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Preparation of barium titanate ultrafine particles from rutile titania by a hydrothermal conversion
Powder Technology 141 (2004) 69 – 74 www.elsevier.com/locate/powtec
Preparation of barium titanate ultrafine particles from rutile titania by a hydrothermal conversion Kun-Yuan Chen, Yu-Wen Chen * Department of Chemical Engineering, No. 300, Chung-Ta Rd., National Central University, Chung-Li 32054, Taiwan Received 25 August 2003; received in revised form 5 November 2003; accepted 3 March 2004
Abstract Ultrafine barium titanate (BaTiO3) particles with cubic phase were synthesized by a hydrothermal conversion reaction. The direct thermal hydrolysis of titanium tetrachloride with perchloric acid had been used for producing nanosized TiO2 particles. The size and morphology of the TiO2 particles were controlled by the molar ratio of acid to water (H/Ti ratio) in the solutions. As the H/Ti ratio is 1, the morphology of TiO2 particles was distinct elongated particles with the major axis ca. 80 nm and the minor axis ca. 10 nm. These TiO2 particles were predominated in the rutile phase. These TiO2 particles were then used as a precursor to prepare BaTiO3 by the hydrothermal conversion in a barium hydroxide solution. The crystal structure of the BaTiO3 particles was cubic phase with a lattice constant of 4.072 nm (a-axis). The morphology of the particles was near-spheres with a size distribution of 20 – 50 nm. The reaction temperature and time did not influence the size and the morphology of BaTiO3 particles, but the lattice constant of a-axis shrunk slightly and the impurity of BaCO3 decreased. The Ba/ Ti molar ratio did not influence the morphology and crystal structure, but the mean particle size decreased with increasing the Ba/Ti ratio. The size and morphology of the BaTiO3 particles were different from those of the precursor TiO2 particles, indicating a dissolution – precipitation mechanism for the conversion of TiO2 to BaTiO3. The particles were cubic phase if the calcination temperature was lower than 900 jC and were converted to tetragonal phase with the tetragonality (c/a) of 1.0105 after calcination at 1150 jC for 2 h in air. D 2004 Elsevier B.V. All rights reserved. Keywords: Thermal hydrolysis; Barium titanate; Lattice constant; Cubic phase; Tetragonal phase
1. Introduction Barium titanate (BaTiO3) is a ferroelectric and piezoelectric material, which has a perovskite structure and possesses high dielectric constant and high resistivity. These characteristics promise applications of the material such as for multilayer capacitors (MLC), positive temperature coefficient (PTC), etc [1 –3]. There are many methods for synthesizing high purity, homogeneous, reactive fine BaTiO3 powders at lower temperatures, such as solid reaction [4,5], coprecipitation [6,7], sol –gel method [8,9], hydrothermal method [10,11] and coprecipitation plus inverse microemulsion [12], etc. Ultrafine, crystalline and unagglomerated powders with a narrow size distribution can be synthesized by a hydrothermal method [13,14]. The various kinetics and morphology studies reported in the literature have proposed two possible * Corresponding author. Tel.: +886-3-422-7151; fax: +886-3-4252296. E-mail address: [email protected] (Y.-W. Chen). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.03.002
mechanisms for the hydrothermal conversion of titania to barium titanate [15 – 17]: (1) in situ mechanism: the Ba2 + ions react on the surface of the titania particles to form an BaTiO3 shell around the TiO2 particles, followed by diffusion of Ba2 + ions through the BaTiO3 shell layer and reaction with the titania core. The overall conversion rate could be controlled by either diffusion rate or reaction rate. BaTiO3 particles obtained by this model should maintain their size and morphology similar to those of the precursor TiO2 particles [15]. (2) Diffusion–precipitation mechanism: a low concentration of TiO2 particles dissolves in the form of soluble hydroxytitanium complexes, which then react with Ba2 + ions in solutions to precipitate BaTiO3, followed by recrystallization and growth. BaTiO3 particles obtained by this model are usually different from the precursor TiO2 particles with regard to their size and morphology [16]. In this present work, ultrafine particles of barium titanate were synthesized hydrothermally from aqueous solutions of barium hydroxide with suspended rutile titania fine particles. A ‘‘two-step’’ approach for preparing ultrafine barium titanate particles was used: (1) direct thermal hydrolysis of
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titanium tetrachloride solution with perchloric acid to synthesize fine titania particles; (2) hydrothermal conversion of suspended fine titania particles in barium hydroxide solutions. The relations of the particle size, morphology, size distribution and crystal structure of the BaTiO3 to such as barium-to-titanium (Ba/Ti) molar ratio, hydrothermal temperature and reaction time were investigated systematically.
