Microstructural Characterization of BaTiO3 Ceramic Nanoparticles

Synthesized by the Hydrothermal Technique

Xin Hua Zhu1,2, Jian Min Zhu 1, Shun Hua Zhou 1, Zhi Guo Liu 1,

Nai Ben Ming 1 and Dietrich Hesse 2

1National Laboratory of Solid State Microstructures, Department of Physics,

Nanjing University, Nanjing 210093, P.R.China, 2 Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany

Keywords: BaTiO3 Nanoparticles, Microstructures, Hydrothermal Technique, TEM, HRTEM

Abstract. BaTiO3 (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 BaTiO3

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 d110 = 0.28 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 90 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 (100) and (110) 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 BaTiO3 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

Solid State Phenomena Vol. 106 (2005) pp. 41-46online at http://www.scientific.net© 2005 Trans Tech Publications, Switzerland

Licensed to MPI of Microstructure Physics - Halle/Saale - GermanyAll rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without thewritten permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 195.37.184.165-07/06/05,14:01:56)

calcination and regrinding of BaCO3 and TiO2 powders at temperatures above 1000°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 BaTiO3

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 800-1000°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 (<300°C ). In the past there have been

many investigations concerning the hydrothermal synthesis of nanocrystalline BaTiO3 particles,

generally focusing on the following aspects [6-10]: (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

125∼130°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 BaTiO3 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 100°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 (TiO2, anatase) as starting materials under moderate conditions.

From Nanopowders to Functional Materials42

44.0 44.5 45.0 45.5 46.0

0

10

20

30

40

50

60

2 θ (degree)

Inte

nsi

ty (

Counts

)

BTa

20 30 40 50 60 70 80

0

40

80

120

160

200

(a)

(301

)

(221)

(211)

(22

0)

(201)(2

00)

(111)

(110)

*

(10

0)

2 θ (degree)

Inte

nsi

ty (

Counts

)

44.0 44.5 45.0 45.5 46.0

0

10

20

30

40

50

60

2 θ (degree)

Inte

nsi

ty (

Coun

ts)

20 30 40 50 60 70 80

0

40

80

120

160(b)

(211)

(301)

(221)(220)

(201)

(200)

(111)

(110)

*

(100)

2 θ (degree)

Inte

nsi

ty (

Coun

ts)

KOH was used as an alkaline mineralizer, and the hydrothermal reaction was carried out in an oven at

220°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 80°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 20-80° with a scan step

of 0.04°. 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 CM20 TEM operated at 200 kV, and

HRTEM images from a JEOL 4010 high-resolution electron microscope operated at 400 kV.

Results and Discussion

Figure 1(a) shows the XRD pattern of sample A. The inset represents the enlarged pattern between

2θ = 44.0 and 46.0°. The XRD pattern fits well with the peak positions of the standard cubic phase

BT. Furthermore, only a single diffraction peak at 2θ = 45.25° can be observed in the inset, i.e. no

split of the {200} peaks around 2θ = 45° can be seen. This demonstrates that the BT nanocrystals

prepared by the hydrothermal method at 100°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 (b). The inset

shows that no peak separation of the (200) and (002) 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.

Fig.1. XRD patterns of the BT nanoparticles: (a) sample A synthesized by a modified hydrothermal

technique using as-prepared titanium hydroxide and barium hydroxide as starting materials, and (b)

sample B synthesized using barium hydroxide and titanium dioxide as starting materials. Insets are

the enlarged patterns between 2θ = 44.0 and 46.0°. Peaks marked * are from the powder holder.

The grain sizes and morphologies of samples A and B are shown in Figs.2 (a) and (b) respectively.

Fig.2 (a) shows that sample A has a fairly narrow size distribution and spherical grain morphology. In

sample B shown in Fig 2(b), coarser faceted particles and the bimodal size distribution of larger and

Solid State Phenomena Vol. 106 43

400 nm

(a)

400 nm

(b)

smaller particles can be clearly seen.

Fig.2. SEM images of (a) sample A, and (b) sample B.

Bright-field TEM images of samples A and B are shown in Fig.3 (a) and (b), 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 90 nm for sample B.

Fig.3. Bright-field TEM images of (a) sample A, and (b) sample B. In Fig.3 (a), 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 (a),

whereas the diffraction contrast across a single grain varies in sample B, as shown in Fig.3 (b). 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

500 nm

200 nm

(a) (b)

100 nm

From Nanopowders to Functional Materials44

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].

Fig.4. HRTEM images (a) a typical lattice image of nanocrystalline BT grain of size of 75 nm in

sample A, (b) a surface profile HRTEM image of part of a BT grain with size of 80 nm in sample B.

Fig 4(a) shows a typical lattice image of a 75 nm nanocrystalline grain in sample A. Clear lattice

fringes with d110 = 0.28 nm reveal that well-crystallized BT nanoparticles are formed. The

10 nm

(a)

4 nm

(b)

(110)

(100)

Solid State Phenomena Vol. 106 45

surrounding edges of the particle are very smooth and no surface steps were oberved. Fig.4 (b) shows

a surface profile HRTEM image of part of a 80nm BT grain in sample B. It is noticed that the surface

facets are parallel to the (100) and (110) 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 (b) are due to the thickness variations associated with the fine-scale

surface facets and surface roughness.

Conclusions

Microstructure of BaTiO3 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 90 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 (100) and (110) 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 (2001) 63

[2] J.M.Wilson: Am. Ceram. Soc. Bull. 74 (1975) 106.

[3] H.Shimooka, M.Kuwabara: J. Am. Ceram. Soc. 79 (1996) 2983.

[4] H.S.Potdar, P.Singh, S.B.Deshpande, P.D.Godbole, S.K.Date: Mater. Lett. 10 (1990)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 (2001) 4875.

[8] X.Y.Wang, B.I.Lee, M.Z.Hu, E.A.Payzant, D.A.Blom: J. Mater. Sci: Mater. Electr. 14 (2003)

495.

[9] R.K.Dutta, J.R.Gregg: Chem. Mater. 4 (1992) 843.

[10] S.W.Lu, B.I.Lee, Z.L.Wang, W.D.Samuels: J. Cryst. Growth 219 (2000) 269.

[11] M.H.Frey, D.A.Payne: Phys. Rev. B 54 (1996) 3158.

[12] E.W.Shi, C.T.Xia, W.E.Zhang, B.G.Wang, C.D.Feng: J. Am. Ceram. Soc. 80 (1997) 1567.

From Nanopowders to Functional Materials46