Crystalline Carbon Nitride Research Articles

Highly crystalline carbon nitrogen polymer with a strong built-in electric fields for ultra-high photocatalytic H2O2 production

Efficient photogenerated charge separation and rapid migration provide more charges for surface redox reactions. Herein, the carbon nitride polymer with highly crystalline is prepared for efficient photocatalytic H2O2 generation. The strong built-in electric field induced by its high crystallinity is beneficial for promoting exciton dissociation, resulting in a surface photovoltage of 19.24 mV. Meanwhile, the formed nanocrystals shorten the charge migration distance to facilitate photogenerated charge rapid migration. The abundant surface −CN/−OH groups provide more active sites for the adsorption of O2. As a result, the photocatalytic H2O2 production rate (16.01 mmol∙L−1∙h−1∙g−1) under visible light (λ > 400 nm) was increased 77-fold relative to pristine graphitic carbon nitride, with a quantum efficiency of 25.1% at 405 nm. The photocatalytic H2O2 production reached 80.36 mM after 8 h. The highly crystalline carbon nitride polymer throws prospect on photocatalytic H2O2 production, showing a new perspective for the design of photocatalysts.

Well-Defined Iron Sites in Crystalline Carbon Nitride.

Carbon nitride materials can be hosts for transition metal sites, but Mössbauer studies on iron complexes in carbon nitrides have always shown a mixture of environments and oxidation states. Here we describe the synthesis and characterization of a crystalline carbon nitride with stoichiometric iron sites that all have the same environment. The material (formula C6N9H2Fe0.4Li1.2Cl, abbreviated PTI/FeCl2) is derived from reacting poly(triazine imide)·LiCl (PTI/LiCl) with a low-melting FeCl2/KCl flux, followed by anaerobic rinsing with methanol. X-ray diffraction, X-ray absorption and Mössbauer spectroscopies, and SQUID magnetometry indicate that there are tetrahedral high-spin iron(II) sites throughout the material, all having the same geometry. The material is active for electrocatalytic nitrate reduction to ammonia, with a production rate of ca. 0.1 mmol cm-2 h-1 and Faradaic efficiency of ca. 80% at -0.80 V vs RHE.

The local ordering of polar solvents around crystalline carbon nitride nanosheets in solution.

The crystalline graphitic carbon nitride, poly-triazine imide (PTI) is highly unusual among layered materials since it is spontaneously soluble in aprotic, polar solvents including dimethylformamide (DMF). The PTI material consists of layers of carbon nitride intercalated with LiBr. When dissolved, the resulting solutions consist of dissolved, luminescent single to multilayer nanosheets of around 60-125 nm in diameter and Li+ and Br- ions originating from the intercalating salt. To understand this unique solubility, the structure of these solutions has been investigated by high-energy X-ray and neutron diffraction. Although the diffraction patterns are dominated by inter-solvent correlations there are clear differences between the X-ray diffraction data of the PTI solution and the solvent in the 4-6 Å-1 range, with real space differences persisting to at least 10 Å. Structural modelling using both neutron and X-ray datasets as a constraint reveal the formation of distinct, dense solvation shells surrounding the nanoparticles with a layer of Br-close to the PTI-solvent interface. This solvent ordering provides a configuration that is energetically favourable underpinning thermodynamically driven PTI dissolution. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

A Monolayer Carbon Nitride on Au(111) with a High Density of Single Co Sites.

