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Polymer-assistant hydrothermal synthesis of lead zirconate titanate nanorolls and their strong blue-light emission
Materials Science and Engineering B 133 (2006) 226–230
Short communication
Polymer-assistant hydrothermal synthesis of lead zirconate titanate nanorolls and their strong blue-light emission夽 Qingtao Pan, Jianfeng Jia, Kai Huang, Deyan He ∗ Department of Physics, Lanzhou University, Lanzhou 730000, China Received 21 February 2006; received in revised form 28 May 2006; accepted 3 June 2006
Abstract Well-crystalline Pb(Zr0.52 Ti0.48 )O3 (PZT) nanorolls with typical wall thicknesses in the range of 300–900 nm and the length up to 10 m have been synthesized by a surfactant-assisted hydrothermal method at a low temperature of 190 ◦ C. The “rolling mechanism” for the formation of PZT nanorolls was discussed. X-ray diffraction, micro-Raman spectrometry, and field emission scanning electron microscope demonstrated the PZT nanostructures and morphology. The photoluminescence measurements showed that a strong and narrow blue-light at 453 nm emits and a weak one at 470 nm at room temperature for the samples, indicating that PZT nanorolls have potential applications in nano-optical devices. © 2006 Elsevier B.V. All rights reserved. Keywords: PZT nanorolls; PVA assistant; Blue-light emission
1. Introduction Perovskite-type ferroelectric oxides such as lead titanate zirconate (PZT) are of great technological interest for many applications such as piezoelectric transducers, pyroelectric sensors, and high dielectric constant capacitors [1–4]. They exhibit large nonlinear optical coefficients and dielectric constants, depending on their chemical homogeneity, particle size, and morphology of starting PZT powders. Considerable efforts have been done in the controllable synthesis of PZT powders [5–8]. The morphology of particles obtained by conventional methods including solid state reaction, sol–gel method, and hydrothermal method is usually cubic or agglomerations of cubic particles. A few papers reported that PZT with novel morphologies such as acicular [9], belts [10], wires [11], and rods [12] have been synthesized by unconventional physical or chemical methods. One-dimensional PZT nanorolls system is an object of investigation for obtaining some knowledge about the influence of size and dimension of materials with respect to their collective optical, ferroelectric, and electronic properties. Recently, a solution-phase method with the assistant of surfactants includ夽 This work was supported by the Teaching and Research Award Program for Outstanding Youth Teachers in High Education institutions of MOE, China. ∗ Corresponding author. Fax: +86 931 8913554. E-mail address: [email protected] (D. He).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.06.004
ing long-chain amines, thioglyolic acids, and sodium dodecyl benzenesukfonate has been developed for synthesizing binarysemiconductors and unitary-metal nanostructures [13–16]. In this paper, we report the polymer-assistant hydrothermal synthesis of ternary-PZT nanorolls at a low temperature of 190 ◦ C. We found that the selection of the surfactant as well as its concentration were the crucial factors for the formation of PZT nanorolls. Poly-vinylalcohol (PVA) was selected as the assistant since it was widely used as an additive in the solvothermal synthesis of different nanostructures. 2. Experimental PZT with a composition of Pb(Zr0.52 Ti0.48 )O3 , which is near the morphotropic phase boundary (MPB) and has superior piezoelectric and dielectric properties [17], was synthesized in the present study. The starting materials were lead acetate (Pb(AC)2 , Chengdu Union Chemical Industry Reagent Research Institute), zirconium nitrate (Zr(NO3 )4 ·5H2 O, Tianjin Guangfu Fine Chemical Research Institute) and titanium isopropoxide (Ti(O-C4 H9 )4 , China Medical (Group) Shanghai Chemical Reagent Corporation). Potassium hydroxide (Tianjin Guangfu Fine Chemical Research Institute) and polyvinylalcohol (Mw = 89,000–98,000, Aldrich) were used as mineralizer and additive, respectively. All chemicals were of analytical grade and were used without further purification.
