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Synthesis and characterization of Bi3.15Nd0.85Ti3O12 nanotube arrays
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 4495–4498
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Synthesis and characterization of Bi3.15Nd0.85Ti3O12 nanotube arrays F. Wang a,b, J.B. Wang a,b,, X.L. Zhong a,b, B. Li a,b, Y.C. Zhou a,b a b
Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China Faculty of Materials, optoelectronics and Physics, Xiangtan University, Xiangtan 411105, Hunan, China
a r t i c l e in fo
abstract
Article history: Received 17 March 2009 Received in revised form 20 July 2009 Accepted 11 August 2009 Communicated by H. Fujioka Available online 28 August 2009
Nd-substituted bismuth titanate (Bi3.15Nd0.85Ti3O12, BNT) nanotube arrays are fabricated by means of a sol–gel method utilizing porous anodic aluminum oxide (AAO) template. The morphologies and structures have been determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The diameter and length of these nanotubes are about 200 nm and 60 mm, respectively, and their wall thickness is about 30 nm. The average grain size is around 40 nm. XRD data show that the BNT nanotubes possess bismuth-layered perovskite structure. High-resolution electron microscopy (HRTEM) image demonstrates that the BNT nanotubes are polycrystalline. Polarization–electric field (P–E) response curves of BNT nanotube arrays were measured, and a size induced polarization reduction phenomenon is observed. & 2009 Elsevier B.V. All rights reserved.
PACS: 77.90.+k 81.07.De 81.20.Fw Keywords: A3. Sol–gel template method B1. Nanotube B2. Ferroelectric materials
1. Introduction One-dimensional nanostructures have attracted a great deal of interest because of their novel properties compared to their bulk counterparts and potential applications in nanoelectronics, nanooptics and nanodevices [1–4]. Up to now, many nanostructures (nanowires and nanotubes) made of various oxide materials such as TiO2, SiO2 and In2O3 have been synthesized [5–10]. Among the numerous oxide materials, broad range of properties of ferroelectric oxides, such as spontaneous polarization, high dielectric permittivity as well as piezo- and pyroelectricity, make ferroelectric nanostructures an extremely interesting material class for research. With the rapid development of the contemporary sciences and technologies into nanoscale regime and the miniaturization trend in the device size, low-dimensional ferroelectric nanostructures are of considerable importance. Moreover, compared with the ferroelectric films, one-dimensional finite ferroelectric structures can greatly increase the storage density of nonvolatile ferroelectric random access memories [11–13]. More and more efforts have been made to synthesize and understand one-dimensional ferroelectric nanostructures, and some conven-
Corresponding author at: Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China. Tel./fax: +86 732 8292199. E-mail address: [email protected] (J.B. Wang).
0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.08.019
tional perovskite ferroelectric nanotubes or nanowires have been developed [10,14–16]. Recent studies reveal that Nd-substituted bismuth titanate (Bi3.15Nd0.85Ti3O12, BNT) is the most promising lead-free ferroelectric material for memory applications because of its relatively large remanent polarization, good fatigue endurance and low coercive field [17,18]. However, the fabrication of one-dimensional BNT nanostructures has still rarely been reported [19,20]. And the electrical properties of one-dimensional BNT nanostructures have not been reported. Among many methods, due to the superiority of convenience in experimental equipments and particularly the capability to facilitate the fabrication of complex materials, the sol–gel template method is one of the most common techniques used to prepare one-dimensional ferroelectric nanomaterials. Hernandez et al. [10] have fabricated BaTiO3 and PbTiO3 nanotubes within 200 nm alumina templates by directly dipping the sol into the pores of the templates. Kim et al. [16] have obtained PZT nanotubes with a diameter of around 50 nm by a sol–gel template process with a spin-coating technique. In the present work, we report the synthesis of the BNT nanotubes by using a sol–gel template method. We choose the anodized aluminum oxide (AAO) membranes as the template because of its highly ordered nanoporous structure, controllable pore size and excellent chemical stability at high temperature. Furthermore, the morphologies, structures and ferroelectric properties of the BNT nanotube arrays have been discussed.
