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A new route to synthesis of γ-alumina nanorods
Materials Letters 61 (2007) 1812 – 1815 www.elsevier.com/locate/matlet
A new route to synthesis of γ-alumina nanorods Ming-Guo Ma a,b , Ying-Jie Zhu a,⁎, Zi-Li Xu c a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b Graduate School of Chinese Academy of Sciences, PR China c Beijing Institute of Petro-Chemical Technology, Beijing 102617, PR China Received 10 May 2006; accepted 23 July 2006 Available online 22 August 2006
Abstract γ-Alumina single-crystalline nanorods have been successfully synthesized by thermal decomposition of boehmite precursor which was prepared by solvothermally treating AlCl3 ⁎ 6H2O, NaOH, sodium dodecyl benzene sulfonate in water and dimethylbenzene mixed solvents. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Our experiments show that the surfactant plays an important role in the morphology and assembly of boehmite. Photoluminescence (PL) spectrum of the boehmite nanorods was also investigated. © 2006 Elsevier B.V. All rights reserved. Keywords: Alumina; Nanorods; Solvothermal
1. Introduction Since the discovery of carbon nanotubes [1], one-dimensional (1-D) nanomaterials have been receiving more attention because of their potential applications as important components and interconnects in nanodevices [2–5]. The important polymorphs of alumina are γ-AlOOH, γ-Al2O3 and α-Al2O3, among which γ-Al2O3 is the most important phase and is usually used as adsorbents, catalysts and catalyst supports. Yu et al. [6] reported the photoluminescent properties of the boehmite powders. Recently, the synthesis of alumina with 1-D morphologies has attracted more attention. Synthesis of 1-D alumina using anodic porous alumina membrane was reported [7–14]. Other strategies for synthesis of alumina nanotubes or nanowires include electrochemical synthesis method [15,16], wet etching of ZnO/Al2O3 core/shell nanofibers [17], thermal evaporation method [18], normal and lateral stepwise anodization [19], and ⁎ Corresponding author. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China. Tel.: +86 21 52412616; fax: +86 21 52413122. E-mail address: [email protected] (Y.-J. Zhu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.138
sol–gel [20]. Wet chemistry methods for the synthesis of 1-D alumina nanostructures have been little reported [21–25]. Kuang et al. [22] reported fabrication of boehmite and γ-Al2O3 nanotubes via a soft solution route. Lee et al. [23] synthesized alumina nanostructures such as nanotubes, nanofibers and nanorods based on templating of surfactants by hydrothermal reaction at 423 K for 72 h. Zhu et al. [24] reported the synthesis of alumina with a fiber morphology and large porosity using aluminum hydrate with nonionic poly(ethylene oxide) (PEO) surfactant. Al2O3 nanotubes and nanorods were synthesized by coating and filling of carbon nanotubes with atomic-layer deposition [25]. To develop a simple synthesis route for the control over the morphology of Al2O3 is of great importance for broadening and improving their industrial applications. Herein, we report a novel, simple surfactant-directed solvothermal route to the synthesis of boehmite nanorods, which are used as the precursor for the synthesis of γ-alumina nanorods by thermal decomposition. 2. Experimental All chemicals were of analytical grade reagents and used as received without further purification. All experiments were
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X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/MAX 2550 X-ray diffractometer with a graphite monochromator and Cu Kα radiation (λ = 1.54178 ÅV ). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) micrographs and selected area electron diffraction (SAED) patterns were recorded on a JEOL JEM-2100F field emission transmission electron microscope with an accelerating voltage of 200 kV. Photoluminescence (PL) spectra were obtained at room temperature on a Fluorolog-3 spectrophotometer (JOBIN YVON) using a Xe lamp with an excitation wavelength of 300 nm. 3. Results and discussion Fig. 1. XRD pattern of γ-Al2O3 nanorods.