Q. Pan et al. / Materials Science and Engineering B 133 (2006) 226–230
In a typical synthesized processing, zirconium and titanium were added in the form of the coprecipitated hydroxide Zr0.52 Ti0.48 (OH)4 (ZTOH). Firstly, a stoichiometric amount of Ti(O-C4 H9 )4 was dissolved in isopropyl alcohol (Beijing Chemical Factory) to form a 0.1 mol L−1 solution, and a stoichiometric amount of Zr(NO3 )4 ·5H2 O was dissolved in distilled water to form a 0.05 mol L−1 solution. Then the 0.1 mol L−1 Ti4+ solution was dropwise added into the 0.05 Zr4+ mol L−1 solution with vigorous stirring. After continuous stirring of the mixture for several hours, a transparent solution was formed. With vigorous stirring, the mixture solution was added dropwise to 100 mL of ammonia solution (1.2:98.8). The pure coprecipitated hydroxide ZTOH was filtered and washed with distilled water and absolute ethanol several times. Subsequently, the refresh ZTOH precipitate was dissolved in distilled water with vigorous stirring, followed by the addition of Pb(AC)2 and KOH pallets. Aqueous PVA solution (about 0.5 wt.%) was prepared by dissolving PVA powder in distilled water and heating at 80 ◦ C with stirring for 2 h. Then it was introduced into the refresh ZTOH mixed solution. ZTOH and Pb concentration of 0.08 mol L−1 , KOH concentration of 1.8 mol L−1 , and PVA concentration of 0.36 g L−1 were obtained in the final suspensions. The solutions were transferred into 45 mL Teflon-lined autoclaves and heated at 190 ◦ C for 12 h. The as-synthesized powder was then collected and washed several times with distilled water and absolute ethanol. Finally, it was dried at 60 ◦ C for 12 h. The phase purity and structure of the prepared PZT powders were determined by X-ray diffraction (XRD) (using a Ragaka RINT2400 X-ray diffractometer with Cu K␣ radiation) and Raman spectroscopy (LabRam HR800, Jobin Yvon). The scanning electron microscopy (SEM) images of the samples were obtained by using a field emission scanning electron microscope (FESEM S-4800, Hitach). Photoluminescence (PL) spectra were observed on a combined steady state and lifetime spectrometer (FLS 920T, UK) at room temperature. 3. Results and discussion XRD analysis was used to determine the phases of the samples. Fig. 1 shows a typical XRD pattern of the as-prepared samples. All the peaks can be indexed to tetragonal perovskite PZT ˚ and c = 4.1487 A, ˚ phase with the lattice constants a = 3.9483 A which are close to the data in JCPDS Card No. 33-0784. No peak of any other phases was detected, indicating that the sample has a high purity. The strong and sharp reflection peaks suggest that the as-prepared samples are well crystallized. Fig. 2 shows Raman spectrum of an as-prepared PZT sample measured at room temperature. Obvious scattering peaks corresponding to different characteristic vibration modes of PZT [18,19] can be seen and summarized in Table 1. The Raman and XRD measurements give consistent evidence for the formation of perovskite-type PZT. Fig. 3 shows SEM images of the PZT synthesized by hydrothermal method assisted with PVA (0.36 g L−1 ). Examining SEM images of the samples prepared at the same condition, we found that almost all the particles are nanorolls. It indicates
227
Fig. 1. XRD pattern of an as-prepared Pb(Zr0.52 Ti0.48 )O3 sample.
Fig. 2. Raman scattering spectrum of an as-prepared sample.
that well-crystalline PZT nanorolls structures can be obtained under the present experimental conditions. The nanorolls are with a uniform length up to 10 m as shown in Fig. 3a. Fig. 3b shows that the cross sections of the samples are typically smooth, and the nanorolls are parallel to each other. We can also find that some samples show a micro-tube structure. Fig. 3c shows an erect PZT micro-tube with the inner diameter of ∼2 m and the outer diameter of ∼2.5 m. In order to investigate the influence of PVA on the formation of PZT nanorolls, the samples were synthesized without surfactant and examined by FESEM. As shown in Fig. 4, in the Table 1 Vibration modes of the Raman scattering peaks shown in Fig. 2 Peak position (cm−1 )
Mode
153 210 254 290 537 780
A1 (1TO) E(2TO) E(3TO + 2LO) + B1 SILENT E(3TO) A1 (3LO)
228
Q. Pan et al. / Materials Science and Engineering B 133 (2006) 226–230
Fig. 4. SEM images of PZT prepared by hydrothermal method without surfactant.
