Harvesting Solar Light with Crystalline Carbon Nitrides for Efficient Photocatalytic Hydrogen Evolution

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.

Thermal reflux to synthesise C3N4 nanostructures with high yield and enhanced photocatalytic activity in hydrogen evolution

Graphitic carbon nitride (g-C3N4) is an attractive photocatalyst and a thermal reflux method was used to significantly increase the yield of g-C3N4 nanostructures and enhance their photocatalytic activity for H2 production. When raw material was placed in an inclined quartz tube with the bottom in the high temperature zone and the top in the low temperature zone, the vapor of the raw material was condensed from the top and refluxed to the bottom to then participate in the reaction again. This method can significantly improve the yield of g-C3N4 with an increase of 28.2%. When used as the photocatalyst in water splitting, these g-C3N4 nanostructures exhibit excellent activity with an H2 production of 2102 μmol h-1 g-1 under visible irradiation, which is 43.2% higher than the sample prepared by a traditional thermal polymerization method. This method is also promising to use for the synthesis of similar two-dimensional nanostructures for effective H2 production.

In-situ one-step construction of poly(heptazine imide)/poly(triazine imide) heterojunctions for photocatalytic hydrogen evolution

The construction of heterojunctions is challenging, requiring atomic-level contact and interface matching. Here, we have achieved atomic-level interfacial matching by constructing poly(heptazine imide)/poly(triazine imide) crystalline carbon nitride heterojunctions in an in-situ one-step method. The content of poly(triazine imide) in heterojunctions is positively related to the proportion of lithium chloride in potassium chloride and lithium chloride mixed-salts. The optimized heterojunction achieves an apparent quantum efficiency of 48.34 % for photocatalytic hydrogen production at 420 nm, which is at a good level in polymeric carbon nitride photocatalysts. The proposed ion-thermal assisted heterojunction construction strategy contributes to the development of polymeric carbon nitride photocatalysts with high crystallization and high charge separation efficiency.

Strategies for Improving the Photocatalytic Hydrogen Evolution Reaction of Carbon Nitride-Based Catalysts

Due to the depletion of fossil fuels and their-related environmental issues, sustainable, clean, and renewable energy is urgently needed to replace fossil fuel as the primary energy resource. Hydrogen is considered as one of the cleanest energies. Among the approaches to hydrogen production, photocatalysis is the most sustainable and renewable solar energy technique. Considering the low cost of fabrication, earth abundance, appropriate bandgap, and high performance, carbon nitride has attracted extensive attention as the catalyst for photocatalytic hydrogen production in the last two decades. In this review, the carbon nitride-based photocatalytic hydrogen production system, including the catalytic mechanism and the strategies for improving the photocatalytic performance is discussed. According to the photocatalytic processes, the strengthened mechanism of carbon nitride-based catalysts is particularly described in terms of boosting the excitation of electrons and holes, suppressing carriers recombination, and enhancing the utilization efficiency of photon-excited electron-hole. Finally, the current trends related to the screening design of superior photocatalytic hydrogen production systems are outlined, and the development direction of carbon nitride for hydrogen production is clarified.

Combined DFT-D3 Computational and Experimental Studies on g-C3N4: New Insight into Structure, Optical, and Vibrational Properties

Graphitic carbon nitride (g-C₃N₄) has emerged as one of the most promising solar-light-activated polymeric metal-free semiconductor photocatalysts due to its thermal physicochemical stability but also its characteristics of environmentally friendly and sustainable material. Despite the challenging properties of g-C₃N₄, its photocatalytic performance is still limited by the low surface area, together with the fast charge recombination phenomena. Hence, many efforts have been focused on overcoming these drawbacks by controlling and improving the synthesis methods. With regard to this, many structures including strands of linearly condensed melamine monomers, which are interconnected by hydrogen bonds, or highly condensed systems, have been proposed. Nevertheless, complete and consistent knowledge of the pristine material has not yet been achieved. Thus, to shed light on the nature of polymerised carbon nitride structures, which are obtained from the well-known direct heating of melamine under mild conditions, we combined the results obtained from XRD analysis, SEM and AFM microscopies, and UV-visible and FTIR spectroscopies with the data from the Density Functional Theory method (DFT). An indirect band gap and the vibrational peaks have been calculated without uncertainty, thus highlighting a mixture of highly condensed g-C₃N₄ domains embedded in a less condensed "melon-like" framework.