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F. Wang et al. / Journal of Crystal Growth 311 (2009) 4495–4498
Fig. 1. SEM images of BNT nanotubes after the removal of AAO template. (a) side view of the BNT nanotube arrays; (b) close side view of the arrays; (c) top view of the arrays; (d) EDX spectrum of BNT nanotubes, inset: the atomic ratio of products.
3. Results and discussion Fig. 1 shows the SEM images of BNT nanotube arrays after the removal of AAO template. Fig. 1(a) is a side view of the BNT nanotube arrays and shows that nanotubes bundle together after the removal of the template. The length of nanotubes is about
10
20
(220)
(008) (111)
(006)
(020)
Intensity (a. u.)
The commercially available AAO membranes (Whatman Anodisc 25, nominal pore diameter 200 nm) were used as templates in our experiments. And the BNT nanotubes were synthesized by a sol–gel method utilizing the AAO templates. The preparation of BNT precursor solution is described in early work [21]. The final concentration of precursor solution was adjusted to be 0.2 M by adding 2-methoxyethanol and acetic acid. The AAO templates were immersed into the precursor solution for 5 h. Then the templates containing the precursor were calcined in air at 720 1C for 5 h using a tube furnace. The morphologies and structures of the BNT nanotubes were determined by the field-emission scanning electron microscope (SEM, LEO-1525), X-ray diffractometer (XRD, Rigaku D/max-rA), transmission electron microscope (TEM, TEM-2100) and highresolution TEM (HRTEM). For SEM and TEM observations, the AAO template embedded with BNT nanotubes was soaked in a 6 M NaOH solution to remove the alumina completely, and then the resulting product was washed with the deionized water for several times. For TEM investigation, the BNT nanotubes were put in the deionized water and subjected to the ultrasonic treatment for several minutes, and then a drop of suspension was placed on the carbon-coated copper grid to carry out TEM studies. To measure the P–E hysteresis loops of the BNT nanotube arrays, both surfaces of the arrays with the template were firstly polished carefully with sand papers until BNT nanotubes were emerged, and then a layer of Pt with a thickness of 100 nm was sputtered on both sides of the template as electrode. The measurement was performed at a Radiant Technologies Precision Workstation ferroelectric test system.
(117)
2. Experimental
30
40
50
60
2θ (°) Fig. 2. XRD pattern of the BNT nanotube arrays.
60 mm, consistent with the thickness of template. A close side view image of BNT nanotube arrays is given in Fig. 1(b). From this picture, the transparent tubular structure can be clearly seen. The particles at the tubes are a result of incomplete removal of BNT sol from template surface and the fractures resulted from sample preparation process. Fig. 1(c) is a top view of the nanotubes, the nanotubes collapse into nanotube bundles because of the high aspect ratio and it can be seen that the nanotubes are open-ended. The average outer diameter and wall thickness of nanotubes are about 200 and 30 nm, respectively. And the outer diameter of the nanotubes corresponds exactly to that of the template. Fig. 1(d) shows a representative energy-disperse X-ray (EDX) spectrum of BNT nanotubes. It is clear that the nanotubes consist of Bi, Nd, Ti and O. And Al comes from the AAO template. The atomic ratio of
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4497
Fig. 3. (a) TEM image of BNT nanotubes. (b) HRTEM image of BNT nanotube. (c) TEM image of single BNT nanotube after etching.
Polarization (μC/cm2)
8 10V 20V 30V
4
0
-4
-8 -6
-4
-2
0
2
4
6
Electric field ( kV/cm) Fig. 4. P–E hysteresis loops of BNT nanotube arrays measured with different voltage.
