Facile Synthesis of Millimeter-Scale Vertically Aligned Boron Nitride

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Facile Synthesis of Millimeter-Scale Vertically Aligned Boron Nitride Nanotube Forests by Template-Assisted Chemical Vapor Deposition Roland Yingjie Tay, Hongling Li, Siu Hon Tsang, Lin Jing, Dunlin Tan, Mingwei Wei, and Edwin Hang Tong Teo Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03300 • Publication Date (Web): 29 Sep 2015 Downloaded from http://pubs.acs.org on October 4, 2015

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Chemistry of Materials

Facile Synthesis of Millimeter-Scale Vertically Aligned Boron Nitride Nanotube Forests by Template-Assisted Chemical Vapor Deposition Roland Yingjie Tay,1,2, ¶ Hongling Li,1,3, ¶ Siu Hon Tsang,2 Lin Jing,4 Dunlin Tan,1,3 Mingwei Wei,1 Edwin Hang Tong Teo,1,4,* 1

School of Electrical and Electronic Engineering, Nanyang Technological University, 50

Nanyang Avenue, Singapore 639798, Singapore 2

Temasek Laboratories@NTU, 50 Nanyang Avenue, Singapore 639798, Singapore

3

CNRS-International NTU Thales Research Alliance CINTRA UMI 3288, Research Techno

Plaza, 50 Nanyang Drive, Singapore 637553, Singapore 4

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

Avenue, Singapore 639798, Singapore ¶

These two authors contribute equally to this work.

* Corresponding author. Tel: +65 67906371. E-mail address: [email protected]

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ABSTRACT There is an increasing amount of research interest in synthesizing boron nitride nanotubes (BNNTs) as well as BN coatings to be used for various applications due to its outstanding mechanical, electrical and thermal properties. However, vertically aligned (VA) BNNTs are difficult to synthesize and the longest VA-BNNTs achieved to date are up to several tens of microns. Here, we report the synthesis of over millimeters long multi-walled BN coated carbon nanotubes (BN/CNT) and BNNT forests via a facile and effective two-step route involving template-assisted chemical vapor deposition at a relatively low temperature of 900 °C and subsequent annealing process. The as-prepared BN/CNTs and BNNTs retain the highly ordered vertically aligned structures of the CNT templates as identified by scanning electron microscopy. The structure and composition of the resulting products were studied using transmission electron microscopy, electron energy-loss spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy and thermogravimetric analysis. This versatile BN coating technique and the synthesis of millimeter-scale BN/CNTs and BNNTs pave a way for new applications especially where the aligned geometry of the NTs is essential such as for field-emission, interconnects and thermal management.


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INTRODUCTION Boron nitride nanotubes (BNNTs) are structural analogues of carbon nanotubes (CNTs) with alternating B and N atoms bonded in a hexagonal configuration.1-5 Due to the strong covalent B– N bonds, BNNTs exhibit various outstanding properties similar to CNTs such as exceptionally high mechanical strength (Young’s modulus of ~1.2 TPa)6,

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and thermal conductivity.8,

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However, in contrast to CNTs, the binary nature of BNNTs induces sublattice symmetry breaking, leading to a large bandgap of ~5 to 6 eV,1, 10 and thus are electrically non-conducting.11 Moreover, the bandgap of BNNTs is independent of the diameter and chiralities of the nanotubes.1, 10 It is well-known that BNNTs possess excellent chemical and thermal stability,12 with a very high resistance to oxidation of up to 800 °C under air and 2800 °C in inert atmosphere.13 As such, BNNTs are very attractive and promising for a wide range of applications. For example, they have been applied as protective capsules for environmental-sensitive nanoparticles from oxidation and contamination by the surrounding.14 In addition, they are also successful candidates for structural reinforcement and as nanofillers to enhance the thermal conductivity of polymeric films and fibers.15, 16 Furthermore, BNNTs have been found to be useful in other prospective applications including gas absorbents,17 protein immobilization,18 spintronic devices,19 as well as interconnects for nanoscale electronics and radiation stoppers.2 However, synthesis of BNNTs is still a challenging task and most of the synthesis approaches for BNNTs are adopted from the growth techniques of CNTs such as arc-discharge method,6 chemical vapor deposition (CVD),20-23 pressurized vapor/condenser (PVC),24 laser ablation,25 ball milling26 and other high temperature and plasma assisted processes.27-31 Although vertically or partially aligned BNNTs have been achieved previously, the lengths of these BNNTs are restricted to tens of microns.23, 30, 32, 33 On the other hand, the development for CNTs growth is 3 ACS Paragon Plus Environment


