Synthesis of aluminium borate nanowires by sol gel method

Transcription

1 Materials Research Bulletin 40 (2005) Synthesis of aluminium borate nanowires by sol gel method Jun Wang a, Jian Sha b,a, Qing Yang a, Youwen Wang c, Deren Yang a, * a State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou , PR China b Department of Physics, Zhejiang University, Hangzhou , PR China c Center of Analysis and Measurement, Zhejiang University, Hangzhou , PR China Received 3 August 2004; received in revised form 20 February 2005; accepted 18 April 2005 Abstract A sol gel process followed by annealing was employed to fabricate single crystal aluminium borate (Al 4 B 2 O 9 and Al 18 B 4 O 33 ) nanowires. The diameter of Al 4 B 2 O 9 nanowires synthesized at 750 8C annealing is ranging from 7 to 17 nm, and that of Al 18 B 4 O 33 nanowires synthesized at C annealing is about 38 nm. Instead of the wellknown vapor liquid solid (VLS) mechanism, self-catalytic mechanism was used to explain the growth of the nanowires. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Sol gel chemistry; C. Electron microscopy; C. X-ray diffraction 1. Introduction One-dimensional nanoscale materials, such as elemental semiconductors, oxides, carbides, and nitrides, have been widely studied due to their interesting optical, electrical, and mechanical properties [1 8]. Aluminium borate is a remarkable ceramic material with high elastic modulus and tensile strength, excellent resistance to corrosion, and chemical stability [9,10]. Aluminium borate whiskers have greater potential in oxidation-resistant, whisker-reinforced composites than those observed in corresponding macroscopic single crystals due to a reduction in the number of defects per unit length (compared with larger structures) that lead to mechanical failure [11 13]. It is thus reasonable to consider aluminium borate nanowires will exhibit greater strengths than previously observed in micrometer-diameter * Corresponding author. Tel.: ; fax: address: mseyang@zju.edu.cn (D. Yang) /$ see front matter # 2005 Elsevier Ltd. All rights reserved. doi: /j.materresbull

2 1552 J. Wang et al. / Materials Research Bulletin 40 (2005) whiskers. Furthermore, the nanowires potentially have considerable applications, such as high strength materials, reinforced composites materials and electronic ceramics. So we try to synthesize aluminium borate nanowires due to their unique features. There are many methods to synthesize quasi-nanowires, such as template-directed synthesis, vapor liquid solid method, solvothermal method, arc discharge method, laser ablation method, physical evaporation method, and chemical vapor deposition method. Among those methods, chemical vapor deposition and physical evaporation of metal oxide powders are the most widely used methods to synthesize binary oxide nanowires. Recently, nanowires of aluminium borate (Al 4 B 2 O 9 and Al 18 B 4 O 33 ) have been synthesized by thermal evaporation method [14,15] and catalytic synthesis [16]. Single-crystal Al 18 B 4 O 33 microtubes have also been synthesized [17]. Here we synthesized two kinds of aluminium borate nanowires (Al 4 B 2 O 9 and Al 18 B 4 O 33 ) by sol gel process and following annealing process. 2. Experimental details Commercial high purity (>99%) Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:3, 1:4 and 1:6 were mixed together in deionized water, respectively. Citric acid was added to serve as ferment, which plays an important role on the synthesis of aluminium borate nanowires. The mixture solution was evaporated at 150 8C in an oven for 10 h. Then the gel was got. The above gels were put in crucibles, respectively. The crucibles were heated in a muffle at 750 8C for 4 h under ambient atmosphere. The gel resultant of the solution of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:3 was also heated at C for 4 h. After heating, white powders were obtained. All solid powders were centrifuged in distilled water to remove the impurities possibly remaining in the final products, and finally dried at 60 8C in air. The structure of the products was examined by X-ray diffraction (XRD, D/max ra, with Cu Ka radiation). Some powders were also ultrasonically dispersed in chemical reagent ethanol solutions and then transferred onto copper grids covered with carbon. The morphology and subtle structure of the nanowires were characterized by transmission electron microscopy (TEM, CM200/Philips, 200 kv accelerating voltage) and high-resolution TEM (HRTEM). To understand the function of citric acid, the mixture of Al(NO 3 ) 3 and H 3 BO 3 were directly being heated in mol ratio of 1:1 at C for 4 h. The products were also checked by means of XRD and TEM. 3. Results and discussion Fig. 1 shows the XRD pattern and the TEM image of the product synthesized by annealing the gel of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:3 at 750 8C for 4 h. Curve a shows the XRD pattern of the assynthesized product before being centrifuged in deionized water. As marked in the figure, the spectrum consists of two sets of peaks. According to the standard JCPDS cards, one set corresponds to the orthorhombic structure with a nominal composition Al 4 B 2 O 9, the second to cubic B 2 O 3 that resulted from the decomposition of H 3 BO 3. After centrifuged in deionized water, the sample became pure orthorhombic aluminium borate Al 4 B 2 O 9, and the boron oxide was dissolved in deionized water so that no relative peaks could be found in the spectrum, as curve b in Fig. 1a. The fitting crystalline parameters are a = Å, b = Å and c = Å (JCPDS, ). Furthermore, it can be seen in the TEM image of Fig. 1b that most Al 4 B 2 O 9 nanowires are about 7 nm in diameter. From the HRTEM image

