Surface Terminations of MXene: Synthesis, Characterization, and Properties

Abstract

MXene, 2D transition metal carbides, nitrides, and carbonitrides with a unique 2D structure, inspired a series of function applications related to energy storage and conversion, biometrics and sensing, lighting, purification, and separation. Its surface terminations are confined by the adjacent MXene layers, and form the 2D planar space with symmetrical surfaces, which is similar to a 2D nanoreactor that can be utilized and determined MXene’s function. Based on the working principle, surface and interface play critical roles in the ion intercalation, physical/chemical adsorption, and chemical reaction process, and show significant effects on MXene’s properties and functions. Although there have been some reviews on MXene, less attention has been paid to the underlying principle of the involved surface chemistry, controllable design, and resultant properties. Herein, the regulation methods, characterization techniques, and the effects on properties of MXene surface terminations were summarized to understand the surface effects, and the relationship between the terminations and properties. We expected this review can offer the route for a series of ongoing studies to address the MXene surface environment and the guidelines for MXene’s application.

1. Introduction

Since the first 2D layered Ti₃C₂T_x in 2011, transition metal carbides, nitrides, or carbonitrides (MXene), as a family of 2D layered structure materials, have rigorously emerged [1]. It is well-established that the ternary carbides and nitrides with a M_n+1AX_n formula, M is an early transition metal, A is an A-group (mostly groups 13 and 14) element, and X is C and/or N, forming laminated structures with anisotropic properties. These, the so-called MAX phases are layered hexagonal (space group P63/mmc), with two formula units per unit cell. The near-close-packed M-layers are interleaved with pure A-group element layers, with the X-atoms filling the octahedral sites between the former. Over 60 MAX phases are currently known to exist. Upon the development of the initial synthesis methods and the new synthetic precursor of MAX, a variety of stable phases MXene (more than 200 kinds, mainly Ti₃C₂T_x, Ti₂CT_x, Nb₂CT_x, Mo₂CT_x, Ti₄N₃T_x, Ta₄C₃T_x, Cr₂TiC₂T_x, V₂CT_x, Zr₃C₂T_x, (Nb_0.8Zr_0.2)₄C₃T_x and so on) have been determined and predicted [2], as shown in Figure 1. Owing to the unusual electronic [3], mechanical [4] and optical properties, numerous functions were explored in batteries [5], supercapacitors [6], photocatalysts [7], catalysts [8], transparent conducting films [9], electromagnetic interference shielding [10], sensors [11], adsorption agents [12], and flexible high-strength composites [13].

However, due to the presence of a large number of exposed metal atoms on the surface of MXene, the structure of MXene is easily destroyed by oxidation [17]. MXene is also prone to irreversible lamellar stacking in preparation and application [18]. Mechanical strength is insufficient [19], poor solubility in non-polar polymer or weakly polar polymer [20], low energy density, poor multiplier performance [21], and poor environmental stability [22].

MXene surface terminations have a significant influence on the stability, electronic structure, and other physical and chemical properties of MXene [23]. The controllable regulation of MXene surface terminations is an effective prerequisite to obtaining MXene materials with differential structural characteristics and properties. At present, the regulation of MXene surface terminations mainly starts from material preparation and post-treatment, and then selects different types or proportions of functional groups according to different application scenarios. For example, the lithium-ion storage capacity of MXene is largely determined by the properties of MXene surface terminations [23]. MXene surface terminations can be modified to remove -F and -OH surface groups that hinder the transport of Li ions, to obtain electrode materials with high lithium storage capacity and high-rate performance. In addition, by modifying MXene surface terminations T_x (-OH, -F, and -O) to adjust the carrier transport behavior of Ti₂CT_x MXene with high mobility and appropriate bandgap can expand the application range of MXene [8,9,10,11]. Moreover, high surface electrical loading of MXene, the physical modification of organic groups, or the loading functional agents on the surface of MXene have proved that the appropriate surface modification and functionalization can improve the performance of MXene in nanomedicine and broaden its potential biomedical application [24]. Moreover, the surface functional groups can affect the interlayer distance, energy band structure, and work function of MXene, which determine several properties and functions. The targeted surface modification and functionalization have proved that it can improve the performances of MXene in nanomedicine, electrocatalysis, thermodynamics, electromagnetism, biology, tribological and fluid mechanics. For example, the -O functional group is HER active site, and the high -F surface coverage is more conductive. Therefore, adjusting the type and proportion of functional groups is a strategy to improve the performance of electrocatalysis.

MXene surface terminations play an important role in interface-related applications such as thermodynamics, electromagnetism, biology, tribological and fluid mechanics. The different surface terminations of MXene own the preparation method and the experimental requirement, which need to be strictly controlled. Moreover, the successful preparation of MXene with different surface terminations requires a large number of comparison tests and characterization, and there may be differences in physical and chemical properties due to the uncertainty of the number of functional groups. Therefore, the key point of using MXene surface terminations regulation is to develop its regulatory methods, characterization techniques, and understand the structure-activity relationship between MXene terminations and properties and performance.

This review summarizes the synthesis route and characterization methods of MXene with differentiated terminations, as well as the effects of different terminations on the mechanical, optical, and magnetic properties of MXene. In addition, the role of MXene termination for its application function is also discussed. This is summarized in Figure 2. At the same time, the key research of MXene surface termination is prospected.

2. Surface Termination Regulation

MXene prepared by fluoride-containing etching method can obtain these surface terminations of -O, -F, and -OH. Owing to the preparation method, the types of MXene surface terminations are very simple, which limits the research on MXene surface terminations and the interaction between MXene surface terminations and properties [25,26]. Therefore, the key to expanding MXene surface termination is to develop the regulation method of MXene surface termination and new MXene preparation technology. With the development of the fluorine-free etching method and Lewis acid molten salt etching method [27], Ti₃C₂ MXene with single -Cl, -Br, -I, -O, and -S as terminations were developed. Ti₃C₂ MXene with binary (-ClBr, -ClI, and -BrI), and ternary (-ClBrI) halogen terminations were also synthesized through the Lewis-acidic-melt etching route [28]. In addition, the surface termination of MXene can be eliminated, replaced, and esterified by hydrothermal and solvothermal methods. Ti₃C₂, Ti₂C, and Nb₂C MXene with the surface terminations of -Se, -Te, -NH, and phosphorous and ester were developed [29,30]. As shown in Figure 3, the most common processes for preparing MXene are summarized. The development of the preparation technology of MXene terminations provides material science basis for the research on MXene surface terminations, and also provides more technical routes for the performance regulation of MXene as functional materials.

2.1. HF/Fluoride Etching

In 2011, as a sign of MXene materials emergence, Yury Gogotsi and Michel W. Barsoum reported the pioneering work of preparing multilayer Ti₃C₂F₂ and Ti₃C₂(OH)₂ by etching Ti₃AlC₂ MAX phase ceramic materials with HF [31]. This is also one of the most used methods to synthesize MXene. As shown in Figure 4a, the HF etching reaction and surface functionalization mechanism are described as follows [25,32]:

$${\mathrm{M}}_{\mathrm{n}+1}{\mathrm{AX}}_{\mathrm{n}}+3\mathrm{HF}\to {\mathrm{AF}}_3+1.5{\mathrm{H}}_2+{\mathrm{M}}_{\mathrm{n}+1}{\mathrm{X}}_{\mathrm{n}}$$

(1)

$${\mathrm{M}}_{\mathrm{n}+1}{\mathrm{X}}_{\mathrm{n}}+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{M}}_{\mathrm{n}+1}{\mathrm{X}}_{\mathrm{n}}{\left(\mathrm{OH}\right)}_2+{\mathrm{H}}_2$$

(2)

$${\mathrm{M}}_{\mathrm{n}+1}{\mathrm{X}}_{\mathrm{n}}+2\mathrm{HF}\to {\mathrm{M}}_{\mathrm{n}+1}{\mathrm{X}}_{\mathrm{n}}{\mathrm{F}}_2+{\mathrm{H}}_2$$

(3)

In Equation (1), the M-A in the MAX phase ceramics is a metal bond, which is weaker than the M-X bond. Due to the difference in the binding energy, HF reacts with the A-layer elements in the MAX phase to generate AF₃, H_2, and the corresponding M_n+1X_n phase [24]. However, M_n+1X_n, which has lost elements in the A-layer, reacts with the surrounding water or HF to form M_n+1X_n(OH)₂ with OH or/and F functional groups, corresponding to Equations (2) and (3). Figure 4b shows the representative steps of Ti₃C₂T_x MXene synthesis from Ti₃AlC₂, and the structure after surface functional group modification [33].

Figure 4. (a) Schematic diagram of the synthesis of MXene by HF etching [24]. (b) The synthesis and structure diagram of Ti₃C₂T_x MXene [25].

Figure 4. (a) Schematic diagram of the synthesis of MXene by HF etching [24]. (b) The synthesis and structure diagram of Ti₃C₂T_x MXene [25].

Another reaction method is to use acid/fluoride salt solution etching MAX phase ceramics, such as HCl/LiF [34,35,36], HCl/NH₄F, HCl/FeF₃ [37,38] and H₂SO₄/LiF [39,40]. As shown in Figure 5, Lipatov et al. added LiF powder to concentrated HCl and stirred it to dissolve completely. Subsequently, Ti₃AlC₂ MAX powder was slowly added to the LiF/HCl dispersion, and the etching process was carried out in a water bath at 35 °C for 24 h under magnetic stirring. The resulting mixture was washed repeatedly with deionized water until a dark supernatant with a pH of approximately 6. After mild ultrasonic treatment, the mixture was centrifuged, and the supernatant was collected and freeze-dried to obtain MXene powder with -O, -OH, and -F functional groups [41].

