Nanjing University, Nanjing , P.R.China, 2. Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D Halle, Germany

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1 Solid State Phenomena Vol. 16 (25) pp online at 25 Trans Tech Publications, Switzerland Microstructural Characterization of BaTiO 3 Ceramic Nanoparticles Synthesized by the Hydrothermal Technique Xin Hua Zhu 1,2, Jian Min Zhu 1, Shun Hua Zhou 1, Zhi Guo Liu 1, Nai Ben Ming 1 and Dietrich Hesse 2 1 National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 2193, P.R.China, 2 Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-612 Halle, Germany Keywords: BaTiO 3 Nanoparticles, Microstructures, Hydrothermal Technique, TEM, HRTEM Abstract. BaTiO 3 (BT) nanoparticles were prepared by the hydrothermal technique using different starting materials and the microstructure examined by XRD, SEM, TEM and HRTEM. X-ray diffraction and electron diffraction patterns showed that the nanoparticles were the cubic BaTiO 3 phase. The BT nanoparticles prepared from the starting materials of as-prepared titanium hydroxide and barium hydroxide have spherical grain morphology, an average size of 65 nm and a fairly narrow size distribution. A uniform diffraction contrast across each single grain is observed in the TEM images, and the clear lattice fringes (with d 11 =.28 nm) observed in HRTEM images reveal that well-crystallized BT nanoparticles are synthesized by the hydrothermal method. The edges of the particles are very smooth, with no surface steps. BT nanoparticles with average grain size of 9 nm, synthesized using barium hydroxide and titanium dioxide as the starting materials, show surface facets. In this case a bimodal size distribution of large faceted and smaller particles is observed. Diffraction contrast variation across the particles caused by high strains within the particles is clearly observed. The high strains obviously stem from structural defects formed during hydrothermal synthesis, presumable in the form of lattice OH ions and their compensation by cation vacancies. HRTEM images demonstrate that surface facets parallel to the (1) and (11) planes and small islands with 3 ~ 4 atomic layer thickness are frequently observed around the edge of the particles. Introduction Barium titanate (BT) has good dielectric and ferroelectric properties, and is widely used in thermistors, multilayer ceramic capacitors (MLCs), and electro-optic devices. Recent developments in microelectronic and communication technology involve the miniaturization of MLCs. To achieve this and to make the next advance, high dielectric constant ceramic particles of better quality and small, uniform size are needed [1]. High permittivities and miniaturization can be achieved by controlling the microstructure, which depends on the homogeneity, composition, surface area and particle size of the starting powders. To manufacture reliable MLCs, high purity, agglomerate-free, highly crystalline and superfine ceramic are required [2]. Although the bulk properties of BT ceramics have been widely investigated, more recently there has been renewed interest in nano-scale particles of the material because the electrical properties are strongly dependent on the grain size and crystalline structure. Because tetragonal BaTiO 3 is used in ferroelectrics and cubic BT is used in capacitors a better understanding of the nanostructure of BT ultrafine particles of both phases is of interest as well as the correlation of properties with particle size. Traditionally BT powders are produced by the mixed oxide route, which involves repeated Licensed to MPI of Microstructure Physics - Halle/Saale - Germany All rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /6/5,14:1:56)

2 42 From Nanopowders to Functional Materials calcination and regrinding of BaCO 3 and TiO 2 powders at temperatures above 1 C. However, this method produces BT particles with uncontrolled and irregular morphologies, which affects the electrical properties of the resulting sintered ceramics. Therefore, wet and novel chemical routes have been developed to produce high-quality BT nanoparticles, possessing great advantages over micrometer-sized ceramic powders, suitable for use in MLCs. Recently nanocrystalline BaTiO 3 particles have been prepared by wet chemical methods [3-6] such as sol-gel, coprecipitation, and hydrothermal methods. However, the products obtained by (co)precipitation or the sol-gel method are either amorphous or precursor compounds. Calcination at 8-1 C, followed by milling, is usually required to form crystalline BT powders. Thus, powder quality is not significantly improved because the production process is similar to the solid-state reaction method. The hydrothermal method provides an alternative method to produce fine, high purity, highly crystalline oxide powders having a well-defined composition and narrow range of grain size with controlled characteristics, directly from aqueous solutions at relatively low temperatures (<3 C ). In the past there have been many investigations concerning the hydrothermal synthesis of nanocrystalline BaTiO 3 particles, generally focusing on the following aspects [6-1]: (1) optimization of preparation parameters (e.g. type of precursors, Ba/Ti ratio, reaction temperature and time, ph value, and so on), (2) understanding reaction kinetics and nanocrystal formation mechanisms of BT, (3) doping with other elements during the hydrothermal synthesis process, morphology control of BT powders and the sintering behavior of green bodies made from hydrothermally produced BT, (4) structural, microstructural, and chemical characterization. Hydrothermally produced BT nanopowders show a number of structural characteristics not seen in powders prepared by conventional solid-state reaction at high temperature. X-ray diffraction of hydrothermal BT powders, particularly those synthesized at lower temperatures, reveals a cubic structure that is normally only observed above the ferroelectric Curie temperature of C. The reasons for the appearance of the cubic structure and the non-ferroelectric properties of fine BT nanocrystals are not well understood, although some possible causes are discussed by Frey et al [11]. However, few detailed nanostructure analyses, at the atomic level, of BT nanopaticles prepared by the hydrothermal method have been reported. It is well known that the physical properties of BT nanoparticles are dependent on the microstructure, e.g., grain boundaries, point and extended defects, as well as surface morphology. It is considered important to investigate the microstructure of BT nanoparticles to obtain a better understanding of size effects on the physical properties. In this work, BT nanoparticles were prepared by the hydrothermal technique using different starting materials. Their microstructure, crystal structure, grain size and distribution, grain morphology, and microstructural defects, were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and (high-resolution) transmission electron microscopy (HRTEM), and the results are presented and discussed. Experimental Procedure Two kinds of BaTiO 3 nanoparticles prepared by the hydrothermal technique using different starting materials were studied. Sample A consisted of BT nanoparticles was synthesized by a modified hydrothermal technique using the as-prepared titanium hydroxide and barium hydroxide as starting materials. These were mixed in the ratio Ba:Ti = 1:1 by stirring and reacted in an autoclave at 1 C for 5 h. After the reaction, the product was washed several times with organic acids and deionized water, and finally dried in an oven for 24 h at 85 C. Sample B was synthesized using barium hydroxide and titanium dioxide (TiO 2, anatase) as starting materials under moderate conditions.

