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Large-Scale Synthesis of Boron Nitride Nanotubes with Iron-Supported Catalysts Ching-Yuan Su,† Wen-Yi Chu,† 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, National Synchrotron Radiation Research Center, Taiwan, and The Center for Nano Science and Technology in the UniVersity System of Taiwan, Hsinchu, Taiwan ReceiVed: May 16, 2009; ReVised Manuscript ReceiVed: June 14, 2009
Boron nitride nanotubes (BNNTs) were synthesized in a large scale with iron-supported catalysts (Fe/SiO2-Al2O3) at low temperatures (900 °C) in a plasma-assisted chemical vapor deposition system. The structural morphology, chemical composition, and optical and photoluminescence properties of BNNTs were characterized. The obtained BNNTs are crystalline and with tubular structures, and the preferential zigzag arrangement of BNNTs was discovered for the first time for BNNT grown at low temperature (1500 °C) thermal CVD. This implies that the zigzag morphology is energetically preferable under these conditions. Although theoretical predictions based on molecular dynamics calculations performed by Blase´ et al.45 favored the growth of arm-chair BN tubes in contrast to most experimental results, it was noted that the synthesis temperatures were very different in the theoretical and experimental studies (i.e., ∼2700 °C in Blase´’s work and 1500-1700 °C in other published experimental results). Therefore, synthesis temperature appears to be a dominant factor influencing atomic arrangement of as-grown BN tubes. Our work is the first to demonstrate a preferential zigzag arrangement of BNNTs synthesized with a plasmaassisted catalytic CVD method, suggesting that this is also an energetically preferable atomic configuration at temperatures lower than 1000 °C. Figure 3a shows the electron diffraction (ED) pattern of an individual multiwalled BNNT, with the bright spots indicating hexagonal (002) and (101) BN planes. Figure 3b depicts the EELS spectrum of a single BNNT, showing the K-edges of boron and nitrogen at 188 and 401 eV, respectively.46 The sharp
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Figure 6. EELS elemental mapping acquired from the tip of an as-grown BNNT. (a) Bright-field images; (b) boron maps; (c) nitrogen maps; (d) iron maps. (e) Schematic illustration of the growth model of BNNTs: (i) Formation of boron-containing catalyst (i.e., Fe2B); (ii) boron segregating on the catalyst surface; (iii) boron on the catalyst surface reacting with NHz radicals, producing a BN layer; (iv) incorporation of nitrogen atoms on the boron-containing catalyst surface, leading to growth of the tube structure. It is worth noting that structure damaged as shown in (a) after a long time of electron-beam radiation.
peaks of π* and σ* on the K-edge of B and N (see inset) revealed hybridized sp2-bonded B and N atoms in the h-BN layer. In addition, a quantitative analysis by EELS47 yielded a B:N ratio of about 0.97 (with an estimated 20% error due to baseline corrections), consistent with the stoichiometric ratio of h-BN. 3.2. Optical Transitions and VUV-PL Characteristics. The optical absorption spectrum of as-grown BNNTs is displayed in Figure 3a. The as-grown materials were dispersed in an ethanol solution, necessitating a background correction of the optical absorption spectra (see Figure S6 of Supporting Information for a detailed description). As shown in Figure 3a, the absorption spectrum displays two absorption lines approximately located at 280 nm (4.45 eV) and 225 nm (5.50 eV). These two lines, attributed to BNNT transitions, have already been observed in single-walled48 and multiwalled BNNTs49 and were further assigned respectively to impurity-levels and near-edge excitonic absorption. The two lines were not present in the optical absorption spectra of bulk h-BN materials and, therefore, were attributed to the rolling up of h-BN sheets.48 Figure 4b shows the PL spectra of pristine iron-supported powder before BNNT growth and the corresponding as-grown materials after BNNT growth. The PL intensity at 234 nm was attributed to excitonic recombination,51 corresponding to the near-edge excitonic absorption at 225 nm in Figure 4a. The luminescence band observed around 300 nm was assigned to the impurity center (possibly attributed to carbon or oxygen impurities50), corresponding to the impurity-level at 280 nm indicated in Figure 4a. Recently, Jaffrennou et al. investigated near-bandedge recombination in multiwalled BNNTs51and detailed the relationship between nanostructure (using HRTEM images) and luminescence peaks (from PL and CL). They directly confirmed that the luminescence band around 300 nm indicated impurity level and not interband transitions. In addition, three peaks located at 418, 474, and 543 nm have not been observed previously and could not be attributed to BNNTs. Spectra collected from the pristine supporting powder showed no peaks in these regions. Therefore, we attributed these peaks to structures formed during the growth process (e.g., AlN or silicide may be formed in our process).
