Bulk Carbon Nitride Research Articles

Defect‐Induced Activity Enhancement of Self‐Exfoliated Carbon Nitrides for Solar Hydrogen Evolution

AbstractThe recombination of photogenerated electron‐hole pairs severely limits the efficiency of solar‐driven hydrogen evolution in carbon nitrides (CN) system. Herein, a K+ doped and cyanamide defects modified CN (CCNK) with free‐standing nanosheets in water was synthesized by a bottom‐up self‐exfoliated strategy. Further experimental characterizations reveal that the cyanamide moiety and K+ in the framework play an important role in the enhancement of hydrophilicity and efficiently suppress electron‐hole pair recombination. CCNK exhibits an impressive photocatalytic hydrogen evolution activity (132.6 μmol h−1), which is 22 times higher than bulk CN (6.0 μmol h−1), and far surpassing other CN‐based photocatalysts. This work provides an innovative facile self‐exfoliated strategy for bottom‐up preparation of ultrathin CN nanosheets without using any acid, base, or solvents, paving the way for the development of high‐performance photocatalysts using CN‐based powders.

In situ loading amorphous CoSx on N-doped g-C3N4 through light assisted synthesis for enhanced photocatalytic hydrogen generation

Loading co-catalysts are an effective strategy to break the confinement of bulk carbon nitride in photocatalysis. Employing this strategy, N-doped g-C3N4 decorated with CoSx was successfully prepared through a photochemical synthesis route. The optimum hydrogen evolution performance of N-CN-CoSx-4 was 1757 μmol g−1 h−1 under visible light irradiation. Superior interfacial carrier transfer properties and improved light absorption of N-CN-CoSx-4 could elucidate its better photocatalytic activity. This research offers a reference for the construction of high-efficiency, stable and low-priced photocatalysts.

Facilitating carrier separation of hierarchical carbon nitride by a nucleation processable strategy in photocatalytic H2 evolution

The adjustment of hierarchical micro-nanostructure and morphology has been identified as an effective strategy to optimize photocatalytic performance of carbon nitrides (CNs). Nevertheless, environmental and additive-free construction of CNs with hierarchical structures is still challenging. Herein, a nucleation processable strategy is proposed to construct hierarchical CNs with unique morphologies (planar, donuts-like, flower-like), free of harmful additions. In this strategy, nucleation process and crystal growth degree of supramolecular precursors (cyanuric acid-melamine aggregates) can be controlled via tuning the initial supersaturation in the crystallization. Moreover, the formation mechanism of unique morphologies is illustrated in this article, based on the understanding of nucleation and crystal growth. Among the as-prepared CNs, owing to its hierarchical architecture and unique morphology, the flower-like CN0.25 display the optimal charge separation and excellent photocatalytic hydrogen evolution performance of 1880 μmol h−1 g−1, which is 27 times higher than that of the bulk CN. Additionally, the apparent quantum yield of CN0.25 is achieved 6.7% at 420 nm. The experimental results demonstrate that the nucleation processable strategy developed herein may provide a new pathway in the design of hierarchically structured CNs with desirable photocatalytic performance.

First principles study on thermal conductivity of nitrogen substituted diamane

2D materials have been in the spotlight due to their impressive properties since graphene was successfully synthesized. A single-layered diamond or diamane, which has high thermal conductivity and high stiffness inherited from its bulk counterpart, is a candidate for heat dissipating applications in nanodevices. Unfortunately, non-passivated dangling bonds on the surface of pristine diamane can cause its structural instability. Recent theoretical studies have suggested that site-specific substitution of N for C could stabilize the diamane’s structure without surface termination. Beside its superhard properties inherited from bulk carbon nitrides, thermal properties of N-substituted diamane have not been revealed. In this work, we investigate thermal properties of N-substituted diamane through first principles study. Our results show that there is an emergence of flexural phonon branch, which is regularly seen in 2D materials, and it contributes large thermal conductivity to N-substituted diamane. The thermal conductivity of N-substituted diamane is decreasing as the temperature increasing. At room temperature, it is 33% lower than that of diamond and 15-35% higher than that of hydrogen passivated diamane (H-diamane).