FeNC Oxygen Reduction Electrocatalyst with High Utilization Penta-Coordinated Sites

Atomic Fe in N-doped carbon (FeNC) electrocatalysts for oxygen (O₂ ) reduction at the cathode of proton exchange membrane fuel cells are the most promising alternative to platinum-group-metal catalysts. Despite recent progress on atomic FeNC O₂  reduction, their controlled synthesis and stability for practical applications remain challenging. A two-step synthesis approach has recently led to significant advances in terms of Fe-loading and mass activity; however, the Fe utilization remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, this issue is addressed by coordinating Fe in a highly porous nitrogen-doped carbon support (≈3295 m²  g^-1 ), prepared by pyrolysis of inexpensive 2,4,6-triaminopyrimidine and a Mg²⁺ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54 × 10¹⁹  sites g_FeNC ^-1  and a record 52% FeN_x electrochemical utilization based on in situ nitrite stripping are achieved. The Fe single atoms are characterized pre- and post-electrochemical accelerated stress testing by aberration-corrected high-angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X-ray absorption spectroscopy and low-temperature Mössbauer spectroscopy suggest the presence of penta-coordinated Fe sites, which are further studied by density functional theory calculations.

Partially cross-linked carbon nitride with unimpeded charge transfer between different chains for boosting photocatalytic hydrogen production

A heptazine-based, partially cross-linked carbon nitride (PC-CN) was successfully prepared via a solid-chloride-salt-assisted polycondensation method. The cross-linking favors the charge transport between different chains, thus dramatically boosting the photocatalytic hydrogen evolution activity of PC-CN, up to 29.3 times that of traditional 1D-CN.

The Construction of Phosphorus-Doped g-C3N4/Rh-Doped SrTiO3 with Type-II Band Alignment for Efficient Photocatalytic Hydrogen Evolution

It is of great importance to promote charge separation in photocatalysts for enhanced photocatalytic activity under visible light irradiation. In this work, a type-II heterostructured photocatalyst was constructed by compositing phosphorus-doped g-C₃N₄ (P-CN) and Rh-doped SrTiO₃ (Rh-STO) via a thermal calcination treatment. A series of characterizations were conducted to investigate the structure of heterostructured P-CN/Rh-STO. It was found that Rh-STO interacted with in situ generated P atoms from the decomposition of P-CN during the calcination process, thus leading to the formation of heterojunction of P-CN/Rh-STO. Compared with the single component, i.e., P-CN or Rh-STO, the obtained P-CN/Rh-STO showed superior photocatalytic activity to that of both P-CN and Rh-STO due to the effective charge separation across the heterojunction between P-CN and Rh-STO.

Synthesis of transition metal ions doped-ZrO2 nanoparticles supported g-C3N4 hybrids for solar light-induced photocatalytic removal of methyl orange and tetracycline pollutants

Photodegradation is an eco-friendly degradation process routinely employed for the removal of various pollutants produced by pharmaceutical and textile industries. In this work, g-C₃N₄ sheets (g-CN) supported with Fe-doped ZrO₂ nanoparticles have been prepared via a facile hydrothermal method as photocatalysts for the effective photodegradation of methyl orange (MO) and tetracycline (TC). The as-prepared photocatalysts were characterized by using a wide range of techniques to understand the origin of their superior photodegradation performance. Structurally, Fe-doped ZrO₂ nanoparticles were found to be uniformly superficially distributed on g-C₃N₄. The addition of Fe-doped ZrO₂ nanoparticles was also found to improve the surface area and light absorption capacity of pure g-CN. It was further revealed that the development of heterojunctions between g-C₃N₄ and Fe-doped ZrO₂ nanoparticles effectively reduced the recombination rate of electron and hole pairs within the photocatalyst system, resulting in improved photocatalytic activity. Previous studies have pointed at the superoxide radical anions (˙O₂^-) and (OH·) as being primarily responsible for the degradation of MO and TC species, leading us to hypothesize that the g-FZ composite works via a possible free-radical based catalytic mechanism to support the photodegradation process.

