Observation of DC field-evaporated ion species extracted from transition metal nitride thin film deposited on tungsten-tip

The ion species extracted from transition metal nitride thin films were investigated in order to understand the field evaporation mechanism of nitrogen in atom probe analysis. Nitrides of group IV transition metal, ie, titanium (Ti), zirconium (Zr), and hafnium (Hf) nitrides, were chosen for analysis, owing to their good electrical conductivities. The samples were prepared by sputtering deposition of nitride thin film on a tungsten needle. Measurements were performed at 3 different direct current voltages, and for each voltage, we observed different ion species. For TiN and ZrN, both atomic metal ions and molecular ions were detected and TiN and ZrN tended to evaporate in the form of triple-charged molecular ions. For ZrN, Zr²⁺, Zr³⁺, ZrN³⁺, and (ZrN)₂³⁺ were observed at lower direct current voltages. For a higher tip voltage, N⁺ ions were detected in addition to these ions. These results suggest that the evaporation field of nitrogen is higher than those of Zr³⁺ and (ZrN)₂³⁺. In the analysis of an HfN tip, no ions could be detected. These results can be explained in terms of the differences between evaporation fields that were roughly estimated from the work functions and the bond energies of the analyzed nitrides.     

Ruthenium(II)−Bipyridine/NanoC3N4 Hybrids: Tunable Photochemical Properties by Using Exchangeable Alkali Metal Cations

A series of nanoporous carbon nitrides that contained a range of alkali metal cations (M@nanoC₃N₄: M=Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) have been successfully synthesized from as‐synthesized g‐C₃N₄ by delamination with concentrated sulfuric acid, followed by neutralization with aqueous solutions of the corresponding alkali metal hydroxides. Tris(2,2′‐bipyridine)ruthenium(II) complexes, [Ru(bpy)₃]²⁺, were grafted onto the carbon nitrides in an effort to explore the physicochemical properties of the deposited [Ru(bpy)₃]²⁺, as well as its photocatalytic activity in the aerobic photooxidation of phenylboronic acid and H₂ production from aqueous media in the presence of a Pt co‐catalyst under visible‐light irradiation. Highly porous nanoC₃N₄ could significantly enhance photocatalytic activity, because of its high surface area, owing to its unique porous structure. More interestingly, the photoluminescence intensities of [Ru(bpy)₃]²⁺ complexes that were associated with M@nanoC₃N₄ increased in the presence of lighter alkali metal cations, which correlated with increased photocatalytic activities for both reactions. This study demonstrates that M@nanoC₃N₄ are fascinating supports, in which the local environment of an immobilized metal complex can be precisely controlled by varying the alkali metal cation from Li⁺ to Cs⁺.     

Chemical looping of metal nitride catalysts: low-pressure ammonia synthesis for energy storage

The activity of many heterogeneous catalysts is limited by strong correlations between activation energies and adsorption energies of reaction intermediates. Although the reaction is thermodynamically favourable at ambient temperature and pressure, the catalytic synthesis of ammonia (NH₃), a fertilizer and chemical fuel, from N₂ and H₂ requires some of the most extreme conditions of the chemical industry. We demonstrate how ammonia can be produced at ambient pressure from air, water, and concentrated sunlight as renewable source of process heat via nitrogen reduction with a looped metal nitride, followed by separate hydrogenation of the lattice nitrogen into ammonia. Separating ammonia synthesis into two reaction steps introduces an additional degree of freedom when designing catalysts with desirable activation and adsorption energies. We discuss the hydrogenation of alkali and alkaline earth metal nitrides and the reduction of transition metal nitrides to outline a promoting role of lattice hydrogen in ammonia evolution. This is rationalized via electronic structure calculations with the activity of nitrogen vacancies controlling the redox-intercalation of hydrogen and the formation and hydrogenation of adsorbed nitrogen species. The predicted trends are confirmed experimentally with evolution of 56.3, 80.7, and 128 μmol NH₃ per mol metal per min at 1 bar and above 550 °C via reduction of Mn₆N_2.58 to Mn₄N and hydrogenation of Ca₃N₂ and Sr₂N to Ca₂NH and SrH₂, respectively.     