Although the direct etching of precursor MAX phase by HF is the most widely used method to prepare MXene, it is potentially dangerous to humans and will produce a large amount of waste acid and liquid, which is not conducive to environmental protection. Therefore, it is necessary to develop a more moderate preparation method. Moreover, in an aqueous liquid environment, it is difficult for M of MXene to avoid contact with -OH and -F in a liquid environment during the reaction process, so the surface terminations formed by the discussed above are mostly -O, -OH, and -F. The ratio of these three functional groups is difficult to control. Due to the competition of metal ions, the content of termination -F on the surface of MXene via fluoride salt is relatively less. In addition, the fresher the sample is, the higher the -OH content is, with the increase of storage time, some -OH transform into -O.

2.2. Molten Salt Etching Method

To date, tremendous efforts have been made to develop synthetic methods of MXene. Based on the principle that transition metal halides are electron acceptors and can undergo displacement reaction with atomic layer A of MAX phase in the molten state, MXene materials can be prepared by etching MAX phase with transition metal halides in a molten state. As shown in Figure 6, taking Ti₃AlC₂ MAX as an example, the transition element atom (Zn) occupies the A site in the preexisting MAX phase structure. A new MAX phase Ti₃ZnC₂ was synthesized by the substitution reaction between the Zn element in molten ZnCl₂ and the Al element in the precursor of MAX phase (Ti₃AlC₂). The formation mechanism can be expressed as the following simplified reaction:

$${\mathrm{Ti}}_3{\mathrm{AlC}}_2+1.5{\mathrm{ZnCl}}_2\to {\mathrm{Ti}}_3{\mathrm{ZnC}}_2+0.5\mathrm{Zn}+{\mathrm{AlCl}}_3\uparrow$$

(4)

$${\mathrm{Ti}}_3{\mathrm{AlC}}_2+1.5{\mathrm{ZnCl}}_2\to {\mathrm{Ti}}_3{\mathrm{C}}_2+1.5\mathrm{Zn}+{\mathrm{AlCl}}_3\uparrow$$

(5)

$${\mathrm{Ti}}_3{\mathrm{C}}_2+\mathrm{Zn}\to {\mathrm{Ti}}_3{\mathrm{ZnC}}_2$$

(6)

It is well known that the melting point of ZnCl₂ is about 280 °C, and in the molten state, it ionizes into Zn²⁺ and ZnCl₄²⁻ tetrahedra [42,43]. Coordination unsaturated Zn²⁺ is a strong acceptor of Cl⁻ and electrons, which act as Lewis acidic in ZnCl₂ molten salts. In this acidic environment, the weakly bonded Al atoms in Ti₃AlC₂ can be easily converted to Al³⁺ by a redox reaction (reaction 5). The resulting Al³⁺ will further combine with Cl⁻ to form AlCl₃, which has a boiling point of about 180 °C and is expected to evaporate rapidly at the reaction temperature (550 °C). At the same time, the in-situ reduced zinc atoms intercalated into the Ti₃C₂ MXene layer and filled the A site of the MAX phase, thus forming Ti₃ZnC₂. The evaporation of AlCl₃ provides the driving force for the outward diffusion of Al atoms, while the a-site vacancies of the MAX phase enable the insertion of Zn atoms into the Ti₃C₂ MXene layer. When excessive amounts of ZnCl₂ are used, Ti₃ZnC₂ subsequently spall off to form -Cl terminating MXene (Ti₃C₂Cl₂) due to the strong Lewis acidity of molten ZnCl₂. Figure 7 shows the theoretical calculation of the synthesis mechanism of Ti₃C₂Cl₂ by electrochemical corrosion assisted by molten salt.

Compared with HF etching method, the chemical process of forming eutectic in Al-Zn system is safer and cleaner [27]. This kind of MXene with -Cl as termination prepared by molten salt also provides an experimental and theoretical basis for the subsequent development of more functional groups of MXene. Based on obtaining -Cl termination MXene, it can also be changed into other functional groups by the molten salt method [44]. As shown in Figure 8, -Cl termination MXene was firstly prepared by LiCl-KCl salt, heating, and electrolysis, etc. Then Li₂O or Li₂S was added to the melting system, and -O or -S termination MXene was obtained after high-temperature cleaning. The synthesis mechanism is as follows:

$${\mathrm{Ti}}_3{\mathrm{AlC}}_2+3{\mathrm{Cl}}^{-}\to {\mathrm{AlCl}}_3\uparrow +{\mathrm{Ti}}_3{\mathrm{C}}_2+3{\mathrm{e}}^{-}$$

(7)

$${\mathrm{Ti}}_3{\mathrm{C}}_2+2{\mathrm{Cl}}^{-}\to {\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{Cl}}_2+2{\mathrm{e}}^{-}$$

(8)

$$\mathrm{Cathode}:{\mathrm{Li}}^{+}\left({\mathrm{K}}^{+}\right)+{\mathrm{e}}^{-}\to \mathrm{Li}\left(\mathrm{K}\right)$$

(9)

$${\mathrm{Ti}}_3{\mathrm{AlC}}_2+5\mathrm{LiCl}\left(\mathrm{KCl}\right)\to {\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{Cl}}_2+5\mathrm{Li}\left(\mathrm{K}\right)+{\mathrm{AlCl}}_3\uparrow$$

(10)

This simple and environmentally friendly process to prepare MXene with controllable functional groups is extremely important. Since electrons are used as reactants during the reaction, cathodic reduction and anodic etching can be spatially isolated. Therefore, there are no metal impurities in Ti₃C₂Cl₂ MXene. In addition, surface terminations were modified in situ from -Cl to -O and/or -S by adding various inorganic salts, which greatly shortened the modification step and enriched the variety of surface terminations. In addition, no acidic waste liquid is produced after synthesis, and the salt can be reused, making MS-E etching a sustainable method [45,46,47,48].

In addition, MXene with single or multiple halogen terminations can be prepared by the molten salt method. Taking Ti₃C₂ MXene powder as an example, Ti₃C₂ MXene with halogen termination group was prepared by molten salt reaction between Ti₃AlC₂ and the corresponding copper halide molten salt.

Table 1. Composition of the starting materials for preparing the halogenated MXene [28].

Samples	Molar Ratio
Ti3C2Cl2	Ti3AlC2/CuCl2 = 1/4
Ti3C2Br2	Ti3AlC2/CuBr2 = 1/4
Ti3C2I2	Ti3AlC2/CuI = 1/6
Ti3C2(ClBr)	Ti3AlC2/CuCl2/CuBr2 = 1/1/4
Ti3C2(ClI)	Ti3AlC2/CuCl2/CuI = 1/1/6
Ti3C2(BrI)	Ti3AlC2/CuBr2/CuI = 1/1/6
Ti3C2(ClBrI)	Ti3AlC2/CuCl2//CuBr2/CuI = 1/1/1/6

lists the composition of the initial material. Thoroughly mix the starting material with mortar under nitrogen protection in the glove box. The resulting powder mixture was then removed from the glove box and placed in an alumina crucible. The alumina crucible was loaded into a tubular furnace and heat treated at 700 °C for 7 h under argon protection to complete the reaction. Note that the reaction product is a mixture of MXene, residual salt, and copper as a by-product. The product was washed with deionized water to remove residual CuCl₂ and CuBr₂, and then further washed with NH₄Cl/NH₃·H₂O solution to remove copper and CuI. The copper removal process had no significant effect on the composition and morphology of MXene. After washing the NH₄Cl/NH₃·H₂O solution, the products were dried under vacuum at 40 °C to obtain Ti₃C₂ MXene with halogen terminations.

Talapin et al. used Lewis acidic CdBr₂ to extend the molten salt etching route beyond chloride, and Ti₃C₂Br₂ and Ti₂CBr₂ were prepared. The experimental results show that MXene with -Cl and -Br as surface terminations can exchange with other free ions, so that the surface chemical environment of MXene materials can be regulated. MXene surface exchange reactions typically require temperatures of 300 °C to 600 °C, which are difficult to achieve by using conventional solvents. In contrast, these molten alkali metal halides as solvents can overcome this problem and have high solubility for various ionic compounds. For example, Ti₃C₂Br₂ dispersed in CsBr/KBr/LiBr eutectic reacts with Li₂Te and Li₂S to form new Ti₃C₂Te and Ti₃C₂S MXene, respectively. Ti₃C₂Cl₂ and Ti₃C₂Br₂ react with Li₂Se, Li₂O and NaNH₂ to produce Ti₃C₂Se, Ti₃C₂O and Ti₃C₂(NH) MXene [29], respectively. The reactions of Ti₃C₂Br₂ and Ti₂CBr₂ with LiH at 300 °C produced bare Ti₃C₂□₂ and Ti₂C□₂, where □ stands for the vacancy site. The process can be described as the reduction and elimination of hydride groups after the exchange reaction. Finally, MXene with -O, -NH, -S, -Cl, -Se, -Br, and -Te surface terminations and without surface terminations were successfully synthesized by substitution and elimination reactions. The strategies for synthesis and removal of surface groups are shown in Figure 9.

The preparation of MXene by the molten salt method has mild experimental conditions, but it needs to be etched at high temperatures for a long time under the protection of inert gas, which is not conducive to large-scale preparation. Therefore, it is still of great research significance to explore a safe and efficient preparation method for MXene.

