The co-ammonolysis of V(NMe2)4 and Si(NHMe)4 with ammonia in THF and in the presence of ammonium triflate ([NH4][CF3SO3]) leads to the formation of monolithic gels. Pyrolysing these gels produces mesoporous composite materials containing nanocrystalline VN and amorphous silicon imidonitride. Elemental mapping indicated a thorough distribution of VN with no evidence of large cluster segregation. Whilst not active for ammonia synthesis, the silicon nitride based materials were found to possess activity for the COx-free production of H2 from methane, which makes them candidates for applications in which the presence of low levels of CO in H2 feedstreams is detrimental.

The effects of surface oxidation on the capacitance of titanium nitride electrode surfaces, produced by reaction of titanium foils with ammonia, are examined. Thermal oxidation and electrochemical oxidation both increase the amount of redox active oxide at the surface, but electrochemical oxidation is found to be more successful in increasing the capacitance.

New approaches to produce nanocrystalline TiN materials with high conductivity and their application as conductive coatings on battery materials have been developed. Sol–gel synthesis routes using tetrakis(dimethylamido)titanium(IV) and a propylamine or ammonia cross linking agent, followed by thermal treatment under NH3 or H2 + N2, were found to produce TiN powders of small crystallite size (<10 nm), with good conductivity in selected cases. The most promising synthesis conditions were used to produce even TiN coatings on LiFePO4 particles, and the resulting materials exhibited significantly improved electrochemical performance relative to uncoated LiFePO4, in terms of higher specific capacity, cycle stability and rate capability. The material with the optimum 10 wt% TiN content exhibited a discharge capacity of 159 mA h g−1, that is ∼93% of the theoretical capacity, when charge/discharge rates of 0.1C were applied. The results demonstrate the suitability of this new route to produce TiN coatings, which could also be applied to high voltage materials or for materials to be operated at high temperatures, where corrosion or degradation of other coating materials (e.g. carbon) would be problematic.

•Preceramic and polymer routes allow access to functional amorphous materials based on silicon nitride with controlled morphologies.•Carbon, boron and other elements can be used to modify properties including high temperature strength and oxidative stability.•A range of composite materials may be obtained on high temperature crystallisation.The use of preceramic polymer and sol-gel processing methods in the production of silicon nitride and a number of related materials is reviewed. Amorphous ceramics in this system, which may contain additional carbon, boron and other elements, have a number of promising and/or useful high temperature properties. These include good mechanical properties and oxidation resistance at temperatures that can exceed 1500 °C with some materials, but also useful charge storage capability, catalytic activity and semiconducting or optical properties after doping.

Bulk nanocrystalline Sn3N4 powders were synthesised by a two step ammonolysis process followed by washing with dilute acid. Their performance as Li-ion and Na-ion battery negative electrodes was assessed by galvanostatic cycling in half cells vs. the metal, giving good performance in both cases and remarkable stability in the sodium cells. The effect of carboxymethyl cellulose and sodium alginate binders was examined and the latter found to give superior performance. Capacity and stability were also enhanced via the use of a fluoroethylene carbonate electrolyte additive.

Reactions of Si(NHMe)4 with ammonia are effectively catalysed by small ammonium triflate concentrations, and can be used to produce free-standing silicon imide gels. Firing at various temperatures produces amorphous or partially crystallised silicon imidonitride/nitride samples with high surface areas and low oxygen contents. The crystalline phase is entirely α-Si3N4 and structural similarities are observed between the amorphous and crystallised materials.

The phase behaviour of TiO2 films and powders produced from Ti(OiPr)4-based sols in ethanol modified with diethanolamine is investigated. The anatase/rutile phase concentrations vary with conditions and are correlated with samples from an acid-catalysed sol used in the same way to make films and powders. Diethanolamine is effective in increasing smoothness of films and reducing cracking, but also results in rutile formation when samples are heated at 600 °C. Mass spectrometry and nuclear magnetic spectroscopy were used to interrogate the species present in solution, where specific cluster types were found to be prominent and the diethanolamine was shown to coordinate with titanium via all three donor atoms.

