C2N/BlueP van der Waals hetero-structure: an efficient photocatalytic water splitting 2D material

Strain-induced excellent photocatalytic performance in Z-scheme BlueP/γ-SnS heterostructures for water splitting

Constructing Z-scheme heterojunction photocatalysts with high solar-to-hydrogen (STH) efficiency is a practical alternative to produce clean and recyclable hydrogen energy on a large scale. This paper presents the design of stable Z-scheme blue phosphorene (BlueP)/γ-SnS heterostructures with excellent photocatalytic activities by applying strains. The first-principles calculations show that the BlueP/γ-SnS heterobilayer is a type-I heterojunction with an indirect bandgap of 1.41 eV and strong visible-light absorption up to 105 cm-1. Interestingly, biaxial strains (ε) can effectively regulate its bandgap width (semiconductor-metal) and induce the band alignment transition (type-I-type-II). Compressive and tensile strains can significantly enhance the interfacial interaction and visible-light absorption, respectively. More intriguingly, compressive strains can not only modulate the heterojunction types but also make the band edges meet the requirements for overall water splitting. In particular, the Z-scheme (type-I) BlueP/γ-SnS bilayer at -8% (-2%) strain exhibits a relatively high STH efficiency of 18% (17%), and the strained Z-scheme system (-8% ≤ ε ≤ -6%) also exhibits high and anisotropic carrier mobilities (158-2327 cm2 V-1 s-1). These strain-induced outstanding properties make BlueP/γ-SnS heterostructures promising candidates for constructing economically feasible photocatalysts and flexible nanodevices.

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.

Rational Design of a Two-Dimensional Janus CuFeO2+δ Single Layer as a Photocatalyst and Photoelectrode

The exfoliation of 2D nanomaterials from 3D multimetal oxides with a stable structure is a great challenge. Herein, a delafossite CuFeO_2+δ nanosheet becomes an open-layered structure by introducing excess oxygen so that the 2D Janus CuFeO_2+δ single layer can be further obtained by aqueous ultrasonic exfoliation. The 2D Janus CuFeO_2+δ single layer breaks the limitation of mirror symmetry, which is very beneficial to the effective separation of photogenerated electron-hole pairs. Serving as both a photoelectrode and a photocatalyst, the 2D Janus CuFeO_2+δ single layer/few layer remarkably enhances the photocatalytic activity with long-term stability: the photocurrent density is increased by 2-fold, and the rate of H₂ evolution is increased by 1.5-fold, in comparison with the counterpart of unexfoliated CuFeO_2+δ nanosheets. This work demonstrates that 2D nanomaterials can be directly exfoliated from 3D nanomaterials by rational composition and microstructure design, which is helpful in promoting the development of bimetallic-oxide-ene (BMOene) as a novel functional material.

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

Photocatalytic water splitting mechanism boosted by two-dimensional catalyst materials has become the vibrant field of research toward clean energy initiative. Here, we propose a new two-dimensional (2D) van der Waals type-II heterostructure based on C6N6/C2N composites as an efficient photocatalyst. The structural, electronic and optical properties of the free-standing monolayer as well as their heterostructure have been investigated by first principles based density functional theory (DFT). The band edge positions of C6N6/C2N heterostructures satisfy the photocatalytic water splitting requirements. The potential drop at the interface of the heterostructure induces a large built-in electric field directed from the C6N6 layer to the C2N layer, thereby facilitating the charge transfer from C2N to C6N6 layer. The higher hole mobility as compared to that of electrons aids in separation of charge carriers and thus prohibiting the recombination of the photogenerated charge carriers by separating them in different layers. This is also reflected in the planar averaged charge density difference and partial charge density calculation. The wide band gap semiconductor (C6N6) in combination with a moderate band gap semiconductor (C2N) allows one to harness solar energy efficiently in the visible region for water splitting as also confirmed in the optical absorption spectra. The favorable band edge position with respect to the water redox potential makes it an ideal substrate for the visible light induced oxygen evolution reaction and the hydrogen evolution reaction.

