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J. Phys. Chem. C 2007, 111, 1136-1139
Coating Aluminum Borate (Al18B4O33) Nanowire Webs with BN H. S. Song, J. Zhang, J. Lin, S. J. Liu, J. J. Luo, Y. Huang, E. M. Elssfah, A. Elsanousi, X. X. Ding, J. M. Gao, and Chengcun Tang* Department of Physics, Central China Normal UniVersity, Wuhan, 430079, People’s Republic of China ReceiVed: NoVember 8, 2006
A facile method was proposed to directly synthesize the completely coated aluminum borate (Al18B4O33) nanowire webs with BN by an epitaxial growth process. The BN-Al18B4O33 nanocable webs could be obtained by the reaction of Al18B4O33 nanowire webs, B2O3, and ammonia at 1200 °C. The high-temperature stable Al18B4O33 nanowire webs acted as a template for the epitaxial coating of BN. The coating was characterized by the means of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and infrared spectrum (IR). High-resolution transmission electron microscopy (HRTEM) clearly shows the BN coating layers closely attached to the inner nanowires. The typical thickness of the BN shells is ∼5 nm. The growth mechanism of the present BN-coated aluminum borate (Al18B4O33) nanostructures was also proposed.
Introduction Reinforcing metal matrix alloys provide possibility to obtain composite materials with excellent mechanical properties.1 The general method adopts some whiskers or nanowires with structure strength and chemical inertness to incorporate into the matrix. In recent years, a new oxide whisker/nanowire composed of aluminum borate (Al18B4O33 or 9Al2O3‚2B2O3) as reinforcement has been investigated because of their high elastic modulus, tensile strength, and low thermal expansion coefficient.2 Furthermore, the lower cost (only 1/10 to 1/20 of those made form SiC) is another incentive to recommend aluminum borate nanowires in composite communities.3 Therefore, aluminum borate-reinforced composites have attracted more and more attention in reinforcing industries. However, there is one main problem that we cannot neglect: the interfacial reactions between matrix and aluminum borate nanowires. Especially to aluminum borate nanowires, at the high working temperature they can easily take the interfacial reactions with the matrix because of their low dimensionality and high surface-to-volume ratio. This chemical reactivity leads to the nanowire oxidation and contamination, resulting in the decrement of the composite strength.4 Various methods have been reported to control the interfacial reactions,4-7 and a coating with chemically stable materials on the aluminum borate nanowires should be a feasible strategy to hinder or avoid the interfacial reactions. On the other hand, hexagonal boron nitride, a covalently bonded compound, displays stable insulating properties (a ∼5.5 eV band gap) independent of its morphology.8 In addition, graphitelike BN is chemically inert and remarkably thermally stable.9 Therefore, boron nitride thin film coatings show us a great deal of interest for modifying the interface in fiberreinforced composites to improve fiber pullout and to prevent interfacial reactions because of their unique chemical and physical properties, such as low density and high thermal conductivity in a wide range of temperatures.5 The previous work of our group6 has successfully synthesized BN-coated * To whom correspondence should be addressed. Tel: +86-27-67861185. Fax: + 86-27-67861185. E-mail:
[email protected].
Al18B4O33 nanowires by using high-temperature unstable Al4B2O9 nanowires as the reactant precursor. Zhu et al. also obtained BN-Al18B4O33 nanocables at high temperature (1750 °C) using B-N-O precursor and fumed alumina.7 Considering the main commercial applications in metal matrix composites, the synthetic method of BN-coated Al18B4O33 nanowire should be in large scale, at low temperature, and using a simple and easily controllable procedure. In this paper, we design a facile method to synthesize those composite nanowires to meet the above requirements. High-temperature stable aluminum borate Al18B4O33 nanowires rather than Al4B2O9 nanowires were used as the precursor; dispersed B2O3 and high chemical active ammonia were selected as the BN source to decrease the reaction temperature to 1200 °C (the lowest temperature for the formation of a crystalline-layer h-BN10). Fortunately, the first time we successfully coated Al18B4O33 nanowire webs with BN in large scale; the whole nanowire webs were directly coated at low temperature. We expect that our method is particularly useful for the further commercial applications of aluminum borate nanostructured materials. Especially, it takes a great deal of meaningfulness to extend to other refractory nanowires or nanotubes. Experimental BN-coated Al18B4O33 nanowire webs were prepared by the reactions between the as-prepared Al18B4O33 nanowire webs11 and the boron oxide in the presence of ammonia. B2O3 methanol solution (1.67 M) was prepared by dissolving B2O3 powder in methanol under mechanically stirring. Two grams of aluminum borate nanowire webs was slowly impregnated with 5 mL of B2O3 methanol solution and was dried in air to form the reaction precursor for BN-coated Al18B4O33 nanowire webs. Subsequently, the as-prepared precursor was placed in an alumina crucible located in an alumina tube, which was mounted in a traditional resistance-heating furnace. The system was first purged with a high purity of N2 gas for 30 min. Then, the N2 gas was replaced by a N2/NH3 flow (N2 30 mL/min, NH3 120 mL/min). The furnace was heated to 1200 °C in 1 h and was kept at that temperature for 2 h. A white powder product was
10.1021/jp067393u CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006
Coating Aluminum Borate Nanowire Webs with BN
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1137
Figure 2. SEM images describe the morphologies of the pure aluminum borate and the coated aluminum borate nanowire webs. (a, b) The low-magnification and high-magnification images of Al18B4O33, respectively. (c, d) The corresponding low- and high-magnification images of BN-Al18B4O33 webs.
