Having received the first representative of the new family of two-dimensional materials with unique properties, MXenes have become one of the most popular areas of modern science [
117]. Their active study led to the rapid discovery of new transition metal carbides/nitrides/carbonitrides with various chemical compositions, structures, and characteristics. It has been found that, due to the wide range of properties inherent to MXenes, they can be successfully used for various applications. For example, the outstanding electrical and optical properties of these two-dimensional materials may be of interest to design sensors [
118]. They also demonstrated strong absorption in the near-infrared region, low cytotoxicity, high biocompatibility, and even some selectivity towards cancer cells, making them attractive candidates for bioimaging, photothermal therapy, theranostics, and drug delivery in the human body [
81]. The development of electrical power systems causes a need for new dielectrics, which must exhibit low dielectric loss and high dielectric constant to be suitable for electrostatic film capacitors with high energy density, flexibility, and high breakdown strength. It was found that incorporation of MXene nanosheets to polymer allows achieving the optimal combination of high dielectric constant and low dielectric loss [
119]. For instance, poly(vinylidene fluoride) (PVDF)-based percolative composites with 2D Ti
3C
2T
x nanosheets as fillers reached a dielectric constant as high as 10
5 near the percolation limit (15.3 wt.% MXene) [
120]. Moreover, the dielectric loss of the MXene/P(VDF-TrFE-CFE) composite increased from 0.06 to 0.35 (5-fold), but the dielectric constant increased by 25 times within 0–10 wt.% MXene composition range. The origin of the substantial permittivity enhancement is primarily due to the microscopic dipoles formed by the accumulation of charges at the interfaces between the MXene fillers and the polymer matrix. Compared with other fillers (hydrothermally reduced graphene oxide, copper phthalocyanine, functionalized graphene nanosheets), MXenes provided the best dielectric constant/loss factor trade-off. The other example of such composite is multilayered Ti
3C
2T
x/PVDF films fabricated by three steps: spin coating, spray coating, and hot-press methods [
119]. This structure consisted of overlapped layers of MXene and PVDF placed on each other to provide enhanced interfacial interactions due to Ti
3C
2T
x and good charge carrier insulation through the ferroelectric PVDF layer. The dielectric constant of the multilayer 4MXene/5PVDF (four layers of Ti
3C
2T
x and five layers of PVDF) measured at 1 kHz was about 41, higher than that of pure PVDF (10.5). An increase was caused by enhanced Maxwell–Wagner–Sillars (MWS) interfacial polarization due to the difference in dielectric performances of Ti
3C
2T
x and PVDF. The dielectric loss at 1 MHz was suppressed below 0.2. Additionally, MXene/PVDF films had low conductivity of <10
9 S m
−1 at 1 kHz. The frequency-dependent alternating current (AC) conductivity suggested an insulating behavior of the composite films. It also demonstrated a superior dielectric constant to loss factor ratio of about 1464.3. Therefore, composite MXene-based films are promising broadband dielectric material for high-frequency capacitors. Another example of the use of MXene phases as reinforcement in ceramic composites is Ti
3C
2T
x/Al
2O
3, which at 2 wt.% of Ti
3C
2T
x showed a 300%, ~150%, and ~300% improvement of the fracture toughness, bending strength, and hardness, respectively [
121]. The ZnO-Ti
3C
2 composite produced by the cold sintering process improved the electrical conductivity of the oxide matrix by 1–2 orders of magnitude and showed a 150% increase in hardness and elastic modulus [
122]. The enhancement of mechanical properties of silicon carbide modified with Ti
3C
2T
x was also reported by Petrus et al. [
123]. The Ti
3C
2T
x MXene composite films with segregated polystyrene inclusions studied by Iqbal et al. [
124] showed superior electromagnetic interference efficiency, making them a promising shielding material with tunable electromagnetic wave absorption properties. Furthermore, the combination of MXenes as nanofillers with polymers allows preparing the flame retardant nanocomposites [
125]. The unique layered structure and versatile interface chemistry of 2D MXenes make it possible to improve polymer properties [
126]. For instance, the addition of 2.0 wt.% Ti
3C
2Tx into unsaturated polyester resin (UPR) resulted in the formation of nanocomposite with an enhanced fire safety property: the peak heat release rate, the total smoke production, and carbon monoxide production were reduced by 29.56%, 25.26%, and 31.58%, respectively [
127]. Yu et al. manipulated the surface of Ti
3C
2T
x with 3-aminopropylheptaisobutyl-polyhedral oligomeric silsesquioxane (AP-POSS) through electrostatic interactions and incorporated the POSS-Ti
3C
2T
x into polystyrene (PS). The resultunt PS nanocomposites exhibited 39.1%, 54.4%, and 35.6% reductions in the peak heat release rate, the CO production rate, and the CO
2 production rate, respectively [
128]. Thus, MXenes are popular materials with many possible applications in various industries. Some of them are schematically shown in
Figure 25.
