MXenes are two-dimensional (2D) materials with versatile applications in optoelectronics, batteries, and catalysis. To unlock their full potential, it is crucial to characterize MXene interfaces and intercalated species in more detail than is currently possible with conventional optical spectroscopies. Here, we combine ultra-broadband ellipsometry and transmission spectroscopy from the mid-infrared (IR) to the deep-ultraviolet (UV) to probe quantitatively the composition, structure, transport, and optical properties of spray-coated Ti₃C₂T_x MXene thin films with varying material properties. We find film thickness heterogeneity and surface roughness in the low-nanometer range as well as depth-dependent conductivity properties, which we quantify with a graded Drude model. The optically determined sheet resistance is confirmed by four-point probe measurements. Furthermore, we employ density-functional-theory calculations to assign the observed absorption bands in the MXene dielectric function to various interband transitions from mixed MXene surface terminations. The prominent 1.48 eV (833 nm) spectral feature is found to be related to oxygen termination. Additional plasmonic effects are also suggested. Finally, we leverage the chemical sensitivity of state-of-the-art IR ellipsometry to separate the fingerprints of intercalated species within the MXene from the dominant Drude contributions, presenting for the first time a set of infrared optical constants of intercalated water. This work lays the foundation for optical metrology for interface engineering of MXene and other 2D materials.

Link: https://pubs.acs.org/doi/full/10.1021/acs.jpcc.4c06906

The high electrical conductivity and good chemical stability of MXenes offer hopes for their use in many applications, such as wearable electronics, energy storage, and electromagnetic interference shielding. While their optical, electronic, and electrochemical properties have been widely studied, information on the thermal properties of MXenes is scarce. In this study, we investigate the heat transport properties of Ti₃C₂T_x MXene single flakes using scanning thermal microscopy and find exceptionally low anisotropic thermal conductivities within the Ti₃C₂T_x flakes, leading to an effective thermal conductivity of 0.78 ± 0.21 W m^–1 K^–1. This observation is in stark contrast to the predictions of the Wiedemann–Franz law, as the estimated Lorenz number is only 0.25 of the classical value. Due to the combination of low thermal conductivity and low emissivity of Ti₃C₂T_x, the heat loss from it is 2 orders of magnitude smaller than that from common metals. Our study explores the heat transport mechanisms of MXenes and highlights a promising approach for developing thermal insulation, two-dimensional thermoelectric, or infrared stealth materials.

Link: https://pubs.acs.org/doi/full/10.1021/acsnano.4c08189

MXenes, i.e., two-dimensional transition metal carbides and nitrides, have been reported as promising materials for various applications, including energy storage, biomedicine, and electronics. The family of MXenes has proliferated, and the chemical space of synthesized MXenes has expanded to 13 transition metals and a dozen elements in surface terminations. The diverse chemistry of MXenes enables systematical tuning of MXene properties to meet the needs of target applications. However, synthesizing new MXene compositions largely relies on a trial-and-error approach. To overcome it, computational predictions of MXene compositions that are thermodynamically stable are crucial to rationalize experimental efforts. Here, we report a comprehensive computational screening for thermodynamically stable MXenes across 29 transition metals and 11 surface terminations. Density functional theory calculations are employed to compute the energy above the convex energy hull as a descriptor of thermodynamic stability. The results are analyzed to explore factors crucial for determining the thermodynamic stability of MXenes, by which the chemistry of surface terminations is found to play a crucial role. The insights on the chemistry of 998 MXene compositions predicted to be (meta)stable are given to systematically guide further research on MXene synthesis and application.

Link: https://pubs.acs.org/doi/10.1021/acs.chemmater.4c02274

(Research materials provided by M-STAR.)

Two-dimensional transition metal carbides and nitrides, commonly known as MXenes, are a novel class of 2D materials with high free carrier densities, making them highly attractive candidates for plasmonic 2D materials. In this study, we use multiphoton photoemission electron microscopy (nP-PEEM) to directly image the plasmonic near fields of multilayers of the prototypical MXene, Ti₃C₂T_x, with mixed surface terminations (T_x = F, O, and OH). Photon-energy dependent nP-PEEM reveals a dispersive surface plasmon polariton between 1.4 eV and 1.9 eV on MXene flakes thicker than 30 nm and waveguide modes above 1.9 eV. Combining experiments with finite difference time domain (FDTD) simulations, we reveal the emergence of a visible surface plasmon polariton in MXenes, opening avenues for exploration in polaritonic phenomena in MXenes in the visible portion of the electromagnetic spectrum.

