Reversible Assembly of Graphitic Carbon Nitride 3D Network for Highly Selective Dyes Absorption and Regeneration

Carbonaceous adsorbents in wastewater treatment: From mechanism to emerging application

Adsorption is of great significance in the water pollution control. Carbonaceous adsorbents, such as carbon quantum dots, carbon nanotubes, graphene, and activated carbon, have long been deployed in sustainable wastewater treatment due to their excellent physical structure and strong interaction with various pollutants; these features allow them to spark greater interest in environmental remediation. Although numerous eye-catch researches on carbon materials in wastewater treatment, there is a lack of comprehensive comparison and summary of the vivid structure-activity-application relationships of different types of carbonaceous adsorbents at the molecular and atomic level. Herein, this review aims to scrutinize and contrast the adsorption mechanisms of carbonaceous adsorbents with different dimensions, analyzing the qualitative differences in adsorption capacity from microscopic perspectives, structural diversity caused by preparation methods, and environmental external factors affecting adsorption occurrence. Then, a quantitatively in-depth critical appraisal of traditional and emerging contaminants in wastewater treatment using carbonaceous adsorbents, and innovative strategies for enhancing their adsorption capacity are discussed. Finally, in the context of growing imposed circularity and zero waste wishes, this review offers some promising insights for carbonaceous adsorbents in achieving sustainable wastewater treatment.

Cholesterol Based Organogelators in Environmental Remediation: Applications in Removal of Toxic Textile Dyes and Oil Spill Recovery

Oil spills in the ocean and textile dyes have a catastrophic impact on the environment, economy, and ecosystem. Phase-selective organic gelator dye sorption and oil separation for oil adsorption should meet certain criteria such as facile synthesis, low cost, effective gelation, and recyclability. This study has discovered that an aliphatic chain synthetic amphiphile based on cholesterol can produce organogels in a variety of organic solvents. Numerous methods, such as X-ray diffraction, Fourier-transform infrared spectroscopy, high-resolution scanning electron microscopy, and rheology, have been used extensively to examine and describe these organogels. An environmentally acceptable technique for achieving hazardous dye separation is presented here. For the sustainable filtration of dye-contaminated water, a new, straightforward, one-step method driven by gravitational force has been employed by using a gel column. This approach has shown excellent stability and reusability with repeated use, and it is easily scalable for the effective removal of a wide range of hazardous dyes. Furthermore, because the oil fraction was absorbed in the gel, the study showed how well it might be used to apply phase selectivity to separate the oil-water mixture from marine accidents. Furthermore, a straightforward distillation method can be used to quantitatively recover the oils contained in the gel and gelator molecules in phase-selective gelation. This low-tech, ecofriendly, and highly effective method also offers valuable insights into the development of advanced materials for separating toxic dyes and oil from water.

Graphitic Carbon Nitride Enters the Scene: A Promising Versatile Tool for Biomedical Applications

Graphitic carbon nitride (g-C3N4), since the pioneering work on visible-light photocatalytic water splitting in 2009, has emerged as a highly promising advanced material for environmental and energetic applications, including photocatalytic degradation of pollutants, photocatalytic hydrogen generation, and carbon dioxide reduction. Due to its distinctive two-dimensional structure, excellent chemical stability, and distinctive optical and electrical properties, g-C3N4 has garnered a considerable amount of interest in the field of biomedicine in recent years. This review focuses on the fundamental properties of g-C3N4, highlighting the synthesis and modification strategies associated with the interfacial structures of g-C3N4-based materials, including heterojunction, band gap engineering, doping, and nanocomposite hybridization. Furthermore, the biomedical applications of these materials in various domains, including biosensors, antimicrobial applications, and photocatalytic degradation of medical pollutants, are also described with the objective of spotlighting the unique advantages of g-C3N4. A summary of the challenges faced and future prospects for the advancement of g-C3N4-based materials is presented, and it is hoped that this review will inspire readers to seek further new applications for this material in biomedical and other fields.

