Springer 2006
Journal of Nanoparticle Research (2006) 8:959–963 DOI 10.1007/s11051-006-9069-6
Perovskite barium zirconate titanate nanoparticles directly synthesized from solutions J.Q. Qi1,2,*, Y. Wang1, W.P. Chen1, L.T. Li2 and H.L.W. Chan1 1 Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, 11111, China; 2Department of Materials Science and Engineering and the State Key Laboratory of Fine Ceramics and New Processing, Tsinghua University, Beijing, 100084, China; *Author for correspondence (Tel.: +86-852-27667797; Fax: +86-852-23337629; E-mail:
[email protected]) Received 16 March 2005; accepted in revised form 22 December 2006
Key words: nanoparticle synthesis, barium zirconate titanate, perovskite, colloids, ceramics
Abstract Perovskite barium zirconate titanate nanoparticles (25–20 nm in diameter) were synthesized at low temperatures and under ambient pressure using titanium alkoxide, zirconium alkoxide and barium hydroxide as the starting materials. Microstructural analyses by X-ray diffraction and transmission electron microscopy indicated that the powders were nano-scaled, well crystallized, and had a perovskite phase. It is proposed that an acid–base neutralization reaction is the key mechanism behind the formation of such nanoparticles.
Introduction In recent years, there has been increased interest in the chemical synthesis of nano-scaled ferroelectric oxides (Morrison et al., 2003). For example, various nano-structures of the typical ferroelectric material barium titanate (BaTiO3, abbreviated as BTO) – nanoparticles, nano-rods, nano-tubes and ultra-thin films – have been developed by means of the sol–gel technique and hydrothermal method, and so forth (Mazdiyasni, 1984; Frey & Payne, 1995; Slamovich & Aksay, 1996; Suchanek et al., 1998; Clark et al., 1999; Veith et al., 2000; O’Brien et al., 2001; Hernandez et al., 2002; Urban et al., 2003). In a chemical synthesis, the structure of the product is largely dependent on the chemical route and processing parameters, especially on the temperature and pressure. For example, while a higher synthesis temperature and pressure may help to
improve the crystallinity of nano-structure products, they can also lead to a higher level of inconvenience in processing and more difficulties in controlling the size of the products. Therefore, a continuous effort has been seen in literature to develop techniques for making high-quality nanostructures under milder conditions. With regard to BaTiO3, the typical synthesis temperature in the literature ranges from 100 to 280C. A great deal of effort is still being made to further lower the synthesis temperature and pressure. In this paper, we report on the chemical synthesis of barium zirconate titanate (Ba(ZrxTi1-x)O3, where 0 £ x £ 1, abbreviated as BZT) nanoparticles at low temperatures (40–80C) and under atmospheric pressure. BZT is a solid solution of BaTiO3 and BaZrO3. BZT ceramics have long been studied for their applications in multilayer capacitors and frequency-agile microwave devices (Yu et al.,
960
2002). By comparison, the research on BZT nanostructures has so far been limited (Suchanek, 1998; Tohma, 2002). The chemical route in this work is designed mainly based on the fact that titanium alkoxides are completely soluble in an alcoholic solvent but become hydrolyzed in water. By employing appropriate chemical precursors (barium hydroxide, zirconium isopropoxide–isopropanol and tetrabutyl titanate, etc.) and allowing them to react via an acid-base neutralization route, perovskite BZT nano-powders were prepared under very mild conditions.
Experimental The synthesis: First, a base solution was prepared by dissolving barium hydroxide into warm water. The concentration of the solution was 0.4 M. After this, a small amount of ammonia was added to the solution. In the meantime, an organic solution was prepared by dissolving tetrabutyl titanate and zirconium isopropoxide–isopropanol into isopropanol. In this solution, the concentration of titanium and zirconium were adjusted in terms of the desired composition of the final products. To prepare Ba(Zr0.50Ti0.50)O3 powders, for example, the Ti and Zr concentration were both 0.2 M. Second, the organic solution was slowly added to the base solution (temperature = T1) while being vigorously stirred. An instantaneous formation of white precipitation was observed. After the reaction was completed, the white precipitation was filtered out of the solution and baked in an oven (temperature =T2) for 24 h. The dried powders obtained via the above process were the final products. Microstructural characterization: X-ray diffraction (XRD) was performed to identify the phase and estimate the particle size of the products. Transmission electron microscopy (TEM) was employed to observe the shape and estimate the size of the particles.