2. Experimental procedure 2.1. Synthesis of titania particles The ice-cold deionized water was added drop-wise to icecold titanium tetrachloride (TiCl4; 99%, Hayashi Chemical Industries, Japan) and stirred for 6 h at 5 jC. The solution became yellow and gel-like during the dissolution step but gradually cleared up within 30 min. The concentration in the prepared flask solution was adjusted to 1.0 M. The 10 ml of 1 M TiCl4 aqueous solution was mixed with perchloric acid (3 N) to adjust the mole ratio of hydrogen to titanium (mole ratio of H/Ti) and the final volume was controlled to be 50 ml by adding deionized water. The mole ratio of H/Ti was controlled to be 0.5,1, 1.5 and 2, respectively. The mixed solution was stirred for 30 min at room temperature and then was heated at 100 jC in an oven for 24 h. All syntheses were performed in a 50 ml glass beaker, which had been carefully washed with concentrated hydrochloric acid and deionized water in order to avoid the alkaline solution in leaching metals from the glass or the presence of sources of nucleation that are able to influence the crystalline structure of the precipitate. After reacting for 24 h, the mother liquor was neutralized (pH c 7) by adding 6 ml of 4N NH4OH solution to avoid redissolution of the precipitates into the mother liquor during the centrifugation and washing period. The precipitates obtained from the above procedure were centrifuged (at 5000 rpm for 3-min intervals) and washed twice with deionized water and dried in an oven at 100 jC for 24 h.
itate was filtered, washed and dried in an oven overnight at a temperature of 80 jC to form particles. 2.3. Characterization Phase identification and crystallite size of the particle samples were analyzed by X-ray powder diffraction (XRD) using a Siemens D&8 automatic powder diffractometer system (RTXRD, 40 kV, 29 mA, using Cu Ka radiation). The particle size and morphology of the particle samples were analyzed by a SEM (Hitachi S-800, 20 kV) and TEM (JEOL JEM-2000 FX Dc). Particle size distribution was measured by the light scattering method (Zetasizer 3000). The obtained particles were pressed under 250 MPa of pressure into discs with polyvinyl alcohol (PVA). The compacted discs were dried at 550 jC in a furnace for 1 h to remove PVA. Sintered pellets were made using a box furnace (Model DF20) at a temperature of 1300 jC for 2 h with a heating rate of 10 jC/min. Densities of the sintered pellets were measured by using the Archimedes principle.
3. Results and discussion 3.1. Synthesis of ultrafine titania particles Perchloric acid was used as a catalyst to increase the rates of hydrolysis and crystallization. If the acid amounts were very large, the crystallization rate would be very fast. Different H/Ti ratios were used to control the rate of hydrolysis and crystallization. The experimental conditions of mother solution were 10 ml of the 1 M TiCl4 solution with H/Ti ratio of 1 and were reacted at 100 jC for 24 h. Fig. 1A shows the TEM micrograph of the primary particles prepared by perchloric acid with H/Ti ratio of 1. As shown in Fig. 1A, nanosized titania particles had successfully been synthesized by using the perchloric acid as a catalyst. The size and morphology of the primary particles prepared by perchloric acid was distinct elongated particles with the
2.2. Synthesis of barium titanate particles A predetermined quantity of barium hydroxide [98% Ba(OH)28H2O] was added to the titania sol in deionized water at various Ba/Ti molar ratios in the range from 1.2 to 2.0; the solution was stirred in a 50 ml autoclave. After stirring, the pH of the solution was 10– 12 (depending on the concentration of the barium hydroxide), which is sufficiently high for the formation of barium titanate. The solution was heated at various temperatures in the range from 80– 200 jC for 3 to 24 h. After the hydrothermal conversion, the precipitate settled to the bottom and was filtered, washed and then stirred in formic acid solution (2 moldm 3) at 50 jC for 30 min, in order to remove unfavorable barium salts such as barium carbonate and barium hydroxide. After formic acid treatment, the precip-
Fig. 1. TEM and SEM images of the ultrafine TiO2 particles prepared with percloric acid at H/Ti ratio of 1; (A) primary particles (TEM images); (B) secondary particles (SEM images).
