Fundamental aspects and recent advances in transition metal nitrides as electrocatalysts for hydrogen evolution reaction: A review

With increasing human population, sustainable energy production has become one of the most persistent and significant problems of the current century. Hydrogen is considered to be the best clean fuel for future energy requirements. As a substitute of fossil fuels, hydrogen is readily provided by an electrocatalytic hydrogen evolution reaction that splits water molecules. Conventional electrocatalysts based on noble metals are scarce and considerably expensive for large-scale hydrogen production, necessitating the search for low-cost earth abundant alternatives. In this context, transition metal nitrides have gained considerable attention as competent electrocatalytic materials for water splitting. This review presents recent advancements and progress on transition metal nitrides as efficient and cost-effective electrocatalysts for hydrogen production. After overviewing the fundamental aspects of the hydrogen evolution reaction (HER), the review discusses various synthetic strategies for developing transition metal nitrides. Discussed herein are titanium nitrides, vanadium nitrides, iron nitrides, nickel nitrides, molybdenum nitrides, tungsten nitrides, and their composite electrocatalysts employed in HER applications. Some design viewpoints for improving the electrocatalytic activity are systematically proposed. Finally, the review discusses challenges and future perspectives for the advancement of non-noble metal-based electrocatalysts.     

Electrocatalytic/photocatalytic properties and aqueous media applications of 2D transition metal carbides (MXenes)

The 2D transition metal carbides (MXenes) are increasingly considered among of the most promising 2D nanomaterials, because of their unique properties such as hydrophilic nature, metallic conductivity, large surface-area-to-volume ratio, and active surface functionalities. This has led to their growing utilization in water/wastewater treatment and environmental remediation applications, including water purification membranes, heavy metal removal, capacitive deionization, and bactericidal agents. This account will focus on the key characteristic properties of MXenes such as high metallic/electronic conductivity, and catalytic activity, and their utilization for the electrocatalytic and photocatalytic-based environmental remediation applications. We will also address the key challenges facing MXene-based materials in aqueous media and possible mitigation routs.     

Synthesis of transition metal-doped tungsten oxide nanostructures and their optical properties

In the present work, we report the solvothermal synthesis of transition metal-doped tungsten oxide nanostructures and their optical properties. Uniform, well-defined Co-doped tungsten oxide nanoblocks consisting of ultrathin nanowires, lenticular bundled Zn-doped tungsten oxide nanorods, and plate-shaped or flower-like Fe-doped tungsten oxide particles have been successfully prepared with the addition of different dopants. The doping of Co and Zn ions may prohibit the oriented growth of the tungsten oxide nanowires and favor their self-assembly, leading to the formation of bundled nanoblocks and nanorods. The Fe-doped tungsten oxide flowers may result from the ordered arrangement of single nanoplates. The band gaps of undoped, Fe-, Zn-, and Co-doped tungsten oxides are found to be 2.92, 2.78, 2.62, and 2.52 eV, respectively.     

Preparation of some transition metal sulphides

Ternary sulphides, LnMS₃ (Ln=rare earth metal; M=first row transition metal) and MV₂S₄ (M=Mg, Fe, Co, Ni and Zn) have been prepared by the reaction of appropriate ternary metal oxides with H₂S or CS₂/N₂ vapours at elevated temperatures. Chemical analysis and x-ray powder diffraction of the products indicate formation of single-phases with unique crystal structures in many cases. Communication No. 117 from the Solid State and Structural Chemistry Unit.     

Work function measurement of transition metal nitride and carbide thin films

Work functions of rf-magnetron sputter-deposited transition metal nitride and carbide thin films, including TiN, HfN, VN, NbN, TaN, HfC, VC, NbC, TaC, Cr₃C₂, and WC, were measured by Kelvin probe in air. Thin films of the above materials were prepared on (1 0 0)-oriented silicon substrate by rf-magnetron sputtering of a compound target. As a result, it was found that the work function of nitride was similar to or slightly lower than that of carbide. For nitride films, those with heavier metal atoms such as Hf and Ta, showed lower work function. The work function depended upon the Period to which the metal atom belongs in the periodic table.     

Thermodynamic properties of ni nitrides and phase stability in the Ni-N system

The thermodynamics of the Ni-N system is poorly known from experiments, and there is a need of information on the stability of the various nitride phases and the Ni-N phase diagram. This kind of information has been obtained by us, by combining the few measurements available with predictions, based on recently reported regularities in bonding properties and vibrational entropy of 3d transition metal compounds. A calculated Ni-N phase diagram is presented. A certain range of homogeneity for the hexagonal nitride phase is obtained, which is comparable to that of other 3d transition metal-nitrogen systems. The question of the possible existence of a stable Ni₄N phase is examined. According to our results, Ni₄N is metastable in the Ni-N system.     

