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Preparation and Environmental Applications of 2D Nanomaterial MXenes

Kang Chun , Lin Yanxin , Jing Yuanju , Wang Xinbo

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (10) : 2239 -2253.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (10) : 2239-2253 DOI: 10.7536/PC220122
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Preparation and Environmental Applications of 2D Nanomaterial MXenes
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Abstract

MXene is a new type of two-dimensional layered nanomaterial obtained by delamination of Mn+1AXn phase material (MAX phase), which is constructed with transition metal carbide, nitride or carbonitride. MXenes have attracted ever-increasing interest due to their unique characteristics such as high surface area, excellent metal conductivity, mechanical stability, as well as their magnetic properties, and have been widely applied in the field of energy storage, catalysis, adsorption, and many other fields. Herein, we summarize the recent advances in MXenes preparation strategies and their applications in terms of environmental purpose, including adsorption of heavy metals, radioactive metals and organic compound, selective adsorption of carbon dioxide, photocatalysis, electrocatalysis, membrane separation, sensor, biological activity, electromagnetic absorption and shielding, and so on. Finally, the current challenges and future opportunities of MXenes to put forwards real applications are discussed.

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MXenes / 2D materials / environment / adsorption / catalysis

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Kang Chun, Lin Yanxin, Jing Yuanju, Wang Xinbo. Preparation and Environmental Applications of 2D Nanomaterial MXenes. Progress in Chemistry, 2022, 34(10): 2239-2253 DOI:10.7536/PC220122

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Contents

1 Introduction
2 Synthesis of Mxenes
2.1 Fluorine-containing etching method
2.2 Fluorine-free peeling method
3 Applications in environmental fields
3.1 Adsorption of heavy metals
3.2 Adsorption of radioactive metals
3.3 Adsorption of organic compound
3.4 Selective adsorption of carbon dioxide
3.5 Photocatalysis
3.6 Electrocatalysis
3.7 Membrane separation
3.8 Sensor
3.9 Bioactivity
3.10 Electromagnetic absorption and shielding
4 Conclusion and outlook

1 Introduction

Two-dimensional materials are ultrathin materials containing a single or few layers of atomic thickness, such as nano-films, quantum wells, etc., with a thickness of nanometers or smaller, while the other two dimensions are in a larger size. Because of their unique layered structure, excellent physical, optical and electrical properties, and large electroactive surface, two-dimensional materials have become rapidly developing emerging materials. According to their chemical composition and structural configuration, they can be classified as metallic, semi-metallic, semiconducting, insulating or superconducting materials[1,2]. In 2004, Geim et al. Successfully exfoliated a sheet composed of only one layer of carbon atoms from graphite by mechanical exfoliation, that is, two-dimensional graphene structure, which opened a new era of two-dimensional materials[3]. However, graphene has only one element carbon, and the van der Waals force between layers is strong, and the preparation process is complex, which limits its wide application to a certain extent. Since then, scientists have discovered two-dimensional structural materials such as hexagonal boron nitride, transition metal sulfides and oxides, and silicene, which have greatly expanded the composition of two-dimensional materials.
MXenes is a new type of two-dimensional material which was first reported in 2011. Because its structure is similar to graphene, it has attracted worldwide attention since it was reported. MXenes material is a kind of two-dimensional metal carbide, nitride or carbonitride, which is obtained from Mn+1AXn phase material (MAX phase for short) by etching to remove the A intercalation layer and then stripping. MAX phases belong to a large family of solid materials, consisting of about 130 different components, where "n" takes values between 1 and 3, M represents early transition metals (Sc, Ti, Zr, V, Cr, Mn, Nb, Hf, Ta, Mo), A is usually an element from groups 12 to 16 of the periodic table (Cd, Al, Si, P, S, Ga, Ge, As, In, Sn, Tl, Pb), and X is C, N, or both[4,5]. In 2011, Gogotsi et Al. Of Drexel University in the United States used HF selective etching to remove Al atoms in Ti3AlC2, stripped two-dimensional crystal Ti3C2Tx, and successfully prepared MXenes phase materials for the first time. Since then, more than 20 different MXenes materials have been prepared by similar methods[6]. Because the synthesis of MXenes often uses liquid phase etching method, functional groups such as hydroxyl and fluoride ions will be attached to the surface of the material, so MXenes are usually also expressed as Mn+1XnTx(T for hydroxyl, fluoride ions, etc.).
Because of its large specific surface area, excellent conductivity and hydrophilicity, as well as abundant surface groups (conducive to modification, surface modification, deposition, etc.), MXenes have shown excellent application potential in many fields such as environment, information, energy, biology and so on. In this paper, the preparation methods of MXenes and the research progress of their application in the environmental field are reviewed, and the existing problems and development prospects of MXenes are analyzed and prospected.

2 Preparation of MXenes

2.1 Fluorine etching method

2.1.1 Hydrofluoric acid etching

MXenes materials are generally obtained by exfoliating the A component in the MAX phase with HF solution. HF is selective to the etching of MAX phase, and will prefer the M-A bond with weaker bond energy, so that the M-X layer connected by A atoms is gradually separated to form the corresponding MXenes phase material. At the same time, the metal elements in the M-X layer are exposed by the stripping of the A atomic layer, so that the metal elements can form complexes such as fluoride salts with functional groups such as hydroxyl and fluoride ions which are rich in the solution[7].
The possible reactions during the reaction are as follows:
M n + 1 A X n + 3 H F → A F 3 + 3 2 H 2 + M n + 1 X n
M n + 1 X n + 2 H 2 O → M n + 1 X n ( O H ) 2 + H 2
M n + 1 X n + 2 H F → M n + 1 X n F 2 + H 2
Reaction (2) and reaction (3) occur in parallel, so that a large number of hydroxyl and fluoride ions are attached to the surface of the generated MXenes.

2.1.2 Modified acid etching

High concentration of HF is highly toxic and corrosive, and the experimental process is highly dangerous. In 2014, Ghidiu et al. First proposed to use a mixed solution of LiF and HCl instead of HF to etch Ti3AlC2, and successfully obtained Ti3C2-MXenes, which opened up a new idea for the preparation of MXenes[8]. In the same year, Halim et al. Used NH4HF2 to replace HF to etch the MXenes phase, resulting in Ti3C2Tx-MXenes materials[9]. In 2017, Wang et al. Etched Ti2AlC with a mixed solution of FeF3 and HCl to obtain Ti2CTx-MXenes materials[10].

2.1.3 Molten salt etching method

The N atom will further enhance the metallic properties of MXenes, so that nitrides have some potential advantages over carbide MXenes: higher conductivity, which is more suitable for electrochemical capacitor electrodes[11][12,13]. However, the dissociation of aza-MAX materials often requires higher energy, and Tin+1Nn materials have low stability and will degrade in HF solution, so Ti4N3Tx is difficult to obtain by fluorine etching. Urbankowski et Al. Heat treated Ti4AlN3 with three fluorine-containing fusible salts (KF, LiF and NaF) at 550 ° C for 30 min under argon flow to selectively remove Al atom intercalation, and then used sulfuric acid to remove impurities, followed by washing and centrifugation to obtain few-layer Ti4N3Tx nanosheets[14]. The key of molten salt etching is atmosphere protection and temperature. If the temperature is too high or the product is heated directly in air without atmosphere protection, the product may be cubic phase structure.One of the disadvantages of molten salt etching is that the fluoride in the molten salt product is difficult to be completely removed, and the crystallinity of the MXenes prepared by molten salt etching is poor compared with that prepared by hydrofluoric acid etching.

