Selective Growth of Boron Nitride Nanotubes by the Plasma-Assisted

Jul 22, 2009 - Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China, National Synchrotr...
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J. Phys. Chem. C 2009, 113, 14681–14688

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Selective Growth of Boron Nitride Nanotubes by the Plasma-Assisted and Iron-Catalytic CVD Methods Ching-Yuan Su,† Zhen-Yu Juang,† Ko-Feng Chen,† Bing-Ming Cheng,‡ Fu-Rong Chen,† Keh-Chyang Leou,†,§ and Chuen-Horng Tsai*,†,§ Department of Engineering and System Science, National Tsing Hua UniVersity, Hsinchu, Taiwan 300, Republic of China, National Synchrotron Radiation Research Center, Hsinchu, Taiwan 300, Republic of China, and Center for Nano Science and Technology, UniVersity System of Taiwan, Hsinchu, Taiwan 300, Republic of China ReceiVed: May 12, 2009; ReVised Manuscript ReceiVed: June 25, 2009

Boron nitride nanotubes (BNNTs) were selectively synthesized on patterned bilayer (Fe/Al) catalysts by the plasma-assisted chemical vapor deposition (PACVD) method. The as-grown nanotubes comprise both coaxial and cup-stacking tubular structures. Optical transitions on cup-stacking BNNTs are investigated for the first time. The observed red-shift of free excitonic luminescence was attributed to the excitonic recombination in terms of defect trapping in the tube’s surface. The O2 additives during the synthetic process were found to balance the excess H radicals that in turn enhance the growth yield of BNNTs. Moreover, our elemental mapping results provide direct evidence of the metal catalytic mechanism and the influence of the as-formed Al2O3 underlayer. Introduction Over the past decade, one-dimensional nanostructured materials have attracted great attention. The rapid development of carbon nanotubes (CNTs) has stimulated intensive investigation of boron nitride nanotubes (BNNTs). BNNTs were theoretically predicted1 and first synthesized2 in 1994 and 1995, respectively. The BNNTs exhibit remarkable chemical and physical properties that are different from CNTs; for instance, BNNTs have a wide band gap of about 5 eV that is independent of diameter and chirality.3 Moreover, the oxidation resistance of BNNTs is up to 800 °C, which is much higher than that of CNTs.4,5 Furthermore, the BNNTs have other advantageous properties such as chemical stability,6 high thermal conductivity (∼350 WmK-1),7 and outstanding mechanical strength (∼1.1-1.3 TPa).8,9 All of these unique properties give BNNTs great potential in electronics,10,11 optoelectronics,12–14 energy storage,15,16 and nanoelectromechanical systems.17,18 Several synthesis techniques have been developed to grow BNNTs, including arc-discharge,19 laser ablation,20 ball-milling,21 plasma-jet,22 substitution reactions with CNTs playing the role of templates,23,24 and chemical vapor deposition (CVD).25 However, most of these synthesis methods were carried out at high temperatures ranging from 1100 to 2700 °C and contained undesirable impurities (e.g., amorphous B particles and BN bulky flakes) in the as-grown products. These two drawbacks limit the use of BNNTs in realistic applications. Recently, C-doped BNNTs (i.e., B-C-N nanotubes)26,27 have been proposed as a promising material to replace CNTs in electronic/ optoelectronic devices because of both their tunable band gap between the metallic CNTs and the dielectric BNNTs, which is mainly dependent on the stoichiometry of BxCyNz28–30 and their insensitivity to a structure’s chirality. However, with * To whom correspondence should be addressed. E-mail: d947108@ oz.nthu.edu.tw (C.-Y.S.); [email protected] (C.-H.T.). † National Tsing Hua University, Hsinchu. ‡ National Synchrotron Radiation Research Center. § University System of Taiwan.

