1 Introduction
Since Andre Geim and Konstantin Novoselov first successfully separated graphene from graphite in 2004, it has been proved that two-dimensional materials can exist stably at room temperature and pressure, thus opening the prelude to human research on two-dimensional materials. In recent years, two-dimensional nanomaterials such as transition metal sulfides, Phosphorene and boron nitride (h-BN) have emerged (Figure 1) and become the research frontier of nanoscience and technology
[1][2][3]. Among them, graphene, as the most representative two-dimensional material, has attracted wide interest in the fields of energy and biomedicine due to its excellent electrical, thermal and mechanical properties
[4,5][6][7]. China's graphene research has formed a considerable scale, more than 40 graphene industrial parks, innovation centers and research institutes have been built or under construction, and more than 2000 enterprises are engaged in the development of graphene raw materials and products, and these data are still growing.
In the frontier research of graphene, in order to improve the performance of graphene, post-functionalization has become a powerful strategy. Among them, nitrogen-doped graphene has attracted wide attention. Nitrogen atoms are closest to carbon atoms in size and have five outermost electrons, which are easy to integrate into the conjugated skeleton of graphene in the form of sp
2 hybridization, showing high compatibility and stability
[8]. However, the existing nitrogen-doped graphene methods have the disadvantages of high randomness, inaccurate structure and low nitrogen content
[9].
Therefore, the construction of nitrogen-rich infinite network structure through the polymerization of nitrogen-rich monomers is a powerful means to achieve precise nitrogen doping. In 2009, Thomas et al. In Germany prepared the covalent triazine framework material CTF-1 by cyclotrimerization of nitrile groups under ionothermal conditions (as shown in Figure 1)
[10]. Baek et al. Developed a new preparation method of CTF-1 based on the non-metallized cyclotrimerization strategy of cyano group
[11]. Tan Bien and Jin Shangbin improved the synthesis of CTFs based on the copolycondensation of aldehyde and amidine, which has good universality
[12]. The synthesized CTFs show good application performance in photocatalysis and energy storage.
The preparation of nitrogen-rich infinite network structures containing pyrazine structure through the condensation reaction of o-diketone and o-diamine has also been greatly developed in recent years. In 2011, based on the polycondensation of 1,2,4,5-tetraaminobenzene and cyclohexane hexanone, Jiang Donglin et al. Prepared aza-CMP (as shown in Figure 1), which has a specific surface area of up to 1227 m
2·g
-1 after heat treatment
[13]. In 2017, Wang Changchun and Guo Jia realized the self-polymerization of hexaazabenzonaphthalene (HATNA) along the coplanar direction, and the new material aza-MGP (Fig. 1) showed microporosity and high specific surface area, which could promote anhydrous proton conduction and catalyze CO
2 cycloaddition reactions
[14]. New nitrogen-rich infinite network structures are emerging, which fully verifies the feasibility of bottom-up preparation method.
Bottom-up synthesis of carbon nitride materials with specific stoichiometry has become an important research direction, among which graphite phase carbon nitride (g-C
3N
4, see Figure 1 for structure) is the most representative. The g-C
3N
4 is stable at room temperature and pressure and has a graphite-like layered structure, and the C and N atoms are hybridized by sp
2 to form a π-conjugated infinite extension plane. In 2009, Wang Xinchen et al. reported for the first time that g-C
3N
4 could be used to produce hydrogen by photolysis of water under visible light. The g-C
3N
4 has a suitable semiconductor side band position (band gap ~ 2.7 eV) and can absorb blue-violet light with a wavelength less than 475 nm, which meets the requirements of water photolysis and can be used as a visible light photocatalyst for solar energy conversion
[15][16]. The formation of hydrogen bonds between g-C
3N
4 and water can induce the generation of interfacial excitons, which can separate photogenerated electrons and holes
[17]. The specific surface area of g-C
3N
4 prepared by traditional methods is small, and the photocatalytic activity is difficult to meet the requirements. The light absorption can be improved by modification. The common method is to compound g-C
3N
4 with other materials
[18][19,20].
In addition, other special stoichiometric carbon nitride materials, such as C
3N, C
4N and C
2N (see fig. 1), can also be synthesized by bottom-up strategy with specific precursors
[21,22][23]. The nitrogen content of that carbon nitride material is fixed, the nitrogen atom are periodically distributed, and the nitrogen doping positions are precise. Taking the two-dimensional honeycomb crystal C
3N material as an example, it contains uniformly distributed nitrogen atoms, and its absorption spectrum can be adjusted to cover the entire visible light range. C
3N doped with a small amount of hydrogen show ferromagnetism at low temperatures (less than 96 K), which is a rare non-metallic ferromagnet
[24].
In order to make up for the unadjustable conduction band of graphene, Baek and others in Korea have creatively synthesized porous nitrogen-containing graphene materials (C
2N) and found that they have very good field-effect transistor effect
[25]. Since then, research work on C
2N materials in energy storage, gas separation, optoelectronic devices and other fields has emerged. This review focuses on the recent advances in the properties, synthesis and potential applications of C
2N materials.