η-Mn3N2 is produced using solvothermal reactions between MnCl2 and LiNH2 in benzene. At 350 °C nanocapsule structures are obtained, whereas at higher temperatures samples consisted of isotropic nanoparticulates. In aqueous KOH solution capacitances were up to 300 F g−1 (2 mV s−1, potential window 0.3 to −0.6 V vs. Hg/HgO), but capacitance fell to relatively low values on cycling. In lithium cells a reversible first cycle capacity of 600 mA h g−1 was obtained, and this decayed to 340 mA h g−1 after 50 cycles. In sodium cells a reversible first cycle capacity of 156 mA h g−1 was obtained and was maintained well on cycling (127 mA h g−1 in the 50th cycle).

The production of thin mesoporous silica films with small (∼2–3 nm) pores oriented perpendicular to a titanium nitride growth surface is demonstrated using two methods. These are the growth from a Stöber silica solution with surfactant ordering at the surface of the electrode, and electrochemically assisted growth from an acidic sol achieved by polarisation of the electrode surface. The thickness, pore order and pore size that can be achieved with these two methods is contrasted. A number of methods to vary the pore size by using different surfactants and swelling agents are explored. The advantage of applying these growth methods on titanium nitride surfaces is that it provides access to a wider electrochemical window for nanowire growth and sensor applications with non-aqueous electrolytes whilst retaining good film growth and adhesion properties.

•Hexagonal δ1-MoN < 500 °C or cubic γ-Mo2N > 600 °C from a MoCl5-derived polymer.•High double layer capacitance in aqueous electrolyte with good cycling stability.•Phase behaviour of molybdenum nitride from a Mo(NMe2)4-derived polymer.•Good capacitance and stable cycling in relatively low surface area materials.•Redox peaks in the electrochemical data suggesting a largely redox-based process.Reactions of MoCl5 or Mo(NMe2)4 with ammonia result in cubic γ-Mo2N or hexagonal δ1-MoN depending on reaction time and temperature. At moderate temperatures the cubic product from Mo(NMe2)4 exhibits lattice distortions. Fairly high surface areas are observed in the porous particles of the chloride-derived materials and high capacitances of up to 275 F g−1 are observed when electrodes made from them are cycled in aqueous H2SO4 or K2SO4 electrolytes. The cyclic voltammograms suggest charge is largely stored in the electrochemical double layer at the surface of these materials. Amide-derived molybdenum nitrides have relatively low surface areas and smaller capacitances, but do exhibit strong redox features in their cyclic voltammograms, suggesting that redox capacitance is responsible for a significant proportion of the charge stored.

The phase changes that occur during discharge of an electrode comprised of LiFePO4, carbon, and PTFE binder have been studied in lithium half cells by using X-ray diffraction measurements in reflection geometry. Differences in the state of charge between the front and the back of LiFePO4 electrodes have been visualized. By modifying the X-ray incident angle the depth of penetration of the X-ray beam into the electrode was altered, allowing for the examination of any concentration gradients that were present within the electrode. At high rates of discharge the electrode side facing the current collector underwent limited lithium insertion while the electrode as a whole underwent greater than 50% of discharge. This behavior is consistent with depletion at high rate of the lithium content of the electrolyte contained in the electrode pores. Increases in the diffraction peak widths indicated a breakdown of crystallinity within the active material during cycling even during the relatively short duration of these experiments, which can also be linked to cycling at high rate.

