Efficient Emission Facilitated by Multiple Energy Level Transitions in Uniform Graphitic Carbon Nitride Films Deposited by Thermal Vapor Condensation

Premise setting for sustainable developing adsorption in environmental remediation using graphitic carbon nitride@agar-derived porous carbon composite

In the adsorption process for wastewater treatment, the adsorbent plays an important role. A composite adsorptive material composed of graphitic carbon nitride and agar-derived porous carbon (CNPC) was fabricated from simple precursors (melamine, thiourea, and agar) and through a facile procedure with different melamine and thiourea ratios. Characterization of CNPC proved a successful formation of a porous structure consisting of mesopores and macropores, wherein CNPC holds distinctive electrochemical (lowered resistance and higher specific capacity) and photochemical properties (lowered bandgap to 2.33 eV) thanks to the combination of graphitic carbon nitride (CN) and agar-derived porous carbon (PC). Inheriting the immanent nature, CNPC was subjected to the adsorption of methylene blue (MB) dye in an aqueous solution. The highest adsorption capacity was 133 mg/g for CNPC-4 which was prepared using a melamine to thiourea ratio of 4:4 - equivalent to the removal rate of 53.2 % and following the pseudo-I-order reaction rate. The effect of pH points out that pH 7 and 9 were susceptible to maximum removal and pretreatment is not required while the optimal ratio of 7.5 mg of MB and 30 mg of material was also determined to yield the highest performance. Furthermore, the reusability of the material for three consecutive cycles was evaluated based on two methods pyrolysis at 200 °C and photocatalytic degradation by irradiation under visible light. In general, the photocatalytic regeneration pathway is more ample and efficient than pyrolysis in terms of energy efficiency (saving energy over 10 times) and adsorption capacity stability. As a whole, the construction of accessible regenerative and stable adsorbent could be a venturing step into the sustainable development spearhead for industries.

TiO2-Based Heterostructure Containing g-C3N4 for an Effective Photocatalytic Treatment of a Textile Dye

Water pollution has become a serious environmental issue. The textile industries using textile dyes are considered to be one of the most polluting of all industrial sectors. The application of solar-light semiconductor catalysts in wastewater treatment, among which TiO2 can be considered a prospective candidate, is limited by rapid recombination of photogenerated charge carriers. To address these limitations, TiO2 was tailored with graphitic carbon nitride (g-C3N4) to develop a heterostructure of g-C3N4@TiO2. Herein, a simple hydrothermal synthesis of TiO2@g-C3N4 is presented, using titanium isopropoxide (TTIP) and urea as precursors. The morphological and optical properties and the structure of g-C3N4, TiO2, and the prepared heterostructure TiO2@g-C3N4 (with different wt.% up to 32%), were analyzed by various laboratory methods. The photocatalytic activity was studied through the degradation of methylene blue (MB) aqueous solution under UV-A and simulated solar irradiation. The results showed that the amount of g-C3N4 and the irradiation source are the most important influences on the efficiency of MB removal by g-C3N4@TiO2. Photocatalytic degradation of MB was also examined in realistic conditions, such as natural sunlight and different aqueous environments. The synthesized g-C3N4@TiO2 nanocomposite showed superior photocatalytic properties in comparison with pure TiO2 and g-C3N4, and is thus a promising new photocatalyst for real-life implementation. The degradation mechanism was investigated using scavengers for electrons, photogenerated holes, and hydroxyl radicals to find the responsible species for MB degradation.

Recent Progress on Photoelectrochemical Water Splitting of Graphitic Carbon Nitride (g−CN) Electrodes

Graphitic carbon nitride (g−CN), a promising visible-light-responsive semiconductor material, is regarded as a fascinating photocatalyst and heterogeneous catalyst for various reactions due to its non-toxicity, high thermal durability and chemical durability, and “earth-abundant” nature. However, practical applications of g−CN in photoelectrochemical (PEC) and photoelectronic devices are still in the early stages of development due to the difficulties in fabricating high-quality g−CN layers on substrates, wide band gaps, high charge-recombination rates, and low electronic conductivity. Various fabrication and modification strategies of g−CN-based films have been reported. This review summarizes the latest progress related to the growth and modification of high-quality g−CN-based films. Furthermore, (1) the classification of synthetic pathways for the preparation of g−CN films, (2) functionalization of g−CN films at an atomic level (elemental doping) and molecular level (copolymerization), (3) modification of g−CN films with a co-catalyst, and (4) composite films fabricating, will be discussed in detail. Last but not least, this review will conclude with a summary and some invigorating viewpoints on the key challenges and future developments.