2 Structure and Properties of C2N Material
Properties, as the basis of application, are important criteria for evaluating the use value of C2N materials. In view of the excellent properties of existing two-dimensional materials, C2N materials are also expected. This paper will focus on the mechanical, optical absorption, thermal conductivity and electromagnetic properties of C2N materials. Due to the difficulty of peeling off the single-layer C2N crystal, the following properties are calculated by first-principles.
2.1 Single Layer Structure
The monolayer C
2N crystal has a planar two-dimensional structure (Fig. 2), containing one C — N bond (1.3434 Å) and two nonequivalent C — C bonds, a long bond facing the hole (1.4747 Å) and a short bond located inside (1.4343 Å), so that each benzene ring is slightly deformed by the surrounding N atoms. The length of C — C short bond is similar to that of C — N bond, which indicates that the crystal structure of C
2N is constructed by strong carbon-nitrogen hybridization, and both the original benzene ring and the new pyrazine ring have graphitization characteristics. It is worth noting that the semiconductor behavior of C
2N crystals with a large band gap (1.96 eV) is observed, which is the result of strong carbon-nitrogen hybridization. The C — N — C bond angle is 118 °, which means that the C
2N cavity is approximately a hexagon with a radius of 5.5252 Å. The cavity provides a series of reaction sites for the binding of adatoms or molecules, creating a large number of opportunities for further functionalization
[26]. All the holes are periodically distributed in the fused ring aromatic network structure, and the interior of the crystal can be regarded as a two-dimensional honeycomb lattice connected by nitrogen atoms, with a lattice constant of 8.3333 Å found by geometric optimization
[27].
2.2 Mechanical Properties
The aromatic structure of C
2N materials endows them with special mechanical properties. The cohesive energy of each atom was calculated to be 7.64 eV,C
2N and the in-plane stiffness to be 9.27 eVÅ
-2, indicating a strong bond between the C and N atoms. A single layer of C
2N material has a Poisson's ratio of 0.26, meaning that when the material is compressed in one direction, it expands in the other
[28]. Compared with graphene (0.16), the Poisson's ratio of C
2N material is larger, which is attributed to its porous structure. At 0 K, the elastic modulus is (400 ± 5) GPa, and the elastic modulus and tensile strength decrease with the increase of temperature.The elastic modulus and tensile strength of the C
2N material decrease to (335 ± 5) GPa and 60 GPa at room temperature, which are close to the g-C
3N
4 of a single layer
[29].
In order to explore the elastic limit of C
2N materials, Yao Yugui et al calculated the strain-stress relationship
[27]. It is observed that the C
2N material exhibits a linear elastic response when subjected to a uniaxial strain of about 8%. The uniaxial tensile strain limits in the armchair and zigzag directions (fig. 3) are 13% and 17%, respectively, when the material is subjected to biaxial strains up to 12%. Due to the presence of holes and weak C — N bonds, these values are slightly lower than those of graphene and hexagonal boron nitride (h-BN), but much higher than those of traditional three-dimensional materials. For slight deformation, the elasticity of a two-dimensional material can be characterized by an in-plane stiffness constant of about 71 N/m calculated C
2N along the chair and sawtooth directions. It is lower than that of graphene (about 340 ± 40 N/m), MoS
2(140 N/m) and h-BN (267 N/m), indicating that the single-layer C
2N material has good flexibility, which is helpful to improve its properties by using strain, and provides a theoretical basis for adjusting the band gap and optical absorption range.
2.3 Optical properties
Photocatalysis is based on the energy band theory of N-type semiconductors. The single-layer C
2N material has a semiconductor band gap of 1.96 eV, which is higher than the theoretical lowest energy band width (1.23 eV) for water photolysis. It can be used as a photocatalyst and mainly responds to near-infrared to green light
[30]. Compared with the g-C
3N
4 with similar structure, the single-layer C
2N material has a smaller band gap (C
2N:~2.0 eV,g-C
3N
4:~2.7 eV) than the g-C
3N
4.It shows strong and extensive optical absorption in the visible region, so C
2N materials may perform better than g-C
3N
4 in solar energy conversion
[31]. The absorption and reflection spectra of the C
2N nanosheet have a broad peak at about 2.6 eV, with a maximum absorption of 3.2×10
-5cm
-1 and a reflectivity of about 0.27%. More interestingly, the application of biaxial or uniaxial strain to single-layer C
2N nanosheets can adjust their band gap values in the range up to 1 eV, thus changing their optical properties and achieving strong optical absorption in a wider wavelength range, which has great application prospects in the field of optoelectronic devices
[32][27].