synthesis temperatures above 1000 °C, several typical metals will be unsustainable and therefore restrict the development of BN-based NT devices. Another critical issue in the device’s application is the selective growth of BNNTs in predefined positions. However, related investigations of BNNTs (such as developing a suitable synthesis approach and a selective method for the device) are still lacking, and the catalytic mechanisms are not fully understood. Therefore, to develop a low-temperature, selective synthesis method for BNNTs with a higher production yield is the highest priority for their practical application. A number of alternative approaches have been proposed to overcome these two obstacles. One of the methods reported growth of multi-walled BNNTs by plasma-enhanced pulsedlaser deposition (PE-PLD) at a temperature of 600 °C with a Fe catalyst.31 Nevertheless, the shortcomings of the laser-assisted process presented limitations for scaling up to a large area, poor uniformity, and unstable laser power. In another method, an attempt to use anodic aluminum oxide (AAO) as a template for microwave plasma-enhanced CVD (MPCVD)32 at temperatures below 520 °C could only attain amorphous BNNTs, which do not possess interesting properties as compared to their crystalline counterpart. Recently, a novel approach33 was demonstrated to synthesize BNNTs at a temperature of 800 °C on a nickel-coated SiO2 substrate in a MPCVD by gas reaction of B2H6-NH3-H2. The crystalline and selective growth of BNNTs was demonstrated by this method, which suggests that a combination of both plasma-assisted and catalytic CVD might be an efficient rule to obtain low-temperature growth and highquality BNNTs. However, the role of catalyst is still open to investigation. In this work, a home-built plasma-assisted CVD (PACVD)34 was developed to grow crystalline BNNTs at 900 °C with the feed gases B2H6, NH3, O2, and Ar. A parametric study of the growth was performed, and the material characterizations were comprehensively carried out. Moreover, the growth mechanism as well as the role of the metal catalyst will be further discussed.

10.1021/jp904402h CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

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Experimental Section First, a p-type Si(100) substrate was cleaned and oxidized to form a thermal oxide with a 100 nm thickness. Then, a spunon photoresistant (PR) and conventional photolithography technique was used to define arrays of 10 × 10 µm2 square patterns. An Al underlayer with a 10 nm thickness and an Fe catalyst layer with a 1 nm thickness were sequentially deposited onto the patterned substrate by electron-beam evaporation. A lift-off process was then used to define the catalyst pads. In our PACVD system, a quartz tubular furnace was used as a reactor to grow BNNTs, while a copper coil was placed 20 cm away from the center of the furnace to produce the remote inductively coupled plasmas (ICP) (supplied by a 13.56 MHz rf power generators). For growing the BNNTs, the patterned substrates were placed in a quartz holder that was then loaded into the center of a quartz tubular furnace, which was attached with a thermocouple (about 5 mm above the sample’s surface) to measure the local temperature close to the substrate’s surface (refer to Figure S1 of the Supporting Information for a detailed facility setup). The chamber was evacuated to a base pressure of ∼13 mTorr. A pretreatment process was carried out under an ammonia (NH3) atmosphere at a temperature of 900 °C for 5 min to form the active catalytic nanoparticles.35 During the growth process, the residual NH3 gas was evacuated back to the base pressure. A mixed gas of B2H6, NH3, O2, and Ar was then introduced into the furnace at a temperature of 900 °C. The growth time was set to be 30 min, and the total gas pressure was fixed at 900 mTorr. For a parametric study, the plasma power was varied from 50 to 200 W, and the flow rate ratio of the feed gases, B2H6/NH3, was regulated from 1/4 to 2/1 (the B2H6 gas flow rate was set to 10 or 20 sccm, while the O2 and Ar rates were fixed at 10 and 20 sccm, respectively). Moreover, the influence of the oxygen additives in the growth process was studied by varying the oxygen’s flow rate from 0 to 15 sccm, while the other conditions were fixed as the baseline (B2H6/ NH3/Ar: 20/80/20 sccm). Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) were used to examine the morphology and nanostructures of the as-grown BNNTs. The nanobeam energy-dispersive spectroscopy (nanobeam EDX) under HRTEM was used to identify elements in the BNNTs (such as metal catalyst). The chemical bonding and atomic composition of the as-grown samples were characterized by micro-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Furthermore, the local composition and stoichiometry of an individual BNNT were identified by electron energy loss spectroscopy (EELS) and nano-Auger electron spectroscopy (nano-AES). It is worth noting that, when preparing samples for Raman and EELS experiments, the as-grown BNNTs were harvested from the substrate and then transferred onto a TEM grid by placing a drop of ethanol on the sample’s surface and then carefully transferring the surface product onto the grid. The purpose of this procedure is to prevent any effects of impurities in the substrate. For the optical transition analysis, photoluminescence (PL) was performed at room temperature on the as-synthesized materials. PL spectra were excited with VUV light that was produced in the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The facility, with a VUV synchrotron-radiation source, was used for the experiments and is described in detail elsewhere.36,37 In this work, the scan range is 600-185 nm with a step of 1 nm every 3 s, while the spectral resolution of the setup is 2.5 nm.