Collaborative enhancement in charge separation and photon harvesting of 2D/2D heterojunction photocatalyst by horizontal loading of SnS2 nanosheets on g-CN films

Highly efficient photocatalysts based on Van der Waals 2D type of heterojunction were fabricated by horizontal loading of ultrathin hexagonal SnS2 nanosheets on the surface of g-CN film via spin...

Simultaneous layer exfoliation and defect activation in g-C3N4 nanosheets with air-water interfacial plasma: spectroscopic defect probing with tailored optical properties

Defect-activated ultrathin graphitic carbon nitride nanosheets (g-C₃N₄) show an enhanced visible light absorption, better charge-separation, and facile charge transport properties. These are requisites for the designing of an active photocatalyst. Conventional methods used for layer exfoliation and defect activation require strong acids, reducing agents, or ultrasonic treatment for a sufficiently long duration. Furthermore, single-step approaches for layer exfoliation and defect incorporation have hardly been reported. Herein, we have shown atmospheric plasma enabled fabrication of g-C₃N₄ nanosheets. This approach is simple, low-cost, less time-consuming, and a green approach to exfoliate layers and activate multiple defects concurrently. The protocol involves plasma discharging at an air-water interface at 5 kV for 30-150 min. Atomic force microscopy (AFM) reveals a layer thickness of 96.27 nm in bulk g-C₃N₄. The thickness becomes 3.78 nm after 150 min of plasma treatment. The exfoliated layers emerge with nitrogen-vacancy sites and self-incorporated defects as probed by positron annihilation spectroscopy (PAS) and X-ray photoelectron spectroscopy (XPS). The defect activated layers show visible light absorption extended up to 600 nm. It is demonstrated that a non-uniform change in the band gap with the plasma treatment time results from quantum confinement in thin layers and Urbach tailing due to defects acting in opposition. Further, steady-state and time-resolved spectroscopy shows the contribution of multiple defect sites for a prolonged lifetime of photoinduced carriers. These defect-activated ultrathin nanosheets of CN serve as an active photocatalyst in the degradation of rhodamine B (RhB) under white LED illumination.

Visible-Light-Driven Photocatalytic Water Disinfection Toward Escherichia coli by Nanowired g-C3N4 Film

Graphitic carbon nitride (g-C3N4) as metal-free visible light photocatalyst has recently emerged as a promising candidate for water disinfection. Herein, a nanowire-rich superhydrophilic g-C3N4 film was prepared by a vapor-assisted confined deposition method. With a disinfection efficiency of over 99.99% in 4 h under visible light irradiation, this nanowire-rich g-C3N4 film was found to perform better than conventional g-C3N4 film. Control experiments showed that the disinfection performance of the g-C3N4 film reduced significantly after hydrophobic treatment. The potential disinfection mechanism was investigated through scavenger-quenching experiments, which indicate that H2O2 was the main active specie and played an important role in bacteria inactivation. Due to the metal-free composition and excellent performance, photocatalytic disinfection by nanowire-rich g-C3N4 film would be a promising and cost-effective way for safe drinking water production.

Advanced Tri-Layer Carbon Matrices with π-π Stacking Interaction for Binder-Free Lithium-Ion Storage

Enabling materials with distinct features toward achieving high-performance energy storage devices is of huge importance but highly challenging. Commercial carbon cloth (CC), because of its appealing chemical and mechanical properties, has been proven to be an excellent conductive substrate for active electrode materials. However, its performance is notably poor when directly used as an electrode in energy storage, due to its low theoretical capacity and surface area. Herein, we successfully endow the CC with enhanced storage capacity via formation of a π-π stacking interaction by integrating electrochemically activated CC (denoted CC/ACC) with biomass-derived carbon (BMDC) (denoted π-CC/ECC@BMDC). The π-CC/ECC@BMDC electrode displays excellent storage performance with a high capacity of 2.53 mAh cm^-2 under 0.2 mA cm^-2 when used as anode material for lithium ion batteries (LIBs). Due to the induction energy, the negatively charged molecules of the CC/ACC functional groups interact with the BMDC during carbonization, creating the π-π stacking interaction. Based on first-principles calculations, the structural design of the tri-layer carbon enables the movement of electrons around the π-π stacking interaction, which significantly facilitates rapid transportation of electrons, creates three-dimensional (3D) ion tunnels for fast transportation of ions, and improves the electrode's mechanical and electronic properties.