The special band gap adjustability of the C
2N material can adjust other two-dimensional materials, for example, the C
2N substrate can generate an electrostatic potential on the surface of the graphene, which can open the band gap of the graphene, and the maximum band gap can reach 0.239 eV.Compared with a pure single-layer C
2N, the graphene /C
2N shows enhanced optical activity and becomes a candidate material for photovoltaic devices
[33].
In addition, the luminescent properties of C
2N materials have also attracted much attention. Yu Dingshan et al. Creatively dispersed the newly prepared C
2N quantum dots (C
2N-QD, size < 5 mm) stably in water, which not only showed blue single-photon luminescence (OPL) under ultraviolet (UV) excitation, but also showed green two-photon induced luminescence (TPL) in the near infrared (NIR) wide excitation range of 750 ~ 900 nm, and could be directly used as a new fluorescent ink (Figure 4)
[34].
2.4 Thermal Conductivity
The intrinsic thermal conductivity of C
2N is 64.8 W·m
-1·K
-1(300 K), and the effective phonon mean free path is 34. 0 nm, which are smaller than those of graphene
[29]. The lattice thermal conductivity of the C
2N material at room temperature is about 82.22 W·m
-1·K
-1, which is almost two orders of magnitude lower than that of graphene (about 3500 W·m
-1·K
-1).Due to the introduction of nitrogen atoms and the porous structure, the phonon transport rate is reduced and the lifetime is shortened, resulting in a decrease in thermal conductivity.The thermal conductivity also decreases with increasing temperature and tensile strain
[35][36]. Although the thermal conductivity of the C
2N material is not as good as that of graphene, some studies have confirmed that its thermal management capability is superior to that of graphene. The thermal conductivity of the C
2N is 2% -3% of that of graphene, but the interfacial thermal conductivity between the C
2N and silicon dioxide is about 171% of that of grapheme, which can be used as an excellent thermal interface material in the field of chip thermal management (as shown in Figure 5)
[37].
2.5 Lectromagnetic property
The semiconductor and electromagnetic properties of C
2N have always been the most concerned. All multilayer C
2N materials have a direct bandgap regardless of the stacking order and number of layers. Because the interlayer coupling of multilayer C
2N is mainly in orbital interaction, the band structure depends on the stacking order between layers, and the band gap decreases monotonically with the increase of the number of layers. When an external electric field is applied to the multilayer C
2N, the band gap of the multilayer C
2N is reduced along with the increase of the electric field, and even a transition from a semiconductor to a conductor is triggered; and the adjustable range of the band gap is wide, so that the layered C
2N material can be applied to a nano-scale photoelectric device, and the defect that the conduction band of graphene and other materials cannot be adjusted is successfully overcome
[30].
In order to explore the transition of C
2N materials from semiconductor to conductor, scientists cut single-layer C
2N into nanoribbons (C
2NNR), including chair (A) or zigzag (Z)C
2N nanoribbons. The AC
2NNR is a nonmagnetic semiconductor with a direct band gap and is very sensitive to edge modification. When the edge is modified by H/O or F/O simultaneously, it turns into a magnetic conductor
[38,39]. While the ZC
2NNR is a magnetic semiconductor or conductor with an indirect bandgap that decreases with increasing width
[38][40]. In addition, the width and the symmetry of the ZC
2NNR can be adjusted, thereby having application value in the fields of photoelectricity and spin electronic devices
[41].
C
2N material is a rare non-metallic ferromagnet at room temperature. When a single layer of porous C
2N is doped with a low concentration of carriers (electrons or holes), it will show spontaneous ferromagnetism. If electrons are doped, the carriers are fully spin-polarized in the doping range of (4~8)×10
13cm
-2, and the Curie temperature is predicted to reach 320 K
[42]. If doped with holes, the C
2N can be induced into ferromagnetism at a very low critical concentration (5×10
13cm
-2), and increasing the hole density can also increase both the magnetic moment and the magnetic coupling
[43]. C
2N is the first room-temperature magnetic two-dimensional nonmetallic material with a tunable gate. When the C content reaches a certain value, the Curie temperature drops to about 297 K, which can be used in room-temperature spintronic devices
[44]. The above results indicate that the layered C
2N materials are promising for use as nanoscale magnetic spintronic devices.
To sum up, the special structural properties of C2N materials determine that they have excellent mechanical, optical absorption, thermal conductivity and electromagnetic properties, which provide a solid foundation for C2N materials as membranes, electrical devices and photocatalysts. In order to verify more structural and performance advantages, more C2N samples are needed for characterization and verification, so simple and feasible synthesis methods are required.
3 Synthesis and Characterization of C2N Material
At present, the synthesis methods of C2N materials are based on nitrogen-rich organic compounds through various polymerization reactions, which basically follow the bottom-up synthesis strategy. For C2N, a carbon material that does not exist in nature, the bottom-up method has the advantages of easy operation, low cost and high production efficiency, but it lacks precise control over the shape and size of particles, so it is necessary to synthesize more morphologies of C2N materials by top-down strategy in order to realize the multi-functional application of C2N.