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well advanced over the past two decades, with commercial CVD systems and mass production of CNT forests are readily available.34, 35 In addition, the lengths of these vertically aligned (VA) CNTs can easily reach over millimeter ranges (exceeding two orders of magnitude longer than the previously reported length of VA-BNNTs) and they can be synthesized under impressive speed of within a few minutes.36-39 Therefore, instead of revising the current techniques for CNTs growth and redeveloping them for BNNTs, a more straightforward alternative would be to use these VA-CNTs as templates. There are two methods to utilize CNTs as templates to obtain BNNTs. The first is to employ a chemical conversion process by substitution of boron (B) and nitrogen (N) atoms in replacement for carbon (C),40-45 and the second is by coating the CNTs with BN layers.46-50 The former, however, requires very high temperature of above 1500 °C and the longest BNNTs formed by conversion could extend up to several microns.41, 42 It should be noted that this method mostly yields semiconducting BN doped CNTs at lower temperatures.43-45 For the latter, enhanced field emission properties have been observed for BN coated CNTs (BN/CNTs),47, 48, 50 and the BN encapsulation can act as a protective layer against oxidation for CNTs.47, 49 However, to the best of our knowledge, fabrication of millimeters-long VA-BNNTs through such coating technique has not been explored. Here, we demonstrate a facile and effective two-step approach to obtain up to over millimeters long highly ordered VA-BNNT forests from CNTs with a relatively low growth temperature of 900 °C. Due to the highly coherent atomic configuration and lattice constants of hexagonal boron nitride (h-BN) and graphene shells for BNNTs and CNTs, respectively, few-layer h-BN can be deposited onto the surface of the as-grown CNTs. The CNT templates can be subsequently removed by burning in air, and thus forming VA-BNNTs. The obtained BNNTs retained the highly ordered geometry of the VA-CNTs as determined by scanning electron microscopy 4 ACS Paragon Plus Environment

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(SEM). Transmission electron microscopy (TEM) combined with electron energy-loss spectroscopy (EELS) mapping verified the coating mechanism of BN layers as well as the crystalline structure of the BNNTs. The BN/CNTs and BNNTs are further characterized with Xray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). EXPERIMENTAL SECTION CVD growth of VA-CNT forests. VA-CNTs were grown on Si substrate via thermal CVD. Firstly, 6 nm Al, 4 nm Al2O3 and 1 nm Fe were sequentially deposited on the Si substrate as catalyst by using an electron beam deposition. H2 (100 sccm, using as etching gas), N2 (80 sccm, using as carrier gas) and C2H2 (20 sccm, using as carbon source) were filled into the quartz tube till the pressure is 720 mbar. The furnace was then ramped up to 720 °C and kept for 20 min to sustain VA-CNT growth. When growth was finished, the gas intakes were turned off and furnace was cooled down to room temperature. VA-CNT forests with an average length of ~0.9 mm were prepared through above method. BN coating on VA-CNT forests by CVD. BN coated VA-CNTs were prepared by CVD under atmospheric pressure. Boric acid powder was loaded at one end of the quartz tube with the amount of ~20 times the CNTs. The tube was first heated up to 150 °C and remained at this temperature for 30 min to get rid of the residual air and moisture in the tube with an Ar flow of 300 sccm. Then the tube was further heated up to 830 °C at a heating rate of 7.5 °C/min and held for 30 min. Meanwhile, boric acid was heated up to ~300 °C and ammonia gas was introduced into the system with a flow rate of 50 sccm while the Ar flow rate was reduced to 250 sccm. The



