Efficient mixed metal oxide routed synthesis of boron nitride nanotubes

Abstract

Boron nitride nanotubes (BNNTs) were successfully synthesized by a simple annealing process. Amorphous boron powder (B) was used as boron source to react with various metal oxide mixtures (V₂O₅/Fe₂O₃ and V₂O₅/Ni₂O₃). V₂O₅ acts as an efficient promoter in the synthetic process due to its highly oxidizing and reducing properties. The Fe₂O₃ and Ni₂O₃ act as catalysts in combination with the B/V₂O₅ system to achieve highly crystalline BNNTs at 1100 °C. The morphology and crystalline nature of the BNNTs were characterised by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. The observations revealed the hexagonal-BN (h-BN) phase of the BNNTs, with a highly crystalline tubular structure. This method proved to be simple and economical, using B/V₂O₅/Fe₂O₃ and B/V₂O₅/Ni₂O₃ mixtures for the large scale production of BNNTs.

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1. Introduction

One dimensional (1D) nanomaterials, including nanotubes, nanowires, nanorods and nanobelts, have drawn extensive attention in various areas such as electronics, electrical, pharmaceuticals, cosmetics and biomedical applications.^1,2 Among various 1D nanostructures, the most exciting classes of nanotubes are carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs). Owing to their structural similarities, BNNTs possesses several advantages over CNTs.³ BNNTs exhibit excellent mechanical, electrical and electronic properties, thermal stability, resistance to oxidation and a wide band gap, independent of geometric parameters.⁴ The theoretical perspective of BNNTs was first proposed by Rubio et al. in 1994, and in the subsequent year BNNT production was achieved by Chopra et al.^5,6

The most popular methods, such as arc-discharge, laser ablation, ball milling, template assisted synthesis and plasma jet synthesis, have failed to synthesis BNNTs of high purity and in a high yield.⁷ The chemical vapor deposition (CVD) technique is considered to be a simple method for the production of BNNTs using various liquid precursors such as diborane (B₂H₆), borazine (B₃N₃H₆) and trimethyl borate (C₃H₉BO₃).⁸⁻¹¹ The drawback in this reaction process is the in situ generation of liquid vapors. To date, the boron oxide chemical vapor deposition (BOCVD) method yields large quantities of BNNTs. This method mainly utilizes MgO as a support (B/MgO, B/MgO/Fe₂O₃ and B/MgO/SnO) for the growth of BNNTs.¹² Pakdel et al. (2012) reported the formation of well structured BNNTs by the annealing of B/FeO/MgO.¹³ Further, Ozmen et al. (2013) explained the formation of BNNT over amorphous B and Fe₂O₃.¹⁴ Li et al. (2011) have reported the synthesis of BNNT over a stainless steel substrate by annealing amorphous B and Fe₂O₃.¹⁵

Apart from using metal oxides as supports, there have also been reports of the production of BNNTs using the self propagation high temperature synthesis (SHS) method,¹⁶ and the formation of multi-walled and double walled BNNTs by catalytic chemical vapor deposition (CCVD) aided by a floating nickel catalyst.¹⁷ A nano-templated reaction, using single-walled carbon nanotubes (SWCNTs) as a template and ammonia borane complex (ABC) precursors was also reported by Nakanishi et al.¹⁸ Colemanite (Ca₂B₆O₁₁·5H₂O) mixed with ZnO, Al₂O₃, Fe₃O₄, and Fe₂O₃ has also produced BNNTs.¹⁹ However, all these reported approaches have various disadvantages, either in the selection of precursors with complex operational equipment setup or the requirement of growth temperatures >1200 °C. In the current scenario, BNNT synthesis on a large scale is considered a necessity for various commercial applications in catalysis, drug delivery, storage, nanoencapsulation, transport of various molecules and medical diagnosis.^20–22