2.2 Fluoride-free stripping

2.2.1 Alkaline etching

Although the direct use of HF can be avoided by using the modified acid etching method, the H+ and F- present in the system may still lead to the release of HF. Moreover, the F- attack on the A layer is only suitable for the removal of alkaline or amphoteric elements. Therefore, the development of new fluorine-free methods to remove the A-layer is crucial to the development of a safe and green route for the preparation of MXenes. Li et al. Utilized a NaOH-assisted hydrothermal process to etch Ti3AlC2 to prepare Ti3C2Tx(T=OH,O)[15]. The results show that Al can be selectively removed from Ti3AlC2 in 27.5 M NaOH solution at 270 ° C to produce OH- and O-terminated multilayer Ti3C2Tx with a purity of 92 wt%.

2.2.2 Late transition metal substitution method

In the preparation and dissociation process of MAX phase, it has been found that the A-layer elements dominated by transition metals such as Al can be replaced by other elements such as late transition metals such as Zn and Ni, which can enrich the members of MAX family and provide ideas for the preparation of MXenes with different terminal groups X. Huang et Al. Found that in ZnCl2 molten salt at 550 ° C, Al in the Ti3AlC2 can react with Zn to form a Ti3ZnC2-MAX phase.And by increasing the amount of ZnCl2, Ti3ZnC2 can be further converted into Ti3C2Cl2-MXenes with Cl element as the terminal group (Fig. 1)[16].

2.2.3 Lewis acid redox method

Huang et al. Found that Lewis acid cations with higher redox potential can effectively oxidize and etch the A-layer atoms with lower redox potential in the MAX phase, and proposed a general strategy for the preparation of MXenes materials by Lewis acid molten salt etching[17]. The Ti3SiC2-MAX phase was immersed in CuCl2 molten salt at a temperature of 750 ° C, and the redox reaction between Ti3SiC2 and CuCl2 occurred, in which the interlayer Si was oxidized to Si4+,It enters the gas phase as SiCl4 gas, and Cu2+ is reduced to Cu (reaction 4), while the produced Ti3C2 further reacts with excess CuCl2 to produce Ti3C2Cl2 and Cu (reaction 5). Using this strategy, a large number of MAX phases (such as Al, Si, Zn, Ga, etc.) Can be etched by selecting the appropriate Lewis acid, which demonstrates the universality of this method.
T i 3 S i C 2 + 2 C u C l 2 → T i 3 C 2 + S i C l 4 ( g ) + 2 C u
T i 3 C 2 + C u C l 2 → T i 3 C 2 C l 2 + C u

2.2.4 Iodine etching

In hydrofluoric acid etching, F- can selectively react with Al to effectively remove Al intercalation. Similarly, the etching effect can also be obtained by the selective reaction of other elements and Al. Feng et Al. Found that elemental iodine in anhydrous acetonitrile (CH3CN) could selectively react with Al in Ti3AlC2 to form AlI3, thus playing an etching role[18]. After iodine etching and delamination in hydrochloric acid solution, fluoride-free Mxenes-Ti3C2Tx can be synthesized, in which most of the flakes are less than 5 nm thick. According to the study, the process roughly undergoes the following chemical reactions (Equations 6 to 10):
T i 3 A l C 2 + ( x + 3 ) / 2 I 2 → T i 3 C 2 - I x + A l I 3
T i 3 C 2 - I x + x / 2 O 2 → T i 3 C 2 - O x + x / 2 I 2
T i 3 C 2 - I x + x H 2 O → T i 3 C 2 - ( O H ) x + x H I
A l I 3 + 3 H 2 O → A l ( O H ) 3 + 3 H I
A l ( O H ) 3 + 3 H C l → A l C l 3 + 3 H 2 O

2.2.5 Electrochemical etching

The fundamental reason why chemical etching can selectively remove A-intercalated atoms is that the M — Al bond is more chemically active than the M — C bond, which is essentially an electrochemical process involving the transfer of charge from the target material to other substances. Therefore, the electrochemical etching method should also be a feasible strategy for the selective etching process. However, in practical operation, the selection of electrochemical medium and parameters is very important, and improper selection will cause the overall corrosion and damage of materials. Feng et Al. Found that in the alkaline binary mixed aqueous solution of NH4Cl and tetramethylammonium hydroxide (TMAOH), Cl- could selectively etch Al atoms and insert N H 4 + between the layers at the same time, thus obtaining large-size lamellar Ti3C2Clx[19]. Hao et al. Proposed a method to fabricate MXenes by thermally assisted electroetching. In the presence of dilute hydrochloric acid etchant, mild heating can accelerate the electrochemical etching efficiency of MAX materials, which makes it easy to prepare MXenes such as Cr2CTx and V2CTx, which are difficult to dissociate[20].

2.2.6 Microenvironmental law

The removal of A intercalation in the MAX phase is only the first step in the preparation of MXenes, and the MXenes flakes must be stably dispersed into the solvent. However, it is difficult for multilayer MXenes prepared by HF acid etching to form a high concentration and stable dispersion in water. The common way to solve this problem is to add TMAOH plasma liquid as a stabilizer in the solution, which is inserted between the sheets. However, the addition of these reagents can affect the performance of MXenes, and the presence of water is not conducive to the requirement of non-hydration for some potential applications. Therefore, it is very important to prepare anhydrous, stable and high concentration organic solvent MXenes dispersion. Gogotsi et al. systematically studied the dispersion of Ti3C2Tx in organic solvents. By comparing the correlation between the concentration of MXene solution and surface tension, boiling point, polarity, etc., they found that although the "good" solvent for dispersing Ti3C2Tx nanosheets could not be predicted by a single solvent property,But polar solvents with high surface tension, high boiling point and high dielectric constant usually have better dispersion stability for Ti3C2Tx (Fig. 2), thus successfully dispersing Ti3C2Tx in various organic solvents, such as: N, N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO) and so on[21]. Li Hui et al. Of Shanghai Normal University have systematically studied how to further improve the dispersion concentration of Ti3C2Tx in organic solvents and developed a method of tuning the microenvironment (TMM)[22]. According to the scheme, a small amount of TBA + is added to carry out intercalation on the multilayer Ti3C2Tx, so that the surface of the Ti3C2Tx is modified, the microenvironment of the surface of the material is changed, the hydrophilicity and hydrophobicity are improved, and the organic solvent has better solvent parameters, so that the MXene can be highly dispersed in the organic solvent. By adopt that scheme, the high-concentration Ti3C2Tx organic solvent dispersion can be rapidly prepared, the yield of the disper monolayer MXenes can reach 90%, the monolayer MXenes has a clean surface and a uniform thickness, excellent oxidation resistance is shown even under the aerobic condition at room temperature, and the electrochemical capacity is doubled compare with that of the MXenes prepared in a water phase when the monolayer MXene is used for a sodium-ion battery.