Fig. 3. SEM images of PZT nanorolls prepared by hydrothermal method. (a) Low-magnification SEM image showing the PZT nanorolls structures. (b) Higher magnification SEM images showing the cross sections of the PZT nanorolls. (c) The SEM image of a typical erect single PZT micro-tube.
absence of PVA, only cubic-like PZT particles were observed, implying that PVA played a key role in the formation of the PZT nanorolls. Recently, a few studies reported that the growth of one-dimensional nanostructures (nanorods and nanowires) was related to selective absorption of organic surfactants onto particular crystallographic facets of growing crystal. This selective absorption controls the growth rates of various crystallographic faces and sequentially yields anisotropies in the kinetic limit [20,21]. In the present conditions, the polymer of PVA was believed to be adsorbed on the PZT particle facets by hydrogen bounding. It minimizes the surface energy [11]. The preferential growth surface of the material was determined by the surface energy. Thus, nanosheets were formed as a result of the decreasing of the surface energy. The relief of built-in strain due to noncentrosymmetry architecture in individual sheets or the change of intrinsic bonding character between neighboring layers [22] made the nanosheets rolled as rolling up a thin film [23], which is called the “rolling mechanism”. Some nanosheets even rolled into micro-tubes. Because of the thickness of the nanosheets, there are creases in the wall of the nanorolls. Han and coworkers [11] have used PVA-assistant hydrothermal methods to synthesize PZT nanowires. In that report, the weight-average molecular weight of PVA was not mentioned. The different morphology of the obtained PZT via a similar method might be attributed to the different polymerization degree of PVA. In addition, we also found that PVA was unique in the formation of PZT nanorolls, and the PVA concentration was a crucial factor too. When poly-vinylpyrrolidone (PVP, Mw = 1,300,000, Aldrich, 0.36 g L−1 ) was used instead of PVA, quadrel-like particles were obtained (Fig. 5a). This might be attributed to carbon chain length, which resulted in the difference in modulating the growth kinetics of PZT crystals. When other reaction parameters were kept constant and the concentration of PVA was increased to 0.72 g L−1 , the products contained many agglomerated spherical particles. This might be that the PVA was attached to the whole surfaces of PZT because of the high concentration, which did not lead to the special growth directions. The exact role of
Q. Pan et al. / Materials Science and Engineering B 133 (2006) 226–230
229
Fig. 6. PL spectrum of the PZT nanorolls measured with 304 nm photoexcitation.
band structure, which may give the sharp blue-light emission. Because of the high aspect ratio of the products, there are lots of surface defects of Ti3+ and Zr3+ dangling bonds, which can also contribute to the blue-light emission. 4. Conclusions In conclusion, well-crystalline PZT nanorolls have been synthesized by a polymer-assistant hydrothermal process. The surfactant is a key for the formation of PZT nanorolls. Under 304 nm photoexitation the strong and narrow blue-light emissions are obtained at room temperature, which is of great important for applications in nano-optical devices. Fig. 5. SEM images of PZT prepared by hydrothermal methods (a) in the presence of PVP instead of PVA and (b) under an increased PVA concentration (0.72 g L−1 ).