In order to circumvent the above problems, the present investigation focuses on the preparation of BNNTs at relatively lower temperatures and with simpler processes compared to existing approaches. In this context, a thermal CVD method was selected for the production of high quantities of BNNTs by heating boron with metal oxides. Here we have investigated a pair of reaction mixtures, B/V₂O₅/Fe₂O₃ and B/V₂O₅/Ni₂O₃, for the synthesis of BNNTs. V₂O₅, which can act as a promoter, oxidizer and a support, is introduced in the reactions. The motivation for the selection of V₂O₅ lies in the creative work of Goldberg et al.²³ Fe₂O₃ and Ni₂O₃ are selected as catalysts due to their high catalytic efficiency towards nanotube formation.²⁴ Metastable Ni₂O₃ was chosen because it can be easily reduced to NiO or Ni, which leads to the formation of BNNTs at lower temperatures.²⁵ Already our group has been successful in the production of BNNTs using MoO₃ as a promoter and using Fe₂O₃ and Ni₂O₃ as catalysts.²⁶

In the present investigation, V₂O₅ undergoes reduction and also oxidizes boron at low temperatures. Simultaneously, boron reacts with Fe₂O₃/Ni₂O₃ and generates B₂O₂ vapor. The large amount of B₂O₂ vapor formed during this process reacts with NH₃, forming BN. Fe/Ni, which attains a liquid state, facilitates the formation of BNNTs. The color of the amorphous boron mixture (B/V₂O₅/Fe₂O₃ and B/V₂O₅/Ni₂O₃) changes from dark brown to white, and this observation further confirms the formation of BNNTs. The effect of reaction temperature on the growth of BNNTs was studied and morphological changes, such as the tube structure and tube diameter, were also investigated using TEM studies. The role of V₂O₅ in the synthetic process was also explained in detail.

2. Experimental

2.1 Materials

Amorphous boron powder (Loba Chemie. Pvt. Ltd), Fe₂O₃ (Central Drug House), V₂O₅ and Ni₂O₃ (Sisco Research Laboratories Pvt. Ltd) were purchased and used as received. NH₃ and N₂ gases were used as the N source and carrier gas, respectively.

2.2 Synthesis of boron nitride nanotubes

In this synthesis, a simple annealing process was adopted as reported by Tang et al.²⁷ The reactions were performed in a horizontal tubular furnace, consisting of an alumina tube. The reactant mixtures, B/V₂O₅/Fe₂O₃ or B/V₂O₅/Ni₂O₃, were mixed adequately with the weight ratio 2 : 1 : 1 and loaded in an alumina boat. The boat was placed at the center of the tubular furnace. The ends of the alumina tube were closed with steel lids, carrying a gas inlet and outlet. Initially, the furnace was programmed with a ramp rate of 5 °C min⁻¹ and heated to 900 °C under N₂ gas flow (50 sccm) to eliminate the residual air. Then the temperature was raised to 1000–1200 °C and maintained for 1 h in an NH₃ atmosphere (50 sccm). After this time, the furnace was slowly cooled to room temperature under an N₂ atmosphere.

The as-synthesized samples were weighed and purified by stirring with 2 M HCl solution for 6 h at room temperature. The product was further treated with 1 M HNO₃ for 24 h at 50 °C, to remove all the metal particles and amorphous boron powder. Finally, it was filtered, washed with distilled water and dried at 100 °C.²⁸