2.3. Ammoniation

Peng et al. introduced the termination of amino (-NH₂) on the surface of MXene, and the schematic is shown in Figure 10. First, in a Teflon beaker, dissolve LiF in HCl and stir. Then, Ti₃AlC₂ powder was slowly added to the LiF+HCl solution and stirred continuously at 35 °C for 24 h. The resulting mixture was then centrifuged and continuously rinsed with deionized water until the pH of 6. Subsequently, the ammonia solution was added to the obtained grout and sonicated after manual shaking. The suspension was centrifuged again to remove unpeeled Ti₃C₂ MXene. The suspension of layered aminated Ti₃C₂ MXene was sonicated under nitrogen protection. Subsequently, the mixer was transferred to an autoclave and heated to 70 °C for 4 h. Finally, the supernatant was centrifuged and continuously rinsed with deionized water until the pH of the supernatant was approximately 7. The sediment was freeze-dried to obtain MXene-NH₂ nanosheets [49,50]. The team proposed a substitution reaction to explain the amination process, and speculated that the positive charge in NH₄⁺ could partially replace the -OH termination group. It is well known that the formation energy of Ti-N is higher [29,51]. Considering the moderate amination process, Ti₃C₂ cannot directly obtain the Ti-N bond in an ammonia solution. In addition, the surface of MXene was coated with -OH, -O, and -F after etching with a mixed solution of LiF and HCl. According to previous reports, the binding energy of Ti-OH was the lowest [26]. It can be inferred that under mild conditions it is easily replaced by other groups. Therefore, the substitution of the hydroxyl group by an amino group is a possible reaction pathway for the formation of Ti-NH₂. In Figure 9, the positively charged NH₄⁺ attacks the negatively charged -OH group to the Ti element with lower binding energy. Finally, part of the -OH group is replaced by the -NH₂ group to form H₂O [49].

2.4. Displacement and Esterification Method

The surface modification can also be carried out by substitution reaction, esterification reaction, and other methods. For example, Jacob et al. reported an important surface modification method by covalently attaching the polymer of PEG6-COOH to MXene nanosheets through an esterification reaction, as shown in Figure 11. Firstly, the aqueous dispersions of Ti₃C₂ MXene were transferred to dimethylformamide (DMF) by repeated operations of ultrasound and centrifugation until completely dispersed. DMF dispersion of Ti₃C₂ MXene binds PEG6-COOH and 4-dimethyl aminopyridine (DMAP). The combination of these components results in the appearance of small particles due to the assembly of -OH and -F formed by the intermolecular hydrogen bonds between PEG6-COOH and the Ti₃C₂ surface. Then, under the environment of nitrogen, a chloroform solution of dicyclohexyl carbon diimide (DCC) was added to initiate esterification. After continuous agitation, the small particles disappear, forming an optically transparent solution. Covalent surface modification with ω-functionalized PEG6 ligands (ω-PEG6-COOH, ω: -NH₂, -N₃, -CH=CH₂) can regulate the types and quantities of MXene functional groups. Moreover, covalent ester bonds are chemically more stable than nitrogen bonds and electrostatic interactions [38]. On the surface of PEG6-functionalized Ti₃C₂, most of the hydrogen bond interactions between the sheets disappear due to ester bond formation.

2.5. Phase Transfer Method

Daesin et al. prepared stable Ti₃C₂T_x MXene dispersions in non-polar organic solvents by interfacial chemical grafting reaction and phase transfer method [30]. As shown in Figure 12, the alkyl phosphonic acid (CnPA) ligand was chemically grafted to the -OH terminus of Ti₃C₂T_x MXene sheet at room temperature through interface nucleophilic addition and sequential condensation reaction to form covalent Ti-O-P bond:

$$\mathrm{Ti}-\mathrm{OH}+\mathrm{P}-\mathrm{OH}\to \mathrm{Ti}-\mathrm{O}-\mathrm{P}+{\mathrm{H}}_2\mathrm{O}$$

(11)

Unlike conventional surface chemical modification methods with complex steps, this is a simple synthesis process that can prepare surface-modified Ti₃C₂T_x MXene in a variety of organic solvents ranging from polar to non-polar. The dispersion of Ti₃C₂T_x MXene in non-polar chloroform also has strong oxidation resistance and stable long-term storage.

3. Characterization of MXene’s Terminations

Accurate characterization of MXene surface terminations is the prerequisite for studying the regulatory methods and the influence of properties and performance. Because the surface termination of MXene presents two-dimensional plane distribution [52] and the collision probability of atoms, molecules, electrons, and optical signals is low. MXene prepared by chemical methods has certain impurity content [53,54]. In addition, most surface terminations do not exist in a stable state, which leads to the difficulty of the accurate characterization of MXene surface terminations. At present, XPS, EDX, XAS and EELS are often used for qualitative and quantitative analysis of MXene surface terminations.

3.1. XRD

Although X-ray diffraction (XRD) cannot directly capture the differences of elements in the surface terminations of MXene, the evolution of MXene interlayer space induced by the dissimilar surface terminations can be deduced via the calculated MXene lattice parameters. As a basic crystal structure characterization technology, the statistical analysis of MXene crystal structure by XRD can indirectly reflect the modulating action of MXene surface termination on MXene crystal structure and the interlayer environment. Based on the information of new peak appearance, peak position shift, and half-peak width change, the surface termination atom arrangement and its induced structural evolution can be analyzed indirectly.

Figure 13a shows the XRD patterns of the initial phase Ti₃AlC₂ and intermediate product Ti₃ZnC₂. The XRD peaks of Ti₃ZnC₂, such as (103) and (104), shift to a lower angle than those of Ti₃AlC₂, indicating that the substitution of Al atoms by Zn atoms results in a larger lattice constant. The relative intensities of peaks (004) and (006) increase, while the relative intensities of peaks (002) decrease, resulting from structural factor changes caused by the substitution of A atoms [27]. Figure 13b shows the XRD patterns of the reaction products Ti₃C₂Cl₂ and HCl rinsed products. Except for the diffraction peak characteristics of zinc, the XRD patterns of the reaction products are similar to those of Ti₃C₂ MXene prepared by the HF etching method. The (104) peak of Ti₃AlC₂ is significantly weakened or disappeared, in addition to the downward shift of the (002) peak to a lower Angle, attributed to the increased C lattice parameters. The C lattice parameter is consistent with the theoretical value calculated by DFT and larger than Ti₃C₂T_x generated by HF etching [2]. Note that unlike the typical wide (00L) peak of Ti₃C₂T_x produced by HF etching, the (00L) peak of Ti₃C₂Cl₂ is sharp and intense, indicating an ordered crystal structure. Therefore, MXene prepared by this method with -Cl as the surface termination has a stronger XRD peak and a more orderly crystal structure. Other MXene with different functional groups also have different manifestations in the XRD. As shown in Figure 13c, the (002) characteristic peaks of Ti₃C₂-O and Ti₃C₂-S obtained by treating molten salt-assisted electrochemical (MS-E) etched Ti₃C₂Cl₂ with Li₂O and Li₂S were further shifted from 8.18° to lower angles of 7.48° and 6.88°. This is attributed to the C lattice parameters ranging from 21.9 Å to 23.6 Å and 25.9 Å [44].

Figure 13d shows the XRD patterns of MXene and Ti₃C₂ (OF) at halogen terminations. It is found that Ti₃C₂Cl₂ and Ti₃C₂ (OF) have strong (002) peaks, corresponding to the C values of 20.90 and 22.22 Å, respectively. The (002) peaks of Ti₃C₂Br₂ and Ti₃C₂I₂ are not very clear, but the (004) peaks are strong enough. Sufficient to determine their respective C values of 23.33 and 25.00 Å, which are slightly larger than the theoretical values calculated by density functional theory (DFT) (23.02 and 24.24 Å). This indicates that the layer spacing of MXene connecting halogen groups increases successively with the radius of halogen groups. The increased layer spacing in Ti₃C₂Br₂ and Ti₃C₂I₂ can be interpreted as an increase in the atomic radius of the halogen group from fluorine to iodine. Different termination atoms will lead to different structure factors of MXene, resulting in different relative intensities of the (00L) peak. The c-value of binary and ternary halogen-terminated MXene is between that of monoid halogen-terminated MXene. For example, the C value of Ti₃C₂ (BrI) (24.06 Å) is larger than that of Ti₃C₂Br₂, but smaller than that of Ti₃C₂I₂ [28].

Figure 13e shows the XRD patterns of conventional MXene and MXene with amino surface functionalization. Compared with the peak (002) of MXene at 6.21°, the peak (002) shifted to a large angle of 6.63° after -NH₂ functionalization. It shows that their D-spacing decreases from 1.42 nm to 1.33 nm. The decrease in D-spacing is due to the introduction of positively charged amino groups on the surface of MXene-NH₂, which reduces the repulsive force of the original negative charge on the MXene nanosheet. Therefore, the small D-spacing in MXene-NH₂ can be judged by XRD [55].

3.2. SEM and EDS

Figure 14a,b show SEM images of Ti₃AlC₂ and Ti₃ZnC₂ powders. The Ti₃AlC₂ precursor exhibits a typical MAX phase layered structure. In contrast, the layered structure of Ti₃ZnC₂ becomes less pronounced, probably due to the dissolution of the powder edges in the molten salt. Ti₃C₂Cl₂ was obtained by adding excessive zinc chloride. Figure 14c,d shows the semi-quantitative analysis of SEM of Ti₃C₂Cl₂ and the corresponding EDS (energy dispersive spectroscopy). Typical accordion-like morphology was found, and the main elements were Ti (43.2 at.%), C (21.5 at.%), and Cl (25.3 at.%). As well, small amounts of Zn (0.7 at.%), Al (2.9 at.%), and O (6.3 at.%). The presence of Zn, Al, and O is plausible because ubiquitous O-containing compounds, such as Al(OH)₃, are hydrolysates of AlCl₃.

