MXenes as Emerging Materials: Synthesis, Properties, and Applications

Due to their unique layered microstructure, the presence of various functional groups at the surface, earth abundance, and attractive electrical, optical, and thermal properties, MXenes are considered promising candidates for the solution of energy- and environmental-related problems. It is seen that the energy conversion and storage capacity of MXenes can be enhanced by changing the material dimensions, chemical composition, structure, and surface chemistry. Hence, it is also essential to understand how one can easily improve the structure–property relationship from an applied point of view. In the current review, we reviewed the fabrication, properties, and potential applications of MXenes. In addition, various properties of MXenes such as structural, optical, electrical, thermal, chemical, and mechanical have been discussed. Furthermore, the potential applications of MXenes in the areas of photocatalysis, electrocatalysis, nitrogen fixation, gas sensing, cancer therapy, and supercapacitors have also been outlooked. Based on the reported works, it could easily be observed that the properties and applications of MXenes can be further enhanced by applying various modification and functionalization approaches. This review also emphasizes the recent developments and future perspectives of MXenes-based composite materials, which will greatly help scientists working in the fields of academia and material science.


Introduction
Global attention is currently focused on energy and environmental challenges. Electric vehicles and stationary batteries both require high-capacity energy conversion/storage systems, while the present energy conversion and storage techniques such as batteries, water splitting, and supercapacitors are being advanced. Thus, further research is required because simultaneous high energy conversion and power density is difficult to attain, and rechargeable technologies are expensive and limited in application [1][2][3][4][5][6]. Because of their unique physicochemical features, two-dimensional (2D) materials such as MXenes have received a lot of interest. MXenes are also very interesting in this context as they are relatively safe, have broad interlayer spacing, are environmentally flexible, and have excellent biocompatibility [7]. MXenes, a broad family of the 2D transition-metal carbides or nitrides, was recently developed by Drexel University researchers. The generic formula for MXenes is M n+1 X n T x (n = 1-3), where M stands for the transition metals (i.e., Ti, Zr,

Synthesis Methods
The development of MXene synthesis pathways had a significant impact on electrical characteristics, physicochemical features, and a wide range of applicatio general, there are three types of MXene synthesis techniques: Etching, top-dow bottom-up. In the etching method, the "A" elements from the MAX phases of the three-dimensional (3D) layer result in the MXenes layered structures. Until now, th prevalent approach for synthesizing MXenes has been recognized as the top strategy (see Figure 2). Unlike the top-down fabrication approach, which usually re a large number of precursor materials, the bottom-up synthesis fabrication route re few organic or inorganic molecules/atoms when carefully constructing the stru Using a crystal growth method, the precursors can be assembled into a definite 2D constituting the MXene structures. The advantages of the bottom-up approach all exquisite control of the size distribution, shape, and surface terminal functionali MXenes in an appropriate way [31].
Normally, MXenes refers to MnXn-1 (n = 2, 3, and 4) layers formed by remov terlayer "A" atoms from the metal-ceramic MAX phase (where M stands for the tion metals, A is the group IIIA or IVA elements, and X represents the C and N elem The M and X atoms stack to form a hexagonal lattice in the MAX phase, with the at X occupying the M octahedral cage center shared by its edges. When the atoms of removed from the layer of MnXn-1, the hexagonal-lattice of MX, rather than the cub tice, is preserved. Hence, the layer of MnXn-1 can be produced by removing the A-As with its predecessor MAX, MXenes thin sheets are frequently oriented horizo

Synthesis Methods
The development of MXene synthesis pathways had a significant impact on their electrical characteristics, physicochemical features, and a wide range of applications. In general, there are three types of MXene synthesis techniques: Etching, top-down, and bottom-up. In the etching method, the "A" elements from the MAX phases of the parent three-dimensional (3D) layer result in the MXenes layered structures. Until now, the most prevalent approach for synthesizing MXenes has been recognized as the top-down strategy (see Figure 2). Unlike the top-down fabrication approach, which usually requires a large number of precursor materials, the bottom-up synthesis fabrication route requires few organic or inorganic molecules/atoms when carefully constructing the structure. Using a crystal growth method, the precursors can be assembled into a definite 2D order constituting the MXene structures. The advantages of the bottom-up approach allow for exquisite control of the size distribution, shape, and surface terminal functionalities of MXenes in an appropriate way [31].
Normally, MXenes refers to M n X n-1 (n = 2, 3, and 4) layers formed by removing interlayer "A" atoms from the metal-ceramic MAX phase (where M stands for the transition metals, A is the group IIIA or IVA elements, and X represents the C and N elements). The M and X atoms stack to form a hexagonal lattice in the MAX phase, with the atoms of X occupying the M octahedral cage center shared by its edges. When the atoms of A are removed from the layer of M n X n-1 , the hexagonal-lattice of MX, rather than the cubic lattice, is preserved. Hence, the layer of M n X n-1 can be produced by removing the A-atoms. As with its predecessor MAX, MXenes thin sheets are frequently oriented horizontally. The bulk of MXenes have good mechanical features and are likely to be quite durable [32].

Etching Method
MXenes can be prepared in a number of different ways. Due to changes in their etching methods, various terminal functionalities can be attached to the M atoms to complete their coordination spheres, so as to lower their surface Gibbs free energy. As a result, the MXenes surface features have a great influence on their fabrication. Herein, different types of the preparation methods are discussed.

Etching with Hydrofluoric Acid (HF)
Due to the in-depth research on MXenes, etching procedures have been widely employed, especially the HF acid etching method, which is still the most commonly used procedure. Naguib et al. [33] proposed the fabrication of the Ti 3 AlC 2 MAX phase via HF acid etching in 2011. Using a simple displacement process, the HF acid removes Al layers from the Ti 3 AlC 2 MAX phase with H 2 production. In addition, the reaction of deionized water with the HF acid solution produces Ti 3 C 2 T x (where T represents the -O, -F, and -OH) as well as H 2 . The HF acid etching was used to successfully strip the Ti 2 AlC, (Ti 0.5 Nb 0.5 ) 2AlC, Ti 3 AlCN, Ta 4 AlC 3 , (V 0.5 Cr 0.5 ) 3AlC 2 , Nb 2 AlC, and a series of the MAX complexes of Zr 3 Al 3 C 5 , Ti 3 SiC 2 , and Mo 2 Ga 2 C, into Mxenes [34]. The HF acid etching has continuously been the most useful synthesis process for MXenes materials from 2011 to date. In the HF acid etching process, the time period, temperature, and F ions density play an important role in the design of high-quality MXenes layers. Alhabeb et al. [35] confirmed that Ti 3 C 2 T X , with high concentration HF acid, produces an excellent layered structure, which is difficult to achieve with other acid solutions. MXenes obtained via the HF acid etching approach retained their unique surface characteristics, along with the -O, -OH, and -F functionalities. Recently, Kim et al. [36] studied the etching behavior of MAX-phase Ti 3 AlC 2 in various etching agents at the atomic scale employing ion-beam and electron microscopy. They investigated the change in the structure of the Ti 3 AlC 2 phase as a function of the etching agents and etching time, revealing that the edge Al atoms at the mid layers of MAX-phase Ti 3

Modified Acid Etching Approach
Acid fluoride solutions have corrosive and poisonous properties, and due to these reasons, researchers are finding techniques to prevent HF acid from being used directly to extract the Al layers from MAX phases. The foremost approach, known as original-location HF acid etching, replaces the HF acid with fluoride salts (i.e., LiF, NH 4 HF 2 , FeF 3 , KF, and NaF) and HCl [37]. Usually, during the synthesis of MXenes via the etching of Al or Ga layers of the MAX phase by HF acid, an undesired byproduct (i.e., AlF 3 ·3H 2 O) is formed. To synthesize MXenes free of this impurity, it is necessary to illuminate the factors that initiate its formation. Thus, modified etching approaches are widely employed. For instance, Cockreham et al. [38] deduced the conditions that led to the formation of the byproduct AlF 3 ·3H 2 O, while proceeding the etching process with cobalt fluorides (i.e., CoF 2 /CoF 3 ). The SEM micrograph of the CoF 3 /MAX sample (Figure 3b) do not show any AlF 3 ·3H 2 O impurity. The MXenes interlayer distance formed via the modified acid-etching approach is improved because of the cation's intercalation, which minimizes the inner force between layers, and potentially delaminates the material layers during ultrasonication. As a result, this approach shortens the previously noted time-consuming multi-step synthesis process, enabling the synthesis of few-layer MXene in a single step [39].

Modified Fluoride-Based Acid Etching
Researchers have worked hard to find better means to remove the atom layers from MAX in order to prevent the significant toxicity that HF etching causes. Apart from HF, etching MAX precursors can also be performed with a mix of fluoride salts (i.e., KF, NaF, LiF, and NH 4 F) and strong acids. Fluoride salts and strong acids have been discovered to selectively etch atoms and led to the cation's intercalation (i.e., K + , Na + , Li + , and NH 4 + ) in situ. The introduction of water between MXene layers increases the interlayer space while lowering MXene layer contact. It is worth noting that the amount of fluoride salt and strong acid in the final MXene fragments can affect their quality and size. For example, the multilayered Ti 3 C 2 generated by the clay approach (5M LiF/6M HCl) requires a subsequent sonication step for delamination into single flakes, which frequently results in tiny defective MXene flakes [40].

Molten Salts Etching
MXene can also be produced by heating the MAX phases, such as Ti 4 AlN 3 , in a molten fluoride salt mixture (i.e., LiF, NaF, KF (29:12:59 weight ratio %)) at 550 • C under argon shielding to make Ti 4 N 3 . The etching procedure can be completed in 30 min. Because the Ti n N n-1 has a lower stability than Ti n C n-1 , it can be dissolved in HF or other fluoridebased acids used as the etching agent. As a result, the molten-salts etching method has the benefit of a relatively quick processing time. Additional cleaning (washing with DI H 2 O and H 2 SO 4 ) and delamination (in TBAOH solution) are compulsory. The crystallinity of delaminated Ti 4 N 3 is lower than that of the MXene obtained via the HF etching, as indicated by the XRD patterns of the resultant Ti 4 N 3 . The TiO 2 phase is also visible in the obtained product. In comparison to the HF and fluoride-based acids etching, the molten salts etching process has the benefit of producing MXenes with limited stability in the HF or fluoride-based acids solution. This technique, on the other hand, has the following disadvantages: (a) The etching process consumes a significant amount of heat and energy; (b) the resultant MXenes reveal low purity and crystallinity; (c) the resultant MXenes have abundant surface defects and vacancies [41]. Very recently, Liu et al. [42] reported the synthesis of MS-Ti 3 C 2 T x via the intercalation of the tetrabutylammonium hydroxide (TBAOH), followed by layer separation via sonication. The as-obtained Cl-terminated MS-Ti 3 C 2 T x was utilized as an anode in a Li-ion battery, which achieved a high specific capacity and exceptional rate capability. The TEM micrograph (Figure 3c) reveals clear Ti 3 C 2 T x nanosheets with distinct edges, with a lateral size of~600 nm. The SAED pattern ( Figure 3d) reveals sharp reflections corresponding to the hex agonal crystal symmetry, suggesting that the TBAOH treatment did not altered the crystallinity of Ti3C2Tx. The HR-TEM micrograph ( Figure 3e) confirmed a 2.05 nm thick ness, equivalent to two layers, considering the monolayer thickness of ∼1.03 nm. Besides TBAOH, some other organic molecules such as isopropyl amine and dimethyl sulfoxide (DMSO) have also been found to be effective intercalants for the delamination o HF-MXenes.