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major axis ca. 80 nm and the minor axis ca. 10 nm. However, these primary particles agglomerated into secondary particles in the solution as shown in the SEM photos. As shown in Fig. 1B, the primary particles agglomerated to form secondary particles in a spherical shape with ca. 0.2 Am in diameter. The XRD patterns of the TiO2 particles synthesized with HClO4 at different H/Ti ratios are shown in Fig. 2. Note the peaks attributed to the anatase TiO2 phase at 2h = 25.3j (101), 2h = 37.8j (004) and to the rutile phase at 2h = 27.5j (110), 2h = 36.1j (101) are present. It should be noted, however, that TiO2 particles synthesized with HClO4 at the H/Ti ratios of 0.5, 1, 1.5 and 2, respectively, were rutile phase and did not change when the temperature was increased from room temperature to 700 jC for 2 h. Fig. 3 shows the SEM images of the TiO2 particles prepared with perchloric acid at various H/Ti ratios, indicating that the morphology of TiO2 particles were not influenced by the H/Ti ratio. It retained elongated particles, but the size of the particles was influenced by the H/Ti ratio. Fig. 4 shows the mean size of the primary and secondary particles measured by the dynamic light scattering (DLS) method. The results showed that the mean size of TiO2 prepared with HClO4 had a minimum value (27 nm) as the H/Ti ratio was 0.5. It increased slowly until H/Ti ratio was 1.5, and then increased significantly with increasing H/Ti ratio. These TiO2 particles prepared with HClO4 at the H/Ti ratio of 0.5 were converted to form ultrafine BaTiO3 by a hydrothermal conversion in a barium hydroxide solution. The as-prepared TiO2 particles were pressed under 250 MPa of pressure into a disc with PVA. The compacted disc was dried at 550 jC in a furnace for 1 h to remove PVA. Sintered pellets were formed at a temperature of 1150 jC for 1 h with a heating rate of 10 jC/min. The mean densities of the five pellets sintered at 1150 jC was 4.177 g/cm3 for the HClO4-derived samples. The theoretical density of TiO2
Fig. 2. XRD patterns of TiO2 particles synthesized with HClO4 at different H/Ti ratios.
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Fig. 3. SEM images of ultrafine TiO2 particles prepared with perchloric acid at various H/Ti ratios. (A) H/Ti = 1; (B) H/Ti = 2.
with rutile phase is 4.25 g/cm3 and the samples accounted for 98.28% of the theoretical densities. 3.2. Synthesis of BaTiO3 particles Barium titanate particles were synthesized by using hydrothermal conversion of the home-made titania particles. A predetermined quantity of barium hydroxide [Ba(OH)28H2O] was added to titania suspensions in deionized water at various Ba/Ti molar ratios in the range from 1.2 to 2.0; the solution was stirred in a 50 ml autoclave, followed by heating in an oven in the range from 80 to 200 jC. Fig. 5 shows the TEM and SEM micrographs of the sample with an initial Ba/Ti ratio of 1.2 after hydrothermal conversion at 120 jC for 6 h. As shown in Fig. 5, the particles of barium titanate were near-spherical with approximately 20 –50 nm in diameter, and the precursor titania particles were distinct elongated particles with the major axis ca. 80 nm and the minor axis ca. 10 nm (Fig. 1A). Barium titanate particles obtained by this route were different from the precursor titania particles with regard to their size and shape, indicating that the formation of barium titanate was possibly through the
Fig. 4. Mean particle size of TiO2 prepared with various H/Ti ratios (measured by DLS).
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Fig. 5. TEM and SEM images of ultrafine BaTiO3 particles prepared at the conditions of H/Ti ratio of 1.2, 120 jC and 24 h. (A) TEM image; (B) SEM image. Fig. 7. XRD patterns of the BaTiO3 sample particles treated with and without formic acid.
dissolution – precipitation mechanism [16]. As in dissolution – precipitation model, it was believed to involve dissolution of TiO2 into Ti(OH)4x x species, precipitative BaTiO3 nucleation (heterogeneous or homogeneous in nature) by reaction with barium ions/complexes in solution, followed by recrystallization and growth. On the basis of this model, it would be expected that the size and morphology of the converted BaTiO3 particles would be different from those of the precursor TiO2 particles. The effects of reaction temperature can be investigated by using various temperatures in the range of 80 –200 jC, while other process parameters remained the same (TiO2 particles prepared with HClO4 at the H/Ti ratio of 0.5, Ba/Ti ratio of 1.2, 24 h). Fig. 6 shows the TEM micrographs of the as-prepared particles, which were synthesized at various temperatures. As shown in Fig. 6, the as-prepared particles were near-spherical shape with ca. 20– 50 nm in diameter. The size and morphology of the as-prepared particles were not influenced by the reaction temperature. The XRD data (Fig. 7) of the same samples show the presence of the cubic barium titanae phase in the samples. However, there was a minor unwanted impurity peak at 2h = f 24j, which was
barium carbonate in the witherite form, as reported by Eckert et al. [15]. In order to obtain high-purity barium titanate, the sample was washed with formic acid in the final step to remove the barium carbonate. This was confirmed by the complete disappearance of barium carbonate peaks in the XRD spectrum (Fig. 7). The effects of reaction time was elucidated by using various time in the range of 3 –24 h, while the other process parameters remained the same (TiO2 particles prepared with HClO4 at the H/Ti ratio of 0.5, Ba/Ti ratio of 1.2, 160 jC), which show the same trends as the effect of the reaction temperature. The size and morphology of BaTiO3 particles were not influenced by reaction time, but as shown in Fig. 8, the peaks shifting to higher 2h value became more significant when reaction time and temperature increased. This occurrence was probably due to the release of the hydroxyl group and barium vacancy in the structure at the higher kinetics. As reviewed by Hennings and Schreinemacher [18], the enlarge-
Fig. 6. TEM micrographs of the BaTiO3 particles prepared with HClO4 at different temperatures: (A) 120 jC; (B) 160 jC.