conducted under air atmosphere. In a typical synthesis experiment, AlCl3·6H2O (2.41 g) was dissolved in mixed solvents of water (30 ml) and dimethylbenzene (15 ml) under vigorous magnetic stirring at room temperature; sodium dodecyl benzene sulfonate (SDBS) (1 g) was added to the above solution under vigorous stirring. Then 25 ml NaOH solution (1 mol dm− 3) was added to the above solution under stirring. The obtained suspension was transferred into a separatory funnel and was set without shaking to form two liquid phases. The upper organic liquid phase was transferred into a 20-mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 200 °C for 24 h without stirring and shaking, and then was allowed to cool to room temperature naturally. The product was separated from the solution by centrifugation, washed by water and ethanol several times and dried at 60 °C in vacuum. Finally, white powder (boehmite) was obtained. The boehmite powder was put into an alumina crucible in a tube furnace and heated to 500 °C in air with a heating rate of 3 °C/min and kept at 500 °C for 3 h.
Fig. 1 shows the XRD pattern of a typical sample after calcination at 500 °C for 3h. The sample consisted of a single phase of well-crystalline γ-Al2O3 with a cubic structure (JCPDS 10-0425). No peaks from impurities were observed. The (400) peak has a stronger intensity than the other peaks, indicating that the (400) planes may be the preferential growth direction. The morphologies of the samples were investigated by TEM. Fig. 2 shows TEM micrographs of the boehmite, from which one can see arrays of nanorods and individual nanorods. The sizes of the nanorods were relatively uniform. The nanorods had diameters ranging from 15 to 25 nm and lengths ranging from 170 to 320 nm. Single nanorods and needle-like nanostructures were also observed as a minor morphology (Fig. 2b and c). Fig. 2d and e shows the typical TEM micrographs of the arrays of nanorods. The formation of the arrays of nanorods may be related to the templating of SDBS. These boehmite nanorods have been successfully used as the precursor and template for the synthesis of γ-Al2O3 nanorod arrays by thermal decomposition, as shown below. Fig. 3 displays TEM micrographs of γ-Al2O3 nanorods prepared by the thermal decomposition of the boehmite nanorods. The size and morphology of γ-Al2O3 are similar to those of the boehmite precursor, indicating that the size and morphology can be well preserved during the transformation from boehmite to γ-Al2O3, which is consistent with
Fig. 2. TEM micrographs of boehmite nanorods. (a) A typical TEM micrograph, (b and c) individual nanorods, (d and e) arrays of nanorods.
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Fig. 3. TEM micrographs of γ-Al2O3 nanorods. The insets of (b), (c) and (d) show the SAED patterns of individual nanorods. (d–h) Display arrays of nanorods assembled from two, five, six, seven and more nanorods, respectively.
previous reports [26,27]. Arrays of nanorods and individual nanorods were observed. The SAED pattern of an individual nanorod (the insets of Fig. 3b,c,d) indicates the single-crystalline structure of the nanorod. Fig. 3d–h displays the arrays of nanorods assembled from two, five, six, seven and more nanorods, respectively. Fig. 4a shows the HRTEM micrograph of the γ-Al2O3 nanorod. The corresponding Fast Fourier Transform (FFT) pattern and the reconstructed image for the squared area in Fig. 4a are given in Fig. 4b and c, respectively. From Fig. 4c, one can see defects (indicated by the lines) in the nanorods. Fig. 5 shows the schematic representation of the formation process of arrays assembled from γ-Al2O3 nanords. We propose that the formation mechanism of nanorod arrays is similar to those proposed for the
synthesis of silica nanotubes [28,29] and alumina nanofibers [24]. The maximum number of hydrogen bonds with the OH groups on boehmite surfaces was achieved by the surfactant micelles, reducing the free energy of the boehmite crystallites with low dimensions and allowing them to grow along one direction to give a 1-D morphology. Further assembly of nanorods assisted by the surfactant SDBS led to the formation of nanorod arrays. The detailed mechanisms for the formation of nanorod arrays of boehmite and γ-Al2O3 need to be investigated further. Fig. 6 shows the photoluminescence (PL) spectrum of the boehmite nanorods, with excitation wavelength at 300 nm. Two strong peaks at 373 nm (3.32 eV) and 389 nm (3.18 eV), and a broad weak peak
Fig. 4. (a) HRTEM micrograph of the γ-Al2O3 nanorod, (b) Fast Fourier Transform (FFT) pattern of the squared area in (a), (c) the reconstructed image of the squared area in (a), the defects are highlighted by the lines.
