Recent trends in synthesis of 2D MXene-based materials for sustainable environmental applications

Abstract

The unique properties of two-dimensional (2D) materials have piqued the interest of the technical community. Titanium carbide (MXene) is a member of a rapidly expanding family of 2D materials with exceptional physiochemical characteristics and a wide range of uses in the environmental field. 2D MXene has long been a topic of interest in environmental applications, including wastewater treatment, electromagnetic interference (EMI) shielding, photocatalysis, and hydrogen evolution reactions (HER) due to its high conductivity, varied band gap, hydrophilic nature, and exceptional structural stability. This study covers important developments in 2D MXene and discusses how design, synthetic methods, and stability have changed over time. In this review paper, we have discussed the strategy synthesizing of conventional, affordable heterojunctions and Schottky junctions, as well as the development, mechanisms, and trends in the deterioration of environmental organic contaminants, HER, and EMI Shielding. We also explore the obstacles and restrictions that prevent the scientific community from producing practical MXene with regulated characteristics and structures for environmental applications and analyzing its present usage. The hazardous-environmental aspects of MXene-based materials and the problems and future possibilities of these applications are also examined and emphasized. This review paper focused on environmental applications such as heavy metal detection and removal, EMI shielding, and hydrogen generation using MXenes. The issues related to wastewater, electromagnetic interference, and clean energy production are very persistent in the environment, and a better material is required to address these challenges. Thus, MXene is a kind of material that could be a better alternative to address these persistent issues, and hence, this review becomes very important, which can pave the way for the development of MXene-based materials to address these issues.

Graphical abstract

1 Introduction

Graphene’s discovery and fascinating physical features have sparked a flurry of interest in the study of 2D materials, which has led to an increase in the study of massive flat materials like transition metal dichalcogenides (TMDCs) and boron nitride. These materials are created by exfoliating the bulk 3D structure into thin 2D layers, which are connected by Van der Walls forces that are weak [1]. The remarkable electrical and optical characteristics and the mechanical properties of 2D materials eventually cleared the door for in-depth research for diverse applications over the past 10 years [2,3,4,5,6,7]. These materials also have potential as basic components in a wide range of technologies, such as membranes, layered structures, and compounds, which exhibit a wide variety of applications [8,9,10,11]. Few single-element 2D compounds have been developed, including but not limited to phosphorene, silicene, germanene, and graphene. On the other hand, the majority of such compounds consist of two or more elements, like dichalcogenides, clays, and oxides. MXenes were added to the family in 2011 by Gogotsi et al. [12]. The rapid development of several synthetic compositions has been the goal of intense scientific research in this area. Subsequently, a growing number of approximately 30 MXene-related compositions have been documented, and various others have been subject to computational investigations [13].

The unique structural characteristics of MXene can be achieved due to the addition of two transition metals. Furthermore, transition metals can create ordered 2D structures within the plane, as seen in (Mo_2/3Y_1/3)₂CT_x, or as atomic sandwiches like Mo₂TiC₂T_x, resulting in a single-layered MXene with well-organized morphology. However, the ordered MXenes were created in 2014 and revised in 2015 [14]. Starting in 2017, there has been significant development of diverse MXene composites. Researchers have directed their attention toward designing structured double transition metal MAX phases and thoroughly investigating their distinctive traits, including their magnetic properties [15, 16]. Several potential MXene frameworks have been calculated using computational results on MXenes and derivatives based on their precursor [17, 18]. The formation of stable combinations on the M and X locations enables the synthesis of a considerable number of non-stoichiometric MXenes with meticulously regulated characteristics. This includes the incorporation of carbon nitrides or mixed transition metals [19]. Presently, researchers are striving to develop and integrate additional X elements into 2D boride structures. Due to the termination of hydroxyl, fluorine, and oxygen groups, the surfaces of carbonitrides are endowed with hydrophilic properties [20, 21]. As a result of their uncomplicated processing and minimal stabilization demands, they can be transformed into films, devices, and coatings [22, 23]. Based on the observed optical bandgap of 0.9 eV for Ti₂C [13, 20] with oxygen termination, it is anticipated that MXenes will exhibit fascinating electrical and thermoelectric properties. However, Mo₂CT_x and Mo₂TiC₂T_x exhibit semiconductor behavior, while Ti₃C₂T_x MXene has excellent metallic conductivity [24]. Harmful waste has been produced due to the rapid growth of the global economy and industrialization over the last several decades, and this garbage has negatively affected the environment and human health. A major environmental challenge is the presence of contaminants in water streams, such as salts, heavy metal ions, medicines, aromatic compounds, and dyestuffs. For living things, the majority of them are mostly fatal and extremely destructive [25, 26]. Aerobic/anaerobic digestion, membrane filtration, adsorption, and biochar treatment are popular biological and physiochemical procedures used to remove diverse environmental pollutants [27, 28]. Nanomaterials are also becoming a cost-effective and ecologically benign choice for efficient environmental cleanup [29,30,31].

These very effective functional materials for environmental remediation, such as semiconductors, carbon-based nanostructures, metal-organic frameworks (MOFs), and ceramics, had improved efficiency, selectivity, mechanical stability, and sensitivity [33, 34]. M_n+1X_nT_x, where n can vary from one to three, is the formula for MXenes; here T represents surface functional groups like fluoride, oxygen, hydroxide, and chlorine, M denotes the transition metal, X represents either N or C, and x represents the total amount of these groups (Fig. 1) [35, 36]. According to the value of n in M_n+1X_nT_x, variability in MXene width is generally less than 1 nm [37].

Fig. 1

MXenes compositions as shown in a periodic table. The building blocks of MXenes are designated by color. On the bottom, you’ll find schematic representations of four common MXene structures [32]

Due to its distinctive features, 2D-MXenes have been investigated for possible applications as an EMI shielding material. Electromagnetic interference has become a constant issue in recent years due to the advancement of wireless communication technology and flexible wearables [38, 39]. The features of currently employed metal materials—susceptibility to corrosion, high density, and low flexibility—make encountering the rising EMI requirements challenging. So far, the creation of lightweight EMI shielding materials has been the endeavor of previous researchers [40, 41]. Materials that can shield against electromagnetic interference (EMI) while maintaining good flexibility and high conductivity are greatly desired, specifically if they can be conveniently manufactured into films. Due to their distinctive properties, in the upcoming generation of EMI shielding materials, MXene materials will be a formidable adversary [42, 43]. To fulfill the mechanical property demands of EMI shielding materials used in electronic products, it is of utmost importance to develop new EMI shielding materials that possess exceptional mechanical properties.

MXenes have also been a potential electrocatalyst for green energy generation and many other environment-related applications [13, 44,45,46]. Many people see hydrogen as a way out of the current worldwide energy dilemma and the damage it does to the environment. Hydrogen has several advantages over other energy sources, including a high energy density, the absence of pollution and greenhouse gas emissions, and the possibility of recycling. With its advantages of security, efficiency, cheap cost, durability, and environmental friendliness, it has the potential to work well in the green energy storage and conversion industries. In addition to using fossil fuels and biomass feedstock to produce hydrogen, other methods include fermenting organic waste and marsh gas. This includes the water-splitting reactions, which have shown to be a cost-effective and green option. There is a lot of research going on in this area across the world. Saxena et al. [47] showed how 2-D and 3-D MXenes could be synthesized by environmentally friendly green method and their utilization in batteries and supercapacitors. In another article, Chaudhuri et al. [48] discussed the applications of graphene oxide/MXene in supercapacitors, lithium-ion batteries, lithium-sulfur batteries, and sodium and potassium-ion batteries. Janjhi et al. [49] showed MXene-based materials to remove antibiotics and heavy metals from water. The researchers also examined the use of MXene and MXene-based materials for removing environmental pollutants, removing toxic metals, biosensors, and air purification [50,51,52,53,54]. These trends show that MXene and MXene-based materials are of the utmost importance, and further analysis is required. Keeping this in mind, in this review paper, we have discussed the synthesis of MXene and MXene-based materials in detail. We have also discussed the properties of these materials, like structure, stability, electronic properties, optical properties, and magnetic properties. The review presents the application of MXene and MXene-based materials in adsorption, where the removal of organic pollutants, heavy metals, and nuclear contaminants is discussed in detail. In this way, the review is unique and presents the removal of each kind of pollutant at a single platform. Moreover, we have also shown the photocatalytic degradation efficiency of these kinds of materials, bringing a new aspect to this review. In addition, this review also presents the application of MXene and based materials for EMI shielding applications. Also, the utilization of MXene and based materials in hydrogen generation and production is discussed thoroughly. Thus, the review paper presents the solution for a major environmental challenge by bringing major portions like clean energy demand, water pollution, and electromagnetic pollution to a single platform. This review will pave the way for utilizing MXene and MXene-based materials for the various other kinds of pollution prominent in the environment. This will also pave the way for further research on these materials and their applications in different aspects of science.

