Surface functionalization of MXenes

MXenes, a new family of two-dimensional (2D) transition metal carbides and nitrides, have received great research attention in diverse applications, including biomedicine, sensing, catalysis, energy storage and conversion, adsorption, and membrane-based separation, owing to their extraordinary physicochemical properties and various chemical compositions. In recent years, many surface functionalization strategies, such as surface-initiated polymerization and single heteroatom doping, have been cleverly developed to expand the potential of MXenes in diﬀerent fields. This article reviews and puts into perspective the recent advances in MXene surface functionalization and various applications of the modified MXenes. The eﬀect of surface functionalization on MXene properties (such as electronic, magnetic, mechanical, optical, and hydrophilicity/hydrophobicity) is discussed. Potential applications of pristine and functionalized MXenes in diﬀerent fields are discussed. Finally, challenges and future directions in MXene surface modification are examined.


Introduction
In recent years, two-dimensional (2D) nanomaterials, such as MXenes, black phosphorus nanosheets, molybdenum disulfide, and graphene, have received considerable attention 1-4 due to their unique structures and attractive chemical and physical properties. As such, they have shown potential for various applications in the fields of biomedicine, sensing, environment, and energy. [5][6][7][8] In particular, MXenes have received considerable attention around the world. 9,10 The first discovered MXene (Ti 3 C 2 ) was synthesized by selective etching of Al elements using hydrofluoric acid (HF). 11 The general chemical formula of MXenes is M n+1 X n T x (n = 1-4), where M, X, and T represent an early transition metal (including Mo, Nb, Cr, V, Ti, and Hf), carbon or nitrogen, and surface functional groups, respectively. 10,12,13 The integer x is the number of functional groups. 11 While many MXenes with various compositions have been proposed, only a limited number of them have been synthesized. The diversity of MXene compositions implies that a variety of MXenes with desired properties can be synthesized. 14,15 The chemical formula of the reactant, the MAX phase (as a kind of ternary ceramic material), from which a MXene is synthesized, is M n+1 AX n , which has a layered hexagonal structure. 16 The MAX phase is formed by covalent bonds between the M and X atoms, and the M-X layer is alternately ordered with an A atomic layer via a metallic bond between A and M, although M-A bonds are weaker compared to M-X bonds. 17 Owing to their promising mechanical, chemical, and physical properties, and large surface area, 18 MXenes are being applied in various applications such as adsorption, [19][20][21] membrane separation, [22][23][24][25] catalysis, 26,27 energy storage, [28][29][30] sensing, 26,[31][32][33] and biomedicine. [34][35][36] Furthermore, the existence of many functional groups such as oxygen (-O), fluorine (-F), hydroxyl (-OH), and chloride (-Cl) on their surface allows for their surface functionalization, which increases their versatility further. [37][38][39][40] For example, adsorption of metal ions was investigated using -O-containing composites of Ti 3 C 2 , which were produced by the conversion of -OH groups on their surface. 41 Besides, because of their extraordinary biocompatibility, 42 MXenes have shown great potential for biomedical applications.
Different methods of synthesizing MXenes have been introduced. 43 They include (i) top-down methods (selective etching of MAX and non-MAX phases), 44,45 (ii) bottom-up methods (chemical vapor deposition (CVD) and salt-templated growth), and (iii) chemical transformations. The single-layer and multilayer nanostructure of MXenes can be manufactured using these methods. On the other hand, poor stability in the presence of molecular oxygen, easy restacking, and low mechanical flexibility are the disadvantages of MXenes that have limited the applications of pristine MXenes. [46][47][48][49] Despite the numerous merits of MXenes, they still lack properties needed in several fields. 50 Additionally, pristine MXenes tend to precipitate and accumulate in water due to their poor water dispersibility. 51 Hence, the surface functionalization and modification can substantially enhance their performance and stability. 52 Generally, the surface properties of MXenes can be modified and improved by the type and fraction of the T group during the production process. 53 Recently, functionalized MXenes have been synthesized by surface modification of pristine MXenes with covalent and non-covalent modifications. The non-covalent surface modification is obtained by a combination of van der Waals forces, hydrogen bonding, and electrostatic attraction. 35,[54][55][56][57] Covalent surface functionalization methods are classified into three groups: [58][59][60] (i) those that involve small molecules such as alkali metal hydroxides, acid halide or acid anhydride, epoxy compounds, and organic amines, [61][62][63][64] (ii) those that involve surface-initiated polymerization by macromolecules, 65,66 and (iii) those that involve single heteroatoms. 16 In this article, recent developments in the functionalization of MXenes are reviewed. First, strategies for MXene surface functionalization are reviewed and discussed. The properties of the surface modified MXenes and their potential applications in separation, biomedical, sensing, energy storage and conversion, and catalysis are discussed. Finally, the future perspective of functionalized MXenes, and challenges and opportunities in this field are highlighted.

Methods of surface functionalization
Plentiful -OH and -O containing groups on the surface of MXenes can be utilized as active sites for covalent binding by means of controlling the surface terminations, surface-active initiators, small molecules, and polymers. [67][68][69][70][71] The performance of MXenes can be enhanced easily by suitable functional groups because of the hydrophilic nature of MXenes. 16 Many have studied the chemical surface functionalization of MXenes. Table 1 lists the various modified MXenes and the surfacefunctionalization methods used to prepare them.

Controlling surface terminations
The surface modification of MXenes through controlling the T groups allows for adjusting the physical properties of MXenes, including the concentrations and types of electrochemical active sites and electronic structures. 83 -O and -F surface terminations can be easily achieved by conventional etching techniques such as fluoride-containing salts or HF. More importantly, the concentrations and types of surface terminations can be tuned by adjusting the etching time and the concentration of etchants, and using various post-treatments such as hydrazine and annealing in diverse atmospheres. 84,85 For instance, Schultz et al. studied the effect of various surface terminations on the physical properties. 86 Their measurement of the electronic structure indicated that Ti 3 C 2 has a high (above 1 eV) dispersion due to its Ti-O bonds. Annealing at 380 1C in vacuum increases the work function from 3.9 to 4.8 eV, because of the desorption of -OH species, contaminants, and water. The work function decreased to 4.1 eV with fluorine desorption at higher temperatures from 500 to 750 1C. Moreover, density functional theory (DFT) has been used to evaluate the effect of surface terminations on the electronic conductivity. 87 It was found that the Fermi level density of states (DOS) of an MXene is affected by surface terminations. The concentration of -F terminations decreased by annealing Ti 3 C 2 from 300 to 775 1C, which was supported by measurements from in situ electron energy loss spectroscopy (Fig. 1a). The significant enhancement of electronic conductivity was ascribed to the decrease of -F terminations during annealing at 700-775 1C (Fig. 1b). Furthermore, Tang et al. investigated the saturation magnetization of Ti 3 C 2 through Ti-C vacancy production under H 2 annealing. 88 They showed a similar level of -O concentration after annealing in H 2 from 100 to 500 1C, while the C-Ti-OH functional groups transformed to O-Ti-O and C-Ti-O. The Ti-C vacancy with C and Ti vacancies formed in the annealing process (Fig. 1c). The fully terminated Ti 3 C 2 MXenes were theoretically nonmagnetic, while the pristine Ti 3 C 2 was ferromagnetic. The influence of Ti-C vacancy on the improvement of saturation magnetization was clearly confirmed from DFT calculations and magnetization enhancement (Fig. 1d).