Etching without Fluoride
Although several etching conditions for synthesizing MXenes have been confirmed most synthesis techniques need HF or fluoride-based chemicals, which may result in the creation of -O and -F terminations on the interface of MXene. In particular, -F termina tions decrease the electrochemical performance of MXenes-based supercapacitors [43] Thus, fluoride-free fabrication techniques are necessary to provide acceptable electro chemical performances. Li et al. [44] developed an alkali-assisted hydrothermal etching technique for the fabrication of Ti3C2 MXene using a NaOH solution as the etching agent Because of the strong interaction between Al and alkali, alkali might be used as an etch ant for Ti3AlC2 MAX phase. Obtaining high-purity multi-layered MXenes is still a big challenge. In this case, the Bayer procedure was used to etch Al layers while avoiding damage to the Ti3C2 MXene skeleton by using a high alkali concentration (i.e., 27.5 M and a high temperature (i.e., 270 °C). Very recently, Chen et al. [45] reported the fabrica tion of a fluoride-free and chloride-containing Ti3C2Tx MXene via electrochemical etching as revealed in Figure 4a. In the synthesis process, Ti3C2Tx was delaminated via sonication in the absence of any toxic organic intercalant. The thickness of the resultant Ti3C2T nanoflakes was ∼3.9 nm (see Figure 4b), and their dispersion in an aqueous medium was very highly stable. The SAED pattern of the Ti3C2Tx (inset Figure 4b) displays hexagona symmetry as a result of the hexagonal arrangement of the Ti-atoms at the 002 surface o The SAED pattern (Figure 3d) reveals sharp reflections corresponding to the hexagonal crystal symmetry, suggesting that the TBAOH treatment did not altered the crystallinity of Ti 3 C 2 T x . The HR-TEM micrograph ( Figure 3e) confirmed a 2.05 nm thickness, equivalent to two layers, considering the monolayer thickness of ∼1.03 nm. Besides TBAOH, some other organic molecules such as isopropyl amine and dimethyl sulfoxide (DMSO) have also been found to be effective intercalants for the delamination of HF-MXenes.

Etching without Fluoride
Although several etching conditions for synthesizing MXenes have been confirmed, most synthesis techniques need HF or fluoride-based chemicals, which may result in the creation of -O and -F terminations on the interface of MXene. In particular, -F terminations decrease the electrochemical performance of MXenes-based supercapacitors [43]. Thus, fluoride-free fabrication techniques are necessary to provide acceptable electrochemical performances. Li et al. [44] developed an alkali-assisted hydrothermal etching technique for the fabrication of Ti 3 C 2 MXene using a NaOH solution as the etching agent. Because of the strong interaction between Al and alkali, alkali might be used as an etchant for Ti 3 AlC 2 MAX phase. Obtaining high-purity multi-layered MXenes is still a big challenge. In this case, the Bayer procedure was used to etch Al layers while avoiding damage to the Ti 3 C 2 MXene skeleton by using a high alkali concentration (i.e., 27.5 M) and a high temperature (i.e., 270 • C). Very recently, Chen et al. [45] reported the fabrication of a fluoride-free and chloride-containing Ti 3 C 2 T x MXene via electrochemical etching as revealed in Figure 4a. In the synthesis process, Ti 3 C 2 T x was delaminated via sonication in the absence of any toxic organic intercalant. The thickness of the resultant Ti 3 C 2 T x nanoflakes was ∼3.9 nm (see Figure 4b), and their dispersion in an aqueous medium was very highly stable. The SAED pattern of the Ti 3 C 2 T x (inset Figure 4b)  as is clear from the HR-TEM micrograph (Figure 4c). Based on the theoretical predictions and experimental results, the surface-attached −F remarkably hinder the electrolyte ions' transportation and sacrifices the electrochemically active sites, which results in low performance of MXenes for applications in Li-ion batteries and supercapacitors [46,47]. Thus, the fabrication of MXenes using F-free methods is highly desirable. planes of the Ti3C2Tx MXene as is clear from the HR-TEM micrograph (Figure 4c). Based on the theoretical predictions and experimental results, the surface-attached −F remarkably hinder the electrolyte ions' transportation and sacrifices the electrochemically active sites, which results in low performance of MXenes for applications in Li-ion batteries and supercapacitors [46,47]. Thus, the fabrication of MXenes using F-free methods is highly desirable.

Top-Down Approaches
For a long time, the top-down strategy has been well-established, especially in the synthesis and design of nanomaterials. These methods often involve the cleavage of 2D or 3D bulky precursor material into essential minute quantum particles [48][49][50]. Graphite, carbon nanotubes, graphene, MoS2 crystals, WS2 powder, black phosphorus, g-C3N4, and other three-dimensional and two-dimensional bulk precursors have been effectively transformed into quantum dots utilizing the top-down technique. Ball-milling [51], liquid exfoliation, chemical etching, electrochemical [52], intercalation, hydrothermal/solvothermal treatment, ultra-sonication, microwave irradiation, and other procedures are among them. The majority of the top-down approaches establish the initial O2-containing functionalities on the surface of catalysts, which facilitate the creation of defects in the catalysts [53]. The surface defects serve as reactive sites and allow the bulk molecules to split into tiny quantum particles [54]. This technique is of great significance because it can be operated at low temperatures. Further, copious raw materials can be used in this technique, and large-scale production can be achieved. The low yield and the need for particular treatments are, however, drawbacks. The most extensively utilized top-down approaches for the fabrication of MXene include ultrasonication, ball-milling, intercalation, hydrothermal, solvothermal, micro-explosion, and acid reflux. In this section, the details of top-down approaches for the synthesis of MXenes are discussed.

Top-Down Approaches
For a long time, the top-down strategy has been well-established, especially in the synthesis and design of nanomaterials. These methods often involve the cleavage of 2D or 3D bulky precursor material into essential minute quantum particles [48][49][50]. Graphite, carbon nanotubes, graphene, MoS 2 crystals, WS 2 powder, black phosphorus, g-C 3 N 4 , and other three-dimensional and two-dimensional bulk precursors have been effectively transformed into quantum dots utilizing the top-down technique. Ball-milling [51], liquid exfoliation, chemical etching, electrochemical [52], intercalation, hydrothermal/solvothermal treatment, ultra-sonication, microwave irradiation, and other procedures are among them. The majority of the top-down approaches establish the initial O 2 -containing functionalities on the surface of catalysts, which facilitate the creation of defects in the catalysts [53]. The surface defects serve as reactive sites and allow the bulk molecules to split into tiny quantum particles [54]. This technique is of great significance because it can be operated at low temperatures. Further, copious raw materials can be used in this technique, and large-scale production can be achieved. The low yield and the need for particular treatments are, however, drawbacks. The most extensively utilized top-down approaches for the fabrication of MXene include ultrasonication, ball-milling, intercalation, hydrothermal, solvothermal, micro-explosion, and acid reflux. In this section, the details of top-down approaches for the synthesis of MXenes are discussed.

Hydrothermal Approach
This is a heterogeneous reaction approach, which involves the heating of aqueous solutions above the boiling point of water within a high-pressure autoclave containing precursor materials. The synergetic effect of the elevated temperature and pressure and the solution pH can be employed to alter the QD size, shape, morphology, and properties [55]. In addition, the pH of the solution, reaction temperature, and time period play a vital role in the production of MXene. To synthesize MXene, a reaction temperature of 100 to 180 • C and pH of 6 to 9 are kept standard [56], because the reaction duration is influenced by the pH and temperature. Furthermore, by adjusting the conditions of the hydrothermal reaction, the material size, properties, and thickness can be altered. Xue et al. [57] employed a hydrothermal technique to create water-soluble Ti 3 C 2 MXene and observed that by altering the hydrothermal reaction temperature to 79 • C, the properties, thickness, and size of MXene can be tailored. The MXenes obtained at 100, 120, and 150 • C resulted in particles with average diameters of 2.9, 3.7, and 6.2 nm, respectively, and average thickness of 0.99, 0.91, and 0.89 nm, showing that the monolayers make up the bulk of particles. At the time of reaction, the Ti 3 C 2 QDs exhibits the -NH surface functionalities, and at a lower temperature (i.e., 100 • C), a new MXene structure is formed in which the d-spacing value can be confirmed. On the other hand, the MXENE formed at 120 • C exhibits a fusion structure with CTi in the core and TiO 2 on the surface. However, because most of the Ti atoms were etched away at an elevated temperature (i.e., 150 • C), an amorphous MXENE structure was formed. According to Xiao and et al. [58], below 100 • C, the MXene structure cannot be produced. They made MXene (Ti 3 C 2 T x ) by varying the reaction temperature from 60 to 80 and 100 • C. Instead of MXene, abundant nanoribbons, including a few Ti 3 C 2 T x nanoflakes, were formed at a temperature of 60 • C. By increasing the temperature to 80 • C, the shrinkage of the particles size occurred, and various-sized nanodots were formed. Ultrafine nanodots are generated and uniformly spread on the nanosheets at 100 • C. More MXenes have been synthesized by applying different hydrothermal conditions [59,60]. By adapting alternative hydrothermal conditions, MXene with doped heteroatoms have also been produced, utilizing the elements' consequent precursors. Using Nb 2 C nanosheets precursor material and L-cysteine as a source of sulfur and nitrogen, Xu and his co-workers fabricated heteroatom-co-doped Nb 2 C MXene (S, NMXene) via the hydrothermal method at 160 • C. The particles size of MXene was between 2.6 and 4.7 nm. The fabricated S, N-MXene has a lateral size of 3.56 nm, remarkably smaller than those of the MXene (i.e., 2.4 nm) and N-MXene (i.e., 2.66 nm). The S, N-MXene had an average thickness of 1.74 nm, indicating that a monolayer was developed [61]. Recently, Peng et al. [62] fabricated 2D MXene (i.e., h-Ti 3 C 2 ) via the hydrothermal method using low-toxicity etchants (i.e., NaBF 4 , HCl). The TEM micrograph of the resultant h-Ti 3 C 2 MXene ( Figure 5a) revealed a layered structural morphology. As is clear from Figure 5b, the 2D h-Ti 3 C 2 nanoflakes are very thin and transparent. The selected area HR-TEM micrograph of the h-Ti 3 C 2 nanoflakes ( Figure 5c) revealed d-spacing of~0.264 nm and~0.155 nm, corresponding to the (0−110) and (0−210) planes of the Ti 3 C 2 , respectively. The FFT pattern revealed hexagonal symmetry as can be seen from the inset of Figure 5c. It is important to note that the hydrothermal etching technique not only avoids the use of HF acid but is also an efficient method for the preparation of Ti 3 C 2 nanoflakes.