Ionic liquid assisted construction of B and P double doped porous g-C3N4 nanosheets with significantly enhanced photocatalytic H2 production

N-type B and P double doped porous g-C3N4 (CN–B–P) nanosheets is successfully prepared by using melamine and cyanuric acid as precursors, [Emin]BF4 and [Bmim]PF6 as B and P sources. The reason for obtaining a porous structure is that the raw material is decomposed to produce gas molecular etching materials such as CO2 and NH3 in the process of pyrosynthesis. The formed porous structure provides largest specific surface area (BET), which reach 137.49 m2g−1. A largest BET and porous morphology are conducive to providing more reactive sites. It short the electron transfer path and accelerate the electron transfer efficiency. Under the influence of B and P doping and porous structure, the H2 production of CN–45B–45P reach 9443.38 ​μmol/g/h, which is 12.34 times more than bulk carbon nitride CN (765.17 ​μmol/g/h), and is better than best sample (N–CN–45B–45P) without platinum as a cocatalyst (1187.34 ​μmol/g/h). In addition, the cyclic and continuous hydrogen production test results shows that the sample has excellent stability and durability.

Fabricating metal-free Z-scheme heterostructures with nitrogen-deficient carbon nitride for fast photocatalytic removal of acetaminophen

Photocatalytic technology has been increasing explored for the elimination of pollutants from aquatic environments. However, constructing powerful photocatalyst to drive rapid decontamination remains challenging. Here, we assemble boron-doped nitrogen-deficient carbon nitride (BNCN) into metal-free Z-scheme heterojunction via a facile approach to boost photocatalytic degradation of acetaminophen. Incorporating boron dopants and nitrogen defects to C3N4 significantly increase the photoabsorption. Furthermore, the constructed heterojunction exhibited decreased band gap and remarkably enhanced separation efficiency of photogenerated carriers compared with individual BNCN. Owing to the modulation of band structures and the better separation of photogenerated carriers, acetaminophen conversion approaching 100 % was achieved in just 30 mins under simulated sunlight, outperforming equivalent quantities of bulk carbon nitride and commercial TiO2 by 3.1 and 2.7 times, respectively. Two possible photodegradation pathways were proposed with hydroxylation and ring-opening reactions as dominant degradation reactions. Our findings provide an instructive catalyst design strategy to boost photocatalytic degradation efficiency.

Controllable adsorption groups on amine-functionalized carbon nitride for enhanced photocatalytic CO2 reduction

CO2 photoreduction by carbon nitride (CN) is an appealing idea, but its current efficiency remains daunting for practical viability. The amine-functionalized CN with controllable adsorption groups was prepared by carboxylation and acylation reactions. The carboxylation and amine-functionalization process preserved the crystalline structure of CN but exfoliated bulk CN to thin layers avoiding its agglomeration. With the successful graft of EDA, the amounts of –NH2 groups for 24-CN-EDA increased remarkably compared with bulk CN. EDA-functionalization benefited CO2 reduction activity of CN with varying amounts of amino groups. The CO yield of 24-CN-EDA was 5.92 folds to bulk CN. The enhancement was mainly triggered by the enlarged CO2 adsorption capacity and reduced charge transfer resistance, rather than band structure and light absorption. Moreover, DFT calculations suggest that the graft of EDA on CN nanosheets possesses reduced energy barrier for CO2 reduction into CO via enhancing CO* desorption and CO formation.

