Efficient Route to Synthesize Transition Metal Nitrides Nanoparticle by Arc Discharge

1. Introduction

Transition metal nitrides (TMNs) represented by titanium nitride (TiN), zirconium nitride (ZrN), hafnium nitride (HfN), etc., are a kind of mesenchymal compound because element nitrogen is inserted into transition metal lattices [1]. Due to the insertion of element nitrogen, the expansion of the metal lattice and the increase of the cell constant weaken the interaction force between metal atoms, resulting in the corresponding d-band shrinkage modification and the redistribution of state density near the Fermi level [2]. This modulation gives transition metal nitrides unique physical and chemical properties. Besides, nanosized TMNs have become scientifically and technologically important, in part because of their high surface area and smaller crystallite sizes, which led to changes in the electronic structure, thereby resulting in new interesting optical and electronic properties [3,4]

Researchers have lately been successful in producing TMNs nanoparticles (NPs) utilizing several methods. These approaches can be divided into physical methods, including laser ablation [5], arc discharge [6], and laser plasma sputtering [7], and chemical methods, consisting of direct nitride of transition metals or transition metal oxides [8], ammonia pyrolysis of transition metal chlorides [9], solvothermal [10], and thermal decomposition of polymer precursors [11]. As to these chemical methods, supreme-quality TMNs NPs could be synthesized when the raw materials are properly chosen, and meanwhile, synthesis processes are easy to control [12]. Nevertheless, the synthesis process usually involves multi-step chemical reactions and requires complicated manipulation, including catalysts, toxic stabilizers, surfactants, or reducing agents in most cases. In addition, in order to improve particle purity and crystallinity, post-treatments consisting of separation, washing, heating, or annealing are required, which are time-consuming and laborious. As a kind of physical method for preparing TMNs NPs, arc discharge synthesis has been proven to be a promising route to efficient production of TMNs NPs because plasma can significantly accelerate reaction rates and even motivate the chemistry that is hardly realized by conventional routes [13,14]. From a practical perspective, arc discharge synthesis offers a low-cost and environmentally friendly alternative and has been demonstrated to be suitable for scaling up NPs throughput. The production rate could linearly scale with reasonable adjustment of synthesis parameters, while the primary particle size remains on the nanoscale [15].

The arc discharge process of synthesizing TMNs NPs is usually optimized by adjusting experimental parameters such as nitrogen pressure and arc current to provide different plasma parameters and spatial distribution, as well as nucleation and growth in the space and time domains. For example, Fu et al. [16] reported that titanium nitride (TiN) NPs were synthesized by direct current (DC) arc discharge under mixed N₂ and Ar at atmospheric pressure and different arc currents. With increasing nitrogen pressure and arc current, the production rate of TiN NPs can be enhanced. Moreover, the crystallite size of TiN NPs increases by almost 20 nm with increasing N₂ pressure, while the increase in arc current has a limited impact on the primary particle size. The reason for the increased productivity may be the enhancement in power input as nitrogen pressure and arc current increase, which causes more material evaporation and in turn leads to an enhancement of the particle production rate. However, Bendavid et al. [17] found that although increasing the arc current would increase the amount of material evaporated, larger TiN NPs were produced at higher arc currents when TiN NPs were synthesized by the arc plasma to elucidate the effect of arc current on TiN NPs production. Besides, Naddaf et al. [18] demonstrated that the average texturing coefficient of (111) orientation and the grain size of both TiN and ZrN increased as N₂ partial pressure increased in arc plasma. Based on the above analysis, N₂ pressure and arc current play a vital role in the synthesis process of TMNs NPs by arc discharge, but the effect mechanism of the two experimental parameters on the formation of TMNs NPs has not been unified, which greatly limits the controlled preparation of TMNs NPs by arc discharge.

Hence, TiN, ZrN, and HfN were environmentally friendly and one-step synthesized by DC arc discharge under different N₂ pressures and currents to elucidate the effect of N₂ pressure and current on TMNs NPs production. The effect of N₂ pressure and current on the formation of TMNs NPs is investigated, and the growth mechanism of TMNs NPs under N₂ pressure and current is achieved. The production rate and cost of TiN, ZrN, and HfN NPs were investigated.