Solvothermal Approach
The solvothermal process is simple and more efficient than the hydrothermal method and has been widely utilized to make MXene. It is similar to the hydrothermal process with the mere distinction that, instead of water, a non-aqueous organic solvent is used to make the precursor solution. Because the size, distribution, and material crystallinity can be precisely regulated as needed, the solvothermal method is regarded to be more successful and adaptable as compared to the hydrothermal method [63]. Furthermore, adjusting experimental parameters such as the reaction's temperature, time duration, and solvent type, the desired result can be fine-tuned. Feng et al. [64] used a solvothermal technique to fabricate nitrogen-doped Ti 3 C 2 MXene. In conclusion, the exfoliated MXene was used to make three different forms of MXene QDs with varied amine doses. To fabricate NTi 3 C 2 QDs, di-ethylene-triamine, di-methyl-formamide (DMF), and the exfoliated MXene were mixed in equal amounts at 150 • C for 8 h. NTi 3 C 2 QDs with an average particle size of 6.2 nm and a width of 1 nm were obtained, indicating the monolayer formation. To synthesize the QDs, the solvothermal reaction time was optimized. The particle size decreased with the increase in the solvothermal reaction time. Recently, Feng et al. [64] demonstrated the fabrication of in-situ N-doped Ti 3 C 2 MXene QDs via the amine-assisted solvothermal method. The SEM micrograph of pristine MXene (Figure 5d) confirmed multi-layers with a thickness of approximately 5 µm. The TEM micrograph ( Figure 5e) reveals a remarkable reduction in the particle size of MXene (i.e., from 600 nm to 6 nm), confirming the formation of N-MXene QDs. As is clear from the HR-TEM micrograph (Figure 5f), the resultant QDs exhibit high crystallinity with a lattice spacing of 0.193 nm (101 plane). The resultant QDs revealed diverse fluorescence quenching responses toward the detection of various metal cations. Similarly, Niu et al. [65] used the solvothermal approach to fabricate three types of Ti 3 C 2 T x QDs from the Ti 3 C 2 MXene precursor, labelling them e-Ti 3 C 2 , f-Ti 3 C 2 , and s-Ti 3 C 2 QDs. Their particle size varied depending on the solvent, and the average particle sizes for e-Ti 3 C 2 , f-Ti 3 C 2 , and s-Ti 3 C 2 QDs were observed to be 2.5-0.2, 3.3-0.2, and 1.8-0.1 nm, respectively. The particles' thickness was in the range of 1.0-2.5 nm. The particles' thickness and lateral sizes were influenced by the polarity of the solvent, oxidation reaction, and boiling point. The disadvantage of this technique is the generation of carbon quantum dots (CQDs) from organic precursors. The undesirable CQDs have an impact on the product's optical and catalytic capabilities [66].

Ball-Milling Approach
Different QDs have been successfully fabricated via the ball-milling approach, which is an extensively used top-down method for reducing the size of nanoparticles. The physical and morphological features of the produced materials are influenced by a variety of factors such as the milling type (wet or dry), speed of the milling, the ratio of the ball to powder weight, and milling time duration. Zhang et al. [67] used the ball-milling procedure to fabricate various-sized Ti 3 C 2 T x MXene nanodots (TNDs) by mixing Ti 3 C 2 T x with diverse solid-state precursors such as red P, S, C, and Si (see Figure 6a). Ball-milling of the mixture was performed at 550 rpm for 48 h under an argon environment, while keeping the powder-to-ball weight ratio at roughly 1:20. The of FE-TEM image of the Ti 3 C 2 T x ( Figure 6b) showed a µm-long thin layered structure. The red-P-assisted ball-milling of Ti 3 C 2 T x results in QDs with a lateral diameter and thickness in the range of 6-25 nm ( Figure 6c). The HR-TEM micrograph of Ti 3 C 2 T x (Figure 6d) revealed an interlayer spacing of~0.91 nm, corresponding to the (0002) planes, which confirms the development of MXene nanodots. During this process, the TiOX (X = C, P, S, Si) bond formation occurs, causing the delamination and reduction in the particle size of Ti 3 C 2 T x MXene, which is governed by the strength of the OX link. The stronger the -OX interaction, the smaller the particle size will be. Due to the strong TiOX connection, a solid-state element existed in the product. This is an ideal method used to fabricate nanocomposites.

Ultrasonic Approach
The ultrasonic technique is an environmentally friendly and risk-free technique used for the fabrication of MXene QDs. Due to the solvents' acoustic, cavitation, and reverberation, the ultrasonication process changes the layered, as well as nonlayered, materials into QDs. In this technique, the inherent features of materials are effectively retained. Solvents such as DMSO, DMF, N-methyl-2-pyrrolidone (NMP), and tetra-butylammonium hydroxide (TBAOH), with their high boiling points and surface energies, could overcome the binding force of bulk materials and transform them into highly dispersed, small-sized QDs. To produce the Ti 3 AlC 2 MXene directly from the MAX phase, Yu et al. [68] used etching-assisted exfoliation and mechanical-force-assisted liquid exfoliation approaches with the solvent TBAOH. The bulk particles of Ti 3 AlC 2 break down into smaller particles with new edges and surface sites during ultrasonic treatment. By treating the OH group with a solvent, these sites were removed as Al(OH) 4 . Meanwhile, the formation of isolated MXene sheets took place [68]. These sheets were then transformed into MXene via the bath sonication. The thickness and average lateral size of the obtained MQDs were 0.6 and 4.9 nm, respectively. Before being utilized in aqueous media via the ultrasonication technique, the organic solvents with high boiling points require extensive post-treatment. High-temperature rotary evaporation as post-treatment affects the quantum dots' optical and catalytic capabilities. As a result, methods for synthesizing MXene in an aquatic environment gained a great deal of attention. Furthermore, widespread utilization is challenging due to the lengthy sonication method and low output yield. As a result, combining the ultrasonication technique with hydrothermal and solvothermal ones is highly effective to shorten the reaction time and increase the yield. To fabricate V 2 C MXene, Huang et al. [69] used an ultrasonication-hydrothermal method. Throughout the synthesis procedure, the V 2 C sheets were ultrasonically treated for 1 h in an Ar environment, and the suspension was treated hydrothermally at 120 • C for 6 h. The SEM micrograph of V 2 C MXene ( Figure 6e) revealed a layered structure. The average size of the generated V 2 C MXene was 4.13 nm, and it was 2.33 nm thick (see Figure 6f). The lattice fringes with d-spacing of 0.247 nm ( Figure 6g) were accredited to the (01 10) plane of the V 2 C MXene. Another way to fabricate MXene is through intercalation ultrasonication. To synthesize Ti 3 C 2 QDs, Wang et al. [70] used the ultrasonic intercalation technique. The Ti 3 C 2 nanosheets were treated for 24 h with 25% aqueous TMAOH to produce guest intercalation and the host-layer fragment. After that, the resultant intercalated Ti 3 C 2 nanosheets were sonicated in an Ar atmosphere for 10 h.

Ultrasonic Approach
The ultrasonic technique is an environmentally friendly and risk-free technique used for the fabrication of MXene QDs. Due to the solvents' acoustic, cavitation, and reverberation, the ultrasonication process changes the layered, as well as nonlayered, materials into QDs. In this technique, the inherent features of materials are effectively retained. Solvents such as DMSO, DMF, N-methyl-2-pyrrolidone (NMP), and tetra-butylammonium hydroxide (TBAOH), with their high boiling points and surface energies, could overcome the binding force of bulk materials and transform them into highly dispersed, small-sized QDs. To produce the Ti3AlC2 MXene directly from the MAX phase, Yu et al. [68] used etching-assisted exfoliation and mechanical-forceassisted liquid exfoliation approaches with the solvent TBAOH. The bulk particles of the ultrasonic intercalation technique. The Ti3C2 nanosheets were treated for 24 h with 25% aqueous TMAOH to produce guest intercalation and the host-layer fragment. After that, the resultant intercalated Ti3C2 nanosheets were sonicated in an Ar atmosphere for 10 h.

Bottom-Up Approaches
In contrast to the top-down procedures, which employ bulk material as precursors the bottom-up approaches use molecular material as precursors. Bottom-up approaches could also be used to make MQDs from tiny precursors of organic and inorganic mole cules. Bottom-up approaches have a number of advantages, including greater atomic usage, structural and morphological control, and faster functionalization, among others which give QDs better structures and properties [71][72][73]. The previous investigations provide an excellent foundation for bottom-up MXene preparation. However, for large-scale fabrication, simple, highly efficient precursors with low toxicity, gentle reac tion conditions, excellent crystallinity, monodispersity, and high yields must be ad dressed. Meanwhile, because of their relatively simple operating conditions compared to the top-down methods, one-pot bottom-up methodologies will likely be employed to prepare MXene in the future to fulfil the incremental application requirements. As there is a little research on the bottom-up methodologies for MQD fabrication, the study is promising, and great stress should be given to these strategies [56].

Molten Salt Synthesis
Due to its ease of use and short reaction time, the molten salt synthesis process has lately gained favor. Using molybdenum acetylacetonate as a precursor, Cheng et al. [74 synthesized a Mo2C QDs/carbon nanosheet (Mo2C/C) composite. In a typical synthesis the molybdenum acetyl-acetonate precursor, NaCl, and a sucrose mixture were calcined for 2 h at 800 °C under an air environment as depicted in Figure 7a. The SEM scan cor roborated the nanosheet structure (Figure 7b), demonstrating the importance of the method. The TEM micrograph further confirmed the uniform and ultrathin structura

Bottom-Up Approaches
In contrast to the top-down procedures, which employ bulk material as precursors, the bottom-up approaches use molecular material as precursors. Bottom-up approaches could also be used to make MQDs from tiny precursors of organic and inorganic molecules. Bottom-up approaches have a number of advantages, including greater atomic usage, structural and morphological control, and faster functionalization, among others, which give QDs better structures and properties [71][72][73]. The previous investigations provide an excellent foundation for bottom-up MXene preparation. However, for large-scale fabrication, simple, highly efficient precursors with low toxicity, gentle reaction conditions, excellent crystallinity, monodispersity, and high yields must be addressed. Meanwhile, because of their relatively simple operating conditions compared to the top-down methods, one-pot bottom-up methodologies will likely be employed to prepare MXene in the future to fulfil the incremental application requirements. As there is a little research on the bottom-up methodologies for MQD fabrication, the study is promising, and great stress should be given to these strategies [56].

Molten Salt Synthesis
Due to its ease of use and short reaction time, the molten salt synthesis process has lately gained favor. Using molybdenum acetylacetonate as a precursor, Cheng et al. [74] synthesized a Mo 2 C QDs/carbon nanosheet (Mo 2 C/C) composite. In a typical synthesis, the molybdenum acetyl-acetonate precursor, NaCl, and a sucrose mixture were calcined for 2 h at 800 • C under an air environment as depicted in Figure 7a

Pyrolysis Approach
Pyrolysis is a well-known bottom-up methodology for MXene since it is a time-saving, easy, and ecologically benign procedure. Wang et al. [75] fabricated the Mo2C QDs-carbon polyhedron composites via a simple pyrolysis technique (see Figure  8a). In summary, the Mo/ZIF-8 was synthesized via the solvothermal method and then pyrolyzed at 700 °C for 2 h in an Ar environment before being acid etched. The SEM micrograph ( Figure 8b) clearly indicated the carbonization of organic frameworks and the development of Mo2C as an individual polyhedron. The TEM micrograph (Figure 8c) revealed uniformly dispersed ultra-small black dots in the carbon matrix. The lattice fringe (i.e., 0.24 nm) of cubic-Mo2C was aligned with the (111) direction plane, as revealed in the HR-TEM image (inset Figure 8c). Despite the fact that significant works have been reported on the synthesis of MQDs using diverse synthetic techniques, further research is needed to make these bottom-up synthesis routes easy, efficient, and commercial. As a result, in bottom-up approaches, the molecular precursors with the choice of elements sources, as well as parameters of the reaction such as reaction time, temperature, and precursor concentration, are crucial in the QDs synthesis. This approach is efficient, simple, and monodisperse in comparison to the top-down ones, with high yield, improved crystallinity, and normal conditions required for the reactions. Still, very little research has been conducted on the synthesis of MXene. These approaches will receive more attention in the near future in order to address the needs of incremental applications. Carbon nanotubes have a conductivity of 15 100 S cm −1 , whereas reduced graphene oxide has a conductivity of 15 100 S cm −1 [76]. As a result, the strong metallic and electrical conductivity of Ti3C2Tx allowed it to work better as an electrocatalyst. In the case of surface functionalization and composition, Ti2COm (1.03 × 10 4 cm 2 V -1 s -1 ) has a better carrier mobility than pure graphene (0.3-1.2 × 10 3 cm 2 V -1 s -1 ) [43]. In addition, the surface functional group of Ti3C2Tx (x = -O, -OH, -F) provides a large number of sites and influences electrocatalytic efficiency. Owing to the substantial density of state (DOS) around the Fermi level, they show strong electrocatalytic activity [43].

