2. MXene-Organic Hybrids through Covalent Interaction

Benefiting from the surface atomic layers of early transition metals with empty d orbital, the MXenes skeleton usually presents a strong affinity to electron donors. When the organic molecules with electron-donating groups are introduced, the original T groups can be replaced. To date, numerous small organic molecules are found to be able to covalently connect on MXenes.

Silane (RSiX_n), one of the most commonly used organic coupling agents, has been widely applied for surface modification of inorganic materials. For silane, the R represents the functional group, such as an amino, alkane, aromatic group, epoxy group, and so on; X stands for hydrolysis group, containing Cl, OMe, OEt, etc. [28]. During the so-called “hydrolytic condensation” process (Figure 2a), X groups will first be hydrolyzed to form a siloxane (–SiOH) structure. Later, the siloxane goes through a dehydration condensation reaction to form oligomers. The –OH groups in oligomers will react with –OH groups on MXenes, forming an intermediate state. Finally, the Ti–O–Si covalent bonds are formed by dehydration condensation [28,29]. Meanwhile, different properties inherited from R groups can be introduced into the inorganic phase. In 2018, Zhao et al. reported a trimethoxy (1H,1H,2H,2H-perfluorodecyl) silane (PFDTMS) modified Ti₃C₂T_x MXene membrane prepared via a simple wet route [29], in which the water contact angle increased from 38.8° to 102.0° after PFDTMS grafting. Later, Sehyeong et al. have systematically studied the surface polarity regulation of Ti₃C₂T_x MXene by silane with different R groups. After replacing the surface T groups of Ti₃C₂T_x MXene with lipophilic octyltriethoxysilanes (OTS) [30], the OTS@MXene showed a water contact angle of 102.6° and exhibited excellent dispersion stability in non-polar hexane for more than four weeks. When methyltriethoxysilane (MTS), propyltriethoxyislane (PTS), and hexyltriethoxysilane (HTS) was selected, the surface hydrophobicity of these silane-MXene hybrids would decrease with shortening the alkyl chain. Beyond the randomly surface grafting, Zhang et al. designed a well-ordered vertically aligned Janus MXene-based aerogel, which has different wettability belong to two sides named VA-MXA [31]. After modifying the Ti₃C₂T_x MXene by fluorinated alkyl silane under vacuum conditions, the water contact angle of the upper layer increased from 44.6° to 133.2°. Also, the excessive concentration of silane clogged the available surface and decreased the hydroxyls on the surface of Ti₃C₂T_x, which can bring novel surface physicochemical properties [28]. In Tran and his co-workers’ work [32], the N,N-(diethylamino)dithiocarbamoyl-benzyl(trimethoxy) silane (SBDC) was used to functional Ti₂CT_x. Interestingly, the SBDC-MXene hybrid was applied as both substrate and iniferter, which allowed further polymerization of thermoresponsive polymer (poly(2-(dimethylamino)ethyl methacrylate (PDMAEMA)) on the surface of Ti₂CT_x. On the other hand, the silane molecules can not only change the surface hydrophilcity, but also alter the surface charged state (zeta potential) of MXenes. For instance, the aminosilane with positively charged amino group was demonstrated to have a critical role on the surface charge of aminosilane-Ti₃C₂T_x hybrids [33,34]. As reported by Hossein et al. the [3-(2-aminoethylamino)propyl]-trimethoxy silane (AEAPTMS)-Ti₃C₂T_x hybrids had a positive zeta potential of +62 mV at pH = 2.58, which is the highest zeta potential reported for MXenes to date. Similar to silane, the alkylphosphoric acid can also form covalent bonds with MXene through dehydration condensation reaction. Kim et al. demonstrated a simultaneous interfacial chemical grafting and phase transfer method for alkylphosphoric acid (C_nPA, n = 3, 6, 8, 10, 12) grafting on the Ti₃C₂T_x flakes [35]. During the reaction, Ti–O–P bonds formed by interfacial nucleophilic addition and sequential condensation reaction between hydroxyl groups from Ti₃C₂T_x and phosphate group from C_nPA. The pH and concentration of C_nPA are found to influence the interfacial chemical reaction and phase transfer. Whereas the dispersion stability of the Ti₃C₂T_x-C_nPA hybrids in nonpolar organic medium can be attributed to the steric stabilization and strong nonpolar interaction of long alkyl chains. Very recently, Sun et al. proposed new evidence on the existence of hydrogen bonds between Ti₃C₂T_x nanosheets and C₈PA [36]. Therefore, it seems that the surface interaction between MXenes and coupling agents may be more complicated than we think.

