Recent Progress in Polymorphs of Carbon Nitride: Synthesis, Properties, and Their Applications

Effective photocatalytic chromium (VI) detoxification with metal-free heterojunction based on g-C3N5 and ZIF-8 carbon under visible light

A metal-free heterojunction (CNZ8C) photocatalyst prepared using g-C3N5 (CN) and zeolitic imidazolate framework (ZIF)-derived carbon (Z8C) is reported for sequential reduction and elimination of toxic Cr (VI) from water. Recombination of photogenerated charge carriers in CN was evicted completely through heterojunction formation of CN with Z8C. This is evidenced from the transient photocurrent responses (TPR), photoluminescence (PL) measurements and electrochemical impedance spectroscopy (EIS). The band structure analysis from the valence band X-ray spectroscopy and diffused reflectance spectroscopy revealed that the synergy in the synthesized CN-Z8C heterojunction is responsible for the reduction of CN band gap. The experimental results obtained confirm that the positions of the valence and conduction bands are aligned to more negative values facilitating the Cr (VI) reduction. Incorporation of the highly conducting porous CZ8 to CN improves the charge separation and also brings the Cr (VI) ions close to CZ8 surface via adsorption, which together favors the overall reduction and removal of reduced ions. The rate of reduction/removal significantly increased with CNZ8C 5:5 (with nearly 100 % removal efficiency) heterojunction compared to that of CN. This heterojunction can be reutilized up to 5 cycles without any significant loss in efficiency. Mechanistic studies revealed that proper bandgap engineering leads to increased reduction and removal of chromium. The post treatment X-ray photoelectrons spectroscopy of the heterojunction confirms the +3 state of chromium. These observations suggest that the successful synergy between the CN and Z8C will bring new insights into chromium remediation.

Carbon- and Nitrogen-Based Complexes as Photocatalysts for Prebiotic and Oxygen Chemistry during Earth Evolution

Sunlight has long served as primary energy source on our planet, shaping the behavior of living organisms. Extensive research has been dedicated to unraveling the evolutionary pathways involved. When the formation of Earth atmosphere, it primarily consisted of small gas molecules, which are considered crucial for the emergence of life. Recent demonstrations have shown that these molecules can also be transformed into semiconductors, with the potential to harness solar energy and catalyze chemical reactions as photocatalysts. Building upon this research, this minireview focuses on the potential revolutionary impact of photocatalysis on Earth. Initially, it examines key reactions, such as the formation of prebiotic molecules and the oxygen evolution reaction via water oxidation. Additionally, various C-N complexes in photocatalysts are explored, showcasing their roles in catalyzing chemical reactions. The conclusion and outlook provide a potential pathway for the evolution of Earth, shedding light on the significance of metal-free photocatalysts in development of Earth.

Bio-Inspired Supramolecular Self-Assembled Carbon Nitride Nanostructures for Photocatalytic Water Splitting

Fast production of hydrogen and oxygen in large amounts at an economic rate is the need of the hour to cater to the needs of the most awaited hydrogen energy, a futuristic renewable energy solution. Production of hydrogen through simple water splitting via visible light photocatalytic approach using sunlight is considered as one of the most promising and sustainable approaches for generating clean fuels. For this purpose, a variety of catalytic techniques and novel catalysts have been investigated. Among these catalysts, carbon nitride is presently deemed as one of the best candidates for the visible light photocatalysis due to its unique molecular structure and adequate visible-range bandgap. Its bandgap can be further engineered by structural and morphological manipulation or by doping/hybridization. Among numerous synthetic approaches for carbon nitrides, supramolecular self-assembly is one of the recently developed elegant bottom-up strategies as it is bio-inspired and provides a facile and eco-friendly route to synthesize high surface area carbon nitride with superior morphological features and other semiconducting and catalytic properties. The current review article broadly covers supramolecular self-assembly synthesis of carbon nitride nanostructures and their photocatalytic water-splitting applications and provides a comprehensive outlook on future directions.

Synthesis of two-dimensional triazine covalent organic frameworks at ambient conditions to detect and remove water pollutants

Synthesis of fully triazine frameworks (C3N3) by metal catalyzed reactions at high temperatures results in carbonized and less-defined structures. Moreover, metal impurities affect the physicochemical, optical and electrical properties of the synthesized frameworks, dramatically. In this work, two-dimensional C3N3 (2DC3N3) has been synthesized by in situ catalyst-free copolymerization of sodium cyanide and cyanuric chloride, as cheap and commercially available precursors, at ambient conditions on gram scale. Reaction between sodium cyanide and cyanuric chloride resulted in electron-poor polyfunctional intermediates, which converted to 2DC3N3 with several hundred micrometers lateral size at ambient conditions upon [2 + 2+2] cyclotrimerization. 2DC3N3 sheets, in bulk and individually, showed strong fluorescence with 63% quantum yield and sensitive to small objects such as dyes and metal ions. The sensitivity of 2DC3N3 emission to foreign objects was used to detect low concentration of water impurities. Due to the high negative surface charge (-37.7 mV) and dispersion in aqueous solutions, they demonstrated a high potential to remove positively charged dyes from water, exemplified by excellent removal efficiency (>99%) for methylene blue. Taking advantage of the straightforward production and strong interactions with dyes and metal ions, 2DC3N3 was integrated in filters and used for the fast detection and efficient removal of water impurities.

