Two-dimensional C2N Research Articles

P-d Orbital Hybridization Induced by Asymmetrical FeSn Dual Atom Sites Promotes the Oxygen Reduction Reaction.

With more flexible active sites and intermetal interaction, dual-atom catalysts (DACs) have emerged as a new frontier in various electrocatalytic reactions. Constructing a typical p-d orbital hybridization between p-block and d-block metal atoms may bring new avenues for manipulating the electronic properties and thus boosting the electrocatalytic activities. Herein, we report a distinctive heteronuclear dual-metal atom catalyst with asymmetrical FeSn dual atom sites embedded on a two-dimensional C2N nanosheet (FeSn-C2N), which displays excellent oxygen reduction reaction (ORR) performance with a half-wave potential of 0.914 V in an alkaline electrolyte. Theoretical calculations further unveil the powerful p-d orbital hybridization between p-block stannum and d-block ferrum in FeSn dual atom sites, which triggers electron delocalization and lowers the energy barrier of *OH protonation, consequently enhancing the ORR activity. In addition, the FeSn-C2N-based Zn-air battery provides a high maximum power density (265.5 mW cm-2) and a high specific capacity (754.6 mA h g-1). Consequently, this work validates the immense potential of p-d orbital hybridization along dual-metal atom catalysts and provides new perception into the logical design of heteronuclear DACs.

The potential of 2D carbon nitride monolayer as an efficient adsorbent for capturing mercury: A DFT study

Owing to its large surface area and the close similarity between nitrogen and mercury (Hg) in C3N, two-dimensional C3N monolayer has the potential of controlling the release of gaseous Hg0. Using density functional theory (DFT), we investigated the adsorption and conversion mechanism of Hg species on this monolayer surface. The findings demonstrated that C4N2 site on the surface of the C3N monolayer is more capable of adsorbing Hg0 than C6 site. Also, the recovery time of the C3N monolayer for Hg0 was approximately 17.63 s and 3.49 μs at 298 and 400 K, respectively. The analyses of electronic structure, charge transport, geometric structure, and adsorption stability demonstrated that the C3N monolayer is an encouraging material for Hg0 adsorption. The C3N monolayer could adsorb the oxidizing species of mercury (namely HgBr, HgCl, HgS, and HgO,) more easily than Hg0, with higher adsorption energy. Overall, the C3N monolayer is a promising sorbent material, which is capable of controlling gaseous mercury in an efficient way.

Removal of As2O3 gaseous by using C3N monolayer adsorbent: DFT study

Due to high affinity and extensive surface area between arsenic and nitrogen in C3N, the two-dimensional C3N monolayer has considerable the potential to effectively adjust the release of gaseous As2O3. By performing density functional theory (DFT) calculations, the adsorption of As2O3 on the surface of C3N monolayer was investigated. The results revealed that site C4N2 on the surface of the C3N monolayer has greater ability than site C6 to strongly adsorb As2O3. Also, the recovery time of the C3N monolayer for As2O3 were about 25.64 s and 5.23 μs at 298 and 400 K, respectively. The results related to the charge transfer (CT), adsorption stability, electronic and geometric structure confirmed that the C3N monolayer was an appropriate choice for adsorption of As2O3. Overall, the C3N monolayer can be considered as one of the most suitable adsorbents that can be used to effectively control gaseous As2O3.

Understanding the Activity and Design Principle of Dual-Atom Catalysts Supported on C2N for Oxygen Reduction and Evolution Reactions: From Homonuclear to Heteronuclear

Using large-scale ab initio calculations and taking the two-dimensional C2N monolayer as a substrate, we sampled a large combinatorial space of C2N-supported homonuclear and heteronuclear dual-atom catalysts and built a detailed view of catalytic activity and stability toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The results indicate that regulating combinations of metal pairs could widely tune the catalytic performance. Pd2-, Pt2-, and PdPt-C2N could effectively balance the adsorption strength of intermediates and achieve optimal bifunctional activity. The favorable catalytic performance could also be realized on GaPd-C2N for the ORR and PdRh-C2N for the OER, surpassing corresponding homonuclear counterparts. The thermodynamic and electrochemical stability simulations reveal that these metal pairs can be stably anchored onto the C2N matrix. Multiple-level descriptors, including Gibbs free energy, d-band center, and bonding/antibonding orbital population, are established to track the activity trend and reveal the origin of activity, indicating that catalytic activity is intrinsically governed by the d-band center of metal pairs.