Figure 1. SEM images of as-grown BNNTs at 900 °C: (a) Asgrown BNNTs on an array with a 10 × 10 µm2 pattern. (b) A magnified view of a 10 µm × 10 µm bit that shows highly dense BNNTs that were grown on a catalyzed pattern without any impurity byproducts around.

Results and Discussion Characterization of the Morphology and Structure of Synthesized BN Nanotubes. Cup-Stacking, Bamboo-like, and Tubular BNNTs. Figure 1a is a SEM image that shows a high density of BNNTs that have been grown on an array with a 10 µm × 10 µm catalyst pattern at a temperature of 900 °C. The plasma power was set to 200 W, and the gas flow rates of B2H6, NH3, O2, and Ar were set to 10, 40, 10, and 20 sccm, respectively. Figure 1b is a magnified view of a 10 µm × 10 µm bit. There are no obvious impurity byproducts (e.g., amorphous BN powders, flakes, boron-oxide particles, etc.) outside of the patterned areas, which indicates that the high density of BNNTs can be selectively synthesized on the designated positions by using a conventional patterning technique. The diameter of this BNNT is around 25 nm, and the length is up to several micrometers. The TEM images in Figure 2a show a BNNT with a bamboo-like tubular structure that has repeat compartments indicated by the white arrows. The inset of Figure 2a is a HRTEM image that was recorded from the side wall of this BNNT which suggests that this BNNT exhibits a crystalline structure. The lattice spacing is around 0.33 nm (0.328 ( 0.0014 measured from six positions), which is close to the intershell spacing of the {0002} lattice plane in the hexagonal phase of BN(h-BN). Furthermore, a nanoparticle, which is indicated by a red arrow, located on the tip of this nanotube is further

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Figure 2. HRTEM images and the corresponding diffraction patterns (DPs) of an as-grown BNNT: (a) The as-grown BNNT exhibits a bamboolike tubular structure with repeat compartments (as marked with white arrows). The inset shows a high-resolution image on the wall portion of a BNNT. (b) A magnified image indicates the angle (labeled as θ) between the tube’s axis and the BN layer in the side wall to be 19.2°. Therefore, the cup angle between the BN layers at both nanotube walls is ∼38.4° (i.e., 2θ). (c) The corresponding diffraction pattern (taken from the point that is marked by a star in part b). Explanatory sketches of the diffraction spot patterns are included in the inset (the red, hollow circles indicate BN(001), while the green, hollow circles indicate BN(002). The BN(002) spots form an angle of ∼38° (labeled as φ), which was consistent with the measured cup angle in part b.

identified to be pure Fe by nanobeam EDX, which suggests that the PACVD-grown BNNTs grew by the metal-catalyzed tip-growth mechanism.2,38 However, further studies (a detailed discussion is shown elsewhere)39 of elemental mapping with EELS show that it was not pure Fe but boron- containing Fe catalyst. The detailed growth mechanism is mentioned in the next section. In the TEM observations, the majority of the tubes reveal the cup-stacking structure, which is shown in Figure 2b, while a few BN tubes (ca. 20%) appear to have a coaxial structure (i.e., BN layer that parallels the tube axis) (see Figure S2 of the Supporting Information). Recently, it was found that ring defects, with a specific arrangement in a hexagonal lattice, will lead to closed cage structures.40 For BNNTs, the stability of the ring defect corresponds to the formation of a cup-stacking structure with an angle of 38.9° (the so-called 240° disclination).40 A similar apex angle was also observed (about 36°) in other reports by using Fe4N/boron powder as a catalyst at a temperature of 1000 °C.41 As shown in Figure 2b, our as-grown BNNTs also revealed a cup-stacking structure with a cup angle of ∼38.4°. These results are consistent with the results of the diffraction pattern that are shown in Figure 2c and are in good agreement with the reported cup angle of the cup-stacking BNNTs.40,41 It is noteworthy that several BNNTs with a larger cup angle were observed as well in this work (see Figure S3 of the Supporting Information). The large-angle (∼52°) BNNTs were synthesized and characterized to be a special structure called a BN hollow conical helix (BN HCH) in an earlier report.42 However, in this work, we observed many BNNTs with a larger angle from 40 to 67°, and the average cup angle was measured from different BNNTs to be about 57 ( 10°. The BNNTs with a large cup angle are seldom contained in as-grown

