Hydrothermal Synthesis of Barium Titanate: Effect of Titania Precursor

Feb 16, 2008 - Department of Chemical Engineering, Nanocatalysis Research Center, National ... The lattice hydroxyl release is believed to be the reas...
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Hydrothermal Synthesis of Barium Titanate: Effect of Titania Precursor and Calcination Temperature on Phase Transition Natarajan Sasirekha, Baskaran Rajesh, and Yu-Wen Chen* Department of Chemical Engineering, Nanocatalysis Research Center, National Central UniVersity, Chung-Li 320, Taiwan, Republic of China

Nanosized barium titanate powders were synthesized by a hydrothermal method. The effect of titania precursors on the phase transition of BaTiO3 with respect to Ba/Ti ratio, reaction temperature, reaction time, and calcination temperature was investigated. The synthesized materials were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. BaTiO3 in pure cubic phase with spherical morphology was observed with a lower calcination temperature, Ba/Ti ratio, reaction temperature, and time. Increase in the tetragonal phase was ascertained in treatments at higher reaction temperature with a longer reaction time. The lattice hydroxyl release is believed to be the reason for tetragonality at high reaction and calcination temperatures. To prepare tetragonal BaTiO3 using HClO4-TiO2, the optimum synthesis conditions viz., Ba/Ti ratio, reaction temperature, and reaction time, are 1.2, 160 °C, and 3 h, respectively, at a calcination temperature of 1150 °C. The reaction time and reaction temperature for the cubic-tetragonal phase transformation of BaTiO3 shifted toward shorter reaction time and lower reaction temperature when TiO2 was synthesized by hydrolysis using HClO4 as the acid catalyst. 1. Introduction Barium titanate (BaTiO3), one of the most well-known ferroelectrics, has played an important part in the modern ceramic industry since the discovery of ferroelectric properties in the tetragonal phase of BaTiO3 during the 1940s.1 It has been broadly used as a dielectric material in multilayer ceramic capacitors (MLCCs),2-4 printed circuit boards (PCBs),5,6 dynamic random access memory (DRAM), positive temperature coefficient of resistance thermistors (PTCRs), piezoelectric sensors for ultrasonic and measuring devices, pressure transducers, infrared detectors, and electrooptic devices7-9 due to its unique perovskite structure (ABO3) and exceptionally high dielectric [(2-5) × 103]10 and piezoelectric properties at room temperature. However, these outstanding behaviors mainly depend on the crystal structure, shape, size, stoichiometry, homogeneity, and surface properties of BaTiO3, which in turn depends on the synthesis method. Among the crystal structures of BaTiO3, the cubic phase exhibits paraelectric properties, while the tetragonal phase shows ferroelectric properties. The direct generation of tetragonal BaTiO3 is of considerable interest. The conventional synthesis of barium titanate compounds typically involves high-temperature (∼1200 °C) calcinations of a BaCO3 and TiO2 powder mixture, which often results in low purity and polydispersity due to high reaction temperature and heterogeneous solid-phase reaction.11,12 Nevertheless, fine BaTiO3 ceramics can be prepared by wet-chemistry synthesis techniques, including the coprecipitation method,13 coprecipitation in combination with the inverse microemulsion method,14,15 sol-gel processing,16,17 the hydrothermal method,18-21 spray pyrolysis,22 the oxalate route,23 the high-temperature ceramic route, the microwave hydrothermal method,24-27 and the polymeric precursor method.28 Hydrothermal synthesis for the preparation of crystalline BaTiO3 has gained popularity recently.29-33 It involves the * To whom correspondence should be addressed. Tel.: (886) 3 422751, ext 34203. Fax: (886) 3 4252296. E-mail: [email protected]. edu.tw.

