1 Introduction
Since the industrial age, the rapid development of human society has brought us material affluence, but at the same time, it also makes us face unprecedented environmental pollution problems. Volatile organic compounds (VOCs), inorganic harmful gases (such as NO
x, SO
x, CO, H
2S, etc.), heavy metal ions and various harmful organic compounds (such as pesticides, organic dyes, industrial wastewater, etc.) Are typical environmental pollutants.Pathogen pollution caused by bacteria, viruses and other microorganisms is also a kind of pollution with great harm, and its threat and impact on human health, safety and production activities can not be ignored
[1][2][3][4][5,6]. Considering the damage to the ecological environment caused by human beings in the past and the sustainable development pursued by human society, how to control pollution efficiently and cheaply has become an eternal problem facing mankind.
In recent decades, the structure, synthesis and characterization of nanomaterials have been studied in depth, which has greatly promoted the development and optimization of new environmental pollution control processes. Since Novoselov et al. Obtained a single atomic layer of graphite material, graphene, by mechanical exfoliation in 2004 and called it a "two-dimensional semi-metallic material", the concept of two-dimensional nanomaterials has attracted great interest
[7]. As the name implies, two-dimensional nanomaterials are similar to two-dimensional planar morphology, only nanoscale or even several atomic layers thick, and most of them are crystals
[8]. Such as nanoribbons with a certain width, nanorings, nanoflakes with irregular shape or nanoflakes with regular geometry, etc
[9][10][11][12]. Most two-dimensional nanomaterials have corresponding Bulk materials, for example, the Bulk material corresponding to graphene is graphite. Top-down is one of the common methods to prepare two-dimensional nanomaterials by means of mechanical exfoliation, ultrasonic exfoliation, shear-assisted liquid exfoliation, ion intercalation exfoliation and selective etching. On the contrary, the Bottom-up method, which uses small molecules as raw materials to synthesize nanomaterials, is also suitable for the preparation of some two-dimensional nanomaterials. These methods are not limited to chemical vapor deposition or wet chemical synthesis methods such as hydrothermal method, solvothermal method and surfactant-assisted synthesis
[13⇓~15]. Because two-dimensional nanomaterials have unique microstructure, large specific surface area and high exposure of active sites, their inherent properties are often better than those of the corresponding bulk materials, and even new properties that bulk materials do not have
[8,16,17]. Two-dimensional nanomaterials derived from various bulk materials have different physicochemical, optoelectronic properties and band structures, which have been favored in many fields such as energy, catalysis, optoelectronic materials and so on
[18⇓⇓~21].
There are many kinds of two-dimensional nanomaterials, and there are still new two-dimensional nanomaterials being synthesized and studied (Figure 1), but in the field of practical application, only those materials with excellent performance and can be really put into use can be commercialized, otherwise these materials are only of theoretical value. In order to make two-dimensional nanomaterials truly "mature", related research and development are more based on the original materials, using various modification methods to improve their performance or endow them with new characteristics. Still taking graphene as an example, the mobility of graphene can reach 200 000 cm
2/(V·s), which is a semimetal, but graphane with insulator properties can be obtained by treating graphene with low-temperature hydrogen plasma, while doping graphene with heteroatoms such as nitrogen, sulfur or boron can not only endow it with new catalytic ability, but also retain its inherent physicochemical properties
[22][23][16]. It can be seen that modification means can bring positive modification to materials, which is essential for the improvement and development of two-dimensional nanomaterials.
In this paper, we will first give a comprehensive overview of the types of two-dimensional nanomaterials, and briefly introduce the properties of some classical materials and new materials. The modification methods of various two-dimensional nanomaterials and their modification effects are emphatically described, and the application and principle of the modified two-dimensional nanomaterials in pollutant treatment such as catalytic degradation of pollutants, sterilization, filtration, adsorption and detection of pollutants are emphatically introduced. Finally, this paper will discuss the development prospects of two-dimensional nanomaterials and their modification, as well as the limitations of two-dimensional nanomaterials in practical applications such as environmental governance.
2 Kinds of two-dimensional nanomaterials.
2.1 Single-element two-dimensional nanomaterials and their derivatives
Graphene is undoubtedly the most widely and deeply studied material among the non-metallic single-element two-dimensional nanomaterials. Graphene has good optical, thermal, chemical and electrochemical stability, low preparation cost, good electrical conductivity, thermal conductivity, optical transparency and many other excellent physical properties
[24][16][7][25][26]. In addition to its own use as an efficient catalyst or catalyst support, graphene has also been largely used to synthesize derivatives such as graphene oxide (GO), reduced graphene oxide (rGO), fluorinated graphene, etc
[16,27].
Black phosphorene is another popular material in recent years. Although the synthesis of black phosphorus was reported as early as 1914, it was not until 2014 that the corresponding two-dimensional nanomaterial, black phosphorene, was synthesized, and black phosphorus began to attract the attention of researchers
[28][29]. Black phosphorene is a direct band gap semiconductor, and its band gap can be increased by reducing the thickness, which is not possessed by graphene. At the same time, because of its stability, non-toxicity and other advantages, black phosphorene is widely used in many fields comparable to graphene
[29,30][31].
In addition, graphyne, silicene, germane, purple phosphorene, arsenene and other materials belong to this category, and some metals and alloys can also be prepared into two-dimensional nanomaterials
[32][33][34][35][36]. However, due to poor stability, the need for carriers or high cost, the application of some of these materials is still very limited, even at the theoretical level, and their properties can only be predicted by theoretical calculations.
2.2 Inorganic compound two-dimensional nanomaterial
Materials that can be prepared into this class include, but are not limited to, metal oxides, layered double hydroxides (LDHs), transition metal chalcogenides (TMDs), nitrides, inorganic perovskite-type materials, inorganic zeolites, clay minerals, and the like. Among them, MoS2, which belongs to TMDs, hexagonal boron nitride (h-BN), which belongs to nitrides, and graphitic carbon nitride (g-C3N4) are representative materials that are often studied, while MXenes and two-dimensional perovskite-type materials, which are inorganic compounds with rapid development in recent years, also show considerable potential for development.
