Kinetics and Mechanism of Synthesis of BaTIO3

Kinetics and Mechanism of Aqueous Chemical Synthesis of BaTiO3 Particles Andrea Testino,† Maria Teresa Buscaglia,‡ Vincenzo Buscaglia,*,‡ Massimo Viviani,‡ Carlo Bottino,‡ and Paolo Nanni† Institute for Energetics and Interphases, Department of Genoa, National Research Council, I-16149 Genoa, Italy, and Department of Process and Chemical Engineering, University of Genoa, I-16129 Genoa, Italy Received September 17, 2003. Revised Manuscript Received November 24, 2003 A systematic kinetic investigation on the chemical synthesis of BaTiO3 particles from aqueous solutions of BaCl2 and TiCl4 at T < 100 °C and at pH 14 has been performed. Initially, a viscous suspension of a Ti-rich gel phase is obtained at room temperature. Later, formation of BaTiO3 is induced by heating above 70 °C and the gel phase is gradually converted to the crystalline perovskite. The isothermal formation kinetics of BaTiO3 and the evolution of crystal size and particle size during the course of reaction are significantly influenced by temperature, concentration, and barium-to-titanium ratio of the solution. The early stages of reaction (yield < 1%) are dominated by primary nucleation, and slow formation of single nanocrystals of BaTiO3 was observed by HRTEM. At a later stage, formation of polycrystalline particles occurs by secondary nucleation of BaTiO3 on the surface of already existing crystals. During this stage, the reaction rate increases by 1 order of magnitude. When the yield exceeds 50%, nucleation becomes less important and the reaction is dominated by growth. Final particles have a diameter in the range 0.3-1.6 ím, depending on the processing parameters. 1. Introduction Barium titanate, BaTiO3, is a ferroelectric ceramic massively used in electric and electronic applications. As a dielectric, it is utilized in multilayer ceramic capacitors (MLCCs) and for embedded capacitance in printed circuit boards. Recent developments in micro- electronics and communication have led to the minia- turization of MLCCs. To achieve this goal and to make the next step forward, powders with improved quality and small and uniform size (<300 nm) are required.1 Many chemical methods have been proposed for the synthesis of high-quality BaTiO3 powders.2 Among them, the hydrothermal route has some advantages because it leads directly to fine, pure, crystalline powders at relatively low temperatures without the need of a further thermal treatment. The hydrothermal process is generally carried out by suspending TiO2 particles or a TiO2 gel in an aqueous Ba(OH)2 solution and then autoclaving at 150-300 °C. Thermodynamic modeling of the Ti-Ba-H2O-CO2 system3 has been used to assess the conditions corresponding to the quantitative formation of BaTiO3. In the absence of CO2 and at pH >12, BaTiO3 is the stable phase over a wide range of barium concentrations. The calculated stability diagrams have also shown that quantitative formation of BaTiO3 is possible even at temperatures <100 °C. Therefore, synthesis of BaTiO3 using more conven- tional chemical precipitation processes has also been explored.4-8 The size distribution of the particles that grow from solution depends, in general, on the rates of nuclei formation and crystallite growth. Crystal agglomeration can also contribute substantially the overall particle growth process.9-10 The relative importance of nucle- ation and growth is determined by the supersaturation of the solution. In turn, supersaturation is very sensitive to temperature, concentration, and mixing conditions. Despite the large potential interest, there is scanty information available on the influence of the above parameters on the formation kinetics of BaTiO3 par- ticles.11-14 The evolution of particle morphology during hydrothermal synthesis was reported by several * To whom correspondence should be addressed. E-mail: v.buscaglia@ge.ieni.cnr.it. † University of Genoa. ‡ National Research Council. (1) Rae, A.; Chu, M.; Ganine, V. In Ceramic Transactions Dielectric Ceramic Materials; Nair, K. M., Bhalla, A. S., Eds.; The American Ceramic Society: Westerville, OH, 1999; Vol. 100, p 1. (b) Venigalla, S. Am. Ceram. Soc. Bull. 2001, 6, 63. (2) Nanni, P.; Viviani, M.; Buscaglia, V. In Handbook of Low and High Dielectric Constant Materials and Their Applications; Nalwa, H. S., Ed.; Academic Press: San Diego, CA, 1999; Vol. 1, p 429. (3) Lencka, M. M.; Riman, R. E. Chem. Mater. 