Small molecules
Performance characteristics of MXenes such as mechanical stability, electrical properties, and solution stability can be further enhanced by surface modification of MXenes with a variety of small molecules, which are easy to process and are inexpensive. 61,63 To illustrate, Xia et al. introduced a non-ionic surfactant, hexaethylene glycol monododecyl ether (C 12 E 6 ), on the surface of Ti 3 C 2 MXenes (C 12 E 6 @Ti 3 C 2 ) to improve molecular interactions, resulting in enhanced packing symmetry (Fig. 2a). 55 In the C 12 E 6 @Ti 3 C 2 composite, strong hydrogen bonds are formed between the -O or -F groups on the MXene's surface and the -OH groups of C 12 E 6 . A novel fan-like texture under an optical microscope was used to confirm the introduction of C 12 E 6 between MXene nanosheets (Fig. 2b). In addition, a one-step direct reaction of Ti 3 C 2 sheets with diazonium salt was reported by Lei et al. who prepared a sulfonic functionalized MXene, denoted as Ti 3 C 2 -SO 3 H. 89 The results of SEM characterization demonstrated that the pristine Ti 3 C 2 MXenes have packed stacked lamellar layered structures. By contrast, granular structures can be seen on the surface of Ti 3 C 2 -SO 3 H ( Fig. 2c and d). Jin et al. used a simple solvothermal/hydrothermal method to modify the surface of Nb 2 C MXenes for developing the electromagnetic wave (EMW) absorbing performance. 73 Deionized (DI) water or ethanol (Et) was used in the solvothermal treatment process instead of DMF. The results presented superior EMW performance for Et-treated Nb 2 C MXenes because of increased surface functional groups, enlarged interlayer spacing, and improved dielectric loss. Larger interlayer spacing can lead to multiple reflections between Nb 2 C sheets, resulting in enhanced EMW absorption. In addition, the functional groups on the edge and surface of Nb 2 C can act as polarization sites to attenuate the energy of EMW. In addition, Yang et al. were able to increase the molecular interactions of MXene nanosheets using hexaethylene glycol monododecyl ether (C 12 E 6 ) as a non-ionic surfactant. 55 MXenes have also been functionalized using other small molecules such as bismuth ferrite (BiFeO 3 ) 92,93 and sodium aluminum hydride (NaAlH 4 ). 94 A chemical coupling method was applied to produce amino groups on the MXene's surface (Fig. 2e), and the resultant functionalized MXene nanosheets can be used as a platform to immobilize different biomolecules. 90 To conduct large-scale delamination of MXenes, Wang et al. developed a method of diazonium surface chemistry, 61 which weakens the M-X bonds and expands the interlayer spacing by the formation of aryl-surface linkage using diazonium and sulfanilic Fig. 1 Effect of surface terminations on the conductivity of MXenes; (a) loss of -F terminations under annealing by in situ electron energy loss spectroscopy of Ti 3 C 2 , and (b) comparison of -F concentration (green triangles) and the post-anneal room temperature resistance of Ti 3 C 2 (black circles). Reproduced with permission. 87 Copyright 2019, Nature. (c) Representation scheme and high-resolution transmission electron microscopy of structural evolution after annealing in H 2 , and (d) indicating the comparison of magnetic properties as a function of the applied field H measured at 2 K. Reproduced with permission. 88 Copyright 2020, Elsevier.
salts. In another study, diazonium ions were used for the in situ modification of the MXene's surface, 91 as shown in Fig. 2f, where it is proven that the covalent grafting of C 6 H 4 SO 3 H groups on the surface of Ti 3 C 2 improves the electrical properties.
It should be noted that the -OH groups on the MXene's surface can react with trimesoyl chloride, epichlorohydrin, and silane agents to conduct etherification and esterification. 95,96 During the production of MXene/cellulose hydrogels, epichlorohydrin served as a cross-linker between MXenes and cellulose via the formation of covalent bonds with the existing -OH groups on the surface of MXenes.

Surface-initiated polymerization
The numerous active functional groups on the surface of MXenes allow polymers to be ex situ blended or in situ polymerized. 65,74,83,[97][98][99][100][101] MXene/polymer composites are usually produced using an ex situ blending approach, which provides some merits such as well-defined polymer structures and tunable compositions. Hence, the interaction between polymers and MXenes can be efficiently improved by hydrogen bonding 102 and electrostatic interactions. 75,76 For instance, Sun et al. produced highly conductive MXene@polystyrene (PS) nanocomposites (MXene@PS) using the electrostatic assembling of positive PS and negative MXene nanosheets, followed by compression molding (Fig. 3a). 75 Fig. 3b shows the perfect and clean microstructure core-shell MXene@PS hybrid. The abundant functional groups (-F, -O, and -OH) on the surface of MXenes allow them to assemble on the PS by van der Waals forces, hydrogen bonding, and electrostatic interactions. At lower contents of Ti 3 C 2 MXenes, assembling occurred through electrostatic interactions, whereas at higher contents, the van der Waals forces and hydrogen bonds are the main driving forces for the assembling of Ti 3 C 2 MXenes on the surface of the PS. In addition, Ling et al. reported the fabrication of two Ti 3 C 2 /polymer composites: Ti 3 C 2 /polyvinyl alcohol (PVA) and Ti 3 C 2 /polydiallyldimethylammonium (PDDA) (Fig. 3c). 76 A PVA/Ti 3 C 2 composite can be easily obtained by hydrogen bonding owing to the abundant hydrogel groups on the structure of PVA. As Ti 3 C 2 MXenes are negatively charged flakes and PDDA is a cationic polymer, the PDDA/Ti 3 C 2 composite can be easily formed by electrostatic interactions. The TEM image of the PVA/Ti 3 C 2 composite (Fig. 3e) clearly shows the intercalation of PVA layers, and Fig. 3d shows the orderly stacked layers of the PDDA/Ti 3 C 2 composite formed by vacuumassisted filtration.
Strong covalent bonds between MXenes and polymers can be formed via in situ polymerization with monomers such as e-caprolaction 103 and pyrrole. 65,104 For example, Boota et al. developed a method for in situ polymerization of pyrrole between Ti 3 C 2 layers 65 (Fig. 4a). As shown in Fig. 4b, a periodic pattern was produced, which can be ascribed to the wellaligned polypyrrole (PPy) between Ti 3 C 2 layers along with appropriate intercalation of PPy chains. In the case of PPy/ Ti 3 C 2 composites, the interaction between Ti 3 C 2 and polymer chains is formed by hydrogen bonds between the -O groups on the surface of Ti 3 C 2 and the H and N of PPy. Moreover, soybean phospholipid (SP) consists of biomacromolecules that are extensively used for surface functionalization due to their biocompatible characteristics. 16 Lin et al. reported that surface functionalization of Ti 3 C 2 with SP ( Fig. 4c) 35 improves blood circulation and biodistribution in tumors in physiological environments. Furthermore, enhanced stability of MXenes in physiological solutions was observed using poly(ethylene glycol) (PEG). 105 Polydopamine (PDA) is another suitable agent for MXene surface modification, because there are abundant amino and catechol groups in its backbone, which can form non-covalent and covalent interactions with organic and inorganic surfaces. 60,107 To produce a Ti 3 C 2 @carbon nanotube (CNT) composite, cobalt was adsorbed on the surface of Ti 3 C 2 , and CNTs were then formed after the formation of PDA layers on the surface of Ti 3 C 2 . 16 The capacitive performance of Ti 3 C 2 improved with PDA modification. 108 Surface-initiated polymerization is another method for surface functionalization of MXenes. For instance, Fig. 4d illustrates the surface-initiated polymerization of V 2 C with poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) using -OH groups as a photoactive site. 98 Additionally, a peroxide decoration method was reported by Tao et al., where the -OH acts as an initiator for surface polymerization of monomers. 106 Polymerization occurs by adding nitrogen to a mixture of an MXene and an acrylic monomer (Fig. 4e). The surface functionalization with macromolecules is a promising strategy to improve MXenes' pristine properties tailored for various applications.
Furthermore, strong bonds between inorganic and organic groups can be made by silane reagents with different functional groups such as phenyl, chloro, vinyl, epoxy, methacrylate, amine, and diamine. 13 Silanol groups can be obtained by the hydrolysis of alkoxy groups and the presence of water and moisture on the surface of nanomaterials, which can cause the formation of covalent bonds between the silanol groups and available -OH groups on the surface of nanomaterials by a dehydration reaction. Amino-silane functionalization of TiO 2 was reported by Shamsabadi et al., 109 which inspired the amino-silane modification of Ti 3 C 2 by the same group 13 because of the similarity of -OH groups in TiO 2 nanoparticles and Ti 3 C 2 . [3-(2-Aminoethylamino)propyl]trimethoxysilane (AEAPTMS) was grafted on the surface of Ti 3 C 2 via chemical reactions between the -OH groups of Ti 3 C 2 and AEAPTMS: the amine group ends leading to electrostatic interactions, and the silanol group ends leading to covalent bonding (Fig. 4f). By amino functionalization of MXene materials, numerous opportunities of subsequent reactions are created. These include reactions with curing agents for epoxy resins and surface-initiated atom transfer radical polymerization due to the existence of free amine groups. These reactions make MXene materials suitable for use in energy-storage devices, electrochemical biosensors, fuel cells, catalysis, metal-ion batteries, antibacterial coating, drug delivery, and dye adsorbents. [110][111][112][113][114]