Pyrolysis Approach
Pyrolysis is a well-known bottom-up methodology for MXene since it is a time-saving, easy, and ecologically benign procedure. Wang et al. [75] fabricated the Mo 2 C QDs-carbon polyhedron composites via a simple pyrolysis technique (see Figure 8a). In summary, the Mo/ZIF-8 was synthesized via the solvothermal method and then pyrolyzed at 700 • C for 2 h in an Ar environment before being acid etched. The SEM micrograph (Figure 8b) clearly indicated the carbonization of organic frameworks and the development of Mo 2 C as an individual polyhedron. The TEM micrograph (Figure 8c) revealed uniformly dispersed ultra-small black dots in the carbon matrix. The lattice fringe (i.e., 0.24 nm) of cubic-Mo 2 C was aligned with the (111) direction plane, as revealed in the HR-TEM image (inset Figure 8c). Despite the fact that significant works have been reported on the synthesis of MQDs using diverse synthetic techniques, further research is needed to make these bottomup synthesis routes easy, efficient, and commercial. As a result, in bottom-up approaches, the molecular precursors with the choice of elements sources, as well as parameters of the reaction such as reaction time, temperature, and precursor concentration, are crucial in the QDs synthesis. This approach is efficient, simple, and monodisperse in comparison to the top-down ones, with high yield, improved crystallinity, and normal conditions required for the reactions. Still, very little research has been conducted on the synthesis of MXene. These approaches will receive more attention in the near future in order to address the needs of incremental applications. Carbon nanotubes have a conductivity of 15 100 S cm −1 , whereas reduced graphene oxide has a conductivity of 15 100 S cm −1 [76]. As a result, the strong metallic and electrical conductivity of Ti 3 C 2 T x allowed it to work better as an electrocatalyst. In the case of surface functionalization and composition, Ti 2 CO m (1.03 × 10 4 cm 2 V -1 s -1 ) has a better carrier mobility than pure graphene (0.3-1.2 × 10 3 cm 2 V -1 s -1 ) [43]. In addition, the surface functional group of Ti 3 C 2 T x (x = -O, -OH, -F) provides a large number of sites and influences electrocatalytic efficiency. Owing to the substantial density of state (DOS) around the Fermi level, they show strong electrocatalytic activity [43].

Challenges in MXenes Synthesis
The quality of MXenes is related to the synthesis procedures. In an ideal synthesis technique, the MAX phase is the precursor material, and its quality governs the MXenes product. Thus, choosing a high-purity MAX phase without unreacted substrates is vital. Further, it is essential to prepare the MAX phase with negligible impurities originating from mechanical milling. Yet, the additives can be detached as residue during delamination and etching. Additionally, if these are acid soluble, then they easily wash out through rinsing.
During MXenes etching, the MAX phase undergoes severe conditions to solubilize and remove the inclosing aluminum layers. In early methods, poisonous HF was utilized, and the most substantial task was associated with the researcher's safety. Later, based on optimal conditions and less-toxic etchants, the MILD technique was developed. Nevertheless, the method was still based on the utilization of HF obtained from the LiF and HCl mixture. More innovation, by utilizing ionic liquids or molten-salts, allows for safe and less hazardous synthesis routes of MXenes. Yet, their upscaling is extremely restricted owing to the expensive chemicals.
The oxidation of MXenes is crucial, and it is important to adjust features of the MAX phase and the processing time. Thus, the more serious feature of the MAX phase is their grain sizes. For example, a MAX phase with large size grains produces unetched MXenes, which settle in the colloidal solution and require a second step of etching to remove Al entirely. On the other hand, a MAX phase with small grains size etches very easily but needs a long milling time and results in additional impurities, which come from the ball milling and vessel. Furthermore, a MAX phase with a small grain size produces small flakes of MXenes. Thus, a MAX phase with a small grain size results in low-quality MXenes.
A comparable deliberation can also be made for the delamination method. For example, MXenes with a large grain size will not delaminate entirely and requires a longer time for delamination. Further, if the delaminated flakes are not reduced via sonication, they would have large dimensions. From this perspective, the adjustment of the materials' parameters is a challenging task as the fabrication methods produce diverse results in terms of the flakes' parameters.
To characterize the as-prepared MXenes, several parameters including the surface-zeta potential, conductivity, and Raman and XRD analyses should be carefully considered. For examples, different MXenes functionalities would give similar Raman spec-

Challenges in MXenes Synthesis
The quality of MXenes is related to the synthesis procedures. In an ideal synthesis technique, the MAX phase is the precursor material, and its quality governs the MXenes product. Thus, choosing a high-purity MAX phase without unreacted substrates is vital. Further, it is essential to prepare the MAX phase with negligible impurities originating from mechanical milling. Yet, the additives can be detached as residue during delamination and etching. Additionally, if these are acid soluble, then they easily wash out through rinsing.
During MXenes etching, the MAX phase undergoes severe conditions to solubilize and remove the inclosing aluminum layers. In early methods, poisonous HF was utilized, and the most substantial task was associated with the researcher's safety. Later, based on optimal conditions and less-toxic etchants, the MILD technique was developed. Nevertheless, the method was still based on the utilization of HF obtained from the LiF and HCl mixture. More innovation, by utilizing ionic liquids or molten-salts, allows for safe and less hazardous synthesis routes of MXenes. Yet, their upscaling is extremely restricted owing to the expensive chemicals.
The oxidation of MXenes is crucial, and it is important to adjust features of the MAX phase and the processing time. Thus, the more serious feature of the MAX phase is their grain sizes. For example, a MAX phase with large size grains produces unetched MXenes, which settle in the colloidal solution and require a second step of etching to remove Al entirely. On the other hand, a MAX phase with small grains size etches very easily but needs a long milling time and results in additional impurities, which come from the ball milling and vessel. Furthermore, a MAX phase with a small grain size produces small flakes of MXenes. Thus, a MAX phase with a small grain size results in low-quality MXenes.
A comparable deliberation can also be made for the delamination method. For example, MXenes with a large grain size will not delaminate entirely and requires a longer time for delamination. Further, if the delaminated flakes are not reduced via sonication, they would have large dimensions. From this perspective, the adjustment of the materials' parameters is a challenging task as the fabrication methods produce diverse results in terms of the flakes' parameters.
To characterize the as-prepared MXenes, several parameters including the surface-zeta potential, conductivity, and Raman and XRD analyses should be carefully considered. For examples, different MXenes functionalities would give similar Raman spectra and XRD patterns because these techniques are precise to the materials' composition and physical nature of the bulk. However, MXenes nanosheets might differ in their physical nature.
Thus, it is a challenging task to regulate and track changes in the chemistry and dimension of flakes (surface functional-group arrangement) via cutting-edge and complementary materials characterizations.
The elimination of Al etching by-products is also a crucial step and vanishes when the solution pH reaches 6 or 7. If the concentration of the flake's dispersion is high, a substantial quantity of the adsorbed inorganic Li + or Clions or organic TMA + ions might be difficult to wash out, and several washing, centrifugation, or filtration steps are required to attain the desired pH.
MXenes types and their stoichiometry also effect their synthesis. For example, etching and delamination of Ti 3 C 2 T x MXene in LiF/HCl is an easy process. On the other hand, the Nb 2 CT x MXene can firstly be etched with HF and further delaminated with TBAOH. This is because the LiF/HCl method is less effective for such types of MXenes. Thus, Ti 3 C 2 T x fabrication could be up-scaled. It is difficult to etch and delaminate several types of MXenes, such as Cr 2 CT x , V 2 CT x , and Ti 2 NT x , into single flakes because of their high oxidation tendency and low stability. Thus, the resultant MXenes would be multi-layered in these phases.
The problem related to the structural preservation of MXenes is due to their oxidation stability. MAX phases exhibit high stability due to the existence of interleaving "A" layers and eliminating this layer via etching can markedly decrease the multi-layered structures' stability. Nevertheless, the multi-layers still exhibit enough stability to endure the preparation of samples for different analysis and industrial applications. In addition, the MXenes structures are destabilized with delamination to a single or few layers, and after several hours, these layers oxidize under ambient conditions and further decompose after a few months. Subsequently, the materials should be kept in a dark and oxygen-free atmosphere at a low temperature.
The above issues also effect the characterization of the materials. For example, the materials will be immediately characterized after fabrication because of the rapid oxidation. The majority of the techniques operate under ambient environments, (i.e., in air, or water presence), and materials are oxidized during preparation for characterization. These changes may be pursued by varying the XRD and Raman patterns. Thus, to avoid the oxidation of MXenes, the appropriate laser intensity for Raman analysis should be chosen. Regarding the surface chemistry, surface changes can be evaluated via the zeta-potential or XPS techniques. By measuring the sample immediately after fabrication, its zeta-potential value would be extremely negative. The immediate XPS analysis after fabrication would detect the metallic bonding with hydroxide groups and fluorine without any metal oxide. Upon oxidation of the sample, the zeta-potential would be close to zero and the XPS would confirm the existence of metal oxides [77][78][79][80].

Properties of MXenes
Because of the abundance of electrons associated with transition metal atoms, MXenes have a broad and changeable surface interaction. The fascinating properties of MXenes, including the physical, chemical, magnetic, mechanical, electrical, surface, thermal, as well redox, are controlled by the MAX phase, etching and raw materials, and delamination and etching strategies. Furthermore, the MXenes properties can be modified for appropriate applications by changing its composition, adopting alternative elements of "M" and "X", and altering the surface terminal functionalities. Recently, theoretical analyses utilizing computer approaches have characterized the features of MXenes. The most attractive qualities linked to energy storage are discussed below [81].

Structural Properties
Due to their exceptional conductivity and abundant surface functionalities, MXenes with high structural and chemical stability can act as both carriers of inherent active and/or other functional materials for various applications including energy conversion and storage [82]. MXenes, on the other hand, have the drawbacks of ultrathin 2D materials [83], namely a significant tendency to restack and the lack of a restricted porosity structure. Over the last three years, many efforts have been devoted to designing and modifying porous MXenes. Until now, a variety of porous MXenes with acceptable topologies have been fabricated using a variety of synthetic techniques and used in a wide range of applications, of which all revealed enhanced performances. The goal of Fanxing Bu pioneer is to bring a new understanding to the relationship between the fundamental synthesis, composition, structure, and performance of porous MXenes, which will spur further progress and serve as a strong platform for practical applications [15]. Due to their beneficial structures, 2D porous MXenes have also been utilized for various applications, inspired by their porous structures and tunable physicochemical properties. Table 1 explains, in detail, the role of porous structures, as well as the particular functions of pores in the associated applications [15].

Optical Properties
The optical features of Ti 3 C 2 T x MXenes include high transparency and a photothermal effect. Ti 3 C 2 T x films have a strong surface plasmonic effect, which is an important feature. Ying et al. [84] achieved transparency of more than 75% in the visible and near-infrared ranges, despite the fact that transparency decreases as the spin rate of the coating decreases. The generated film is very transparent at 550 nm, according to a V 2 CT X transmittance investigation, and the transmittance decreases as the film thickness increases. They attained 89% maximum transmittance for an 11-nm-thick layer. The absorbance coefficient of the film was 1.22 ± 0.05 × 10 5 /cm at 550 nm. This value is lower than those of the Ti 2 CT x (i.e., 2.7 ± 0.1 × 10 5 /cm) and Ti 3 C 2 T x (i.e., 2.4 ± 0.1 × 10 5 /cm) MXenes. According to this investigation, the annealing temperature of the MXenes film has no effect on the transmittance or absorption coefficient. This is in direct opposition to the conductivity findings. Zhang et al. [85] made 2D titanium carbide MXene with a transparency of >90%. The synthesis method involves an irreversible color shift that occurred during the Ti 3 C 3 T Z intercalation stage. However, there has been little research on the effect of synthesis on optical characteristics [86].