In the first half of this manuscript, various synthesis techniques, the delamination process, and the most intriguing characteristics, including stability and structural, mechanical, optical, and electronic, are covered in detail. The second half of the paper is focused on promising environmental applications, including wastewater treatment (organic removal, heavy metal ions, and radionuclides), EMI shielding, and HER. The future possibilities for MXenes, which have been at the vanguard of environmental remediation, were finally projected. The development of novel MXene-based materials is projected to be a significant research area for several years owing to their remarkable characteristics and prospective applications in the environmental field. Timeline of MXene: A journey from 2011 to 2022 is depicted by a schematic diagram in Fig. 2.

Fig. 2

Timeline of MXene: investigation and exploration

2 Synthesis of MXene

For the fabrication of MXene materials, there are diverse fabrication processes previously reported in the literature [55, 56], and one fabrication technique may be better than the other process for a particular application. Before synthesizing any material, the main goal is to engineer the material so that the resultant product has desired properties required for the targeted application. In this section, we have discussed the etching technique, delamination process, and other bottom-up techniques for fabricating MXenes, as depicted in Fig. 3. Depending upon the fabrication route followed for the fabrication process, MXenes inherit different properties. In general, the production process of MXene may require limited hours to several days, which is influenced by factors such as the etching temperature and the concentration of HF used for etching. In the case of Ti₃C₂T_x, for instance [57, 58], employed as little as 3 wt% of etching agent containing HF and found that the stability, purity, and characteristics of MXenes are all affected by the concentration of HF in Ti₃C₂T_x flakes. For Ti₃C₂T_x, the technique of synthesis that is chosen has a direct impact on the end product’s size, flake quality (such as the quantity and type of flaws), and surface functionalization [59]. The inherent properties and characterization methods have also been discussed in this section.

Fig. 3

Various synthesis routes to fabricate MXenes

2.1 Etching

2.1.1 Etching using fluorine-containing acid

Generally, the synthesis of MXene starts with chemical etching of the MAX phase by selectively targeting a layer element, i.e., Al from Ti₃AlC₂. Through the etching process, terminated multilayered MXenes are obtained, linked by weak Van-der walls forces and hydrogen bonding [60]. The termination of MXenes with functional groups like -F, -O, and -OH can also be done while etching. The acidic solution can be removed or separated from multilayered MXenes by centrifugation after the etching process is completed. A safe pH~6 can be achieved after repeated washing of the mixture and removing reaction salts and residual acid. In order to extract multilayered MXene from the acidic solution, washing is often done through repeated centrifugation and filtration of the acidic supernatant. After this, the multilayered MXenes can be achieved by high-speed centrifugation or vacuum-assisted filtration.

The A-element layer on MAX phases is often removed selectively using HF, the most used etching agent [62]. Irrespective of quantity, it is crucial to be familiar with risk management and necessary safety procedures when dealing with hazardous HF. For the most part, the methods described in the literature, Naguib et al. used 10 or 50 weight percent of HF [63]. Despite the fact that a lower HF concentration, as will be described later enough to separate Al from Ti₃AlC₂. Naguib et al. used 30, 10, and 5 wt% HF concentrations. They demonstrated that an etching duration of 5 h is sufficient when utilizing a concentration of HF below 30% and that eliminating Al can be accomplished with as little as 5 W%, as depicted in Fig. 4. High-temperature etching and high concentrations of HF for a longer time generally lead to smaller lateral sizes and more defects in MXenes [57]. Besides HF, ammonium bifluoride and the combination of fluoride salts and acids have also been recently used as etching agents [63, 64]. It is best to avoid using a high concentration of HF because of the significant dangers associated with its extreme corrosiveness. The in situ HF approach has various benefits, including the use of less toxic substances than fluoride-based acids, intercalation, and etching processes happening simultaneously, and some researchers also found out that sonication is not required in the case of in situ etching [61, 65]. Figure 4 explains the general map toward the fabrication of Ti₃C₂ using etching acid methods like HF or in situ HF etching process. Concentrated HF has frequently been used as the preferred etchant to eliminate Al from Ti₃AlC₂, as stated in most MXene literature [66]. Previously, many researchers have used diluted HF with 5 wt% concentration to prevent the use of highly concentrated HF. Therefore, as a substitute, HF production during reaction from hydrogen fluoride or fluoride salts can be used using substances like NH₄HF₂ or LiF to create etchants with a 3–5 wt% HF content [64, 67]. Delamination methods have also been shown in Fig. 4 with different intercalating materials where sonication can also be used depending upon the application for which MXenes must be synthesized. The in situ HF etching brings more properties for MXenes than HF etching. These properties include larger interlayer distance and flake size, more -O functionalized groups, low concentration of defects, and less -F terminated groups. The lattice parameter was 2.0 when HF was used as the etchant, but it was reduced to 30–60 min when LiF/HCl was used [68]. It has also been suggested that using a greater LiF/HCl mixture lessens the need for sonication. Figure 5 is a schematic showing how MXene may be synthesized via chemical etching.

Fig. 4

Schematic diagram showing synthesis of Ti₃C₂ MXene [61]

Fig. 5

Schematic representation of Ti₃C₂ MXene synthesis via chemical etching

To synthesize carbonitride and carbide MXenes, the most adaptable technique is the exfoliation of MAX in the solution containing HF. Nevertheless, in the case of the synthesis of nitride MXenes, this etching technique is usually not suggested. This is because, in HF, nitride MXenes form very high energy and have very poor stability [69]. In previously reported work, MXenes based on nitride have been synthesized by the molten fluoride salt etching method [70]. Multilayered Ti₂NT_x has been successfully synthesized previously [71].

The selective removal of Al in Ti₂AlN was attained by submerging it in a KF-HCl solution. However, this leads to the deformation in the layered structure of MXenes. To delaminate MXenes into 2D sheets, which are isolated by weakening the interaction between layers, the role of intercalating compounds has gained a huge interest. It has been considered an important step (Fig. 6). The surface area of MXenes grows when their interlayer gap expands due to the addition of ions and molecules between the layers during delamination. -F and -OH termination groups are often found in etched MXene. Following intercalation, certain functional groups in the etched MXene are replaced by cations from the intercalants, promoting the preferential adsorption of the designated contaminants [72]. In most cases, intercalating substances are utilized, and they are polar organic molecules like dimethyl sulfoxide (DMSO), urea, or hydrazine [14]. Using a mechanical vibrator or ultra sonicator may create a colloidal monolayer or multilayer MXenes solution. Previous research shows that DMSO causes c-LP values to grow to 35.04±0.02 while urea decreases to 25±0.02. After ultra-sonication, the resultant dispersed 2D MXene sheets are electrically stabilized and free from aggregation and accumulation and can be processed further.

Fig. 6

Two possible strategies for extracting MXenes from MAX phases and similar layered compounds are depicted schematically here. Acids containing fluoride ions are used to selectively etch the MAX phase in the first method. Method two involves doing a selective etching in molten salts of the MAX phase. The result is often MXene particles with many layers, which can be intercalated to create monolayers [72]

When formulating with salts based on fluorine, it becomes important to keep in mind that the efficiency, size, and capacity of Ti3C2Tx will be affected by the amount of HCl and LiF employed to produce HF during in situ process [57, 58]. Ghidiu et al. previously reported a clay method in which multilayered Ti₃C₂T_x (Fig. 4) were confined into single flakes through sonication, frequently generating small and defective MXene flakes [70, 73]. In addition, a simple handshake was required to transform a multilayered powder of Ti3C2Tx into big, unbroken flakes [74]. Minimally invasive layer delamination (MILD) is the term given to this improved technique to differentiate it from the previous LiF/HCl technique. Researchers have used the MILD technique, where bigger flakes are preferred [75, 76].