Figure 2. (a) Surface modification process of d-Ti₃C₂T_x nanosheets by PFDTMS; Reproduced with permission [29]. Copyright 2018, Royal Society of Chemistry. (b) Proposed mechanism of amidoxime functionalization for MXene Ti₃C₂T_x; Reproduced with permission [38]. Copyright 2020, American Chemical Society.

Diazonium salt is another commonly used reagent to covalently connect organic functional groups on MXenes. In 2020, Zhang et al. first proposed a detailed reaction mechanism between aryl diazonium salts and Ti₃C₂T_x MXene, in which the aromatic primary amine can undergo a diazotization reaction and the produced diazonium salts will covalently bond with MXenes. As shown in Figure 2b, the detailed reaction mechanism can be divided into three steps [37,38]: (1) an electron was transferred from Ti₃C₂T_x MXene to aryl diazonium salt, which resulted in the cleavage of diazonium to nitrogen and aromatic free radical; (2) the aromatic free radical received an additional H atom from Ti₃C₂T_x MXene, generating a Ti–O radical on the surface of MXene; (3) strong Ti–O–C covalent bonds formed between Ti–O radical and aromatic free radical. In fact, sulfanilic acid diazonium salts were introduced to get large-scale delaminated Ti₃C₂T_x multilayers as early as 2016 [39,40,41]. Compared with the pristine Ti₃C₂T_x, the phenylsulfonic acid grafted Ti₃C₂T_x MXene becomes amphiphilic material, which shows an increased solubility and excellent stability in water for more than one month. However, in consideration of the potential aromatic π interaction and the oxygen from the sulfonic group, the works did not give a detailed mechanism and solid evidence on the formation of MXene-aryl (Ti–O–C) linkages. Recently, Muhammad et al. compared the noncovalent and covalent interaction mechanism of 1-aminoanthraquinone (AQ) on Ti₃C₂T_x MXene (denoted as AQ@Ti₃C₂T_x and AQ-Ti₃C₂T_x, respectively) [42]. It is found that after the diazonium reaction, the amino group disappeared, and the covalent bond formed between AQ and Ti₃C₂T_x. As observed, the covalently attached AQ on Ti₃C₂T_x MXene offered better charge transportation, thus electrochemical performance. According to the abovementioned works, the diazonium reaction can be a useful way for the covalent connection of organic molecules onto the MXene nanosheets and further expands the applications of MXene-organic hybrids.

Taking advantage of the electrical conductivity and layered structure of MXenes, the MXene-polymer hybrids usually show improved properties when compared with the pure polymer, such as mechanical properties [43,44,45,46], flame retardancy [47,48], and responsiveness [32,49]. In early 2015, an MXene-organic hybrid with CO₂ and temperature dual-responsive property was reported by Chen et al. [49]. The hybrid consisted of V₂CT_x MXene and self-initiated photo grafting and photopolymerized (SIPGP) poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes. The hydroxyl groups on V₂CT_x MXene were acting as the photoactive sites, which allowed further growth of polymer brushes via SIPGP. When pre-modified MXene by SBDC [32], PDMAEMA brushes can also be connected onto the MXene surface. Zhang et al. [50] utilized in situ free radical polymerization to prepare Ti₃C₂T_x/polyacrylamide (PAA) hybrid with acrylamide monomer and potassium persulfate as redox initiator, in which the Ti₃C₂T_x MXene was introduced as the crosslinker.

3. MXene-Organic Hybrids through Electrostatic Interaction

In addition to utilizing T groups of MXenes to covalently graft organic molecules by chemical reactions, the electrostatic interactions on negatively charged MXenes surface can also be utilized. Cui et al. synthesized the (3-aminopropyl) triethoxylsilane (APTES) grafted Si nanoparticles first [51], in which the covalently X groups on silane were consumed. However, by directly using the positively charged Y group (here, amino), the grafted Si nanoparticles were still self-assembled with Ti₃C₂T_x through electrostatic interaction. Xu et al. reported microcontact printing to get ultrathin MXene micropatterns [52]. The MXene ink was applied to a polydimethylsiloxane (PDMS) film with patterns of alternating grooves. Subsequently, transfer printing it onto APTES-modified coverslips. In contrast, Ti₃C₂T_x adsorbed onto coverslips by electrostatic interaction with amino groups.

