High-pressure bulk synthesis of crystalline C(6)N(9)H(3).HCl: a novel c(3)n(4) graphitic derivative

Chloride Ion-Induced Spatial Separation and Long Recombination Time of Photogenerated Electrons and Holes in Crystalline Carbon Nitride

Poly(triazine imide)·Li+Cl- (PTI/Li+Cl-) as one of the most reported crystalline carbon nitrides has shown exciting potential for photocatalysis. However, understanding the role of Li+/Cl- in the photoexcited charge transfer in the time and space of PTI is a challenging problem. Here, we have investigated the nonradiative charge recombination of series ion intercalated PTI systems (PTI/Li+X-, where X = F, Cl, Br, and I) by performing the ab initio nonadiabatic molecular dynamics simulations. The results indicate that the intercalated anions in PTI/Li+Cl- and PTI/Li+Br- have the potential to trap holes, separate the electrons and holes, and prolong the nonradiative electron-hole recombination. In particular, ∼70% of holes in PTI/Li+Cl- can transport among interlayers toward the {0001} planes, while most of the electrons are transferred to the {101̅0} planes, exhibiting different transport pathways and directions. Furthermore, PTI/Li+Cl- has an electron-hole recombination time as long as 136 ns, which explains its excellent optoelectronic properties. This work provides a theoretical baseline for the reported facet engineering improvement of crystalline carbon nitride for overall water splitting.

Amphoteric dissolution of two-dimensional polytriazine imide carbon nitrides in water

Crystalline two-dimensional carbon nitrides with polytriazine imide (PTI) structure are shown to act amphoterically, buffering both HCl and NaOH aqueous solutions, resulting in charged PTI layers that dissolve spontaneously in their aqueous media, particularly for the alkaline solutions. This provides a low energy, green route to their scalable solution processing. Protonation in acid is shown to occur at pyridinic nitrogens, stabilized by adjacent triazines, whereas deprotonation in base occurs primarily at basal plane NH bridges, although NH2 edge deprotonation is competitive. We conclude that mildly acidic or basic pHs are necessary to provide sufficient net charge on the nanosheets to promote dissolution, while avoiding high ion concentrations which screen the repulsion of like-charged PTI sheets in solution. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Insights into engineered graphitic carbon nitride quantum dots for hazardous contaminants degradation in wastewater

Increased environmental pollution is a critical issue that must be addressed. Photocatalytic, adsorption, and membrane filtration methods are suitable in environmental governance because of their high selectivity, low cost, environment-friendly nature, and excellent treatment efficiency. Graphitic carbon nitride (g-C₃N₄) quantum dots (QDs) have been considered as photocatalysts, adsorbents, and membrane materials for wastewater treatments, owing to their stability, adsorption capacity, photochemical properties, and low toxicity and cost. This review summarizes g-C₃N₄ QD synthesis techniques, operating parameters affecting the removal performance in the treatment process, modification effects with other semiconductors, and benefits and drawbacks of g-C₃N₄ QD-based materials. Furthermore, this review discusses the practical applications of g-C₃N₄ QDs as adsorbents, photocatalysts, and membrane materials for organic and inorganic contaminant treatments and their value-added product formation potential. Modified g-C₃N₄ QD-based material adsorbents, photocatalysts, and membranes present potentially applicable effects, such as removal of most waterborne contaminants. Excellent results were obtained for the reduction of methyl orange, bisphenol A, tetracycline, ciprofloxacin, phenol, rhodamine B, E. coli, and Hg. Overall, this paper provides comprehensive background on g-C₃N₄ QD-based materials and their diverse applications in wastewater treatment, and it presents a foundation for the enhancement of similar unique materials in the future.

Removing unreacted amino groups in graphitic carbon nitride through residual heating to improve the photocatalytic performance

In most of the research about graphitic carbon nitride (g-C₃N₄), g-C₃N₄ is prepared through the calcination of nitrogen-rich precursors. However, such a preparation method is time-consuming, and the photocatalytic performance of pristine g-C₃N₄ is lackluster due to the unreacted amino groups on the surface of g-C₃N₄. Therefore, a modified preparation method, calcination through residual heating, was developed to achieve rapid preparation and thermal exfoliation of g-C₃N₄ simultaneously. Compared with pristine g-C₃N₄, the samples prepared by residual heating had fewer residual amino groups, a thinner 2D structure, and higher crystallinity, which led to a better photocatalytic performance. The photocatalytic degradation rate of the optimal sample for rhodamine B could reach 7.8 times higher than that of pristine g-C₃N₄.