1993, 5, 61. (b) Lencka, M. M.; Riman, R. E. Ferroelectrics 1994, 151, 159. (4) Kiss, K.; Magder, J.; Vukasovich, M. S.; Lockhart, R. J. J. Am. Ceram. Soc. 1966, 49, 291. (5) Her, Y.-S.; Matijevic, E.; Chon, M. C. J. Mater. Res. 1995, 10, 3106. (b) Her, Y.-S.; Lee, S.-H.; Matijevic, E. J. Mater. Res. 1996, 11, 156. (6) Leoni, M.; Viviani, M.; Nanni, P.; Buscaglia, V. J. Mater. Sci. Lett. 1996, 15, 1302. (7) Kumar, V. J. Am. Ceram. Soc. 1999, 82, 2580. (8) Grohe, B.; Miehe, G.; Wegner, G. J. Mater. Res. 2001, 16, 1901. (9) Dirksen, J. A.; Ring, T. A. Chem. Eng. Sci. 1991, 46, 2389. (10) Zukoski, C. F.; Rosenbaum, D. F.; Zamora, P. C. Chem. Eng. Res. Des. 1996, 74A, 723. (b) Van Hyning, D. L.; Klemperer, W. G.; Zukoski, C. F. Lamgmuir 2001, 17, 3128. (11) Hertl, W. J. Am. Ceram. Soc. 1988, 71, 879. 1536 Chem. Mater. 2004, 16, 1536-1543 10.1021/cm031130k CCC: $27.50 © 2004 American Chemical Society Published on Web 03/23/2004 KICET 高亮 KICET 高亮 groups,13-24 but the observations were mainly qualita- tive. Analysis of the above-cited literature shows that many fundamental aspects of the formation of BaTiO3 particles from solution are not yet well defined. In general, the formation rate of BaTiO3 increases and the particle size decreases with increasing barium concen- tration, but these dependencies were never studied on a systematic basis. The effect of temperature is uncer- tain. The particles are often polycrystalline, but it is unclear whether this is a consequence of secondary nucleation or originates from colloidal aggregation of smaller crystals. In any case, heterogeneous nucleation is reported to be an important factor at the early stages of reaction. Tendency to formation of dendritic particles was also observed when the barium concentration is decreased. The study described here was aimed to gain a better understanding of the formation of barium titanate particles from dilute (e0.1 M) aqueous metal chloride solutions at temperatures <100 °C. Synthesis of BaTiO3 particles was carried out according to the overall reac- tion where (aq) denotes a salt in an aqueous solution. According to our observations, reaction 1 is rather fast at T g 80 °C and cation concentration >0.1 mol dm-3, and formation of a white BaTiO3 precipitate can be readily observed. However, at lower concentration, the reaction proceeds in two steps: (i) initial, rapid forma- tion of an amorphous Ti-rich gel phase and (ii) slower reaction between the gel phase and the Ba2+ ions left in solution with formation of crystalline BaTiO3 par- ticles. While step (i) is very fast at any temperature, step (ii) is hindered below �70 °C. The isothermal crystallization kinetics and the evolution of morphology, crystallite size, and particle size were determined as a function of concentration at two different temperatures. The influence of the barium-to-titanium ratio was investigated for a given combination of temperature and Ti concentration. 