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In this work, we employ Density Functional Theory to study the effects of decoration with transition metal (TM) atoms—Ag, Au, Cu, Sc, Ti and Pd—on the H 2 adsorption properties of tin carbide monolayers (SnC-ML), as a prospective material for hydrogen storage. The results indicate that TM adatoms are strongly bonded to the SnC-ML and that electronic charge is transferred from the adatoms to the SnC-ML. In particular, it is found that Sc and Ti are chemisorbed on SnC-ML with strong binding energies. The most stable adsorption site for these metal atoms is above Sn atoms of the SnC-ML. Also, these TM atoms exhibit the higher hydrogen-storage capacities with up to four hydrogen molecules per adatom. In contrast, the other studied metals have at most 2 hydrogen molecules adsorbed. Approximate temperature- and pressure-dependent curves suggest that, to storage hydrogen, Sc- and Ti-decorated SnC-ML should be cooled under freezing temperatures, or kept at 1 MPa and 2.5 MPa, respectively, which are much lower pressures than those currently used in vehicular tanks, which attain pressures of 35 MPa. These results indicate that Sc and Ti decorated SnC-ML can be useful as hydrogen-storage solid-state devices. [Display omitted] • SnC monolayers with transition metal adatoms have been investigated as hydrogen-storage materials. • Sc and Ti atoms are strongly bonded to the carbon atom of the SnC monolayers. • SnC monolayers with Sc and Ti atoms have a high capacity for hydrogen storage. • SnC monolayers with Ag and Au atoms have poor hydrogen storage capacities. • H 2 is physisorbed on transition metal-decorated SnC monolayers. [ABSTRACT FROM AUTHOR]

[Display omitted] Carbide nano-precipitates are commonly used to improve mechanical properties of steel. It has been experimentally observed that TiC, NbC, and VC carbide precipitates initially form as 'plate-like' particles oriented in the {1 0 0} planes of the ferrite lattice. These platelets share similarities with Guinier-Preston zones in Al-Cu alloys. The clustering of group IV and V transition metal atoms (M = Ti, Zr, Hf, V, Nb, Ta) in ferrite is studied using density functional theory. It is deduced that the transition metal carbides all form in a similar way. Furthermore, the transition from an initial M–C cluster to a NaCl-structured platelet to a NaCl-structured precipitate is examined through atomistic simulations using Modified Embedded Atom Method potentials. A route is established along which transition metal carbides form and transform into precipitates that possess the Baker-Nutting orientation relation with the ferrite matrix. [ABSTRACT FROM AUTHOR]

The crystal structure and composition of the zeta phase in the group VB transition metal carbides are not completely understood despite decades of experimental studies. As such, the phase rarely appears on phase diagrams of the group VB transition metal carbides. There is currently renewed interest in this phase, as tantalum carbide composites exhibit high fracture toughness in the presence of this phase. This work extends the initial computational study using density functional theory of the phase stability of the zeta phase in the tantalum carbide system, where the tantalum carbide zeta‐phase crystal structure and stability were determined, to the niobium and vanadium carbides. It is shown that the zeta phases in the three systems share the same crystal structure and it is an equilibrium phase at low temperatures. The carbon atom ordering in the three different phases is explored and it is demonstrated that the zeta phase in the tantalum carbides prefers to order carbon atoms differently than in the niobium and vanadium carbide zeta phases. Finally, the properties of this crystal are computed, including elastic constants, electronic densities of states and phonon dispersion curves, to illustrate that this crystal structure is similar to other transition metal carbides. [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]

Experimental results have shown that two-dimensional early transition metal carbides such as Nb 2 C also can be terminated by S atoms. In this study, 20 types of sulfur terminated MXenes (M 2 XS 2 , where M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W; X = C, N) are investigated using the density functional theory method. Phonon frequency analysis and molecule dynamic simulations upon 1000 K results show that V 2 CS 2 , Cr 2 NS 2 , Nb 2 CS 2 , Hf 2 CS 2 , and Ta 2 CS 2 are dynamically stable. Formation enthalpy calculations indicate that two-dimensional V 2 CS 2 , Nb 2 CS 2 , Ta 2 CS 2 , Hf 2 CS 2 , and Cr 2 NS 2 structures have the highest stability and may be easily synthesized. It indicates that MXenes can also be terminated by S atoms, which is influenced by the synthesis process. The binding energies for V 2 CS 2 , Nb 2 CS 2 , and Cr 2 NS 2 bilayer systems are smaller than 0.18 eV/atom, which indicates they can be easily separated into single layers. In contrast, the binding energies for Ta 2 CS 2 and Hf 2 CS 2 bilayers systems are up to 0.326 and 0.36 eV/atom, respectively. This indicates that Ta 2 CS 2 and Hf 2 CS 2 are relatively difficult to separate into single layers, which can be attributed to the localized p-orbitals of S atoms. Ta 2 CS 2 is a direct band gap semiconductor with band gap of 0.36 eV. Cr 2 NS 2 system is ferromagnetic material among MXenes, with the average magnetic moment of Cr atoms of 1.60 μ B . This research provides new viewpoints for the synthesis and surface termination of MXenes. [ABSTRACT FROM AUTHOR]

