Rheology-Guided Assembly of a Highly Aligned MXene/Cellulose Nanofiber Composite Film for High-Performance Electromagnetic Interference Shielding and Infrared Stealth

Two-Dimensional MXene-Based Electrocatalysts: Challenges and Opportunities

MXene, regarded as cutting-edge two-dimensional (2D) materials, have been widely explored in various applications due to their remarkable flexibility, high specific surface area, good mechanical strength, and interesting electrical conductivity. Recently, 2D MXene has served as a ideal platform for the design and development of electrocatalysts with high activity, selectivity, and stability. This review article provides a detailed description of the structural engineering of MXene-based electrocatalysts and summarizes the uses of 2D MXene in hydrogen evolution reactions, nitrogen reduction reactions, oxygen evolution reactions, oxygen reduction reactions, and methanol/ethanol oxidation. Then, key issues and prospects for 2D MXene as a next-generation platform in fundamental research and real-world electrocatalysis applications are discussed. Emphasis will be given to material design and enhancement techniques. Finally, future research directions are suggested to improve the efficiency of MXene-based electrocatalysts.

Ultrastrong MXene film induced by sequential bridging with liquid metal

Assembling titanium carbide (Ti3C2Tx) MXene nanosheets into macroscopic films presents challenges, including voids, low orientation degree, and weak interfacial interactions, which reduce mechanical performance. We demonstrate an ultrastrong macroscopic MXene film using liquid metal (LM) and bacterial cellulose (BC) to sequentially bridge MXene nanosheets (an LBM film), achieving a tensile strength of 908.4 megapascals. A layer-by-layer approach using repeated cycles of blade coating improves the orientation degree to 0.935 in the LBM film, while a LM with good deformability reduces voids into porosity of 5.4%. The interfacial interactions are enhanced by the hydrogen bonding from BC and the coordination bonding with LM, which improves the stress-transfer efficiency. Sequential bridging provides an avenue for assembling other two-dimensional nanosheets into high-performance materials.

Supergravity-Steered Generic Manufacturing of Nanosheets-Embedded Nanocomposite Hydrogel with Highly Oriented, Heterogeneous Architecture

Designing nanocomposite hydrogels with oriented nanosheets has emerged as a promising toolkit to achieve preferential performances that go beyond their disordered counterparts. Although current fabrication strategies via electric/magnetic force fields have made remarkable achievements, they necessitate special properties of nanosheets and suffer from an inferior orientation degree of nanosheets. Herein, a facile and universal approach is discovered to elaborate MXene-based nanocomposite hydrogels with highly oriented, heterogeneous architecture by virtue of supergravity to replace conventional force fields. The key to such architecture is to leverage bidirectional, force-tunable attributes of supergravity containing coupled orthogonal shear and centrifugal force field for steering high-efficient movement, pre-orientation, and stacking of MXene nanosheets in the bottom. Such a synergetic effect allows for yielding heterogeneous nanocomposite hydrogels with a high-orientation MXene-rich layer (orientation degree, f = 0.83) and a polymer-rich layer. The authors demonstrate that MXene-based nanocomposite hydrogels leverage their high-orientation, heterogeneous architecture to deliver an extraordinary electromagnetic interference shielding effectiveness of 55.2 dB at 12.4 GHz yet using a super-low MXene of 0.3 wt%, surpassing most hydrogels-based electromagnetic shielding materials. This versatile supergravity-steered strategy can be further extended to arbitrary nanosheets including MoS2, GO, and C3N4, offering a paradigm in the development of oriented nanocomposites.

Large-Scale, Mechanically Robust, Solvent-Resistant, and Antioxidant MXene-Based Composites for Reliable Long-Term Infrared Stealth

MXene-based thermal camouflage materials have gained increasing attention due to their low emissivity, however, the poor anti-oxidation restricts their potential applications under complex environments. Various modification methods and strategies, e.g., the addition of antioxidant molecules and fillers have been developed to overcome this, but the realization of long-term, reliable thermal camouflage using MXene network (coating) with excellent comprehensive performance remains a great challenge. Here, a MXene-based hybrid network comodified with hyaluronic acid (HA) and hyperbranched polysiloxane (HSi) molecules is designed and fabricated. Notably, the presence of appreciated HA molecules restricts the oxidation of MXene sheets without altering infrared stealth performance, superior to other water-soluble polymers; while the HSi molecules can act as efficient cross-linking agents to generate strong interactions between MXene sheets and HA molecules. The optimized MXene/HA/HSi composites exhibit excellent mechanical flexibility (folded into crane structure), good water/solvent resistance, and long-term stable thermal camouflage capability (with low infrared emissivity of ≈0.29). The long-term thermal camouflage reliability (≈8 months) under various outdoor weathers and the scalable coating capability of the MXene-coated textile enable them to disguise the IR signal of various targets in complex environments, indicating the great promise of achieved material for thermal camouflage, IR stealth, and counter surveillance.