Fig. 8. XRD patterns of the BaTiO3 particles prepared with HClO4 at various reaction times.
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ment of the barium titanate lattice was due to the presence of hydroxyl group in the lattice, and concluded that these lattice defects significantly influenced the Coulomb attractive forces, which control ionic bonding strength. These lattice defects can reduce the Coulomb attractive forces between ions. As a result, the bond length can increase, and thus, the lattice can expand. This indicated that the less the hydroxyl group or barium vacancy, the stronger the Coulomb attractive forces were between ions due to the shrinkage of the lattice. Table 1 shows the lattice constants of a-axis at various reaction temperature and time. The crystal structure of the BaTiO3 particles was assigned as cubic and the lattice constant of a-axis was shrunk slightly; it should be due to the release of hydroxyl group or barium vacancy at higher reaction temperature and longer reaction time. Further analysis by XRD patterns of the treated BaTiO3 samples, as shown in Fig. 8, showed the decreasing of the peak at 2h = f 24j, indicating that a higher reaction temperature and longer reaction time tend to decrease the concentration of unfavorable salt (BaCO3). The effects of Ba/Ti ratio was studied by using various Ba/Ti ratio in the range of 1.2 – 2.0, while the other process parameters remained the same (TiO2 particles prepared with HClO4 at the H/Ti ratio of 0.5, 160 jC, 24 h). The morphology and crystal structure of BaTiO3 particles were not influenced by Ba/Ti ratio. However, as shown in Fig. 9, the size of the particles decreased slightly with an increasing of Ba/Ti ratio. As reported by Wada et al. [14], the average particle size decreased with increasing Ba/Ti molar ratio in the starting materials, and above the Ba/Ti molar ratio of 20, the particle size was almost constant at about 20 nm. The results of this study showed the same trend as that of Wada et al.—the higher the Ba/Ti molar ratio was, the more defects in the structure were. But an excess of Ba+ 2 ions in the starting solution was employed (Ba/Ti = 1.2) in an attempt to remove any unreacted TiO2, and therefore drive the reaction to completion. However, the Ba/Ti molar ratio of 1.2 was used in this work to prevent the more defects in the structure and drive the reaction to completion. Particles prepared at the conditions of 160 jC, Ba/Ti ratio of 1.2 and 24 h were near-sphere with a size distribution of 20 –50 nm and had a lattice constant of 4.072 nm. To evaluate the stability of the barium titanate at high temperature, the sample prepared at Ba/Ti ratio of 1.2 was heated to 900 and 1150 jC, respectively, in a box furnace for 2 h. The barium titanate peaks were not altered below Table 1 Lattice constant (a) of BaTiO3 particles prepared at different hydrothermal conditions (nm) Time (h)
Lattice constant (a)
Temp. (jC)
Lattice constant (a)
Ba/Ti
Lattice constant (a)
3 6 12 24
4.0091 4.0072 4.0063 4.0045
80 120 160 200
4.0100 4.0081 4.0072 4.0063
1.2 1.4 1.6 1.8
4.0072 4.0072 4.0072 4.0072
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Fig. 9. SEM micrographs of the BaTiO3 particles prepared with HClO4 at the Ba/Ti ratios of (A) 1.2 and (B) 1.8.
900 jC (cubic phase) but were altered above 1150 jC (tetragonal phase). It transformed to the tetragonal phase with the tetragonality (c/a) of 1.0105 after calcination at 1150 jC for 2 h in air.
4. Conclusions An approach for the formation of ultrafine BaTiO3 particles by two hydrothermal processing steps was used in this study. Firstly, the direct thermal hydrolysis of titanium tetrachloride solution with perchloric acid was used to synthesize fine titania particles. The hydrothermal conversion of titania particles in barium hydroxide solutions was used to form barium titanate. The size and morphology of the TiO2 particles was controlled by the molar ratio of acid to titania (H/Ti) in the solutions. In the hydrothermal process, the size and morphology of the converted particles were different from those of the precursor TiO2 particles, indicating a dissolution – precipitation mechanism for the conversion of TiO2 to BaTiO3. The reaction temperature and time did not influence the size and morphology of BaTiO3 particles, but the lattice constant of a-axis was shrunk slightly and the concentration of BaCO3 impurity was decreased. The Ba/Ti molar ratio did not influence the morphology and crystal structure, but the mean particle size decreased with the increasing of Ba/Ti ratio. Particles prepared at the conditions of 160 jC, Ba/Ti ratio of 1.2 and 24 h were near-sphere with a size distribution of 20– 50 nm and had a lattice constant of 4.072 nm. These assynthesized particles were cubic phase if the calcination temperature was below 900 jC and converted to tetragonal phase with the tetragonality (c/a) of 1.0105 after calcination at 1150 jC for 2 h in air.