3 Application of MXenes in Environmental Field

3.1 Adsorption of heavy metal

Heavy metal pollution in water has become a serious environmental problem worldwide due to its toxicity and carcinogenicity. In particular, some toxic heavy metals are extremely dangerous even in trace amounts, which can cause serious damage to the tissues of animals and plants, so they must be removed from the water body before discharge, and two-dimensional materials with large specific surface area and abundant active sites are considered to be ideal adsorbents for environmental remediation[23,24]. During the preparation of MXenes, new dangling bonds such as M — O, M — OH, M — F and M — Cl were formed with the cleavage of M — a bond, which provided a large number of active sites for the adsorption of metal ions. At the same time, the abundant surface functional groups of MXenes make it have negative Zeta potential, good hydrophilicity and negative surface charge, so the spontaneous intercalation adsorption will occur when metal cations contact with MXenes in aqueous solution, and MXenes has large specific surface area and chemical stability, which makes it an ideal adsorbent for metal ions[25].
Peng et al. Prepared Ti3C2(OH/ONa)xF2-x material by chemical stripping and alkalization intercalation method, and found that it had a strong adsorption effect on lead ions (PbⅡ), with an adsorption capacity of 140 mg/G and chemical equilibrium within 120 s, and the adsorption was not affected by other common competitive ions such as MgⅡ and CaⅡ, and the lead residue after adsorption was lower than the drinking water standard recommended by the World Health Organization (10 μg/L)[26]. Guo et al. discussed the reasons for the excellent adsorption of heavy metal ions by the material through first-principle calculations, and found that under alkaline conditions,Because the hydrogen atoms and oxygen atoms of the hydroxyl functional groups on the surface of the Ti3C2(OH)2 form a potential well with the adsorbed heavy metal, the surface hydroxyl and oxygen atoms form a stable chemical bond (such as Pb — O bond) with the metal, thus enhancing the adsorption capacity of the metal atoms[27]. In addition, the calculation also found that the terminal heteroatoms such as F can reduce the adsorption capacity of MXenes, while the intercalation of Li+, Na+ and K+ in MXenes can enhance the adsorption capacity of MXenes for heavy metal ions. Inspired by this, Gu et al. Prepared Ti3C2ONa nanosheets without fluorine substitution by strong alkali etching, and the adsorption capacity of PbⅡ was increased to 286 mg/G, showing potential practical value[28].
In addition to PbⅡ,The energy barriers for some other heavy metal ions such as CuⅡ, CdⅡ, ZnⅡ, HgⅡ, BaⅡ to form M — O bonds on the surface of MXenes are also only − 1.0 to − 3.3 eV,Indicating good adsorption. Atieh et Al. Prepared multilayer Ti3C2Tx-MXenes by HF etching to remove the Al layer, and then further intercalated under KOH alkaline conditions to expand the interlayer spacing to 26 26 Å, thereby promoting the diffusion and adsorption of heavy metal ions, and thus realized the electrochemical monitoring of CuⅡ, CdⅡ, PbⅡ, and HgⅡ[29]. Shahzad et al. Studied the adsorption/desorption process of Ti3C2Tx-MXenes nanosheets on CuⅡ, and the results showed that the performance of Ti3C2Tx-MXenes nanosheets exfoliated into thin layers was much better than that of multilayer structure, with the removal rate of CuⅡ up to 80% within 1 min and the absorption capacity up to 78. 5 mg/G, which was about three times that of commercial activated carbon[30]. The Ti3C2Tx thin layer sheet also has a good adsorption effect on BaⅡ, and Atieh et al. Found that the adsorption capacity was as high as 9.3 mg/G in an aqueous solution with an initial BaⅡ concentration of 55 ppm, which was significantly higher than that of other adsorbents such as dolomite (3.958 mg/G) and expanded perlite (2.486 mg/G)[29]. The effect of pH value on the adsorption capacity and removal rate was further studied. It was found that under acidic conditions, the surface of the adsorbent had a positive charge and a low affinity for cations. Under alkaline conditions, the surface of the adsorbent has a negative charge, and the adsorption capacity of the adsorbent for Ba ions is enhanced. However, too high pH will lead to the precipitation of barium ions by the formation of hydroxide complexes, so the optimal adsorption pH is between 6 and 7. They found that Ti3C2Tx-MXenes has a strong affinity for a variety of divalent metal ions, and the order of affinity for different metal ions is Ba > Sr > Ca > Pb > Cr > As, so the selective adsorption of coexisting metals can be achieved under optimized conditions, for example, the removal rate of barium can reach 98% under optimized conditions, while the removal rate of coexisting As (III) is about 70%. Lee et al. Used a hydrothermal method to prepare magnetic Ti3C2Tx-MXenes/Fe3O4 composite materials. The introduction of MGMX,Fe3O4 nanoparticles, on the one hand, provides a high specific surface area, and on the other hand, it can realize the convenient recycling of materials because of its magnetism[31]. Experiments show that the material has obvious hydrophilicity and high M2+ ion affinity, and shows excellent HgⅡ adsorption performance under acidic and alkaline conditions (pH = 3-9), and the adsorption capacity reaches 1128.41 mg·g-1, which is obviously higher than that of the previously reported two-dimensional nanomaterials and nanocomposites.Moreover, the material is not influenced by interfering ions such as MgⅡ, CaⅡ and the like, has high HgⅡ selectivity, and has no obvious performance degradation after five times of continuous adsorption-desorption cycle reuse, which reflects the great application potential of the material in removing mercury ions from wastewater.
The adsorption of MXenes on low-valence metals is mainly based on physical adsorption and chemical chelation, while the redox reaction between high-valence oxidizing metal ions, such as CrVI, and MXenes has a significant impact on the adsorption process. Ying et al. Found that Ti3C2Tx nanosheets had excellent adsorption capacity for Cr[32]. At pH = 2, CrVI ions are first adsorbed on the surface of protonated nanosheets, and with the increase of pH value, electron transfer occurs between the surface adsorbed CrVI and the host of MXenes, and the CrVI is reduced to CrⅢ.Some of them form Cr(OH)3 precipitates, and some of them are adsorbed on the surface of MXenes in the form of Ti—O—CrⅢ. At pH = 5, Cr ions in the solution are almost completely removed, the adsorption capacity is up to 250 mg/G, and the quality of the effluent meets the WHO standard. Other oxidative metal pollutants, such as K3[Fe(CN)6], KMnO4 and NaAuCl4, can also be effectively removed by similar redox reactions with MXenes, which further illustrates the feasibility of MXenes to remove high-valence oxidative metal pollutants.