these four elements is given in the inset of Fig. 1(d), showing that Bi:Nd:Ti:O is 3.15:0.86:2.92:12.04. This demonstrates that the BNT nanotubes have an ideal stoichiometric ratio. The XRD spectrum of BNT nanotubes after dissolving away the surface alumina is shown in Fig. 2. The reflection peaks were indexed according to the standard diffraction pattern data of Bi4Ti3O12 phase compiled in the JCPDS card, which shows that the obtained BNT nanotubes are polycrystalline and have a single phase of bismuth-layered perovskite structure. The broad XRD reflection peaks are ascribed to the small grain size, and there is an amorphous background due to the amorphous AAO template. And it is noticed that the lattice spacing of (117) is 2.9685 A˚ ˚ Sun [22] has which is a little less than the standard value 2.9707 A. explained the lattice contraction with the bond-order-length-
strength (BLOS) correlation which indicates that the atomic coordination imperfection causes the remaining bonds of the under-coordinated atom to contract spontaneously associated with bond strength gain and the intra-atomic trapping potential well depression. Fig. 3(a) shows the TEM image of BNT nanotubes after the alumina has been completely dissolved away. It is clearly seen that the outer diameter of the BNT nanotubes is about 200 nm, which corresponds exactly to the observation of SEM. The bismuth-layered perovskite structure is further confirmed by using a HRTEM image of a single BNT nanotube, as shown in Fig. 3(b), where the well-recognized lattice spacing of 2.97, 2.58 and 3.28 A˚ correspond to the (117), (2 0 4) and (0 0 1 0) crystal planes, respectively. Fig. 3(b) reveals the polycrystalline structure nature of the BNT nanotubes. After etching in a 6 M NaOH solution, the fine nanocrystalline structure of single BNT nanotube became obvious in Fig. 3(c), with an average grain size of around 40 nm. Fig. 4 shows the P–E hysteresis loops for the AAO template filled with BNT nanotubes. Considering that the contact area between Pt electrode and BNT nanotubes is about 12.75% of the overall Pt electrode, the measured polarization values are normalized to the area fraction of BNT (12.75%). The P–E loops are unambiguous proof of the ferroelectricity of the tubes. The remanent polarization (Pr) and the coercive electric field (Ec) obtained from the P–E hysteresis loops are about 1.47 mC/cm2 and 0.91 kV/cm, respectively. The values of Pr and Ec are much lower than those of sol–gel derived polycrystalline BNT films [18]. This could be explained by two reasons. One reason may be the expected grain size induced effects [23]. As mentioned lattice contraction in the measurement of XRD, Huang et al. [24,25] concluded that the surface bond contraction induces a compressive stress on the inner part of a grain and this effect should be taken into account for ferroelectric materials in the nanometre size range. The induced stress causes decreases of Curie temperature and spontaneous polarization with decreasing grain size. And the average grain size of BNT nanotube is about
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F. Wang et al. / Journal of Crystal Growth 311 (2009) 4495–4498
40 nm. A size driven dielectric anomaly can be seen due to the surface bond contraction. There are many researches reported [26,27] that the polarization of the bulk and thin films decrease with reducing grain size. A change of grain size, likewise a change of temperature or an external stress, produces a variation of the phase transition and the related ferroelectric properties [28]. Furthermore, in the nanotube structure, the grain size effects are also influential. And a similar conclusion was reported by Hernandez et al. [23] that the grain size contributed to a change in ferroelectricity of PbTiO3 nanotube prepared by sol–gel templating. The PbTiO3 nanotubes with the grain size of 11 nm have no indication of thermal hysteresis associated with displacive phase transformation. Another reason may be that the nanotubes are different with the bulk and thin films in geometry [23]. These polarization values of nanotubes cannot be directly compared to those of bulk and thin films, since the measurements were performed with a nanotube embedded in AAO template that has a relatively intricate field distribution [29]. A further study to investigate the influence of the grain size and the stress in nanotubular structures on the ferroelectric behaviors is in progress.
4. Conclusion In summary, BNT nanotube arrays are prepared via a sol–gel template approach. The morphologies and structures are investigated by SEM, XRD and TEM in detail. Furthermore, P–E loops of BNT nanotube arrays are measured, and the hysteresis clearly demonstrates the ferroelectricity of the nanotubes, indicating BNT nanotube arrays is a potential media for ferroelectric information storage.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 10802072, 50702048 and 50772093), the Hunan Provincial Natural Science Foundation of China (No. 09JJ1006), the Program for New Century Excellent Talents in University (No. NCET-08-0687), and the Specialized
Research Fund for the Doctoral Program of Higher Education (Nos. 200805301020 and 20070530010).