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temperature was further increased to 900 °C in 40 min and was held for another 1 h. Finally, the system was cooled down naturally to room temperature under Ar flow. Preparation of VA-BNNT forests. The as-prepared VA-BN/CNT forests were loaded into a quartz tube and the temperature was increased to 700 °C at a heating rate of 1.9 °C/min and was held at this temperature for 4 h in air. Upon completion, the system was cooled down naturally and the distinctive white colored VA-BNNT forests were obtained. Characterization. Field emission scanning electron microscopy (FESEM, LEO 1550 GEMINI) and transmission electron microscopy (TEM, Tecnai G2 F20 X-Twin) were used to characterize the morphologies and structures of the various NTs. TEM equipped with electron energy-loss spectroscopy (EELS) mapping was performed to determine the coating mechanism. X-ray photoelectron spectroscopy (XPS, Model PHI Quantera SXM) analysis and SEM coupled with energy-dispersive X-ray microanalysis (SEM/EDX, JSM-5600LV) were used to determine the chemical composition of the samples. XPS was measured using Al Kα as the excitation source (1486.6 eV), and binding energy calibration was based on C1s at 284.8 eV. Raman spectroscopy (WITEC CRM200 Raman system) was performed under ambient environment using a 532 nm laser (2.33 eV). Thermogravimetric analysis (TGA, Shimadzu DTG-60H thermal analyzer) was carried under a constant flow of air (100 mL/min) and heated from 30 to 1300 °C at a heating rate of 10 °C/min. Fourier transform infrared spectroscopy (FT-IR, IRPrestige-21 spectrometer) was performed within the wavenumber ranging from 4000 to 400 cm−1. RESULTS AND DISCUSSION Figure 1a shows the schematic illustration of the two-step fabrication process to obtain VABNNTs. Firstly, VA-CNT forest templates with an average length of ~0.9 mm are prepared by 6 ACS Paragon Plus Environment

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thermal CVD. Few-layer h-BN is then coated onto the outer walls of the CNTs by a separate CVD process using boric acid and ammonia gas as precursors for B and N, respectively. During the BN growth process, boric acid was sublimated at ~300 °C forming boron oxide (B2O3) and water vapor (H2O), and ammonia gas (NH3) was injected into the system simultaneously. The detailed BN coating process is provided in the experimental section and in Figure S1 (Supporting Information). The proposed coating mechanism of BN is described in two stages: (i) Nucleation of BN on the outer surface of the CNT and (ii) growth of BN layers.46, 47 During the first stage, an intermediate ternary h-BNC surface layer is formed relies on the fact that the B and N atoms substitute for C atoms under CNT oxidation by B2O3 vapor in a flowing ammonium atmosphere along with the reactions2, 43, 47, 51: B2O3 + 2C (nanotubes) + 2NH3 B2O3 + BNC + 2NH3

2BNC + 3H2O

3BN + 2H2O + CO + H2

(1) (2)

Gong et al. proposed that for such a conversion process, the nucleation of BN is initiated by the inclusion of a N atom at a defective site or at a functionalized C atom by hydroxyl groups.51 This is quickly followed by substitution of B and N atoms around the embedded N atoms which grow and merge with neighboring BN domains to form a continuous layer of h-BN.51 Upon the completion of the h-BN barrier layer on the surface of the CNT, further reaction with the CNT is ceased. At the same time, this newly formed h-BN layer also acts as a seed layer for BN growth.46 During the second stage of growth, the BN layer grows in thickness when B2O3 vapor directly reacts with ammonia gas: B2O3 + 2NH3

2BN + 3H2O

(3)