Enhanced Fenton-like catalysis via interfacial regulation of g-C3N4 for efficient aromatic organic pollutant degradation

For the efficient degradation of organic pollutants with the goal of reducing the water environment pollution, we employed an alkaline hydrothermal treatment on primeval g-C3N4 to synthesize a hydroxyl-grafted g-C3N4 (CN-0.5) material, from which we engineered a novel Fenton-like catalyst, known as Cu-CN-0.5. The introduction of numerous hydroxyl functional groups allowed the CN-0.5 substrate to stably fix active copper oxide particles through surface complexation, resulting in a low Cu leaching rate during a Cu-CN-0.5 Fenton-like process. A sequence of characterization techniques and theoretical calculations uncovered that interfacial complexation induced charge redistribution on the Cu-CN-0.5 surface. Specifically, some of the π electrons in the tris-s-triazine units were transferred to the copper oxide particles along the newly formed chemical bonds (C(π)-O-Cu), forming a π-deficient area on the tris-s-triazine plane near the complexation site. In a typical Cu-CN-0.5 Fenton-like process, a stable π-π interaction was established due to the favorable positive-negative match of electrostatic potential between the aromatic pollutants and π-deficient areas, leading to a significant improvement in Cu-CN-0.5's adsorption capacity for aromatic pollutants. Furthermore, pollutants also delivered electrons to the Cu-CN-0.5 Fenton-like system via a "through-space" approach, which suppressed the futile oxidation of H2O2 in reducing the high-valent Cu2+ and significantly improved the generation efficiency of •OH with high oxidative capacity. As expected, Cu-CN-0.5 not only exhibited an efficient Fenton degradation for several typical aromatic organic pollutants, but also demonstrated both a low metal leaching rate (0.12 mg/L) and a H2O2 utilization rate exceeding 80%. The distinctive Fenton degradation mechanism substantiated the potential of the as-prepared material for effective wastewater treatment applications.

Organocatalysis with carbon nitrides

Carbon nitrides, a distinguished class of metal-free catalytic materials, have presented a good potential for chemical transformations and are expected to become prominent materials for organocatalysis. This is largely possible due to their low cost, exceptional thermal and chemical stability, non-toxicity, ease of functionalization, porosity development, etc. Especially, the carbon nitrides with increased porosity and nitrogen contents are more versatile than their bulk counterparts for catalysis. These N-rich carbon nitrides are discussed in the earlier parts of the review. Later, the review highlights the role of such carbon nitride materials for the various organic catalytic reactions including Knoevenagel condensation, oxidation, hydrogenation, esterification, transesterification, cycloaddition, and hydrolysis. The recently emerging concepts in carbon nitride-based organocatalysis have been given special attention. In each of the sections, the structure-property relationship of the materials was discussed and related to their catalysis action. Relevant comparisons with other catalytic materials are also discussed to realize their real potential value. The perspective, challenges, and future directions are also discussed. The overall objective of this review is to provide up-to-date information on new developments in carbon nitride-based organic catalysis reactions that could see them rising as prominent catalytic materials in the future.

Photocatalytic Activities of g-C3N4 (CN) Treated with Nitric Acid Vapor for the Degradation of Pollutants in Wastewater

In this article, we reported a novel setup treatment using nitric acid vapor to treat g-C₃N₄ (CN). By treatment with nitric acid vapour, the basic structure of the CN has not been destroyed. These adoptive treatments enhanced the photocatalytic performance of CN and were reflected in the elimination of rhodamine B (RhB) as well as tetracycline hydrochloride (TC). In comparison to CN, CN-6 demonstrated the highest photocatalytic yield for the breakdown of RhB (99%, in 20 min). Moreover, the excellent reuse of CN-6 for breaking down RhB was also demonstrated. This clearly demonstrated that treatment with nitric acid vapor promoted a blue shift, positively extended its valence band position, and increased the oxidizability of the holes. This also caused CN to disperse better into the aqueous phase, introducing more oxygen-containing functional groups. Thus, treatment with nitric acid vapor has the potential to be applied to delaminate the CN in order to enhance photocatalytic activity.

