The SHS method (or named as combustion synthesis) is used to describe a process in which initial reagents spontaneously transform into products due to the exothermic heat of reaction, once ignited (Subrahmanyam and Vijayakumar, 1992). Due to the fast-release of heat from the exothermic reaction, the time to meet the combustion temperature is short and the conversion rate to products is high (Merzhanov, 2004). For example, a divalent europium-doped nitride phosphors Ca_1-x Eu _x AlSiN₃ (x = 0–0.2) were prepared by heating Ca_1-x Eu _x AlSi alloy powders under continuous N₂ flow at 1050°C (Piao et al., 2007). Wang’ groups reported the selective synthesis of four types of BN nanotubes by using a porous precursor through the SHS method (Wang et al., 2011). SHS is a practical method for large-scale preparation of nitride powders, but it is difficult to achieve the controllable preparation of nitride nanomaterials.

Solid state metathesis has been widely used to produce bulk or nano-sized metal nitrides. The core design of SSM is the replacement reactions between metal halides and alkali (or alkaline earth) nitrides (E.g., Li₃N and NaN₃), yielding corresponding metal nitrides (Wiley and Kaner, 1992; Zhu et al., 2011; Karaballi et al., 2019), as described in Eq. 1. When producing low thermal stability nitrides (E.g., GaN, InN, Zn₃N₂, Ta₃N₅), the synthesis temperature need to be controlled (Gillan and Kaner, 1994). MC l x +x /3 L i 3 N → MN + x LiCl + (x -3)/6 N 2 (1) A high-pressure solid-state metathesis (HPSSM) reaction was reported between boron nitride (BN) and ternary metal oxide A _x M _y O _z (A = alkaline or alkaline-earth metal and M = main group or transition metal), as presented in Eq. 2. Then, a series of well crystallized metal nitrides (Fe₃N, Re₃N, VN, GaN, CrN, and W _x N) were produced (Lei et al., 2013). In subsequent reports, high pressure was confirmed in SSM to benefit the contact diffusion and ion-exchange of the solid reactions (Lei and Zhang, 2018). Well-crystallized W _x N nanomaterials were synthesized via the HPSSM, where solid-state ion exchanges and a nitrogen pressure of 5 GPa were involved (Wang et al., 2012). According to DFT calculations of thermodynamic investigations, high pressure could reduce the reaction enthalpy △H. A x M y O z + ( 2z-a x ) /3 BN → y /w M w N + 0.5 [ ( 2z-a x/3- y/w ) ] N 2 + A x B (2z-a x)/3 O z (2) Moreover, kinetically controlled SSM reactions provide an avenue toward the synthesis of nitride materials (Rognerud et al., 2019). For example, Mn₃N₂ micro-/nano-crystals were prepared by a kinetically controlled SSM reactions. The further developments of SSM for inorganic nitride nanomaterials would be focused on lower pressure and the kinetic control, where slightly mild and mild kinetics are helpful for the controllable preparation of nanostructures in a exothermic reaction.

Chemistry Vapor Deposition (CVD) method is widely favored by chemists, physicists, and material scientists, by which nitride nanomaterials (mainly thin layers, geometric nano-sheets, nano-column, or nano-flowers, etc.) were fabricated by heating volatile metal halides under flow gas (E.g., N₂+H₂ or NH₃+H₂, etc.) at high temperatures (above 850°C) (Fix et al., 1993; Zerr et al., 2003), as illustrated in Eq. 3. The replacement of metal chlorides by metal-organic salts could reduce the reaction temperatures to 200–450°C (Fix et al., 1996). MC l x + N 2 /N H 3 + H 2 →MN + x HCl (3) Recent reports focused on the kinetic control and the nano-structural evolutions during the CVD processes. For CVD method, the substrate places an important role for the growth of nitride crystals in terms of the phase and the crystal morphology. In a vapor-liquid-solid case, high-quality GaN nanowires were synthesized via metal-initiated metalorganic CVD (Kuykendall et al., 2003). For another example, high-quality h-BN mono-layer was prepared by using a folded Cu-foil enclosure (Song et al., 2015). Smooth Cu surface effectively reduced the precursor feeding rate, leading to a drastic decrease in the nucleation density. Density functional theory (DFT) calculations demonstrated the crystallographic orientation of the Cu substrate strongly correlated with the orientation of h-BN. Similar phenomena appear in the silicon-assisted growth of centimeter-scale MoSi₂N₄ monolayers (Hong et al., 2020). It can be predicted that more and more nitrides of the 2D thin-layered family will be synthesized by CVD.