A First‐Cycle Coulombic Efficiency Higher than 100 % Observed for a Li2MO3 (M=Mo or Ru) Electrode

The lithiation/de‐lithiation behavior of a ternary oxide (Li₂MO₃, where M=Mo or Ru) is examined. In the first lithiation, the metal oxide (MO₂) component in Li₂MO₃ is lithiated by a conversion reaction to generate nano‐sized metal (M) particles and two equivalents of Li₂O. As a result, one idling Li₂O equivalent is generated from Li₂MO₃. In the de‐lithiation period, three equivalents of Li₂O react with M to generate MO₃. The first‐cycle Coulombic efficiency is theoretically 150 % since the initial Li₂MO₃ takes four Li⁺ ions and four electrons per formula unit, whereas the M component is oxidized to MO₃ by releasing six Li⁺ ions and six electrons. In practice, the first‐cycle Coulombic efficiency is less than 150 % owing to an irreversible charge consumption for electrolyte decomposition. The as‐generated MO₃ is lithiated/de‐lithiated from the second cycle with excellent cycle performance and rate capability.     

Präkeramische Polyazane über die Sol‐Gel‐Route im Ammonosystem und aus molekularen Einkomponentenvorläufern — ein Leistungsvergleich

Preceramic Polyazanes via Sol Gel Route in the Ammono System and via Molecular Single Source Precursors — a Comparison of Performance All steps of the sol gel process in the oxo system can be transferred analogously to the ammono system. For this purpose, element alkyl amides of titanium, zirconium, tantalum and boron, have been co‐ammonolised with silicon alkyl amides in aprotic solvents. Pyrolysis of the resulting polyazanes yields multinary nitridic ceramic powders. For the model systems Si(NHMe)₄/Ti(NMe₂)₄, Si(NHMe)₄/Zr(NMe₂)₄, Si(NHMe)₄/Ta(NMe₂)₅, and Si(NHMe)₄/B(NMe₂)₃ the corresponding polymeric polyboro‐, polytitano‐, polyzirkono‐ and polytantalosilazanes were synthesised. Pyrolysis in flowing ammonia at 1000°C results in ternary amorphous silicon nitrides; further heating in nitrogen leads to partly crystalline composites. The process of pyrolysis as well as the morphology and composition of the final pyrolysis products were analysed by means of NMR, MAS‐NMR, FT‐IR, DTA‐TG‐MS, XRD, SEM, EDX, and elemental analyses. In order to verify this new and convenient access to multinary nitride ceramics, single source precursors were synthesised and, via polymer intermediates, processed to ceramic powders. A specially developed reaction sequence is generally applicable to syntheses of single source precursors for various ternary metal silicon nitrides. Dilithiated silazanes of the type Si(NMe₂)₂(NLiR)₂, with R = t‐Butyl, SiMe₃, CH₃, synthesised and structurally characterised for the first time, are good silicon synthones. While forming nitrido bridges, those compounds react with e.g. two‐ or four valent transition metal chlorides, to silicon transition metal adducts. Actually, we have analysed the reaction of Si(NMe₂)₂(NLiR)₂ and TiCl₄, ZrCl₄, TaCl₅, CrCl₃, MnCl₂, and ZnCl₂, respectively. The adducts as formed were crosslinked with ammonia and pyrolysed. In the case of the chlorides of the 4.—6. group metals, amorphous ceramics were obtained. Treatment at higher temperatures results in composites of transition metal nitride and an amorphous matrix. MnSiN₂ and a new hexagonal modification of ZnSiN₂ were obtained in the systems Mn/ Si/ N and Zn/ Si/ N. Surprisingly, during the synthesis of ZnSiN₂, ZnCN₂ was obtained as a side product.     

A green route for microwave synthesis of sodium tungsten bronzes NaxWO3 (0<x<1)

A green route has been developed for microwave synthesis of sodium tungsten bronzes Na_xWO₃ (0<x<1) from Na₂WO₄, WO₃ and tungsten powder. The hybrid microwave synthesis was carried out in argon atmosphere using CuO powder as the heating medium. Tungsten powder is used as the reducing agent instead of the alkali metal iodides previously used for the microwave synthesis of oxide bronzes. The prepared samples were characterized by powder X-ray diffraction, energy-dispersive X-ray analysis and scanning electron microscopy, and their phase constitutions, crystal structures and morphologies are in consistence with that in the literature. This synthesis method is simple, green and atom economic, and promising for preparation of other oxide bronzes and related compounds.     

United in Nitride: The Highly Condensed Boron Phosphorus Nitride BP3N6

Owing to intriguing materials properties non‐metal nitrides are of special interest for both, solid‐state chemistry and materials science. Mixed ternary non‐metal nitrides, however, have only been sparsely investigated, as preparative chemistry lacks a systematic access, yet. Herein, we report on the highly condensed boron phosphorus nitride BP₃N₆, which was synthesized from (PNCl₂)₃, NH₄N₃ and h‐BN in a high‐pressure high‐temperature reaction. By increasing partial pressure of HCl during synthesis using NH₄Cl, single‐crystals of BP₃N₆ up to 80 μm in length were obtained. The unprecedented framework‐type structure determined by single‐crystal XRD blends structural motifs of both, α‐P₃N₅ and c‐BN, rendering BP₃N₆ a double nitride. The compound was further investigated by Rietveld refinement, EDX, temperature‐dependent PXRD, FTIR and solid‐state NMR spectroscopy. The formation of BP₃N₆ through use of reactive precursors exemplifies an innovative access to mixed non‐metal nitrides.     

