Introduction

Boron nitride nanotube (BNNT), a structural analogue of hexagonal boron nitride (h-BN), is receiving great concerns among material researchers, because of its unusual material properties such as high thermal conductivity [1], high Young’s modulus (up to 1.3 TPa) [2], and high electrical resistance [3], which have never been demonstrated with classical bulky materials, and it is believed to shape the future of mechanical and functional applications.

BNNTs are structurally similar with carbon nanotubes (CNTs), so many researches have tried to apply synthesis methods which was successful with CNTs. To date, various methods have been introduced to synthesize highly crystalline BNNTs such as chemical vapor deposition (CVD) [4, 5], ball milling [6], arc discharge [7], and laser ablation [8]. However, these methods are limited to synthesizing BNNTs in only gram-level amounts. BNNT synthesis mcethods developed so far are mostly based on seeded-growth mechanism, where boron containing solidphase source are reacted with gas-phase nitrogen which results in successive formation of B-N bonds with hexagonal alternating one-by-one arrangement of boron and nitrogen atoms. In contrast to CNTs, the high-throughput and high-efficiency synthesis of BNNTs was difficult, primarily due to [1] lack of vapor-phase boron precursors, and [2] highly strong bond of nitrogen molecules which hindered fast and efficient reaction between boron and nitrogen atoms. Further study needs to be employed for scalable synthesis of BNNTs along with cost-controlling methods.

The plasma process system provides a new pathway to the synthesis of scalable and size controllable BNNTs [9]. Extremely high temperature of thermal plasma (exceeding 10000 K) ensures vaporization of boron [10], and strong electric field generated in the plasma torch breaks N-N bond, and makes reactive nitrogen species facilitating B-N reaction. In plasma synthesis of BNNTs, firstly boron nanoparticles are formed via nucleation from highly concentrated boron vapors just after passing high-temperature plasma region. Boron nanoparticles formed during the plasma process act as the seed material for the growth of BNNTs. Because the growth of BNNT starts with the formation of boron seed and ends by the solidification of the boron, the growth of BNNTs is restricted only to temperature range of 2000~4000°C. Boron seed resides in this region very shortly, less than several tens of ms, so there is no plenty of reaction time. Furthermore, most of dissociated nitrogen atoms tend to recombine into N₂ just after passing the high-temperature region of plasma, thus the frequency of active reaction between B and N is far below than expected and the growth of BNNTs is disturbed.

Here, hydrogen plays a big role in impeding the recombination by forming N-H bonds and thus consequently aiding the growth of BNNT as a vital N-source. Kim et al proved the theory through their thermal plasma synthesis of BNNTs with the precursor h-BN and H₂ gas [9]. BNNTs made from such precursors showed high yield of nanotubes compared to BNNTs fabricated from h-BN alone. However, direct use of H₂ gas as a precursor material is not strongly recommended as it is extremely flammable, having a wide explosive/flammability range (4% - 74% in air), meaning that even a small leak can cause a hazardous fire [11]. Therefore, use of other precursors is recommended for safety reasons, especially when synthesizing materials with high temperature processes.

Although many methods for synthesizing BNNTs are being developed, further study in BNNT based applications/ devices has not yet been explored extensively due to its difficulty in dispersing in aqueous solution [12]. BNNT solubility region lies at a low hydrogen bonding range, implying its hydrophobicity, thus lacking interaction with solvents with hydroxyl groups [13]. In order to fabricate BNNT composites or to apply to devices, it is crucial to be able to prepare BNNT in uniform and stable dispersions [14].

Here, we present plasma synthesis of BNNTs in the help of catalytic solid-sate ammonium compounds without using explosive hydrogen. We have chosen NH₄Cl as an ammonium compound, because NH₄Cl does not leave any solid state byproduct during thermal decomposition, and NH₄Cl itself is well dissolvable in pure water. In this work, BNNTs were synthesized via RF-ITP with precursors h-BN, h-BN+H₂, and h-BN+NH₄Cl. Sheet-like BNNTs were collected from the collector filter only from the two combinations of precursors, h-BN+H₂ and h- BN+ NH₄Cl, implying successful fabrication of nanotubes. Also meaning that the hydrogen from NH₄Cl has substituted the role of H₂ gas (impeding the recombination of N₂) and contributing to the synthesis of BNNTs. As-synthesized BNNTs were dispersed in distilled water to test for dispersion ability in aqueous solution.

