Two-dimensional Carbon Nitride Research Articles

Permselective and transparent wooden membrane with artemisinin-imprinted nanocages based on a MOFs@C3N4 self-assembly design

Molecularly imprinted membranes have superior selective separation capacity, rebinding capacity, and flux due to their microporous substrates and tightly packed molecularly imprinted polymers. In current study, artemisinin (Ars), the most effective antimalarial drug, is used as template molecule. The permeation-selectivity wooden membranes based on Ars-imprinted nanocages designed by metal organic framework@carbon nitride self-assembly process (MDWMs) has been successfully prepared: (i) based on three-dimensional mesoporous basswood membranes, dopamine-based auto-polymerization process was developed on basswood to for harvest the first layer imprinting and secondary reaction platform. (ii) The two-dimensional sheet carbon nitride modified by metal-organic framework is stably loaded on the surface of the polydopamine-modified basswood membrane, providing a rich irregular point array structure for the formation of recognition sites. Based on elaborate designs and optimizations, selectivity, rebinding capabilities, and flux are enhanced on the developed MDWMs (βAre/Ars = 6.49, βAru/Ars = 4.22, βDH-Ars/Ars = 7.42), and high regeneration rate (>94% after 10 cycles) indicates that MDWMs have superior selectivity and stability. The MDWMs and preparation strategies developed here will provide a completely new design methodology for subsequent selective separation of Ars.

Enhanced visible-light photocatalytic hydrogen evolution using two-dimensional carbon nitride sheets with the removal of amine groups

Two-dimensional (2D) carbon nitride sheets (CNs) with atomically thin structures are regarded as one of the most promising materials for solar energy conversion. However, due to their substantially enlarged bandgap caused by the strong quantum size effect and their incomplete polymerisation with a large number of non-condensed surface amino groups, the practical applicability of CNs in photocatalysis is limited. In this study, CNs with broad visible-light absorption were synthesised using a 5-min fast thermal annealing. The removal of uncondensed amine groups reduces the bandgap of CNs from 3.06 eV to 2.60 eV, increasing their absorption of visible light. Interestingly, the CNs were distorted after annealing, which can differentiate the spatial positions of electrons and holes, enhancing the visible-light absorption efficiency. As a result, when exposed to visible light, the photocatalytic hydrogen production activity of atomically thin 2D CNs rose by 8.38 times. This research presents a dependable and speedy method for creating highly effective visible-light photocatalysts with narrowed bandgaps and improved visible-light absorption.

Reversible hydrogen storage tendency of light-metal (Li/Na/K) decorated carbon nitride (C9N4) monolayer

The emergence of hydrogen (H2) as a future carrier of energy and the issue related to its storage leads to the exploration of two-dimensional materials, which have the potential to be explored as H2 storage materials. In this regard, potential of alkali metals (Li/Na/K) decorated two-dimensional carbon-nitride (C9N4) monolayer is studied for H2 storage by performing first-principles density functional theory (DFT) computations. Metal dopants, Li, Na, and K show strong binding interactions with the C9N4 due to the transfer of charges from the formers to the later. In addition to strong bindings with C9N4 high diffusion barriers further nullify the cluster formation among metal dopants. The effect of temperature on the stability of alkali metal decorated C9N4 is studied in terms of ab-initio molecular dynamics (AIMD) simulations. The effect of metal decoration on electronic as well as magnetic properties of C9N4 are studied in terms of partial density of states (PDOS) plots. We find that each Li of Li-doped C9N4 (Li@C9N4) can stably binds six H2 molecules without disintegration and results in the average adsorption energy of −0.20 eV, leading to a notably high H2 storage capability of 11.9 wt%. Similarly, the storage capacities and average H2 adsorption energies in the case of Na@C9N4 and K@C9N4 complexes fulfil the US Department of Energy (DOE) criteria. The electronic, AIMD, and thermodynamics analysis in this study provide an insight into that alkali metal decorated C9N4 as reversible H2 storage material.

Design of a Four-Atom Cluster Embedded in Carbon Nitride for Electrocatalytic Generation of Multi-Carbon Products.