Copper(I) nitride, produced by the ammonolysis of copper(II) pivalate at 250 °C, shows a competitive capacity and stable cycling behavior in sodium cells with a NaPF6/ethyl carbonate/diethyl carbonate electrolyte. Ex situ X-ray diffraction studies suggest that this material acts as a conversion electrode, with Cu3N reduced to copper metal, but that these reactions occur only at the surfaces of the particles. A higher capacity is observed in lithium cells, again with stable cycling behavior. Hydrolysis results in nanocrystalline CuO, which has a higher sodium cell capacity. However, this capacity gradually decays on cycling and, after 30 cycles, is similar to that observed with Cu3N.

Hf3N4 in nanocrystalline form is produced by solution phase reaction of Hf(NEtMe)4 with ammonia followed by low-temperature pyrolysis in ammonia. Understanding of phase behavior in these systems is important because early transition-metal nitrides with the metal in maximum oxidation state are potential visible light photocatalysts. A combination of synchrotron powder X-ray diffraction and pair distribution function studies has been used to show this phase to have a tetragonally distorted fluorite structure with 1/3 vacancies on the anion sites. Laser heating nanocrystalline Hf3N4 at 12 GPa and 1500 K in a diamond anvil cell results in its crystallization with the same structure type, an interesting example of prestructuring of the phase during preparation of the precursor compound. This metastable pathway could provide a route to other new polymorphs of metal nitrides and to nitrogen-rich phases where they do not currently exist. Importantly it leads to bulk formation of the material rather than surface conversion as often occurs in elemental combination reactions at high pressure. Laser heating at 2000 K at a higher pressure of 19 GPa results in a further new polymorph of Hf3N4 that adopts an anion deficient cottunite-type (orthorhombic) structure. The orthorhombic Hf3N4 phase is recoverable to ambient pressure and the tetragonal phase is at least partially recoverable.

Nickel nitride is synthesised by high temperature ammonolysis of nickel(II) hexamine and tris(ethylenediamine) salts. Its electrochemical characteristics are examined in half-cells vs. lithium and sodium. Samples with high surface area are found to have significant reversible charge storage capacity in sodium cells and hence to be a promising negative electrode material for sodium-ion batteries.

Reactions of Ta(NMe2)5 and n-propylamine are shown to be an effective system for sol-gel processing of Ta3N5. Ordered macroporous films of Ta3N5 on silica substrates have been prepared by infiltration of such a sol into close-packed sacrificial templates of cross-linked 500 nm polystyrene spheres followed by pyrolysis under ammonia to remove the template and crystallize the Ta3N5. Templates with long-range order were produced by controlled humidity evaporation. Pyrolysis of a sol-infiltrated template at 600 °C removes the polystyrene but does not crystallize Ta3N5, and X-ray diffraction shows nanocrystalline TaN plus amorphous material. Heating at 700 °C crystallizes Ta3N5 while retaining a high degree of pore ordering, whereas at 800 °C porous films with a complete loss of order are obtained.

Ammonolysis of solutions containing tetrakis(methylamido)silane and either pentakis(dimethylamido)tantalum(V) or tetrakis(dimethylamido)molybdenum(IV) results in gels that can be pyrolysed in ammonia to yield metal-containing amorphous silicon nitride compositions. The tantalum-containing samples remain amorphous to 1000 °C, though there is evidence of some metal clustering, whereas the molybdenum-containing samples crystallise molybdenum nitride nanorods at 1000 °C.

The formation of silicon nitride films using a sol–gel process and dip-coating is reported. The effect of a trifluoromethanesulfonic acid catalyst on condensation is investigated, and affects the behaviour of gels on heating. Smooth films can be obtained on silicon wafer substrates, and these can be built up using multiple dippings as the gelation process is irreversible. Firing at 1000 °C produces amorphous silicon nitride films, though these contain some carbon and hydrogen and are sensitive to surface oxidation.

