Effect of vacancy defect and strain on the structural, electronic and magnetic properties of carbon nitride 2D monolayers by DFTB method

We investigate the electronic and magnetic properties of CnNm(C₆N₆, C₂N, C₃N and C₃N₄) using density functional tight-binding (DFTB) method. We find that these compounds are dynamically stable and their electronic band gaps are in the range of 0.59-3.28 eV. We show that the electronic structure is modulated by strain and the semiconducting behavior is well preserved except for C₃N at +5% biaxial strain, where a transition from semiconductor to metal was observed. Under +3% biaxial strain, C₃N₄undergoes a transition from an indirect (K-Γ) to a direct (Γ-Γ) band gap. Moreover, band gap of C₂N transforms from direct (Γ-Γ) to indirect (M-Γ) under +4% biaxial strain. However, no change in the nature of the band gap of C₆N₆. Further, when the studied materials under uniaxial tensile strain, their bandgaps reduce. Our theoretical study showed that an indirect-to-direct nature transition may occur for C₆N₆and for C₃N₄, which broadens their applications. On the other hand, magnetism is observed in all N-vacancy defected CnNm, which encourages its application in spintronic. Moreover, calculations of formation energies indicate that N-vacancy is energetically more favorable than C-vacancy in both C₂N and C₃N₄. Opposite behavior found for C₆N₆and C₃N.

Two-dimensional AlN/g-CNs van der Waals type-II heterojunction for water splitting

A type-II van der Waals heterojunction photocatalyst is not only an ideal material for hydrogen production by water splitting, but also an important way to improve efficiency and produce low-cost clean energy. In this work, we unexpectedly found that monolayers of AlN and C₂N, g-C₃N₄, and C₆N₈ all formed type-II heterojunctions according to density functional theory, and we report a comparison of their photocatalytic performance. Among them, the AlN/C₂N heterojunction has an appropriate band gap value of 1.61 eV for visible light water splitting. It has higher carrier mobility than the AlN/g-C₃N₄ heterojunction (electron 253.1 cm² V^-1 s^-1 > 31.6 cm² V^-1 s^-1 and hole 11043.4 cm² V^-1 s^-1 > 524.7 cm² V^-1 s^-1), and an absorption peak similar those of monolayer C₂N in visible light (8 × 10⁴ cm^-1) and monolayer AlN in ultraviolet light (11 × 10⁴ cm^-1). The Bader charge shows that the charge transfer number of the AlN/g-C₃N₄ heterojunction is higher than that of the AlN/C₂N heterojunction, and its Gibbs free energy (-0.22 eV) is smaller than that of single-layer g-C₃N₄ (-0.30 eV). The AlN/C₆N₈ heterojunction also has a perfect band gap of 2.16 eV and an absorption peak of over 10 × 10⁴ cm^-1 in the UV region. Since a type-II heterojunction can effectively promote the separation of photogenerated electron-hole pairs and prevent their rapid recombination, the above heterojunctions are promising candidates for new photocatalysts.

Interface Engineering in 2D/2D Heterogeneous Photocatalysts

Assembling different 2D nanomaterials into heterostructures with strong interfacial interactions presents a promising approach for novel artificial photocatalytic materials. Chemically implementing the 2D nanomaterials' construction/stacking modes to regulate different interfaces can extend their functionalities and achieve good performance. Herein, based on different fundamental principles and photochemical processes, multiple construction modes (e.g., face-to-face, edge-to-face, interface-to-face, edge-to-edge) are overviewed systematically with emphasis on the relationships between their interfacial characteristics (e.g., point, linear, planar), synthetic strategies (e.g., in situ growth, ex situ assembly), and enhanced applications to achieve precise regulation. Meanwhile, recent efforts for enhancing photocatalytic performances of 2D/2D heterostructures are summarized from the critical factors of enhancing visible light absorption, accelerating charge transfer/separation, and introducing novel active sites. Notably, the crucial roles of surface defects, cocatalysts, and surface modification for photocatalytic performance optimization of 2D/2D heterostructures are also discussed based on the synergistic effect of optimization engineering and heterogeneous interfaces. Finally, perspectives and challenges are proposed to emphasize future opportunities for expanding 2D/2D heterostructures for photocatalysis.

Enhanced H2evolution performance by carbonized SiC/g-C3N4heterojunction under visible-light illumination

In this study, carbonized silicon carbide/graphitic carbon nitride ((SiC/C)/g-C₃N₄) composites were fabricated via a facile calcination method. The optimal SiC/C/g-C₃N₄composite shows an excellent visible-light photocatalytic activity for water splitting, with the highest hydrogen evolution amount being 200.2μmol, which is four times higher than that of pure g-C₃N₄when triethanolamine and platinum (1.0 wt%) are used as the sacrificial agent and cocatalyst, respectively. With an intimate interface between SiC/C and g-C₃N₄, the energy band structure of g-C₃N₄was well engineered for photocatalytic H₂production. This study provides a novel method for fabricating g-C₃N₄-based heterojunctions for application in environmental conservation.