chemical reactions of Ba(OH)2, TiO2, or gels of Ba-Ti acetate mixtures at a high temperature. It has the advantage of producing finer particles with more uniform size. In addition, the interaction between the solid and fluid phases determines the physical characteristics of BaTiO3, and hence, the synthesis allows one to control the particle size by adjusting the synthesis parameters, such as the reaction temperature, time, and pH values. The synergistic effect of solvent, temperature, and pressure on the ionic reaction equilibrium in the hydrothermal reaction medium can stabilize the formation of BaTiO3 and retards the formation of impurities. Also, the precursors for the preparation of BaTiO3 by hydrothermal synthesis are readily available, inexpensive, and easy to handle, which makes the hydrothermal synthesis an easy and effective method to adopt for the synthesis of BaTiO3. Although the hydrothermal synthesis has the abovementioned advantages, the formation and growth mechanisms in BaTiO3 synthesis have not been well understood. There are a few articles about the study of phase transformation of BaTiO3 with preparation conditions. However, so far there is no report about the effect of titania precursor, which plays a vital role in the growth mechanism of BaTiO3 synthesis, on the phase transformation of BaTiO3. In this paper, a systematic investigation on the effect of titania precursor on phase transformation of BaTiO3 with respect to Ba/Ti ratio, reaction temperature, reaction time, and calcination temperature is presented. 2. Experimental Section 2.1. Hydrothermal Synthesis of BaTiO3. Barium titanate powders were synthesized by hydrothermal method. All the reagents used were of analytical grade. Hydrothermally produced TiO2 particles34 prepared using hydrochloric acid and perchloric acid as acid precursors were used as the titania source. The detailed characteristics of these materials have been presented in our previous paper.34 Ba(OH)2‚8H2O (Showa Chemical Co., Ltd.) and TiO2 powder (rutile)34 were mixed with a Ba/Ti ratio (1.2, 1.4, 1.6, 1.8, 2.0) in a 50 mL autoclave with 45 mL of deionized water. The autoclave was sealed, shaken, and placed in an oven at 160 °C for a variable reaction period ranging from

10.1021/ie070986m CCC: $40.75 © 2008 American Chemical Society Published on Web 02/16/2008

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Figure 1. XRD patterns of BaTiO3 synthesized at various Ba/Ti ratios using (a) HCl-TiO2 and (b) HClO4-TiO2 precursors and washed with formic acid followed by calcination at 900 °C (2 h).

3 to 24 h. After cooling naturally to room temperature, the contents of the autoclave were diluted in 90 mL of 0.1 M formic acid with stirring for 5 min in an attempt to remove any BaCO3 formed and the addition of excess Ba2+ to the starting solution. The mixture was filtered and washed with distilled water (500 mL) three times, and the residue was dried in oven at 100 °C for 24 h. 2.2. Characterization. The crystalline phase of BaTiO3 was analyzed by powder X-ray diffraction (XRD) using a Siemens D500 automatic powder diffractometer. Nickel-filtered Cu KR radiation (λ ) 0.154 18 nm) was used with a generator voltage of 40 kV and a current of 29 mA. Bragg-Brentano focusing geometry was employed with an automatic divergence slit (irradiated sample length was 12.5 nm), a receiving slit of 0.1 nm, a fixed slit of 4°, and a proportional counter as a detector. It was operated in the step scan mode, at scanning speeds of 0.1° 2θ/s and 1 s step time in the range 20-80° for barium titanate. Scherrer’s equation35 was used to calculate the crystallite size of barium titanate crystals from the full width at halfmaximum of the XRD peak. The morphology of the particles was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images were acquired with a Hitachi S-800 field emission microscope using an acceleration voltage of 20 kV. The samples were coated with Au prior to analysis and imaged directly. TEM images were obtained on a JEOL JEM2000FX Π microscope using an electron beam generated by a tungsten filament and an accelerating voltage of 160 kV, a beam diameter of approximately 1-2 µm, and an objective lens aperture of 20 µm. The sample grids were prepared via sonication of powdered sample in ethanol for 10 min and evaporating 1 drop of the suspension onto a carbon-coated, porous film supported on a 3 mm, 200-300 mesh copper grid. TEM images were recorded at a magnification of 100000400000×. The magnification was calibrated in pixels per nanometer on the camera. 3. Results and Discussion 3.1. Effect of Ba/Ti Ratio. BaTiO3 powder, prepared with Ba/Ti ratio in the range of 1.2-2.0, was characterized by XRD, SEM, and TEM techniques to study the cubic-tetragonal phase transformations and surface modifications of BaTiO3. The ratio of Ba/Ti was chosen to be greater than 1 to avoid contamination of BaTiO3(s) with excess TiO2(s) under the conditions of hydrothermal synthesis.24 Furthermore, Ba/Ti > 1 increases the pH of the solution, which is an important thermodynamic