A multi-scale layered helical structure composite using the co-dispersion of cellulose nanocrystals and the micro-nano Al sheets and its efficient near-infrared stealth performance

To design flexible functional materials with high efficiency, light weight, less metal consumption, stable structure for the thermal infrared stealth materials is a great challenge. We hypothesized that the use of crystal materials with different sizes to design composites with a chiral layered helical structure, the layered structures can repeatedly reflect infrared ray. Here, we reported the novel multi-scale layered helical chiral structure composite by self-assembly using the co-dispersion of cellulose nanocrystals (CNC) and micro-nano Al sheets. A new stable interlocking supermolecular structure is formed between the positively charged metal sheet and the negatively charged CNC photonic crystals. Metal sheets and CNC organic crystals were hybridized at the molecular level and form the Pickering-like CNC-Al co-dispersion system. The metal sheets in CNC chiral helical layered structure greatly improve its near-infrared reflection and stealth camouflage. Surprisingly, the CNC/Al composite on the heated glass substrate enabled the temperature drop 23 °C, and made its emissivity in the range of 7-14 μm significantly reduce. The synergetic effect of the Al sheets and the CNCs helical structure greatly improved the thermal infrared reflection and heat insulation properties. It is expected to provide a chiral layered material for the infrared stealth, and pattern camouflage fields.

Sandwich-Structured Mxene/Waste Polyurethane Foam Composites For Highly Efficient Electromagnetic Interference, Infrared Shielding and Joule Heating

Electromagnetic interference (EMI) shielding and infrared (IR) stealth materials have attracted increasing attention owing to the rapid development of modern communication and military surveillance technologies. However, to realize excellent EMI shielding and IR stealth performance simultaneously remains a great challenge. Herein, a facile strategy is demonstrated to prepare high-efficiency EMI shielding and IR stealth materials of sandwich-structured MXene-based thin foam composites (M-W-M) via filtration and hot-pressing. In this composite, the conductive Ti3C2Tx MXene/cellulose nanofiber (MXene/CNF) film serves as the outer layer, which reflects electromagnetic waves and reduces the IR emissivity. Meanwhile, the middle layer is composed of a porous waste polyurethane foam (WPUF), which not only improves thermal insulation capacity but also extends electromagnetic wave propagation paths. Owing to the unique sandwich structure of "film-foam-film", the M-W-M composite exhibits a high EMI shielding effectiveness of 83.37 dB, and in the meantime extremely low emissivity (22.17%) in the wavelength range of 7-14 µm and thermal conductivity (0.19 W m-1 K-1), giving rise to impressive IR stealth performance at various surrounding temperatures. Remarkably, the M-W-M composite also shows excellent Joule heating properties, capable of maintaining the IR stealth function during Joule heating.

Revealing Surface/Interface Chemistry of the Ordered Aramid Nanofiber/MXene Structure for Infrared Thermal Camouflage and Electromagnetic Interference Shielding

The past decade has witnessed the advances of infrared (IR) thermal camouflage materials, but challenges remain in breaking the trade-off nature between emissivity and mechanical properties. In response, we identify the key role of a moderate reprotonation rate in the aramid nanofiber (ANF)/MXene film toward a surface-to-bulk alignment. Theoretical simulation demonstrates that the ordered ANF/MXene surface eliminates the local high electric field by field confinement and localization, responsible for the low IR emissivity. By scrutinizing the surface/interface chemistry, the processing optimization is achieved to develop an ordered and densely stacked ANF/MXene film, which features a low emissivity of 16%, accounting for sound IR thermal camouflage performances including a wide camouflage temperature range of 50-200 °C, a large reduction in radiation temperature from 200.5 to 63.6 °C, and long-term stability. This design also enables good mechanical performance such as a tensile strength of 190.8 MPa, a toughness of 12.1 MJ m-3, and a modulus of 7.9 GPa, responsible for better thermal camouflage applications. The tailor-made ANF/MXene film further attains an electromagnetic interference (EMI) shielding effectiveness (40.4 dB) in the X-band, manifesting its promise for IR stealth compatible EMI shielding applications. This work will shed light on the dynamic topology reconstruction of camouflage materials for boosting thermal management technology.

