VB Transition Metal Research Articles

The effects of refractory elements on the properties of quaternary high entropy carbides—A first-principles and experiment study

In this work, the effects of refractory transition metal elements on the properties of quaternary equimolar ratio high entropy carbides composed of ⅣB, ⅤB and ⅥB transition metals and C element were studied by first-principles calculations and experiments.The calculation results show that all 75 kinds of high entropy carbides were thermodynamically and mechanically stable and can be formed spontaneously. Among them, Hf can promote the synthesis of high entropy carbides, while Cr has the opposite effect. In addition, single phase solid solution structures can be formed for all 73 HECs except (TiZrHfCr)C and (ZrHfVCr)C. Thereafter, the mechanical properties of these 75 quaternary HECs were compared. The calculation results show that W, Mo, and Ta have a positive effect on improving the elastic modulus, fracture toughness, hardness and melting point of high entropy carbides, while Cr has the opposite effect. The calculation results of phonon spectra show that the six high entropy carbides represented by (TiHfNbTa)C, (TiVNbTa)C, (TiZrNbTa)C, (TiZrNbCr)C, (TiZrNbMo)C and (TiZrNbW)C have lattice dynamic stability. (TiHfNbTa)C, (TiVNbTa)C, (TiZrNbTa)C, (TiZrNbMo)C and (TiZrNbW)C face-centered cubic high entropy carbides were prepared by spark plasma sintering and the microstructure and properties were tested. The experimental results are basically consistent with the calculated results.

Formation mechanism of two-dimensional hexagonal silica on SiO2/Si substrate

Owing to their remarkable electronic properties, silica ultrathin films have been utilized as an insulating layer in nanoelectronics systems. Silica films have been epitaxially grown on different substrates using various synthesis methods. Among all fabrication approaches, chemical vapor deposition has long been an advanced method for synthesizing two-dimensional (2D) materials due to its ability to ensure precise stacking control and minimize contamination between layers. This study harnessed the potential of CVD to atomically fabricate thin layered 2D silica on a SiO2/Si substrate. Significantly, a unique combination of multiple transition metals and salt as the catalysts aided the formation of 2D silica for the first time. Salt is a crucial catalyst in promoting the evaporation of high-melting-point metal catalysts, resulting in hexagonal nucleation sites on the SiO2/Si wafer. By meticulously controlling growth parameters, a distinctive hexagonal structure was obtained. Correspondingly, this work delves into the growth mechanism of 2D silica, as evidenced by experiments involving salt alone and individual transition metals. Group VB transition metals played a prominent role in achieving the hexagonal structure compared to their group IVB counterparts. This research offers insight into the formation and growth mechanism of 2D silica, expanding the understanding of silica nanostructures.

High density of stacking faults strengthened TaN/TiN multilayer

Multilayer coatings with individual layer thickness on the nanoscale exhibit superior mechanical properties because of their high interface density. However, compared to artificial phase interfaces, planar defects like twin and stacking faults (SFs) are hardly investigated in ceramic multilayer systems. We choose to combine the group VB transition metal nitride (TMN) TaN with its extremely low stacking faults energy (SFE) and TiN to form a superlattice (SL). TiN/TaN multilayers with a bilayer period (Λ) from 8 nm to 100 nm were prepared by dc magnetron sputtering. These as-deposited films show a peak hardness 36 ± 2.4 GPa at Λ=20 nm. The extensive high-resolution transmission electron microscopy (HRTEM) observations reveal that the dissociation of full dislocations results in the network of SFs and the formation of Lomer-Cottrell lock arrays inside the TaN layer. Meanwhile, further dislocation analysis indicated the Shockley partials cross slip at the interface. These findings provide us with a new perspective for designing TMN multilayers with planar defects.