Experimental

BNNTs were synthesized via TEKNA PL-35LS with precursors h-BN, H₂ gas and NH₄Cl. Precursors h-BN and NH₄Cl were injected from the powder feeder to the plasma torch at feeding rate 4 g/min with 5 slpm of Ar carrier gas. BNNTs were first synthesized from h-BN only. Ar gas was first purged and filled into the system followed by 70 slpm N₂ of sheath gas. The second sample of BNNTs was synthesized from precursors h-BN and H₂ gas with 5 slpm of Ar carrier gas. The sheath gas was programmed to 65 slpm N₂, 15 slpm H₂ and 5 slpm of Ar carrier gas. The third sample of BNNTs was also synthesized at the same conditions with a precursor ratio of 1:1 for h-BN and NH₄Cl but this time without H₂ gas. The two types of powders were manually mixed and were put into the hopper to be injected from the powder feeder. The as-synthesized BNNTs were easily collected from the collector filter as the BNNTs were fabricated into sheet-like form. The aqueous dispersion test of BNNTs was performed by bath sonication for 15 minutes.

The structural morphology of BNNTs were examined by Field Emission Scanning Electron Microscopy (Tescan, MIRA3 LM) and X-Ray Diffraction (Rigaku, D/ Max-2500VL/PC). Distilled water was used to investigate the dispersion ability of BNNTs in aqueous solution. Vacuum filtration was performed with PTFE filter with sub-20 nm pores.

Results and Discussion

The RF-ITP system (TEKNA PL-35LS) is arranged into four major parts: the plasma torch, reactor chamber, cyclone chamber, and collector chamber as shown in Figure 1 [15]. Precursor materials are injected into the injection probe and into the thermal plasma created in the plasma torch induced by the induction coil. Figure 2 shows the mechanism scheme of BNNT synthesis via thermal plasma. As shown in figure 2(a), hydrogen from H₂ gas plays a role in impeding the recombination of N₂ molecule and forming highly reactive N-H molecules, which contribute to the synthesis of BNNTs. Figure 2(b) also shows a similar scheme, however, this time reactive N-H molecules come directly from dissociation of NH₄Cl and contribute to the synthesis of BNNTs. Meaning, when NH₄Cl is used along with h-BN as a precursor material, H₂ gas is no longer needed, solving the safety issues of using H₂ gas in extreme temperature synthesis such as thermal plasma. Figure 3 shows photographs of BNNTs synthesized from precursors h-BN+ NH₄Cl. Figure 3(a) shows how the sheet-like BNNTs were peeled off from the collector filter. As the sheets are peeled off, the sheets roll up and are easily peeled off from the surface of the filter. Figure 3(b) shows the peeled off and free-standing bundle of sheets of BNNTs. While the BNNTs exhibit grayish color when bundled up, it appears to be whiter when only one layer of BNNTs is peeled off.

Fig. 1

Schematic illustration of RF-ITP system employed in this work.

Fig. 2

Mechanism scheme of BNNT synthesis with precursors (a) h-BN+H₂ (b) h-BN+NH₄Cl via thermal plasma.

Fig. 3

Photographs of as-synthesized BNNTs from precursors h-BN+ NH₄Cl via thermal plasma. (a) As-synthesized BNNTs collected on metal filter, and (b) free-standing as-synthesized BNNT sheets peeled off from collector filter. Photographs of BNNTs synthesized from (c) h-BN (d) h-BN+H₂ (e) h-BN+NH₄Cl.

Figure 3 also shows photographs of as-synthesized BNNTs, each synthesized from three different combinations of precursors. Figure 3(c) shows BNNTs synthesized from h-BN only while figure 3(d) and 3(e) shows BNNTs synthesized from h-BN+H₂ and h-BN+NH₄Cl respectively. A difference can be seen between BNNTs synthesized from only h-BN and when synthesized from H₂ or NH₄Cl as figure 3(c) does not show any sheet-like properties, and were solely collected in a powder form. However, figure 3d show that BNNTs can fully be grown in our thermal plasma system with the precursor h-BN and H₂ gas as reported by Kim et al. [9]. Figure 3(e) also shows that BNNTs can fully be grown from the aid of only NH₄Cl. The sheet-like and rolling properties seen in figure 3(b) and 3(c) implies the successful synthesis of nanotubes [16]. The synthesis of BNNTs was possible without H₂ gas as NH₄Cl played a role in providing hydrogen radicals to form N-H bonds (impeding the recombination of N₂ molecule) and thus taking over the role of H₂ gas and contributing to the growth of nanotubes. FE-SEM images shown in figure 4(a) and 4(b) confirms the unsuccessful growth of BNNTs from only h-BN as no nanotubes are found in the images. Figure 4(c) and 4(d) show successful growth of BNNTs from h- BN+H₂ and same goes with figure 4(e) and 4(f). Figure 4(e) and 4(f) however show a slightly larger portion of nanoparticles distributed between the nanotubes compared to Figure 4(c) and 4(d). While nanotubes in figure 4(c) and 4(d) are entangled with each other, nanotubes in figure 4(e) and 4(f) are more widely distributed, which can improve its dispersion properties when dispersed in aqueous solutions.

