**3. Synthetic procedure for metal nitride based catalyst**

The most widely used method for the preparation of nanostructured metal nitrides is via heating the corresponding oxides and hydroxides in the presence of different nitrogen sources such as NH3, N2, NH2NH2, urea, and dicyanamide [7–9]. For the synthesis of binary metal nitrides such as Mo2N, and FexN etc., NH3 is frequently used as the nitrogen source with heat treatment between 400 and 1000°C. The heating rate, gas flow rate and reaction time typically controls the composition of resulting catalyst. For example, heating molybdenum oxide with NH3 at a flow rate of 100 mL/min at 700°C for 2 h produced Mo2N [10]. In this procedure, the metal salt along with a polymer (PVP) was dispersed in DMF to form an uniform coating on the substrate before heat treatment. As per the SEM and TEM data, the coating of nitride catalyst on the Ni foam was uniform. The procedure was effective to produce crystalline Fe and Ni nitrides. Importantly, optimization in reaction condition is necessary to control the nanostructure of the catalyst, that is important for improved catalytic activity. For example, liquid exfoliation and templating are used to prepare ultrathin 2D nanosheets [11]. Liquid exfoliation is a relatively simpler technique, in which the bulk metal nitride synthesized is added to a high polar solvent such NMP, and the mixture is ultrasonicated to exfoliate the catalyst to nanosheets. For example, a solvent exfoliated atomically thin MoN nanosheet demonstrated improved HER activity compared to that of the as prepared catalyst [12].

Similarly, N2 is another nitrogen source used for synthesis of metal nitride via calcination process and plasma treatment [13]. The ternary metal nitrides are prepared from the ternary metal oxides via treatment with a nitrogen source [14]. In many of the reports, the metal oxides are utilized as the precursor for the corresponding nitrides and the resulting nitride catalysts have mimicked the structure of the oxide precursor. For example, the electrocatalytic Ni-Mo nitride nanotubes were synthesized by heating NiMoO4 nanotubes under NH3 atmosphere at 550°C. The first step involved the synthesis of Ni-Mo bimetallic oxide nanorods under heat treatment in presence of air. Subsequently, the NiMoO4 was converted to NiMoN in presence of NH3. Metal hydroxides are another option for being used as the precursor in metal nitride synthesis. The advantage with the hydroxides is that the metal hydroxides require lower temperature compared to that of the oxide precursors for conversion. For example, Ni3FeN was successfully synthesized by nitridation reaction with the corresponding double hydroxide precursors in presence of NH3. The Ni3Fe layered double hydroxides were heated at 400°C to prepare the Ni3FeN nanoparticles. The SEM images supported a change in surface morphology after ammonia treatment and the TEM displayed lattice spacing of 0.217 nm consistent with the 111 plane of the catalyst supporting the synthesis of Ni3FeN nanocatalyst.

Some specific procedures include 1-methylimidazole (1-MD)-fixation" strategy to support nano/micro-sized nitrides on carbon materials [15]. Covalent organic frameworks (COFs) were utilized to support Ni3N nanoparticles through a solid state synthesis to prepare COF-Ni3N composite [16]. This approach has two advantages, the first one is, this procedure allows nanoscale confinement of the metal nitride catalyst which is otherwise difficult to achieve. The second advantage is a π-conjugated support further aids the conductivity of nitride catalyst, a property desirable for OER. Though, volatile source rich with nitrogen is treated at high temperature with metal oxides with a programmed temperature ramp is an established procedure to generate the corresponding metal nitrides, plasma treatment is also used in literature to convert Ni(OH)2 to corresponding nitride [17]. This procedure allows the synthesis of catalyst at a relatively low temperature of 250°C using N2-H2 plasma as the source. In the above work, Li et al. utilized the plasma treatment to synthesize Ni3N nanocatalysts of 30 nm size. The XPS and XRD spectra revealed quantitative conversion of the hydroxides to nitrides in a fairly short duration of 1 h. Nitrates [Ni(NO3)2] can also be utilized as a source to prepare the nitride catalyst as reported in literature. For example, recently a mixture of Ni(NO3)2·6H2O and (NH4)6Mo7O24·4H2O is treated with NH3 as the "N" source to synthesize ammonium nickel molybdate [18]. The XRD data of the resulting catalyst NiMo4N5 revealed 111 plane accountable to the FCC lattice of Ni, whereas the planes for crystalline phase related to Mo was absent suggesting uniform distribution of the two metals. Derivation of catalyst from a rigid MOF precursor is recently utilized to develop catalyst with controlled nanostructure. For example, Co-Mo2N was synthesized by heat treatment of ZIF-67/Mo-MOFs-2 at 500°C for 3 h. The resulting catalyst mimicked the shape of MOF precursor [19]. The XPS data showed the peaks assigned to Mo and N present in Mo-N linkage supporting the synthesis of catalyst.

