Introduction

Motivated by the novel applications of diverse nanometer-scale materials, extensive research has been carried out on the design and preparation of nanostructures with different shapes and sizes because of their corresponding novel properties and potential uses (Buha et al. 2007; Gomathi and Rao 2006; Zhang and Gao 2004; Fischer et al. 2007; Wu et al. 2005). Recently, the preparation of group four (for example, TiN) and group five (for example, NbN and TaN) transition metal nitrides has attracted extensive attention among synthetic chemists owing to their special physical and chemical properties. They exhibit metallic conductivity and some show superconductivity (Toth 1971; Storms 1972; Joshi et al. 2005; Hu et al. 2000). These nitrides are generally resistant to chemical attack and are stable at high temperatures under inert or reducing atmospheres (Blocher 1956). The important applications of these materials are as hard, protective coatings for cutting tools (Buhl et al. 1981; Sundgren 1985). Dense crucibles of ZrN and TiN serve as vessels for the melting of metal. Recently, the superconductivity of NbN has been exploited to make superconducting coatings in radiofrequency cavities (Fabbricatore et al. 1989), and TiN has shown promising results as a diffusion barrier for microelectronics (Ostling et al. 1986; Gillan and Kaner 1994). Tantalum nitride coatings are used in electronics industry as a diffusion barrier (Aryasomayajula et al. 2006). Niobium nitride thin films can be used for corrosion protective applications (Fenker et al. 2003). Recently, Serro et al. studied the TiN films for biomedical applications (Gispert et al. 2007).

There are several methods quoted in the literature for the preparation of metal nitrides (Carmalt et al. 2003; Tenne et al. 1992; Margulis et al. 1993; Tenne 1995; Troitskiy et al. 2003; Li and Gao 2003; O’Loughlin et al. 2001). In the current article, single-crystalline wide band gap semiconductors, TiN, NbN, and TaN nanocrystals, are prepared by using a mixture of metal chlorides and sodium azide via a convenient low temperature reaction under autogenic pressure at elevated temperature (RAPET) technique. There is only one reference that reports the synthesis of titanium nitride at ambient temperature (Griffiths et al. 2001). From the literature search, this is the lowest temperature ever reported for the fabrication of NbN and TaN nanocrystals. The RAPET approach used for the fabrication of TiN, NbN, and TaN is uncomplicated, solvent less, and environment friendly. We had employed the RAPET technique for nitride formation, emphasizing the relatively low temperature under which these nitrides are prepared.

Experimental

Reagents

TiCl4, NbCl5, TaCl5 (Aldrich, 99.9%), and NaN3 (Sigma, anhydrous) used as-received.

Synthesis

The synthesis of TiN, NbN, and TaN nanocrystals was carried out by the thermal decomposition of a mixture of TiCl4 and NaN3, purchased from Aldrich Co. and used as-received. The 3-mL closed vessel cell was assembled from stainless steel Letlok parts (manufactured by HAM-LET Co., Israel). A 1/2″ union part was plugged from both sides by standard caps. For the synthesis, 0.25 mL of TiCl4 and 0.59 g of NaN3 were introduced into the cell at room temperature. The filled cell was closed tightly by the other plug and then placed inside an iron pipe in the middle of the furnace. The temperature was raised at a heating rate of 10 °C/min. The closed-vessel cell was heated at 350 °C for 12 h. The reaction took place under the autogenic pressure of the precursors. The Letlok was gradually cooled (~1–2 h) to room temperature, and after opening a mixture of brown powder and white salt was obtained. We have followed the above synthetic strategy for the synthesis of NbN and TaN nanomaterials by the RAPET method. For the synthesis of NbN, 0.4 g of NbCl5 and 0.5 g of NaN3 were introduced into the cell at room temperature. For the synthesis of TaN, 0.2 g of TaCl5 and 0.18 g of NaN3 were introduced into the cell at room temperature The as-prepared products synthesized through RAPET synthesis were further washed with water and diluted HCl in order to remove the NaCl. The washing removes the NaCl and leads to the formation of neat TiN, NbN, and TaN nanocrystals. The experimental results are summarized in Table 1.

Table 1 Summary of experimental data, including reaction temperature and times, material color, the identity of the structure comprising the reaction product after washing the as-prepared material with diluted HCl and distilled water
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Warning

Exceeding this amount of reactants inside the 3-mL capacity of the stainless steel Letlok is not advisable. In other words, care should be taken to perform these solid-state metathesis reactions on a small scale first (less than 1 g of reactant mixture), taking adequate safety precautions. The Letlok parts are not dangerous to open it after the reaction.