2.1.2 Fluoride-free etching

The procedure of etching using fluoride ion acid posed environmental risks. Additionally, the fluorine-containing acid etching method can produce additional -F groups that might negatively impact environmental sustainability. This has led to the development of numerous fluoride-free etching techniques for fabricating MXenes. In this part, etching techniques based on fluorine-free salts, such as chemical vapor deposition, Lewis acidic, and electrochemical etching, have been discussed.

Alkali-based etching process has been studied previously by many researchers due to the amphoteric character of Al [77, 78]. Organic bases like n-butylamine or tetrabutylammonium hydroxide (TBAOH) might deteriorate the interactions among M-X layers, as was found by Naguib et al. [61]. They are thus advantageous for the widespread delamination of stacked MXenes. The above intercalated MXenes had shortened source material in -F groups as well as broad d-spacing [61]. In order to specifically target Allayers, Al(OH)⁴⁻ reconfigured and TMA⁺-intercalated T₃C₂ resulted from TBAOH’s use as an intercalation agent by Xuan et al. [79]. In order to create Ti₃C₂ with a distinctive hierarchical highly porous, the conjunction of ball milling was done using etching based on organic bases [80]. Despite the fact that these techniques for etching remove the creation of -F end groups, the significant amount of base necessary for the process still poses a considerable risk, thereby limiting its widespread use in manufacturing.

Many researchers have reviewed electrochemical etchings previously [81, 82]. One of its benefits is the electrochemical etching process’s relatively quick etching time. Typically, the electrochemical etching method requires two phases: targeted removal of the Al layer from an electrode is accomplished by either (1) electrochemical etching or (2) hydrolysis to produce a MAX phase predecessor with reduced lateral parameters [83]. Additionally, Yang et al. created a simple approach that relies on a lithiation intercalation alloying-expansion-micro explosion process to generate single or small layers of Ti₃C₂T_x MXene [84]. A general method for etching MAX phases that have been published by Li et al. [85]. The process of production is parallel to the etching based on fluoride-based salts. In the MAX material, the M atoms are coordinated with Lewis salts cations and Cl⁻ ions that function as H⁺ and F⁻ ions, respectively. In addition to chemical vapor deposition, wet etching has been investigated to make MXenes [86, 87]. Scanning electron microscopic images of the compact layered structure of Ti₃AlC₂ powder and 30F-Ti₃C₂T_x are depicted in Fig. 7. SEM image displays the compact layered structure of Ti₃AlC₂ powder and the multilayered 30F-Ti₃C₂T_x powder created with 30 wt% HF. Thus, it can be concluded that the variation in the etching process and various methods can produce variation in the morphology of the MXene layers.

Fig. 7

SEM images of a display the compact layered structure of Ti₃AlC₂ powder, and of b illustrate the multilayered 30F-Ti₃C₂T_x powder created with 30 wt% HF, highlighting MXene layers that are expanded and exhibit an accordion-like structure when viewed at higher magnification [88]

2.2 Bottom-up techniques

As was previously mentioned, MXene materials are often fabricated using selective etching processes. The template approach, atomic layer, and chemical vapor deposition (CVD) are only a few bottom-up processes that have emerged in recent years [89, 90]. Bottom-up procedures, especially chemical vapor deposition (CVD), produce materials with superior crystalline purity to those produced by selective etching techniques. These approaches have only synthesized multilayer ultrathin films rather than single-layered MXenes [91]. By using magnetron sputtering to deposit elements like Ti, Al, and C onto a sapphire substrate, Halim et al. successfully created a thin-film Ti₃AlC₂ MAX phase. In addition, direct magnetron sputtering can be used to create both non-MAX (Mo₂Ga₂C) and MAX phase (Mo₂GaC) thin films, which can then be used to synthesize epitaxial Mo₂C films via straightforward selective chemical etching. Another novel method for directly synthesizing MXene materials is chemical vapor deposition (CVD) [90]. For instance, CVD synthesis using methane over an alloyed surface containing Cu and Mo above 1085 °C yielded ultrathin -Mo₂C having high quality [87, 89]. Using chemical vapor deposition (CVD), Lipatov et al. have recently manufactured thin flakes of Ti₃C₂T_x [92].

Template techniques for creating 2D MXene materials have also been developed previously by many researchers [93]. Using the template method, Joshi et al. fabricated an MoN film having a hexagonal structure [93]. Jia et al. utilized the dicyandiamide dopant and MoO₂ template to synthesize N-doped Mo₂C (N-Mo2C) in an ultrathin form [94]. In order to manufacture different MXene films and subsequently analyze their mechanical, optical, and electrical properties, it is crucial to do further research into these bottom-up procedures.

2.3 Intercalation methods

To break down MXene into separate 2D sheets, intercalation materials have been used to weaken the bonds among the interconnected layers and to widen the interlayer distance between the flakes of MXenes [61]. A proper solvent is required for the intercalation and delamination, and procedure. Also, a stage is included to mix the intercalant into 2D MXene sheets, particularly ultra-sonication, that determines the concentration and size of the flakes. After that, the solution is centrifuged to remove the intercalation material from the multilayered MXene [94]. Functional and processable 2D sheets of MXenes, which are electrically stabilized, also free from clumping and aggregation, are obtained in the final colloidal solution. Besides, the targeted application, the etching method, and the concentration required will decide whether sonication is needed. Long-time sonication with high power leads to smaller flake sizes having defects and different concentrations than the non-sonicated solutions. The type of intercalated material used and the synthesis process determine the concentration of MXene sheets in the solution. Following are some reports on previously used intercalation materials and their effect on the delamination of MXenes:

• Dimethyl sulfoxide: dimethyl sulfoxide (DMSO), a type of large organic molecule, was among the initial intercalants used to expand the spacing between the layers in Ti₃C₂T_x synthesized using fluorine-based salts [95]. Ti₃C₂T_x may also have DMSO or another organic solvent added to it. Without ultrasonic treatment, the stacked sheets of Ti₃C₂T_x will settle to the bottom of the vial when DMSO or other organic solvents are used to dissolve them. When using these techniques, flake sizes typically range from a few hundred to a few thousand nanometers.

• Tetraalkylammonium hydroxides: TBAOH and TMAOH are two examples of tetraalkylammonium compounds used to exfoliate oxides with a layered structure [96]. Multilayer oxides and other materials can be delaminated with this method based on the ion exchange leading to delamination and expansion with just a shaking process [96]. MXenes Nb₂C, V₂C, Mo₂C, and Ti₃CN have all been exfoliated using TBAOH. On the other hand, even ultra-sonication was ineffective in TBAOH delaminating Ti₃C₂T_x [61]. Other research claims that by first treating the MXene surface with twenty to thirty wt% HF for a short time, Ti₃C₂T_x can be produced and exfoliated employing TMAOH [79]. Since the breakdown temperature of TMAOH is more than 200 °C, a greater degree of heat is necessary to eliminate the intercalated substances [97, 98].

• Lithium ions: Using lithium-containing etchants leads to delamination caused by the intercalation of Li⁺ ions. Although sonication of the intercalated MXene is considered necessary for the clay approach but not for the MILD method, as shown in Fig. 4. These flakes are bigger and more defect-free than those produced by sonication. Since the MILD approach produces flakes directly proportional to the shape and grain size of the beginning precursor powder, the MXene synthesis process is top-down. That is why larger MXene flakes require larger crystals of MAX phases.

3 Properties

3.1 Structure and stability

Novel stable compounds are often highlighted using computer modeling to examine their structural characteristics. These studies revealed that there are six distinct forms of MXenes, as shown in Fig. 8: (1) solid solutions; (2) single-M elements; (3) double-M elements that are out-of-plane ordered with the transition metals occupying the outer layer; (4) in-plane ordered; (5) ordered vacancies; and (6) randomly distributed vacancies [100,101,102,103]. The structure of MXenes remains like MAX (Fig. 9).

Fig. 8

Different forms of MXene structures. Reprinted with permission from [99] (Elsevier, License No. 5435971332920)

Fig. 9

Schematic illustration of the structure of MXene [104]

MXenes’ characteristics and stabilities may be modified by their terminal groups. The impact of different functional groups and configurations has been described in only a handful of publications [105]. Six bonds between X and M atoms produce M₂XT₂; this has been confirmed by computational research [106]. This is the signature six coordination of transition metals. When comparing the positions of the terminated T atoms to those of the X and M atoms, it was discovered that diverse configurations were often the most stable [107]. In certain MXenes, to improve their electrical interface, the T elements were sited above the X atoms [108]. Experiments confirmed that, as predicted by DFT calculations, termination groups in V₂CT_x and Ti₃C₂T_x MXenes are distributed randomly.