Acknowledgements This research was supported by the National Science Council of the Republic of China.
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Preparation of barium titanate ultrafine particles from rutile titania by a hydrothermal conversion Kun-Yuan Chen, Yu-Wen Chen * Department of Chemical Engineering, No. 300, Chung-Ta Rd., National Central University, Chung-Li 32054, Taiwan Received 25 August 2003; received in revised form 5 November 2003; accepted 3 March 2004
Abstract Ultrafine barium titanate (BaTiO3) particles with cubic phase were synthesized by a hydrothermal conversion reaction. The direct thermal hydrolysis of titanium tetrachloride with perchloric acid had been used for producing nanosized TiO2 particles. The size and morphology of the TiO2 particles were controlled by the molar ratio of acid to water (H/Ti ratio) in the solutions. As the H/Ti ratio is 1, the morphology of TiO2 particles was distinct elongated particles with the major axis ca. 80 nm and the minor axis ca. 10 nm. These TiO2 particles were predominated in the rutile phase. These TiO2 particles were then used as a precursor to prepare BaTiO3 by the hydrothermal conversion in a barium hydroxide solution. The crystal structure of the BaTiO3 particles was cubic phase with a lattice constant of 4.072 nm (a-axis). The morphology of the particles was near-spheres with a size distribution of 20 – 50 nm. The reaction temperature and time did not influence the size and the morphology of BaTiO3 particles, but the lattice constant of a-axis shrunk slightly and the impurity of BaCO3 decreased. The Ba/ Ti molar ratio did not influence the morphology and crystal structure, but the mean particle size decreased with increasing the Ba/Ti ratio. The size and morphology of the BaTiO3 particles were different from those of the precursor TiO2 particles, indicating a dissolution – precipitation mechanism for the conversion of TiO2 to BaTiO3. The particles were cubic phase if the calcination temperature was lower than 900 jC and were converted to tetragonal phase with the tetragonality (c/a) of 1.0105 after calcination at 1150 jC for 2 h in air. D 2004 Elsevier B.V. All rights reserved. Keywords: Thermal hydrolysis; Barium titanate; Lattice constant; Cubic phase; Tetragonal phase
1. Introduction Barium titanate (BaTiO3) is a ferroelectric and piezoelectric material, which has a perovskite structure and possesses high dielectric constant and high resistivity. These characteristics promise applications of the material such as for multilayer capacitors (MLC), positive temperature coefficient (PTC), etc [1 –3]. There are many methods for synthesizing high purity, homogeneous, reactive fine BaTiO3 powders at lower temperatures, such as solid reaction [4,5], coprecipitation [6,7], sol –gel method [8,9], hydrothermal method [10,11] and coprecipitation plus inverse microemulsion [12], etc. Ultrafine, crystalline and unagglomerated powders with a narrow size distribution can be synthesized by a hydrothermal method [13,14]. The various kinetics and morphology studies reported in the literature have proposed two possible * Corresponding author. Tel.: +886-3-422-7151; fax: +886-3-4252296. E-mail address: [email protected] (Y.-W. Chen). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.03.002
mechanisms for the hydrothermal conversion of titania to barium titanate [15 – 17]: (1) in situ mechanism: the Ba2 + ions react on the surface of the titania particles to form an BaTiO3 shell around the TiO2 particles, followed by diffusion of Ba2 + ions through the BaTiO3 shell layer and reaction with the titania core. The overall conversion rate could be controlled by either diffusion rate or reaction rate. BaTiO3 particles obtained by this model should maintain their size and morphology similar to those of the precursor TiO2 particles [15]. (2) Diffusion–precipitation mechanism: a low concentration of TiO2 particles dissolves in the form of soluble hydroxytitanium complexes, which then react with Ba2 + ions in solutions to precipitate BaTiO3, followed by recrystallization and growth. BaTiO3 particles obtained by this model are usually different from the precursor TiO2 particles with regard to their size and morphology [16]. In this present work, ultrafine particles of barium titanate were synthesized hydrothermally from aqueous solutions of barium hydroxide with suspended rutile titania fine particles. A ‘‘two-step’’ approach for preparing ultrafine barium titanate particles was used: (1) direct thermal hydrolysis of
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titanium tetrachloride solution with perchloric acid to synthesize fine titania particles; (2) hydrothermal conversion of suspended fine titania particles in barium hydroxide solutions. The relations of the particle size, morphology, size distribution and crystal structure of the BaTiO3 to such as barium-to-titanium (Ba/Ti) molar ratio, hydrothermal temperature and reaction time were investigated systematically.
