Growth of Boron Nitride Nanotube Over Al‐Based Active Catalyst and its Application in Thermal Management

The effective identification of the active catalytic phase is essential to elucidate the growth mechanism of boron nitride nanotubes (BNNTs) and realize their controllable and scalable synthesis. However, owing to the complexity of chemical reactions during BNNT growth via chemical vapor deposition (CVD) and the lack of techniques for in situ characterization at high temperatures (1100–1300 °C), identifying the true catalyst during BNNT growth is challenging. Herein, an aluminum (Al)‐based active catalyst for BNNT growth via CVD is investigated. The initial Al2O3 nanoparticle catalyst precursor is transformed into an Al‐B phase prior to BNNT growth. Based on our density functional theory‐based molecular dynamic simulations of BNNT nucleation, AlB x (x = 1.5 to 2) shows catalytic activity for the formation of BN chains and BN six‐membered rings. Confirmatory experiments demonstrate that AlB2 is the active Al‐based catalyst during BNNT growth. A nanocomposite is prepared from cellulose nanocrystal, and purified BNNTs exhibited a high in‐plane thermal conductivity of 13.33 W m−1 K−1 at 20 wt% BNNTs. A further application for light‐emitting diode chip cooling demonstrates excellent heat‐dissipation performance of the nanocomposite film. Thus, this study can guide the controllable synthesis of high‐quality BNNTs and facilitate their use in thermal interface materials.

The effective identification of the active catalytic phase is essential to elucidate the growth mechanism of boron nitride nanotubes (BNNTs) and realize their controllable and scalable synthesis. However, owing to the complexity of chemical reactions during BNNT growth via chemical vapor deposition (CVD) and the lack of techniques for in situ characterization at high temperatures (1100-1300°C), identifying the true catalyst during BNNT growth is challenging. Herein, an aluminum (Al)-based active catalyst for BNNT growth via CVD is investigated. The initial Al 2 O 3 nanoparticle catalyst precursor is transformed into an Al-B phase prior to BNNT growth. Based on our density functional theorybased molecular dynamic simulations of BNNT nucleation, AlB x (x ¼ 1.5 to 2) shows catalytic activity for the formation of BN chains and BN six-membered rings. Confirmatory experiments demonstrate that AlB 2 is the active Al-based catalyst during BNNT growth. A nanocomposite is prepared from cellulose nanocrystal, and purified BNNTs exhibited a high in-plane thermal conductivity of 13.33 W m À1 K À1 at 20 wt% BNNTs. A further application for light-emitting diode chip cooling demonstrates excellent heat-dissipation performance of the nanocomposite film. Thus, this study can guide the controllable synthesis of high-quality BNNTs and facilitate their use in thermal interface materials.
MgB 2 phases were directly detected at the tip of the BNNTs, presumably because Mg has a low boiling point (1104°C) and is easily boronized and nitrided, [29] while MgB 2 was possibly converted into MgO or magnesium borate owing to the high oxygen content in the reactions. [14] Furthermore, using nonequilibrium molecular dynamics (MD) simulations, Ben et al. [30,31] reported that the formation of H 2 and O has an important effect on the nucleation of BNNT on Ni catalyst nanoparticles (NPs). Overall, owing to the complexity of catalyst phase changes during CVD-based BNNT growth and the lack of techniques for in situ characterization at high temperatures (1100-1300°C), the research on the mechanism of the catalytic growth of BNNTs is still in its infancy.
Aluminum (Al) is close to Mg in the periodic table, and both are quite similar in several basic properties. [32] Moreover, both Al-based particles (Al and Al 2 O 3 ) and Mg have been reported to effectively catalyze carbon nanotube growth. [33,34] More importantly, Al has a higher boiling point (2520°C) than Mg and also forms stable boride (AlB 2 and AlB 12 ) and nitride (AlN) phases; [35][36][37] hence, Al-based materials have the potential to more efficiently catalyze BNNT growth than Mg-based materials.