Thermal and Electrical Properties
Thermal conductivity and thermal oxidation are both high in Ti 3 AlC 2 . The thermal conductivity of the Ti 3 C 2 F x monolayer is approximately 108 W/(mK), while the Ti 3 C 2 O x monolayer is approximately 11 W/m K [87]. However, with Ti 3 C 2 T x films, it is found to be approximately 2.84 W/(mK) in experiments. The metallic conductivity of Ti 3 C 2 T x sheets is 2400 S cm −1 and is extremely flexible. V 2 C is a kind of MXene that has been investigated and found to have a conductivity of 3300 S cm −1 . Its versatility makes it perfect for applications such as wearable electronics [88]. The thickness of the layers and calcination temperature have a great effect on the sheet conductivities. According to Ying et al. [84], after annealing, the conductivity increased, and the sheet resistance for thicker films was lower than for thinner films.

Mechanical Properties
The mechanical characteristics of MXenes can vary significantly based on the surface terminations. Magnuson et al. [89] discovered that the surface terminal groups weaken the Ti-C bond by removing the charge from them. They discovered that the Ti-C bond in Ti 2 C-T x is longer than in Ti 3 C 2 -T x , which might affect the elastic properties of the materials. The scientists also proposed that modifying the binding strength could help to improve flexibility. Because of their higher mechanical strength, Zha et al. [90] indicated that MXenes with an O-terminal functionality should be the first choice for structure materials, supercapacitors, and other applications.
MXenes have one distinctive characteristic, i.e., their excellent mechanical strength. Despite the significant improvements in the fabrication of MXene nanostructures, Ti 3 C 2 T x has been extensively explored for a variety of applications owing to its exceptional conductivity, mechanical capabilities, and high electro-magnetic shielding effect [91]. MXenes revealed high mechanical ion adsorption capabilities in numerous investigations, paving the path for more research into their possible usage in sensors and flexible electronics [92]. The MXenes' mechanical properties with composition variation are summarized in Table 2 [93]. In the current 5G era, electronic gadgets on the market are growing at an exponential rate, resulting in significant electromagnetic pollution that endangers human health and device operation. As a result, lightweight EMI-shielding materials with exceptional mechanical strength and a high shielding efficiency (SE) have received huge attention. Due to their hydrophilic nature, exceptional specific surface area, and superior conductivity, 2D transition metal carbides and carbonitrides achieved excellent performance in the field of EMI-shielding. In one study, Ti 3 C 2 T x MXene was mixed with a formaldehyde and resorcinol solution in a sol-gel technique to generate a hybrid gel and Ti 3 C 2 T x MXene/C hybrid-foam (MCF) via freeze-drying and heat reduction. The vacuum-assisted impregnation and curing methods were utilized to make MCF/epoxy EMI-shielding composites. The conductivity, EMI performance, and mechanical strength of the MCF/epoxy EMI-shielding composites were thoroughly investigated. The structure of the resulting MCF was investigated using Raman, XPS, and SEM. On this premise, the impact of the Ti 3 C 2 T x MXene mass fraction on the electrical conductivity, EMI performance, and mechanical strength of the MCF/epoxy EMI-shielding composites was studied in depth [94].

Applications of MXenes
MXenes gained tremendous research interest due to their unique properties, including their superior electrical conductivity [76], high volumetric-electrochemical capacitance [95], tunable plasmonic features and work function [95], optical transparency and electrochromic in thin films [96], high thermal stability [97], good mechanical strength [98], and their capability to form highly stable colloidal solutions in various polar solvents, especially alcohols and water [95]. MXenes have been utilized in a variety of fields, such as energy storage, electronics, structural components, and environmental cleanup, due to their attractive qualities. Energy storage was the first application of MXenes and is the most wide-ranging to date [99], but it has since been extended to include supercapacitors and all types of batteries [99][100][101]. Even in clear 40-50 nm layers, MXenes showed higher efficiency than graphene and carbon-based composite materials and were found to be effective as metals in communication [102] and electromagnetic interference (EMI) screening [103]. The removal of heavy metals, desalination, and the adsorption of pollutants are all examples of how MXenes are being employed in environmental remediation [104,105]. Other potential applications of MXenes include catalysis, electronics, and biological applications such as photo-dynamic cancer therapy [105], dialysate-regeneration [87], implanted electrodes [106], and intraocular-lenses [107]. In medical technology, when MXene films substitute an Au or Pt metal electrode, it not only enhances the performance but also saves money. In terms of their applications, the industry should focus on the use of Mxenes [95].

Photocatalysis
Photocatalysis is a catalytic approach used for solar energy conversion into chemical fuels and the decontamination of various pollutants from the environment [108][109][110][111][112][113][114][115][116][117]. The process occurs in various steps: (i) Charge carrier's generation, (ii) the separation and migration of photogenerated charge carriers to the surface of the photocatalyst, and (iii) redox reactions via the aid of photoinduced electrons and holes (see Figure 9a-d) [118]. The charge carrier's separation and transfer is regarded as the rate-determining step in photocatalysis because rapid charge carrier's recombination greatly reduces the photoactivity and solar-to-chemical energy conversion efficiency of the catalysts [119]. In light of these exciting qualities, the design and fabrication of novel 2D materials with unique optoelectronic structures has received tremendous interest in photocatalysis.
MXene (transition metal carbide, nitride, and carbonitride), a novel class of 2D materials, was first reported in 2011 and has steadily grown in popularity as a photocatalytic material [120]. MXene was initially investigated for solar fuel conversion and storage devices (i.e., supercapacitors and batteries) owing to its large theoretical gravimetric capacitance and super conductivity [30,121,122]. Since 2014, MXene has been intensively investigated in photocatalysis, with a significant increase in literature based on MXenebased photocatalysts. The following factors are mostly responsible for MXenes' effective photocatalysis applications: (i) The abundance of functional groups produced by the wet chemical etching technique is advantageous for making a close-contact boundary between MXene and the coupled semiconducting material, (ii) the MXene surface chemistry can be tweaked to change the bandgap alignment, and (iii) the highly conductive metal centers of MXene's multilayer structure improve its metallic conductivity and electron-accepting capabilities [123]. As a result, MXene is seen as a viable alternative to other reported 2D materials, and it has been extensively investigated in photocatalysis for various applications such as CO 2 conversion, water splitting, pollutant oxidation, and N 2 fixation. MXene could improve photocatalytic activity in these applications by performing a variety of functions, including boosting charge carrier's separation and transfer, acting as a strong support, restricting photocatalyst size, and promoting adsorption of reactants [118]. An effective synergetic interface could improve photocatalytic reaction processes and could greatly affect the performance of 2D/2D MXene-based heterostructure photocatalysts due to the noticeable "face-to-face" interactive contact and different interface areas' exposure [125]. In fact, the two-dimensional MXene structure can provide a large number of surface groups for creating 2D/2D heterojunctions with other two-dimensional photocatalysts. Close "face to face" contact in a 2D/2D MXene-based heterojunction offers a great deal of potential for increasing photo-induced charge carrier separation and transfer across the heterojunction interface. Photo-induced electrons triggered by the 2D heterostructure catalysts could aggregate onto the surface via electron transfer, thanks to the strong photocurrent and outstanding electronic conductivity of MXene's. The 2D structural platform of MXene can act as an electron mediator, enabling the fast extraction of electrons from 2D materials for better photocatalytic activities [124]. Photocatalytic activities of MXene-based composites are summarized in Table 3.  As a new class of layered material, MXene has a variety of unique photochemical features and infancy variables that must be further researched and investigated in order to produce useful photocatalytic effects. Such advantages of MXene-based composites could lead to the enhancement in photocatalysis for practical applications. As a result, a thorough examination of MXene could provide useful insight into the development of MXene-based heterostructure materials. This perception analyzes the underlying role of MXene in promoting photoactivity and presents the recent progress made in the design of MXene-based heterostructure photocatalysts [124].
An effective synergetic interface could improve photocatalytic reaction processes and could greatly affect the performance of 2D/2D MXene-based heterostructure photocatalysts due to the noticeable "face-to-face" interactive contact and different interface areas' exposure [125]. In fact, the two-dimensional MXene structure can provide a large number of surface groups for creating 2D/2D heterojunctions with other two-dimensional photocatalysts. Close "face to face" contact in a 2D/2D MXene-based heterojunction offers a great deal of potential for increasing photo-induced charge carrier separation and transfer across the heterojunction interface. Photo-induced electrons triggered by the 2D heterostructure catalysts could aggregate onto the surface via electron transfer, thanks to the strong photocurrent and outstanding electronic conductivity of MXene's. The 2D structural platform of MXene can act as an electron mediator, enabling the fast extraction of electrons from 2D materials for better photocatalytic activities [124]. Photocatalytic activities of MXene-based composites are summarized in Table 3.

MXenes-Based Electrocatalysts for HER
Hydrogen energy development provides a cost-effective response to the current environmental and energy problems. In the proposed hydrogen economy, it is necessary to design HER (hydrogen evolution reaction) catalysts with excellent performance, such as high stability, superior conductivity, and selectivity [136]. The electrocatalysts with high performance can reduce the required overpotential for HER, thereby increasing the efficiency [137]. The hydrogen adsorption Gibbs free energy (GH), which is regarded to be the descriptor of HER activity as a first approximation [138], can be calculated using density functional theory (DFT). The best HER activity is achieved when the GH is close to thermal-neutral. MXenes has better potential than other NPM-based HER electrocatalysts and exhibit excellent physicochemical properties. For example, (a) MXene surfaces contain a large number of -O and -OH groups, which can form strong bonds with other semiconductors surfaces. Furthermore, (b) the super electrical conductivity of MXenes allows for efficient charge-carrier transport. Additionally, (c) MXenes have abundant exposed metal sites at their terminals, which allow them to reveal higher redox activity than carbon compounds. (d) MXenes also exhibit high hydrophilicity, which guarantees they make appropriate contact with water molecules. Lastly, (e) MXenes exhibit exceptional chemical and structural stability in aqueous media. In addition, HER electrocatalysts based on MXenes received tremendous attention in recent years, and a great deal of experimental and theoretical work has been performed. Through structural engineering, MXenes have been optimized intrinsically and extrinsically in terms of terminal groups' modification, metal doping, hybridization, and nanoarchitecture. As HER electrocatalysts, MXenes and their composites show great potential in substituting Pt-based catalysts (Table 4) [139].