Link: https://chemrxiv.org/engage/chemrxiv/article-details/66ba644a5101a2ffa81dc23b

Achieving both high redox activity and rapid ion transport is a critical and pervasive challenge in electrochemical energy storage applications. This challenge is significantly magnified when using large-sized charge carriers, such as the sustainable ammonium ion (NH₄⁺). A self-assembled MXene/n-type conjugated polyelectrolyte (CPE) superlattice-like heterostructure that enables redox-active, fast, and reversible ammonium storage is reported. The superlattice-like structure persists as the CPE:MXene ratio increases, accompanied by a linear increase in the interlayer spacing of MXene flakes and a greater overlap of CPEs. Concurrently, the redox activity per unit of CPE unexpectedly intensifies, a phenomenon that can be explained by the enhanced de-solvation of ammonium due to the increased volume of 3 Å-sized pores, as indicated by molecular dynamic simulations. At the maximum CPE mass loading (MXene:CPE ratio = 2:1), the heterostructure demonstrates the strongest polymeric redox activity with a high ammonium storage capacity of 126.1 C g⁻¹ and a superior rate capability at 10 A g⁻¹. This work unveils an effective strategy for designing tunable superlattice-like heterostructures to enhance redox activity and achieve rapid charge transfer for ions beyond lithium.

Link: https://onlinelibrary.wiley.com/doi/full/10.1002/aenm.202402715

Due to the ubiquity of textiles in the lives, electronic textiles (E-textiles) have emerged as a future technology capable of addressing a myriad of challenges from mixed reality interfaces, on-garment climate control, patient diagnostics, and interactive athletic wear. However, providing sufficient electrical power in a textile form factor has remained elusive. To address this issue, different approaches are discussed, starting with supercapacitors’ advantages and limitations and material choices for textile-based supercapacitors before discussing proper data analysis and design considerations of textile-based energy storage to power wearable electronics.

Link: https://onlinelibrary.wiley.com/doi/full/10.1002/aenm.202402367

Surface chemistry of MXenes is of great interest as the terminations can define the intrinsic properties of this family of materials. The diverse and tunable terminations also distinguish MXenes from many other 2D materials. Conventional fluoride-containing reagents etching approaches resulted in MXenes with mixed fluoro-, oxo-, and hydroxyl surface groups. The relatively strong chemical bonding of MXenes’ surface metal atoms with oxygen and fluorine makes post-synthetic covalent surface modifications of such MXenes unfavorable. In this minireview, we focus on the recent advances in MXenes with uniform surface terminations. Unconventional methods, including Lewis acidic molten salt etching (LAMS) and bottom-up direct synthesis, have been proven successful in producing halide-terminated MXenes. These synthetic strategies have opened new possibilities for MXenes because weaker surface chemical bonds in halide-terminated MXenes facilitate post-synthetic covalent surface modifications. Both computational and experimental results on surface termination-dependent properties are summarized and discussed. Finally, we offer our perspective on the opportunities and challenges in this exciting research field.

Link: https://onlinelibrary.wiley.com/doi/10.1002/anie.202409480

Fourier-transform infrared (FTIR) spectroscopy characterization is a powerful and easy-to-use technique frequently employed for the characterization and fingerprinting of materials. Although MXenes are a large and fastest growing family of inorganic 2D materials, the lack of systematic FTIR spectroscopy studies hinders its application to MXenes and often leads to misinterpretation of the results. In this study, we report experimental and calculated FTIR spectra of 12 most typical carbide and carbonitride MXenes with different compositions (5 transition metals) and all four basic structures, including Ti₂CT_x, Nb₂CT_x, Mo₂CT_x, V₂CT_x, Ti₃C₂T_x, Ti₃CNT_x, Mo₂TiC₂T_x, Mo₂Ti₂C₃T_x, Nb₄C₃T_x, V₄C₃T_x, Ta₄C₃T_x, and Mo₄VC₄T_x. The measurements were performed on delaminated MXene flakes incorporated in KBr pellets in the 4000–400 cm^–1 range. We provide detailed instructions for sample preparation, data collection, and interpretation of FTIR spectra of MXenes. Background correction and spectra smoothing are applied to obtain clear FTIR peaks corresponding to bond vibrations in MXenes. Density functional theory calculations were used for the precise assignment of all characteristic FTIR peaks and an in-depth analysis of the vibration modes. This work aims to provide the 2D material community with the FTIR spectroscopy technique as a reliable method for identifying and analyzing MXenes.