Single heteroatom method
The use of MXenes in Na-ion batteries (NIBs) and Li-ion batteries (LIBs) is still challenging 115 because of their unfavorable electrochemical properties, which can be addressed by increasing the interlayer spacing of MXenes via single heteroatoms. 116 Significant improvement in the electrochemical performance of MXenes has been obtained using P, S, and N doping approaches. 81,82,117,118 For instance, N-doped Nb 2 C MXene was produced by Liu et al. by means of a hydrothermal method. 81 The layered structure of N-doped Nb 2 C can be seen in Fig. 5a, where it can be concluded that the morphology of Nb 2 C does not change with N-doping, as confirmed by the uniform distribution of C, O, and Nb elements (Fig. 5b). Furthermore, p-doped V 2 C was synthesized by Yoon et al. using heat treatment via triphenylphosphine (TPP). p-Doped V 2 C with a large interlayer spacing (1.03 nm) was obtained, while pristine V 2 C had an interlayer spacing of 0.97 nm (Fig. 5c). Besides, N-doped V 2 C with different N contents was produced, as the heat treatment temperature was varied from 350 to 550 1C. The N-doped V 2 C was produced by annealing in an ammonia atmosphere, resulting in the uniform distribution of elements (such as N, C, and V). Other recent efforts to enhance the performance of MXenes include the syntheses of cation (NH 4+ , Al 3+ , Mg 2+ , Ca 2+ , and K + )-doped Ti 3 C 2 , germanium-and silicondoped Sc 2 C(OH) 2 , 119 boron-doped W 2 CO 2 and Mo 2 CO 2 , 120 and palladium/platinum-doped Nb 2 C. 121

Electronic properties
MXenes have shown diverse electronic properties such as semiconductivity, metallicity, and topological insulativity, owing to their flexible thickness tunability, different surface functionalization possibilities, and various compositions. 122,123 It should be noted that the electronic characteristics of MXenes can be improved by surface modification. [124][125][126] However, as most MXenes are metallic, 127 their electronic properties are mainly determined by the M atoms. As has been predicted, the Mo-and W-based MXenes are insulators. 128,129 Some metallic MXenes may become semiconductors after surface modification. 130 For instance, F-and I-terminated Ti 3 C 2 have shown semiconductivity, while pristine Ti 3 C 2 131 has metallic properties.
Dong et al. reported Hf 2 VC 2 , Hf 2 MnC 2 , and Ti 2 MnC 2 MXenes ( Fig. 6a and b). 132 Ti 2 MnC 2 is metallic or a semiconductor based on the type of the functional groups. For example, it is a semiconductor with -O terminations. Moreover, Du et al. reported that some metallic MXenes with -OH and -O terminations (such as Nb 4 C 3 , Ti 3 C 2 , Nb 2 C, V 2 C, and Ti 2 C) have shown significant charge transfer, 133 while Ti 3 CO 2 exhibited semiconductor properties due to the generation of a large gap induced by the downshifting of the Fermi level (Fig. 6c). In TiCO 2 , by increasing the donation of electrons between carbon, oxygen, and titanium atoms, the Fermi level decreases. This phenomenon was confirmed by another study by Pandey et al. which evaluated the electronic properties of M n C nÀ1 O 2 MXenes (n = 2, 3, 4, and M = W, Ta, Hf, Mo, Nb, Zr, Y, Mn, Cr, V, Ti, Sc). 134 Apart from several O-terminated MXenes, including Hf 3 C 2 O 2 , Hf 2 CO 2 , Zr 3 C 3 O 2 , ZrCO 2 , and TiCO 2 , other F-or OH-terminated MXenes were metallic. Furthermore, the electronic properties of i-MAX phases, such as (V 2/3 Zr 1/3 ) 2 AlC, (W 2/3 Y 1/3 ) 2 AlC, (W 2/3 Sc 1/3 ) 2 AlC, (Mo 2/3 Y 1/3 ) 2 AlC, and (Mo 2/3 Y 1/3 ) 2 AlC, have been studied. 135 The pristine MXenes synthesized from (Mo 2/3 Y 1/3 ) 2 AlC and OH-and F-terminated of (Mo 2/3 Y 1/3 ) 2 C are metallic, while the O-terminated sample becomes a semiconductor (Fig. 6d). Chen et al. investigated the conductivity properties of pristine Ti 3 C 2 and Ti 3 C 2 /glycine hybrid. 54 Their results (Fig. 6e) indicated that the conductivity of the hybrid diminished by increasing the amount of poor conductive glycine in the hybrid.
In addition to experimental studies, theoretical studies have also been performed recently to assess the DOS, band structure, and electronic properties of pristine and functionalized MXenes. 136 As examples, several MXenes such as Hf 2 CO 2 , Zr 2 CO 2 , Ti 2 CO 2 , and Sc 2 CT x (T = O, F, OH) were reported with a band gap ranging from 0.24 to 1.8 eV, 130 which shows the remarkable effect of the surface groups of MXenes (Fig. 7a). The band structure of Sc 2 CO 2 is different from those of Sc 2 CF 2 and Sc 2 C(OH) 2 , and this remarkable effect can be ascribed to the fact that an -OH or -F group is stabilized by receiving an electron, while an -O group is stabilized by receiving two electrons. The electronic structures of pristine and functionalized MXenes were investigated to understand the mechanism of being semiconducting after the surface functionalization. 130 From Fig. 7b and c, there is a tiny band gap between the p and d bands of X elements. Particularly, O terminations caused large downward shifting of the Fermi level energy resulting in the synthesis of the Ti 2 CO 2 semiconductor, while F functionalization led to small downward shifting. These mechanisms are supported by DOS analysis (see Fig. 7).

Magnetic properties
The application of 2D materials in spintronics has been limited because 2D materials are mostly nonmagnetic. 138 Hence, many studies have recently been carried out to improve the magnetic properties of MXenes. Fortunately, several MXenes such as Mn 2 C, Cr 2 N, Cr 2 C, Ti 2 N, and Ti 2 C are inherently magnetic. It has been found that Mn 2 C 139 and Cr 2 N 140 are antiferromagnetic, but Cr 2 C, 141 Ti 2 N, 142 and Ti 2 C 140 are ferromagnetic. Recently, magnetic MXenes with half-metallic features have been studied, where one spin-channel is semiconducting, while the other one is metallic; thereby resulting in entirely spin-polarized electrons at the Fermi level. 138 The band structure of Cr 2 C (a half-metallic MXene) is presented in Fig. 8a, which shows an insulating spin-down channel and a metallic spin-up channel. 141 Surface modification can also affect the magnetic properties of MXenes. It can lead to a surface-functionalized MXene with poorer magnetic properties, which are induced by unpaired surface electrons of transition metal elements. 138 For instance, modification with -Cl, -OH, -H, and -F groups changes the ferromagnetic half metal of pristine Cr 2 C to an antiferromagnetic semiconductor. 143 However, modification with -O groups endows Cr 2 NO 2 MXene with a ferromagnetic half metal feature. 140 Furthermore, Mn 2 CCl 2 MXene displays an antiferromagnetic insulator characteristic, while the pristine Mn 2 C is an antiferromagnetic metal. Interestingly, a ferromagnetic half metal of Mn 2 CF 2 with a sizeable and half metallic gap, and a high Curie temperature, was fabricated. 144 Moreover, as reported by Khazaei et al., functionalized Cr 2 N and Cr 2 C MXenes are ferromagnetic, 130 and this could be related to the d orbitals of Cr, as it has been theoretically indicated that Cr atoms might have magnetic moments of (0.45m B /Cl) in MAX phases. 145 The magnetic properties of functionalized MXenes with asymmetrical functionalization groups have been widely evaluated. As reported, Cr 2 CFOH, Cr 2 CHF, Cr 2 CHCl, Cr 2 CClBr, and Cr 2 CFCl are antiferromagnetic semiconductors with a Néel temperature of almost 400 K. 144 Ferromagnetic or antiferromagnetic properties of asymmetrical functionalized Cr 2 C MXenes are similar to symmetrical functionalized materials. 144 Fig. 8b shows the DOS of Cr 2 CFCl and Cr 2 CF 2 , where Cr 2 CFCl exhibited bipolar magnetic semiconductor characteristics, indicating that the valence band maximum and the conductive band minimum show opposite spin polarization in comparison to Cr 2 CF 2 with symmetrical functionalization groups.

Mechanical properties
Surface terminations of MXenes are considered as a main factor for their mechanical properties. 146 Generally, O-terminated MXenes show higher stiffness compared to -OH and -F terminated MXenes. 147 This phenomenon can be attributed to the various lattice properties of MXenes with diverse termination groups. 148 For example, Guo et al. reported that the surface functionalization of Ti 2 C leads to increased flexibility, 149 increased strain, but decreased Young's modulus. 149 The critical strain increases, and the collapse of Ti layers decelerates, because the different functional groups on the surface of MXenes act as a delimiter under tensile deformation. Another factor that influences the mechanical properties is the number of atomic layers of MXenes based on their chemical formula. In this regard, Borysiuk et al. found that the hardness and tensile strength of an M n+1 X n MXene increase with decreased n. 150 Furthermore, the mechanical properties of MXenes can be improved by incorporating CNTs and polymerizing with various types of polymers. 146 It has been reported that MXene-polymer composites have higher compressive and tensile strengths, toughness, and flexibility, compared to pristine MXenes. For instance, the Ti 3 C 2 /PVA composite exhibited larger tensile strength compared to pristine Ti 3 C 2 . 146 Additionally, Ti 3 C 2 / ultrahigh molecular weight polyethylene (UHMWPE) 151 and Ti 3 C 2 /polyacrylamide (PAM) 152 composites have higher yield strength and toughness compared to pristine Ti 3 C 2 .

