2D MoO3-xSx/MoS2 van der Waals Assembly: A Tunable Heterojunction with Attractive Properties for Photocatalysis

Evolution of structural and electronic properties standardized description in rhenium disulfide at the bulk-monolayer transition

The structural and electronic properties of ReS2 different forms - three-dimensional bulk and two-dimensional monolayer - were studied within density functional theory and pseudopotentials. A method for standardizing the description of bulk unit cells and "artificial" slab unit cells for DFT research has been proposed. The preference of this method for studying zone dispersion has been shown. The influence of the vacuum layer thickness on specified special high-symmetry points is discussed. Electron band dispersion in both classical 3D Brillouin zones and transition to 2D Brillouin zones in the proposed two-dimensional approach using the Niggli form of the unit cell was compared. The proposed two-dimensional approach is preferable for low-symmetry layered crystals such as ReS2. It was established that the bulk ReS2 is a direct gap semiconductor (band gap of 1.20 eV), with the direct transition lying in the X point of the first Brillouin zone, and it is in good agreement with published experimental data. The reduction in material dimension from bulk to monolayer was conducted with an increasing band gap up to 1.45 eV, with a moving direct transition towards the Brillouin zone center. The monolayer of ReS2 is a direct-gap semiconductor in a wide range of temperatures, excluding only a narrow range at low temperatures, where it comes as a quasi-direct gap semiconductor. The transition, situated directly in the Γ-point, lies 3.3 meV below the first direct transition located near this point. The electronic density of states of ReS2 in the bulk and monolayer cases of ReS2 were analyzed. The molecular orbitals were built for both types of ReS2 structures as well as the electron difference density maps. For all types of ReS2 structures, an analysis of populations according to Mulliken and Voronoi was carried out. All calculated data is discussed in the context of weak quantum confinement in the 2D case.

Two-Dimensional Layered Heterojunctions for Photoelectrocatalysis

Two-dimensional (2D) layered nanomaterial heterostructures, arising from the combination of 2D materials with other low-dimensional species, feature a large surface area to volume ratio, which provides a high density of active sites for catalytic applications and for (photo)electrocatalysis (PEC). Meanwhile, their electronic band structure and high electrical conductivity enable efficient charge transfer (CT) between the active material and the substrate, which is essential for catalytic activity. In recent years, researchers have demonstrated the potential of a range of 2D material interfaces, such as graphene, graphitic carbon nitride (g-C3N4), metal chalcogenides (MCs), and MXenes, for (photo)electrocatalytic applications. For instance, MCs such as MoS2 and WS2 have shown excellent catalytic activity for hydrogen evolution, while graphene and MXenes have been used for the reduction of carbon dioxide to higher value chemicals. However, despite their great potential, there are still major challenges that need to be addressed to fully realize the potential of 2D materials for PEC. For example, their stability under harsh reaction conditions, as well as their scalability for large-scale production are important factors to be considered. Generating heterojunctions (HJs) by combining 2D layered structures with other nanomaterials is a promising method to improve the photoelectrocatalytic properties of the former. In this review, we inspect thoroughly the recent literature, to demonstrate the significant potential that arises from utilizing 2D layered heterostructures in PEC processes across a broad spectrum of applications, from energy conversion and storage to environmental remediation. With the ongoing research and development, it is likely that the potential of these materials will be fully expressed in the near future.

Influence of shear strain on the electronic properties of monolayers MoS2, WS2, and MoS2/WS2 vdW heterostructure

CONTEXT

This study investigates the dynamic stability of monolayers MoS2, WS2, and MoS2/WS2 van der Waals heterostructures (vdWHs) and the influence of shear strain on their electronic properties. The computational results of the binding energy and phonon dispersion demonstrate the excellent dynamic stability of MoS2/WS2 vdWHs. The MoS2/WS2 vdWH, with a type-II band alignment and an indirect bandgap, reduces electron-hole recombination, enhancing the efficiency and performance of optoelectronic devices. Under shear strain, the bandgap size and type of monolayers MoS2, WS2, and MoS2/WS2 vdWHs were effectively modulated, along with the interlayer charge redistribution in the MoS2/WS2 vdWHs. This work reveals the tunability of the electronic properties of monolayers MoS2, WS2, and MoS2/WS2 vdWHs under shear strain, offering new possibilities and solutions for developing optoelectronic devices, sensors, and related fields.

METHODS

This work employed the CASTEP module within the Materials Studio software package for first-principles calculations. Ultrasoft pseudopotentials were employed during geometry optimizations to account for ion-electron interactions using the GGA-PBE functional for exchange-correlation potentials. The electronic configurations of the S, Mo, and W atoms were chosen as their typical arrangements: (3s2p4), (4s2p6d55s1), and (5s2p6d46s2), respectively. A vacuum layer of 20 Å was added to avoid interactions between the atomic layers. A cutoff energy of 500 eV was set for structural optimization and self-consistent calculations, with k-point grids of 6 × 6 × 1 and 9 × 9 × 1. During the structural optimization process, the energy convergence criterion was set to 1 × 10-5 eV, and the thresholds for interatomic forces and stresses were set to 0.01 eV/Å and 0.01 GPa, respectively. Grimmer's DFT-D2 correction accounted for the interlayer vdW interactions in the MoS2/WS2 vdWH, while the phonon dispersion was calculated using the linear response method.