Screening of transition metal doped two-dimensional C2N (TM-C2N) as high-performance catalyst for the non-oxidative propane dehydrogenation

Since last decade, single atom catalysts have massive development in the field of propane dehydrogenation (PDH). In this study, both the catalytic activity and reaction mechanism of 3d, 4d, and 5d transition metal atom anchored on C2N (TM-C2N) for non-oxidative PDH are studied in detail using the density functional theory methods. Firstly, six stable catalysts (Cr-, Mn-, Fe-, Co-, Cu-, and Ag-C2N) are screened out by calculating the difference between the formation energy of the single atom and the cohesive energy of the corresponding bulk metal. Combining the results of adsorption energy and density of states, it is demonstrated that propane and propylene can be stably adsorbed on all screened TM-C2N. The calculated activation barrier and reaction energy of elementary reactions involved in PDH indicate that the initial C−H bond breaking step is determined as the rate-determining step on almost all the screened TM-C2N catalysts. Among all studied TM-C2N, the Fe-C2N possesses excellent catalytic activity even better than other promising catalysts. In addition, the d-band center values of transition metal atoms in screened TM-C2N show a stronger liner relationship with the interaction energy, suggesting that the d-band center may be served as a priority descriptor for the interaction energy between reaction species and catalyst surface rather than for the adsorption energy of reaction species.

Theoretical study on the synthesis of vinyl acetate from acetylene and acetic acid over nonmetallic catalysts with different carbon-nitrogen ratios

At present, the acetylene acetic acid reaction catalysts studied are metal catalysts. Whether there are non-metallic materials that can catalyze the reaction is very worth exploring. In this study, we used density functional theory (DFT) to calculate the feasibility of preparing vinyl acetate (VAc) on four CN non-metallic materials (C2N, C3N, C4N and C5N) under the reaction conditions of 1 atm, 393.15–493.15 K at B3LYP/6-31G(d, p) level. It is found that after considering the reaction temperature, only C2N and C3N show good adsorption capacity for acetylene and acetic acid, and C3N has stronger adsorption capacity for reactants. After that, we calculated and simulated the reaction process of acetylene acetic acid reaction catalyzed by four CN materials. It is found that among the four CN materials, the energy barrier of the reaction rate control step of C3N is the lowest (the free energy barrier of the rate control step is only 30.57 kcal/mol at 393.15 K), and the interior of CN non-metallic materials has higher activity than the boundary of hydrogen sealing. We also found that the reaction process on CN non-metallic material is consistent with the acid catalysis mechanism, which shows that the non-metallic catalyst is theoretically feasible for catalyzing the acetic acid acidification of acetylene, and the two-dimensional C3N material is likely to be a potential non-metallic catalyst for the preparation of vinyl acetate by acetylene gas phase method.

Substitutional doping effect of C3N anode material: A first principles calculations study

Two-dimensional C3N has attracted various attention owing to its excellent electrical and mechanical properties. However, a critical issue is that the trap of Li, introduced by the vacancy defect during experimental preparation, will weaken the performance of the batteries. In this work, atom-doped regulation is proposed to repair the vacancy and further improve the performance of pristine C3N. The calculated formation energy indicates that C and B atoms are more favorable to appear as compared to other atoms such as Si, P, and N atom. Moreover, the Li ionic conductivity and the binding energy of Li on doped C3N are greatly improved compared to pristine C3N without the sacrifice of mechanical properties, and migration capability. These studies could provide theoretical guidance for the realization of high-performance C3N anode materials.

Bandgap engineering of two-dimensional C3N bilayers

Bandgap engineering of two-dimensional C3N bilayers

Dual-atom active sites embedded in two-dimensional C2N for efficient CO2 electroreduction: A computational study

Double-atom catalysts (DACs) have emerged as an enhanced platform of single-atom catalyst for promoting electrocatalytic CO2 reduction reaction (CO2RR). Herein, we present a density-functional theory study on CO2RR performance of seven C2N-supported homo- and heteronuclear DACs, denoted as M2@C2N. Our results demonstrate that there exists substantial synergistic effect of dual-metal-atom N2M2N2 active site and C2N matrix on OCO bond activation. The dual-atom M2 sites are able to drive CO2RR beyond C1 products with low limiting potential (UL). Specifically, C2H4 formation is preferred on FeM@C2N (M = Fe, Co, Ni, Cu) versus CH4 formation on CuM@C2N (M = Co, Ni, Cu). Furthermore, *CO+*CO co-binding strength can serve as a descriptor for CO2RR activity for making C2 products such that the moderate binding results in the lowest UL. Remarkably, C-affinity matters most to CC bond coupling and C2H4 formation while both C- and O-affinity control CH4 formation. Our results provide theoretical insight into rational design of DACs for efficient CO2RR.