Charge Carrier Dynamics of CsPbBr3/g-C3N4 Nanoheterostructures in Visible-Light-Driven CO2-to-CO Conversion

The photon energy-dependent selectivity of photocatalytic CO₂-to-CO conversion by CsPbBr₃ nanocrystals (NCs) and CsPbBr₃/g-C₃N₄ nanoheterostructures (NHSs) was demonstrated for the first time. The surficial capping ligands of CsPbBr₃ NCs would adsorb CO₂, resulting in the carboxyl intermediate to process the CO₂-to-CO conversion via carbene pathways. The type-II energy band structure at the heterojunction of CsPbBr₃/g-C₃N₄ NHSs would separate the charge carriers, promoting the efficiency in photocatalytic CO₂-to-CO conversion. The electron consumption rate of CO₂-to-CO conversion for CsPbBr₃/g-C₃N₄ NHSs was found to intensively depend on the rate constant of interfacial hole transfer from CsPbBr₃ to g-C₃N₄. An in situ transient absorption spectroscopy investigation revealed that the half-life time of photoexcited electrons in optimized CsPbBr₃/g-C₃N₄ NHS was extended two times more than that in the CsPbBr₃ NCs, resulting in the higher probability of charge carriers to carry out the CO₂-to-CO conversion. The current work presents important and novel insights of semiconductor NHSs for solar energy-driven CO₂ conversion.

Coordination-Assistant Chiral Agent Anchoring on Amphiphilic Graphitic Phase Carbon Nitride Membrane for Multiple Molecular Separation

Membranes composed of two-dimensional (2D) materials suffer from low stability and structural swelling and are usually restricted to applications in aqueous systems. Among various 2D materials, graphitic phase carbon nitride (GCN, g-C3N4) has shown great application potential owing to its structural tunability. Herein, we develop a coordination-assisted strategy to regulate the GCN layer spacing and chemical environment via copper ion (Cu2+) coordination-assisted intercalation of enantiopure (1S,2S)-(-)-1,2-diphenyl-1,2-ethanediamine (DPE) between GCN nanosheets. The obtained GCN-Cu-DPE membrane is continuous and intact, free of cracks and pinholes, stable under acidic and alkaline conditions, and exhibits water permeability above 215 L m-2 h-1 bar-1 and a high rejection rate to dye molecules. The membrane is amphiphilicity and thus allows both polar solvent (water) and nonpolar solvent (hexane) to freely pass through. Remarkably, the permeation rate is proportional to the viscosity of the solvent. Benefiting from the chiral space between nanosheets, the GCN-Cu-DPE membrane shows selective permeation of aspartic acid racemate in aqueous systems and limonene racemate in the organic phase. Our work demonstrates a general and promising strategy for chiral membrane fabrication toward high-value-added chiral separation, especially in the pharmaceutical industry.

Attenuating metal-substrate conjugation in atomically dispersed nickel catalysts for electroreduction of CO2 to CO

Atomically dispersed transition metals on carbon-based aromatic substrates are an emerging class of electrocatalysts for the electroreduction of CO2. However, electron delocalization of the metal site with the carbon support via d-π conjugation strongly hinders CO2 activation at the active metal centers. Herein, we introduce a strategy to attenuate the d-π conjugation at single Ni atomic sites by functionalizing the support with cyano moieties. In situ attenuated total reflection infrared spectroscopy and theoretical calculations demonstrate that this strategy increases the electron density around the metal centers and facilitates CO2 activation. As a result, for the electroreduction of CO2 to CO in aqueous KHCO3 electrolyte, the cyano-modified catalyst exhibits a turnover frequency of ~22,000 per hour at -1.178 V versus the reversible hydrogen electrode (RHE) and maintains a Faradaic efficiency (FE) above 90% even with a CO2 concentration of only 30% in an H-type cell. In a flow cell under pure CO2 at -0.93 V versus RHE the cyano-modified catalyst enables a current density of -300 mA/cm2 with a FE above 90%.

Preparation of Cu modified g-C3N4 nanorod bundles for efficiently photocatalytic CO2 reduction

Carbon nitride-based photocatalysts for CO₂ reduction have received great attention. The introduction of transition metals can effectively improve the photocatalytic efficiency of carbon nitride. However, how to introduce transition metals into carbon nitride in more ways remains a challenge. Herein, the Cu modified g-C₃N₄ nanorod bundles (CCNBs) were prepared by chemical vapor co-deposition using the mixture of urea and chlorophyllin sodium copper salt as precursor. The prepared CCNBs exhibited excellent photocatalytic activity for CO₂ reduction. The unique hierarchical structure was beneficial to enhance light harvesting. Besides, the introduction of uniformly dispersed Cu further improved the absorption capacity of visible light, increased active sites, and promoted the separation and transfer of carriers. The CO yield of CCNBs was 5 times higher than that of bulk g-C₃N₄, and showed excellent stability in cycle experiments. This work provides a strategy to prepare carbon nitride-based photocatalysts for efficient CO₂ reduction.