Characterization

The XRD patterns of TiN, NbN, and TaN samples are recorded using a Bruker D8 diffractometer with Cu Kα radiation. C, H analysis was carried out on an Eager 200 CE instrument and an EA 1110 Elemental Analyzer. The morphologies of the as-prepared sample are studied by a scanning electron microscope (SEM), and SEM coupled with energy dispersive X-ray analysis (EDX). Transmission electron microscopy (TEM) studies are carried out on a JEOL 2000 electron microscope. High-resolution TEM (HRTEM) images are taken using a JEOL 2010 with a 200-kV accelerating voltage. Samples for the TEM and HRTEM measurements are obtained by placing a drop of the suspension from the as-sonicated reaction product in ethanol onto a carbon-coated copper grid, followed by drying under air to remove the solvent. Energy-dispersive X-ray analysis (EDX) studies are carried out on a JEOL micrograph (JEOL 2010 operated at 200 kV).

Results

Powder X-ray diffraction (PXRD), elemental (C and H) analysis, SEM, TEM, HRTEM, and EDX analysis

The PXRD patterns of the thermally decomposed mixture of MCl n [M = Ti, Nb and Ta; n = 4 or 5] and NaN3 at 350 °C in a closed Letlok cell under air atmosphere are presented in Fig. 1a–c.

Fig. 1
figure 1

PXRD pattern of the thermally decomposed mixture of a TiCl4 + NaN3, b NbCl5 + NaN3, and c TaCl5 + NaN3 at 350 °C under air atmosphere and further washed with water and diluted HCl

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The elemental (C, H, N, S) analysis detected 0% carbon and 0% hydrogen in the TiN, NbN, and TaN products after the washing process. The calculated elemental (wt) percentages of N, Ti, Na, and Cl in the mixture of TiCl4 and NaN3 before a reaction are 16%, 17%, 9.2%, and 56%, respectively. We could determine the nitrogen content in the TiN sample with an elemental [C, H, N, and S] analyzer. The measured amount of nitrogen in the three samples, TiN, NbN, and TaN, are 21 wt% (theoretically 23%), 13 wt% (theoretically, 14%), and 8 wt% (theoretically 8%), respectively. The XRD analyses of the three metal nitride nanocrystals are shown in Table 2.

Table 2 The obtained products, crystal structure, morphology, and lattice parameters are illustrated
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The morphologies of titanium nitride, niobium nitride, and tantalum nitride crystals

The morphologies of the TiN, NbN, and TaN nanocrystals are observed by SEM, TEM, and HRTEM analysis. The morphologies of the neat TiN nanocrystals obtained after washing the as-prepared sample with water and diluted HCl under air atmosphere are primarily investigated by SEM measurements. Figure 2a demonstrates the SEM image of neat TiN crystals. The neat TiN sample shows various morphologies, including nanocrystals, a few crystals, as well as disordered nanoplate particles. The mean diameter of the various nanosized particles is ~180 nm. The stacking of two or more nanocrystals is also observed in the SEM picture. The diameter of the sintered TiN nanocrystals varies in the range of 250–350 nm. In addition to the nanocrystals there are disordered plates, and the size of these nanoparticles varies in the range of 350–600 nm in diameter. Gillan et al. demonstrated the synthesis of cupric nitride nanoparticles with nearly cubic shape morphology via a solvothermal route (Choi and Gillan 2005).

Fig. 2
figure 2

a SEM images of TiN nanocrystals obtained after washing the as-prepared sample with water and a diluted HCl aqueous solution. b SEM images of NbN nanocrystals. c SEM images of TaN nanocrystals obtained after washing the as-prepared sample with water and diluted HCl

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Figure 2b demonstrates the SEM image of neat NbN nanocrystals obtained after washing the as-prepared NbN sample with a diluted HCl aqueous solution. Although many of the nanocrystals are still observed in the SEM picture (Fig. 2b), the disordered nanosheets are also detected along with the nanocrystals. The size of the NbN nanocrystals varies in the range of 80–90 nm in diameter. The stacking of two or more nanocrystals, and nanosheets, also appears in the SEM image. This type of typical assembly might be due to the sintering of the nanoparticles. The diameter of the sintered NbN nanoparticles varies in the range of 230–350 nm.

Figure 2c demonstrates the SEM images of TaN nanocrystals. The washed sample shows crystalline particles. The diameter of the crystalline particles is in the range of 200–500 nm. In addition to the nanocrystals in the washed product, we have found a fine powder with a morphology suggestive of agglomerations of hundreds of small spherical nanoparticles. The diameter of the agglomerates is in the range of 280–570 nm. EDX measurements carried out on individual TiN, NbN, and TaN nanocrystals of samples washed with diluted HCl and water indicate the presence of only Ti/N, Nb/N, and Ta/N, respectively, and no other impurities are detected. The M/N, atomic ratio in the MN [M = Ti, Nb, and Ta] nanocrystals, obtained from EDX analysis is ~1:1, which is in agreement with the MN formula.