Carbon nitrides that expose atomically dispersed single-atom metals in the form of M-N-C (M = metal) sites are attractive earth-abundant catalyst materials that have been demonstrated in electrocatalytic conversion reactions. The catalytic performance is determined by the abundance of N-doped sites and the type of metal coordination to N, but challenges remain to synthesize pristine carbon nitrides with a high concentration of the most active sites and prepare homogeneously doped materials that allow for in-depth characterization of the M-N-C sites and quantitative evaluation of their catalytic performance. Herein, we have synthesized and characterized a well-defined monolayer carbon nitride phase on a Au(111) surface that exposes an exceedingly high concentration of Co-N4 sites. The crystalline monolayer carbon nitride, whose formation is controlled by an on-surface reaction between Co atoms and melamine on Au(111), is characterized by a dense array of 4- and 6-fold N-terminated pockets, whereof only the 4-fold pocket is found to be holding Co atoms. Through detailed characterization using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory modeling, we determine the atomic structure and chemical state of the carbon nitride network. Furthermore, we show that the monolayer carbon nitride structure is stable and reactive toward the electrocatalytic oxygen reduction reaction in alkaline electrolyte, with a quantitative performance metric that significantly exceeds comparable M-N-C-based catalyst types. The work demonstrates that high-density active catalytic sites can be created using common precursor materials, and the formed networks themselves offer an excellent platform for onward studies addressing the characteristics of M-N-C sites.

Effects of potassium ions and platinum cocatalysts on the photocatalytic performance of Poly(Heptazine Imides)

In the synthesis of highly crystalline carbon nitride, alkali metal halides are used as high-temperature solvents and structural guiding agents, but the effect of alkali metal ions on the catalytic activity of materials is still unclear. In addition, the shape and size of the cocatalyst also significantly affect the catalytic activity of the material. Therefore, we have carried out the density functional theory calculations to reveal the differences in the structure, electronic, and optical properties, as well as the photocatalytic hydrogen evolution performance of one kind of crystalline carbon nitride called potassium poly(heptazine imide) (K-PHI) and its proton-exchanged counterpart (H-PHI) loaded with different sizes of platinum cocatalysts. The results show that potassium ions have the effect of aggregation electrons in K-PHI, and potassium ions have short-range induced dipole effect on nearby Pt atoms, resulting in slightly better photocatalytic hydrogen evolution activity at these reaction sites. Additionally, Pt cocatalyst with larger load size has better photocatalytic hydrogen evolution activity than small size. This work provides a theoretical basis for the rational design of high-performance metal cocatalysts and exploration of the functionality of alkali metal ions in carbon nitrides.

Copper single atoms incorporated in crystalline carbon nitride for efficient photocatalytic activation of peroxymonosulfate toward bisphenol A removal with visible light

The activation of peroxymonosulfate (PMS) over carbon nitride-based photocatalysts towards refractory organic removal has received growing attention in wastewater treatment. In this study, we successfully synthesized copper single atoms incorporated into crystalline carbon nitride (Cu-CCN) and applied it to activate PMS towards bisphenol A (BPA) removal. The characterization and theoretical calculation results indicated that Cu-CCN had good crystallinity, outstanding charge separation and transfer ability, and numerous PMS adsorption and activation sites provided by Cu single atoms. Consequently, Cu-CCN demonstrated an enhanced ability to activate PMS for BPA degradation. The activity performance of Cu-CCN was affected by the experimental conditions at different levels, and the catalyst had good stability in structure and activity during cyclic runs. A plausible reaction mechanism was also proposed according to free radical trapping and quenching experiments. This oxidation process can efficiently degrade organic pollutants as well as reduce the toxicity. The findings of this work elucidated an efficient photocatalyst for activating PMS towards organic removal and might guide the design of other metal single-atom modified photocatalysts.

Construction of Type-II Heterojunctions in Crystalline Carbon Nitride for Efficient Photocatalytic H2 Evolution.