Fig. 4

(a-f) FE-SEM images of BNNTs synthesized from (a, b) h-BN (c, d) h-BN+H₂ (e, f) h-BN+NH₄Cl. (g) XRD analysis result of BNNTs synthesized from h-BN+H₂ and h-BN+ NH₄Cl.

In order to first distinguish the nanoparticles apart from the nanotubes, the as-synthesized BNNTs were characterized by XRD analysis as shown in Figure 4(g). BNNTs synthesized from h-BN and H₂ gas only shown in figure 4(g) exhibit h-BN peaks at 2θ = 26, 50° implying hexagonal structures of BNNTs [17]. BNNTs synthesized from h-BN+NH₄Cl also exhibit h-BN peaks at 2θ = 26, 50o. In addition, peaks at 2θ = 43, 75o are also seen, indicating the presence of BH₁₂N₃O₃ [18] (ammonium borate). The presence of above peaks of nanoparticles implies that the nanoparticles distributed between the nanotubes in Figure 4(e)-4(f) are synthesized through the thermal plasma process along with the synthesis of BNNTs. BH₁₂N₃O₃ [19] possess high solubility in aqueous solution, which may contribute to the dispersion of BNNTs as the distribution of nanoparticles impede the nanotubes from entangling with each other.

The dispersion ability of the BNNTs samples were tested in distilled water as shown in figure 5. BNNTs synthesized from h-BN+H₂ show poor dispersibility in distilled water as shown in figure 5(a). The sheet-like BNNT break into smaller pieces and remain in the solution in a colloidal-like form. BNNTs synthesized from NH₄Cl show improved dispersibility in aqueous solution as shown in figure 5(b). Differences can be seen from the two samples as the dispersed BNNTs exhibit opaque properties in figure 5(b) compared to figure 5(a). It is typical to show such opacity in solutions with well-dispersed nanotubes [20], implying that BH₁₂N₃O₃ nanoparticles played a big role in contributing to the dispersibility properties by impeding the entanglement of BNNTs in the fabrication process. The proposed principle of dispersity enhancement in the help of reaction by product is schematically represented in figure 5(b).

Fig. 5

Dispersion test of BNNTs in distilled water synthesized from (a) h-BN+H₂ (b) h-BN+NH₄Cl, and their schematic representation of dispersion principles.

The dispersed BNNTs can be easily retrieved from the solution by vacuum filtration. Figure 6(a) schematically shows vacuum filtration process employed in this work. BNNT/water solution was poured onto PTFE filter having sub-20 nm pores, and solvents are extracted out via vacuum pumping. Figure 6(b) and 6(c) show optical images of filters poured with solutions dispersed with H₂- catalyzed-synthesized and NH₄Cl-catalyzed-synthesized BNNTs, respectively. Dark color of collected BNNTs is originated from increased volumetric density of boron nanoparticles due to compaction of BNNTs during filtration process. Figure 6(d, e) and Figure 6(f, g) shows SEM images captured from surfaces of filters decorated with H₂-catalyzed-synthesized and NH₄Cl-catalyzed-synthesized BNNTs, respectively. Compacted PNNT fibrils can be clearly seen in the images, and it is difficult to find out the difference in areal density of BNNTs between two samples.

Fig. 6

Vacuum filtration process of BNNT solution. (a) A schematic of vacuum filtration process. (b, c) Optical images of filters poured with solutions dispersed with H₂-catalyzedsynthesized (b) and NH₄Cl-catalyzed-synthesized BNNTs (c). (d-g) SEM images captured from surfaces of filters decorated with H₂-catalyzed-synthesized (d, e) and NH₄Clcatalyzed- synthesized BNNTs (f, g).

Conclusion

BNNTs were synthesized from three different combinations of precursors via thermal plasma. Because of the safety reasons of using H₂ gas in high temperature synthesis, the use of different precursor other than H₂ gas was investigated. h-BN was used as the base precursor with H₂ and NH₄Cl as the independent variable. BNNTs synthesized from NH₄Cl (without H₂ gas) showed successful fabrication of nanotubes with sheet-like and rolling properties. In addition, BNNTs synthesized from NH₄Cl showed improved dispersibility in aqueous solution compared to nanotubes synthesized from h-BN+H₂ gas. The BNNTs successfully retrieved from the solution by vacuum filtration.

Acknowledgements

Acknowledgements

This study was supported financially by Fundamental Research Program of the Korea Institute of Materials Science (KIMS).

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