Charge-Compensated N-Doped π-Conjugated Polymers: Toward both Thermodynamic Stability of N-Doped States in Water and High Electron Conductivity

The understanding and applications of electron-conducting π-conjugated polymers with naphtalene diimide (NDI) blocks show remarkable progress in recent years. Such polymers demonstrate a facilitated n-doping due to the strong electron deficiency of the main polymer chain and the presence of the positively charged side groups stabilizing a negative charge of the n-doped backbone. Here, the n-type conducting NDI polymer with enhanced stability of its n-doped states for prospective "in-water" applications is developed. A combined experimental-theoretical approach is used to identify critical features and parameters that control the doping and electron transport process. The facilitated polymer reduction ability and the thermodynamic stability in water are confirmed by electrochemical measurements and doping studies. This material also demonstrates a high conductivity of 10-2  S cm-1  under ambient conditions and 10-1  S cm-1  in vacuum. The modeling explains the stabilizing effects  for various dopants. The simulations show a significant doping-induced "collapse" of the positively charged side chains on the core bearing a partial negative charge. This explains a decrease in the lamellar spacing observed in experiments. This study fundamentally enables a novel pathway for achieving both thermodynamic stability of the n-doped states in water and the high electron conductivity of polymers.

Recent advances in high-crystalline conjugated organic polymeric materials for photocatalytic CO2 conversion

The photocatalytic conversion of carbon dioxide (CO₂) to high-value-added fuels is a meaningful strategy to achieve carbon neutrality and alleviate the energy crisis. However, the low efficiency, poor selectivity, and insufficient product variety greatly limit its practical applications. In this regard, conjugated organic polymeric materials including carbon nitride (g-C₃N₄), covalent organic frameworks (COFs), and covalent triazine frameworks (CTFs) exhibit enormous potential owing to their structural diversity and functional tunability. Nevertheless, their catalytic activities are largely suppressed by the traditional amorphous or weakly crystalline structures. Therefore, constructing relevant high-crystalline materials to ameliorate their inherent drawbacks is an efficient strategy to enhance the photocatalytic performance of conjugated organic polymeric materials. In this review, the advantages of high-crystalline organic polymeric materials including reducing the concentration of defects, enhancing the built-in electric field, reducing the interlayer hydrogen bonding, and crystal plane regulation are highlighted. Furthermore, the strategies for their synthesis such as molten-salt, solid salt template, and microwave-assisted methods are comprehensively summarized, while the modification strategies including defect engineering, element doping, surface loading, and heterojunction construction are elaborated for enhancing their photocatalytic activities. Ultimately, the challenges and opportunities of high-crystalline conjugated organic polymeric materials in photocatalytic CO₂ conversion are prospected to give some inspiration and guidance for researchers.

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.

Well-Defined Ultrasmall V-NiP2 Nanoparticles Anchored g-C3N4 Nanosheets as Highly Efficient Visible-Light-Driven Photocatalysts for H2 Evolution

Exploring low-cost and highly active, cost-effective cocatalysts is of great significance to improve the hydrogen evolution performance of semiconductor photocatalysts. Herein, a novel ultrasmall V-doped NiP2 nanoparticle, as an efficient cocatalyst, is reported to largely upgrade the photocatalytic hydrogen evolution reaction (HER) of g-C3N4 nanosheets under visible-light irradiation. Experimental results demonstrate that V-NiP2 cocatalyst can enhance the visible-light absorption ability, facilitate the separation of photo-generated electron-hole pairs and boost the transfer ability of electrons of g-C3N4. Moreover, the V-NiP2/g-C3N4 hybrid exhibits prominent photocatalytic HER activity 17 times higher than the pristine g-C3N4 counterpart, even outperforming the 1 wt.% platinum-loaded g-C3N4. This work displays that noble-metal-free V-NiP2 cocatalyst can serve as a promising and efficient alternative to Pt for high-efficiency photocatalytic H2 evolution.