As an encouraging photocatalyst, crystalline carbon nitride (CCN) exhibits unsatisfactory photocatalytic activity and stability due to its rapid recombination of photo-generative carriers. Herein, high-crystalline g-C3N4 was prepared, including CCN obtained in KCl (K-CCN), LiCl-KCl mixture (Li/K-CCN), and LiCl-NaCl-KCl mixture (Li/Na/K-CCN), via the molten salt strategy using pre-prepared bulk carbon nitride (BCN) as a precursor. The obtained BCN sample was formed by heptazine-based units, which convert into triazine-based units for K-CCN. Heptazine and triazine are two isotypes that co-exist in the Li/K-CCN and Li/Na/K-CCN samples. Compared with BCN and other CCN samples, the as-prepared Li/Na/K-CCN sample exhibited the optimal photocatalytic hydrogen evolution rates (3.38 mmol·g-1·h-1 under simulated sunlight and 2.25 mmol·g-1·h-1 under visible light) and the highest apparent quantum yield (10.97%). The improved photocatalytic performance of the Li/Na/K-CCN sample is mainly attributed to the construction of type-II heterojunction and the institution of the built-in electric field between triazine-based CCN and heptazine-based BCN. This work provides a new strategy for the structural optimization and heterostructure construction of crystalline carbon nitride photocatalysts.

A crystalline carbon nitride-based separator for high-performance lithium metal batteries.

Lithium metal anodes with ultrahigh theoretical capacities are very attractive for assembling high-performance batteries. However, uncontrolled Li dendrite growth strongly retards their practical applications. Different from conventional separator modification strategies that are always focused on functional group tuning or mechanical barrier construction, herein, we propose a crystallinity engineering-related tactic by using the highly crystalline carbon nitride as the separator interlayer to suppress dendrite growth. Interestingly, the presence of Cl- intercalation and high-content pyrrolic-N from molten salt treatment along with highly crystalline structure enhanced the interactions of carbon nitride with Li+ and homogenized lithium flux for uniform deposition, as supported by both experimental and theoretical evidences. The Li-Li cell with the modified separator therefore delivered ultrahigh stability even after 3,000 h with dendrite-free cycled electrodes. Meanwhile, the assembled Li-LiFePO4 full-cell also presented high-capacity retention. This work opens up opportunities for design of functional separators through crystallinity engineering and broadens the use of C3N4 for advanced batteries.

Simultaneous photocatalytic biomass conversion and CO2 reduction over high crystalline oxygen-doped carbon nitride

Simultaneous photocatalytic biorefinery and CO2 reduction to co-produce fuels and high value-added chemicals have recently attracted significant attention; however, comprehensive studies are still lacking. Herein, we report the preparation of highly crystalline oxygen-doped carbon nitride nanotubes (O-CNNTs-x) using an ammonium fluoride-assisted hydrothermal/calcination strategy. The hollow structure, high crystallinity, and O incorporation endowed the O-CNNTs-x with photocatalytic activity by considerably improving optical absorption andmodulating the charge carrier motion. The lactic acid yield and CO evolution rate over O-CNNTs-2.0 reached 82.08% and 67.95μmol g-1 h-1, which are 1.57- and 7.37-fold times higher than those of CN, respectively. Moreover, ·OH plays a key role in the oxidation half-reaction. This study offers a facile approach for fabricating highly crystalline element-doped CN with a customizable morphology and electronic properties and demonstrates the viability of co-photocatalytic CO2 reduction and biomass selective oxidation.

Enhanced photocatalytic H2 production independent of exciton dissociation in crystalline carbon nitride

The role of exciton dissociation in charge separation dominated by in-plane crystallinity has been rarely addressed, although it is the decisive step of polymeric carbon nitride (CN) photocatalysis. Here, taking ten kinds of crystalline carbon nitride (CCN) as examples, the relationship between exciton dissociation, carrier mobility, conductivity, charge separation and photocatalytic activity under different in-plane crystallinity has been investigated. Although the in-plane crystallinity dominates charge separation and photocatalytic activity, it does not significantly reduce excitons binding energy (EBE) or improve exciton dissociation efficiency (EDE), only decreasing by 1.5 meV and increasing by 2.3 %, respectively. It is the greatly improved carrier mobility and conductivity that are more responsible for the increased charge separation and photocatalytic activity than the slight change in EDE. To our knowledge, this is the first reported case of overall water splitting in CCN of poly(heptazine imide) structure, resulting from the optimized in-plane crystallinity.