Experimental methods for studying the MXenes structure have included scanning tunneling microscopy (STM) with atomic precision and high-resolution transmission electron microscopy [109, 110]. The structure of MXenes, along with data on their termination groups and intrinsic defects, was all gleaned from these methods. The stoichiometric composition and elemental ratio were also investigated using XPS [111]. NMR spectroscopy has previously been used in certain research to learn about functional groups on MXenes’ surfaces.

Since studies suggested that MXenes might degrade following exposure to moisture or higher temperature, it is important to discuss the durability of MXene materials during storage and application [112]. Extensive research on Nb₂CT_x, for instance, has led researchers to postulate that the structure destabilizes and deteriorates because Nb atoms located at hcp surface locations interact with the exposed oxygen [113]. Even more so, Lipatov et al. estimated that Ti₃C₂T_x’s initial conductivity would decrease by 20% after being exposed to air for more than 70 h [43]. Therefore, to prolong the shelf life of manufactured MXene materials, it is suggested that they be kept in a phenomenon having no oxygen, dissolved in a powerful polar solvent, or filtered using a process such as MXene films [106]. Additionally, the stability of MXene can be enhanced by using optimized etching procedures that reduce the number of adatoms on its surface.

Because each OH/F surface functional group can be eliminated at a distinct operating temperature, thermal treatment has been recommended as a possible strategy for their efficient complete removal [114, 115]. Previously, Ti₃C₂T_x was heated to 750 °C in a vacuum by Persson et al. [116]. As a result of this method, a pure MXene product can be prepared by removing the majority of the oxygen-based terminations. More research is needed to determine the thermal characteristics in air and oxygen environments before they can be used in real-world applications. Although rutile crystals are produced at roughly 500 °C in an oxidative environment, the phase transformation occurs at a significantly higher temperature (for instance, Ti₃C₂T_x has converted to TiC at 1000 °C calcination over 2 h). While Mo-based MXenes have recently demonstrated thermal stabilities up to 530 °C, Zr₃C₂T_x has demonstrated greater (1200 °C) thermal stability relative to Ti₃C₂T_x, allowing it to be used in a number of applications [116, 117].

The MXene characteristics can be drastically changed based on the environment and method of creation. Only one study that compared the thermal characteristics of Ti₃C₂T_x prepared using two distinct methods was identified. The results indicated that MXene synthesized with NH₄HF₂ intercalating agent exhibits a transition temperature higher than the one made through the HF route [118]. However, additional research into alternative approaches is still needed.

3.2 Mechanical properties

The terminations present in MXenes have a significant impact on their mechanical characteristics. The toughness of MXenes with a -O group is generally greater. MXenes display less elastic stiffness with -F and -OH groups than those with -O groups [119]. This may be because the lattice parameters of MXenes with -O functional terminations are usually smaller than those with -OH or -F functional groups [120]. The versatility of MXenes doped with surface functional groups was greater than that of pure MXenes. For instance, Guo et al. [121] demonstrated that the surface functionalization of Ti₂C MXene decreases the material’s Young’s modulus but increases its ability to withstand strain than pure graphene and MXene. Upon tensile stress, the surface termination in Ti₂C shields the material from damage, preventing the layers of Ti from collapsing and increasing the material’s critical strain.

The atomic layer count (n) in MXene (M_n+1Xn) also affects its mechanical characteristics. According to Borysiuk et al. [122], the toughness and durability of surface-functionalized M_n+1X_nT_x are improved as n is decreased. Elastic constant and Young’s moduli of MXenes were calculated by Yorulmaz et al. [123] with the use of classical molecular dynamics in their pure form. To improve their compressive and tensile capabilities, stiffness, and versatility, MXenes can be mixed with other polymers [33, 73]. Polyvinyl alcohol (PVA)-Ti₃C₂T_x composites, for example, exhibited remarkable malleabilities and excellent tensile and compressive capacities. The study demonstrated that the Ti₃C₂T_x-PVA composites had a tensile strength of approximately 4.1 times larger than Ti₃C₂T_x, because of the interfacial interaction between the two materials [124]. The Ti₃C2Tx-UHMWPE material has advantages as well. It has been observed that polyethylene and Ti₃C₂T_x-PAM have higher yield strength and toughness [125, 126].

3.3 Electronic and optical properties

The electrical characteristics of MXenes were found to be influenced by both the termination groups and the transition metal element. Altering the terminations could result in a transition of the MXene from a metallic state to a semiconductor or even a topological insulator [127]. Most MXenes exhibit metallic behavior when surface functional groups are introduced. Work functions (WF) of MXenes are shown in Fig. 10, as calculated because of the modification of functional groups [129]. Compared to pure MXene, MXenes with a -OH group have a greater WF, while MXenes with a -O group have a small work function [130]. -F functionalized MXene decorations exhibit a paradoxical pattern typical of this class of materials. The shift in WF values was mostly due to the effect of the termination on the dipole moment. The WF was found to be reduced for the -OH functional group and increased for the -O group, both of which resulted in a negative dipole moment. Different MXene materials will exhibit positive or negative dipole moments for the -F group. The MXene WF value would be dragged down to the intermediate level by the interaction of -OH, -O, and -F groups [131]. A close correlation exists between the etching and delamination processes used to synthesize MXenes and their conductivity. The conductivity of MXenes is subject to modification by intercalated species and the interaction between the nanosheets. For instance, the conductivity of Mo₂CT_x would decrease if TBAOH was inserted during the synthesis process, as this would result in TBA⁺ intercalated species among the MXene layers, leading to lower conductivity. Applying heat treatment can eliminate TBA⁺ species, leading to a reduction in the interlayer distance and a consequent enhancement in conductivity [116].

Fig. 10

Image depicting work function of bare MXene, Pt, and Sc decorated with functional groups [128]

The absorbance of ultraviolet (UV) and ultraviolet-visible (UV-vis) light, specifically, is closely linked to the photocatalytic activities of MXenes. The transmission was significantly affected by the thickness and intercalation of the MXene layer. The 5-nm-thick Ti₃C₂T_x films described by Hantanasirisakul et al. [128] had up to 92% transmission in the ultraviolet-visible spectrum between 300 and 500 nm. After exposure to urea, hydrazine, and DMSO, the transmittance of Ti₃C₂T_x decreased. However, intercalation of Ti₃C₂T_x with tetramethyl ammonium hydroxide or NH₄HF₂ increased transmission. Furthermore, MXenes’ transmission property was significantly different from that of the MAX phase predecessor. Compared to Ti₃C₂T_x (90%), the transmission of the MAX phase in Ti₃AlC₂ was only about 30% [64]. Researchers found that the surface functionalization of MXenes affected how well they absorbed light. Compared to the light absorption properties of -OH and -F functionalized Ti₂C and Ti₃C₂, pure, -O functionalized Ti₂C and Ti₃C were superior [119].

3.4 Magnetic properties

Studies have expanded their assessments to MXenes’ magnetic characteristics because of the magnetization possibilities, in contrast to MAX phases. Ti₃N₂ [132], Zr₃C₂ [133], Ti₃CN [134], Cr₂C [135], Fe₂C [136], Zr₂C [133], and Ti₄C₃ [70] are just a few examples of the purified compounds that are hypothesized to have magnetic moments. However, each MXene and functionalization group must be examined separately following functionalization. For example, Cr₂NT_x and Cr₂CT_x maintain ferromagnetic at normal temperature with terminated attached [137], whereas Ti₃CNT_x and Ti₄C₃T_x get to be losing the magnetization with the terminations [134], and Mn₂NT_x is ferromagnetic irrespective of the surface functionalization [138]. Previously reported magnetic moments, nonetheless, are still just computer simulations and have not yet been seen in an experimental setting. This is because there is currently a deficiency of control over surface chemistry and inadequate production of pure MXenes [139].

4 Environmental applications

4.1 Adsorption

4.1.1 Removal of organic pollutants

The most important category of aquatic pollutants is organic contamination, distinguished by increased chemical stabilization and tolerance to biological deterioration. The main aqueous organic pollutants include a variety of dyestuffs, phenols, medicines, and insecticides [140,141,142,143,144,145,146]. The occurrence of these organic pollutants immediately impacts our ecology in wastewater. Therefore, it is crucial to remove these organic pollutants properly.