Unblocking Ion-occluded Pore Channels in Poly(triazine imide) Framework for Proton Conduction

Poly(triazine imide) or PTI is an ordered graphitic carbon nitride hosting Å-scale pores attractive for selective molecular transport. AA'-stacked PTI layers are synthesized by ionothermal route during which ions occupy the framework and occlude the pores. Synthesis of ion-free PTI hosting AB-stacked layers has been reported, however, pores in this configuration are blocked by the neighboring layer. The unavailability of open pore limits application of PTI in molecular transport. Herein, we demonstrate acid treatment for ion depletion which maintains AA' stacking and results in open pore structure. We provide first direct evidence of ion-depleted open pores by imaging with the atomic resolution using integrated differential phase-contrast scanning transmission electron microscopy. Depending on the extent of ion-exchange, AA' stacking with open channels and AB stacking with closed channels are obtained and imaged for the first time. The accessibility of open channels is demonstrated by enhanced proton transport through ion depleted PTI.

Recent advances in visible-light graphitic carbon nitride (g-C3N4) photocatalysts for chemical transformations

Graphitic carbon nitride (g-C3N4) has emerged as a new research hotspot, attracting broad interdisciplinary attention in the form of metal-free and visible-light-responsive photocatalysts in the field of solar energy conversion and environmental remediation. These photocatalysts have evolved as attractive candidates due to their non-toxicity, chemical stability, efficient light absorption capacity in the visible and near-infrared regions, and adaptability as a platform for the fabrication of hybrid materials. This review mainly describes the latest advances in g-C3N4 photocatalysts for chemical transformations. In addition, the typical applications of g-C3N4-based photocatalysts involving organic transformation reactions are discussed (synthesis of heterocycles, hydrosulfonylation, hydration, oxygenation, arylation, coupling reactions, etc.).

Photocatalytic degradation of pharmaceuticals by pore-structured graphitic carbon nitride with carbon vacancy in water: Identification of intermediate degradants and effects of active species

Pharmaceuticals are increasingly used in daily life and have been massively discharged to the aquatic environment. The removal of pharmaceuticals from water by various nanomaterials including graphitic carbon nitride (g-C₃N₄) has received extensive attention. Herein, we synthesized a carbon-defective carbon nitride with pore structure through a simple thermal polymerization method for photodegradation of lidocaine, mepivacaine and ropivacaine (typical amide local anesthetics). The results showed that the degradation process conformed to the pseudo-first-order reaction kinetics, and the degradation rate constant of organic pollutants using CCN-600 (i.e., g-C₃N₄ synthesized at 600 °C) reached 5.05 × 10^-2 min^-1, about 2.5 times higher than that of the prototype g-C₃N₄ (2.09 × 10^-2 min^-1). The capture experiment of active species and the electron paramagnetic resonance (EPR) test demonstrated that superoxide radical (O₂^-) played a major role in the degradation process. Based on the possible photodegraded intermediate products identified, the degradation pathways were deduced. This study provides not only a new strategy for fabrication of pore-structured g-C₃N₄ with carbon vacancy, but also a reference method for the treatment of pharmaceuticals in water bodies.

New functionalisation reactions of graphitic carbon nitrides: Computational and experimental studies

The functionalisation of two-dimensional materials is key to modify their properties and facilitate assembly into functional devices. Here, new reactions have been proposed to modify crystalline two-dimensional carbon nitrides of polytriazine imide structure. Both amine alkylation and aryl-nitrene-based reactions have been explored computationally and with exploratory synthetic trials. The approach illustrates that alkylation is unfavourable, particularly at basal-plane sites. In contrast, while initial trial reactions were inconclusive, the radical-addition of nitrenes is shown to be energetically favourable, with a preference for functionalising sheet edges to minimise steric effects.

Effect of Tricarboxylic Acids on the Formation of Hydrogels with Melem or Melamine: Morphological, Structural and Rheological Investigations

Herein, aggregation behaviors of melem or melamine in the presence of three symmetric carboxylic acids (1,3,5-tris(4-carboxyphenyl)benzene (TPCA), 1,3,5-benzene-tri-carboxylic acid (BTA) and 1,3,5-cyclohexane-tri-carboxylic acid (CHTA)) have been performed to check the influence of acid on the formation of aggregated structures which have been investigated by optical microscopy, FESEM, FTIR, XRD and viscoelastic properties have been explored with rheological studies. Interestingly, melem, that has limited solubility in aqueous medium, forms aggregation that leads to the formation of hydrogels with TPCA. More significantly, hydrogel is formed here by matching the size selectivity. Melem forms hydrogel with only large tricarboxylic acid, whereas melamine produces hydrogel with any kind of its counterpart from small to large tricarboxylic acid derivatives. Present investigations and results provide the strategy of design of organic self-assembled materials having two component systems.

