Electronic structures of B1 MoN, fcc Mo2N, and hexagonal MoN

Statistical Design-Guided Synthesis of Nanoarchitectonics of High-Performance NiFeMoN Electrocatalyst through Facile One-Step Magnetron Sputtering

This study presents an innovative, statistically-guided magnetron sputtering technique for creating nanoarchitectonics of high-performing, NiFeMoN electrocatalysts for oxygen evolution reaction (OER) in water splitting. Using a central composite face-centered (CCF) design, 13 experimental conditions are identified that enable precise optimization of synthesis parameters through response surface methodology (RSM), confirmed by analysis of variance (ANOVA). The statistical analysis highlighted a interaction between Mo% and N% in the nanostructured NiFeMoN and found optimizing values at 31.35% Mo and 47.12% N. The NiFeMoN catalyst demonstrated superior performance with a low overpotential of 216 mV at 10 mA cm-2 and remarkable stability over seven days, attributed to the modifications in electronic structure and the creation of new active sites through Mo and N additions. Furthermore, the NiFeMoN coating, when used as a protective layer for a Si photoanode in 1 m KOH, achieved an applied-bias photon-to-current efficiency (ABPE) of 5.2%, maintaining stability for 76 h. These advancements underscore the profound potential of employing statistical design for optimizing synthesis parameters of intricate catalyst materials via magnetron sputtering, paving the way for accelerated advancements in water splitting technologies and also in other energy conversion systems, such as nitrogen reduction and CO2 conversion.

Effect of V/Mo Atomic Ratio on the Microstructure and Mechanical Properties of MoVCuN Coatings

To improve the gas ionization ratio, the Mo-V-Cu-N coatings were deposited by pulsed dc magnetron sputtering with assistance from an anode layer ion source, and the influence of the V/Mo atomic ratio was explored with regard to the microstructure and mechanical properties of the coatings. The findings of this study indicated that the MoVCuN coatings exhibited a solid solution phase of FCC B1-MoVN with a prominent (220) preferred orientation, and the deposition rate was found to decrease from 4.7 to 1.8 nm/min when the V/Mo atomic ratio increased. The average surface roughness of the MoVCuN coatings gradually decreased, and the lowest surface roughness of 6.9 nm was achieved at a V/Mo atomic ratio of 0.31. Due to the enhanced ion bombardment effect, the coatings changed from a coarse columnar to a dense columnar crystal structure, and promoted grain refinement at higher V/Mo atomic ratios, contributing to a gradual improvement in the compressive residual stress, hardness and adhesion strength of the coatings.

Effect of Charge Voltage on the Microstructural, Mechanical, and Tribological Properties of Mo–Cu–V–N Nanocomposite Coatings

As an important high-power impulse magnetron sputtering (HIPIMS) parameter, charge voltage has a significant influence on the microstructure and properties of hard coatings. In this work, the Mo–Cu–V–N coatings were prepared at various charge voltages using HIPIMS technique to study their mechanical and tribological properties. The microstructure was analyzed by scanning electron microscope (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The mechanical and tribological properties were investigated by nano-indentation and ball-on-disc tribometer. The results revealed that all the coatings showed a solid-solution phase of B1-MoVN, the V atoms dissolved into face-centered cubic (FCC) B1-MoN lattice by partial substitution of Mo, and formed a solid-solution phase. Even at a high Cu content (~8.8 at. %), the Cu atoms existed as an amorphous phase. When the charge voltage increased, more energy was put into discharge, and the microstructure changed from coarse structure into dense columnar structure, resulting in the highest hardness of 28.2 GPa at 700 V. An excellent wear performance with low friction coefficient of 0.32 and wear rate of 6.3 × 10−17 m3/N·m was achieved at 750 V, and the wear mechanism was dominated by mild abrasive and tribo-oxidation wear.

Defect-mediated ab initio thermodynamics of metastable γ-MoN(001)

Refractory transition metal nitrides exhibit a plethora of polymorphic expressions and chemical stoichiometries. To afford a better understanding of how defects may play a role in the structural and thermodynamics of these nitrides, using density-functional theory calculations, we investigate the influence of point and pair defects in bulk metastable γ-MoN and its (001) surface. We report favorable formation of Schottky defect pairs of neighboring Mo and N vacancies in bulk γ-MoN and apply this as a defect-mediated energy correction term to the surface energy of γ-MoN(001) within the ab initio atomistic thermodynamics approach. We also inspect the structural distortions in both bulk and surfaces of γ-MoN by using the partial radial distribution function, g(r), of Mo-N bond lengths. Large atomic displacements are found in both cases, leading to a broad spread of Mo-N bond length values when compared to their idealized bulk values. We propose that these structural and thermodynamic analyses may provide some insight into a better understanding of metastable materials and their surfaces.

Formation of buried superconducting Mo2N by nitrogen-ion-implantation

Nitrogen ion implantation is a useful technique to put nitrogen ions into lattices. In this work, nitrogen ion implantation into epitaxial Mo films is performed to create a buried superconducting γ-Mo₂N. Atomically flat epitaxial (110) Mo films are grown on (0001) Al₂O₃. By impinging nitrogen ions, where the beam energy is fixed to 20 keV, we observe (111) γ-Mo₂N diffraction and the formation of a γ-Mo₂N layer from X-ray reflectivity. Magnetization and transport measurements clearly support a superconducting layer in the implanted film. Our strategy shows that formation of a buried superconducting layer can be achieved through ion implantation and self-annealing.

Formation of chemically distinct interfaces, including crystalline buried-superconducting Mo₂N, by low-energy nitrogen ion implantation in an epitaxial molybdenum thin film.