Highly crystalline carbon nitride with small-sized sea urchin-like structure for efficient photocatalytic hydrogen production under visible-light irradiation

Molten-salt method has been deemed as a feasible strategy to increase the crystallinity of graphitic carbon nitride (abbreviated as CN), thereby improving its photocatalytic activity. However, highly crystalline CN prepared by molten-salt method using bulk carbon nitride (BCN) as the precursor still suffers from a small specific surface area, which is unfavorable to the separation of photogenerated carriers. Here, we report a facile approach to obtain highly crystalline CN with a large specific surface area using tubular carbon nitride (TCN) as raw material, and the final sample is named as tubular carbon nitride-molten salt (TCN-M). Compared with bulk carbon nitride-molten salt (BCN-M) sample, the obtained TCN-M sample not only retains the advantage of high crystallinity, but also exhibits a smaller-sized sea urchin-like structure owing to the CN precursor pretreatment process. The experiment results demonstrate that TCN-M displays an excellent hydrogen production activity of 4.9 mmol g -1 h -1 under visible-light, and its hydrogen production performance can be further enhanced to 14.7 mmol g -1 h -1 after adding 0.5 M K 2 HPO 4 . Thus, this work may supply valuable ideas to further optimize the photocatalytic activity of highly crystalline CN prepared by molten-salt method from the perspective of CN precursor pretreatment. • A highly crystalline carbon nitride with small-sized sea urchin-like structure was successfully prepared. • Large specific surface area and the presence of cyano groups greatly improve the photocatalytic H 2 production activity. • Optimizing the photocatalytic activity of highly crystalline CN from the perspective of CN precursor pretreatment. • A typical H + -reduction cycle can be constructed by introducing K 2 HPO 4 into the reaction system.

2D/3D- C3N4/CeO2 S-scheme heterojunctions with enhanced photocatalytic performance

• Successfully developed a novel method to obtain g-C 3 N 4 /CeO 2 composite photo-catalysts via simple, ball milled thermal spreading technique. • Facile control of surface morphology of carbon nitride on ceria. The optical absorption is tuned by morphology control. • The optimum loading was found to be 7.5 wt. %. • Morphology with an intermediate thickness between very thin and bulk carbon nitride on ceria surface is important to achieve maximum efficiency. • High efficiency of methylene blue degradation and good recycling performance. A facile, green and easily scalable method has been developed to facilitate the construction of 2D-C 3 N 4 /3D-CeO 2 heterojunction materials. It uses only cheap urea and ceria in a thermal spreading technique that allows the controlled growth of carbon nitride layers on cerium oxide surface. The exfoliation of bulk carbon nitride and subsequent stabilization of layers on ceria takes place during thermal spreading technique in a single step. Simple variation of carbon nitride allowed to control the morphology of carbon nitride from highly dispersed thin layer up to bulk-like structures over ceria surface. The optical absorption was enhanced with 2D-C 3 N 4 /3D-CeO 2 materials. The variation of structure and consequent differences in the solar powered decomposition of methylene blue has been analyzed. Results reveal that, the optimum loading of 7.5 wt.% carbon nitride can lead to the formation of intermediate structure in between very thin layers and bulk-like structure, that achieves best contact between the two phases making effective heterojuctions. This best catalyst exhibits 98% conversion at a timing of 100 minutes. The radical scavenging studies showed that •O 2− and h+ performed an imperative role. In addition, a plausible mechanism of photo catalytic decomposition is also proposed for 2D-C 3 N 4 /3D-CeO 2 materials.

Application of carbon nitride nanosheets for adsorption of various humic substances from aqueous solutions

Humic substances (HSs) cause serious problems such as membrane fouling and disinfection by-products. Particularly, HSs as the main UV-quenching substances in landfill leachate pose great challenges. In this study, we evaluated the application of carbon nitride nanosheets (CN-Ns) for adsorption of various HSs, as well as leachate nanofiltration concentrates (LNCs). The characterization revealed that as increasing thermal oxidation temperature, the properties of CN-Ns were affected, such as morphology, porosity, and crystallinity. The surface area of CN-N at 575 °C (CN-N-575) was 145.3 m2·g−1, resulting in an enhanced adsorption performance. The adsorption capacity of CN-N-575 for lignite humic acid (LHA) and soil fulvic acid (SFA) was considerably greater than that of bulk CN; the maximum adsorption amounts of LHA and SFA were 143.11 and 63.27 mg C·g−1, respectively. Upon adsorption, the removal rate of LHA and Elliott soil humic acid was about 90 %. Among HSs and LNCs, the humic-like components showed superior affinity; in some cases, the removal rate reached 100 %. The adsorption preference of aromatic compounds in HSs and LNCs was ascribed to π-electron interactions. The adsorption of LHA was more pH-dependent than that of SFA, attributed to the interactions (electrostatic, hydrogen bonding, and complexation) along with a specific conformation effect. The findings proved the great potential of CN-Ns in HSs adsorption, especially in the treatment of landfill leachate.

