Interfacial coupling induced direct Z-scheme water splitting in metal-free photocatalyst: C3N/g-C3N4 heterojunctions

A review on recent advances in g-C3N4-MXene nanocomposites for photocatalytic applications

The solutions for environmental remediation and renewable energy generation have intensified the exploration of efficient photocatalytic materials. Recently, the composites of g-C3N4and MXene have gained considerable interest for their potential applications in photocatalysis. In the g-C3N4-MXene composite, the g-C3N4possesses unique physical, chemical, and optical properties to increase visible light absorption. At the same time, MXene improves conductivity, adsorption of reactant molecules or the active sites, and charge transfer properties. Combining the unique physico-chemical properties of MXene and g-C3N4, the resulting composite exhibits superior photo-responsive behavior and is critical in photocatalytic reactions. Furthermore, the g-C3N4-MXene composite exhibits stability and recyclability, making it a promising candidate for sustainable and scalable photocatalytic material in environmental remediation. This review offers an in-depth analysis of the development and design of g-C3N4-MXene composites through diverse synthesis procedures and a comprehensive analysis of their application in carbon dioxide (CO2) reduction, photocatalytic degradation, water splitting processes, mainly hydrogen (H2) generation, H2O2production, N2fixation, and NOxremoval. The charge transfer mechanism of g-C3N4-MXene composite for photocatalytic application has also been discussed. This review provides insights into the photocatalytic capabilities of g-C3N4-MXene composites, showing their potential to address current environmental challenges and establish a robust foundation for sustainable energy conversion technologies.

Towards the selective growth of two-dimensional ordered CxNy compounds via epitaxial substrate mediation

Two-dimensional (2D) ordered carbon-nitrogen binary compounds (CxNy) show great potential in many fields owing to their diverse structures and outstanding properties. However, the scalable and selective synthesis of 2D CxNy compounds remain a challenge due to the variable C/N stoichiometry induced coexistence of graphitic, pyridinic, and pyrrolic N species and the competitive growth of graphene. Here, this work systematically explored the mechanism of selective growth of a series of 2D ordered CxNy compounds, namely, the g-C3N4, C2N, C3N, and C5N, on various epitaxial substrates via first-principles calculations. By establishing the thermodynamic phase diagram, it is revealed that the individualized surface interaction and symmetry match between 2D CxNy compounds and substrates together enable the selective epitaxial growth of single crystal 2D CxNy compounds within distinct chemical potential windows of feedstock. The kinetics behaviors of the diffusion and attachment of the decomposed feedstock C/N atoms to the growing CxNy clusters further confirmed the feasibility of the substrate mediated selective growth of 2D CxNy compounds. Moreover, the optimal experimental conditions, including the temperature and partial pressure of feedstock, are suggested for the selective growth of targeted 2D CxNy compound on individual epitaxial substrates by carefully considering the chemical potential of carbon/nitrogen as the functional of experimental parameters based on the standard thermochemical tables. This work provides an insightful understanding on the mechanism of selective epitaxial growth of 2D ordered CxNy compounds for guiding the future experimental design.

Progress of charge carrier dynamics and regulation strategies in 2D CxNy-based heterojunctions

Two-dimensional carbon nitrides (CxNy) have gained significant attention in various fields including hydrogen energy development, environmental remediation, optoelectronic devices, and energy storage owing to their extensive surface area, abundant raw materials, high chemical stability, and distinctive physical and chemical characteristics. One effective approach to address the challenges of limited visible light utilization and elevated carrier recombination rates is to establish heterojunctions for CxNy-based single materials (e.g. C2N3, g-C3N4, C3N4, C4N3, C2N, and C3N). The carrier generation, migration, and recombination of heterojunctions with different band alignments have been analyzed starting from the application of CxNy with metal oxides, transition metal sulfides (selenides), conductive carbon, and Cx'Ny' heterojunctions. Additionally, we have explored diverse strategies to enhance heterojunction performance from the perspective of carrier dynamics. In conclusion, we present some overarching observations and insights into the challenges and opportunities associated with the development of advanced CxNy-based heterojunctions.

Theoretical exploration of the structural, electronic and optical properties of g-C3N4/C3N heterostructures

Integration of graphene-like carbon nitride materials is essential for nanoelectronic applications. Using density-functional theory (DFT), we systematically investigate the structural, electronic and optical properties of a s-triazine-based g-C₃N₄/C₃N heterostructure under different modified conditions. The g-C₃N₄/C₃N van der Waals heterostructure (vdWH) formed has an indirect bandgap with type-II band alignment and the band structures can be tuned from type-II band alignment to type-I band alignment by applying biaxial strains and external electric fields (E_field). Compared to single transition metal (TM) atoms at g-C₃N₄/C₃N surfaces, the TM atoms anchored in the interlayer region exhibit more stability, and the corresponding bandgaps are changed from 0.19 eV to 0.61 eV. In addition, the g-C₃N₄/C₃N heterostructure has a strong absorption coefficient in the ultraviolet-visible light region along the x direction. It is found that compressive strain has a large influence on the absorption coefficient of the g-C₃N₄/C₃N system. With the increased compressive strain, the absorption spectra in the visible light region disappeared. Tensile strain has a slight effect on the absorption range, but causes a red shift of the absorption spectrum. In comparison, the light absorption coefficient of the g-C₃N₄/C₃N system remains almost unchanged under the E_field conditions. In summary, the formation of a s-triazine-based g-C₃N₄/C₃N heterostructure has shown potential for applications in nanoelectronic and optoelectronic devices.