Solid State Metathesis Reactions in Various Applications

Solid state metathesis reactions have been studied in fused silica tubes, by differential thermal analysis, and by X‐ray powder diffraction. A selection of reactions between metal (La, Nb, and Ni) chlorides and lithium nitride or lithium acetylide were investigated to get more insight into reaction pathways and intermediate reaction stages that may be adopted on course of the formation of metal nitrides or carbides. Intermediate compounds are considered to be important because they can control the reactivity of a system. Such compounds were traced by changing the molar ratios of reaction partners away from the salt‐balanced binary metal nitride or carbide target compositions. New preparative perspectives are discovered when metal chlorides were reacted with lithium nitridoborate or lithium cyanamide. Due to their reductive nature towards several d‐block metal chlorides, (BN₂)^3‐ and (CN₂)^2‐ react to form metals or metal nitrides plus X‐ray amorphous BN, and probably C₃N₄. With lanthanum chloride they can react to form nitridoborates and nitridocarbonates. The metathesis reaction between lithium cyanamide and cyanuric chloride (C₃N₃Cl₃) instead of metal chloride was studied for the synthesis of C₃N₄.     

Synthesis of orthorhombic LiMnO2 by solid-phase reaction under steam atmosphere and a study of its heat and acid-treated phases

Low crystalline orthorhombic LiMnO₂ (o-LiMnO₂) samples were synthesized by reacting either γ-MnOOH or Mn₂O₃ with LiOH·H₂O in the solid phase under steam atmosphere at 120°C. In the closed system, the vapor arising from LiOH·H₂O may strengthen the reactivity of LiOH at the surface of MnOOH or Mn₂O₃ particles, which may enable slow diffusion of Li⁺ ions forming LiMnO₂. These samples were compared with crystalline o-LiMnO₂ prepared by a solid-state reaction method at 700°C in nitrogen gas. The powder X-ray diffraction patterns of low crystalline samples after heating at 400°C in air revealed the formation of a single phase of cubic Li_1.6Mn_1.6O₄, but the crystalline sample revealed a mixed phase of o-LiMnO₂ and LiMn₂O₄ after heating at 400°C in air. The Li⁺/H⁺ exchange in the Li_1.6Mn_1.6O₄ sample progressed topotactically, while maintaining the crystal structure and morphology of the precursor. But heat-treated crystalline o-LiMnO₂ showed a disproportionation reaction with dissolution of Mn²⁺ ions.     

A New Route to Metal Azides

Beside several other applications, metal azides can be used for the synthesis of nitridophosphates and binary nitrides. Herein we present a novel synthetic access to azides: Several metals, such as main‐group, transition metals, and rare‐earth metals, react with silver azide in liquid ammonia as a solvent giving the corresponding metal azides. In this work Mn(N₃)₂, Sn(N₃)₂, and Eu(N₃)₂, as well as their ammonia complexes were synthesized for the first time through low‐temperature methods. Also a simpler access to Zn(N₃)₂ was possible. At room temperature and the respective vapor pressure of NH₃, it became possible to grow single crystals of the dinuclear holmium azide [Ho₂(μ‐NH₂)₃(NH₃)₁₀](N₃)₃?1.25 NH₃. We are confident that this new route could lead to novel metal azides as well as nitrides of the main‐group, the transition, and the rare‐earth metals upon careful decomposition.     

Preparation of ribavirin analogues by copper- and ruthenium-catalyzed azide-alkyne 1,3-dipolar cycloaddition

In this study, we described the synthesis of 1,4- and 1,5-disubstituted-1,2,3-triazolo-nucleosides from various alkynes with 1′-azido-2′,3′,5′-tri-O-acetylribose using either copper-catalyzed azide-alkyne cycloaddition (CuAAC) or ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC), respectively. Optimized RuAAC conditions were realized with the commercially available [Cp^∗RuCl(PPh₃)₂] under microwave heating, which allows a significant acceleration of the reaction times (from 6 h to 5 min). This reaction can work under water-containing system. RuAAC and CuAAC are useful tools for the synthesis of 1,2,3-triazolyl-nucleosides small libraries.     