2. Experimental procedure 2.1. Synthesis of titania particles The ice-cold deionized water was added drop-wise to icecold titanium tetrachloride (TiCl4; 99%, Hayashi Chemical Industries, Japan) and stirred for 6 h at 5 jC. The solution became yellow and gel-like during the dissolution step but gradually cleared up within 30 min. The concentration in the prepared flask solution was adjusted to 1.0 M. The 10 ml of 1 M TiCl4 aqueous solution was mixed with perchloric acid (3 N) to adjust the mole ratio of hydrogen to titanium (mole ratio of H/Ti) and the final volume was controlled to be 50 ml by adding deionized water. The mole ratio of H/Ti was controlled to be 0.5,1, 1.5 and 2, respectively. The mixed solution was stirred for 30 min at room temperature and then was heated at 100 jC in an oven for 24 h. All syntheses were performed in a 50 ml glass beaker, which had been carefully washed with concentrated hydrochloric acid and deionized water in order to avoid the alkaline solution in leaching metals from the glass or the presence of sources of nucleation that are able to influence the crystalline structure of the precipitate. After reacting for 24 h, the mother liquor was neutralized (pH c 7) by adding 6 ml of 4N NH4OH solution to avoid redissolution of the precipitates into the mother liquor during the centrifugation and washing period. The precipitates obtained from the above procedure were centrifuged (at 5000 rpm for 3-min intervals) and washed twice with deionized water and dried in an oven at 100 jC for 24 h.
itate was filtered, washed and dried in an oven overnight at a temperature of 80 jC to form particles. 2.3. Characterization Phase identification and crystallite size of the particle samples were analyzed by X-ray powder diffraction (XRD) using a Siemens D&8 automatic powder diffractometer system (RTXRD, 40 kV, 29 mA, using Cu Ka radiation). The particle size and morphology of the particle samples were analyzed by a SEM (Hitachi S-800, 20 kV) and TEM (JEOL JEM-2000 FX Dc). Particle size distribution was measured by the light scattering method (Zetasizer 3000). The obtained particles were pressed under 250 MPa of pressure into discs with polyvinyl alcohol (PVA). The compacted discs were dried at 550 jC in a furnace for 1 h to remove PVA. Sintered pellets were made using a box furnace (Model DF20) at a temperature of 1300 jC for 2 h with a heating rate of 10 jC/min. Densities of the sintered pellets were measured by using the Archimedes principle.
3. Results and discussion 3.1. Synthesis of ultrafine titania particles Perchloric acid was used as a catalyst to increase the rates of hydrolysis and crystallization. If the acid amounts were very large, the crystallization rate would be very fast. Different H/Ti ratios were used to control the rate of hydrolysis and crystallization. The experimental conditions of mother solution were 10 ml of the 1 M TiCl4 solution with H/Ti ratio of 1 and were reacted at 100 jC for 24 h. Fig. 1A shows the TEM micrograph of the primary particles prepared by perchloric acid with H/Ti ratio of 1. As shown in Fig. 1A, nanosized titania particles had successfully been synthesized by using the perchloric acid as a catalyst. The size and morphology of the primary particles prepared by perchloric acid was distinct elongated particles with the
2.2. Synthesis of barium titanate particles A predetermined quantity of barium hydroxide [98% Ba(OH)28H2O] was added to the titania sol in deionized water at various Ba/Ti molar ratios in the range from 1.2 to 2.0; the solution was stirred in a 50 ml autoclave. After stirring, the pH of the solution was 10– 12 (depending on the concentration of the barium hydroxide), which is sufficiently high for the formation of barium titanate. The solution was heated at various temperatures in the range from 80– 200 jC for 3 to 24 h. After the hydrothermal conversion, the precipitate settled to the bottom and was filtered, washed and then stirred in formic acid solution (2 moldm 3) at 50 jC for 30 min, in order to remove unfavorable barium salts such as barium carbonate and barium hydroxide. After formic acid treatment, the precip-
Fig. 1. TEM and SEM images of the ultrafine TiO2 particles prepared with percloric acid at H/Ti ratio of 1; (A) primary particles (TEM images); (B) secondary particles (SEM images).