Solvothermal reactions are carried out in organic solvents within an autoclave under a certain temperature and autogenous pressure, where the solvents dissipate the enthalpy of the exothermic metathetical reactions and reduce the diffusion barriers between reactants (Demazeau, 2007). Thus, the solvothermal reactions could be occurred at a relative low temperature and provide better control over the size and morphology of the particles (compared to solid state reactions like SHS or SSM).

In early reports, solvothermal synthesis of nitride nanomaterials mainly explored the regulation of metastable phases, different solvents, and nano morphologies. For example, the reaction between GaCl₃ and Li₃N in benzene could yield hexagonal/rocksalt mixed-phase GaN nano-crystallites at 280°C (Xie et al., 1996). InN could be synthesized by reactions between InCl₃ and Li₃N in xylene at 250°C (Demazeau et al., 2002). Later, CrN, ZrN, TiN and NbN were obtained from reactions of chlorides with NaN₃ in benzene (Mazumder and Hector, 2009). Similarly, solvothermal methods have been applied to synthesize group 5, 6 metal nitrides in different organic solvents (Mazumder et al., 2008; Bugaris and Ibers, 2010). In recent years, many nitride nanomaterials were fabricated and studied for various renewable energy applications. A Cu₃N nanocubes were fabricated in a mixed solvent of ODA and OAm, which exhibited room-temperature ferromagnetism and excellent electrocatalytic activity for ORR and nitrobenzene reduction (Xi et al., 2014). Oxygen-doped g-C₃N₄ hollow nanospheres were synthesized from acetonitrile solvothermal method at 180°C, which showed remarkably photocatalytic activity for environment pollutant purification and splitting water for HER (Wang et al., 2017). Compared to other methods, solvothermal synthesis is energy-efficient and easy-control. Besides, the reaction is carried out in a closed system, which can effectively prevent the oxidation and uneven growth of nanocrystals, then becoming popular for the synthesis of nitride nanostructures.

Besides four traditional synthetic methods, we summarized some other novel methods for inorganic nitride nanomaterials. Generally, thermodynamics driven nanocrystals are easily obtained from additive assisted synthesis; ammonolysis, molecular precursors, sol-gel method and salt-melt synthesis are based on the precursor design with similar principles; microwave synthesis uses the high energy of microwaves for a quickly synthesis; spin-steaming, acid-etching, liquid-exploration, MOFs-assisted synthesis are designed to prepare nitrides with certain characteristic nanostructures (Figure 2).

Inspired by SSM and solvothermal reactions, the key to efficient synthesis of nitride crystals is to select thermodynamically spontaneous reactions and raw materials with high activity. In our previous works, a series of nitride (E.g., TiN, BN, AlN, MgSiN₂, VN) nanocrystals were prepared from metal oxides with the assistant pre-reactions of metallic Mg (or Na) with H₂O (or S) at a low temperature. In detail, TiN, ZrN, BN, and AlN were prepared by using the corresponding elements (Ti, Zr, B, Al), NaN₃ and sulfur as starting materials at 250°C (Yang et al., 2010). The exothermic reaction of additives (Na and S) was taken to prepare TiN, BN, AlN, MgSiN₂, VN nanocrystals, and the raw materials were metal oxides and NaN₃. Due to the additives, the reaction temperature was lowered at 150°C (Chen et al., 2012). And a metal-hydrolysis-assisted synthesis (MHAS) strategy was used to synthesis nitrides (VN, AlN, CrN, MgSiN₂ etc.) from related metal oxides in a low temperature range (120–180°C) (Zhou et al., 2017). Additive-assisted synthesis is very efficient, but the controllability of nanostructures needs further optimization.