Mass spectrometric diagnosis of the surface nitriding mechanism in a d.c. glow discharge

Reactive constituents have been investigated in a molecular beam generated in the cathode surface glow area and surface boundary layer. Mixtures of nitrogen and hydrogen form NH_x(x=0–4) compounds, which are of relevance in heterogeneous, plasma vs. metal nitriding reactions. Ammonia decomposition leads to NH_x(x=2–4). Strong cataphoretic enrichment of hydrogen has been observed in the cathode glow area. Heterogeneous reactions of NH_x with iron lead to the formation of iron nitrides via intermediates such as FeNH_2–3. In a pulsed d.c. glow discharge, increased sputtering and decreased hydrogen enrichment have been observed.     

The influence of phase and morphology of molybdenum nitrides on ammonia synthesis activity and reduction characteristics

The reactivities of a series of ternary and binary molybdenum nitrides have been compared. Data have been obtained for the catalytic synthesis of ammonia at 400 °C and ambient pressure using a 3:1 H₂:N₂ mixture. Amongst the ternary nitrides, the mass normalised activity is in the order Co₃Mo₃N>Fe₃Mo₃N?Ni₂Mo₃N. For the binary molybdenum nitrides, the ammonia synthesis activity is significantly lower than that of Co₃Mo₃N and Fe₃Mo₃N and varies in the order γ-Mo₂N∼β-Mo₂N_0.78?δ-MoN. Nanorod forms of β-Mo₂N_0.78 and γ-Mo₂N exhibit generally similar activities to conventional polycrystalline samples, demonstrating that the influence of catalyst morphology is limited for these two materials. In order to characterise the reactivity of the lattice nitrogen species of the nitrides, temperature programmed reactions with a 3:1 H₂:Ar mixture at temperatures up to 700 °C have been performed. For all materials studied, the predominant form of nitrogen lost was N₂, with smaller amounts of NH₃ being formed. Post-reaction powder diffraction analyses demonstrated lattice shifts in the case of Co₃Mo₃N and Ni₂Mo₃N upon temperature programmed reaction with H₂/Ar. Incomplete decomposition yielding mixtures of Mo metal and the original phase were observed for Fe₃Mo₃N and γ-Mo₂N, whilst β-Mo₂N_0.78 transforms completely to Mo metal and δ-MoN is converted to γ-Mo₂N.     

Preparation and crystal chemistry of some tetrahedral Li3PO4-type compounds

The crystal chemistry of Li₃PO₄, Li₃VO₄ and Li₃AsO₄ are compared. All three have an isostructural low phase, designated β_II, and an isostructural high phase, γ_II, but in Li₃VO₄ and Li₃AsO₄ the high-low transformation proceeds reversibly through one or more transitional phases some of which can be quenched to ambient. The crystal chemistry of derivative Li₃PO₄ phases, including Li₂MgSiO₄, Li₂ZnSiO₄, Li₂CoSiO₄, Li₂MgGeO₄ and Li₂ZnGeO₄ is compared and the occurrence of high, low, and of distorted high and low phases is correlated with the temperature of preparation and rate of cooling. The derivative Li₃PO₄ phases show extensive or complete mutual solubility not only with each other, but with Li₃PO₄, with M₂XO₄ compounds (M = Zn²⁺, Mg²⁺; X = Ge⁴⁺, Si⁴⁺) and also with Li₄XO₄ compounds (X = Ge⁴⁺, Si⁴⁺). The sequence of phase transformations encountered on heating or cooling is quite sensitive to the stoichiometry of the derivative phases.     

Synthesis of Nanocrystalline Zr3N4 and Hf3N4 Powders from Metal Dialkylamides

A simple process to synthesize Zr₃N₄ and nitrogen‐rich Hf₃N₄ powders via ammonolysis of metal dialkylamides, i.e. Zr(NEt₂)₄ and Hf(NEt₂)₄, at temperatures below 700 °C is presented. The obtained nitrides have a rhombohedrally distorted NaCl‐type structure, which has previously been reported only for nitrogen‐rich films of these nitrides. Regardless of the atmosphere (N₂ and He), the Zr₃N₄ starts to decompose above about ～ 650 °C and achieves the highest decomposition rate at about 900 °C, finally yielding the mononitride ZrN. Both Zr₃N₄ and Hf₃N₄ powders are nanocrystalline with the crystal size of about 2 nm.     