Figure 1. (a) XRD pattern of the BN-coated Al18B4O33 nanowire webs. (b) Infrared spectra of the pure Al18B4O33 nanowebs and the coated nanowebs, and the star symbols show the peaks of BN film.
obtained after the furnace was naturally cooled to room temperature under the N2 flow. The product was identified by means of X-ray diffraction (XRD, D/max-rB, Cu KR radiation) analysis. Infrared spectra (IR) were measured on a NICOLET NEXUS470 spectrophotometer. The spectra were recorded on a KBr pressed disk. The overview morphologies of the sample were checked by a field emission scanning electron microscopy (SEM, JSM-6700F, JEOL). The nature of the BN-Al18B4O33 nanowire webs was studied using an ESCALAB MK II X-ray photoelectron spectroscopy. The powder sample was also ultrasonically dispersed in ethanol solution and was then transferred onto a copper grid covered with carbon film for the transmission electron microscopy (TEM, JEM-2010F, JEOL) and the highresolution transmission electron microscope (HRTEM JEM2010FEF, JEOL). Results and Discussion Figure 1a shows the XRD pattern of the product, which can be well indexed as the mixed phase of an orthorhombic Al18B4O33 and a hexagonal BN. The measured pattern is in agreement with the reported XRD patterns (JCPDS 32-003 for Al18B4O33 and JCPDS 26-0773 for BN) concerning both peak intensity and position. This result indicates that BN is successfully contained in the product. Information from the infrared spectra of the samples would be helpful in understanding the
structure of the compounds. Figure 1b shows the IR spectra of the Al18B4O33 nanowire webs and the spectra of BN-Al18B4O33 nanocable webs, respectively. The pure Al18B4O33 nanowires show the same IR characterization as that reported in the literature.12 The vibration absorption region of 1200-1460 cm-1 is due to B-O bond asymmetric stretching of BO3 units, while B-O bond stretching of BO4 units appears in the range 10001100 cm-1.13 From the spectra, the bands in the region of 10001100 cm-1 are very weak, indicating a minor BO4 unit. In addition, the presence of absorption peaks in the range of 900810 cm-1 originates from vibrations mainly involving AlO4 units or BO3 units; the bands in the region of 810-650 cm-1 are attributed to the vibrations of AlO5 units or BO3 units; the bands in the range of 600-500 cm-1 are due to the vibrations of AlO6 units or BO3 units.12 Therefore, Al18B4O33 mainly consists of BO3, AlO4, AlO5, and AlO6 units. Although the main spectra of BN-Al18B4O33 are the same as the ones of Al18B4O33, there is no peak in the region of 1000-1100 cm-1 indicating the absence of BO4 units, especially the spectra containing two characteristic peaks, 1380 cm-1 and 806 cm-1 (marked by the star symbol), which correspond to the IR peaks of hexagonal BN.14 Thus, the IR spectra further indicate the present phases of hexagonal BN and aluminum borate. Figure 2 describes the morphologies of the pure aluminum borate (Figure 2a and Figure 2b) and the coated aluminum borate nanowire webs (Figure 2c and Figure 2d). Figure 2a and Figure 2b displays the overview morphology and the magnified morphology of bare nanowires, respectively. There is little change for the two types of the product judging from the SEM images. This means that the coating is very uniform and thin and did not considerably change the morphology of the original nanowebs. Figure 2c shows a large weblike block made of highly dense and ordered nanowires. Observing the magnified pattern (Figure 2d), it is found that the webs are constructed by multilayer different orientation quasi-arrays, which make a knittinglike structure. To determine the completeness of the coatings and the chemical compositions of the product surface, X-ray photoelectron spectroscopy was followed to test the sample. The B 1s and N 1s binding energies of the coated samples are represented in Figure 3. As can be seen, the N 1s spectrum was detected and the B 1s peak was shifted toward lower binding energy (190.2 eV) compared with the data of bare aluminum borate.15 The energy for the B 1s peak of aluminum borate whiskers is higher (up to 192.3 eV); and the B atoms are bonded with oxygen, the lower energy (190.2 eV) after coating is
1138 J. Phys. Chem. C, Vol. 111, No. 3, 2007
Figure 3. XPS survey scan of BN-Al18B4O33 webs: B 1s and N 1s. The N 1s spectrum and B 1s spectrum show the binding energies of 190.2 and 398.0 eV, respectively. Both of them are in good agreement with values of h-BN.