Constructing porous carbon nitride nanosheets for efficient visible-light-responsive photocatalytic hydrogen evolution

The photocatalytic performance of polymeric carbon nitride (CN) is mainly restricted by the poor mass charge separation efficiency and poor light absorption due to its polymeric nature. The conventional strategies to address these problems involved constructing a nanosheets structure would result in a blue shifted light absorption and increased exciton binding energy. Here, with combination of ammonia etching and selectively hydrogen-bond breaking, holey carbon nitride nanosheets (hCNNS) were constructed, thus widening the light absorption range, and spontaneously shortening the migration distance of electrons and holes in the lateral and vertical directions, respectively. Further analysis also found out the reserved atomic structure order endowed hCNNS with the relatively high redox potential. When irradiated with visible light (λ > 420 nm) and loaded with 3 wt% Pt as the cocatalyst, the hydrogen evolution rate of hCNNS was about 40 times higher than the bulk CN, and the apparent quantum yield (AQY) of hCNNS is 1.47% at 435 ± 15 nm. We expect this research can provide a new sight for achieving highly efficient solar utilization of CN-based photocatalysts.

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.

Selective Photocatalytic CO2 Reduction to CH4 on Tri-s-triazine-Based Carbon Nitride via Defects and Crystal Regulation: Synergistic Effect of Thermodynamics and Kinetics

Realizing the high selectivity of CH₄ from the photocatalytic CO₂ reduction reaction (CO₂ RR) remains a great challenge owing to the lower efficiency of multi-electron transfer and the similar thermodynamic properties of CH₄ and CO. Herein, nitrogen-deficient carbon nitride two-dimensional (2D) nanosheets were prepared via the high-temperature crystalline phase transformation process. Optimizing crystallinity enhances the in-plane polarization along the a-axis. Owing to the increased electron density of the N defect, the kinetic possibilities of CH₄ production have increased. Furthermore, the potential energy of the mid-gap states introduced by the N defect favors the thermodynamics of CH₄ production. The selectivity values of CH₄ based on yield and electrons are 87.1 and 96.4%. This work unravels the mechanism to selectively produce CH₄ from CO₂ photoreduction through the crystalline phase and defect regulation and provides significant guidance for the rational design of CO₂ reduction photocatalysts for selective CH₄ production.

Photocatalytic hydrogen evolution based on carbon nitride and organic semiconductors

Photocatalytic hydrogen evolution (PHE) presents a promising way to solve the global energy crisis. Metal-free carbon nitride (CN) and organic semiconductors photocatalysts have drawn intense interests due to their fascinating properties such as tunable molecular structure, electronic states, strong visible-light absorption, low-cost etc. In this paper, the recent progresses of photocatalytic hydrogen production based on organic photocatalysts, including CN, linear polymers, conjugated porous polymers and small molecules, are reviewed, with emphasis on the various strategies to improve PHE efficiency. Finally, the possible future research trends in the organic photocatalysts are prospected.

Photocatalytic Water-Splitting by Organic Conjugated Polymers: Opportunities and Challenges

The future challenges associated with the shortage of fossil fuels and their current environmental impacts intrigued the researchers to look for alternative ways of generating green energy. Solar-driven water splitting into oxygen and hydrogen is one of those advanced strategies. Researchers have studied various semiconductor materials to achieve potential results. However, it encountered multiple challenges such as high cost, low photostability and efficiency, and required multistep modifications. The conjugated polymers (CPs) have emerged as promising alternatives for conventional inorganic semiconductors. The CPs offer low cost, sufficient light absorption efficiency, excellent photo and chemical stability, and molecular optoelectronic tunable characteristics. Furthermore, organic CPs also present higher flexibility to tune the basic framework of the backbone of the polymers, amendments in the sidechain to incorporate desired functionalities, and much-needed porosity to serve better for photocatalytic applications. This review article summarizes the recent advancements made in visible-light-driven water splitting covering the aspects of synthetic strategies and experimental parameters employed for water splitting reactions with special emphasis on conjugated polymers such as linear CPs, planarized CPs, graphitic carbon nitride (g-C₃ N₄ ), conjugated microporous polymers (CMPs), covalent organic frameworks (COFs), and conjugated polymer-based nanocomposites (CPNCs). The current challenges and future prospects have also been described briefly.