Link: https://pubs.acs.org/doi/full/10.1021/acs.chemmater.4c01536

MXenes are promising passive components that enable lithium-sulfur batteries (LSBs) by effectively trapping lithium polysulfides (LiPSs) and facilitating surface-mediated redox reactions. Despite numerous studies highlighting the potential of MXenes in LSBs, there are no systematic studies of MXenes’ composition influence on polysulfide adsorption, which is foundational to their applications in LSB. Here, a comprehensive investigation of LiPS adsorption on seven MXenes with varying chemistries (Ti₂CT_x, Ti₃C₂T_x, Ti₃CNT_x, Mo₂TiC₂T_x, V₂CT_x, Nb₂CT_x, and Nb₄C₃T_x), utilizing optical and analytical spectroscopic methods is performed. This work reports on the influence of polysulfide concentration, interaction time, and MXenes’ chemistry (transition metal layer, carbide and carbonitride inner layer, surface terminations and structure) on the amount of adsorbed LiPSs and the adsorption mechanism. These findings reveal the formation of insoluble thiosulfate and polythionate complex species on the surfaces of all tested MXenes. Furthermore, the selective adsorption of lithium and sulfur, and the extent of conversion of the adsorbed species on MXenes varied based on their chemistry. For instance, Ti₂CT_x exhibits a strong tendency to adsorb lithium ions, while Mo₂TiC₂T_x is effective in trapping sulfur by forming long-chain polythionates. The latter demonstrates a significant conversion of intermediate polysulfides into low-order species. This study offers valuable guidance for the informed selection of MXenes in various functional components benefiting the future development of high-performance LSBs.

Link: https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202404430

MXenes are a large family of two-dimensional materials that have attracted attention across many fields due to their desirable optoelectronic, biological, mechanical and chemical properties. There currently exist many synthesis procedures that lead to differences in flake size, defects and surface chemistry, which in turn affect their properties. Herein, we describe the steps to synthesize Ti₃C₂T_x—the most important and widely used MXene, from a Ti₃AlC₂ MAX phase precursor. The procedure contains three main sections: synthesis of Ti₃AlC₂ MAX, wet chemical etching of the MAX in hydrofluoric acid/HCl solution to yield multilayer Ti₃C₂T_x and its delamination into single-layer flakes. Three delamination options are described; these use LiCl, tertiary amines (tetramethyl ammonium hydroxide/ tetrabutyl ammonium hydroxide) and dimethylsulfoxide respectively. These procedures can be adapted for the synthesis of MXenes beyond Ti₃C₂T_x. The MAX phase synthesis takes about 1 week, with the etching and delamination each requiring 2 d. This protocol requires users to have experience working with hydrofluoric acid, and it is recommended that users have experience with wet chemistry and centrifugation; characterization techniques such as X-ray diffraction and particle size analysis are also essential for the success of the protocol. While alternative synthesis methods, such as minimally intensive layer delamination, are desirable for certain MXenes (such as Ti₂CT_x) or specific applications, this protocol aims to standardize the more commonly used hydrofluoric acid/HCl etching method, which produces Ti₃C₂T_x with minimal concentration of defects and the highest conductivity and serves as a guideline for those working with MXenes for the first time.