Temperature programmed ammonolysis was developed, which involves heating the metal oxide in flowing NH₃ and slowly raising the temperature (Hausler and Schnick, 2018). Crystallographic orientations, particle size, and geometry are retained from the reactant solid. For example, S. Imran U. reported an ammonolysis reaction of MoCl₅ and [Mo (NMe₂)₄] to form two different polymeric precursors, where the cubic Mo₂N from Mo(NMe₂)₄ exhibited lattice distortions and significant redox properties as the supercapacitor electrode, and hexagonal Mo₂N (observed from the MoCl₅) with higher surface areas a had a high capacitance of 275 F g⁻¹ (Shah et al., 2014). Also, binary Ni₂Mo₃N nanorods were synthesized similarly, which showed excellent electrochemical performance for supercapacitors with a specific capacity of 264 F g⁻¹ at 0.5 A g⁻¹ and 81.4% capacity retention after 1000 cycles (Kumar et al., 2020). Moreover, through this Ammonolysis method, ultrathin 2D GaN (∼2 nm thick) and InN (∼1.3 nm thick) were synthesized from liquid metal (Syed et al., 2019).

In this way, nitrogenous precursors or intermediate products were firstly synthesized, then the precursors/intermediates would follow by a low-temperature decomposition and a high-temperature calcination, through which the size and morphology of the nanocrystals can be controlled by adjusting the temperature and the heating time. For example, nitride (ZrN, TiN, NbN) nanomaterials are pyrolyzed by reactions of Zr⁴⁺, Ti⁴⁺ and Nb⁵⁺ dialkylamines with liquid ammonia to produce insoluble precipitates (Brown and Maya, 1988). Also, group 13 nitrides could be obtained by refluxing urea derivatized AlCl₃, InCl₃ and GaCl₃ precursors in TOA, then hexagonal GaN and cubic AlN, InN are generated (Sardar et al., 2005). It is worth learning that Lee used a urea precursor to prepare mesoporous carbon nitride nanostructures with silica nanospheres as a hard template (Lee et al., 2012).

Sol-gel methods are wildly used for producing nitride nanomaterials, such as nano-powders or nanoporous structures (Giordano and Antonietti, 2011). During the whole process, ions-organics molecular precursors are formed in solution, then heated the reacted to yield a gel, and finally fired to the target nitrides. For example, TiN nanoparticles (NPs) were coated on carbon fibers via a sol-gel method based on self-condensation of titanium alkyl amide species. In this work, the capacity of the coated fibers was increased with significant redox capacitance contributions (Zhang et al., 2018). In another example, ZrN NPs have been prepared by heating the ZrCl₄-Urea sol-gel under an argon flow, which showed better ORR activity than commercial Pt/C catalysts in an alkaline environment (Yuan et al., 2020). In general, the size, morphology, and porosity of the sol-gel prepared nitrides can be controlled by adjusting the organic content, reaction conditions and templates or supports.

Salt-melt synthesis (SMS) is a special synthesis method in which the reaction happen in the liquid molten salts and products will be obtained by washing the cooled salts when the reaction is completed. There are two main advantages of SMS. For the first, SMS can lower the synthesis temperature and shorten the reaction time. For example, by using the SMS, the size and morphology of the nitride nanoparticles can be well controlled even under lower synthesis temperature (Liu et al., 2013). And secondly, SMS is a convenient and efficient way to fabricate 2D films or 3D porous nanostructures. For example, 3D porous N-doped graphene (HNG) was synthesized by SMS method, which display super bifunctional catalytic activity toward both ORR and OER (Cui et al., 2018). Also, a family of 2D layered transition metal nitrides (TMNs, such as MoN_1.2, WN_1.5, and Mo_0.7W_0.3N_1.2 etc.) could be produced under an atmospheric pressure and showed superior performance in HER (Jin et al., 2020). Lastly, the choice of salts is important in SMS, especially for the large-scale (over decagram) synthesis with High purity requirements.