Refractory transition metal carbides have intriguing physical and chemical properties, especially when structured down to nanoscale. The lack of study in defected transition metal carbides, is partially due to the synthesis difficulty of refractory materials. The synthesis of defected carbide and their substitutional alloy will be even more difficult than single phase. First principle theory-based simulation can help to conceptualize the effects of substitutional defects on their mechanical properties. In this paper, we performed density functional theory (DFT) simulation of carbon defected α-MoC(1-x) phases to investigate their formation and mechanical properties for thesis sub-stoichio- metric materials. [ABSTRACT FROM AUTHOR]

Among the investigated systems in this study, W C 2 exhibited the lowest overpotential as 0.42 V, indicating that it can be potentially promising electrocatalyst for oxygen reduction reactions (ORR). [Display omitted] • Oxygen reduction reaction performance of CrC 2 , MnC 2 , FeC 2 , TcC 2 , HfC 2 , and WC 2 was explored. • The overpotential values of CrC 2 , MnC 2 , FeC 2 , TcC 2 , and HfC 2 were 1.04, 1.16, 1.56, 1.39, and 1.69 V, respectively. • WC 2 showed the lowest overpotential as 0.42 V among the other investigated systems. • Dual-active-site mechanism is found to be important for WC 2. Investigating highly efficient electrocatalysts for fuel cells is urgent. Herein, the oxygen reduction reaction performance of a group of MXene-like carbide two-dimensional materials, including C r C 2 , M n C 2 , F e C 2 , T c C 2 , H f C 2 , and W C 2 , was investigated through first-principles calculations. In these systems, transition metals are sandwiched between C 2 dimers at the top and those at the bottom of a M C 2 structure. Among the investigated systems, W C 2 exhibits the low overpotential of 0.42 V compared with the other systems. The overpotential values of C r C 2 , M n C 2 , F e C 2 , T c C 2 , and H f C 2 are 1.04, 1.16, 1.56, 1.39, and 1.69 V, respectively. In addition, two different catalytic mechanisms based on single and double active sites were compared to each other for T c C 2 and W C 2 , revealing that the mechanism based on dual-active-sites is more favorable than the mechanism based on a single-active-site for these two systems. Our study suggests that a W C 2 monolayer can be a potential electrocatalyst for oxygen reduction. [ABSTRACT FROM AUTHOR]

The development of novel transition metal carbides for improved hard coating technologies requires a detailed understanding of the factors influencing their stability and mechanical performance. To this end, we carried out first principles calculations based on density functional theory to tabulate the electronic structures, formation energies, and phonon dispersion curves of 3d transition metal carbides adopting zincblende, rocksalt, and cesium chloride structures. By analyzing the corresponding results, we outline a theoretical framework that describes how valence electron concentration and bonding configuration control the stability of these compounds. Many early transition metal carbides are predicted to be stable in the rocksalt and zincblende structures, enabled by filled bonding states, whereas the cesium chloride structure shows persistent instability. For compounds that are predicted to be stable, mechanical properties were investigated through calculation of elastic tensors, from which observable properties including Vicker's hardness and ductility were derived. A robust mechanical performance is shown to be correlated with complete filling of bonding orbitals as illustrated for rocksalt TiC and VC, which have calculated hardnesses of 25.66 and 22.63 GPa respectively. However, enhanced ductility and toughness can be achieved by allowing partial occupation of the antibonding states as in CrC, which has a relatively low Pugh's ratio of 0.51. [Display omitted] • Early 3d transition metal carbides show robust stability and high hardness. • Complete filling of bonding orbitals is crucial to maintain stability. • Partial antibonding contributes to decreased hardness but enhanced ductility. • Overoccupied antibonding orbitals cause instabilities in the rocksalt structure. • Jahn-Teller distortions cause instabilities in the zincblende structure. [ABSTRACT FROM AUTHOR]