One of the key points of interest in pyrochlore materials containing bismuth derives from the dielectric properties of some such materials that are linked to the displacements of the bismuth atoms from the ideal site. This study uses high pressure to probe the variations in, and causes of, these displacements. Under compression Bi2Ti2O7 does not undergo any phase changes, but Bi2Sn2O7 undergoes a similar series of changes to those observed during heating. The trigonal β-Bi2Sn2O7 structure is solved from high temperature powder neutron diffraction data and hence the sequence of phases observed in Bi2Sn2O7 is discussed for the first time. The variation in Bi displacements can be considered in terms of the frustration of the tetrahedral lattice that accommodates them. It can also be inferred that the main driver for Bi displacement is a deficiency in the bond valence sum of bismuth.

Supercritical chemical fluid deposition (SCFD) from [nBu2In(μ-EtBu2)2InnBu2] precursors (E = P or As) dissolved in a homogeneous hexane/CO2 fluid results in the growth of a thin film of InP or InAs with a thick mat of nanowires attached to the surface. InP films were well-adhered to the substrate and the nanowires can be removed without disrupting the film; however, the InAs films were poorly adhered, being dislodged simply by tapping the substrate. In many cases, the InP nanowires are single crystalline, and both the nanowires and the underlying films exhibit band-edge luminescence. Use of supercritical CHF3 as the delivery solvent also leads to good quality InP films and reduced carbon incorporation into the films. The single-source precursor system was responsible for significant amounts of carbon. This work shows that the SCFD method may be generally applicable to the growth of compound semiconductors, with important implications for growth of these materials in confined environments.

Transition metal chlorides are reacted with lithium amide or ammonia under solvothermal conditions in benzene at temperatures up to 550 °C. The products are metal nitrides with particle sizes of a few nm. VN, NbN, CrN, MoN and WN form with a cubic rocksalt-type structure, whilst Ta3N5 adopts the known orthorhombic structure. Products often contain carbon due to solvent decomposition, the carbon content is higher when ammonia is the nitrogen source, and varied to some extent from metal to metal. Analytical data shows nitrogen-deficient carbonitride compositions. Most samples crystallise with partially aggregated, regular crystallites. Some crystallise with a nanorod morphology and this was most pronounced in Ta3N5, which forms high aspect ratio, single crystal nanorods when synthesised with ammonia.

[{C7F15CO2}2AgAu(PPh3)]2 is obtained in good yield from the reaction of [C7F15CO2Ag] with [ClAuPPh3] in THF solution. The crystal structure shows a zig-zag Au–Ag–Ag–Au core with fluorocarboxylate ligands bridging the Au–Ag and Ag–Ag bonds and triphenylphosphine groups bound to Au. Electrodeposition from acetonitrile yields Au–Ag alloys. The deposited alloys are Ag rich and the composition varies with deposition potential.[{C7F15CO2}2AgAu(PPh3)]2 has a zig-zag Au–Ag–Ag–Au core and electrodeposits Au–Ag alloys.

Most current applications of nitride materials are based on films deposited from the vapour phase. However, a series of other potential uses of nitrides have been envisaged based on properties such as higher conductivity than oxides, hardness, inertness and catalytic or electrochemical activity. Many current applications use nanocrystalline nitrides and increasingly the size and shape dependent properties are of interest. This feature article reviews synthesis methods to make nanocrystalline and nanoparticulate nitride materials, plus it discusses the current applications and several potential ones.

Reactions of VCl4 with one mol equiv. of L–L (L–L = MeSe(CH2)2SeMe, MeSe(CH2)3SeMe, nBuSe(CH2)2SenBu) in anhydrous CH2Cl2 solution at room temperature give [VCl4(L–L)] as very moisture-sensitive dark purple solids. Using VCl4 and excess selenoether in gently refluxing CH2Cl2 leads to reduction to [VCl3(L–L)] (L–L as above and o-C6H4(CH2SeMe)2), while VCl4 reacts with excess SeMe2 at room temperature to give [VCl3(SeMe2)2]. All new complexes were characterised by microanalysis, IR and UV/visible spectroscopy and magnetic measurements. Reaction of [(Cp)2VCl2] with two mol equiv. of LiSetBu in anhydrous thf gives the VIV selenolate complex [(Cp)2V(SetBu)2] as a very moisture-sensitive brown solid. The new complexes have been investigated as possible reagents for deposition of vanadium selenide. While low pressure chemical vapour deposition (LPCVD) experiments showed that the diselenoether complexes were not sufficiently volatile for VSe2 deposition, [VCl3(SeMe2)2] gives very thin deposits of VSe2. LPCVD studies on [(Cp)2V(SetBu)2] at 600 °C produce thicker black films of VSe2. In all cases EDX measurements show that the films are Se deficient.

