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Two-dimensional multiferroic (2D) materials have garnered significant attention due to their potential in high-density, low-power multistate storage and spintronics applications. MXenes, a class of 2D transition metal carbides and nitrides, were first discovered in 2011, and have become the focus of research in various disciplines. Our study, utilizing first-principles calculations, examines the lattice structures, and electronic and magnetic properties of nitride MXenes with intrinsic band gaps, including V2NF2, V2NO2, Cr2NF2, Mo2NO2, Mo2NF2, and Mn2NO2. These nitride MXenes exhibit orbital ordering, and in some cases the orbital ordering induces magnetoelastic coupling or magnetoelectric coupling. Most notably, Cr2NF2 is a ferroelastic material with a spiral magnetic ordered phase, and the spiral magnetization propagation vector is coupled with the direction of ferroelastic strain. The ferroelectric phase can exist as an excited state in V2NO2, Cr2NF2, and Mo2NF2, with their magnetic order being coupled with polar displacements through orbital ordering. Our results also suggest that similar magnetoelectric coupling effects persist in the Janus MXenes V8N4O7F, Cr8N4F7O, and Mo8N4F7O. Remarkably, different phases of Mo8N4F7O, characterized by orbital ordering rearrangements, can be switched by applying external strain or an external electric field. Overall, our theoretical findings suggest that nitride MXenes hold promise as 2D multiferroic materials. [ABSTRACT FROM AUTHOR]

Transition metal carbides and nitrides are interesting non-precious materials that have been shown to replace or reduce the loading of precious metals for catalyzing several important electrochemical reactions. The purpose of this review is to summarize density functional theory (DFT) studies, describe reaction pathways, identify activity and selectivity descriptors, and present a future outlook in designing carbide and nitride catalysts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), nitrogen reduction reaction (N2RR), CO2 reduction reaction (CO2RR) and alcohol oxidation reactions. This topic is of high interest to scientific communities working in the field of electrocatalysis and this review should provide theoretical guidance for the rational design of improved carbide and nitride electrocatalysts. [ABSTRACT FROM AUTHOR]

2D transition metal carbides and nitrides (MXenes) are promising hydrogen evolution reaction (HER) catalysts owing to their metallic conductivity, abundant surface active sites, and high specific surface area. The introduction of a single transition metal atom (TM) at the surface is a good way to improve the HER performance of MXenes. However, the effect of TM on MXenes in previous theoretical studies focused on pure functional groups, and ignored the hybrid‐functionalized ones, which are mostly observed in experiments. Herein, the HER performance of four O/F ratios stable hybrids MXenes, Ti2CTx (T = O, F), is explored. Ti2CO1.33F0.67 exhibits superior HER catalytic activity, comparable to that of platinum metals. Further combinatorial screening of ≈200 TMs based on Ti2CTx structures suggests that Rh, Ti, Ir, and Pt are optimal TM candidates that enhance the sensitivity to strain modulation and reduce the activation barrier for hydrogen generation. A descriptor ψ is used to quantify HER performance and reveals the role of the electron filling of TM to the antibonding orbitals. These findings propose feasible candidates with high HER performance through single‐atom modification for hybrid‐functional MXenes, and a useful descriptor to screen for MXenes with desirable catalytic properties. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, are continuously growing in terms of both crystalline and composition varieties. They have received significant attention in science and technology. The new members of MXenes with in-plane ordered double transition metals have been named as iMXenes. In this study, we have investigated the electronic structures and optical properties of 2D (Mo2/3Y1/3)2CO2 and (W2/3Y1/3)2CO2 iMXene monolayers, using a set of density functional theory calculations. We found that the (Mo2/3Y1/3)2CO2 and (W2/3Y1/3)2CO2 2CO2 are semiconductors with indirect bandgaps of 0.477 eV and 0.655 eV, respectively. To investigate the optical properties, we calculated the absorption spectrum and reflectivity percentage of these structures along x and z directions using the real and imaginary parts of the dielectric function. It is observed that the real and imaginary parts of their dielectric functions possess many peaks in the energy region of less than 3.1 eV. Interestingly, they show high absorption in the visible and UV regions, implying the potential applications of these semiconducting iMXenes in solar cells and optical nanodevices. [ABSTRACT FROM AUTHOR]