3.3. Oxygen-Assisted Growth Mechanism and the Role of Metal Catalysts. Parts a-c of Figure 5 show SEM images of BNNTs grown by PACVD with different oxygen gas rates ranging from 0.00 to 8.33% (0.00 to 10 sccm). For low O2 concentrations (below 4.17%), we observed no BNNT growth or very low growth yields (i.e., growth density), implying that the presence of oxygen was essential for BNNT formation. The added O2 in this synthesis process was supposed to play an important role: enhancing dehydrogenation from the catalyst surface and reducing excess H radicals in the atmosphere. In earlier investigations of PECVD growth of CNTs, added oxygen helped balance C and H, forming OH radicals to suppress formation of redundant amorphous carbon and reducing the etching effect of hydrogen plasma.52 For the catalytic growth of BNNTs by reaction gas mixtures of B2H6-NH3-H2 in an MPCVD system,27 several active species such as H2, BH2, BNH2, B2H2, NH, and NH2 were detected via quadrupole mass spectroscopy during BNNT deposition. In our case, similar H species could have been present during growth due to a similar precursor (B2H6-NH3) and a plasma-assisted system. Therefore, addition of O2 could have helped complete dehydrogenation (even on the catalyst surface or in the atmosphere) and facilitated growth by B and N atoms. It is worthwhile to further discuss the influence of the catalyst in this work. Although it was suspected that the Fe catalyst could have been easily nitrified into inert Fe4N in the presence of ammonia, destroying Fe’s catalytic activity, the Oku group53 showed in 2005 that Fe2B (see the B-Fe phase diagram in a previous work54) is more thermodynamically stable than Fe4N. They also recently30 demonstrated that Fe4N is reduced to Fe by boron at temperatures of 700 °C (see eq 1). This means that, in our case, a boron-iron catalyst was formed at 900 °C
Fe4N + 3B f 2Fe2B + BN
(1)
Parts a-d of Figure 6 illustrates the EELS elemental mapping acquired from the tip of an as-grown BNNT. The boron mapping as shown in Figure 6b suggests that boron aggregated on the catalyst particle surface. This is the first time that such a phenomenon has been demonstrated experimentally, and the
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results agreed with the earlier growth mechanisms hypothesized by Oku,30 who concluded that boron in liquid-phase Fe2B starts to precipitate on the catalyst surface and reacts with nitrogen from NHz radicals to produce BN layers (eq 2). This phenomenon was also consistent with an earlier model55 showing the recombination of boron (from liquid boron particles) and nitrogen (from injected reaction gas) to form the boron nitride structure. Therefore, the metal catalyst in this case acted as a “container”, helping boron efficiently dissolve and react with atmospheric nitrogen to form BN layers on the catalyst surface.
Fe2B + NHz f BN + 2Fe + ZH
(2)
Figure 6e shows the steps of the phenomenological model for formation of BN nanotubes: (i) Formation of the boroncontaining catalyst due to the solid solution of boron in Fe; (ii) boron segregating on the surface of catalyst; (iii) boron on the catalyst surface reacting with NHz radicals, producing a BN layer; (iv) incorporation of nitrogen atoms on the surface of the boron-containing catalyst, leading to growth of the tube structure. We believe that the catalytic mechanism for the formation of the tubular structure of the BN nanotubes may be similar to the CNT model. In 2004, Stig et al.56 observed the dynamitic growth of CNTs by recording a large number of consecutive TEM images. CNTs developed through a reactioninduced reshaping of the Ni catalyst. During CNT growth, the Ni particles obtained a highly elongated shape, and tubular structures formed with the graphene sheets aligned parallel to the tube axis. In our case, the boron-containing catalyst (Fe2B) initially elongated, and boron then reacted with nitrogen to form BN sheets on the catalyst surface. Catalyst elongation appeared to correlate with the formation of more BN sheets at the BNcatalyst interface. Hence, catalyst reshaping helped align the BN-layers into a tubular structure. The catalyst elongation continued until the catalyst surface energy no longer compensated for the energy gained by the binding of the BN layer to the catalyst surface. The catalyst then abruptly contracted into a pear shape, as illustrated in part iii of Figure 5d. The elongation/contraction continued periodically during tube growth. Growth ceased when the BN layers completely encapsulated the catalyst and blocked the nitrogen from the NHz radicals in the growth atmosphere (part iv of Figure 5d). Conclusion Boron nitride nanotubes were synthesized on a large scale with iron-supported catalysts at low substrate temperatures (900 °C) with a PACVD approach using controlled gas-phase precursors (diborane and ammonia). The as-grown BNNTs had crystalline and tubular structures with a stoichiometric ratio of h-BN of approximately 1. In addition, this is the first time that the preferential zigzag arrangement of BNNTs has been demonstrated using a plasma-assisted, catalytic CVD method. The optical transitions of as-grown BNNTs were identified by correlating optical absorption spectrum with VUV-PL characteristics. Addition of O2 in this synthesis process increased the BNNT growth yields. This was attributed to increases in dehydrogenation from the catalyst surface and reductions in excess H radicals in processing atmosphere. Moreover, the role of the metal catalyst in this catalytic CVD process was investigated, and a phenomenological model of the growth mechanisms was presented. These results suggest a new strategy for synthesizing large-scale, high quality BNNTs at low
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