3.2 Adsorbed radioactive metal

The development of nuclear industry not only brings the development of energy economy, but also brings more radioactive pollution, so the efficient removal of radioactive pollutants has become the focus of attention[33]. Uranium is one of the most widely used radioactive pollutants with high biological toxicity. Its half-life in the natural environment is as long as millions of years. If it enters the environment and can not be effectively removed, it will pose a great security risk to animals, plants and humans. In addition, as a key nuclear raw material, effective enrichment and recovery of uranium is of great strategic significance. Strong acidity, strong radioactivity, high mobility and the influence of many coexisting ions are the main challenges for the effective recovery of uranium. Uranium in nature mainly exists in the form of tetravalent UIV and hexavalent uranyl ion (U O 2 2 +), among which U O 2 2 + has good aqueous solubility, so it is easier to diffuse into the environment and has greater harmfulness. At present, various adsorbents are used to remove UVI in wastewater, including activated carbon, molecular sieves, graphite oxide, metal oxides, inorganic nanomaterials, organic porous polymers, metal-organic frameworks, organic frameworks and so on.The mechanism also includes adsorption and oxidation-reduction, and the latter is mainly the reduction of UVI to low-solubility UIV by iron-based or biological bacteria reductants[33⇓~35].
Shi et al. Used density functional theory calculations to find that there are strong chemical interactions and hydrogen bonding between Ti3C2(OH)2 and uranyl ions, which are strong enough to form stable chemical bonds, so that uranyl ions are preferentially adsorbed on hydroxylated titanium carbide nanosheets without the influence of other anionic ligands such as OH-, Cl-, N O 3 -, and dominate the whole adsorption process[36,37]. Experimentally, they treated Ti3C2Tx with NaOH or DMSO to control the interlayer distance, obtained Ti3C2Tx-NaOH and Ti3C2Tx-DMSO with different intercalation, and investigated the effects of intercalation elements and bound water on U adsorption capacity. The results showed that the hydrated Ti3C2Tx-DMSO exhibited the highest uptake capacity (160 mg·g-1) for U, which was more than five times higher than that of Ti3C2Tx-dry(26 mg·g-1), greatly improving the adsorption capacity. The possible reason is that the hydrated Ti3C2Tx has a larger interlayer distance and is flexible and swellable, so it can easily accommodate more uranyl ions inside the MXenes channel. However, dry Ti3C2Tx with a small interlayer spacing may inhibit uranyl ions from entering the MXenes channel, resulting in a reduced uranium uptake capacity.
Through further study of X-ray absorption spectroscopy, diffraction spectroscopy and photoelectron spectroscopy, it is found that there are two mechanisms for the removal of UVI by Ti2CTx-MXenes, as shown in Figure 3.First, uranyl ions and hydroxyl groups on the surface of MXenes form strong hydrogen bonds to adsorb on the surface, and then electrons are transferred from Ti2CTx to U O 2 2 + for redox reaction, and Ti2CTx on the surface is reduced to TiO2,UVI and UIV[25]. Under acidic conditions, the UIV is coordinated on the MXenes adsorbent in the form of mononuclear due to the dissolution of surface TiO2, while under nearly neutral conditions,The surface of Ti2CTx is a layer of disordered TiO2 formed by oxidation, and uranium is adsorbed on the TiO2 in the form of U O 2 + x 2 +. The adsorption capacity of Ti2CTx for UVI was 470 mg/G at pH = 3. Through strong alkali treatment of Ti2CTx-MXenes, their group also prepared hierarchical titanate nanostructure Na/K-HTN intercalated with sodium and potassium ions, and the Na/K ions in the intercalation were exchanged, thus realizing the effective adsorption of another radioactive element ion EuⅢ[38]. The reductive adsorption ability of MXenes also allows it to be used in the removal of other oxidizing ions.

3.3 Adsorbed organic matter

Organic dye wastewater has always been one of the most difficult industrial wastewater to treat because of its deep coloring, miscellaneous components and high harm. Graphene and its oxides, boron nitride, molybdenum disulfide and other two-dimensional materials have large specific surface area, which are considered to be good adsorbent candidates.However, their application in the field of dye wastewater treatment has been greatly limited due to their shortcomings such as single element (graphene), weak force (Van der Waals force), difficult preparation and lack of surface functional groups. However, MXenes materials not only have similar two-dimensional lamellar structure and high specific surface area, but also have abundant surface functional groups and negative surface charges, so that they can combine with dye molecules through electrostatic interaction, van der Waals force, hydrogen bonding, etc., and are particularly suitable for the adsorption of cationic dyes in aqueous solution. Methylene blue (MB) is a widely used cationic dye for staining biological and bacterial tissues, and is also used as an indicator in chemical reactions. Noh et al showed that F-terminated Ti3C2Tx-MXenes had good adsorption performance for methylene blue in wastewater[39]. MB removal efficiency reached 91.9% 5 min after the addition of 40 mg of MXenes to 20 μM MB solution, and it increased with the increase of pH value of MXenes suspension. Gogotsi et al. Used Langmuir and Freundlich isotherms to fit the adsorption of methylene blue by Ti3C2Tx, and the fitting correlations were 0.811 and 0.928, respectively, indicating that the adsorption process followed the Freundlich model more closely[40]. Yoon et al. Combined MXenes with ultrafiltration (UF) to construct a hybrid system of "MXenes-UF"[41]. The results showed that the membrane flux of MXenes-UF for methylene blue and methyl orange was higher than that of single UF system. The hybrid system has less irreversible fouling than ultrafiltration because the dye can be adsorbed on the MXenes and only a small amount of MXenes is deposited on the filter membrane due to electrostatic repulsion. Through the layer-by-layer (LbL) strategy, Peng et al. Prepared a MXenes/polymer mixture MXenes-COOH @ (PEI/PAA) with a layered core-shell structure. by introducing a large number of carboxyl functional groups on the surface, the surface electronegativity of MXenes was further enhanced, thus greatly improving the adsorption capacity of dye molecules such as safranine T, methylene blue, and neutral red[42]. It is worth mentioning that this strategy of surface modification with organic polymers is versatile and can be conveniently used to prepare other MXenes @ polymer structured materials to upgrade the adsorption capacity of single MXenes.
In addition to dyes, another kind of organic molecules that seriously pollute environmental water is the abuse of drug molecules. Drug molecules entering the environment may cause outbreaks of infection by drug-resistant bacteria, as well as food toxicity. Some degradation methods may produce more toxic secondary pollutants due to incomplete degradation, while adsorption treatment has the advantages of low cost and no by-product pollution. Yoon et al. Studied the adsorption capacity of Ti3C2Tx-MXenes for common drug molecules such as amitriptyline, elapamil, carbamazepine, 17α-ethynylestradiol, ibuprofen and diclofenac, and found that amitriptyline, which is easily protonated and positively charged, was more easily adsorbed, and the adsorption capacity reached 58.7 mg·g-1 at pH = 7, which was higher than that of commercially available activated carbon[43].
Similar to other two-dimensional materials, the dissociated multilayer MXenes are also easily intercalated into the layers by small organic molecules, such as urea, dimethyl sulfoxide, tetrabutylamine oxide, and isopropylamine, which means that MXenes are effective adsorbents for such small molecules. Gogotsi et al. Studied the adsorption process of urea by MXenes with different structures, including Ti3C2Tx, Ti2CTx and Mo2TiC2Tx, and the results showed that Ti3C2Tx had the best urea adsorption capacity, which could adsorb 98% of urea in the solution within 4 min, and the adsorption capacity reached 9.7 mg·g-1[44]. The analysis shows that the hydrogen bonding between urea molecules and oxygen-containing functional groups on the surface of MXenes is the main driving force for the adsorption. In addition, the biocompatibility experiment proves that the Ti3C2Tx has good compatibility with 3T3 fibroblasts, which indicates that the Ti3C2Tx can be used in the urea removal of biological related liquid.

3.4 Selective adsorption of carbon dioxide

Global warming caused by the use of a large number of fossil fuels is becoming a huge challenge for human beings. In order to reduce carbon dioxide emissions, carbon adsorption materials with large specific surface area, including carbon materials, zeolites, metal-organic frameworks, porous organic polymers and so on, have been continuously developed. However, the long-term stability and limited CO2/N2 selectivity of adsorption materials are still full of challenges. Persson et al. Used in situ environmental transmission electron microscopy (ETEM) to treat monolayer MXenes at high temperature to desorb the terminal F atoms, then exposed the treated MXenes to hydrogen to remove the terminal O atoms on the surface, and finally exposed the treated MXenes to CO2 gas. The results showed that the first MXenes was terminated by incoherent molecules, and each metal atom could adsorb one carbon dioxide molecule at 100 ℃, that is, the adsorption capacity reached 12 mol/kg[45]. At the same time, the modified Mxenes hardly adsorb nitrogen, thus greatly improving the selectivity of CO2/N2.