Termination Modification
MXene termination modification is thought to help in the progress of HER performance by increasing the conductivity and surface termination. The termination modification of MXenes is widely known for optimizing the electronic structure and constitutionally promoting the HER activity. Thus, MXene termination modification has been studied extensively in recent years, both experimentally and theoretically [130]. Experimentally, the 2D MXenes containing O/OH terminal groups displayed metallic characteristics and indicated good charge transfer. Surface Pourbaix diagrams confirmed the experimental findings, demonstrating that this type of MXenes exhibits high surface chemical stability, resulting in a significantly higher exchange current and enhanced hydrogen evolution. The GH was determined, indicating that their surface terminal oxygen atoms serve as active sites for HER. The contact between 2D MXenes and H was aided by these surface oxygen atoms. Further research revealed that the Heyrovsky mechanism was involved in the HER process over these MXenes. MXenes with various surface functionalities (i.e., -OH, -O, -F) were used to study their HER activity. The O-terminated surface MXenes revealed excellent performance compared to those with OH-and F-terminated surfaces [140]. There have been a number of other research studies on termination modification. Based on the ultrathin O-functionalized Ti 3 C 2 MXenes, Jiang et al. [141] developed an effective HER catalyst as depicted in Figure 10a-e. On the basal plane, the equivalent F-functionalized MXenes were harmful to HER, slowing the kinetics of hydrogen adsorption. Ti 3 C 2 O x MXene was made by scattering Ti 3 C 2 T x in an aqueous KOH electrolyte and reducing the F-termination with the OH ones. The resultant Ti 3 C 2 (OH) x was then annealed at 450 • C in an Ar environment by converting the OH groups to O-terminal ones via the dehydration reaction. The Ti 3 C 2 O x nanosheets were regarded as an excellent HER electrocatalyst. It had a 190 mV overpotential at 10 mA cm 2 , significantly lower than those of Ti 3 C 2 (OH) x (217 mV) and Ti 3 C 2 T x -450 (266 mV). The extremely active O-sites on the Ti 3 C 2 O x basal plane resulted in a 60.7 mV dec −1 Tafel slope. This research establishes the groundwork for altering the surface terminal groups of MXenes-based HER electrocatalysts to improve their performance. In short, the functionalities on the basal plane regulate the HER performance of MXenes. The higher concentration of F on the basal plane showed reduced HER activity for Ti 3 C 2 , Mo 2 C, and Mo 2 Ti 2 C 3 Mxenes [142]. For HER, the O-terminated Ti 3 C 2 MXene is a suitable electrocatalyst. MXenes with other surface functionalities (i.e., -Cl, -S, and -Br) have been synthesized in the past, but their HER performance was not studied [143]. Table 4. Summary of the HER performances of MXene-based electrocatalysts [139].

Nitrogen Fixation
Under mild circumstances, the synthesis of ammonia (i.e., N2 + 3H2 → 2NH3) is an interesting and attractive catalytic reaction [144]. Fritz Haber and Carl Bosch discovered and implemented the catalytic conversion of N2 to NH3 at large-scale, currently known as the Haber Bosch (HB) process. This process uses multi-promoted fused Fe catalysts operating at elevated temperatures (i.e., 400-500 °C) and pressures (i.e., 100-300 bar) for NH3 synthesis [145]. Because of the chemical inertness of N2, the severe circumstances require large, yet essential, energy expenditure. In reality, the HB rate-limiting step is kinetically obstructed dissociation. As a result, establishing an efficient and practically applicable N2 fixation technique is critical. For instance, Carbide and Nitride MXenes with 2:1 stoichiometry revealed exceptional N2 activation capabilities, with substantially exothermic adsorption energies [146]. MXenes have been regarded as emerging catalysts for low-pressure (i.e., 1 bar) N2 dissociation, based on the first-principle density functional calculations and micro kinetic simulation studies. Their activation barrier for N2 dissociation is actually inferior to that of Ru nanoparticles. MXenes can adsorb N2 exothermically with relatively high energy of adsorption (i.e., 1.11-3.45 eV) and 20% extension of the N2 bonding length. Compared to the crystal surfaces of Ru (0001), which have energy barriers in the range of 0.4-2.0 eV due to their active sites, this substantially simplifies the dissociation, with an energy barrier less than 1 eV. MXene (W2N) is projected to be the most appropriate electrocatalyst with the smallest energy barrier (i.e., 0.3 eV), and can be easily managed under ordinary pressure and temperature conditions [147]. For instance, Wei et al. [148] reported the fabrication of Ti3C2 MXene nanosheets, which were then transformed into Ti3C2 nanoribbons. The material was employed in N2 fixation for ammonia synthesis. The SEM micrograph (Figure 11a) of the resultant Ti3C2Tx (T = F, OH) after etching with HF revealed a 2D layered structure, demonstrating the effective exfoliation of Al from the Ti3C2Al MAX phase. The TEM micrograph (Figure 11b) clearly revealed a sheet-like structure with a single-layer nanosheet. From the SAED pattern (inset Figure 11b), it was confirmed that the resultant MXene has a monocrystalline nature. After treating the Ti3C2 nanosheets with KOH, the Ti3C2 nanoribbons were formed as clear from the SEM micrograph (Figure 11c). Further, it was confirmed by the HR-TEM micrograph (Figure 11d) that the Ti3C2 nanoribbons exhibit a diameter of approximately 16 nm. The 3D structure of Ti3C2 MXene nanoribbons led to the significantly enhanced N2 fixation performance due to the surface-exposed Ti−OH sites (Figure 11e). Based on the above discussion, it is confirmed that MXenes have promising applications in N2 fixation.

Nitrogen Fixation
Under mild circumstances, the synthesis of ammonia (i.e., N 2 + 3H 2 → 2NH 3 ) is an interesting and attractive catalytic reaction [144]. Fritz Haber and Carl Bosch discovered and implemented the catalytic conversion of N 2 to NH 3 at large-scale, currently known as the Haber Bosch (HB) process. This process uses multi-promoted fused Fe catalysts operating at elevated temperatures (i.e., 400-500 • C) and pressures (i.e., 100-300 bar) for NH 3 synthesis [145]. Because of the chemical inertness of N 2 , the severe circumstances require large, yet essential, energy expenditure. In reality, the HB rate-limiting step is kinetically obstructed dissociation. As a result, establishing an efficient and practically applicable N 2 fixation technique is critical. For instance, Carbide and Nitride MXenes with 2:1 stoichiometry revealed exceptional N 2 activation capabilities, with substantially exothermic adsorption energies [146]. MXenes have been regarded as emerging catalysts for low-pressure (i.e., 1 bar) N 2 dissociation, based on the first-principle density functional calculations and micro kinetic simulation studies. Their activation barrier for N 2 dissociation is actually inferior to that of Ru nanoparticles. MXenes can adsorb N 2 exothermically with relatively high energy of adsorption (i.e., 1.11-3.45 eV) and 20% extension of the N 2 bonding length. Compared to the crystal surfaces of Ru (0001), which have energy barriers in the range of 0.4-2.0 eV due to their active sites, this substantially simplifies the dissociation, with an energy barrier less than 1 eV. MXene (W 2 N) is projected to be the most appropriate electrocatalyst with the smallest energy barrier (i.e., 0.3 eV), and can be easily managed under ordinary pressure and temperature conditions [147]. For instance, Wei et al. [148] reported the fabrication of Ti 3 C 2 MXene nanosheets, which were then transformed into Ti 3 C 2 nanoribbons. The material was employed in N 2 fixation for ammonia synthesis. The SEM micrograph (Figure 11a) of the resultant Ti 3 C 2 T x (T = F, OH) after etching with HF revealed a 2D layered structure, demonstrating the effective exfoliation of Al from the Ti 3 C 2 Al MAX phase. The TEM micrograph (Figure 11b) clearly revealed a sheet-like structure with a single-layer nanosheet. From the SAED pattern (inset Figure 11b), it was confirmed that the resultant MXene has a monocrystalline nature. After treating the Ti 3 C 2 nanosheets with KOH, the Ti 3 C 2 nanoribbons were formed as clear from the SEM micrograph (Figure 11c). Further, it was confirmed by the HR-TEM micrograph (Figure 11d) that the Ti 3 C 2 nanoribbons exhibit a diameter of approximately 16 nm. The 3D structure of Ti 3 C 2 MXene nanoribbons led to the significantly enhanced N 2 fixation performance due to the surface-exposed Ti−OH sites (Figure 11e). Based on the above discussion, it is confirmed that MXenes have promising applications in N 2 fixation.

Gas Sensing
Gas sensing is an emerging research direction because it is employed in air quality control for monitoring pollution, breath analysis, and diagnosis [149,150]. Aside from highly reactive gases such as NO2 and H2S, the detection of volatile organic compounds (VOCs) at parts per million (ppm) concentrations is crucial for the early identification of stern disorders such as peptic ulcers. Usually, the low electrical noise and high-signal throughput are the two critical problems in developing a sensitive gas sensor [151]. This is likely in the case that the sensing platform can accommodate both the high conduction and abundant active sites for interfacial interactions. A trade-off relationship occurs in traditional materials, such as graphene, metal oxides, and black phosphorous (PB), which classifies them as appropriate but not optimal for sensing applications [152,153]. MXenes provide numerous advantages in gas sensing due to their highly conductive surfaces with appropriate dangling functions and adsorption active sites [154]. MXene (Ti3C2Tx) has been widely researched as a substrate for detecting various gases including acetone (CH3COCH3), ethanol (C2H5OH), ammonia (NH3), propanol (C2H5CHO), and some highly reactive and hazardous gases such as NO2, SO2, and CO2. The gas response was detected in terms of the change in the resistance of the gas adsorption substrate (i.e., Ti3C2Tx), standardized against the N2 gas resistance baseline. Unlike conventional materials, Ti3C2Tx shows a positive change in resistance for all gases, independent of their composition. This indicates that MXene exhibits the universal capability of the adsorption of gases. The molecules of gases interact with the surface functionalities of Ti3C2Tx, altering its resistance and lowering its conductivity. In addition, the maximum responses toward ammonia, propanal, acetone, and ethanol were observed to be 0.8%, 0.88%, 0.97%, and 1.7%, respectively, with lower responses for SO2, NO2, and CO2. Further, the sensitivity response of the sensor was investigated to clarify whether it varies with MXene sheets thickness. The DFT results revealed that the metal-like conductivity and high energy of adsorption of the surface terminal moieties are responsible for the sensor's sensitive response. This was intriguing since it implied that Ti3C2Tx's unique functionalities may be utilized to enable selective gas sensing. W. Yuan and his co-worker converted MXene into a 3D polymer network to improve its sensitivity for VOCs such as ethanol, methanol, and acetone. An electrospinning method was employed to form a 3D network, which involved the mixing of aqueous electrolytes of the positively charge polymers (PVA/PEI) and the negatively charged MXene (Ti3C2Tx). The electrostatic interaction allowed the MXene nano-flakes to self-assemble over the polymer network via