A first-principles study on the electronic and optical properties of a type-II C2N/g-ZnO van der Waals heterostructure

The structural, electronic and optical properties of a new van der Waals heterostructure, C2N/g-ZnO, composed of C2N and g-ZnO monolayers with an intrinsic type-II band alignment and a direct bandgap of 0.89 eV at the Γ point, are extensively studied using first-principles density functional theory calculations. The results indicate that the special optoelectronic properties of the constructed heterostructure mainly originate from the interlayer coupling and electron transfer between the C2N and g-ZnO monolayers, and the photogenerated electrons and holes are located on the C2N and g-ZnO layers, respectively, which reduces the recombination probability of the electron-hole pairs. According to Bader charge analysis, there are 0.029 electrons transferred from g-ZnO to C2N to form a built-in electric field of ∼9.5 eV at the interface. Furthermore, the tunability of the electronic properties of the C2N/g-ZnO heterostructure under vertical strain and electric field is explored. Under different strains, the type-II band alignment properties of the heterostructure are retained and the vertical compressive strain has a greater influence on the bandgap modulation than the vertical stretching strain. The implemented electric field also does not change the type-II band alignment but changes the bandgap of the heterostructure from 1.30 to 0.58 eV when the electric field strength varies from -0.6 to 0.6 V Å-1. In addition, the absorption spectrum of the C2N/g-ZnO heterostructure under solar light is also studied. The absorption range of the heterostructure varies from the ultraviolet to near-infrared region with the absorption intensity in the order of 105 cm-1. All of these studies indicate that the C2N/g-ZnO heterostructure has excellent electronic and optical properties and promising applications in nanoelectronics and optoelectronics.

Tunable electronic properties and band alignments of InS–arsenene heterostructures via external strain and electric field

van der Waals heterostructures (vdWHs) based on two-dimensional (2D) materials have been extensively recognized as promising candidates for fabricating multi-functional novel devices.

C2N: A Class of Covalent Frameworks with Unique Properties

C2N is a unique member of the CnNm family (carbon nitrides), i.e., having a covalent structure that is ideally composed of carbon and nitrogen with only 33 mol% of nitrogen. C2N, with a stable composition, can easily be prepared using a number of precursors. Moreover, it is currently gaining extensive interest owing to its high polarity and good thermal and chemical stability, complementing carbon as well as classical carbon nitride (C3N4) in various applications, such as catalysis, environmental science, energy storage, and biotechnology. In this review, a comprehensive overview on C2N is provided; starting with its preparation methods, followed by a fundamental understanding of structure-property relationships, and finally introducing its application in gas sorption and separation technologies, as supercapacitor and battery electrodes, and in catalytic and biological processes. The review with an outlook on current research questions and future possibilities and extensions based on these material concepts is ended.

Two-Dimensional As/BlueP van der Waals Hetero-Structure as a Promising Photocatalyst for Water Splitting: A DFT Study

Constructing van der Waals (vdW) hetero-structure by stacking different two-dimensional (2D) materials has become an effective method for designing new-type and high-quality electronic and optoelectronic nano-devices. In this work, we designed a 2D As/BlueP vdW hetero-structure by stacking monolayer arsenene (As) and monolayer blue phosphorous (BlueP) vertically, which were recently implemented in experiments, and investigated its structural, electronic, and photocatalytic water splitting properties by using the standard first principles calculation method with HSE06 hybrid exchange-correlation functional. Numerical results show that the As/BlueP vdW hetero-structure is structural robust, even at room temperature. It presents semi-conducting behavior, and the conduction band minimum (CBM) and the valence band maximum (VBM) are dominated by BlueP and As, respectively. The typical type-II band alignment predicts the potential application of the hetero-structure in highly efficient optoelectronics and solar energy conversion. Moreover, the CBM and the VBM straddle the redox potentials of water in acid environment, predicting the possibility of the As/BlueP hetero-structure as a 2D photocatalyst for water splitting. When an in-plane strain is applied, the band edges and, further, the optoelectronic properties of the hetero-structure can be effectively tuned. Especially, when tensile strain is equal to 4.5%, the optical absorption spectrum is effectively broadened in a visible light region, which will largely improve its photocatalytic efficiency, although the pH value of the solution range reduction. This work provides theoretical evidence that the As/BlueP hetero-structure has potential application as a 2D photocatalyst in water splitting.