2.3 Characterization methods

The crystalline structure of the samples was investigated by X-ray diffraction (XRD) analysis using a Cu Kα (λ = 1.54 Å) radiation source at room temperature, operated at 40 kV, 230 mA, at a scan rate of 0.02° (2θ) per s from 10 to 80° (2θ). The Fourier transform-infrared (FT-IR) spectra were recorded on a Perkin Elmer spectrophotometer. The morphological changes of the samples were studied by scanning electron microscopy (SEM, JEOL-840) and their elemental compositions were identified by energy dispersive spectroscopy (EDS). Nanostructures were viewed by a transmission electron microscopy (TEM, JEOL-2100) instrument operated at 200 kV. In the analysis, the samples were ultrasonically dispersed in acetone and dropped onto a carbon-coated copper grid. Raman spectroscopy analysis was performed using a WITec alpha 300R instrument with the 532 nm laser line from a Nd:YAG source. X-ray photoelectron spectroscopy (XPS) was performed using an Omicron Nanotechnology GMBH spectrometer employing a monochromatic Al Kα (1486.6 eV) X-ray source. All XPS profiles were corrected according to the binding energy of carbon (C–C) at 284.6 eV.

3. Results and discussion

3.1 Structural and morphological studies of BNNTs

3.1.1 X-ray diffraction analysis. The crystalline natures of BNNTs were examined by XRD analysis. Fig. 1a–c presents the XRD patterns corresponding to purified BNNTs obtained during the reaction using B/V₂O₅/Fe₂O₃ and NH₃ at various temperatures: 1000, 1100 and 1200 °C. The as-synthesised product obtained at 1100 °C in Fig. 1 (inset pattern) clearly shows a mixture of phases, such as hexagonal BN (h-BN), VO₂, VO and Fe. The peaks related to VO₂ are inferred from XRD patterns at 2θ values of around 31.5°, 33.3°, 35.5° and 65.06° (JCPDS no.: 82-1074). The peaks at 2θ values of 37.7° and 44.67° confirm the VO phase (JCPDS no.: 77-2173). The formation of VO₂ and VO is due to the reduction of V₂O₅. During the annealing process, the reduction of Fe₂O₃ to Fe occurred completely. The presence of Fe is deduced from the observation of peaks at 2θ values of 43.6° and 63.7° (JCPDS no.: 89-4186). The peaks at 2θ values of 26.67°, 41.85°, 53.59°and 76.38° correspond to the h-BN phase. The formation of h-BN is due to the oxidation of B and the reduction process of V₂O₅/Fe₂O₃.

Fig. 1 XRD patterns of BNNT formed over B/V₂O₅/Fe₂O₃ at (a) 1000 °C, (b) 1100 °C, (c) 1200 °C. The inset shows the XRD pattern of the as-synthesized sample obtained at 1100 °C.

The XRD patterns in Fig. 1a–c show the reflections of the h-BN phase at (002), (100), (101), (102), (004), (104) and (110). It is also observed that the crystallinity of the h-BN phase increases with increasing temperature. In Fig. 1b, the diffraction of h-BN at 2θ values of 26.5°, 41.7°, 43.5°, 50.6°, 54.6°, 72.5° and 75.6° corresponds to interplanar spacings of 3.36, 2.16, 2.07, 1.80, 1.67, 1.30 and 1.25 Å respectively. The calculated lattice constant values, a = 2.49 and c = 6.72 Å ,are consistent with JCPDS file no.: 85-1068.

Fig. 2 shows the XRD pattern of the BNNTs formed over B/V₂O₅/Ni₂O₃ at 1000, 1100 and 1200 °C. The inset XRD pattern represents the as-synthesized product obtained at 1100 °C. This XRD pattern showed peaks related to h-BN, NiO, Ni, VO₂ and VO. The formation of the NiO and Ni phases is due to the reduction of Ni₂O₃. The characteristic peaks at 2θ values of 37.3° and 43.2° correspond to the NiO phase (JCPDS file no.: 89-3080). Similarly, the peak at a 2θ value of 44.4° corresponds to Ni (JCPDS file no.: 88-2326). The presence of the other two phases, VO₂ and VO, is due to the reduction of V₂O₅, which was observed as in the case of B/V₂O₅/Fe₂O₃. The diffraction peaks at 2θ values of 26.29°, 42.4° and 76.38° form the fingerprint of the h-BN phase.