Recent Progress in Phase Stability and Elastic Anomalies of Group VB Transition Metals

Recently discovered phase transition and elastic anomaly of compression-induced softening and heating-induced hardening (CISHIH) in group VB transition metals at high-pressure and high-temperature (HPHT) conditions are unique and interesting among typical metals. This article reviews recent progress in the understanding of the structural and elastic properties of these important metals under HPHT conditions. Previous investigations unveiled the close connection of the remarkable structural stability and elastic anomalies to the Fermi surface nesting (FSN), Jahn–Teller effect, and electronic topological transition (ETT) in vanadium, niobium, and tantalum. We elaborate that two competing scenarios are emerging from these advancements. The first one focuses on phase transition and phase diagram, in which a soft-mode driven structural transformation of BCC→RH1→RH2→BCC under compression and an RH→BCC reverse transition under heating in vanadium were established by experiments and theories. Similar phase transitions in niobium and tantalum were also proposed. The concomitant elastic anomalies were considered to be due to the phase transition. However, we also showed that there exist some experimental and theoretical facts that are incompatible with this scenario. A second scenario is required to accomplish a physically consistent interpretation. In this alternative scenario, the electronic structure and associated elastic anomaly are fundamental, whereas phase transition is just an outcome of the mechanical instability. We note that this second scenario is promising to reconcile all known discrepancies but caution that the phase transition in group VB metals is elusive and is still an open question. A general consensus on the relationship between the possible phase transitions and the mechanical elasticity (especially the resultant CISHIH dual anomaly, which has a much wider impact), is still unreached.

Considerations in solute substitution for nanocrystalline thermomechanical behavior

Stabilizing a nanocrystalline material against grain growth makes it possible to effectively use the Hall-Petch strength of the nanostructure. This stabilization is often achieved by alloying, which has motivated previous efforts to guide alloy selection. In this work, we substitute niobium for tantalum, both group VB transition metals, in a copper-based nanocrystalline alloy, with the niobium providing an equivalent thermal stabilizing effect as reported for copper-tantalum alloys. However, the thermomechanical strength of the copper-niobium alloy is found to be diminished in comparison to copper-tantalum. This elevated-temperature-strength difference is a result of the solute-contaminant reactant phases, which arise from alternate equilibrium oxides formed between the two solute types. In particular, the misfit strain with temperature is found to be a critical variable for the copper-niobium alloy's thermomechanical strength. These findings highlight that solute-impurity reactions are crucial to nanocrystalline alloy design, particularly for elevated temperature structural applications.

The role of entropy and enthalpy in high entropy carbides

The thermodynamic stability of equiatomic mixed carbides, commonly referred to as high entropy carbides (HECs), is investigated via the CALculation of PHAse Diagrams (CALPHAD) approach for mixed carbides consisting of the group IVB and VB transition metal carbides as well as tungsten carbide. The Gibbs free energy of the B1-structured mixed carbides is computed using the compound energy formalism while that of the Bh-structured mixed carbides is evaluated using a point-defect model. The required thermodynamic data for the CALPHAD approach are obtained from density functional theory calculations and the Debye-Grüneisen model. The lower temperature limit at which the HECs mix in thermodynamic equilibrium is determined via numerical and analytical approaches. We find that enthalpy of mixing is at least as important as configurational mixing entropy in these mixed transition metal carbide compounds. The lower limit temperature where an equiatomic solid solution is present is largely independent of the number of components with the only exception being solutions containing tungsten carbide, where a weak temperature dependence is noted. Furthermore, the only equiatomic solid solutions that are thermodynamically stable below approximately 1000 K are those stabilized by enthalpy alone, indicating that many currently fabricated HECs are not at equilibrium at room temperature. Collectively, our results demonstrate that the formation of these carbides is controlled by the competition between entropy and enthalpy, or enthalpy alone, and thus these materials should be referred to as multi-principal component carbides since the former terminology can be misleading.