Nanospheres,Nanotubes, Toroids,and Gels with Controlled Macroscopic Chirality

The interaction of a highly dynamic poly(aryl acetylene) (poly‐ 1 ) with Li⁺, Na⁺_, and Ag⁺ leads to macroscopically chiral supramolecular nanospheres, nanotubes, toroids, and gels. With Ag⁺, nanospheres with M helicity and tunable sizes are generated, which complement those obtained from the same polymer with divalent cations. With Li⁺ or Na⁺, poly‐ 1 yields chiral nanotubes, gels, or toroids with encapsulating properties and M helicity. Right‐handed supramolecular structures can be obtained by using the enantiomeric polymer. The interaction of poly‐ 1 with Na⁺ produces nanostructures whose helicity is highly dependent on the solvation state of the cation. Therefore, structures with either of the two helicities can be prepared from the same polymer by manipulation of the cosolvent. Such chiral nanotubes, toroids, and gels have previously not been obtained from helical polymer–metal complexes. Chiral nanospheres made of poly(aryl acetylene) that were previously assembled with metal(II) species can now be obtained with metal(I) species.     

Nanospheres,Nanotubes, Toroids,and Gels with Controlled Macroscopic Chirality

The interaction of a highly dynamic poly(aryl acetylene) (poly‐ 1 ) with Li⁺, Na⁺_, and Ag⁺ leads to macroscopically chiral supramolecular nanospheres, nanotubes, toroids, and gels. With Ag⁺, nanospheres with M helicity and tunable sizes are generated, which complement those obtained from the same polymer with divalent cations. With Li⁺ or Na⁺, poly‐ 1 yields chiral nanotubes, gels, or toroids with encapsulating properties and M helicity. Right‐handed supramolecular structures can be obtained by using the enantiomeric polymer. The interaction of poly‐ 1 with Na⁺ produces nanostructures whose helicity is highly dependent on the solvation state of the cation. Therefore, structures with either of the two helicities can be prepared from the same polymer by manipulation of the cosolvent. Such chiral nanotubes, toroids, and gels have previously not been obtained from helical polymer–metal complexes. Chiral nanospheres made of poly(aryl acetylene) that were previously assembled with metal(II) species can now be obtained with metal(I) species.     

High‐Pressure Synthesis of Metal–Inorganic Frameworks Hf4N20⋅N2, WN8⋅N2, and Os5N28⋅3 N2 with Polymeric Nitrogen Linkers

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one‐step synthesis of metal–inorganic frameworks Hf₄N₂₀?N₂, WN₈?N₂, and Os₅N₂₈?3 N₂ via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf₄N₂₀, WN₈, and Os₅N₂₈) are built from transition‐metal atoms linked either by polymeric polydiazenediyl (polyacetylene‐like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high‐pressure reaction between Hf and N₂ also leads to a non‐centrosymmetric polynitride Hf₂N₁₁ that features double‐helix catena‐poly[tetraz‐1‐ene‐1,4‐diyl] nitrogen chains [?N?N?N=N?]_∞.     

Thermal plasma synthesis of transition metal nitrides and alloys

Applications of arc plasma processing to high-temperature chemistry of Group V nitrides and Si and Ge alloys are studied. The transition metal nitrides -VN, -NbN, and -TaN are directly synthesized in a dc argon-nitrogen plasma from powders of the metals. A large excess of N₂ is required to form stoichiometric -VN, while the Nb and Ta can only be synthesized with a substoichiometric N content. In a dc argon plasma the alloys V₃Si, VSi₂, NbSi₂, NbGe₂, Cr₃Si, and Mo₃Si are obtained from powder mixtures of the corresponding elements. The compounds are identified by x-ray diffraction patterns and particle shape and size are studied by electron microscopy.     

Lithium Imide Synergy with 3d Transition‐Metal Nitrides Leading to Unprecedented Catalytic Activities for Ammonia Decomposition

Alkali metals have been widely employed as catalyst promoters; however, the promoting mechanism remains essentially unclear. Li, when in the imide form, is shown to synergize with 3d transition metals or their nitrides TM(N) spreading from Ti to Cu, leading to universal and unprecedentedly high catalytic activities in NH₃ decomposition, among which Li₂NH? MnN has an activity superior to that of the highly active Ru/carbon nanotube catalyst. The catalysis is fulfilled via the two‐step cycle comprising: 1) the reaction of Li₂NH and 3d TM(N) to form ternary nitride of LiTMN and H₂, and 2) the ammoniation of LiTMN to Li₂NH, TM(N) and N₂ resulting in the neat reaction of 2 NH₃?N₂+3 H₂. Li₂NH, as an NH₃ transmitting agent, favors the formation of higher N‐content intermediate (LiTMN), where Li executes inductive effect to stabilize the TM? N bonding and thus alters the reaction energetics.