Fig. 2 XRD patterns of BNNT formed over B/V₂O₅/Ni₂O₃ at (a) 1000 °C, (b) 1100 °C, (c) 1200 °C. The inset shows the XRD pattern of the as-synthesized sample obtained at 1100 °C.

Fig. 2a–c represents the XRD pattern of purified BNNT formed over B/V₂O₅/Ni₂O₃. The entire diffraction pattern depicts the formation of (002), (100), (004), (104) and (110) planes related to h-BN. It is observed that the peaks related to metal oxide were totally absent. Noteworthy, the intensity of the peaks relating to the h-BN phase increase with increasing temperature. In Fig. 2b, the calculated d-spacing values of 3.39, 2.17, 1.68, 1.30 and 1.25 Å correspond to the (002), (100), (004), (104) and (110) planes, respectively. The lattice constants, a = 2.50 and c = 6.77 Å, were found to match with JCPDS file no.: 34-0421.

The presence of Fe₂O₃ and Ni₂O₃ in the reactant mixture plays a vital role in the formation of BNNTs. In general, Fe and Ni catalyze the reaction and influence the formation of BNNTs. In this study, Fe₂O₃ and Ni₂O₃ completely reduce to Fe and Ni respectively. Although NiO is more stable than Ni₂O₃, the formation of Ni by reduction is more possible at lower temperatures while using Ni₂O₃.²⁵

3.1.2 Scanning electron microscopy analysis. The effect of various reaction temperatures, 1000, 1100 and 1200 °C, on the growth of BNNTs over B/V₂O₅/Fe₂O₃ and B/V₂O₅/Ni₂O₃ was studied. Fig. 3a–c illustrates the SEM images of as-synthesised BNNTs obtained at various temperatures using B/V₂O₅/Fe₂O₃. At 1000 °C, the formation of bulk BN dominates the BNNTs formation. According to a thermodynamic study, VO₂ was found to be in a liquid state above 953 °C.²⁹ In this state, liquid VO₂ and B/Fe₂O₃ (solid) would mix with each other; this initiates the reduction of Fe₂O₃ to Fe and also oxidizes B to B₂O₂ at an earlier stage. The reaction between B₂O₂ and NH₃ initially creates B/N species, and the development in the growth phase resulting in the formation of coiled like structures was shown in Fig. 3a. When the temperature was raised to 1100 °C, the nucleation of BN occurs on the surface of finely dispersed Fe particles and, upon supersaturation, BN shells are precipitated layer by layer.¹³ At this stage, the morphology of BNNTs was observed to be straight rather than as coiled structures, as shown in Fig. 3b. Similar characteristics of the BNNTs were also found at 1200 °C (Fig. 3c). So, based on these results, it is well understood that a reaction temperature of 1100 °C yields high quality BNNTs compared with a temperature of 1000 °C. The EDS spectrum shown in Fig. 3d represents the elemental composition of BNNTs obtained at 1100 °C. The peaks confirmed the existence of B and N in the sample.

Fig. 3 SEM images of BNNTs formed over B/V₂O₅/Fe₂O₃ at (a) 1000 °C, (b) 1100 °C and (c) 1200 °C, and (d) EDAX spectrum obtained of the BNNTs at 1100 °C shown in image (b).

Similarly, the SEM images of as-synthesised products, obtained using B/V₂O₅/Ni₂O₃ at various temperatures, 1000, 1100 and 1200 °C, are shown in Fig. 4. It was observed that BNNTs were grown at 1000 °C. The theoretical background to this reaction process states that the metastable Ni₂O₃ starts to decompose at 600 °C and a liquid to solid state transition (NiO) occurs at an earlier stage. As stated earlier, V₂O₅ decomposed to VO₂ and VO and attained a liquid state above 953 °C. Both these reduction reactions in turn oxidize boron and generate enormous amounts of B₂O₂ vapors. When the temperature reaches 1000 °C, these B₂O₂ vapors react with NH₃ gas and promote the precipitation of BN over the Ni catalyst. The nucleation process initiates the growth of nanotubes by layer deposition on the surface of the catalyst, which tends to form BNNTs as shown in Fig. 4a. The maximum yield of BNNTs was obtained at 1100 °C compared to other temperatures. Fig. 4d presents the EDS spectrum of BNNT obtained, depicting the presence of B and N. The additional peaks in Fig. 4d originate from unreacted species.

