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Since the form of the exact functional in density functional theory is unknown, we must rely on density functional approximations (DFAs). In the past, very promising results have been reported by combining semi-local DFAs with exact, i.e. Hartree–Fock, exchange. However, the spin-state energy ordering and the predictions of global minima structures are particularly sensitive to the choice of the hybrid functional and to the amount of exact exchange. This has been already qualitatively described for single conformations, reactions, and a limited number of conformations. Here, we have analyzed the mixing of exact exchange in exchange functionals for a set of several hundred isomers of the transition metal carbide, Mo4C2. The analysis of the calculated energies and charges using PBE0-type functional with varying amounts of exact exchange yields the following insights: (1) The sensitivity of spin-energy splitting is strongly correlated with the amount of exact exchange mixing. (2) Spin contamination is exacerbated when correlation is omitted from the exchange-correlation functional. (3) There is not one ideal value for the exact exchange mixing which can be used to parametrize or choose among the functionals. Calculated energies and electronic structures are influenced by exact exchange at a different magnitude within a given distribution; therefore, to extend the application range of hybrid functionals to the full periodic table the spin-energy splitting energies should be investigated. [ABSTRACT FROM AUTHOR]

Mass-selected photoelectron velocity-map imaging spectroscopy in conjunction with the density functional theory calculations was employed to investigate the geometrical and chemical bonding properties of NiC3−/0. Both the photoelectron spectrum and photoelectron angular distribution were measured from the spectra, yielding useful geometrical and electronic information about NiC3−/0. The complementary theoretical calculations suggest that the linear and fan-like structures were both populated experimentally in the cluster beam. Further comparative study on the synergistic donor–acceptor interactions in both isomers revealed the side-on coordination-induced bond weakening in the fan-like isomer as compared to the linear isomer. These findings will shed light on the structure-dependent reactivity of transition metal carbides. [ABSTRACT FROM AUTHOR]

The development of spintronic devices urgently requires ultra-thin two-dimensional (2D) ferromagnetic materials with high Curie temperature (Tc), TC however, there are few natural intrinsic ferromagnetic 2D materials. The successful synthesis of Janus monolayer MoSSe in experiments provides a new approach for designing new 2D materials. By replacing transition metal carbides with two different transition metal atoms, we have designed over 70 Janus MXence materials and determined that 30þ materials have ferromagnetic ground states, of which three have robust ferromagnetism through density functional theory analysis. The ferromagnetic coupling in suchmaterials mainly originates from the direct exchange of d-orbitals between transition metal atoms in different layers. Further, using K-nearest neighbours (KNN) machine learning (ML) method, six out of the remaining 360 Janus MXence systems were screened for ferromagnetism, with one system exhibiting strong ferromagnetism. This work provides an alternative and convenient method for developing ultra-thin 2D magnetic materials for next generation spintronic device applications. [ABSTRACT FROM AUTHOR]

Green hydrogen is seen as the fuel of the future as various efforts are made to further implement it in different industries and sectors. Green hydrogen offers a clean and emissions free renewable source of high energy density that helps work towards the global net zero emissions goal. Green hydrogen, which is produced via the hydrogen evolution reaction (HER), requires a technological breakthrough in the electrolyzer; the latter necessitates for the presence of non-precious, stable, and highly active electrocatalysts. Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides (MXenes) are promising future electrocatalysts that have great reported exhibiting potential for commercial scalability due to their excellent intrinsic properties and structural flexibility. First, the synthesis methods are comprehensively summarized and discussed based on their electrocatalytic usage for the HER alongside their electronic properties. Second, this mini-review captures the current status of MXenes as electrocatalysts both experimentally and theoretically. The present MXenes can be separated into three categories: pristine, doped and hybrid structures where comparisons are drawn based on electrochemical descriptors backed by DFT studies. In addition, density functional theory (DFT) screening studies based on the Gibbs free energy of hydrogen adsorption are reported on different MXene variants. Pristine, double transition metal, carbonitride, vacancy induced, transition metal anchored and unique surface terminations MXenes are all reported, showing promising energetics for further experimental expansion in this field. Finally, secondary tools such as molecular dynamics and machine learning are coupled with DFT to explore novel MXene structures and efficiently predict their HER performance. • MXene as HER electrocatalysts are mainly synthesized via top-down methods. • Termination elements significantly impact HER kinetics of MXene electrocatalysts. • MXene heterostructures show lowest overpotentials and Tafel slopes. • HER DFT screening of pristine, carbonitride, double TM, and more MXenes. • MXene DFT exploration for HER is assisted with machine learning. [ABSTRACT FROM AUTHOR]

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]

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]

Confinement plays an important role in determining ion transport in porous materials, which, in turn, may influence the performance of many energy storage and desalination devices. In this work, we combined density functional theory (DFT) with an implicit solvation model and ab initio molecular dynamics (AIMD) to investigate the effects of nanoconfinement on several solvated alkaline metal cations in a single-digit 1T-MoS2 nanochannel. Our DFT calculations with a solvation model indicated that cations with stronger hydration energy introduce a higher number of co-intercalated water molecules into the channel, consistent with early experimental observation obtained for MXene (2D transition metal carbide) channels. The predicted optimal water numbers for the cations were then used for AIMD simulations that explicitly include the effects of the solvent. When compared with the cations in bulk solution, our simulations showed that the hydration structure and coordination number (CN) of the solvated cations confined in the MoS2 channel can be significantly altered. We found that larger cations with weaker hydration energy (K+, Rb+, and Cs+) exhibited a distinctive CN decrease under confinement, while smaller cations (Li+ and Na+) retained a similar hydration shell as in the bulk solution. More specifically, the hydration shell of large cations (K+, Rb+, and Cs+) in MoS2 showed similar features of the coordination angle to the bulk, which suggests the partially broken hydration shell with no geometry change under confinement. Our simulations provided insights into the change of the hydration structure of alkaline metal cations under confinement, which may have important implications on their transport in the 1T-MoS2 channel. [ABSTRACT FROM AUTHOR]