Shen et al. prepared -O termination MXene and -S termination MXene by adding Li₂O or Li₂S to the melting system to modify Ti₃C₂T_x powder. SEM shows that the modified Ti₃C₂T_x powder still maintained accordion-like multilayer morphology (Figure 14e,f) and was tightly stacked. In EDS analysis, element Cl was not detected, and elements O and S were the main components of surface terminations.

Huang et al. prepared Ti₃C₂Cl₂, Ti₃C₂Br₂, Ti₃C₂I₂, Ti₃C₂(ClBr), Ti₃C₂(ClI), Ti₃C₂(BrI) and Ti₃C₂(ClBrI) using halogen atoms as terminations by molten salt method. The morphology and elemental composition were characterized by SEM and EDS, as shown in Figure 14g–m. The SEM confirmed that MXene has typical accordion morphology, and EDS analysis obtained their main composition elements (Ti, C) and end-group elements (Cl, Br, I), which can judge the morphology and preliminary functional success of MXene, indicating that halogen atoms almost completely occupy the termination position of the surface.

Based on the combination of SEM and EDX, the surface terminations of MXene in the microzone can be observed, and the information on element species, distribution, and content can be preliminarily obtained. As a fast and efficient means of characterization, SEM+EDX can achieve high-throughput data collection, which provides support for the study of MXene surface terminations.

3.3. XPS

XPS can accurately measure the inner electron binding energy and chemical shift of atoms, provide information on molecular structure and valence state, and provide information on element composition and content, chemical state, and molecular structure. In addition, because the X-ray beam incident on the sample surface is a photon beam, the damage to the sample is very small. XPS is used as the most conventional means to analyze the surface terminations of MXene. With the accurate representation of XPS elements, the valence states, chemical bonds, and chemical shifts of surface termination elements are collected. Meanwhile, the mechanism of the surface termination formation process can be obtained by comparing the XPS high-resolution spectrogram. Providing support for the development of differentiated surface terminations is one of the most powerful characterization techniques in MXene surface termination research.

XPS spectra of Ti 2p and Cl 2p of Ti₃C₂Cl₂ are shown in Figure 15a. The peaks at 454.4 eV and 455.7 eV are assigned to the Ti-C(I) (2p_3/2) and Ti-C (II) (2p_3/2) bonds [56,57]. The peak at 458.1 eV belongs to the high-valence Ti compound, which is assigned to the Ti-Cl (2p_3/2) bond [58,59]. In addition, the peaks at 460.3 eV, 461.8 eV and 464.1 eV are attributed to Ti-C (I) (2p_1/2), Ti-C (II) (2p_1/2) and Ti-Cl (2p_1/2) bonds, respectively. The right picture of Figure 14a shows its Cl 2p spectrum. The peaks at 198.6 eV and 200.1 eV are consistent with the positions of Cl-Ti (2p_1/2) and Cl-Ti (2p_3/2) bonds [58,59]. This confirms the existence of Ti-Cl bonds. The Ti-Cl ratio determined by XPS analysis was 2.94:2.

High-resolution Ti 2p spectra (Figure 15b) also show that Ti₃C₂Cl₂ forms a Ti-O (2P_3/2) bond after treatment in Licl-KCl-Li₂O, so that there is no Ti-Cl bond at 458.9 eV and a new peak occurs. For Ti₃C₂T_x treated with Li₂S, the Ti-Cl bond was also not visible. There are two peaks at 459.2 eV and 456.8 eV, which are attributed to Ti-O (2p_3/2) and Ti-S bond (2p_3/2), respectively [29]. These results indicate that -Cl surface terminations change to surface terminations with -O and/or -S.

XPS can also confirm the amino groups on the surface of MXene, as shown in Figure 15c. It can be seen that there are only four major elements C, F, Ti, and O in the XPS spectrum of conventional MXene. However, the N 1s peak appears in the measured spectra of MXene-NH₂ films and CMC/MXene-NH₂ composite films. The evolution of the N 1s spectrum shows five peaks that can be attributed to the N-Ti, N-C, -NH₂, -NH₄⁺ and -NH₃⁺ bonds, respectively. -NH₄⁺ and -NH₃⁺ should result from the protonation of the NH₃·H₂O and -NH₂ groups, respectively. The Ti 2p high-resolution spectrum shows two peaks at 455.3 eV and 463.2 eV, originating from the Ti-N and Ti-F bonds, respectively. The C-N bond in MXene-NH₂ was also shown. Thus, the N element in MXene-NH₂ consists of Ti-N, C-N, -NH₂, and -NH₃⁺ forms. Importantly, the appearance of -NH₂, -NH₃⁺ and Ti-N bonds would be attributed to the introduction of amino-termini on the surface of MXene.

The XPS measurement spectra of Ti₃C₂Br₂ and Ti₃C₂Te MXene were compared, as shown in Figure 15d. After surface group exchange in CsBr/LiBr/KBr molten salt, the Br peak was replaced by the Te peak. The Cs 3D peak corresponds to the intercalated Cs⁺ ions or residual Cs⁺. Figure 15e shows the high-resolution XPS spectrum of Ti₃C₂Br₂ MXene and Ti₃C₂Te MXene. The binding energy of the Ti-C component of Ti₃C₂Br₂ MXene shifts to a lower value after Br^- replaces Te^2-. This result is consistent with the fact that Te is less electronegative than Br. Figure 15f shows the XPS spectrum of Ti₃C₂ (NH) MXene, measured with the highlighted N 1s region. The elemental analysis of the measured spectrum shows that the Ti:N ratio is 3:1.1, and the analysis of the high-resolution N 1s spectrum shows that there are three components. The 396.2 eV peak (59%) belongs to the β-N chemisorbed on the Ti surface [61]. The 397.5 eV peak (26%) corresponds to α-N₂ chemically adsorbed on the titanium surface. The 399.9 eV peak (16%) may correspond to the N-H bond. Figure 15g shows the high-resolution XPS spectra of Ti₃C₂□₂ MXene. Compared with Ti₃C₂Br₂ MXene in Figure 14e, the Ti-C component binding energy to a lower value after Br^- has been eliminated with H^-.

In addition, Chen et al. compared the XPS spectra of original Ti₃C₂T_x MXene and MXene modified by -SO₃H (Ti₃C₂Tx-PSS), which prepared Ti₃C₂T_x MXene composite membranes by intercalating MXene nanosheets with sodium 4-styrene sulfonate (PSS), a spacer containing sulfonic acid group (SO₃H). The spacer introduced abundant -SO₃H groups into the membrane, as shown in Figure 16a,b. Compared with the F 1s spectra of Ti₃C₂T_x MXene, the signals of F-C and F-Ti move from 687.00 eV and 684.98 eV to 686.36 eV and 684.94 eV, respectively, indicating that these sites bind protons from the PSS. In addition, the S 2p XPS spectrum of MXene-PSS shows two peaks representing S 2P_1/2 and S 2p_3/2, in contrast to the S 2p spectrum of MXenewithout any peaks. This indicates that a large number of -SO₃H groups were successfully introduced into the membrane.

XPS is used to analyze the MXene surface termination, realizing the accurate confirmation of complex multi-element and hybrid terminations. With the help of high-resolution characteristic element spectrography analysis, combined with computational simulation, the fine structure of MXene surface termination can be obtained. From qualitative to quantitative, provide technical support for MXene surface analysis. However, it is also limited to surface analysis, and is difficult to obtain termination information. It is easy to be affected by the external environment and this peak fitting is subjective.

3.4. TEM

TEM and spherical aberration-corrected electron microscopy can directly observe the atomic composition of MXene at the atomic scale, and the surface terminations of heavy elements can be directly observed [60]. With the help of sample preparation techniques such as frozen section and FBI, ultrathin samples can be obtained, and the arrangement of spatial terminations in the MXene layer can be observed. Depending on the support of testing methods such as EDX and EELS, atomic identification, valence analysis, and coordination relationship judgment can be realized at the atomic scale. In addition, the structure analysis of MXene can be realized by selecting the comparison between electron diffraction and XRD.

Figure 17a is a TEM image of Ti₃C₂Cl₂ MXene with organ-like morphology. Figure 17b is a TEM image of MXene-NH₂ similar to graphene. In addition, the HRTEM image and its selected area electron diffraction pattern (inset) confirm that the crystal structure of the aminated nanosheets is the same as that of conventional MXene. Sardar et al. Modified MXene with PEG6 and observed the morphology by TEM. Figure 17c shows TEM images of pristine Ti₃C₂ MXene flakes and modified Ti₃C₂-PEG6, respectively. Any small nanoparticles formed in the form of TiO₂ nanoparticles in the Ti₃C₂-PEG6 flakes were not observed. In addition, the length of Ti₃C₂-PEG6 flakes is in the order of microns. These results indicate that the chemical modification of the Ti₃C₂ MXene flakes does not change the size of the MXene flakes and does not lead to any oxidation of the MXene flakes. This is an important discovery for maintaining the structural integrity of the Ti₃C₂ MXene flakes. Yu et al. achieved surface amino functionalization of Nb₂CT_x MXene by hydrazine treatment. Figure 17d,e are TEM and HRTEM images of pristine P-Nb₂CT_x and T-Nb₂CT_x MXene with amino functionalization. All the Nb₂CT_x MXene exhibited a typical 2D morphology, and the size and thickness of Nb₂CT_x MXene did not change before and after treatment. HRTEM images of P-Nb₂CT_x and T-Nb₂CT_x MXene nanosheets indicate that the lattice parameters of P-Nb₂CT_x and T-Nb₂CT_x MXene nanosheets are 0.27 nm. In addition, the corresponding selected area electron diffraction (SAED) patterns of P-Nb₂CT_x and T-Nb₂CT_x MXene nanosheets are shown in the inset. This means that the P-Nb₂CT_x and T-Nb₂CT_x MXene nanosheets have hexagonal structures.