materials, which might be attributed to the metastable state due to such a low growth temperature. The bamboo-type (Figure 2a) BNNTs with cup-stacking structures (Figure 2b) were also observed elsewhere in the literature.41 This unique structure will lead to highly reactive properties in the tube’s surface due to the unstable dangling bonds and would be suitable for future application of hydrogen storage.15 Chemical Bonding, Stoichiometry, and Surface Chemical Analysis of Synthesized BNNTs. Raman spectroscopy was carried out with a laser of 514 nm wavelength. To avoid contributions to the signal from other structures (such as the BN “film”) that could possibly form under similar growth conditions as the PACVD system that was reported in our previous work,34 the as-grown BNNTs were carefully harvested and transferred to a lacy carbon film that was supported by a TEM grid. Figure 3 shows the micro-Raman spectra of the h-BN powder (Figure 3a); a standard lacy carbon film is supported by a TEM grid (Figure 3b), and the harvested BNNTs are on the same TEM grid (Figure 3c). Several peaks sitting on the broadened background signals in Figure 3c were indicated by the black arrows. By comparison with the signals in Figure 3b, peaks at 1370 and 1580 cm-1 are revealed to be attributable to the TEM lacy carbon film. It is worth noting that a sharp peak at 1364 cm-1 was observed (see Figure 3c), which was caused by the Raman-active E2g mode that arises due to in-plane atomic displacement of the BN nanotube.43 This characteristic peak of the BNNTs is somewhat red-shifted and broader than that of the h-BN powder (Figure 3a) which is attributed to curvatureinduced softening at the small radius of the tubular structure. This is in good agreement with previously reported work.44 There were several weak peaks that appeared around 1000 cm-1,

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Figure 3. Raman spectra of (a) h-BN powder, (b) a lacy carbon film that is supported by a TEM grid, and (c) BNNTs in the same TEM grid. The inset black arrows indicate peaks at 961, 1074, 1364, 1370, and 1580 cm-1, respectively.

which were attributed to the lattice disorder43 and unwanted byproducts (e.g., boric acid), as reported in the literature.45 Figure 4 shows the XPS spectra of the as-grown BNNTs. A photon energy of 1486.6 eV was used. Strong 1s peaks of boron, nitrogen, carbon, and oxygen were detected, as shown in Figure 4a. The Gaussian-fitted B 1s spectrum in Figure 4b indicates that there are two bonding states: B-N (190.3 eV) and B-O (191.5 eV).46 From Gaussian fitting, Figure 4c also shows that the N 1s spectrum consists of two peaks: N-B (397.8 eV) and N-O (398.3 eV).46 These results suggest that the strong bonding signal of the B-N dominates the atomic bonding structure of as-grown nanotubes. Moreover, the observed bonding structures of B-O and N-O revealed that oxidation occurs in as-grown nanotubes. Further quantification of the XPS intensity revealed that the as-grown materials contained ∼18% oxygen, which was also frequently observed in the Boron-related materials45 due to the ease of boron oxide formation when exposed to the atmosphere. Figure 5 depicts an EELS spectrum that was taken from a single BN nanotube. It shows the K-edges of the boron and nitrogen at 188 and 401 eV, respectively.38 A very weak C K-edge at 288 eV was observed in this spectrum, which may be attributed to marginal contamination effects during EELS spectrum acquisition that are caused by the focused electron beam. The sharp peaks of π* and σ* on the K-edge of B and N, as shown in the inset, reveal hybridized sp2-bonded B and N atoms in the h-BN layer. In addition, a quantitative analysis with EELS47 yields a B:N atomic ratio of about 0.98 (the given ratio has an estimated error of 20% due to baseline corrections), which is closely consistent with the stoichiometric ratio in h-BN. The nano-AES experiment allows a chemical analysis of the surface at the nanometer scale,48 which allows the surface structure of the as-grown sample to be better understood. Figure 6 shows an AES spectrum that was recorded from the position marked with a red star in the inset. The elements of B, N, O, and C are identified. Again, the signal of C may come from the contamination due to the focus electron beam, as it has in the EELS spectrum. The observed O peak was further quantified to be ∼19.7%, which is much closer to the quantified amounts of B-O/N-O in the XPS results. The probe resolution in this case is about 8 nm, which suggests that most of the oxidation was formed in the surface of the as-grown nanotubes. This result is consistent with the frequently observed thin layer of boron-