3 Solid State Phenomena Vol KOH was used as an alkaline mineralizer, and the hydrothermal reaction was carried out in an oven at 22 C for three days. After cooling to room temperature, BT powders were obtained by filtration and washed with organic acids and water several times to remove the absorbed impurities, and finally oven dried at 8 C for 24 h. The phase purity of the BT powders was studied by X-ray diffraction in a Philips X Pert MRD four-circle diffractometer using CuKα radiation collected over a 2θ range of 2-8 with a scan step of.4. Morphology and grain size were investigated by SEM and TEM. The TEM specimens were prepared by dispersing small amounts of BT powders in pure alcohol, mixing it in an ultrasonic generator, and placing a drop of the dispersion on a copper mesh covered with a holey carbon film. Conventional TEM images were obtained from a Philips CM2 TEM operated at 2 kv, and HRTEM images from a JEOL 1 high-resolution electron microscope operated at kv. Results and Discussion Figure 1 shows the XRD pattern of sample A. The inset represents the enlarged pattern between 2θ = 44. and 46.. The XRD pattern fits well with the peak positions of the standard cubic phase BT. Furthermore, only a single diffraction peak at 2θ = can be observed in the inset, i.e. no split of the {2} peaks around 2θ = 45 can be seen. This demonstrates that the BT nanocrystals prepared by the hydrothermal method at 1 C exhibit the characteristics of the cubic phase. This was confirmed by the following selected area electron diffraction patterns of the same sample. Similar X-ray diffraction patterns were also obtained from sample B, as shown in Fig.1. The inset shows that no peak separation of the (2) and (2) peaks around 2θ 45 can be observed. This indicates that the BT nanocrystals are of the cubic phase, which is confirmed by selected area electron diffraction patterns taken from the sample (1) * (11) (111) (2) (21) (211) 6 BTa (22) (221) (31) (1) * (11) (111) (2) (21) (211) (22) (31) (221) Fig.1. XRD patterns of the BT nanoparticles: sample A synthesized by a modified hydrothermal technique using as-prepared titanium hydroxide and barium hydroxide as starting materials, and sample B synthesized using barium hydroxide and titanium dioxide as starting materials. Insets are the enlarged patterns between 2θ = 44. and 46.. Peaks marked * are from the powder holder. The grain sizes and morphologies of samples A and B are shown in Figs.2 and respectively. Fig.2 shows that sample A has a fairly narrow size distribution and spherical grain morphology. In sample B shown in Fig 2, coarser faceted particles and the bimodal size distribution of larger and

4 44 From Nanopowders to Functional Materials smaller particles can be clearly seen. nm nm Fig.2. SEM images of sample A, and sample B. Bright-field TEM images of samples A and B are shown in Fig.3 and, respectively. The particles are found to be single crystals, which was additionally proven by high-resolution lattice images of individual particles. The electron diffraction patterns inserted in Fig.3 also show that the particles are cubic BaTiO3, the diffraction rings corresponding well to the cubic perovskite structure, which agrees with the XRD results. The average particle sizes, based on the SEM and TEM images, were 65 nm for sample A, and 9 nm for sample B. 2 nm 5 nm 1 nm Fig.3. Bright-field TEM images of sample A, and sample B. In Fig.3, the insets are a higher magnified TEM image and a selected area electron pattern, respectively. A uniform diffraction contrast across the single grains in sample A is clearly observed in Fig.3, whereas the diffraction contrast across a single grain varies in sample B, as shown in Fig.3. This indicates that the grains in the sample B have a higher strains than those in sample A. (In a TEM image, large strains are indicated by contrast variation across a particle. If a particle is a single crystal and is strain free, it should be uniform in contrast. However, if the TEM image of a single crystal