Stability and mechanical properties of single-phase quinary high-entropy metal carbides: First-principles theory and thermodynamics

We have employed thermodynamics and first-principles density-functional calculations to investigate the structural stability and mechanical properties of fifty-six quinary high-entropy metal carbides composed of carbon and Groups IVB, VB, and VIB refractory transition metals, Ti, Zr, Hf, V, Nb, Ta, Mo, and W, thirty-eight of which have not yet been synthesized. To determine the stability of the quinary high-entropy metal carbides, we have constructed a three-dimensional phase diagram in terms of the average melting point, mixing enthalpy, mixing entropy, and lattice size difference, from which we predict that it is feasible to synthesize 38 new high-entropy metal carbides. We have further found that all the 56 metal carbides would have unique mechanical properties of high hardness and high fracture toughness. In addition, our study suggests that the brittleness of high-entropy metal carbides steadily decreases with the increase of the valence electron concentration.

Breaking the linear scaling relations in MXene catalysts for efficient CO2 reduction

Electrocatalytic carbon dioxide reduction reaction (CO2RR) toward value-added fuels has attracted increasing attention in carbon-neutral and energy-production fields, but the catalytic efficiency is seriously hindered by the robust linear scaling relations between adsorption energies of intermediates. Herein, we have extensively investigated the effect of a series of group ⅣB, ⅤB, and ⅥB transition metal (TM) atoms substitution for middle Mo in Mo3C2 MXene on the catalytic performance of CO2RR. Our results suggest that the captured CO2 can be selectively reduced to methane (CH4) on both Mo3C2 and Mo2TMC2 bimetal MXenes. We highlight that TM substitution can significantly reduce the limiting potential (UL) of CO2RR from −0.651 V (Mo3C2) to −0.350 V (Mo2TiC2) by decreasing the Gibbs energy difference of rate-determining step (OCH2O* + H++e-= HOCH2O*). The modulation mechanism is illuminated that TM substitution in Mo3C2 MXene gives rise to the upshift of d-band center of Mo atoms, which selectively tunes the adsorption strength of OCH2O* and HOCH2O*, resulting in breaking their linear scaling relations. Further analyses on electron localization function (ELF) visualize the TM substitution induced stronger surface localization lone electrons, which endows the surface Mo with promoted chemical activity. The dynamical stability of Mo2TiC2 has been well verified by phonon dispersion curves and ab initio molecular dynamics (AIMD) simulations, suggesting the robust stability of Mo2TiC2 as an electrocatalyst for CO2RR. Our findings pave the way of MXenes for CO2 capture and pioneer the application of Mo2TiC2 as a novel and efficient catalyst for CO2 to CH4.

Vacancy-cluster and off-lattice metal-atom diffusion mechanisms in transition metal carbides

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.

Water-based lubrication of niobium nitride

Water-based lubrication has attracted wide attention as an oil-free lubrication method owing to its greener and cleaner lubrication means. However, due to operating in the water environment, most moving parts would inevitably suffer from abrasion, rusting, and aging problems. Developing a novel solid-water composite system with ultra-low friction and wear will open new possibilities for innovative lubrication material research and development. Here, we first revealed the water-based lubrication behavior of a high-hardness niobium nitride coating (NbN). In a three-phase contact environment (water, air, and NbN), oxidation and hydrolytic reactions of NbN result in the formation of “colloidal solutions”, containing Nb2O5 colloidal particles between the tribo-pairs. Utilizing the double electric layer repulsion and weak shear action of the “colloidal solution”, NbN achieves ultra-low friction and wear; the corresponding values are as low as 0.058 and 1.79 × 10−10 mm3·N−1·m−1, respectively. In addition, other VB transition metal nitrides (VB TMNs) exhibit the same low friction feature as NbN in the three-phase contact environment; the friction coefficients are even lower than those in an oil-based environment. The water-based lubrication of VB TMNs provides a new reliable scheme for optimizing solid-water composite lubrication systems without additives and is expected to be applied in environments with high humidity or insufficient water coverage.

Recent Advances in 2D Group VB Transition Metal Chalcogenides.