CO2 electroreduction by transition metal-embedded two-dimensional C3N: A theoretical study

Transition metal (TM) and nitrogen co-doped carbon (T M-N -C) materials have emerged as the promising single-atom catalyst (SAC) for the electrocatalytic CO2 reduction reaction (CO2RR), for which the coordination environment of the anchored single atom plays an important role. Inspired by this, the C3N monolayers embedded with the TM single atoms at the CC double vacancy, denoted as M-CC (MTi, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au), have been evaluated for CO2RR by using the first-principles calculation. We find that the M-CC SACs are of high stability and metallic conductivity, beneficial for the electron transport during the electrocatalytic process. More importantly, all M-CC have high selectivity toward CO2RR versus the hydrogen evolution reaction. Especially, Cu-, Co-, Fe-, and Mn-CC have acceptable or ultra-low limiting potentials of −0.68, −0.48, −0.83, and −0.24 V to produce HCOOH, CH2O, CH3OH, and CH4, respectively. The catalytic activities of M-CC are correlated with the adsorption strength of the key intermediates, further rationalized by the bonding/antibonding population analysis. Our work proposes a new material platform to realize TM-C-N SACs for efficiently electrocatalytic CO2RR, which could provide useful insights into the design of such catalysts.

Water Photo-Oxidation Reaction on Clean and Doped Two-Dimensional Graphitic C<sub>2</sub>N

In the search for new efficient photo-catalysts for hydrogen production through water splitting, the main attention has been paid to tuning the band gap width and its position with respect to vacuum level. However, actual electro-catalytic activity for the water oxidation reaction on a catalyst surface is no less important than those quantities. In this work we evaluate from first principles the thermodynamics of the reaction on relatively new candidates for water splitting: two-dimensional C2N and that doped with phosphorus. We find that the 4-step reaction usually expected for water splitting will not proceed on these systems, resulting in oxygen atoms left strongly adsorbed to the surface. Another option, a 3-step reaction, is also found to be unfavorable. We also test an effect of higher oxygen coverage on the reaction thermodynamics, as suggested elsewhere. We find that, indeed, the doubled O-coverage makes the 4-step reaction feasible for the doped C2N. However, an unacceptably high anode potential is required to make this reaction proceed. We thus conclude that the materials under consideration may not be efficient electro-catalysts for water splitting.

Metallic two-dimensional C3N allotropes with electron and ion channels for high-performance Li-ion battery anode materials

Two-dimensional monolayer C3N has attracted much attention in many fields, owing to its unique mechanical, electrical and thermal properties and diverse structure. However, the physical origin of these properties lacks a systematic and clear understanding. In this work, three dynamically stable metallic C3N allotropes were computationally investigated. The cohesive energy of all these structures is within 0.09 eV/atom of that of the experimentally reported semi-conductive C3N structure. Density-functional theory calculations showed that the metallicity of two-dimensional C3N can be attributed to the continuity of the carbon chains, which form channels for passage of free electrons. Furthermore, additional calculations showed that the C3N allotropes possess excellent mechanical properties, good electronic conductivity and Li migration capability (wherein the C-C chains provide a good migration channel for lithium ions). These properties make C3N a promising candidate for Li-ion battery anode materials. Our findings suggest a physical mechanistic basis for modulating the electrochemical properties of such materials by controlling the arrangement of atoms. The study also provides new ideas for the modulation of the structure and properties of two-dimensional materials and points out a direction for the ongoing development and application of C-N materials and devices.

Two-dimensional C3N based sub-10 nanometer biosensor.

Detection and sequencing of various nucleobases are of immense usefulness that can revolutionise future medical diagnostics procedures. In this regard, the newly discovered 2D material, C3N, has demonstrated supreme potential for future nanoelectronic and spintronic developments due to its unique sets of electronic properties and structural similarity to graphene. Herein, we have investigated the effect of various nucleobases in the close vicinity of a C3N nanoribbon. Our extensive calculations revealed significant changes in the transport behaviour in the presence of DNA/RNA molecules. The transport response can be further modified through the (i) incorporation of doping, (ii) presence of defects, (iii) concentration of the adsorbed molecule, etc. Furthermore, in the presence of a gate voltage in a field-effect transistor (FET) geometry, the conductivity response can be improved significantly with an ∼100% change in the presence of an adsorbed molecule. The observation of a negative differential resistance (NDR) in the C3N system has also been reported here for the first time. Our current observation demonstrates the usefulness of the C3N system as a next generation bio-sensor for the sequencing of various nucleobases, offering new leads for future developments in bioelectronics, superior sensing architectures and sustainable designs.