Electrochemical synthesis of transition metal oxide nitrides with ε-TaN, δ-NbN and γ′-Mo2N structure type in a molten salt system

The three nitrides ε-TaN, δ-NbN and γ′-Mo2N have been synthesized electrochemically from the elements at 450°C in a molten salt mixture LiCl/KCl:Li3N. For all compounds the working electrode consisting of a tantalum, niobium or molybdenum foil was anodically polarized and the system was fed with dry nitrogen. The applied constant voltage was 2.5 V (for ε-TaN), 2.2 V (for δ-NbN), and 2.8 V (for γ′-Mo2N). Chemical analysis on N and O resulted in compositions of TaN0.81(1)O0.13(2), NbN1.17(2)O0.28(1) and MoN0.88(1)O0.11(1), respectively. Lattice parameters of ε-TaN refined by the Rietveld method are a =  519.537(4) and c = 291.021(3) pm. The other two nitrides crystallize in the cubic system (rocksalt type) with a = 436.98(2) pm for δ-NbN and with a = 417.25(2) pm for γ′-Mo2N.

Metallic porous nitride single crystals at two-centimeter scale delivering enhanced pseudocapacitance

Pseudocapacitors that originate from chemisorption contain redox active sites mainly composed of transition metal ions with unsaturated coordination in lattice on the electrode surface. The capacitance is generally dictated by the synergy of the porous microstructure, electronic conduction and active sites in the porous electrode. Here we grow metallic porous nitride single crystals at 2-cm scale to enhance pseudocapacitance through the combination of large surface area with porous microstructure, high conductivity with metallic states and ordered active sites with unsaturated coordination at twisted surfaces. We show the enhanced gravimetric and areal pseudocapacitance and excellent cycling stability both in acidic and alkaline electrolyte with porous MoN, Ta5N6 and TiN single crystals. The long-range ordering of active metal-nitrogen sites account for the fast redox reactions in chemisorption while the high conductivity together with porous microstructure facilitate the charge transfer and species diffusion in electrodes.

Discovery of Superconductivity in Hard Hexagonal ε-NbN

Since the discovery of superconductivity in boron-doped diamond with a critical temperature (T_C) near 4 K, great interest has been attracted in hard superconductors such as transition-metal nitrides and carbides. Here we report the new discovery of superconductivity in polycrystalline hexagonal ε-NbN synthesized at high pressure and high temperature. Direct magnetization and electrical resistivity measurements demonstrate that the superconductivity in bulk polycrystalline hexagonal ε-NbN is below ∼11.6 K, which is significantly higher than that for boron-doped diamond. The nature of superconductivity in hexagonal ε-NbN and the physical mechanism for the relatively lower T_C have been addressed by the weaker bonding in the Nb-N network, the co-planarity of Nb-N layer as well as its relatively weaker electron-phonon coupling, as compared with the cubic δ-NbN counterpart. Moreover, the newly discovered ε-NbN superconductor remains stable at pressures up to ∼20 GPa and is significantly harder than cubic δ-NbN; it is as hard as sapphire, ultra-incompressible and has a high shear rigidity of 201 GPa to rival hard/superhard material γ-B (∼227 GPa). This exploration opens a new class of highly desirable materials combining the outstanding mechanical/elastic properties with superconductivity, which may be particularly attractive for its technological and engineering applications in extreme environments.

Hexagonal-structured ε-NbN: ultra-incompressibility, high shear rigidity, and a possible hard superconducting material

Exploring the structural stability and elasticity of hexagonal ε-NbN helps discover correlations among its physical properties for scientific and technological applications. Here, for the first time, we measured the ultra-incompressibility and high shear rigidity of polycrystalline hexagonal ε-NbN using ultrasonic interferometry and in situ X-ray diffraction, complemented with first-principles density-functional theory calculations up to 30 GPa in pressure. Using a finite strain equation of state approach, the elastic bulk and shear moduli, as well as their pressure dependences are derived from the measured velocities and densities, yielding B_S0 = 373.3(15) GPa, G₀ = 200.5(8) GPa, ∂B_S/∂P = 3.81(3) and ∂G/∂P = 1.67(1). The hexagonal ε-NbN possesses a very high bulk modulus, rivaling that of superhard material cBN (B₀ = 381.1 GPa). The high shear rigidity is comparable to that for superhard γ-B (G₀ = 227.2 GPa). We found that the crystal structure of transition-metal nitrides and the outmost electrons of the corresponding metals may dominate their pressure dependences in bulk and shear moduli. In addition, the elastic moduli, Vickers hardness, Debye temperature, melting temperature and a possible superconductivity of hexagonal ε-NbN all increase with pressures, suggesting its exceptional suitability for applications under extreme conditions.

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

Recently, a number of graphene-like early transition metal carbides and nitrides named as MXenes were fabricated by exfoliating MAX phases in hydrofluoric acid at room temperature. From experiments results and theory calculations, MXenes are promising anode materials in batteries as well as in metal-ion capacitors. To the best of our knowledge, experimental or calculated evidence has been supported the existence of more than 70 MAX phases members. Therefore, many counterparts MXene may be exist. Herein, employing density functional theory (DFT) computations, we have systematically examined the relative stability, structure and electronic properties of a series of two-dimensional metal carbides and nitrides including M2C (M=Sc, Ti, V, Cr, Zr, Nb, Hf, Mo and Ta), M2N (M=Ti, V, Cr, Zr, Hf), M3C2(M=Ti, V, Nb, Ta), Ti3N2, M4C3(M=Ti, V, Nb, Ta) and Ti4N3. The results demonstrate that all MXenes are metallic and have the similarly electronic structure with bulk transition metal carbides and nitrides, indicating that MXene may have superior catalysis and adsorption instead of expensive pure transition metal.