As another kind of common-used organic molecules, alkylammonium salts including dodecyl trimethyl ammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB), stearyl trimethyl ammonium bromide (STAB), octadecyl trimethyl ammonium bromide (OTAB), dioctadecyl dimethyl ammonium chloride (DDAC), dihexadecyl dimethyl ammonium bromide (DDAB), and DHT are usually used to insert in MXene layers through electrostatic adsorption. After pillared by alkylammonium, the Ti₃C₂T_x MXene presented a dramatic increase in interlayer spacing [53,54]. Obviously, the interlayer spacing increased along with the increasing length of alkyl chains [55]. Bian et al. proposed that the positively charged ammonium group might mainly be anchored on the surface –O groups from Ti₃C₂T_x MXene [56], and the intercalation of alkylammonium can also affect the wettability of MXene (Figure 3a). Maybe because of the competitive adsorption of cations, the CTA cation pillared Ti₃C₂T_x MXene can only form a stable oil-in-water emulsion in neutral or basic condition. While in acidic conditions, the MXene will aggregate at the oil/water interface. In a current study reported by Michael et al., the DHT not only improved the organophilic nature of Ti₃C₂T_x, produced stable colloidal suspensions of Ti₃C₂T_x in nonpolar solvents but also shielded the MXene flakes from oxidation in water [27].

Figure 3. (a) Top: a possible scheme describing the interaction between cetyltrimethylammonium bromide (CTAB) and Ti₃C₂T_x MXene. Bottom: the emulsion stability as a function of the pH of the aqueous phase at a fixed weight ratio of CTAB to MXene; reproduced with permission [56]. Copyright 2018, Royal Society of Chemistry. (b) Schematic illustration of the interaction of the polymer with Ti₃C₂T_x layers (top) and the synthesized polymers with nonpolar, polar, and polar charged nitrogen lateral chain ends (bottom); Reproduced with permission [57]. Copyright 2017, American Chemical Society.

Amino acids are also studied as the organic part towards MXene-organic hybrids. Elumalai et al. proposed that the electrostatic interaction between –OH and –F groups on Ti₃C₂T_x MXene and the zwitterion consisted of amino acid molecules might be the driving force for the spontaneous intercalation [58]. The MXene hybrids contained negatively charged glycine (Gly), phenylalanine (Phe), tryptophan (Trp), and histidine (His) can form stable suspensions in water due to the interparticle electrostatic repulsion. Whereas aromatic (Phe, Trp, His) intercalation may also produce rutile TiO₂@Ti₃C₂T_x by sonication. On the contrary, combined with first principle calculations (DFT), recently, Chen et al. studied the interaction between Gly and double-layer structure Ti₃C₂O₂ [59]. From their analysis, the Ti and N atoms from Ti₃C₂O₂ and Gly, respectively, were likely to share electrons and lead to the formation of Ti–N bonding. Obviously, the ionizable organic molecules are another good choice for preparing MXene-organic hybrids with controllable properties. However, the unclear interaction mechanism makes it an urgent task to be solved.

Carey and his coworkers prepared the 12-aminolauric acid (ALA), or DHT-modified Ti₃C₂T_x first (denoted as ALA-Ti₃C₂T_x and DHT-Ti₃C₂T_x) [60,61]. After the modification, MXene changed from hydrophilic to organophilic, so the intercalation of the ε-caprolactam monomer and 6-aminocaproic acid catalyst was easier [60]. Additionally, uniform Ti₃C₂T_x MXene/epoxy hybrids can be synthesized by ALA-Ti₃C₂T_x and DHT-Ti₃C₂T_x. The authors proposed that ALA and DHT can speed up the curing reaction of epoxy in the interlayer space, which further enlarged the interlayer space of the MXene layers [61].

Similarly, the aniline monomers are believed to electrostatically adsorb on and between the multilayer MXene nanosheets [62]. Fu et al. reported a graphene-encapsulated Ti₂CT_x@polyaniline (PANI) hybrid named GMP for supercapacitors [63]. The adsorbed aniline monomers can be chemical oxidative polymerized with ammonium persulfate as the oxidant to form Ti₂CT_x@PANI hybrid (Figure 4). For graphene encapsulation, CTAB was utilized to lower the surface energy and change the zeta potential of Ti₂CT_x@PANI. Recently, there is also evidence that the surface functional groups on MXenes will facilitate nucleation during the PANI polymerization [64].

Figure 4. Scheme of the synthesis of graphene-encapsulated Ti₂CT_x@PANI hybrid (GMP); Reproduced with permission [63]. Copyright 2018, American Chemical Society.