Optical properties
Optical properties of MXenes, including refractive index, reflectivity, absorption, and transmittance are of importance in different applications. There have been a few experimental studies on the effect of surface functionalization on optical properties. 153,154 For instance, pristine Ti 3 C 2 showed 77% transmittance in visible light, while the 30% transmittance was observed for a thin Ti 3 AlC 2 MAX phase. 155,156 Ammonium bifluoride (NH 4 HF 2 ) intercalated Ti 3 C 2 thin films (Ti 3 C 2.3 -O 1.2 F 0.7 N 0.2 ) showed 90% transmittance. 156 Optical properties such as reflection, absorption, and transmittance can be investigated theoretically by calculating the imaginary part of the dielectric function tensor at different photon wavelengths. 147,157 Bai et al. reported the optical properties of pristine Ti 2 C and functionalized Ti 2 C with -O, -OH, and -F in both out-plane and in-plane directions (Fig. 9a). It can be seen obviously that the in-plane absorption coefficients of OH-and F-functionalized Ti 2 C MXenes are lower in comparison to pristine and O-functionalized Ti 2 C MXenes. Moreover, the white color indicates the OH-and F-functionalized Ti 2 C according to the refractivity results. Furthermore, the performance of MXenes in visible and UV light absorption is the key factor for the optoelectronic, photovoltaic, and photocatalytic properties, which are affected by the existence of functional groups (Fig. 9b). 75,158,159 The same properties were observed for F-and OH-functionalized MXenes, in contrast to the performance of O-functionalized MXenes. Additionally, in the UV range, the reflectivity improved in comparison with pristine MXenes, while in the visible region, the F-and OH-functionalized MXenes decreased the reflectivity and absorption (Fig. 9c).

Hydrophilicity/hydrophobicity
As mentioned earlier, MXenes have shown superior performance in numerous applications. [160][161][162][163] However, MXenes degrade in the presence of water and oxygen because of their hydrophilic nature induced by -O, -F, and -OH functional groups. [164][165][166][167][168] The surface functionalization of MXenes has been reported as an efficient strategy to enhance their stability as well as their surface characteristics. 169 For example, Ji et al. functionalized Ti 3 C 2 with silylation reagents ((3-aminopropyl)triethoxysilane (APTES)) to tune its hydrophilicity (Fig. 10a). 170 The stability of Ti 3 C 2 improved by APTES functionalization by blocking the contact between water and MXenes. This improvement was also achieved for hexadecyltrimethoxysilane and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (FOTS) (Fig. 10b). The results indicated the change of surface properties from hydrophilic to hydrophobic. Two superhydrophobic self-assembled monolayers (SAMs) [(3-chloropropyl)trimethoxysilane (CPTMS) and FOTS] were used for surface modification of MXenes via formation of covalent bonds between -OH and different surface functional groups (Fig. 10c). 171,172 The water contact angle results (Fig. 10d  showed a superhydrophobicity of 1561 and 961 for the fluorinecontaining (Ti 3 C 2 -F) and chlorine-containing (Ti 3 C 2 -Cl) films, respectively, while the pristine Ti 3 C 2 is basically hydrophilic due to its -O and -OH functional groups (a water contact angle of 331). Hence, the functionalization of Ti 3 C 2 with superhydrophobic reagents such as FOTS and CPTMS can be an effective approach to inhibit the degradation of MXene materials.

Applications
Because of their synergistic properties, functionalized MXenes have shown significantly better performances in comparison to pristine MXenes. 6,[174][175][176][177] To understand the effect of a specific functionalization on a MXene, the structural and applicationspecific properties of functionalized MXenes should be studied.
To this end, herein, applications of surface-modified MXenes in fields such as biomedical application, sensors, catalysis, energy storage and conversion, and membrane-based separation are reviewed.

Biomedical applications
MXenes have been extensively used in biomedical applications owing to their outstanding properties such as magnetic, electronic, optical, etc. MXenes are typically non-toxic and biocompatible due to multiple compositions. 42 The different functional groups (-F, -O, and -OH) on the surface of MXenes make them hydrophilic. Moreover, MXenes have excellent light absorption in the near-infrared (NIR) region, which allows their use in photoacoustic imaging (PAI) and photothermal therapy (PTT) applications. 52 Additionally, the surface of MXenes can be easily modified by various approaches, providing new opportunities for further biomedical applications. 36 35 The UV-vis-NIR absorption spectra of Ti 3 C 2 demonstrated an absorption of around 750-850 nm at l = 808 nm, and the absorption intensity remarkably increased by increasing the concentration of Ti 3 C 2 (Fig. 11a). An extinction coefficient of 25.2 L g À1 cm À1 was achieved, and a linear relationship between the Ti 3 C 2 concentration and the normalized adsorption intensity was observed. The photothermal performance of Ti 3 C 2 was measured at various C concentrations at a power density of 1.5 W cm À2 (Fig. 11b), where the temperature of the solution (containing 72 ppm Ti 3 C 2 MXenes) reached 57 1C, while the temperature of clean water had not changed, indicating the fast conversion of NIR light to thermal energy. Furthermore, the efficient effect of Ti 3 C 2 -SP MXene on the photothermal properties was confirmed under NIR laser irradiation, where by increasing the power density, more cells incubated with SP-modified Ti 3 C 2 were killed compared to cells without Ti 3 C 2 -SP. TdT-mediated dUTP Nick-End labeling (TUNEL) and hematoxylin and eosin (H&E) staining indicated more necrosis of tumor cells after treatment with Ti 3 C 2 -SP under NIR laser irradiation than after treatment with other materials (Fig. 11c). Fig. 11d shows the strong effect of Ti 3 C 2 -SP under NIR laser irradiation on the in vivo proliferative activity of the Ki-67 antibody. 4.1.2. Biosensing. In addition, surface-functionalized MXenes have received a lot of attention in biosensing because of their outstanding biocompatibility, excellent electrical properties, and large surface area. 181 For example, a new organ-like MXene-based material was prepared for biosensing applications to detect H 2 O 2 in the range of 0.1-260 mM. 182 Hereby, hemoglobin (Hb), as a protein, was utilized for the modification of the surface of Ti 3 C 2 MXenes (Fig. 12a). The cyclic voltammogram (CVs) performances of Hb-functionalized Ti 3 C 2 electrodes under the physiological condition of pH 7.0 are shown in Fig. 12b, where the Nafion/MXenes/glassy carbon (GC) electrode (curve a) did not show any redox peak, demonstrating its electro-inactive state in the potential window. Besides, two redox peaks can be observed at À0.327 V and À0.367 V vs. Ag/AgCl for the Nafion/Hb/MXenes/GC electrode (curve b), indicating that an appropriate microenvironment can be achieved for Hb to facilitate the reaction of electron transfer. A smaller redox peak is seen at the Nafion/Hb/GC electrode (curve c), which may be attributed to the unsuitable direct electron transfer (DET) between the electrode and the enzyme. It can be concluded that the DET process between the GC electrode and Hb molecules improved at the Nafion/Hb/ MXene/GC electrode. Moreover, the Nafion/Hb/MXene/GC electrode exhibited good performance in detecting H 2 O 2 by the measurement of amperometric responses (Fig. 12c and d). The nitrite biosensor and glucose biosensor based on MXenes have been reported in the literature. 183,184 Hence, MXene-based materials have shown incredible potential for enzyme conjugation and biosensing applications.
4.1.3. Photothermal therapy and drug delivery system. Cancer disease is a major cause of death worldwide. 185 Recently, many operation treatments have been developed for cancer therapy, although most of them, such as radiotherapy and chemotherapy, not only do not remove cancer tumors completely, but also create side effects. 186 Therefore, photothermal therapy (PTT) has emerged as a highly efficient strategy to prevent the growth of cancer tumors because PTT can transform NIR light to thermal energy. 187 It should be noted that NIR light is classified into two sections, including first biowindow (750-1000 nm) and second bio-window (1000-1350 nm), where a few studies have utilized the second bio-window in PTT applications because there are no adequate materials with high-efficient photothermal conversion and tailored NIR light absorption. 188 Interestingly, MXene-based materials have attracted tremendous attention for PTT applications owing to their high photothermal conversion and strong absorption. For instance, Lin et al. prepared poly(vinylpryrrolidone) (PVP)functionalized Nb 2 C MXenes for using in the PTT of mice tumor xenografts, as depicted in Fig. 13a and b. 99 At first glance, other treatment methods were applied on the tumor cells of mice, while the tumor cells are still large (Fig. 13c). Fig. 13d and e indicate that tumor regions entirely vanished once PTT was used by Nb 2 C-PVP + NIR-I and Nb 2 C-PVP + NIR-II. Fig. 13f depicts the comparison of survival curves for various treatment techniques, where it can be clearly seen that the recovered mice by Nb 2 C-PVP + NIR-I and Nb 2 C-PVP + NIR-II survived for more than 50 days. MXene-based materials provide numerous opportunities to be anchored by therapeutic molecules, so they are widely applied for developing drug delivery systems. 189 Recently, Han et al. synthesized Ti 3 C 2 immobilized by SP for drug delivery of PTT and chemotherapy ( Fig. 14a and b), 37 which led to facile transport in blood vessels and superior water dispersibility. 190 Afterward, electrostatic adsorption was used for the conjugation of anti-cancer drugs (Dox and doxorubicin) on the surface of Ti 3 C 2 -SP MXenes. The results showed that the photothermal conversion of Ti 3 C 2 -SP MXene led to a noticeable improvement of Dox release under laser irradiation. The red fluorescence (Fig. 14c-e) demonstrated more dead cells by combining laser irradiation and Dox@Ti 3 C 2 -SP MXenes compared with other groups (Ti 3 C 2 -SP + laser and Dox@Ti 3 C 2 -SP).