2D materials have received considerable research interest owing to their abundant material systems and remarkable properties. Among them, 2Dgroup VB transition metal chalcogenides (GVTMCs) stand out as emerging 2D metallic materials and significantly broaden the research scope of 2D materials. 2D GVTMCs have great advantages in electrical transport, 2D magnetism, charge density wave, sensing, catalysis, and charge storage, making them attractive in the fields of functional devices and energy chemistry. In this review, the recent progress of 2D GVTMCs is summarized systematically from fundamental properties, growth methodologies to potential applications. The challenges and prospects are also discussed for future research in this field.

Femtosecond ultrafast pulse generation with high-quality 2H-TaS2 nanosheets via top-down empirical approach.

Tantalum disulfide (TaS2), an emerging group VB transition metal dichalcogenide, with unique layered structure, rich phase diagrams, metallic behavior, higher carrier concentration and mobility is emerging as a prototype for revealing basic physical phenomena and developing practical applications. However, its photonics properties and even engineering-related processes are still rare. Here, the top-down experiment demonstration, including synthesis, thickness optimization and nonlinear optical application, has been reported. In addition, the ultrafast (∼373 fs) erbium-doped fiber pulse with a small time-bandwidth product (∼0.34) and long-term stability (∼25 days) was realized using the nonlinear absorption properties of the high-quality 2H-TaS2 nanosheet. These results suggest an experimental route for further ultrafast photonics exploration based on metallic transition metal dichalcogenides.

Probing the origin of group VB transition metal monocarbides for high-efficiency hydrogen evolution reaction: A DFT study

Transition metal carbides have received increasing attention for the application in hydrogen evolution reaction (HER), especially the group VB transition metal monocarbides (GVB-MCs). GVB-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 GVB-MCs and ascertain their catalytic mechanism. The resultant density functional theory (DFT) calculations suggest that the (110) surfaces of GVB-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 (110) surfaces of GVB-MCs. Specifically, the Gibbs free energy of hydrogen adsorption (ΔGH*) of VC (110) 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 (110) surface is only slightly lower than that on the Pt (111) surface. Therefore, this work emphasizes the importance of active sites and provides reliable DFT guidelines for the further design of effective GVB-MCs electrocatalysts.

Statistical study of vacancy diffusion in TiC and TaC

Density functional theory (DFT) simulations, Metropolis Monte Carlo (MMC), and kinetic Monte Carlo (kMC) simulations were performed to understand the mechanisms of mass diffusion in a group IVB transition metal carbide, TiC, and a group VB transition metal carbide, TaC. The DFT calculations were used to obtain the vacancy formation energy and migration energy of a variety of microstates for off-stoichiometric TiC and TaC. MMC simulations, based on our DFT results, were used to determine the ensemble average of the metal vacancy formation energy and determine the average size metal-vacancy clusters present in these materials. kMC simulations were used to determine the ensemble average of the migration energy barrier as well as understand how the vacancies in these materials, on average, migrate. These collective results show that metal vacancy migration in TiC and TaC are quite different, where Ti vacancies should be surrounded by four to five carbon vacancies whereas, on average, tantalum vacancies are surrounded by one or zero carbon vacancies. However, the carbon vacancies substantially contribute to metal vacancy diffusion in both these materials, as the metal vacancy statistically will have a carbon vacancy near it before and after migration. From these results, we find that the activation energy of metal vacancy diffusion is 7.66 eV in ${\mathrm{Ti}}_{0.995}{\mathrm{C}}_{0.97}$ and 6.41 eV in ${\mathrm{Ta}}_{0.995}{\mathrm{C}}_{0.97}$, which agrees reasonably well with experimentally reported activation energies. These results give further insights into the mechanisms associated with mass diffusion within the transition metal carbide families, an important insight needed to better elucidate high temperature diffusional creep responses which is often difficult to assess experimentally.