2. Experimental methods

2.1. Preparation of TMNs NPs

The synthesized equipment of TMNs NPs has been introduced in detail in the published study [19]. The diameter and height of a cylindrical DC arc discharge generator are 300 mm. Ti, Zr, and Hf bulks with 99 % purity (Runde Metal Materials Co., Ltd., China) and 10 mm-diameter 99.99% pure tungsten rods (Runde Metal Materials Co., Ltd., China) are served as the cathode and anode, respectively. The tungsten rod is positioned perpendicular to the anodic center and can move towards it to maintain two electrodes 5-10 mm apart since the anode is continuously consumed during the discharge process.

Prior to injecting 50 kPa of 99.99% pure N₂ (Messe, Germany), the chamber pressure is inflated to less than 3 Pa, and the discharge current is kept between 100 and 200 A. To eliminate the effects of air, the aforementioned procedure is done three times. Initial N₂ with 50-70 kPa pressure as a buffer gas is employed to synthesize TMNs NPs, and the inner wall sediment is collected after the arc discharge.

2.2. Characterization

A transmission electron microscope (TEM, JEM-2100, JEOL, Japan) with an accelerating voltage of 200 kV, a high-resolution transmission electron microscope (HRTEM), and selected area electron diffraction (SAED) are used to study the morphological characteristics, distribution, and particle size of TMNs. X-ray diffraction (XRD, Rigaku D/Max-2500, Rigaku, Japan) measurements with Cu-K radiation in the 2 range of 20 to 80° at the scanning rate of 5 omin-1 are used to characterize the crystalline structure.

3. Results and discussion

Figure 1 shows a schematic diagram of the synthesis process of TMNs NPs by DC arc discharge. In the arc discharge synthesis process, nitrogen molecules are ionized into nitrogen atoms or ions, whereas metal anodes are evaporated into metal atoms due to a high-temperature plasma [20,21,22]. The evaporated transition metal atoms are promptly chilled, react with the flowing nitrogen atoms or ions by random collisions, and then quickly migrate to a quench area, where the homogeneous nucleation, growth, and coagulation of TMNs NPs occur [23,24,25]. TMNs NPs are subsequently deposited in the water-cooled chamber.

To investigate the effect of current and N₂ pressure on the product, a series of experiments on TiN, ZrN, and HfN NPs synthesis at a pressure of 70 kPa N₂ with various currents and at a current of 200 A with various N₂ pressures are carried out, and meanwhile relevant XRD patterns are exhibited in Figure 2, Figure 3, and Figure 4. Obviously, as shown in Figure 2, all XRD patterns show a cubic TiN phase (Joint Committee on Powder Diffraction Standards (JCPDS) card No. TiN 38-1420) and a characteristic peak of TiO (JCPDS card No. TiO 08-0117) in the as-synthesized products [26]. With the rise in N₂ pressure and current, there is no discernible change in the peak intensities of TiN and TiO. According to the process for synthesizing TMNs, the emergence of oxygen in the samples is attributed to the reaction of TiN and oxygen when the sample is exposed to air. This may be the reason for the lack of a remarkable change in the peak intensity of TiN and TiO with increasing N₂ pressure and current. A variety of experiments on ZrN NPs synthesis at 70 kPa N₂ pressures with various currents and at 200 A current with various N₂ pressures are implemented, and relevant XRD patterns are presented in Figure 3. In the as-produced products, all XRD patterns reveal a cubic ZrN phase (JCPDS Card No. ZrN 74-1217), a monoclinic ZrO₂ phase (JCPDS Card No. ZrO₂ 37-1484), and a distinctive peak of the Zr₇O₈N₄ phase (JCPDS Card No. Zr₇O₈N₄ 38-1420) [27,28]. Similar to the synthesis of TiN, the peak intensity of ZrN and Zr₇O₈N₄ has also not been noticeably altered with increasing N₂ pressure and current. A sequence of experiments on HfN NPs synthesis at a pressure of 70 kPa N₂ with various currents and at a current of 200 A with various N₂ pressures are performed, and relevant XRD patterns are illustrated in Figure 4. The surface of the samples synthesized by DC arc discharge using Hf bulk as anode is composed of three phases, including the cubic HfN phase (JCPDS Card No. HfN 33-0592), the HfO₂ phase (JCPDS Card No. HfO₂ 34-0104), and the Hf₂ON₂ phase (JCPDS Card No. Hf₂ON₂ 50-1171) because HfN NPs have a higher activity in reacting with oxygen than that of TiN and ZrN NPs [29].