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major axis ca. 80 nm and the minor axis ca. 10 nm. However, these primary particles agglomerated into secondary particles in the solution as shown in the SEM photos. As shown in Fig. 1B, the primary particles agglomerated to form secondary particles in a spherical shape with ca. 0.2 Am in diameter. The XRD patterns of the TiO2 particles synthesized with HClO4 at different H/Ti ratios are shown in Fig. 2. Note the peaks attributed to the anatase TiO2 phase at 2h = 25.3j (101), 2h = 37.8j (004) and to the rutile phase at 2h = 27.5j (110), 2h = 36.1j (101) are present. It should be noted, however, that TiO2 particles synthesized with HClO4 at the H/Ti ratios of 0.5, 1, 1.5 and 2, respectively, were rutile phase and did not change when the temperature was increased from room temperature to 700 jC for 2 h. Fig. 3 shows the SEM images of the TiO2 particles prepared with perchloric acid at various H/Ti ratios, indicating that the morphology of TiO2 particles were not influenced by the H/Ti ratio. It retained elongated particles, but the size of the particles was influenced by the H/Ti ratio. Fig. 4 shows the mean size of the primary and secondary particles measured by the dynamic light scattering (DLS) method. The results showed that the mean size of TiO2 prepared with HClO4 had a minimum value (27 nm) as the H/Ti ratio was 0.5. It increased slowly until H/Ti ratio was 1.5, and then increased significantly with increasing H/Ti ratio. These TiO2 particles prepared with HClO4 at the H/Ti ratio of 0.5 were converted to form ultrafine BaTiO3 by a hydrothermal conversion in a barium hydroxide solution. The as-prepared TiO2 particles were pressed under 250 MPa of pressure into a disc with PVA. The compacted disc was dried at 550 jC in a furnace for 1 h to remove PVA. Sintered pellets were formed at a temperature of 1150 jC for 1 h with a heating rate of 10 jC/min. The mean densities of the five pellets sintered at 1150 jC was 4.177 g/cm3 for the HClO4-derived samples. The theoretical density of TiO2
Fig. 2. XRD patterns of TiO2 particles synthesized with HClO4 at different H/Ti ratios.
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Fig. 3. SEM images of ultrafine TiO2 particles prepared with perchloric acid at various H/Ti ratios. (A) H/Ti = 1; (B) H/Ti = 2.
with rutile phase is 4.25 g/cm3 and the samples accounted for 98.28% of the theoretical densities. 3.2. Synthesis of BaTiO3 particles Barium titanate particles were synthesized by using hydrothermal conversion of the home-made titania particles. A predetermined quantity of barium hydroxide [Ba(OH)28H2O] was added to titania suspensions in deionized water at various Ba/Ti molar ratios in the range from 1.2 to 2.0; the solution was stirred in a 50 ml autoclave, followed by heating in an oven in the range from 80 to 200 jC. Fig. 5 shows the TEM and SEM micrographs of the sample with an initial Ba/Ti ratio of 1.2 after hydrothermal conversion at 120 jC for 6 h. As shown in Fig. 5, the particles of barium titanate were near-spherical with approximately 20 –50 nm in diameter, and the precursor titania particles were distinct elongated particles with the major axis ca. 80 nm and the minor axis ca. 10 nm (Fig. 1A). Barium titanate particles obtained by this route were different from the precursor titania particles with regard to their size and shape, indicating that the formation of barium titanate was possibly through the
Fig. 4. Mean particle size of TiO2 prepared with various H/Ti ratios (measured by DLS).
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Fig. 5. TEM and SEM images of ultrafine BaTiO3 particles prepared at the conditions of H/Ti ratio of 1.2, 120 jC and 24 h. (A) TEM image; (B) SEM image. Fig. 7. XRD patterns of the BaTiO3 sample particles treated with and without formic acid.
dissolution – precipitation mechanism [16]. As in dissolution – precipitation model, it was believed to involve dissolution of TiO2 into Ti(OH)4x x species, precipitative BaTiO3 nucleation (heterogeneous or homogeneous in nature) by reaction with barium ions/complexes in solution, followed by recrystallization and growth. On the basis of this model, it would be expected that the size and morphology of the converted BaTiO3 particles would be different from those of the precursor TiO2 particles. The effects of reaction temperature can be investigated by using various temperatures in the range of 80 –200 jC, while other process parameters remained the same (TiO2 particles prepared with HClO4 at the H/Ti ratio of 0.5, Ba/Ti ratio of 1.2, 24 h). Fig. 6 shows the TEM micrographs of the as-prepared particles, which were synthesized at various temperatures. As shown in Fig. 6, the as-prepared particles were near-spherical shape with ca. 20– 50 nm in diameter. The size and morphology of the as-prepared particles were not influenced by the reaction temperature. The XRD data (Fig. 7) of the same samples show the presence of the cubic barium titanae phase in the samples. However, there was a minor unwanted impurity peak at 2h = f 24j, which was
barium carbonate in the witherite form, as reported by Eckert et al. [15]. In order to obtain high-purity barium titanate, the sample was washed with formic acid in the final step to remove the barium carbonate. This was confirmed by the complete disappearance of barium carbonate peaks in the XRD spectrum (Fig. 7). The effects of reaction time was elucidated by using various time in the range of 3 –24 h, while the other process parameters remained the same (TiO2 particles prepared with HClO4 at the H/Ti ratio of 0.5, Ba/Ti ratio of 1.2, 160 jC), which show the same trends as the effect of the reaction temperature. The size and morphology of BaTiO3 particles were not influenced by reaction time, but as shown in Fig. 8, the peaks shifting to higher 2h value became more significant when reaction time and temperature increased. This occurrence was probably due to the release of the hydroxyl group and barium vacancy in the structure at the higher kinetics. As reviewed by Hennings and Schreinemacher [18], the enlarge-
Fig. 6. TEM micrographs of the BaTiO3 particles prepared with HClO4 at different temperatures: (A) 120 jC; (B) 160 jC.