3.3. Multifunctional MXene-Based Smart Textiles
The range of MXene applications is extensive, and it is hard to describe all of them in one article. Therefore, we would like to briefly mention one more interesting field where these 2D materials can be effectively implemented: wearable electronics with multifunctional characteristics. Lately, it has been shown that wearable and flexible textile-based electronics can be extremely useful due to their versatility. Such smart textile devices can be successfully applied for healthcare electronics [
185], human motion monitoring [
186], antibacterial [
187,
188], pressure sensors [
189], electromagnetic interference (EMI) shielding [
190], and so forth. Although multifunctional textiles are excellent candidates for wearable electronics, they have some limitations. They are connected with difficulties integrating necessary functions, such as conductivity, into the traditional textile substrates while maintaining their high intrinsic properties: breathability and flexibility [
191,
192]. Therefore, a search for new conductive materials that can be used for smart textiles has led to the recently discovered two-dimensional MXenes. Their unique properties allowed fabricating electronic textiles demonstrating multifunctionality.
For instance, Zhang et al. [
192] reported preparing the MXene-decorated woven cotton fabrics with an interwoven conductive network. Such textile maintained the flexibility and air permeability of cotton and, in addition, gained new exciting characteristics even at the low addition of Ti
3C
2T
x nanosheets. The synthesized MXene-decorated fabric showed remarkable EMI shielding, Joule heating, and strain sensing performances. The cotton textile with vertically interwoven weft and warp yarns was used. That allowed obtaining the vertically interconnected conductive networks that could increase the MXene conductivity. Meanwhile, the Ti
3C
2T
x nanosheets were produced by selective etching of Ti
3AlC
2 MAX-phase in the HCl + LiF solution. Synthesized MXenes were then uniformly deposited onto the cotton fabric with 2−6 wt.% concentration by the spray-drying coating method. The study showed that such modified fabrics could be successfully applied as flexible heaters for warming up any bendable parts of the human body due to their outstanding Joule heating performance with a temperature up to 150 °C at an external voltage of 6 V. The MXene loading content and voltage could easily tailor the temperature of such heaters. Moreover, MXene-decorated fabric was found to be a promising candidate for the application in sensing small-range human activities since sensors on its bases exhibited high stability after 5000 cycles and superior sensitivity even at a bending strain of about 0−2.09%. MXenes with high electrical conductivity of about 48−5 Ω sq
−1 made it possible to turn simple cotton fabrics into multifunctional textiles, which can be used for wearable electronics.
Another way to employ MXene-based flexible material for electronic device-cooling applications was proposed by Liu et al. [
193]. The authors produced a free-standing graphene/MXene film, demonstrating excellent thermal conductivity and heat dissipation for fire retardant ability. This composite film was produced from graphene oxide (GO), synthesized by modified Hummer’s method [
194], and Ti
3C
2T
x, prepared by selective etching of T
i3AlC
2 in HCl + LiF mixture. The vacuum-assisted filtration with a cellulose ester filter membrane was used to synthesize graphen oxide/Ti
3C
2T
x film. After that, to weld the superior flexible composite graphene/MXene (GM) paper, it was reduced through immersion in hydroiodic acid, placed in a warm oil bath at 90 °C, dried at 40 °C, heated to 473 and 1000 K in an Ar atmosphere, and finally cooled down. The GM films demonstrated high thermal conductivity. At the concentration of MXene of about 40 wt.%, the thermal conductivity reached up to 26.49 W m
−1 K
−1, which was 6.71-fold and 2.93-fold higher than graphene oxide and reduced graphen oxide films. After welding graphene-MXene, more effective formation of paths for heat conduction in graphene was observed. Thus, a few reasons contributed to enhanced thermal conductivity of composites: the sp2 carbon structure of graphene and orderly alignment of MXene in the graphene matrix. Moreover, the experiment when GM paper with 40 wt.% of MXene was used as ultra-thin heat sink for LED was conducted. The results showed that a protective layer of nonconductive TiO
2 and graphite, appeared during combustion, provided a good flame retardant ability and outstanding heat-dissipation.
