Advancements in MAX phase materials: structure, properties, and novel applications

The MAX phase represents a diverse class of nanolaminate materials with intriguing properties that have received incredible global research attention because they bridge the divide separating metals and ceramics. Despite the numerous potential applications of MAX phases, their complex structure leads to a scarcity of readily accessible pure MAX phases. As a result, in-depth research on synthesis methods, characteristics, and structure is frequently needed for appropriate application. This review provides a comprehensive understanding of the recent advancements and growth in MAX phases, focusing on their complex crystal structures, unique mechanical, thermal, electrical, crack healing, corrosion-resistant properties, as well as their synthesis methods and applications. The structure of MAX phases including single metal MAX, i-MAX and o-MAX was discussed. Moreover, recent advancements in understanding MAX phase behaviour under extreme conditions and their potential novel applications across various fields, including high-temperature coatings, energy storage, and electrical and thermal conductors, biomedical, nanocomposites, etc. were discussed. Moreover, the synthesis techniques, ranging from bottom-up to top-down methods are scrutinized for their efficacy in tailoring MAX phase properties. Furthermore, the review explores the challenges and opportunities associated with optimizing MAX phase materials for specific applications, such as enhancing their oxidation resistance, tuning their mechanical properties, and exploring their functionality in emerging technologies. Overall, this review aims to provide researchers and engineers with a comprehensive understanding of MAX phase materials and inspire further exploration into their versatile applications in materials science and engineering.

Regulating the Electronic Structure of MAX Phases Based on Rare Earth Element Sc to Enhance Electromagnetic Wave Absorption

MAX phases are highly promising materials for electromagnetic (EM) wave absorption because of their specific combination of metal and ceramic properties, making them particularly suitable for harsh environments. However, their higher matching thickness and impedance mismatching can limit their ability to attenuate EM waves. To address this issue, researchers have focused on regulating the electronic structure of MAX phases through structural engineering. In this study, we successfully synthesized a ternary MAX phase known as Sc2GaC MAX with the rare earth element Sc incorporated into the M-site sublayer, resulting in exceptional conductivity and impressive stability at high temperatures. The Sc2GaC demonstrates a strong reflection loss (RL) of -47.7 dB (1.3 mm) and an effective absorption bandwidth (EAB) of 5.28 GHz. It also achieves effective absorption of EM wave energy across a wide frequency range, encompassing the X and Ku bands. This exceptional performance is observed within a thickness range of 1.3 to 2.1 mm, making it significantly superior to other Ga-MAX phases. Furthermore, Sc2GaC exhibited excellent absorption performance even at elevated temperatures. After undergoing oxidation at 800 °C, it achieves a minimum RL of -28.3 dB. Conversely, when treated at 1400 °C under an argon atmosphere, Sc2GaC demonstrates even higher performance, with a minimum RL of -46.1 dB. This study highlights the potential of structural engineering to modify the EM wave absorption performance of the MAX phase by controlling its intrinsic electronic structure.

Progress and challenges of emerging MXene based materials for thermoelectric applications

To realize sustainable development, more and more countries forwarded carbon neutrality goal. Accordingly, improving the utilization efficiency of traditional fossil fuel is an effective strategy for this great goal. Keeping this in mind, developing thermoelectric devices to recover waste heat energy resulted in the consumption process of fuel is demonstrated to be promising. High performance thermoelectric devices require advanced materials. MXenes are a kind of 2D materials with a layered structure, which demonstrate excellent thermoelectric performance owing to their unique physical, mechanical, and chemical properties. Also, substantial achievement has been gained during the past few years in synthesizing MXene based materials for thermoelectric devices. In this review, the mainstream synthetic routes of MXene from etching MAX were summarized. Significantly, the current state and challenges of research on improving the performance of MXene based thermoelectrics are explored, including pristine MXene and MXene based composites.

Recent advances in 2D metal carbides and nitrides (MXenes): synthesis and biological application

As a new two-dimensional (2D) material, transition metal carbides and nitrides (MXenes) have attracted much attention because of their excellent physical and chemical properties. In recent years, MXenes have been widely applied in the biological field due to their high biocompatibility, abundant surface groups, good conductivity, and photothermal properties. Here, the main synthesis methods of MXenes and the analysis of the advantages and disadvantages of each method are presented in detail. Then, the latest developments of MXenes in the biological field, including biosensing, antibacterial activity, reactive oxygen species (ROS) and free radical scavenging, tissue repair and antitumor therapy are comprehensively reviewed. Finally, the current challenges and future development trends of MXenes in biological applications are discussed.

