Citation

Zhuang Cui-Cui, Feng Jia, Xu Hong, Li Ling, Liu Xiao-Wei.Synthesis of boron nitride nanotube films with a nanoparticle catalyst[J]. Chin. Chem. Lett., 2016,27(06): 871-874  

Zhuang Cui-Cui, Feng Jia, Xu Hong, Li Ling, Liu Xiao-Wei     

Since Rubio theoretically predicated the existence of the stable boron nitride nanotubes (BNNTs) in 1994 ^[1],BNNTs have attracted extensive attention because of their superior mechanical properties,excellent thermal stability and chemical stability,which make them a promising candidate in a wide range of applications ^[2-4]. Several methods have been used to produce BNNTs,such as laser ablation ^{[5, 6]},substitution reactions ^{[7, 8]},chemical vapor deposition (CVD) ^[9-12] and ball milling ^{[13, 14]}. Moreover,the mixtures of iron oxide and other metal oxides,such as the mixture of MgO and FeO,the mixture of MgO and Fe₂O₃,have been used as catalysts under NH₃ atmosphere in recent research. In this article,taking account of remarkable catalytic activity of nanoparticle catalysts,we investigated the growth of BNNT films using nano-Fe₃O₄ as a catalyst based on ball milling and thermal chemical vapor deposition under N₂ with 15% H₂ atmosphere,which is much safer than NH₃ atmosphere.

A crystalline boron powder (B,99%) as a raw material,was milled for 18 h at a rotating speed of 200 rpm in a horizontal-type high energy ball mill under N₂ atmosphere to enhance the reaction activity of the precursor. The balls with a diameter of 6 mm were used to smash and grind B powder. The weight ratio of hardness stainless steel ball to boron powder was 30:1. Then the milled B powder and nano-Fe₃O₄ (diameter: ×20 nm) were dispersed into an ethanol solution to formB ink after 20 min ultrasonication. The mass ratio of the catalyst to B powderwas 0.25:1. Then the inkwas poured into an alumina sintering boat,and several stainless steel substrates were placed on the top of the alumina sintering boat. Moreover,the ink was also smeared on the upper surface of the stainless steels with a brush. The alumina boat covered with stainless steels was placed in a cylindrical crucible,and then was pushed into a tube furnace and annealed at 1150℃ for 1 h under N₂ with 15% H₂. After the furnacewas cooled to roomtemperature naturally,white BNNT films can be found on both sides of the stainless steels.

Themorphology and chemical composition of the as-synthesized BNNT films without purification were studied using X-ray diffraction (XRD,PANalytical Empyrean),scanning electron microscopy (SEM,TescanVEGA3 SBH) and energy dispersive X-ray spectroscopy (EDS,ThermoNORAN) attached to the SEM. The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were performed using a JEOL JEM-2010 instrument.

Fig. 1a presents the image of the as-synthesized BNNT film,showing a snow-white looking. The whole stainless steel substrate with an area of 2 - 1 cm² is covered by BNNT film,confirming the synthesis of a mass of BNNTs. Fig. 2b-d present the SEM images of the as-synthesized BNNT films without purification. High-density and high-quality BNNT films were fulfilled without obvious impurity phases or particles. The as-produced BNNTs have a diameter in the range of 50×200 nm with an average length of more than 40 mm. Besides the morphology that BNNTs grew in random orientations as shown in Fig. 1b,BNNTs also were wellaligned in some regions (Fig. 1c),which could be a result of the direction of brushing ink and the gas flow ^{[15, 16]}. In addition to the BNNTs in bulk quantities being obtained,two-dimensional BNNT nanojunction structures with various shapes,such as ringlike,Y type and treelike,were found as shown by the arrows in Fig. 1d. Moreover,almost all the nanojunction structures were observed on the surface of BNNT films. The generated nanojunctions made of nanotubes may have potential applications in nano devices,such as nano systems and nano circuits ^[17-19].

Figure 1

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Figure 1. Image of the as-synthesized BNNT film: (a) and the SEM images; (b) unordered films; (c) ordered films; (d) nanojunction structures

Figure 2

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Figure 2. EDS spectrum and TEM micrographs of as-synthesized BNNTs: (a) EDS spectrum; (b) TEM image of BNNTs; (c) TEM image of short bamboo structure; (d) TEM image of long bamboo structure; (e) and (f) HRTEM images.

An EDS spectrum taken from the samples is shown in Fig. 2a. It clearly demonstrates that the dominant elements in the films are B (50.62%) and N (45.01%) with an atom ratio of approximating to 1:1,which shows the synthesis of BNNTs. O was found in the EDS profile,which is common in the synthesis of nano materials using CVD. The O may come from the oxidation of milled B powder.