PVA is still unclear. A detailed study of the growth mechanism of the nanorolls is under way. Sun et al. [24] have reported the blue-light emission of PZT and obtained an emission with a rather broad band. For our PZT nanorolls, a strong and sharp blue-light emission band centered at 453 nm and a weak one at 470 nm were observed as shown in Fig. 6. The width of the peak centered at 453 nm is as narrow as 20 nm. According to Liu et al. [25] the energy gap of PZT was estimated to be about 3.2 eV. Since the energy of emitted photons are about 2.74 and 2.64 eV for the present sample, the PL result suggests that the observed emissions are not related to the direct electronic transition between the valence band and the conduction band of PZT. Silva et al. [26] have reported that the displacement of one titanium atom and the breaking of the Ti–O bond can cause some broadening of the valence and conduction bands, leading to the creation of new states in the gap. ˚ The XRD analysis shows that the lattice constant (a = 3.9483 A) of our sample is smaller than that appeared in JCPDS Card. The structure distortion probably leads to the new states in the
References [1] K. Uchino, Ferroelectric Devices, Marce Decker, New York, 2000. [2] S. Gottschalk, H. Hahn, A.G. Balogh, W. Puff, H. Kungl, M.J. Hoffmann, J. Appl. Phys. 96 (2004) 7464. [3] G.H. Haertling, J. Am. Ceram. Soc. 82 (1999) 797. [4] Q. Tan, D. Viehland, J. Am. Ceram. Soc. 82 (1998) 328. [5] S.S. Chandratreya, R.M. Fulrath, J.A. Pask, J. Am. Ceram. Soc. 73 (1981) 422. [6] S. Linardos, Q. Zhang, J.R. Alock, J. Eur. Ceram. Soc. 26 (2006) 117. [7] H. Cheng, J. Ma, B. Zhu, Y. Cui, J. Am. Ceram. Soc. 76 (1993) 625. [8] X.Y. Liu, F.M. Elizabeth, E.M. Larry, M.B. Kate, R. Ramamoorthy, C. Frederic, A.R. George Jr., E.R. Richard, Langmuir 21 (2005) 3207. [9] S.B. Cho, M. Oledzka, E.R. Richard, J. Cryst. Growth 226 (2001) 313. [10] G. Wen, J.F. Li, C.E. Peng, Z.L. Gui, L.T. Li, Adv. Mater. 17 (2005) 1952. [11] G. Xu, Z.H. Ren, P.Y. Du, W.J. Weng, G. Shen, G.R. Han, Adv. Mater. 17 (2005) 907. [12] S.J. Limmer, S. Seraji, M.J. Forbess, Y. Wu, T.P. Chou, C. Nguyen, G.Z. Cao, Adv. Mater. 13 (2001) 1269. [13] J. Zhan, X. Yang, D. Wang, S. Li, Y. Xia, Y. Qian, Adv. Mater. 12 (2000) 1348. [14] D. Kuang, A. Xu, Y. Fang, H. Liu, C. Frommen, D. Fenske, Adv. Mater. 15 (2003) 1747. [15] H. Zhang, Y. Ji, X. Ma, J. Xu, D. Yang, Nanotechnology 14 (2003) 974. [16] Z. Liu, S. Li, Y. Yang, S. Peng, Z. Hu, Y. Qian, Adv. Mater. 15 (2003) 1946. [17] B. Guiffard, M. Troccaz, Mater. Res. Bull. 33 (1998) 1759. [18] G. Burns, B.A. Scott, Phys. Rev. Lett. 25 (1970) 1191.
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[19] P.S. Dobal, R.S. Katiyar, J. Raman Spectrosc. 33 (2002) 405. [20] X.G. Peng, L. Manna, W.D. Yang, J.T. Wichham, E. Scher, A. Kadavanich, A.P. Alivisators, Nature 404 (2000) 59. [21] Z.Y. Tang, N.A. Kotov, M. Giersig, Science 297 (2002) 237. [22] R. Ma, Y. Bando, T. Sasak, J. Phys. Chem. B 108 (2004) 2115. [23] O.G. Schmidt, K. Ebert, Nature 410 (2001) 168.
[24] C.Q. Sun, D. Jin, J. Zhou, S. Li, B.K. Tay, S.P. Lau, X.W. Sun, Appl. Phys. Lett. 79 (2001) 1082. [25] Y. Liu, C.N. Xu, K. Nonaka, H. Tateyama, Ferroelectrics 264 (2001) 331. [26] M.S. Silva, M. Cilense, E. Orhan, M.S. G¨oes, M.A.C. Machado, L.P.S. Santos, C.O. Paiva-Santos, E. Longo, J.A. Varela, M.A. Zaghete, P.S. Pizani, J. Lumin. 111 (2005) 205.