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from Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. We thank the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL200506SIC) from Shanghai Institute of Ceramics, Chinese Academy of Sciences. References
Fig. 5. Schematic representation of the formation process of γ-Al2O3 nanorods.
Fig. 6. PL spectrum of the boehmite nanorods.
centered at about 735 nm were observed. The peaks at 373 nm and 389 nm may be ascribed to the anionic vacancies (F and F+ centers) in the nanorods [6,30]. The broad weak peak centered at about 735 nm belongs to the nonbridging oxygen hole centers [6].
4. Conclusion In summary, γ-alumina arrays assembled from single-crystalline nanorods have been successfully synthesized by thermal decomposition of a boehmite precursor which was prepared by the solvothermal method using AlCl3 ⁎ 6H2O, NaOH, sodium dodecyl benzene sulfonate in water and dimethylbenzene mixed solvents at 200 °C for 24 h. The surfactant plays an important role in the morphology and assembly of boehmite. Acknowledgments Financial support from the National Natural Science Foundation of China (50472014), the Program of Shanghai Postdoctoral Scientific Research Foundation (05R214148), and
[1] S. Iijima, Nature 354 (1991) 56. [2] S. Frank, P. Poncharal, Z.L. Wang, W.A. de Heer, Science 280 (1998) 1744. [3] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [4] H.M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [5] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. [6] Z.Q. Yu, C.X. Wang, X.T. Gu, C. Li, J. Lumin. 106 (2004) 153. [7] Y.F. Mei, G.G. Siu, R.K.Y. Fu, P. Chen, X.L. Wu, T.F. Hung, P.K. Chu, Y. Yang, J. Appl. Phys. 97 (2005) (Art. No. 034305). [8] Y.F. Mei, X.L. Wu, X.F. Shao, G.S. Huang, G.G. Siu, Phys. Lett., A 309 (2003) 109. [9] L. Zhang, B. Cheng, W.S. Shi, E.T. Samulski, J. Mater. Chem. 15 (2005) 4889. [10] L. Zhang, S.W. Yao, W.G. Zhang, H.Z. Wang, Acta Phys. Chim. Sin. 21 (2005) 1254. [11] Y.T. Tian, G.W. Meng, T. Gao, S.H. Sun, T. Xie, X.S. Peng, C.H. Ye, L.D. Zhang, Nanotechnology 15 (2004) 189. [12] Z.L. Xiao, C.Y. Han, U. Welp, H.H. Wang, W.K. Kwok, G.A. Willing, J.M. Hiller, R.E. Cook, D.J. Miller, G.W. Crabtree, Nano Lett. 2 (2002) 1293. [13] Z.H. Yuan, H. Huang, S.S. Fan, Adv. Mater. 14 (2002) 303. [14] Y.F. Mei, X.L. Wu, X.F. Shao, G.G. Siu, X.M. Bao, Europhys. Lett. 62 (2003) 595. [15] Y.T. Pang, G.W. Meng, L.D. Zhang, W.J. Shan, C. Zhang, X.Y. Gao, A.W. Zhao, Y.Q. Mao, J. Solid State Electrochem. 7 (2003) 344. [16] O. Rabin, P.R. Herz, Y.M. Lin, A.I. Akinwande, S.B. Cronin, M.S. Dresselhaus, Adv. Funct. Mater. 13 (2003) 631. [17] J. Hwang, B.D. Min, J.S. Lee, K. Keem, K. Cho, M.Y. Sung, M.S. Lee, S. Kim, Adv. Mater. 16 (2004) 422. [18] Q. Zhao, X. Xu, H. Zhang, Y. Chen, J. Xu, D. Yu, Appl. Phys., A 79 (2004) 1721. [19] L. Pu, X.M. Bao, J.P. Zou, D. Feng, Angew. Chem., Int. Ed. 40 (2001) 1490. [20] S.C. Kuiry, E. Megen, S.D. Patil, S.A. Deshpande, S. Seal, J. Phys. Chem., B 109 (2005) 3868. [21] H.W. Hou, Y. Xie, Q. Yang, Q.X. Guo, C.R. Tan, Nanotechnology 16 (2005) 741. [22] D.B. Kuang, Y.P. Fang, H.Q. Liu, C. Frommen, D. Fenske, J. Mater. Chem. 13 (2003) 660. [23] H.C. Lee, H.J. Kim, S.H. Chung, K.H. Lee, H.C. Lee, J.S. Lee, J. Am. Chem. Soc. 125 (2003) 2882. [24] H.Y. Zhu, J.D. Riches, J.C. Barry, Chem. Mater. 14 (2002) 2086. [25] J.S. Lee, B. Min, K. Cho, S. Kim, J. Park, Y.T. Lee, N.S. Kim, M.S. Lee, S.O. Park, J.T. Moon, J. Cryst. Growth 254 (2003) 443. [26] S.J. Wilson, J. Solid State Chem. 30 (1979) 247. [27] D.L. Trim, A. Stanislaus, Appl. Catal. 21 (1986) 215. [28] M. Adachi, T. Harada, M. Harada, Langmuir 15 (1999) 7097. [29] M. Adachi, T. Harada, M. Harada, Langmuir 16 (2000) 2376. [30] P. Gao, Y. Xie, Y. Chen, L.N. Ye, Q.X. Guo, J. Cryst. Growth 285 (2005) 555.