This review assesses the track record and prospects of non-oxide sol–gel and solvothermal routes to nitride materials for use in catalysis. There is a strongly developing body of synthesis methods that yield highly porous materials, some with engineered pore structures, nanocrystalline materials with high surface areas and anisotropic nanocrystals that have their surface area tuned to a particular crystal face. Most of the existing catalytic work on such solution-derived nitrides has focussed on utilising base properties due to surface bound amide and imide groups in silicon (imido)nitride compositions, but there are opportunities to extend these methods to other interesting nitride compositions.

Solvothermal reactions of TaCl5 with LiNH2 or Mg3N2 lead to amorphous products at temperatures up to 500 °C. Post-reaction annealing under nitrogen yields crystalline products with Ta3N5 (LiNH2) and TaN (Mg3N2) structures. When these reactions are carried out with LiNH2 in refluxing mesitylene, rods of Ta3N5-structured material are obtained. Using commercial LiNH2 these have dimensions of ∼5 × 200 nm but are contaminated with LiTaO3. With high purity LiNH2 a black oxide-free but carbon-containing material presents as rods of 20–50 nm length. Higher temperature reactions in an autoclave lead to isotropic nanocrystals of ca. 10 nm diameter of a nitrogen-deficient or carbide-substituted Ta3N5-type material. Carbon incorporation is attributed to solvent decomposition at the temperatures required for the reactions. The TaN derived from reactions with Mg3N2 consists of nanoparticles of 6–8 nm in diameter.

Monolithic gels have been produced by reaction of Si(NHMe)4 with ammonia in THF solution and converted to high surface area aerogels by critical point drying with ammonia saturated diethylamine solvent.

Solvothermal reactions of TaCl5 with LiNH2 in benzene result in nanocrystalline Ta3N5 at 500 or 550 °C. The ∼25 nm Ta3N5 particles have a band gap of 2.08−2.10 eV. The same reactions in mesitylene resulted in a higher crystallization temperature and large amounts of carbon incorporation due to solvent decomposition. Reactions of Ta(NMe2)5 with LiNH2 under the same conditions resulted in TaN. Rocksalt-type MN phases are obtained for Zr, Hf, or Nb when their chlorides (ZrCl4, HfCl4, or NbCl5) or dialkylamides (M(NEtMe)4, M = Zr, Hf) are reacted with LiNH2 under similar conditions. With the amides, there is some evidence for nitrogen-rich compositions (HfN>1), and carbon is incorporated into the products through pyrolysis of the dialkylamide groups.

A solution phase route to films of nanocrystalline titanium nitride and carbonitride is reported. Transamination of Ti(NMe2)4 with primary amines results in primary amide groups which can self condense forming alkylimide bridges. The use of controlled amounts of amine results in sols in tetrahydrofuran; these can be coated onto silica substrates via a dip coating method. Pyrolysis of these films under ammonia or nitrogen leads to TiN and Ti(C,N) respectively.