2D MXenes (transition metal carbides, Ti 3 C 2 T x) with large surface area and metallic conductivity are widely used as supporters in catalysts. Unfortunately, the single MXene sheets would deteriorate in the presence of oxygen and water, which is almost the same as the environment of oxygen reduction reaction (ORR). Herein, an "armor" was constructed to encapsulate the oxidizable MXene by a confined in-situ oxidative polymerization strategy, employing the electrostatic interaction between the exfoliated MXene sheets (negatively charged) and oxidant (Fe3+, positively charged). The ultra-dispersed binary Fe 3 N and Co sits coupled with the large surface area MXene supporter synergy to endow more exposed active sites, meanwhile, the density functional theory (DFT) studies reveal that a moderate binding strength between the intermediates and the binary active sites was effectively modulated to enhance the ORR performance. The synthesized Fe 3 N/Co–N–C@MXene exhibits an impressive half-wave potential (E 1/2 , 0.871V) and a lower Tafel slope of 52.56 mV dec−1 than that of Pt–C-20% (74.89 mV dec−1). The Fe 3 N/Co–N–C@MXene based ZAB exhibits a four times higher peak power density (189.16 mW cm−2) than the Pt–C-20% based ZABs and superior cycle stability within 320 h. Furthermore, the assembled flexible ZABs shows high flexibility and stability in different bending scenarios. [Display omitted] • Ultra dispersion binary Fe 3 N/Co–N–C encapsulated MXene composites for ORR. • Large surface area and uniformly distributed active sites synergize for a high-performance ORR catalyst. • About four times higher peak power density (189.16 mW cm−2) than that of Pt–C-20%. • High-stability zinc-air battery with a voltage gap merely increased by 36 mV in 320 h. • High flexible zinc-air battery for different bending scenarios. [ABSTRACT FROM AUTHOR]

2D transition metal carbides (MXenes) obtained from bulk Mn+1AXn (n = 1, 2, 3, or 4) phases are an intriguing class of crystalline solids with unique physicochemical properties for promising applications such as batteries, capacitive energy storage, and electrocatalysis. One of the obstacles that must be overcome for technical applications is that MXene flakes delaminated in aqueous conditions suffer from phase transition and/or structural decomposition over time. Herein, a simple but powerful strategy to enhance their stability by passivating vulnerable edges on the delaminated MXene (Ti3C2Tx) with heterocyclic aromatic amines is reported. In particular, pyrrole‐functionalized MXenes are found to facilitate anti‐oxidation in aqueous solutions at room temperature over 700 days, at 70 °C over 42 days, and even with a strong oxidizer (H2O2, 9.70 mmol) over 50 days. On the other hand, the as‐prepared MXene solution lost its color within a month at room temperature, a day at 70 °C, and 5 min in the presence of H2O2 (9.70 mmol). Density functional theory calculations indicate that chemical interactions between MXene and pyrrole are extremely strong and involve the formation of TiC bonds. Furthermore, pyrrole‐functionalized MXenes exhibit higher electrochemical performance than pristine MXenes as a supercapacitor. [ABSTRACT FROM AUTHOR]

A galaxy of new two‐dimensional (2D) transition metal carbides, nitrides, and carbonitrides (MXenes) have outperformed other 2D nanomaterials in numerous promising applications due to their extraordinary properties. However, the synthesis of MAX phase is pertinent as it is the only precursor for the development of 2D MXenes. As many conventional MAX phase synthesis procedures are complex, time‐ and power consuming, we have introduced a novel electric‐field‐assisted flash sintering technique for a rapid synthesis of Ti3SiC2 MAX phase in both air and vacuum, facilitated by Joule heating effect. The flashing event was observed with an uncontrollable rise in current flow (∼255 mA/mm2) on the application of voltage in a range of 35–42 V/cm. Experimental outcome predicted that Ti3SiC2 MAX phase was synthesized by applying an electric field. Here we support the experimental findings through computational approach by using density functional theory calculations. We also report the synthesis of 2D Ti3C2TX MXene from flash‐synthesized Ti3SiC2 MAX phase. The scientific innovation discussed here could be a breakthrough in the rapid synthesis of different MAX phases with great potential for implementation in industrial scale. [ABSTRACT FROM AUTHOR]