3.5 Photocatalysis

Photocatalysis is an advanced catalytic technology to convert solar energy into specific chemical energy, which shows great potential in solving energy shortage and environmental crisis. From the reaction mechanism, the photocatalytic process generally involves: I) generation of charge carriers (electrons and holes) by the photocatalyst after absorption of sufficient incident light energy; ii) separation and migration of photogenerated charge carriers, and iii) reduction and oxidation reactions by consuming photogenerated electrons and holes, respectively[46].
Mxenes are excellent CO2 photoconversion catalysts or catalyst supports. Due to the rapid development of industry, a large number of CO2 produced by the extensive use of fossil fuels such as coal and oil are discharged into nature. Halmann et al. First converted CO2 to HCOOH by photoelectrocatalysis, opening a new chapter in the photocatalytic reduction of CO2[46]. Photocatalytic reduction of CO2 involves five steps: light absorption, charge separation, CO2 adsorption, surface redox reaction, and product desorption. Many catalysts for photocatalytic reduction of CO2 have been developed, among which, p-type Cu2O nanostructures with conduction bands more negative than the redox potential of CO2 and not too wide band gap have been widely used for photoinduced conversion of CO2. However, the main drawback of Cu2O based nanomaterials is that Cu2O is easily oxidized or reduced by photogenerated charge carriers. Zeng et al. Fragmented the layered Ti3C2 to form smaller MXenes quantum dots loaded on the Cu2O nanorods, which greatly enhanced the stability of Cu2O[47]. The theoretical calculation shows that the Fermi level of the Ti3C2 QDs changes from 0. 71 V to -0. 523 V, and the decrease of the Fermi level leads to the migration of photogenerated electrons from the Cu2O to the Ti3C2 QDs for enrichment, thus realizing the multi-electron CO2 reduction reaction. Yu et al. Grew TiO2 nanoparticles on Ti3C2 in situ by calcination method, and the experiment showed that the production efficiency of the optimized TiO2/Ti3C2 composite photocatalytic CH4 was 0.22 μmol/H[48]. 3.7 times higher than the commercially available TiO2. Further study showed that the TiO2/Ti3C2 composite had a unique multilayer structure, which formed a large specific surface area and provided a large number of surface active sites for photocatalytic reaction. At the same time, Ti3C2 can greatly inhibit the electron-hole recombination on TiO2, accumulate electrons to form a multi-electron reduction reaction, and reduce the production of intermediates.
Mxenes can significantly enhance the photodegradation of organic phenols. Liu et al. Studied the process of phenol degradation catalyzed by C3N4 supported Ti3C2 under illumination[49]. During the 30 min adsorption-desorption equilibrium process, less than 5% of phenol was adsorbed and removed by all catalysts, and the concentration of phenol did not change significantly without catalyst under light irradiation. After 180 min of irradiation, 25.0% and 54.6% of phenol were degraded by the monolithic g-C3N4(BCN) and the porous g-C3N4(PCN) catalyst, respectively. 70.4%, 98.0% and 81.9% of phenol were degraded by Ti3C2/PCN-1/10, Ti3C2/PCN-1/5 and Ti3C2/PCN-1/2, respectively. Under the same experimental conditions, the photocatalytic removal efficiency of phenol by Ti3C2/PCN nanocomposite was higher than that of PCN catalyst.
Mxenes has important applications in photocatalytic hydrogen production (H2). Using water as raw material to produce hydrogen by solar photocatalysis is regarded as a promising strategy to solve the global energy problem. Water splitting with semiconductor photocatalysts has shown great potential as a clean, low-cost, and sustainable method for solar H2 production[50]. Ti3C2Tx is the most widely used photocatalyst for hydrogen evolution reaction. The advantages of Ti3C2Tx for photocatalytic hydrogen evolution are as follows: (a) hydrophilic surface functional groups are beneficial to the adsorption of water molecules and promote the reaction; The Gibbs free energy of hydrogen adsorption by (b)Ti3C2Tx is infinitely close to zero, which is beneficial to the reduction of H+[51]. Xiao et al. Successfully synthesized a Schottky junction of one-dimensional CdS nanorods/two-dimensional Ti3C2-MXenes nanosheets, using the electrostatic interaction of Ti deficiency and free Cd2+ on the surface of Ti3C2 to fix the Cd2+ to prepare one-dimensional CdS nanorods, which showed excellent hydrogen production performance and produced 6.68 times more hydrogen than pure CdS[52]. Wang et al. Used Ti3C2Tx as a co-catalyst to synthesize a TiO2/Ti3C2Tx composite photocatalyst, which was four times more efficient than pure TiO2 in hydrogen evolution from water photolysis[53]. The Schottky barrier formed between TiO2 and Ti3C2Tx effectively improves the separation efficiency of electrons and holes. Monolayer Ti3C2Tx or quantum dot Ti3C2Tx showed better activity in HER. However, the use of monolayer Ti3C2Tx as a photocatalyst has several disadvantages: (a) the preparation of monolayer Ti3C2Tx is complicated; (B) the structural stability is low, and the catalyst is easily oxidized in water; (C) Manipulation of monolayer or few-layer structures is not easy[51].
Heavy metal ion Cr (Ⅵ) in wastewater has high fluidity, which is easy to enter cells to produce active oxygen and cause oxidative damage to cell tissues[54,55]. In2S3 has the characteristics of low cost, high toxicity, good visible light absorption and good chemical stability. However, the original In2S3 has some limitations, such as fast recombination of photogenerated carriers and easy agglomeration[56]. Huang et al. Prepared a photocatalyst composed of ultrathin MXenes(Ti3C2) and indium sulfide (In2S3) for the ultrafast photocatalytic reduction of Cr (Ⅵ) to Cr (Ⅲ) under visible light[57]. The prepared heterojunction can completely reduce Cr (Ⅵ) to Cr (Ⅲ) within 6 min, and can ensure the stable existence of Cr (Ⅲ). Yuan et al. Used Ti3C2 quantum dots (TQDs) to construct an all-solid-state Z-scheme photocatalytic system (WO3/TQDs/In2S3) for photocatalytic reduction of Cr (VI) and photocatalytic oxidation of bisphenol A (BPA) under visible light irradiation, which showed extremely strong performance, four and three times higher than that of WO3/In2S3 heterojunction, respectively[58].