Karlsson et al. used STEM-EELS to study the chemical and structural properties of individual Ti₃C₂ MXene sheets with associated surface groups at the atomic level, as shown in Figure 18. MXene show inhomogeneous coverage of surface groups that locally affect chemical properties. According to the EELS measurement results, the average components of Ti, C, O, N and F were 47, 33, 20, 3, and 1 at.%, respectively, ignoring the edge effect of a single MXene sheet. Double layer MXene sheets corresponding to 46, 29, 24, 2, and 1 at.%. For multilayer MXene sheets, 40, 24, 31, 2, and 3 at.%, respectively. This indicates that C decreases while F and O groups strongly increase upon a larger thickness of MXene, and the presence of N is also revealed by EELS. As well, while EELS can recognize O as an element and spatially link O to surface groups, it cannot distinguish between molecules [63,64]. In contrast, XPS was unable to distinguish the number of sheets but identified the presence of Ti-bound O_x, (OH)_x and absorbed H₂O as well as a small fraction of TiO_x.

3.5. Raman and FTIR

Raman spectroscopy is an analytical method based on the Raman scattering effect discovered by the Indian scientist C.V. Raman, which analyzes the scattering spectrum with different frequencies from the incident light to obtain information on molecular vibration and rotation, and is applied to the study of molecular structure. Raman spectrometer features: (1) No contact or damage to the sample, and the sample does not need to be prepared. (2) Rapid analysis and identification of the characteristics and structure of various materials. (3) Suitable for black water and water samples, accurate measurement in high, low temperature, and high-pressure conditions. Application directions of Raman spectroscopy: Raman spectroscopy is a molecular structure characterization technique based on the Raman effect, and its signal comes from the correlation between molecular vibration and rotation. The analysis directions of Raman spectroscopy are as follows:

• Qualitative analysis: Different substances have different characteristic spectra, so qualitative analysis can be carried out by spectra.
• Structure analysis: The analysis of spectral bands is the basis of material structure analysis.
• Quantitative analysis: According to the absorbance characteristics of the spectrum of substances, one can have a good ability to analyze the amount of substances.

MXene spectrum mainly has four characteristic regions: formant, thin section, T_x area, and carbon area. MXene can be characterized according to the change of peak displacement, intensity, width, and other characteristics in the characteristic region. Different surface terminations, intercalated, and adsorbed species may affect the lattice vibrations. Unit cell distortion and distribution of surface groups result in peak shifting and broadening. Both HF-HCl and LiF-HCl etched Ti₃C₂T_x spectra show strong -OH surface groups and similar A_lg (C) peak positions. However, the -O component dominates the HF etched multilayer Ti₃C₂T_x spectrum, and the peak position of A_1g shifts to 711 cm⁻¹, mainly because the addition of HCl reduces the pH value and leads to surface protonation. Talapin et al. measured the Raman spectra of S-terminated MXene using 632 nm laser excitation, and the peak positions of the out-of-plane (A_1g) and in-plane (E_g) vibrational modes corresponding to the surface groups are particularly sensitive to MXene surface chemistry. Raman spectra of Ti₃C₂Cl₂, Ti₃C₂S (Figure 19a), and Ti₃C₂NH (Figure 19b) MXene show the characteristic A_1g and E_g vibrational modes. The Raman spectrum is different from that of Ti₃C₂O MXene (Figure 19c). In particular, for Ti₃C₂Cl₂ MXene, the position of the A_1g mode remains at about 170 cm⁻¹; for the position of the Ti₃C₂S MXene, A_1g mode is kept at 180 cm⁻¹; for the position of the Ti₃C₂NH MXene, A_1g mode remains at about 210 cm⁻¹. The A_1g pattern is completely different from that of Ti₃C₂O MXene at 222 cm⁻¹. The position of the A_1g mode is mainly determined by the atomic mass of the surface group, and Br and Se lead to lower frequencies than Cl and S. The E_g mode corresponds to the in-plane vibration of the surface group and the external Ti atom has a similar tendency [29]. The 153, 235, and 277 cm⁻¹ characteristic peaks of the Ti₃C₂ vibrational E_g, A_1g, and E_u modes appear in the bare Ti₃C₂ MXene Raman spectra. The characteristic peaks of Ti₃C₂ vibration E_g and A_1g modes in Ti₃C₂-NH, Ti₃C₂-O, and Ti₃C₂T_x are blue-shifted. As well, the E_g mode of the in-plane vibration of the surface group does not exist in the region of 300–400 cm⁻¹ where a characteristic peak appears corresponding to the E_g mode of the in-plane vibration of the surface group. The assignment of the bare Ti₃C₂ vibrational modes is based on the work of Hu et al. The appearance of the Raman-forbidden E_u mode at 277 cm⁻¹ may be related to the disorder present in the stack of bare Ti₃C₂ MXene sheets.

Raman spectroscopic analysis of Ti₃C₂-PEG6 flakes is shown in Figure 19d. First, the peak at 123 cm⁻¹ is the result of the incident laser interacting with the plasmon resonance of the material. In addition, the planar peaks of (Ti, C, O) are clearly visible near 203 cm⁻¹ and at 725 cm⁻¹. Second, the Raman peak position of A_1g is slightly shifted to the left when PEG6 is esterified to form a link, and there is a similar change when the surface group is changed. The most important feature is the absence of Raman stretching due to possible oxidation products during the processing and analysis of the Ti₃C₂ MXene. If there is obvious oxidation in the PEG6-functionalized Ti₃C₂ MXene sample, it is expected that a very pronounced Raman stretching will be observed due to the formation of TiO₂ [38]. Raman spectra show that there is no obvious oxidation of Ti₃C₂ MXene flakes during the synthesis process, which is prevented by the extremely mild reaction conditions.

In particular, Raman can not only clarify the molecular structure of materials, but also illustrate the reaction process and energy conversion mechanism. To prove the energy conversion mechanism of halogenated MXene, the structural and compositional changes of Ti₃C₂Br₂ and Ti₃C₂I₂ under different charge-discharge states were studied. As shown in Figure 19e, f, a broad Raman shift at 260–270 cm⁻¹, corresponding to Br_n⁻ (3 ≤ n ≤ 5), was detected in the Ti₃C₂Br₂ electrode upon charging. Considering the reversible reaction, Br⁻+ (n − 1) Br⁰↔ Br_n⁻, the formation of Br_n⁻ indicates the generation of Br⁰ (Br₂) due to the oxidation of Br⁻. Upon discharge, after trapping electrons, the peak of Br_n⁻ disappears because Br⁰ is restored to Br⁻ [28].

In addition, a recent study showed that the low sensitivity of Raman scattering was improved by using gold or silver tips, increasing Raman scattering by six orders of magnitude. Although Raman spectroscopy of monolayer MXene sheets can be studied, Raman imaging of single sheets has not been reported so far.

FTIR is an infrared spectrometer based on the principle of the Fourier transform of infrared light after interference. It can be used for qualitative and quantitative analysis of samples, not only to measure the absorption and reflection spectra of various gas, solid and liquid samples, but also to measure chemical reactions in a short time.

In order to determine the amino end groups on the surface of Ti₃C₂, FTIR spectroscopy was performed. As shown in Figure 20a, MXene-NH₂ also has -F, -O, -OH groups similar to traditional MXene. However, the appearance of two characteristic peaks can be observed. Generally, the characteristic peak at 3313 cm⁻¹ corresponds to the N-H stretching vibration, and the peak at 1617 cm⁻¹ is caused by the N-H bending vibration. Therefore, it can be preliminarily proved that amino groups are successfully introduced on the surface of MXene. In addition, the peak of the 1650 cm⁻¹ CMC/MXene-NH₂ film was assigned to the amide group (-NH-CO-) stretching vibration, which was attributed to the thermal reaction between the carboxyl group in the CMC and the amino group on the MXene-NH₂ film. Thus, it further confirms the amination termination on the surface of MXene-NH₂. This also makes the MXene-NH₂ nanosheets in the CMC/MXene-NH₂ composite film more compactly stacked. These interfacial covalent bonds will also impart enhanced mechanical properties to the CMC/MXene-NH₂ film. Figure 20b shows the FTIR spectra of pure Ti₃C₂ MXene flakes and surface modification with PEG6-COOH. The green box indicates the presence of -OH in the sheet structure, and the green line represents a typical surface group bound to a Ti atom, such as the vibrational mode of Ti-O at ~550 cm⁻¹. The characteristic low-energy vibrational mode (green dashed line) remains unchanged even after the attachment of PEG6 (see blue spectrum). The O-H stretch at ~3400 cm⁻¹ is absent, and it is thought that this may be due to the formation of -COO bonds by the esterification chemistry of all -OH. In addition, the carbonyl stretch of pristine Ti₃C₂ is present in the Ti₃C₂-PEG6 flakes, as well as an additional carbonyl stretch at 1704 cm⁻¹, derived from the ester bond carbonyl. Therefore, the FTIR characterization strongly supports the covalent surface modification of pristine Ti₃C₂ MXene with PEG6-COOH through ester bond formation.

3.6. Others

Figure 21a shows Ti₃C₂ with Cl atoms as surface terminations. Ti₃C₂Cl₂ MXene shows good ordering along the basal plane, which is consistent with the strong (000l) peak in the XRD pattern. The electronic band structure and phonon spectrum calculated by DFT for a single layer of Ti₃C₂Cl₂, which is essentially a metallic layer with a finite density of states at the Fermi level, are shown in Figure 21b. The phonon spectra also show that all phonon frequencies are positive, i.e., Ti₃C₂Cl₂ MXene is dynamically stable [27].

