Facile Synthesis of High-Quality Nano-Size ¹⁰B-Enriched Fibers of Hexagonal Boron Nitride

Abstract

The interesting properties of hexagonal boron nitride (h-BN) and its potential uses in thermo-structural advanced applications have been limited or restricted by its inherent brittleness, which can easily be eliminated by its fibers (h-BN) in nanoscale dimensions. The current study is based on the synthesis of nanoscale ¹⁰B-enriched fibers of h-BN (¹⁰BNNFs) from ¹⁰B in the precursors instead of B in two-hour annealing at 900 °C and one-hour growth at 1000 ^°C. All of the ¹⁰BNNFs are randomly curved and highly condensed or filled from ¹⁰h-BN species with no internal space or crack. XRD peaks reported the ¹⁰h-BN phase and highly crystalline nature of the synthesized ¹⁰BNNFs. ¹⁰h-BN phase and crystalline nature of ¹⁰BNNFs are confirmed from high-intensity peaks at 1392 (cm⁻¹) in Raman and FTIR spectroscopes.

1. Introduction

Nanofibers have a large surface area and porous structure due to which it has characteristic properties suitable for a variety application in the field of environmental sciences, automotive industries, smart textile and biomedical, etc. [1].

Boron nitride (BN) is an important material known as an artificial non-oxide ceramic. Hexagonal or cubical are the most commonly available phases of BN. Both of these polymorphs have interesting properties: high-temperature stability, high electric insulation, the low dielectric constant, large cross-section for thermal neutron and resistance to oxidation at high temperature, etc. [2,3,4]. Among all polymorphs of BN, hexagonal boron nitride (h-BN) is more interesting due to its stability and ease of synthesis or production. However, the inherent brittleness is the only drawback of h-BN which restricts its use in thermos structural advanced applications [2,5]. The brittleness issue can be resolved by preparing h-BN in fibers. Afterward, it can be used as composite materials. Such composites have all the interesting properties of nanoscale materials of h-BN. However, the first step in making such composites is the synthesis of h-BN fibers (BNFs), which are commercially not available in the market [5]. Because of this goal, many researchers have focused on developing a reliable technique for producing fibers of h-BN. Nanosize fibers of h-BN (BNNFs) were claimed with boron oxide as a precursor. For the increased spin ability, polyvinyl butyral (PVB) was added to the solution. The pyrolyzation of the as-formed product resulted in the synthesis of BNNFs with a diameter under 100 nm [6]. Next time, electrospun polyacrylonitrile (PAN) was coated with boron oxide solutions to form BNNFs [7]. Oxygen contents in the final product was one of the very serious issues related to the above techniques. Polyborazine/polyacrylonitrile blend solutions were also used as precursors for the synthesis of BNNFs [8]. The health-risky nature of the precursors and the complexity of the experimental procedure were some of the main problems associated with the above technique. Boric acid and melamine-based polymeric precursors had been used to synthesize BNFs with lengths in the range of 200–500 µm and diameters of 1–2 µm. The adopted procedure was claimed to be cost-effective, simple, and notified to form pure BNFs [2]. However, unlike the claim, the procedure was found to be complex and lengthy with carbon- and nitrogen-based compounds as impurities in the final product. The as-found impurities were almost impossible to eliminate even at a higher temperature of 1600 °C.

Regardless of the pros and cons of the above techniques, a simple and cost-effective technique was needed to synthesize fibers of h-BN. The fibers of h-BN needed to be not only pure and nanoscale but also ¹⁰B-enriched.

¹⁰B is one of the stable isotopes of Boron, which has a thermal neutron cross-section of 3840b. The daughter nuclei of ⁷Li and ⁴He are produced when a thermal neutron is captured by ¹⁰B according to the nuclear reactions given as below [9,10]:

¹⁰B + n → ⁷₃Li + ⁴₂He     2.792 MeV (ground state) 6%

(1)

¹⁰B + n → ⁷₃Li + ⁴₂He     2.310 MeV (excited state) 94%

(2)

¹⁰B-enriched nanofibers of h-BN (¹⁰BNNFs) are large bandgap semiconductors. The emitted daughter’s nuclei when accelerated into the semiconductor layer of ¹⁰BNNFs produce electron-hole pairs. These pairs, when detected by their respective electrodes, can be shown in the form of electrical signals. Thus, ¹⁰BNNFs can be a potential element in solid-state neutron detectors like ¹⁰BNNTs [11] and other nuclear feeding activities. In this reference, it can also be a useful material in boron neutron capture therapy (BNCT), bio-medical and drug delivery, etc.