3.6 Electrocatalysis

Electrocatalytic technology is considered to be the core of clean energy conversion technology in the future, and it is the key and important way to solve the current energy crisis and environmental pollution. The development of non-noble metal catalysts with high performance is a challenge in this field. MXenes based materials have become ideal electrocatalytic materials due to their high activity, high selectivity and long load life, and have been widely used in various fields, including nitrogen reduction reaction (NRR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR) and methanol oxidation reaction (MOR)[59].
Traditional nitrogen reduction mainly relies on the Haber-Bosch process, which promotes the reaction of high-purity hydrogen and nitrogen to produce ammonia through extremely harsh conditions such as high temperature and high pressure, resulting in a large amount of energy consumption and greenhouse gas emissions[60][61]. The electrochemical nitrogen reduction reaction can reduce N2 in air to N H 3 [ 62 ] with high yield by controlling the applied voltage and reaction conditions, which greatly reduces the energy consumption compared with the traditional route, and the high-performance catalyst is the key to realize this process. Zhao et al. Found that Ti3C2Tx(T=F, OH) -MXenes nanosheets could effectively catalyze N2 to NH3 in 0.1 M HCl with a yield of 20.4 mg·h-1·m g c a t - 1 and a faradaic efficiency of 9.3%[63]. Li et al. Systematically investigated a series of single transition metal atoms confined in vacancies of MXenes nanosheets (Mo2TiC2O2)[64]. The results show that Zr, Mo, Hf, Ta, W, Re and Os supported on the Mo2TiC2O2 defect layer can significantly promote the NRR process. Among them, the Zr-doped single-atom catalyst (Mo2TiC2O2-ZrSA) has the lowest barrier (0.15 eV) in the potential-determining step and excellent NRR selectivity. In contrast to NRR, the cleavage of the N — O bond in nitrate requires only 204 kJ·mol-1 of energy and the infinite solubility of the nitrate ion can be used as a new source for ammonia production[65]. Li et al. Showed that MXenes can well disperse and anchor molecular catalysts (CuPc), which helps to construct electrocatalysts (CuPc @ MXenes) and significantly improve the performance of N O 3 - RR[66]. The 10% CuPc @ MXenes had excellent ammonia selectivity (94.0%) and high nitrate conversion (90.5%) and remained stable after seven recovery tests. Sun et al. Used Ti3C2Tx-MXenes as a carrier and Ti source to prepare CuCl-BEF. With the built-in electric field, low-concentration nitrate can be converted efficiently, with high selectivity for ammonia production (98.6%) and low nitrite production (0.6 mg·L-1)[67].
Oxygen evolution reaction (OER) is a key process in many new energy applications, such as fuel cells and water splitting[68]. While many noble metal-based oxides have been shown to be effective OER catalysts, the scarcity and high cost of noble metals have severely hindered their wide range of applications[69]. Yu et al. Prepared a novel active OER catalyst (FeNi-LDH/Ti3C2-MXenes) based on MXenes[70]. The catalyst has high conductivity, high surface activity and high stability, and the electrocatalytic oxygen evolution performance is improved through synergistic effect. The results show that the FeNi-LDH/Ti3C2-MXenes electrocatalyst has lower overpotential, lower Tafel slope, and superior durability in alkaline media than the RuO2 and FeNi-LDH/rGO catalysts.
Hydrogen evolution reaction (HER) is a new clean energy production technology that uses electrocatalytic water splitting to produce hydrogen, which is the key to reduce the use of fossil fuels. Noble metals, especially platinum, are the most active catalysts at present, but the high price of platinum hinders its wide application, so it is urgent to develop stable catalysts based on non-noble metals. Xie et al. Systematically investigated the HER performance on MXenes(Mo2X and W2X,X=C and N) based on DFT calculations, and found that their HER performance largely depends on their composition, hydrogen adsorption configuration, and surface functionalization[71]. Compared with other MXenes, the W2C monolayer has excellent HER activity and near-zero overpotential at high hydrogen density, and hydrogenation can further improve its catalytic performance in a wide range of hydrogen density, while oxidation can significantly reduce its activity. Wang et al. Immobilized highly active nano-/atomic-scale metal platinum on Ti3C2Tx-MXenes sheets (MXenes @ Pt) connected with conductive SWCNTs network to construct hierarchical platinum-MXenes-single-walled carbon nanotubes (SWCNTs) heterostructures for HER catalysis[72]. The as-prepared hierarchical HER catalyst exhibited high stability during 800 H of operation, with a volumetric current density as high as 230 mA·cm-3 at − 50 mV versus reversible hydrogen electrode (RHE), and an overpotential as low as − 62 mV at a current density of -10 mA·cm-2. Considering the properties and structure-activity relationship of Pt-MXenes-SWCNTs heterostructure, other MXenes may show more potential in electrocatalysis.
Converting CO2 into liquid fuels with the help of green energy can make the carbon cycle more effective and realize the green development concept of "artificial carbon cycle". Using MXenes to capture CO2 and convert it into fuels such as methanol and methane has important research significance and prospects. Li et al. And Sun et al., by means of calculation, conducted an in-depth and systematic study on the electrochemical mechanism of capturing CO2 under two system MXenes-Tx(T=OH,O) of MXenes surface without covering functional groups and OH surface and O surface, and found that pure MXenes without functional groups were more conducive to the capture and conversion of CO2 into methanol and methane[73]. Yang et Al. Prepared MXenes-anchored single-atom copper catalyst for electrocatalytic CO2 reduction to methanol by selective etching of hybrid A layer (Al and Cu) in quaternary MAX phase (Ti3(Al1-xCux)C2)[74]. The results show that the as-prepared single-atom Cu catalyst has a high faradaic efficiency of 59.1% for the formation of CH3OH and good electrocatalytic stability.
Direct methanol fuel cells (DMFCs) are a sustainable, environmentally friendly, and efficient energy utilization technology. Methanol selective oxidation (MOR) is the core of DMFCs, and the key challenge is to prepare electrocatalysts with high activity and long lifetime. The two-dimensional layered structure and abundant anchoring sites of MXenes, together with their excellent electrical conductivity and structural stability, make it possible to prepare such catalysts. Xu et al. Reported the nanoscale Pd electrocatalyst Pd/Ti3C2Tx-rGO(1∶1) co-supported by MXenes and reduced graphene oxide, and the mass activity of the catalyst for methanol oxidation was 753 mA/mg, which was 1.7 times higher than that of Pd/C (446 mA/mg)[75]. In addition, due to the synergistic effect of Ti3C2Tx and rGO between Pd and Ti3C2Tx/rGO, which can enhance the catalytic active surface area and rapid mass and charge transfer, the catalyst has excellent cycle stability and long life. The current density of the catalyst was 9 times higher than that of Pd/C after 7200 s of use. Wang et al. Successfully loaded Pt nanoions on the surface of Ti3C2Mxenes, and found that the prepared catalyst had lower initial potential and higher peak current density in the MOR reaction[76].
Chloramphenicol (CAP), as a broad-spectrum antibiotic, is often abused in medicine and animal husbandry, which increases the environmental risk of drug-resistant bacteria and genes[77,78]. Therefore, degradation of CAP before release by mild and efficient electrochemical hydrodechlorination (EHDC) is of great interest. Li et al. Selected Ti3C2Tx-MXene nanosheets as ultra-small Pt NPs support to form Pt @ MXene electrocatalyst that can effectively degrade chloramphenicol[79]. 98.7% of CAP can be removed on 1% Pt @ MXene within 90 min, and the removal rate remains at 86.5% after 25 stability test cycles.