3.1 bottom-up method
3.1.1 Condensation reaction method based on ketone carbonyl and amino group
In 2015, Beak et al. In Korea directly synthesized high-purity and high-stability HAB trihydrochloride from p-nitroaniline through nitration, C-amino substitution and catalytic hydrogenation, with a yield of 92%
[45]. After solving the problem of the source of raw materials, they first obtained two-dimensional layered network C
2N-h2D materials by acid-catalyzed dehydration polycondensation of hexaaminobenzene (HAB) and cyclohexanehexanone (HKH). The polycondensation process was completely spontaneous and the activation energy of the reaction was ΔG=-89.7 kcal·mol
-1[25]. They used elemental analysis and X-ray photoelectron spectroscopy (XPS) to characterize that the molar ratio of C to N in the product was about 2:1. Meanwhile, the dispersion liquid of the C
2N material is spread on a SiO
2 substrate and is subjected to heat treatment at 700 deg C under the protection of argon, so that the C
2N multilayer nanosheet with metallic luster can be collected. C
2N samples were thermally deposited on Cu (111) substrates under ultra-high vacuum (UHV) conditions, and the hexagonal honeycomb-like pores were observed by high-resolution scanning tunneling microscopy (STM). The uniformly distributed nitrogen-rich pores are composed of benzene rings and pyrazine rings alternately, and the diameter of the pores is about 0.83 nm (as shown in Figure 6), which is consistent with the measurement results.
The powder X-ray diffraction (XRD) pattern of the C2N material showed a clear 002 diffraction peak at 27.12 °, corresponding to a layer spacing of 0.328 nm, indicating a highly crystalline layer-packed structure. The interlayer spacing is slightly narrower than that of graphite (0. 335 nm), which is due to the uniform distribution of nitrogen-containing holes, the smaller size of nitrogen atoms, the greater electronegativity and the stronger interlayer interaction force. However, the packing of the C2N material layer is too tight, and the strong hydrogen bonding between the nitrogen atoms and the residual solvent leads to the blockage of the pores, resulting in a significantly low BET surface area, which needs to be improved by heat treatment at 700 ℃ (281 m2·g-1). The uniform distribution of nitrogen-rich pores in C2N materials can give them more potential applications, such as organic non-metallic magnets, selective adsorption of metals or biomacromolecules, and the design of new catalysts.
3.1.2 Ammoniation polymerization based on chloranilic acid
In 2018, Lee and others in Korea took a new approach, using chloranilic acid as the starting material, and obtained three-dimensional C
2N aerogel materials with high specific surface area through a series of chemical conversion reactions and polymerization reactions (Figure 7). They first ammoniated chloranilic acid with ethylenediamine as the nitrogen source to obtain the key monomer HAB. Then HAB is polymerized at high temperature in the presence of chloranilic acid, L-alanine, L-cysteine and the like to form an S-doped π-conjugated polymer structure. Finally, the three-dimensional C
2N aerogel material with the multilevel pore structure is obtained through freeze-drying and high-temperature cracking treatment
[46]. The spongy C
2N aerogel has high BET specific surface area (1943 m
2·g
-1), pore volume (1.56 cm
3·g
-1) and extremely low mass density (only 64 mg/m
3), and can be suspended on dandelion. SEM images show that the interior of the nanobelts is a three-dimensional porous network structure composed of twisted nanobelts, with a width of 150-200 nm and a length of several microns. High-resolution TEM images show that the C
2N material has a (002) lattice plane similar to that of graphitic carbon, and the lattice spacing is about 0. 322 nm; STM images show that the material has a well-distributed hexagonal pore structure with a highly crystalline pore diameter of about 0. 827 nm, which is also close to the theoretical value.
3.1.3 HAT-based condensation polymerization
German Oschatz et al. Used hexacyanohexaazatriphenylene (HAT-6CN) as a precursor for carbonization reaction, that is, small molecules were removed through thermal condensation reaction between phthalocyanines, thus forming a C
2N material with a polypyrazine structure (as shown in Figure 8)
[47]. HAT-6CN is an electron-deficient, rigid aromatic discotic molecule with excellent π-π stacking ability, which is an ideal precursor for polycondensation. Oschatz et al. Directly polymerized HAT-6 CN at different temperatures of 550, 700, 850 and 1000 K without any template, and the polycondensation yields were 50%, 33%, 27% and 25%, respectively. Among them, the former three show a stoichiometric ratio close to the C
2N, only the latter is highly carbonized and the nitrogen content is significantly reduced. The highest BET specific surface area (814 m
2·g
-1) was obtained at 850 K.
