, electrical, and thermodynamical features of 12 low-energy bulk In₂O₃ polytypes. The cubic structure In₂O₃ is majorly used for many of the In₂O₃-based transparent conducting oxides. The objective of this study is to explore other new stable In₂O₃ polytypes that may exist. The structural properties and stability studies are performed using the Vienna ab initio simulation package code. All the In₂O₃ polytypes have semiconductive properties, according to electronic band structure investigations. The full elastic tensors and elastic moduli of all polytypes at 0 K are computed. Poisson's and Pugh's ratio confirms that all stable polytypes are ductile. The phonon and thermal properties including heat capacity are obtained for mechanically stable polytypes. For the first time, we report the Raman and infrared active modes of stable polytypes.

MoS2 as a Co-Catalyst for Photocatalytic Hydrogen Production: A Mini Review

Molybdenum disulfide (MoS₂), with a two-dimensional (2D) structure, has attracted huge research interest due to its unique electrical, optical, and physicochemical properties. MoS₂ has been used as a co-catalyst for the synthesis of novel heterojunction composites with enhanced photocatalytic hydrogen production under solar light irradiation. In this review, we briefly highlight the atomic-scale structure of MoS₂ nanosheets. The top-down and bottom-up synthetic methods of MoS₂ nanosheets are described. Additionally, we discuss the formation of MoS₂ heterostructures with titanium dioxide (TiO₂), graphitic carbon nitride (g-C₃N₄), and other semiconductors and co-catalysts for enhanced photocatalytic hydrogen generation. This review addresses the challenges and future perspectives for enhancing solar hydrogen production performance in heterojunction materials using MoS₂ as a co-catalyst.

Potential transition and post-transition metal sulfides as efficient electrodes for energy storage applications: review

Electrochemical energy storage has attracted much attention due to the common recognition of sustainable energy development. Transition metal sulfides and post-transition metal sulfides have been intensively been focused on due to their potential as electrode materials for energy storage applications in different types of capacitors such as supercapacitors and pseudocapacitors, which have high power density and long cycle life. Herein, the physicochemical properties of transition and post-transition metal sulfides, their typical synthesis, structural characterization, and electrochemical energy storage applications are reviewed. Various perspectives on the design and fabrication of transition and post-transition metal sulfides-based electrode materials having capacitive applications are discussed. This review further discusses various strategies to develop transition and/or post-transition metal sulfide heterostructured electrode-based self-powered photocapacitors with high energy storage efficiencies.

Electrochemical energy storage has attracted much attention due to the common recognition of sustainable energy development.

Crystal facet and phase engineering for advanced water splitting

This review covers the principles and recent advances in facet and phase engineering of catalysts for photocatalytic, photoelectrochemical, and electrochemical water splitting. It suggests the basis of catalyst design for advanced water splitting.

Novel two-dimensional ZnO2, CdO2 and HgO2 monolayers: a first-principles-based prediction

In this paper, the existence of monolayers with the chemical formula XO₂, where X = Zn, Cd, and Hg with hexagonal and tetragonal lattice structures is theoretically predicted by means of first principles calculations.