Microwave synthesis is a modern synthesis technique under the condition of microwave, which has advantages of rapid heating, homogeneity, and selectivity (Tompsett et al., 2006). Since microwaves can penetrate deep into the substance, it only takes one-tenth to one-hundredth of the time of conventional methods to complete the entire heating process. The size of the nitride products is small and its distribution is uniform. For example, Graphitic carbon nitride (g-C₃N₄) sub-microspheres were prepared via a facile microwave synthesis through polymerization reaction between C₃N₃Cl₃ and NaN₃ in acetonitrile solvent (Dai et al., 2013). Guo et al. produced a g-C₃N₄ material after optimizing the microwave reaction time which can effectively generate H₂ under visible-light irradiation (Guo et al., 2016). In this work, the highest H₂ evolution rate achieved was 40.5 mmol h⁻¹, which was two times higher than that of g-C₃N₄ products prepared by other methods.

In addition to the above methods, two novel synthesis methods of nitride materials have been developed recently. One is acid-etching, E.g., multilayered Ti₂N sheets were achieved by the selective etching of ternary layered Ti₂AlN with a mixture of potassium fluoride and hydrochloric acid, which showed excellent SERs efficiency (Soundiraraju and George, 2017). Another method named as liquid exfoliation, E.g., atomically-thin MoN nanosheets were obtained by liquid exfoliation of MoN precursors in N methyl-pyrrolidone (NMP) after ultrasound treatment for about 8 h, which had wonderful HER performance (Xie et al., 2014). It is still quite difficult to prepare high-quality nitrides by traditional methods, because the obtained nitrides always contain a certain number of nitrogen vacancies, and the miscibility of products is common. These problems are the same important as the control of nanostructures. In addition, it is undeniable that the high-pressure effect can effectively improve the high temperature thermal stability of nitrides and inhibit the dissipation of nitrogen atoms. In conclusion, the selection of suitable precursors and substrates, and the control of high-pressure reaction path are still the keys for the synthesis of nitride nanomaterials with high-purity.

Nanostructured nitrides with a large surface area and pore volume are much expected because the confined-space effects inside the pore-structures would improve their chemical and physical performance to meet the demands of electrode/catalysis materials for renewable energy (Wan et al., 2006) As we known, the soft-templating approach is effective and most wildly used to fabricate ordered micro-/meso-porous metal oxides, metal sulfides, metal phosphates, silicates, and zeolites. However, it is difficult to apply soft-templating approach for the preparation of porous metal nitrides due to the lack of proper raw materials for the sol-gel or molecular precursor processes (Yang and Huang, 2005). Present studies mainly adopted the hard-templating method for the development of nitride porous nanostructures (Wan and Zhao, 2007). For example, GaN, VN, and TiN nanoparticles have been synthesized inside the confinement space of porous templet materials (E.g., ordered mesoporous: MCM-41 and SBA-15; disordered mesoporous C _x N), and nitride products with porous composites or isolated nanoparticles would be left after the removal of mesoporous hard templates (Yang and Zhao, 2005; Kogiso et al., 2007; Uedono et al., 2007). Also, self-supported ordered mesoporous metal nitrides (CrN) have been reported (Shi et al., 2008)

Nitrides with defects have shown many unique functions, where defects in nitrides are detrimental to the electrode or catalytic reactions by working as a recombination center of charge carriers (Van de Walle and Neugebauer, 2004; Wang et al., 2018; Takamura et al., 2020). Reasonable control of defects in nitrides is the key to the excellent performance of these nitrides. Li’ groups proposed a method to control the defects of carbon nitrates, which can effectively influence the defects of PHI-type CN nanocrystals (Li H. et al., 2020). Defects in h-BN are promising single-electron emitter materials with the advantages of high brightness and stability under room temperatures (Abidi et al., 2019). In the process of molten-salt polymerization with melamine as the precursor, KOH were added to realize the nitrogen defect regulation of carbon nitride. For the case of CVD, backside boron gettering technology is used to control the diffusion of B into Cu, thus controlling the defect growth in h-BN (Abidi et al., 2019). In addition, ion implantation can also be used to introduce the boron-vacancy, like ion beams (E.g. nitrogen, xenon, and argon) (Kianinia et al., 2020).