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After the CNTs are being coated with h-BN layers, the CNT templates are removed by simply annealing under air at 700 °C. Photographs in Figure 1a provide the most intuitive form of characterization. It is observed that the initially black BN/CNTs, with little or no visible change in appearance from the starting CNTs, dramatically turn white after annealing, indicating the removal of CNTs and that the resulting BNNTs are still in the upright arrangement and did not collapse throughout the process. Figure 1b-d shows the cross-section SEM images of the CNTs, BN/CNTs and BNNTs, respectively. It is apparent that both the BN/CNTs and BNNTs retain the vertically aligned structure of the CNTs. The presence of B, C and N elements in the various NTs can be roughly identified by using SEM/EDX (Figure S2, Supporting Information). However, it is observed that there is a significant decrease in the length of the BNNTs as compared to those of starting CNTs or BN/CNTs (Figure S3, Supporting Information), attributive to the spatially non-uniform densities at different heights of the CNT forests (Figure S4, Supporting Information). As the CNT forests follow a density decaying bottom growth mechanism, the density at the top region is, in general, higher than the bottom.52 The densely populated top region of the CNT forest could impede the arrival of the BN precursors and the CNTs in this region were hard to be coated. Hence, the resultant BNNTs are observed to be shorter than the starting CNTs. The microstructures of the VA-CNTs, BN/CNTs, and BNNTs were examined by TEM. Figure 2a-f shows the representative low and high magnification TEM images of the dispersed CNTs, BN/CNTs and BNNTs on Cu TEM grids, respectively. It is observed that the initial CNT has an average inner diameter of ~5.53 nm and an outer diameter of ~8.19 nm (Figure 2b). The walls thickness of the CNT is ~1.33 nm, corresponding to ~4 C layers.36 After the BN coating process, obvious thickening of the walls is observed on the BN/CNT while the inner diameter of the tube 8 ACS Paragon Plus Environment

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remains similar to that of the starting CNT of ~5.47 nm (Figure 2d). The measured thickness of the wall of the BN/CNT is ~3.35 nm, corresponding to ~9 to 10 (C+BN) layers. After removing the CNT, the inner diameter of the BNNT is ~8.09 nm, similar to that of the outer diameter of the starting CNT (Figure 2f). The measured wall thickness of the BNNTs is ~2.13 nm, corresponding to ~6 BN layers.30, 33 It should be noted that multiple samples were taken to verify the consistency of the TEM measurements (Figure S5, Supporting Information). It is observed that the crystallographic structure of the BNNTs is not fully crystallized which may be due to the relatively low process temperature. The slight variation between the samples could be due to several factors such as the diameters of the starting CNTs and the amounts of BN coated onto the CNTs during the process. TEM/EELS analysis was used to determine the presence of B, C and N elements in various NTs and mapping was done to provide a direct observation of the BN coating mechanism. Figure 3 shows the EELS spectra of the individual CNT, BN/CNT and BNNT, respectively. For CNT, only one distinct absorption peak commencing at 284 eV is observed, corresponding to the sp2 hybridized C–C bonds.48 For BN/CNT, besides having an absorption peak at 284 eV due to the presence of CNT, two additional absorption peaks commencing at 188 eV and 401 eV are observed, corresponding to the B-K and N-K edges, respectively, which are the characteristic peaks for sp2 hybridized B–N bonds.11, 48 For BNNT, a suppressed absorption peak is located at 284 eV indicating the presence of a small amount of residual C. Both the absorption peaks commencing at 188 eV and 401 eV confirm the B–N bonds as expected for BNNT.11 Figure 4a-d shows EELS mappings of elements B, C, N and the composite image of a BN/CNT, respectively. It is observed in the composite image that the B and N elements are located along the outer walls of the CNT, confirming that the CNT is coated with a thin layer of BN. Figure 4e-h shows the 9 ACS Paragon Plus Environment