Detailed microstructure characterizations of the BNNTs are illustrated in TEM images (Fig. 2b-f).TEM images reveal there are two types of bamboo BNNTs,which are similar to that of the BNNTs reported previously ^[20-22]. Both types of BNNTs possess multiwalled and spindle-like structures,in which all the spindles stretch along the same direction. The TEM image (Fig. 2c) of the first type BNNTs with short distance bamboo structures show that the BNNT owns a bended morphology,additionally,the bamboo at the bending point (indicated by arrow) is slightly longer compared to the other bamboos. The internal diameter of bamboo-like structures continuously and steeply varies from point to point,very similar to a spindle,while the outer diameter of BNNTs remains the same. They possess a larger diameter and thicker wall than the second type BNNTs shown in Fig. 2d. The second type BNNTs with long distance bamboo structures exhibit long spindle and the length of individual spindle is about 500 nm. The internal diameter of BNNTs remains almost the same,and only varies on the local sharp terminals. Owing to the accepted correlation between the catalyst size and nanotube diameter and the fact that reducing the catalyst size is found to decrease the final nanotube diameter ^{[21, 23, 24]},we argued that the difference of above mentioned two types of BNNTs is due to different aggregation degree of catalyst particles and agglomeration produces greater particles,increasing the nanotube diameter. Fig. 2e describes the high-resolution TEM (HRTEM) image of the tube wall segment of BNNT in Fig. 2d. The wall with a thickness of 20 nm is built up of layer-by-layer stacked BN graphite-like layers. The wall of BNNT presents clearly aligned lattice fringes with an estimated interplanar spacing of 0.34 nm,which is consistent with the d002 spacing of the hexagonal phase of BN (h-BN) ^{[25, 26]}. Generally speaking,the lattice fringes are continuous and clear,but the lattice fringes are discontinuous in some regions of the wall,as shown in the red circle region in Fig. 2f. Those discontinuous regions may be caused by defects and impurity particles,such as Fe and O.

The XRD pattern taken from the BNNT films is shown in Fig. 3. Some peaks corresponding to h-BN,Fe and Fe₂N,respectively are clearly observed. In addition,other impurity peaks,such as the peaks of FeO,Fe₂O₃ and Fe₃O₄,were not observed in this pattern. It indicates that the reactions involving the dissociation of Fe₃O₄ took place during this experiment.

Figure 3

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Figure 3. The XRD pattern of the BNNT film

Based on the above results and the previous mechanistic studies ^[27-30],the possible synthetic mechanism of BNNTs is suggested to be vapor-liquid-solid (VLS) related ^[31]. The formation and intermediate processes possibly occurred during the synthesis of BNNT are shown below:

At the growth temperature (1150 ℃),milled B powder reacts with nano-Fe₃O₄ generating gaseous B₂O₂,which is essential to react with nitrogen source during the formation of BNNTs on the substrate ^[11]. The nano-Fe₃O₄ is reduced to Fe particles,as illustrated by Fe peaks in Fig. 3. When there is sufficient vapor pressure in the reaction region,BN,a product of B₂O₂ and N₂/H₂,gradually dissolves into the catalyst liquid particles. Once reaching the supersaturated state,BN precipitates and the formation of BNNTs proceeds. The growth of nanojuction structures may be due to the second growth of nanotubes,which is somewhat similar to that of nanojunction previously proposed ^{[32, 33]}. When there is sufficient vapor pressure and reactant on the surface of BNNT film,providing the suitable conditions for growth of BNNTs,Fe particles on the tips or the surfaces of the tubes on the surface of BNNT film drive the secondary growth of the nanojunction structures and lead to the formation of nanojunction structures.

Some researchers have demonstrated that the size of the catalyst particles has a big impact on the diameter of nanotubes and the diameters of the grown nanotubes tend to be slightly smaller than the particle size ^[34]. However,the as-synthesized BNNTs in this experiment have larger diameters and thicker tube walls,not meeting our expectation. An explanation for the deviation between experimental results and theoretical prediction is that the aggregation of nanoparticle catalyst is severe,resulting in greater catalyst particles during the process of BNNT growth. Accordingly,the as-synthesized BNNTs have larger diameters. In addition to large catalyst drops,the nanoparticle catalyst has good catalytic activity and can enhance the chemical reactions,driving the formation of the BN phase and thus the growth of a thicker tube wall.

In summary,BNNT films were synthesized using a nanoparticle catalyst. The as-produced BNNTs with bamboo-like structure have a diameter in the range of 50 ~ 200 nm with an average length of more than 40 μmm. In addition,BNNT nanojunction structures were synthesized. A possible growth mechanism of BNNTs and BNNT nanojunction structures was proposed. Moreover,we propose the aggregation of nanoparticle catalyst results in the formation of BNNTs with larger diameters and thicker tube walls. Further investigations of proper dispersion of nanoparticle catalyst without aggregation are necessary in order to control the diameter of BNNTs,which lays foundation for obtaining the desired shape of BNNTs by simply controlling the size of catalyst. This work will provide a promising approach to obtain the BNNTs with desired diameters and produce branched junctions of BNNTs.