First Principles Calculations of the Relative Stability, Structure and Electronic Properties of Two Dimensional Metal Carbides and Nitrides

Atomic Scale Design of MXenes and Their Parent Materials─From Theoretical and Experimental Perspectives

More than a decade after the discovery of MXene, there has been a remarkable increase in research on synthesis, characterization, and applications of this growing family of two-dimensional (2D) carbides and nitrides. Today, these materials include one, two, or more transition metals arranged in chemically ordered or disordered structures of three, five, seven, or nine atomic layers, with a surface chemistry characterized by surface terminations. By combining M, X, and various surface terminations, it appears that a virtually endless number of MXenes is possible. However, for the design and discovery of structures and compositions beyond current MXenes, one needs suitable (stable) precursors, an assessment of viable pathways for 3D to 2D conversion, and utilization or development of corresponding synthesis techniques. Here, we present a critical and forward-looking review of the field of atomic scale design and synthesis of MXenes and their parent materials. We discuss theoretical methods for predicting MXene precursors and for assessing whether they are chemically exfoliable. We also summarize current experimental methods for realizing the predicted materials, listing all verified MXenes to date, and outline research directions that will improve the fundamental understanding of MXene processing, enabling atomic scale design of future 2D materials, for emerging technologies.

Fabrication of Smart Tantalum Carbide MXene Quantum Dots with Intrinsic Immunomodulatory Properties for Treatment of Allograft Vasculopathy

MXene nanomaterials have sparked significant interest among interdisciplinary researchers to tackle today's medical challenges. In particular, colloidal MXene quantum dots (MQDs) offer the high specific surface area and compositional flexibility of MXene while providing improvements to aqueous stability and material-cell interactions. The current study for the first time reports the development and application of immunoengineered tantalum-carbide (Ta4C3T x ) MQDs for in vivo treatment of transplant vasculopathy. This report comes at a critical juncture in the field as poor long-term safety of other MXene compositions challenge the eventual clinical translatability of these materials. Using rational design and synthesis strategies, the Ta4C3T x MQDs leverage the intrinsic anti-inflammatory and antiapoptotic properties of tantalum to provide a novel nanoplatform for biomedical engineering. In particular, these MQDs are synthesized with high efficiency and purity using a facile hydrofluoric acid-free protocol and are enriched with different bioactive functional groups and stable surface TaO2 and Ta2O5. Furthermore, MQDs are spontaneously uptaken into antigen-presenting endothelial cells and alter surface receptor expression to reduce their activation of allogeneic T-lymphocytes. Finally, when applied in vivo, Ta4C3T x MQDs ameliorate the cellular and structural changes of early allograft vasculopathy. These findings highlight the robust potential of tailored Ta4C3T x MQDs for future applications in medicine.

Systematic investigations of the electron, phonon and elastic properties of monolayer M2C (M = V, Nb, Ta) by first-principles calculations

Based on first-principles calculations and theoretical analysis, we investigated various properties of pristine monolayer M₂C (M  =  V, Nb, Ta). Firstly, we optimized the structures of monolayer M₂C and computed the corresponding electronic band structures, the results show that they are metallic. And there exists Dirac points in the band structure, which make them may being potential candidates for investigating Dirac-physics-based applications. Secondly, we analyzed the phonon spectra combining with the corresponding projected phonon density of states of monolayer M₂C. The results indicate that the three monolayers M₂C are dynamically stable. The large energy gap between the optical phonon ZO and ZO' mode gets wider with the mass of translation metal increasing. Thirdly, the related thermodynamic properties, such as the Raman (E _g, A _1g), infrared active (E _u, A _2u) mode, Debye temperature, sound speed, temperature-dependent heat capacity, entropy, free energy and lattice thermal conductivity were also investigated. Finally, the planar elastic stiffness coefficients and other derived elastic properties of monolayer M₂C were determined. We find that the Y _s value of Nb₂C and Ta₂C is larger than that of monolayer Ti₂C (130 N m^-1). By using the uniaxial tensile, we obtained the stress-strain properties of monolayer M₂C. The monolayer Ta₂C has the strongest peak strength in the direction of armchair. Its maximum stress is 83GP at ε _arm  =  0.19. Thus, those MXene materials can be considered as extremely stiff 2D materials.

The Synthesis Process and Thermal Stability of V₂C MXene

The effect of etching solution on the synthesis process of two-dimensional vanadium carbide (V₂C MXene) was researched. Three etching solutions were used to etch ternary carbide V₂AlC at 90 °C. The three solutions were: lithium fluoride + hydrochloric acid (LiF + HCl), sodium fluoride + hydrochloric acid (LiF + HCl), and potassium fluoride + hydrochloric acid (KF + HCl). It was found that only NaF + HCl solution was effective for synthesizing highly pure V₂C MXene. The existence of sodium (Na⁺) and chloridion (Cl⁻) in etching solution was essential for the synthesis. The thermal stability of the as-prepared V₂C MXene in argon or air was studied. From thermogravimetry and differential thermal analysis, V₂C MXene was found to be stable in argon atmosphere at a temperature of up to 375 °C. As the temperature increased, V₂C MXene was gradually oxidized to form nanoparticles composed of vanadium trioxide (V₂O₃) and a part of V₂C MXene was broken and transformed to vanadium carbide (V₈C₇) at 1000 °C. In air atmosphere, V₂C MXene was stable at 150 °C. At 1000 °C, V₂C MXene was oxidized to form vanadium pentoxide (V₂O₅).