Ultralong Nanostructured Carbon Nitride Wires and Self-Standing C-Rich Filters from Supramolecular Microspheres

Graphitic Carbon Nitride Quantum Dots (g-C3N4 QDs): From Chemistry to Applications

Since their emergence in 2014, graphitic carbon nitride quantum dots (g-C3N4 QDs) have attracted much interest from the scientific community due to their distinctive physicochemical features, including structural, morphological, electrochemical, and optoelectronic properties. Owing to their desirable characteristics, such as non-zero band gap, ability to be chemically functionalized or doped, possessing tunable properties, outstanding dispersibility in different media, and biocompatibility, g-C3N4 QDs have shown promise for photocatalysis, energy devices, sensing, bioimaging, solar cells, optoelectronics, among other applications. As these fields are rapidly evolving, it is very strenuous to pinpoint the emerging challenges of the g-C3N4 QDs development and application during the last decade, mainly due to the lack of critical reviews of the innovations in the g-C3N4 QDs synthesis pathways and domains of application. Herein, an extensive survey is conducted on the g-C3N4 QDs synthesis, characterization, and applications. Scenarios for the future development of g-C3N4 QDs and their potential applications are highlighted and discussed in detail. The provided critical section suggests a myriad of opportunities for g-C3N4 QDs, especially for their synthesis and functionalization, where a combination of eco-friendly/single step synthesis and chemical modification may be used to prepare g-C3N4 QDs with, for example, enhanced photoluminescence and production yields.

Hydrophobic Carbon Nitride Nanolayer Enables High-Flux Oil/Water Separation with Photocatalytic Antifouling Ability

Efficient oil/water separation tackles various issues in occasions of oil leakage and oil discharge, such as environmental pollution, recollection of the oil, and saving the water. Herein, a compact superhydrophobic/superoleophilic graphitic carbon nitride nanolayer coated on carbon fiber networks (CNBA/CF) is designed and synthesized for efficient gravity-driven oil/water separation. The CNBA/CF shows excellent oil absorption and an impressive oil/water filtration separation performance. The flux reaches the state-of-art value of 4.29 × 105 L/m2/h for dichloromethane with separation efficiency up to 99%. Successive oil absorption tests, long-term filtration separation, and harsh conditions experiments confirm the remarkable separation and chemical structure stability of the CNBA/CF filter. Besides, the CNBA/CF demonstrates good photocatalytic antifouling ability thanks to the extended visible light absorption and improved charge separation. This work combines the material surface wettability modulation with a photocatalytic self-cleaning property in the fabrication of efficient oil/water separation materials while overcoming the filter fouling issue.

Developing extended visible light responsive polymeric carbon nitrides for photocatalytic and photoelectrocatalytic applications

Polymeric carbon nitride (CN) has emerged as an attractive material for photocatalysis and photoelectronic devices. However, the synthesis of porous CNs with controlled structural and optical properties remains a challenge, and processable CN precursors are still highly sought after for fabricating homogenous CN layers strongly bound to a given substrate. Here, we report a general method to synthesize highly dispersed porous CN materials that show excellent photocatalytic activity for the hydrogen evolution reaction and good performance as photoanodes in photoelectrochemical cells (PEC): first, supramolecular assemblies of melem and melamine in ethylene glycol and water are prepared using a hydrothermal process. These precursors are then calcined to yield a water-dispersible CN photocatalyst that exhibits beneficial charge separation under illumination, extended visible-light response attributed to carbon doping, and a large number of free amine groups that act as preferential sites for a Pt cocatalyst. The optimized CN exhibits state-of-the-art HER rates up to 23.1 mmol h^-1 g^-1, with an AQE of 19.2% at 395 nm. This unique synthetic route enables the formation of a homogeneous precursor paste for substrate casting; consequently, the CN photoanode exhibits a low onset potential, a high photocurrent density and good stability after calcination.

Visible Light-Driven Reforming of Lignocellulose into H2 by Intrinsic Monolayer Carbon Nitride

The photoreforming of lignocellulose is a novel method to produce clean and sustainable H₂ energy. However, the catalytic systems usually show low activity under ultraviolet light; thus, this reaction is very limited at present. Visible light-responsive metal-free two-dimensional graphite-phased carbon nitride (g-C₃N₄) is a good candidate for photocatalytic hydrogen production, but its activity is hindered by a bulky architecture. Although reported layered g-C₃N₄ modified with active functional groups prepared by the chemical exfoliation enhances the photocatalytic activity, it lost the intrinsic structure and thus is not conducive to understand the structure-activity relationship. Herein, we report an intrinsic monolayer g-C₃N₄ (∼0.32 nm thickness) prepared by nitrogen-protected ball milling in water, which shows good performance of photoreforming lignocellulose to H₂ driven by visible light. The exciton binding energy of g-C₃N₄ was estimated from the temperature-dependent photoluminescence spectra, which is a key factor for subsequent charge separation and energy transfer. It is found that monolayer g-C₃N₄ with smaller exciton binding energy increases the free exciton concentrations and promotes the separation efficiency of charge carriers, thereby effectively improving its performance of photocatalytic reforming of lignocellulose, even the virgin lignocellulose and waste lignocellulose. This result could lead to more active catalysts to photoreform the raw biomass, making it possible to provide clean energy directly from locally unused biomass.