3 J. Wang et al. / Materials Research Bulletin 40 (2005) Fig. 1. XRD pattern (a), TEM images (b), HRTEM (c) and diameter distribution (d) of the product synthesized by annealing the gel of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:3 at 750 8C for 4 h. (Fig. 1c), it can be seen clearly that this nanowire crystallizes well. The distance between the parallel fringes is about 0.48 nm, corresponding to the [1 1 1] planes of Al 4 B 2 O 9. By measuring over 50 nanowires from recorded TEM images, the average diameter of the nanowires was determined to be 7.3 nm. As shown in Fig. 1d, most nanowires are in the range of 6 8 nm, indicating very good diameter uniformity. A sample exhibiting this diameter uniformity and distribution is presented by the TEM image in Fig. 1b. The diameter distribution of other samples synthesized at different experimental conditions was measured by the same method, and it was found that all the nanowires have very good diameter uniformity. Fig. 2a shows the XRD pattern of the product synthesized by annealing the gel of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:6 at 750 8C for 4 h. The pattern can be indexed as orthorhombic aluminium borate Al 4 B 2 O 9. The fitting crystalline parameters are a = 14.8 Å, b = 15.1 Å and c = 5.6 Å (JCPDS, ), which is minor different from the product synthesized by annealing the gel of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:3 at 750 8C for 4 h, might due to the shift of the peaks as the mol ratio of H 3 BO 3 increased.

4 1554 J. Wang et al. / Materials Research Bulletin 40 (2005) Fig. 2. XRD pattern (a) and TEM images (b and c) of the product synthesized by annealing the gel of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:6 at 750 8C for 4 h. The inset shows SAED of a nanowire. Examination by TEM established the presence of one-dimensional nanowire structures, as shown in Fig. 2b and c. The significant proportion of the TEM specimen is Al 4 B 2 O 9 nanowires with average diameter of 17 nm. Some flakes are seen in Fig. 2b. It can also be found that the average diameter of Al 4 B 2 O 9 nanowires increases from 7 to 17 nm as the mol ratio of Al(NO 3 ) 3 and H 3 BO 3 ranges from 1:3 to 1:6, and the quantity of the flakes also increases. We ascribe this phenomenon to the increase of B 2 O 3, which resulted in the nanowires growing continuously for the large sticking coefficient. The inset of Fig. 2c is the selected area electron diffraction (SAED) pattern of an individual nanowire. It shows that this nanowire crystallizes well. Fig. 3a shows the XRD pattern of the product when the mol ratio of Al(NO 3 ) 3 and H 3 BO 3 was 1:3 and the heating temperature was C. The pattern can be indexed as orthorhombic Al 18 B 4 O 33. The fitting crystalline parameters are a = Å, b = Å and c = Å (JCPDS, ). The TEM observation shows that the average diameter of Al 18 B 4 O 33 nanowires is about 38 nm. The inset of Fig. 3b shows a typical SAED pattern taken from a single nanowire that can be indexed as an orthorhombic Al 18 B 4 O 33 phase. The SAED pattern shows that the nanowires are single crystal. In this work, to understand the function of citric acid, the mixture of Al(NO 3 ) 3 and H 3 BO 3 were directly heated in the mol ratio of 1:1 at C for 4 h. For comparison, the gel of Al(NO 3 ) 3 and H 3 BO 3 in the same ratio produced by a sol gel process was also treated at C for 4 h. Fig. 4a shows the XRD pattern of the product when the mol ratio of Al(NO 3 ) 3 and H 3 BO 3 was 1:1 and the annealing temperature was C by direct heating. It shows that the product is orthorhombic Al 18 B 4 O 33. For the sol gel process following by annealing, the same product was received. Fig. 4b and c show the TEM image of the products synthesized by direct heating and sol gel process, respectively. It can be seen that the nanowires synthesized by the sol gel process have the uniform diameter about 100 nm, but those synthesized by direct heating have the diameter from 50 to 300 nm. Furthermore, the nano-particles can