Figure 21c shows the HAADF-STEM image of Ti₃C₂Br₂ and the corresponding EDS elemental map. Br element was uniformly distributed on the particles, and no element segregation was observed. Figure 21d, HR-TEM image of Ti₃C₂Br₂ shows that the d value of the (0002) plane is ~11.77 Å, which is consistent with the XRD result [28]. Figure 21e,f are atomic resolution scanning transmission electron microscopy (STEM) images, and the corresponding EDS elemental maps show the -Br terminations of the Ti₃C₂ surface. The brightness of the atoms depends on the mass-dependent scattering conditions, demonstrating also the successful functionalization of the MXene surface by Br. The atomic and electronic structures of two single-halogen-terminated MXene, Ti₃C₂Br_2, and Ti₃C₂I₂, were investigated by density functional theory (DFT) calculations, as shown in Figure 21g. Atomic and electronic structures of Ti₃C₂Br₂ and Ti₃C₂I₂. -Br and -I preferentially occupy the FCC site, which is similar to -Cl in Ti₃C₂Cl₂ and -O and -F in Ti₃C₂(OF). For the interplanar spacing of Ti₃C₂Br₂ and Ti₃C₂I₂, the halogen terminations and the outermost Ti layer are 1.86 and 2.0606 Å, respectively, corresponding to the strong bonding of Ti-Br with a bond length of 2.6464 Å and the weaker bonding of Ti-I with a bond length of 2.8383 Å [28]. The bonding strength is further confirmed by the formation energy (Ef) of the halogen atoms on the surface of MXene, which is directly related to the bonding strength between the end group and the Ti₃C₂ matrix. The calculated Ef for Ti₃C₂Br₂ and Ti₃C₂I₂ are −5.624 and −3.644 eV, respectively, which are less negative than the previously reported Ti₃C₂Cl₂ value of −6.694 eV, Ti₃C₂F₂ (−7.111 eV) and Ti₃C₂O₂ (−9.589 eV). In addition, the AFM image and size distribution of the obtained MXene-NH₂ nanosheets are shown in Figure 21h [49].

To establish an MXene characterization platform measuring the termination structure, elemental composition, and coordination relationship, and developing accurate quantitative test methods are the key route for MXene research.

However, quantitative analysis of MXene is still challenging due to the lack of accuracy. As well, most of the functional groups are light and difficult to detect and resolve. The development of new characterization methods is necessary for a further study of MXene surface terminations.

4. Properties Depended on Surface

As a new 2D layered material, MXene has the merits of other 2D materials. More importantly, surface functionalization renders them easily achieve improved properties. Meanwhile, the applications of MXene have been expanded, including biomedical, energy storage devices (battery, supercapacitor), sensors, catalysis, and electromagnetic interference shielding [65].

4.1. Electrical Conductivity

MXene has a higher electrical conductivity, with a theoretical electron mobility of up to 10⁴ cm²/(V · s). In 2014, Gogotsi et al. obtained a flexible self-supported Ti₃C₂T_x MXene thin film material by vacuum filtration of the monolithic dispersions of Ti₃C₂T_x MXene, and its electrical conductivity was 1500 S/cm by the four-probe method. Mathis et al. used the MAX phase with more aluminum as the precursor to reduce the defects of the Ti layer on the MXene surface during etching, and obtained Ti₃C₂T_x MXene film with electrical conductivity up to 20,000 S/cm. By adjusting the surface chemical properties of MXene, the electronic conductivity of MXene can also be effectively adjusted. Some surface terminations (especially -F) can be changed or removed, so that the lamellar structure of MXene is more compact and the conductivity is significantly increased.

DFT calculation results show that MXene without surface terminations generally behaves as metal because the d electrons of transition metals have a very high density of states (DOS) near the Fermi level. However, MXene synthesized by experiment often has abundant surface terminations, and the number and type of surface terminations are difficult to accurately control. In addition, there is still a lack of advanced characterization methods for the conductivity test of MXene monolayer nanosheets, which poses a challenge to the study of the intrinsic electronic properties of MXene. According to the calculation results of Perdew-Burke-Ernzerhof (PBE) and HSE06, as shown in Figure 22, both Ti₂X and MAX phases exhibit metal properties, while the density of states of Ti₂XT₂ with surface terminations at the Fermi level decreases significantly.

With the introduction of surface terminations, the band structure of MXene changes significantly. Due to the hybridization between the p orbital of the end group and the d orbital of the transition metal M, the peak density of states is significantly reduced, and it is possible to raise the d orbital above the Fermi level, resulting in a band gap. In addition, a larger atomic number of M usually produces a larger bandgap value [65]. Due to the generation of the band gap, the MAX phase and the MXene (M_n+1X_n) without termination groups, which originally exhibited metal properties, are transformed into semiconductors after the introduction of termination groups (M_n+1X_nT_x), as shown in

Table 2. Termination semiconductor MXene and its bandgap calculated based on PBE and HSE06 [66].

MXene	Termination	PBE (eV)	HSE06 (eV)
Sc2C	O	1.8 [67], 1.84 [68], 1.86 [69]	2.90 [70], 2.92 [71], 3.01 [69]
	F	1.0 [69,72], 1.03 [67,68], 1.05 [73]	1.64 [73], 1.84 [70], 1.88 [69]
	OH	0.34 [69], 0.44 [68], 0.45 [67], 0.71 [72]	0.71 [69], 0.74 [70]
	Cl	0.88 [70]	1.64 [70]
Ti2C	O	0.17 [72], 0.24 [74], 0.33 [75]	0.78 [76], 0.88 [74], 0.92 [77]
Zr2C	O	0.66 [72], 0.88 [67], 0.95 [75]	1.54 [77]
Hf2C	O	0.8 [72], 1.00 [67,75]	1.657 [78,79], 1.75 [77]
V2C	F	0.56 [80]
	OH	0.44 [80]
Cr2C	O
	F	0.22 [72]	3.15 [79], 3.49 [81]
	OH	0.03 [72]	1.39 [79], 1.76 [81]
	Cl	0.15 [72]	2.56 [81]
Mo2C	O
	F	0.25 [72]
	OH	0.1 [72]
	Cl	0.15 [72]
W2C	O	0.194 [82]	0.472 [82]
(Mo2/3Sc1/3)2C	O	0.04 [83]	0.58 [83]
(Mo2/3Y1/3)2C	O	0.45 [83]	1.23 [83]
(W2/3Sc1/3)2C	O	0.675 [83]	1.3 [83]
(W2/3Y1/3)2C	O	0.625 [83]	1.3 [83]
Mo1.33C	O2/3F1/3	0.5 [84]
Hf3C2	O		0.155 [85]
Hf2MnC2	O	0.238 [86]
	F	1.027 [86]
Hf2VC2	F	0.4 [87]	0.9 [87]
Mo2TiC2	O	0.041 [88], 0.052 [89]	0.119 [88], 0.125 [89]
Mo2ZrC2	O	0.069 [88], 0.087 [89]	0.125 [88], 0.147 [89]
Mo2HfC2	O	0.153 [88], 0.213 [89]	0.238 [88], 0.301 [89]
W2TiC2	O	0.136 [88]	0.290 [88]
W2ZrC2	O	1.170 [88]	0.280 [88]
W2HfC2	O	0.285 [88]	0.409 [88]
Cr2TiC2	F		1.35 [79]
	OH		0.85 [79]

.

The results show that MXene with a larger n value is more likely to exhibit metal properties, and the introduction of surface termination has little effect on its metal properties. For example, Ti₃C₂, Zr₃C₂, Ti₄C₃, Nb₄C₃ and Ta₄C₃ MXene exhibit metal properties no matter which is the surface termination. In the ordered bimetallic MXene structure, the density of states near the Fermi level largely depends on the D electrons of the surface metal atoms. Therefore, compared with the inner metal, the outer transition metals have a greater effect on the electronic properties. Compared to carbide MXene, nitride and carbonic nitride MXene generally have higher total state densities at Fermi levels due to stronger Ti-N bonds (compared to Ti-C bonds) and extra nitrogen electrons [66].

The effect of amino termination on the electronic behavior of MXene-NH₂ was studied by UPS spectroscopy. The finite density of states of MXene and MXene-NH₂ nanosheets shown in Figure 23a at the Fermi level (set at 0 eV) indicates that MXene-NH₂ exhibits metallic conductive behavior similar to conventional MXene. The secondary electron cutoff regions of the UPS spectra of the two nanosheets are shown in Figure 23b. The results show that the work function of MXene-NH₂ increases from 6.28 eV to 6.33 eV. It has also been shown that NH₄⁺ is an electron acceptor during amino substitution [90]. Although NH₄⁺ replacing the -OH reduce the electron carrier density of MXene-NH₂, which has a higher electron transport. As shown in Figure 23c, the conductivity of pure MXene-NH₂ film reaches 21,100 S/cm, which is still a considerable value, although it is 82.5% lower than that of conventional MXene film.

Since the surface terminations of MXene can provide wider layer spacing and more space for ion shuttling [91], its application in various energy storage can be improved by controlling the interconversion between different functional groups. For example, among various MXene, the surface-modified Ti₃C₂T_x dominates the current solar cell research due to its high conductivity, carrier mobility, and adjustable work function (WF) [91]. Transition metals tend to be less electronegative than functional groups and carbon atoms, with -OH and -F accepting one electron and -O accepting two electrons from the transition metal atom. In addition, Ti, Zr, and Hf are located in the same column of the periodic table and have the same shell electron configuration, therefore, after -O functional modification, a similar trend of metal-semiconductor characteristic transition is found in the corresponding M₂C MXene [66]. This property can be used in capacitors to achieve energy storage. In addition, MXene can be used as a cocatalyst for the separation and transfer of charge in photocatalytic reactions due to its good conductivity [92]. After the modification of functional groups, it showed excellent performance and reduced the reaction cost.