The structure of the carbon nitride material obtained by the method is plane accumulation with smooth surface. Unlike the high crystallinity of the former two C2N, there is no sharp peak in the powder XRD pattern, which confirms the amorphous structure of graphite-like stacking. The C2N material obtained by the method has a large number of nitrogen atoms, so that the structure is strongly polarized and specific binding sites can be provided, and the interaction enthalpy between the adsorbent and the guest is greatly improved. For example, there is a strong interaction with Lewis acidic carbon dioxide and water which is easy to form hydrogen bonds. The maximum adsorption capacity of C2N-700 K for carbon dioxide can reach 5.34 mmol·g-1(273 K,1 bar), and the maximum adsorption capacity for water is 10.22 mmol·g-1(p/p0=0.2).
3.1.4 Comparison and evaluation
It is worth mentioning that all the above synthesis methods do not need any template assistance, and only through the polymerization of organic compound monomers to construct two-dimensional or three-dimensional C2N materials. The synthesis methods and product comparison of the three products are shown in Table 1. Among them, the first two schemes are wet synthesis, although the steps are relatively more, but the overall operation is highly controllable, they provide a simple and efficient scheme for the construction of fused ring aromatic two-dimensional network structure, and promote the development of more functional two-dimensional materials. The method (3) is a dry synthesis without any solvent, and the self-polymerization is initiated only by heating, so that the purpose of accurately constructing the structure is difficult to achieve, the product finally has the appearance of an amorphous porous carbon material, and the most striking property of the amorphous porous carbon material is only microporosity and strong adsorption effect on molecules such as water and carbon dioxide, so that other application values are difficult to develop. Comparing the two wet synthesis methods in detail, there are still differences. First of all, in the synthesis method of monomer HAB, Baek and others use TATB reduction method. TATB is a well-known energetic material, which is difficult to obtain or synthesize. However, with the help of ammoniation of chloranilic acid, the starting materials of chloranilic acid, ethylenediamine and L-alanine are easier to obtain, which reduces the difficulty and cost of synthesis. However, in terms of polymerization methods, Baek et al. Reported the condensation polymerization of ketone carbonyl and amino groups, which can obtain a very precise C2Nstructure. The product is a two-dimensional crystal with metallic luster, semiconductor band gap and FET effect. However, Lee et al. Used many kinds of reactants, complicated steps and longer time-consuming, resulting in very complex polymerization direction and path, resulting in three-dimensional aerogel morphology of C2N materials, inaccurate structure and low synthesis efficiency. As for the pore properties of the product, unlike the multi-layer two-dimensional C2N material synthesized by Baek et al., the three-dimensional C2N aerogel material obtained by Lee et al. Is formed by cross-linking polymerization of two-dimensional porous C2N material structure in three-dimensional space, showing a higher BET specific surface area, but its three-dimensional cross-linking structure leads to its modifiability is not as good as the former, which ultimately limits the scope of application.
At present, the bottleneck of C
2N synthesis lies in the difficulty of obtaining monomer HAB and the time-consuming and energy-consuming polymerization conditions. Our group has solved the problem of safe and efficient preparation of HAB, but the polymerization conditions still need to be improved
[48,49]. Comparing the three methods, the method reported by Baek et al is the easiest to realize and the product morphology is better, which is the main method to prepare C
2N materials at present, and the C
2N materials prepared by this method are mostly used in subsequent modification or application.
3.2 Top-down method (bottom-up)
Baek et al. Successfully synthesized C2N materials through a bottom-up method, but the latest research is still mainly focused on C2N bulk crystals or aerogels, which are difficult to synthesize some special morphologies, and there is polydispersity of shape and size, so the relevant research is still in the early stage, which to some extent hinders the discussion of their inherent characteristics and potential uses. Therefore, the top-down strategy is used to synthesize C2N materials with specific morphologies, which is expected to expand their functions.
Xu Hangxun et al. Successfully prepared C
2N nanosheets (C
2NNS), and found that acidification and ultrasonic treatment significantly improved the exfoliation rate of C
2N materials. The exfoliation rate of the C
2N material was only 7% in pure water, but increased to 44% in 12 M hydrochloric acid. The TEM images of the exfoliated C
2N material confirmed the ultrathin sheet structure. The average thickness of the exfoliated ultrathin C
2N nanosheet was less than 5 nm as measured by atomic force microscopy. The principle is that the high protonation of the heterocyclic nitrogen atoms in the framework leads to the formation of strong hydrogen bonds between the surface and water molecules, which promotes the exfoliation process, and the resulting ultra-thin nanosheets of C
2N are confirmed to be new and efficient catalysts for water photolysis (Fig. 9)
[50].