Fluorescent graphitic carbon nitride with photocatalytic oxidase-like activity for anti-counterfeiting application

Herein bulk phenyl- and carbon-modified graphitic carbon nitride (PCCN) powders with tunable fluorescent emission from green-color to yellow-color were prepared by copolymerization of 2,4-diamino-6-phenyl-1,3,5-triazine and 2,2,6-triaminopyrimidine. The corresponding nanosheets with blue-color to green-color fluorescence were obtained by the oxidation of their bulk powders in sulfuric or nitric acid and then ultrasonic exfoliation. The typical PCCN_0.6 nanosheets not only displayed strong green-color fluorescence but also exhibited photocatalytic oxidase-like activity, which can catalyze the oxidation of substrates 3,3',5,5'-tetramethylbenzidine and Amplex UltraRed by O₂ to produce blue-color colorimetric product and pink-color fluorescent product, respectively. By taking advantage of green-color fluorescence and photocatalytic activity of PCCN_0.6 nanosheets, a prototype for high-level anti-counterfeiting application was demonstrated by using the mixture of PCCN_0.6 nanosheets and Amplex UltraRed as the fluorescent ink.

High Photocatalytic Oxygen Evolution via Strong Built-In Electric Field Induced by High Crystallinity of Perylene Imide Supramolecule

A highly crystalline perylene imide supramolecular photocatalyst (PDI-NH) is synthesized via imidazole solvent method. The catalyst shows a breakthrough oxygen evolution rate (40.6 mmol g^-1 h^-1 ) with apparent quantum yield of 10.4% at 400 nm, which is 1353 times higher than the low crystalline PDI-NH. The highly crystalline structure comes from the ordered self-assembly process in molten imidazole solvent via π-π stacking and hydrogen bonding. Further, the excellent performance ascribes to the robust built-in electric field induced by its high crystallinity, which greatly accelerates the charge separation and transfer. What is more, the PDI-NH is quite stable and can be reused over 50 h without performance attenuation. Briefly, the crystalline PDI-NH with strong built-in electric field throws light on photocatalytic oxygen evolution, showing a new perspective for the design of organic photocatalysts.

Recent Advancement of the Current Aspects of g-C3 N4 for its Photocatalytic Applications in Sustainable Energy System

Being one of the foremost enticing and intriguing innovations, heterogeneous photocatalysis has also been used to effectively gather, transform, and conserve sustainable sun's radiation for the production of efficient and clean fossil energy as well as a wide range of ecological implications. The generation of solar fuel-based water splitting and CO₂ photoreduction is excellent for generating alternative resources and reducing global warming. Developing an inexpensive photocatalyst can effectively split water into hydrogen (H₂ ), oxygen (O₂ ) sources, and carbon dioxide (CO₂ ) into fuel sources, which is a crucial problem in photocatalysis. The metal-free g-C₃ N₄ photocatalyst has a high solar fuel generation potential. This review covers the most recent advancements in g-C₃ N₄ preparation, including innovative design concepts and new synthesis methods, and novel ideas for expanding the light absorption of pure g-C₃ N₄ for photocatalytic application. Similarly, the main issue concerning research and prospects in photocatalysts based g-C₃ N₄ was also discussed. The current dissertation provides an overview of comprehensive understanding of the exploitation of the extraordinary systemic and characteristics, as well as the fabrication processes and uses of g-C₃ N₄ .

A novel electrochemical sensor for the detection of fipronil and its toxic metabolite fipronil sulfone using TiO2-polytriazine imide submicrostructured composite as an efficient electrocatalyst