Metal (M = Ru, Pd and Co) embedded in C2N with enhanced lithium storage properties

The improvement of the high-efficiency electrode materials for lithium ion batteries is one of the research priorities. The research on the carbon with M − N active sites (M-N-C) as anodes for lithium ion batteries is still quite rare. Herein, we report a series of highly crystalline M@C2N hybrids in which the metal (M = Ru, Pd and Co) are uniformly embedded in the two-dimensional C2N networks. Together with the unique structural features (e.g., uniform two-dimensional nanohole structure, high surface area and enlarged distance between C2N nanosheets), rich N-M coordination homogenously distributing through the M@C2N structure are favorable for the brisk infiltration of electrolyte, swift transportation of Li+/electrons and more storage of Li+, thereby leading to the superior lithium storage properties.

Two-dimensional carbon nitride (C3N) nanosheets as promising materials for H2S and NH3 elimination: A computational approach

Abstract Motivated by the increased emission of pollutant in the atmosphere, we explore the promise of pristine and defect carbon nitride (C 3N) monolayers for H2S and NH3 capture by means of van der Waals corrected density functional theory (DFT) calculation. Our results reveal that the adsorption energies ( Eb) of pristine C3N towards H2S and NH3 are 0.610 eV and 0.444 eV, respectively. However, a spontaneous partial dissociation of H2S is observed in case of single C and N vacancy in C3N monolayer with minimum Eb values of 1.90 and 1.577 eV, respectively. Furthermore, by using transition states (TS) calculation, we predicted a complete dissociation path of H2S by ab-initio molecular dynamics simulation at 1000 K. Results show complete dissociation of H2S molecule on C3N–C system. In addition, the trend of our results has demonstrated that O2 and H2O contaminations in our sample could barrier H2S adsorption. However, we also observed a physisorption phenomenon in the case of the interaction between NH3 and C3N. In addition, the binding energy of NH3 at C and N vacancies resulted in Eb values of 0.45 and 0.30 eV, respectively. This proves that the mono vacancy defect is highly selective toward H 2S gas than NH3. This study reveals a future for Two-Dimensional C3N nanosheets as H2S sensor.

Phonon-mediated superconductivity in charge doped and Li-deposited two dimensional C3N

By using first-principles calculations, we investigate the electron-phonon interaction and phonon-mediated superconductivity of charge doped and Li-deposited monolayer C3N. No superconductivity is found for 0.2 e/cell doped C3N due to a small electron-phonon coupling (EPC) strength which is less that 0.1. Surprisingly, when Li atom is deposited the monolayer, much more low lying optical phonon modes especially the out-of-plane vibration mode participate the EPC process. This results in a pronounced boost of EPC constant to 0.86 and the transition temperature Tc obtained according to isotropical McMillan–Allen–Dynes formula reaches 13.03 K, which is higher than 8.1 K of Li-deposited graphene and 1.4 K of Ca-deposited graphene. Our findings not only widening the potential application of the intriguing two-dimensional material C3N, but also enrich the world of two dimensional superconductors with a relatively high Tc.

Thermal conductivity of two-dimensional BC3: a comparative study with two-dimensional C3N.

The thermal conductivities of single-layer BC3 (SLBC) sheets and their responses to environmental temperature, vacancy defects and external strain have been studied and compared with those of single-layer C3N (SLCN) sheets by molecular dynamics (MD) simulations. We found that SLBC and SLCN are isotropic in the basal plane and that their predicted thermal conductivities for infinite length sheets are 488.54 W m-1 K-1 and 799.87 W m-1 K-1, respectively. Despite many similar features in the structures of these materials, SLBC exhibits a lower thermal conductivity than SLCN due to stronger flexural acoustic phonon-defect scattering rates and weaker interatomic bonding stiffnesses. The vibrational density of states (VDOS) are calculated in both structures to elucidate their thermal conductivity differences. SLBC exhibits a more substantial redshift phenomenon in the high- and low-frequency domains than SLCN. In addition, the thermal conductivities of these materials exhibit decreasing trends in response to increases in temperature and defect ratio, and the temperature effect in SLBC is more substantial than that in SLCN, while the defect effect in SLBC is less substantial than that in SLCN. The influences of uniaxial compressive and tensile strains on the thermal conductivities of these materials are analysed separately. These two deformation modes cause different effects on the thermal transport behaviours of SLBC and SLCN: the effect of uniaxial compressive strain is slightly negative, while the effect of uniaxial tensile strain is initially positive and then negative. Moreover, the biaxial strains result in a more severe reduction in thermal conductivity than the uniaxial strains. Remarkably, the impact of uniaxial and biaxial tensile strains on thermal transport was stronger in SLBC than in SLCN. We propose that SLBC nanomembranes are promising candidates for various thermal applications.