Water is an indispensable material for human life. Unfortunately, the development of industrial activities has reduced the quality of water resources in the world. Meantime, heavy metals are an important factor in water pollution due to their toxicity. This study highlights the method for the capture of heavy metal ions from wastewater using the procedure of adsorption. The adsorption of toxic heavy metal ions (Pb2+, Hg2+, and Cd2+) on Ca 2 C as well as Cr 2 C carbide-nitride MXene monolayers is investigated using the density functional theory. We have carried out the optimization of the considered MXenes by nine DFT functionals: PBE, TPSS, BP86, B3LYP, TPSSh, PBE0, CAM-B3LYP, M11, and LC-WPBE. Our results have shown a good agreement with previously measured electronic properties of the Ca 2 C and Cr 2 C MXene layers and the PBE DFT method. The calculated cohesive energy for the Ca 2 C and Cr 2 C MXene monolayers are −4.12 eV and −4.20 eV, respectively, which are in agreement with the previous studies. The results reveal that the adsorbed heavy metal ions have a substantial effect on the electronic properties of the considered MXene monolayers. Besides, our calculations show that the metal/MXene structures with higher electron transport rates display higher binding energy as well as charge transfers between the metal and Ca 2 C and Cr 2 C layers. Time-dependent density functional analysis also displayed "ligand to metal charge transfer" excitations for the metal/MXene systems. The larger Ebin for the Pb@Ca 2 C as well as Pb@Cr 2 C are according to larger redshifts which are expected (Δ λ = 45 nm and 71 nm, respectively). Our results might be helpful for future research toward the application of carbide-nitride MXene materials for removing wastewater pollutants. [Display omitted] • The DFT study on the adsorption of pollutant Cd2+, Hg2+, and Pb2+ on MXene layers. • Analysis of possible charge transfers between the cations and the MXenes. • TDDFT analysis showed "ligand to metal charge transfer" excitations for the complexes. [ABSTRACT FROM AUTHOR]

[Display omitted] • A theoretical framework for the evaluation of flexible anode materials is established. • The mechanical property of Janus MXene is comparable with that of pristine MXene. • A constant diffusion barrier on Ti 2 COF under strain is suitable for a flexible anode. Two-dimensional transition metal carbides (MXenes) have received tremendous attention because of their great promise in flexible energy storage. Although surface engineering plays an important role in regulating the properties of MXenes with the development of synthesis technology, an atomistic design of surface diversity for desired functionalities is generally limited by purely unified surface termination. Herein, the Janus MXenes consisting of different surface terminations are proposed for flexible anodes through first-principles calculations. Taking Ti 2 C TT' (T , T' = O, F, or OH) as an illustration of Janus MXenes, critical strains, ideal strengths, Li-ion diffusion barriers, equilibrium voltages, and theoretical capacities are determined and used as descriptors to evaluate the properties of flexible anodes. These Janus MXenes exhibit high critical strains and ideal strengths, indicating their mechanical flexibility. The strain-independent Li-ion diffusion barrier of Ti 2 COF suggests its fast Li-ion transport even suffering from mechanical deformation, endowing its application potential in flexible anodes. These results establish a theoretical framework for the comprehensive evaluation of flexible anode materials, providing a theoretical scheme for designing flexible anode materials. [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]

We use first‐principles‐based density functional theory (DFT) calculations to investigate the structural, elastic, and electronic properties of various pristine and oxygen (O)‐functionalized double transition metal (DTM) MXenes with general formulas of M2′M′′C2 and M2′M′′C2O2, where M′ = Mo, Cr and M′′ = Ti, V, Nb, Ta. The dynamic stability of the DTM MXenes are assessed and elastic stiffness constants (Cij) are used to investigate the mechanical stability and properties of the compositions. The calculated elastic properties of the pristine Mo‐based MXenes are found to be superior compared to Cr‐based compounds. Furthermore, the O‐functionalized MXenes exhibit improved in‐plane elastic constants, Young's moduli, and shear moduli compared to their pristine counterpart. We observe that the hybridization of the energy states results in stronger covalent interactions as such increased elastic properties for the M2′M′′C2O2 MXenes. Ashby plot clearly demonstrates superior materials properties of O‐functionalized Mo‐based DTM MXenes compared to other commonly known two‐dimensional materials. All the MXenes exhibit metallic character evident from the density of states (DOS) calculations. Additionally, the work functions are studied and the calculated values are higher in the case of O‐functionalized MXenes. Overall, this work will be a guide for future investigations on the mechanical properties of DTM MXenes for their targeted applications in structural nanocomposites. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) transition metal carbides/nitrides (MXenes) materials exhibit potential as catalysts for the hydrogen evolution reaction (HER) due to their large surface area, excellent conductivity, and notable electrical properties. Despite these advantages, pristine MXenes often display insufficient catalytic activity compared to noble metal electrocatalysts. Enhancing the HER performance of MXenes by introducing nonmetal atoms on the surface has proven effective. However, understanding the precise regulation of the HER performance by nonmetal atoms remains challenging. In this study, we utilized density functional theory calculations to explore how the introduction of fourteen distinct nonmetal atoms into Mo 2 CO 2 catalysts (NM-Mo 2 CO 2) influences the catalytic behavior in HER. The results indicated that nonmetal atom doping significantly activates oxygen atoms, substantially enhancing catalytic activity. Notably, the addition of nonmetal atoms like chlorine (Cl), bromine (Br), and iodine (I) onto the Mo 2 CO 2 surface effectively improved HER activity, showcasing their potential as HER catalysts. Our study proved that nonmetal atom doping not only causes localized structural distortions but also charge redistribution, influencing the HER performance of NM-Mo 2 CO 2. Furthermore, we developed two descriptors to quantify the regulation mechanism of nonmetal atoms, successfully predicting the HER activity of NM-Mo 2 CO 2 and extending these insights to NM-Cr 2 CO 2 and NM-V 2 CO 2. These findings offer a comprehensive understanding of how nonmetal atom doping enhances HER activity, providing valuable insights for innovative catalyst design. [Display omitted] • NM doping activates oxygen atoms, boosting catalytic activity in Mo 2 CO 2 for HER. • Chlorine, bromine, and iodine enhance Mo 2 CO 2 , showcasing potential as HER catalysts. • Study unveils localized structural distortions and charge redistribution in NM-Mo 2 CO 2. • Two descriptors predict and extend insights to NM-Cr 2 CO 2 and NM-V 2 CO 2. [ABSTRACT FROM AUTHOR]