Solution phase reactions between tetrakisdimethylamidotitanium (Ti(NMe2)4) and ammonia yield precipitates with composition TiC0.5N1.1H2.3. Thermogravimetric analysis (TGA) indicates that decomposition of these precursor materials proceeds in two steps to yield rocksalt-structured TiN or Ti(C,N), depending upon the gas atmosphere. Heating to above 700 °C in NH3 yields nearly stoichiometric TiN. However, heating in N2 atmosphere leads to isostructural carbonitrides, approximately TiC0.2N0.8 in composition. The particle sizes of these materials range between 4–12 nm. Heating to a temperature that corresponds to the intermediate plateau in the TGA curve (450 °C) results in a black powder that is X-ray amorphous and is electrically conducting. The bulk chemical composition of this material is found to be TiC0.22N1.01H0.07, or Ti3(C0.17N0.78H0.05)3.96, close to Ti3(C,N)4. Previous workers have suggested that the intermediate compound was an amorphous form of Ti3N4. TEM investigation of the material indicates the presence of nanocrystalline regions <5 nm in dimension embedded in an amorphous matrix. Raman and IR reflectance data indicate some structural similarity with the rocksalt-structured TiN and Ti(C,N) phases, but with disorder and substantial vacancies or other defects. XAS indicates that the local structure of the amorphous solid is based on the rocksalt structure, but with a large proportion of vacancies on both the cation (Ti) and anion (C,N) sites. The first shell Ti coordination is approximately 4.5 and the second-shell coordination ∼5.5 compared with expected values of 6 and 12, respectively, for the ideal rocksalt structure. The material is thus approximately 50% less dense than known Tix(C,N)y crystalline phases.Amorphous and nanocrystalline titanium nitrides and carbonitrides with a very defect-rich rocksalt structure.

A series of samples of Ta3N5 (Cmcm  , a=3.89Å, b=10.22Å, c=10.28Å) have been produced by high-temperature ammonolysis of amorphous tantalum oxide under various temperature and heating time regimes. These were characterised by powder diffraction (X-ray and neutron), combustion microanalysis, thermogravimetric analysis and transmission electron microscopy. All samples were found to contain oxide in the 3-coordinate anion site and the b-axis length was sensitive to compositional variation. Samples heated for longer times at higher temperatures were anion deficient. The compositions of the samples have been related to their optical band gap UV–visible spectra. This work highlights the importance of careful compositional analysis when samples of nitride materials are produced for real-world applications.One crystallographic nitrogen site in Ta3N5 varies markedly in composition with preparation conditions, the effect of this is seen in the b-axis length and optical properties.

An unusual k = [½ ½ ½] magnetic structure is reported in Sr2FeO3F at low temperature. The magnetic structures of LaBaFeO4, LaSrFeO4, LaCaFeO4, Ca2FeO3Cl, Ca2FeO3Br, Sr2FeO3F, Sr2FeO3Cl, Sr2FeO3Br and solid solutions in the series Sr2Fe1−xCoxO3Cl have been examined and only Sr2FeO3F exhibits this structure. The La2CuO4-type spin arrangement dominates in all these phases, with antiferromagnetic ordering in the xy planes. The k = [½ ½ ½] magnetic phase in Sr2FeO3F occurs below 100 K and requires La2CuO4-type stacking across the SrO layers and La2NiO4-type stacking across the SrF layers. It has been possible to show that this phase grows in domains which have purely La2CuO4-type stacking at higher temperatures. In the Sr2Fe1−xCoxO3Cl series the La2CuO4-type magnetic structure in Sr2FeO3Cl and the La2NiO4-type structure in Sr2CoO3Cl coexist and structural and magnetic behaviour indicate phase segregation into cobalt- and iron-rich regions.

Bi2Ti2O7 has been synthesized using a co-precipitation route from H2O2/NH3(aq) solutions of titanium with aqueous bismuth nitrate. The stoichiometric material crystallizes into a pale yellow cubic pyrochlore phase. A powder X-ray diffraction study showed this crystallization to be very temperature sensitive, the pure phase can only be obtained within a few degrees of 470°C. Time-of-flight powder neutron diffraction studies of Bi2Ti2O7 (Space group , a=10.37949(4) Å at ambient temperature, Z=8, Rp=3.95%, Rwp=4.75%) revealed positional disorder in the bismuth site and in the O′ oxide site both at ambient temperature and at 2 K.