The development of efficient and stable catalysts for the electrocatalytic CO2 and CO reduction reactions (CORR) is under active investigation, but the problems of poor selectivity and low efficiency for C2 products still exist. We design a two-dimensional carbon nitride material (C5N2H2) that contains an eight N-atom structure capable of coordinating four-metal atom clusters and supporting simultaneously two carbon oxide molecules needed for the C2 coupling. The designed material has excellent electrical conductivity and stability. After high-throughput screening of catalytic performance of multiple four-metal clusters embedded into the framework, we systematically investigate the CORR process of 11 candidates. We find that Cu4-C5N2H2 has superior selectivity and low limiting potential for generating ethylene, while Cu2Zn2-C5N2H2 is selective and efficient to synthesize ethanol. Further, we discover a novel type of descriptor related to 2D material flexibility to evaluate the potential-determining step for generating ethylene. Our report both broadens the possibilities for few-atom CO reduction and demonstrates a novel substrate flexibility-related descriptor to predict the catalytic performance of materials.

Reversible Hydrogen Storage Media by g-CN Monolayer Decorated with NLi4: A First-Principles Study

A two-dimensional graphene-like carbon nitride (g-CN) monolayer decorated with the superatomic cluster NLi4 was studied for reversible hydrogen storage by first-principles calculations. Molecular dynamics simulations show that the g-CN monolayer has good thermal stability at room temperature. The NLi4 is firmly anchored on the g-CN monolayer with a binding energy of -6.35 eV. Electronic charges are transferred from the Li atoms of NLi4 to the g-CN monolayer, mainly due to the hybridization of Li(2s), C(2p), and N(2p) orbitals. Consequently, a spatial local electrostatic field is formed around NLi4, leading to polarization of the adsorbed hydrogen molecules and further enhancing the electrostatic interactions between the Li atoms and hydrogen. Each NLi4 can adsorb nine hydrogen molecules with average adsorption energies between -0.152 eV/H2 and -0.237 eV/H2. This range is within the reversible hydrogen storage energy window. Moreover, the highest achieved gravimetric capacity is up to 9.2 wt%, which is superior to the 5.5 wt% target set by the U.S. Department of Energy. This study shows that g-CN monolayers decorated with NLi4 are a good candidate for reversible hydrogen storage.

Interfacial engineering to construct 2D-2D NiCo-LDH/g-C3N4 heterojunctions for enhanced photocatalytic hydrogen production performance

Photocatalytic water splitting provides a green method to solve the energy shortage issue. Combine two-dimensional carbon nitride nano sheets with other two-dimensional semiconductors can effectively increase the construct area and improve the utilization of photogenerated charges. Herein, 2D-2D NiCo-LDH/g-C3N4 composites were successfully prepared by a simple hydrothermal method. The lamellar NiCo-LDH was grown in situ on g-C3N4, in which way the hydrogen production rate was enhanced by about 21 times, reaching 755 μmol·g−1·h−1. According to the results of density functional theory （DFT） calculations, an S-type heterojunction is successfully constructed, which achieves the spatial separation of semiconductor photogenerated electron-hole with guaranteed strong redox capability. This work emphasizes that effective transport channels for transfer and separation of photogenerated charges can be created through efficient interfacial regulation strategy.

Spatially restricted strategy to construct crystalline carbon nitride nanosheet assists exciton dissociation to enhance photocatalytic hydrogen evolution activity

Two-dimensional carbon nitride nanosheets continue to attract attention in photocatalysis, but inefficient exciton dissociation and charge separation limit further improvement of their photocatalytic performance. Herein, we have prepared highly crystalline carbon nitride sheets using a integrated strategy of spatially restricted growth in solid salt media and hydrochloric acid induced exfoliation. In the confined space of the solid salt, K+ induce the ordered growth of carbon nitride and diffuse into the carbon nitride bulk phase with thermodynamically feasible paths. Pickling was also found to control the K+ concentration gradient distribution and regulate the charge characteristic of cyanoamino, thereby controlling the quantity and activity of trapped long-lived electrons. The storage free electron behavior of carbon nitride dual-functionalized with cyanoamino and K+ (KCCN) greatly facilitates the dissociation of excitons, resulting in photocatalytic hydrogen evolution efficiency of 10.78 mmol·g−1·h−1, which was 49 times higher than that of ordinary carbon nitride. KCCN had a hydrogen production quantum efficiency of 27.79%, higher than the majority of the similar materials reported. This study provides new insights into the design of more excellent carbon nitride-based photocatalysts through exciton engineering.

2D carbon nitride as a support with single Cu, Ag, and Au atoms for carbon dioxide reduction reaction.