Phosphate based new organic polymer networks for efficient dye sorption and catalyst loading for chemo-selective reactivity

Phosphate-based porous organic polymers adsorb small cationic dyes efficiently and host gold nanoparticles for catalytic reduction of electron rich nitroaromatics.

From 1D to 3D Graphitic Carbon Nitride (Melon): A Bottom-Up Route via Crystalline Microporous Templates

Herein, we present a novel bottom-up preparation route for heptazine-based polymers (melon), also known as graphitic carbon nitride. The growth characteristics of isolated 1D melon strings in microporous templates are presented and studied in detail. Removal of the microporous silicate template via etching is accompanied by the self-assembly of a 1D melon to stacked 3D structures. The advantages and limitations of the bottom-up approach are shown by using microporous templates with different pore sizes (ETS-10, ZSM-5, and zeolite Y). In accordance with the molecular size of the heptazine units (0.67 nm), a 1D melon can be deposited in ETS-10 with a pore width of about 0.78 nm, whereas its formation in the smaller 0.47 nm pores of ZSM-5 is sterically impeded. The self-assembly of isolated 1D melon to stacked 3D structures offers a novel experimental perspective to the controversial debate on the polymerization degree in 2D sheets of graphitic carbon nitride as micropore sizes below 1 nm confine the condensation degree of heptazine to isolated 1D strands at a molecular level. The growth characteristics and structural features were investigated by X-ray diffraction, N2 physisorption, scanning transmission electron microscopy/energy-dispersive X-ray analysis, 13C CP-NMR spectroscopy, and attenuated total reflection-infrared spectroscopy.

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.

Emerging graphitic carbon nitride-based membranes for water purification

Membrane separation is a promising technology that can effectively remove various existing contaminants from water with low energy consumption and small carbon footprint. The critical issue of membrane technology development is to obtain a low-cost, stable, tunable and multifunctional material for membrane fabrication. Graphitic carbon nitride (g-C₃N₄) has emerged as a promising membrane material, owing to the unique structure characteristics and outstanding catalytic activity. This review paper outlined the advanced material strategies used to regulate the molecule structure of g-C₃N₄ for membrane separation. The presentative progresses on the applications of g-C₃N₄-based membranes for water purification have been elaborated. Essentially, we highlighted the innovation integration of physical separation, catalysis and energy conversion during water purification, which was of great importance for the sustainability of water treatment techniques. Finally, the continuing challenges of g-C₃N₄-based membranes and the possible breakthrough directions in the future research was prospected.

Hydroxyl/amino and Fe(III) co-grafted graphite carbon nitride for photocatalytic removal of volatile organic compounds

Hydroxyl/amino and Fe(III) co-grafted graphite carbon nitride (CN) is fabricated via alkaline hydrothermal treatment and followed by an impregnation adsorption process. In this unique fabrication, hydroxyl and amino groups enriched on the surface play a vital role in improving the adsorption capacity for volatile organic compounds (VOCs), while the grafted amorphous Fe(III) clusters could dominantly regulate the path of molecular oxygen activation via photo-Fenton reaction, and change the selectivity of intermediate reactive oxygen species (ROS) with the assistant of the rich surficial hydroxyl groups. Meanwhile, both the grafted functional groups and Fe(III) clusters can serve as photogenerated charge acceptors for collaboratively accelerating carriers' separation. Besides, the Fe(III)-mediated interfacial charge transfer effect (IFCT) also could extend visible light absorption and boost carriers' generation. Benefiting from the virtues of the complementary and synergy of the grafted hydroxyl/amino and Fe(III), the dual-functionalized CN is qualified as an efficient photocatalyst for removal of VOCs, which exhibits 22 and 18 times isopropanol (IPA) adsorption capacity and CO₂ production than of pristine CN during photocatalytic IPA removal, respectively. Moreover, this work provides a new strategy of surficial group-cluster bifunctionalization for systematically improving sustainable solar-to-chemical energy conversion towards VOCs mineralization.

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.