Sensors
MXenes have been found to have promising potential in sensing applications, such as electrochemical sensors, 33 strain sensors, [191][192][193] biosensors, [194][195][196] and gas sensors, 31,197 due to their piezoelectric and thermoelectric response, rich surface groups, large surface area, and excellent anisotropy of conductivity. For instance, Shi et al. prepared layered Ti 3 C 2 -Ag nanowire-PDA/Ni 2+ nanocomposite film-based strain sensors using a microscale ''brick-and-mortar'' architecture. 192 They incorporated either Ni 2+ or PDA into Ti 3 C 2 -Ag nanowires. As shown in Fig. 15a, the Ti 3 C 2 -Ag nanowire-PDA/Ni 2+ nanocomposite exhibited superior performance of 83% stretchability compared to those of the pristine Ti 3 C 2 -Ag nanowire (32%), Ti 3 C 2 -Ag nanowire-Ni 2+ (47%), and Ti 3 C 2 -Ag nanowire-PDA (75%). The output signals of Ti 3 C 2 -Ag nanowire-PDA/Ni 2+ sensors were measured at various strains (Fig. 15b), and at a strain of 80%, the steady electrical responses of the sensors resulted in a strain rate of 1.2-8 mm S À1 . Furthermore, the practical potential of the Ti 3 C 2 -Ag nanowire-PDA/Ni 2+ strain sensor was verified by the attachment of two individual strain sensors on the skin of the knee and wrist joint of a volunteer (Fig. 15c). Fig. 15d shows the electrical signals generated by the strain sensor on the wrist when the volunteer is walking. The radial waves of a single heartbeat, including percussion (P), tidal (T), and diastolic (D),  were isolated. The strain sensor detected motion signals in running, demonstrating the durability and reliability of the sensors (Fig. 15e). Kumar et al. reported a biofunctionalized Ti 3 C 2 (f-Ti 3 C 2 ) via APTES for the detection of cancer. 90 Besides, the deposition of TiO 2 nanoparticles on the surface of MXenes may offer further advantages for the detection of H 2 O 2 . 198 Therefore, the shielding microenvironment for increasing the stability and activity of enzymes can be achieved by loading TiO 2 with exceptional biocompatibility on the surface of MXenes. The active sites of enzyme adsorption significantly increased for TiO 2 -Ti 3 C 2 compared with pristine Ti 3 C 2 . Moreover, Chen et al. reported that the dispersion of MXenes could be adjusted by varying the temperature and CO 2 concentration, which changes the conductivity and transmittance of V 2 C, 98 where PDMAEMA was used to modify the surface of V 2 C by self-initiated photografting and photopolymerization. By increasing the temperature of the solution above the lower critical solution temperature (LCST), the hydrophilic nature of PDMAEMA changes to hydrophobic, resulting in the enhancement of the transmittance of V 2 C@PDMAEMA. Besides, when CO 2 was added to water to form charged ammonium bicarbonate, the conductivity of V 2 C@PDMAEMA increased because of the CO 2 responsive characteristic of V 2 C@PDMAEMA, demonstrating that V 2 C@PDMAEMA is sensitive to CO 2 and temperature.

Catalysis
Recently, the utilization of MXene-based materials in catalysis applications has received considerable attention. This is due to the appealing properties of MXenes such as (i) the abundant active sites and rich surface groups, (ii) the adjustable band gap of MXenes by the arrangement of surface chemistries, and (iii) the exceptional carrier mobility of MXene-based materials, which can lead to improving the migration and separation of photogenerated electron-hole pairs. 161,199,200 For example, Ramalingam et al. reported the coordination interactions between Ti 3 C 2 and ruthenium single atoms (Ru SA ) through the S and N heteroatom doping method. 201 The polarization curves of the hydrogen evolution reaction (HER) of MXenebased materials (including Ru SA -N-S-Ti 3 C 2 , Ru SA -Ti 3 C 2 , N-S-Ti 3 C 2 , Ti 3 C 2 , and carbon paper) (Fig. 16a) revealed that Ru SA , N, and S co-doped Ti 3 C 2 have great potential for electrocatalytic HER performance; they showed approximately remarkable zero onset potential and the smallest overpotential of 76 and 237 mV to reach 10 and 100 mA cm À2 , respectively. The HER baseline of Ru SA -doped Ti 3 C 2 and Ru SA , N, and S co-doped Ti 3 C 2 catalysts are not at zero potential; this may be due to the underpotential hydrogen adsorption effect of Ru and the capacitance effect of the nanocarbons of Ti 3 C 2 MXenes. 202 The slope of Tafel plots was calculated to evaluate the reaction kinetics of the catalysts in the HER process (Fig. 16b), where this slope for the Ru SA , N, and S co-doped Ti 3 C 2 catalysts was about 90 mV dec À1 , demonstrating that the Volmer-Heyrovsky mechanism is followed by these electrocatalysts which includes two steps: (i) fast initial discharge reaction (reaction (I)) and (ii) slow electrochemical desorption reaction (reaction (II)). 203 Additionally, the significant activity of the Ru SA , N, and S co-doped Ti 3 C 2 electrocatalysts was confirmed by the measurements of turnover frequency (ToF), showing the exceptional performance of these electrocatalysts compared to other Ru SAbased electrocatalysts.
Interfacial engineering of MXene-based materials can be easily coupled with other nano-materials by the combination of large exposed basal plane and good flexibility of two phases, resulting in the formation of highly active catalysts. 205 Besides, the reaction kinetics can be improved via the change in the electronic structure of MXenes. 205 Recently, many studies have been performed on the effect of the interfacial structure of Ti 3 C 2 MXene and TiO 2 nanoparticles on the photocatalytic reactions, 88,206,207 where the TiO 2 /Ti 3 C 2 was prepared by the in situ growth of TiO 2 on the surface of Ti 3 C 2 by a hydrothermal strategy. 208 As Fig. 16c shows, the TiO 2 /Ti 3 C 2 exhibited superior photocatalytic degradation (almost 98%) compared to pristine TiO 2 (42%) in 30 min; this can be related to the effective electron-hole separation. In addition, a mild oxidation method has been applied for the homogeneous growth of Nb 2 O 5 on the surface of Nb 2 C MXenes in order for the HER. 209 Charge carriers were generated at the interface of Nb 2 O 5 /Nb 2 C due to the contact between conductive Nb 2 O 5 and Nb 2 C. Wang et al. formed a Schottky barrier in MIL-100(Fe)/Ti 3 C 2 (Fig. 16d). 210 The efficient visible light photocatalytic properties obtained for MIL-100(Fe)/Ti 3 C 2 were three and four times higher than those for pristine MIL-100 and Ti 3 C 2 , respectively.
Moreover, FeNi-LDH nanoplates were interfacially coupled on the surface of Ti 3 C 2 , 211 leading to an increase in the positive charge of FeNi-LDH/Ti 3 C 2 owing to the strong electronic junction between the Ti 3 C 2 MXenes and FeNi-LDH. This indicated the significant improvement of the oxygen evolution reaction (OER) performance, as the Tafel slope decreased from 62 to 43 mV dec À1 in comparison to FeNi-LDH ( Fig. 16e and f). The HER activity of Nb-doped Ti 3 C 2 was enhanced by surface modification with an Ni/Co alloy, which led to the weakening of the M-H affinity. 212 DFT calculations showed that better catalysts can be fabricated by replacing Ti with Co or Ni, which can be attributed to the synergistic effect of Ni/Co@Nb-doped Ti 3 C 2 .
MXene-based materials have also found applications in photocatalytic water splitting, a novel technology of clean energy conversion. 138 Traditional catalysts such as precious metal-based oxides have shown low efficiency due to their high cost and scarcity. 213,214 However, recently MXene-based materials have been widely utilized in photocatalytic water splitting technology because of their rich exposed metal sites, suitable hydrophilicity, and excellent electronic conductivity. The OER is considered as a key reaction in water splitting because it allows the production of highly pure O 2 . The high catalytic performance of MXene-based materials can be obtained by adjusting their geometric properties. 205 It was found that N-doped Ti 3 C 2 (denoted as Ti 3 C 1.6 N 0.4 ) possessed high electrical conductivity in comparison to pristine Ti 3 C 2 (Fig. 17b). 215 The smaller contact angle with the KOH electrolyte can be seen for the prepared film of Ti 3 C 1.6 N 0.4 MXene, confirming its suitability as an electrolyte for the OER (Fig. 17a). As depicted in Fig. 17c and d, the Tafel slope and onset potential of the Ti 3 C 1.6 N 0.4 film were 216.4 mV dec À1 and 245.8 mV, respectively, showing better electrocatalytic properties compared to the pristine Ti 3 C 2 . Furthermore, the surface structures of Mo 2 C were tuned by incorporating isolated cobalt atoms into the lattice of Mo 2 C MXenes, leading to a lower overpotential. 216 Moreover, using an electrodeposition approach, Pt atoms were anchored on the surface of Mo 2 TiC 2 . The resulting material showed high HER performance, which can be ascribed to the efficient active site of Pt atoms for the HER. 217 DFT calculations exhibited strong atomic interactions in Mo 2 TiC 2 -Pt SA , resulting in the enhancement of d-electron domination near the Fermi level and also conductivity, and consequently, leading to improved catalytic activity. Besides, the strong bonding between the Mo 2 TiC 2 and Pt atoms led to improved stability properties.