3.7 Membrane separation

Membrane separation technology is one of the most promising technologies in gas separation and wastewater treatment because of its low energy consumption, high efficiency and easy large-scale application. Among the nanomaterials used for membrane preparation, two-dimensional materials are ideal materials for separation membranes because they have abundant and uniform nanochannels, which can break the trade-off between permeability and selectivity in the separation process. Graphene and its derivatives graphene oxide (GO), exfoliated hexagonal boron nitride (h-BN), graphitic carbonitride (g-C3N4), transition metal dichalcogenides (TMD) and MXenes, as several typical classes of 2D nanomaterials with strong structural, physical and/or chemical similarities, have been used for membrane separation process studies. In addition, different two-dimensional materials provide different material properties to meet the needs of different applications. MXenes are of particular interest because of their rich surface chemistry and unique physicochemical properties, and have been widely studied in gas separation, water purification, and other fields[80].
Wang and Gogotsi et al. Reported a two-dimensional layered molecular sieve membrane MXenes with highly ordered sub-nanometer channels, which uses the abundant and uniform terminal groups on the surface of MXenes nanosheets to support the formation of highly ordered two-dimensional layered nanopores, thus achieving rapid and accurate gas separation capacity :H2 permeation flux up to 160 2200 Barrer,H2/CO2 separation selectivity, breaking through the trade-off limit[81]. Jin et al. () effectively achieved the regulation of the membrane spacing of MXenes by crosslinking MXenes nanosheets with borate and polyethylene diamine (PEI), and the prepared borate/PEI/MXenes hybrid membrane had significantly enhanced permeability to CO2[82]. Thanks to its good thermal stability, MXenes membranes can be used for gas separation at high temperatures. Liu et al. Studied the separation performance of MXenes membrane for H2/N2 at 320 ° C, with selectivity up to 41, and showed no signs of degradation up to 200 H of operation[83].
High surface charge, hydrophilicity, ion adsorption ability, chemical tunability and antibacterial properties make MXenes attractive materials for water treatment. Xing et al. Utilized a solvent as a template to crumple flat Ti3C2Tx-MXenes nanosheets by a freeze-drying method[84]. After assembling the wrinkled Ti3C2Tx nanosheets into a layered film, the obtained c-Ti3C2Tx-MXenes film has a large number of interfacial voids and enlarged interlayer channels compared with the unwrinkled Ti3C2Tx-MXenes film. The water and organic solvent permeabilities were significantly enhanced, and the permeabilities for water and acetone were 5460 and 3745 L/m2·h·bar, which were about 14 and 27 times higher than those of the Ti3C2Tx membrane with the same mass loading. Xu et al. Incorporated the synthesized 2D Ti3C2Tx-MXenes nanosheets into chitosan (CS) to fabricate a new MXenes/CS mixed matrix membrane for solvent dehydration via pervaporation process[85]. The results showed that the incorporation of MXenes nanosheets did not increase the surface adsorption, but the assembled MXenes layer with interlayer channels greatly enhanced the penetration of water molecules through the membrane. Therefore, the flux and separation coefficient of the CS membrane were improved by introducing MXenes nanosheets. Yang et al. Prepared a new type of Fe3O4@MXenes composite membrane by vacuum filtration method with commercial calcium membrane as support, which significantly increased the pure water flux of MXenes[86]. Under the synergistic effect of composite film sieving and adsorption of Fe3O4NPs and MXenes nanosheets, the removal rates of heavy metal ions Cu2+, Cd2+ and Cr6+ in the solution were 63.2%, 64.1% and 70.2%, respectively. After HCl solution washing and three cycles of continuous filtration experiments, the membrane flux and heavy metal removal rate were well restored. Li et al. Used Ti3C2Tx nanosheets with high aspect ratio to construct a highly regular interlayer channel two-dimensional membrane, which has strong separation performance in the treatment of water-soluble antibiotics and ethanol-soluble antibiotics[87]. Compared with the organic nanofiltration membrane, the permeation flux of the membrane can be improved by more than one order of magnitude.

3.8 Sensor

Two-dimensional layered transition metal dichalcogenides (2DTMD) are considered as promising sensing materials due to their high surface area to volume ratio, good adsorption properties, a large number of redox active sites and high surface reactivity, which can be integrated into chemical sensors to detect various harmful gases, such as NH3, NO2 and VOCs[88]. However, gas sensing devices using only 2D TMD materials usually have high electrical resistance and lack selectivity and recovery for target analytes, especially at room temperature, which hinders their practical sensing applications in the Internet of Things. MXenes possess stable and easily tunable microstructure, high conductivity, large chemically active surface, and tunable hydrophilicity, and combining TMD with MXenes may be an effective strategy to further improve the room-temperature sensing performance of devices[88]. Chen et al. Fabricated a Ti3C2Tx/WSe2 hybrid sensor for selective detection of various volatile organic compounds by liquid phase exfoliation and inkjet printing[88]. The Ti3C2Tx/WSe2 hybrid sensor has the advantages of low noise, ultrafast response and recovery time, and good flexibility for various volatile organic compounds. Compared with the original Ti3C2Tx, the sensitivity of the hybrid sensor to ethanol is improved by more than 12 times, and the gas response and recovery can be repeated and fast. To further understand the advantages of Ti3C2Tx/WSe2 in detecting various VOCs, the sensors were exposed to 40 ppm of ethanol, methanol, acetone, hexane, benzene, and toluene, and for each individual target analyte, the Ti3C2Tx sensor showed a small positive response value, whereas the Ti3C2Tx/WSe2 sensor showed a negative and much higher response value.
Sensitive gas detectors are becoming increasingly important in detecting toxic gases contained in the air for pollution monitoring and therapeutic diagnosis through breath analysis. Detection of volatile organic compounds (VOCs) in exhaled air at levels below minus one part per million (ppm) is essential for early diagnosis of disease[89]. For example, for the diagnosis of peptic ulcer caused by Helicobacter pylori, there is an urgent need to detect ammonia levels between 50 and 2000 ppb[90]. To distinguish diabetic patients from normal patients, acetone levels of about 300 to 1800 ppb should be detected[91,92]. Gogotsi et al. Showed that Ti3C2Tx could detect acetone gas as low as 50 ppb, ethanol gas as low as 100 ppb and ammonia gas at room temperature. The study showed that Ti3C2Tx was sensitive to VOCs and could provide key conditions for early treatment and diagnosis of diseases[93].
As a new research method, PEC technology has broad application prospects because of its high sensitivity, high efficiency, high response speed, low noise and other characteristics. BiVO4 is widely used in PEC technology because of its suitable band gap, excellent water stability and low toxicity. Jiang et al. Found that Ti3C2-MXenes materials could trap holes generated by slight oxidation, and the transfer direction of holes could be controlled under the interface modification of Ti3C2 to realize the spatial directional distribution of charges, which made up for the characteristics of low electron mobility, limited light absorption capacity and few active sites of BiVO4[94]. The results show that Ti3C2Tx, as a metal-like material, can form Schottky junction with BiVO4 semiconductor, which enhances the photoelectric conversion efficiency and promotes the charge transfer.

3.9 Antibacterial activity

In recent years, the potential of MXenes in biological activity is gradually being discovered and valued. Rasool et al. Studied the growth of Gram bacteria on the surface of MXenes[95]. The results showed that Ti3C2Tx exhibited excellent antibacterial activity against both Gram (+) and Gram (-) bacteria. As the concentration of Ti3C2Tx increased, the loss of bacterial cells gradually increased. The viability of E. Coli and B. Subtilis was 92.53% and 93.96%, respectively, at 2 μg/mL of Ti3C2Tx. Moreover, at lower concentrations, the Ti3C2Tx dispersion had a greater effect on B. Subtilis than on E. Coli. Compared with graphene oxide (GO), Ti3C2Tx-MXenes has higher antibacterial activity. At a concentration of 200 μg/mL, Ti3C2Tx showed more than 98% cell inactivation for both bacterial strains, whereas GO had an inactivation rate of about 90% at the same concentration.
Based on the excellent antibacterial properties of MXenes, antibacterial and bactericidal medical materials mediated by MXenes have been developed. Li Jiebo and Hong Weili of Beijing University of Aeronautics and Astronautics, and Chen Xiangmei of Peking University Health Science Center, combined MXenes with near-infrared laser to achieve rapid and efficient killing of bacteria and biofilm within 20 minutes[96]. Zhang Qiuyu of Northwestern Polytechnical University has designed and developed a 2D Ti3C2TxMXenes scaffold with self-healing, conductivity and tissue adhesion, which has hemostatic ability, good biocompatibility and excellent wound healing ability of MRSA infection, and is expected to solve the problem of bacterial infection in wound healing and skin regeneration[97].