MXenes is a general term for two-dimensional transition metal carbides, nitrides, and carbonitrides, which are formed by selectively etching MAX bulk materials (M is most of the early transition metals such As Sc, Ti, Zr, V, Cr, Mn, Nb, Hf, Ta, or Mo, and A is some of the elements from group IIB to group VIA such as Cd, Al, Si, P, S, Ga, Ge, As, In, Sn, Tl, or Pb.X represents C, N or both), and the surface of the material contains a large amount of functional groups such as -F (or -Cl, depending on the preparation method), -O, -OH and the like due to the exfoliation process, so the MXenes is a hydrophilic material and has the characteristics of high conductivity, unique physicochemical properties, layered structure, easy modification and the like. Ti
3C
2T
x was the first to be synthesized and is currently the most studied MXenes (where T represents the functional group generated upon exfoliation), and since its inception in 2011, MXenes have been popular in the fields of sensors, photoelectrocatalysis, energy storage, or supercapacitor electrodes
[37⇓⇓⇓⇓~42].
Perovskite-type materials are a class of materials with the same crystal structure as calcium titanate (CaTiO
3) in natural mineral perovskite, which can be generally expressed as ABX
3, where X represents an anion, A and B represent cations, and the ionic radius of A is larger than that of B
[43⇓~45]. Many perovskite-type materials have efficient photoelectric conversion performance, so they are often used to develop the third generation solar cells with great industrial potential.However, three-dimensional perovskite materials often have the disadvantages of small band gap and easy decomposition in high temperature and high humidity environment, which hinder their application. If they are prepared into two-dimensional morphology, these problems can be effectively improved
[46][44,47,48]. Common 2D inorganic perovskite-type materials are complex oxides (e.g., SrTiO
3, Ca
2Nb
3 ) and lead halide perovskites (CsPbX
3,X=Cl, Br, or I), etc
[49⇓~51][52]. Considering the toxicity of lead element in lead halide perovskites, halide perovskites such as Cs
2AgBiX
6, Cs
3Bi
2X
9 and Cs
3Sb
2X
9 have been developed, but their properties in various aspects still need to be improved
[53].
2.3 Organic two-dimensional nanomaterials
In the family of porous materials, metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are highly ordered crystal phase materials, which are formed by the combination of organic ligands and structural units. These organic frameworks are characterized by large specific surface area, controllable pore size and diversified structural design. The three-dimensional structure of some MOFs and COFs is formed by the superposition and combination of two-dimensional framework units, so through the rational design of the molecular structure and combination of organic ligands and structural units, combined with the "bottom-up" or "top-down" synthetic ideas,A wide variety of two-dimensional MOFs and two-dimensional COFs can be prepared to realize the application of organic framework materials in the fields of medicine, biology and the like, and at the same time, the general properties of two-dimensional nanomaterials can be endowed
[54⇓⇓⇓~58]. In recent years, in order to develop materials with both the function of metal ions in MOFs and the excellent stability of COFs, metal-covalent organic frameworks (MCOFs) have emerged, which can also be prepared into two-dimensional nanomaterials by planar stacking
[59].
Two-dimensional organic-inorganic perovskite-type materials also belong to this class. The use of various organic cationic groups to synthesize perovskite-type materials can avoid the problem of using highly toxic lead to prepare lead halide perovskites, and at the same time, the regulation of the properties of the materials themselves has become more diversified, which brings a broader prospect for the mature research and development of perovskite-type materials
[43].
3 Strategies for modifying two-dimensional nanomaterials.
In terms of modification strategies, two-dimensional nanomaterials have many similarities with general nanomaterials, but the characteristics of two-dimensional nanomaterials in size, dimension and large specific surface area make their modification strategies more flexible. It should be noted that in some material systems, some modification strategies are only used to help other modification strategies, and simply using them can not directly modify the material, so some material systems often use a variety of modification strategies to achieve the "1 + 1 > 2" modification effect.
3.1 Material compounding
Compositing other materials with 2D nanomaterials is the most commonly used modification strategy. The materials used for composites can be various nanomaterials, macromolecules, small molecules and even ions, and the ways of combination between different materials can also vary greatly. Wan et al. Reported the synthesis of Au/Bi
2WO
6-MoS
2 heterojunction photocatalyst, in which S — O covalent bond was formed between the interface of MoS
2 and Bi
2WO
6, thus ensuring their close contact, while Au, which can produce local surface plasmon resonance (LSPR) effect, was attached to its edge through the dangling bond of Bi
2WO
6 nanosheets
[60]; Wang et al. Synthesized black phosphorus nanosheet (BPNS) -polyethyleneimine (PEI) -porphyrin dye composite, in which dye molecules with highly conjugated structure can be adsorbed on the surface of BPNS-PEI through π-π stacking interaction and electrostatic force
[62]. Dong et al. Synthesized GO/Co(OH)
2 nanosheets and used them to prepare nanofiltration membranes. The electrostatic force between GO and Co(OH)
2 and the coordination between carboxyl and cobalt ions made the GO/Co(OH)
2 nanosheets have good stability in water
[63]. Wang et al. Complexed sodium alginate to Ti
3C
2T
x rich in oxygen-containing functional groups through hydrogen bonds, overlapped and assembled them through Van der Waals force, and then cross-linked them with different metal cation solutions.Lamellar nanofiltration membranes with different ion selectivities can be obtained, in which the alginate hydrogel columns formed between the layers by different crosslinking reactions can maintain the layer gap at 7. 4 ± 0.22 Å, thus ensuring the stability of the lamellar structure
[64].
The strategy of using material composite can play a synergistic role, or combine multiple functions. For example, if you want the material to be easy to collect after use, you can consider compounding the material that responds to the external environment or the carrier of macroscopic size on the material that plays a major role. Fu et al. Constructed a poly ((C
3N
3S
3)/rGO) composite embedded with magnetic Fe
3O
4 nanoparticles, which is based on rGO with strong affinity for metal ions, and the poly (C
3N
3S
3) on its surface adsorbs Pb (II) and Hg (II) in water through the coordination ability of sulfur and nitrogen elements.The two play a synergistic role, and after the adsorption reaches equilibrium, the material can be easily enriched and recovered by magnetic force, and the adsorption capacity of the material can be restored by EDTA solution
[65].
