Behavior of Barium Acetate and Titanium Isopropoxide during the

1581 (Ti−OAc, bridging acetate), 1014 [ν(O−C−C) of Ti−OPI], and 1131 cm-1 .... To characterize the formation of the intermediate and fina...
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Ind. Eng. Chem. Res. 2004, 43, 728-734

Behavior of Barium Acetate and Titanium Isopropoxide during the Formation of Crystalline Barium Titanate Un-Yeon Hwang,* Hyung-Sang Park, and Kee-Kahb Koo Department of Chemical Engineering, Sogang University, Seoul 121-742, Korea

The behavior of Ba and Ti sources in the reaction mixture during the formation of crystalline barium titanate (BaTiO3) powders from titanium isopropyl alkoxide (TIP)/acetic acid/barium acetate/KOH solutions has been investigated. From Fourier transform infrared (FT-IR) analysis, it was found that two acetate groups per Ti atom participate in TIP modification and titanium oxo bridges (Ti-O-Ti) are generated by the alcohol and isopropyl acetate forming condensation reaction. The low dissociation constant of barium acetate in the strong alkaline aqueous solution is responsible for the formation of nonstoichiometric BaTiO3 particles when the molar ratio of barium acetate to TIP is equal to 1 in reaction mixtures. From FT-IR and X-ray diffraction analysis of as-prepared powders and pure barium acetate, it was found that barium oxalates, barium carbonate, and potassium carbonate intermediate phases were formed during the thermal treatment of as-prepared powders because of the decomposition of unreacted pseudo barium acetate phases coexisting in as-prepared powders. The pseudo barium acetate is needlelike and easily removed by washing. Pure crystalline BaTiO3 powders were obtained by calcination at 900 °C for 1 h. Because of the nonhomogeneity of as-prepared particles, the crystallinity of BaTiO3 particles calcined at 900 °C decreased with increasing molar [AcOH]/[TIP] ratios. 1. Introduction Barium titanate (BaTiO3) is an attractive material for industrial applications including multilayer capacitors, pyroelectric detectors, ferroelectric memory, and positive temperature coefficient sensors. Two crystalline phases of BaTiO3 are especially important for applications in the microelectronics industry. The tetragonal phase is used in a broad array of electronic devices because of its ferroelectric property, and the cubic form, although not ferroelectric, has a high dielectric constant that makes it suitable for capacitors.1 The conventional processing of ceramic powders formation such as BaTiO3 or SrTiO3 involves mixing oxides and/or carbonates using a ball mill and the calcination of the resulting mixture at an appropriate temperature. While the conventional method is suitable for large batch processing of those electroceramics, the process has inherent disadvantages including inhomogeneous mixing, impurities introduced during milling, broad particle size distribution, incomplete carbonate burnout during calcination, etc. To resolve the problems arising from the conventional ceramic techniques and to produce homogeneous, spherical, and stoichiometric BaTiO3, wet techniques such as homogeneous precipitation,2-4 an oxalate process,5,6 a sol-gel process,7 a resin intermediate method,8,9 the chemical modification of the alkoxides,10,11 and hydrothermal synthesis12-14 have been developed. However, the distinct chemical behavior of Ti and Ba atoms in reacting solution leads to some other problems such as spontaneous self-condensation between the Ti-OH groups15 and stoichiometric deviation.6 For most applications, the controlled preparation of mixed oxides is necessary to ensure reproducible properties of mixed * To whom correspondence should be addressed. Tel.: 82-2-705-8475. Fax: 82-2-3272-0331. E-mail: huy1012@ hanmail.net.

oxides. The preparation method is usually selected based upon the desired properties of the final product. Titanyl acylate [Ti(OiPr)4-x(OAc)x] and barium acetate [Ba(OAc)2] have been widely used for the formation of the alkaline-earth-metal titanates as the starting materials of Ti and Ba, respectively, because Ti(OiPr)4-x(OAc)x is water-soluble and easy to handle and Ba(OAc)2 is relatively inexpensive compared with other organic alkoxide precursors.6,10,16,17 Most investigators have placed their aim mainly on (1) the control of the particle size, particle size distribution, and morphology and (2) the increase of the composition homogeneity and the crystallinity of the final particles. Therefore, understanding the individual reaction steps of the transformation from the mixture of Ti(OiPr)4-x(OAc)x and Ba(OAc)2 to BaTiO3 particles in a strong alkaline solution is very important in order to control the physical properties of the final product. Nevertheless, there have been few studies that dealt with the reaction mechanism of the starting materials for the formation of BaTiO3 particles in solution. The main goal of this paper is to investigate the reaction mechanism of titanium isopropoxide [Ti(OiPr)4, TIP] with Ba(OAc)2 during the formation of spherical crystalline BaTiO3 using the sol-gel method. From various analyses for the pure Ba(OAc)2 solid, we report the experimental results on (1) the behavior of Ba(OAc)2 in pure water and a strong alkaline solution, (2) the possible reason for nonstoichiometric BaTiO3 powders formed in a mixture of [Ba(OAc)2]/[TIP] of ratio 1, and (3) the effect of an unreacted pseudo-Ba(OAc)2 phase on the transition mechanism of the intermediate phase during the heat treatment. 2. Experimental Section 2.1. Materials and BaTiO3 Synthesis. All of the reagents were analytical grade and were used without further purification. Ba(OAc)2 (99%, Aldrich) and TIP