Link: https://www.nature.com/articles/s41596-024-00969-1

Efficient catalysts with minimal content of catalytically active noble metals are essential for the transition to the clean hydrogen economy. Catalyst supports that can immobilize and stabilize catalytic nanoparticles and facilitate the supply of electrons and reactants to the catalysts are needed. Being hydrophilic and more conductive compared with carbons, MXenes have shown promise as catalyst supports. However, the controlled assembly of their 2D sheets creates a challenge. This study established a lattice engineering approach to regulate the assembly of exfoliated Ti₃C₂T_x MXene nanosheets with guest cations of various sizes. The enlargement of guest cations led to a decreased interlayer interaction of MXene lamellae and increased surface accessibility, allowing intercalation of Pd nanoparticles. Stabilization of Pd nanoparticles between interlayer-expanded MXene nanosheets improved their electrocatalytic activity. The Pd-immobilized K⁺-intercalated MXene nanosheets (PdKMX) demonstrated exceptional electrocatalytic performance for the hydrogen evolution reaction with the lowest overpotential of 72 mV (@10 mA cm^–2) and the highest turnover frequency of 1.122 s^–1 (@ an overpotential of 100 mV), which were superior to those of the state-of-the-art Pd nanoparticle-based electrocatalysts. Weakening of the interlayer interaction during self-assembly with K⁺ ions led to fewer layers in lamellae and expansion of the MXene in the c direction during Pd anchoring, providing numerous surface-active sites and promoting mass transport. In situ spectroscopic analysis suggests that the effective interfacial electron injection from the Pd nanoparticles strongly immobilized on interlayer-expanded PdKMX may be responsible for the improved electrocatalytic performance.

Link: https://pubs.acs.org/doi/full/10.1021/acsnano.3c09640

MXenes produced by Lewis acid molten salt (LAMS) etching of MAX phases have attracted the community’s attention due to their controllable surface chemistry. However, their delamination is challenging due to the hydrophobicity of the produced multilayer MXene and strong interactions between the halogen-terminated MXene sheets. The current delamination method involves dangerous chemicals such as n-butyllithium or sodium hydride, making scale-up difficult and limiting the practical application of this class of MXenes. In this work, we present a simple and efficient method for the delamination of MXenes from the LAMS synthesis while maintaining their surface chemistry. LiCl salt and anhydrous polar organic solvents are used for delamination. Films produced from the delaminated MXene are flexible and have an electrical conductivity of 8000 S/cm, which is maintained after a week of exposure to 95% humidity. This successful delamination, preservation of inherent surface properties, and stability under high-humidity conditions dramatically expand the range of MXene chemistries available for research and potential applications.

Link: https://pubs.acs.org/doi/full/10.1021/acs.chemmater.3c02872

We are entering the era of new materials: materials that can be assembled from nanoscale building blocks, unlike all previous material generations from the Stone Age to the Silicon Age. Two-dimensional (2D) materials provide nanometer and subnanometer-thin “bricks” for such assembly. If needed, organic molecules and polymers can serve as mortar, but van der Waals (vdW) or electrostatic forces can also provide a strong bonding between the 2D layers. To make this vision real and start assembling materials, structures, and devices from nanoparticles, we need many building blocks with a large variety of physical and chemical properties. … The advancement of MXenes toward industrial use would require the development of scalable, low-cost, safe, and environmentally friendly synthesis processes, as well as extensive and thorough studies of their toxicity and fate in the environment. All new materials follow this path. Based on the unique and tunable properties of MXenes as well as their enormous compositional diversity and tunability of properties, we are confident that the field of MXenes has a bright and exciting future.

Link: https://pubs.acs.org/doi/full/10.1021/acs.chemmater.3c02491

Since their discovery in 2011, the family of 2D transition metal carbides and nitrides, MXenes, has made remarkable progress. While their initial development was relatively slow compared to their current growth and some other 2D materials, MXenes have gained momentum over the past seven years. We can expect accelerating progress as the critical mass of scientists and engineers focusing on MXenes is being accumulated. Still, since MXenes are developing into the largest known family of inorganic low-dimensional materials with an enormously broad range of applications in almost every engineering field, as well as healthcare and analytical chemistry, there is plenty of room for discovery for any number of researchers entering the field of these fascinating materials. Moreover, considering the transformational role that MXenes are expected to play in the fields from communication to optoelectronics, healthcare, energy and environment, countries that seize the opportunity earlier may gain a major technological advantage.

Link: https://link.springer.com/article/10.1007/s41127-023-00067-1