For the first time, a simple and sensitive electrochemical sensor based on a screen printed electrode (SPE) modified with titanium dioxide (TiO₂) and polytriazine imide submicrostructured composite (TiO₂-PTI) has been developed for the simultaneous detection of fipronil (FIP) and its toxic metabolite fipronil sulfone (FIP-S). The submicrostructured composite material based on TiO₂ and PTI was obtained by simple hydrothermal treatment of the Ti peroxocomplexes in the presence of pristine. This carbon nitride allotrope has better crystallinity and conductivity than its graphitic analog. It was found that the TiO₂-PTI submicrostructured composite enhanced the electrochemical sensing of the SPE electrode towards FIP and its metabolite FIP-S in 0.1 M Britton-Robinson buffer (pH 10) at the oxidation potentials of 0.82 V and 0.94 V, respectively. In addition, it showed good stability and reproducibility for the determination of both analytes. Under optimal conditions, the peak currents by square wave voltammetry were found to vary linearly with FIP and FIP-S concentrations in the range from 0.01 to 10 μM and from 10 to 50 μM, with a detection limit of 8.42 nM, 3.6 μg/kg for FIP and 9.72 nM, 4.04 μg/kg for FIP-S. This sensor was successfully used to detect FIP and FIP-S in eggs and water samples with good recoveries of 90%-106.6%.

Fully Condensed Poly (Triazine Imide) Crystals: Extended π-Conjugation and Structural Defects for Overall Water Splitting

Conventional polymerization for the synthesis of carbon nitride usually generates amorphous heptazine-based melon with an abundance of undesired structural defects, which function as charge carrier recombination centers to decrease the photocatalytic efficiency. Herein, a fully condensed poly (triazine imide) crystal with extended π-conjugation and deficient structure defects was obtained by conducting the polycondensation in a mild molten salt of LiCl/NaCl. The melting point of the binary LiCl/NaCl system is around 550 °C, which substantially restrain the depolymerization of triazine units and extend the π-conjugation. The optimized polymeric carbon nitride crystal exhibits a high apparent quantum efficiency of 12 % (λ=365 nm) for hydrogen production by one-step excitation overall water splitting, owing to the efficient exciton dissociation and the subsequent fast transfer of charge carriers.

Amphiphilicity of Intricate Layered Graphene/g-C3N4 Nanosheets

The hybrid heterostructure of the tri-s-triazine form of graphitic carbon nitride (g-C₃N₄), a stable two-dimensional material, results from intricate layer formation with graphene. In this material, g-C₃N₄, an amphiphilic material, stabilizes Pickering emulsions as an emulsifier and can effectively disperse graphene. Due to the various technological applications of the hybrid nanosheets in an aqueous environment, it is essential to study the interaction of water molecules with graphene and g-C₃N₄ (Gr/g-C₃N₄)-combined heterostructure. Although few studies have been performed signifying the water orientation in the interfacial layer, we find that there is a lack of detailed studies using various dynamical and structural properties of the interfacial water molecules. The interface of the Gr/g-C₃N₄ hybrid structure, one of the rarely found amphiphilic interfaces (on the g-C₃N₃ side), is appropriate for exploring the water affinity due to the availability of heterogeneous interfacial aqueous interactions. We adopted classical molecular dynamics simulations using two models for water molecules to study the structure and dynamics of an aqueous interface. We have correlated the structural properties to dynamics and spectral properties to understand the overall behavior of the amphiphilic interface. Our results branch into two significant hydrogen bond (HB) properties in HB count and HB strength among the water molecules in the different layers. The HB counts in the different layers of water are correlated using the average distance distribution (P_rO4), tetrahedral order parameters, HB donor/acceptor count, and total HBs per water molecule. A conspicuous difference is found in the HB count and related dynamics of the system. The HB lifetime and diffusion coefficient hint at the equivalent strength of HBs in the different layers. All the findings conclude that the amphiphilicity of the Gr/g-C₃N₄ interface can help in understanding various interfacial physical and chemical processes.

Donor Bandgap Engineering without Sacrificing the Reduction Ability of Photogenerated Electrons in Crystalline Carbon Nitride

Crystalline carbon nitride (CCN) with a light response up to 700 nm has been seldom reported but is significant for the artificial photocatalysis. In this study, it is proposed that, unlike acceptors, introducing donors can effectively narrow the bandgap without sacrificing the reduction ability of photogenerated electrons, which is more advantageous to photocatalytic reduction reactions. Hence, a series of heptazine-based K⁺ -implanted CCN (KCN) with a narrow bandgap (2.87-1.86 eV) are constructed by copolymerization of pyrimidine donors. The optimized photocatalysts can extend the light response to 700 nm and account for approximately 122- and 33-fold enhancements in H₂ production (λ>500 nm) in comparison to CN and KCN, respectively. The apparent quantum efficiency (AQE) can reach 8.2 % at 500 nm and is comparable to the top-level CN- and CCN- based materials. Its photoactive wavelength has significant advantages over previously reported CCN-based photocatalysts. This method offers a universal donor bandgap engineering strategy towards photocatalytic reduction reactions.