Photocatalytic composites such as MoS
2/ poly (N-isopropylacrylamide) (PNIPAM) and MoS
2-HPC/HPC (hydroxypropyl cellulose) hydrogel were also synthesized by our research group
[66][67]. The former is responsive to ambient temperature, and when the temperature is higher than the lower critical solution temperature of PNIPAM and the material is dispersed in water or toluene, the material will agglomerate, thus facilitating filtration and collection; The latter uses HPC hydrogel as a macroscopic carrier, in which MoS
2-HPC nanosheets with the ability of photocatalytic degradation of organic pollutants are uniformly dispersed. When the adsorption of hydrogel in water reaches equilibrium,It can be directly taken out and placed in sunlight to degrade the adsorbed organic matter, and the large specific surface area and anionic force of MoS
2-HPC nanosheets can also play a role in adsorption, so there is also a synergistic effect between hydrogel and nanosheets.
To a large extent, the purpose of building composites determines the specific way of building and the kind of materials. For example, in semiconductor composite materials such as heterojunction photocatalysts, compared with the interface bonded to each other only by Van der Waals force, the interface formed by chemical bonds between different materials has smaller spacing and stronger interaction.Therefore, in the latter, carriers can be transported more effectively, and the separation efficiency of electron-hole pairs is relatively higher, so that the photocatalytic efficiency of the material is improved
[60,68,69]; For some heterojunction photocatalysts (such as p-n junction, Schottky junction and S-type heterojunction), the built-in electric field formed at the interface can also promote the directional transport of carriers, which also requires the tight bonding of the interface
[70⇓⇓⇓⇓~75]. Or considering the stability of two-dimensional nanomaterials, if the composite material is easily decomposed and lost due to the external environment, the nanomaterials can be coated with specific materials (such as small molecules, polymers or other nanomaterials) to provide protection
[76][62][77]; From the thermodynamic point of view, all nanomaterials have the tendency to reduce the specific surface area and anisotropy, which is manifested by the agglomeration of nanomaterials, so different material composite methods can be used to avoid this phenomenon of losing the advantages of nanomaterials
[78,79]. As shown in Figure 2, for two-dimensional nanomaterial-based heterojunction photocatalysts, the functional characteristics of each composite method are different, and the synthesis methods (such as in-situ growth, ex-situ assembly, solvothermal method, hydrothermal method, etc.) are also different. However, the so-called "concrete analysis of specific problems", without considering the limitations of synthesis steps, whether it is heterojunction photocatalyst or other materials, different material combinations should be selected as far as possible to maximize the advantages of materials or make composite materials have specific functions, rather than blindly pursuing the advantages of a certain composite method.
3.2 Element modification
The global addition or substitution of elements to materials can often change the electron density, crystal structure and chemical properties of catalysts, thus affecting their catalytic performance
[80,81]. Zou et al. Calcined graphyne at high temperature in chlorine atmosphere to obtain porous graphyne nanosheets uniformly doped with chlorine element, in which the doped chlorine element reduces the reaction barrier of N
2 reduction to NH
3, so that the material has superior electrocatalytic nitrogen reduction performance at room temperature
[82]; Banerjee et al. Reported a method to grow vertically aligned ZnO or Mg-ZnO nanosheets on a variety of substrates, in which the band gap of Mg-ZnO is slightly increased due to the doping of Mg, and the photocatalytic degradation efficiency of dyes is also higher
[83]. Xun et al. Synthesized a Si/TiO
2/2H-MoS
2 photocathode for photoelectrocatalytic water splitting. Interestingly, in the process of in situ photoelectrocatalysis, part of the sulfur atoms on the basal plane of the electrode MoS
2 will be replaced by oxygen atoms, thus greatly improving the activity of autocatalytic hydrogen evolution reaction
[84]. Wu et al. Prepared g-C
3N
4 doped with Pt
2+ ions, which can be used as an excellent photosensitizer and used to effectively adsorb and degrade a variety of antibiotics because the metal-ligand charge transfer and heavy atom effects of Pt
2+ ions promote the light absorption of g-C
3N
4 and the generation of reactive oxygen species (ROS), respectively
[85].
Broadly speaking, ions themselves can also be regarded as a kind of material, so the addition and modification of elements is actually a modification means to realize the composite strategy of materials. Ding et Al. Used concentration diffusion method to intercalate Al
3+ ions into Ti
3C
2T
x, and assembled it into seawater desalination membrane by vacuum filtration method. Because of the formation of Al — O ionic bonds between Al
3+ and oxygen-containing groups of Ti
3C
2T
x, the strong interaction makes the membrane not easy to swell, and the stability and water flux are also improved
[87]. The La
3+-GO nanofiltration membrane reported by Nie et al. Also used a similar strategy, in which La
3+ as a crosslinking agent coordinated with the oxygen-containing groups of GO to increase the interlayer gap of GO in an endohedral manner, which not only achieved size-dependent ultrafast selective molecular transport, but also improved the use stability of the membrane, as shown in Figure 3
[86].
For MOF, COF, MCOF and other materials, although the physical and chemical properties such as pore size and catalytic performance can be adjusted by the design of structural units at the initial synthesis,However, on the basis of the original material, it is still possible to modify the elements, such as the replacement and addition of metal central ions or other ions, and the change of the spin state of metal ions, which can change its adsorption and catalytic properties
[59][88,89][90].
3.3 Group modification
Grafting, replacing or removing the original groups on the surface of materials can also significantly change the physical and chemical properties of materials. This modification method is often used in two-dimensional nanomaterials such as GO, rGO and MXenes, which contain surface functional groups. It can not only directly endow materials with new properties, but also be used as a means of material compounding. Yan et al. Used natural kaolinite to prepare SiO
2-AlOOH porous nanosheets rich in hydroxyl groups on the surface, and 3-aminopropyltriethoxysilane (APTES) was uniformly grafted on the surface of the nanosheets through these hydroxyl groups. Because the amino group of APTES was further protonated by the hydroxyl group, the obtained nanosheets had a strong adsorption effect on anionic dyes such as Congo red
[92]. Lu et al. First used the carboxyl group contained in nanofibrillated cellulose (NFC) to compound NFC with polyethyleneimine (PEI), and then the obtained PEI-NFC was compounded with the negatively charged oxygen-containing functional group of GO through the positive charge carried by PEI.After the subsequent high temperature treatment, the final aerogel has high hydrophobicity due to the increase of — NH — and C — C structures and the pyrolysis of hydrophilic functional groups, and can be used as an adsorption material for organic solvents and an oil-water separation material
[93]; He et al. Synthesized nanoscale zero-valent iron (nZVI)/alkalized Ti
3C
2 composite for adsorption and reduction of Cr (VI), in which the fluorine atoms on the surface of Ti
3C
2 nanosheets were replaced by hydroxyl groups after alkalization and intercalation treatment, so that the Fe
2+ could be more uniformly attached to the surface or interlayer gap of the nanosheets, and the nZVI could grow uniformly without agglomeration in the subsequent reduction preparation process
[94].