Photoinduced Self-Assembly of Carbon Nitride Quantum Dots

The study of nanocrystal self-assembly into superlattices or superstructures is of great significance in nanoscience. Carbon nitride quantum dots (CNQDs), being a promising new group of nanomaterials, however, have hardly been explored in their self-organizing behavior. Here we report of a unique irradiation-triggered self-assembly and recrystallization phenomenon of crystalline CNQDs (c-CNQDs) terminated by abundant oxygen-containing groups. Unlike the conventional self-assembly of nanocrystals into ordered superstructures, the photoinduced self-assembly of c-CNQDs resembles a "click reaction" process of macromolecules, in which the activated -OH and -NH₂ functional groups along the perimeters initiate cross-linking of adjacent QDs through a photocatalytic effect. Our findings unveil fundamental physiochemical features of CNQDs and open up new possibilities of manipulating carbon nitride nanomaterials via controlled assembly. Prospects for potential applications are discussed as well.

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.

An Overview of the Recent Progress in Polymeric Carbon Nitride Based Photocatalysis

Recently, polymeric carbon nitride (g-C₃ N₄ ) as a proficient photo-catalyst has been effectively employed in photocatalysis for energy conversion, storage, and pollutants degradation due to its low cost, robustness, and environmentally friendly nature. The critical review summarized the recent development, fundamentals, nanostructures design, advantages, and challenges of g-C₃ N₄ (CN), as potential future photoactive material. The review also discusses the latest information on the improvement of CN-based heterojunctions including Type-II, Z-scheme, metal/CN Schottky junctions, noble metal@CN, graphene@CN, carbon nanotubes (CNTs)@CN, metal-organic frameworks (MOFs)/CN, layered double hydroxides (LDH)/CN heterojunctions and CN-based heterostructures for H₂ production from H₂ O, CO₂ conversion and pollutants degradation in detail. The optical absorption, electronic behavior, charge separation and transfer, and bandgap alignment of CN-based heterojunctions are discussed elaborately. The correlations between CN-based heterostructures and photocatalytic activities are described excessively. Besides, the prospects of CN-based heterostructures for energy production, storage, and pollutants degradation are discussed.

Research development on graphitic carbon nitride and enhanced catalytic activity on ammonium perchlorate

Recently, graphitic carbon nitride (g-C3N4) has attracted considerable attention due to its attractive features including excellent electrochemical performance, suitable band gap, nontoxicity, and high mechanical and thermal stability. Such unique advantages endow it with promising applications in batteries, photocatalysts, photodegradation, and so on. In particular, it has been applied to catalyze the thermal decomposition of ammonium perchlorate (AP) and has shown excellent performance. In this review, the structure, preparation methods and exfoliation strategies of g-C3N4 are comprehensively introduced. Furthermore, the enhanced catalytic mechanism for the thermal decomposition of AP is discussed in detail, and a future research direction is also proposed.

Polymerization Mechanism of Nitrogen-Containing Heteroaromatic Compound Under High-Pressure and High-Temperature Conditions

Hydrogenated carbon nitride is synthesized by polymerization of 1,5-naphthyridine, a nitrogen-containing heteroaromatic compound, under high-pressure and high-temperature conditions. The polymerization progressed significantly at temperatures above 573 K at 0.5 GPa and above 623 K at 1.5 GPa. The reaction temperature was relatively lower than that observed for pure naphthalene, suggesting that the reaction temperature is considerably lowered when nitrogen atoms exist in the aromatic ring structure. The polymerization reaction largely progresses without significant change in the N/C ratio. Three types of dimerization are identified; naphthylation, exact dimerization, and dimerization with hydrogenation as determined from the gas chromatograph-mass spectrometry analysis of soluble products. Infrared spectra suggest that hydrogenation products were likely to be formed with sp³ carbon and NH bonding. Solid-state ¹³C nuclear magnetic resonance reveals that the sp³/sp² ratio is 0.14 in both the insoluble solids synthesized at 0.5 and 1.5 GPa. Not only the dimers but also soluble heavier oligomers and insoluble polymers formed through more extensive polymerization. The major reaction mechanism of 1,5-Nap was common to both the 0.5 and 1.5 GPa experiments, although the required reaction temperature increased with increasing pressure and aromatic rings preferentially remained at the higher pressure.

Metal-doped carbon nitrides: synthesis, structure and applications

This perspective provides a comprehensive overview of the latest progress of M–CN, which would promote further development, such as for single-atom catalysis and nanozymatic reactions.