5 Solid State Phenomena Vol shows dark-bright variation in contrast, it is likely that the grain is highly strained). In these cubic BT nanocrystals, the distortion of the TiO6 structure, resulting in a cubic-to-tetragonal phase transition when cooling the sample through the Curie temperature, has obviously not taken place. A possible reason is the size of the BT nanocrystals which are so small that the structural defects in the particles prevent the completion of the structural transition. This has created high strains in the nanocrystals which has caused some distortion of the cubic structure but it is obviously not sufficient to result in the formation of the tetragonal phase. It is well known that structural defects of BT nanoparticles form during hydrothermal synthesis primarily in the form of lattice OH ions and their compensation by cation vacancies [6,12]. Clark et al. observed that as-prepared BT powders contain many defects, primarily in the form of lattice OH ions [6]. Shi et al. reported that stabilization of the cubic phase of BT prepared by the hydrothermal method is caused by surface defects including OH defects and barium vacancies [12]. 1 nm (1) 4 nm (11) Fig.4. HRTEM images a typical lattice image of nanocrystalline BT grain of size of 75 nm in sample A, a surface profile HRTEM image of part of a BT grain with size of 8 nm in sample B. Fig 4 shows a typical lattice image of a 75 nm nanocrystalline grain in sample A. Clear lattice fringes with d11 =.28 nm reveal that well-crystallized BT nanoparticles are formed. The

6 46 From Nanopowders to Functional Materials surrounding edges of the particle are very smooth and no surface steps were oberved. Fig.4 shows a surface profile HRTEM image of part of a 8nm BT grain in sample B. It is noticed that the surface facets are parallel to the (1) and (11) planes. Small islands of 3 ~ 4 atom layer thickness were frequently observed around the edge of the particle. The surface roughness of the grains in sample B is much higher than that in sample A. This may be caused by the high strains in the grains. The contrast variations across Fig.4 are due to the thickness variations associated with the fine-scale surface facets and surface roughness. Conclusions Microstructure of BaTiO 3 nanoparticles prepared by the hydrothermal technique have been examined by XRD, SEM, TEM and HRTEM. XRD results indicated that the BT nanoparticles were of cubic phase, which was confirmed by electron diffraction. SEM and TEM images show that the BT nanoparticles prepared using the as-prepared titanium hydroxide and barium hydroxide as starting materials have a fairly narrow size distribution and a spherical grain morphology, with an average grain size of 65 nm. A uniform diffraction contrast across single grains was observed. Preliminary results show that well-crystallized BT nanoparticles are synthesized by the hydrothermal method. The surrounding edges of the particles are very smooth, no surface steps were observed. The BT nanoparticles synthesized using barium hydroxide and titanium dioxide as the starting materials have surface facets and a bimodal size distribution. The average grain size was measured to be 9 nm, and contrast variations across the particles were observed, indicating high strain caused by lattice defects. HRTEM images show that the surface facets were parallel to the (1) and (11) planes and small islands with 3 ~ 4 atomic layer thickness were frequently observed around the edge of the particle. Acknowledgements This work is financially supported by the opening project of National Laboratory of Solid State Microstructures, Nanjing University and a grant for State Key Program for Basic Research of China. One author, (X.H.Zhu), acknowledges financial support by the Alexander von Humboldt Foundation. References: [1] S.Venigalla: Am. Ceram. Soc. Bull. 6 (21) 63 [2] J.M.Wilson: Am. Ceram. Soc. Bull. 74 (1975) 16. [3] H.Shimooka, M.Kuwabara: J. Am. Ceram. Soc. 79 (1996) [4] H.S.Potdar, P.Singh, S.B.Deshpande, P.D.Godbole, S.K.Date: Mater. Lett. 1 (199)112. [5] S.Kumar, G.L.Messing, W.B.White: J. Am. Ceram. Soc. 76 (1993) 617. [6] I.J.Clark, T.Takeuchi, N.Ohtori, D.C.Sinclair: J. Mater. Chem. 9 (1999) 83. [7] E.Ciftci, M.N.Rahaman, M.Shumsky: J. Mater. Sci. 36 (21) [8] X.Y.Wang, B.I.Lee, M.Z.Hu, E.A.Payzant, D.A.Blom: J. Mater. Sci: Mater. Electr. 14 (23) 495. [9] R.K.Dutta, J.R.Gregg: Chem. Mater. 4 (1992) 843. [1] S.W.Lu, B.I.Lee, Z.L.Wang, W.D.Samuels: J. Cryst. Growth 219 (2) 269. [11] M.H.Frey, D.A.Payne: Phys. Rev. B 54 (1996) [12] E.W.Shi, C.T.Xia, W.E.Zhang, B.G.Wang, C.D.Feng: J. Am. Ceram. Soc. 8 (1997) 1567.