The method for the synthesis of ¹⁰BNNFs in the current study can be traced back to the Boron oxide chemical vapor deposition (BOCVD) technique for the synthesis of BNNTs. In BOCVD, Boron (B) and Magnesium oxides (MgO) were developed as carbon-free precursors for the synthesis of BNNTs in high quality [12]. The BOCVD precursors for the BNNTs synthesis were further developed by the addition of iron oxides (FeO). As a result, the mixture of B, MgO, and FeO was developed as effective precursors. The precursors resulted in the synthesis of high-quality of BNNTs [13]. The as-introduced precursors were obtained in nano-size with γ-Fe₂O₃ instead of FeO. Such a precursor’s mixture has successfully been optimized for the synthesis of nanotubes, nanowires, nanosheets, and microtubes, etc., of h-BN [14]. The optimization also resulted in a simple technique for the synthesis of BNFs. The work on the synthesis of BNNTs showed that the replacement of B by ¹⁰B resulted in enriched boron nitride nanotubes (¹⁰BNNTs) [15]. The logics of ¹⁰BNNTs and BNFs lead to the synthesis of enriched boron (¹⁰B) nitride nanofibers. The detail of the methodology thus developed for the synthesis of ¹⁰BNNFs is given in the next section.

2. Materials and Methods

A 100 mg mixture of Magnesium oxides (MgO) and Iron Oxides in a 1:1 ratio is uniformly mixed with enriched Boron (¹⁰B) powder of 100 mg in weight. The mixture is put in an alumina boat and annealed in the Argon atmosphere at 900 °C for 2-hours. Afterward, the annealed mixture in the boat is partially covered via a few Silicon (Si) substrates and pushed inside one end closed quartz tube already placed in the quartz tube chamber (beneath the heating filament) of the horizontal tube furnace. The furnace is seal closed and afterward flushed with Argon gas with a flow rate of 100 sccm. Consequently, the furnace is programmed to heat the precursor’s mixture with a heating rate of 10 °C/min. During heat-up, Argon gas flow (100 sccm) is maintained as a source of an inert atmosphere. In such a condition, the precursor’s mixture is heated up to 800 °C [16]. At 800 °C, NH₃ (200 sccm) is introduced into the system as a reactive gas. Under such a condition, the precursor’s mixture is further heated up to 1000 °C. Growth of the final material was assumed to occur at 1000 °C. Therefore, 1000 °C is fixed as the final temperature. At the final temperature, the system is maintained for 1-hour to complete the growth. Subsequently, the reactive gas (NH₃) flow is stopped whereas the inert (Argon) atmosphere was maintained till the system was brought to room temperature.

3. Results and Discussions

B, MgO, and FeO were introduced as effective precursors for the synthesis of BNNTs [13]. The replacement of enriched Boron ( ¹⁰B) in the precursors with MgO and γ-Fe₂O₃ in the precursors resulted in the synthesis of enriched Boron nitride nanotubes (¹⁰BNNTs) [15]. Along with the precursors, variations in the experimental parameters have also been found to play a crucial role in the synthesis of different nano (10⁻⁹) or micro (10⁻⁶) structures of hexagonal boron nitride (h-BN) [14,17]. As a strategy, experimental procedures were successfully optimized in two stages of annealing and growth duration. As a result, a technique was developed for the synthesis of micron-scale fibers of h-BN [18]. In that technique, the precursor’s mixture (B, MgO, and γ-Fe₂O₃) was first annealed (in the inert atmosphere) for two hours at 900 °C. Afterward, the annealed mixture was first softened in the Argon atmosphere up to 800 °C. The growth was initiated at 800 °C with NH₃ as a reactive gas and then continued for two hours at 1000 °C [18]. In the present work, the goal was to synthesize not only fibers of h-BN in nano-size but also ¹⁰B-enriched. Therefore, changes had to be brought not only in precursors but also in the experimental parameters. As for precursors, ¹⁰B was introduced as a precursor with MgO and γ-Fe₂O₃ instead of Natural B whereas, for the reduction of size, the growth duration was reduced to one hour instead of two. The rest of the procedure remains the same as for the synthesis of micron size BNFs [18]. The changes result in the synthesis of high-quality nano-size ¹⁰B-enriched fibers of h-BN (¹⁰BNNFs). Figure 1 shows the as-synthesized ¹⁰BNNFs characterized by Field emission scanning electron microscopy (FESEM). Figure 1a shows a relatively lower magnification micrograph of the as-synthesized ¹⁰BNNFs. All of the ¹⁰BNNFs are randomly curved and dispersed at the top of the Si-substrate. In general, the ¹⁰BNNFs create a view of cooked noodles from the top. All the ¹⁰BNNFs are of variable size or diameter in the range of greater than 20 nm and smaller than 100 nm. The smaller size ¹⁰BNNFs lay among or beside larger size ¹⁰BNNFs. Some of them seem to stick with others and form clusters. The region with a cluster-like appearance is further magnified and viewed in higher magnification as shown in Figure 1b. In a higher magnification micrograph, most of the ¹⁰BNNFs seem isolated. A cluster-like appearance is a place in the micrograph where a higher density of smaller size ¹⁰BNNFs is present. Both smaller and larger size ¹⁰BNNFs have a variable diameter and randomly curved morphology with an average length of greater than five microns. No other species or morphologies can be seen in higher magnification. To internally look into the structure, the ¹⁰BNNFs sample is analyzed with the help of a high-resolution transmission electron microscope (HRTEM).