3.10 Electromagnetic absorption and shielding

With the development of economy, electronic equipment is widely used in daily life, especially with the advent of the 5G era, causing more serious electromagnetic pollution, which not only affects human health, but also causes the failure and degradation of civil or military electronic equipment. Electromagnetic absorption materials with excellent performance are also the key to improve the stealth performance of equipment[98]. Conventional electromagnetic shielding (EMI) materials are made of highly conductive and permeable metals or metal oxides. However, its application is limited due to its high density, narrow absorption bandwidth, poor impedance and easy corrosion[99,100]. As the Al layer is etched, a large number of defects are generated on the surface of MXenes, which can provide excellent conductivity and dielectric properties of MXenes, together with its two-dimensional lamellar structure and excellent stability, making it an ideal electromagnetic absorption and shielding material.
Cui et al. Found that different synthesis conditions have a great influence on the electromagnetic absorption properties of MXenes[101]. The Ti3C2Tx etched by HCl-LiF for 24 H has the minimum reflection loss of -27.5 dB at the thickness of 4 mm. However, under the same etching time and loading, the Ti3C2Tx obtained by HF etching can achieve an electromagnetic loss of-42. 5 dB at a thickness of 1. 7 mm, and the tunable absorption bandwidth can reach 5 GHz. This is because of HCl-LiF etching, resulting in fewer layers of MXenes; While multilayer MXenes prepared from HF have stronger interlayer reflection.
MXenes based materials have great potential for lightweight. Weng et al. Prepared layered MXenes/AgNW sponges using dip-coating and one-way freeze-drying techniques[102]. Due to the synergistic effect of AgNW and Ti3C2Tx-MXene, the density of the mixed sponge is only 49.5 mg·cm-3, while its RL can reach − 52.6 dB. Liang et al. Prepared a three-dimensional MXenes/graphene composite aerogel anchored by magnetic Ni nanochains by directional freezing and hydrazine vapor reduction[103]. The special orientation structure and the heterointerface of dielectric/magnetic components are beneficial to obtain excellent microwave absorbing properties, and have good impedance matching, multiple polarization and electric/magnetic coupling effect, with the minimum reflection loss of-75. 2 dB and the widest effective frequency of 7. 3 GHz. In addition, the excellent structural robustness and mechanical properties of the aerogel, as well as its high hydrophobicity and thermal insulation properties, ensure its stable and durable EMA application performance, and can resist deformation, water or humid environment, and high temperature attack.
The introduction of magnetic materials such as ferrite, carbonyl iron, iron oxide or nickel oxide can further increase the absorption bandwidth of MXenes and improve the microwave absorption performance. Feng et al. Synthesized Ni-modified MXenes by alkalization and surface nickel ion modification of Ti3C2-MXenes, which significantly enhanced the absorption capability in the 2 – 18 GHz band[104]. Liang et al. Prepared MXenes/Ni hybrid materials, consisting of one-dimensional Ni nanochains and two-dimensional Ti3C2Tx nanosheets[105]. At 11.9 GHz, the MXenes/Ni hybrid with a thickness of 1.75 mm has a minimum RL of − 49.9 dB and an effective absorption bandwidth of 2.1 GHz, and the electromagnetic shielding efficiency is further improved with the increase of Ti3C2Tx content. Wang et al. Synthesized the hierarchical structure of Ti3C2/Fe3O4/PANI composite through HF etching, coprecipitation and in situ polymerization route, and the minimum RL was − 40.3 dB at 15.3 GHz, which was much higher than that of Fe3O and Ti3C2/Fe3O4[106]. Moreover, it shows an effective absorption bandwidth of 5.2 GHz (12.8 ∼ 18 GHz).
In 2016, Gogotsi et al. First found that MXenes materials containing Ti have better electromagnetic shielding performance (Fig. 4), and proposed the mechanism of multiple reflection and surface dipole absorption[108]. In 2020, they reported a class of nitrogen-doped two-dimensional transition metal carbide Ti3CNTxMXenes, which were prepared by vacuum-assisted filtration and showed higher electromagnetic shielding capability than undoped Ti3CTx or metal film materials with the same thickness, reaching -61 dB[107]. Furthermore, the electromagnetic shielding effect is improved to -77 dB, -99 dB and -116 dB by thermal calcination at 150 ℃, 250 ℃ and 350 ℃ in Ar atmosphere, which reflects a good application prospect. The improvement of electromagnetic absorption capacity brought about by hot calcination may be due to the emergence of metamaterial structures during calcination, which improves the dielectric constant and permeability.

4 Conclusion and prospect

Researchers at home and abroad have synthesized MXenes phase materials by various methods, and have carried out theoretical and computational analysis on them. Studies have shown that two-dimensional materials MXenes have unique layered structure, excellent conductivity and hydrophilicity, and have a wide range of applications in the adsorption of radioactive elements, heavy metals and organic compounds, photocatalytic reduction of CO2 and degradation of organic compounds, environmental monitoring and biological activity. However, compared with the research of two-dimensional materials such as graphene, the understanding of MXenes phase is still in the initial stage, and there are still the following problems:
(1) Preparation of MXene phase. At present, most of the preparation methods take the corresponding MAX phase material as the precursor, the synthesis method is relatively single, the preparation in large quantities is difficult, and the prepared material will produce structural defects due to etching, which will affect its performance. Generally, most of the products are multilayer structures, and it is difficult to prepare MXenes phase materials with few layers or even single layers.
(2) The study of MXenes is not mature enough. At present, most of the research is still in the basic stage, and its surface functional groups make its structure more complex and more difficult to study. The research lacks a large number of experimental data to support, and the scope of practical application is narrow, so that its research can not be developed rapidly.
(3) Stability of MXenes. The functional groups on the surface are easy to be removed during calcination, which destroys the relative activity of the surface, and when the temperature is high, they will be oxidized into metal oxides by O2, thus losing the original performance.
To sum up, since the first report of MXenes in 2011, researchers have invested tremendous and continuous efforts to explore the preparation, properties, and application potential of MXenes. Especially in recent years, applications based on MXenes have made many innovative and meaningful progress in various fields. However, the current research is still in its infancy, and it is still challenging to design and clarify the mechanism of action of material structures for different application purposes. Our summary only includes a small part of them, but we still hope that it can help to better understand the current challenges and opportunities of MXene-based materials, and provide some reference for researchers in different fields.

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Funding

National Natural Science Foundation of China(21908018)

National Natural Science Foundation of China(22078174)

Youth Innovation Program of Universities in Shandong Province(2019KJD007)

support from the State Key Laboratory of Fine Chemicals, Dalian University(KF2114)

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