The differences in anode mass between before and after arc discharge were counted in order to obtain the production rates of TiN, ZrN, and HfN NPs synthesized by the DC arc discharge under various N₂ pressures and currents, as shown in Figure 5. Accordingly, as shown in Figure 5 (a), the production rate of TiN rises progressively with increasing current and N₂ pressure, at 100 A for 0.78 g·min^–1, 150 A for 1.31 g·min^–1, 200 A for 1.35 g·min^–1, 50 kPa for 1.22 g·min^–1, and 60 kPa for 1.28 g·min^–1, respectively. However, the upward trend of the production rate in synthesizing TiN NPs has slowed down when N₂ pressure is 70 kPa and the current is 200 A. The production rate of ZrN also rises gradually with increasing current and N₂ pressure, at 100 A for 0.84 g·min^–1, 150 A for 0.93 g·min^–1, 200 A for 1.0 g·min^–1, 50 kPa for 0.63 g·min^–1, and 60 kPa for 0.9 g·min^–1, respectively, as displayed in Figure 5 (b). As displayed in Figure 5 (c), the production rate of HfN also increases gradually with increasing current and N₂ pressure, at 100 A for 0.59 g·min^–1, 150 A for 0.92 g·min^–1, 200 A for 1.1 g·min^–1, 50 kPa for 0.89 g·min^–1, and 60 kPa for 1.08 g·min^–1, respectively. As a result of increased current and N₂ pressure, both the temperature in the center of the arc and the anodic energy input accelerate the volume evaporation rate of the anode metal. Convective cooling vapor rate, metal vapor supersaturation, and nucleation growth rate all rise with an increase in the vapor density of anodic metal bulk [30]. Hence, the production rate gradually rises as current and N₂ pressures increase.

The consequences demonstrate that N₂ pressure and current are closely related to the morphology of the products for the preparation of nanomaterials by DC arc discharge. Therefore, as presented in Figure 6, Figure 7, and Figure 8, the morphology of TiN, ZrN, and HfN synthesized by DC arc discharge under different N₂ pressures and currents is manifested by TEM. Figure 6 shows TEM images of TiN synthesized by DC arc discharge under different N₂ pressures and currents, in which it can be clearly seen that all TiN NPs are regular cubic structures and possess good dispersion. Contrasted with TiN in morphological characteristics, Figure 7 exhibits TEM images of ZrN NPs that have a near-spherical morphology with few intergrowths and a good dispersion. Similar to ZrN in morphological characteristics, Figure 8 (b) shows TEM images of HfN, in which HfN NPs have a near-spherical morphology with few intergrowths and a good dispersion. Remarkably, since the production rate of ZrN and HfN NPs under 70 kPa N₂ pressure with a current of 100 A is extremely low, which can be confirmed by Figure 5, the morphology of samples has not been analyzed. On the basis of the above analysis, the optimum conditions for the preparation of TiN, ZrN, and HfN by DC arc discharge are 200 A current with 70 kPa nitrogen pressure.

Figure 9 (a) shows a TEM image of TiN in which it can be clearly seen that all TiN NPs show normal cubic structure, good dispersion, and uniform particle size. Based on the HRTEM image in Figure 9 (b), an initial oxide layer with a thickness of around 0.6 nm on the surface of as-synthesized TiN NPs is generated by the reaction between TiN and air.