Gas Sensing
Gas sensing is an emerging research direction because it is employed in air quality control for monitoring pollution, breath analysis, and diagnosis [149,150]. Aside from highly reactive gases such as NO 2 and H 2 S, the detection of volatile organic compounds (VOCs) at parts per million (ppm) concentrations is crucial for the early identification of stern disorders such as peptic ulcers. Usually, the low electrical noise and high-signal throughput are the two critical problems in developing a sensitive gas sensor [151]. This is likely in the case that the sensing platform can accommodate both the high conduction and abundant active sites for interfacial interactions. A trade-off relationship occurs in traditional materials, such as graphene, metal oxides, and black phosphorous (PB), which classifies them as appropriate but not optimal for sensing applications [152,153]. MXenes provide numerous advantages in gas sensing due to their highly conductive surfaces with appropriate dangling functions and adsorption active sites [154]. MXene (Ti 3 C 2 T x ) has been widely researched as a substrate for detecting various gases including acetone (CH 3 COCH 3 ), ethanol (C 2 H 5 OH), ammonia (NH 3 ), propanol (C 2 H 5 CHO), and some highly reactive and hazardous gases such as NO 2 , SO 2 , and CO 2 . The gas response was detected in terms of the change in the resistance of the gas adsorption substrate (i.e., Ti 3 C 2 T x ), standardized against the N 2 gas resistance baseline. Unlike conventional materials, Ti 3 C 2 T x shows a positive change in resistance for all gases, independent of their composition. This indicates that MXene exhibits the universal capability of the adsorption of gases. The molecules of gases interact with the surface functionalities of Ti 3 C 2 T x , altering its resistance and lowering its conductivity. In addition, the maximum responses toward ammonia, propanal, acetone, and ethanol were observed to be 0.8%, 0.88%, 0.97%, and 1.7%, respectively, with lower responses for SO 2 , NO 2 , and CO 2 . Further, the sensitivity response of the sensor was investigated to clarify whether it varies with MXene sheets thickness. The DFT results revealed that the metal-like conductivity and high energy of adsorption of the surface terminal moieties are responsible for the sensor's sensitive response. This was intriguing since it implied that Ti 3 C 2 T x 's unique functionalities may be utilized to enable selective gas sensing. W. Yuan and his co-worker converted MXene into a 3D polymer network to improve its sensitivity for VOCs such as ethanol, methanol, and acetone. An electrospinning method was employed to form a 3D network, which involved the mixing of aqueous electrolytes of the positively charge polymers (PVA/PEI) and the negatively charged MXene (Ti 3 C 2 T x ). The electrostatic interaction allowed the MXene nano-flakes to self-assemble over the polymer network via the electrospinning process and resulted in the development of a 3D porous fibrous network. This interconnected porous nature of the network allowed the diffusion of large molecules of gas and VOCs detection, even at low concentrations of 0.1-0.17 ppm, with detection up to 50 ppb. Further, the sensing ability of the 3D porous architecture was compared with that of the bare MXene, which was exposed to 5 ppm of the VOC gases, and its signal response was twice that of the bare Ti 3 C 2 T x . This reveals the potential significance of the 3D network [155]. In another report, MXene (V 2 CT x ) was used to construct a sensor via a more contemporary technique. Though the majority of early work on gas sensors was based on Ti 3 C 2 T x , this study revealed that MXenes with abundant compositions can be employed as promising materials for gas sensing applications. De-laminated V 2 CT x sheets (Figure 12a) were used to make an inter-digitated platinum electrode that could detect polar and non-polar gases such as H 2 and CH 4 . In addition, the responses for ethanol, acetone, NH 3 , CH 4 , H 2 , and H 2 S were 0.0816, 0.0226, 0.0166, 0.0167, 0.2435, and 0.005 (Figure 12b,c). Further, the real-time sensing response of the V 2 CT x sensor towards varying concentrations of different gases, including (d) hydrogen, (e) acetone, (f) methane, and (g) H 2 S, is depicted in Figure 12d-g. MXenes based on vanadium exhibit high sensitivity towards H 2 gas rather than the Ti 3 C 2 T x MXene (highly sensitive towards NH 3 ). Thus, altering the mixture of MXene materials could form different interfacial contacts, affecting the selectivity toward various gases. Another technique was to investigate the interlayer swelling of Ti 3 C 2 T x to clarify how it effects the capability of gas sensing [156]. The gas sensing performance of MXene-based composites is summarized in Table 5. the electrospinning process and resulted in the development of a 3D porous fibrous network. This interconnected porous nature of the network allowed the diffusion of large molecules of gas and VOCs detection, even at low concentrations of 0.1-0.17 ppm, with detection up to 50 ppb. Further, the sensing ability of the 3D porous architecture was compared with that of the bare MXene, which was exposed to 5 ppm of the VOC gases, and its signal response was twice that of the bare Ti3C2Tx. This reveals the potential significance of the 3D network [155]. In another report, MXene (V2CTx) was used to construct a sensor via a more contemporary technique. Though the majority of early work on gas sensors was based on Ti3C2Tx, this study revealed that MXenes with abundant compositions can be employed as promising materials for gas sensing applications.
De-laminated V2CTx sheets (Figure 12a) were used to make an inter-digitated platinum electrode that could detect polar and non-polar gases such as H2 and CH4. In addition, the responses for ethanol, acetone, NH3, CH4, H2, and H2S were 0.0816, 0.0226, 0.0166, 0.0167, 0.2435, and 0.005 (Figure 12b,c). Further, the real-time sensing response of the V2CTx sensor towards varying concentrations of different gases, including (d) hydrogen, (e) acetone, (f) methane, and (g) H2S, is depicted in Figure 12d-g. MXenes based on vanadium exhibit high sensitivity towards H2 gas rather than the Ti3C2Tx MXene (highly sensitive towards NH3). Thus, altering the mixture of MXene materials could form different interfacial contacts, affecting the selectivity toward various gases. Another technique was to investigate the interlayer swelling of Ti3C2Tx to clarify how it effects the capability of gas sensing [156]. The gas sensing performance of MXene-based composites is summarized in Table 5.

Supercapacitors
Due to their exceptional metallic conductivity, MXenes are frequently used in supercapacitor applications to assure fast electron transport; particularly, Ti 3 C 2 T x is the most promising candidate. It was discovered that metal ions can spontaneously or electrochemically intercalate 2D MXenes, indicating that MXene has the capacity to store energy [164]. The ions span the shallow and deep adsorption sites in MXenes and could be accommodated initially at low adsorption sites along the particle's edges, and subsequently, at the deeper sites in the particle's interior, which exhibit high activation energy of the ion adsorption. As a result, the ions may be adsorbed quickly on the shallower absorption sites at high scan speeds to confirm an exact volume of charge storage. This makes MXenes the most attractive choice with increased rate performance. Further, the cation intercalation may affect the fluctuation of the water confined between layers. The nanosheets of MXenes are interpolated with kosmotropic ions (i.e., Mg 2 + , Li + , and Al 3 + ) particularly in the partially hydrated form without any dehydration, whereas the chaotropi -ions (i.e., TEA + and Cs + ) effectively dehydrate the Mxenes [165]. During cycling, fluctuations in c-values are frequently caused by the ions' extraction from and insertion into aqueous media. In contrast to the interlayer gap extension in graphite, different ions in MXene cause different deformation behaviors. If the interpolated cations exhibit higher charges and a short ionic radius, the interlayer gap between the Ti 3 C 2 T x MXene nanosheets will be reduced, whereas cations with low charges and a large ionic radius would result in the extension of interlayers in MXene.

Nonaqueous Systems
As mentioned earlier, the hydrated-cations intercalation in electrodes of MXene occurs the in aqueous media. Inside the insignificant potential window of aqueous media, an electric double-layer is formed by the hydrated cations in the interlayer section, which results in a distinctive capacitance performance. Nevertheless, the solvated cations are interpolated into the space region of interlayers in the primary phase of the charging cycle. While employing the non-aqueous electrolytes, collapse of the solvation shell occurs because of the significant difference in the inner potential of the interlayer space region. Thus, the desolvated ions entirely interpolate upon successive charging, and overlapping of their atomic orbitals occurs with MXenes orbitals to form a donor band. The expansion of this donor band reduces MXenes and results in intercalation pseudo-capacitance owing to the transfer of charges from these ions to the MXene sheets [166].
When compared to the aqueous environments, the electrochemical behavior of Ti 3 C 2 T x MXene in nonaqueous fluids is radically different. Nonaqueous environments such as organic electrolytes and ionic liquids have been extensively studied due to the broad potential windows, resulting in high energy densities. For instance, Lin et al. [167] investigated the Ti 3 C 2 T x MXene electrochemistry in an ionic-liquid electrolyte (i.e., EMITFSI in acetonitrile). At 0.2 V and 0.6 V, the cyclic voltammograms displayed large peaks. In fact, MXene with these terminal groups is electrochemically inactive in ionic-liquid electrolytes, and the pseudo-capacitance features are not expected to occur in aqueous systems as compared to the Ti 3 C 2 T x Mxene [88]. These peaks result from the intercalation of the EMI + and TFSI ions in Ti 3 C 2 T x MXene in both negative and positive voltage windows. At the Ti 3 C 2 T x /electrolyte (LiPF 6 /EC-DMC) interface, partial desolvation of Li + ions are seen, following the charge storage (redox reactions) after intercalation. These redox reactions take place over a widespread range of voltage values (i.e., 0.05 V to 3 V). It happens in a row, resulting in a series of large redox peaks related to the intercalation of Li + ions.

MXenes-Composites
Approximately 30 different types of MXene have been reported, and several types have been utilized for applications in supercapacitors. Many considerations, such as the scalability, electrochemical activity, etc., must be addressed when utilizing MXene as an active material. Thus, different modification strategies of MXene, such as surface termination, interlayer spacing, electrodes topologies, and constructing composites, are critical in determining the supercapacitor electrochemical performance. MXenes are mostly made by selective etching of their precursor layers, and the MA bond energy in various MAX-phases limits the etching environments. The bond energies of the 413 MAX (V 4 AlC 3 ) phases, for example, are frequently higher than those of the 312 or 211 MAX (V 2 AlC) phases [168]. In order to etch a layer of the 413 MAX phase, this disparity leads to a longer duration and higher HF concentrations. The M 4 X 3 T x also has a low specific capacitance because of the moderately high molecular mass, and for that reason, it is rarely used as an electrode material. However, despite the multiple layers, V 4 C 3 T x exhibits poor performances. It has a higher capacitance than the delaminated V 2 CT x and the multi-layered Ti 3 C 2 T x and Ti 2 CT x . It is worth noting that, by increasing the "n" value of M n+1 X n T x MXene, its cyclic stability can be remarkably improved. In fact, the Ti 3 C 2 T x MXene has the best electrochemical performance among all different types, although its fabrication method is too simple. Thus, it is of the utmost importance to investigate other types of MXenes.
Ti 3 C 2 T x is the most-investigated MXene in supercapacitor applications [169]. The volumetric capacitance of the Ti 3 C 2 T x electrode in neutral, as well as basic, media was found to be 300-400F/cm 3 . Hence, the outstanding results outperform the current purecarbon-based state-of-the-art EDLC and are even comparable to activated graphene-based electrode materials (i.e., capacitance 350F/cm 3 ) [170]. Nevertheless, the cyclic voltammograms' profile varies depending on the positive ions, referred to as the "capacitors" [171]. Subsequently, the protons, as the smallest cations, have contact with the largest number of electro-chemically active sites, and a volumetric capacitance (>900 F/cm 3 ) was obtained in 1 M H 2 SO 4 for pure laminated Ti 3 C 2 T x clay electrodes. The Li + ions avoid the MXene sheets, assembling with interpolated H 2 O molecules during fabrication [172]. The MXene-based composites for supercapacitor applications are summarized in Table 6.