Short communication
Polymer-assistant hydrothermal synthesis of lead zirconate titanate nanorolls and their strong blue-light emission夽 Qingtao Pan, Jianfeng Jia, Kai Huang, Deyan He ∗ Department of Physics, Lanzhou University, Lanzhou 730000, China Received 21 February 2006; received in revised form 28 May 2006; accepted 3 June 2006
Abstract Well-crystalline Pb(Zr0.52 Ti0.48 )O3 (PZT) nanorolls with typical wall thicknesses in the range of 300–900 nm and the length up to 10 m have been synthesized by a surfactant-assisted hydrothermal method at a low temperature of 190 ◦ C. The “rolling mechanism” for the formation of PZT nanorolls was discussed. X-ray diffraction, micro-Raman spectrometry, and field emission scanning electron microscope demonstrated the PZT nanostructures and morphology. The photoluminescence measurements showed that a strong and narrow blue-light at 453 nm emits and a weak one at 470 nm at room temperature for the samples, indicating that PZT nanorolls have potential applications in nano-optical devices. © 2006 Elsevier B.V. All rights reserved. Keywords: PZT nanorolls; PVA assistant; Blue-light emission
1. Introduction Perovskite-type ferroelectric oxides such as lead titanate zirconate (PZT) are of great technological interest for many applications such as piezoelectric transducers, pyroelectric sensors, and high dielectric constant capacitors [1–4]. They exhibit large nonlinear optical coefficients and dielectric constants, depending on their chemical homogeneity, particle size, and morphology of starting PZT powders. Considerable efforts have been done in the controllable synthesis of PZT powders [5–8]. The morphology of particles obtained by conventional methods including solid state reaction, sol–gel method, and hydrothermal method is usually cubic or agglomerations of cubic particles. A few papers reported that PZT with novel morphologies such as acicular [9], belts [10], wires [11], and rods [12] have been synthesized by unconventional physical or chemical methods. One-dimensional PZT nanorolls system is an object of investigation for obtaining some knowledge about the influence of size and dimension of materials with respect to their collective optical, ferroelectric, and electronic properties. Recently, a solution-phase method with the assistant of surfactants includ夽 This work was supported by the Teaching and Research Award Program for Outstanding Youth Teachers in High Education institutions of MOE, China. ∗ Corresponding author. Fax: +86 931 8913554. E-mail address: [email protected] (D. He).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.06.004
ing long-chain amines, thioglyolic acids, and sodium dodecyl benzenesukfonate has been developed for synthesizing binarysemiconductors and unitary-metal nanostructures [13–16]. In this paper, we report the polymer-assistant hydrothermal synthesis of ternary-PZT nanorolls at a low temperature of 190 ◦ C. We found that the selection of the surfactant as well as its concentration were the crucial factors for the formation of PZT nanorolls. Poly-vinylalcohol (PVA) was selected as the assistant since it was widely used as an additive in the solvothermal synthesis of different nanostructures. 2. Experimental PZT with a composition of Pb(Zr0.52 Ti0.48 )O3 , which is near the morphotropic phase boundary (MPB) and has superior piezoelectric and dielectric properties [17], was synthesized in the present study. The starting materials were lead acetate (Pb(AC)2 , Chengdu Union Chemical Industry Reagent Research Institute), zirconium nitrate (Zr(NO3 )4 ·5H2 O, Tianjin Guangfu Fine Chemical Research Institute) and titanium isopropoxide (Ti(O-C4 H9 )4 , China Medical (Group) Shanghai Chemical Reagent Corporation). Potassium hydroxide (Tianjin Guangfu Fine Chemical Research Institute) and polyvinylalcohol (Mw = 89,000–98,000, Aldrich) were used as mineralizer and additive, respectively. All chemicals were of analytical grade and were used without further purification.