Recently, two-dimensional (2D) transition metal carbides and nitrides known as MXenes, have gained a lot of attention because of their tunable electronic and magnetic properties depending on surface functionalization. In the present work, the structural, electronic, and magnetic properties of both T and H phases of bare Ti2 C and fully surface terminated Ti2 CT2 (T = -F, = O, -OH) are calculated using a set of first principles calculations. The ground state structures of Ti2 CT2 are computed in two and four different configurations for both H and T phases, respectively. We demonstrate that while H phase of Ti2 C exhibits half-metallic behavior with magnetic moments of 2 µB per formula unit, it displays metallic behavior with magnetic moments of 1.27 µB, 0.25 µB per formula unit, and semiconductor behavior with 0.35 eV band gap in -F, -OH, and =O surface functionalization, respectively. We also show that while T phase of Ti2 C exhibits metallic behavior with magnetic moment of 1.89 µB per formula unit, it stays in metallic nonmagnetic behavior in both -F and -OH. Meanwhile, it displays semiconductor behavior with 0.25 eV band gap in -O surface functionalization. We expect that our results can advance the future applications of MXenes from energy storage to spintronic. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Interstitial transition metal nitrides and carbides based on simple crystal structures with partial occupation of certain sites by N and C frequently show long-range ordering. In this work the focus is on the important group of superstructures based on a face-centered cubic (fcc)-based arrangement of the metal atoms and octahedral site occupation by N or C. The ordering tendencies are explored for various Ni 4 N and Ni 4 C superstructures using density-functional-theory. The ordering tendencies differ strikingly as revealed by the low-energy superstructures encountered for Ni 4 N and for Ni 4 C: in the case of Ni 4 N 2nd-next-neighbor N-N pairs appear favorable whereas in Ni 4 C 1st-next neighbor C-C pairs are preferred. This can be explained by an interplay between elastic interactions favoring 1st-next neighbor pairs of N and C atoms in an fcc-based arrangement of metal atoms and chemical/Coulomb interactions favoring 2nd-next neighbor (i.e. more distant) pairs. While the elastic interactions are expected to be similar for N and C, the chemical/Coulomb interactions are stronger for N-N as compared to C-C and are thus able to explain the different ordering tendencies. [ABSTRACT FROM AUTHOR]

Density functional theory calculations are employed to systematically investigate the trend of hydrogen evolution reaction (HER) performance of oxygen-terminated MXenes. By studying 30 transition-metal carbides and 30 transition-metal nitrides, Mn+1CnO2 and Mn+1NnO2 (M = Sc, Cr, Hf, Mo, Nb, Ta, Ti, V, W, Zr; n = 1, 2, 3), the tendency of oxygen desorption after hydrogen adsorption is elucidated to play a key role in HER performance of oxygen-terminated MXenes. On the basis of these observations, we propose a suitable HER descriptor, oxygen vacancy formation energy (Ef), which scales linearly with the adsorption free energy of hydrogen, ΔGH. In addition, this new descriptor is linearly correlated with the lithium binding strength on oxygen-terminated MXenes. Therefore, Ef is a universal descriptor for identifying the trend of adsorption processes where adsorbed species donate electrons to oxygen-terminated MXenes. This work provides a general guideline for large-scale screening of promising MXene-based materials for energy storage and conversion. [ABSTRACT FROM AUTHOR]

We investigated the effect of shock impact on some transition metal carbides and nitrides {MC/N (M = Zr, Nb, Ta, Ti)} within the Debye-Gruneisen theory. Parameters required for the model were obtained from first principles by fitting the total free energy vs. volume data obtained from DFT to the equation of state (EOS) proposed by Li. The EOS model was used to predict shock Hugoniot curves for these compounds to verify the effect of increasing pressures and temperatures on the EOS. Reasonably good agreement with experimental data was noted for NbC, ZrC, TaC and ZrN and TiN up to ∼ 200 GPa considering the metals to exist in B 1 (NaCl) phase. However, in case of ZrC, TaC and ZrN, single Hugoniot data points were available at higher pressures ( ∼ 400 GPa). In all three cases, the predicted Hugoniot corresponding to B 1 phase did not match with the experimental data point at that pressure. This was attributed to a structural phase transition occurring in the metals. We found all the compounds to be stable in the rock salt structure ( B 1) and transform to CsCl ( B 2) structure at high pressures (beyond ∼ 200 GPa) and 0 K. Since experiments are performed at finite temperatures, we investigated this transition at finite temperatures. From our analysis, we concluded that B 1 → B 2 transition does not occur at high pressures under shock loading. Experimental evidence of the transition has also not been reported for these compounds as per our knowledge. The only other possibility of a structural phase transition was solid-liquid transition i.e. melting. We found that all the compounds melt under shock loading at high pressures beyond ∼ 200 GPa. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ag and Au electrocatalysts supported on transition metal carbides (TMCs) and nitrides (TMNs) were synthesized and evaluated for CO 2 RR. • TaC and TaN were identified as promising supports for Ag and Au catalysts for electroreduction of CO 2 to synthesis gas. • ΔG (*HOCO) was identified as a key descriptor for CO 2 RR based on a linear correlation between ΔG (*HOCO) and J ECSA (CO). • Electron transfer from TMC/TMN substrates to Ag/Au played a dominant role in controlling the CO 2 RR activity. Synthesis gas produced from the electrochemical carbon dioxide reduction reaction (CO 2 RR) using renewable energy represents a promising solution toward carbon neutrality. Both high activity and controlled CO/H 2 ratios are crucial for coupling CO 2 RR with downstream thermochemical reactions. In the current study, transition metal carbides (TMCs) and nitrides (TMNs) of Group VB elements (V, Nb and Ta) were used as supports to tune the CO 2 RR activity and CO/H 2 ratios over Ag- and Au-based electrocatalysts. Combined electrochemical measurements and density functional theory (DFT) calculations identified the Gibbs free energy for adsorption of the *HOCO intermediate (ΔG (*HOCO)) as a potential descriptor for CO 2 RR, which could be adjusted by using different TMC and TMN supports. As opposed to structural strain effects, the electronic modification was found to be the key controlling factor in the interactions between TMC/TMN and supported Ag/Au. TaC and TaN have been identified as promising supports for Ag- and Au-based electrocatalysts for CO 2 RR to synthesis gas production. [ABSTRACT FROM AUTHOR]