Defected mesoporous carbon nitride with quantum confinement effect for NO purification

Combining mesoporous structure and defect engineering is an effective way to optimize the photocatalytic activity of graphitic carbon nitride. Herein, a unique N-deficient mesoporous carbon nitride (nmpg-C3N4), constructed by silica-template method, possesses efficient and stable photocatalytic purification of NO, far exceeding the bulk carbon nitride (B-C3N4). In nmpg-C3N4, the mesoporous structure not only exposes more active sites, but also the resulting quantum confinement effect widens the band gap of nmpg-C3N4 and enhances its redox ability. While N defects constitute catalytic active sites and induces effective separation of photogenerated carriers. Under the synergistic effect of mesoporous structure and defect engineering, nmpg-C3N4 produces more •O2− and •OH to remove NO in the air. Its photocatalytic activity reaches 60.61% higher than B-C3N4 (35.51%) and exposes better stability. Moreover, a reasonable NO purification path was proposed using in-situ DRIFTS technology. Our research proves that utilizing the quantum confinement effect of mesoporous materials combined with defect engineering can effectively extend the energy band and make the catalyst have more active sites. This study provides new enlightenment for the modification of carbon nitride.

Directing Quasiparticle Movement in Graphitic Carbon Nitride through Spatial Engineering for Enhanced Photocatalytic Hydrogen Evolution

The inherently low dielectric properties of polymeric semiconductors lead to high exciton binding energy, impeding the photogenerated electron and hole separation. The aim of this work is to regulate the spacing of charge through spatial engineering in graphitic carbon nitride (CN), which performs photocatalysis through dual routes, where ruthenium phosphide (RuxP) and hydroxide ions (OH–) serve as the electron acceptor and hole extractor, respectively. The unique heterojunction of RuxP incorporated in the bulk CN (B-RuxP-CN) shows a lower exciton binding energy (61 meV) than bare CN and CN surface-deposited RuxP (S-RuxP-CN). This favors high carrier density and rapid escape of active electrons from bound excitons. The photocatalytic hydrogen (H2) evolution rate of B-RuxP-CN increases with increasing pH. However, S-RuxP-CN maintains an almost invariable H2 production rate even with similar increases in pH. We reasonably ascribe the different H2 evolution performances of both photocatalysts to their contrasting structures and discrepancy in quasiparticle relaxation dynamics. The wrapped structure of B-RuxP-CN endows it with a prolonged charge separation lifetime (189 ns) and an enhanced H2 evolution rate (32.0 μmol/h) in an alkaline scavenger solution. This work provides a controllable procedure for quasiparticles’ directional movement in polymeric photocatalysts.

Facilitating Molecular Activation and Proton Feeding by Dual Active Sites on Polymeric Carbon Nitride for Efficient CO2Photoreduction

AbstractPhotoreduction of CO2provides an appealing way to alleviate the energy crisis and manage the global carbon balance but is limited by the high activation energy and the rate‐limiting proton transfer. We now develop a dual‐site strategy for high‐efficiency CO2conversion through polarizing CO2molecules at pyridine N vacancies and accelerating the intermediate protonation by protonated pyridine N adjacent to nitrogen vacancies on polymeric carbon nitride. Our photocatalysts with atomic‐level engineered active sites manifest a high CO production rate of 1835 μmol g−1 h−1, 183 times higher than the pristine bulk carbon nitride. Theoretical prediction and experimental studies confirm that such excellent performance is attributed to the synergistic effect between vacant and protonated pyridine N in decreasing the formation energy of the key *COOH intermediates and the efficient electron transfer relay facilitated by the defect‐induced shallow trap state and homogeneous charge mediators.