Harmonic generation in transition metal dichalcogenides and their heterostructures

Few layered transition metal dichalcogenides (TMD), with an absence of crystal inversion symmetry and outstanding optical characteristics, are frequently applied in studies of nonlinear optics (NLO) for harmonic generation. Related materials are regarded as potential candidates for many optoelectronics applications. In order to enhance and manipulate the intrinsically weak NLO responses, TMD’s have been fabricated into heterostructures in recent years. The basic physics of harmonic generation and of TMD optical responses, as well as the interactions in TMD hybrid structures are introduced briefly, and the current state-of-the-art in the performance of TMDs in harmonic generation are reviewed. A particular focus is made on heterostructure studies to enhance and manipulate the response, which represent the core issues for devices and applications.     

Synthesis and electronic structure of proton-type partially substituted birnessite by period-four transition metal

Partially substituted proton-type birnessite were prepared by solid state reaction and their structures were refined. The formed birnessite with no substitution is identified to rhombohedral phase. In the case of substitution treatment by V and Cr for Mn, birnessite phase was not formed. On substituting Fe, hexagonal phase increased with increase of the amount of the Fe. For Co and Ni-substitution, monoclinic phase emerged at substitution ratio of around 0.37 and 0.02, respectively. For the substitution of Cu, only the monoclinic birnessite formed irrespective of the ratio. The electric conductivity of the partially substituted birnessites was examined at room temperature. The general trend is lower conductivity with increasing ratio of contained substituents. On several mol% of the substitution by Ni and Cu, the conductivity slightly increased. From DOS calculation of these compounds, the partially substitution for Mn by Fe, Co and Ni in the birnessite poses splitting of crystal field to emerge new bands at around −1 and +1 eV by Mn(IV) 3d orbital.     

Synthesis,characterization and photoresponse study of undoped and transition metal (Co,Ni, Mn) doped ZnO thin films

The synthesis, characterization and photoresponse studies of undoped and transition metal doped zinc oxide thin films are carried out in this work, in prospect of visible light photo detection and sensor applications. The undoped and transition metal ions such as, Co, Ni and Mn doped ZnO films in this study were synthesized by chemical solution deposition, involving spin-coating. We have characterized the deposited films using X-ray diffraction, scanning electron microscopy, photoluminescence and UV–vis spectroscopy studies. The devices of the films for photoresponse study were fabricated by top Ag contacts on the film surface in metal–semiconductor–metal configuration. The current–voltage characteristics and switching measurements of these devices were studied under the illumination of an incandescent lamp. We found a high ON/OFF ratio of 8 and highest photocurrent density of 0.7 mA/cm² for Ni doped ZnO film.     

碳纳米管负载金属氮化物催化剂的制备及其性能研究

采用等体积浸渍法结合程序升温还原技术制备出一系列负载型过渡金属氮化物催化剂。利用XRD、BET、TPR等表征手段,结合氨分解制氢反应,研究了它们的表面性质和反应性能。发现改性碳纳米管负载的氮化钴催化剂具有较大的比表面积,对氨分解制氢反应的催化活性最高。新鲜态CoNx/CNTs催化剂的比表面积可达151.85m2/g,在650℃时氨转化率为80.3%,850℃达到完全转化。     

Fully strained low-temperature epitaxy of TiN/MgO(001) layers using high-flux, low-energy ion irradiation during reactive magnetron sputter deposition

Epitaxial TiN layers, 0.3 μm thick, are grown on MgO(001) in the absence of applied substrate heating using very high flux, low-energy (below the lattice atom displacement threshold), ion irradiation during reactive magnetron sputter deposition in pure N₂ discharges. High-resolution x-ray diffraction, reciprocal lattice maps, and transmission electron microscopy analyses reveal that the TiN(001) films grow with an (001)_TiN||(001)_MgO and [100]_TiN||[100]_MgO orientation relationship to the substrate. The layers are fully coherent with no detectable misfit dislocations. For comparison, TiN/MgO(001) films grown at temperatures of 700-850 °C under similar conditions, but with no intentional ion irradiation, are fully relaxed with a high misfit dislocation density. Thus, the present results reveal that intense low-energy ion irradiation during film growth facilitates high adatom mobilities giving rise to low-temperature epitaxy, while the low growth temperature quenches strain-induced relaxation and suppresses misfit dislocation formation.     

Large-scale, hot-filament-assisted synthesis of tungsten oxide and related transition metal oxide nanowires

A scalable and versatile method for the large-scale synthesis of tungsten trioxide nanowires and their arrays on a variety of substrates, including amorphous quartz and fluorinated tin oxide, is reported. The synthesis involves the chemical-vapor transport of metal oxide vapor-phase species using air or oxygen flow over hot filaments onto substrates kept at a distance. The results show that the density of the nanowires can be varied from 10(6)-10(10) cm(-2) by varying the substrate temperature. The diameter of the nanowires ranges from 100-20 nm. The results also show that variations in oxygen flow and substrate temperature affect the nanowire morphology from straight to bundled to branched nanowires. A thermodynamic model is proposed to show that the condensation of WO(2) species primarily accounts for the nucleation and subsequent growth of the nanowires, which supports the hypothesis that the nucleation of nanowires occurs through condensation of suboxide WO(2) vapor-phase species. This is in contrast to the expected WO(3) vapor-phase species condensation into WO(3) solid phase for nanoparticle formation. The as-synthesized nanowires are shown to form stable dispersions compared to nanoparticles in various organic and inorganic solvents.     