η-Mn3N2 is produced using solvothermal reactions between MnCl2 and LiNH2 in benzene. At 350 °C nanocapsule structures are obtained, whereas at higher temperatures samples consisted of isotropic nanoparticulates. In aqueous KOH solution capacitances were up to 300 F g−1 (2 mV s−1, potential window 0.3 to −0.6 V vs. Hg/HgO), but capacitance fell to relatively low values on cycling. In lithium cells a reversible first cycle capacity of 600 mA h g−1 was obtained, and this decayed to 340 mA h g−1 after 50 cycles. In sodium cells a reversible first cycle capacity of 156 mA h g−1 was obtained and was maintained well on cycling (127 mA h g−1 in the 50th cycle).

The production of thin mesoporous silica films with small (∼2–3 nm) pores oriented perpendicular to a titanium nitride growth surface is demonstrated using two methods. These are the growth from a Stöber silica solution with surfactant ordering at the surface of the electrode, and electrochemically assisted growth from an acidic sol achieved by polarisation of the electrode surface. The thickness, pore order and pore size that can be achieved with these two methods is contrasted. A number of methods to vary the pore size by using different surfactants and swelling agents are explored. The advantage of applying these growth methods on titanium nitride surfaces is that it provides access to a wider electrochemical window for nanowire growth and sensor applications with non-aqueous electrolytes whilst retaining good film growth and adhesion properties.

The co-ammonolysis of V(NMe2)4 and Si(NHMe)4 with ammonia in THF and in the presence of ammonium triflate ([NH4][CF3SO3]) leads to the formation of monolithic gels. Pyrolysing these gels produces mesoporous composite materials containing nanocrystalline VN and amorphous silicon imidonitride. Elemental mapping indicated a thorough distribution of VN with no evidence of large cluster segregation. Whilst not active for ammonia synthesis, the silicon nitride based materials were found to possess activity for the COx-free production of H2 from methane, which makes them candidates for applications in which the presence of low levels of CO in H2 feedstreams is detrimental.

Reactions of Si(NHMe)4 with ammonia are effectively catalysed by small ammonium triflate concentrations, and can be used to produce free-standing silicon imide gels. Firing at various temperatures produces amorphous or partially crystallised silicon imidonitride/nitride samples with high surface areas and low oxygen contents. The crystalline phase is entirely α-Si3N4 and structural similarities are observed between the amorphous and crystallised materials.

Monolithic gels have been produced by reaction of Si(NHMe)4 with ammonia in THF solution and converted to high surface area aerogels by critical point drying with ammonia saturated diethylamine solvent.

A solution phase route to films of nanocrystalline titanium nitride and carbonitride is reported. Transamination of Ti(NMe2)4 with primary amines results in primary amide groups which can self condense forming alkylimide bridges. The use of controlled amounts of amine results in sols in tetrahydrofuran; these can be coated onto silica substrates via a dip coating method. Pyrolysis of these films under ammonia or nitrogen leads to TiN and Ti(C,N) respectively.

Most current applications of nitride materials are based on films deposited from the vapour phase. However, a series of other potential uses of nitrides have been envisaged based on properties such as higher conductivity than oxides, hardness, inertness and catalytic or electrochemical activity. Many current applications use nanocrystalline nitrides and increasingly the size and shape dependent properties are of interest. This feature article reviews synthesis methods to make nanocrystalline and nanoparticulate nitride materials, plus it discusses the current applications and several potential ones.