The Rise of 212 MAX Phase Borides: DFT Insights into the Physical Properties of Ti2PB2, Zr2PbB2, and Nb2AB2 [A = P, S] for Thermomechanical Applications

In this article, ab initio calculations of unexplored Ti₂PB₂, Zr₂PbB₂, and Nb₂AB₂ [A = P, S] were performed wherein Ti₂PB₂ along with its 211 boride phase Ti₂PB was predicted for the first time. The stability was confirmed by calculating the formation energy, phonon dispersion curve, and elastic stiffness constants. The obtained elastic constants, elastic moduli, and Vickers hardness values of Ti₂PB₂, Zr₂PbB₂, and Nb₂AB₂ [A = P, S] were found to be significantly larger than those of their counterparts 211 borides and carbides. The studied compounds are brittle, like most MAX and MAB phases. The electronic band structure and density of states revealed the metallic nature of the titled borides. Several thermal parameters were explored, certifying the suitability of Ti₂PB₂, Zr₂PbB₂, and Nb₂AB₂ [A = P, S] to be used as efficient thermal barrier coating materials. The response of Ti₂PB₂, Zr₂PbB₂, and Nb₂AB₂ [A = P, S] to the incident photon was studied by computing the dielectric constant (real and imaginary parts), refractive index, absorption coefficient, photoconductivity, reflectivity, and energy loss function. In this work, we have explored the physical basis of the improved thermomechanical properties of 212 MAX phase borides compared to their existing carbide and boride counterparts.

MXene: Evolutions in Chemical Synthesis and Recent Advances in Applications

Two-dimensional materials have secured a novel area of research in material science after the emergence of graphene. Now, a new family of 2D material-MXene is gradually growing and making itsmark in this field of study. MXenes since 2011 have been synthesized and experimented on in several ways.The HF treatment although successful poses some serious problems that gradually propelled the ideas of new synthesis methods. This review of the literature covers the major breakthroughs of MXene from the year of its discovery to recent endeavors, highlighting how the synthesis mechanisms have been developed over the years and also the importance of good characterization of data. Results and properties of this class of materials arealso briefly discussed alongwith recent advance in applications.

Role of the A-Element in the Structural, Mechanical, and Electronic Properties of Ti3AC2 MAX Phases

Combining metallic and ceramic properties, and as precursors for MXenes, MAX phases have attracted extensive attention. In recent years, A-element substitution has been demonstrated as an effective scheme to enrich the MAX family. To explore more possible MAX members, the structural, mechanical, and electronic properties and stabilities of 31 Ti₃AC₂ (A = Al, Si, P, S, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Os, Ir, Pt, Au, Hg, TI, Pb, Bi, and Po) configurations are investigated in this work. Moreover, the interfacial strength implicating the possibility of exfoliating MAX into MXenes is examined. The A-element plays a crucial role in the lattice parameters and mechanical strength of Ti₃AC₂, and their variations are well explained by the synergistic effects of d-d and p-d hybridizations between the valence orbitals of Ti and A. Ti₃SC₂ presents the largest Young's modulus of 360 GPa, which is 6.82% higher than that in the well-studied Ti₃SiC₂. Ti₃SbC₂ is a mechanical quasi-isotropic configuration. After checking the mechanical, dynamical, and thermodynamic stability, Ti₃AC₂ (A = Al, Si, P, S, Ga, Ge, As, Cd, In, Sn, Sb, Au, Hg, Pb, TI, and Po) are stable, while Ti₃AC₂ (A = Fe, Co, Zn, Se, Ru, Rh, Pd, Ag, Te, Ir, Pt, and Bi) are metastable. Compared to Ti₃AlC₂, Ti₃AC₂ (A = Ag, Sb, Te, Bi, and Po) exhibit much lower interfacial strength in Ti-A interfaces and larger ratios between the interfacial strengths of neighboring Ti-C and Ti-A interfaces. This implies that these configurations are promising precursors for the synthesis of Ti₃C₂T_x (T_x denotes surface groups) with a large flake size. All of the configurations are metallic, and Ti₃AC₂ (A = Fe and Co) are magnetic. Based on the phonon dispersion and electronic structure, these Ti₃AC₂ configurations might have potential applications in phononic crystals and topological materials.