Table 1. Effect of Ba/Ti Ratio and Calcination Temperature on the Crystalline Phase of BaTiO3

Ti precursor HCl-TiO2

HClO4-TiO2

HCl-TiO2

HClO4-TiO2

HCl-TiO2

HClO4-TiO2

calcination temp (°C)

Ba/Ti ratio

synthesis temp (°C)

synthesis time (h)

900

1150

crystalline phase

1.2 1.4 1.6 1.8 2.0 1.2 1.4 1.6 1.8 2.0 1.2 1.4 1.6 1.8 2.0 1.2 1.4 1.6 1.8 2.0 1.2 1.4 1.6 1.8 2.0 1.2 1.4 1.6 1.8 2.0

160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

yes yes yes yes yes yes yes yes yes yes -

yes yes yes yes yes yes yes yes yes yes

cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic tetragonal tetragonal tetragonal tetragonal tetragonal

variable for the synthesis of perovskite materials, and helps to avoid the addition of an alkaline mineralizer to facilitate the formation of BaTiO3. According to the thermodynamic calculations of stability diagrams for the hydrothermal Ba-Ti system, high pH and Ba/Ti > 1 are necessary for the synthesis of highpurity BaTiO3 crystals.28 XRD patterns of the as-synthesized BaTiO3, with Ba/Ti ratios of 1.2, 1.4, 1.6, 1.8, and 2.0, showed the characteristic peaks of both cubic BaTiO3 (JCPDS File No. 79-2263) and those of BaCO3 for both TiO2 precursors. Modest BaCO3 contamination was noted in almost all the samples due to the introduction of airborne CO2, which would dissolve as CO32- and reacts with Ba2+ to form BaCO3 during the posttreatment.26,36 The formation of BaCO3, observed in this work, is quite common in the hydrothermal processing as BaCO3 can precipitate at lower pH values than those needed to precipitate BaTiO3.24 According to

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Figure 2. XRD patterns of BaTiO3 synthesized at various Ba/Ti ratios using (a) HCl-TiO2 and (b) HClO4-TiO2 precursors and calcined at 1150 °C (2 h).

Figure 3. SEM micrographs of as-synthesized BaTiO3 prepared using HCl-TiO2 at various Ba/Ti ratios: (a) 1.2, (b) 1.4, (c) 1.6, and (d) 1.8.

the thermodynamic stability diagram of Ba-Ti systems, BaCO3 precipitates at lower pH values than those needed to precipitate BaTiO3.28 Moreover, the formation of BaCO3 is more predominant in BaTiO3 prepared using HClO4-TiO2, which indicates its lower pH compared with those prepared using HCl-TiO2. The relative pH of HClO4 is lower than that of HCl as the pKa values of HClO4 and HCl are -10 and -7, respectively.37 The XRD results of the as-synthesized BaTiO3 illustrate the absence of apparent peak splitting at 2θ ) 45°, which corresponds to the tetragonal phase of BaTiO3 (JCPDS Card No. 5-0626) and hence confirmed the cubic structure with symmetry Pm3m. In order to remove BaCO3, the powders were washed with formic acid and calcined at 900 °C for 2 h. Figure 1 shows XRD patterns of BaTiO3 chemically treated with formic acid and calcined at 900 °C, which confirms the removal of BaCO3 from BaTiO3 prepared using HCl-TiO2 and HClO4-TiO2. Moreover, it indicates that there is no change in the cubic phase of BaTiO3 upon treatment with formic acid. The effect of Ba/Ti ratio and calcination temperature on the crystal structure of BaTiO3 is summarized in Table 1. It can be observed that, even at a Ba/Ti molar ratio of 2 without heat treatment, the tetragonal splitting of the diffraction peaks corresponding to the (200) and (002) planes of the perovskite BaTiO3 could not be distinguished, indicating the presence of pure cubic crystalline phase. Shi et al.36 observed tetragonal BaTiO3 crystallites when the precursor with high Ba/Ti molar ratio of 3 was used, which reduces the probability of forming barium vacancies and stabilized tetragonal phase. The influence of calcination temperature at various Ba/Ti ratios on the cubic-tetragonal phase transition of BaTiO3 can be observed by comparing Figures 1 and 2. Figure 2 shows the