Novel B-N-Co surface bonding states constructed on hollow tubular boron doped g-C3N4/CoP for enhanced photocatalytic H2 evolution

Graphitic carbon nitride (g-C₃N₄) is a promising photocatalyst for water hydrogen evolution. Nonetheless, fast recombination of photogenerated electron-hole pairs and the slow kinetics of hydrogen production result in the unsatisfactory efficiency of solar hydrogen production. we address this issue by anchoring the cobalt phosphide (CoP) cocatalyst onto the one-dimensional boron doped g-C₃N₄ nanotube (B-CNNT) to construct B-N-Co surface bonding states in the B-CNNT/CoP photocatalyst. Spectroscopic measurement and density functional theory (DFT) calculations demonstrated that the B-N-Co bonds optimize the local electronic distribution of bonded Co and adjacent P atoms, strengthen the electrons' delocalization capacity of Co atoms for high electrical conductivity and accelerate the photogenerated carrier transfer between B-CNNT and CoP, which lead to the enhanced photocatalytic activity of the B-CNNT/CoP photocatalyst for hydrogen evolution. B-CNNT/CoP-2.45% achieved a remarkable photocatalytic hydrogen production rate of 784 μmol g^-1h^-1 with an apparent quantum efficiency of 5.32% at 420 nm, which is significantly higher than demonstrated by CNNT/CoP-2.45% (153 μmol g^-1h^-1). Our findings provide insights into as well as establish theoretical and practical grounds for the development of low-cost, high-performance photocatalytic materials for hydrogen evolution.

Molecular Level Design of Nitrogen-Doped Well-Defined Microporous Carbon Spheres for Selective Adsorption and Electrocatalysis

Nitrogen-doped porous carbon spheres have attracted great interest in diversified fields owing to their unique physical and chemical properties. However, the synthesis of nitrogen-doped porous carbon spheres with hierarchical superstructures and refined micropore structures is still a challenge. Herein, we develop a molecular-scale silica templating strategy to prepare nitrogen-doped microporous carbon spheres (MCSSs) with high porosity and a well-defined micropore structure. Octa(aminophenyl) polyhedral oligomeric silsesquioxane is used as a building block in MCSS precursors to provide precise molecular-scale templating and nitrogen doping. The morphology of MCSSs can be easily tuned by choosing the proper solvent. The as-synthesized MCSS with a large surface area (2036 m² g^-1), narrow micropore size distribution, nitrogen doping, and hierarchical geometry can serve as an efficient selective adsorbent for CO₂ and organic pollutants. Furthermore, the MCSS decorated with Fe-N-C active sites (MCSS-Fe) shows enhanced electrocatalytic ORR activity in alkaline solution. This novel approach may open a new avenue for controllable fabrication of porous carbon spheres with desired geometry and well-designed pore structure and show potential applications in selective adsorption and catalysis.

An "on-off-super on" photoelectrochemical sensor based on quenching by Cu-induced surface exciton trapping and signal amplification of copper sulfide/porous carbon nitride heterojunction

In this work, we report an "on-off-super on" photoelectrochemical sensor for probing hydrogen sulfide due to its toxicity in water environment by using porous carbon nitride as photoelectric transducers. Synthesized by an alkaline-assisted hydrothermal method, the porous carbon nitride photoanode exhibited a remarkable photocurrent on the initial "on" state. Cu²⁺ immobilized on the surfaces of porous carbon nitride could significantly decrease the charge transfer efficiency and quench the photoelectrochemical signal in the "off" state. In addition, the introduction of S^2- ions could eliminate the influence of Cu-induced surface exciton trapping and amplify the photoelectrochemical signal due to the formation of carbon nitride/copper sulfide heterojunction, thus leading to the achievement of the ''super on'' state and subsequently detection of hydrogen sulfide. More importantly, this photoelectrochemical sensor shows the excellent performance for probing hydrogen sulfide in terms of stability, selectivity, sensitivity and fabrication cost. Enabled by a unique "on-off-super on" strategy, it could serve as a reference for developing the new class of photoelectrochemical sensor.

Ultrafine Fe nanoparticles embedded in N-doped carbon nanotubes derived from highly dispersed g-C3N4 nanofibers for the oxygen reduction reaction

The improved performance was attributed to the synergetic interaction between Fe, h-CN and CNTs.