Recent studies on the adsorption of pollutants, like urea, amitriptyline (AMT), AB80, phenol, and methylene blue, by materials based on MXene have been conducted through a small group of researchers. The most current adsorption of organic pollutants employing different MXene-based composites is summarized in Table 1. An example of MXene production was given by Meng et al., who used the in-situ technique to synthesize MXene [156]. Wu et al. [149] reported that Ti₃C₂T_x displayed exceptional adsorption capabilities towards phenol, with recyclability demonstrating excellent duplicability and long-lasting sustainability. The charged state of organic contaminants significantly impacts the adsorption kinetics of MXene compounds. The adsorption of multiple pharmaceutical compounds, including 17-ethinyl estradiol, diclofenac, carbamazepine, ibuprofen, AMT, and verapamil, was conducted for the first time by Kim et al. [27] using Ti₃C₂T_x under three different solution pHs.

Table 1 MXene-based composites for organic removal

Moreover, MXenes have demonstrated remarkable adsorption capabilities toward various types of dyes. Wei et al. [155] inspected the adsorption behavior of methylene blue using pure Ti₃C₂T_x and three different alkaline-treated Ti₃C₂T_x MXenes. Their research indicated that Ti₃C₂T_x treated with NaOH displayed an adsorption capacity of around 189 mg/g, more than the other layered two-dimensional materials [156]. Fe₃O₄ was painted onto the Ti₃C₂-MXene surface by Zhu et al. and used as an adsorbent toward MB dye at various temperatures [152]. Their studies showed that, owing to hydrogen bonding and strong electrostatic contact between Ti-OH and MB, the adsorption capacity of Ti₃C₂T_x increased with an increase in temperature [152]. Adding terminated groups containing sulfone also significantly improved the adsorption capacity of Ti₃C₂T_x [157]. The etching method used to produce MXenes also affects their adsorption efficiency. Hydrothermally produced Ti3C2 showed a higher ability to bind MB dye than the conventional etching method based on fluorine-based salt, likely due to the larger specific surface area created by the hydrothermal method [147]. Self-assembled Ti₃C₂-Co₃O₄ MXene nanocomposites were found to effectively adsorb RhB and MB dyes, while Ti₃C₂T_x MXene coupled with terephthalate showed a high adsorption capability against MB, conceivably due to the accessible carboxylate groups and greater separation between MXene [158].

4.1.2 Heavy metal removal

Elements with a density of more than 5g/cm³ and high atomic weight of up to 200 come under heavy metals [159]. The release of industrial waste into the water sources results in excess heavy metals in the environment. Heavy metals do not have the properties of degradation and incline to gather in living organisms. Cr, Pb, Ni, Hg, Zn, Cd, As, and Cu are frequently discovered heavy metals in industrial wastewater [160]. MXenes demonstrated the ability to adsorb different heavy metal ions from industrial effluent effectively.

One of the significant advantages of MXene nanosheets in capturing heavy metal ions is their small interlayer gap, which is less than 2Å, making them suitable for capturing heavy metal ions [161]. Moreover, the adsorption performance of MXenes was enhanced by surface-functionalized characteristics. Titanium-based MXenes, particularly Ti₃C₂T_x nanosheets, are the most commonly used adsorbents for heavy metal ions. Table 2 discusses the MXene-based composites for heavy metal removal. Ti₃AlC₂ is exfoliated by Fard et al. [161] to create Ti₃C₂T_x nanosheets, used to adsorb barium ions. MXenes also have the unique ability to remove pollutants via an in situ reduction-adsorption method. In addition to removing the reduced Cr(III) ions, Ti₃C₂T_x alongside reduced Cr(VI) ions Cr(III) [171]. Delaminated Ti₃C₂T_x MXene nanosheets were used in a different study to use the reduction-adsorption method to remove Cu(II) ions [172]. Ti₃C₂T_x MXene was also effectively used by Pandey et al. [173] to convert BrO₃ ions in water to Br.

Table 2 MXene-based composites for heavy metal removal

The surface-functionalized properties of MXenes were found to impact the capability to adsorb heavy metals. Researchers created Ti₃C₂(OH/ONa)_xF_2-x with an OH group to remove Pb(II) from wastewater [167]. The MXene exhibited a high Pb(II) ion removal capacity, and the symmetry for adsorption-desorption was touched in less than 2 min. Additionally, the sorption capability of Ti₃C₂T_x for Pb(II) ions was further enhanced by terminating with a coupling agent silane coupling, increasing to 147.3 mg/g.

Additionally, MXene compounds have effectively removed several heavy metal ions through adsorption. Hybrid Fe₂O₃/Ti₃C₂T_x blends were created by Shahzad et al. [164] and cast off to remove mercury ions. C-TiO₂ composites MXene were compared for their ability to absorb Cr(VI) ions by Zou et al. [174]. Similar improvements in Cu(II) ion elimination from wastewater were also seen in the levodopa-decorated Ti₃C₂T_x MXene-based films also capable of eliminating Au(III), Pd(II), Ag(I), and Cr(VI) ions [175]. These were reused after being cleaned with HCl and NaOH solution.

4.1.3 Removal of nuclear contaminants

The pollution brought on by nuclear waste has drawn attention due to the nuclear industry’s rapid expansion due to its detrimental environmental consequences. Chemisorption is still the most popular and efficient way to handle the detoxification of nuclear waste out of all the methods for dealing with it. MXenes have distinguished themselves among the various adsorbents used to detoxify nuclear waste as being particularly effective against the adsorption of Cs+, palladium, Th(IV), and uranium U(VI) [176, 177]. Both experimental results and theoretical predictions made using DFT calculations were published for the efficient removal of uranyl ions by V₂C(OH)₂ nanosheets [178, 179]. The elimination of U(VI) ions by the V₂CT_x nanosheets was exceptionally high (174 mg/g). However, the sorption capacity of V₂C(OH)₂ nanosheets, as estimated by DFT, was substantially greater than the actual value (89.9 mg/g) [178].

Using the hydrated intercalation approach, Wang et al. [180] created DMSO intercalated Ti₃C₂T_x nanosheets to remove uranium ions. The investigational outcomes revealed that the adsorption capacity increased significantly up to 160 mg/g due to hydration and intercalation stimulation. In an alternative study, Wang et al. [181] reported similar observations using Ti₃C₂T_x MXene to remove U(VI) via reduction adsorption techniques. The hydrated-Ti₂CT_x MXene displayed stronger specificity and sorption capability against Ti(IV) ions than the dry-Ti₂CT_x, according to research by Li et al. [182]. According to the sorption mechanism analysis, ion exchange and electrostatic contact were the primary mechanisms for removing radionuclides, as shown in Fig. 11. In addition, Ti-OH and Th(IV) exhibit a high affinity, according to their XPS study.

Fig. 11

Adsorption isotherm of methane over Ti₃C₂ manufactured by using HCl from 0 to 60 bar and at 25°C: (1) LiF, (2) NaF, (3) KF, and (4) NH₄F. Reprinted with permission from [183] (Elsevier, License No. 5435980355724)

Mu et al. [184] fabricated Ti₃C₂T_x by treating it with a fluorine-based salt process, exfoliating it at various temperatures, and using it as an adsorbent for Pd(II). Their outcomes indicated that the samples that underwent more intense exfoliation at higher temperatures had advanced adsorption capacities and good regeneration characteristics. Hierarchical titanate nanostructures (HTNs) were created from MXene using an enhanced hydrothermal oxidation and alkalization method by Zhang et al. Hierarchical titanate nanostructures had a higher adsorption capacity for europium ions, primarily due to ion swapping with the terminating groups at the interlayer. MXenes also efficiently remove Cs+ ions from wastewater [177, 179]. As shown in Fig. 11 (2), Ti₃C₂T_x nanosheets quickly reached equilibrium. The improved adsorption ability was attributed to the MXene having layers and the terminating groups attached. Recent research by Jun et al. [176] associated eliminating radioactive cesium ions from nuclear wastewater using porous activated carbon and Ti₃C₂T_x. The greater amount of surface negative charges on MXene than on PAC was thought to be responsible for its higher adsorption capability. The suggested removal mechanism involved electrostatic contact between the adsorbent and the cesium ions.

