Large-scale fabrication of boron nitride nanotubes with high purity via solid-state reaction method
© Pan and Chen; licensee Springer. 2014
Received: 1 August 2014
Accepted: 23 September 2014
Published: 7 October 2014
An effective solid-state reaction method is reported for synthesizing boron nitride nanotubes (BNNTs) in large scale and with high purity by annealing amorphous boron powder and ferric chloride (FeCl3) catalyst in ammonia atmosphere at elevated temperatures. FeCl3 that has rarely been utilized before is introduced not only as a catalyst but also as an efficient transforming agent which converts boron powder into boron chloride (BCl3) vapor in situ. The nanotubes are bamboo in shape and have an average diameter of about 90 nm. The effect of synthetic temperatures on nanotube morphology and yield is investigated. The photoluminescence (PL) measurement shows emission bands of the nanotubes at 354, 423, 467, and 666 nm. A combined growth mechanism of vapor–liquid-solid (VLS) and solid–liquid-solid (SLS) model is proposed for the formation of the BNNTs.
As an isostructural and isoelectronic analog to carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs) have attracted more and more attention on account of their superb mechanical property , outstanding thermal conductivity and stability , excellent radiation shielding , strong light emissions , and hydrogen storage capacity especially the bamboo-shaped BNNTs . In addition, recent theory simulations reveal that metal-doped (e.g., Fe, Al) BNNTs seem to be more sensitive to some specific absorbates [6, 7]. BNNTs are not cytotoxic and can be functionalized for interaction with proteins and cells . Partially vertically aligned BNNT films deposited on Si substrate exhibit strong water repellency . All these fascinating properties make BNNTs promising applications in many areas such as lasing action, gas detective sensors, nanocoating for composites, reinforcement additive , hydrogen storage devices , and biomedical materials.
Since the discovery of BNNTs in 1995 , the main approaches known for the growth of CNTs have been modified to synthesize BNNTs as well, for instance, arc discharge , laser ablation , and chemical vapor deposition . Unfortunately, however, the yield or purity of BNNTs prepared by such methods is normally disappointing in comparison with that of CNTs. The demands of property investigation and applications are required for an effective route to mass production of BNNTs with high purity. Chen et al.  developed a ball milling and annealing method as potential for large-scale production of BNNTs, which gave a conception that a solid-state reaction method would be an alternative technique in bulk synthesis of BNNTs. Nonetheless, particles usually coexisted with BNNTs in the product, which needed further purification process before application. In the current study, we report an effective solid-state reaction method which offers BNNTs with both large quantity and high purity by annealing a highly homogenous precursor composed of amorphous boron (B) powder and ferric chloride (FeCl3) in flowing ammonia (NH3) atmosphere. To the best of our knowledge, FeCl3 has never been employed as the catalyst for the synthesis of BNNTs. Furthermore, FeCl3 could also act as a crucial transforming agent that converted solid-state B powders to gaseous boron chloride (BCl3) in situ. The gaseous BCl3 possessed high reaction activity and guaranteed the formation of BNNTs under NH3 flow. The effect of synthetic temperature on the morphology and the growth mechanism of the BNNTs was discussed in detail. The photoluminescence (PL) property of the synthesized BNNTs was also investigated.
Synthesis of BNNTs
In a typical procedure, amorphous B powders and FeCl3 · 6H2O with a molar ratio of 1:0.05 were selected as the raw materials. Firstly, FeCl3 · 6H2O was dissolved in absolute ethyl alcohol, then B powders were added into the solution. The mixture was stirred and heated in the water bath at 40°C for 2 h in order to evaporate the solvent. Then the obtained paste-like mixture was dried at 55°C to thoroughly remove the ethanol. After, a highly homogeneous distributed precursor was prepared. The precursor was loaded into an alumina boat which was placed at the center of a tube furnace. Before heating up, high-purity NH3 flow was introduced to flush out the residual air in the chamber. Then the furnace was heated to 1,200°C at 50 mL min−1 NH3 flow and maintained for 5 h. Finally, the furnace was naturally cooled to ambient temperature under the protection of N2 flow.