Colloidal properties of the metal-free semiconductor graphitic carbon nitride

The metal-free, polymeric semiconductor graphitic carbon nitride (g-CN) family is an emerging class of materials and has striking advantages compared to other semiconductors, i.e. ease of tunability, low cost and synthesis from abundant precursors in a chemical environment. Efforts have been done to improve the properties of g-CN, such as photocatalytic efficiency, designing novel composites, processability and scalability towards discovering novel applications as a remedy for the problems that we are facing today. Despite the fact that the main efforts to improve g-CN come from a catalysis perspective, many fundamental possibilities arise from the special colloidal properties of carbon nitride particles, from synthesis to applications. This review will display how typical colloid chemistry tools can be employed to make 'better g-CNs' and how up to now overseen properties can be levered by integrating a colloid and interface perspective into materials chemistry. Establishing a knowledge on the origins of colloidal behavior of g-CN will be the core of the review.

Smart Removal of Dye Pollutants via Dark Adsorption and Light Desorption at Recyclable Bi2O2CO3 Nanosheets Interface

The adsorbents for water treatment and purification are commonly not recyclable because of the lack of a reagent-less "switch" to readily release the adsorbed compounds. Herein, the interface of Bi₂O₂CO₃ (BOC) nanosheets is designed, synthesized, and modified with citric acid, namely, modified Bi₂O₂CO₃ (m-BOC). The m-BOC is able to selectively adsorb methylene blue (MB) in the dark and the adsorbed MB could be released in the light from m-BOC without the addition of any chemicals. The adsorption mechanism is attributed to the electrostatic attraction between positively charged MB and the negatively charged surface of m-BOC. In contrast, the desorption of MB has resulted from the photo-induced charge redistribution on the surface of m-BOC, which unlocks the coordination bond between m-BOC and the carboxylic group. As a result, BOC is recycled. Such a mechanism was verified by both experimental investigation and DFT calculation. This work provides a promising interfacial engineering strategy for the remediation of dye-polluted water and smart separation in chemical engineering.

Nanostructured Carbon Nitrides for CO2 Capture and Conversion

Carbon nitride (CN), a 2D material composed of only carbon (C) and nitrogen (N), which are linked by strong covalent bonds, has been used as a metal-devoid and visible-light-active photocatalyst owing to its magnificent optoelectronic and physicochemical properties including suitable bandgap, adjustable energy-band positions, tailor-made surface functionalities, low cost, metal-free nature, and high thermal, chemical, and mechanical stabilities. CN-based materials possess a lot of advantages over conventional metal-based inorganic photocatalysts including ease of synthesis and processing, versatile functionalization or doping, flexibility for surface engineering, low cost, sustainability, and recyclability without any leaching of toxic metals from photocorrosion. Carbon nitrides and their hybrid materials have emerged as attractive candidates for CO₂ capture and its reduction into clean and green low-carbon fuels and valuable chemical feedstock by using sustainable and intermittent renewable energy sources of sunlight and electricity through the heterogeneous photo(electro)catalysis. Here, the latest research results in this field are summarized, including implementation of novel functionalized nanostructured CNs and their hybrid heterostructures in meeting the stringent requirements to raise the efficiency of the CO₂ reduction process by using state-of-the-art photocatalysis, electrocatalysis, photoelectrocatalysis, and feedstock reactions. The research in this field is primarily focused on advancement in the synthesis of nanostructured and functionalized CN-based hybrid heterostructured materials. More importantly, the recent past has seen a surge in studies focusing significantly on exploring the mechanism of their application perspectives, which include the behavior of the materials for the absorption of light, charge separation, and pathways for the transport of CO₂ during the reduction process.

Post-annealed graphite carbon nitride nanoplates obtained by sugar-assisted exfoliation with improved visible-light photocatalytic performance

Two-dimensional (2D) graphitic carbon nitride (g-C₃N₄) nanoplates (CNNP) have become a hot research topic in photocatalysis due to their small thickness and large specific surface area that favors charge transport and catalytic surface reactions. However, the wide application of 2D g-C₃N₄ nanoplates prepared by ordinary methods suffers from increased band gaps with a poor solar harvesting capability caused by the strong quantum confinement effect and reduced conjugation distance. In this paper, a facile approach of exfoliation and the following fast thermal treatment of the bulk g-C₃N₄ is proposed to obtain a porous few-layered g-C₃N₄ with nitrogen defects. Due to the preferable crystal, textural, optical and electronic structures, the as-obtained porous CNNP demonstrated a significantly improved photocatalytic activity towards water splitting than the bulk g-C₃N₄ and even the 3 nm-thick CNNP obtained by sugar-assisted exfoliation of the bulk g-C₃N₄. The difference in the enhancement factors between the H₂O splitting and organic decomposition has revealed the effect of N defects. This study offers insightful outlooks on the scalable fabrication of a porous few-layered structure with a promoted photocatalytic performance.