When turning to the other common-used conducting polymer, poly(3,4-ethylene dioxythiophene) (PEDOT), Gogotsi and his co-workers revealed the mechanism of charge-transfer-induced polymerization of EDOT on the surface of Ti₃C₂T_x MXene [65]. Verified by the theoretical calculations, the parallel orientation of EDOT monomer on Ti₃C₂T_x surface presented the most stable configuration with the lowest binding energy of −1.02 eV. After adsorption, 0.34 electrons were transferred from EDOT to Ti₃C₂O₂, which initiated the in situ polymerization of EDOT on Ti₃C₂O₂. With the energy supplement via electrochemical reaction, Qin et al. demonstrated a one-step in situ electrochemical polymerization for Mo_1.33C/PEDOT and Ti₃C₂T_x/PEDOT films with controlled thickness and micropattern [66]. However, due to the insoluble nature of PEDOT, the commercial product usually contains a mixture of poly(styrene sulfonate) (PSS). In consideration of the bicomponent structure and the ionic chain nature, the ex-suit blending method was more popular than in-suit polymerization towards MXene-PEDOT:PSS hybrids and the electrostatic interaction between PSS and MXene promoted the formation of 3D interconnected and reticulated structure. The hydrogen from –SO₃H on PEDOT:PSS with –OH,–F on Ti₃C₂T_x and the electrostatic interaction between PSS and MXene were also found to jointly shield the coulombic attraction between PEDOT and PSS, which enhanced the inter-chain charge transport between PEDOT chains [67]. The Ti₃C₂T_x/(PEDOT:PSS) film can be fabricated by drop-casting [68], vacuum-assisted filtration [69], and simply filtration [70,71]. Recently, MXene/(PEDOT:PSS) hybrid was also fabricated into a fiber-shaped supercapacitor by a one-step wet-spinning approach [72].

By the ex-situ blending method, plenty of MXene/polymers hydrides can be prepared. Firstly reported by Gogotsi et al., charged poly-diallyl dimethyl ammonium chloride (PDDA) and electrically neutral polyvinyl alcohol (PVA) were blended with Ti₃C₂T_x by vacuum-assisted filtration [73]. Subsequently, the dispersibility of MXene sheets in the PVA matrix and the dielectric performance of MXene/PVA hybrids in X-band frequency was studied [74]. Boota et al. synthesized polyfluorene (PFO) derivatives containing nonpolar, polar nitrogen-containing, and charged nitrogen-containing lateral chains by Suzuki polycondensation reaction (Figure 3b) [57]. They have pointed out that both the electrostatic interactions and hydrogen bonds may be involved between MXene and PFO derivatives. However, the bulky methyl groups from polar polymers with charged nitrogen-containing ends prohibited the formation of the hydrogen bond. A physical vapor deposition technique, resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE), was performed by Ajnsztajn et al. to produce Ti₃C₂T_x/PFO transparent hybrid electrode [75]. Via RIR-MAPLE, the film exhibited the minimal phase segregation.

4. MXene-Organic Hybrids through Hydrogen Bonds and Other Supermolecular Interactions

Except for the strong inorganic-organic correlation provided by covalent and electrostatic interaction, the supermolecular interactions are also critical to the formation of MXene-organic hybrids. In 2016, a Ti₃C₂T_x/polypyrrole (PPy) hybrid with highly aligned PPy chains in between the Ti₃C₂T_x layers was prepared by unconventional oxidant-free polymerization (Figure 5a) [76]. Specifically, the key factor in the alignment process was the hydrogen bond, which might originate from the N–H group of the pyrrole ring and terminating oxygen or fluorine on the Ti₃C₂T_x surface. What is more, the fluorine on Ti₃C₂T_x will be doped in PPy chains, which can further enhance their electrochemical activity. Different from the oxidant-free chemical polymerization, Zhu et al. [77] fabricated Ti₃C₂T_x/PPy hybrid film via electrophoretic deposition and electrochemical polymerization. The pronounced acidic character of the Ti₃C₂T_x surface generated hydrogen bonding between Ti₃C₂T_x and PPy during electrochemical polymerization. However, for the most traditional pyrrole polymerization [78], FeCl₃·6H₂O was applied as an oxidant, and the strong oxidizing Fe³⁺ ions will result in the oxidation of Ti₃C₂T_x. Therefore, Zhang et al. only found the 3D TiO₂@NC/Fe₇S₈ hybrids by in situ polymerization of pyrrole monomer with alkalized Ti₃C₂T_x.