Multifunctional carbon nitride nanoarchitectures for catalysis

Catalysis is at the heart of modern-day chemical and pharmaceutical industries, and there is an urgent demand to develop metal-free, high surface area, and efficient catalysts in a scalable, reproducible and economic manner. Amongst the ever-expanding two-dimensional materials family, carbon nitride (CN) has emerged as the most researched material for catalytic applications due to its unique molecular structure with tunable visible range band gap, surface defects, basic sites, and nitrogen functionalities. These properties also endow it with anchoring capability with a large number of catalytically active sites and provide opportunities for doping, hybridization, sensitization, etc. To make considerable progress in the use of CN as a highly effective catalyst for various applications, it is critical to have an in-depth understanding of its synthesis, structure and surface sites. The present review provides an overview of the recent advances in synthetic approaches of CN, its physicochemical properties, and band gap engineering, with a focus on its exclusive usage in a variety of catalytic reactions, including hydrogen evolution reactions, overall water splitting, water oxidation, CO2 reduction, nitrogen reduction reactions, pollutant degradation, and organocatalysis. While the structural design and band gap engineering of catalysts are elaborated, the surface chemistry is dealt with in detail to demonstrate efficient catalytic performances. Burning challenges in catalytic design and future outlook are elucidated.

Carbon-Based Electrochemical (Bio)sensors for the Detection of Carbendazim: A Review

Carbendazim, a fungicide widely used in agriculture, has been classified as a hazardous chemical by the World Health Organization due to its environmental persistence. It is prohibited in several countries; therefore, detecting it in food and environmental samples is highly necessary. A reliable, rapid, and low-cost method uses electrochemical sensors and biosensors, especially those modified with carbon-based materials with good analytical performance. In this review, we summarize the use of carbon-based electrochemical (bio)sensors for detecting carbendazim in environmental and food matrixes, with a particular interest in the role of carbon materials. Focus on publications between 2018 and 2023 that have been describing the use of carbon nanotubes, carbon nitride, graphene, and its derivatives, and carbon-based materials as modifiers, emphasizing the analytical performance obtained, such as linear range, detection limit, selectivity, and the matrix where the detection was applied.

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 Air Purification Using Functional Polymeric Carbon Nitrides

The techniques for the production of the environment have received attention because of the increasing air pollution, which results in a negative impact on the living environment of mankind. Over the decades, burgeoning interest in polymeric carbon nitride (PCN) based photocatalysts for heterogeneous catalysis of air pollutants has been witnessed, which is improved by harvesting visible light, layered/defective structures, functional groups, suitable/adjustable band positions, and existing Lewis basic sites. PCN-based photocatalytic air purification can reduce the negative impacts of the emission of air pollutants and convert the undesirable and harmful materials into value-added or nontoxic, or low-toxic chemicals. However, based on previous reports, the systematic summary and analysis of PCN-based photocatalysts in the catalytic elimination of air pollutants have not been reported. The research progress of functional PCN-based composite materials as photocatalysts for the removal of air pollutants is reviewed here. The working mechanisms of each enhancement modification are elucidated and discussed on structures (nanostructure, molecular structue, and composite) regarding their effects on light-absorption/utilization, reactant adsorption, intermediate/product desorption, charge kinetics, and reactive oxygen species production. Perspectives related to further challenges and directions as well as design strategies of PCN-based photocatalysts in the heterogeneous catalysis of air pollutants are also provided.

Ab-Initio Spectroscopic Characterization of Melem-Based Graphitic Carbon Nitride Polymorphs

Polymeric graphitic carbon nitride (gCN) compounds are promising materials in photoactivated electrocatalysis thanks to their peculiar structure of periodically spaced voids exposing reactive pyridinic N atoms. These are excellent sites for the adsorption of isolated transition metal atoms or small clusters that can highly enhance the catalytic properties. However, several polymorphs of gCN can be obtained during synthesis, differing for their structural and electronic properties that ultimately drive their potential as catalysts. The accurate characterization of the obtained material is critical for the correct rationalization of the catalytic results; however, an unambiguous experimental identification of the actual polymer is challenging, especially without any reference spectroscopic features for the assignment. In this work, we optimized several models of melem-based gCN, taking into account different degrees of polymerization and arrangement of the monomers, and we present a thorough computational characterization of their simulated XRD, XPS, and NEXAFS spectroscopic properties, based on state-of-the-art density functional theory calculations. Through this detailed study, we could identify the peculiar fingerprints of each model and correlate them with its structural and/or electronic properties. Theoretical predictions were compared with the experimental data whenever they were available.