XRD patterns of BaTiO3 prepared using HCl-TiO2 and HClO4TiO2, subjected to calcination at 1150 °C. For BaTiO3 samples prepared using HCl-TiO2, the XRD results show that the crystalline phase is metastable cubic phase for all the samples at 900 and 1150 °C. The peaks are very sharp, indicating that the crystalline structure is well developed. Generally, the tetragonality of BaTiO3 is deduced from the plane spacing of (002) over that of (200); the corresponding peak appears near 45° in the XRD patterns. Because the peak splitting at 45° is a predominant one to confirm the formation of tetragonal phase, tetragonal splitting of the peaks corresponding to (200) and (002) planes have been chosen to verify the formation of tetragonal phase. Normally, BaTiO3 cubic-tetragonal phase changes begin at 900 and 1150 °C, but for BaTiO3 prepared using HCl-TiO2 the phase transformation does not proceed to completion even at 1150 °C. In the case of BaTiO3 prepared using HClO4-TiO2 and calcined at 900 °C, the XRD patterns are almost identical irrespective of Ba/Ti ratio and cubic phase is observed. However, tetragonal phase was observed at 2θ ) 45° for the powders synthesized at Ba/Ti ratios of 1.2, 1.4, 1.6, 1.8, and 2.0 and calcined at 1150 °C for 2 h, as shown in Figure 2b. A slight increase in the intensity of tetragonal phase with Ba/Ti ratio may be due to the removal of barium vacancies (charge compensator of OH- defect) by excess barium content, which stabilizes tetragonality. Figures 3 and 4 show the SEM pictures of BaTiO3 samples prepared using HCl-TiO2 and HClO4-TiO2, respectively, at various Ba/Ti ratios of 1.2, 1.4, 1.6, and 1.8. The particles agglomerated in a spherical shape with ca. 0.05-0.15 µm (ca. 50-150 nm) diameters. A possible mechanism for the formation of BaTiO3 by hydrothermal synthesis is the dissolution-

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Figure 4. SEM micrographs of as-synthesized BaTiO3 prepared using HClO4-TiO2 at various Ba/Ti ratios: (a) 1.2, (b) 1.4, (c) 1.6, and (d) 1.8.

Figure 5. TEM micrographs of BaTiO3 prepared using (a) HCl-TiO2, Ba/Ti ) 1.2; (b) HCl-TiO2, Ba/Ti ) 1.4; (c) HClO4-TiO2, Ba/Ti ) 1.2; and (d) HClO4-TiO2, Ba/Ti ) 1.4.

Figure 6. XRD patterns of BaTiO3 synthesized at various temperatures using (a) HCl-TiO2 and (b) HClO4-TiO2 precursors calcined at 900 °C (2 h).

precipitation method,38 in which there is a chemical equilibrium between TiO2 and [Ti(OH)x]4-x. [Ti(OH)x]4-x, which is a highly active species, can combine with Ba2+ to form a new nucleus, and hence, with an increase in Ba/Ti ratio the chance for the formation of a new nucleus by [Ti(OH)x]4-x increases and leads to a decrease in the particle size of BaTiO3. Figure 3 shows the decrease in the particle size of BaTiO3 with an increase in Ba/ Ti ratio, which is in accord with the dissolution-precipitation mechanism. TEM results show that the primary particles of the sample prepared at Ba/Ti ratios of 1.2 and 1.4 using HCl-TiO2 as precursor are spherical in shape with 50-60 nm diameters, as shown in Figure 5a,b, whereas the particle sizes of BaTiO3 prepared using HClO4-TiO2 are in the range of 40-50 nm diameters (Figure 5c,d). Moreover, it can be ascertained from