References [1] M. Remskar, Adv. Mater. 16 (2004) 1497. [2] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003) 5. [3] Z.L. Wang, Adv. Mater. 15 (2003) 1487. [4] K. Shantha Shankar, A.K. Raychaudhuri, Mater. Sci. Eng. C 25 (2005) 738. [5] P. Hoyer, Adv. Mater. 8 (1994) 857. [6] B.B. Lakshmi, C.J. Patrissi, C.R. Martina, Chem. Mater. 9 (1997) 2544. [7] H. Nakamura, Y. Matsui, J. Am. Chem. Soc. 117 (1995) 2651. [8] A. Michailowski, D. AlMawlawi, G.S. Cheng, M. Moskovits, Chem. Phys. Lett. 349 (2001) 1. [9] Y. Lei, L.D. Zhang, G.W. Meng, G.H. Li, X.Y. Zhang, C.H. Liang, W. Cheng, S.X. Wang, Appl. Phys. Lett. 78 (2001) 1125. [10] B.A. Hernandez, K.S. Chang, E.R. Fisher, P.K. Dohout, Chem. Mater. 14 (2002) 480. [11] I.I. Naumov, L. Bellaiche, H. Fu, Nature 432 (2004) 737. [12] I.I. Naumov, H. Fu, Phys. Rev. Lett. 95 (2005) 247602. [13] J.F. Scott, Nat. Mater. 4 (2005) 13. [14] X.Y. Zhang, X. Zhao, C.W. Lai, Appl. Phys. Lett. 85 (2004) 4190. [15] Y. Yang, X.H. Wang, C.F. Zhong, C.K. Sun, L.T. Li, Appl. Phys. Lett. 92 (2008) 1. [16] J. Kim, S. A Yang, Y.C. Choi, J.K. Han, K.O. Jeong, Y.J. Yun, D.J. Kim, S.M. Yang, D. Yoon, H. Cheong, Ki-Seog Chang, T.W. Noh, S.D. Bu, Nano Lett. 8 (2008) 1813. [17] U. Chon, H.M. Jang, M.G. Kim, C.H. Chang, Phys. Rev. Lett. 89 (2002) 087601. [18] X.L. Zhong, J.B. Wang, Y.C. Zhou, J.J. Liu, X.J. Zheng, J. Cryst. Growth 277 (2004) 233. [19] M. Liao, X.L. Zhong, J.B. Wang, H.L. Yan, J.P. He, Y. Qiao, Y.C. Zhou, J. Cryst. Growth 304 (2007) 69. [20] W. Cai, X.M. Lu, D. Li, H.F. Bo, R.W. Peng, X.B. Wu, Y.F. Liu, J.S. Zhu, Appl. Phys. Lett. 94 (2009) 1. [21] X.L. Zhong, J.B. Wang, X.J. Zheng, Y.C. Zhou, G.W. Yang, Appl. Phys. Lett. 85 (2004) 5661. [22] C.Q. Sun, Prog. Solid State Chem. 35 (2007) 1. [23] B.A. Hernandez, K.-S. Chang, M.T. Scancella, J.L. Burris, S. Kohli, E.R. Fisher, P.K. Dorhout, Chem. Mater. 17 (2005) 5909. [24] H.T. Huang, C.Q. Sun, P. Hing, J. Phys.: Condens. Matter 12 (2000) 127. [25] H.T. Huang, C.Q. Sun, T.S. Zhang, P. Hing, Phys. Rev. B 63 (2001) 184112. [26] M.H. Frey, Z. Xu, P. Han, D.A. Payne, Ferroelectrics 206 (1998) 337. [27] T. Takeuchi, C. Capiglia, N. Balakrishnan, Y. Takeda, H. Kageyama, J. Mater. Res. 17 (2002) 575. [28] Z. Zhao, V. Buscaglia, M. Viviani, M.T. Buscaglia, L. Mitoseriu, A. Testino, M. Nygren, M. Johnsson, P. Nanni, Phys. Rev. B 70 (2004) 024107. [29] Y. Luo, I. Szafraniak, N.D. Zakharov, V. Nagarajan, M. Steinhart, R.B. Wehrspohn, J.H. Wendorff, R. Ramesh, M. Alexe, Appl. Phys. Lett. 83 (2003) 440.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Synthesis and characterization of Bi3.15Nd0.85Ti3O12 nanotube arrays F. Wang a,b, J.B. Wang a,b,, X.L. Zhong a,b, B. Li a,b, Y.C. Zhou a,b a b
Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China Faculty of Materials, optoelectronics and Physics, Xiangtan University, Xiangtan 411105, Hunan, China
a r t i c l e in fo
abstract
Article history: Received 17 March 2009 Received in revised form 20 July 2009 Accepted 11 August 2009 Communicated by H. Fujioka Available online 28 August 2009
Nd-substituted bismuth titanate (Bi3.15Nd0.85Ti3O12, BNT) nanotube arrays are fabricated by means of a sol–gel method utilizing porous anodic aluminum oxide (AAO) template. The morphologies and structures have been determined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The diameter and length of these nanotubes are about 200 nm and 60 mm, respectively, and their wall thickness is about 30 nm. The average grain size is around 40 nm. XRD data show that the BNT nanotubes possess bismuth-layered perovskite structure. High-resolution electron microscopy (HRTEM) image demonstrates that the BNT nanotubes are polycrystalline. Polarization–electric field (P–E) response curves of BNT nanotube arrays were measured, and a size induced polarization reduction phenomenon is observed. & 2009 Elsevier B.V. All rights reserved.