Figure 2a shows an individual ¹⁰BNNF from the as-synthesized ¹⁰BNNF sample. The ¹⁰BNNF looks like a round and solid concrete pillar, which is highly condensed or filled with no internal space or crack. The HRTEM shows a non-uniform or rough outer surface of the as-synthesized ¹⁰BNNF. The rough or non-uniform outer surface confirms the non-uniform or variable diameter of ¹⁰BNNF in the sample. To further confirm this fact and others, another ¹⁰BNNF is arranged on the TEM copper grid and characterized via HRTEM. The micrograph of this ¹⁰BNNF is shown in Figure 2b. It shows that the ¹⁰BNNF is curved from various points. These curves in the structure confirm the curved morphology of the ¹⁰BNNFs shown in Figure 1 via FESEM. Besides this, the ¹⁰BNNF has a solid or condensed structure. However, the condensation of the ¹⁰h-BN species in the current ¹⁰BNNF is different from the one shown in Figure 2a. Unlike the previous, the ¹⁰h-BN species in the current ¹⁰BNNF seems to be condensed from out to inside. As a result of this condensation, the ¹⁰BNNF attains a rope-like morphology. Some crack-like spots can also be seen near the center. However, these are not the cracks. These are the points where ¹⁰h-BN species from different directions meet and construct the final morphology. All the condensed species have visible lattice fringes with an interlayer spacing of ~0.34 nm, as shown in the inset on the upper right hand of Figure 2b. According to the available literature, these layer spacings are the characteristics of the h-BN lattice with a highly crystalline nature [19].

The possible plan for the formation (during different growth stages) of ¹⁰BNNF due to condensation of ¹⁰h-BN species is shown via a sketch in Figure 3 The sketch shows (a) the formation, (b) attraction or assembling (c) condensation, and finally (d) deposition of condensed ¹⁰h-BN species on Si substrate forming ¹⁰BNNF.

The compositions, crystalline nature, and phase of the as-synthesized ¹⁰BNNF sample are checked with X-ray diffraction (XRD). The as-obtained XRD pattern is shown in Figure 4. The peaks in the XRD pattern are spotted at different 2-theta values of 26.7°, 41.8°, 43°, 55°, 76.2°. The sharp and high-intensity peak at 26.7° stands for (002) planes in ¹⁰h-BN with a highly crystalline nature. The peaks with lower intensities reported at 41.8°, 43°, 55°, 76.2° correspond to (100), (101), (004) and (110) planes in ¹⁰h-BN [13,20]. The intensities of the peaks on one side indicate the crystalline nature of the sample and on the other side reflect the higher number of planes in that particular sample. Based on this logic, the majority of planes present in the current ¹⁰BNNF sample are (002).

Non-destructive Raman spectroscopy was used to further check the contents, phase, and crystalline nature of the synthesized ¹⁰BNNFs. Raman spectroscopy of the sample is carried out in the spectral range of 800–1600 (cm⁻¹) with a laser beam with a wavelength of 514 nm. The obtained Raman spectrum of the sample is displayed in Figure 5. The spectrum shows a high-intensity peak in the displayed spectrum in 1392 (cm⁻¹). This high-intensity peak according to the available literature corresponds to the E_2g mode of vibrations in ¹⁰h-BN (with B−10 content) or ¹⁰BNNFs [15,21]. During Raman spectroscopy of the sample, a small peak can also be seen in the spectrum at 1150 (cm⁻¹). Such a lower intensity peak in the spectrum can be assigned to the formation of boric acid. It might have been formed due to sample exposure to oxygen in the air or ¹⁰B₂O₃ contents in the sample with laser interaction during Raman spectroscopy [22].

Fourier transformed infrared (FTIR) spectroscopy of the ¹⁰BNNF sample is performed in the spectral range of 400–3500 (cm⁻¹). The as-obtained FTIR spectrum is shown in Figure 6. The FTIR spectrum shows a lower intensity peak at 800 (cm⁻¹). It is a weak absorption and corresponds to an out-of-plane R-mode. In this mode of vibrations, Boron and Nitrogen atoms move radially in or outward. Along with lower intensity, a higher intensity peak was also spotted in the FTIR spectrum at 1392 (cm⁻¹). It is a strong absorption corresponds to in-plane stretching in ¹⁰h-BN. In this mode, the atoms vibrate along longitudinal or tube axis called L-mode [21].

4. Conclusions

As the use of B, MgO, and γ-Fe₂O₃ as precursors mixture worked for the synthesis of nanotubes, nanowires, nanosheets, and microtubes, etc., of hexagonal boron nitride with different experimental parameters. Likewise, the same precursor’s mixture with two hours annealing and two hours growth duration resulted in micron size fibers of hexagonal boron nitride. This is because the ¹⁰B-enriched boron nitride nanotubes can be synthesized by using ¹⁰B instead of B in precursors with MgO and γ-Fe₂O₃. Similarly, the same precursor’s mixture (¹⁰B, MgO, and γ-Fe₂O₃), with a growth duration of one hour instead of two, resulted in enriched nanoscale fibers of h-BN. ¹⁰B enrichment, nanoscale properties, and semiconductor nature enable ¹⁰BNNFs a potential material to be used as a sensing element in solid-state neutron detectors, BNCT and biomedical applications, amongst others