Figure 4. XPS spectrum of as-grown BNNTs using a photon energy of 1486.6 eV: (a) 1s peaks of boron, nitrogen, carbon, and oxygen are detected. (b) The B1s XPS spectrum consists of two Gaussian-fitted peaks at 190.3 and 191.5 eV, which indicates the existence of chemical bonding in the B-N (green line) and B-O (blue line), accordingly. (c) The N 1s XPS spectrum also consists of two Gaussian-fitted peaks at 397.8 and 398.3 eV, which indicates the existence of N-B (green line) and N-O (blue line) bonds, respectively. In parts b and c, the original data points are represented as black circles, while the sum of the fitted peaks is shown as a red line.

oxide in the surface of the BN/BCN nanostructures.49,50 However, the percentage of absorbed oxygen in the surface of the nanotubes is much higher, which might be attributed to a large number of dangling bonds in the surface of the nanotube that is due to the cup-stacking structure (see Figure 2b). Characterization of Optical Transitions by PL Spectroscopy Measurements. Figure 7 shows the PL spectroscopy that was recorded from the as-grown BNNTs at room temperature with an excitation at 157 nm. The PL spectrum was composed

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Figure 5. EELS spectrum recorded from a single BNNT (the measured point was taken from the point that is marked by a star; see Figure 2b). It shows the two distinct K-edges of the boron and nitrogen at 188 and 401 eV, respectively. The inset shows a magnified view of the K-edge of B and N that composes the π* and σ* structure features and implies the existence of sp2 hybridization.

Figure 6. Nano-AES spectrum of a single BNNT with a 20 keV electron beam (probe resolution of about 8 nm); the measured point was the point that is marked with a red star in the inset. B, N, C, and O are identified.

of two luminescence bands: a strong UV band at 227 nm (5.48 eV) and a broad band at around 338 nm (3.68 eV). These two lines were typically attributed to the transitions of BNNTs and have been observed for both single-walled BNNT51 and multiwalled BNNTs.52 The UV luminescence at 227 nm is attributed to the near-edge excitonic absorption, while the luminescence band at around 338 nm is assigned to the impurity levels (possibly attributed to the carbon or oxygen impurities53). However, the UV peak is at 227 nm instead of the free excitonic luminescence of the BN nanotube, which is located at 215 nm, indicating that red-shifted emission occurred. This phenomenon is most intense at 300 K and has frequently been observed in earlier studies.54–56 Recently, Jaffrennou et al.57 contributed an investigation on the near-band-edge recombination in multiwalled BNNTs and discussed in detail the relation between the nanostructure (identified by HRTEM imaging) and the luminescence bands that are recorded from PL and CL. They concluded that this red-shifted emission was likely attributed to the excitonic recombinations in terms of excitons bound to the structural defects such as grain boundaries and dislocations.54,56,58 This indicates that the red-shifted emission in our case is most likely from the large defect states in the surface of the as-grown

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Figure 7. PL spectra of the as-grown BNNTs, carried out at room temperature (300 K) with an excitation of 154 nm. A luminescence line at 227 nm and a deep blue luminescence band at about 338 nm were observed.