A two-dimensional h-BN/C2N heterostructure as a promising metal-free photocatalyst for overall water-splitting

The construction of a heterostructure (HS) is an effective strategy to modulate the desired properties of two-dimensional (2D) materials and to extend their applications. In this paper, based on the density functional theory, we predict a metal-free type-II HS formed by h-BN and C₂N single layers. The h-BN/C₂N HS possesses a smaller bandgap than individual h-BN and C₂N single layers, and it exhibits excellent visible light absorption. Importantly, its band edge positions satisfy the requirements for spontaneous water-splitting. With the assistance of the built-in electric field across the HS and the band offset, the photoinduced carriers can be readily spatially separated. Free energy calculations indicate the high catalytic activity for water oxidation and reduction reactions. The performance can be further enhanced by strain, which modulates the bandgap and the band edge positions of the HS. The band alignment may undergo a transition from type-I to type-II under strain, offering an effective switch for the reaction. The appropriate bandgap, suitable band edge positions, and effective carrier separation make the h-BN/C₂N HS a promising candidate for use as a photocatalyst in water-splitting.

A two-dimensional CdO/CdS heterostructure used for visible light photocatalysis

Based on hybrid density functional calculations, the geometrical and electronic structures of a two-dimensional (2D) CdO/CdS heterostructure (HT) formed by a CdO monolayer (ML) and a CdS ML are investigated. The formation of the CdO/CdS HT is exothermic, and the CdO/CdS HT shows excellent ability for visible light absorption. The CdO/CdS HT with a rotation angle of 0° possesses the characteristics of type-II band alignment and strong built-in electric field across the interface, which boost the photogenerated carrier separation. Besides, the band edge positions of the CdO/CdS HT of 0° are energetically favorable for overall water-splitting processes with the pH scope of 0-3.6. Therefore, the CdO/CdS HT is a promising photocatalyst to split water.

Two-dimensional few-layered PC3 as a promising photocatalyst for overall water splitting

Recently, 2D carbon phosphides (PCs) have attracted much attention due to their superior electronic and photovoltaic properties suitable for potential applications in field effect transistors and photodetectors. In this work, we systematically investigate the stability, electronic properties, optical absorption and photocatalytic water splitting performance of few-layered PC3 by using the first principles calculation method. Numerical results indicate that both monolayered and bilayered PC3 can serve as efficient photocatalysts for overall water splitting due to their high stability, moderate band gaps, suitable band edge positions, anisotropic high carrier mobilities and strong capacity of solar absorption. Compared with monolayered PC3, bilayered PC3 displays higher carrier mobilities (2500-23 000 cm2 V-1 s-1) and a wider optical absorption spectrum. Moreover, by applying an in-plane biaxial strain, the utilization of solar energy and the pH range suitable for overall water splitting can be improved effectively for both monolayered and bilayered PC3. Our work reliably expands the potential application of 2D few-layered PC3 in the field of nano-electronics and nano-optoelectronics.

Zn2GeO4−x/ZnS heterojunctions fabricated via in situ etching sulfurization for Pt-free photocatalytic hydrogen evolution: interface roughness and defect engineering

The Zn₂GeO_4−x/ZnS intimate heterojunctions were synthesized by in situ etching sulfurization, which realized photocatalytic H₂ evolution from water without the Pt co-catalyst by the synergism between interface topology and oxygen defect control.

Exceptional mechano-electronic properties in the HfN2 monolayer: a promising candidate in low-power flexible electronics, memory devices and photocatalysis

The response of the electronic properties of the HfN₂ monolayer to external perturbation such as strain and electric fields has been investigated using density functional theory calculations for its device-based applications and photocatalysis.