Boosting the photocatalytic hydrogen evolution performance of monolayer C2N coupled with MoSi2N4: density-functional theory calculations

Very recently, an important two-dimensional material, MoSi₂N₄, was successfully synthesized. However, pure MoSi₂N₄ has some inherent shortcomings when used in photocatalytic water splitting to produce hydrogen, especially a low separation rate of photogenerated electron-hole pairs and a poor visible light response. Interestingly, we find that the MoSi₂N₄ can be used as a good modification material, and it can be coupled with C₂N to form an efficient heterojunction photocatalyst. Here, using density functional theory, a type-II heterojunction, C₂N/MoSi₂N₄, is designed and systematically studied. Based on AIMD simulations and phonon dispersion verification, C₂N/MoSi₂N₄ shows sufficient thermodynamic stability. As well as its perfect interface electronic properties, its large interlayer charge transfer and good visible light response lay the foundation for its excellent photocatalytic performance. In addition, the oxidation and reduction potentials of the C₂N/MoSi₂N₄ heterojunction not only can meet the requirements of water splitting well but can also maintain a delicate balance between oxidation and reduction reactions. More importantly, the |ΔG_H*| value of the C₂N/MoSi₂N₄ heterojunction is very close to zero, indicating great application potential in the field of photocatalytic water splitting. In brief, our research paves the way for the design of future MoSi₂N₄-based efficient heterojunction photocatalysts.

Layer-dependent photocatalysts of GaN/SiC-based multilayer van der Waals heterojunctions for hydrogen evolution

The interlayer interaction has a great influence on the formation of type-II heterojunctions, which can efficiently decompose water.

A novel design of SiH/CeO2(111) van der Waals type-II heterojunction for water splitting

Searching for economical low-dimensional materials to construct the highly efficient type-II heterojunction photocatalysts for splitting water into hydrogen is very strategic. In this study, using the first-principles calculations, we construct a novel SiH/CeO2(111) type-II heterojunction with a very small lattice mismatch of less than 1%. Based on AIMD simulation and phonon dispersion calculations, the SiH/CeO2(111) heterojunction reveals sufficient stability, and is easy to synthesize. Due to the vdW interaction between SiH and CeO2(111) components, electron and hole accumulation regions form at the heterojunction interface, which is very conducive to the separation of photoexcited electron-hole pairs. Besides, the SiH/CeO2(111) heterojunction has good visible light response, and even a strong absorption peak of up to 8.7 × 105 cm-1 in the high-energy visible region. More importantly, the SiH/CeO2(111) heterojunction exhibits good OER and HER performance because its oxidation and reduction potentials well meet the requirements of water splitting. Consequently, SiH/CeO2(111) is a potential photocatalyst for splitting water to hydrogen.

Two dimensional CdS/ZnO type-II heterostructure used for photocatalytic water-splitting

The electronic structures of two dimensional (2D) CdS/ZnO heterostructure (CdZnHT) consisting of CdS singlelayer (SL) and ZnO SL are explored based on hybrid density functional calculation. The negative interface formation energies suggest the formation of CdZnHT is exothermic. The bandgap of CdZnHT is favorable for absorbing visible light, and the decent band edge position makes it thermodynamically feasible for spontaneous generation of oxygen and hydrogen. The formed electric field across the interface induced by charge transfer will reduce photogenerated carrier recombination and promote carrier migration. Particularly, CdZnHT is a type-II heterostructure. Oxygen generation takes place at ZnO layer and hydrogen production occurs at CdS layer, which will also promote the effective separation and migration of phogogenerated carriers and enhance photocatalytic performance. These findings suggest that 2D CdZnHTs are possible candidates as water-splitting photocatalysts.

Tuning the electronic structures of monolayer triphosphides MP3 (M = Sn and Ge) for CO2 electroreduction through interface engineering: a theoretical prediction

Interface engineering by integrating various two-dimensional materials to form heterostructures can not only preserve the desired properties of individual components, but also induce new functions. Herein, by means of density functional theory (DFT) computations, we have investigated the effects of interface engineering of the graphene substrate on the electronic structures of monolayer triphosphides MP3 (M = Sn and Ge) and their catalytic performance for the electroreduction of carbon dioxide (CO2ER). Our results revealed that the MP3/graphene interfaces exhibit good structural stability, enhanced electrical conductivity, superior CO2ER performance, and obvious suppressing effects on hydrogen evolution due to the charge transfer at the interface. Thus, our results suggested that SnP3/graphene and GeP3/graphene heterostructures can be utilized as promising CO2ER catalysts with high-efficiency and high-selectivity, offering cost-effective opportunities to convert CO2 for renewable energy supply via interface engineering.

Nitrogen deficient carbon nitride for efficient visible light driven tetracycline degradation: a combination of experimental and DFT studies

The narrow visible-light absorption range and a high recombination rate of photo-excited electrons and holes are the main reasons for the confined photocatalytic performance of graphitic carbon nitride (g-C₃N₄).

2D layered SiC/C2N van der Waals type-II heterostructure: a visible-light-driven photocatalyst for water splitting

2D layered SiC/C₂N type-II heterostructure is an effective photocatalyst for hydrogen production from water splitting by visible light.