Infiltration of C-ring into crystalline carbon nitride S-scheme homojunction for photocatalytic hydrogen evolution

Enhancing the carrier separation in graphitized carbon nitride (g-C3N4) is advantageous for improving its photocatalytic activity. Herein, we propose a feasible method for preparing CN-C by thermal polymerization to gradually infiltrate carbon rings (C-rings) into the surface of crystalline carbon nitride (CN), enabling photogenerated electrons to be transferred rapidly between the CN inner layers and the CN/C outer layer. Successful penetration and distribution of carbon rings into carbon nitride were confirmed by secondary ion mass spectroscopy using ratio analysis of C and N elements at various depths of the prepared photocatalyst. Theoretical calculations indicated that CN and CN/C in the molecule generated different Fermi levels to form an S-scheme homojunction, establishing appropriate built-in electric fields and thus enabling interlayer charge migration. Moreover, the overlap of the conjugate plane of C-rings with carbon nitride led to the formation of photogenerated in-plane charge transfer tunnels. The two-electron transfer tunnels greatly improved the dissociation efficiency of photogenerated electrons. The prepared sample loaded with 3 wt% Pt as a co-catalyst for hydrogen production under visible light irradiation, and the prepared optimal sample CN-C showed a maximum quantum efficiency of 15.56% for photocatalytic H2 evolution at 385 nm. This research introduces a new idea for constructing a directional transfer path for charge carriers in-plane and intralayer.

Mechanism insights into visible light-induced crystalline carbon nitride activating periodate for highly efficient ciprofloxacin removal

The development of suitable and efficient periodate activation methods is currently a hot research topic. In this study, a novel efficient photocatalytic activation strategy using crystalline carbon nitride (CCN) and visible light (Vis) to activate PI for pollutant degradation was proposed. In the CCN/PI/Vis system, 100% ciprofloxacin hydrochloride (CIP) degradation was achieved within 10 min which was much higher than polymerized carbon nitride (PCN)/PI/Vis system. Besides, CCN/PI/Vis system demonstrated outstanding CIP removal activity in a wide range of pH, anions and organic matter, and natural water environments. Mechanistic studies demonstrated that the CCN/PI/Vis system decomposed CIP via nonradical/radical mixing processes dominated by 1O2 and electron transfer. The excellent degradation ability depends on the establishment of an electron cycle among CCN, PI, and CIP. In which, CCN generated photogenerated electrons, CIP acted as an electron donor, and PI served as an electron acceptor. Due to the extended in-plane periodicity and reduced interlayer distance and the formation of ordered electron transport channels caused by the increase of crystallinity, CCN had stronger light absorption and more efficient charge carriers separation and mobility than PCN, resulting in more durable and efficient photogenerated electron and thus much enhanced synergetic photocatalytic and PI activation ability of CCN.

Construction of S-Scheme 2D/2D Crystalline Carbon Nitride/BiOIO3 van der Waals Heterojunction for Boosted Photocatalytic Degradation of Antibiotics.

Utilization of semiconductor photocatalyst materials to degrade pollutants for addressing environmental pollution problems has become a research focus in recent years. In this work, a 2D/2D S-scheme crystalline carbon nitride (CCN)/BiOIO3 (BOI) van der Waals heterojunction was successfully constructed for effectively enhancing the degradation efficiency of antibiotic contaminant. The as-synthesized optimal CCN/BOI-3 sample exhibited the highest efficiency of 80% for the photo-degradation of tetracycline (TC, 20 mg/L) after 120 min visible light irradiation, which was significantly higher than that of pure CCN and BOI. The significant improvement in photocatalytic performance is mainly attributed to two aspects: (i) the 2D/2D van der Waals heterojunction can accelerate interface carriers' separation and transfer and afford sufficient active sites; (ii) the S-scheme heterojunction elevated the redox capacity of CCN/BOI, thus providing a driving force for the degradation reaction. The degradation pathways of TC for the CCN/BOI composite were investigated in detail by liquid chromatography-mass spectrometry (LC-MS) analysis. This work provides a design idea for the development of efficient photocatalysts based on the 2D/2D S-scheme van der Waals heterojunctions.