For materials without surface functional groups, if we want to modify them, we can consider material composite and other modification methods to graft functional groups. For example, the surface of TMDs prepared by the ion intercalation exfoliation method contains abundant defects such as sulfur vacancies, and Zhou et al. Utilized the property that sulfhydryl groups can be combined with sulfur vacancies by means of covalent bonds to graft organic molecules containing sulfhydryl groups and target functional groups onto the surface of MoS
2 or WS
2 nanosheets, while maintaining the chemical activity and stability of the target functional groups (Fig. 4)
[95,96][91]. These target functional groups enable functionalized MoS
2 or WS
2 nanosheets to undergo different reactions, such as esterification, ring-opening polymerization, radical polymerization, and dispersion reduction of metal ions in situ, thus pioneering a facile method to functionalize the surface of layered TMDs. Inspired by this, our group also synthesized MoS
2 nanosheets modified by different groups and used for the synthesis of different materials, for example, 2-mercaptoethanol was grafted onto the surface of MoS
2 nanosheets exfoliated by lithium ion intercalation method, the obtained MoS
2-OH could be closely combined with polyvinyl alcohol (PVA), and the final conjugated polyvinyl alcohol (CPVA)/MoS
2-OH could be used for photocatalytic degradation of Cr (VI)
[97]; In addition, a similar method was also used in the previously mentioned MoS
2/PNIPAM and MoS
2-HPC/HPC. In the former, the MoS
2 nanosheets were grafted with 2-aminoethanethiol, while in the latter, thioglycolic acid was first grafted and acyl chlorinated, and then condensed with HPC to obtain MoS
2-HPC. In both cases, the group modification of the MoS
2 nanosheets was for subsequent material compounding.
Considering the organic composition of materials such as MOF, COF and MCOF, it is not surprising that flexible and diverse group modifications can be carried out before and after the synthesis of these materials. The pore size of organic framework materials is one of their important physical properties, which can be precisely adjusted by changing the connection mode of structural units, the size of structural units or the size of their side chain groups
[57,59]. There are many examples of changing the physicochemical properties of organic framework materials by group modification, for example, a conductive 2D MCOF(M=Cu
2+ or Zn
2+) synthesized from tert-butyl pyrene tetraone (
tBu-PT) and copper phthalocyanine (or zinc phthalocyanine) exhibits dislocated AA stacking due to the steric hindrance of the tert-butyl group,This affects the transport of charge carriers between surfaces, so that its conductivity is only about 5×10
-7S/cm, while if the tert-butyl group in the
tBu-PT unit is removed, the 2D Ni-COF prepared in this way can achieve completely coincident AA stacking, and its conductivity is increased to 2.51×10
-5S/cm, and the pore size is also increased (Fig. 5)
[98,99]; Wen et al. Synthesized a GO-COF composite for adsorption and removal of uranium and plutonium in water, and used hydroxylamine hydrochloride to oximate the ketone group of COF in the material. Compared with the original GO-COF, the oximated GO-COF showed stronger selective adsorption due to its strong chelating effect on
, and its acid resistance and radiation resistance were also better
[100].
3.4 Defect Engineering
For crystal materials, defect engineering in a narrow sense refers to the modification of lattice defects such as dangling bonds, vacancies or dislocations, which can change many properties of materials.Such as the adsorption ability, light absorption ability, band structure, carrier mobility, phase state of the interface, and some defects themselves can also act as active sites or adsorb heterogeneous atoms to carry out specific reaction processes
[101⇓⇓⇓~105]. Because one of the characteristics of crystals is the long-range order of atomic arrangement, we can even regard the surface and edge of crystalline materials as defects, and the atoms in these sites exhibit different properties due to their surrounding environment.For example, in the hydrodesulfurization reaction or hydrogen evolution reaction, the edge of MoS
2 nanosheets has stronger electrocatalytic activity than its basal plane, and as mentioned in the previous section, the sulfur vacancy of TMDs can be combined with sulfhydryl groups, and this unique property enables defect engineering to serve other modification methods
[106].
It should be emphasized here that the doping of heteroatoms and the introduction of surface groups can also cause the destruction of crystal structure. These modification methods can be attributed to defect engineering in a broad sense, but considering that doping and group modification have been discussed separately in this paper, this section will only deal with defect engineering in a narrow sense. At the same time, it should be noted that defects are not the more the better, too many defects may lead to material reconstruction or instability, and the modification caused by defects is not necessarily positive, for example, electrons have a longer mean free path in intact graphene, which makes it more difficult to recombine with holes
[107].
In fact, defects are inevitably introduced during the synthesis of two-dimensional nanomaterials, and even if the whole material is intact, its surface and edge often show the characteristics of defects. Some nanomaterial synthesis methods for defect engineering have also been developed to deliberately introduce a large number of defects into two-dimensional nanomaterials or increase their surface area, thus highlighting the advantages of defects. These synthesis methods are mainly realized by top-down methods such as ultrasonic liquid-phase assisted exfoliation, mechanical exfoliation and ion intercalation exfoliation.Ion beam bombardment, chemical etching or adjusting the stoichiometric ratio of reactants for material synthesis are also commonly used methods, which enable the defect engineering of nanomaterials to enter the application field, rather than being limited to the model of calculation software
[84,101,103]. Chen et al. Used intercalation exfoliation and in situ oxidation to prepare oxygen vacancy-rich Bi
2O
3 nanosheets, and used this as a model.Density functional theory (DFT) calculation shows that the oxygen vacancy confined in the atomic layer can reduce the adsorption energy of CO
2 molecules on the reaction site, so that the material can carry out photocatalytic conversion of CO
2 to dimethyl carbonate at room temperature
[108]. Wu et al. Synthesized alkali-metal-doped and nitrogen-vacancy-introduced g-C
3N
4 as photocatalysts for the generation of H
2O
2, and confirmed through characterization that the nitrogen vacancy therein can capture photogenerated electrons, while the doped alkali metal reduces the band gap, making it easier for photogenerated carriers to be generated, thus playing a synergistic role with the nitrogen vacancy
[109].