A new water-insoluble coordination polymer as efficient dye adsorbent and olefin epoxidation catalyst

A new water-insoluble bi-metallic coordination polymer was simply prepared via polymerization-precipitation of molybdenum complex building blocks with Zn²⁺ cation. The linker was a di-carboxylic acid consisting of two coordination sites i.e. N,O and COO^- suitable for coordinating to MoO₂ unit and Zn²⁺, respectively. Characterization of the prepared coordination polymer was carried out with various physicochemical methods which confirmed the proposed structure. The prepared coordination polymer preferentially adsorbed methylene blue (more than 92% of methylene blue after 2 min) relative to methyl orange and can be reused at least four times without any loss of adsorption efficiency. The adsorption process of both dyes followed the pseudo-second order kinetic equation. Additionally, the obtained coordination polymer catalyzed epoxidation of olefins with tert-butylhydroperoxide (TBHP) quantitatively with excellent selectivity (>99%) under mild reaction conditions.

Water-Soluble Polymeric Carbon Nitride Colloidal Nanoparticles for Highly Selective Quasi-Homogeneous Photocatalysis

Heptazine-based polymeric carbon nitrides (PCN) are promising photocatalysts for light-driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom-up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi-homogeneous conditions. The superior performance of water-soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2 O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4-methoxybenzyl alcohol and benzyl alcohol or lignocellulose-derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re-dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.

Introducing graphitic carbon nitride nanosheets as supersandwich-type assembly on porous electrode for ultrasensitive electrochemiluminescence immunosensing

Biomarkers in blood or tissue provide essential information for clinical screening and early disease diagnosis. However, increasing the sensitivity of detecting biomarkers remains a major challenge in a wide variety of electrochemical immunoassays. Herein, we present an electrochemiluminescence (ECL) immunosensing strategy with 1: Nⁿ amplification ratio (target-to-signal probe) for biomarkers detection on a porous gold electrode. The high porosity of the electrode surface provides enough bonding sites for capturing the target biomolecules and thus many DNA labels can be introduced. On the basis of this concept, a great number of graphitic carbon nitride (g-C₃N₄) nanosheets are employed to create a supersandwich-type assembly on a porous electrode via the DNA hybridization process. Furthermore, compared with the traditional sandwich immunoassay (the ratio of target-to-signal probe is 1 : 1), the supersandwich construction can introduce a large number of signal probes, thus resulting in a highly improved sensitivity. The proposed ECL immunosensor exhibits an excellent performance in a concentration range from 0.01 fg mL^-1 to 1 μg mL^-1 with an ultralow detection limit of 0.001 fg mL^-1 (S/N = 3) and excellent selectivity. This sensing strategy could be developed into a real-time assay for the disease-related molecular targets, with many practical applications in biotechnology and life science.

Amino-Assisted NH2-UiO-66 Anchored on Porous g-C3N4 for Enhanced Visible-Light-Driven CO2 Reduction

Constructing heterostructured photocatalysts is an efficient method to improve photocatalytic carbon dioxide (CO₂) reduction. Herein, holey g-C₃N₄ (HGN) with rich amino groups (-NH_x) was hybridized with NH₂-UiO-66 (NUZ) via a facile in situ growth method. NUZ nanocrystals were anchored on HGN via NHx-Zr-O chemical bonding, leading to the uniform dispersion and avoiding the leaching of NUZ, thus showing excellent stability in photocatalysis. The chemically bonded interfacial charge transfer effect originated from the NHx-Zr-O formation efficiently accelerated the separation and migration of charge carriers, improving the photoactivity. Benefiting from the NHx-Zr-O formation, the optimized NUZ/HGN-35% heterojunctions exhibited outstanding activity in the photoreduction of CO₂ to CO (31.6 μmol g^-1 h^-1), which was about 2 and 3 times higher than that of pure NUZ and HGN under visible-light irradiation. This study is expected to provide useful insights for constructing composites with strong interaction for CO₂ reduction, H₂ production, and N₂ reduction.