Graphitic Carbon Nitride Films: Emerging Paradigm for Versatile Applications

Graphitic carbon nitride (g-C₃N₄) is a well-known two-dimensional conjugated polymer semiconductor that has been broadly applied in photocatalysis-related fields. However, further developments of g-C₃N₄, especially in device applications, have been constrained by the inherent limitations of its insoluble nature and particulate properties. Recent breakthroughs in fabrication methods of g-C₃N₄ films have led to innovative and inspiring applications in many fields. In this review, we first summarize the fabrication of continuous and thin films, either supported on substrates or as free-standing membranes. Then, the novel properties and application of g-C₃N₄ films are the focus of the current review. Finally, some underlying challenges and the future developments of g-C₃N₄ films are tentatively discussed. This review is expected to provide a comprehensive and timely summary of g-C₃N₄ film research to the wide audience in the field of conjugated polymer semiconductor-based materials.

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.

g-C3N4 synthesized from NH4SCN in a H2 atmosphere as a high performance photocatalyst for blue light-driven degradation of rhodamine B

Graphitic carbon nitride (g-C₃N₄) was prepared by a simple thermal polymerization method in this work. The effects of precursor type, thermal polymerization temperature, constant temperature time and atmosphere on the crystal structure, morphology, elemental composition, valence distribution, light absorption properties and photocatalytic activity of the prepared photocatalytic materials were investigated. Taking rhodamine B (RhB) as the target degradant, the blue light catalytic activity of the photocatalytic material was studied in detail. The experimental results showed that the final pyrolysis temperature and constant temperature time are positively related to the adsorption characteristics and photocatalytic ability of the prepared materials. In addition, the adsorption capacity and photocatalytic activity of the products obtained in Ar and H₂ atmospheres are better than those produced in CO and CH₄, which can be attributed to the combined effect of large specific surface area and structural defects of the materials. The sample's large specific surface area, wide band gap, and excellent photogenerated carrier separation and transfer capabilities make the adsorption performance and photocatalytic performance of the products obtained with ammonium thiocyanate and thiourea as precursors better than those prepared from melamine and dicyandiamide. g-C₃N₄ prepared by using ammonium thiocyanate as precursor at 550 °C for 5 h under a hydrogen atmosphere showed the best catalytic activity for the degradation of RhB under blue light. It was demonstrated that g-C₃N₄ prepared exhibited good stability and reusability after four repeat experiments. The active components that play major roles in the degradation of RhB described herein were holes and superoxide radicals, which was inferred by free radical trapping experiments. This work provides a theoretical basis for the idea of converting the mixed salts of desulfurization waste liquid containing ammonium thiocyanate into an excellent photocatalyst g-C₃N₄ with visible light response.

Shine Bright Like a Diamond: New Light on an Old Polymeric Semiconductor

Brilliance usually refers to the light reflected by the facets of a gemstone such as diamond due to its high refractive index. Nowadays, high-refractive-index materials find application in many optical and photonic devices and are mostly of inorganic nature. However, these materials are usually obtained by toxic or expensive production processes. Herein, the synthesis of a thin-film organic semiconductor, namely, polymeric carbon nitride, by thermal chemical vapor deposition is presented. Among polymers, this organic material combines the highest intrinsic refractive index reported so far with high transparency in the visible spectrum, even reaching the range of diamond. Eventually, the herein presented deposition of high quality thin films and their optical characteristics open the way for numerous new applications and devices in optics, photonics, and beyond based on organic materials.

Facile assembly of a graphitic carbon nitride film at an air/water interface for photoelectrochemical NADH regeneration

A graphitic carbon nitride film electrode could be assembled at an air/water interface from nanosheets which exhibits improved photoelectrochemical coenzyme regeneration by further coupling with graphene during the interfacial self-assembly.

Grafting Polymers onto Carbon Nitride via Visible-Light-Induced Photofunctionalization

Metal-free graphitic carbon nitride (g-CN) has attracted significant attention recently due to its multiple applications, such as photocatalysis, energy storage and conversion, and biomaterials, albeit formation of g-CN films is challenging. Herein, a “grafting to” route to graft polymer brushes onto g-CN via visible-light irradiation is described. Afterward, g-CN/polymer films can be obtained through spin coating on glass substrates. As such, the present material provides an improved process toward further application of g-CN in thin films. Moreover, an improved dispersibility in organic solvent was realized after grafting and functional groups (such as epoxides) were introduced to g-CN. Subsequently, the epoxy groups were utilized for further functionalization to adjust the surface polarity.