In an era in which sustainable energy sources and electric vehicles are prevalent, the development of efficient anode materials is paramount to address the pressing demand for improved energy storage technologies. After the graphene revolution, many other 2D materials such as graphene, MXene (atomically thin layer of Transition Metal Carbide/Nitride/Carbonitride), TMDC (Transition Metal Dichalcogenide), and MBene (atomically thin layer of Transition Metal Borides) have emerged as potential high-performance anode materials. A recent study demonstrated the possibility of synthesizing chromium-based MBenes for the first time, and the electrochemical performance of Cr 2 B 2 as a potential anode material has also been studied theoretically. Inspired by this, the current study utilized density functional theory and ab initio molecular dynamics simulations to probe the role of Cl and F-functionalization of Cr 2 B 2 MBenes as anode materials for rechargeable lithium-ion batteries (LIB) and sodium-ion batteries (NIB). The electronic structure, adsorption, diffusion, and storage capacity of Li (Lithium) and Na (Sodium) were systematically investigated. This study revealed that Cr 2 B 2 F 2 and Cr 2 B 2 Cl 2 exhibit excellent conductivity due to their intrinsic metallic properties. In addition, they have low diffusion energy barriers for Li and Na ions (0.13/0.14 and 0.20/0.20 eV for Cr 2 B 2 F 2 and Cr 2 B 2 Cl 2 , respectively) facilitating rapid charge/discharge rates. The calculations further unveiled a notable Li-ion storage capacity of 818 mAh g-1 for 2D Cr 2 B 2 Cl 2 , accompanied by a moderate open-circuit voltage (OCV) range of 0.38 V to 0.98 V, illustrating its potential as a superior anodic material for LIBs. Similarly, Cr 2 B 2 F 2 demonstrated a substantial Na-ion storage capacity of 655 mAh g-1, coupled with a reasonably ranged average OCV of 0.20 V to 0.71 V, positioning it as a compelling candidate for NIBs. In all, this investigation has identified two superior electrode materials for alkali-ion batteries, which can spark additional interest in exploring MBene as a promising candidate for advancing next-generation anode materials. [Display omitted] • Cr 2 B 2 Cl 2 and Cr 2 B 2 F 2 are both mechanically, thermally, and dynamically stable auxetic materials. • Surface functionalization of Cr-MBene by Cl/F significantly lowers the diffusion barrier of Li/Na-ions. • Cr 2 B 2 Cl 2 has a high charge storage capacity of 818 mAh g−1 for Li-ion battery. • Cr 2 B 2 F 2 is an outstanding candidate for Na-ion battery with an ultra-high storage capacity of 655 mAh g−1. [ABSTRACT FROM AUTHOR]

[Display omitted] • MXenes synthesized using molten salts (fluorinated and non-fluorinated). • Mechanical, optical, and electronic properties of MXenes are discussed. • Bandgap of MXenes is tuned for improved photoelectrochemical properties. • MXene can be applied as photoelectrochemical sensors (chemical, biological). MXene is a newly discovered family of 2D transition metal carbides, nitrides, or carbonitrides that is predominantly conductive. The most common MXene, titanium carbide (Ti 3 C 2), can be used as a conductive material to enhance the photocurrent generated by photoelectrochemical (PEC) sensors. In order to produce MXene sheets that are of high quality for PEC sensors and other light-based applications, it is imperative that the synthesis methods for MXene production continue to be improved and that an extensive understanding of its properties is gained. Several fluorinated and non-fluorinated MXene synthesis methods from MAX are discussed in this review. Detailed description of the mechanical, optical, topological, electronic, and electrochemical properties of the resultant MXenes are also presented. A critical analysis of the density functional theory (DFT) data for pure MXene, as well as the changes in the optical bandgap of MXene when doped with other heteroatoms to form heterojunctions or ternary nanocomposites, are extensively described. Additionally, a systematic exposition is done on the current MXene-based PEC sensors, highlighting the critical parameters that yield a high performing sensor. The review concludes with an opinion on the future direction of MXenes in PEC sensing and the electrochemical field. [ABSTRACT FROM AUTHOR]

Hydrogen production from water splitting is one of the most promising approaches to achieve carbon neutrality when high‐performance electrocatalysts are ready for the sluggish hydrogen evolution reaction (HER). Although earth‐rich and cheap transition metal carbides (TMCs) are potential HER electrocatalysts, their platinum‐like electronic structures are severely hampered by their strong binding with hydrogen intermediates (H*). Here, a universal "balance effect" strategy is proposed, where nitrogen‐doped graphene (NG) is introduced to weaken the interactions of TMCs (M = Mo, W, Ti, and V) with H*. Hydrogen binding energies calculated by the density functional theory show that the TMCs coupled with NG appear to be thermo‐neutral. Stemming from different work functions of TMCs and NG, partial electrons transfer from TMC to the NG surface, resulting in optimized electronic structures of these electrocatalysts. These optimized electronic structures balance hydrogen adsorption and desorption, leading to synergistically‐enhanced HER kinetics. The overpotentials and Tafel slopes of the HER on these TMC@NG electrocatalysts are thus pronouncedly reduced in both acidic and alkaline solutions. This universal strategy provides a novel approach to design effective and stable TMCs as superior HER electrocatalysts. It can be expanded to other electrocatalysts for sustainable hydrogen production in different media. [ABSTRACT FROM AUTHOR]