Synergy of intermolecular Donor-Acceptor and ultrathin structures in crystalline carbon nitride for efficient photocatalytic hydrogen evolution

Crystalline carbon nitride is regarded as the new generation of emerging metal-free photocatalysts as opposed to polymeric carbon nitride (g-C₃N₄) because of its high crystalline structure and ultrahigh photocatalytic water splitting performance. However, further advances in crystalline g-C₃N₄ are significantly restricted by the sluggish separation of charge carriers and limited active sites. In this study, we demonstrate the successful synthesis of heptazine-triazine donor-acceptor-based ultrathin crystalline g-C₃N₄ nanosheets (UCCN) using a combined hot air exfoliation and molten salt (NaCl/KCl) copolymerization approach. The synergy of the donor-acceptor heterojunction and the ultrathin structure greatly accelerated the separation of the charge carriers and enriched the active sites. Accordingly, the superior hydrogen evolution activity and an ultrahigh apparent quantum efficiency of 73.6% at 420 nm under a natural photosynthetic environment were achieved by UCCN, positioning this material at the top among reported conjugated g-C₃N₄ materials. This study provides a novel paradigm for the development of donor-acceptor-based ultrathin crystalline layered materials.

Construction 0D/2D heterojunction by highly dispersed Ag2S quantum dots (QDs) loaded on the g-C3N4 nanosheets for photocatalytic hydrogen evolution

In recent years, the use of quantum dots (QDs) cocatalysts to improve the hydrogen evolution activity from the water splitting of photocatalysts has become a popular research topic. Herein, we successfully prepared a novel 0 dimension/2 dimension (0D/2D) heterojunction nanocomposite (denoted Ag₂S quantum dots (QDs)/g-C₃N₄) with excellent photocatalytic performance by anchoring the Ag₂S QDs cocatalyst on the surface of g-C₃N₄ through a self-assembly strategy. Ag₂S QDs with an average particle size of approximately 5.8 nm were uniformly and tightly modified on g-C₃N₄. The Ag₂S QDs/g-C₃N₄ composite with 0.5 wt% Ag₂S QDs loading achieved the highest hydrogen evolution rate of 471.1 μmol·g^-1·h^-1 with an apparent quantum efficiency (AQE) of 1.48% at 405 nm. Such remarkable hydrogen evolution activity far exceeded that of undoped g-C₃N₄ and Ag₂S nanoparticles (NPs)/g-C₃N₄. Moreover, it was 2.04 times the activity of Pt/g-C₃N₄ with Pt as the cocatalyst. The enhanced photocatalytic performance was attributed to the energy band broadening of Ag₂S QDs caused by the quantum size effect and the convenient and effective charge transfer between g-C₃N₄ and Ag₂S QDs cocatalysts. The mechanism underlying the enhanced photocatalytic H₂ evolution activity was further proposed. This study demonstrates that semiconductor-based quantum dots are strong candidates for excellent cocatalysts in photocatalysis.

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.

Photoredox-Catalyzed Simultaneous Olefin Hydrogenation and Alcohol Oxidation over Crystalline Porous Polymeric Carbon Nitride

Booming of photocatalytic water splitting technology (PWST) opens a new avenue for the sustainable synthesis of high-value-added hydrogenated and oxidized fine chemicals, in which the design of efficient semiconductors for the in-situ and synergistic utilization of photogenerated redox centers are key roles. Herein, a porous polymeric carbon nitride (PPCN) with a crystalline backbone was constructed for visible light-induced photocatalytic hydrogen generation by photoexcited electrons, followed by in-situ utilization for olefin hydrogenation. Simultaneously, various alcohols were selectively transformed to valuable aldehydes or ketones by photoexcited holes. The porosity of PPCN provided it with a large surface area and a short transfer path for photogenerated carriers from the bulk to the surface, and the crystalline structure facilitated photogenerated charge transfer and separation, thus enhancing the overall photocatalytic performance. High reactivity and selectivity, good functionality tolerance, and broad reaction scope were achieved by this concerted photocatalysis system. The results contribute to the development of highly efficient semiconductor photocatalysts and synergistic redox reaction systems based on PWST for high-value-added fine chemical production.