A new route to synthesis of γ-alumina nanorods Ming-Guo Ma a,b , Ying-Jie Zhu a,⁎, Zi-Li Xu c a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b Graduate School of Chinese Academy of Sciences, PR China c Beijing Institute of Petro-Chemical Technology, Beijing 102617, PR China Received 10 May 2006; accepted 23 July 2006 Available online 22 August 2006
Abstract γ-Alumina single-crystalline nanorods have been successfully synthesized by thermal decomposition of boehmite precursor which was prepared by solvothermally treating AlCl3 ⁎ 6H2O, NaOH, sodium dodecyl benzene sulfonate in water and dimethylbenzene mixed solvents. The products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Our experiments show that the surfactant plays an important role in the morphology and assembly of boehmite. Photoluminescence (PL) spectrum of the boehmite nanorods was also investigated. © 2006 Elsevier B.V. All rights reserved. Keywords: Alumina; Nanorods; Solvothermal
1. Introduction Since the discovery of carbon nanotubes [1], one-dimensional (1-D) nanomaterials have been receiving more attention because of their potential applications as important components and interconnects in nanodevices [2–5]. The important polymorphs of alumina are γ-AlOOH, γ-Al2O3 and α-Al2O3, among which γ-Al2O3 is the most important phase and is usually used as adsorbents, catalysts and catalyst supports. Yu et al. [6] reported the photoluminescent properties of the boehmite powders. Recently, the synthesis of alumina with 1-D morphologies has attracted more attention. Synthesis of 1-D alumina using anodic porous alumina membrane was reported [7–14]. Other strategies for synthesis of alumina nanotubes or nanowires include electrochemical synthesis method [15,16], wet etching of ZnO/Al2O3 core/shell nanofibers [17], thermal evaporation method [18], normal and lateral stepwise anodization [19], and ⁎ Corresponding author. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China. Tel.: +86 21 52412616; fax: +86 21 52413122. E-mail address: [email protected] (Y.-J. Zhu). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.138
sol–gel [20]. Wet chemistry methods for the synthesis of 1-D alumina nanostructures have been little reported [21–25]. Kuang et al. [22] reported fabrication of boehmite and γ-Al2O3 nanotubes via a soft solution route. Lee et al. [23] synthesized alumina nanostructures such as nanotubes, nanofibers and nanorods based on templating of surfactants by hydrothermal reaction at 423 K for 72 h. Zhu et al. [24] reported the synthesis of alumina with a fiber morphology and large porosity using aluminum hydrate with nonionic poly(ethylene oxide) (PEO) surfactant. Al2O3 nanotubes and nanorods were synthesized by coating and filling of carbon nanotubes with atomic-layer deposition [25]. To develop a simple synthesis route for the control over the morphology of Al2O3 is of great importance for broadening and improving their industrial applications. Herein, we report a novel, simple surfactant-directed solvothermal route to the synthesis of boehmite nanorods, which are used as the precursor for the synthesis of γ-alumina nanorods by thermal decomposition. 2. Experimental All chemicals were of analytical grade reagents and used as received without further purification. All experiments were