Hydrothermal synthesis of transition metal oxide nanomaterials in HEPES buffer solution

Various transition metal oxide (Co₃O₄, Mn₃O₄ and ZnO) nanostructures were readily synthesized in HEPES buffer solution (200 mmol/L, pH 7.40) via a simple hydrothermal method. The products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX). It was found that the morphologies of the as-obtained transition metal oxide nanostructures were affected by the reaction temperature, HEPES/metal salt molar ratio and metal precursor. HEPES with two free nitrogen atoms (piperazine group) and terminal hydroxyl groups plays a critical role as a reactant and surfactant to prevent the transition metal oxide nanomaterials from aggregation.     

A comparative study of proton transport properties of zirconium phosphate and its metal exchanged phases

A new phase of amorphous zirconium phosphate (ZrP), an inorganic ion exchanger of the class of tetravalent metal acid (TMA) salt, is synthesized by sol-gel method. The protons present in the structural hydroxyl groups indicate good potential for TMA salts to exhibit solid state proton conduction. Cu²⁺ and Li⁺ are exchanged onto ZrP to yield CuZrP and LiZrP exchanged phases. All these materials were characterized for elemental analysis (ICP-AES), thermal analysis (TGA, DSC), X-ray analysis and FTIR spectroscopy. The transport properties of these materials were explored and compared by measuring conductance at different temperatures using an impedance analyser. It is observed that conductivity decreases with increasing temperature in all cases and mechanism of transportation is proposed to be Grotthuss type. Conductivity performance of ZrP, CuZrP and LiZrP is discussed based on conductivity data and activation energy.     

Stacking of lamellae in Mg/Al hydrotalcites: Effect of metal ion concentrations on morphology

A hybrid nanocomposite based on the intercalation of carbonate anion has been synthesized through co-precipitation technique. Powder X-ray diffraction patterns (PXRD) showed pure layered double hydroxide (LDH) phases having crystallite size around 20 and 13 nm in ‘a’ and ‘c’ crystallographic directions, respectively. Fourier transform infrared and Raman spectroscopy measurements exhibit shifting of bands with increase of divalent metal ion concentration and it further suggests the presence of carbonate anions. Differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA) exhibit the three stages of thermal degradation, which is characteristic behaviour of layered double hydroxide. CHN and energy dispersive X-ray analysis support the PXRD and spectroscopy results. The nature of charge observed through Zeta potential analyzer is positive. Transmission electron microscope (TEM) exhibits the characteristic LDH platelet morphology with the platelets stacked one above the other.     

Synthesis, characterization and electrical properties of quaternary selenodiphosphates: AMP2SE6 with A = Cu, Ag and M = Bi, Sb

The new quaternary selenophosphate phases AMP₂Se₆ (A=Cu, Ag and M=Bi, Sb) were synthesized by ceramic methods at 1023 K. These phases were characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray analysis (EDX) and a.c. and d.c. electrical conductivity measurements. The phases all show values of electrical conductivity, σ, of about 10⁻⁴ Ω⁻¹ cm⁻¹ at 303 K and photoconductive effect. The conductivity is nearly five orders of magnitude larger than that of related phases.     

The growth and thermodynamical feasibility of tungsten diselenide single crystals using chemical vapour transport technique

Thermodynamical feasibility study and the growth of layer structured transition metal dichalcogenide single crystals of WSe₂, using iodine as transporting agent, has been reported in this paper. The characterization of the grown samples have been done by X-ray analysis.     

Tribological properties of transition metal di-chalcogenide based lubricant coatings

Transition metal di-chalcogenides MX₂ (X= S, Se, Te; and M= W, Mo, Nb, Ta) are one kind of solid lubricant materials that have been widely used in industry. The lubricant properties of such lubricant coatings are dependent not only on microstructure, orientation, morphology, and composition of the coatings, but also on the substrate, the interface between substrate and lubricant coatings, and the specific application environment. In this review, the effects of parameters on tribological properties of such kind of lubricant coatings were summarized. By comparing advantages and disadvantages of those coatings, the special treatments such as doping, structural modulation and post-treatment were suggested, aiming to improve the tribological performance under severe test conditions (e.g. high temperature, oxidizing atmosphere or humid condition).     

Effects of a ferromagnetic metal stripe and a Schottky metal stripe on the electron transport in a nanostructure

In this paper, the spin-dependent electron transport in a two-dimensional electron gas (2DEG) modulated by a ferromagnetic (FM) metal stripe and a Schottky metal (SM) stripe is in detail studied. It is found that the position and the width of the SM stripe as well as the incident energy of electron play an important role on the spin polarization. It is also found that the spin polarization is obviously dependent on the electric-barrier height induced by an applied voltage to the SM stripe and such a device can be used as a voltage-tunable electron-spin filter.