Modified MAX Phase Synthesis for Environmentally Stable and Highly Conductive Ti3C2 MXene

High-temperature stability and phase transformations of titanium carbide (Ti3C2Tx) MXene

Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, are under increasing pressure to meet technological demands in high-temperature applications, as MXenes can be considered to be one of the few ultra-high temperature 2D materials. Although there are studies on the stability of their surface functionalities, there is currently a gap in the fundamental understanding of their phase stability and transformation of MXenes' metal carbide core at high temperatures (>700 °C) in an inert environment. In this study, we conduct systematic annealing of Ti₃C₂T_xMXene films in which we present the 2D MXene flake phase transformation to ordered vacancy superstructure of a bulk three-dimensional (3D) Ti₂C and TiC_ycrystals at 700 °C ⩽T⩽ 1000 °C with subsequent transformation to disordered carbon vacancy cubic TiC_yat higher temperatures (T> 1000 °C). We annealed Ti₃C₂T_xMXene films made from the delaminated MXene single-flakes as well as the multi-layer MXene clay in a controlled environment through the use ofin situhot stage x-ray diffraction (XRD) paired with a 2D detector (XRD²) up to 1000 °C andex situannealing in a tube furnace and spark plasma sintering up to 1500 °C. Our XRD²analysis paired with cross-sectional scanning electron microscope imaging indicated the resulting nano-sized lamellar and micron-sized cubic grain morphology of the 3D crystals depend on the starting Ti₃C₂T_xform. While annealing the multi-layer clay Ti₃C₂T_xMXene creates TiC_ygrains with cubic and irregular morphology, the grains of 3D Ti₂C and TiC_yformed by annealing Ti₃C₂T_xMXene single-flake films keep MXenes' lamellar morphology. The ultrathin lamellar nature of the 3D grains formed at temperatures >1000 °C can pave way for applications of MXenes as a stable carbide material 2D additive for high-temperature applications.

Optical-Transparent Self-Assembled MXene Film with High-Efficiency Terahertz Reflection Modulation

The modulation of the terahertz (THz) wave is fundamental for its applications in next-generation communications, biological imaging, sensing, and so forth. Searching for higher efficient modulation is still in progress, although plenty of materials have been explored for tuning THz wave. In this work, optical-transparent self-assembled MXene films are used to modulate the THz reflection at the SiO₂/MXene/air interface based on the impedance matching mechanism. By adjusting the number of stacked MXene layers/concentrations of MXene dispersions, the sheet conductivity of the MXene films will be changed so that the impedance at the SiO₂/MXene/air interface can be tuned and lead to a giant modulation of THz reflection. Particularly, we demonstrate that the MXene films have highly efficient THz modulation from antireflection to reflection-enhancing with a relative reflection of 27% and 406%, respectively. This work provides a new pathway for developing the MXene films with the combination of optical-transparency and high smart THz reflection characteristics, and the films can be applied for THz antireflection or reflection-enhancing.

Solution-Processed Ti3 C2 Tx MXene Antennas for Radio-Frequency Communication

Highly integrated, flexible, and ultrathin wireless communication components are in significant demand due to the explosive growth of portable and wearable electronic devices in the fifth-generation (5G) network era, but only conventional metals meet the requirements for emerging radio-frequency (RF) devices so far. Here, it is reported on Ti₃ C₂ T_x MXene microstrip transmission lines with low-energy attenuation and patch antennas with high-power radiation at frequencies from 5.6 to 16.4 GHz. The radiation efficiency of a 5.5 µm thick MXene patch antenna manufactured by spray-coating from aqueous solution reaches 99% at 16.4 GHz, which is about the same as that of a standard 35 µm thick copper patch antenna at about 15% of its thickness and 7% of the copper weight. MXene outperforms all other materials evaluated for patch antennas to date. Moreover, it is demonstrated that an MXene patch antenna array with integrated feeding circuits on a conformal surface has comparable performance with that of a copper antenna array at 28 GHz, which is a target frequency in practical 5G applications. The versatility of MXene antennas in wide frequency ranges coupled with the flexibility, scalability, and ease of solution processing makes MXene promising for integrated RF components in various flexible electronic devices.

Electrochemical Impedance Spectroscopy on 2D Nanomaterial MXene Modified Interfaces: Application as a Characterization and Transducing Tool

This review presents the basic characteristics of MXene, a novel 2D nanomaterial with many outstanding properties applicable to electrochemical sensing and biosensing. The second part deals with electrochemical impedance spectroscopy (EIS) and its beneficial features applicable to ultrasensitive electrochemical sensing and label-free biosensing. The main part of the review presents recent advances in the integration of MXene to design electrochemical interfaces. EIS was used to evaluate the effect of anodic potential on MXene and the effect of the MXene preparation route and for characterization of MXene grafted with polymers. It also included the application of EIS as the main transducing tool for antibody- and aptamer-based biosensors or biosensors integrating molecularly imprinted polymers.

Exfoliation and Defect Control of Two-Dimensional Few-Layer MXene Ti3C2Tx for Electromagnetic Interference Shielding Coatings

Defect-controlled exfoliation of few-layer transition-metal carbide (f-Ti₃C₂T_x) MXene was demonstrated by optimizing chemical etching conditions, and electromagnetic interference (EMI) shielding coatings were explored. The structural features such as layer morphology, lateral size, layer thickness, defect density, and mechanical stability of the exfoliated f-Ti₃C₂T_x were strongly dependent on exfoliation conditions. By selecting appropriate exfoliation conditions, moderate etching time leads to the formation of quality f-Ti₃C₂T_x with lesser defects, whereas longer etching time can break the layer structure and increase defect density, structural misalignment, and oxidative products of f-Ti₃C₂T_x. The resultant fabricated free-standing flexible f-Ti₃C₂T_x films exhibited electrical conductivity and electromagnetic interference (EMI) shielding effectiveness (SE) in the X-band of about 3669 ± 33 S/m and 31.97 dB, respectively, at a thickness of 6 μm. The large discrepancy in EMI SE performance between quality (31.97 dB) and defected (3.164 dB) f-Ti₃C₂T_x sheets is attributed to interconnections between f-Ti₃C₂T_x nanolaminates interrupted by defects and oxidative products, influencing EMI attenuation ability. Furthermore, the demonstrated solution-processable high-quality f-Ti₃C₂T_x inks are compatible and, when applied for EM barrier coating on various substrates, including paper, cellulose fabric, and PTFE membranes, exhibited significant EMI shielding performance. Moreover, controlling defects in f-Ti₃C₂T_x and assembly of heterogeneous disordered carbon-loaded TiO₂-Ti₃C₂T_x ternary hybrid nanostructures from f-Ti₃C₂T_x by tuning etching conditions could play an enormous role in energy and environmental applications.