BNNTs that occur due to the revealed cup-stacking and bamboolike structure. This red-shifting phenomenon was also observed in CL measurement of the bamboo-like BNNTs57 and was also attributed to the excitons that are bound to different types of defect traps. Our work is the first to characterize luminescence in such a BNNT structure (i.e., cup-stacking and bamboo-like). The results reveal that the optical transitions of the as-grown BNNTs preserved the typical absorption properties (as reported for different BNNT structures in other works), while the redshift in the UV band is correlated with the cup-stacking and bamboo-like structure that formed due to the exciton’s trapping mechanism in the surface defects. Effect of Oxygen Additives during Growth, Influence of Fe/Al Bilayer Catalysts, and Role of Metal Catalysts. To optimize the process of obtaining high-yield and high-purity BNNTs at lower temperature, the processing parameters (such as plasma power, gas flow ratio, and process temperature) were systematically investigated. Consequently, it was found that the growth density of the BNNTs increased with increasing plasma power from 100 to 200 W (Figure 1 shows the case for 200 W). It was believed that increasing the plasma power would increase the density of B and N radicals and therefore provide more energetic radicals for the growth of BNNTs. The other two parameters, such as gas flow ratio and process temperature (results not shown here), are also quite sensitive. When the gas flow ratio of B2H6/NH3 was regulated to be higher or lower than 1/4, few or even no BNNTs grew, while the same tendency also occurred for the temperature parameter (fewer BNNTs grew at 700 °C), which means that, by this method, the reaction will only take place with sufficient energy and at specific radical compositions. It is worthwhile to note that the sample, in the absence of oxygen gas, exhibits very low growth yields; no BNNTs were observed in most cases (Figure 8a), which implies that the presence of oxygen is crucial to form BNNTs. The growth density significantly increased when the introduced oxygen increased from 5 (Figure 8b) to 10 sccm (Figure 8c). When the oxygen further increased to 15 sccm (Figure 8d), the observed morphology showed no difference, as compared to the 10 sccm of oxygen. The O2 additive in this synthesis process was assumed to play an important role in enhancing the dehydrogenation from the catalyst surface and to reduce the excess H radicals in the process atmosphere. In earlier investiga-

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Figure 8. SEM image of as-grown BNNTs with different oxygen gas flow rates and other fixed-growth conditions (B2H6/NH3/Ar: 20/80/20): (a) Growth without O2 and growth with O2 at (b) 5 sccm, (c) 10 sccm, and (d) 15 sccm.

tions of CNT growth with the PECVD system, the oxygen additive also appeared to play a role in balancing the C and H by forming OH radicals to suppress the formation of redundant amorphous carbon and to reduce the etching effect of the hydrogen plasma.59 For the catalytic growth of BNNTs by the reaction of gas mixtures of B2H6-NH3-H2 in a MPCVD system,60 several active species (such as H2, BH2, BNH2, B2H2, NH, NH2, etc.) were detected with quadrupole mass spectroscopy (QMS) during the growth of BNNTs. In our case, similar H species could be reasonably presented during the growth process due to a similar precursor (B2H6-NH3) and the plasmaassisted system. Therefore, the O2 additives could help to enhance the dehydrogenation process (even at the surface of the catalyst or in the atmosphere), and facilitate the growing mechanism that is mainly carried with B and N atoms. Another interesting phenomenon to be noted is that the bilayer metal catalyst can yield a higher density of BNNTs as compared with the use of the thin-film catalyst (Fe) under the same growth conditions (see Figure S4 of the Supporting Information), which implies that the bilayer catalyst system might provide a promising rule by which to synthesize a high yield and high quality of BNNTs. In earlier work on the synthesis of CNTs, the multilayer catalyst system (Al/Fe/Mo)61 and the supporting catalyst (Fe/Al2O3-SiO2)62 have been demonstrated to yield high-purity SWNTs. We believed that the bilayer catalyst provided more active nucleation sites and prevented aggregation of the catalyst particles at elevated temperatures in the CVD process. In our work, this physical mechanism was as depicted in Figure 9a: the Al layer was transformed into aluminum oxide (AlxOy islands) during the pretreatment step at 900 °C due to interdiffusion between the Al layer and the underlying SiO2. Simultaneously, the initial Fe layer was transformed into nanoparticles and uniformly distributed on the surface of the AlxOy underlayer (see the SEM image shown in Figure S4b of