The electrochemical conversion of CO2 into value-added chemicals is an important approach to recycling CO2. In this work, we have combined the most efficient metal catalysts for this reaction, namely Cu, Ag, and Au, as single-atom particles dispersed on a two-dimensional carbon nitride support, with the aim of exploring their performance in the CO2 reduction reaction. Here, we report density functional theory computations showing the effect of single metal-atom particles on the support. We found that bare carbon nitride needed a high overpotential to overcome the energy barrier for the first proton-electron transfer, while the second transfer was exergonic. The deposition of single metal atoms enhances the catalytic activity of the system as the first proton-electron transfer is favored in terms of energy, although strong binding energies were found for CO adsorption on Cu and Au single atoms. Our theoretical interpretations are consistent with the experimental evidence that the competitive H2 generation is favored due to the strong CO binding energies. Our computational study paves the road to finding suitable metals that catalyze the first proton-electron transfer in the carbon dioxide reduction reaction and produce reaction intermediates with moderate binding energies, promoting a spillover to the carbon nitride support and thereby serving as bifunctional electrocatalysts.

A chemically bonded and plasmonic Z-scheme junction for high-performance artificial photosynthesis of hydrogen peroxide

Chemically bonded and plasmonic Z-scheme junction as a model material was produced byin situassembly of nonstoichiometric W18O49(WO) onto two-dimensional carbon nitride nanosheets (CNs) for high-performance artificial photosynthesis of H2O2.

Reinforced Photogenerated Electrons in Few-Layer C3N5 for Enhanced Catalytic NO Oxidation and CO2 Reduction

Two-dimensional (2D) polymer carbon nitrides are cutting-edge environment-friendly photocatalysts relying on their extraordinary properties of cost-effectiveness, low toxicity, visible-light response, and chemical stability. Here, we report few-layer C3N5 with a thickness of around 1.2 nm as a case study, whose photogenerated carriers’ dynamics achieve a significant increase proven by the transient diffuse reflectance spectra. In addition, few-layer C3N5 changes to mainly exhibit the behavior of photogenerated electrons in comparison to bulk C3N5, whose lifetime extends 3.25-fold and is even 3.72-fold of that for g-C3N4. Density functional theory calculations further verify the superior adsorption and activation of small molecules of NO, O2, and CO2 over few-layer C3N5. The abovementioned unique intrinsic properties endow few-layer C3N5 with a significantly enhanced photocatalytic capability in air NO oxidation and CO2 reduction with respect to bulk C3N5 and traditional C3N4. This work may help in developing efficient 2D metal-free catalysts for photocatalytic applications.

Efficient Pb(II) adsorption in aqueous solution by hierarchical 3D/2D TiO2/CNNS nanocomposite

A multifunctional nanomaterial for contaminants’ removal and solar fuel production is designed, for solving environmental and energy problems. In this research, hierarchical three-dimensional (3D)TiO2 doped with porous graphite-like two-dimensional (2D) carbon nitride (g-C3N4) nanosheets (TiO2/CNNS) composite was fabricated by solid interfacial reaction through a facile acid hydrothermal route. Interestingly, the produced catalyst showed extraordinarily selective adsorption towards Pb (II) metal ions through strong electrostatic attraction mainly due to the enlarged surface areas and pore volume, besides increased adsorption/active sites. The obtained batch mode adsorption measurements reveal a high removal efficiency of give 378 mg.g−1 in a very short period of time of 7 min. This study highlights the importance of designing 3D/2D multifunctional g-C3N4-based photocatalysts for environmental heavy metals decontamination.

Constructing metal-free heterophotocatalyst using two-dimensional carbon nitride sheets and violet phosphorene for highly efficient visible-light photocatalysis

Two-dimensional (2D) carbon nitride sheets (CNs) with nanoscale thickness have great potential in solar energy conversion owing to their intrinsic bandgap. However, their photocatalytic efficiency falls far below the practical requirements because the generated electron-hole pairs rapidly recombine. We construct a metal-free 2D p–n junction heterophotocatalyst from CNs and violet phosphorene (VP) using a simple electrostatic self-assembly method. The experimental results and density functional theory calculations show that the built-in electric field of the p–n junction promotes photogenerated carrier transport at the heterojunction interface, thereby promoting the separation of photogenerated electron-hole pairs. Under visible-light illumination, the visible-light photocatalytic hydrogen and CO production rates of the CNs/VP heterojunction were increased by 5.85 and 1.51 times, respectively, when compared to that of CNs. This study provides new insights into the design of metal-free heterogeneous photocatalysts with high catalytic activity.