2. Experimental Section 2.1. Synthesis of BaTiO3. To obtain reproducible results corresponding to a well-defined condition, precipitation experi- ments were carried out by premixing the reactant solutions at room temperature, where formation of BaTiO3 is kinetically inhibited, and then heating the resulting gel suspension to the reaction temperature. Several precipitation experiments (see Table 1) were performed in a 500 mL vessel using �250 mL of an aqueous solution of TiCl4 (Acros, 99.9%) and BaCl2Æ2H2O (Aldrich, 99.9%). The chloride solution was quickly mixed (20 s) with the same volume of a NaOH solution with immediate formation of a highly viscous, gelatinous suspension of a Ti- rich gel phase. The concentration of the NaOH solution was that required to have [OH-] ) 1 mol dm-3 after quantitative precipitation of BaTiO3, according to reaction 1. The experi- ments were conducted at two different temperatures, 82 and 92 °C, and the barium concentration in the reactor (i.e. the concentration referred to the total volume after mixing of the reactant solutions) was varied from 0.02 to 0.12 mol dm-3. Unless otherwise stated, the [Ba]/[Ti] molar ratio, R, in the solution was 1.11. The vessel was then closed, heated, and kept at constant temperature with stirring. The closed environment avoids formation of BaCO3. The temperature inside the reactor was measured by means of a Pt100 sensor. Progressive formation of crystalline BaTiO3 occurs during aging. The transformation was visually indicated by the transition from a translucent viscous medium to a white and opaque suspen- sion. The head of the reactor was equipped with 12 plastic (PEEK) tubes and valves. Each tube was connected to a (12) Vivekanandan, R.; Philip, S.; Kutty, T. R. N. Mater. Res. Bull. 1986, 22, 99. (b) Kutty, T. R. N.; Padmini, P. Mater. Chem. Phys. 1995, 39, 200. (13) Eckert, J. O., Jr.; Hung-Houston, C. C.; Gersten, B. L.; Lencka, M. M.; Riman, R. E. J. Am. Ceram. Soc. 1996, 79, 2929. (14) Moon, J.; Suvaci, E.; Morrone, A.; Costantino, S. A.; Adair, J. H. J. Eur. Ceram. Soc. 2003, 23, 2153. (15) Hennings, D.; Rosenstein, G.; Schreinemacher, H. J. Eur. Ceram. Soc. 1991, 8, 107. (16) Dutta, P. K.; Gregg, J. R. Chem. Mater. 1992, 4, 843. (b) Dutta, P. K.; Asiaie, R.; Akbar, S. A.; Zhu, W. Chem. Mater. 1994, 6, 1542. (17) Kumazawa, H.; Annen, S.; Sada, E. J. Mater. Sci. 1995, 30, 4740. (18) Chien, A. T.; Speck, J. S.; Lange, F. F.; Daykin, A. C.; Levi, C. G. J. Mater. Res. 1995, 10, 1784. (19) Slamovich, E. B.; Aksay, I. A. J. Am. Ceram. Soc. 1996, 79, 239. (20) Zhao, L.; Chien, A. T.; Lange, F. F.; Speck, J. S. J. Mater. Res. 1996, 11, 1325. (21) Choi, J. Y.; Kim, C. H.; Kim, D. K. J. Am. Ceram. Soc. 1998, 81, 1353. (22) Bagwell, R. B.; Sindel, J.; Sigmund, W. J. Mater. Res. 1999, 14, 1844. (23) Hu, M. Z.-C.; Kurian, V.; Payzant, E. A.; Rawn, C. J.; Hunt, R. D. Powd. Technol. 2000, 110, 2. (24) MacLaren, I.; Ponton, C. B. J. Eur. Ceram. Soc. 2000, 20, 1267. Table 1. Summary of Precipitation Experiments and Properties of Final Powdersa expt [Ba] (mol dm-3) [Ba]/[Ti] gel phase duration (min) final yield (%) t1/2b (min) final d50 (ím) (d90 - d10)/d50 final dBET (ím) final dXRD (ím) 82 °C 1 0.035 0.63 1320 92 185 2.2c 1.0 >0.3 2 0.044 0.63 150 100 47.5 0.86 0.75 0.29 0.26 3 0.052 0.66 66 100 13.4 0.55 0.51 0.25 0.22 4 0.070 0.64 48 100 2.6 0.36 0.49 0.20 0.22 5 0.121 30 100 <1 0.28 0.56 0.16 0.10 6 0.045 0.63 37 100 8.5 0.51 0.47 0.24 0.21 7 0.047 0.64 41 100 4.5 0.49 0.49 0.25 0.27 92 °C 8 0.023 0.57 270 91 62 1.57 0.96 >0.3 9 0.029 0.56 65 93 6.9 0.77 0.61 0.22 0.14 10 0.035 0.53 34 100 6.1 0.57 0.58 0.21 0.21 11 0.0405 0.53 14.5 100 3.8 0.47 0.69 0.19 0.12 12 0.059 30 100 <1 0.36 0.46 0.18 0.10 a For expts 1-5 and 8-12, R ) 1.11. For expt 6, R ) 1.155. For expt 7, R ) 1.2. R is the [Ba]/[Ti] ratio in the reactor. b Half-transformation time for formation of BaTiO3. c d50 is 1.38 ím after 270 min. BaCl2(aq) + TiCl4(aq) + 6NaOH(aq) f BaTiO3(s) + 6NaCl(aq) + 3H2O(l) (1) Aqueous Chemical Synthesis of BaTiO3 Particles Chem. Mater., Vol. 16, No. 8, 2004 1537 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 EricHexin Highlight syringe on one side and immersed in the suspension on the other side. At given times (time zero corresponds to the instant the temperature reaches the reaction temperature) 20 mL of suspension was collected, cooled, and sealed in small bottles. Each syringe was used only once. The aliquot collected at time zero was used to determine the Ba/Ti ratio in the gel phase. After quantitative precipitation of BaTiO3 (or when conversion attains a constant value), the suspension remaining in the reactor was centrifuged, and the powder was washed and finally freeze-dried. 