Characterization of BNNTs
Afterwards, it was found that the color of the upper layered staring materials changed from brown into white; the white product was characterized to be BN nanosheet self-assembled microwires and will be discussed elsewhere. Beneath the white product, a gray product was obtained in the boat. The gray product was collected and characterized by scanning electron microscopy (SEM, Zeiss Merlin; Carl Zeiss AG, Oberkochen, Germany), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, JEOL JEM-2011; JEOL Ltd., Tokyo, Japan) equipped with X-ray energy dispersive spectrometer (EDS), and electron energy loss spectroscopy (EELS, FEI Titan 80–300; FEI, Hillsboro, OR, USA). The PL property of the product was measured at room temperature (Edinburgh FLS920, 300-nm excitation; Edinburgh Instruments, Hertfordshire, UK).
Results and discussion
At first, FeCl3 is reduced to FeCl2 by NH3. Along with heating up, FeCl2 goes through a series of transformation from solid phase to liquid and gas phase (Equations 1, 2, and 3). Proved by the thermodynamic calculations, FeCl2 boils up mildly, and liquid-phase FeCl2 is more reactive than gas-phase FeCl2 in transforming solid B powders into BCl3 vapor at experimental temperatures (Equations 4 to 5). When the temperature reaches to 1150 to 1,250°C, Fe melts down partially and absorbs the surrounding vapors of BCl3 and NH3 to form Fe-B-N alloy droplets. When the concentrations of the BN species in the droplets are greater than the saturation threshold, BN shells begin to precipitate layer by layer around the droplets (Equation 6). The thickness of BN shell gradually increases, and the diameter of newly formed BN shell decreases with the inward growth of the BN shells. The increasing curvature caused by smaller diameter of the inner BN shell results in growing stress energy between the shells and the catalyst core . When the stress energy reaches a certain degree, the molten core will be expulsed from the defective region of the BN shells and sequentially shrinks to locate at the door of the torn elongated BN shell. The expulsed catalyst core keeps on absorbing gas species and the next growth proceeds via the same way. The whole process is extremely similar to the VLS growth of bamboo-shaped BNNTs reported in literatures [27, 28]. Particularly worth mentioning in the current study is that the catalyst core will be expulsed out wholly or partially depending on the value of stress energy, which can explain the TEM observation of discontinuous encapsulated particles within the compartment cavities of the nanotubes. As the described procedure occurs in cycles, bamboo-shaped structures are finally formed. It can be found that the growth process of the nanotubes not only involves the absorption and dissolution of NH3 and BCl3 from gas phase but also includes the relatively slow diffusion of B from solid phase to the alloy droplets, which may determine the final growth rate of the nanotubes. Therefore, the growth of BNNTs in the current study follows a combination mechanism of VLS and solid–liquid-solid (SLS) models [27, 29].
When the temperature lowers to 1,150°C, the reactions (Equations 4 and 5) could still be carried out but the reaction rates reduce significantly. Moreover, the diffusion rate of B/N atoms on alloy droplet surfaces also decreases. Thus, the insufficiency of BCl3 vapor results in the formation of particles rather than nanotubes. As the reaction temperature rises to 1,250°C, both the diffusion rates of B/N atoms through the surface and the bulk of the catalyst droplets increase. However, the surface diffusion rate enhances significantly , which favors the formation of nanotube walls rather than the compartment layers. In addition, the crystalline perfection of the nanotubes improves with the rise of temperature and the compartment will endure stronger stress. Hence, a quasi-cylindrical structure with less strain will be more favorable based on the principle of minimum free energy. However, the nitridation reaction (Equation 6) performs very fast and the concentration of BCl3 vapor within the chamber increases so quickly within a short period of time that some of them will be flushed out of the chamber by NH3 flow. Therefore, the yield of the BNNTs diminishes.
High-purity BNNTs are successfully synthesized in large quantity by annealing FeCl3 and amorphous boron in NH3 atmosphere. FeCl3 not only provides catalyst Fe but also reacts with boron powders to generate BCl3 vapor in situ, which is vital for the formation of BNNTs. The BNNTs are bamboo in shape with an average diameter of about 90 nm. A combination growth mechanism of VLS and SLS models is proposed to govern the formation of BNNTs. The synthetic temperature affects the morphology and yield of the BNNTs greatly through influencing the generation of BCl3 vapor. With the rise of temperature, the structure of the BNNTs has a tendency to form from bamboo toward cylindrical, but the yield of the BNNTs tends to be lower. The BNNTs exhibit strong PL emissions at 354 nm, suggesting their potential applications in optical and electronic devices.
The project is supported by the National Science Foundation of China (Grant Nos. 51162001 and 51362008), Hainan University Scientific Research Funding (Grant No. kyqd1240), and local service project (Grant No. HDSF201308). The work makes use of the resources of the Beijing National Center for Electron Microscopy.
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