Apart from spray-drying [
190,
191] and vacuum-assisted filtration [
195] strategies, there are also dip-coating [
196] and 3D printing [
197] methods. An example of the dip-coating approach is a superhydrophobic and breathable intelligent textile device with a four-core shell structure [
196]. The consisting parts are the following: (1) a cheap modified elastic polypropylene textile substrate, (2) an interface polydopamine (P) layer for creating active sites for the MXene and better bonding between the MXene layer and substrate, (3) a functional MXene layer (M) of Ti
3C
2T
x nanosheets, and (4) a protective polydimethylsiloxane (PDMS) layer for decreasing the MXene surface energy, preventing their oxidation, and providing a superhydrophobic ability. All the steps were performed by the dip-coating method through immersion to a certain solution regarding the layer.
Figure 30 demonstrates possible applications of a fabricated MXene-based PM/PDMS textile. This composite is expected to remarkably perform as waterproof all-in-one wearable electronics capable of tracking body motion and temperature changes due to the superior superhydrophobicity, breathability, photo-thermal and electro-thermal effects, strain and temperature sensing performance. An example of using the Ti
3C
2T
x MXene/polyurethane composite fibers in the one-piece elbow sleeve, prepared by a commercial-scale flat-bed knitting machine, for tracking different movements through strain sensing and sending the signal by Bluetooth to a personal computer is presented in
Figure 30d,e [
198]. This single jersey knit of MXene/polyurethane four-ply yarn could recognize strains up to 200% and maintain stable work during 1000 stretching deformation cycles.
Figure 30. (
a) Schematic illustration of the PM/PDMS textile attached to the joints of the human body to monitor motion actions. (
b) A photograph of the volunteer wrist with MXene-based textile serving as a smart strain sensing device; the inset shows the photograph of one black ring textile. (
c) Promising applications and superior properties. Reprinted with permission from [
196] 2021 Elsevier. (
d) One-piece sleeve for elbow knitted using four-ply yarn of Ti
3C
2T
x MXene/polyurethane fiber in as-prepared and functioning states. (
e) Strain sensing response of the elbow sleeve collected during continuous bending and straightening the elbow at two different frequencies (0.2 and 0.7 Hz) through a wireless Bluetooth connection. Reprinted with permission from [
198] 2020 John Wiley and Sons.
3D printing can also be employed as a strategy to synthesize ideal flexible and multifunctional textiles. Cao et al. [
197], for the first time, reported on the fabrication of a flexible smart textile with hybrid composite inks of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxylradical)-mediated oxidized cellulose nanofibrils (TOCNFs) and Ti
3C
2 MXene. They prepared the Ti
3C
2 nanosheets from Ti
3AlC
2 powder by selective etching in LiF and HCl. The inks for 3D printing were obtained by mixing dispersions of TOCNFs and MXenes and magnetic stirring for 12 h at room temperature. These inks then could be transformed to continuous and stable gel TOCNFs/Ti
3C
2 fibers through extruding and injecting to an ethanol coagulation bath from a narrow nozzle of a 3D printer within seconds. Since these fibers possess good flexibility, they could be easily knotted after drying without causing any damage. The TOCNFs/Ti
3C
2 composite fibers also demonstrated enhanced mechanical properties. That was due to the numerous functional surface groups of MXenes providing many hydrogen bonds between the Ti
3C
2 and TOCNFs. Thus, breaking such fibers would need a large amount of energy. The TOCNFs/Ti
3C
2 composite fibers and textiles showed remarkable properties through photothermal and electrothermal functions in response to multiple external photon, electron, or strain stimuli.