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X-ray powder diffraction (XRD) patterns were recorded on a Rigaku D/MAX 2550 X-ray diffractometer with a graphite monochromator and Cu Kα radiation (λ = 1.54178 ÅV ). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) micrographs and selected area electron diffraction (SAED) patterns were recorded on a JEOL JEM-2100F field emission transmission electron microscope with an accelerating voltage of 200 kV. Photoluminescence (PL) spectra were obtained at room temperature on a Fluorolog-3 spectrophotometer (JOBIN YVON) using a Xe lamp with an excitation wavelength of 300 nm. 3. Results and discussion Fig. 1. XRD pattern of γ-Al2O3 nanorods.
conducted under air atmosphere. In a typical synthesis experiment, AlCl3·6H2O (2.41 g) was dissolved in mixed solvents of water (30 ml) and dimethylbenzene (15 ml) under vigorous magnetic stirring at room temperature; sodium dodecyl benzene sulfonate (SDBS) (1 g) was added to the above solution under vigorous stirring. Then 25 ml NaOH solution (1 mol dm− 3) was added to the above solution under stirring. The obtained suspension was transferred into a separatory funnel and was set without shaking to form two liquid phases. The upper organic liquid phase was transferred into a 20-mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 200 °C for 24 h without stirring and shaking, and then was allowed to cool to room temperature naturally. The product was separated from the solution by centrifugation, washed by water and ethanol several times and dried at 60 °C in vacuum. Finally, white powder (boehmite) was obtained. The boehmite powder was put into an alumina crucible in a tube furnace and heated to 500 °C in air with a heating rate of 3 °C/min and kept at 500 °C for 3 h.
Fig. 1 shows the XRD pattern of a typical sample after calcination at 500 °C for 3h. The sample consisted of a single phase of well-crystalline γ-Al2O3 with a cubic structure (JCPDS 10-0425). No peaks from impurities were observed. The (400) peak has a stronger intensity than the other peaks, indicating that the (400) planes may be the preferential growth direction. The morphologies of the samples were investigated by TEM. Fig. 2 shows TEM micrographs of the boehmite, from which one can see arrays of nanorods and individual nanorods. The sizes of the nanorods were relatively uniform. The nanorods had diameters ranging from 15 to 25 nm and lengths ranging from 170 to 320 nm. Single nanorods and needle-like nanostructures were also observed as a minor morphology (Fig. 2b and c). Fig. 2d and e shows the typical TEM micrographs of the arrays of nanorods. The formation of the arrays of nanorods may be related to the templating of SDBS. These boehmite nanorods have been successfully used as the precursor and template for the synthesis of γ-Al2O3 nanorod arrays by thermal decomposition, as shown below. Fig. 3 displays TEM micrographs of γ-Al2O3 nanorods prepared by the thermal decomposition of the boehmite nanorods. The size and morphology of γ-Al2O3 are similar to those of the boehmite precursor, indicating that the size and morphology can be well preserved during the transformation from boehmite to γ-Al2O3, which is consistent with
Fig. 2. TEM micrographs of boehmite nanorods. (a) A typical TEM micrograph, (b and c) individual nanorods, (d and e) arrays of nanorods.