Energy storage and conversion
4.4.1. Energy storage. MXene-based materials have great potential for energy storage and conversion applications, because of their appropriate mechanical stability, metallic conductivity, and good hydrophilic surface. 6,115 The low capacity and tendency of restacking of pristine MXenes have restricted their applications in energy storage and conversion, which can be overcome by surface functionalization of MXenes. 26 In the case of battery performance, the volume variation of charge and discharge cycles can be substantially diminished, while modified MXenes have exhibited improved performance. 218,219 For instance, a melamine formaldehyde template method was employed to fabricate N-doped Ti 3 C 2 for lithium (Li)-sulfur (S) chemistry, as a highly efficient electrocatalyst. 220 The process of conversion and capture of lithium polysulfides (LiPSs) by porous N-doped Ti 3 C 2 Fig. 17 (a) The contact angle measurements of Ti 3 C 2 , Ti 3 C 1.8 N 0.2 , and Ti 3 C 1.6 N 0.4 films in a 1M KOH electrolyte. (b) Electrical conductivity of the three films. (c) Tafel plots of the three electrocatalysts. (d) Comparison of the Ti 3 C 2 , Ti 3 C 1.8 N 0.2 , and Ti 3 C 1.6 N 0.4 electrocatalysts at an onset overpotential of 1.0 mA cm À2 and a mass activity of 350 mV. Reproduced with permission. 215 Copyright 2020, The Royal Society of Chemistry. Fig. 18a, where numerous active sites are provided for the conversion and adsorption of LiPSs because MXenes have high electrical conductivity, large surface area, and porous structure. Besides, the interfacial interactions between the porous N-doped Ti 3 C 2 MXenes and the Li 2 S cluster were improved by the N doping process, which led to the degradation of the Li 2 S dissociation barrier on the porous Ndoped Ti 3 C 2 MXenes and as well as the acceleration of the nucleation kinetics of Li 2 S, thereby the decomposition reaction of Li 2 S increases. Moreover, the sulfur redox kinetics are notably facilitated by the low diffusion barrier of Li atoms on the porous N-doped Ti 3 C 2 . The results showed that the prepared S-porous N-doped Ti 3 C 2 cathode has higher capacities compared to the S/Ti 3 C 2 cathode and pristine S cathode (Fig. 18b). Fig. 18c shows the initial capacity and coulombic efficiency, where S-porous N-doped Ti 3 C 2 MXenes exhibited superior performance at 0.5 C in comparison to those of S/Ti 3 C 2 and pristine S cathode. 220 4.4.1.1. Electrochemical capacitors. Electrochemical capacitors have attracted great interest because of their high power densities, long cycle life, and fast charge/discharge capabilities. 221 Among the various materials, MXene-based materials have received considerable attention owing to their superior electrical conductivity, excellent intercalation effects, and electrochemical energy storage characteristics. 159,222 It should be noted that the capacitive performance can be significantly affected by surface modification. 223 (c) Cycling performance of S/N-doped Ti 3 C 2 at 0.5 C. Reproduced with permission. 220 Copyright 2020, Elsevier. Electrochemical properties of pristine Ti 3 C 2 , 500N-Ti 3 C 2 , 700N-Ti 3 C 2 , 900N-Ti 3 C 2 , and 900N-Ti 3 C 2 without NH 2 CN electrodes with 6M KOH as the electrolyte; (d) comparison of cyclic voltammetry curves at a scan rate of 100 mV S À1 . (e) Discharge current densities as a function of scan rate. (f) Gravimetric specific capacitance at different current densities. Reproduced with permission. 118 Copyright 2018, Willey.

Materials Advances Review
Open Access Article. Published on 25 October 2021. Downloaded on 6/10/2023 9:10:12 PM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.
For example, an N-doped Ti 2 C MXene with various contents of N doping was prepared by carbon nitride decomposition and sequential intercalation via exfoliation and thermal annealing approach. 118 The capacitance performance of all pristine Ti 2 C and N-doped Ti 2 C demonstrated that as the annealing temperature increases, the cyclic voltammetry (CV) curve shape becomes more and more rectangular and the related surface areas increase (Fig. 18d). Indeed, the largest area observed for N-doped Ti 2 C MXene at 9001 suggests the high efficiency of the doping strategy. The discharge current densities of N-doped Ti 2 C exhibit an approximately linear relationship with the scan rate, indicating that the fast electrochemical reaction occurred on the entire surface of the electrode (Fig. 18e). The highest specific capacitance of the N-doped Ti 2 C electrode can attain 327 AE 1 F g À1 at 900 1C which is higher than those of the pristine Ti 2 C and other N-dopedbased MXene electrodes at different temperatures (Fig. 18f).