TEM images that the Ba/Ti ratio increases the cluster size of primary particles. Besides, it should be noted that the stability of cubic and tetragonal phases depends on the critical size of BaTiO3 particles and the critical size was reported to be ∼50 nm.39 The crystallite size of BaTiO3 is a principal factor controlling tetragonality because the surface defects of nanocrystallites are predominant over the bulk ones below a certain critical size of BaTiO3. The surface defects can prevent the completion of phase transformation, leading to high strains within the crystal. Increase in the cluster size of primary particles reduces the strain within the cubic structure for distortion. It can be concluded that the phase transition of cubic BaTiO3 occurs at 1150 °C irrespective of Ba/Ti ratio when HClO4-TiO2 was used as the TiO2 precursor. Moreover, the primary particle

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Table 2. Effect of Synthesis Temperature and Calcination Temperature on the Crystalline Phase of BaTiO3

Ti precursor HCl-TiO2

HClO4-TiO2

HCl-TiO2

HClO4-TiO2

HCl-TiO2

HClO4-TiO2

calcination temp (°C)

Ba/Ti ratio

synthesis temp (°C)

synthesis time (h)

900

1150

crystalline phase

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

80 120 160 180 200 80 120 160 180 200 80 120 160 180 200 80 120 160 180 200 80 120 160 180 200 80 120 160 180 200

6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6

yes yes yes yes yes yes yes yes yes yes -

yes yes yes yes yes yes yes yes yes yes

cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic tetragonal tetragonal cubic cubic tetragonal tetragonal tetragonal

size of BaTiO3 prepared using HClO4-TiO2 is smaller than that prepared using HCl-TiO2, which can be ascribed to the smaller particle size of TiO2 prepared by HClO4.34 The agglomeration of BaTiO3 nanoparticles at higher calcination temperature promotes the stability of the tetragonal phase. 3.2. Effect of Synthesis Temperature. To study the influence of synthesis temperature on the phase of BaTiO3 and particle morphology, BaTiO3 was prepared at 80, 120, 160, 180, and 200 °C while keeping the rest of the process parameters as Ba/ Ti ) 1.2 and the synthesis time as 6 h. The XRD patterns of BaTiO3 obtained at different reaction temperatures and calcined at 900 °C are given in Figure 6. The XRD results illustrated well-developed cubic crystalline phase and the intensity of the peaks increased with reaction temperature. There is a possibility of decrease in the unit-cell volume and decrease in density with an increase in the reaction temperature due to the release of lattice hydroxyls.20 The influence of calcination temperature with

synthesis temperature is compiled in Table 2. Figure 7 depicts the XRD patterns of BaTiO3 calcined at 1150 °C. With increased synthesis temperature, the diffraction peaks related to the (200) and (002) planes of the tetragonal BaTiO3 separated and the c/a ratio of the lattice increased, confirming the cubic-tetragonal phase transition with synthesis temperature. The splitting of the (200) reflection is apparent for the samples synthesized at 180 and 200 °C, suggesting the tetragonal phase. The intensity ratio of 45° peaks significantly increased as the synthesis temperature increased. The presence of shoulders at 80 and 120 °C represents the formation of partially tetragonal phase in cubic BaTiO3. The stabilization of tetragonal phase with increase in temperature may be due to the removal of hydroxyl groups in the BaTiO3 lattice. Figure 8 depicts the SEM micrographs of BaTiO3 prepared using HCl-TiO2 at 80, 120, 180, and 200 °C for 24 h. The particles agglomerated into a spherical shape, and the particle sizes estimated from the SEM micrographs are within 0.050.15 µm in diameter. When the synthesis temperature increased from 80 to 200 °C and with reacting for 24 h, the particle size of BaTiO3 increased to 0.09-0.15 µm. The shape of the particles was observed to be spherical independent of treated temperatures. The increase in the synthesis temperature leads to an increase in the particle size, which may explain the stronger agglomeration at higher temperature. However, for BaTiO3 prepared using HClO4-TiO2 (Figure 9), the secondary particle size increased from 0.05 to 0.10 µm with an increase in the reaction temperature from 80 to 200 °C for 24 h, confirming that the particle size of BaTiO3 was dependent on synthesis temperature. The overall shape of the agglomerated secondary particle size was estimated to be 0.05-0.10 µm in diameter. As shown in Figure 10, the particles consist of near-monodisperse spherical nanoparticles of BaTiO3. The agglomeration of primary particles with an increase in the reaction temperature can be identified from TEM images, as shown in Figure 10. The clusters of primary particles observed for HClO4-TiO2 precursor are more than those of HCl-TiO2. This result suggested that the crystallite size is one of the vital factors that control tetragonality. At a higher reaction temperature, the phase transition has occurred from cubic to tetragonal. 3.3. Effect of Calcination Temperature. The stability of the cubic phase in BaTiO3 prepared by hydrothermal synthesis at room temperature may be accounted for by the presence of weakly bound water molecules absorbed onto the surface of the particles and the more strongly bonded structural water as lattice OH- ions. The content of barium vacancy as well as OH- defects in the cubic crystallites is higher than that in