Creation of carbon defects and in-plane holes with the assistance of NH4Br to enhance the photocatalytic activity of g-C3N4

A new holey defect-rich g-C₃N₄ photocatalyst from an NH₄Br-assisted programmed heating method exhibits an enhanced photocatalytic performance.

Aquaporin-like water transport in nanoporous crystalline layered carbon nitride

Designing next-generation fuel cell and filtration devices requires the development of nanoporous materials that allow rapid and reversible uptake and directed transport of water molecules. Here, we combine neutron spectroscopy and first-principles calculations to demonstrate rapid transport of molecular H2O through nanometer-sized voids ordered within the layers of crystalline carbon nitride with a polytriazine imide structure. The transport mechanism involves a sequence of molecular orientation reversals directed by hydrogen-bonding interactions as the neutral molecules traverse the interlayer gap and pass through the intralayer voids that show similarities with the transport of water through transmembrane aquaporin channels in biological systems. The results suggest that nanoporous layered carbon nitrides can be useful for developing high-performance membranes.

New nitrides: from high pressure-high temperature synthesis to layered nanomaterials and energy applications

We describe work carried out within our group to explore new transition metal and main group nitride phases synthesized using high pressure-high temperature techniques using X-ray diffraction and spectroscopy at synchrotron sources in the USA, UK and France to establish their structures and physical properties. Along with previously published data, we also highlight additional results that have not been presented elsewhere and that represent new areas for further exploration. We also describe new work being carried out to explore the properties of carbon nitride materials being developed for energy applications and the nature of few-layered carbon nitride nanomaterials with atomically ordered structures that form solutions in polar liquids via thermodynamically driven exfoliation. This article is part of the theme issue 'Fifty years of synchrotron science: achievements and opportunities'.

Formation of an ion-free crystalline carbon nitride and its reversible intercalation with ionic species and molecular water

Crystalline layered carbon nitrides can be inter-converted by simple ion exchange process allowing their properties to be tuned.

The development of processes to tune the properties of materials is essential for the progression of next-generation technologies for catalysis, optoelectronics and sustainability including energy harvesting and conversion. Layered carbon nitrides have also been identified as of significant interest within these fields of application. However, most carbon nitride materials studied to date have poor crystallinity and therefore their properties cannot be readily controlled or easily related to their molecular level or nanoscale structures. Here we report a process for forming a range of crystalline layered carbon nitrides with polytriazine imide (PTI) structures that can be interconverted by simple ion exchange processes, permitting the tunability of their optoelectronic and chemical properties. Notable outcomes of our work are (a) the creation of a crystalline, guest-ion-free PTI compound that (b) can be re-intercalated with ions or molecules using “soft chemistry” approaches. This includes the intercalation of HCl, demonstrating a new ambient pressure route to the layered PTI·xHCl material that was previously only available by a high-pressure-high-temperature route (c). Our work also shows (d) that the intercalant-free (IF-) PTI material spontaneously absorbs up to 10 weight% H₂O from the ambient atmosphere and that this process is reversible, leading to potential applications for membranes and water capture in dry environments.

Structure and reactivity of 2,4,6-tricyano-1,3,5-triazine under high-pressure conditions

2,4,6-Tricyano-1,3,5-triazine is an attractive precursor for the synthesis of extended layered and nanoporous carbon nitrides.

Investigation on the Stability of Derivative Melam from Melamine Pyrolysis under High Pressure

Although various kinds of carbon nitride precursors have been proposed, s-triazine-based structures are hardly reported because of their unfavorable energy, higher than that of heptazine-based ones. In this study, we investigate the thermal stability of s-triazine-based melam processed at a high pressure of 5 GPa and a temperature of 400–700 °C and complete the analyses of the composition and structure of the treated samples through X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and elemental analyses (EA). Results show that melam can stably exist up to 600 °C at 5 GPa. XRD and FTIR analyses reveal that residual melamine can be pyrolyzed into melam as temperature increases from 400 °C to 600 °C at a high pressure, suggesting that melam may be purified through high-pressure pyrolysis. Further melam polymerization at a higher pressure is a promising strategy for the preparation of s-triazine-based carbon nitride precursors used for bulk carbon nitride synthesis.