In this study, using Al 2 O 3 NPs as a catalyst precursor, we synthesized large-scale, high-quality BNNTs on the surface of a SiO 2 /Si wafer in a horizontal tubular furnace. Growth characterization showed the existence of AlB 31 crystal particles at the BNNT tips, while theoretical calculations indicated that AlB x (x ¼ 1.5 to 2) was a potentially active catalyst for BNNT growth. To confirm this theoretical prediction, AlB 2 and B/AlB 2 were applied as precursors to grow BNNTs, and the effective growth of BNNTs confirmed the high catalytic efficiency of AlB 2 .
Furthermore, we investigated the thermally conductive performance of cellulose nanocrystal (CNC) composite filled with purified BNNTs. The as-obtained nanocomposite showed an in-plane thermal conductivity of 13.33 W m À1 K À1 at 20 wt% BNNTs. By utilizing the CNF/20% BNNT nanocomposite film as a thermal interface material, we have demonstrated effective heat dissipation for a light-emitting diode (LED) chip. Our work paves the way for the controllable synthesis of high-quality BNNTs and facilitates their use in electronic device-cooling applications. Figure 1 shows the as-grown BNNTs synthesized in an NH 3 atmosphere using B/Al 2 O 3 NPs as boron precursors and catalysts. As shown in Figure 1a, abundant snow-white BNNTs occurred on the surface of the SiO 2 /Si wafer. Furthermore, most of the white BNNTs lumps were collected from the inner wall of the BN boat. Based on the ratio of BNNTs output to precursors input, the yield of BNNTs is about 10%. The scanning electron microscopy (SEM) image in Figure 1b shows dense and pure BNNTs, indicating the high catalytic efficiency of the Al-based catalysts and precursors. Figure 1c displays the transmission electron microscopy (TEM) image of a typical BNNT with an external diameter of %36 nm. The high-resolution TEM (HRTEM) image in the Figure 1c inset shows lattice fringes on both sides of the nanotube wall and an intertube distance of 0.35 nm, corresponding to the (002) plane of h-BN. [38] The Raman spectrum ( Figure 1d) features a strong peak at 1367 cm À1 , attributable to the E 2g in-plane vibration mode of h-BN, suggesting the high crystallinity of the BNNTs. [39][40][41]   From the XRD pattern ( Figure S1a, Supporting Information) of as-grown BNNTs, the strong (002) peak around %27°can be indexed to h-BN (JCPDS No. 73-2095). No impurity phase was detected within the detection limit. The surface elements and chemical states of the as-prepared samples were verified via X-ray photoelectron spectroscopy (XPS). The full XPS spectrum features four peaks attributable to B 1s , N 1s , O 1s , and C 1s ( Figure S1b, Supporting Information). The B 1s and N 1s spectra (Figure 1e,f ) exhibit peaks at 190.6 and 398.2 eV, respectively, attributable to the N-B bonds in h-BN. [42][43][44] The Al 2p spectrum shows no obvious characteristic peak ( Figure S1c Figure S2, Supporting Information). The B/Al 2 O 3 NP ratio had little effect on the growth of large-area BNNTs, which is conducive to the scalability of CVD experiments. The optimal growth temperature and NH 3 flow rate to obtain high-purity BNNTs were 1300-1320°C and 30 sccm, respectively ( Figure S3-S4, Supporting Information).

Results and Discussion
To gain insight into the growth mechanism of BNNTs, we investigated a particle at the tip of a BNNT via TEM. The high-angle annular dark-field scanning TEM (HAADF-STEM) image reveals that the catalytic NP was completely wrapped by BN layers (Figure 2a). In Figure 2a, the elemental mapping images within the white solid square show that Al was mainly distributed in the center of the particle. Figure  According to the above analysis, AlB 31 occurred in the grown BNNTs, which was formed via the β-rhombohedral B with inclusions of Al atoms in the unit cell; [36,45,46] and therefore, the AlB 31 is unlikely to be highly active in the synthesis of a large quantity of BNNTs because of the abundance of β-B with high  thermodynamic and chemical stability. [47,48] The observed AlB 31 phase and encapsulated h-BN layers were likely formed during the cooling process through the reactions among the aluminum boride catalyst, boron, and NH 3 . It is highly possible that the true Al-B active catalyst was a B-rich substance, such as AlB 2 and AlB 12 , as shown in the Al-B phase diagram. [49] To determine the active Al-B catalyst, MD simulations based on density functional theory (DFT) were used to simulate BNNT nucleation over an Al x B y NP catalyst. Based on the above analysis, the reaction between B powder and Al 2 O 3 NPs during BNNT synthesis can form Al x B y NPs with varying x:y ratios. To determine the structure of the active Al-B catalyst, we constructed various Al x B y nanoclusters with to form a more stable structure. Moreover, the N atoms preferentially bond with B atoms, to form some N-B-N units (the red dashed circles in Figure 3a). The MD trajectories of the above growth process at different time steps are detailed in Figure S5, Supporting Information.