Wang et al. [185] reported the development of a new 3D structured Ti₂CT_x composite to eliminate perrhenate better. They incorporated poly(diallyl dimethylammonium chloride) to adjust charges at the surface and enhance Ti₂CT_x stability, efficiently eliminating rhenium up to 363 mg/g. In a different investigation, Deng et al. [186] created nanocomposites of Ti₃C₂/SrTiO₃ by partially oxidizing a multilayered Ti₃C₂ based on hydrothermal crystallography. These materials were then used to remove U(IV) ions.

4.2 Photocatalytic degradation

With CO₂ and H₂O as the end products, photocatalysis, a powerful and affordable photo-redox method, has demonstrated tremendous potential in wastewater purification from many organic contaminants [187]. In order to develop the potential applications, a lot of work has been done to use SC-based photocatalysts to degrade pollutants [188]. However, one of the major issues with most semiconductor photocatalysts was the charge recombination too quickly. The MXenes are the ideal alternative because they have a greater surface area, narrower bandgap energy, and the capacity to operate as electron acceptors, accelerating the separation of charges induced by the Schottky barrier. For instance, Ti₃C₂ and TiSO₄ were used as precursors in the hydrothermal method of TiO₂/Ti₃C₂ preparation by Gao et al. [189]. TiO₂ was also evenly dispersed, which increased the number of available active sites in the photodegradation process. A large number of active sites and high electrical conductivity lead to the effective degradation of MO than pure MXene.

Additionally, TiO₂/Ti₃C₂ (TiO₂(1 1 1)/Ti₃C₂) using the NH₄F-assisted hydrothermal oxidation procedure was reported by Chao et al. [190]. To upsurge the ratio of TiO₂, the NH₄F contents introduced throughout the fabrication was carefully regulated. According to the photodegradation data, increasing the exposed ratio upon the surface of titania optimized the MB dye degradation capability, mostly attributed to the much higher charge carrier efficiency. Additionally, the hydrazine hydrate treatment produced the -O vacancy, creating an excess of active sites for the photodegradation activity. Other semiconductors have been attached to the surface of Ti₃C₂ in addition to TiO₂ to enhance photocatalytic efficacy towards organic contaminants. CeO₂/Ti₃C₂ was created using an easy hydrothermal method by Zhou et al. [191], who also looked into the photocatalytic effectiveness of the material against RhB dye degradation. Researchers concluded that enhanced light utilization and enhanced charge carrier separation efficiency were responsible for CeO₂/Ti₃C₂’s superior photocatalytic activity compared to pure Ti₃C₂ and CeO₂. Table 3 summarizes the results of photocatalytic degradation tests conducted on diverse pollutants using MXene-based composites.

Table 3 MXene-based composites for degradation of inorganic and organic contaminants

In a different study, Cao et al. investigated the photocatalytic activities of CuFe₂O₄/Ti₃C₂ toward the breakdown of the antibiotic sulfamethazine [203]. Compared to pure CuFe₂O₄ and Ti₃C₂, the produced CuFe₂O₄/Ti₃C₂ displayed a greater photocurrent density, indicating remarkable photogenerated charge carrier separation. As a result, the CuFe₂O₄/Ti₃C₂ combination outperformed pure CuFe₂O₄ and Ti₃C₂ regarding photocatalytic activity for the decomposition of sulfamethazine (Fig. 12a). Using liquid chromatography, as shown in Fig. 12b, to learn more about the sulfamethazine breakdown route. Characteristically, a peak at 277.09 was noted, attributed to sulfamethazine’s dissociation. However, in addition to the peak described above, 7 more peaks were also observed at 134.87, 146.97, 155.46, 160.26, 187.84, 223.80, and 312.73. Based on these mass peaks, the photodegradation mechanism of sulfamethazine was hypothesized and shown in Fig. 12c. The hydroxyl (^•OH) radicals then mineralized these mediators into CO₂, H₂O, and other molecules. Future research might benefit from this thorough investigation of the photodegradation route employing nanomaterials based on MXene.

Fig. 12

a Sulfamethazine photocatalytic degradation over photolysis of pure and blended materials, b HPLC-MC spectrum of process of degradation, c proposed degradation mechanism of SMZ. Reprinted with permission from [203] (Elsevier, License No. 5435980643762)

According to Jiao et al. [204], post-treating MXene-based photocatalysts could be a very effective way to increase their photoactivity. They created a MoS₂/Ti₃C₂ nanocomposite using a hydrothermal technique, subjected it to friction treatment and assessed its photocatalytic activity for degrading MO dye. According to the findings, pure Ti₃C₂ and MoS₂ did not display the MoS₂/Ti₃C₂ nanocomposite has greatly improved photocatalytic activity. Rough surface leading to extra active sites for the photodegradation process was attributed to the improved photocatalytic activity. Ti₃C₂/Ag₂WO₄ nanocomposites were created by Fang et al. [205], and their adsorption and photocatalytic degradation effectiveness against sulfadimidine (SFE) and tetracycline hydrochloride were examined (TC). According to dark adsorption, Ag₂WO₄ and Ti₃C₂ appeared to have essentially nonexistent removal capacities against SFE and TC. However, the homogeneous integration of Ti₃C₂ improved the ability of Ag₂WO₄ to adsorb contaminants by reducing aggregation and decreasing surface adsorption of Ag₂WO₄. Ag₂WO₄/Ti₃C₂ demonstrated noticeably better photocatalytic activity against SFE and TC thanks to improved adsorption capacity and charged carrier separation efficiency. A 2D/2D Fe₂O₃/Ti₃C₂ magnetic nanocomposite was created by Zhang et al. [206] and used as a visible light active photocatalyst to degrade RhB dye. More than 90% of RhB dye can be removed in 60 min of visible light illumination, thanks to the discovery that 2D-Fe₂O₃ might simply be introduced into the Ti₃C₂ layered structure to prevent agglomeration, resulting in enhanced charge carrier capability and increase in surface area. In addition to better charge carrier separation, the increased surface area was a key factor in increased photocatalytic activity.

Making ternary composites is another useful strategy for increasing the degradation action of materials based on MXene for pollutant removal. A ternary In₂S₃/TiO₂/Ti₃C₂ composite was produced by Wang et al. [198] for the photodegradation of MO dye. EIS studies revealed that the TiO₂/In₂S₃/Ti₃C₂ nanocomposite was considerably more effective than pure In₂S₃ in removing photogenerated charge carriers. Type-2 heterojunction resulted in high removal efficacy leading to the enhanced transfer of electrons to holes. The TiO₂ surface’s gathered electrons may also be transferred to those of Ti₃C₂. The photocatalytic activity data in Fig. 13a shows that the In₂S₃/TiO₂/Ti₃C₂ combination was more effective at photodegrading MO dye than the In₂S₃/CNT (58.4%), In₂S₃/rGO (19.7%), In₂S₃/TiO₂ (77.8%), and In₂S₃/MoS₂ (75.6%). The Z-scheme photocatalytic approach is among the best techniques to increase charge separation. The z-scheme consists of holes in the valence band and electrons in the conduction band, which provide higher oxidizing and reducing potential, respectively [192]. A Ti₃C₂/TiO₂/CdS photocatalyst following the z-scheme by Liu et al. [192] was fabricated to investigate the functions. Photogenerated charge carrier separation efficiency was analyzed using the PL spectra (Fig. 13b). Their findings showed that the Z-scheme heterojunction development caused a decrease in PL intensity following CdS injection. Additionally, the produced compounds’ photocatalytic effects on phenol, RhB, MB, and sulfachloropyridazine were investigated. According to the findings (Fig. 13c), MXene can act as a bridge for the transportation of charge carriers between semiconductor devices.