On the basis of the above work, Yu Dingshan et al. Converted the C
2N material into zero-dimensional quantum dots (C
2NQD) by top-down strategy, and the average particle size of the prepared C
2NQD was less than 5 nm, which could be adjusted by changing the time of acid etching and ultrasonic treatment
[34]. The C
2NQD has abundant oxygen-containing functional groups and dense active edges, showing good post-modification ability. The C
2NQD has excellent dispersibility in water, the concentration of the aqueous solution can reach about 0.47 mg·mL
-1, and even after 2 months, the solution remains quite stable without obvious precipitation under ambient conditions. C
2NQD is not only expected to be used as a new fluorescent ink due to its quantum effect, but also as a metal-free catalyst to promote the redox kinetics of polysulfides in Li-S batteries. It can be seen that the success of C
2NQD with porous properties enriches the family of C
2N materials, and also provides many opportunities for the research of C
2N in optics, photocatalysis, biological imaging, energy storage and conversion.
It can be seen that the internal structure and physical properties of C2N materials synthesized by top-down method are consistent with the bulk, but the special morphology further broadens its application field. At the same time, due to the abundant hexagonal nitrogen-rich pores in the interior, the C2N material can stably adsorb or combine gas molecules, metal ions and biological macromolecules, and becomes a prominent adsorption and separation material and a biological transport material. The potential application value of C2N materials has been fully confirmed by the existing research.
4 Application of C2N Material
4.1 Lectrical device
4.1.1 Lithium/sodium ion battery electrode material
The conjugated skeleton structure of the C
2N material is stable and can efficiently conduct positive charges, and the C
2N electrode has high reversible capacity and cycle stability
[51]. The nitrogen atoms have strong trapping ability to light metals and can be used as electrode materials of lithium/sodium ion batteries. Hankel et al. calculated by DFT theory and found that the nitrogen atoms in the monolayer C
2N have strong interaction with lithium, sodium and calcium ions, which shows high initial storage capacity, the maximum theoretical capacity for Na
+ and Li
+ is 6 to 8 times that of graphite, the cycle stability is high, and the battery can be charged/discharged quickly
[52][53]. If the transition metals (Ru, Pd and Co) are embedded into the two-dimensional C
2N network, the interlayer spacing of the C
2N can be widened, the specific surface area can be increased, more space can be provided for Li
+/e
- transport, and the maximum capacity can be greatly improved
[54].
The C
2N material is also expected to be used as a cathode material for a lithium-sulfur battery (LSB). The intermediate products (polysulfides) of lithium ion batteries are easily dissolved in organic electrolytes, which increases the viscosity of electrolytes and reduces the ionic conductivity, resulting in the loss of active materials and the waste of electric energy. The calculation results of Wang et al show that polysulfides can adhere to the C
2N matrix in a thermodynamically stable cluster state and are no longer soluble in the electrolyte
[55]. The effect is better when the single-layer C
2N is embedded with transition metals, not only the conductivity is improved, but also the adsorption of lithium polysulfide (LiPS) is enhanced and the decomposition of Li
2S is promoted. Among them, the monolayer Co@C
2N has the highest LiPS adsorption energy and the lowest Li
2S decomposition energy, which is the best choice for LSB cathode material
[56]4.1.2 Optoelectronic device
The C
2N materials mentioned above have considerable light absorption range and capacity, and the band gap can be adjusted. The layered C
2N materials are often combined with other two-dimensional materials to form vertical heterostructures. So far, 2D materials including GeC, MoS
2, GaTe, α-Te, h-BN, MoSe
2, WSe
2, β-Sb, blue phosphorus, borene and graphene (Figure 10) have been used to form heterostructures, and these combinations can effectively separate photogenerated electrons and holes
[57][58][59⇓~61][62][63][64, 65][66][67][68][69,70]. The heterojunction shows strong absorption in the visible and near ultraviolet regions, which is stronger than that of the single component. In addition, the substitution of homologue elements (P, As) for the monolayer C
2N can also improve the optical absorption properties
[71]4.1.3 Non-metallic magnet
C
2N materials become rare nonmetallic magnets when they are doped. The results of first-principles calculations show that the C
2N materials doped with H and B have a finite magnetic moment
[72]. C
2N materials doped with Cl, Br and I will become non-metallic magnetic semiconductors due to the induced magnetism of the local magnetic moment of halogen atoms
[73]. The ferromagnetic transition metal atoms (such as Fe, Co, Ni, etc.) bound to the C
2N material become metallic and are spin-polarized, especially the Fe-C
2N can induce a ferromagnetic coupling moment of 3. 91 μB, which is expected to be used as a nanoscale spintronic device
[74]. To sum up, different types of heteroatoms combined with or substituted for C
2N materials may produce different electromagnetic effects (conductors, magnetic semiconductors, or ferromagnets) for specific applications
[75].