Metal-ion batteries.
In the area of metal-ion batteries, including LIBs and SIBs, MXene-based materials have received considerable attention due to their kinetics and thermodynamics of metal-ion diffusion and intercalation in the lamellar space of MXenes. However, there are still unknowns such as the storage mechanism of metal ions, appropriate surface functional groups, and optimal MXene/metal ratios. 224,225 The specific capacities of MXene-based materials are much lower than those of other electrode materials. 226 Hence, surface modification is a promising method to enhance the performance in SIBs and LIBs via combining the advantages of active sites and the conductivity of MXenes. For instance, the rate capability and the specific capacity of MXenes have been improved via surface modification through hydrothermal oxidation of TiO 2 ; 227-229 a specific capacity of 291 mA h g À1 for LIBs after 2000 cycles at a current density of 100 mA g À1 230 was achieved. Liu et al. synthesized a novel N-doped Nb 2 C MXene via a thermal reaction with urea. 81 Their N-doped Nb 2 C MXene presented superior cycling stability and an excellent capacity of 360 mA h g À1 due to the improved specific surface area and electrical conductivity caused by the extension of the c-lattice parameter. Moreover, the surface chemistry of MXenes can be adjusted by cation modification. 29 The ion capacity of Sn 4+ decorated V 2 C MXenes was boosted to 1284 mA h g À1 at a density of 0.1 A g À1 , and the cycling stability reached a remarkable value of 1262.9 mA h g À1 after 90 cycles at the same density. 231 These improvements can be ascribed to the V-O-Sn bond and enlarged interlayer space, resulting in the fast charge transfer and interface ionic diffusion. Moreover, Guo et al. fabricated a 2D phosphorene/MXene hybrid anode with an in situ formed fluorinated interphase for LIB applications. 232 Their theoretical and experimental investigations showed that the combination of Ti 3 C 2 and the 2D phosphorene heterostructure not only enhances the sodium diffusion but also alleviates the volume change of phosphorene. Furthermore, an in-depth X-ray photoelectron spectroscopy analysis demonstrated that the -F terminated MXene could enhance the Coulombic efficiencies and cyclic performances of the phosphorene/MXene hybrid anode by forming fluorine-rich compounds. As a result, the phosphorene/Ti 3 C 2 F 2 presented excellent cycling performance (343 mA h g À1 after 1000 cycles at 1 A g À1 ) and a superior reversible capacity of 535 mA h g À1 at 0.1 A g À1 compared to the fluorine-free electrolyte. Natu et al. reported that Ti 3 AlC 2 can be etched and delaminated in various organic solvents in the presence of NH 4 HF 2 . 233 They found that the electrode capacity of the Ti 3 C 2 etched in propylene carbonate (PC) was almost two times more than the same MXene synthesized in water when tested in NIB and that Ti 3 C 2 rich in -F terminations can be synthesized using NH 4 HF 2 -containing polar organic solvents in the etching process.
Chen et al. investigated the experimental and theoretical influence of glycine functionalization of Ti 3 C 2 O 2 MXenes on charge storage because organic molecules can lead to enhanced interlayer space and hinder restacking. 54 The strong chemical reaction between glycine and Ti 3 C 2 O 2 MXenes was ascribed to the formation of the Ti-N bond via shared electrons between them. The interlayer spacing of pristine Ti 3 C 2 O 2 increased from 10.16 to 14.06 Å by surface functionalization (Fig. 19a and b), which leads to enhanced cycling performance, improved charging rate, and better ion accessibility. Surface functionalization of Ti 3 C 2 O 2 led to a high capacitance of 324 F g À1 at a scan rate of 10 mV s À1 (Fig. 19c), and also to an improved retention capacitance of 140 F g À1 for Ti 3 C 2 O 2 /glycine electrodes, which is double the value for pristine Ti 3 C 2 O 2 .
4.4.2. Energy conversion. The use of abundant solar energy allows the existing challenges in energy and environmental pollution to be addressed. 234 Solar energy can be directly converted to heat energy using 2D materials such as MXenes. 235 Photothermal energy conversion is extensively used in self-heating textiles, 236 seawater desalination, 237 and tumor treatment. 238 For instance, Yang et al. prepared PPy-modified Ti 3 C 2 MXenes by in situ polymerization on the surface of Ti 3 C 2 to enhance the antioxidant capacity and performance of photothermal energy conversion. 239 They then sprayed the PPy-modified MXenes (MXe-ne@PPy) on a transparent film heater. Superior photothermal conversion of MXene@PPy can be obtained because of the synergistic effect of Ti 3 C 2 and PPy. A comparison of the photothermal energy conversion performances of the pristine MXene and MXene@PPy films ( Fig. 20a and b) indicated that under 200 mW cm À2 irradiation, the pristine MXene film showed tiny light-to-heat conversion along with a negligible increment in the temperature. In contrast, MXene@PPy was able to convert light to thermal energy more efficiently. The surface temperature increased from 25 to 61 1C. This enhancement in temperature confirmed the efficient photothermal conversion ability of MXe-ne@PPy compared to the pristine MXene. After grafting PPy onto the MXene surface, the photothermal conversion performance is stable for approximately 7 months in air atmosphere. Du et al. synthesized a polymerized PDA-decorated MXene (Ti 3 C 2 @PDA) by in situ grafting of PDA on the surface of Ti 3 C 2 . 240 Fig. 20c illustrates the solar-thermal conversion performance of Ti 3 C 2 @ PDA by calculation of the temperature evolution curves under 250 mW cm À2 irradiation. The solar-thermal efficiencies of a pristine film and an MXene-based film with 2%wt content of Ti 3 C 2 @PDA were reported to be 64.2% and 90.1%, respectively.
It is obvious that the solar-thermal conversion performance of the organic phase change materials (PMCs) enhanced with the incorporation of Ti 3 C 2 @PDA. Furthermore, Fan et al. reported the introduction of silver nanoparticles (AgNPs) onto the surface of MXene for transparent device applications. 241 The obtained AgNP@MXene hybrid demonstrated a substantial temperature gain of 111 and 148 1C under the irradiation of 600 and 800 mW cm À2 , respectively. The effect of surface modification was evaluated by measuring the surface temperature of pristine MXene and AgNP@MXene hybrid at different time instants under vis-IR light (Fig. 20d). The significant discrepancy in the temperature of the AgNP@MXene hybrid shows the efficient conversion of light to thermal energy owing to the thermal conductivity and photothermal effect of MXenes with the plasmonic properties of AgNPs in conjunction.

Membrane-based separation
In the past few decades, many conventional techniques, including filtration, condensation, crystallization, and distillation, have been widely utilized in separation. However, membrane-based processes have emerged as a promising candidate for use in different applications such as water and wastewater treatment, and pharmaceutical and chemical industries. 242,243 Recently, MXene-based materials have been employed in membranebased separation with high separation performance because of their large surface area, superior flexibility, and abundant surface functional groups. 13,244,245 This section reviews the effect of MXene-based materials on the separation performance of membranes and puts the advances into perspective.
4.5.1. Pervaporation. Membrane-based separation processes have attracted much attention in solvent dehydration because of their high energy efficiency and simplicity. 246,247 MXenebased materials have shown excellent separation performance, due to their good mechanical strength and excellent metallic conductivity. 248,249 It should be noted that enough care should be taken to achieve a defect-free layered structure for MXenebased membranes along with highly efficient solvent dehydration. Liu et al. prepared an ordered laminar structure of Ti 3 C 2 MXene membrane by introducing positively charged polyelectrolytes (including PDDA, polyallylamine hydrochloride (PAH), and polyethylene (PEI)) to create electrostatic interactions with negatively charged Ti 3 C 2 nanosheets (Fig. 21a) for pervaporation dehydration of isopropanol. 250 The lowest value of water contact angle for MXene-PDDA indicated that it can absorb more water molecules and facilitate the transport pathway of water, showing better separation performance compared to those of pristine MXene, MXene-PAH, and MXene-PEI membranes. As revealed in Fig. 21b, the MXene-PDDA membrane showed water content of B99.5 wt%, while it was B57, B63, and B18.9 wt% for the MXene-PAH, MXene-PEI, and pristine MXene membranes, respectively. The separation mechanism of the pervaporation process was described according to solutiondiffusion; 251 the diffusion selectivity improved by the formation of regular packing of MXenes induced by PDDA introduction and the enhancement of water content may be related to the improvement of adsorption selectivity toward the water. Moreover, water flux decreased when the PDDA concentrations increased from 0 to 0.36 mg mL À1 , suggesting an increase of transport resistance due to the occupation of transport pathways and enhanced membrane thickness. 252 However, the separation factor first increased and then decreased because of the promotion of selective diffusion and preferential sorption of water rather than isopropanol caused by the improvement of either the packing order of MXenes or hydrophilicity.
4.5.2. Nanofiltration. In another study, functionalized Ti 3 C 2 MXenes were employed in thin-film nanocomposite membranes (TFNs) to improve selectivity and flux for alcoholbased solvents. 253 Four modifiers -APTES, (dodecyl)triethoxysilane (DCTES), (methyl aniline)triethoxysilane (MATES) and (g-methacryloxypropyl)trimethoxysilane (MPTMS) -were employed to prepare functionalized Ti 3 C 2 -NH 2 , Ti 3 C 2 -C 12 H 26 , Ti 3 C 2 -C 6 H 6 , and Ti 3 C 2 -COOR, respectively (Fig. 21c). The area swelling and solvent uptake of the prepared TFN membranes were evaluated with polar solvents (ethyl acetate and isopropanol) and non-polar solvents (n-heptane and toluene). As a result, the incorporation of Ti 3 C 2 MXene led to the reduction of area swellings and solvent uptakes except for isopropanol due to its non-swelling characteristics. However, the storage and adsorption of isopropanol were promoted by the -OH groups on Ti 3 C 2 through H-interaction. For the same reason, Ti 3 C 2 -COOR and Ti 3 C 2 -NH 2 embedded TFN membranes demonstrated higher area swelling and solvent uptakes for polar solvents, while Ti 3 C 2 -C 6 H 6 and Ti 3 C 2 -C 12 H 26 exhibited higher area swelling and solvent uptakes toward non-polar solvents. All Ti 3 C 2 -M incorporated TFN membranes improved the affinity for a specific solvent based on the polarity properties of solvents and functional groups. This phenomenon can be ascribed to the fact that each of the four functional groups acts as a carrier and facilitates molecular transport through the TFN membranes. The affinity of Ti 3 C 2 -NH 2 and Ti 3 C 2 -COOR incorporated membranes can be improved towards polar solvents. For example, -NH 2 can facilitate the transport of isopropanol molecules by acting as carrier sites. Similarly, embedding Ti 3 C 2 -C 12 H 26 and Ti 3 C 2 -C 6 H 6 enhances the transport of non-polar solvents. Adding a Ti 3 C 2 -M into the polymer matrix improves the rejection ability, and this could be related to the Ti 3 C 2 and Ti 3 C 2 -M blocking the transport of molecules. 4.5.3. Desalination. MXene-based materials have provided a great opportunity to prepare membranes for seawater desalination. 255,256 MXene-based membranes have been synthesized to separate salts according to ionic charges and the hydration radius, 257 where the interlayer spacing can play a key role in salt separation. The surface modification of MXene nanosheets enhances the interlayer spacing, resulting in the improvement of selective separation. 258 Furthermore, it has been predicted that MXenes can show significant performance as a photothermal material along with the conversion of phototo-thermal characteristics, 259  MXenes into the polymeric network of cellulose acetate. 254 Li-intercalated MXene (denoted as Li-MXene) possesses a high absorption rate (Fig. 21d) with high structural similarity to the pristine MXene. This may be attributed to the high photon absorption of Li-MXene in its structure and also to its black color, where lower lights are reflected. Measurement of the mass change demonstrated a remarkable enhancement of steam generation by the CAM membrane, which are ascribed to the superior photothermal characteristics of Li-MXene. Fig. 21e shows the energy conversion of the CAM membrane, where an efficiency of 92.1% was obtained after 11 cycles.
In addition, a salt rejection of 97.7% was achieved for high salinity synthesized seawater (Na + concentration of 18 000 mg L À1 ) (Fig. 21f). These results showed that CAM membranes are good candidates for solar desalination applications because of their excellent robustness, scalability, and effectiveness.

