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Subjects:
Physics, Condensed Matter
MXenes are two-dimensional layered materials containing early transition metal carbides, nitrides, and carbonitrides. This entry shows how they can be used as coatings on textile fabrics or in fibrous form as electromagnetic shielding materials.
- shielding effectiveness
- conductive coating
- magnetic properties
1. Different MXenes and Their Preparation
MXenes are two-dimensional layered materials containing early transition metal carbides, nitrides, and carbonitrides [27]. They are prepared by starting from so-called three-dimensional MAX phases, where MAX is the abbreviation of Mn+1AXn with (n = 1, 2, 3), and M denotes an early d-block transition metal (i.e., Ti, Sc, V, Cr, Ta, Nb, Zr, Mo, or Hf), A means a main group sp element especially from the groups 13 and 14, and X represents C and/or N [28]. By etching the sp element layers out of the MAX phases, two-dimensional MXenes (without the “A”) remain [29]. MXenes have additional terminated functional groups (e.g., -OH, -O, -F) named “T”, resulting in their general formula Mn+1XnTx [30].
More than 60 of these MXenes have been found, yet with different metal or ceramic properties, depending on the chemical constitution [31]. One of the problems of MXenes is their susceptibility to oxidation in humid or aqueous environments, necessitating either excluding water vapor to reach them or increasing their stability against oxidation [32].
2. MXene Coatings
Due to their two-dimensional nature, MXenes are mostly applied in the form of coatings on textile fabrics, either solely or together with intrinsically conductive polymers, metals, or carbon-based fillers. Li et al. reported electromagnetic interference shielding combined with the potential applications of photothermal conversion and solar water evaporation, using a layer-by-layer assembly method on a textile fabric [33]. They combined SiO2 nanoparticles/poly(dimethylsiloxane) (PDMS) and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) to reach superhydrophobicity (i.e., a water contact angle larger than 160°) as well as MXene to reach a high conductivity of 1200 S/m, resulting in an EMI shielding of 36 dB.
Zheng et al. prepared bark-shaped MXene/textiles which showed not only high EMI shielding, but also good Joule heating and good piezoresistive sensing [34]. The bark-shape was suggested to enhance multiple interfaces scattering of EM waves to improve the EMI shielding effectiveness. To reach this shape, the authors used the pad-drying technology normally used in fabric dyeing to apply MXene flakes on a cellulose nonwoven. In their study, they synthesized Ti3C2Tx MXene sheets, immersed a cellulose nonwoven into an aqueous MXene solution and used a padder to remove the excess water before drying, repeating this cycle 1–9 times. By increasing the number of pad-drying cycles from 3–9, the EMI shielding effectiveness could be increased from 3.2 dB to 36.3 dB in a range from 8.2 GHz–12.4 GHz (X-band) due to improving single conductive paths towards a conductive network, which was also visible in the decreased sheet resistance for larger numbers of pad-drying cycles.
Using a spray-drying procedure, Zhang et al. reported good electrical conductivity and low sheet resistance of 5 Ω already for low MXene loading of 6 wt% on a woven cotton fabric, resulting in efficient EMI shielding and well-balanced Joule heating as well as the possibility of using this fabric as a strain sensor to detect human motion [35]. Besides EMI shielding and thermal heating, Yu et al. also mentioned bactericidal activity of their Mxene-decorated, polydopamine (PDA) modified cellulose nonwovens [36]. They reached an EMI shielding of 38.6 dB in the X-band, good heating performance, and a very high bactericidal efficacy of more than 99.99% against E. coli and S. aureus.
Working on basalt fiber fabrics, Yu et al. used multilayer spray-drying to coat Ti3C2Tx nanosheets and Ti3C2Tx/natural rubber layers, the latter protecting the inner Ti3C2Tx coating and additionally formed conductive connections between the conductively coated basalt fibers [37]. The Mxene phase was prepared by etching Al from commercially available Ti3AlC2 powder, using LiF, followed by exfoliation by ultrasonication in an ice bath under Ar flow. With this procedure, a sheet resistance of (5 ± 3) Ω was reached with the maximum tested Ti3C2Tx amount of 4 mg/cm2, resulting in EMI shielding up to 41 dB in the X-band.
Yao et al. also combined Mxene with a polymer network, here PDMS, to give a textile fabric EMI shielding, electro-thermal and photo-thermal conversion as well as pressure-sensing properties [38]. While different numbers of dip-coating cycles into a suspension of Ti3C2Tx Mxene were examined, these coated textile fabrics were dipped into low cross-linked PDMS and thermally cured to reach adhesive properties. While samples without Mxene showed nearly no EMI shielding in the X-band, Mxene coated samples with 3–9 dip-coating cycles reached more than 30 dB along the whole X-band, mostly due to absorption.