Fig. 13

a Time-dependent photocatalytic degradation of MO (reprinted with permission from [198] Elsevier, License No. 5435980910611), b PL spectra of pristine Ti₃C₂@TiO₂ along with 1 to 4% composite with CdS, c proposed degradation mechanism of Ti₃C₂@TiO₂ loaded with CdS [192]

Scientists’ attention has begun to grow on producing reusable magnetic photocatalysts. For the first instance, a magnetic MXene-based photocatalyst for RhB dye photodegradation was synthesized by Zhang et al. [207] and contained Ti₃C₂, -Fe₂O₃, and ZnFe₂O₄. Initially synthesized individually, the Ti₃C₂ and -Fe₂O₃/ZnFe₂O₄ were subsequently joined using an ultrasonic-assisted method. The photocatalytic findings indicate that -Fe₂O₃/ZnFe₂O₄/Ti₃C₂ had a benefit over -Fe₂O₃/ZnFe₂O₄ in terms of increased photocatalytic activity of -Fe₂O₃/ZnFe₂O₄, having a photocatalytic rate constant that was approximately three times higher. Additionally, the magnetic characteristics of -Fe₂O₃, -ZnFe₂O₄, and -Ti₃C₂ helped to separate them from the aqueous solution enabling simple recycling across four cycles.

4.3 EMI shielding

Because of its intriguing properties, MXene has been observed to act as a lightweight material for EMI shielding. These properties include solution processability, low density, outstanding metallic conductivity, tunable surface chemistry, large specific surface area, and superior shielding performance. Therefore, several MXene composites and hybrids have been studied to advance the EMI shielding characteristics of MXenes. The innovation and rapid increase of electronic systems have grown quickly due to the downsizing of contemporary electronic circuits and equipment. Unfortunately, these tiny gadgets produce unwanted EMI, which can impair the functionality of electrical systems [208]. Long-term exposure to electromagnetic (EM) radiation is bad for human health since it can lead to cancer, eye difficulties, nausea, headaches, and severe effects on the growth of a child’s brain [209]. In addition, some surgical implants and equipment are prone to failure in a fluctuating electromagnetic field [210].

Due to the high EMI vulnerability of conventional combat, it is important to safeguard personnel and equipment against EM attack or contamination [211]. As a result, materials science has shifted its focus to figuring out how to stop or lessen the effects of hazardous electromagnetic radiation. Metals like copper, aluminum, Ag, and steel are broadly cast off to fight electromagnetic pollution because of their high conductivity [212]. Nevertheless, the use of MXene in highly combined contemporary portable electronics has been constrained by their high density, challenging processing ability, and high vulnerability to corrosion. For EMI shielding applications, a variety of heterogeneous composites with conducting fillers, such as 1D fillers, 2D fillers, magnetic fillers, and dielectric fillers. These composites have benefits like reduced weight, increased environmental resilience, and superior anticorrosive capabilities [213, 214]. Their poor shielding capacity, however, has restricted their application. There are a number of properties that make MXene an ideal EMI shielding material [43]. Because of their malleable surface chemistry, MXenes and composites may be fabricated into various forms. Due to the outstanding EMI shielding of 2D Ti3C2Tx, MXenes have quickly risen to the position of leading lightweight EMI shielding material, with a correspondingly increasing number of research papers [43].

Due to the mismatch in the air and shield impedance values, the shield interacts with the incoming em wave, and part of the wave gets reflected on the rear and front surfaces. Any residual power after attenuation or transmission is stored as heat in the shield (PT) [215]. The EMI SE is defined by Eq. (1) as the logarithmic ratio of the transmitted and incident powers and is used to characterize the degree to which a shield attenuates incident EM waves.

$${\textrm{SE}}_{\textrm{T}}\left(\textrm{dB}\right)=10\log {P}_{\textrm{T}}/{P}_1=20\log {E}_T/{E}_1$$

(1)

where P and E represent the intensity of the electric field and the power of the electromagnetic waves that are incident and transmitted, respectively [36]. According to Schelkunoff’s hypothesis, total EMI shielding effectiveness is represented by Eq. (2), which incorporates attenuation from reflection (SER), multiple reflection, and absorption [36].

$${\textrm{SE}}_{\textrm{T}}={\textrm{SE}}_{\textrm{R}}+{\textrm{SE}}_{\textrm{A}}+{\textrm{SE}}_{\textrm{M}}$$

(2)

It has been claimed that single metal Ti₃C₂T_x ordered double metal Mo₂TiC₂T_x and Mo₂Ti₂C₃T_x. Due to its greater electrical conductivity for Mo₂TiC₂T_x and Mo₂Ti₂C₃T_x, respectively, Ti₃C₂T_x demonstrated the highest EMI shielding effectiveness. According to the reported literature, the electrical conductivity of Ti₃C₂T_x MXene was several orders of magnitude greater than that of other conductive materials at the same thickness level. Ti₃C₂T_x MXene was shown to be the most effective material for lightweight EMI shielding. EMI shielding mechanism by layered MXene structure has been proposed by schematic representation in Figure 14. Incoming EM waves are striking an MXene flake’s surface. Because of the large number of charges on the highly conducting surface, some of the incident EM waves that travel through the MXene structure are instantly reflected off the surface. Local dipoles created by functionalized groups instead aid the absorption of incident waves. The same procedure is subsequently applied to waves that have less energy when they come into contact with another MXene flake, leading to numerous internal reflections as well as increased absorption. As a result, an EM wave’s intensity significantly decreases each time it passes through an MXene flake, leading to an overall attenuated or entirely erased EM wave.

Fig. 14

Schematic of EMI shielding mechanism by layered MXene

Due to its high electrical conductivity and laminate architecture, made feasible by the alignment of 2D flakes, Ti₃C₂T_x MXene films provide exceptional EMI SE. After strongly interacting with the high electron density MXene layers in the layered structure of MXene, the residual EM waves are diminished by a combination of eddy current and ohmic losses. A subsequent study, where the Fresnel formula and attenuation rule presented theoretical calculations, provided more support for the good EMI shielding results of the Ti₃C₂T_x MXene [216]. The theoretical calculations supported the experimental findings and highlighted Ti₃C₂T_x MXene’s exceptional potential as an EMI shielding material.

Ti₃C₂Tx MXene was synthesized using two alternative methods, and its EMI shielding abilities were assessed (as seen in Fig. 15). The M- Ti₃C₂T_x composite included more F terminations than the U- Ti₃C₂T_x composite’s preponderance of O terminations. Ti₃C₂Tx MXenes were cold pressed at a pressure of 5 MPa after being mixed with an EM transparent SiO₂ nanoparticle matrix at varied mass ratios. U- Ti₃C₂Tx composite, with 60 wt% MXenes, has higher conductivity than M-Ti₃C₂Tx (6.3 105 S cm⁻¹). The 1-mm-thick 80 wt% U-Ti₃C₂Tx MXene composite has an EMI SE of 58 dB in the 8.2–12.4 GHz frequency band. U-Ti₃C₂Tx MXene composites fared better than M-Ti₃C₂Tx in EMI shielding because they had more conductive networks, greater surface area, and higher conductivity. Attenuation and dipolar polarization loss were further increased by the U-Ti₃C₂Tx MXene composite’s larger exposed surface area, many surface terminations, and point defects.

Fig. 15

Proposed EM interaction of Ti₃C₂T_x composites [217]

MXene hybrids provide additional magnetic or conducting components to improve the shielding efficiency. The engagement between incident electromagnetic waves and the structure is enhanced by several phases, which can also increase mechanical qualities. For EMI shielding applications, thin, lightweight TiO₂-Ti₃C₂T_x/graphene hybrid laminate sheets have been produced [218]. Magnetic Ni chains and MXenes combine to produce outstanding EMI shielding performance. MXene laminates can be created at the nanoscale to fulfil the needs of commercial goods and offer superior EMI SE with a relatively low thickness. For aerospace and military applications, aerogels and porous foams provide effective, lightweight EMI shielding at the thickness loss. There is still much to learn about the new MXenes and their capacity to act as shields because the study of MXenes is still beginning.

4.4 HER

Because of the unique properties MXenes have, it has shown prodigious potential in hydrogen generation. Hydrogen has come to the forefront as a possible resolution to the global energy crisis and ecological dilapidation. Hydrogen has numerous advantages, including low greenhouse gas emissions and low pollution, a high energy density, and the ability to be recycled. In addition, due to its many positive characteristics, including little environmental impact, high reliability, low cost, and long lifespan, it may be easily integrated into green energy storage and conversion systems.