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EELS mappings of elements B, C, N and the composite image of a BNNT. It is observed that the BNNT consists mainly of B and N elements with a small fraction of residual C. The composite image shows that the residual C is originated from the CNT template as it is located mostly along the inner wall of the BNNT. The chemical composition of the as-prepared CNTs before and after BN coating as well as the BNNTs samples were further identified using XPS. Figure 5a shows the XPS survey spectra of the three different types of NTs. CNTs show a single peak corresponding to C 1s, while the presence of B, C, N and O are observed for both the BN/CNTs and the BNNTs. The weak C 1s peak observed for the BNNTs indicates a significantly lower amount of C concentration due to the removal of CNTs. The elemental ratios of the CNTs, BN/CNTs and BNNTs, which are extracted based on the integral intensities of each element peak, are displayed in Table 1. It is observed that after the annealing process of the BN/CNTs, the C concentration is reduced from 37.85 % to 1.08 % and the O concentration increased from 3.74 % to 10.57 %. This suggests that the CNTs are effectively burned away and the BNNTs are partially oxidized. The B/N ratio of the BN/CNTs and BNNTs remained identical of 0.934 and 1.07, respectively, which is consistent to the ideal stoichiometry of h-BN of 1:1.11, 23 High-resolution XPS spectra of C 1s, B 1s, N 1s and the corresponding peaks fitting are shown in Figure 5b-h. Figure 5b shows the C 1s spectrum for the CNTs. Four deconvoluted peaks centered at 284.7, 285.8, 287.5, and 290.9 eV, arising from C-C/C=C, C-O, C=O, and O-C=O bonds are identified, respectively.47 Likewise for the BN/CNTs, the four similar peaks are observed (Figure 5c). However, a relatively stronger peak intensity at 285.8 eV indicates that formation of C-N bond could have occur during the coating process.47 Two deconvoluted peaks in the B 1 s (Figure 5d) and N 1s (Figure 5e) spectra located at 190.8 and 192.1 eV, and 398.4, and 399.9 eV, are attributed to B-N and B-O bonds, and N-B 10 ACS Paragon Plus Environment

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and N-C bonds, respectively.47 This clearly indicates the presence of BN coatings on the CNTs. Figure 5f shows the C 1s spectrum for BNNTs with four deconvoluted peaks located at 283.6, 284.8, 285.9 and 287.8 eV, attributed to C-B, C-C/C=C, C-O/C-N, and C=O bonds, respectively, due to the small quantity of residual C. Three deconvoluted peaks located at 190.8 eV, 191.9 eV and 193.1 eV in the B 1s spectrum (Figure 5g) and 398.4, 399.5 and 400.5 eV in the N 1s spectrum (Figure 5h) are identified corresponding to B-N, B-O and B-C bonds, and N-B, N-C/NO and N-C bonds, respectively.47 It is observed that both the peaks in the B 1s and N 1s spectra of the BNNTs are broader than those of the BN/CNTs due to the increase in O concentration which is induced during annealing process under air. Figure 6a shows the Raman spectra of the CNTs, BN/CNTs and BNNTs. Characteristic D, G and 2D peaks are observed for both the CNTs and BN/CNTs, which confirms that the crystal structure of the CNTs are not altered by the coating process.48 The presence of h-BN can only be identified by Raman spectroscopy after removing the CNTs due to its relatively weaker intensity as compared to graphitic C. A peak at ~1366 cm-1 observed for the BNNTs is attributed to the E2g vibration mode of h-BN.23, 48 Figure 6b shows the FT-IR spectra of the various NTs. For CNTs, the curve shows the characteristic C=C stretching at 1643 cm-1, while for the BN/CNTs, additional B-N and B-N-B stretching vibrations are observed at 1358 and 771 cm-1, respectively. These two peaks further shift to 1389 and 801 cm-1, respectively, after the annealing process and are typical characteristics of BNNTs.23 In order to evaluate their thermal behavior, the CNTs, BN/CNTs and BNNTs samples are subjected to TGA under air at a heating rate of 10 °C/min. As shown in Figure 6c, when the temperature is lower than 500 °C, the weight loss of the all the three samples is small. For CNTs, this minute weight loss is attributed to the presence of water as revealed in the FT-IR spectrum and carbonaceous impurities, while for BN/CNTs and BNNTs, 11 ACS Paragon Plus Environment