The particle sizes of TiN, NbN, and TaN nanocrystals calculated by the X-ray measurements are smaller than that of calculated by the SEM measurements. Aggregation of small particles is responsible for these discrepancies.

TEM and HRTEM measurements

The structures of the TiN, NbN, and TaN nanocrystals are further studied by TEM and HRTEM measurements. The TEM image of TiN nanocrystals is illustrated in Fig. 3a. The TiN nanocrystals have dimensions in the range of 100–350 nm.

Fig. 3
figure 3

a TEM image of TiN nanocrystals obtained after washing the as-prepared sample with water and diluted HCl aqueous solution. b HRTEM image of the edge portion of one individual TiN nanocrystal with diffraction pattern (inset (a))

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The TEM image reveals that at least small parts of the TiN nanoparticles are formed in a cubic shape, while the rest are crystalline particles. The TiN nanocrystals obtained by RAPET differ from the spherical-shaped TiN nanoparticles obtained by Rao’s synthetic strategy (Gomathi and Rao 2006). Rao and coworkers have designed and synthesized spherical TiN nanoparticles by the reaction of TiCl4 with urea using an acetonitrile solvent, and the reaction was maintained in an autoclave at 900–1,000 °C for 3 h in N2 atmosphere. Our experimental approach in this article involves a similar precursor, TiCl4, which requires neither solvents nor a nitrogen atmosphere for the formation of TiN nanocrystals. The TiN nanocrystals produced under Letlok reaction conditions have lengths ranging from 160 to 350 nm and are larger than those of the TiN nanoparticles (25 nm) produced by Rao’s synthetic approach. Perhaps each of these larger structures is an agglomerate of 30 nm particles, a number calculated by PXRD for the particle’s size.

In order to obtain the detailed structure and composition of individual TiN, NbN, and TaN nanocrystals, we have carried out a HRTEM image, selected area electron diffraction (SAED), and selected area energy dispersive X-ray spectroscopy (SAEDS) measurements. Figure 3b demonstrates the HRTEM image of the edge of a single octahedral TiN nanocrystal. The measured distance between the fringes is 3.8 nm which is of course much larger than the planes separation of the face-centered cubic lattice of the TiN. Therefore, the unique TiN shape exhibited in Fig. 3a and b is interpreted as an inorganic fullerene (IF)-like structure. The first IFs, MoS2, and WS2 were discovered by Tenne et al. They have shown that triangular and rectangular onion-like shapes exist (Hershfinkel et al. 1994). Although bulk synthesis of the IFs normally yields quasi-spherical nanoparticles with at least 20 molecular layers and outer diameters of greater than 30 nm, the formation of hollow MoS2 clusters with octahedral or tetrahedral shapes was often observed (Tenne et al. 1992; Margulis et al. 1993; Tenne 1995). We observe in the TiN structure at least six molecular layers and more are perhaps hidden. The corresponding SAED pattern is demonstrated in inset (a) of Fig. 3b, featuring a single crystalline TiN. In order to identify the composition of nanocrystals, we have measured a selected area EDS analysis of an individual TiN crystalline particle (figure not represented here). The measurements demonstrate the existence of 78.4 wt% of Ti and 22.5 wt% of N, which is very close to the theoretical value of TiN (Ti = 77 wt% and N = 23 wt%).

Figure 4 depicts the TEM image of the morphology of the neat NbN particles. Nanocrystals are the major structure observed in the picture. They are obtained after washing the as-prepared product with water and a diluted HCl aqueous solution. The face of the neat NbN nanocrystals has a length in the range of 75–100 nm. The average size of the agglomerated nanocrystals is 205 nm.

Fig. 4
figure 4

a TEM image of NbN nanocrystals obtained after washing the as-prepared sample with water and a diluted HCl aqueous solution. b HRTEM image of the edge portion of one individual NbN nanocrystal with diffraction pattern (inset (a))

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Figure 4b demonstrates the HRTEM image of the edge of a single NbN nanocrystal. The image is recorded along the [111] zone. The measured distance between these (111) lattice planes is 0.22 nm, which is very close to the distance between the planes reported in the literature (0.25 nm) for the face-centered cubic lattice of the NbN [powder diffraction file (PDF) no. 01-089-5128]. The corresponding SAED pattern is demonstrated in inset (a) of Fig. 4b, featuring a single crystal of NbN nanocrystals.