4.2. Magnetic Properties

Most MAX phases reported so far are not magnetic, while some MAX phases containing Cr or Mn are magnetic. Theoretical calculation shows that most of the unterminated MXene obtained after removing the A atomic layer in the MAX phase are magnetic. To determine the magnetic moment of the unit cell of MXene, the factors to be considered mainly include the transition metal oxidation state, the coordination number of the transition metal, and the number of d electrons. Moreover, C⁴⁻, N³⁻, F⁻, OH⁻ and O²⁻ can also contribute a small amount of magnetic moment. The unbonded d orbitals in MXene are formed near the Fermi level between the bonded and anti-bonded states; therefore, the electrons occupying the unbonded D orbitals play a major role in magnetism [93].

According to the theoretical prediction, most of MXene without termination groups have magnetic properties, but when surface termination groups are introduced, the magnetic properties of MXene may change greatly, as shown in

Table 3. Magnetic moment of MXene predicted by theoretical calculation [65]. (Unit: μ_B/unit cell).

MXene	Magnetic Moments (Pristine) (μB)	Magnetic Moments (Termination Group) (μB)
		-O	-F	-OH
Ti3C2	1.8~1.93	—	Nonmagnetic	Nonmagnetic
Ti2N	1.0~1.1	—	—	—
Ti2C	1.9~1.91	—	—	—
Ti3N2	0.34/Ti atom	—	—	—
V2C	0.16	—	—	—
V2N	Nonmagnetic	—	—	—
Fe2C	3.95	—	—	—
Zr2C	1.90	—	—	—
Zr3C2	1.73	—	—	—
Mn2N	—	7.0	9.0	8.8
Cr2C	0.54/Cr atom	Nonmagnetic	2.71/Cr atom	2.24
Cr2N	—	5.6/Cr atom	3.23/Cr atom	3.01/Cr atom
Sc2N	—	1.00	—	—
Mn2N	—	7.0	9.0	8.8
(Ti2Mn)C2	—	2.97	4.24	3.90
(Hf2Mn)C2	—	3.00	5.00	4.84
(Hf2V)C2	—	1.00	1.27	1.33
Ti4N3	7.00	0.37	0.88	Nonmagnetic
(TiMn2)C2	16.3	—	4.0	—
(TiCr2)C2	3.4	1.8	3.3	3.0

[65]. In addition, the magnetic properties of MXene of different transition metals usually show different changes when they are combined with different types of surface end groups. For example, some magnetic MXene, such as Ti₃C₂T_x and Ti₄C₃T_x, change to non-magnetic after surface termination. When the surface terminations of Cr₂CT_x are -OH and -F, the ferromagnetism remains at room temperature, and the magnetism disappears when the surface termination groups of Cr₂CT_x are -O. However, Mn₂NT_x always remains ferromagnetic, independent of surface termination. Monolithic V₂C is an antiferromagnetic metal, and after -F or -OH termination, V₂C transforms into a small bandgap antiferromagnetic semiconductor. The gratuitous group Cr₂C is a ferromagnetic semi-metal; however, with the end-group of -F, -Cl, -OH, or -H, Cr₂CT_x becomes an antiferromagnetic semiconductor. Cr₂N has more valence electrons than Cr₂C, and the ground state of Cr₂N is antiferromagnetic, but it becomes semi-metallic with -F, -O, or -OH surfaces.

Some magnetic MXene exhibit nonmagnetic properties after binding to surface termination, which is mainly due to the formation of p-d bonds between surface transition metal atoms and end groups, leading to the disappearance of magnetism [94]. Orbital splitting theory can also be used to explain the phenomenon of magnetic properties change of MXene: For MXene with surface termination, each transition metal atom is surrounded by C/N atoms and surface termination, forming an octahedral cage around the transition metal. The resulting crystalline field near the octahedron divides the d orbital of the transition metal into t_2g (d_xy, d_yz, d_xz) and e_g (d_x2−y2, d_z2) orbitals. Because of the different orbital shapes, the energy of the e_g orbital manifold is higher than that of the t_2g orbital manifold; Therefore, the electrons occupy the t_2g orbital first. The valence electrons of transition metals are different, and the electron configurations of d orbitals are different, so the magnetic behavior of MXene is different.

MXene with asymmetric surface termination, has also been studied. Cr₂CFCl, Cr₂CHF, Cr₂CFOH and V₂CFOH are antiferromagnetic semiconductors [95]. In Sc₂CTT ′(T, T′=H, O, OH, F, and Cl,) with different asymmetric combinations, these containing -O are ferromagnetic ordered semi-metal, while the others are non-magnetic semiconductors. The magnetic properties of MXene vary greatly due to different termination combinations. Therefore, MXene can afford various magnetic properties by adjusting termination, and the application of MXene will be expanded into the field of magnetism.

4.3. Optical Properties

In the visible range, the presence of -F and -OH decreases the absorption and reflectance of MXene, while in the UV region, the presence of surface terminations increases the reflectance compared to bare MXene. In the infrared and visible light ranges, -O has a higher ε2 (ω) than -F with -OH, and therefore has a stronger optical absorption performance, which is mainly attributed to the formation of oxidation states close to the Fermi level.

The surface modification of MXene using magnetic transition metals, such as Fe, CO, Ni, etc., can improve the optical properties of Ti₃C₂ MXene material and increase the optical absorption coefficient of Ti₃C₂ in the ultraviolet region, visible region, and near-infrared short-wave region [96]. The optical absorption coefficient of Ni/Ti₃C₂ composites increases by 50% in the visible and ultraviolet light range, and the optical absorption coefficient of Fe/Ti₃C₂ increases by more than 100% compared with that before modification, especially in the infrared region. This significant enhancement of light absorption may be related to the change of electronic structure after doping magnetic atoms. DFT calculations show that Ti₃C₂ MXene retains its metallic properties after Fe, Co, and Ni modification, and the d electron orbital plays a dominant role in the electronic states near the Fermi level. A strong peak in the range of −4~−1 eV appears in the d orbital, resulting in a spin-polarized state, leading to a great change in the optical properties.

4.4. Solubility and Dispersion

Ti₃C₂ powder with -Cl as termination prepared by the molten salt method is black, as shown in Figure 24a. These substances were well dispersed in absolute alcohol or water (0.17 mg mL⁻¹) and exhibited a pronounced Dundar effect. In addition, the effect of amino closure on the dispersion of MXene-NH₂ solution was also pointed out, and the synthesized Ti₃C₂ MXene-NH₂ nanosheets were tested for Zeta potential. As shown in Figure 24b, compared with traditional MXene, the Zeta potential of MXene-NH₂ ranges from −51.3 mV to −30.3 mV in water and from −6.68 mV to −5.80 mV in 75% ethanol solution. This is mainly due to the introduction of an amino terminus on the surface of MXene. The amination of MXene-NH₂ reduces the electronegativity of the nanosheets. So, its Zeta potential goes down. However, MXene-NH₂ nanosheets also showed excellent dispersion in aqueous suspensions. After standing for 2 weeks, only a small amount of MXene-NH₂ precipitation was observed at the bottom of its aqueous suspension, as shown in Figure 24c. Good dispersion and the same Zeta potential are the keys to the preparation of homogeneous hybrid films by vacuum-assisted filtration. The excellent dispersibility of the solution provides a feasible preparation method for MXene-NH₂ nanosheets [49].

4.5. Mechanical and Tribological Properties

Ti₃C₂T_x MXene clays synthesized by the LiF/HCl method show good compression properties and can be rolled into films with tensile strength up to 568 ± 24 MPa [97]. To study the effect of different surface terminations on the mechanical properties of MXene, the bond energy of M − T_x tends to be maximized in the early transition metal groups, which is interpreted as the tendency of the surface terminations (halides) to contribute electrons to most of the unfilled D orbitals in the early transition metal groups on the surface. This reduces the electronegativity difference between the surface termination and the transition metal in the late transition metal group [98]. In general, -O₂ terminations resulted in higher 2D stiffness of MXene, whereas -F₂, -(OH)₂, and -Cl₂ terminations resulted in a slight increase in stiffness (Figure 25). The effect of different surface terminations on mechanical properties can be attributed to the changed M-X structure [99,100,101]. In the M_n+1 X_n structure, the addition of strong mixed ion/covalent M-O bonds reduces the contribution of M-M interactions [98], which helps to increase the strength of the M-X bond. However, the major ionic bonds of M-F, M-Cl, or M-(OH), did not result in a reduction in M-M interactions compared to the M-O bond. An exception can be found at the −Cl₂ end of the posterior transition metal, which is the result of covalent bonds, due to the length of the M-Cl bond being longer than that of the M-F bond and the increased ionization energy of the right transition metal in the periodic table. This could explain the similar mechanical properties of V₂CCl₂ and Cr₂CCl₂ compared to V₂CF₂ and Cr₂CF₂ [102].

The friction of the amino-functionalized Ti₃C₂T_x MXene-reinforced epoxy composite was reduced by about 35%, and the wear rate was reduced by 72%. This is attributed to the improved dispersion and compatibility of functionalized MXene in the organic matrix, resulting in increased storage modulus and interfacial strength, and reduced plastic deformation and microcracking [103]. The hydrophilicity of the surface terminations of MXene reduces the stability of its dispersions in polar solvents. Thus, we can use the existing functional groups (−O₂, −(OH)₂, −F_2, and −Cl₂) as anchors for hydrophobic molecules to progressively manipulate their hydrophobic/hydrophilic properties. This significantly improves the stability of dispersants in hydrophobic liquids and reduces agglomerating, thus improving the resulting frictional properties.