The MD simulations show that N atoms tend to react with liquid Al x B y nanoclusters with x:y ¼ 1:1.5 or 1:2, and the liquid state of the cluster allows the complicated reaction to easily reach thermal equilibrium. To further explore BNNT nucleation and growth, more N atoms were added to the surface of the nanoclusters to simulate the subsequent growth process. The last frame of the above MD simulation was taken as the initial structure for the next simulation, in which 21 N atoms were added to simulate BNNT growth catalyzed by the Al 16 B 24 (x:y ¼ 1:1.5) and Al 14 B 28 (x:y ¼ 1:2) nanoclusters. The MD trajectory of the simulated Al 14 B 28 N 21 (x:y ¼ 1:2)-catalyzed CVD is shown in Figure 3b. As the simulation proceeded, the BN chains transformed into six-membered rings, indicating the initial formation of BNNT (the red dashed circles in Figure 3b). Figure 3c shows the total energy as a function of time for the Al 14 B 28 N 21 model. The energy of the nanocluster decreased with time, and the whole system gradually stabilized. Furthermore, we conducted similar MD simulations for Al 16 B 24 (x:y ¼ 1:1.5) nanoclusters. The number and length of BN chains increased over time, which suggests BNNT formation (the red dashed circles in Figure S6a, Supporting Information). The system stability increased as the simulation progressed ( Figure S6b, Supporting Information).  According to the MD simulation results of the Al x B y -catalyzed BNNT nucleation process, the true Al-B active phases are AlB x (x ¼ 1.5 to 2). Verifying the results of the above MD simulations via experiments is crucial to accurately identify the active phases of Al-based compounds. Here, AlB 2 was first used as a precursor to grow a certain number of BNNTs under vapor-liquid-solid (VLS) growth mechanism model ( Figure S7a, Supporting Information). However, unlike the traditional VLS mechanism, AlB 2 particles can not only decompose to produce B components but also act as catalysts to transport N components from vapor to solid. Such metal borides as both B source and catalyst to grow BNNTs have been reported in the literature, such as Fe-B NPs and MgB 2 . [26,50,51] Furthermore, the quantity of BNNTs collected on the surface of the SiO 2 /Si wafer was relatively small, mainly owing to the low content of the B source ( Figure S7a, Supporting Information, inset). Amorphous B powder was added in order to combine with Al during CVD to form AlB 2 nanoclusters that have catalytic activity and can also provide B component. To obtain more BNNTs, mixtures of B/AlB 2 with molar ratios of 10:1 and 20:1 were used as precursors for CVD experiments in an NH 3 atmosphere (Figure 4a and S7b, Supporting Information). The TEM image shows a BNNT with an outer diameter of %27 nm and clean tube walls, demonstrating its high purity (Figure 4b). The interplanar spacing of %0.35 nm can be regarded as the (002) lattice plane of h-BN (Figure 4b, inset). Moreover, Raman spectroscopy and XPS analysis confirmed the structural and chemical states of the as-synthesized samples. The Raman spectrum ( Figure S8a, Supporting Information) features a strong peak at 1367 cm À1 , attributable to the E 2 Â g in-plane vibration mode of h-BN. The high-resolution XPS spectra of B 1s and N 1s are consistent with the characteristic peaks of BN ( Figure S8b-8d, Supporting Information). Overall, the Raman analysis and XPS results demonstrate the high crystallinity and high purity of BNNTs prepared over AlB 2 as a catalyst.
