Mxenes Derived Laminated and Magnetic Composites with Excellent Microwave Absorbing Performance

Tuning the Work Function of MXene via Surface Functionalization

MXenes, a class of two-dimensional materials, have garnered significant attention due to their versatile surface chemistry and customizable properties. In this study, we investigate the work function (WF) tuning capabilities of MXene Ti3C2Tx, where Tx denotes the surface termination, synthesized via both conventional hydrogen fluoride-etched and recently reported molten salt-etched routes. When MXene samples are subjected to gas phase reactions, WF variations exceeding 0.6 eV are achieved, highlighting the potential for precise WF control. Notably, the WF increases from ∼4.23 eV (in N-doped MXene etched using molten salt) to ∼4.85 eV (N-doped MXene etched using HF). Complementary density functional theory (DFT) calculations reveal WF tuning across a >1 eV range via modification of the surface with different terminal groups (bare metal, F*, O*, N*, and Cl*). These changes in WF are attributed to surface termination modifications and the formation of TiO2 and TiN phases during annealing. DFT calculations further unveil an inverse correlation between the WF and the electron affinity of surface terminations. The findings from this comprehensive study provide insights into the tunable WF of MXenes, paving the way for their potential applications as interfacial layers in photovoltaic, energy conversion, and storage technologies.

A Novel Nano-Laminated GdB2C2 with Excellent Electromagnetic Wave Absorption Performance and Ultra-High-Temperature Thermostability

A novel nano-laminated GdB2C2 material was successfully synthesized using GdH2, B4C, and C via an in situ solid-state reaction approach for the first time. The formation process of GdB2C2 was revealed based on the microstructure and phase evolution investigation. Purity of 96.4 wt.% GdB2C2 was obtained at a low temperature of 1500 °C, while a nearly fully pure GdB2C2 could be obtained at a temperature over 1700 °C. The as-obtained GdB2C2 presented excellent thermal stability at a high temperature of 2100 °C in Ar atmosphere due to the stable framework formed by the high-covalence four-member and eight-member B-C rings in GdB2C2. The GdB2C2 material synthesized at 1500 °C demonstrated a remarkably low minimum reflection loss (RLmin) of -47.01 dB (3.44 mm) and a broad effective absorption bandwidth (EAB) of 1.76 GHz. The possible electromagnetic wave absorption (EMWA) mechanism could be ascribed to the nano-laminated structure and appropriate electrical conductivity, which facilitated good impedance matching, remarkable conduction loss, and interfacial polarization, along with the reflection and scattering of electromagnetic waves at multiple interfaces. The GdB2C2, with excellent EMWA performance as well as remarkable ultra-high-temperature thermal stability, could be a promising candidate for the application of EMWA materials in extreme ultra-high temperatures.

MXene titanium carbide synthesized by hexagonal titanium aluminum carbide with high specific capacitance and low impedance

The electrochemical properties of the MXene titanium carbide, Ti₃C₂, which has received much attention in the application of electrode materials for supercapacitors, are affected by the different morphologies of its precursor. In particular, the increase of layer spacing and specific capacitance, as well as the decrease of impedance and the dynamics analysis of Ti₃C₂ etched from hexagonal Ti₃AlC₂ precursors, are still not clear, and need to be further studied and explored. In this work, MXene Ti₃C₂ was synthesized efficiently in 2 hours by microwave assisted selective etching with hexagonal Ti₃AlC₂ as the precursor material. The specific capacitance of the Ti₃C₂ electrode is up to 357.85 F g^-1, while the ohmic resistance R_Ω of the whole electrochemical energy storage system is 0.234 ohm and the charge transfer resistance R_ct is 0.875 ohm. By analyzing the structural evolution and electrochemical properties from hexagonal Ti₃AlC₂ to Ti₃C₂, it is revealed that Ti₃C₂ prepared with hexagonal Ti₃AlC₂ as the precursor material has larger atomic layer spacing, more active sites, smaller diffusion impedance and higher energy storage efficiency than that prepared with ordinary Ti₃AlC₂. These lay a structural foundation for improving the energy storage performance of Ti₃C₂ supercapacitor electrodes.