Recent advances in TEMPO-oxidized cellulose nanofibers: Oxidation mechanism, characterization, properties and applications

Cellulose is the richest renewable polymer source on the earth. TEMPO-mediated oxidized cellulose nanofibers are deduced from enormously available wood biomass and functionalized with carboxyl groups. The preparation procedure of TOCNFs is more environmentally friendly compared to other cellulose, for example, MFC and CNCs. Due to the presence of functional carboxyl groups, TOCNF-based materials have been studied widely in different fields, including biomedicine, wastewater treatment, bioelectronics and others. In this review, the TEMPO oxidation mechanism, the properties and applications of TOCNFs are elaborated. Most importantly, the recent advanced applications and the beneficial role of TOCNFs in the various abovementioned fields are discussed. Furthermore, the performances and research progress on the fabrication of TOCNFs are summarized. It is expected that this timely review will help further research on the invention of novel material from TOCNFs and its applications in different advanced fields, including biomedicine, bioelectronics, wastewater treatment, and the energy sector.

Diverse Structural Design Strategies of MXene-Based Macrostructure for High-Performance Electromagnetic Interference Shielding

There is an urgent demand for flexible, lightweight, mechanically robust, excellent electromagnetic interference (EMI) shielding materials. Two-dimensional (2D) transition metal carbides/nitrides (MXenes) have been potential candidates for the construction of excellent EMI shielding materials due to their great electrical electroconductibility, favorable mechanical nature such as flexibility, large aspect ratios, and simple processability in aqueous media. The applicability of MXenes for EMI shielding has been intensively explored; thus, reviewing the relevant research is beneficial for advancing the design of high-performance MXene-based EMI shields. Herein, recent progress in MXene-based macrostructure development is reviewed, including the associated EMI shielding mechanisms. In particular, various structural design strategies for MXene-based EMI shielding materials are highlighted and explored. In the end, the difficulties and views for the future growth of MXene-based EMI shields are proposed. This review aims to drive the growth of high-performance MXene-based EMI shielding macrostructures on basis of rational structural design and the future high-efficiency utilization of MXene.

Scalable Production of Catecholamine-Densified MXene Coatings for Electromagnetic Shielding and Infrared Stealth

Processing transition metal carbides/nitrides (MXenes) inks into large-area functional coatings expects promising potential for electromagnetic interference (EMI) shielding and infrared stealth. However, the coating performances, especially for scalable fabrication techniques, are greatly constrained by the flake size and stacking manner of MXene. Herein, the large-area production of highly densified and oriented MXene coatings is demonstrated by engineering interfacial interactions of small MXene flakes with catecholamine molecules. The catecholamine molecules can micro-crosslink MXene nanosheets, significantly improving the ink's rheological properties. It favors the shear-induced sheet arrangement and inhibition of structural defects in the blade coating process, making it possible to achieve high orientation and densification of MXene assembly by either large-area coating or patterned printing. Interestingly, the MXene/catecholamine coating exhibits high conductivity of up to 12 247 S cm-1 and ultrahigh specific EMI shielding effectiveness of 2.0 ×10 5  dB cm2 g-1 , obviously superior to most of the reported MXene materials. Furthermore, the regularly assembled structure also endows the MXene coatings with low infrared emissivities for infrared stealth applications. Therefore, MXene/catecholamine coatings with ultraefficient EMI shielding and low infrared emissivity prove the feasibility of applications in aerospace, military, and wearable devices.

Applications of X-Ray-Based Characterization in MXene Research

X-rays are a penetrating form of high-energy electromagnetic radiation with wavelengths ranging from 10 pm to 10 nm. Similar to visible light, X-rays provide a powerful tool to study the atoms and elemental information of objects. Different characterization methods based on X-rays are established, such as X-ray diffraction, small- and wide-angle X-ray scattering, and X-ray-based spectroscopies, to explore the structural and elemental information of varied materials including low-dimensional nanomaterials. This review summarizes the recent progress of using X-ray related characterization methods in MXenes, a new family of 2D nanomaterials. These methods provide key information on the nanomaterials, covering synthesis, elemental composition, and the assembly of MXene sheets and their composites. Additionally, new characterization methods are proposed as future research directions in the outlook section to enhance understanding of MXene surface and chemical properties. This review is expected to provide a guideline for characterization method selection and aid in precise interpretation of the experimental data in MXene research.