Hybrid density functional study on band structure engineering of ZnS(110) surface by anion–cation codoping for overall water splitting

The (Ru + C)-codoped ZnS(110) surface is predicted to be a potential candidate for solar-driven water splitting.

Ultrathin structure of oxygen doped carbon nitride for efficient CO2photocatalytic reduction

Photocatalytic conversion of carbon dioxide into fuels and valuable chemicals is a promising method for carbon neutralization and solving environmental problems. Through a simple thermal-oxidative exfoliation method, the O element was doped while exfoliated bulk g-C₃N₄into ultrathin structure g-C₃N₄. Benefitting from the ultrathin structure of g-C₃N₄, the larger surface area and shorter electrons migration distance effectively improve the CO₂reduction efficiency. In addition, density functional thory computation proves that O element doping introduces new impurity energy levels, which making electrons easier to be excited. The prepared photocatalyst reduction of CO₂to CO (116μmol g^-1h^-1) and CH₄(47μmol g^-1h^-1).

Preparation, Characterization of Graphitic Carbon Nitride Photo-Catalytic Nanocomposites and Their Application in Wastewater Remediation: A Review

Energy crisis and environmental pollution are the major problems of human survival and development. Photocatalytic technology can effectively use solar energy and is prospective to solve the above-mentioned problems. Carbon nitride is a two-dimensional polymer material with a graphite-like structure. It has good physical and chemical stabilities, unique chemical and electronic energy band structures, and is widely used in the field of photocatalysis. Graphitic carbon nitride has a conjugated large π bond structure, which is easier to be modified with other compounds. thereby the surface area and visible light absorption range of carbon nitride-based photocatalytic composites can be insignificantly increased, and interface electron transmission and corresponding photogenerated carriers separation of streams are simultaneously promoted. Therefore, the present study systematically introduced the basic catalytic principles, preparation and modification methods, characterization and calculation simulation of carbon nitride-based photocatalytic composite materials, and their application in wastewater treatment. We also summarized their application in wastewater treatment with the aid of artificial intelligence tools. This review summarized the frontier technology and future development prospects of graphite phase carbon nitride photocatalytic composites, which provide a theoretical reference for wastewater purification.

Latest progress in constructing solid-state Z scheme photocatalysts for water splitting

Artificial Z scheme photocatalysis has been considered as a promising strategy for producing the clean energy source of hydrogen gas. The core of the Z scheme is a two-step excitation process in a tandem structured photosystem aiming to satisfy both the criteria of wide range solar spectrum absorption and strong thermodynamic driving force for photolysis reactions. Therefore, efficient connection and matching between the two photosystems is the key to improve the photocatalytic activity. Recently, new progress has been achieved concerning the principles and applications of state-of-the-art solid-state Z schematic systems to enhance the photocatalytic efficiency and repress competitive reactions. This review summarizes the latest approaches to all-solid-state Z schemes for photocatalytic water splitting, including new tandem structures, new morphologies, and new connection modes to improve light absorption as well as carrier transportation. The challenges for developing novel high performance Z scheme photocatalysts are also discussed.

Designing a Novel Monolayer β-CSe for High Performance Photovoltaic Device: An Isoelectronic Counterpart of Blue Phosphorene

Using the first-principles method, an unmanufactured structure of blue-phosphorus-like monolayer CSe (β-CSe) was predicted to be stable. Slightly anisotropic mechanical characteristics in β-CSe sheet were discovered: it can endure an ultimate stress of 5.6 N/m at 0.1 along an armchair direction, and 5.9 N/m at 0.14 along a zigzag direction. A strain-sensitive transport direction was found in β-CSe, since β-CSe, as an isoelectronic counterpart of blue phosphorene (β-P), also possesses a wide indirect bandgap that is sensitive to the in-plane strain, and its carrier effective mass is strain-dependent. Its indirect bandgap character is robust, except that armchair-dominant strain can drive the indirect-direct transition. We designed a heterojunction by the β-CSe sheet covering α-CSe sheet. The band alignment of the α-CSe/β-CSe interface is a type-II van der Waals p-n heterojunction. An appreciable built-in electric field across the interface, which is caused by the charges transfering from β-CSe slab to α-CSe, renders energy bands bending, and it makes photo-generated carriers spatially well-separated. Accordingly, as a metal-free photocatalyst, α-CSe/β-CSe heterojunction was endued an enhanced solar-driven redox ability for photocatalytic water splitting via lessening the electron-hole-pair recombination. This study provides a fundamental insight regarding the designing of the novel structural phase for high-performance light-emitting devices, and it bodes well for application in photocatalysis.

Simply blending Ni nanoparticles with typical photocatalysts for efficient photocatalytic H2 production

A photocatalytic system construction method of simply blending as-prepared Ni nanoparticles (Ni NPs) with g-C₃N₄ was carried out to greatly improve the photocatalytic activity of g-C₃N₄.