Carbon-Coated Three-Dimensional MXene/Iron Selenide Ball with Core-Shell Structure for High-Performance Potassium-Ion Batteries

Two-dimensional (2D) MXenes are promising as electrode materials for energy storage, owing to their high electronic conductivity and low diffusion barrier. Unfortunately, similar to most 2D materials, MXene nanosheets easily restack during the electrode preparation, which degrades the electrochemical performance of MXene-based materials. A novel synthetic strategy is proposed for converting MXene into restacking-inhibited three-dimensional (3D) balls coated with iron selenides and carbon. This strategy involves the preparation of Fe₂O₃@carbon/MXene microspheres via a facile ultrasonic spray pyrolysis and subsequent selenization process. Such 3D structuring effectively prevents interlayer restacking, increases the surface area, and accelerates ion transport, while maintaining the attractive properties of MXene. Furthermore, combining iron selenides and carbon with 3D MXene balls offers many more sites for ion storage and enhances the structural robustness of the composite balls. The resultant 3D structured microspheres exhibit a high reversible capacity of 410 mAh g^-1 after 200 cycles at 0.1 A g^-1 in potassium-ion batteries, corresponding to the capacity retention of 97% as calculated based on 100 cycles. Even at a high current density of 5.0 A g^-1, the composite exhibits a discharge capacity of 169 mAh g^-1.

Design and Synthesis Strategies: 2D Materials for Electromagnetic Shielding/Absorbing

Two-dimensional (2D) materials possess special physical and chemical properties. They have been proved to have potential application advantage in the microwave absorption (MA) and electromagnetic interference (EMI) shielding. Particularly, they exhibit positive shielding and absorbing response to EMI. Here, the research progress of preparation, electromagnetic performance and microwave shielding/absorbing mechanisms of 2D composite materials are introduced. Effective preparation routes including introducing heteroatoms, constructing unique structures and 2D composite materials are described. Furthermore, the application prospects and challenges for the development of novel EMI materials are expatiated.

A high operating voltage micro-supercapacitor based on the interlamellar modulation type Ti3C2Tx MXene

The increasing demand for miniaturized, wearable, and flexible electronics has promoted the development of micro power sources such as microsupercapacitors (MSCs). This work reports a high-performance MSC based on Ti₃C₂T_x-layer/MnO₂-nanorod with an ionic liquid gel electrolyte, achieving a high areal capacitance of 24.7 mF cm^-2 within a wide voltage window of 2.5 V. The specific layer-rod interlaced structure of Ti₃C₂T_x/MnO₂ is designed to solve the inaccessibility of large-sized ions in ionic liquids into Ti₃C₂T_x layers. As a result, the structure modification provides an enhanced capacitance because the expanded interspace enables a sufficient number of large-sized ions to intercalate/deintercalate. This work provides insightful guidance for the interlaminar modification of Ti₃C₂T_x MXene to accommodate high operating voltage electrolyte with large-sized ions to obtain high-performance MSCs.

Strain Engineering of a MXene/CNT Hierarchical Porous Hollow Microsphere Electrocatalyst for a High-Efficiency Lithium Polysulfide Conversion Process

Tensile-strained Mxene/carbon nanotube (CNT) porous microspheres were developed as an electrocatalyst for the lithium polysulfide (LiPS) redox reaction. The internal stress on the surface results in lattice distortion with expanding Ti-Ti bonds, endowing the Mxene nanosheet with abundant active sites and regulating the d-band center of Ti atoms upshifted closer to the Fermi level, leading to strengthened LiPS adsorbability and accelerated catalytic conversion. The macroporous framework offers uniformed sulfur distribution, potent sulfur immobilization, and large surface area. The composite interwoven by CNT tentacle enhances conductivity and prevents the restacking of Mxene sheets. This combination of tensile strain effect and hierarchical architecture design results in smooth and favorable trapping-diffusion-conversion of LiPS on the interface. The Li-S battery exhibits an initial capacity of 1451 mAh g^-1 at 0.2 C, rate capability up to 8 C, and prolonged cycle life.