Much higher EMI shielding values were even reported by Uzun et al. who used dip-coating of cotton and linen fabrics in Ti3C2Tx Mxene dyes [39]. While 4 dip-coating cycles resulted in approx. 40 dB shielding in the X-band, 24 dip-coating cycles increased this value to approx. 80 dB. Interestingly, these values were decreased by only 8% and 13% for cotton and linen fabrics, respectively, after storing them for 2 years under ambient conditions.
Even higher values were reported from groups who combined Mxene coating with intrinsically conductive polymers, such as polyaniline (Pani), polypyrrole (Ppy), polythiophene, polyphenyl sulfide, polyacetylene, polyphenylene, polyphenylene vinylene, or poly (3,4-ethylene dioxythiophene) (PEDOT) [40]. Wang et al., e.g., applied Ppy-modified Mxene sheets on poly(ethylene terephthalate) (PET) textiles and subsequently coated them with silicone, resulting in high electrical conductivity around 1000 S/m and EMI shielding efficiency of approx. 90 dB as well as good Joule heating performance [41].
Combining Mxene with Pani nanowires on a carbon fiber fabric followed by PDMS coating resulted in a good conductivity of 325 S/m and EMI shielding effectiveness around 35 dB [42]. A 3D nanoflower structure from Ti3C2Tx/Pani was prepared by Li et al. by polymerization of aniline monomer on single-layer Ti3C2Tx nanosheets, resulting in a shielding effectiveness of 52 dB in the X-band [43]. Previously, combining Mxenes and Pani in a layer-by-layer assembly, Yin et al. reached a conductivity of 25 S/m and an EMI shielding efficiency of 26 dB [44].
Other authors combined Mxene with metal, e.g., with Ag nanowires (Ag NWs), reaching an EMI shielding efficiency of 54 dB in the X-band [45], or with Fe3O4 hollow nanospheres, resulting in low sheet resistance of about 5 Ω and high EMI shielding effectiveness of 33 dB, enabling tuning the shielding mechanism between absorption and reflection [46]. Alternatively, combining MXene with carbon-based conductive materials is reported, e.g., a Ti3C2Tx/carbon nanotube (CNT) coated thermoplastic polyurethane nonwoven, leading to high EMI shielding around 43 dB [47].
As these examples show, there are many possibilities to apply MXene coatings on textile fabrics to prepare EMI shielding fabrics. However, only few reports about MXene fibers can be found in the literature. They are discussed in the next section.
3. MXene Fibers
One possibility to prepare MXene fibers is to use them as core or shell in coaxially spun fibers, as described by Liu et al. [48]. The authors prepared a cellulose spinning solution by dissolving cotton linter pulp with LiOH solution, urea, and distilled water, and received regenerated cellulose after putting the cellulose dispersion into an acetic acid coagulation bath. Ti3C2Tx MXene was mixed with graphene oxide (GO) to prepare the other spinning solution. Both solutions were coaxially spun into a rotating bath, with cellulose or MXene/GO building the core. In this way, it was possible to prepare meter-long hollow fibers from regenerated cellulose and GO/MXene which could lift a mass of 100 g. These fibers were found to have conductivities up to 105 S/m. The EMI shielding effectiveness depended on the mesh grid spacing of woven or sewn structures prepared from these fibers, showing values around 27–33 dB for a single layer with the smallest grid spacing and up to more than 100 dB for 3 layers, building an only 12 µm thick MXene film. The same group also showed coaxial spinning of core-shell fibers with MXene core and aramid nanofiber shell [49]. In this way, they reached a conductivity of 3 × 105 S/m and an EMI shielding efficiency of 83 dB.
Zhou et al. suggested preparing compact MXene fibers by combining wet spinning from a MXene-glutaraldehyde (GA) solution with thermal drawing, resulting in significantly increased tensile strength and toughness, conductivity around 8 × 105 S/m and EMI shielding effectiveness of 50–60 dB in the X-band [50].
Instead of these filament-based approaches, Xiong et al. used short MXene fibers, produced by wet-spinning, to produce a MXene nonwoven by wet-assembly [51]. In this way, a strong interfiber bonding was reached. This MXene nonwoven showed high conductivity around 70,000 S/m and an EMI shielding effectiveness of 75 dB in the X-band.
Another path was suggested by Zheng et al. who prepared a core–shell aerogel from reduced graphene oxide (rGO) with MXene by wet-spinning and freeze-drying [52]. In this way, they reached an EMI shielding effectiveness up to 83 dB which degraded by only 17% after 120 days.
As these few examples show, MXene fibers are challenging to produce and thus are less often prepared for EMI shielding applications. Carbon fibers and metal wires, on the other hand, are commercially available in diverse qualities and diameters; however, EMI shielding is nevertheless mostly reached by functionalizing textile fabrics with carbon or metal containing coatings, as the full paper Textile Fabrics as Electromagnetic Shielding Materials—A Review of Preparation and Performance describes.
This entry is adapted from the peer-reviewed paper 10.3390/fib11030029
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