Figure 7. XRD patterns of BaTiO3 synthesized (Ba/Ti ) 1.2) at various temperatures using (a) HCl-TiO2 and (b) HClO4-TiO2 precursors and calcined at 1150 °C (2 h).

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Figure 8. SEM micrographs of BaTiO3 synthesized using HCl-TiO2 at various temperatures: (a) 80, (b) 120, (c) 180, and (d) 200 °C.

Figure 9. SEM micrographs of BaTiO3 synthesized using HClO4-TiO2 at various temperatures: (a) 80, (b) 120, (c) 180, and (d) 200 °C.

Figure 10. TEM micrographs of BaTiO3 prepared using (a) HCl-TiO2, 120 °C; (b) HCl-TiO2, 180 °C; (c) HClO4-TiO2, 120 °C; and (d) HClO4-TiO2, 180 °C.

tetragonal phase. The cubic-tetragonal phase transformation at higher reaction temperature and calcination temperature is due to the elimination of OH- vacancies from the lattice with heat treatment, which leads to the tetragonal stability. At the Curie point, where BaTiO3 undergoes a phase transition, the relative displacement of cation sublattice to O2- sublattice causes the phase transition of BaTiO3 from cubic to tetragonal. The oxygen vacancies have significant mobility above 650 °C, whereas the cation vacancies acquire measurable mobility only above 1050 °C.24 Moreover, the decrease in the lattice parameter of the crystallites with temperature led to the conclusion that the removal of OH- defects caused the enlargement of the unit cell and released the lattice strain to form the tetragonal phase. 3.4. Effect of Synthesis Time. The effect of synthesis time on the formation of crystalline BaTiO3 was also studied by performing the experiments at different reaction times ranging

from 3 to 24 h at 160 °C with Ba/Ti ) 1.2. The crystalline form at shorter periods of time, viz., 3 and 6 h, is primarily the metastable cubic form. SEM micrographs indicated no significant difference in the morphology. The cluster size was larger by extending the processing time, but the particle size has no significant difference. BaTiO3 powders prepared by using HClO4-TiO2 as the precursor resulted similarly to those prepared by using HCl-TiO2 as the precursor. The XRD patterns as shown in Figure 11 confirm the cubic phase of BaTiO3 prepared using HCl-TiO2 and HClO4-TiO2 calcined at 900 °C. Figure 12a shows the typical phase transformation of BaTiO3 prepared using HClTiO2. When BaTiO3 prepared using HCl-TiO2 was treated for 12 h, a noticeable peak splitting appeared. With increasing reaction time from 12 to 24 h, the intensity and sharpness of the tetragonal peak splitting increased, indicating an increase in the crystallinity of the tetragonal phase along with an increase

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Figure 11. XRD patterns of BaTiO3 synthesized at various reaction times using (a) HCl-TiO2 and (b) HClO4-TiO2 precursors calcined at 900 °C (2 h).