PACS: 77.90.+k 81.07.De 81.20.Fw Keywords: A3. Sol–gel template method B1. Nanotube B2. Ferroelectric materials
1. Introduction One-dimensional nanostructures have attracted a great deal of interest because of their novel properties compared to their bulk counterparts and potential applications in nanoelectronics, nanooptics and nanodevices [1–4]. Up to now, many nanostructures (nanowires and nanotubes) made of various oxide materials such as TiO2, SiO2 and In2O3 have been synthesized [5–10]. Among the numerous oxide materials, broad range of properties of ferroelectric oxides, such as spontaneous polarization, high dielectric permittivity as well as piezo- and pyroelectricity, make ferroelectric nanostructures an extremely interesting material class for research. With the rapid development of the contemporary sciences and technologies into nanoscale regime and the miniaturization trend in the device size, low-dimensional ferroelectric nanostructures are of considerable importance. Moreover, compared with the ferroelectric films, one-dimensional finite ferroelectric structures can greatly increase the storage density of nonvolatile ferroelectric random access memories [11–13]. More and more efforts have been made to synthesize and understand one-dimensional ferroelectric nanostructures, and some conven-
Corresponding author at: Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, Xiangtan University, Xiangtan 411105, Hunan, China. Tel./fax: +86 732 8292199. E-mail address: [email protected] (J.B. Wang).
0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.08.019
tional perovskite ferroelectric nanotubes or nanowires have been developed [10,14–16]. Recent studies reveal that Nd-substituted bismuth titanate (Bi3.15Nd0.85Ti3O12, BNT) is the most promising lead-free ferroelectric material for memory applications because of its relatively large remanent polarization, good fatigue endurance and low coercive field [17,18]. However, the fabrication of one-dimensional BNT nanostructures has still rarely been reported [19,20]. And the electrical properties of one-dimensional BNT nanostructures have not been reported. Among many methods, due to the superiority of convenience in experimental equipments and particularly the capability to facilitate the fabrication of complex materials, the sol–gel template method is one of the most common techniques used to prepare one-dimensional ferroelectric nanomaterials. Hernandez et al. [10] have fabricated BaTiO3 and PbTiO3 nanotubes within 200 nm alumina templates by directly dipping the sol into the pores of the templates. Kim et al. [16] have obtained PZT nanotubes with a diameter of around 50 nm by a sol–gel template process with a spin-coating technique. In the present work, we report the synthesis of the BNT nanotubes by using a sol–gel template method. We choose the anodized aluminum oxide (AAO) membranes as the template because of its highly ordered nanoporous structure, controllable pore size and excellent chemical stability at high temperature. Furthermore, the morphologies, structures and ferroelectric properties of the BNT nanotube arrays have been discussed.