Carbon Nitride Materials for Water Splitting Photoelectrochemical Cells

Graphitic carbon nitride materials (CNs) have emerged as suitable photocatalysts and heterogeneous catalysts for various reactions thanks to their tunable band gap, suitable energy-band position, high stability under harsh chemical conditions, and low cost. However, the utilization of CN in photoelectrochemical (PEC) and photoelectronic devices is still at an early stage owing to the difficulties in depositing high-quality and homogenous CN layer on substrates, its wide band gap, poor charge-separation efficiency, and low electronic conductivity. In this Minireview, we discuss the synthetic pathways for the preparation of various structures of CN on substrates and their underlying photophysical properties and current photoelectrochemical performance. The main challenges for CN incorporation into PEC cell are described, together with possible routes to overcome the standing limitations toward the integration of CN materials in PEC and other photoelectronic devices.

In situ growth of triazine–heptazine based carbon nitride film for efficient (photo)electrochemical performance

Carbon nitride polymer film with a triazine–heptazine network on FTO as a bifunctional electrode shows boosted (photo)electrochemical performance for water splitting.

Mechanism of Charge Separation and Frontier Orbital Structure in Graphitic Carbon Nitride and Graphene Quantum Dots

Graphene quantum dots (GQDs) and carbon nitride quantum dots (CNQDs), the latest addition to the carbon material family, are promising materials for numerous novel applications in optical sensing, photocatalysis, biosensing, and photovoltaics. However, understanding the photocatalytic capability of CNQDs compared to GQDs requires investigations of the charge behavior on the excited state energy surface. In this work, through time-dependent density functional tight binding (TD-DFTB) calculations, we show that CNQDs exhibit superior ground state frontier orbitals (FOs) localization. Strong localization of the FOs and excited state charge separation observed in the first excited state are caused by the relaxation of the structure. Excited energy surface investigations reveal spatial confinement of FOs to the stretched C-N bonds due to excited state structural relaxation. On the other hand, no such localized FOs structure was found for GQDs, presumably caused by its strong π-conjugated configuration not allowing large structural changes upon excitation. The optical absorption and emission of CNQDs is sensitive to size and does not show large variations with the shape of the QD. Our approach provides an explanation for the origin of the enhanced photocatalytic performance of CNQDs over GQDs and their characteristic FOs localization.

Malic Acid Carbon Dots: From Super-resolution Live-Cell Imaging to Highly Efficient Separation

As-synthesized malic acid carbon dots are found to possess photoblinking properties that are outstanding and superior compared to those of conventional dyes. Considering their excellent biocompatibility, malic acid carbon dots are suitable for super-resolution fluorescence localization microscopy under a variety of conditions, as we demonstrate in fixed and live trout gill epithelial cells. In addition, during imaging experiments, the so-called "excitation wavelength-dependent" emission was not observed for individual as-made malic acid carbon dots, which motivated us to develop a time-saving and high-throughput separation technique to isolate malic acid carbon dots into fractions of different particle size distributions using C₁₈ reversed-phase silica gel column chromatography. This post-treatment allowed us to determine how particle size distribution influences the optical properties of malic acid carbon dot fractions, that is, optical band gap energies and photoluminescence behaviors.

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

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

In situ decoration of plasmonic Au nanoparticles on graphene quantum dots-graphitic carbon nitride hybrid and evaluation of its visible light photocatalytic performance

This work spotlights the development of a plasmonic photocatalyst showing surface plasmon induced enhanced visible light photocatalytic (PC) performance. Plasmonic Au nanoparticles (NPs) are decorated over the hybrid nanosystem of graphitic carbon nitride (GCN) and graphene quantum dots (GQD) by citrate reduction method. Surface plasmon resonance (SPR) induced enhancement of Raman G and 2D band intensity is encountered on excitation of the Plasmonic hybrid at 514.5 nm, which is near to the 532 nm absorption band of Au NPs. Time-resolved photoluminescence and XPS studies show charge transfer interaction between GQD-GCN and Au NPs. Plasmonic hybrid exhibits an enhanced PC activity over the other catalysts in the photodegradation of methylene blue (MB) under visible light illumination. Plasmonic photocatalyst displays more than 6 fold enhancement in the photodecomposition rate of MB over GQD and nearly 2 fold improvement over GCN and GQD-GCN. GQD-GCN absorbs mostly in the near visible region and can be photoexcited by visible light of wavelength ([Formula: see text]) < 460 nm. Plasmon activation in Au NPs decorated GQD-GCN could exploit the entire UV-visible light for photocatalysis. Furthermore, plasmonic Au act as antennas for accumulation and enhancement of localized electromagnetic field at the interface with GQD-GCN, and thereby promotes photogeneration of large numbers of carriers on GQD-GCN. The carriers are separated by charge transfer migration from hybrid to Au NPs. Finally, the carriers on the plasmonic Au nanostructures initiate MB degradation under visible light. Our results have shown that plasmon decoration is a suitable strategy to design a carbon based hybrid photocatalyst for solar energy conversion.