The lattice spacing of 0.212 nm, which is in accordance with the lattice fringes of the cubic TiN (200) crystal plane, and the interplanar distance of cubic TiN NPs are shown in Figure 9 (c), suggesting that the as-synthesized TiN has good crystallinity [31]. According to the SAED pattern, the exposed crystal planes of the as-obtained TiN are (111), (200), (220), and (311) [32,33]. A TEM picture of ZrN is shown in Figure 9(d), which indicates that ZrN NPs have a nearly spherical morphology with few intergrowths and good dispersion. As can be deduced from the HRTEM image of ZrN NPs in Figure 10 (a), the initial oxidation layer of about 1.2 nm thickness on the surface of the as-synthesized ZrN NPs is produced by the reaction between ZrN and air. It is found that the as-synthesized ZrN NPs have the obvious crystal lattices, signifying good crystallinity, and the lattice spacing is 0.264 nm, corresponding to the (111) crystal plane of ZrN [34,35]. According to SAED analysis of ZrN, the exposed crystal planes are (111), (200), (220), and (311), as shown in the inset in Figure 10 (b) [36,37]. A TEM picture of HfN with a nearly spherical shape, few intergrowths, and good dispersion is shown in Figure 10 (c). HRTEM image of HfN in Figure 10 (d) shows the oxidation layer of around 1.5 nm thickness that resulted from the reaction between HfN and air and the obviously crystal lattices. Additionally, HfN NPs exhibit good crystallinity, and the lattice spacing is 0.269 nm, which corresponds to the (111) crystal plane [38,39]. According to the SAED of HfN, as illustrated in the inset of Figure 11 (a), the exposed crystal planes are (111), (200), (220), and (311) [40,41]. Through TEM analysis of more than 200 particles, the size distribution of TiN, ZrN, and HfN NPs is investigated. The size range of TiN in Figure 11 (b) is 40-100 nm, with an average size of 81.6 nm. It can be seen from the size distribution in Figure 11 (c) that the size range of the as-synthesized ZrN NPs has an average size of 10.1 nm and a size range of 5-20 nm. As can be observed from Figure 11 (d), the size range of HfN NPs is 5-20 nm, and the average size is 10.5 nm. In general, all the lattices are perfectly symmetric in cubic microstructure and reveal the (111) crystal plane.

In order to evaluate the economic benefit of the production of TiN, ZrN, and HfN NPs from Ti, Zr, and Hf bulks with 99 % purity in this work, as listed in Table 1, the cost of preparing TiN, ZrN, and HfN NPs by arc discharge is analyzed. Taking 1 kg of the products as an example, according to the above analysis, the cost of producing TiN and ZrN is much lower than that of HfN NPs because the cost of Hf bulk is much higher than that of Ti and Zr bulks. It is evident that utilizing Ti and Zr bulks as the raw material for the production of TiN and ZrN NPs through arc plasma technology not only prepares TMNs NPs in a good environment but also has good economic benefits.

4. Conclusions

In conclusion, TMN NPs, such as TiN, ZrN, and HfN, are efficient, sustainable, and produced in one step by DC arc discharge. Additionally, the effects of gas pressure and current on the morphology and purity of TMNs NPs generated by arc discharge were investigated. Although as-synthesized TiN, ZrN, and HfN NPs have an oxidation layer thickness of about 0.6, 1.2, and 1.5 nm, respectively, it does not affect the purity of TMNs NPs, which is only inevitable when TMNs NPs with a highly active surface come into contact with the air. Due to the higher energy and more nitrogen atoms and ions involved in the formation of metal nitride reactions, the TMNs NPs yield is higher at higher current and gas pressure, but the morphology and purity of TMN NPs are unaffected. According to a series of experiments, the optimum conditions for the preparation of TiN, ZrN, and HfN were determined, which refers to a 200 A current with 70 kPa N₂ pressure. Specifically, the average size of TiN NPs with a regular cubic structure is 81.6 nm. ZrN NPs with a near-spherical structure are 10.1 nm in size on average. The average size of HfN NPs with a near-spherical morphology is 10.5 nm. Finally, the economic benefit of the production of TiN, ZrN, and HfN NPs from Ti, Zr, and Hf bulks with 99 % purity was evaluated. The cost of producing TiN and ZrN NPs is much lower than the production cost of HfN NPs because the cost of Hf bulk is much higher than that of Ti and Zr bulks. The work offers a practical method for producing TMNs NPs with high purity and good dispersion, opening up new avenues for technological development.