Figure 12. XRD patterns of BaTiO3 synthesized at various reaction times using (a) HCl-TiO2 and (b) HClO4-TiO2 precursors. Conditions: reaction temperature ) 160 °C; Ba/Ti ) 1.2; calcination temperature ) 1150 °C (2 h). Table 3. Effect of Synthesis Time and Calcination Temperature on the Crystalline Phase of BaTiO3

Ti precursor HCl-TiO2

HClO4-TiO2

HCl-TiO2

HClO4-TiO2

HCl-TiO2

HClO4-TiO2

calcination temp (°C)

Ba/Ti ratio

synthesis temp (°C)

synthesis time (h)

900

1150

crystalline phase

1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2

160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160 160

3 6 12 24 3 6 12 24 3 6 12 24 3 6 12 24 3 6 12 24 3 6 12 24

yes yes yes yes yes yes yes yes -

yes yes yes yes yes yes yes yes

cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic cubic tetragonal tetragonal tetragonal tetragonal tetragonal tetragonal

in the particle size of BaTiO3. The results are summarized in Table 3. The tetragonal BaTiO3 can be synthesized from HClTiO2 at reaction times of 12 and 24 h using Ba/Ti ratio of 1.2, synthesis temperature of 160 °C, and calcination temperature at 1150 °C for 2 h. However, even at 3 h reaction time, BaTiO3

prepared using HClO4-TiO2 achieved the cubic-tetragonal phase transformation (Figure 12b). The reason can be ascribed to the acidic nature of HClO4, which leads to the formation of BaTiO3 with fewer defects so as to stabilize the tetragonal phase apart from the possible influence of Cl- ions present in the reaction mixture. The presence of chloride ions is speculated to influence the diffusion of Ba2+ ions and retard the crystal growth process, thereby stabilizing the tetragonal phase by forming larger crystals.40 In the early stage of reaction, chloride ions produce more nuclei and form smaller particles, which grow larger at prolonged time. Sun and Li41 reported that BaTiO3 particles synthesized in the presence of chloride ions are slightly larger than the particles synthesized in the absence of chloride ions, however, with an enhanced tetragonality compared to the latter. The physicochemical properties of TiO2 prepared from HCl and HClO4 make the difference in the properties of BaTiO3.34 Therefore, it is concluded that the BaTiO3 tetragonal phase can be successfully synthesized using HClO4-TiO2 as the precursor at a [H+]/[Ti4+] ratio of 1.2, synthesis temperature of 160 °C, and calcination temperature of 1150 °C (2 h).34 4. Conclusion In the present study, the morphology and phase transformation of BaTiO3 prepared using HCl-TiO2 and HClO4-TiO2 with respect to reaction temperature, reaction time, Ba/Ti ratio, and calcination time were investigated. Increase in Ba/Ti ratio, temperature, and reaction time increases the possibility of cubic phase transformations. Well-crystallized tetragonal BaTiO3 powders of high purity were obtained using HCl-TiO2 as the precursor at optimum conditions of Ba/Ti ratio ) 1.2, temper-