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Fig. 3. TEM micrographs of γ-Al2O3 nanorods. The insets of (b), (c) and (d) show the SAED patterns of individual nanorods. (d–h) Display arrays of nanorods assembled from two, five, six, seven and more nanorods, respectively.
previous reports [26,27]. Arrays of nanorods and individual nanorods were observed. The SAED pattern of an individual nanorod (the insets of Fig. 3b,c,d) indicates the single-crystalline structure of the nanorod. Fig. 3d–h displays the arrays of nanorods assembled from two, five, six, seven and more nanorods, respectively. Fig. 4a shows the HRTEM micrograph of the γ-Al2O3 nanorod. The corresponding Fast Fourier Transform (FFT) pattern and the reconstructed image for the squared area in Fig. 4a are given in Fig. 4b and c, respectively. From Fig. 4c, one can see defects (indicated by the lines) in the nanorods. Fig. 5 shows the schematic representation of the formation process of arrays assembled from γ-Al2O3 nanords. We propose that the formation mechanism of nanorod arrays is similar to those proposed for the
synthesis of silica nanotubes [28,29] and alumina nanofibers [24]. The maximum number of hydrogen bonds with the OH groups on boehmite surfaces was achieved by the surfactant micelles, reducing the free energy of the boehmite crystallites with low dimensions and allowing them to grow along one direction to give a 1-D morphology. Further assembly of nanorods assisted by the surfactant SDBS led to the formation of nanorod arrays. The detailed mechanisms for the formation of nanorod arrays of boehmite and γ-Al2O3 need to be investigated further. Fig. 6 shows the photoluminescence (PL) spectrum of the boehmite nanorods, with excitation wavelength at 300 nm. Two strong peaks at 373 nm (3.32 eV) and 389 nm (3.18 eV), and a broad weak peak
Fig. 4. (a) HRTEM micrograph of the γ-Al2O3 nanorod, (b) Fast Fourier Transform (FFT) pattern of the squared area in (a), (c) the reconstructed image of the squared area in (a), the defects are highlighted by the lines.
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from Chinese Academy of Sciences under the Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. We thank the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (SKL200506SIC) from Shanghai Institute of Ceramics, Chinese Academy of Sciences. References
Fig. 5. Schematic representation of the formation process of γ-Al2O3 nanorods.
Fig. 6. PL spectrum of the boehmite nanorods.
centered at about 735 nm were observed. The peaks at 373 nm and 389 nm may be ascribed to the anionic vacancies (F and F+ centers) in the nanorods [6,30]. The broad weak peak centered at about 735 nm belongs to the nonbridging oxygen hole centers [6].