[Display omitted] Machine learning (ML) models utilizing structure-based features provide an efficient means for accurate property predictions across diverse chemical spaces. However, obtaining equilibrium crystal structures typically requires expensive density functional theory (DFT) calculations, which limits ML-based exploration to either known crystals or a small number of hypothetical crystals. Here, we demonstrate that the application of Bayesian optimization with symmetry constraints using a graph deep learning energy model can be used to perform "DFT-free" relaxations of crystal structures. Using this approach to significantly improve the accuracy of ML-predicted formation energies and elastic moduli of hypothetical crystals, two novel ultra-incompressible hard MoWC 2 (P 6 3 / mmc) and ReWB (Pca 2 1 ) were identified and successfully synthesized via in situ reactive spark plasma sintering from screening 399,960 transition metal borides and carbides. This work addresses a critical bottleneck to accurate property predictions for hypothetical materials, paving the way to ML-accelerated discovery of new materials with exceptional properties. [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]

The electrocatalytic performance of nitrogen reduction reaction (NRR) is seriously hindered by the lack of cost-effective electrocatalysts with high-efficiency and high-selectivity. In this work, the NRR catalytic activity of single carbon (C) atom embedded into two-dimensional (2D) transition metal carbides (M 2 CO 2 , M = Ti, Zr, Hf, Nb, Ta, Mo, and W) with oxygen vacancy was systematically evaluated by means of comprehensive density functional theory (DFT) computations. Our results revealed that the embedded single C atom possesses good durability due to its strong interaction with metal atoms around vacancy in these MXenes. Interestingly, through high-throughput screening, the single C atoms anchored on Nb 2 CO 2 , Mo 2 CO 2 , and W 2 CO 2 nanosheets are identified as promise NRR catalysts with high-activity due to their low limiting potentials (–0.14 to –0.38 V) via a distal mechanism and outstanding selectivity again the competing hydrogen evolution reaction. Remarkably, the intrinsic activity of the C single atom supported by these MXenes mainly originates from the activation degree of the adsorbed N 2 molecule, which is greatly dependent on the electron filling degree of p z orbital in C atom. Thus, by carefully choosing suitable substrates, the single C catalyst can be utilized as ideal NRR catalysts for NH 3 synthesis. [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]

Transition metal carbides (TMCs) feature high catalytic activity and superior stability for the hydrogen evolution reaction (HER). However, their platinum‐like HER catalytic performance is heavily hindered, due to their strong interaction with hydrogen. Herein, Ni activation of TMCs (M = V, Fe, Cr, and Mo) is proposed through introducing adsorbed nickel atoms on the TMC surface (Ni/TMC). In both acidic and alkaline solutions, a sharp decrease of both overpotentials and Tafel slopes of the Ni/TMC catalysts for HER is achieved. At 10 mA cm−2, the overpotentials of the Ni/vanadium carbide (VC) and Ni/Fe3C catalysts are 128 and 93 mV in 1 m KOH, 111 and 112 mV in 0.5 m H2SO4, respectively. Even at 150 mA cm−2, they exhibit the overpotentials of as low as 270 and 291 mV, respectively. In the alkaline solutions, the performance of these Ni/TMC catalysts is even superior to a Pt/C catalyst. As confirmed from density functional theory calculations and X‐ray absorption fine structure analysis, such adsorbed Ni atoms effectively optimize the d‐electron structure and improve HER performance. As a versatile strategy, this work provides a universal route to activate TMCs for highly efficient HER in different media. [ABSTRACT FROM AUTHOR]

The seeking of the intrinsically ferromagnetic materials has been extended to the two-dimensional transition metal carbide MXene in recent years. However, as the first synthesized member in this family, the experimental study of the magnetism on Ti 3 C 2 T x is rare. Herein, monolayer MXene Ti 3 C 2 T x was prepared by LiF/HCl etching method and annealed in H 2. The magnetic properties of the as-prepared and annealed Ti 3 C 2 T x were investigated. The results show that the as-prepared Ti 3 C 2 T x exhibits the very weak paramagnetism. After H 2 annealing, the significant enhancement in the saturation magnetization is observed, and the ferromagnetism appears after annealing at 500 °C. Density functional theory simulations show that Ti–C vacancy pairs created during the exfoliation and H 2 annealing are the major magnetic sources. Image 1 [ABSTRACT FROM AUTHOR]

Electrocatalytic Ammonia Synthesis Process on the Mo 2 TiC 2 MXene. • Nineteen different possible pathways (five association pathway and fourteen dissociation pathway) analyzed by DFT calculation and Gibbs free energy calculation. • Valid N 2 -philicity, N≡N triple bond of the N 2 molecule (the optimal distance) is sufficiently activated, from 1.11 Å to 1.268Å • Mo 2 TiC 2 MXene can reduce the overpotential by changing the reaction pathway. • Mo 2 TiC 2 as an ordered, double transition metals carbides is an eligible electrocatalyst for the NRR Electrocatalytic ammonia synthesis provides an energy-efficient alternative to the Haber−Bosch process. The aim is to find promising electrocatalysts which are able to change the reaction pathway and reduce the overpotential. Here, based on density functional theory, a comprehensive mechanism study of the N 2 activation and NH 3 synthesis on the Mo 2 TiC 2 MXenes is presented. For catalytic reaction mechanism, nineteen different possible pathways are screened for the lowest overpotential, where the corresponding potential-determining step are compared by Gibbs free energy calculation. The result reveals Mo 2 TiC 2 MXenes exhibit both valid N 2 -philicity and high catalytic activity for electrocatalytic ammonia synthesis through a dissociation mechanism with a low overpotential of 0.26 V. Further, the competing reaction of H 2 evolution is simultaneously suppressed which shows a relatively high potentials of 0.74 V. This study shows a brand new material for catalyzing NH 3 synthesis under ambient conditions and provides the theory background to reduce the overpotential by changing the reaction pathway. [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]