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(97%, Aldrich) were used as the starting materials. A Ti(OiPr)4-x(OAc)x precursor was prepared by the mixing of TIP and glacial acetic acid (AcOH; J. T. Baker) for 1 h at 45 °C with a [AcOH]/[TIP] ratio of 3 to 8. A Ba(OAc)2 aqueous solution was prepared by dissolving Ba(OAc)2 in distilled water. For the preparation of the Ba-Ti solution, a Ba(OAc)2 aqueous solution was added into the Ti(OiPr)4-x(OAc)x precursor with agitation until the mixture became clear. The molar ratio of barium and titanium ions was controlled at 1:1. Thereafter, a concentrated solution of KOH (3 N, pH > 13) was added to the Ba-Ti solution and agitated vigorously for the formation of BaTiO3 for 3 h at 45 °C. All of those experiments were conducted under a N2 atmosphere. The BaTiO3 powders formed in the reaction mixture were filtered and dried for 24 h at 50 °C. The powders were then divided into several groups and subjected to various heat-treatment processes. 2.2. Characterization of BaTiO3 Powders. Fourier transform infrared (FT-IR) spectra for various solutions were measured by placing a droplet between two KRS5 windows and were recorded on a Bio-Rad spectrometer in the 4000-350 cm-1 frequency range. FT-IR spectra of the dried and calcined powders were obtained by the KBr pellet method. A pellet was prepared by mixing 0.01 g of the sample with 2 g of KBr powder and formed by a uniaxial press. Each sample was fired to 900 °C. The heating rate was 5 °C/min, and the samples were held at the desired temperature for 1 h. The crystallinity was examined by a powder X-ray diffractometer (XRD; Rigaku, Tokyo, Japan; scanning speed ) 1°/min, power ) 50 kV, 150 mA, 14° e 2θ e 80°) using Cu KR radiation monochromated by a graphite. Thermal gravimetric analyses (TGA; DuPont 2000) for pure Ba(OAc)2 were performed from room temperature to 800 °C in air at a heating rate of 5 °C/min using about 20 mg of samples. The shape and size of the particles were evaluated by a scanning electron microscope (SEM; Philips 535M, Eindhoven, The Netherlands).

Figure 1. FT-IR spectra of the molecular precursors: (A) TIP alkoxides; (B) 2-propanol; (c) isopropyl acetate ester; (D) acetic acid.

3. Results and Discussion

Figure 2. FT-IR spectra of AcOH/TIP mixtures with a reaction time of 1 h at 45 °C and pure TIP.

3.1. Reaction between TIP and AcOH. To aid in interpreting the FT-IR spectra for various mixtures prepared in the present study, the FT-IR spectra for the important pure constituent chemicals [TIP, isopropyl alcohol (iPrOH), isopropyl acetate ester (iPrOAcE), and AcOH] are shown in Figure 1. iPrOH is generated from AcOH/TIP mixtures by the acetate replacement of isopropyl groups bonded to titanium ions, and iPrOAcE is also generated (by the various reactions described below) as a byproduct. Figure 2 shows the FT-IR spectra for pure TIP and AcOH/TIP mixtures with the reaction time of 1 h at 45 °C. Two absorption bands at 1125.5 and 1004.9 cm-1 in pure TIP are assigned to the stretching vibrations ν(O-C-C) and ν(C-C), respectively, of isopropyl groups [OCH(CH3)2].18,19 In the spectra for the [AcOH]/[TIP] ratio of 0.5 to 3, we can see that, with an increase in the [AcOH]/[TIP] ratio, the relative intensities of two absorption bands at 1125.5 and 1004.9 cm-1 decrease and their positions shift gradually to 1131.9 and 1014.3 cm-1, respectively. Also, two absorption bands at 1741.0 and 1247.0 cm-1, which are the main absorption bands of iPrOAcE (see Figure 1), are clearly shown to be in all AcOH/TIP mixtures (Figure 2B-F). However, in the mixtures with a [AcOH]/[TIP] ratio of above 3, the positions of the absorption bands due to ν(O-C-C) and

