Designing C6N6/C2N van der Waals heterostructures for photogenerated charge carrier separation

Electrical contact property and control effects for stable T(H)-TaS2/C3B metal-semiconductor heterojunctions

Metal-semiconductor heterojunctions are the basis for developing new electronic devices. Here, T(H)-TaS2/C3B metal-semiconductor heterostructures are constructed by different phase T- and H-TaS2 monolayers combined with the C3B monolayer. The calculated corrected binding energies, phonon band structures, elastic constants, and molecular dynamics simulations indicated that both heterojunctions are highly stable, meaning that T(H)-TaS2/C3B heterojunctions possibly exist in experiments. The electronic property calculations showed that the intrinsic T(H)-TaS2/C3B heterojunction is an n(p)-type Schottky contact with a low Schottky barrier height (SBH), which is very important for the design of high-performance field-effect transistors. The electronic properties of the T(H)-TaS2/C3B heterojunctions can be controlled by varying the vertical strain and external electric field; however, the strain only resulted in a small change in the heterojunction SBH. Nevertheless, under external electrical field control, the T-TaS2/C3B heterojunction could manage a transition from an n-type Schottky contact to an n-type Ohmic contact and the H-TaS2/C3B heterojunction could be altered from a p-type Schottky contact to a p-type Ohmic contact. These findings provide theoretical insights into the electronic and electrical contact properties of the T(H)-TaS2/C3B heterojunction, which could be beneficial for developing n-type MOS and p-type MOS transistors.

The direct Z-scheme character and roles of S vacancy in BiOCl/Bi2S3-(001) heterostructures for superior photocatalytic activity: a hybrid density functional investigation

Given some current speculations and controversies regarding the type of BiOCl/Bi2S3-(001) heterostructure in experiments, it is of great importance to clarify these controversies and further explain the relevant experimental results. In this work, based on first-principles hybrid density functional calculations, it is verified that the BiOCl/Bi2S3-(001) heterostructure is a direct Z-scheme photocatalyst with high photo-generated carrier separation efficiency and strong redox ability that can react with O2 and OH- to produce photocatalytic active species of superoxide ions (˙O2-) and hydroxyl radicals (˙OH), respectively. This is consistent with the experimental findings and explains the excellent photocatalytic performance of the BiOCl/Bi2S3-(001) heterostructure in experiments. Besides, excitingly, it is found that the optical absorption, built-in electric field intensity, interlayer recombination probability, hydrogen evolution reaction ability, and the difference in electron-hole mobility are further enhanced via S vacancy introduction in BiOCl/Bi2S3-(001). Therefore, the significant roles of S vacancy in further improving the photocatalytic properties of the BiOCl/Bi2S3-(001) heterostructure are profoundly revealed. This work can provide valuable theoretical insights for designing the superior direct Z-scheme BiOCl/VS-Bi2S3-(001) heterostructure with promising photocatalytic properties.

First-principles Calculations Reveal Frictional Advantage for C2 N/C6 N6 van der Waals Heterostructures

Friction at the atomic scale is determined for three different carbon nitride structures namely C2 N/C2 N, C6 N6 /C6 N6 and C6 N6 /C2 N employing ab-initio density functional theory (DFT). The sliding path along the lowest energy corrugations determines the static frictional forces. Both the homo-layer structures (C2 N/C2 N and C6 N6 /C6 N6 ) have higher corrugation energy and correspondingly higher static lateral forces with respect to the hetero-layer structure (C2 N/C6 N6 ). The corrugation energy for the C2 N/C6 N6 heterostructure (δ c o r r ${{\delta }_{corr}}$ =0.29 meV/atom) is one-order lower than C2 N/C2 N (δ c o r r ${{\delta }_{corr}}$ =2.08 meV/atom) and C6 N6 /C6 N6 (δ c o r r ${{\delta }_{corr}}$ =4.37 meV/atom). Such a significantly lower corrugation energy for the heterostructure arises due to the reduced fluctuation in the interfacial charge density along the sliding pathway. Moreover, the change in the interlayer distance along the sliding pathway is only 0.2 Å for the heterostructure while its 0.3 Å and 0.4 Å for C2 N and C6 N6 homo-layers respectively. The friction coefficients (FL /FN , FL =static lateral force; FN =normal force) decrease with increasing load for all the systems with the lowest value (0.04) for C2 N/C6 N6 at 2 GPa. The van der Waals heterostructures are, therefore, predicted to be highly efficient lubricant materials for reducing friction at the atomic scale.