assigned to B atoms bonded with nitrogen atoms.15,16 Furthermore, the N 1s spectrum (398.0 eV) suggests that nitrogen present in the chemical component is neither contained in NH3 nor in AlN, and the binding energy of 398.0 eV agrees very well with the reported value of 398.0 eV for BN film.16 From the spectra of XPS, we detect that no boron atom bonded with other atoms except nitrogen atoms; in addition, the binding energies for B 1s and N 1s are in good agreement with values of h-BN. Therefore, we can get the conclusion that aluminum borate nanowire webs were completely coated by BN film.
Song et al. It is much more obvious to observe the morphology and the thickness of the coatings by means of the TEM techniques. Although the sample for TEM characterization underwent a strong sonication process, the web morphologies of the product were not affected (Figure 4a). This means that the weblike morphology was very steady after the surface coating and that the coating functioned the weblike nanowires into a whole piece of web. Figure 4b and Figure 4c displays the edge and the center structures of the nanowire arrowed in Figure 4a, respectively. From Figure 4b, it is clear to observe that the BN layers uniformly and completely coated the aluminum borate nanowires. The sheathed BN layer has a uniform coating thickness ∼5 nm, independent of the diameter of the inner Al18B4O33 nanowires. A typical interlayer spacing of 0.33 nm of the coating agrees very well with the interplanar distance of the (002) planes of hexagonal BN. After the coating process, the matrix aluminum borate nanowires still stayed intact with the interplanar spacing of ∼0.54 nm (Figure 4c) along the axis direction [001]. The corresponding selective area electron diffraction analysis (SAED) pattern was shown in the inset of Figure 4c, which can be indexed as the orthorhombic Al18B4O33 single crystal recorded from the [-210] zone axis with the crystalline parameters the same as the calculated results from the XRD measurement. On the basis of the experiments and analyses, we believe that the BN-coated Al18B4O33 nanowire webs reported here were formed via an epitaxial growth process on the Al18B4O33 nanowire template. Boron oxide methanol solution was used to achieve the monodispersed boron oxide onto the surface of matrix nanowires. After the evaporation of the methanol, the boron oxide adhered on the surface of the nanowires. At higher temperature, part of the boron oxide formed a thin film on the nanowires because of the excellent wetting ability of boron oxide.17 Subsequently, further ammonia-thermal reaction resulted in the gradual phase transition of the thin film to B-N-O intermediate product.18 The selective absorption of B2O3 and B-N-O on the surface of Al18B4O33 nanowires was due to the chemical affinity effect between B2O3 and aluminum borate nanowires and favors the BN sheaths preferentially growing along the surface of aluminum borate wires.10 At the reaction temperature (∼1200 °C), the intermediate reacts with ammonia, epitaxially growing along the wires to get the BN sheaths coated on the surface of Al18B4O33. The reaction can be described as 2NH3 + B2O3 f 2BN + 3H2O.6 In conclusion, an effective method is developed for the generation of high-temperature-stable BN-sheathed nanostruc-
Figure 4. TEM images of BN-Al18B4O33 webs. (a) Low-magnification TEM image showing the web structure with uniform coating of BN, and the arrow pointing to the nanowire that was further tested by the following HRTEM. (b) HRTEM image of the nanowire edge section clearly showing the layers of BN coating. (c) HRTEM image of the nanowire central section and the corresponding SAED pattern recorded along the [-210] zone axis.