Fig. 4 SEM images of BNNT formed over B/V₂O₅/Ni₂O₃ at (a) 1000 °C, (b) 1100 °C and (c) 1200 °C and (d) EDAX spectrum taken for BNNTs at 1100 °C shown in image (b).

3.1.3 Transmission electron microscopy analysis. The morphologies of the BNNTs synthesised from B/V₂O₅/Fe₂O₃ at 1100 °C were analyzed by TEM as shown in Fig. 5. The tubes were observed to be multiwalled. Fig. 5a represents the TEM image of as-synthesised BNNTs. It is clearly evidenced that the tubes were grown from the solid support. The morphologies of the purified BNNTs were displayed in Fig. 5b–e. The selected area electron diffraction (SAED) pattern in Fig. 5d (inset image), taken from the central part of the nanotubes, corresponds to (002), (100), (101), (104) and (110) of hexagonal BN. The crystalline nature of BNNTs formed at 1100 °C, confirmed by the SAED pattern, is consistent with the XRD pattern. It was observed from Fig. 5e that the BNNTs possess inner and outer diameters of 7.67 and 17.2 nm, respectively.

Fig. 5 TEM images of BNNTs formed over B/V₂O₅/Fe₂O₃ at 1100 °C. Low magnification image indicating as-synthesised BNNTs grow from solid precursors (a), high resolution TEM images of purified BNNTs (b–e), selected area electron diffraction pattern taken from the tube surface inset in image (d).

The morphologies of BNNTs synthesised from B/V₂O₅/Ni₂O₃ at 1100 °C were studied by TEM analysis as shown in Fig. 6. The TEM images of the as-synthesised BNNTs are shown in Fig. 6a. We can see from Fig. 6a that the tubes are straight in shape and have originated from the support. These tubes were comparatively longer with a smaller diameter than the tubes formed over B/V₂O₅/Fe₂O₃. Fig. 6b–e denotes the TEM images of purified BNNTs taken at various magnifications. The lengths of the tubes were found to be >1 μm. The bunching of BNNTs and multi-walled layers can be clearly identified from the TEM images. The tubes had both opened and closed ends. The inset SAED pattern shown in Fig. 6c corresponds to the (002), (100), (004), (104) and (110) crystalline planes of the h-BN structure, which coincide with the XRD analysis. These results confirm the formation of highly crystalline BNNTs. The BNNTs obtained in our case were similar in morphology to those reported by Zhi et al.³⁰ The inner and outer diameters of the tubes were found to be 5.16 and 10.96 nm, respectively.

Fig. 6 TEM images of BNNTs formed over B/V₂O₅/Ni₂O₃ at 1100 °C, low magnification image indicating as-synthesised BNNTs grown from solid precursors (a), high resolution TEM images of purified BNNTs (b–d) and corresponding selected area electron diffraction pattern taken from the tube surface, inset in image (d).

3.1.4 Fourier transform-infrared spectroscopy analysis. The FT-IR spectra of the purified BNNTs formed over B/V₂O₅/Fe₂O₃ and B/V₂O₅/Ni₂O₃ at 1100 °C are shown in Fig. 7. The spectra indicate the formation of h-BN. The two broad peaks at 1384 and 1382 cm⁻¹ correspond to the B–N stretching vibrations, and the sharp peaks centered at 788 and 786 cm⁻¹ were due to the B–N–B bending vibrations.^31,32 The peak at 2300 cm⁻¹ was assigned to boron and that at 2518 cm⁻¹ was due to CO₂ adsorption from the atmosphere. The weak bands at 3412 and 3413 cm⁻¹ were due to the O–H stretching vibrations of water.^14,33

Fig. 7 FT-IR spectra of purified BNNTs formed over B/V₂O₅/Fe₂O₃ (a) and B/V₂O₅/Ni₂O₃ (b) at 1100 °C.