Regulation of Polymerization Kinetics to Improve Crystallinity of Carbon Nitride for Photocatalytic Reactions.

Carbon nitride (CN) polymers exhibit tunable and fascinating physicochemical properties and are thus an essential class of photocatalytic materials with potential applications. Although significant progress has been made in the fabrication of CN, the preparation of metal-free crystalline CN via a straightforward method remains a considerable challenge. Herein, we describe a new attempt to synthesize crystalline carbon nitride (CCN) with a well-developed structure through regulation of the polymerization kinetics. The synthetic process involves the pre-polymerization of melamine to remove most of the ammonia and further calcination of the pre-heated melamine in the presence of copper oxide as an ammonia absorbent. Copper oxide can decompose the ammonia produced by the polymerization process, thereby promoting the reaction. These conditions facilitate the polycondensation process while avoiding carbonization of the polymeric backbone at high temperatures. Owing to the high crystallinity, nanosheet structure, and efficient charge-carrier transmission capacity, the as-prepared CCN catalyst shows much higher photocatalytic activity than its counterparts. Our study provides a novel strategy for the rational design and synthesis of high-performance carbon nitride photocatalysts by simultaneously optimizing polymerization kinetics and crystallographic structures.

S-scheme heterojunction of crystalline carbon nitride nanosheets and ultrafine WO3 nanoparticles for photocatalytic CO2 reduction

Highly crystalline carbon nitride polymers have shown great opportunities in overall water photosplitting; however, their mission in light-driven CO2 conversion remains to be explored. In this work, crystalline carbon nitride (CCN) nanosheets of poly triazine imide (PTI) embedded with melon domains are fabricated by KCl/LiCl-mediated polycondensation of dicyandiamide, the surface of which is subsequently deposited with ultrafine WO3 nanoparticles to construct the CCN/WO3 heterostructure with a S-scheme interface. Systematic characterizations have been conducted to reveal the compositions and structures of the S-scheme CCN/WO3 hybrid, featuring strengthened optical capture, enhanced CO2 adsorption and activation, attractive textural properties, as well as spatial separation and directed movement of light-triggered charge carriers. Under mild conditions, the CCN/WO3 catalyst with optimized composition displays a high photocatalytic activity for reducing CO2 to CO in a rate of 23.0 µmol/hr (i.e., 2300 µmol/(hr·g)), which is about 7-fold that of pristine CCN, along with a high CO selectivity of 90.6% against H2 formation. Moreover, it also manifests high stability and fine reusability for the CO2 conversion reaction. The CO2 adsorption and conversion processes on the catalyst are monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), identifying the crucial intermediates of CO2*−, COOH* and CO*, which integrated with the results of performance evaluation proposes the possible CO2 reduction mechanism.

Alkali-assisted deep-deamination to improve the crystallinity of poly(heptazine imide) for boosted photocatalytic H2 evolution

The poly(heptazine imide) (also known as heptazine-based crystalline carbon nitride, abbreviated as PHI) has drawn wide attention in solar energy conversion, which is profited from its improved photoinduced charge separation compared with melon-based carbon nitride. Nevertheless, the crystallinity of the samples synthesized by direct heat-treating precursors is low and further impede electron transport. This situation is especially prominent if precursors are used in large quantities and the reason is also unclear at present. Herein, this crucial issue was preliminarily revealed: the by-product HCl formed in the synthesis process has seriously restricted the deamination of PHI if it cannot be removed in time. Based on this, an original alkali-assisted method was proposed and demonstrated to realize deep-deamination for improving the crystallinity and extending π-conjugation of PHI. Benefit from this, alkali-assisted synthesized PHI illustrates significantly enhanced electron transport performance and further improved photocatalytic H2 evolution activity. The optimized PHI exhibits highly boosted photocatalytic H2 evolution activity of 2223 μmol h−1, which is 18 times that of original PHI synthesized without alkali-assistance. Additionally, the mechanism was further verified by density-functional theory (DFT) computations and the universality of alkali-assisted method was verified by using different kinds of alkali.