Cancer Therapy
MXenes have recently been used as multifunctional biological material for a variety of purposes, including fluorescence imaging [57], computed-tomography imaging, photoacoustic imaging [181], photo-thermal therapy, as well as photo-dynamic therapy [182,183]. The X-ray, PL, and near-infrared (NIR) light adsorption capabilities of MXenes are based on their tunable quantum confinement, localized-surface-plasmon resonance, as well as edge. The Ti 3 C 2 MXene was firstly employed in photothermal therapy, which revealed an exceptional in vivo photothermal therapy efficacy. Since MXenes are composed of flexible elements that occupy the in-layer space, early translation metals could be chosen precisely to improve the element-based multi-functionalities of MXenes nanomaterials. As a rarely investigated MXene type, the two-dimensional Ta 4 C 3 contains a biocompatible Ta element and has exceptional performance in the photothermal therapy of tumors. The Ta 4 C 3 MXene has a broad absorption spectrum and exhibits a strong absorption band, almost similar to those of the two-dimensional graphene and MoS 2 semiconductors. Thus, this material provides the necessary photo-absorption characteristics for the photothermal transduction process and exhibits the exceptional photothermal conversion efficiency of 44.7%, which is higher than most of the reported inorganic photothermal materials [184][185][186]. Some of the MXene-based nanomaterials were utilized as active theranostic platforms by integrating photo-acoustic computed tomography imaging with photo-thermal therapy. Integrating chemical therapy with photothermal therapy will help increase their cancer therapeutic efficiency even more. However, because there is no restricted region on the surface of MXenes, loading the medicine is problematic. Moreover, there is a lack of amine functionalities on MXenes for further functionalization, making the probable targeting alterations and surface engineering for precise goals such as target delivery/therapy more difficult. MXene coatings with biocompatible porous nanomaterials are one of the possible approaches in this scenario. For the first time, Li et al. [187] and his colleagues fabricated core@shell MXene/mesoporous silica. The silica layers with a mesoporous structure not only carry the therapeutic molecules DOX but also allow for the alteration of cancer cells using polyethylene glycol (PEG) and active target materials such as the three amino acid peptides, i.e., arginine, glycine, ad aspartic acid (RGD). They created a new theranostic nanoplatform, Ti 3 C 2 @mMSNs-RGD, by combining the photothermal effect of MXene (Figure 13a). DOX-Ti 3 C 2 @mMSNs-RGD exhibits pH-and NIR-activated release behavior (Figure 13b,c), and the acidity responsive characteristic allows for the controlled release of the DOX in tumors, where the micro-environment is slightly acidic (i.e., pH 5.8-7.1) and the intra-cellular environment is highly acidic (i.e., pH 5.0). Concurrently, the NIR-activated drug release is mostly restricted to the tumor cells, enhancing the outcome of chemical therapy. DOXTi 3 C 2 @mMSNs-RGD has a stronger cancer therapeutic impact than DOX, Ti 3 C 2 @mMSNs, and Ti 3 C 2 @mMSNs-RGD due to its targeted delivery capabilities. This work paves the road for aMXene/ordered mesoporous structure composite production and application. Afterward, these researchers improved the platforms of MXene and mesoporous silica by altering loading molecules and MXenes, thereby replacing the DOX with mesopores, creating agent such as CTAC to streamline the synthesis process and swapping Ti 3 C 2 for Nb 2 C to obtain an NIR-II photothermal response [188]. They loaded the thermally decomposable radicals initiator into the mesopores to design a new thermodynamically cancer therapeutic system [189]. In fact, the MXenes' combination with mesoporous materials offer a promising strategy for the design and fabrication of new emerging theranostic nanoplatforms for the efficient treatment of tumor cells.
cancer therapeutic system [189]. In fact, the MXenes' combination with mesoporous materials offer a promising strategy for the design and fabrication of new emerging theranostic nanoplatforms for the efficient treatment of tumor cells.

Summary and Outlook
MXenes, or 2D transition metal carbides and nitrides, have received tremendous research attention in in the field of environmental science, biomedical technology, catalysis, energy conversion and storage, etc. The exceptional surface chemistry (i.e., termination with oxides and hydroxides functional groups), interesting optoelectronic properties, layered structure, and high charge carriers' density of MXenes allowed for excellent performances in supercapacitors and next-generation energy conversion and storage devices. In summary, the unique features of MXenes and their prospective uses are mostly determined by manufacturing processes, structure, morphology, composition, and the number and variety of surface functionalities. Optimizing the synthesis techniques results in the improvement of MXenes with tunable and controllable properties. Progress, on the other hand, is foreseen, justifiable, and required to sustain the extension of MXenes. In the last five years, 2D carbides, nitrides, and carbonitrides have been fabricated by the selective exfoliation and etching of the layered ternary precursors, which resulted in a broad family of 2D MXenes. The hydrofluoric acid (HF) etching, modified-acid etching, molten salts etching, modified fluoride-based acid etching, and fluoride-free etching methods lead to the synthesis of MXenes with hydrophilic surfaces. In water-based colloidal solutions with high surface charges (i.e., negative zeta-potential greater than 30 mV), they are stable and do not need any surfactant for stabilization. This allows MXenes to be used in applications of device printing and coatings and films production. Furthermore, because of their exceptional physicochemical and opto-electronic features, MQDs have become a more popular branch of QD materials. Many investigations have revealed that QDs can be synthesized in a variety of methods. The top-down and bottom-up approaches are the most common in the synthesis of MQDs. The precursors of 2D MXenes were frequently industrialized by etching the MAX phases in high concentrations of acidic solutions (i.e., HF or LiF + HCl), which is highly toxic and harmful to human health. Thus, it is vital to explore the fluoride-free and environmentally benign etchants alternative to the conventional HF for MXenes fabrication. Meanwhile, there has been little investigation into bottom-up approaches for the fabrication of MQDs. Nevertheless, several of its benefits, such as the improved atomic usage, morphology and dimension modification, and better structural control, imply that it is a

Summary and Outlook
MXenes, or 2D transition metal carbides and nitrides, have received tremendous research attention in in the field of environmental science, biomedical technology, catalysis, energy conversion and storage, etc. The exceptional surface chemistry (i.e., termination with oxides and hydroxides functional groups), interesting optoelectronic properties, layered structure, and high charge carriers' density of MXenes allowed for excellent performances in supercapacitors and next-generation energy conversion and storage devices. In summary, the unique features of MXenes and their prospective uses are mostly determined by manufacturing processes, structure, morphology, composition, and the number and variety of surface functionalities. Optimizing the synthesis techniques results in the improvement of MXenes with tunable and controllable properties. Progress, on the other hand, is foreseen, justifiable, and required to sustain the extension of MXenes. In the last five years, 2D carbides, nitrides, and carbonitrides have been fabricated by the selective exfoliation and etching of the layered ternary precursors, which resulted in a broad family of 2D MXenes. The hydrofluoric acid (HF) etching, modified-acid etching, molten salts etching, modified fluoride-based acid etching, and fluoride-free etching methods lead to the synthesis of MXenes with hydrophilic surfaces. In water-based colloidal solutions with high surface charges (i.e., negative zeta-potential greater than 30 mV), they are stable and do not need any surfactant for stabilization. This allows MXenes to be used in applications of device printing and coatings and films production. Furthermore, because of their exceptional physicochemical and opto-electronic features, MQDs have become a more popular branch of QD materials. Many investigations have revealed that QDs can be synthesized in a variety of methods. The top-down and bottom-up approaches are the most common in the synthesis of MQDs. The precursors of 2D MXenes were frequently industrialized by etching the MAX phases in high concentrations of acidic solutions (i.e., HF or LiF + HCl), which is highly toxic and harmful to human health. Thus, it is vital to explore the fluoridefree and environmentally benign etchants alternative to the conventional HF for MXenes fabrication. Meanwhile, there has been little investigation into bottom-up approaches for the fabrication of MQDs. Nevertheless, several of its benefits, such as the improved atomic usage, morphology and dimension modification, and better structural control, imply that it is a viable method to which more attention should be devoted. Furthermore, the stability and monodisperse nature are critical for improving the physicochemical features and practical use of MQDs, requiring the development of new technologies. Furthermore, precisely controlling the surface chemical properties of MQDs, as well as the formation of pure MQDs, is difficult. This is because that the fabrication process is mostly direct and suitable for adapting MQDs properties for specific applications.
In addition, MXenes have been suggested to exhibit optical, thermoelectric, and mechanical capabilities in addition to their appealing structure. Even due to the existence of surface functionalities and H 2 O water molecules between their layers, Ti 3 C 2 T x has been found to have high metallic conductivity. On the other hand, Mo 2 CT x and Mo 2 TiC 2 T x behave similarly to semiconductors. For oxygen-terminated Ti 2 C, a bandgap of roughly 0.9 eV was estimated, and the Dirac cones are predictable for the fluorine-terminated Ti 2 C MXene. However, the prediction of many other properties is still awaiting experimental confirmation.
Furthermore, the photocatalytic hydrogen generation over a Ti 3 C 2 MXene photocatalyst provides new options for sustainable, clean, and renewable energy sources. Enormous efforts have been devoted to enhancing their solar-to-energy conversion efficiency. The unique structure and outstanding electrical features of Ti 3 C 2 MXene allow it the distinctive ability for the generation of a Schottky barrier for electron trapping and employment as a cocatalyst to derive H 2 generation. It is worth noting that the electrical and electronic features of Ti 3 C 2 MXene play a vital role in boosting its efficiency. It is hypothesized that the incorporation of Ti 3 C 2 MXene in various Ti 3 C 2 MXene-based composites with metal-organic frameworks could remarkably enhance the physicochemical properties of the semiconductors in different ways, serving as effective support materials, lowering potential barriers, promoting carrier dynamics, and as electron trapping agents [190,191]. Thus, MXenes and MXene-based composite materials will have potential applications in various research fields including energy conversion and storage, catalysis, gas sensing, and phototherapy.

Future Perspectives
Future studies may comprise the effect of various precursor materials on the performance and fabrication of flawless surfaces of free functionality MXenes, the design and characterization of various carbide and nitride MXenes, and methods to optimize the top-down and the bottom-up approaches by selectively etching the layered precursors without the element's "A".
In addition, understanding the impact of MXene's structural design, such as the surface chemistry, interlayer gaps, and nanolayers stacking, on the charge storage mechanisms is of great significance. The process of increasing the gap between the nanolayers of MXenes should be further studied to promote the dynamics of ion diffusion and interpolation.
Further, the energy-storage mechanism of MXene-based materials is the main focus of future research, and the impact of thermal and chemical stability for energy storage performance is also of the utmost importance. The advancement in the fabrication of MXenes-based composites will reveal potential applications in the energy-storage field. Experimental studies on Ti 3 C 2 MXene's termination groups should be further extended. This is to clearly illustrate the importance of each terminal group in promoting MXenes' stability and photocatalytic performance. For example, it should be clarified which terminating group is best suited to promoting the photocatalytic H 2 evolution or other reactions.
For supercapacitor applications, the MXenes surface area is lower than that of graphene. If its specific surface area is brought to the level of graphene, this would likely increase its energy density and specific capacitance. The improvement can be made by adopting new approaches such as treatment with a concentrated acid and base to introduce pores. This would allow more space for hosting the pseudocapacitive particles and accommodate high-voltage electrolytic ions to enhance the energy density. The current surge in various metal (i.e., Li, Na, K, Zn) ions-based capacitors should be pushed forward to realize this high voltage.
It is investigated that the fabrication of MXenes is scalable, which means no property loss occurs when its batch size is increased from 1 to 50 g. However, it is of great significance to improve its batch size on the kilogram scale. For this, a chemical engineering technique should be designed to further scale-up MXenes' production. Improvements should be made to yield precursors of the MAX phase in a cost-effective fashion. Additionally, cost-efficient synthesis methods should be utilized, i.e., self-propagating high-temperature fabrication from minerals or oxides precursors.
For Ti-based MXenes (Ti, Al, C), the components are quite reasonable. However, for other MXenes (Ta, Hf, V), it is necessary to adjust the precursor cost, otherwise these MXenes will have limited industrial applications. It is vital to stress that MXenes are materials, not chemicals, and there is a direct relationship between their production and physicochemical properties, such as environmental stability and electrical conductivity. Thus, it is necessary to understand the relationship between the fabrication and processing of the MAX-phase precursors and MXenes properties. The current state-of-the-art can be upgraded by recognizing less aggressive etchants and accelerating the fabrication process of MXenes.
Through operand characterization techniques, Ti 3 C 2 MXene matrix materials can be deeply investigated, thereby obtaining basic knowledge of the relationship between the molecular structure and activity of Ti 3 C 2 MXene and its selectivity. This technology can conduct extensive research on the stability of Ti 3 C 2 MXenes under different reaction conditions, understand the properties of the catalytic site of Ti 3 C 2 MXenes' reaction mechanism, and help improve the role of Ti 3 C 2 MXene in the field of photocatalysis.