Coating Aluminum Borate Nanowire Webs with BN tures. Al18B4O33 nanowire webs have been successfully coated by the epitaxially oriented BN sheaths on the closely packed plane. The coating relies on the following three factors: the excellent wetting ability of B2O3, the conversion of B-N-O, and the expitaxial growth on the matrix. We expect that the strategy reported here is effective in the formation of BN coatings on other refractory nanowires or nanotubes, including ZnO, SiC, magnesium borate, and so forth. Acknowledgment. This work was supported by the Fok Ying Tong Education Foundation (grant no. 91050) and the National Natural Science Foundation of China (grant no. 50202007). References and Notes (1) (a) Ibrahim, I. A.; Mohamed, F. A.; Lavernia, E. J. Mater. Sci. 1991, 26, 1137. (b) Wa, Y.; Lavernia, E. J. Metall. Trans. A 1992, 23A, 2923. (2) (a) Liaw, P. K.; Greggi, J. G. J. Mater. Sci. 1987, 22, 1613. (b) Wada, H.; Sakane, K.; Kitamura, T.; Hata, H.; Kambara, H. J. Mater. Sci. Lett. 1991, 10, 1078. (3) (a) Hu, J.; Fei, W. D.; Li, C.; Yao, C. K. J. Mater. Sci. Lett. 1994, 13, 1797. (b) Ray, S. P. J. Am. Ceram. Soc. 1992, 75, 2605. (4) (a) Ding, D. Y.; Rao, J. C.; Wang, D. Z.; Ma, Z. Y.; Geng, L.; Yao, C. K. Mater. Sci. Eng., A 2000, 279, 138. (b) Ding, D. Y.; Wang, J. N.; Ning, C. Q.; Dai, K. R. Mater. Sci. Eng., A 2003, 358, 159. (5) (a) Sahu, S.; Kavecky, S.; Illesova, L.; Madejova, J.; Bertoti, I.; Szepvolgyi, J. J. Eur. Ceram. Soc. 1998, 18, 1037. (b) Singh, R. N.; Brun, M. K. AdV. Ceram. Mater. 1998, 3, 235.
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1139 (6) Zhang, J.; Huang, Y.; Lin, J.; Ding, X. X.; Tang, C. C. J. Phys. Chem. B 2005, 109, 13060. (7) Zhu, Y. C.; Bando, Y.; Ma, R. Z. AdV. Mater. 2003, 15, 1377. (8) Blase, X.; Rudio, A.; Louie, S. G.; Cohen, M. L. Europhys. Lett. 1994, 28, 335. (9) Synthesis and properties of boron Nitride; Pouch, J. J., Alterovitz, A., Eds.; Trans Tech: Zurich, Switzerland, 1990. (10) (a) Han, W. Q.; Zettl, A. Appl. Phys. Lett. 2002, 81, 26. (b) Zhu, Y. C.; Bando, Y.; Xue, D. F.; Xu, F. F. J. Am. Chem. Soc. 2003, 125, 14226. (11) Aluminum borate (Al18B4O33) nanowire webs were prepared by heating micron scale alumina, B2O3, and boron powder at 1100 °C in Ar atmosphere for 2 h and were quenched to room temperature. (12) (a) You, H. P.; Hong, G. Y. J. Phys. Chem. Solids 1999, 60, 325. (b) You, H. P.; Hong, G. Y.; Wu, X. Y. Chem. Mater. 2003, 15, 2000. (13) Kamitsos, E. I.; Kiarakassides, M. A.; Chryssikos, G. D. J. Phys. Chem. 1987, 91, 1073. (14) (a) Hu, J. Q.; Lu, Q. Y.; Tang, K. B.; Yu, S. H.; Qian, Y. T.; Zhou, G. E.; Liu, X. M.; Wu, J. X. J. Solid State Chem. 1999, 148, 325. (b) Hubacek, M.; Sato, T.; Ishii, T. J. Solid State Chem. 1994, 109, 384. (15) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1979. (16) (a) Riviere, P.; Pacaud, Y.; Cahoreau, M. Thin Solid Films 1993, 227, 44. (b) Wang, P. S.; Malghan, S. G.; Hsu, S. M. J. Mater. Res. 1995, 10 (2), 302. (c) Liu, H.; Bertolet, D. C.; Rogers, J. W. Surf. Sci. 1995, 340, 88. (17) (a) Perez, E.; Ramos, M. A.; Vieira, S. Phys. ReV. B 1997, 56 (1), 32. (b) Cho, K. J.; Ryu, J. T.; Lee, S. Y. Solid-State Electron. 2003, 47, 633. (18) (a) Sato, T. Proc. Jpn. Acad. Ser. B 1985, 61, 459. (b) Ishii, T.; Sato, T.; Sekikaw, Y.; Iwata, M. J. Cryst. Growth 1981, 52, 285.