Facilitating Molecular Activation and Proton Feeding by Dual Active Sites on Polymeric Carbon Nitride for Efficient CO2 Photoreduction.

Photoreduction of CO2 provides an appealing way to alleviate the energy crisis and manage the global carbon balance but is limited by the high activation energy and the rate-limiting proton transfer. We now develop a dual-site strategy for high-efficiency CO2 conversion through polarizing CO2 molecules at pyridine N vacancies and accelerating the intermediate protonation by protonated pyridine N adjacent to nitrogen vacancies on polymeric carbon nitride. Our photocatalysts with atomic-level engineered active sites manifest a high CO production rate of 1835 μmol g-1 h-1 , 183 times higher than the pristine bulk carbon nitride. Theoretical prediction and experimental studies confirm that such excellent performance is attributed to the synergistic effect between vacant and protonated pyridine N in decreasing the formation energy of the key *COOH intermediates and the efficient electron transfer relay facilitated by the defect-induced shallow trap state and homogeneous charge mediators.

Potassium-doped carbon nitride: Highly efficient photoredox catalyst for selective oxygen reduction and arylboronic acid hydroxylation

Photocatalytic two-electron oxygen reduction reaction for hydrogen peroxide production, a promising liquid energy carrier and an environmentally friendly oxidant, is regard as a promising process owing to its low-cost and environment-friendly advantages compared to the state-of-art anthraquinone oxidation for H2O2 production. Herein, a potassium-doped carbon nitride photocatalyst has been successfully synthesized by the copolymerization of self-assembly supramolecular cyanuric acid and melamine aggregates with introduction of potassium chloride (KCMCN), wherein its band structure was carefully tuned and a new channel for charge transfer was created. Notably, DFT calculations pointed out that a lower absorption energy of oxygen (Eads = 0.27 eV) over KCMCN was estimated than that of MCN (Eads = 1.02 eV) with the introduction of K into carbon nitride framework, which is beneficial for superior H2O2 evolution. The H2O2 yield of KCMCN (317.9 µmol·L−1·h−1) was about 10.7 times higher than bulk carbon nitride at low concentration of sacrificial agent (0.5% isopropanol aqueous solution) and the apparent quantum efficiency (AQE) of KCMCN reached to 7.5% at 420 nm. Furthermore, in-situ photoinduced superoxide radical anion (·O2−) species were generated from dioxygen in KCMCN, which were reacted with arylboronic acids to produce corresponding aryl alcohols through a hydroxylation process under visible light irradiation. This work opens up prospects for providing guidance for other organic synthesis based on the oxidative effect of H2O2 and exploring novel heterogeneous photocatalysts in the field of value-added products formation.

Recent Advances of Doping and Surface Modifying Carbon Nitride with Characterization Techniques

As a non-metallic organic semiconductor photocatalyst, graphitic carbon nitride (g–C3N4, CN) has become a research hotspot due to its excellent performance in organic degradation, CO2 reduction and water splitting to produce hydrogen. However, the high recombination rate of electron-hole pairs, low specific surface area and weak light absorption of bulk CN synthesized by the traditional one-step thermal polymerization method seriously restrict its photocatalytic performance and practical application. To enhance the photocatalytic performance of CN, doping and surface modification strategies are usually employed to tune the band gap of carbon nitride and improve the separation of carriers. In this paper, the research progress of different methods to modify CN in recent years is introduced, and the mechanisms of improving the photocatalytic performance are mainly analyzed. Typical modification methods are mainly divided into metal doping, non-metal doping, co-doping and surface-functionalized modification. Some characterization methods that can analyze the doping state and surface modification are also discussed as examples. Finally, the difficulties that need to be addressed through modified CN photocatalysts and the directions for future research are pointed out.

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.