Mechanical deformation induced charge redistribution to promote the high performance of stretchable magnesium-ion batteries based on two-dimensional C2N anodes.

Stretchable batteries play a central role in stretchable electronics such as healthcare devices and sensors. However, challenges in stretchable batteries, such as unstable performance during the deformation process and their mechanical resistances, slow down their applications. In this paper, by adopting state-of-the-art first-principles calculations, the high performance of two-dimensional (2D) nitrogenated holey graphene C2N based stretchable magnesium-ion batteries (MgIBs) and its origin are determined. Their maximum capacity and average open circuit voltage are 1175 mA h g-1 and 0.447 eV, respectively, in the strain-free state, which are much more superior to most of the previously reported 2D anode materials. Mechanical activation is a facile and effective approach to induce charge redistribution to promote their high performance, i.e. their capacities are promoted under large compressive and tensile strains, whereas their high capacities are maintained under small compressive and tensile strains. The origin of the high performance of stretchable MgIBs is also proposed based on the evidence of electron localization function and charge density difference. Compressive and tensile strains remodulate the structures of electrode materials at the atomic scale and redistribute the electrons at the electronic scale, resulting in the reactivation of adsorption sites in electrode materials and maintaining the high performance of stretchable MgIBs. Additionally, large compressive strains enhance the stability of the C2N/Mg system, further increasing its capacities. The stretchable battery shows a two-stage diffusion mechanism in strain-free and strained states, i.e. in the first stage, the out-of-plane Mg ions diffuse quickly, and in the second stage, the in-plane Mg ions migrate moderately. Compared with the stretchable battery in the strain-free state, its barrier energies are lowered in the strained states. This study provides new insights and microscale mechanisms for non-stretchable and stretchable batteries with high performance and facilitates the innovation of low cost non-stretchable and stretchable batteries with 2D material anodes.

Adsorption mechanisms of different volatile organic compounds onto pristine C2N and Al-doped C2N monolayer: A DFT investigation

Volatile organic compounds (VOCs) are a class of air pollutants which seriously affect the ecological environment and threaten human health. Adsorption of VOCs is considered as a promising technology for VOCs enriching, separation and utilization. In this work, density functional theory (DFT) calculations are employed to investigate the adsorption properties of typical VOCs molecules onto C2N monolayer and Al-doped C2N system due to the huge specific surface and porous structure of newly-reported holey two-dimensional C2N. The results demonstrate that three typical VOCs: formaldehyde, benzene and trichloroethylene, can be absorbed onto pristine C2N monolayer with weak adsorption energy, while the adsorption ability can be improved with the formation of a strong chemical bond. Especially for the formaldehyde molecule, the best adsorption capacity on the Al-doped C2N surface with adsorption energy of −2.754 eV, nearly 5 times larger than the pristine C2N system. The PDOS analysis and electronic distribution results also show that doped Al atom can act as a bridge to link VOCs molecules with C2N monolayer and strengthen their interaction, which can significantly enhance the adsorption capacity. Therefore, Al doped C2N monolayer is demonstrated to be a promising adsorbent for VOCs enriching and utilization.

Structural characteristics and strain behavior of two-dimensional C3N : First principles calculations

In this study, structural characteristics and strain behaviors of C3N are investigated by first principles calculations. First, we investigate the stacking behaviors of C3N, and find the most stable order. Then, the band structure of C3N is investigated. We attempt to explain the disagreements between theoretical and experimental band gaps by impurity phase and biaxial strain. The band alignment at α−Al2O3/C3N interface is presented. The valence band offset (VBO) and conduction band offset (CBO) are larger than 1.49 eV, which suggest that the interface is a promising candidate used on short channel transistors. Finally, the strain behavior of C3N is studied. It is found that the monolayer C3N at 273 K can withstand up to a strain level of ε = 12% for zigzag and armchair uniaxial strain, and ε = 10% for ab biaxial strain. The band edge position, effective mass and band gap under the strain are presented. The CBO increases with the strain, which suggests that appropriate tensile strain can effectively prevent the leakage current of the device. Our investigation can provide valuable information for insight into the novel C3N materials.