ARTICLE IN PRESS 4496
F. Wang et al. / Journal of Crystal Growth 311 (2009) 4495–4498
Fig. 1. SEM images of BNT nanotubes after the removal of AAO template. (a) side view of the BNT nanotube arrays; (b) close side view of the arrays; (c) top view of the arrays; (d) EDX spectrum of BNT nanotubes, inset: the atomic ratio of products.
3. Results and discussion Fig. 1 shows the SEM images of BNT nanotube arrays after the removal of AAO template. Fig. 1(a) is a side view of the BNT nanotube arrays and shows that nanotubes bundle together after the removal of the template. The length of nanotubes is about
10
20
(220)
(008) (111)
(006)
(020)
Intensity (a. u.)
The commercially available AAO membranes (Whatman Anodisc 25, nominal pore diameter 200 nm) were used as templates in our experiments. And the BNT nanotubes were synthesized by a sol–gel method utilizing the AAO templates. The preparation of BNT precursor solution is described in early work [21]. The final concentration of precursor solution was adjusted to be 0.2 M by adding 2-methoxyethanol and acetic acid. The AAO templates were immersed into the precursor solution for 5 h. Then the templates containing the precursor were calcined in air at 720 1C for 5 h using a tube furnace. The morphologies and structures of the BNT nanotubes were determined by the field-emission scanning electron microscope (SEM, LEO-1525), X-ray diffractometer (XRD, Rigaku D/max-rA), transmission electron microscope (TEM, TEM-2100) and highresolution TEM (HRTEM). For SEM and TEM observations, the AAO template embedded with BNT nanotubes was soaked in a 6 M NaOH solution to remove the alumina completely, and then the resulting product was washed with the deionized water for several times. For TEM investigation, the BNT nanotubes were put in the deionized water and subjected to the ultrasonic treatment for several minutes, and then a drop of suspension was placed on the carbon-coated copper grid to carry out TEM studies. To measure the P–E hysteresis loops of the BNT nanotube arrays, both surfaces of the arrays with the template were firstly polished carefully with sand papers until BNT nanotubes were emerged, and then a layer of Pt with a thickness of 100 nm was sputtered on both sides of the template as electrode. The measurement was performed at a Radiant Technologies Precision Workstation ferroelectric test system.
(117)
2. Experimental
30
40
50
60
2θ (°) Fig. 2. XRD pattern of the BNT nanotube arrays.
60 mm, consistent with the thickness of template. A close side view image of BNT nanotube arrays is given in Fig. 1(b). From this picture, the transparent tubular structure can be clearly seen. The particles at the tubes are a result of incomplete removal of BNT sol from template surface and the fractures resulted from sample preparation process. Fig. 1(c) is a top view of the nanotubes, the nanotubes collapse into nanotube bundles because of the high aspect ratio and it can be seen that the nanotubes are open-ended. The average outer diameter and wall thickness of nanotubes are about 200 and 30 nm, respectively. And the outer diameter of the nanotubes corresponds exactly to that of the template. Fig. 1(d) shows a representative energy-disperse X-ray (EDX) spectrum of BNT nanotubes. It is clear that the nanotubes consist of Bi, Nd, Ti and O. And Al comes from the AAO template. The atomic ratio of
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4497
Fig. 3. (a) TEM image of BNT nanotubes. (b) HRTEM image of BNT nanotube. (c) TEM image of single BNT nanotube after etching.
Polarization (μC/cm2)
8 10V 20V 30V
4
0
-4
-8 -6
-4
-2
0
2
4
6
Electric field ( kV/cm) Fig. 4. P–E hysteresis loops of BNT nanotube arrays measured with different voltage.
