The increasing electromagnetic pollution necessitates the development of advanced microwave absorbers. Although MAX phases exhibit chemical stability and electrical conductivity, their absorption performance is limited by a singular loss mechanism. Here, we propose a “pre-placed vacancy and isomorphous occupancy” strategy to engineer A-site high-entropy (HE) MAX phases, achieving unprecedented incorporation of large-radius elements (Ag and Bi). The optimized absorber delivers exceptional microwave absorption performance, with a minimum reflection loss of −71.6 dB (at 3.05 mm) and a broad effective absorption bandwidth of 4.1 GHz (at just 1.25 mm), outperforming both reported MAX phase variants and commercial absorbers. These remarkable properties stem from three synergistic mechanisms: A-site composition tailoring optimized impedance matching, HE-induced lattice distortion enhanced dipolar polarization, and A-site entropy engineering increased conduction loss. Our work pioneers a novel method for manipulating electromagnetic response in MAX phases through atomic-scale entropy engineering, paving the way for next-generation electromagnetic protection materials.

Polycyclic conjugated hydrocarbons have acquired increased interests recently because of their potential applications in electronic devices. On metal surfaces, the selective synthesis of four- and five-membered carbon rings remains challenging due to the presence of diverse reaction pathways. Here, utilizing the same precursor molecule, we successfully achieved substrate-controlled highly selective cycloaddition reactions towards four- and five-membered carbon rings. A 97 % yield for four-membered carbon rings on Au(111), while a 96 % yield towards five-membered carbon rings is achieved on Ag(111). The detailed topological structures of the reaction products are carefully examined by bond-resolving scanning tunneling microscopy (BR-STM) imaging with a CO functionalized tip. The underlying mechanism of the novel surface-directed reaction selectivity is elucidated by extensive density functional theory (DFT) calculations. Our study paves the way for high selective synthesis of polycyclic conjugated hydrocarbons with non-benzenoid rings.

High-entropy (HE) MAX phases represent an emerging family of multi-constituent solid solutions that provide large compositional variations and, therefore, a wide variety of properties. Here, we report the experimental realization of M2AlC (M = Ti/Ta/V/Nb/Cr) MAX phase by an energy-efficient self-propagating high-temperature synthesis, which enables facile scalability to an environmentally friendly industrial production. The HE-MAX phase was developed according to crystal size, electronegativity, and valence electron concentration of corresponding metals required to form a substitutional single-phase material. Variations in initial mixture composition, inert gas pressure, additive amount and sample diameter played a decisive role in HE-MAX formation. The combustion of the stoichiometric mixture favors the formation of the HE-carbide. Deviation from the stoichiometry has resulted in the formation of 211 and/or 413 type HE-MAX phases. Fine-tuning the aluminum and carbon content in the initial mixture, facilitated the formation of a layered structure, characteristic of MAX phases. DSC/TG analysis proved an enhanced oxidation resistance of HE-MAX phases, which outperforms conventional MAX phases and several previously studied HE-MAX phases.

Selective etching has emerged as a key method for synthesizing 2D materials, with the conversion of MAX phases to MXenes being by far the most widely studied and reported example. While traditional methods rely on etching in primarily acidic aqueous media, molten salts offer an intriguing alternative. However, the current understanding of MAX phase reactivity in molten salts is limited, restricting our ability to predict reaction outcomes. In this study, we present a computational framework that uses process-specific phase diagrams to model A-element substitution and MXene formation, as well as competing side reactions. Applying this approach to Ti3AlC2, V2AlC, and Ti2AlN in ZnCl2 molten salt, we reveal distinct reaction behaviors despite identical redox potentials-defined here by the Al-to-Zn exchange-of key processes. Our findings underscore the limitations of predicting reactions based solely on redox potentials and show that our model can capture key trends in MXene synthesis. Beyond MXenes, our methodology lays the groundwork for identifying new 2D materials accessible through molten salt etching.

Achieving large two-dimensional (2D) sheets of any metal is challenging due to their tendency to coalescence or cluster into 3D shapes. Recently, single-atom-thick gold sheets, termed goldene, was reported. Here, we ask if goldene can be extended to include multiple layers. The answer is yes, and trilayer goldene is the magic number, for reasons of electronegativity. Experiments are made to synthesize the atomically laminated phase Ti4Au3C3 through substitutional intercalation of Si layers in Ti4SiC3 for Au. Density functional theory calculations suggest that it is energetically favorable to insert three layers of Au into Ti4SiC3, compared to inserting a monolayer, a bilayer, or more than three layers. Isolated trilayer goldene sheets, ~100 nanometers wide and 6.7 angstroms thick, were obtained by chemically etching the Ti4C3 layers from Ti4Au3C3 templates. Furthermore, trilayer goldene is found in both hcp and fcc forms, where the hcp is ~50 milli–electron volts per atom more stable at room temperature from ab initio molecular dynamic simulations.

Nanostructured titania, TiO2, holds significant importance in various scientific fields and technologies for their distinctive properties and multipurpose characteristics. In this article, the facile, economical, and scalable synthesis of 1D lepidocrocite, 1DL, titania nanostructures derived from a water-soluble Ti precursor, titanium oxysulfate (with oxidation of Ti+4) at temperature <100 degrees C under atmospheric pressure is discussed. Titanium oxysulfate with tetramethyl ammonium hydroxide, TMAH, is simply reacted to yield individual lepidocrocite titania-based chain-forming nanofilaments, NFs, 6 x 6 & Aring;(2) in minimal cross-section and aspect ratios of approximate to 20 1DLs. If only ethanol is used for washing, the 1DL self-assemble into approximate to 10 <mu>m, porous mesostructured particles, PMPs. If water is used, quasi-2D sheets form instead. Characterization of the resulting powders showed them to be quite similar to those derived from TiB2, and other water-insoluble Ti precursors. The 1DL bandgap energies are approximate to 4 eV, due to quantum confinement. They adsorbed rhodamine 6G. The latter also sensitized the 1DLs and allowed for dye degradation using only visible light. Used as electrodes in supercapacitors, the 1DLs can be cycled over 1.6 V and result in high power densities (300 W kg(-1)). Stronger birefringence started to appear in samples with concentrations >15 gL(-1) indicating the formation of a liquid crystal phase. This new synthesis protocol enables the cheaper scalable production of 1DLs with significant implications across various fields.