Metal-organic frameworks (MOFs)-derived metal oxides usually have porous nanostructures and have broad applications in gas-sensing. MXene, as transition metal carbide, has received extensive attention due to its unique properties. In this article, the MOFs-derived In 2 O 3 /ZnO hollow nanotubes were successfully combined with Ti 3 C 2 T X MXene to realize the detection of ethanol under room temperature (RT). The morphology and composition of MOFs-derived In 2 O 3 /ZnO/Ti 3 C 2 T X MXene were determined by characterization strategies. The sensing performance measurement results show that the response of the sensor to 10 ppm ethanol gas at 25 ºC and 30 %RH is 2.15, and the response-recovery time is 85/112 s. In addition, the gas selectivity of In 2 O 3 /ZnO/Ti 3 C 2 T X MXene was analyzed by density functional theory (DFT) calculations. Therefore, this study is expected to present a new strategy for sensors to detect ethanol gas at RT. [Display omitted] • In 2 O 3 /ZnO/Ti 3 C 2 T X MXene ternary nanocomposites present enhanced ethanol-sensing performance at room temperature. • Enhanced sensing response is attributed to synergistic effect of metal oxide heterojunction/2D hybrid materials. • The sensing mechanism is discussed in detail in conjunction with density functional theory calculations. [ABSTRACT FROM AUTHOR]

The exfoliation behavior and superior photothermal conversion performance of six different MXenes have systematically revealed by combining experimental and theoretical studies, which will shed lights onto the effective design of diverse MXene materials and promote their applications from wearable devices, biomedical technology to solar water desalination. [Display omitted] • The different mono-M and double-M MXenes have been successfully synthesized. • The exfoliation behavior of MAX phases have been understood by theoretical study. • All the MXenes display fast-response and superior photothermal conversion ability. • The V 2 CT x MXene exhibits best saturation temperature, which outperforms Ti 3 C 2 T x. • The results promote the rapid development of MXenes in photothermal applications. Two dimensional transition metal carbides (MXenes) have attracted significant interest due to their unique layer structure, chemical diversity and superior physiochemical performances. However, most studies have focused on Ti 3 C 2 T x , and the synthesis process and photothermal conversion capability of other MXenes remain unclear. In this work, we have systematically investigated the exfoliation behavior of six different MXenes, including mono-transition-metal (Ti 3 C 2 T x , V 2 CT x , and Nb 2 CT x) and double-transition-metal (TiVCT x , TiNbCT x , and NbVCT x) MXenes derived from MAX phases, integrated with their photothermal conversion performance by combining experimental and theoretical studies. The phase, morphology, composition and surface groups of the different MXenes have been analyzed using XRD, SEM, XPS, and HRTEM techniques. Our results indicate a systematic variation in etching difficulty from difficult to easy, as Nb 2 AlC > NbVAlC > V 2 AlC > TiNbAlC > TiVAlC > Ti 3 AlC 2 , which correlates with the chemical composition and etching duration. Furthermore, the exfoliation factor m from density functional theory calculations reveals that the bond energies of M−A bonds are much weaker than M−X bonds in MAX phases, which agrees well with the experimental results. Interestingly that, all the obtained MXenes exhibit remarkable photothermal conversion performance with fast response, where the MXenes beyond Ti 3 C 2 T x , such as V 2 CT x , NbVCT x , Nb 2 CT x , TiVCT x , and TiNbCT x , display higher saturation temperature of 95, 88, 81, 78, and 71 °C, respectively, than that of Ti 3 C 2 T x (65 °C). These findings could potentially facilitate the efficient design and development of diverse MXene materials with outstanding photothermal conversion performance for applications in wearable devices, biomedical technology and solar water desalination. [ABSTRACT FROM AUTHOR]

The effects of pressure on structure, the density of states, mechanical properties and elastic anisotropy of Sc 3 TC 4 (T = Ni,Os,Ru) were investigated using the CASTEP modular that is based on density functional theory. The results show that Sc 3 TC 4 (T = Ni,Os,Ru) has relatively high compressibility under pressure, and are mechanically stable. As the pressure increases, the anisotropic elastic constants and elastic moduli of the three materials increase. The stability of the three compounds increases with increase of pressure and satisfies: Sc 3 NiC 4 >Sc 3 OsC 4 >Sc 3 RuC 4. • The effects of pressure on structural, mechanical properties and elastic anisotropy of Sc 3 TC 4 (T = Ni,Os,Ru) are studied. • Sc 3 TC 4 (T = Ni,Os,Ru) are mechanically stable under pressure. • The mechanical properties of Sc 3 TC 4 (T = Ni,Os,Ru) can be improved by increasing the pressure. • This study fills the research gap in the mechanical properties of Sc 3 TC 4 series compounds. [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]

Density functional theory calculations have been performed to study the structures and energetics of coherent and semicoherent TiC/Fe interfaces. A systematic method for determining the interfacial energy of the semicoherent interface with misfit dislocation network has been developed. The obtained interfacial energies are used to evaluate the aspect ratio for the plate-like precipitate and a quantitative agreement with the experimental results is reached. Based on the obtained interfacial energies and atomic structure details, we propose two scenarios for heterogeneous nucleation on an edge dislocation, shedding light on the thermodynamics of precipitate nucleation and growth. The present method can be easily applied to any heterophase interfaces between metals and oxides/carbides/nitrides. [ABSTRACT FROM AUTHOR]