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the weight loss is due to the small traces of carbonaceous residues due to the growth and annealing process, respectively.47 It should be noted that carbon residues are observed in both the EELS and XPS spectra for the as-fabricated BNNTs. The CNTs show a steady decline in weight as a result of oxidation starting at 608 °C, with nearly 0% weight residue at over 700 °C. For BN/CNTs, the BN coatings promote the oxidation temperature to 625 °C and significant weight loss takes place later at 700 to 800 °C, with over 40% weight residue at 760 °C. These results suggest that the thermal stability can be effectively improved by coating BN onto the CNTs. The TGA profile of the BN/CNTs exhibits a slight increase in weight when BN oxidized into B2O3 at ~900 °C, and followed by a gradual weight loss when the B2O3 sublimates at higher temperatures. However, there is still over 34% weight residue at 1200 °C. The BNNTs are stable up to 975 °C under air oxidation with a slight increase in weight at higher temperatures, suggesting good thermal stability of BNNTs.2, 8 Consequently, by combing the SEM, TEM/EELS, XPS, Raman, FT-IR and TGA results, we can reasonably conclude that the VA-BN/CNT and BNNT forests were successfully fabricated by using a template-assisted CVD process. The synthesis of BNNTs using this coating process and subsequent annealing have been determined to be feasible even for CNTs with over millimeters length. However, the commercially available millimeters long VA-CNT forests are often highly dense as shown in the cross-sectional SEM image in Figure 7a. The CNTs were not able to be coated properly and the resultant BNNTs shrink significantly in size. In order to reduce the density of the CNTs, we deliberately use O2 plasma to etch away part of the CNTs. Figure 7b shows the cross-sectional SEM image of the VA-CNTs after O2 plasma treatment. Noticeable reduction of the CNTs density is evident as observed by the increment in inter-spacing distances between the CNTs. In addition, it is noteworthy that O2 plasma functionalizes the CNTs with hydroxyl groups which


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could further aid the nucleation of BN.51 Figure 7c,d shows the photographs of as-prepared BN/CNTs (top) and BNNTs (bottom) fabricated using pristine VA-CNTs and O2 plasma treated VA-CNTs, respectively. Due to the dense structure of the pristine VA-CNTs, BN coatings could only occur on the CNTs located at the outer exposed regions of the free-standing CNT forest. Significant reduction in size is observed on the BNNTs as the uncoated CNTs were burned away during the annealing process. In contrast, when the VA-CNTs are treated with O2 plasma prior to BN coating and subsequent annealing, the resultant BNNTs retain the tall structures of the CNTs without any observable difference in length. CONCLUSIONS In summary, over millimeters long BNNT forests are fabricated via a template-assisted CVD coating process and subsequent removal of the CNT templates. The CNTs are firstly coated with a thin layer of h-BN at a relatively low temperature of 900 °C and the CNTs can be subsequently removed by simply annealing under air at 700 °C. The obtained BN/CNTs and BNNTs retained the vertically aligned structures of the starting CNTs as determined by SEM. The coating mechanism of BN layers and the crystalline structure of the BNNTs are verified by TEM and EELS analysis. The chemical composition and structure of BN/CNTs and BNNTs are further identified with XPS, Raman, and FT-IR. TGA results show that the as-prepared BNNTs are stable up to 975 °C under air, indicating excellent thermal stability. The highly versatile BN coating technique to produce both the highly ordered millimeters long VA-BN/CNT and BNNT forests could open up a wide range of applications such as for enhanced field emission, nanoscale electronics interconnects, protective encapsules for other nanomaterials and thermal management.



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ASSOCIATED CONTENT Supporting Information Figures S1 – S5, showing the experiment details for CVD, additional SEM, TEM images and EDX elemental mapping of the various NTs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author *Email: [email protected] ACKNOWLEDGEMENTS The authors would like to acknowledge the funding support from NTU-A*STAR Silicon Technologies Centre of Excellence under the program grant No. 1123510003 and Singapore Ministry of Education Academic Research Fund Tier 2 No. MOE2013-T2-2-050.