Q. Pan et al. / Materials Science and Engineering B 133 (2006) 226–230
In a typical synthesized processing, zirconium and titanium were added in the form of the coprecipitated hydroxide Zr0.52 Ti0.48 (OH)4 (ZTOH). Firstly, a stoichiometric amount of Ti(O-C4 H9 )4 was dissolved in isopropyl alcohol (Beijing Chemical Factory) to form a 0.1 mol L−1 solution, and a stoichiometric amount of Zr(NO3 )4 ·5H2 O was dissolved in distilled water to form a 0.05 mol L−1 solution. Then the 0.1 mol L−1 Ti4+ solution was dropwise added into the 0.05 Zr4+ mol L−1 solution with vigorous stirring. After continuous stirring of the mixture for several hours, a transparent solution was formed. With vigorous stirring, the mixture solution was added dropwise to 100 mL of ammonia solution (1.2:98.8). The pure coprecipitated hydroxide ZTOH was filtered and washed with distilled water and absolute ethanol several times. Subsequently, the refresh ZTOH precipitate was dissolved in distilled water with vigorous stirring, followed by the addition of Pb(AC)2 and KOH pallets. Aqueous PVA solution (about 0.5 wt.%) was prepared by dissolving PVA powder in distilled water and heating at 80 ◦ C with stirring for 2 h. Then it was introduced into the refresh ZTOH mixed solution. ZTOH and Pb concentration of 0.08 mol L−1 , KOH concentration of 1.8 mol L−1 , and PVA concentration of 0.36 g L−1 were obtained in the final suspensions. The solutions were transferred into 45 mL Teflon-lined autoclaves and heated at 190 ◦ C for 12 h. The as-synthesized powder was then collected and washed several times with distilled water and absolute ethanol. Finally, it was dried at 60 ◦ C for 12 h. The phase purity and structure of the prepared PZT powders were determined by X-ray diffraction (XRD) (using a Ragaka RINT2400 X-ray diffractometer with Cu K␣ radiation) and Raman spectroscopy (LabRam HR800, Jobin Yvon). The scanning electron microscopy (SEM) images of the samples were obtained by using a field emission scanning electron microscope (FESEM S-4800, Hitach). Photoluminescence (PL) spectra were observed on a combined steady state and lifetime spectrometer (FLS 920T, UK) at room temperature. 3. Results and discussion XRD analysis was used to determine the phases of the samples. Fig. 1 shows a typical XRD pattern of the as-prepared samples. All the peaks can be indexed to tetragonal perovskite PZT ˚ and c = 4.1487 A, ˚ phase with the lattice constants a = 3.9483 A which are close to the data in JCPDS Card No. 33-0784. No peak of any other phases was detected, indicating that the sample has a high purity. The strong and sharp reflection peaks suggest that the as-prepared samples are well crystallized. Fig. 2 shows Raman spectrum of an as-prepared PZT sample measured at room temperature. Obvious scattering peaks corresponding to different characteristic vibration modes of PZT [18,19] can be seen and summarized in Table 1. The Raman and XRD measurements give consistent evidence for the formation of perovskite-type PZT. Fig. 3 shows SEM images of the PZT synthesized by hydrothermal method assisted with PVA (0.36 g L−1 ). Examining SEM images of the samples prepared at the same condition, we found that almost all the particles are nanorolls. It indicates
227
Fig. 1. XRD pattern of an as-prepared Pb(Zr0.52 Ti0.48 )O3 sample.
Fig. 2. Raman scattering spectrum of an as-prepared sample.
that well-crystalline PZT nanorolls structures can be obtained under the present experimental conditions. The nanorolls are with a uniform length up to 10 m as shown in Fig. 3a. Fig. 3b shows that the cross sections of the samples are typically smooth, and the nanorolls are parallel to each other. We can also find that some samples show a micro-tube structure. Fig. 3c shows an erect PZT micro-tube with the inner diameter of ∼2 m and the outer diameter of ∼2.5 m. In order to investigate the influence of PVA on the formation of PZT nanorolls, the samples were synthesized without surfactant and examined by FESEM. As shown in Fig. 4, in the Table 1 Vibration modes of the Raman scattering peaks shown in Fig. 2 Peak position (cm−1 )
Mode
153 210 254 290 537 780
A1 (1TO) E(2TO) E(3TO + 2LO) + B1 SILENT E(3TO) A1 (3LO)
228
Q. Pan et al. / Materials Science and Engineering B 133 (2006) 226–230
Fig. 4. SEM images of PZT prepared by hydrothermal method without surfactant.