3.5 Morphology control
Two-dimensional nanomaterials themselves are known for their large specific surface area, but reducing the number of atomic layers or making large pieces of two-dimensional nanomaterials more "fragmented" can further enhance this advantage, and the more extreme way is to create holes in two-dimensional nanomaterials. A typical example of morphology control modification is that the decrease of the number of atomic layers of black phosphorus nanosheets and TMDs nanosheets will lead to the increase of their band gap, and the band gap of the latter will change from indirect band gap to direct band gap
[29,30,111].
The above modification methods are often accompanied by the generation of material defects, and the specific synthesis methods are similar to those of defect engineering. For example, the works of Ran et al. And Zhao et al. Involved the synthesis of ReS
2 nanosheets with intact basal plane and antimony nanosheets, respectively, the former controlled the exfoliation thickness of ReS
2 by ultrasound-assisted liquid phase exfoliation and found that thin ReS
2 nanosheets had stronger photocatalytic hydrogen evolution ability due to more edge sites,The activity of the edge sites of the nanosheets was illustrated, and the latter introduced a large number of antimony vacancies and oxygen vacancies during the exfoliation synthesis, and the activity of these vacancies in visible light nitrogen fixation was confirmed by the synthesis control of the thickness and area of the nanosheets (Fig. 6)
[112][110]. Liu et al. Synthesized WO
3 porous nanosheets, and combined with DFT analysis, we know that these pores play the role of defects. Because the high-momentum electrons in this region are more easily excited, the WO
3 nanosheets rich in pores can be used for photocatalytic induction of nitrogen to nitrate
[113].
In the aspect of permeable membrane materials, the pores of the material itself or the gaps between materials can become the channels of specific molecules, at this time, the pore size and two-dimensional size of the material become the factors that need to be regulated. Nie et al. Also prepared two kinds of GO nanosheets with different sizes by controlling the degree of ultrasonic exfoliation when synthesizing the La
3+-GO nanofiltration membrane, and found through experiments that the La
3+-GO nanofiltration membrane assembled by small GO sheets improved the nanofiltration flux while maintaining the organic dye cut-off rate due to shorter and smoother pores (Fig. 3);Sapkota et al. Reported the peptide-modified MoS
2 porous nanosheet/nanodisc composite membrane, in which the composite nanodisc and the increase of the pore size of the porous nanosheet help to increase the flow channel of the solvent.O as to increase the flux of osmosis and reverse osmosis, and although the permeation rejection rate is correspondingly reduced, the rejection rate of the composite membrane to ions and charged molecules can be improved by compounding the charged polypeptide molecules
[86][114].
In addition, the phase state of the material can also have a significant impact on its performance, for example, the stable 2H phase TMDs are semiconductors, while the metastable 1T phase TMDs have good conductivity similar to metals, and the active sites in the basal plane are very suitable for hydrogen evolution reaction
[80,115]. 2H phase nano MoS
2 with different morphologies and phase composition were prepared by ultrasonic exfoliation method, hydrothermal method and lithium ion intercalation method respectively in our research group, among which that MoS
2 nanosheet synthesized by lithium ion intercalation method has the relatively smallest morphology size,At the same time, it contains 37. 9% of 1 T phase component, which is much higher than that of the MoS
2 prepared by the other two methods, so the photocatalytic sterilization performance of the MoS
2 nanosheet is more significant
[116]. Xie et al. Synthesized vertically distributed partially amorphous Ni-Fe LDH nanosheets on the surface of spongy Ni-Fe alloy, which introduced stable Ni
3+ ions due to a large number of crystal-amorphous interfaces, and also had more electrocatalytic reaction sites compared with completely amorphous and highly crystalline Ni-Fe LDH
[117]. Based on the previous synthesis of monolayer graphite-phase ZnO (g-ZnO), Zhang et al. used hybrid density functional calculation to simulate the adsorption trend of monolayer g-ZnO on various gases. Interestingly, these adsorbed gas molecules also have an impact on the electronic structure, work function, optical absorption and other properties of monolayer g-ZnO, which is expected to develop a sensor for gas detection
[118].
4 Application of Two-dimensional Nanomaterials in Environmental Pollution Control
4.1 Classification and Detection of Environmental Pollutants
According to the environmental factors of pollutants, environmental pollutants can be roughly divided into water pollutants, air pollutants and other types. Among them, common water pollutants can be divided into various harmful ions, organic pollutants, pathogens and their genes. Air pollutants include VOCs, nitrogen oxides, sulfides and various suspended particulate matter. In the face of the environmental pollution that has been caused, it is important to deal with it in time, but the idea of prevention should also be kept in mind, so the detection of pollutants in the atmosphere and water environment is also an area that can not be ignored.
Considering the complex and changeable environment when detecting pollutants, it is obvious that the sensitivity and selectivity of sensors need to be considered first. The direct detection of specific molecules with very low concentration in gas samples requires very strict performance such as the detection limit of the sensor, but the pre-concentration of specific molecules can be achieved without considering the polarity of the molecules by using porous materials such as MOF to selectively adsorb and desorb the samples at high temperature before the detection end
[131]. Some materials can also be enriched for specific target analytes in other ways. Li et al. Reported a MOF composite nanosheet ZIF-8/NH
2-MIL-53(Al) integrating detection and treatment, which can selectively adsorb various tetracyclines in water by hydrogen bonding, π-π stacking and chemical adsorption, and can also efficiently detect tetracyclines in a fluorescence sensing manner
[121]. Another way to improve the detection selectivity or detection limit is to use porous materials as molecular sieves. Jang et al. Prepared PdO-WO
3 nanosheets, an electrochemical sensing material capable of detecting a variety of small gas molecules and VOCs, and synthesized porous GO nanosheets with a pore size of 0.3 ~ 0.4 nm by a simple and controllable chemical etching method and loaded them on cellulose filter paper.His research group combined these two materials, so that H
2S with a kinetic diameter of 0.36 nm could pass through the filter paper and be detected by PdO-WO
3 nanosheets, while other macromolecular gases could not pass through the filter paper, thus improving the selectivity of H
2S
[126].