Recent Advances in Multidimensional (1D, 2D, and 3D) Composite Sensors Derived from MXene: Synthesis, Structure, Application, and Perspective

With the advent of the era of intelligent manufacturing, sensors, with various detection objects, have set off a wave of enthusiasm and reached new heights in medical treatment, intelligent industry, daily life, and so on. MXene, as an emerging family of 2D transition metal carbides/nitrides, possesses impressive electrical conductivity, outstanding structural controllability, and satisfying universality with other substrates. Consequently, MXene-based sensors with various functions show a booming growth based on great research potential of MXene. To promote the orderly and efficient development of MXene application in sensors, and further accelerate market-scale application of ideal sensors, in this review, a full range research effort on current MXene-based sensors is summarized. Starting with various synthesis methods of the raw material MXene, a comprehensive summary work along with 1D, 2D, or 3D MXene-based sensors on most recent works is put forward, including the preparation method, characteristic structure, and potential sensing application of each type of MXene-based composite sensors. Ultimately, insights of the opportunities and challenges on the strength of the current reported MXene-based sensor are given.

Tuning the Chemical Properties of Co-Ti3 C2 Tx MXene Materials for Catalytic CO2 Reduction

MXenes, a novel family of 2D materials, are energy materials that have gained considerable attention, particularly for their catalytic applications in emerging areas such as CO₂ and N₂ hydrogenation. Herein, for the first time, it is shown that the surface reducibility of Ti₃ C₂ T_x MXene can be tuned by N doping, which induces a change in the catalytic properties of supported Co nanoparticles. Pristine Co-Ti₃ C₂ T_x MXene favors CO production during CO₂ hydrogenation, whereas CH₄ production is favored when the MXene is subjected to simple N doping. X-ray photoelectron spectroscopy and transmission electron microscopy (TEM) reveal that surface rutile TiO₂ nanoparticles appear on the Ti₃ C₂ T_x support upon N doping, which interact strongly with the supported Co nanoparticles. This interaction alters the reducibility of the supported Co nanoparticles at the interface with the TiO₂ nanoparticles, shifting the product selectivity from CO to CH₄ . This study successfully showcases a practical strategy, based on surface chemistry modulation of 2D MXenes, for regulating product distribution in CO₂ hydrogenation.

Single-Atom-Thick Active Layers Realized in Nanolaminated Ti3(AlxCu1-x)C2 and Its Artificial Enzyme Behavior

A Ti₃(Al_xCu_1-x)C₂ phase with Cu atoms with a degree of ordering in the A plane is synthesized through the A site replacement reaction in CuCl₂ molten salt. The weakly bonded single-atom-thick Cu layers in a Ti₃(Al_xCu_1-x)C₂ MAX phase provide actives sites for catalysis chemistry. As-synthesized Ti₃(Al_xCu_1-x)C₂ presents unusual peroxidase-like catalytic activity similar to that of natural enzymes. A fabricated Ti₃(Al_xCu_1-x)C₂/chitosan/glassy carbon electrode biosensor prototype also exhibits a low detection limit in the electrochemical sensing of H₂O₂. These results have broad implications for property tailoring in a nanolaminated MAX phase by replacing the A site with late transition elements.

Phase formation of nanolaminated Mo2AuC and Mo2(Au1-xGax)2C by a substitutional reaction within Au-capped Mo2GaC and Mo2Ga2C thin films

Au-containing nanolaminated carbides Mo₂AuC and Mo₂(Au_1-xGa_x)₂C were synthesized by a thermally induced substitutional reaction in Mo₂GaC and Mo₂Ga₂C, respectively. The Au substitution of the Ga layers in the structures was observed using cross-sectional high-resolution scanning transmission electron microscopy. Expansion of c lattice parameters was also observed in the Au-containing phases compared to the original phases. Energy dispersive spectroscopy detected residual Ga in Au-substituted layers of both phases with a peculiar Ga in-plane ordering for Au : Ga = 9 : 1 ratio along the Au-Ga layers in Mo₂(Au_1-xGa_x)₂C. These results indicate a generalization of the Au substitution reaction for the A elements in MAX phases.