Anchoring Tailored Low-Index Faceted BiOBr Nanoplates onto TiO2 Nanorods to Enhance the Stability and Visible-Light-Driven Catalytic Activity

In this work, a fantastic one-dimensional (1D) BiOBr/TiO₂ nanorod (NR) heterojunction composite was rationally proposed and designed from the perspective of molecular and interface engineering. The fabricated intimately connected interfacial heterojunction between two-dimensional BiOBr nanoplates and 1D TiO₂ NRs acts as an interfacial nanochannel to promote efficient interfacial charge migration and separation of photogenerated electron-hole pairs. As a result, 1D BiOBr/TiO₂ NR heterojunctions exhibited outstanding visible-light photocatalytic activities and sustained cycling performance. Under visible-light irradiation for 120 min, the reduction efficiency of Cr(VI) over the TB-2 sample (molar ratio: n(Ti)/n(Bi) = 2:1) is as high as 95.4% without adding any scavengers. Furthermore, the sample also shows excellent photodegradation activity of RhB with a much higher apparent rate constant of 0.49 min^-1 and 88.5% total organic carbon removal ratio. Furthermore, the corresponding mechanism of enhanced photocatalytic activity is proposed according to comprehensively investigated results from photoluminescence spectroscopy, photoelectrochemical measurement analysis, and radical trapping experiments. This study provides an attractive avenue to design and fabricate highly efficient 1D NR heterojunction photocatalysts, which possessed a high application value in the field of environmental remediation, especially for wastewater purification.

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

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

Graphitic Carbon Nitride Film: An Emerging Star for Catalytic and Optoelectronic Applications

Graphitic carbon nitride (g-CN) is a unique organic semiconductor that has been widely applied as a visible-light-driven photocatalyst. However, these applications are primarily based on g-CN powders. Applications of g-CN in devices are hindered because of difficulties associated with the synthesis of high-quality g-CN films. This work reviews the latest advances in g-CN films. The deposition methods are summarized and the structural, optical, and electronic properties of g-CN films and their applications in catalysis, solar cells, and light-emitting diodes are outlined. Moreover, the challenges remaining in this field are also discussed.

Combinatorial Vibration-Mode Assignment for the FTIR Spectrum of Crystalline Melamine: A Strategic Approach toward Theoretical IR Vibrational Calculations of Triazine-Based Compounds

Although polymeric graphitic carbon nitride (g-C3N4) has been widely studied as metal-free photocatalyst, the description of its structure still remains a great challenge. Fourier transform infrared (FTIR) spectroscopy can provide complementary structural information. In this paper, we reconsider the representative crystalline melamine and develop a strategic approach to theoretically calculate the IR vibrations of this triazine-based nitrogen-rich system. IR calculations were based on three different models: a single molecule, a 4-molecule unit cell, and a 32-molecule cluster, respectively. By this comparative study the contribution of the intermolecular weak interactions were elucidated in detail. An accurate and visualized description on the experimental FTIR spectrum has been further presented by a combinatorial vibration-mode assignment based on the calculated potential energy distribution of the 32-molecule cluster. The theoretical approach reported in this study opens the way to the facile and accurate assignment for IR vibrational modes of other complex triazine-based compounds, such as g-C3N4.

In situ growth of graphitic carbon nitride films on transparent conducting substrates via a solvothermal route for photoelectrochemical performance

Graphitic carbon nitride films werein situgrownviasolvothermal route with significantly improved photoelectrochemical performance.

Ultra-thin coating of g-C3N4 on an aligned ZnO nanorod film for rapid charge separation and improved photodegradation performance

Aligned 1D ZnO/g-C₃N₄ films were fabricated by simple refluxing with thermal vapor condensation for rapid charge separation and recyclable test.

Isotype heterostructure of bulk and nanosheets of graphitic carbon nitride for efficient visible light photodegradation of methylene blue

Isotype heterostructure of bulk and nanosheets of graphitic carbon nitride with effective band gap of 2.62 eV and charge carrier mean lifetime of 21 ns exhibits an efficient visible light photocatalysis.