2.2. Particle Characterization and Measurement of Particle Size Distribution. A 5-10 mL portion of suspension was washed and filtered on a filter paper. The X-ray diffraction (XRD) pattern of the precipitate was collected directly from the surface of the filter after drying with a Philips PW1710 diffractometer (Co KR radiation). The powder patterns, includ- ing (110) and (111) reflections, were recorded in the range 35- 50° 2ı using a 2ı step of 0.03° and a sampling time (for each step) up to 60 s for samples containing a small amount of crystalline phase. The crystallite size (dXRD) of BaTiO3 was estimated from the broadening of the (111) XRD peak by means of the Scherrer equation, after correction for instru- mental broadening, assuming negligible microstrain broaden- ing. The (hhh) reflections are not subjected to splitting during the cubic to tetragonal distortion of the BaTiO3 unit cell. The drawback of using the (111) reflection is its low relative intensity, only 30%. Therefore, for yield values <10%, mean- ingful crystal size measurements could not be carried out because the signal-to-noise ratio was too low. Since the broadening effect for particles >150 nm is rather small, size measurements of the coarser crystallites may be affected by a large uncertainty. Above 300 nm, broadening is negligible and size measurements are no longer possible. Because of the limitations described above, the crystallite size data should be considered as semiquantitative values. The number particle size distribution (PSD) was obtained from the measurement of the diameter of �1000 particles by a Philips 515 scanning electron microscope (SEM). For this purpose, 5 mL of suspension was centrifuged, and the pre- cipitate was washed and finally treated with a 5% HNO3 solution to dissolve the gel phase, leaving the BaTiO3 particles. SEM observation before and after the treatment has shown no appreciable variation of particle morphology. Three param- eters, d10, d50, and d90, were obtained from the PSD. In general, dp is the diameter corresponding to the percentage p of particles in the cumulative particle size distribution. The average particle size was defined as the number median diameter (d50) of the PSD. The powder obtained after completion of reaction was also characterized. The density, F, was measured by helium pic- nometry (model Accupyc 1330, Micromeritics, Norcross, GA). The specific surface area, SBET, was determined by nitrogen physisorption (BET method, model ASAP 2010, Micromeritics, Norcross, GA). The equivalent BET diameter, dBET, was calculated with the formula dBET ) 6/FSBET. 2.3. Determination of the Reaction Kinetics. Method 1. A 10 mL portion of suspension was centrifuged and the barium concentration in the supernatant was determined by gravimetric titration as BaSO4. Use of conductometric titration gave equivalent results. The progress of reaction was calcu- lated by the equation where [Ba] is the barium concentration in the supernatant, [Ba]T the total barium concentration, and Rg the Ba/Ti ratio in the gel phase (measured on the suspension withdrawn at time zero), assumed to be constant during the course of reaction. This hypothesis was verified by determining the precipitation kinetics at a barium concentration of 0.035 mol dm-3 by a second independent method. Method 2. The suspension was centrifuged, the precipitate washed, and finally the gel phase was removed by the same treatment adopted for PSD measurement. The BaTiO3 pre- cipitate was finally dissolved using a 3:1 mixture of concen- trated HCl and HNO3, and the amount of barium was determined by gravimetric titration. Although SEM observa- tions showed that the treatment for removal of the gel phase has not an appreciable influence on particle morphology, leaching of barium ions from the surface of BaTiO3 could not be completely avoided and a small correction of the raw kinetic data was required for method 2. This correction was deter- mined from the amount of barium dissolved from a reference BaTiO3 powder prepared with the same methodology and subjected to the same treatment. As shown in Figure 1, the kinetic data obtained by the two methods result to be nearly coincident. However, method 1 was preferred because it is faster and more reliable. 