Transition metal carbides (TMC) have intriguing physical and chemical properties, especially when structured down to nanoscale. The temperature and chemical resilience of this family of materials made it ideal candidate for electrochemical applications in harsh environment. Among the TMC family, molybdenum carbide has been demonstrated as efficient catalyst for hydrogen evolution reactions (HER). This paper will use density function theory to investigate of the efficiency of molybdenum and tungsten carbide as catalyst for HER in neutral and alkane environment. [ABSTRACT FROM AUTHOR]

Transition metal carbides (TMC) have intriguing physical and chemical properties, especially when structured down to nanoscale. The temperature and chemical resilience of this family of materials made it ideal candidate for electrochemical applications in harsh environment. Among the TMC family, molybdenum carbide has been demonstrated as efficient catalyst for hydrogen evolution reactions (HER). This paper will use density function theory to investigate of the efficiency of molybdenum and tungsten carbide as catalyst for HER in neutral and alkane environment. [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]

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]

First-principles calculations based on density functional theory was performed to analyse the structural stability of transition metal carbides TMC (TM = Ru, Rh, Pd, Os, Ir, Pt). It is observed that zinc-blende phase is the most stable one for these carbides. Pressure-induced structural phase transition from zinc blende to NiAs phase is predicted at the pressures of 248.5 GPa, 127 GPa and 142 GPa for OsC, IrC and PtC, respectively. The electronic structure reveals that RuC exhibits a semiconducting behaviour with an energy gap of 0.7056 eV. The high bulk modulus values of these carbides indicate that these metal carbides are super hard materials. The high B/G value predicts that the carbides are ductile in their most stable phase. [ABSTRACT FROM AUTHOR]

Abstract: Two‐dimensional (2D) heterostructured materials, combining the collective advantages of individual building blocks and synergistic properties, have spurred great interest as a new paradigm in materials science. The family of 2D transition‐metal carbides and nitrides, MXenes, has emerged as an attractive platform to construct functional materials with enhanced performance for diverse applications. Here, we synthesized 2D MoS2‐on‐MXene heterostructures through in situ sulfidation of Mo2TiC2Tx MXene. The computational results show that MoS2‐on‐MXene heterostructures have metallic properties. Moreover, the presence of MXene leads to enhanced Li and Li2S adsorption during the intercalation and conversion reactions. These characteristics render the as‐prepared MoS2‐on‐MXene heterostructures stable Li‐ion storage performance. This work paves the way to use MXene to construct 2D heterostructures for energy storage applications. [ABSTRACT FROM AUTHOR]

Three MXenes, namely Fe 2 N, Co 2 N, and Ni 2 N in both bare and surface-passivated forms were investigated theoretically using density functional theory. Fe 2 N(OH) 2 , Fe 2 NO 2 , Co 2 NO 2 , Ni 2 NF 2 , Ni 2 N(OH) 2 , and Ni 2 NO 2 are found to be intrinsic half-metals, while other structures have antiferromagnetic ground states. Notably, the half-metallicity of Ni 2 NT 2 (T = F, OH, and O) does not depend on the type of surface terminations. This is interesting because the synthesized MXenes usually have mixed surface terminations of F, OH, and O. The half-metallic band gaps calculated with the HSE06 hybrid density functional are 2.03, 1.81, and 1.01 eV, respectively. This implies that thermally induced spin flip should be suppressed. The ferromagnetic coupling between Ni atoms is explained by a superexchange interaction mechanism mediated by spin polarized N atoms. Based on the Heisenberg model, large intra- and inter-layer exchange parameters were obtained. Monte Carlo simulation indicates that the Curie temperatures of Ni 2 NF 2 , Ni 2 N(OH) 2 , and Ni 2 NO 2 are 1800, 2400, and 3300 K respectively, all much higher than room temperature. Ab initio molecular dynamics simulations also indicate that the ferromagnetic Ni 2 NT 2 compounds are stable at room temperature. Therefore, we predict that the half-metallicity of Ni 2 NT 2 should be relatively easy to be achieved and Ni 2 NT 2 is a good candidate for spintronics. [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]

To effectively utilize glycerol as a fuel for electrochemical fuel cells, it is necessary to optimize catalysts for effective C-C bond cleavage and complete oxidation of reaction intermediates to achieve maximum efficiency. The current work showed that the synergistic interactions of platinum (Pt) with transition metal carbide (TMC) substrates, such as tungsten carbide (WC) and tantalum carbide (TaC), fulfilled these criteria. The TMC-supported Pt catalysts showed higher activity and selectivity for complete glycerol oxidation than commercial 10 wt% Pt/C. In-situ FTIR analysis revealed that 5 wt% Pt/WC was the most effective catalyst among those tested for complete glycerol oxidation at 0.9 V vs RHE. In-situ X-ray absorption fine structure characterization and density functional theory calculations provided additional insight into the synergistic interactions for glycerol oxidation over Pt/TMC catalysts. [Display omitted] • Pt-modified transition metal carbides (TMCs) were compared with Pt/C for complete glycerol electrooxidation. • In-situ FTIR revealed Pt/TMC enhanced CO 2 selectivity and lowered onset potential. • Synergy between Pt and TMCs enhanced glycerol binding energy and reduced CO binding energy. [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]