Figure 9. Schematic models for the growth of BNNTs: (a) The initial bilayer is transformed during the pretreatment process into an AlxOy island, where nanosized Fe particles were uniformly distributed on the surface of the AlxOy clusters. (b) A four-step model depicts the growth of the BN nanotube as follows: (i) Boron atoms dissolve in the Fe catalyst. (ii) Boron segregates onto the particle’s surface. (iii) Boron reacts with nitrogen to form BN layers. (iv) The dynamic growth of catalyst reshaping combined with precipitation of BN layers forms the cup-stacking and bamboo-like BN nanotube.

the Supporting Information). A concern is that iron may also oxidize (FexOy) during the growth and therefore destroy the catalyst activity. The generation of pure Fe nanoparticles was proposed to be related to a reduction process, where FexOy loses its oxygen atom to the Al underlayer63 and transforms it into more stable AlxOy64 particles. Therefore, the AlxOy underlayer plays a critical role in providing stable support for the Fe nanoparticles (i.e., it preserves the catalyst activity and prevents the aggregation effect, as shown in Figure S4 of the Supporting Information), which react with the boron and nitrogen species. In addition, it is worthwhile to further investigate the metal catalytic mechanism in this synthesis approach. During this

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process, there is the possibility that the Fe catalyst may be nitrified to Fe4N in the presence of ammonia, which becomes chemically inert and then destroys the catalyst activity. However, it appears that the Fe2B is thermodynamically more stable than Fe4N,65,66 and further work67 also demonstrated that Fe4N was sufficiently reduced to Fe by B when the temperature reached up to 700 °C (the reaction is expressed in eq 1). This suggests that the boron-containing Fe catalyst was formed when the temperature reached 900 °C in our synthesis process.

Fe4N + 3B f 2Fe2B + BN

(1)

Moreover, it was found that boron tends to aggregate at the surface of the Fe2B catalyst particles and then starts to react with nitrogen from the NH radicals to produce the BN layers (see the reaction that is expressed in eq 2). This is the first report that experimentally explains the growth mechanism by EELS mapping images.68 This result is in good agreement with the earlier hypothesis of BNNT growth with Fe4N/B mixed powder.65

Fe2B + NHz f BN + 2Fe + ZH

(2)

Therefore, the role of the metal catalyst in this case is like a container that helps the boron efficiently dissolve in and then react to form BN layers on the surface of the catalyst with nitrogen from the atmosphere. Figure 9b shows the steps of the phenomenological model for the formation of BN nanotubes: (i) formation of the boron-containing catalyst due to the solid solution of boron in Fe; (ii) the boron starts to segregate onto the surface of the catalyst; (iii) the boron on the catalyst surface will react with NHz radicals, which produces the BN layer; (iv) incorporation of the nitrogen atoms on the surface of the boroncontaining catalyst that achieves the growth of the tube structure (a detailed description of each steps is found elsewhere39). Moreover, it is noteworthy that steps iii and iv, as shown in Figure 9b, depict the formation of a BN nanotube with cupstacking and bamboo-like structures. We believed that the reshaping catalysts (as the pear shape that is illustrated in Figure 9b-iii) appear to be correlated with the formation of BN sheets at the BN-catalyst interface. In agreement with earlier works on metal catalytic growth of BNNTs, it appears that the Fe catalysts were frequently observed to be enclosed at the tip of the BNNTs68,69 and often led to the formation of nanotube with a cup-stacking and bamboo-like structure. It was believed that the dynamic catalyst-reshaping phenomenon70 during the growth process might govern the cup-stacking angles and the compartment of the BN nanotube, as shown in parts b and a of Figure 2, respectively. In other words, the cup-stacking structure was formed when the catalyst was elongated to be a pear shape in the initial step followed by precipitation of the BN layers with a specific angle (see Figure S3 of the Supporting Information) of the cup-stacking structure. Moreover, there were only a few amounts of BN nanotubes that revealed a cylinder and hollow structure, as shown in Figure S2 of the Supporting Information. It is believed that the dynamic reshaping of the catalyst during the growth process also played a crucial role in the formation of this structure (a detailed discussion of the formation of cylinder nanotube structures is mentioned elsewhere39). The highly elongated shape of the catalyst particle led to the tubular structure with the precipitated BN layers aligned parallel to the tube’s axis.