5 J. Wang et al. / Materials Research Bulletin 40 (2005) Fig. 3. XRD pattern (a) and TEM images (b) of the product synthesized by annealing the gel of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:3 at C for 4 h. The inset shows SAED pattern taken from a single nanowire, and the patterns agree well with orthorhombic Al 18 B 4 O 33 phase. be also observed in Fig. 4b. Thus, it is concluded that citric acid can uniform the diameter of the nanowires. The same conclusion has been achieved for the products of Al(NO 3 ) 3 and H 3 BO 3 with different ratios at 750 and C in the direct heating and sol gel process. Moreover, it can be also found that the quantity of H 3 BO 3 should be excessive corresponding to the stoichiometry of Al 4 B 2 O 9 and Al 18 B 4 O 33. No nanowires could be got as the quantity of H 3 BO 3 decreases, e.g. the mol ratio of Al(NO 3 ) 3 :H 3 BO 3 is 3:1. This indicates that the mol ratio of Al(NO 3 ) 3 and H 3 BO 3 plays a key role on the growth of nanowires. The possible growth mechanism of the as-synthesized aluminum borate nanowires might be as follows: in our experiments no tips were found at the end of the nanowires, which is the character of vapor liquid solid mechanism [18,19]. So the VLS mechanism is not suitable for the as-prepared nanowire growth. Here we use the self-catalytic mechanism to explain the growth of our single crystal nanowires. In our method, the reaction temperature is 750 and C. During the process, H 3 BO 3 will decompose into B 2 O 3 at about 250 8C, then B 2 O 3 will melt at about 450 8C. With the increase of temperature, Al(NO 3 ) 3 will decompose into Al 2 O 3 grains at about 500 8C. The reactions are as following: 4H 3 BO 3! 2B 2 O 3 ðsolidþ þ6h 2 O ðgasþ (1) B 2 O 3 ðsolidþ!b 2 O 3 ðliquidþ (2)

6 1556 J. Wang et al. / Materials Research Bulletin 40 (2005) Fig. 4. XRD pattern (a) and TEM image (b) of the product synthesized by directly heating the mixture of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:1 at C for 4 h; TEM image (c) of the product synthesized by annealing the gel of Al(NO 3 ) 3 and H 3 BO 3 in mol ratio of 1:1 at C for 4 h. 2AlðNO 3 Þ 3! Al 2 O 3 ðsolidþ þ6no 2 ðgasþ (3) 2Al 2 O 3 þ B 2 O 3! Al 4 B 2 O 9 ðsolidþ (4) 9Al 2 O 3 þ 2B 2 O 3! Al 18 B 4 O 33 ðsolidþ (5) At 750 and C, small Al 2 O 3 grains will dissolve into molten droplets of B 2 O 3. All these small Al 2 O 3 grains will serve as nuclei for the growth of nanowires just like Au catalysts used in VLS growth. Then Al 2 O 3 grains will react with B 2 O 3. The Al 2 O 3 grains absorb the liquid B 2 O 3 and subsequently yield Al 4 B 2 O 9 and Al 18 B 4 O 33 nanowires at 750 and C, respectively. According to the principle of the lowest energy, the mixture will form Al 4 B 2 O 9 nanowires at 750 8C but Al 18 B 4 O 33 nanowires at C. And the citric acid can uniform the diameter of the nanowires. 4. Conclusion In conclusions, the single crystalline aluminium borate nanowires with the diameter about 7 17 and 38 nm (Al 4 B 2 O 9 and Al 18 B 4 O 33 ) were fabricated at different temperature by the sol gel process following by annealing at different temperatures. From XRD and TEM results, it is found that the nanowires are single crystal with uniform diameter. The growth mechanism of the nanowires might be self-catalytic mechanism.

7 J. Wang et al. / Materials Research Bulletin 40 (2005) Acknowledgement This work was supported by the National Natural Science Foundation of China (Nos and ) and the Key Project of Chinese Ministry of Education. References [1] H.J. Dai, E.W. Wong, Y.Z. Lu, S.S. Fan, C.M. Lieber, Nature (London) 375 (1995) 769. [2] P. Yang, C.M. Lieber, Science 273 (1996) [3] W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) [4] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [5] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) [6] Y. Wu, H. Yan, M. Huang, B. Messer, J.H. Song, P. Yang, Chemistry 8 (2002) [7] Y. Xia, P. Yang, Adv. Mater. 15 (2003) 351. [8] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353. [9] L.M. Peng, S.J. Zhu, Z.Y. Ma, J. Mi, F.G. Wang, H.R. Chen, D.O. Northwood, Mater. Sci. Eng. A 265 (1999) 63. [10] D. Jaque, O. Enguita, J.G. Sole, A.D. Jiang, Z.D. Luo, Appl. Phys. Lett. 76 (2000) [11] C. Herring, J.K. Galt, Phys. Rev. 85 (1952) [12] W.W. Webb, W.D. Forgeng, Acta Metall. 6 (1958) 462. [13] A.P. Levitt, in: A.P. Levitt (Ed.), Whisker Technology, Wiley-Interscience, New York, 1970, pp [14] R. Ma, Y. Bando, T. Sato, Appl. Phys. Lett. 81 (2002) [15] Y. Liu, Q. Li, S. Fan, Chem. Phys. Lett. 375 (2003) 632. [16] C. Cheng, C. Tang, X.X. Ding, X.T. Huang, Z.X. Huang, S.R. Qi, L. Hu, Y.X. Li, Chem. Phys. Lett. 373 (2003) 626. [17] R. Ma, Y. Bando, T. Sato, C. Tang, F. Xu, J. Am. Chem. Soc. 124 (2002) [18] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (1964) 89. [19] X.F. Duan, J.F. Wang, C.M. Lieber, Appl. Phys. Lett. 76 (2000) 1116.