In most of the synthesis, it has been observed that the shape of the precursor material is retained after nitridation in presence of NH3 at high temperature. In some cases, the nitridation also induces porosity in the samples. For example, Co3FeNx porous nanowires were synthesized from Co3Fe double hydroxide nanofibers using NH3 as the nitrogen source at 623 K [20]. The resulting catalysts were effective against both OER and HER and current density values of 20 mA/cm2 and 10 mA/cm2 were achievable at 222 mV and 23 mV overpotentials for OER and HER respectively. Sometimes color can be used as an indicator of the surface coating and functional group conversion. For example, NiFe(OH)x was grown in-situ on bare Ni

#### *Recent Trends in Development of Metal Nitride Nanocatalysts for Water Electrolysis Application DOI: http://dx.doi.org/10.5772/intechopen.95748*

foam and the color changed from gray to brown. Further the sample was calcined in presence of NH3 to convert the hydroxides to corresponding nitrides. The color of the surface changed to black suggesting the functional group conversion [21]. In this case, the Ni foam not only serves as a substrate but also acts as a precursor during the hydroxide growth process via redox itching of Fe precursor. A distinct shift in the Ni binding energy from 853 to 855.7 eV was observed in the XPS spectra suggesting conversion of hydroxides (Ni-OH) to the corresponding nitrides (Ni-N). Recently, a non-stoichiometric NiNx was directly synthesized from Ni foam by plasma treatment. A piece of Ni foam was exposed to N2 plasma initiated by microwave for in-situ growth of nickel nitride nanostructures. The SEM and TEM images displayed change in surface morphology after exposure to plasma. The XPS data displayed a peak at 398 eV accountable to the "N" of Ni-N linkage supporting the formation of nanocatalyst on surface of Ni foam [22].

One of the problems that researchers have frequently faced is the weak bonding between the substrate and nanocatalyst. To address this, a strategy was recently utilized via use of inks. Importantly, the strategy worked with a number of metal catalysts and the method was relatively convenient. In this approach, the metal salts were dissolved in an organic solvent to prepare the ink. Subsequently, the substrate was dipped in the ink to soak the salt on the surface. The sample was then heated at 500°C under NH3 for nitridation. The strategy allowed uniform distribution of the catalyst on surface and the durability of the system was adequate, which will be discussed in the subsequent section. Another challenging aspect of catalyst synthesis is to have a nanocomposite coating of two elements on the metal surface. For example, polymerization-pyrolysis-evaporation strategy was utilized to synthesize nitride doped porous carbon anchored on atomically dispersed FeN4. The synthetic strategy involved two steps; in the first step bimetallic Zn/Fe polyphthalocyanine was synthesized and in the second step the above polymer was pyrolyzed to produce the final catalyst. The HAADF-STEM data revealed uniform distribution of C and N on Fe surface. The catalytic sites were further demonstrated by 57Fe Mössbauer transmission spectra [23]. Sputtering technique is also used in literature to deposit metal nitrides on base material such as carbon black and Ni foam. The temperature of the sputtering chamber controlled the stoichiometry of the resulting catalyst. For example, at 90°C Ni4N was synthesized, whereas at 180°C Ni3N formed on the surface of carbon cloth at 18 mTorr. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy revealed the bands for both Ni4N and Ni3N supporting the stoichiometric control in the above synthesis. Some other specific modes of synthesis of metal nitrides is by utilizing a reactive ammonia species to decrease the reaction temperature. For example, the manganese and iron oxide were treated with NaNH2 to synthesize the corresponding metal nitrides at 240°C [24].