Photoinduced post-modification of graphitic carbon nitride-embedded hydrogels: synthesis of 'hydrophobic hydrogels' and pore substructuring

Hydrogels are a special class of crosslinked hydrophilic polymers with a high water content through their porous structures. Post-modifications of hydrogels propose an attractive platform so that a variety of fresh functions, which are not arising from initial monomers, could be accessible on a parental network. Photoinduced post-modification of hydrogels by embedding semiconductor nanosheets would be of high interest and novelty. Here, a metal-free semiconductor graphitic carbon nitride (g-CN)-embedded hydrogel as an initial network was synthesized via redox-couple initiation under dark conditions. Post-photomodification of so-formed hydrogel, thanks to the photoactivity of the embedded g-CN nanosheets, was exemplified in two scenarios. The synthesis of 'hydrophobic hydrogel' is reported and its application in delayed cation delivery was investigated. Furthermore, pores of the initial hydrogel were modified by the formation of a secondary polymer network. Such a facile and straightforward synthetic protocol to manufacture functional soft materials will be of high interest in near future by the means of catalysis and agricultural delivery.

Synergistic Doping and Surface Decoration of Carbon Nitride Macrostructures by Single Crystal Design

Tailored design of hybrid carbon nitride (CN) materials is quite challenging because of the drawbacks of the solid-state reaction, and the utilization of single crystals containing C-N monomers as reactants for the high-temperature reaction has been proven to imprint a given chemical composition, morphology, or electronic structure. We report the one-pot synthesis of alkali-containing CN macrostructures with ionic crystals on its surface by utilizing a tailored melamine-hydrochloride-based molecular single crystal containing NaCl and KCl as reactants. Structural and optical investigations reveal that upon calcination, molecular doping with Na⁺ and K⁺ is achieved, and additionally, the ionic species remain on the surface of the materials, resulting in an enhanced H₂ evolution performance through water splitting owing to a high ionic strength of the reaction media. Additionally, the most stable configuration of the alkaline metals in the CN lattice is evaluated by DFT calculations. This work provides an approach for the rational design of CN and other related metal-free materials with controllable properties for energy-related applications and devices.

Emerging Chemical Functionalization of g-C3N4: Covalent/Noncovalent Modifications and Applications

Atomically 2D thin-layered structures, such as graphene nanosheets, graphitic carbon nitride nanosheets (g-C₃N₄), hexagonal boron nitride, and transition metal dichalcogenides are emerging as fascinating materials for a good array of domains owing to their rare physicochemical characteristics. In particular, graphitic carbon nitride has turned into a hot subject in the scientific community due to numerous qualities such as simple preparation, electrochemical properties, high adsorption capacity, good photochemical properties, thermal stability, and acid-alkali chemical resistance, etc. Basically, g-C₃N₄ is considered as a polymeric material consisting of N and C atoms forming a tri-s-triazine network connected by planar amino groups. In comparison with most C-based materials, g-C₃N₄ possesses electron-rich characteristics, basic moieties, and hydrogen-bonding groups owing to the presence of hydrogen and nitrogen atoms; therefore, it is taken into account as an interesting nominee to further complement carbon in applications of functional materials. Nevertheless, g-C₃N₄ has some intrinsic limitations and drawbacks mainly related to a relatively poor specific surface area, rapid charge recombination, a limited light absorption range, and a poor dispersibility in both aqueous and organic mediums. To overcome these shortcomings, numerous chemical modification approaches have been conducted with the aim of expanding the range of application of g-C₃N₄ and enhancing its properties. In the current review, the comprehensive survey is conducted on g-C₃N₄ chemical functionalization strategies including covalent and noncovalent approaches. Covalent approaches consist of establishing covalent linkage between the g-C₃N₄ structure and the chemical modifier such as oxidation/carboxylation, amidation, polymer grafting, etc., whereas the noncovalent approaches mainly consist of physical bonding and intermolecular interaction such as van der Waals interactions, electrostatic interactions, π-π interactions, and so on. Furthermore, the preparation, characterization, and diverse applications of functionalized g-C₃N₄ in various domains are described and recapped. We believe that this work will inspire scientists and readers to conduct research with the aim of exploring other functionalization strategies for this material in numerous applications.