Using density functional theory and the Peierls–Nabarro model, we calculated the shear strength and generalized stacking fault energy of TiN and TiC as prototypes of transition metal nitrides and carbides. In spite of its higher elastic moduli, TiN was found to have a lower plastic resistance as compared to TiC due to the higher shear strength and stacking fault energy of TiC. A mechanism based on the development of a deformed electronic structure is proposed to explain the origin of this difference in plastic resistance. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal carbides and nitrides, known as MXenes, have gained notable attention recently because these super-strong materials have many promising applications. In this study, computational analysis is conducted to explore the in-plane elastic constants, 2D stiffness and shear modulus of oxygen terminated M 4 X 3 MXenes by means of the density functional theory (DFT) calculations. Our results reveal that the binding site of the oxygen atoms can greatly affect the mechanical properties. The crystal orbital Hamilton population (COHP) analysis suggests that the impact of the oxygen-binding site is because the M − O bonding strength can significantly influence their mechanical stiffness. Our outcomes may, therefore, provide the theoretical foundation for the advance of MXene-based applications in mechanical engineering. [Display omitted] • 2D M 4 X 3 O 2 materials are super-strong materials and possess outstanding mechanical properties. • Introducing the oxygen termination groups to the surface of MXenes improve their mechanical strength. • The influence of transition metal on the mechanical properties of MXenes has been investigated. • The COHP analysis indicates that the M–O bonding strength mainly determines the mechanical strength of M 4 X 3 O 2. [ABSTRACT FROM AUTHOR]

MXenes, transition metal carbides and nitrides, are a bourgeoning class of two-dimensional (2D) materials due to their tunable electronic and magnetic structures, rich surface chemistry and thermal stability. Here, we perform structural, electronic, vibrational and thermal properties of six transition-metal carbides (Re 2 C, Tc 2 C, Mo 2 C, W 2 C, Os 2 C, Ru 2 C) both 2H and 1T phases by first-principle calculations within density functional theory. Although physical and thermal properties of some pristine MXenes put forward to understanding tunable electronic and magnetic properties, many of 1T-phase and almost never of 2H-phase are not investigated in detailed. Firstly, the atomic structure of six MXenes both 1T and 2H phases are optimized, and their respective dynamical stabilities are discussed. Secondly, electronic band structure calculations reveal that the pristine MXenes are metallic. Finally, phonon calculations of pristine MXenes are computed with the density functional perturbation theory and reported. Raman-active modes are predicted and to assign them to specific atomic motions. Ab-initio molecular dynamic simulations (AIMD) are also performed to check the thermal stability of these MXenes. Our results clearly proved that while 2H-W 2 C, 2H-Mo 2 C, 2H-Re 2 C and 1T-W 2 C can keep the stabilities at very high temperature (1500 K). The results suggest that these pristine MXenes combining with outstanding properties such as non-magnetic metallic state and high-temperature thermal stability would provide promising candidates for using from energy storage to spintronic applications. • The new 2D MXenes have been predicted by first-principle calculations. • Structural, electronic, vibrational and thermal properties are performed. • AIMD are proved thermal stabilities at the different temperature. • MXenes would provide promising candidates for future applications. [ABSTRACT FROM AUTHOR]

In this paper we introduce a stacking-fault based model to understand the energetics of formation of the nanolamellar-based metal carbide and nitride structures. The model is able to reproduce the cohesive energies of the stacking fault phases from density functional theory calculations by fitting the energy of different stacking sequences of metal layers. The model demonstrates that the first and second nearest metal-metal neighbor interactions and the nearest metal-carbon/nitrogen interaction are the dominant terms in determining the cohesive energy of these structures. The model further demonstrates that above a metal to non-metal ratio of 75%, there is no energetic favorability for the stacking faults to form a long-range ordered structure. The model’s applicability is demonstrated using the Ta-C system as its case study from which we report that the interfacial energy between ζ-Ta4C3 and TaC or Ta2C is negligible. Our results suggest that the closed packed planes of these phases should be aligned and that precipitated phases should be thin, which is in agreement with experiments. [ABSTRACT FROM AUTHOR]