Controlled synthesis of organic single-crystalline nanowires via the synergy approach of the bottom-up/top-down processes

The controlled fabrication of organic single-crystalline nanowires (OSCNWs) with a uniform diameter in the nanoscale via the bottom-up approach, which is just based on weak intermolecular interaction, is a great challenge. Herein, we utilize the synergy approach of the bottom-up and the top-down processes to fabricate OSCNWs with diameters of 120 ± 10 nm through stepwise evolution processes. Specifically, the evolution processes vary from the self-assembled organic micro-rods with a quadrangular pyramid-like end-structure bounded with {111}s and {11-1}s crystal planes to the "top-down" synthesized organic micro-rods with the flat cross-sectional {002}s plane, to the organic micro-tubes with a wall thickness of ∼115 nm, and finally to the organic nanowires. Notably, the anisotropic etching process caused by the protic solvent molecules (such as ethanol) is crucial for the evolution of the morphology throughout the whole top-down process. Therefore, our demonstration opens a new avenue for the controlled-fabrication of organic nanowires, and also contributes to the development of nanowire-based organic optoelectronics such as organic nanowire lasers.

Voids padding induced further enhancement in photocatalytic performance of porous graphene-like carbon nitride

Design of 2-Dimensional nanostructured photocatalyst is an effective way to improve the photocatalytic activity of its bulk counterpart. However, the remaining (or newborn) drawbacks, such as enlarged band gap and the surface recombination of photogenerated charge carries, extremely limited the practical application of nanosheeted photocatalysts in solar energy conversion. In this study, we demonstrated that the voids padding with NH₄Cl can eliminate part of quantum size effect to reduce the band gap of nanosheeted carbon nitride. In addition, the padded NH₄Cl can create conjugate center and interface electric field in nanosheeted carbon nitride, and therefore to inhibit the surface recombination of photogenerated charge carries. This work not only provides a facile strategy to eliminate the drawbacks of nanosheeted carbon nitride, but also paves a new way to further improve the photocatalytic activity of other nano-sheeted materials.

Photocatalytic overall water splitting by conjugated semiconductors with crystalline poly(triazine imide) frameworks

Photocatalytic water splitting is an ideal pathway to produce hydrogen for the future energy supply due to the sustainability of solar energy and the mild reaction conditions. In the past four decades, many inorganic semiconductor photocatalysts have been studied for this purpose. In recent years, conjugated polymers, in particular covalent carbon nitride frameworks, have rapidly emerged as a new family of photocatalysts. However, the use of conjugated photocatalysts in overall water splitting in the absence of sacrificial agents has been much less reported. Herein, we used surface kinetic control to photocatalyze overall water splitting by a covalent carbon nitride semiconductor with a crystalline poly(triazine imide) (PTI) frameworks. Our study demonstrates that the loading of a Pt co-catalyst on the PTI surface plays the key role in inducing overall water splitting. The co-deposition of a cobalt species can effectively increase the photocatalytic activity and adjust the ratio of H2 and O2 produced, as well as enhancing the stability of the photocatalyst. The optimal sample with the dual co-catalysts shows an apparent quantum yield of 2.1% for the overall water splitting reaction.

A quasi-hexagonal prism-shaped carbon nitride for photoreduction of carbon dioxide under visible light

A quasi-hexagonal prism-shaped carbon nitride (H-C₃N₄) was synthesized from urea-derived C₃N₄ (U-C₃N₄) using an alkaline hydrothermal process. U-C₃N₄ decomposition followed by hydrogen bond rearrangement of hydrolyzed products leads to the formation of a quasi-hexagonal prism-shaped structure. The H-C₃N₄ catalysts displayed superior activity in the photoreduction of CO₂ with H₂O compared to U-C₃N₄. The enhanced photocatalytic activities can be attributed to the promotion of incompletely coordinated nitrogen atom formation in the C₃N₄ molecules. Graphical abstract ᅟ.

Carbon nitrides: synthesis and characterization of a new class of functional materials

We examine the characterization of carbon nitrides and provide a perspective on their functional properties as next-generation materials.

Two-dimensional graphitic carbon nitride nanosheets for biosensing applications

Two-dimensional graphitic carbon nitride nanosheets (CNNSs) with planar graphene-like structure have stimulated increasingly research interest in recent years due to their unique physicochemical properties. CNNSs possess superior stability, high fluorescence quantum yield, low-toxicity, excellent biocompatibility, unique electroluminescent and photoelectrochemical properties, which make them appropriate candidates for biosensing. In this review, we first introduce the preparation and unique properties of CNNSs, with emphasis on their superior properties for biosensing. Then, recent advances of CNNSs in photoelectrochemical biosensing, electrochemiluminescence biosensing and fluorescence biosensing are highlighted. An additional attention is paid to the marriage of CNNSs and nucleic acids, which exhibits great potentials in both biosensing and intracellular imaging. Finally, current challenges and opportunities of this 2D material are outlined. Inspired by the unique properties of CNNSs and their advantages in biological applications, we expect that more attention will be drawn to this promising 2D material and extensive applications can be found in bioanalysis and diseases diagnosis.