Synthesis of vacant graphitic carbon nitride in argon atmosphere and its utilization for photocatalytic hydrogen generation

Graphitic carbon nitride (C3N4) was synthesised from melamine at 550 °C for 4 h in the argon atmosphere and then was reheated for 1–3 h at 500 °C in argon. Two band gaps of 2.04 eV and 2.47 eV were observed in all the synthetized materials. Based on the results of elemental and photoluminescence analyses, the lower band gap was found to be caused by the formation of vacancies. Specific surface areas of the synthetized materials were 15–18 m2g−1 indicating that no thermal exfoliation occurred. The photocatalytic activity of these materials was tested for hydrogen generation. The best photocatalyst showed 3 times higher performance (1547 μmol/g) than bulk C3N4 synthetized in the air (547 μmol/g). This higher activity was explained by the presence of carbon (VC) and nitrogen (VN) vacancies grouped in their big complexes 2VC + 2VN (observed by positron annihilation spectroscopy). The effect of an inert gas on the synthesis of C3N4 was demonstrated using Graham´s law of ammonia diffusion. This study showed that the synthesis of C3N4 from nitrogen-rich precursors in the argon atmosphere led to the formation of vacancy complexes beneficial for hydrogen generation, which was not referred so far.

Preparation of porous C doped g-C3N4 nanosheets controlled by acetamide for photocatalytic H2 evolution

Photocatalytic production H2 of splitting water is an important branch of the application of photocatalytic materials. In this paper, porous C doped carbon nitride was successfully synthesized by calcining urea and acetamide at high temperature, and then analyzed by various characterization methods. XRD and FT-IR spectra show that the crystal structure of carbon nitride modified by acetamide are similar to bulk carbon nitride. SEM and TEM spectra show that the morphology and structure of carbon nitride and carbon nitride was modified by acetamide, which presents a porous nanosheets structure. XPS and EDS show that the content of carbon element in carbon nitride modified by acetamide increased significantly. The PL spectrum is obviously red shift, and the light absorption range is obviously enlarged, which is consistent with the absorption edge of UV–vis Spectrum. The BET of the optimized sample CN2-6 is 122.79 m2g-1, which is 3.04 times higher than that of bulk carbon nitride CN1 (40.456 m2g-1). In addition, the hydrogen production of CN2-6 is 13169.04 μmol/g/h under 300 W Xenon lamp, which is 14.95 times more than CN1 (880.82 μmol/g/h) hydrogen production rate. When the light source is a 10 W LED lamp, the apparent quantum efficiency (AQE) of sample CN2-6 reaches the maximum (2.91) under the wavelengths of 430 nm, which is 1.27 times more than CN1 under the same conditions. Meanwhile, prepared samples have excellent stability and durability under visible light.

Supramolecular self-assembly coupled with alkali metal molten salts to construct Nv-carbon nitride for efficient photocatalytic H2O2 production

H2O2 production over carbon nitride photocatalyst is an emerging alternative and ecological solar-driven application. However, bulk carbon nitride (BCN) has stubbornly weak visible light absorption and easy recombination of carriers. Here, a sodium/potassium co-doped N vacancy (Nv) rich carbon nitride was synthesized by a one-step melamine and cyanuric acid self-assembly coupled with alkali metal molten salts. The hybridization of sodium is found to locate on edge site of CN layers, which introduces Nv structure and cyano structure on CN layers, hence modulates the migration of in-plane electrons. The potassium is found to intercalate between CN layers, which promotes carrier transportation between the interlayers. Both effects induce higher carrier transfer efficiency of catalysts and the co-doping of Na and K bring us the optimal sample (CNNK) with an ultra-high production rate (6289.6 μmol/g/h) and apparent quantum yield (44.70 %, 380 nm), which is 124.5 and 10.4 times higher than BCN during the hydrogen peroxide process, respectively. Mechanistic analysis confirms the two-step single-electron oxygen reduction reaction pathway of H2O2 production. This strategy provides an achievable coupling strategy idea to design photocatalysts with elaborate structures for energy conservation.