MXene-based textiles, apart from the above-described functions, can also provide the EMI shielding effect [
199]. The electronics industry is developing rapidly as electronic devices have become an integral part of medicine, communications, computations, space, and automation. The numerous systems located close to each other cause electromagnetic interference issues [
200]. EMI occurs in the radio frequency range of the electromagnetic spectrum within 10
4–10
12 Hz [
201]. Computers, fluorescent lamps, radio transmitters, and electric motors mainly radiate in the microwave range (1–40 GHz). EMI is suspected to be harmful to the human body [
202,
203]. Therefore, humans should be protected from the possible adverse effects of electromagnetic radiation. The way to control the influence of everyday exposure to EMI pollution is by using smart textiles capable of effective shielding [
204].
EMI shielding materials first have been represented by metal-based foils, nanowires, and fibers from copper, nickel, stainless steel fiber, etc. However, the fact that such materials are heavy and susceptible to corrosion contributed to the emergence of a new class of EMI shielding materials: carbon-based composites and foams. Nevertheless, at low thickness, their protective abilities were found to be limited [
205]. Therefore, the latest trend is introducing 2D MXenes into various types of fibers by different methods to achieve superior shielding characteristics [
206]. The high electrical conductivity of MXenes enables the material with excellent shielding of electromagnetic waves [
207].
For instance, high-elasticity and abrasion-resistant polyethylene terephthalate (PET) textiles that were dipped intoTi
3C
2T
x MXene solution demonstrated superb EMI shielding properties [
187]. When MXene content was about 17.2 wt.%, the modified textile showed shielding efficiency of 42.1 dB in the X-band at a small thickness of 340 μm. The primary mechanism of EMI shielding in this textile was an absorption-dominant one. This is favorable since, in such a way, the secondary EMI pollution from reflection can be significantly reduced.
Figure 31a shows a schematic representation of the MXene-decorated textile’s functioning. First, after the incident electromagnetic wave (EMW) strikes its surface, the high electrical conductivity of MXene nanosheets contributes to the reflection of a small part of the EMW. The rest EMW that entered the textile is trapped in the MXene nanosheets and is absorbed or dissipated in the form of heat within the material (textile itself has weaving and a porous structure providing an interface for effective reflection). If some part of the incident EMW is left, the multiple reflections are further absorbed and dissipated as MXene nanosheets have a large surface and interface area. The results of the electromagnetic radiation test, confirming the effectiveness of such a mechanism of EMI shielding in Ti
3C
2T
x/PET textile, are presented in
Figure 31b,c. The electromagnetic radiation tester was used to record the radiation from a far-infrared radiation (FIR) therapeutic lamp and cellphone before and after covering in Ti
3C
2T
x/PET textile. As it can be seen, without the textile, the tester demonstrated 3703, 1378, and 433 µW cm
−2 depending on the distance from the lamp. However, after using the MXene-decorated textile, the tester showed 0 µW cm
−2 regardless of the distance. The same behavior was observed with a cellphone that generates severe electromagnetic radiation. The EM radiation was reduced from 4989 to 0 µW cm
−2 by simply wrapping the cellphone.
Figure 31. (
a) Schematic of electromagnetic microwave dissipation in the MXene-decorated textiles. A electromagnetic radiation tester used to check the radiation before and after applying Ti
3C
2T
x/PET textile on a FIR lamp (
b) and a cellphone (
c). Reprinted with permission from [
187] 2020 Royal Society of Chemistry.
The multifunctional MXene-decorated textiles are very helpful since they can be used for heating to keep warmth, protect from electromagnetic radiation generated from daily life, and sense human movements and temperature while being stretchable, breathable, and flexible. Nevertheless, assembling microscopic MXene nanosheets into macroscopic textiles can be challenging due to the irregular shape and size of the exfoliated MXenes. Many papers are devoted to Ti
3C
2T
x MXene, but other members of a big family are still not discovered for smart textiles for wearable multifunctional electronics applications. The obtained results are promising, and there are plenty of effects to be explained. MXenes have remarkable mechanical properties with an ultra-high elastic modulus and excellent electrical conductivity that makes them the leading materials for smart fibers and textiles so far [
208].