The combination of two-dimensional nanomaterials and other theoretical technologies can complement each other and have excellent performance in the field of environmental pollutant detection. Hussain et al., combined with electron localization function and DFT calculations, theoretically analyzed nitrided porous graphene (C
2N), graphyne and heterojunctions composed of them, respectively.It is found that VOCs such as acetone, ethanol, propionaldehyde and toluene have strong van der Waals binding effects with monolayer C
2N and AB stacking C
2N/ graphyne van der Waals heterojunction (C
2N…GDY), which can also change the electronic properties and charge density of the materials
[132]. Li et al. Fabricated a virtual sensor array (VSA) that can be used at room temperature by loading a Ti
3C
2T
x film on an interdigitated electrode, and the VSA can produce different broadband impedance spectral responses in different gas environments.By applying principal component analysis and linear discriminant analysis to process the spectrum to obtain the specific data pattern of each VOC, the qualitative and quantitative detection of various VOCs and the prediction of the content of a certain VOC in a variable environment can be realized
[133].
Table 1 representatively shows a variety of sensing systems containing two-dimensional nanomaterials and some performance indicators. When developing material systems for sensing, different detection methods and principles are needed for different pollutants, but in the final analysis.Performance parameters such as sensitivity, detection range or linear range, response time, anti-interference and working environment determine the practical prospects of the sensing system, and these indicators need to be constantly broken through for high-performance sensors.
4.2 Water pollution control
4.2.1 Control of harmful ion pollution
Heavy metal ions are the most common harmful ions in water, and the common strategy is to adsorb them by electrostatic force or coordination between materials and metal ions, and some materials can also capture metal ions by ion exchange, so the advantages of large specific surface area of two-dimensional nanomaterials can be brought into full play. Shamsayei et al. Reported a Mg-Al LDH/δ-MnO
2 composite assembled by histidine, which can adsorb Pb
2+ and
in water through the electrostatic force generated by two kinds of nanosheets and the coordination of histidine functional groups. Its efficient adsorption performance can even be fully utilized in seawater or industrial wastewater, and it can be desorbed and regenerated after being treated by HCl solution and NaOH solution
[134]. Liu et al. Compounded curly CaSiO
3 nanosheets on the surface of Fe
3O
4 nanospheres,These nanosheets adsorb heavy metal ions such as Cu
2+, Ni
2+ and Cr
3+ through ion exchange of Ca
2+, coordination formed by oxygen atoms and surface polarity.The material can not only be collected by a magnetic field, but also be desorbed and regenerated by a triethylenetetramine solution
[135].
Harmful ions include not only heavy metal ions, but also non-heavy metal elements such as beryllium and aluminum, which have strong toxicity to organisms, can also be removed by material adsorption
[87,136]. The research and development of materials for the adsorption of radioactive elements or isotopes is also a research direction that has attracted much attention. These adsorption materials need strong stability to prevent the materials from being damaged or decomposed due to strong ionizing radiation. In addition, the treatment of harmful ions may be slightly different, for example, when dealing with some heavy metal ions with strong oxidizing properties, such as Cr
2 or
, it often involves the catalytic reduction process of light, electricity, chemistry and other ways to convert them into Cr (Ⅲ) or low-valent manganese elements to reduce toxicity and carcinogenicity.
4.2.2 Control of organic pollution
Industrial wastewater, dyes, drugs and so on are common organic pollutants in water, among which antibiotics, persistent organic pollutants (POPs) and other stubborn pollution are difficult to control. In general, these pollutants can be decomposed or removed by applying various advanced oxidation processes (AOP), filtration, or adsorption.
As an advanced oxidation process, photocatalysis is popular because of its low secondary pollution, clean energy utilization and efficient and powerful oxidation ability, so the development of photocatalysts is also very important. In addition, combined AOPs, such as photoelectrocatalysis, sonophotocatalysis and ozonophotocatalysis, have also been widely studied. These AOPs can often "mineralize" organic pollutants, that is, completely oxidize them into water, CO
2, salt ions and other inorganic substances, without leaving other relatively harmful organic by-products
[137⇓⇓⇓~141]. The essence of photocatalyst is semiconductor, so the photocatalytic performance can be improved from the following aspects: (1) increase the degree of light absorption, and let the photocatalyst use common light sources such as sunlight or visible light as far as possible; (2) reducing the recombination of photogenerated carriers and promoting their separation and migration; (3) Increase the contact degree and surface reaction rate between the photocatalyst and the target degradation product. Element doping and defect engineering can introduce impurity energy levels into the band gap of photocatalysts and change the electron density of materials, so they can control the performance of photocatalysts from the first two aspects.The third angle can be achieved by using electrostatic force, hydrophilic or hydrophobic interaction or reducing the agglomeration of two-dimensional nanomaterials, at this time, modification means such as group modification or more "universal" material composite can be used
[109,142].
When the modification method of material composite is adopted, the two-dimensional nanomaterial is suitable to be used as the carrier of the photocatalyst due to the morphology characteristic; If two-dimensional nanomaterials are used as catalyst components and form heterojunction or Schottky junction composites, the band structure of the composites can also be optimized to improve the photocatalytic ability of the composites. Schottky junctions formed by semiconductors and conductors, interface charge transfer mechanisms, and heterojunctions or mechanisms with reasonable band structures such as p-n junctions, type II, type Z, and type S can separate electron-hole pairs to varying degrees and retain their respective redox capabilities for subsequent chemical reactions, as shown in Figure 7
[71,72,74,75,80,143]. When constructing these band structures, the band gap of the semiconductor should be adjusted according to the reaction required for photocatalysis, or the appropriate components should be selected for material recombination.Only in this way can the ingenious band structure design be successfully realized, and the redox potential of electrons and holes exceed the potential of the target reaction, so that the whole photocatalytic process can be carried out smoothly. For example, when metals are selected as conductors to form Schottky junctions, some metals can be used as co-catalysts because of their catalytic activity, just as metallic 1T phase TMDs are selected for hydrogen evolution reactions. Graphene as a conductor can make the material have photothermal effect as a whole
[71,80]. Fei et al. Loaded NiFe-LDH nanosheet /Co
3O
4 nanowire composite with p-n junction on nickel foam and used it as an electrode for photoelectrocatalysis, and the catalytic system could simultaneously oxidize bisphenol A and reduce Cr (Ⅵ) in water; Liu et al. Prepared Ti
3C
2T
x nanosheet /g-C
3N
4 nanosheet composite,They combine with each other by van der Waals force and form a Schottky junction, in which the Ti
3C
2T
x can transfer and store the photogenerated electrons generated by the g-C
3N
4, so the catalyst can continue to degrade a variety of organic pollutants for up to 2 H after 5 H of illumination, which can be called "the initial realization of night photocatalytic process"
[144][145].