Figure 5. (a) Schematic illustration of pyrrole polymerization using MXene; reproduced with permission [76]. Copyright 2016, WILEY-VCH. (b) Schematic drawing depicting the preparation steps of OMPDA/Ti₃C₂T_x hybrid: (i) mixing the DAmi with Ti₃C₂T_x nanosheets, (ii) packing of DAmi and subsequent, (iii) direct polymerization of polydopamine (PDA) micelles on the surface of Ti₃C₂T_x nanosheets, and (iv) obtaining the OMPDA/Ti₃C₂T_x after heat treatment; Reproduced with permission [81]. Copyright 2020, American Chemical Society.

As confirmed by previous studies, quinones are important redox-active centers for the chemical reaction [79]. However, there are only a limited amount of quinones reserved in polydopamine (PDA), which is positively correlated to the oxidation state of PDA [80]. Interestingly, Ti₃C₂T_x/PDA hybrids were found to be an excellent precursor for o-benzoquinone after low-temperature heat treatment at 300 °C [79]. After that, Li et al. further prepared vertically oriented ordered mesopores (OM) PDA/Ti₃C₂T_x hybrid (OMPDA/Ti₃C₂T_x) by using the PS-b-PEO block polymer as a soft-template (Figure 5b) [81]. In their report, the hydrogen bond also played a crucial role in the assembly of dopamine/PS-b-PEO micelles and Ti₃C₂T_x. Recently, Du and coworkers utilized Ti₃C₂T_x@PDA prepared by in situ polymerization as starting materials to hybrid with poly(ethylene glycol) (PEG)-based polyurethane (PU) [82]. The PDA is found to be a medium molecule that facilitated the interfacial compatibility of Ti₃C₂T_x flakes in PEG-based PU, where the Ti₃C₂T_x@PDA covalently bonded with PU during the process of copolymerization.

Beyond anchoring the organic species on the MXene surface, the hydrogen bond also functions as a flexible junction between the inorganic MXene and organic part. Hybrid consisting of Ti₃C₂T_x and polyacrylamide (PAM) was fabricated by Niu et al. [50]. In 2020, the exfoliated Ti₃C₂T_x nanosheets were reported for the first time that acted as a crosslinker instead of traditional organic molecules. In the hybrid, hydrogen bonding between the –CONH₂ groups from PAM chains and the hydrophilic groups (–OH and –F) from the Ti₃C₂T_x nanosheets were observed. Zhang et al. simply prepared MXene hydrogel hybrids by mixing Ti₃C₂T_x MXene and commercial polyvinyl alcohol (PVA) hydrogel [83]. They indicated the secondary crosslinking between MXene and PVA and a dense network yielded by polymer chain entanglements. Xuan et al. improved this method by added MXene flakes into a homogeneous PVA solution [84], with the borate solution acting as the crosslinking agent, MXene bonding with PVA chains, and tetra-functional borate ion covalently. A similar interaction was also found in hydrophobically associated dry polyacrylamide (HAPAM) hydrogel [85]. In HAPAM hydrogel, hydrogen bonds between MXene and amide group of HAPAM became additional crosslinking points, which contributed to the hybrid double-network with N,N’-methylene diacryl amide (MBAA) crosslinker and affected the self-healing ability and temperature sensibility. When there were two or more different kinds of monomers or polymers together with MXene nanosheets, MXene-copolymeric hydrogels can also be prepared [86,87,88].

On the aspect of ex situ blending, Le et al. intertwined Ti₃C₂T_x MXene particles in tangled PPy nanowires through hydrogen bond [89]. The intercalated PPy chains between the conductive Ti₃C₂T_x sheets prevented the dense stacking of Ti₃C₂T_x. Meanwhile, the intercalation further improved the structural stability of PPy backbones. Moreover, Ti₃C₂T_x MXene and PANI hybrid was also prepared by mechanical blending [90]. The lightweight Ti₃C₂T_x/PANI hybrids for EMI shielding was obtained by simply mixing PANI powder with MXene powder. The authors suggested that an equal concentration of both materials would be beneficial for enhanced interfacial interaction between them.

Except for hydrogen, the polymer chain entanglements, ionic interactions, covalent bonding were also reported, and the role of MXene sheets in hydrogel varies [86]. MXene polymer hydrogel was gradually developed with improved performance. The typical interactions between MXene and organic modified species are summarized in Table 1.