these four elements is given in the inset of Fig. 1(d), showing that Bi:Nd:Ti:O is 3.15:0.86:2.92:12.04. This demonstrates that the BNT nanotubes have an ideal stoichiometric ratio. The XRD spectrum of BNT nanotubes after dissolving away the surface alumina is shown in Fig. 2. The reflection peaks were indexed according to the standard diffraction pattern data of Bi4Ti3O12 phase compiled in the JCPDS card, which shows that the obtained BNT nanotubes are polycrystalline and have a single phase of bismuth-layered perovskite structure. The broad XRD reflection peaks are ascribed to the small grain size, and there is an amorphous background due to the amorphous AAO template. And it is noticed that the lattice spacing of (117) is 2.9685 A˚ ˚ Sun [22] has which is a little less than the standard value 2.9707 A. explained the lattice contraction with the bond-order-length-
strength (BLOS) correlation which indicates that the atomic coordination imperfection causes the remaining bonds of the under-coordinated atom to contract spontaneously associated with bond strength gain and the intra-atomic trapping potential well depression. Fig. 3(a) shows the TEM image of BNT nanotubes after the alumina has been completely dissolved away. It is clearly seen that the outer diameter of the BNT nanotubes is about 200 nm, which corresponds exactly to the observation of SEM. The bismuth-layered perovskite structure is further confirmed by using a HRTEM image of a single BNT nanotube, as shown in Fig. 3(b), where the well-recognized lattice spacing of 2.97, 2.58 and 3.28 A˚ correspond to the (117), (2 0 4) and (0 0 1 0) crystal planes, respectively. Fig. 3(b) reveals the polycrystalline structure nature of the BNT nanotubes. After etching in a 6 M NaOH solution, the fine nanocrystalline structure of single BNT nanotube became obvious in Fig. 3(c), with an average grain size of around 40 nm. Fig. 4 shows the P–E hysteresis loops for the AAO template filled with BNT nanotubes. Considering that the contact area between Pt electrode and BNT nanotubes is about 12.75% of the overall Pt electrode, the measured polarization values are normalized to the area fraction of BNT (12.75%). The P–E loops are unambiguous proof of the ferroelectricity of the tubes. The remanent polarization (Pr) and the coercive electric field (Ec) obtained from the P–E hysteresis loops are about 1.47 mC/cm2 and 0.91 kV/cm, respectively. The values of Pr and Ec are much lower than those of sol–gel derived polycrystalline BNT films [18]. This could be explained by two reasons. One reason may be the expected grain size induced effects [23]. As mentioned lattice contraction in the measurement of XRD, Huang et al. [24,25] concluded that the surface bond contraction induces a compressive stress on the inner part of a grain and this effect should be taken into account for ferroelectric materials in the nanometre size range. The induced stress causes decreases of Curie temperature and spontaneous polarization with decreasing grain size. And the average grain size of BNT nanotube is about
ARTICLE IN PRESS 4498
F. Wang et al. / Journal of Crystal Growth 311 (2009) 4495–4498
40 nm. A size driven dielectric anomaly can be seen due to the surface bond contraction. There are many researches reported [26,27] that the polarization of the bulk and thin films decrease with reducing grain size. A change of grain size, likewise a change of temperature or an external stress, produces a variation of the phase transition and the related ferroelectric properties [28]. Furthermore, in the nanotube structure, the grain size effects are also influential. And a similar conclusion was reported by Hernandez et al. [23] that the grain size contributed to a change in ferroelectricity of PbTiO3 nanotube prepared by sol–gel templating. The PbTiO3 nanotubes with the grain size of 11 nm have no indication of thermal hysteresis associated with displacive phase transformation. Another reason may be that the nanotubes are different with the bulk and thin films in geometry [23]. These polarization values of nanotubes cannot be directly compared to those of bulk and thin films, since the measurements were performed with a nanotube embedded in AAO template that has a relatively intricate field distribution [29]. A further study to investigate the influence of the grain size and the stress in nanotubular structures on the ferroelectric behaviors is in progress.
4. Conclusion In summary, BNT nanotube arrays are prepared via a sol–gel template approach. The morphologies and structures are investigated by SEM, XRD and TEM in detail. Furthermore, P–E loops of BNT nanotube arrays are measured, and the hysteresis clearly demonstrates the ferroelectricity of the nanotubes, indicating BNT nanotube arrays is a potential media for ferroelectric information storage.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 10802072, 50702048 and 50772093), the Hunan Provincial Natural Science Foundation of China (No. 09JJ1006), the Program for New Century Excellent Talents in University (No. NCET-08-0687), and the Specialized
Research Fund for the Doctoral Program of Higher Education (Nos. 200805301020 and 20070530010).