In addition to the advanced oxidation process, filtration and adsorption are also efficient treatment methods. At this time, the interaction of electrostatic force, hydrophilicity or hydrophobicity will also have a great impact on the effect of filtration and adsorption. For filtration membrane materials, the pore size has a direct impact on the interception of pollutant molecules. Yang et al. Prepared a strong and tough porous large-area graphite nanomesh/carbon nanotube nanofiltration membrane by precise O
2 plasma etching technology. Because the average pore size of the membrane is only 0.63 nm,It has a good rejection effect on charged small molecules, ions and neutral small molecules with a solvation radius of more than 1 nm, and can achieve the filtration of small drug molecules which is difficult to achieve by commercial membrane materials at present
[146]. The GO-halloysite nanotube composite membrane was prepared by a simple suction filtration method. The composite membrane is durable and can be used for efficient oil-water separation of pump-in-water emulsion stabilized by surfactant. It is expected to be used for the treatment of industrial wastewater containing oil or marine oil spill pollution
[147]. When designing adsorption materials and filtration materials, it can also combine the functions of photocatalytic degradation and pollutant detection, so that the materials can be used more widely
[67,121,148].
4.2.3 Control of pathogen contamination
At present, the treatment of pathogens is mainly aimed at pathogenic bacteria, including drug-resistant bacteria and multi-drug resistant bacteria, which are relatively difficult to kill, and the drug-resistant genes carried by them are persistent pollutants, which can be transmitted through horizontal gene transfer between microorganisms and absorption of microorganisms, making drug-resistant bacteria more likely to run rampant
[149⇓⇓~152]. The application of AOP methods such as photocatalysis to the elimination and degradation of pathogens and drug-resistant genes is a promising response.
Photocatalytic sterilization mainly depends on ROS, such as hydroxyl radical · OH, superoxide radical ·
, hydrogen peroxide H
2O
2, hydrogen peroxide radical HO
2·, singlet oxygen
1O
2, etc. These active species can oxidize bacterial enzymes, lipids, nucleic acids and other biological macromolecules, destroy the cell structure of bacteria, and thus inactivate bacteria
[153⇓⇓⇓~157]. However, the lifetime of ROS is short, for example, the lifetime of
1O
2 in water is only microseconds. Therefore, when two-dimensional nanomaterials are used for sterilization, in order to increase the inactivation ability of ROS to bacteria, electrostatic force or hydrophobicity can be used to make the materials close to or coat the surface of bacteria, so that ROS can play a more direct and effective role in oxidizing bacteria
[158,159]. In addition, the two-dimensional nanomaterial itself can inhibit the proliferation of bacteria or directly inactivate bacteria in the following ways: (1) the sharp edge of the two-dimensional nanomaterial can directly damage the membrane of bacteria and cause the death of bacteria; (2) Two-dimensional nanomaterials can wrap bacteria and block the active sites on the membrane, and the large-area nanosheets have a stronger effect on inhibiting the physiological activities of bacteria due to their better wrapping effect
[154,160,161]. The
/C composite nanosheets prepared by Zhang et al. Have a strong absorption of infrared light due to the oxygen holes and the coated carbon, and the resulting photothermal catalysis can effectively destroy the cell membrane of Escherichia coli, resulting in the reduction of its enzyme activity and the leakage of bacterial contents, while the material itself has almost no biological toxicity
[162]. Guo et al. Loaded a layer of anatase TiO
2 film on a quartz plate, and used the quartz plate for photocatalytic inactivation of methicillin-resistant Staphylococcus aureus and multidrug-resistant Pseudomonas aeruginosa, and photocatalytic degradation of the drug-resistant genes mecA and ampC contained in these two bacteria, respectively.The final concentration of the drug-resistant bacteria and the drug-resistant gene after photocatalysis is reduced by 4 to 5 orders of magnitude, and the synthesis strategy not only utilizes the high catalytic activity of the anatase phase of the TiO
2, but also avoids the agglomeration of the TiO
2[151].
4.3 Air pollution control
4.3.1 Treatment of Gaseous Pollutants
Common gaseous pollutants are VOCs, H
2S, NO
x, SO
x, etc., among which the concentration of VOCs volatilized into the air is only ppb or ppm level, but long-term exposure to such a low concentration of VOCs is still enough to pose a threat to health, especially in confined spaces where harmful gases are easily accumulated due to lack of air circulation. Unconventional pollutants are not limited to radon and its decay products, mercury vapor, etc., and their adsorption treatment has also been studied experimentally or theoretically
[131,163].
Catalytic decomposition or adsorption are commonly used to control gas pollution. Wang et al. Prepared a GO/MnO
x/g-C
3N
4 composite film by suction filtration assembly. When irradiated by visible light at room temperature, the photothermal effect of GO can initiate the thermal catalytic reaction of MnO
x and promote the migration of photogenerated carriers and surface reaction of g-C
3N
4.The synergistic effect of light and thermal catalysis enables the composite film to effectively degrade formaldehyde vapor, and has the potential to be applied to an air purifier for treating indoor decoration pollutants
[164]; Wang et al. Synthesized two-dimensional variable valence oxide CeCuO
x with unchanged morphology but crystal strain and grain boundary defects by heat treatment using bimetallic MOF flaky crystals with two-dimensional size up to micron as precursors. The thermal catalyst can completely oxidize methanol, acetone, toluene or o-xylene with a concentration of 500 ppm in air below 200 ℃.The conversion and gas space velocity of the methylated polyethyleneimine /g-C
3N
4 composite synthesized by using 0.15 G of the catalyst are respectively more than 90%, and the adsorption capacity of the methylated polyethyleneimine /g-C
3N
4 composite synthesized by using 23 000 h
-1;Lan and the like for SO
2 gas at normal temperature and pressure is 11.82 to 15.88 mmol/G, and the composite has high selective adsorption property relative to CO
2 and N
2, and the material can be desorbed and regenerated when heated to 100 deg C in an inert gas atmosphere
[165][166].