Novel electronic properties of monoclinic MP4 (M = Cr, Mo, W) compounds with or without topological nodal line

Transition metal phosphides hold novel metallic, semimetallic, and semiconducting behaviors. Here we report by ab initio calculations a systematical study on the structural and electronic properties of hbox {MP}_4 (M = Cr, Mo, W) phosphides in monoclinic C2/c (C_{2h}^6) symmetry. Their dynamical stabilities have been confirmed by phonon modes calculations. Detailed analysis of the electronic band structures and density of states reveal that hbox {CrP}_4 is a semiconductor with an indirect band gap of 0.47 eV in association with the p orbital of P atoms, while hbox {MoP}_4 is a Dirac semimetal with an isolated nodal point at the Gamma point and hbox {WP}_4 is a topological nodal line semimetal with a closed nodal ring inside the first Brillouin zone relative to the d orbital of Mo and W atoms, respectively. Comparison of the phosphides with group VB, VIB and VIIB transition metals shows a trend of change from metallic to semiconducting behavior from hbox {VB-MP}_4 to VIIB-hbox {MP}_4 compounds. These results provide a systematical understandings on the distinct electronic properties of these compounds.

Niobium disulfide as a new saturable absorber for an ultrafast fiber laser.

Group VB transition metal dichalcogenides (TMDCs) are emerging two-dimensional materials and have attracted significant interests in the fields of physics, chemistry, and material sciences. However, there are very few reports about the optical characteristics and ultrafast photonic applications based on group VB TMDCs so far. In this work, we have calculated the niobium disulfide (NbS2) band structure by the density functional theory (DFT), which has revealed that NbS2 is a metallic TMDC. In addition, we have prepared an NbS2-microfiber device and the nonlinear optical characteristics have been investigated. The modulation depth, saturation intensity and non-saturable loss have been measured to be 13.7%, 59.93 MW cm-2 and 17.74%, respectively. Based on the nonlinear optical modulation effect, the Er-doped fiber (EDF) laser works in the soliton mode-locking state with the pump power of 94-413 mW. The pulse duration of 709 fs and the maximum average output power of 23.34 mW have been obtained at the pump power of 413 mW. The slope efficiency is as high as 6.79%. Compared to the recently reported studies based on TMDCs comprehensively, our experimental results are better. These experimental results demonstrate that NbS2 with excellent nonlinear optical properties can be used as a promising candidate to advance the development of ultrafast photonics.

Niobium diselenide nanosheets for a vector soliton fiber laser

Niobium diselenide (NbSe2), an emerging group VB transition metal dichalcogenide, has been widely used in materials science, chemistry, and physics, while the applications of NbSe2 in ultrafast photonics are still rare.

The crystal structure and phase stability of the zeta phase in the group VB transition metal carbides: a computational investigation.

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.

Group VB transition metal dichalcogenides for oxygen reduction reaction and strain-enhanced activity governed by p-orbital electrons of chalcogen

Developing alternative oxygen reduction reaction (ORR) catalysts to replace precious Pt-based metals with abundant materials is the key challenge of commercial application of fuel cells. Owing to their various compositions and tunable electronic properties, transition metal dichalcogenides (TMDs) have the great potential to realize high-efficiency catalysts for ORR. Here, various 3R-phase dichalcogenides of group VB and VIB transition metals (MX2, M = Nb, Ta, Mo, W; X = S, Se, Te) are investigated for ORR catalysts by using density functional theory calculations. The computed over-potentials of group VB TMDs are much less than those of group VIB TMDs. For group VB TMDs, a volcano-type plot of ORR catalytic activity is established on the adsorption energies of *OH, and NbS2 and TaTe2 exhibit best ORR activity with an over-potential of 0.54 V. To achieve even better activity, strain engineering is performed to tune ORR catalytic activity, and the minimum over-potential of 0.43 V can be realized. We further demonstrate that the shift of p orbital center of surface chalcogen elements under strain is responsible for tuning the catalytic activity of TMDs.

Planar transition metal oxides SERS chips: a general strategy

Group IVB, VB and VIB transition metal oxides planar SERS chips can achieve a low limit of detection below 10−9 M.