4.2 Adsorption separation
4.2.1 Seawater desalination
The C
2N material contains abundant hexagonal nitrogen-rich pores (8.33 Å in diameter), which is expected to be a new nanofiltration material. In 2016, Zhao Mingwen et al. Predicted that single-layer C
2N could be used as high-efficiency filters for seawater desalination through first-principle calculations. The monolayer C
2N has strong water permeability and salt filtration capacity, and the water permeability is almost one order of magnitude higher than that of graphene. Due to the hydrogen bonds formed between the C
2N material and water molecules, the tensile strain can precisely adjust the on-off state of the monolayer C
2N filter. When the biaxial tensile strain is greater than 4%, the C
2N filter can only permeate water molecules and is completely impermeable to salt ion
[76][77]4.2.2 Isotope separation
In 2016, Zhao Mingwen and others theoretically confirmed that C
2N materials can achieve effective separation of
3He/
4He through quantum sieving effect, and moderate tensile strain can effectively adjust the quantum sieving ability of C
2N materials.The C
2N material is also applicable to that separation of other isotope combination such as H
2/D
2, H
2/T
2 and the like, opens up a new way for the separation of light isotope in industry, and has great significance for enriching isotopes in industrial application
[78]4.2.3 Gas adsorption, detection and separation
The nitrogen-rich pores of C
2N materials endow them with considerable potential for gas separation. The DFT calculation shows that the monolayer C
2N material has a pore size and an energy barrier (0.13 eV) suitable for He molecules in a wide temperature range, and can highly selectively adsorb He in the mixed systems of He/Ne(Ar
4, CH
4, CO
2, N
2, etc., thus becoming an excellent candidate for separating He from natural gas
[79].
Carbonaceous materials are the best hydrogen storage materials at present, with high storage efficiency, safety and reliability, and the results show that C
2N materials have better hydrogen storage potential. The calculation proves that the C
2N sheet has extremely high selective adsorption capacity for H
2, and the adsorption capacity is as high as 10.5 wt%, which is superior to that of the nano-polymer membrane and the silicon dioxide membrane
[80]. Ion doping or metal modification (Fig. 11) can further improve the hydrogen storage capacity. For example, the hydrogen storage capacity of Li-C
2N combination at room temperature is high (13 wt%), which is 40% and 30% higher than that of MgH
2 and C
60, and much higher than that of alkali-modified graphene, MoS
2 and phosphene
[81][82].
The N atoms contained in the pores of the C
2N have affinity for the CO
2, and the effect is better after metal modification. The Ca-C
2N is considered to be a strong adsorbent for the CO
2 (as shown in Figure 11). The simulation results show that at room temperature, the maximum adsorption capacity of the Ca-C
2N reaches 50 wt% at 30 bar and 23 wt% at 1 bar, which is higher than that of all two-dimensional materials
[83]. For similar polluting gases, Co
2-C
2N and Ni
2-C
2N can adsorb and degrade SF
6, and monolayer C
2N can also effectively trap HF, HCN and H
2S (Fig. 11). Electrostatic interaction and hydrogen bonding dominate the adsorption process
[84][85]. Al-C
2N can adsorb formaldehyde, benzene and vinyl chloride
[86]. However, the conductivity of the C
2N material adsorbing NH
3 or NO changes significantly. This unique sensitivity to the adsorbate makes the C
2N hopeful to be used as a new type of gas sensor (as shown in Figure 11). If it is replaced by other gases or shows weak physical adsorption, the conductivity is almost unchanged. Or unable to desorb
[87].
4.3 Green Catalysis
4.3.1 Immobilized metal catalyst
C
2N materials benefit from abundant active sites and porous structure, and can be used to prepare nano-catalyst TM
x@C
2N with different dispersion degrees by complexing transition metals with nitrogen-doped graphene and g-C
3N
4[88][89]. Baek et al. In Korea introduced a variety of transition metals into the framework of C
2N, such as Ru@C
2N catalysts, which have excellent stability in acid/alkaline media, and the catalytic effect of water electrolysis is not weaker than that of platinum catalysts
[90]. The Co@C
2N has excellent catalytic activity on that hydrolysis of alkaline NaBH
4, the maximum hydrogen production rate is close to that of a noble metal catalyst, and the nitro group can be catalyze to be reduced into an amino group in situ unde the assistance of the NaBH
4[91]; The stability and ORR activity of the Fe@C
2N catalyst in acid/alkali media are superior to that of the Pt/C catalyst, and the C
2N protects the active Fe nanoparticles and reduces direct contact
[92].
The simulation results show that TM
x@C
2N catalysts are basically conductive and can be used as non-noble metal catalysts for water electrolysis (as shown in Figure 12), with low cost and high durability. They can effectively accumulate surface polarization charges, transfer them to adsorbed oxygen molecules, and stimulate the catalytic performance of metals
[93][94,95]. TM
x@C
2N catalysts are also suitable for the electrochemical reduction of CO
2 (CO
2RR), and metals can activate inert CO
2 molecules.Cu
2@C
2N is a typical example (Figure 12), and heterometallic Ni-Co@C
2N catalysts can even reduce CO
2 to CH
4[96][97].