Wearable electronics are some of the most promising technologies with the potential to transform many aspects of human life such as smart healthcare and intelligent communication. The design of self-powered fabrics with the ability to efficiently harvest energy from the ambient environment would not only be beneficial for their integration with textiles, but would also reduce the environmental impact of wearable technologies by eliminating their need for disposable batteries. Herein, inspired by classical Archimedean spirals, we report a metastructured fiber fabricated by scrolling followed by cold drawing of a bilayer thin film of an MXene and a solid polymer electrolyte. The obtained composite fibers with a typical spiral metastructure (SMFs) exhibit high efficiency for dispersing external stress, resulting in simultaneously high specific mechanical strength and toughness. Furthermore, the alternating layers of the MXene and polymer electrolyte form a unique, tandem ionic-electronic coupling device, enabling SMFs to generate electricity from diverse environmental parameters, such as mechanical vibrations, moisture gradients, and temperature differences. This work presents a design rule for assembling planar architectures into robust fibrous metastructures, and introduces the concept of ionic-electronic coupling fibers for efficient multimodal energy harvesting, which have great potential in the field of self-powered wearable electronics. In this work, a concept of ionic-electronic coupling fibers by integrating a 2D MXene and a polymer electrolyte to fabricate spiral metastructures is proposed to realize multimodal power generation from various sources simultaneously.

Sulfur-terminated single sheet (ss-)MXene was recently achieved by delamination of multilayered van der Waals bonded (vdW)-MXenes Nb2CS2 and Ta2CS2 synthesized through solid-state synthesis, rather than via the traditional way of selectively etching A-layers from the corresponding MAX phase. Inspired by this, we perform a computational screening study of vdW-MXenes M(2)CCh(2) isotypical to Nb2CS2 and Ta2CS2, with M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, or W and Ch = S, Se, or Te. The thermodynamic stability of each vdW-MXene M(2)CCh(2) is assessed, and the dynamical stability of both vdW- and ss-MXene is considered through phonon dispersions. We predict seven stable vdW-MXenes, out of which four have not been reported previously, and one, V2CSe2, incorporates a new transition metal element into this family of materials. Electronic properties are presented for the vdW- and ss-forms of the stable vdW-MXenes, suggesting that the materials are either metallic, semimetallic, or semiconducting. In previous experimental reports the vdW-MXene Nb2CS2 is synthesized by manipulation of the corresponding M(2)AX phase Nb2SC. Therefore, we also evaluate the thermodynamic stability of the corresponding M(2)AX phases, identifying 15 potentially stable phases. Six of these are experimentally reported, leaving nine new M(2)AX phases for future experimental investigation.

The creation of vacancies and/or pores into two-dimensional materials, like graphene and MXenes, has shown to increase their performance for sustainable applications. However, a simple and affordable method with controlled and tailorable vacancy concentration and/or pores size remains challenging. Herein, a simple and reproducible method is presented for controlled synthesis of Mo1.74CTz MXene with randomly distributed vacancies and pores, obtained from selective etching of both Ga and Cr in the Cr-alloyed MAX-phase like precursor Mo1.74Cr0.26Ga2C. Structural and compositional analysis of the 3D alloy show approximate to 13% Cr on the metal site, homogeneously distributed between different particles and within the atomic structure. After etching, it translates to Mo1.74CTz MXene, exhibiting defect-rich sheets. Notably, the incorporation of Cr facilitates a shorter etching time with an improved yield compared to Mo2CTz. The Mo1.74CTz MXene displays excellent electrochemical properties, almost doubling the capacitance values (1152 F cm(-3) and 297 F g(-1) at 2 mV s(-1) scan rate), compared to its pristine counterpart Mo2CTz. The presented method and obtained results suggest defect engineering of MXenes through precursor alloying as a pathway that can be generalized to other phases, to further improve their properties for various applications.

Assembly of two-dimensional (2D) nanomaterials into well-organized architectures is pivotal for controlling their function and enhancing performance. As a promising class of 2D nanomaterials, MXenes have attracted significant interest for use in wearable electronics due to their unique electrical and mechanical properties. However, facile approaches for fabricating MXenes into macroscopic fibers with controllable structures are limited. In this study, we present a strategy for easily spinning MXene fibers by incorporating polyanions. The introduction of poly(acrylic acid) (PAA) into MXene colloids has been found to alter MXene aggregation behavior, resulting in a reduced concentration threshold for lyotropic liquid crystal phase. This modification also enhances the viscosity and shear sensitivity of MXene colloids. Consequently, we were able to draw continuous fibers directly from the gel of MXene aggregated with PAA. These fibers exhibit homogeneous diameter and high alignment of MXene nanosheets, attributed to the shear-induced long-range order of the liquid crystal phase. Furthermore, we demonstrate proof-of-concept applications of the ordered MXene fibers, including textile-based supercapacitor, sensor and electrical thermal management, highlighting their great potential applied in wearable electronics. This work provides a guideline for processing 2D materials into controllable hierarchical structures by regulating aggregation behavior through the addition of ionic polymers.