3.1.5 Raman spectroscopy studies. The crystalline natures of BNNTs were further analyzed by Raman spectroscopy. Fig. 8a and b correspond to the Raman spectra of purified BNNTs formed over B/V₂O₅/Fe₂O₃ and B/V₂O₅/Ni₂O₃ at 1100 °C. Both the spectra exhibit only one sharp peak at 1367 cm⁻¹. The peak corresponds to the in-plane vibrational Raman active E_2g mode of h-BN. In this mode, the B and N atoms were moving against each other within a plane. Lee et al. have also reported a similar observation for BNNTs synthesized from B, MgO and FeO mixtures.³⁴ This result confirms that the purified BNNTs were crystalline and free from impurities.

Fig. 8 Raman spectra of purified BNNTs formed over B/V₂O₅/Fe₂O₃ (a) and B/V₂O₅/Ni₂O₃ (b) at 1100 °C.

3.1.6 X-ray photoelectron spectroscopy analysis. The binding nature of h-BN in BNNTs were investigated using XPS spectroscopy. Fig. 9 presents XPS spectra of the purified BNNTs obtained using B/V₂O₅/Fe₂O₃ at 1100 °C. The XPS spectra corresponding to purified BNNTs obtained from B/V₂O₅/Ni₂O₃ were shown in Fig. 10. The presence of carbon and oxygen peaks in both the cases are due to surface contamination during the exposure of the sample to the air before analysis.³⁵ The survey spectrum in Fig. 9a shows the presence of boron and nitrogen in BNNTs. Fig. 9b shows the XPS profile related to B 1s at 190.7 and 188.2 eV. These peaks were ascribed to B–N bonding. The XPS spectrum showing the N 1s peak is shown in Fig. 9c. The origin of the peaks at 398.4 eV and 396.7 eV can be ascribed to N atoms in the N–B bonds of typical h-BN species, as reported in the literature.^36,37

Fig. 9 XPS spectra of purified BNNTs formed over B/V₂O₅/Fe₂O₃ at 1100 °C; (a) the survey spectrum, (b) B 1s and (c) N 1s.

Fig. 10 XPS spectra of purified BNNTs formed over B/V₂O₅/Ni₂O₃ at 1100 °C; (a) the survey spectrum, (b) B 1s and (c) N 1s.

Fig. 10a shows the survey spectrum of BNNTs formed over B/V₂O₅/Ni₂O₃. This indicates the presence of boron and nitrogen in the BNNTs. The individual spectra of boron and nitrogen are shown in Fig. 10b and c, respectively. The peaks at 190.1 and 188.9 eV correspond to the B 1s band and are attributed to B–N bonding. The N 1s peaks at 397.9 and 396.5 eV indicate N–B bonding of h-BN.³⁸

3.1.7 Yield and purity of the synthesized BNNTs. The yields and purities of the BNNTs grown from B/V₂O₅/Fe₂O₃ and B/V₂O₅/Ni₂O₃ at 1100 °C were summarized in Table 1. About 0.805 g of the reactant mixture (B/V₂O₅/Fe₂O₃) yielded 0.836 g of product after reaction. Consequently, after acid purification, 0.818 g of BNNTs were obtained. Similarly, 0.802 g of fine mixtures of B/V₂O₅/Ni₂O₃ produced 0.846 g of product, yielding 0.834 g after acid purification. The percentage yields after purification were found to be 97.8 and 98.5% in the case of B/V₂O₅/Fe₂O₃ and B/V₂O₅/Ni₂O₃, respectively. The results obtained from XRD, TEM, Raman and XPS analyses account for the purity of the formed nanotubes. The XRD patterns of purified BNNTs in Fig. 1 and 2 clearly show the hexagonal BN phase of the nanotubes formed, and this was further confirmed by the Raman spectra with the E_2g vibrational mode of h-BN. The TEM images of BNNTs evidenced that BNNTs were free from impurities. Consequently, this also confirms the formation of pure BNNTs.