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Figure captions Figure 1. (a) Schematic illustration of the fabrication process of VA-BN/CNT and BNNT forests using CNT templates. The insets show the photographs of the respective as-grown VA-NTs on Si as labeled. BN layers are coated onto the CNTs by CVD in step 1 and the CNT templates are removed by annealing in air in step 2. SEM images of (b) CNTs, (c) BN/CNTs and (d) BNNTs displaying their vertically aligned structures. Figure 2. Representative low and high magnification TEM images of dispersed (a,b) CNTs, (c,d) BN/CNTs and (e,f) BNNTs. The measurements of the respective inner (in blue) and outer (in red) average tube diameters are displayed in (b,d,f). Figure 3. TEM/EELS spectra of an individual CNT, BN/CNT and BNNT. Figure 4. Elemental B, C, N and composite maps generated by TEM/EELS mapping of an individual (a-d) BN/CNT and (e-h) BNNT, respectively. Note that the violet color in the composite images is due to the combination of red and blue, representing B and N elements, respectively. Figure 5. XPS spectra of the CNTs, BN/CNTs and BNNTs: (a) survey spectra; (b) C 1s highresolution spectrum of CNTs; (c) C 1s, (d) B 1s and (e) N 1s high-resolution spectra of BN/CNTs; (f) C 1s, (g) B 1s and (h) N 1s high-resolution spectra of BNNTs. Figure 6. (a) Raman, (b) FT-IR and (c) TGA spectra of the CNTs, BN/CNTs and BNNTs. Figure 7. Magnified cross-sectional SEM images of VA-CNTs (a) before and (b) after oxygen plasma treatment. The inset in (a) shows the photograph of a free-standing over millimeters long



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VA-CNT forest. (c,d) Photographs of the free-standing CNT forests after BN coating process (top) and after further annealing in air at 700 °C (bottom). Table caption Table 1. Elemental ratios of CNTs, BN/CNTs and BNNTs.


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Figure 1. (a) Schematic illustration of the fabrication process of VA-BN/CNT and BNNT forests using CNT templates. The insets show the photographs of the respective as-grown VA-NTs on Si as labeled. BN layers are coated onto the CNTs by CVD in step 1 and the CNT templates are removed by annealing in air in step 2. SEM images of (b) CNTs, (c) BN/CNTs and (d) BNNTs displaying their vertically aligned structures.



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Figure 2. Representative low and high magnification TEM images of dispersed (a,b) CNTs, (c,d) BN/CNTs and (e,f) BNNTs. The measurements of the respective inner (in blue) and outer (in red) average tube diameters are displayed in (b,d,f). 22 ACS Paragon Plus Environment

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Figure 3. TEM/EELS spectra of an individual CNT, BN/CNT and BNNT.



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Figure 4. Elemental B, C, N and composite maps generated by TEM/EELS mapping of an individual (a-d) BN/CNT and (e-h) BNNT, respectively. Note that the violet color in the composite images is due to the combination of red and blue, representing B and N elements, respectively.


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Figure 5. XPS spectra of the CNTs, BN/CNTs and BNNTs: (a) survey spectra; (b) C 1s highresolution spectrum of CNTs; (c) C 1s, (d) B 1s and (e) N 1s high-resolution spectra of BN/CNTs; (f) C 1s, (g) B 1s and (h) N 1s high-resolution spectra of BNNTs.



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Figure 6. (a) Raman, (b) FT-IR and (c) TGA spectra of the CNTs, BN/CNTs and BNNTs.


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Figure 7. Magnified cross-sectional SEM images of VA-CNTs (a) before and (b) after oxygen plasma treatment. The inset in (a) shows the photograph of a free-standing over millimeters long VA-CNT forest. (c,d) Photographs of the free-standing CNT forests after BN coating process (top) and after further annealing in air at 700 °C (bottom).



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Table 1. Elemental ratios of CNTs, BN/CNTs and BNNTs.

Sample

B (at.%)

C (at.%)

N (at.%)

O (at.%)

CNTs

0

99.96

0

0.03

BN/CNTs

28.22

37.85

30.2

3.74

BNNTs

45.63

1.08

42.71

10.57


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TOC graphic:


Facile Synthesis of Millimeter-Scale Vertically Aligned Boron Nitride

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