TM
x@C
2N catalysts can also be used to degrade pollutants. Taking carbon monoxide (CO) as an example, the catalysts can degrade CO by two ways of normal temperature oxidation and electro-reduction, respectively. Cu
2@C
2N, Fe@C
2N and Fe-Cu@C
2N can adsorb and activate CO and O
2 to promote the oxidation of CO at normal temperature
[98][99][100]. The Mn@C
2N activates the adsorbed CO and electroreduces the CO to methane, ethanol, and propylene
[101].
The TM
x@C
2N catalyst can also be used as a novel artificial nitrogen fixation catalyst, the nitrogen-containing pores and the transition metal cooperate to activate N
2 molecules, the nitrogen fixation process is thermodynamically stable (as shown in the figure), and can directly electrocatalytically reduce N
2 into NH
3[102][103]4.3.2 Photocatalyst
Doping elements or compounding other materials can change the band gap of C
2N materials and enhance the photocatalytic ability. For example, B, O or B, S co-doped C
2N materials with band gaps of 2. 19 and 2. 11 eV, respectively, are expected to be directly used as photocatalysts for water splitting
[104]. The monolayer C
2N with I
2, Si/Ge substitution and P/As substitution can absorb visible light, and the energy collection ability is enhanced
[105][106][107]. The C
2N material is compounded with other two-dimensional materials (WS
2, blue phosphorus, g-C
3N
4, BCN, InSe, GaTe/InTe, CdS and the like) to form a heterojunction, can inhibit the recombination of photogenerated electrons and holes and enhance the absorption of visible light, and is more suitable for being used as a water splitting catalyst than a single component
[108][109][110][111][112][113][114].
4.4 Biological Application
Two-dimensional materials represented by graphene have high specific surface area, can carry drugs or biomacromolecules, and are increasingly applied in the field of biomedicine, but graphene and other materials have poor water solubility.It has low utilization efficiency, high preparation difficulty and cost, induces the increase of active oxygen free radicals in cells, stimulates autophagy, has potential toxicity to living cells, and needs to find better substitutes.
Studies have shown that C
2N materials are biocompatible. Zhou Ruhong et al. Found that the C
2N can effectively adsorb double-stranded DNA (Figure 13), the two are tightly bound by Van der Waals and electrostatic interaction, the DNA can not move translationally, the hydrogen bonds between the two strands are almost not destroyed, and the structure remains intact
[115]. C
2N materials can also adsorb different single-stranded DNA sequences, which has potential applications in nucleic acid transport and template repair
[116]. The C
2N material can also stably bind proteins (as shown in fig. 13). Due to the electrostatic potential generated by the periodic holes of the C
2N, the charged amino acids can be adsorbed, the binding sites are very strong, there is almost no translational movement, and the structure remains unchanged
[117].
The compatibility of two-dimensional nanomaterials with cell membranes is determined by both morphological characteristics and surface physicochemical properties. By comparing the compatibility of C
2N and graphene oxide with red blood cells, it was found that graphene oxide caused the rupture of red blood cells and the outflow of hemoglobin at the same dosage, while C
2N was well compatible and no rupture of red blood cells was observed under the microscope
[118]. In vitro experiments showed that C
2N had a slight toxicity to human umbilical vein endothelial cells, which originated from its oxidative capacity and led to the accumulation of reactive oxygen species in cells, but to a much lesser extent than graphene oxide (Figure 13). The physical damage of C
2N materials to cell membranes is almost negligible, which can be used as a substitute for graphene and its derivatives
[119].
5 Conclusion
Porous graphene nitride C2N materials have attracted great attention from academia since they were reported by Baek et al in 2015, showing great potential for application. Through precise nitrogen-doped graphene, the C2N material perfectly obtains the tunable band gap that graphene does not have, and can realize various changes from semiconductor to conductor, non-magnet to magnet, and has considerable electromagnetic and optical absorption properties. In addition, the inherent porosity of C2N materials is expected to realize many applications such as seawater desalination, isotope separation and gas adsorption. The strong complexation of multiple nitrogen atoms with metal atoms in C2N materials has shown great potential in the field of fuel cells. The rare biocompatibility and low nano-toxicity of C2N materials will make them expected to become the latest drug delivery carriers and nucleic acid sequencing materials. Due to the unique structure and physical and chemical properties, C2N materials have considerable application prospects in many application fields, and their synthesis strategies inspire more [120~122] for the synthesis of carbon nitride materials, which is of great significance for the synthesis of new two-dimensional materials.
However, the synthesis of C2N materials requires high monomer and polymerization process, and the preparation of monolayer C2N materials is a great challenge, which makes it difficult to prepare in large quantities. Therefore, the study of the properties of C2N materials is mainly based on first-principles calculations. It can be predicted that more practical synthesis methods will determine the research progress of C2N materials, the acquisition of monolayer C2N materials will help to reveal their precise chemical structure and many properties, and the application research of more C2N materials will promote the sustainable development of many application fields, which will make porous graphene nitride C2N materials become a research hotspot in this field.
the National Natural Science Foundation of China(21772013)
Beijing Natural Science Foundation(2202049)
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