Fig. 8. XRD patterns of the BaTiO3 particles prepared with HClO4 at various reaction times.
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ment of the barium titanate lattice was due to the presence of hydroxyl group in the lattice, and concluded that these lattice defects significantly influenced the Coulomb attractive forces, which control ionic bonding strength. These lattice defects can reduce the Coulomb attractive forces between ions. As a result, the bond length can increase, and thus, the lattice can expand. This indicated that the less the hydroxyl group or barium vacancy, the stronger the Coulomb attractive forces were between ions due to the shrinkage of the lattice. Table 1 shows the lattice constants of a-axis at various reaction temperature and time. The crystal structure of the BaTiO3 particles was assigned as cubic and the lattice constant of a-axis was shrunk slightly; it should be due to the release of hydroxyl group or barium vacancy at higher reaction temperature and longer reaction time. Further analysis by XRD patterns of the treated BaTiO3 samples, as shown in Fig. 8, showed the decreasing of the peak at 2h = f 24j, indicating that a higher reaction temperature and longer reaction time tend to decrease the concentration of unfavorable salt (BaCO3). The effects of Ba/Ti ratio was studied by using various Ba/Ti ratio in the range of 1.2 – 2.0, while the other process parameters remained the same (TiO2 particles prepared with HClO4 at the H/Ti ratio of 0.5, 160 jC, 24 h). The morphology and crystal structure of BaTiO3 particles were not influenced by Ba/Ti ratio. However, as shown in Fig. 9, the size of the particles decreased slightly with an increasing of Ba/Ti ratio. As reported by Wada et al. [14], the average particle size decreased with increasing Ba/Ti molar ratio in the starting materials, and above the Ba/Ti molar ratio of 20, the particle size was almost constant at about 20 nm. The results of this study showed the same trend as that of Wada et al.—the higher the Ba/Ti molar ratio was, the more defects in the structure were. But an excess of Ba+ 2 ions in the starting solution was employed (Ba/Ti = 1.2) in an attempt to remove any unreacted TiO2, and therefore drive the reaction to completion. However, the Ba/Ti molar ratio of 1.2 was used in this work to prevent the more defects in the structure and drive the reaction to completion. Particles prepared at the conditions of 160 jC, Ba/Ti ratio of 1.2 and 24 h were near-sphere with a size distribution of 20 –50 nm and had a lattice constant of 4.072 nm. To evaluate the stability of the barium titanate at high temperature, the sample prepared at Ba/Ti ratio of 1.2 was heated to 900 and 1150 jC, respectively, in a box furnace for 2 h. The barium titanate peaks were not altered below Table 1 Lattice constant (a) of BaTiO3 particles prepared at different hydrothermal conditions (nm) Time (h)
Lattice constant (a)
Temp. (jC)
Lattice constant (a)
Ba/Ti
Lattice constant (a)
3 6 12 24
4.0091 4.0072 4.0063 4.0045
80 120 160 200
4.0100 4.0081 4.0072 4.0063
1.2 1.4 1.6 1.8
4.0072 4.0072 4.0072 4.0072
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Fig. 9. SEM micrographs of the BaTiO3 particles prepared with HClO4 at the Ba/Ti ratios of (A) 1.2 and (B) 1.8.
900 jC (cubic phase) but were altered above 1150 jC (tetragonal phase). It transformed to the tetragonal phase with the tetragonality (c/a) of 1.0105 after calcination at 1150 jC for 2 h in air.
4. Conclusions An approach for the formation of ultrafine BaTiO3 particles by two hydrothermal processing steps was used in this study. Firstly, the direct thermal hydrolysis of titanium tetrachloride solution with perchloric acid was used to synthesize fine titania particles. The hydrothermal conversion of titania particles in barium hydroxide solutions was used to form barium titanate. The size and morphology of the TiO2 particles was controlled by the molar ratio of acid to titania (H/Ti) in the solutions. In the hydrothermal process, the size and morphology of the converted particles were different from those of the precursor TiO2 particles, indicating a dissolution – precipitation mechanism for the conversion of TiO2 to BaTiO3. The reaction temperature and time did not influence the size and morphology of BaTiO3 particles, but the lattice constant of a-axis was shrunk slightly and the concentration of BaCO3 impurity was decreased. The Ba/Ti molar ratio did not influence the morphology and crystal structure, but the mean particle size decreased with the increasing of Ba/Ti ratio. Particles prepared at the conditions of 160 jC, Ba/Ti ratio of 1.2 and 24 h were near-sphere with a size distribution of 20– 50 nm and had a lattice constant of 4.072 nm. These assynthesized particles were cubic phase if the calcination temperature was below 900 jC and converted to tetragonal phase with the tetragonality (c/a) of 1.0105 after calcination at 1150 jC for 2 h in air.
Acknowledgements This research was supported by the National Science Council of the Republic of China.
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