temperature and on concentration of the solution. In all cases, however, the diameter fell within the range of 25–120 nm. Figure 1 shows a typical TEM image of the Ba(Zr0.50Ti0.50)O3 sample (prepared at T1 = T2 = 60C). The particles are uniform and the average diameter is 40 nm. Figure 2 shows the XRD patterns of the BZT powders with different Zr/Ti ratios (x = 0, 0.1, 0.2, 0.5, 0.7, and 1.0, respectively). All of the powders were prepared at 60C (i.e., T1 = T2 = 60C). The shape and intensity of the peaks indicate that the particles are well crystallized. In most cases, the powders have a pure perovskite phase. In some of the compositions (especially when x = 0 and 0.1), a small amount of barium carbonate (identified from the XRD peaks located at 25, as shown in Figure 2) was found to coexist with the major perovskite phase. While systematic work is still being undertaken to achieve a better understanding of the kinetic and thermodynamic issues in synthesis processing, in this paper we only offer a qualitative explanation for the formation of crystalline BZT. We believe that the synthesis can be essentially regarded as an acid–base neutralization reaction. Barium hydroxide is a typical base. Zirconium isopropoxide–isopropanol and tetrabutyl titanate are not acids; however, when their organic
Results and Discussions TEM observations revealed that all of the products, regardless of Zr/Ti ratio, were uniform nanoparticles. The average diameter of the nanoparticles was dependent on the processing
Figure 1. TEM observation of the powders with a nominal composition of Ba(Zr0.5Ti0.5)O3.
961
x= =1.0
I (a.u.)
x= =0.7
=0.5 x= x= =0.2 #
#
#
20
#
30
x= =0.1 x= =0
40
50
60
2-theta Figure 2. X-ray diffraction of nanoparticles with a nominal composition of Ba(ZrxTi1-x)O3. The peak marked with ‘‘#’’ is identified as a BaCO3 peak (JCPDS Card 45–1471). All other peaks are perovskite peaks.
solutions were dripped into the base solution while being stirred, they were immediately hydrolyzed, leading to the formation of intermediate products – Zr(OH)4 and Ti(OH)4. Because of the high concentration of alcohol inside the droplets of Zr and Ti solutions, the freshly formed Zr(OH)4 and Ti(OH)4 were further dehydrated and converted correspondingly into the weak acids H2ZrO3 and H2TiO3. The acid–base neutralization reaction between H2ZrO3, H2TiO3, and Ba(OH)2 finally yielded Ba(Zr,Ti)O3:
dissolved into the solutions and reacted with Ba(OH)2. This phase of impurity can be easily suppressed or completely removed by using inert gas (such as argon or nitrogen) to protect the fabrication process until the dry powders are obtained. A further analysis of the XRD data in Figure 2 has revealed that: (1) The lattice constant of the nanoparticles increases as the Zr/(Zr+Ti) ratio increases, a trend that is consistent with that in BZT ceramics, and (2) BZT nanoparticles always have a cubic symmetry, regardless of the Zr/ (Zr+Ti) ratio. The second feature is very different from that of BZT ceramics, whose lattice symmetry changes from tetragonal (when Zr/(Zr+Ti) = 00.02) to pseudo-monoclinical (0.02–0.06), then to rhombohedral (0.06–0.20), and finally to cubic (0.20–1.0) (Landolt-Bornstern, 1981). A comparison of the lattice symmetry of the nanoparticles and BZT ceramics is given in Figure 3. This lattice symmetry abnormality in our nanoparticles is consistent with observations made in the literature on many other nano-sized ferroelectric oxides (Frey & Payne, 1993, 1996; Bottcher, 2000). In perovskite-structured oxides, such as barium titanate (BaTiO3) and lead zirconate titanate (Pb(Zr,Ti)O3), the lattice symmetry at room temperature has been found to change from tetragonal or rhombohedral to cubic when the grain size of the oxides is reduced to around tens of nanometers in diameter, resulting in the absence of ferroelectricity at the same temperature.
2BaðOHÞ2 þ 2H2 ZrO3 þ 2H2 TiO3 ð1Þ
The precipitation of BZT made the reactions irreversible. The process is analog in principle to the literature work where amorphous b-titanic acid (H2TiO3) and Ba(OH)2 were used as precursors for the synthesis of BaTiO3 nano-powders (Clark et al., 1999). The use of different ‘‘start’’ materials for the neutralization is the major difference – in the literature, commercial titanic acid was used while in our process, H2ZrO3 and H2TiO3 which had been freshly generated in a precedent reaction were used. The influence of the different processing on the structure of the final product is still under investigation. As about the barium carbonate phase in the final product, we believe it was formed because a small amount of carbon dioxide in the air had
0.425 Lattice constant (nm)
¼ 2Ba(Zr,Ti)O3 þ 5H2 O
0.420
our powders cubic (a=b=c)
0.415 0.410
tetragonal (a=b