Density Functional Theory Study on the Enhancement Mechanism of the Photocatalytic Properties of the g-C3N4/BiOBr(001) Heterostructure

The van der Waals heterostructures fabricated in two semiconductors are currently attracting considerable attention in various research fields. Our study uses density functional theory calculations within the Heyd-Scuseria-Ernzerhof hybrid functional to analyze the geometric structure and electronic structure of the g-C₃N₄/BiOBr(001) heterojunction in order to gain a better understanding of its photocatalytic properties. The calculated band alignments show that g-C₃N₄/BiOBr can function as a type-II heterojunction. In this heterojunction, the electrons and holes can effectively be separated at the interface. Moreover, we find that the electronic structure and band alignment of g-C₃N₄/BiOBr(001) can be tuned using external electric fields. It is also noteworthy that the optical absorption peak in the visible region is enhanced under the action of the electric field. The electric field may even improve the optical properties of the g-C₃N₄/BiOBr(001) heterostructure. Given the results of our calculations, it seems that g-C₃N₄/BiOBr(001) may be significantly superior to visible light photocatalysis.

Computational Insight into TM-N x Embedded Graphene Bifunctional Electrocatalysts for Oxygen Evolution and Reduction Reactions

Due to the energy crisis, development of bifunctional electrocatalysts for both oxygen evolution and reduction reactions is highly demanding. In this study, we have systematically investigated the bifunctional activity of metal (Co/Rh/Ir) and N co-doped graphene systems with varying N-dopant concentrations (TM-N _x @G, x = 0, 2, 4) using first-principles calculations. Charge transfer from the metal sites to the adsorbed intermediates and the adsorption free energy of the intermediates play important roles to help understand the potential-determining step and overpotential values for oxygen evolution reaction (OER)/oxygen reduction reaction (ORR). A dual volcano plot for all the systems using a common descriptor ΔG _OH* has been constructed. We find that the systems having ΔG _OH* values in the range of 0.40-0.70 eV can act as bifunctional electrocatalysts. Our study not only highlights the importance of metal and non-metal co-doped graphene as bifunctional catalysts but also can serve as a promising strategy for the design of efficient OER/ORR electrocatalysts.

Promoted photocarrier separation by dipole engineering in two-dimensional perovskite/C2N van der Waals heterostructures

Due to the aggravation of environmental pollution and the energy crisis, it is urgent to develop and design environment-friendly and efficient photocatalysts for water splitting. van der Waals heterostructures composed of different two-dimensional materials offer an easily accessible way to combine properties of individual materials for applications. Herein, a novel Cs₃Bi₂I₉/C₂N heterostructure is proposed through first-principles calculations. The structural, electronic, and optical properties, as well as the charge transfer mechanism at the interface of Cs₃Bi₂I₉/C₂N are systematically investigated. Due to the difference between the work functions of Cs₃Bi₂I₉ and C₂N monolayers, when they are constructed into heterostructures, redistribution of charge occurs in the whole structure, and some of the charge transfer occurs at the interface due to the formation of an internal electric field. The band structure of Cs₃Bi₂I₉/C₂N has type-II band alignment, and the band edge position as well as the band-gap value of the heterostructure are suitable for visible light water splitting. The in-plane biaxial strain, interfacial spacing, and external electric field can effectively modulate the electronic structure and photocatalytic performance of the heterostructure. Under certain conditions, the heterostructure can be changed from type-II to type-I band alignment, accompanied by the transition from an indirect band-gap semiconductor to a direct band-gap semiconductor. Moreover, the intrinsic anion defect (I vacancy) at different positions, as donor defects, can introduce defect levels near the conduction band edge, which affects the transition of photogenerated carriers in these systems. Our findings provide a theoretical design for strategies to improve the performance of two-dimensional perovskites/C₂N in photocatalytic and optoelectronic applications.

Band alignment tuning of heptazine-g-C3N4/g-ZnO vdW heterostructure as a promising water-splitting photocatalyst

Van der Waals (vdW) heterostructures of two-dimensional monolayers are a relatively new class of materials with highly tunable band alignment, bandgap energy, and bandgap transition type. In this study, we performed density functional theory calculations to investigate how a vdW heterostructure of heptazine-based graphitic carbon nitride (hg-C₃N₄) and graphitic zinc oxide (g-ZnO) monolayers is formed (hg-C₃N₄/g-ZnO). This heterostructure is a potential solar-driven photocatalyst for the water-splitting reaction. Upon the formation of the heterostructure, a type-I indirect bandgap (E_g = 2.08 eV) is created with appropriate conduction band minimum and valence band maximum levels relative to the oxidation/reduction potentials for the water-splitting reaction. In addition, a very large electrostatic potential difference of 11.18 eV is generated across the heterostructure, leading to a large, naturally-formed, built-in electric field directing from hg-C₃N₄ to g-ZnO. The produced electric field forces photogenerated electrons in g-ZnO to transfer toward hg-C₃N₄, leading to a decrease in the electron-hole recombination rate. We also found that both g-ZnO and hg-C₃N₄ synergistically lead to higher light absorption of the heterostructure (λ_max = 387 nm). Furthermore, band alignment, bandgap energy, and transition type of the heterostructure can be tuned by applying external perpendicular electric fields and biaxial strains. It was found that a strain of +2% leads to a Z-scheme band alignment (E_g = 2.34 eV, direct) and an electric field of 1 V Å^-1 leads to a type-II heterostructure (E_g = 2.29 eV, indirect), which are both beneficial for efficient water-splitting photocatalysis.