It is worth mentioning that two-dimensional nanomaterials also have excellent performance in the fields of catalytic reduction, separation and adsorption of CO
2. Although CO
2, which is a primary pollutant in the atmosphere, is more often classified as a greenhouse gas in environmental science, no matter how it is classified, catalytic reduction of CO
2 to synthesize high value-added chemical raw materials such as CO and CH
4 is considered to be an excellent means to solve the energy crisis and global warming
[167⇓⇓~170].
4.3.2 Treatment of suspended particulate matter
The suspended particles in the atmosphere include PM
2.5, PM
10, viruses, bacteria and other pathogens. In air purification, protective equipment such as masks and other aspects of indoor or closed spaces such as carriages, the common strategy is to use a filter layer composed of fibers to filter the air.Nanomaterials can be used in the filtration layer to improve the filtration performance or endow it with special functions, for example, two-dimensional polymer nanomesh can be compounded in the filtration layer to improve the mechanical properties and pore size distribution of the filtration layer, and the latter has a direct impact on the resistance and efficiency of filtration. Or compounding the nano-photocatalyst to the filter layer to kill the entrapped pathogens. The application of the above strategies can more effectively deal with the health threats caused by haze
[171⇓⇓⇓~175]. Koo et al. Reported a synthesis strategy of multiple growth of sheet-like MOF on a variety of substrates, which can grow micron-sized flower-like zeolite imidazolate framework (ZIF) on spongy polyurethane, non-woven fabrics and other substrates.The research group synthesized the micron-sized polypropylene fiber modified by the flower-like ZIF, and prepared a washable filtration membrane for efficient filtration of PM
2.5 and PM
10[176]. Huang et al. Used laser-induced graphene (LIG) for mask fabrics, so that masks have the functions of photothermal sterilization and self-utility detection, and LIG with different hydrophilicity and other properties can be prepared by using different precursors and laser-induced conditions, and the precursors used can even be wood or A4 paper, so the large-scale application of LIG has broad prospects
[177].
5 Conclusion and prospect
To sum up, the combination of two-dimensional nanomaterials and material composite, element or group modification, defect engineering, morphology control and other modification strategies makes two-dimensional nanomaterials play an outstanding role in the field of environmental pollution control. Among them, composite modification can conveniently add new functions and properties to two-dimensional nanomaterials, and there is a lot of room for modification. The modification of elements, groups and defects can effectively change the physical and chemical properties of two-dimensional nanomaterials and make the materials have excellent properties in some aspects. In order to achieve these modification methods, theoretical calculation and simulation can often play a great guiding role. Morphology modification can further optimize the properties of materials on the basis of the above modification strategies.
Unfortunately, although great progress has been made in the preparation, performance research and application of two-dimensional nanomaterials in the field of environmental governance, most of the reported achievements have not yet "gone out of the laboratory". Most of these achievements are small-scale or short-term tests combined with the actual environment, and the scale of the experiments is often only milligrams or milligrams.In addition, the cost of raw materials, synthesis process, safety, service life, the cost required during the use period and the amplification effect of the process are all factors that need to be considered comprehensively in practical application and commercialization, so most of the idealized laboratory results are still difficult to apply to complex practical scenarios.
In order to make two-dimensional nanomaterials more promising for large-scale practical applications, the author believes that we should focus on the following perspectives to overcome the application bottleneck.
(1) Optimize the process of synthesis, preparation and modification of two-dimensional nanomaterials, pursue the simplicity, safety and environmental protection of the process, and ensure that the raw materials are easy to obtain and can be produced on a large scale, which is particularly important in the field of pollutant treatment where a large number of materials are used. Layered minerals such as kaolinite and montmorillonite, which are abundant in nature, can be prepared into two-dimensional nanomaterials and used for two-dimensional substrates. After calcination, etching and other treatments, the charges or functional groups on their surfaces can be used for subsequent synthesis and modification, which is an example of solving the cost of raw materials
[92,178,179]. As for the industrial large-scale preparation of two-dimensional nanomaterials, shear-assisted liquid phase exfoliation is expected to play an important role in this respect. At present, many two-dimensional nanomaterials, such as TMD, Mxenes, and few-layer graphene (1-3 layers), have been reported by using a kitchen mixer
[180][181][182]. At present, some scholars are committed to developing material systems that do not use precious metals or heavy metals, or exploring simple and efficient new synthesis methods, such as Kang et al. Using bacterial etching at room temperature to strip bulk g-C
3N
4 into nanosheets and use them for visible light disinfection of water bodies.Although the optimal etching time is up to 2 days, the g-C
3N
4 nanosheets prepared by this method have abundant unpaired electrons, and tens of grams of exfoliation preparation can be achieved in the laboratory, which is expected to be scaled up to kilogram-scale
[183].
(2) Reasonable and flexible use of theoretical calculation and model fitting to explore new material systems and analyze their mechanisms, which can reduce the cost of synthetic research and development, or explore the impact of environmental factors on the performance of material systems, so as to pave the way for practical application. For example, Cen et al. Designed a new type of two-dimensional A
2BO
3N(A=Ca, Sr or Ba; B = Ta or Nb) perovskite oxynitride by using first principles and GW calculation, and found that the heterojunction composed of monolayer Sr
2NbO
3N/ and monolayer Ba
2NbO
3N has efficient electron-hole pair transmission ability and separation ability
[68]. Yazdankish et al. Reported the excellent adsorption and desorption performance of g-C
3N
4/Mg-Co-Al-LDHs composite material for
131I in medical wastewater, and analyzed the optimal adsorption conditions of the material in different environments by fitting various models of experimental data
[184].
(3) In-depth study of the problems that may be encountered in the practical application of two-dimensional nanomaterials, such as material agglomeration, decomposition, toxicity, stability, recycling, etc., and overcome these problems in the design of material systems and even specific equipment.
In fact, both the number of papers and the depth of research from the above perspectives have achieved considerable results. As the so-called "quantitative change leads to qualitative change", it is time for us to trigger "qualitative change" which has a direct contribution to practical application, rather than blindly pursuing the simple accumulation of the number of papers. In a word, the optimization and improvement of the material system based on the existing achievements are more hopeful to make these materials complete the transition from theory and experiment to application and practice, and bring breakthroughs and innovations to the related application fields. Only in this way can the title of this paper have real practical significance.
Anhui Provincial Natural Science Foundation(2008085ME138)
Hefei Municipal Natural Science Foundation(2021034)
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