2D nitride materials (E.g., graphene, BN, MoN, and MXene, etc.) have received widespread attentions, because the electron migration and heat diffusion are confined to nano-sized two-dimensional planes, further leading to excellent electrical, optical, and mechanical properties (Naguib et al., 2014; Anasori et al., 2017). For example, 2D transition metal nitrides (MXene) have great application prospects in energy storage, like lithium and sodium ion batteries (Urbankowski et al., 2017). And 2D BN nanomaterials show highly stable structure and unique photoelectric characteristics as graphene, which are mainly prepared by exfoliation, CVD, and vapor phase epitaxy (Zhang et al., 2017). MoN nanosheets was prepared via a liquid exfoliation of annealed and can be used as effective HER electrocatalyst (Xie et al., 2014). Nitride-based MXene was prepared by etching the A atomic layer of the layered precursor MAX phase by HF. For example, 2D Ti₄N₃ is obtained by etching the Al atomic layer in Ti₄AlN₃ with molten fluoride under a condition of high temperatures (Soundiraraju and George, 2017). Many 2D nitride nanomaterials show great application potentials in energy storage (Xiao et al., 2019).

Ternary nitrides are usually formed by combining alkali metals or alkaline earth metals and transition metals with nitrogen elements. In such combinations, the cations can stabilize the high oxidation state N³⁻ anions (Balbarin et al., 1996). No more than 400 kinds of ternary metal nitrides have been synthesized (Sun et al., 2019). Compared with binary nitrides, ternary nitrides exhibit more extensive functions and adjustable plasmonic performance (Ran et al., 2019). As the ammonia generated by ammonolysis can damage the test instrument during characterization process. The common synthetic methods for synthesizing ternary nitrides are: HPSSM, solvothermal, ammonolysis, sol-gel, etc. For example, the Ti_0.5Cr_0.5N was synthesized by ammonizing a solid metal oxide (ZnMO, M = Cr, Ti). By using the HPHT method, a series of ternary materials (Mo_0.23Nb_0.77N_0.66 and Mo_0.66Nb_0.34N_0.60) were synthesized (Tareen et al., 2019). And a template reaction method was reported to prepare continuously adjustable metal composition ternary nitride nanoparticles (Al-Ga-N and Ti-V-N) by using mesoporous graphite carbon nitride as the nanoreactor and reactant (Fischer et al., 2008). It seems that sol-gel method is preferred for the synthesis of ternary nitrides.

Graphite phase carbon nitride (g-C₃N₄) with few nanometers can be named as g-C₃N₄ quantum dots (g-CNQDs), and they always exhibit enhanced photo-absorption and photo-response comparing with the bulk g-C₃N₄ materials due to the quantum confinement effects (Albolkany et al., 2020). Because of these properties, g-CNQDs showed wide range of applications that include photocatalysis, fluorescence probes, drug delivery, bioimaging, and security Ink, etc. (Zhang et al., 2014; Yin et al., 2017; Dong et al., 2018; Patir and Gogoi, 2018) Basically, g-CNQDs could be prepared utilizing two methods: the top-to-down way, or the bottom-to-up way. The top-to-down strategy might involve the fragmentation of the bulk g-C₃N₄ by chemical exfoliation, thermal treatment or ultrasonication into g-CNQDs (Song et al., 2016); and the bottom-to-up strategy is usually based on thermal treatment of nitrogen-rich organic precursors into well dispersed g-CNQDs (Barman and Sadhukhan, 2012; Tang et al., 2014). It can be expected that g-CNQDs will receive growing attentions in the field of functional nanomaterials.