Dye removal.
Dyes are the major pollutants of discharged wastewater from industries, which can threaten human health issues because of their high carcinogenicity. 260 Therefore, the removal of dyes from wastewater should be considered as a major priority from an environmental viewpoint. Functionalized MXene-based membranes have exhibited promising potential for the removal of dyes from water and wastewater. 261,262 In this area, the interlayer spacing of MXenes plays a vital role in pressure-driven membrane processes because molecules larger than the interlayer spacing are rejected, and solvents are facilely penetrated. 22 Pandey et al. synthesized Ag@MXene membranes by grafting Ag on the surface of MXenes to enhance the interlayer spacing. 263 Accordingly, water flux increased by introducing Ag nanoparticles on the MXene surface compared to the pristine MXene-based membrane. This promotion can be due to the interlayer voids of additional nanochannels by the presence of Ag nanoparticles, leading to facile water pathways. Besides, the Ag@M-Xene membranes exhibited high rejection of 100%, 92%, and 79.9% for bovine serum albumin (BSA), methyl green (MG), and rhodamine, respectively ( Fig. 22a and b). Flux recovery of membranes needs to be evaluated to understand the membrane fouling properties. The Ag@MXene membranes after exposure to BSA and MG indicated 91% and 97% of flux recovery compared to 81% and 86% for the pristine MXene membrane. 4.5.5. Gas separation. Another field that MXene-based materials have impacted is the area of membrane-based gas separation, which is growing due to the low energy consumption and high efficiency of these processes. 265 Different kinds of 2D materials, such as metal-organic frameworks (MOFs), 266,267 covalent-organic frameworks (COFs), 268 zeolites, 269 and graphene, 270 have been extensively employed for improving the gas separation performance of membranes by surpassing the trade-off relationship between permeability and selectivity. 271 MXene-based materials are attractive to further improve the gas separation performance by tuning the interlayer spacing, as gas molecules larger than the interlayer spacing are prevented from passing through. 272 The extraordinary separation performance of MXene-based membranes, such as favorable mechanical stability and good reproducibility due to the existence of ordered channels in lamellar MXene membranes, has been reported. 272 The surface modification of MXene nanosheets can improve the interlayer space, resulting in enhanced selective molecular separation. Shen et al. prepared a MXene-based nanofilm on an anodic aluminum oxide (AAO) substrate for H 2 /CO 2 separation (Fig. 22c). 264 The pristine MXene nanofilm with 20 nm thickness showed high permeance of 1584 GPU and H 2 /CO 2 selectivity of 27, which exceeded the Robeson upper-bound. Besides, it is worth noting that this kind of membrane exhibited stable performance during 100 h of continuous long-term operation (Fig. 22d). Borate and polyethylenimine (PEI) were introduced between the MXene nanosheets to assess the gas transport behavior of CO 2 capture. Borate can not only facilitate the transport of CO 2 molecules with reversible reactions for the enhancement of CO 2 permeance, but can also crosslink with the -O groups on MXenes to decrease the interlayer spacing for promoting the selectivity of CO 2 /N 2 and CO 2 /CH 4 . Furthermore, strong electrostatic interactions can be formed between the positively charged PEI and negatively charged MXene with the possible reaction of Ti suboxides and amine groups on PEI. Excellent gas separation was obtained for functionalized MXene nanofilms (Fig. 22e). The investigation of gas separation mechanism of pristine and functionalized MXene nanofilms demonstrated that pristine MXene membranes are diffusion-controlled, while borate and PEI functionalized MXene nanofilms indicated CO 2 selective separation (Fig. 22f) because of the CO 2 -philic nature of borate and PEI. Currently, there is a need for studies to understand how factors such as gas mixture composition, the size of MXene nanosheets, the interaction between MXene nanosheets and gas molecules, and various surface terminations affect gas selectivity and permeability.

Conclusion and perspective
In the past decade, MXenes have received considerable attention because of their appealing properties such as physicochemical, mechanical, magnetic and electronic characteristics, abundant surface terminations, and great surface area. As such, they have found applications in many areas such as biomedicine, sensing, catalysis, energy storage and conversion, and membrane-based separation. Surface functionalization of MXenes allows the alteration of various properties of MXenes, further increasing their versatility and enabling their weaknesses such as poor stability in the presence of water and molecular oxygen, facile restacking, and low flexibility to be addressed. This article reviewed the recent advances in MXene surface functionalization methods, MXeneaffected properties, and the practical applications of the resulting surface-modified MXenes. Surface-modified MXenes have demonstrated great potential for use in different applications. Previous studies indicate that for every specific application, a modified MXene that outperforms the pristine MXene can be fabricated. In spite of many reported studies on surface functionalization of MXenes, there are still challenges and opportunities in this area. Examples are as follows: 1. An MXene with desired concentrations of termination groups and interlayer spacing is essential to prepare surfacemodified MXene materials for different applications.
2. In catalysis, the exact mechanisms via which pristine and surface-modified MXenes promote reactions and the role of active sites on their surface are not well understood. Hence, computational and experimental studies are needed to improve the understanding, allowing the invention of novel catalysts.
3. In biomedical applications, MXene-based materials have shown great potential and low toxicity. However, the effect of toxicity of pristine and surface-modified MXenes on humans and the environment has not been investigated yet. 4. The water dispersity of MXenes can be improved via surface modification with hydrophilic polymers. However, these surface-modified MXenes are not stable adequately under some excessive conditions. Thus, better functionalization approaches are needed to form covalent and dynamic bonds. Besides, the effect of surface functionalization on biocompatibility needs to be explored further.
5. For application in sensors and photodetectors, performances of pristine and surface functionalized MXenes such as sensitivity and responsivity are still limited. Furthermore, the weight and size of pristine and surface functionalized MXenebased sensors need to be reduced for practical applications, especially for biomedical sensors. Therefore, energy-efficient, highly sensitive, super-small, and intelligent sensors and photodetectors should be developed. 6. It has been demonstrated that the interlayer spacing and surface functional groups of pristine and surface-modified MXenes have a great influence on their performance and structure. However, the impact of every single functional group on the performance of supercapacitors and batteries has not been studied; the relationship between the physicochemical characteristics and every single termination group is very important to understand. 7. For LIBs, the first cycle coulombic efficiency should be adequately enhanced by either modifying the anode/electrode interface or developing the surface chemistry of MXenes. The interface functionalization can be performed by providing an artificial layer of solid electrolyte interface on the MXene electrode, which leads to hindering undesirable reactions and does not prevent the Li + ion diffusion.
8. Pristine and surface-modified MXenes are promising candidates for separation applications due to their outstanding features. However, easy tuning of the interlayer spacing and reducing the degradation of MXenes materials should be addressed. 9. It is essential to master tailoring of the surface chemistry of MXenes by post-etch processing, which facilitates the surface modification of MXenes and enhances MXene properties. The effect of individual functional group in each application needs to be investigated further via ex situ and in situ characterization studies.

Conflicts of interest
There are no conflicts to declare.