4.6. Water Purification

The ultrathin two-dimensional structure and high surface area of MXene lead to the exposure of more internal atoms, which leads more likely to the formation of defects and in turn provide more surface active sites, which will facilitate its application in the adsorption field [104]. Therefore, MXene materials have been used to remove inorganic pollutants and dye compounds. The adsorption mechanism can be interpreted as pure MXene adsorbs pollutants with positive and negative charges due to its surface charge. However, the surface of MXene is rich in functional groups that can interact with opposite charges of compounds in water. In addition, due to the large layer spacing of pure MXene, pollutants can be embedded in MXene and then removed [105].

According to density functional theory (DFT), the bond lengths of termination groups (such as -OH, -F, =O) on the surface of MXene are about 0.97 Å for Ti-OH, 2.1 Å for Ti-F and 1.9 Å for Ti-O. Compared with -OH, -H, -F, and -Cl, =O has the largest adsorption energy. This will affect the adsorption properties of MXene [105].

4.7. Biological Characteristics

The antimicrobial mechanism of MXene may be the direct physical interaction between the sharp edge of the nanosheet and the bacterial membrane, and the highly hydrophilic surface with a negative charge can enhance the contact between bacteria and the membrane surface, and contact killing leads to microbial inactivation. In addition, the antibacterial activity of MXene was related to particle size and action time [106].

Wu et al. immobilized the enzyme on the -OH surface termination to maintain the biological activity and stability of MXene on the media-free phenol biosensor platform [107]. It was found that the electrostatic adsorption of lysozyme [108] and collagen [109] on the surface of Ti₂C and Ti₃C₂ resulted in changes in physicochemical and biological properties of MXene. Liu et al. obtained similar results by modifying the surface of Ti₃C₂ with hyaluronic acid, thereby achieving active tumor targeting and aggregation and enhancing the stability of the multifunctional nanoplatforms [110]. Notably, polyethylene glycol (PEG) can also be used to modify Ti₂C to stabilize the photothermal ablation of cancer cells and minimize negative effects on nonmalignant cells [111]. Other studies have used soy phospholipid-stabilized Ta₄C₃ MXene [112] or polyvinylpyridol (PVP) -stabilized Nb₂C [113] for dual-mode photoacoustic/CT imaging and photothermal treatment of cancer without significant in vitro and in vivo toxicity.

4.8. Catalytic Performance

The unique optical characteristics and heterojunction structure of MXene are conducive to improving the photocatalytic performance of MXene, which plays a crucial role in the application of MXene in the field of catalysis. Heterogeneous structures terminated by van der Waals (vdW) forces are important for photocatalysis [114]. The heterostructures are classified as type I, type II, and type III, which represent the crossover gap, interleaved gap, and fragmentation gap, respectively. MXene and transition metal dichalogenates can form a variety of heterostructures, such as MoS₂/Ti₂CF₂ [115], graphene/Ti₂C [116], and so on [117]. Scientists found that MXene heterostructure can enhance the photocatalytic performance of hydrogen evolution reaction (HER) [118,119], N₂ reduction reaction, water splitting, and photocatalytic CO₂ reduction. Cao et al. found that Ti₃C₂/Bi₂WO₆ produced nearly five times more CH₄ and CH₃OH than Bi₂WO₆ [120].

ROS plays a decisive role when considering the photocatalytic degradation process [121,122]. In this regard, titanium dioxide (TiO₂), which is of course very much related to Ti-based MXene and MBenes, offers excellent photocatalytic properties, as well as low cost and low toxicity. When TiO₂ is used in a photocatalytic process, the absorbed energy induces an electron-hole pair due to electron transfer from the valence guide band [123,124]. Their interaction with surrounding water or oxygen induces the formation of ROS. In addition to the important effects of TiO₂ particle size, size, surface area, and crystallinity, the photocatalytic activity is also quite sensitive to the amount of ROS generated [125,126]. This has also confirmed that TiO₂’s material properties, such as crystal phase and size, as well as defect density, significantly affect the formation of ROS, thus affecting photocatalytic activity [126,127].

The important effect of ROS on the photocatalytic performance of MXene nanosheets has also recently been demonstrated [128]. Rosales et al. verified by XPS that a small amount of active TiO₂ sites on the surface of Ti₃C₂ nanosheets could form electron/hole pairs under UVA irradiation [129]. Electrons undergo a transfer from the TiO₂ lead to the surface of the nanosheet, thereby helping to separate light-generated charge carriers. The existing surface terminations and the excellent electrical conductivity of MXene reduce the possibility of recombination, thus significantly facilitating the photocatalytic process. It can be hypothesized that complex redox reactions (-OH surface terminations and oxygen or water) are likely to induce ROS generation. Due to the increased TiO₂ content on the surface of monolayer Ti₃C₂ nanosheets compared with multilayer nanosheets, the monolayer Ti₃C₂ nanosheets showed higher ROS generation capacity [130].

Gogotsi found that Ti₃C₂T_x degraded 81% of MB and 62% of AB80 in wastewater within 5 h of UV irradiation, while only four times MB was degraded in the dark. This study is the first to explore the application of MXene in photodegradation, but does not explain this complex process [131]. MXene is unstable in aquatic environments, and Ti₃C₂T_x will be oxidized to TiO₂ to enhance photodegradation performance [132]. In 2020, scientists proposed that the cleavage process of HgClOH adsorbed on Ti₃C₂ would produce hydroxyl radical (·OH), which could promote the conversion of Ti₃C₂ to TiO₂/C nanocomposites. The nanocomposites show good photocatalytic performance. This distinctive mechanism is applicable for mercury (II) removal [105].

Since the catalytic performance mainly depends on the instability of MXene, it is still a major challenge to improve the reversible and cyclic performance of MXene. In addition, further studies are needed to investigate the performance mechanism of MXene in the catalytic region. In practice, it is still inconvenient to use and recycle low-dimensional materials. How to obtain the macroscopic MXene materials and how to effectively separate MXene materials remain to be studied.

4.9. Others

In addition to the above properties, the influence of surface chemistry on the properties of MXene is also reflected in many other fields due to the unique structure of MXene which affords excellent physicochemical properties. For example, MXene is easily oxidized in air or water, which reduces its physical and chemical properties, thus greatly limiting its application scope [133]. Surface functionalization of Ti₃C₂ MXene sheets by various organic ligands has been performed in various ways, including catechol [134], silane [135,136], isocyanate [137,138], amino acid [139,140], and phosphonic acid [30]. Although these methods have been successful, there are still many limitations in the oxidization of MXene. For example, the attachment of amino acids requires an aqueous phase synthesis route, which may result in the oxidation of Ti₃C₂ sheets to TiO₂ due to the presence of dissolved oxygen or water itself. The surface terminations of MXene can be tailored toward certain redox chemistries. Based on the aforementioned characteristics, modified MXene with high electronic/ionic transport is becoming a promising electrode material [30].

MXene-based materials are used to develop sensors for gas detection, strain, pressure, and humidity currently. The main working mechanism behind all of these MXene-based sensors is the change in the electrical conductivity of external stimuli. However, although a large number of studies have shown that MXene is a potential two-dimensional material, there are still many challenges to fully apply MXene in sensing, such as difficulty to achieving green preparation of materials, insufficient stability, and in complete control of surface functional groups [141,142]. However, some studies have pointed out that cetyl trimethyl ammonium bromide (CTAB) or octadecyl trimethyl ammonium bromide (STAB) can be used to change the surface terminations of Ti₃C₂T_x, and the interlayer spacing can be adjusted [30]. This adjustable interlayer spacing makes it more prominent in applications such as gas sensing and gas separation.

In addition, Khaledialidusti et al. compared the M-B, B-B, and M-M bond strengths of MBene and proves that the B-B is the strongest bond, while M-M tends to be the weakest one. This tendency also holds true for bare and terminated MXene with the order of B-B > M-O > M-F ≈ M-OH > M-B > M-M [46,62,143]. We speculate that the existence of surface terminations on the outer MBenes’ surface and between the layers is inevitable. Considering the available knowledge about MXenes’ surface terminations, we anticipate that similar surface terminations including -O, -OH, -F, and -Cl groups will be present for MBenes. Nair et al. summarized the electronic properties of various surface-terminated MBenes [50,144]. It indicates that the electronic properties of MBenes are affected by surface terminations. The same property may be applied to MXene.

The future applications of MXene are expanding and have the potential to change the world. To truly realize the potential of the entire MXene family, basic research on these necessary properties will be required in addition to clarifying the effects of structure and composition on MXene [145,146,147].

5. Conclusions and Perspective

This account reviews these investigations of the MXene surface terminations, and provides fundamental information on regulation methods, characterization techniques, and the relevance among terminations, properties, and applications.

Moreover, some issues were proposed as research emphasis and difficulties of MXene surface terminations.

(I)

How to observe these surface terminations of MXene at the atomic level? Employing spherical aberration-corrected electron microscopy combined with advanced spectral characterization techniques may offer new sights for MXene’s surface studies. Through cross-section sample preparation, the interlayer of MXene can be observed.

(II)

How to prepare the bare Mxene without surface terminations? It is required for understanding the detailed bonding nature between surface species of bare MXene, the regulation rule between surface metal species and the surface functional groups, and the interfacial interaction of MXene and hybrid phases.

(III)

And how to construct the unsymmetrical MXene surface terminations on both sides of the single MXene nanosheets? There are key issues to improve to study the physical properties of MXene and support an accurate model for theoretical calculation and simulation.

(IV)

Developing MXene with a new organo-functional group and analyzing the interface between MXene and other hybrids.

To address these issues, devoting to understanding the MXene surface terminations is the route to improving the MXene’s function to practical application.