To improve the performance of the lithium ion batteries (LIBs), one of the hottest topics in this field is searching high-performance electrode candidates. Recently, a new kind of MXene, named Cr 2 N monolayer attracted our interests, which can be successfully exfoliated from the bulk Cr 2 GaN structure by removing the layers of Ga. Since the surface of Cr 2 N monolayer is full of active sites, it is interesting to investigate the possibility of Cr 2 N monolayer and its derivatives as electrode materials for LIBs. Here, we performed first-principles calculations basing on density functional theory (DFT) to study the adsorption, diffusion and capacity of Li atoms on the Cr 2 N monolayer and its derivatives. Our results show that Cr 2 NF 2 and Cr 2 NO 2 monolayers are promising candidates of LIBs. The diffusion energy barriers of Li atom on Cr 2 NF 2 and Cr 2 NO 2 monolayer are only 0.18 eV and 0.58 eV, which ensures the excellent charge-discharge rates. More importantly, the theoretical specific capacity of Cr 2 NF 2 monolayer is calculated to be about 583 mA h•g-1 and that of Cr 2 NO 2 monolayer is about 556 mA h•g-1, which are high enough to support commercial applications. • We firstly demonstrated that the Cr2NF2 and Cr2NO2 monolayer are potential candidates for the electrode material of high-performance LIBs. • The theoretical specific capacities of Cr 2 NF 2 and Cr 2 NO 2 monolayer are calculated to be about 583 mA h·g−1 and 556 mA h·g−1, which are much higher than that of most of 2D transition metal compounds. [ABSTRACT FROM AUTHOR]

MXenes, a class of two-dimensional transition metal carbides and nitrides, have shown promise experimentally and computationally for use in energy storage applications. In particular, the most lightweight members of the monolayer MXene family (M = Sc, Ti, V, or Cr) are predicted to have gravimetric capacities above 400 mAh/g, higher than graphite. Additionally, intercalation of ions into multilayer MXenes can be accomplished at low voltages, and low diffusion barriers exist for Li diffusing across monolayer MXenes. However, large discrepancies have been observed between the calculated and experimental reversible capacities of MXenes. Here, dispersion-corrected density functional theory calculations are employed to predict reversible capacities and other battery-related properties for six of the most promising members of the MXene family (O-functionalized Ti- and V-based carbide MXenes) as bilayer structures. The calculated reversible capacities of the V2CO2 and Ti2CO2 bilayers agree more closely with experiment than do previous calculations for monolayers. Additionally, the minimum energy paths and corresponding energy barriers along the in-plane [1000] and [0100] directions for Li travelling between neighboring MXene layers are determined. V4C3O2 exhibits the lowest diffusion barrier of the compositions considered, at 0.42 eV, but its reversible capacity (148 mAh/g) is dragged down by its heavy formula unit. Conversely, the V2CO2 MXene shows good reversible capacity (276mAh/g), but a high diffusion barrier (0.82 eV). We show that the diffusion barriers of all bilayer structures are significantly higher than those calculated for the corresponding monolayers, advocating the use of dispersed monolayer MXenes instead of multilayers in high performance anodes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ti 3 C 2 /G-MWCNTs/ZnO nanocomposites have been applied to sensor for the first time. • G-MWCNTs and ZnO have enhanced the chemical stability and electrocatalytic properties of Ti 3 C 2. • This sensor has low LOD of 3.3 nM and high sensitivity of 16 A/M. • The sensor showed satisfactory stability and accuracy in human serum samples. • The redox process and the electron transfer mechanism of DA have been investigated by DFT. Dopamine is one of the most important neurotransmitters in the human body, and its sensitive detection and quantification are important for the diagnosis, prevention, and treatment of related neurological diseases. Herein, a highly-sensitive dopamine electrochemical sensor has been constructed based on the two-dimensional transition metal carbide Ti 3 C 2 MXene, graphitized multi-walled carbon nanotubes and ZnO nanospheres. Dopamine presents a standard reversible redox reaction on the sensor, and density flooding theory has been successfully applied to reveal the redox process of dopamine. Under optimal experimental conditions, the sensor exhibits a wide linear range (0.01–30 μM), low limit of detection (3.2 nM) and high sensitivity (16 A/M). Furthermore, the sensor has excellent stability and anti-interference ability, satisfactory accuracy in human serum samples, which can be used for the detection of actual samples. This work has provided a novel method for detecting dopamine and a new idea for the application of MXene and metal oxide semiconductor in electrochemical sensors. [ABSTRACT FROM AUTHOR]

Catalytic reduction of CO2 requires active, selective and low-cost catalysts. Results of this study show that transition metal carbides are a class of promising catalysts and their activity is correlated with oxygen binding energy and reducibility as shown by DFT calculations and in situ measurements. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal carbides (termed MXenes) are a new family of compounds generating considerable interest due to their unique properties and potential applications. Intercalation of ions into MXenes has recently been demonstrated with good electro-chemical performance, making them viable electrode materials for rechargeable batteries. Here we have performed global screening of the capacity and voltage for a variety of intercalation ions (Li+, Na+, K+, and Mg2+) into a large number of M2C-based compounds (M = Se, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta) with F-, H-, 0-, and OH-functionalized surfaces using density functional theory methods. In terms of gravimetric capacity a greater amount of Li+ or Mg2+ can be intercalated into an MXene than Na+ or K+, which is related to the size of the intercalating ion. Variation of the surface functional group and transition metal species can significantly affect the voltage and capacity of an MXene, with oxygen termination leading to the highest capacity. The most promising group of M2C materials in terms of anode voltage and gravimetric capacity (>400 mAh/g) are compounds containing light transition metals (e.g., Se, Ti, V, and Cr) with nonfunctionalized or O-terminated surfaces. The results presented here provide valuable insights into exploring a rich variety of high-capacity MXenes for potential battery applications. [ABSTRACT FROM AUTHOR]