2.4. TEM Observation. The growth of BaTiO3 nanocrystals during the early reaction stages (yield < 2 mol %) could be observed by conventional and high-resolution transmission electron microscopy (TEM and HRTEM) with a JEOL J2010 microscope operated at 200 kV. A precipitation experiment was yield ) R([Ba] - [Ba]T) + Rg[Ba]T (Rg - 1)[Ba]T Figure 1. Formation kinetics of crystalline BaTiO3 from aqueous solutions of BaCl2 and TiCl4. The legend of (a) and (b) indicates the barium concentration. (a) At 82 °C and [Ba]/ [Ti] ) 1.11. (b) At 92 °C and [Ba]/[Ti] ) 1.11. (c) At 82 °C and [Ti] ) 0.039 mol dm-3 for different values of [Ba]/[Ti]. Open symbols represent kinetic data obtained by titration of super- natant. Full symbols represent kinetic data obtained by titration of the BaTiO3 precipitate after removal of the Ti-rich gel phase. 1538 Chem. Mater., Vol. 16, No. 8, 2004 Testino et al. KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 KICET 高亮 performed at 82 °C at a barium concentration of 0.044 mol dm-3. Aliquots of the suspension were collected at different times. The samples were transferred in small bottles and sealed. Immediately before observation, a drop of the suspen- sion was deposited onto a carbon-coated copper grid and dried. Identification of BaTiO3 crystals was carried out using an energy-dispersive electron microprobe and electron diffraction (ED). 3. Results 3.1. Reaction Kinetics. The synthesis conditions are given in Table 1. The Ba/Ti molar ratio in the gel phase at the beginning of reaction (time zero) is practically independent of concentration and equal to 0.64 at 82 °C and 0.55 at 92 °C. The formation kinetics of BaTiO3 is shown in Figure 1. All the curves have a sigmoidal shape and the reaction rate is strongly dependent on concentration. At 82 °C (Figure 1a), as the barium concentration is increased by a factor 2 (from 0.035 to 0.07 mol dm-3), the half-transformation time, t1/2, decreases by a factor 70 (from 185 to 2.6 min). At 92 °C (Figure 1b), the reaction is �20 times faster at [Ba] ) 0.041 mol dm-3 than at [Ba] ) 0.023 mol dm-3. The effect of temperature can be illustrated by comparing the two experiments (1 and 10) performed at a barium concentration of 0.035 mol dm-3. As the temperature is lowered from 92 to 82 °C, t1/2 increases by a factor 30 (from 6 to 185 min). The strong slowing down of the crystallization kinetics induced by a 10 °C decrease of temperature explains why formation of BaTiO3 is not observed near room temperature. At low concentration, the precipitation of BaTiO3 is not quantitative, even after times much longer than the half-transformation time (see Table 1, expts 1, 8, 9). At the highest concentration for both temperatures (expts 5 and 12), formation of BaTiO3 is rather fast and the reaction has already started before the temperature attained the preset value. The corresponding kinetic data were thus discarded. The influence of the [Ba]/[Ti] ratio on the crystallization of BaTiO3 at 82 °C is shown in Figure 1c. The experiments were conducted at a constant Ti concentration of 0.039 mol dm-3. It can be observed that upon increasing R from 1.11 to 1.2, t1/2 decreases from 47.5 to 4.5 min. This is a significant effect considering that the barium concentration only changes by 7%. 3.2. Characterization of Final Particles: Mor- phology, PSD, and Crystallite Size. Mean particle size, size distribution, crystallite size, and equivalent BET diameter of