Phenyl-Bridged Graphitic Carbon Nitride with a Porous and Hollow Sphere Structure to Enhance Dissociation of Photogenerated Charge Carriers and Visible-Light-Driven H2 Generation

Graphitic carbon nitride (CN) suffers from rapid recombination of photoexcited charges due to the existing highly symmetrical tri-s-triazine ring and long charge diffusion path, resulting in moderate photocatalytic activity. The bridged phenyl embedded in the CN structure was used to reduce the symmetry of the tri-s-triazine ring. In addition, the CN material was constructed with a porous and hollow sphere structure to shorten the diffusion path of charge carriers. Herein, simple thermal polymerization of a trimesic acid-doped melamine-cyanuric acid (MCA) supramolecular was employed to construct phenyl-bridged graphitic carbon nitride (Ph-CN-MCA) with a hollow sphere structure composed of porous nanosheets for visible-light catalytic H₂ evolution. The porous and hollow sphere-structured Ph-CN-MCA possessed increased degree of polymerization, more negative conduction band potential, enlarged Brunauer-Emmett-Teller (BET) surface area, and shortened charge diffusion path. In addition, bridged phenyl embedded in the Ph-CN-MCA structure not only accelerated the dissociation of photogenerated carriers but also narrowed the band gap and extended the visible-light absorption. Further, the separated highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of Ph-CN-MCA facilitated the spatial dissociation of photogenerated charges, which was also confirmed by theoretical calculations. As a consequence, compared with the reference CN-MA catalyst prepared from melamine, Ph-CN-MCA showed approximately 48.42 times the photocatalytic H₂ evolution under visible-light irradiation. The developed synthetic method herein highlights that phenyl-bridged graphitic carbon nitride with a porous and hollow sphere structure could provide an efficient platform to boost the dissociation of photoexcited charge carriers and photocatalytic H₂ evolution.

Modulation of Polymeric Carbon Nitrides through Supramolecular Preorganization for Efficient Photocatalytic Hydrogen Generation

Supramolecular pre-assembly by design is an effective strategy to adapt the physicochemical properties of polymeric carbon nitrides (PCNs) to improve their solar conversion performance. A new supramolecular preorganization protocol, which employs H₂ O as the self-assembly medium and sodium persulfate as a modifier, is proposed to modulate the textural and photoelectronic features of PCNs for efficient visible-light H₂ evolution. Sodium persulfate is revealed to precisely tailor the final carbon nitride polymers with unusual porous layered structures and promoted charge separation and migration kinetics. As a result, the modulated PCNs with optimized structures show a greatly enriched activity for photocatalytic H₂ generation compared to the analogous materials derived from melamine without the modifier.

Tailoring carbon nitride properties and photoactivity by interfacial engineering of hydrogen-bonded frameworks

The rational synthesis of carbon nitride materials, ranging from polymeric carbon nitride to nitrogen-doped carbon, by supramolecular preorganization of their monomers is a powerful tool for the design of their morphology and photophysical and catalytic activities. Here we show a new facile and scalable approach for the synthesis of ordered CN materials with excellent photoactivity, which consists of supramolecular interfacial preorganization of CN monomers at the interface of two non-miscible solvents. Molecular dynamic simulations supported by experimental results reveal that an appropriate choice of monomers and solvents leads to the formation of a supramolecular assembly solely at the interface of the solvents. As a proof of concept, we show that the properties of the CN materials after thermal condensation can be tuned by adding an additional monomer to one solvent only. The advantages of the new method are demonstrated here through the tunable morphologies and surface area, the formation of new electronic junctions and high activity as a photocatalyst for hydrogen evolution and pollutant degradation of the CN materials.

Monomer sequence design at two solvent interface enables the synthesis of highly photoactive carbon nitride

Structural modifications in carbon nitrides and related carbon-based materials have been achieved in recent years by organizing their monomers into versatile supramolecular structures that serve as reactants for the high temperature solid-state reaction. To date, the organization is usually carried out in one solvent where the building blocks must be dispersed. Here, we show the utilization of a molecule with both hydrogen bond donor and acceptor sites for constructing hydrogen bonded frameworks in interfacial systems. The chemical and electronic properties of the carbon nitride materials after calcination are strongly altered showing enhanced photocatalytic performance in different model reactions. This work shows a new large-scale pathway for the synthesis of highly photoactive carbon nitride with tailored properties and morphology by employing novel supramolecular assemblies prepared in the interface between two solvents, and furthermore opens new opportunities in the rational design of different carbon–nitrogen based materials utilizing supramolecular structures.

The design of a supramolecular assembly in a two solvent interface is used to tailor the morphology, chemical and electronic properties of carbon nitride. This approach opens many opportunities for the design of C–N based materials.

Polymer grafted graphitic carbon nitrides as precursors for reinforced lubricant hydrogels

Carbon nitride-based hydrogels are formed in a two-step procedure and feature significant toughness, compressibility and lubricant properties.