Regulating the Assembly of Precursors of Carbon Nitrides to Improve Photocatalytic Hydrogen Production

Two-dimensional graphitic carbon nitrides (2D g-C3N4) are promising photocatalysts for water splitting to hydrogen due to their non-toxicity and high stability. However, the bulk g-C3N4 has some intrinsic drawbacks, such as rapid electron–hole recombination and low charge-carrier mobility, resulting in poor photocatalytic activity. Here, 2,4-diamine-6-phenyl-1,3,5-triazine was employed as a precursor to regulating the assembly of melamine and cyanuric acid in water. The resulting g-C3N4 not only improved the visible light absorption and electron–hole separation but also provided more catalytic sites for enhanced photocatalytic hydrogen evolution. The modified g-C3N4 (CNP10-H) showed a hydrogen-releasing rate of 2184 μmol·g−1·h−1, much higher than the bulk g-C3N4.

Higher-than-common temperature short-time processed polymeric carbon nitride nanosheets as an efficient photocatalyst for H2 production

Two-dimensional polymeric carbon nitride nanosheets exhibiting unique optical properties are considered promising materials for visible light active photocatalysts. Here, we synthesized thin polymeric carbon nitride nanosheets via the higher-than-common temperature (710 °C) pyrolysis of a polymeric carbon nitride intermediate for 20 min. The exfoliation efficiency of the nanosheets was dramatically improved by the higher-than-common temperature treatment. Notably, compared with the pristine polymeric carbon nitride nanosheets, the synthesized thin polymeric carbon nitride nanosheets exhibited a reduced optical bandgap and more negative conduction band (ECB) positions, which ensured enhanced visible-light absorption and reductive ability. When applied to photocatalytic water-splitting reaction for H2 evolution, the thin polymeric carbon nitride nanosheets exhibited enhanced activity under visible light irradiation, accounting for an optimal H2-evolution rate of 8187 µmol·g−1·h−1 with a Pt loading of 1 wt%, which was approximately six-fold higher than that obtained with the urea-derived pristine polymeric carbon nitride nanosheets. This work provides a rapid, affordable and facile method for synthesizing polymeric carbon nitride nanosheets with high photocatalytic performances.

Numerical Investigation of Photo-Generated Carrier Recombination Dynamics on the Device Characteristics for the Perovskite/Carbon Nitride Absorber-Layer Solar Cell.

The nitrogenated holey two-dimensional carbon nitride (C2N) has been efficaciously utilized in the fabrication of transistors, sensors, and batteries in recent years, but lacks application in the photovoltaic industry. The C2N possesses favorable optoelectronic properties. To investigate its potential feasibility for solar cells (as either an absorber layer/interface layer), we foremost detailed the numerical modeling of the double-absorber-layer-methyl ammonium lead iodide (CH3NH3PbI3) -carbon nitride (C2N) layer solar cell and subsequently provided in-depth insight into the active-layer-associated recombination losses limiting the efficiency (η) of the solar cell. Under the recombination kinetics phenomena, we explored the influence of radiative recombination, Auger recombination, Shockley Read Hall recombination, the energy distribution of defects, Band Tail recombination (Hoping Model), Gaussian distribution, and metastable defect states, including single-donor (0/+), single-acceptor (-/0), double-donor (0/+/2+), double-acceptor (2/-/0-), and the interface-layer defects on the output characteristics of the solar cell. Setting the defect (or trap) density to 1015cm-3 with a uniform energy distribution of defects for all layers, we achieved an η of 24.16%. A considerable enhancement in power-conversion efficiency ( η~27%) was perceived as we reduced the trap density to 1014cm-3 for the absorber layers. Furthermore, it was observed that, for the absorber layer with double-donor defect states, the active layer should be carefully synthesized to reduce crystal-order defects to keep the total defect density as low as 1017cm-3 to achieve efficient device characteristics.

Controllable construction 2D-CN photocatalyst for degradation MBT and mechanism insights

A nonmetal two-dimensional carbon nitride (2D-CN) with different thickness is constructed via the facile hydrothermal (adjusting the solution pH to 4.0, 7.0 and 10.0) and calcination method. The 2D-CN-7.0 displays the dramatically enhanced photocatalytic activity (82%) for degrading 2-mercaptobenzothiazole (MBT), originating from the ultrathin 2D structural improved transfer efficiency of charge carriers, greatly decrease the diffusion path of electrons/holes to MBT, accelerating the reaction rate. More importantly, we find that there are no rules can be followed between the band gap and the thickness of CN by density functional theory (DFT) computation theoretically and experimentally. Also, the possible mechanism for the enhancement of photocatalytic performance, the degradation pathway of MBT, photogenerated carrier transfer behaviors are also discussed in depth. This research provides a new strategy to control the thickness of CN to improve photocatalytic activity.