4. Conclusion In summary, γ-alumina arrays assembled from single-crystalline nanorods have been successfully synthesized by thermal decomposition of a boehmite precursor which was prepared by the solvothermal method using AlCl3 ⁎ 6H2O, NaOH, sodium dodecyl benzene sulfonate in water and dimethylbenzene mixed solvents at 200 °C for 24 h. The surfactant plays an important role in the morphology and assembly of boehmite. Acknowledgments Financial support from the National Natural Science Foundation of China (50472014), the Program of Shanghai Postdoctoral Scientific Research Foundation (05R214148), and
[1] S. Iijima, Nature 354 (1991) 56. [2] S. Frank, P. Poncharal, Z.L. Wang, W.A. de Heer, Science 280 (1998) 1744. [3] X. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 66. [4] H.M. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [5] Y.N. Xia, P.D. Yang, Y.G. Sun, Y.Y. Wu, B. Mayers, B. Gates, Y.D. Yin, F. Kim, H.Q. Yan, Adv. Mater. 15 (2003) 353. [6] Z.Q. Yu, C.X. Wang, X.T. Gu, C. Li, J. Lumin. 106 (2004) 153. [7] Y.F. Mei, G.G. Siu, R.K.Y. Fu, P. Chen, X.L. Wu, T.F. Hung, P.K. Chu, Y. Yang, J. Appl. Phys. 97 (2005) (Art. No. 034305). [8] Y.F. Mei, X.L. Wu, X.F. Shao, G.S. Huang, G.G. Siu, Phys. Lett., A 309 (2003) 109. [9] L. Zhang, B. Cheng, W.S. Shi, E.T. Samulski, J. Mater. Chem. 15 (2005) 4889. [10] L. Zhang, S.W. Yao, W.G. Zhang, H.Z. Wang, Acta Phys. Chim. Sin. 21 (2005) 1254. [11] Y.T. Tian, G.W. Meng, T. Gao, S.H. Sun, T. Xie, X.S. Peng, C.H. Ye, L.D. Zhang, Nanotechnology 15 (2004) 189. [12] Z.L. Xiao, C.Y. Han, U. Welp, H.H. Wang, W.K. Kwok, G.A. Willing, J.M. Hiller, R.E. Cook, D.J. Miller, G.W. Crabtree, Nano Lett. 2 (2002) 1293. [13] Z.H. Yuan, H. Huang, S.S. Fan, Adv. Mater. 14 (2002) 303. [14] Y.F. Mei, X.L. Wu, X.F. Shao, G.G. Siu, X.M. Bao, Europhys. Lett. 62 (2003) 595. [15] Y.T. Pang, G.W. Meng, L.D. Zhang, W.J. Shan, C. Zhang, X.Y. Gao, A.W. Zhao, Y.Q. Mao, J. Solid State Electrochem. 7 (2003) 344. [16] O. Rabin, P.R. Herz, Y.M. Lin, A.I. Akinwande, S.B. Cronin, M.S. Dresselhaus, Adv. Funct. Mater. 13 (2003) 631. [17] J. Hwang, B.D. Min, J.S. Lee, K. Keem, K. Cho, M.Y. Sung, M.S. Lee, S. Kim, Adv. Mater. 16 (2004) 422. [18] Q. Zhao, X. Xu, H. Zhang, Y. Chen, J. Xu, D. Yu, Appl. Phys., A 79 (2004) 1721. [19] L. Pu, X.M. Bao, J.P. Zou, D. Feng, Angew. Chem., Int. Ed. 40 (2001) 1490. [20] S.C. Kuiry, E. Megen, S.D. Patil, S.A. Deshpande, S. Seal, J. Phys. Chem., B 109 (2005) 3868. [21] H.W. Hou, Y. Xie, Q. Yang, Q.X. Guo, C.R. Tan, Nanotechnology 16 (2005) 741. [22] D.B. Kuang, Y.P. Fang, H.Q. Liu, C. Frommen, D. Fenske, J. Mater. Chem. 13 (2003) 660. [23] H.C. Lee, H.J. Kim, S.H. Chung, K.H. Lee, H.C. Lee, J.S. Lee, J. Am. Chem. Soc. 125 (2003) 2882. [24] H.Y. Zhu, J.D. Riches, J.C. Barry, Chem. Mater. 14 (2002) 2086. [25] J.S. Lee, B. Min, K. Cho, S. Kim, J. Park, Y.T. Lee, N.S. Kim, M.S. Lee, S.O. Park, J.T. Moon, J. Cryst. Growth 254 (2003) 443. [26] S.J. Wilson, J. Solid State Chem. 30 (1979) 247. [27] D.L. Trim, A. Stanislaus, Appl. Catal. 21 (1986) 215. [28] M. Adachi, T. Harada, M. Harada, Langmuir 15 (1999) 7097. [29] M. Adachi, T. Harada, M. Harada, Langmuir 16 (2000) 2376. [30] P. Gao, Y. Xie, Y. Chen, L.N. Ye, Q.X. Guo, J. Cryst. Growth 285 (2005) 555.