Conclusion In conclusion, the crystalline BNNTs were selectively grown on a bilayer catalyst at a temperature of 900 °C in a PACVD system. The as-grown BNNTs reveal a crystalline and bamboolike tubular structure with a stoichiometric ratio of h-BN. The unstable dangling bonds have a unique structure, which in turn leads to the highly reactive properties of the tube’s surface. Promising applications may be found in hydrogen storage and sensing devices. We were the first to contribute the optical transition on cup-stacking and bamboo-type BNNTs. From the results, it appears that the excitonic recombination is due to defect trapping on the tube’s surface. With our synthesized method, the O2 additives in the process were found to help enhance the dehydrogenation process and facilitate the growth mechanism of BNNTs. Moreover, the role of the metal catalyst and influence of the Fe/Al underlayer in the growth of BNNTs suggested a boron container and a suitable supporting layer for preventing catalyst aggregation, respectively. These results make a new strategy for achieving BN-nanotube-based devices possible by taking advantage of BNNT’s selective growth and low synthesis temperature. Acknowledgment. This work was financially supported by the National Science Council of Taiwan under Contract No. NSC 95-2120-M-007-001 and the Center of Nano-Science and Technology (CNST) in the University System of Taiwan (UST). Supporting Information Available: Figures showing the experimental setup of the PACVD system (S1), HRTEM images of the coaxial BNNTs (S2) and the cup-stacking BNNTs (S3), and SEM images of the bilayer catalyst system after the pretreatment step and the growth process (S4). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Rubio, A.; Corkill, J. L.; Cohen, M. L. Phys. ReV. B 1994, 49, 5081. (2) Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966. (3) Blase´, X.; Rubio, A.; Louie, S. G.; Cohen, M. L. Europhys. Lett. 1994, 28, 335. (4) Chen, Y.; Zou, J.; Campbell, S. J.; Caer, C. L. Appl. Phys. Lett. 2004, 84, 2430. (5) Golberg, D.; Bando, Y.; Kurashima, K.; Sato, T. Scr. Mater. 2001, 44, 1561. (6) Pouch, J. J.; Alterovitz, A. Synthesis and Properties of Boron Nitride; Trans Tech Publications: Zurich, Switzerland, 1990. (7) Chang, C. W.; Fennimore, A. M.; Afanasiev, A.; Okawa, D.; Ikuno, T.; Garcia, H.; Li, D.; Majumdar, A.; Zettl, A. Phys. ReV. Lett. 2006, 97, 85901. (8) Smith, B. W.; Monthioux, M.; Luzzi, D. Nature 1988, 396, 323. (9) Chopra, N. G.; Zettl, A. Solid State Commun. 1998, 105, 297. (10) Radosavljevic, M.; Appenzeller, J.; Derycke, V.; Martel, R.; Avouris, P.; Loiseau, A.; Cochon, J. L.; Pigache, D. Appl. Phys. Lett. 2003, 82, 4131. (11) Radosavljevic, M.; Appenzeller, J.; Derycke, V.; Martel, R.; Avouris, P.; Loiseau, A.; Cochon, J. L.; Pigache, D. Appl. Phys. Lett. 2003, 82, 4141. (12) Cumings, J.; Zettl, A. AIP Conf. Proc. Series in Electronic Properties of Molecular Nanostructures; American Institute of Physics: New York, 2001; p 577. (13) Golberg, D.; Dorozhkin, P. S.; Bando, Y.; Dong, Z. C.; Grobert, N.; Reyes, R.; Terrones, H.; Terrones, M. Appl. Phys. Lett. 2003, 82, 1275. (14) Chen, H.; Chen, Y.; Liu, Y.; Xu, C. N.; Williams, J. S. Opt. Mater. 2007, 29, 1295. (15) Lim, S. H.; Luo, J.; Ji, W.; Jianyi, L. Catal. Today 2007, 120, 346. (16) Shevlin, S. A.; Guo, Z. X. Phys. ReV. B 2007, 76, 24104. (17) Yum, K.; Yu, M. F. Nano Lett. 2006, 6, 329. (18) Aktas, A. C.; Stubbins, J. F.; Zuo, J. M. J. Appl. Phys. 2007, 102, 24310. (19) Loiseau, A.; Willaime, F.; Demoncy, N.; Hug, G.; Pascard, H. Phys. ReV. Lett. 1996, 76, 4737.

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