References [1] M. Remskar, Adv. Mater. 16 (2004) 1497. [2] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003) 5. [3] Z.L. Wang, Adv. Mater. 15 (2003) 1487. [4] K. Shantha Shankar, A.K. Raychaudhuri, Mater. Sci. Eng. C 25 (2005) 738. [5] P. Hoyer, Adv. Mater. 8 (1994) 857. [6] B.B. Lakshmi, C.J. Patrissi, C.R. Martina, Chem. Mater. 9 (1997) 2544. [7] H. Nakamura, Y. Matsui, J. Am. Chem. Soc. 117 (1995) 2651. [8] A. Michailowski, D. AlMawlawi, G.S. Cheng, M. Moskovits, Chem. Phys. Lett. 349 (2001) 1. [9] Y. Lei, L.D. Zhang, G.W. Meng, G.H. Li, X.Y. Zhang, C.H. Liang, W. Cheng, S.X. Wang, Appl. Phys. Lett. 78 (2001) 1125. [10] B.A. Hernandez, K.S. Chang, E.R. Fisher, P.K. Dohout, Chem. Mater. 14 (2002) 480. [11] I.I. Naumov, L. Bellaiche, H. Fu, Nature 432 (2004) 737. [12] I.I. Naumov, H. Fu, Phys. Rev. Lett. 95 (2005) 247602. [13] J.F. Scott, Nat. Mater. 4 (2005) 13. [14] X.Y. Zhang, X. Zhao, C.W. Lai, Appl. Phys. Lett. 85 (2004) 4190. [15] Y. Yang, X.H. Wang, C.F. Zhong, C.K. Sun, L.T. Li, Appl. Phys. Lett. 92 (2008) 1. [16] J. Kim, S. A Yang, Y.C. Choi, J.K. Han, K.O. Jeong, Y.J. Yun, D.J. Kim, S.M. Yang, D. Yoon, H. Cheong, Ki-Seog Chang, T.W. Noh, S.D. Bu, Nano Lett. 8 (2008) 1813. [17] U. Chon, H.M. Jang, M.G. Kim, C.H. Chang, Phys. Rev. Lett. 89 (2002) 087601. [18] X.L. Zhong, J.B. Wang, Y.C. Zhou, J.J. Liu, X.J. Zheng, J. Cryst. Growth 277 (2004) 233. [19] M. Liao, X.L. Zhong, J.B. Wang, H.L. Yan, J.P. He, Y. Qiao, Y.C. Zhou, J. Cryst. Growth 304 (2007) 69. [20] W. Cai, X.M. Lu, D. Li, H.F. Bo, R.W. Peng, X.B. Wu, Y.F. Liu, J.S. Zhu, Appl. Phys. Lett. 94 (2009) 1. [21] X.L. Zhong, J.B. Wang, X.J. Zheng, Y.C. Zhou, G.W. Yang, Appl. Phys. Lett. 85 (2004) 5661. [22] C.Q. Sun, Prog. Solid State Chem. 35 (2007) 1. [23] B.A. Hernandez, K.-S. Chang, M.T. Scancella, J.L. Burris, S. Kohli, E.R. Fisher, P.K. Dorhout, Chem. Mater. 17 (2005) 5909. [24] H.T. Huang, C.Q. Sun, P. Hing, J. Phys.: Condens. Matter 12 (2000) 127. [25] H.T. Huang, C.Q. Sun, T.S. Zhang, P. Hing, Phys. Rev. B 63 (2001) 184112. [26] M.H. Frey, Z. Xu, P. Han, D.A. Payne, Ferroelectrics 206 (1998) 337. [27] T. Takeuchi, C. Capiglia, N. Balakrishnan, Y. Takeda, H. Kageyama, J. Mater. Res. 17 (2002) 575. [28] Z. Zhao, V. Buscaglia, M. Viviani, M.T. Buscaglia, L. Mitoseriu, A. Testino, M. Nygren, M. Johnsson, P. Nanni, Phys. Rev. B 70 (2004) 024107. [29] Y. Luo, I. Szafraniak, N.D. Zakharov, V. Nagarajan, M. Steinhart, R.B. Wehrspohn, J.H. Wendorff, R. Ramesh, M. Alexe, Appl. Phys. Lett. 83 (2003) 440.