A high-cyano groups-content amorphous-crystalline carbon nitride isotype heterojunction photocatalyst for high-quantum-yield H2 production and enhanced CO2 reduction

Polymeric carbon nitride suffers terrible recombination of photo-generated carriers and thus usually results in limited photocatalytic activity. Improving crystallinity and creating novel structure can effectively alleviate the aforementioned problem. Herein, a high-cyano groups-content and amorphous-crystalline carbon nitride isotype heterojunction (BCN-NaK) was obtain via a post treatment strategy in eutectic salt. The cyano groups acted as electron acceptors, and the intimate isotype heterojunction constructed by amorphous carbon nitride and crystalline poly-heptazine-imide built interfacial electric field. Both synergistically enabled BCN-NaK more efficient charge transfer than other amorphous and crystalline carbon nitrides. Comprehensive and systematic analysis demonstrated that the promoted charge transfer made the predominant contribution to the ultrahigh photocatalytic H2-production activity of BCN-NaK with an excellent apparent quantum yield of 68.9 % at 405 nm. In addition, BCN-NaK exhibited a good photocatalytic CO2-to-CO conversion rate of 22.8 μmol h−1 gcat−1 without adding sacrificial agent or loading metal under visible light.

Single-atom cobalt-incorporating carbon nitride for photocatalytic solar hydrogen conversion: An X-ray spectromicroscopy study

The use of carbon nitride-based materials and light to drive catalytic water splitting has enormous potential for the production of hydrogen. Revealing the processes of molecular conjugation, nucleation, and crystallization in crystalline carbon nitride is expected to enhance the photocatalytic activity through the creation of isotype heterojunctions and active sites. In this work, the addition of cobalt salts in ionothermal synthesis was found to promote the phase transition of heptazine-based crystalline carbon nitride (CCN) to triazine-based poly (triazine imide) (PTI), resulting in the formation of a single-atom cobalt-doped coordinated isotype CCN/PTI heterojunction. The new hybrid orbital modulates the atomic/electronic structure and the band gap of the CCN/PTI heterojunction, and synergistically increases the absorption of visible light, accelerating the separation and transfer of photoexcited electrons and holes. Synchrotron-based X-ray spectroscopy and microscopy are used to identify the origin of the improved performance of the single-atom cobalt-doped CCN/PTI heterojunction in the photocatalytic hydrogen evolution reaction. This work demonstrates that synchrotron X-ray spectroscopy is a promising tool for designing materials aimed at enhancing photocatalytic activity in solar energy conversion applications.

Synthesis of crystalline carbon nitride with molten salt thermal treatment for efficient photocatalytic reduction and removal of U(VI)

Synthesis of crystalline carbon nitride with molten salt thermal treatment for efficient photocatalytic reduction and removal of U(VI)

Designing Black Phosphorus and Heptazine-Based Crystalline Carbon Nitride Composites for Photocatalytic Water Splitting

Black phosphorus (BP) and heptazine-based crystalline carbon nitride (KPHI) composite photocatalysts were synthesized by molten salt and ultrasound-assisted liquid phase exfoliation methods. The structure, morphology and optical properties of the as-prepared BP/KPHI composites were evaluated by various characterization techniques. In addition, the photocatalytic performance of BP/KPHI composites for hydrogen production was investigated. The photocatalytic activity of BP/KPHI composite catalysts could be modulated by changing the loading mass ratio of BP. The BP/KPHI composite photocatalyst with a mass ratio of 10% exhibited the highest photocatalytic activity with the hydrogen production rate of 4.3 mmol g−1 h−1, about three times higher than that of pristine KPHI. Benefiting from the advantages of simplicity, rapidity, high yield and good controllability, this nanocomposite photocatalyst has the potential to serve as an excellent photocatalytic material for solar energy conversion.