H₂ can be generated in many ways, including the combustion of fossil fuels, the fermentation of marsh gas, or the use of organic waste. In addition, an HER based on water splitting is beneficial economically and ecologically [219]. In order to produce hydrogen, the electrocatalyst is crucial, and costly metal-based electrocatalysts perform well. However, lack of resources and rising costs have made it difficult to meet the growing demand for electrocatalysts based on precious metals. It has therefore been a pressing task to continue developing high-efficient NPM-based HER electrocatalysts for producing hydrogen. Since then, a lot of advancements have been achieved, leading to the creation of the electrode materials phosphides, carbides, sulfides, nitrides, oxides, and nanocarbon free of metal particles [220,221,222,223,224]. However, two major issues remain the low electrical conductivity, which causes the active coating to thicken, and activity to drop rapidly. In addition, the issue is that these NPM-based materials do not exhibit adequate electrochemical reaction stability in aqueous solutions. MXenes are two-dimensional (2D) materials that mimic graphene and may offer solutions to the problems mentioned above.

The creation of hydrogen energy is an efficient response to the present energy and environmental concerns. Hydrogen evolution reaction (HER) is a vital method for creating hydrogen and water splitting. It is crucial to develop HER catalysts with high stability, conductivity, and selectivity for use in the planned hydrogen economy. The high-performance electrocatalyst enables a reduced overpotential, increasing HER efficiency [225, 226]. Calculations using density functional theory (DFT) have shown that the Gibbs free energy of hydrogen adsorption (GH) is a good indication of HER activity [227]. When GH is most strongly linked to thermoneutrality, HER activity is at its highest. Compared to other NPM-based HER electrocatalysts, MXenes have a greater potential, and they also have other fantastic physical and chemical qualities: (a) MXenes rich in -OH and -O groups on their surface may form stable bonds with a variety of semiconductors. Because of their high electrical conductivity, MXenes (b) improve the efficiency of charge-carrier transfer. (c) MXenes may display more potent redox activity than carbon compounds due to the exposed metal sites at their terminals. (d) MXenes can contact water molecules appropriately thanks to their exceptional hydrophilicity. (e) The chemical and structural stability of MXenes in aqueous solutions is exceptional. In recent years, there has been a rise in theoretical and experimental study of HER electrocatalysts based on MXenes. Figure 16 shows how structural engineering has been used to enhance MXenes in hybridization, nanostructure creation, metal-atom doping, and termination modification. When compared to Pt-based catalysts, MXene-based materials show promise as electrocatalysts

Fig. 16

Strategies for enhancing MXene-based HER electrocatalyst performance. Descriptive diagram summarizing numerous strategies for better results

Energy storage, conversion, and utilization rely on developing high-performance electrode materials and environmentally friendly NPM. Because of their remarkable hydrophilicity, strong metallic conductivity, and huge surface area, MXenes may be used in a variety of electrode applications [228]. Although substantial progress has been made over the last several decades in developing MXene-based electrocatalysts, there is still room for advancement if we are to build highly active, commercially viable electrocatalysts that are better than platinum-based materials. The surface functional groups of MXene are technique-dependent, and so remain a challenge to tune. This complexity of the surface environment has led to a continued debate about the catalytic process. Because of their limited yield, MXene nanostructures may be impractical for widespread use.

5 Summary and future prospective

Due to their unique properties, malleable structure, and component particles, MXenes are among the most inventive 2D materials developed for a wide range of applications. Adsorption, membrane filtration, and photocatalytic degradation of MXenes were intensively studied as potential applications in environmental cleanup. In order to make them more effective in wastewater treatment, recent studies have focused on improving their recyclable qualities, biocompatibility, and structural stability in both aqueous and ambient air environments. Surface chemistry, including selective oxidation and functionalization, may be precisely controlled to produce MXene analogues. Due to issues with long-term stability and aggregation in pure MXenes, a variety of polymeric MXene composites have been developed. If MXenes is to take the lead in environmental remediation, more research is needed to find solutions to the present problems. It is safe to say that MXenes are the materials of the future, having huge potential in fields like waste management and water purification.

While adsorption and catalytic degradation are two of the most common approaches to clean polluted water. The exciting discovery of extraordinary properties and performance of graphene-based nanomaterials, layered double hydroxides (LDHs), transition metal dichalcogenides, etc. have spurred a fast-developing interest in employing MXene-related materials for environmental applications. It is also shown that these materials have inherent challenges, such as the hydrophobicity of MoS₂, the expensive cost of graphene, and the low stability of LDHs under complex settings. Scholarly efforts have helped mitigate these difficulties, but resolving them remains challenging. MXenes and similar composites have established themselves in the literature as cutting-edge substance materials with significant promise in eliminating pollutants due to their superior performance. MXene-based materials are interesting for environmental applications because of their distinctive morphology, which includes properties such as changeable layer thickness, high hydrophilicity, adaptable structural design, a large specific surface area, and a wide compositional range. Among the numerous new uses for MXene-based materials are the adsorption and catalytic degradation of various pollutants. These pollutants include oxidative metal ions, pigments, antibiotics, radioactive contaminants, organic compounds, heavy metal ions, and waste gases. Recently developed MXenes and MXene-based hybrids have been examined for their potential in environmental adsorption and catalytic degradation. The study of MXenes, in contrast to other two-dimensional materials, is in its infancy. Therefore, there are many open issues about their characteristics and their uses. At some point, conducting a thorough analysis of these concerns will be important. The technologies used for HF etching and the huge amounts of acidic gas and waste liquid produced during the preparation process must be improved. Even if a lot of time and energy has already been invested, there may be further avenues to explore. The search for more environmentally friendly preparation techniques continues. Research on the surface characteristics of MXene-based materials is sparse. Modifying MXene surfaces and doing more in-depth research into how surface properties affect MXene’s stability in aqueous solution are two examples of what is needed to expand MXene’s uses. Particularly for pollutant adsorption, surface engineering may significantly impact adsorption efficacy. Extensive research into MXenes in biological applications has led to early signs of their low toxicity; however, the toxicity and impacts of MXenes and their composites on the environment and people have not been well examined, nor are the toxicity mechanisms acknowledged. However, if these issues are addressed, MXenes will perform much better in environmental applications.

There have been considerable advances, but many obstacles remain, so developing high-performance MXene-based nanomaterials is urgent. Although there are now more ways to make MXene-supported semiconductor photocatalysts, many more are in the works. There are benefits and drawbacks to trying out novel synthetic approaches from scratch, such as having more say over product quality. The MXene family series may be expanded with the MXene family and vice versa. There are numerous conceivable heterogeneous structures between the structure of MXene and its photocatalytic performance, although many of its processes are still contested; MXene is quickly oxidized when kept for extended periods, making developing novel storage solutions vital; for instance, the efficiency of its electrical and photocatalytic systems will be impacted by quantum dot doping and surface modification. This issue has only been the focus of a few research studies in the last several years.

The high price of the photocatalytic MXene components employed in the preparation and the poor yield purity make it difficult to reach g-level mass manufacturing. This paves the way for further research on its preliminary theoretical foundations. Massive synthesis approaches are currently unrealistic. To prevent restacking, maintain a high surface area, and provide active attachment sites for target pollutants, the appropriate spacing between MXene-based layers must be changeable. This being the case, large-scale manufacturing of high-quality MXene of consistent size is essential for the future.

It is important to consider how MXene and nanomaterials made from it could affect our energy use and the natural world. Recent advances in MXene-based nanomaterials show promise in developing photocatalytic semiconductors, but there is still a long way to go before stable nanocatalysts can be created. Although MXene-based nanomaterials show much potential, they must undergo rigorous ecotoxicological testing and life-cycle analysis first. The kinetics and thermodynamic control technologies for MXene photocatalyst production need to be investigated in the future; MXene should be adjusted in tandem with theoretical analysis and experimental use. Theoretical calculations (density functional theory and basic principles) should be included in the catalyst design process to predict future structural performance, comprehend the whole MXene photocatalytic process, and perform a micro-level analysis of the mechanism.

Given its significant photocatalytic efficiency in removing water-borne organic pollutants, it is reasonable to expect that the MXene-based composite photocatalyst may also remove air pollutants (including volatile organic compounds, NO, SO₂, H₂S), NO₂, and other exhaust fumes. Despite MXene’s promising photocatalytic performance, there is still an opportunity for advancement in integrating theory and experiment. There is an essential need to address the worldwide environmental crisis of air pollution. Few studies and publications address the potential of MXene-based composite photocatalysts for air purification; therefore, further research is needed. Researchers face new problems and opportunities as they explore MXene’s vast potential across diverse sectors. We anticipate that our evaluation will contribute to the logical design of MXene-based materials, improving their efficiency and durability for real-world uses.