Table 1 Summary of characterization techniques, indicating the yield and the purity of BNNTs

Samples	% yield	XRD	TEM	Raman	XPS
B/V2O5/Fe2O3	97.8	h-BN phase	Inner dia. – 7.67 nm, outer dia. – 17.2 nm	h-BN	h-BN
B/V2O5/Ni2O3	98.5	h-BN phase	Inner dia. – 5.17 nm, outer dia. – 10.9 nm	h-BN	h-BN

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3.1.8 Catalytic activity and its influence on the growth of BNNTs. The catalytic activity of Fe₂O₃ and Ni₂O₃ in the growth of BNNTs was investigated by the addition of V₂O₅. When the reaction mixtures composed of B/V₂O₅/Fe₂O₃ were subjected to heating, V₂O₅ first started to melt at 690 °C.³⁹ The thermodynamic study of VO₂–V₂O₅ suggests that V₂O₅ is reduced to VO₂ at higher temperatures. It was reported that at 953–1081 °C, liquid and solid curves of VO₂ exist.²⁹ Due to the low decomposition temperature, the reduction of V₂O₅ was achieved by a combination of diffusion, coalescence and stabilization processes.⁴⁰ In this study, at 900 °C, NH₃ gas was passed to reduce V₂O₅ to VO₂ and VO. Similarly, Fe₂O₃ was also reduced to Fe as shown in eqn (A1). The presence of Fe, VO₂ and VO was confirmed by XRD analysis. Both of these reduction processes enhance the oxidation of boron to form B₂O₂ vapors. Based on the vapor–liquid–solid (V–L–S) mechanism, it was suggested that the B/N nucleation process occurred when B₂O₂ vapors started to react with NH₃, and led to the formation of the BN at the initial stage.⁴¹ When the temperature was raised from 1000–1200 °C, generated B₂O₂ vapors react with NH₃ to form BNNTs, as predicted by previous reports. Such processes facilitated the formation of BNNTs.⁴² Here, we suggest that the B₂O₂ vapors might have reacted with NH₃ to form intermediate HO–B=N–H vapors, as shown in eqn (A2). These vapors might be adsorbed and activated by the Fe/Ni clusters in the molten liquid to form BNNTs. The clusters decomposed HO–B=N–H to form BN, which might subsequently react on the surface of the catalyst to form BNNTs.

10B_(s) + 2V₂O_5(s) + 2Fe₂O_3(s) ⇄ 5B₂O_2(g) + 2VO_2(s,l) + 2VO_(s,l) + 4Fe (A1)

Also, the mechanism of formation of BNNTs from B/V₂O₅/Ni₂O₃ was similar to the above process, except for the formation of NiO and Ni. The formation of NiO and Ni was confirmed by XRD. Hence, this study establishes that V₂O₅ is an efficient oxidizing agent.

4. Conclusion

In summary, through a simple annealing approach, BNNTs were successfully synthesized. These BNNTs have distinct characteristics with controlled diameters and long lengths. The formed BNNTs were found to be of the h-BN phase and of high crystallinity. This method can be extended to the synthesis of various BN based nanostructures. The synthesized BNNTs can be used as nanovectors in the future due to their non-toxic nature.

Acknowledgements

One of the authors, Jeghan Shrine Maria Nithya, is thankful to the AICTE for the award of the NDF (National Doctorial Fellowship) and also to the Department of Chemistry, for providing the facilities sponsored by the DST (FIST).

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