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ature ) 180 °C, synthesis time ) 6 h, and calcination at 1150 °C for 2 h. However, the phase transformation of BaTiO3 prepared using HClO4-TiO2 occurred at lower reaction temperature (160 °C) and synthesis time (3 h) than BaTiO3 particles prepared using HCl-TiO2. BaTiO3 particles are agglomerated to a spherical shape with ca. 80-90 nm and BaTiO3 particles synthesized with HClO4-TiO2 were smaller than those prepared by HCl-TiO2. The stabilization of cubic BaTiO3 is caused by defects including OH- defects and barium vacancies. The formation of tetragonal BaTiO3 is promoted by the use of high reaction temperature, calcination temperature, and reaction time, which reduces the probability of forming OH- vacancies. In summary, the precursor has a strong influence on the size and morphology of BaTiO3. BaTiO3 prepared from HClO4-TiO2 indeed increases the transformation of cubic-to-tetragonal phase at lower reaction conditions without significant particle growth. Acknowledgment This research was supported by the National Science Council, Taiwan, Republic of China. Literature Cited (1) von Hippel, A. Ferroelectricity, domain structure, and phase transitions of barium titanate. ReV. Mod. Phys. 1950, 22, 221. (2) Zhu, W.; Akbar, S. A.; Asiaie, R.; Dutta, P. K. Sintering and dielectric properties of hydrothermally synthesized cubic and tetragonal BaTiO3 powders. Jpn. J. Appl. Phys. 1997, 36, 214. (3) Kiss, K.; Magder, J.; Vukasovich, M. S.; Lockhart, R. J. Ferroelectrics of ultrafine particle size: I, synthesis of titanate powders of ultrafine particle size. J. Am. Ceram. Soc. 1996, 49, 291. (4) Bruno, S. A.; Swanson, D. K. High-performance multilayer capacitor dielectrics from chemically prepared powders. J. Am. Ceram. Soc. 1993, 76, 1233. (5) Yogo, T.; Yamamoto, T.; Sakamoto, W.; Hirano, S. In situ synthesis of nanocrystalline BaTiO3 particle-polymer hybrid. J. Mater. Res. 2004, 19, 3290. (6) Rao, Y.; Wong, C. P. Material characterization of a high-dielectricconstant polymer-ceramic composite for embedded capacitor for RF applications. J. Appl. Polym. Sci. 2004, 92, 2228. (7) Huybrechts, B.; Ishizaki, K.; Takata, M. The positive temperature coefficient of resistivity in barium titanate. J. Mater. Sci. 1995, 30, 2463. (8) Choi, G. J.; Lee, S. K.; Woo, K. J.; Koo, K. K.; Cho, Y. S. Characteristics of BaTiO3 particles prepared by spray-coprecipitation method using titanium acylate-based precursors. Chem. Mater. 1998, 10, 4104. (9) Hu, M. Z. C.; Miller, G. A.; Payzant, E. A.; Rawn, C. J. Homogeneous (co)precipitation of inorganic salts for synthesis of monodispersed barium titanate particles. J. Mater. Sci. 2007, 35, 2927. (10) Wul, B. M.; Goldman, I. M. Dielectric constants of titanates of metals of the second group. C. R. Acad. Sci. URSS 1945, 46, 139. (11) Haertling, G. H. Ferroelectric ceramics: history and technology. J. Am. Ceram. Soc. 1999, 82, 797. (12) Boulos, M.; Guillemet-Fritsch, S.; Mathieu, F.; Durand, B.; Lebey, T.; Bley, V. Hydrothermal synthesis of nanosized BaTiO3 powders and dielectric properties of corresponding ceramics. Solid State Ionics 2005, 176, 1301. (13) Testino, A.; Buscaglia, M. T.; Buscaglia, V.; Viviani, M.; Bottino, C.; Nanni, P. Kinetics and mechanism of aqueous chemical synthesis of BaTiO3 particles. Chem. Mater. 2004, 16, 1536. (14) Beck, C.; Hartl, W.; Hempelman, R. Size-controlled synthesis of nanocrystalline BaTiO3 by a sol-gel type hydrolysis in microemulsionprovided nanoreactors. J. Mater. Res. 1998, 13, 3174. (15) Wang, J.; Fang, J.; Ng, S. C.; Gan, L. M.; Chew, C. H.; Wang, X.; Shein, Z. Ultrafine barium titanate powders via microemulsion processing routes. J. Am. Ceram. Soc. 1999, 82, 873. (16) Kobayashi, Y.; Nishikata, A.; Tanase, T.; Konno, M. J. Size effect on crystal structures of barium titanate nanoparticles prepared by a sol-gel method. Sol-Gel Sci. Technol. 2004, 29, 49. (17) Harizanov, O.; Harizanova, A.; Ivanova, T. Formation and characterization of sol-gel barium titanate. Mater. Sci. Eng., B 2004, 106, 191.

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ReceiVed for reView July 20, 2007 ReVised manuscript receiVed November 30, 2007 Accepted December 23, 2007 IE070986M