Figure 5 represents the TEM image of TaN nanocrystals. The TEM image shows the agglomerated TaN nanocrystals. The TaN nanocrystals are the major structure observed in the picture and have an average diameter in the range of 25–65 nm. The diameter of the agglomerated spherical nanocrystals is in the range of 150–180 nm.

Fig. 5
figure 5

TEM image of TaN nanocrystals obtained after washing the as-prepared sample with water and a diluted HCl aqueous solution. Inset (a) of this figure demonstrates the HRTEM image of one spherical TaN nanocrystal

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Inset (a) of Fig. 5 demonstrates the HRTEM image of a single TaN nanocrystal. The image is recorded along the [111] zone. The measured distance between these (111) lattice planes is 0.23 nm, which is very close to the distance between the planes reported in the literature (0.25 nm) for the face-centered cubic lattice of the TaN [powder diffraction file (PDF) no. 01-089-5196].

Discussion

The mechanism suggested herein was based on the obtained analytical data and also on previously published data. From XRD, EDX, elemental (C, H, N, S) analysis, SEM, TEM, and HRTEM analysis, it was clear that the product, i.e., MN (M = Ti, Nb and Ta) nanocrystals, is obtained as a result of the RAPET reaction of TiCl4 and NaN3 under Letlok reaction conditions. The Letlok cell in our reaction is heated to a temperature of 350 °C, which is sufficiently hot to start a reaction between metal chlorides and azide (Gillan and Kaner 1994). The NaN3 decomposes to Na and N2 at 365 °C (Gillan and Kaner. 1994). The boiling points of TiCl4, NbCl5, and TaCl5 used in this study are 137, 254, and 242 °C, respectively, and these precursors decompose to metals and chlorine atoms below 500 °C. As the heating continues, the thermal decomposition of reactants (Eq. 1) takes place producing transition metal, Na, Cl, and N atoms in the gaseous phase. The second step involves the initial formation of the by-product, salt, with a reaction between chlorine and sodium, followed by a reaction of finely divided metal and nitrogen gas in a molten salt flux, as shown in Eq. 2. In the second step, the thermodynamically favored by-product, NaCl, serves as a potent driving force, enhancing the nitride formation process (Gillan and Kaner 1994).

$$ {\text{MCl}}_{x} + x{\text{NaN}}_{ 3} \to {\text{M}} + {\text{Na}} + n{\text{N}}_{ 2} + n{\text{Cl}}_{ 2} \quad [x = 4,\;5] $$
(1)
$$ {\text{M}} + {\text{Na}} + n{\text{N}}_{2} + n{\text{Cl}}_{2} \to {\text{MN}} + {\text{NaCl}} $$
(2)

Previously, Kaner and coworkers had initiated the solid-state metathesis reaction between metal chlorides (M = Ti, Nb, and Ta) and NaN3 or Li3N by employing the hot filament for a few seconds producing the metal nitrides, MN (Gillan and Kaner 1994). Parkin had filled the metal chloride and sodium azide into the Pyrex ampoule and heated the ampoule in the furnace in the temperature range of 300–400 °C for the production of metal nitrides (Hector and Parkin 1995). However, the RAPET reaction reported in this article was operated at 350 °C. The variety of RAPET reactions addressed in references (George et al. 2007; George and Gedanken 2008; Pol et al. 2004a, b, c, 2006) generates nanocrystals with distinct morphologies. It is evident that the growth of one-, two-, and three-dimensional nanostructures, such as nanorods, nanotubes, nanofilms, 3-D nanorods, 3-D nanotubes, and 3-D nanofibers, occurs under the influence of temperature and pressure. Similarly, we suggest that under the application of autogenic pressure, the metathesis reaction can be conducted at a lower temperature due to the high pressure of N2 and Cl2 pushing the reaction towards products. In addition, the low temperature favors the formation of TiN and NbN nanocrystals. In the current study, the RAPET technique yielded a 76%, 92%, and 96% (wt) recovery of the starting reaction mixture for TiN, NbN, and TaN, respectively.

Conclusions

In summary, by adjusting the metal halides to the sodium azide ratio, the temperature, and the reaction time, a series of metal nitrides (TiN, NbN, and TaN) can be selectively synthesized in the absence of a solvent, a catalyst, or surfactant through a convenient, low temperature RAPET method. This work is valuable for the understanding of the formation of metal nitrides via a simple, convenient, and low temperature RAPET technique. This article has provided a general and effective method to control the composition and phase structure of transition metal nitrides, which will be very important for inorganic synthesis methodology and further applications of metal nitrides.