[Display omitted] • Group IVB and VB transition metal carbides have different mass diffusion mechanisms. • Vacancy clustering is much more favorable in group IVB than VB metal carbides. • Off-lattice diffusion mechanisms have lower activation energies than on-lattice. • Off-lattice diffusion might be the most dominant mechanism in the group IVB TMCs. Using ab initio simulations, we report potential metal atom diffusion mechanisms in the group IVB and VB transition metal carbides. By computing the metal vacancy formation energies of vacancy clusters, we find that a metal vacancy surrounded by six carbon vacancies is the lowest metal vacancy formation energy structure for the group IVB carbides while the lowest energy for the group VB carbides is a metal vacancy surrounded by two carbon vacancies. The vacancy cluster mechanisms reveal activation energies that are consistent with experiments in both TiC (group IVB) and TaC (group VB). We also report that an off-lattice diffusion mechanism, that is only energetically favorable in the group IVB transition metal carbides, has a lower formation energy than the regular vacancy cluster mechanism. This new mechanism shows lower formation energies for a given carbon vacancy concentration which indicates this off-lattice mechanism might be the most dominant metal-atom diffusion mechanism among the existing mechanisms proposed for the group IVB metal carbides. [ABSTRACT FROM AUTHOR]

Abstract: The segregation energies of 3d (Sc–Cu), 4d (Y–Ag) and 5d (La–Au) transition metal impurities on the (100) surface of TiC have been obtained using first-principles electronic structure calculations. The results are in agreement with available experimental data and show that the difference in atomic size between the impurity and host species, as well as the difference in surface energies determines if the impurity will segregate towards the surface or not. The results indicate that the difference in size is the dominant factor for the trends in segregation of transition metal impurities towards the (100) surface of TiC. [Copyright &y& Elsevier]

• We theoretically report the low-miller-index surfaces of G VB -MCs for HER performance for the first time. • VC has a small absolute ΔG H* value of 0.011 eV, which is better than Pt −0.085 eV. • The (1 1 0) surfaces of G VB -MCs exhibit outstanding HER performance due to the proper d-band center and low work function. • Electronic transfer and bond length analysis indicate that metal–metal bridge site serves as the best active site. Transition metal carbides have received increasing attention for the application in hydrogen evolution reaction (HER), especially the group VB transition metal monocarbides (G VB -MCs). G VB -MCs display excellent HER activity in electrochemical water splitting which can even be favorably comparable with Pt in acidic condition, whereas it still remains as a challenge to illustrate their inherent catalytic mechanism. Herein, we use first-principle study to get insight into the HER activity of G VB -MCs and ascertain their catalytic mechanism. The resultant density functional theory (DFT) calculations suggest that the (1 1 0) surfaces of G VB -MCs have high stability and good conductivity. More importantly, we discover the highly active sites of the metal–metal bridge for the first time, which play an important role in guaranteeing the excellent HER activity on the (1 1 0) surfaces of G VB -MCs. Specifically, the Gibbs free energy of hydrogen adsorption (ΔG H*) of VC (1 1 0) surface (0.011 eV) is superior to that of precious metal Pt, and the calculated activation energy (0.667 eV) for the Tafel reaction on VC (1 1 0) surface is only slightly lower than that on the Pt (1 1 1) surface. Therefore, this work emphasizes the importance of active sites and provides reliable DFT guidelines for the further design of effective G VB -MCs electrocatalysts. [ABSTRACT FROM AUTHOR]

• MXenes show great potential for catalyzing industrially relevant processes. • MXenes (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo; W; X = C; N) exothermically adsorb water. • Activation energies for the first O H bond cleavage in H 2 O are below 0.44 eV. • MXenes dissociate water more effectively than surfaces containing noble metals. Two-dimensional few-layered transition-metal nitrides and carbides, called MXenes, have attracted a great interest given their large surface areas and their unique physicochemical properties. Motivated by the known reactivity of surfaces of bulk transition metal carbides on the mechanism behind the water-gas shift (WGS) reaction, density functional theory (DFT) calculations were employed to investigate the bonding of water and its dissociation on a set of eighteen M 2 X MXene (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W, while X = C or N) surfaces. Here it is shown that all the studied MXenes exothermically adsorb water, with adsorption energies ranging from -1.43 to -2.94 eV, and greatly facilitate its dissociation, with energy barriers below 0.44 eV. These results reinforce the role of MXenes in promoting water dissociation, effectively suggesting their potential as catalysts for industrially relevant processes such as the WGS reaction. [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]