Fig. 3. SEM images of PZT nanorolls prepared by hydrothermal method. (a) Low-magnification SEM image showing the PZT nanorolls structures. (b) Higher magnification SEM images showing the cross sections of the PZT nanorolls. (c) The SEM image of a typical erect single PZT micro-tube.
absence of PVA, only cubic-like PZT particles were observed, implying that PVA played a key role in the formation of the PZT nanorolls. Recently, a few studies reported that the growth of one-dimensional nanostructures (nanorods and nanowires) was related to selective absorption of organic surfactants onto particular crystallographic facets of growing crystal. This selective absorption controls the growth rates of various crystallographic faces and sequentially yields anisotropies in the kinetic limit [20,21]. In the present conditions, the polymer of PVA was believed to be adsorbed on the PZT particle facets by hydrogen bounding. It minimizes the surface energy [11]. The preferential growth surface of the material was determined by the surface energy. Thus, nanosheets were formed as a result of the decreasing of the surface energy. The relief of built-in strain due to noncentrosymmetry architecture in individual sheets or the change of intrinsic bonding character between neighboring layers [22] made the nanosheets rolled as rolling up a thin film [23], which is called the “rolling mechanism”. Some nanosheets even rolled into micro-tubes. Because of the thickness of the nanosheets, there are creases in the wall of the nanorolls. Han and coworkers [11] have used PVA-assistant hydrothermal methods to synthesize PZT nanowires. In that report, the weight-average molecular weight of PVA was not mentioned. The different morphology of the obtained PZT via a similar method might be attributed to the different polymerization degree of PVA. In addition, we also found that PVA was unique in the formation of PZT nanorolls, and the PVA concentration was a crucial factor too. When poly-vinylpyrrolidone (PVP, Mw = 1,300,000, Aldrich, 0.36 g L−1 ) was used instead of PVA, quadrel-like particles were obtained (Fig. 5a). This might be attributed to carbon chain length, which resulted in the difference in modulating the growth kinetics of PZT crystals. When other reaction parameters were kept constant and the concentration of PVA was increased to 0.72 g L−1 , the products contained many agglomerated spherical particles. This might be that the PVA was attached to the whole surfaces of PZT because of the high concentration, which did not lead to the special growth directions. The exact role of
Q. Pan et al. / Materials Science and Engineering B 133 (2006) 226–230
229
Fig. 6. PL spectrum of the PZT nanorolls measured with 304 nm photoexcitation.
band structure, which may give the sharp blue-light emission. Because of the high aspect ratio of the products, there are lots of surface defects of Ti3+ and Zr3+ dangling bonds, which can also contribute to the blue-light emission. 4. Conclusions In conclusion, well-crystalline PZT nanorolls have been synthesized by a polymer-assistant hydrothermal process. The surfactant is a key for the formation of PZT nanorolls. Under 304 nm photoexitation the strong and narrow blue-light emissions are obtained at room temperature, which is of great important for applications in nano-optical devices. Fig. 5. SEM images of PZT prepared by hydrothermal methods (a) in the presence of PVP instead of PVA and (b) under an increased PVA concentration (0.72 g L−1 ).
PVA is still unclear. A detailed study of the growth mechanism of the nanorolls is under way. Sun et al. [24] have reported the blue-light emission of PZT and obtained an emission with a rather broad band. For our PZT nanorolls, a strong and sharp blue-light emission band centered at 453 nm and a weak one at 470 nm were observed as shown in Fig. 6. The width of the peak centered at 453 nm is as narrow as 20 nm. According to Liu et al. [25] the energy gap of PZT was estimated to be about 3.2 eV. Since the energy of emitted photons are about 2.74 and 2.64 eV for the present sample, the PL result suggests that the observed emissions are not related to the direct electronic transition between the valence band and the conduction band of PZT. Silva et al. [26] have reported that the displacement of one titanium atom and the breaking of the Ti–O bond can cause some broadening of the valence and conduction bands, leading to the creation of new states in the gap. ˚ The XRD analysis shows that the lattice constant (a = 3.9483 A) of our sample is smaller than that appeared in JCPDS Card. The structure distortion probably leads to the new states in the
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