Facile preparation of flame-retardant cellulose composite with biodegradable and water resistant properties for electronic device applications

The aim of the present study is to produce flexible, flame-retardant, water-resistant and biodegradable composite materials. The ultimate goal of this research is to develop simple processes for the production of bio-based materials capable of replacing non-degradable substrates in printed circuit board. Cellulose was chosen as a renewable resource, and dissolved in 1-ethyl-3-methylimidazolium acetate ionic liquid to prepare a cellulosic continuous film. Since flame retardancy is an important criterion for electronic device applications and cellulose is naturally flammable, we incorporated ammonium polyphosphate (APP) as a flame-retardant filler to increase the flame retardancy of the produced materials. The developed material achieved a UL-94 HB rating in the flammability test, while the cellulose sample without APP failed the test. Two hydrophobic agents, ethyl 2-cyanoacrylate and trichloro(octadecyl)silane were applied by a simple dip-coating technique to impart hydrophobicity to the cellulose-APP composites. Dynamic mechanical analysis indicated that the mechanical properties of the cellulosic materials were not significantly affected by the addition of APP or the hydrophobic agents. Moreover, the biodegradability of the cellulosic materials containing APP increased owing to the presence of the cellulase enzyme. The hydrophobic coating slightly decreased the biodegradability of cellulose-APP, but it was still higher than that of pure cellulose film.

Improved Mechanical Strength of Dicatechol Crosslinked MXene Films for Electromagnetic Interference Shielding Performance

Pristine MXene films express outstanding excellent electromagnetic interference (EMI) shielding properties. Nevertheless, the poor mechanical properties (weak and brittle nature) and easy oxidation of MXene films hinder their practical applications. This study demonstrates a facile strategy for simultaneously improving the mechanical flexibility and the EMI shielding of MXene films. In this study, dicatechol-6 (DC), a mussel-inspired molecule, was successfully synthesized in which DC as mortars was crosslinked with MXene nanosheets (MX) as bricks to create the brick-mortar structure of the MX@DC film. The resulting MX@DC-2 film has a toughness of 40.02 kJ·m^-3 and Young's modulus of 6.2 GPa, which are improvements of 513% and 849%, respectively, compared to those of the bare MXene films. The coating of electrically insulating DC significantly reduced the in-plane electrical conductivity from 6491 S·cm^-1 for the bare MXene film to 2820 S·cm^-1 for the MX@DC-5 film. However, the EMI shielding effectiveness (SE) of the MX@DC-5 film reached 66.2 dB, which is noticeably greater than that of the bare MX film (61.5 dB). The enhancement in EMI SE resulted from the highly ordered alignment of the MXene nanosheets. The synergistic concurrent enhancement in the strength and EMI SE of the DC-coated MXene film can facilitate the utilization of the MXene film in reliable, practical applications.

Multilayer Ultrathin MXene@AgNW@MoS2 Composite Film for High-Efficiency Electromagnetic Shielding

Structure and material composition is crucial in realizing high electromagnetic interference (EMI) shielding effectiveness (SE). Herein, an ultrathin MXene@AgNW@MoS₂ (MAM) composite film that resembles the structure of a pork belly and exhibits superior EMI shielding performance was fabricated via the vacuum-assisted suction filtration process and atomic layer deposition (ALD). The staggered AgNWs form skeletons and intersperse in MXene sheets to build a doped layer with three-dimensional network structures, which improves the electrical conductivity of the film. Based on the optimal dispersion concentration of Ag in doped and single layers, the MXene/AgNW doped layer and AgNW single layer are alternately vacuum-assisted-filtered to obtain laminated structures with multiple heterogeneous interfaces. These interfaces generate interface polarization and increase multiple reflection and scattering, resulting in the increased electromagnetic (EM) wave losses. On the other hand, MoS₂ outer nanolayers fabricated precisely by ALD effectively increases the absorption proportion of electromagnetic waves, reduces the secondary reflection, and improves the stability of EMI shielding properties. Ultimately, an ultrathin MAM film (a thickness of 0.03 mm) with five alternating internal layers and MoS₂ outer layers exhibits an excellent EMI SE of 86.3 dB in the X-band.