Improved atomic hydrogen desorption by Cu3N with suitable electronic structure to enhance photocatalytic H2 evolution

Photocatalytic hydrogen evolution reaction (HER) using solar energy as a driving force is considered to be one of the most promising approaches to alleviate energy and environmental problems. However, the problem of low solar conversion efficiency resulted from the easy recombination of photogenerated electron-hole pairs in semiconductor catalysts needs to be solved urgently. Theoretical calculations were firstly taken to prove that copper nitride (Cu3N) exhibits a matched D-band center position and hydrogen adsorption Gibbs free energy (ΔGH∗), which can improve the H2 adsorption/desorption balance and then achieve an effective promotion of photogenerated charge kinetics. Cu3N was herein chosen as a co-catalyst to enhance photocatalytic HER performance. Two-dimensional carbon nitride (2D CN) was chosen as the catalyst due to the layer structure which could shorten electronic transmission distance. The results showed that the HER activity of Cu3N/2D CN is 117 times higher than that of 2D CN (2721.55 vs. 23.18 μmol/g/h) and an apparent quantum yield of 3.78% at 420 nm. Compared with other Cu(I)-based composite catalysts, Cu3N/2D CN also exhibits the highest HER activity. This work investigates the transfer process of photogenerated carriers and reveals the photocatalytic mechanism through a combination of theoretical calculations and experimental characterization.

Two-dimensional polymerized carbon nitride coupled with (0 0 1)-facets-exposed titanium dioxide S-scheme heterojunction for photocatalytic degradation of norfloxacin

Poor visible light response and low separation efficiency of photogenerated carriers restricted the availability of titanium dioxide for antibiotic photodegradation, and the fabrication of two-dimensional/two-dimensional (2D/2D) heterojunction by coupling to a narrower bandgap semiconductor was an effective approach. Herein, a simple solvothermal method was used to grow 2D (001)-facets-exposed titanium dioxide ((001)-TiO2) nanosheets in situ on the surface of 2D polymeric carbon nitride (PCN) nanosheets to obtain PCN/(001)-TiO2 heterojunction for the photodegradation of norfloxacin. The synthesized heterojunction composites exhibited superior photodegradation performance, and the norfloxacin degradation rate reached up to 99% under simulated solar for 90 min. The efficient photocatalytic performance was attributed to the reduction of the bandgap and the formation of S-scheme heterojunctions, which enhanced the absorption of visible light and the carrier migration separation efficiency. Futhermore, the hierarchical 2D/2D nanosheet structure and the built-in electric field formed also sped up the charge transfer at the interface. This work presented a new viable solution to the problem of antibiotic contamination.

Strong Tribocatalytic Nitrogen Fixation of Graphite Carbon Nitride g-C3N4 through Harvesting Friction Energy.

Mechanical energy derived from friction is a kind of clean energy which is ubiquitous in nature. In this research, two-dimensional graphite carbon nitride (g-C3N4) is successfully applied to the conversion of nitrogen (N2) fixation through collecting the mechanical energy generated from the friction between a g-C3N4 catalyst and a stirring rod. At the stirring speed of 1000 r/min, the tribocatalytic ammonia radical (NH4+) generation rate of g-C3N4 can achieve 100.56 μmol·L−1·g−1·h−1 using methanol as a positive charge scavenger, which is 3.91 times higher than that without any scavengers. Meanwhile, ammonia is not generated without a catalyst or contact between the g-C3N4 catalyst and the stirring rod. The tribocatalytic effect originates from the friction between the g-C3N4 catalyst and the stirring rod which results in the charges transfer crossing the contact interface, then the positive and negative charges remain on the catalyst and the stirring rod respectively, which can further react with the substance dissolved in the reaction solution to achieve the conversion of N2 to ammonia. The effects of number and stirring speed of the rods on the performance of g-C3N4 tribocatalytic N2 fixation are further investigated. This excellent and efficient tribocatalysis can provide a potential avenue towards harvesting the mechanical energy in a natural environment.

Functionalized Two-Dimensional Carbon Nitride Nanodots Detect and Reverse Lead Toxicity in the Physiological Milieu

Functionalized Two-Dimensional Carbon Nitride Nanodots Detect and Reverse Lead Toxicity in the Physiological Milieu