Solvothermal reactions of TaCl5 with LiNH2 or Mg3N2 lead to amorphous products at temperatures up to 500 °C. Post-reaction annealing under nitrogen yields crystalline products with Ta3N5 (LiNH2) and TaN (Mg3N2) structures. When these reactions are carried out with LiNH2 in refluxing mesitylene, rods of Ta3N5-structured material are obtained. Using commercial LiNH2 these have dimensions of ∼5 × 200 nm but are contaminated with LiTaO3. With high purity LiNH2 a black oxide-free but carbon-containing material presents as rods of 20–50 nm length. Higher temperature reactions in an autoclave lead to isotropic nanocrystals of ca. 10 nm diameter of a nitrogen-deficient or carbide-substituted Ta3N5-type material. Carbon incorporation is attributed to solvent decomposition at the temperatures required for the reactions. The TaN derived from reactions with Mg3N2 consists of nanoparticles of 6–8 nm in diameter.

Nickel nitride is synthesised by high temperature ammonolysis of nickel(II) hexamine and tris(ethylenediamine) salts. Its electrochemical characteristics are examined in half-cells vs. lithium and sodium. Samples with high surface area are found to have significant reversible charge storage capacity in sodium cells and hence to be a promising negative electrode material for sodium-ion batteries.

Transition metal chlorides are reacted with lithium amide or ammonia under solvothermal conditions in benzene at temperatures up to 550 °C. The products are metal nitrides with particle sizes of a few nm. VN, NbN, CrN, MoN and WN form with a cubic rocksalt-type structure, whilst Ta3N5 adopts the known orthorhombic structure. Products often contain carbon due to solvent decomposition, the carbon content is higher when ammonia is the nitrogen source, and varied to some extent from metal to metal. Analytical data shows nitrogen-deficient carbonitride compositions. Most samples crystallise with partially aggregated, regular crystallites. Some crystallise with a nanorod morphology and this was most pronounced in Ta3N5, which forms high aspect ratio, single crystal nanorods when synthesised with ammonia.

Bulk nanocrystalline Sn3N4 powders were synthesised by a two step ammonolysis process followed by washing with dilute acid. Their performance as Li-ion and Na-ion battery negative electrodes was assessed by galvanostatic cycling in half cells vs. the metal, giving good performance in both cases and remarkable stability in the sodium cells. The effect of carboxymethyl cellulose and sodium alginate binders was examined and the latter found to give superior performance. Capacity and stability were also enhanced via the use of a fluoroethylene carbonate electrolyte additive.

The formation of silicon nitride films using a sol–gel process and dip-coating is reported. The effect of a trifluoromethanesulfonic acid catalyst on condensation is investigated, and affects the behaviour of gels on heating. Smooth films can be obtained on silicon wafer substrates, and these can be built up using multiple dippings as the gelation process is irreversible. Firing at 1000 °C produces amorphous silicon nitride films, though these contain some carbon and hydrogen and are sensitive to surface oxidation.

New approaches to produce nanocrystalline TiN materials with high conductivity and their application as conductive coatings on battery materials have been developed. Sol–gel synthesis routes using tetrakis(dimethylamido)titanium(IV) and a propylamine or ammonia cross linking agent, followed by thermal treatment under NH3 or H2 + N2, were found to produce TiN powders of small crystallite size (<10 nm), with good conductivity in selected cases. The most promising synthesis conditions were used to produce even TiN coatings on LiFePO4 particles, and the resulting materials exhibited significantly improved electrochemical performance relative to uncoated LiFePO4, in terms of higher specific capacity, cycle stability and rate capability. The material with the optimum 10 wt% TiN content exhibited a discharge capacity of 159 mA h g−1, that is ∼93% of the theoretical capacity, when charge/discharge rates of 0.1C were applied. The results demonstrate the suitability of this new route to produce TiN coatings, which could also be applied to high voltage materials or for materials to be operated at high temperatures, where corrosion or degradation of other coating materials (e.g. carbon) would be problematic.

The effects of surface oxidation on the capacitance of titanium nitride electrode surfaces, produced by reaction of titanium foils with ammonia, are examined. Thermal oxidation and electrochemical oxidation both increase the amount of redox active oxide at the surface, but electrochemical oxidation is found to be more successful in increasing the capacitance.