Polymeric Carbon Nitride-Derived Photocatalysts for Water Splitting and Nitrogen Fixation

Photocatalysis is a promising energy conversion and environmental restoration technology. The main focus of photocatalysis is the development and manufacture of highly efficient photocatalysts. Semiconductor-based photocatalysis technology based on harnessing solar energy is considered as an attractive approach to solve the problems of global energy shortage and environmental pollution. Since 2009 pioneering work has been carried out on polymeric carbon nitride (PCN) for visible photocatalytic water splitting, thus PCN-based photocatalysis has become a hot research topic, demanding significant research attention. This article reviews the physical and chemical properties, synthesis methods, and the methods to control the morphology, heteroatom doping, and construction of heterojunctions to improve the performance of PCN-based photocatalysts in water splitting and nitrogen fixation. Through different design strategies, the photo-generated electron-hole pair separation efficiency of PCN materials can be effectively improved, thereby improving their photocatalytic performance. Finally, the challenges of PCN-based photocatalysts in water splitting and nitrogen fixation applications are discussed herein. It is strongly believed that through different design strategies, efficient PCN-based photocatalysts can be constructed for both water splitting and nitrogen reduction. These excellent modification strategies can be used as a guiding theory for photocatalytic reactions of other promising catalysts and further promote the development of photocatalysis.

Tunable linear donor–π–acceptor conjugated polymers with a vinylene linkage for visible-light driven hydrogen evolution

Extending the in-plane conjugation or/and increasing the electron push–pull interaction of linear D–π–A polymers with a vinylene linkage could broaden the visible-light absorption band, promote the charge separation and transfer and the photocatalytic hydrogen production.

Regulating morphological and electronic structures of polymeric carbon nitrides by successive copolymerization and stream reforming for photocatalytic CO2 reduction

Greatly reinforced photocatalytic CO₂ reduction is realized with carbon nitride polymers tailored via a coupled copolymerization and stream reforming strategy.

Bio-inspired honeycomb-like graphitic carbon nitride for enhanced visible light photocatalytic CO2 reduction activity

Graphitic carbon nitride (g-C₃N₄) is paying attention lately owing to its interesting characteristics and substantial application in improving environmental and energy concerns. Nevertheless, the photocatalytic activity of g-C₃N₄ is constrained by the inertness of the surface and particle aggregation during photocatalytic activity. Herein, we report the preparation of g-C₃N₄ with honeycomb-like morphology (HC-C₃N₄) via thermal condensation of prepared SiO₂ templates and dicyandiamide. The etching out of the SiO₂ templates by NH₄HF₂ created hollow or macropores in the C₃N₄ matrix resulting in its structural changes. Similar, to the bulk C₃N₄, the HC-C₃N₄ exhibited higher photocatalytic CO₂ reduction in hydrocarbons. This improved photocatalytic achievement is associated with higher specific surface area, excellent visible light absorption capability, higher electron donor density, easy mass diffusion of materials for surface reaction, and effective segregation of photogenerated charge carriers. Furthermore, the HC-C₃N₄ honeycomb structure was deposited with Ni(OH)₂ clusters which showed remarkable CO₂ reduction activity of 1.48 μmolh^-1 g^-1 of CH₄ and 0.73 μmolh^-1 g^-1 of CH₃OH generation which is 3.5 and 4.3 times higher CO₂ reduction activity compared with bulk C₃N₄ clustered with Ni(OH)₂ particles. This comprehensive study demonstrated that HC-C₃N₄ nanostructured polymeric semiconductor is envisaged to have great potential in the application of a variety of fields such as photocatalysis, sensor technology, and nanotechnology.