Stabilizing tetramethylammonium lead iodide perovskite and exploring its electronic and optical absorption for solar cell absorber application

Modeling perovskites as solar cell absorbers has become popular due to the breakthrough of methylammonium lead iodide (CH3NH3PbI3). In this study, we modeled a tetramethylammonium lead iodide (CH3)4NPbI3 structure. We further confirmed the stability of the structure by determining the phonon dispersion using density functional perturbation theory. We calculated the spin-orbit and non-spin-orbit coupling-based electronic structure using the Perdew-Burke-Ernzerhof exchange-correlation functional within the generalized gradient approximation of the density functional theory and the self-consistent GW quasiparticle methods. Similarly, the absorption spectra were calculated from the real and imaginary parts of the dielectric tensor obtained from solving the Bethe-Salpeter equation using the GW quasiparticle database. The solar cell absorber spectroscopic limited maximum efficiency was calculated at 293.15 K. The self-consistent GW method without spin-orbit coupling reported bandgaps of 2.63 eV and 2.89 eV for GW0 and GW methods, respectively, in agreement with experimental reports. The phonon dispersion showed positive phonon modes across the high symmetry point, which attest to its thermodynamic stability. The absorption coefficient on the order of 105 was reported along the ultraviolet region. The standard limited maximum efficiency between 7% and 12% was recorded at 293.15 K between 0.01 and 100 μm absorber thicknesses. The thermodynamic stability, high absorption coefficient, and low transmittance indicated exciting prospects for a non-transparent (CH3)4NPbI3 solar cell absorber.

A first-principles study on the electronic, piezoelectric, and optical properties and strain-dependent carrier mobility of Janus TiXY (X ≠ Y, X/Y = Cl, Br, I) monolayers

Janus transition metal dichalcogenide monolayers (TMDs) have attracted wide attention due to their unique physical and chemical properties since the successful synthesis of the MoSSe monolayer. However, the related studies of Janus monolayers of transition metal halides (TMHs) with similar structures have rarely been reported. In this article, we systematically investigate the electronic properties, piezoelectric properties, optical properties, and carrier mobility of new Janus TiXY (X ≠ Y, X/Y = Cl, Br, I) monolayers using first principles calculations for the first time. These Janus TiXY monolayers are thermally, dynamically, and mechanically stable, and their energy bands near the Fermi level (E_F) are almost entirely contributed by the central Ti atom. Besides, the Janus TiXY monolayers exhibit excellent in-plane and out-of-plane piezoelectric effects, especially with an in-plane piezoelectric coefficient of ∼4.58 pm V^-1 for the TiBrI monolayer and an out-of-plane piezoelectric coefficient of ∼1.63 pm V^-1 for the TiClI monolayer, suggesting their promising applications in piezoelectric sensors and energy storage applications. The absorption spectra of Janus TiXY monolayers are mainly distributed in the visible and infrared regions, implying that they are fantastic candidates for photoelectric and photovoltaic applications. The obtained carrier mobilities revealed that TiXY monolayers are hole-type semiconductors. Under uniaxial compressive strain, the hole mobilities of these monolayers are gradually improved, indicating that TiXY monolayers have potential applications in the field of flexible electronic devices.

Development of defective molybdenum oxides for photocatalysis, thermal catalysis, and photothermal catalysis

The localized surface plasmon resonance (LSPR) of noble metals has been investigated for decades for applications in various catalysis reactions and optical research studies, but its development has been hampered by inefficient light absorption and high costs. In comparison, the creation of less expensive semiconductors (metal oxides) with strong plasmonic absorption is an appealing option, particularly defective molybdenum oxide (H_xMoO_3-y) has received considerable attention and investigation as a promising plasmonic material for a variety of catalytic reactions (photocatalysis, thermocatalysis, photothermal catalysis, etc.).The LSPR effect of H_xMoO_3-y can be tuned throughout a broad spectrum range from visible to near-infrared (NIR) by altering the doping amount by electrochemical control, chemical reduction, or photochemical control. Notably, defects (oxygen vacancies) in H_xMoO_3-y arise in conjunction with the LSPR effect, resulting in the formation of unique and useful active sites in a range of catalytic processes. In this review, we explore the formation mechanism of H_xMoO_3-y with plasmonic features and discuss its applications in photocatalysis, thermocatalysis, and photothermal catalysis.

Electronic structures of the MoS2/TiO2 (anatase) heterojunction: influence of physical and chemical modifications at the 2D- or 1D-interfaces

To tackle the challenge of CO₂ photoreduction, semiconducting layered transition metal dichalcogenides like MoS₂ have attracted much attention due to their tunable 2D nano-structures. By using advanced periodic density functional theory calculations (HSE06 functional), we provide a systematic quantification of the optoelectronic properties of various interfacial heterostructures composed of 2H-MoS₂ and anatase TiO₂. We systematically determine the band gaps, and conduction band (CB) and valence band (VB) positions to figure out the nature of the heterojunction. Two main surface orientations of anatase TiO₂ particles, (101) and (001), are considered with 2D-MoS₂ nanosheets or nanoribbons forming either a 2D physical (van der Waals) or through a 1D chemical interface. The possibility to chemically modify the MoS₂/TiO₂ interface, either by sulfidation or hydration, and its effect on the electronic structure are deeply investigated. These modifications in the heterostructure lead to important changes in the electronic properties and charge transfer between the two materials which impact both photon absorption properties and charge carrier dynamics suspected to influence in turn the photocatalytic activity. While a type I hetrojunction is found for the 1D chemical interface, a type II heterojunction with appropriate CB/VB positions for CO₂ reduction and H₂O oxidation is identified for the 2D physical interface which could lead to the targeted Z-scheme mechanism with strong potential interest in photocatalysis applications.

First-principle calculations of 0D/2D GQDs-MoS2 mixed van der Waals heterojunctions for photocatalysis: a transition from type I to type II

The fabrication of type II heterojunctions is an efficient strategy to facilitate charge separation in photocatalysis. Here, mixed dimensional 0D/2D van der Waals heterostructures (graphene quantum dots (GQDs)-MoS2) for generating...