Filler-Free Conducting Polymers as a New Class of Transparent Electromagnetic Interference Shields

Transparent electromagnetic interference (EMI) shields are increasingly in demand for medical, military, wireless networks, aerospace electronics, and navigation control systems. To date, researchers have mixed pristine and/or doped conductive polymers with carbon allotropes and metallic fillers to increase the total shielding effectiveness, compromising the transparency, amount of the materials used, and weight of the shields. Obtaining cost-effective and transparent EMI shields without the need to incorporate fillers is extremely desirable. Herein, we implement a design strategy for fabricating a gigahertz (GHz) highly transparent shield made of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The total EMI shielding effectiveness of 15 dB is achieved in the X-band frequency range for a 50 nm ultrathin film with a high transparency of 97.1%. The fabricated filler-free EMI shield holds a record thickness-specific shielding figure-of-merit of 300 dB μm^-1-far exceeding the best values for micron-thick silver-, carbon-, and MXene-based composite material shields-with even a higher transparency. The feasibility of the developed filler-free shield for large-scale applications is validated by its integration into a cell phone display glass, as a prototype, in which the EMI shielding effectiveness elevates to 18.3 dB.

Ti3C2Tx MXenes as thin broadband absorbers

Atomically multilayered two-dimensional transition-metal carbides have abundant interfaces, and are very promising as outstanding electromagnetic absorbing materials at thin thickness. Here, a Ti₃C₂T_x MXene was prepared by hydrofluoric acid etching method, and has typical multilayered morphology with stacks of nanosheets. The microwave dielectric behaviours of the Ti₃C₂T_x with efficient microwave absorption were investigated. The Ti₃C₂T_x presents good impedance matching, achieved with effective absorption bandwidth covering from 12.4 GHz to 17.1 GHz, with thickness of only 1.5 mm, which nearly covers the whole Ku band. The microwave absorption performance was adjusted, and the Ti₃C₂T_x has a minimum reflection loss of -34.4 dB at 12 GHz at only 1.7 mm. This study demonstrates the real potential of Ti₃C₂T_x MXene materials as electromagnetic wave thin broadband absorbers.

Super strong 2D titanium carbide MXene-based materials: a theoretical prediction

The discovery of strong materials is essential in materials science and engineering. It becomes more significant to the practical applications of two-dimensional (2D) materials. In this study, the mechanical properties of all known 2D titanium carbide-based MXene monolayers have been systematically investigated by means of the density functional theory computations. Both the impacts of the thickness of the MXenes and the surface functionalization have been considered. Our results reveal that the in-plane planar elastic constants, Young's moduli and Shear moduli increase over the thickness. Moreover, they are enhanced by the terminal groups of surface functionalization. And the oxygen terminal group has the largest influence. As a result, the 2D Ti₄C₃O₂ is the strongest one among all 2D titanium carbide-based MXene, which is even stronger than the graphene. Our prediction provides the theoretical foundation for the specific application of MXenes that demands superior mechanical properties.

Sandwich-Like Fe&TiO2@C Nanocomposites Derived from MXene/Fe-MOFs Hybrids for Electromagnetic Absorption

Electromagnetic pollution has been causing a series of problems in people's life, and electromagnetic absorbers with lightweight and broad absorbing bandwidth properties are widely desired. In this work, novel sandwich-like 2D laminated Fe&TiO2 nanoparticles@C nanocomposites were rationally designed and successfully developed from the MXene-MOFs hybrids. The formation of Fe and rutile-TiO2 nanoparticles sandwiched by the two-dimensional carbon nanosheets provided strong electromagnetic energy attenuation and good impedance matching for electromagnetic wave (EMW) absorption. As expected, the nanocomposites achieved a broad effective absorption bandwidth of 6.5 GHz at a thickness of only 1.6 mm and the minimum reflection loss (RL) value of - 51.8 dB at 6.6 GHz with a thickness of 3 mm. This work not only provides a good design and fabricating concept for the laminated metal and functional nanoparticles@C nanocomposites with good EMW absorption, but also offers an important guideline to fabricate various two-dimensional nanocomposites derived from the MXene precursors.