To further understand the catalytic mechanism of AlB 2 , the NPs at the BNNT tips were characterized (Figure 4c). Similar to the characterization results for BNNTs prepared using Al 2 O 3 as the catalyst precursor, the Al element was mainly distributed in the center of the tip, indicating the catalytic role of Al-based compounds. Moreover, the powder samples left in the BN boat after the CVD process were analyzed via HRTEM and XRD. The HRTEM image shows that the BNNT had the same composition as the powder sample produced using the Al 2 O 3 catalyst precursor (Figure 4d Figure 4f illustrates the process of BNNT growth over the AlB 2 catalyst. The results indicate that the AlB 31 phase with residual B could easily form after BNNT growth. However, the high melting point of AlB 31 hinders it from occurring as an active phase for catalyzing the growth of a large quantity of BNNTs. Thus, AlB 2 was the true active catalyst for BNNT growth during the CVD process. Considering the high thermal conductivity of BNNTs, we studied the thermally conductive performance of CNC composite filled with purified BNNTs. The cross-section SEM image of the CNC/20% BNNT nanocomposite shows well-organized layered structures (Figure 5a). The XRD patterns of pure CNC film and CNC/20% BNNT nanocomposite film are shown in Figure 5b. The pure CNC film shows three characteristic peaks  These results reveal that the CNC polymer matrix has little effect on the intact crystal lattice of BNNTs. Figure 5c shows the in-plane (K ‖ ) and out-of-plane (K ⊥ ) thermal conductivities of pure CNC film and CNC/20% BNNT nanocomposite film. The CNC/20% BNNT nanocomposite film exhibits a high in-plane thermal conductivity of 13.33 W m À1 K À1 , which is 31.7 times higher than that of the pure CNC film. [52] The CNC/20% BNNT nanocomposite film exhibits a slight increase in out-of-plane thermal conductivity (0.11 W m À1 K À1 ). Thus, the CNC/20% BNNT nanocomposite film exhibits a highly anisotropic thermal conductivity feature. To evaluate the performance of the CNC/20% BNNT nanocomposite film in cooling electronic devices, we used the film as a thermal interface material for heat dissipation of a LED chip (Figure 5d). Figure 5e,f shows the central temperature evolution of the LED chip as a function of running time and the infrared thermal images, respectively. The central temperature with the CNC/20% BNNT nanocomposite film reaches 77.4°C when the LED chip is powered on for 60 s, which is much lower than the 112.1°C of pure CNC film. After turning off the LED chip, the central temperature with the CNC/20% BNNT nanocomposite film drops to 49.4°C within 10 s, whereas the central temperature with pure CNC film is 74.1°C, demonstrating the excellent heat-dissipation ability of the CNC/20% BNNT nanocomposite film. Overall, the CNC/20% BNNT film with highly anisotropic thermal conductivity (K ‖ /K ⊥ ≫ 1) can diffuse thermal energy from small hot spots of the LED chip to a larger area, making more efficient use of thermal convection and radiation from the heat sink surface than the neat CNC film, resulting in the dramatic cooling of the LED chip.

Conclusions
In summary, the CVD-based growth of BNNTs over an Al-based catalyst was investigated. The results revealed that the initial Al 2 O 3 NPs precursor transformed into the Al-B phase structure before BNNT growth. According to the MD simulation of the BNNT nucleation process, the true active catalytic phase was AlB x (x ¼ 1.5 to 2). Subsequent CVD experiments indicated that www.advancedsciencenews.com www.small-structures.com AlB 2 could act as both a B source and an active catalyst for BNNT growth. Moreover, the increase in the B source concentration considerably increased the amount of grown BNNT. The confirmatory experiments and MD simulations demonstrate that AlB 2 is an effective Al-based catalyst phase during BNNT growth. Moreover, a nanocomposite prepared from CNC and purified BNNTs exhibited a high in-plane thermal conductivity of 13.33 W m À1 K À1 at 20 wt% BNNTs. A further application for LED chip cooling demonstrated excellent heat-dissipation performance of the nanocomposite film. Thus, our work identifies the active catalytic phase of BNNT over Al-based compounds, paving the way for the controllable and scalable growth of BNNTs, and facilitating their use in electronic device-cooling applications.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.