ν(C-C) of isopropyl groups coordinated to the Ti atoms are shown to be fixed and an absorption band at 1294.2 cm-1 due to AcOH appears. The relative intensity ratios of the adsorption band at 1294.2 cm-1 (AcOH) with that at 1247.0 cm-1 (iPrOAcE) are observed to increase continuously with increasing [AcOH]/[TIP] ratio. Those results imply that two acetate groups per Ti atom participate in the TIP modification. Therefore, when [AcOH]/[TIP] is larger than 2, Ti(OiPr)2(OAc)2 molecules are expected to coexist with iPrOH, which are generated by the reaction of TIP with AcOH, and unreacted AcOH in the reaction mixture. Now let us consider further the role of the unreacted AcOH in the reaction mixture. The absorption band at 1052 cm-1, which appeared at a [AcOH]/[TIP] ratio larger than 1.5, can be assigned to a OiPr ligand bonded to titanium atoms by a bridging mode.20 The acetate ligand generated by the reaction of AcOH with metal ion has several modes of coordination, including monodentate and bidentate (chelating and bridging).21,22 Those bonding modes of the acetate group are identified to some extent by means of infrared spectroscopy because the infrared spectrum of a carboxyl group, COO-, shows the characteristic doublet absorption due to the asymmetric [νas(COO-)] and symmetric [νs(COO-)] stretching vibrations in the wavenumber ranging from 1200 to 1700 cm-1. The frequency

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separation (∆ν) between the two peaks strongly depends on the bonding mode such as the largest (∆ν ) 425 cm-1) in the monodentate ligand and the smallest (∆ν ) 130∼80 cm-1) in the chelating one. The bridging acetate groups have an intermediate value (∆ν ) 140-160 cm-1) that is slightly larger than that of the chelating mode. Therefore, the three bands at 1581, 1550, and 1440 cm-1 in Figure 2 are due to acetate ligands.23,24 The frequency separation (∆ν ) 140 cm-1) between νas(COO-) at 1581 cm-1 and νs(COO-) at 1440 cm-1 suggests that an acetate group (AcO-) is bonded to titanium as a bridging ligand. The frequency separation (∆ν < 110 cm-1) between νas(COO-) at 1550 cm-1 and νs(COO-) at 1440 cm-1 suggests that an acetate group is bonded as a chelating ligand. The intensity of an absorption band at 1600 cm-1 due to the bending vibration of a Ti-OH bond25 increased with an increase in the [AcOH]/[TIP] ratio from 1.5 to 4. In AcOH/TIP mixtures, a Ti-OH bond can be generated by the following reactions:

Ti-OiPr + AcOH f iPrOAcE + Ti-OH

(1)

Ti-OAc + iPrOH f iPrOAcE + Ti-OH

(2)

Parts G-J of Figure 2 show that the intensities for the four absorption bands at 1600 (Ti-OH), 1581 (Ti-OAc, bridging acetate), 1014 [ν(O-C-C) of Ti-OPI], and 1131 cm-1 [ν(C-C) of Ti-OPI] decrease gradually with an increase in the [AcOH]/[TIP] ratio, and an absorption band at 660 cm-1 increases as a result of the stretching vibration of the ν(Ti-O-Ti) bond. Therefore, it is expected that titanium oxo bridges (Ti-O-Ti) are generated by the following two condensation reactions:

Ti-OiPr + HO-Ti f iPrOH + Ti-O-Ti

(3)

Ti-OAc (bridging mode) + iPrO-Ti f i PrOAcE + Ti-O-Ti (4) From Figure 2, it was found that two isopropyl groups per TIP are substituted by acetate groups, bridging and chelating acetate modes coexist in Ti(OiPr)2(OAc)2 molecules, and titanium oxo bridges (Ti-O-Ti) are generated by the alcohol and isopropyl acetate forming condensation reaction. 3.2. Behavior of Ba(OAc)2 and Ti(OiPr)4-x(OAc)x in an Aqueous Solution. To investigate the behavior of Ba(OAc)2 and Ti(OiPr)4-x(OAc)x under the reaction condition, the FT-IR spectra for various mixtures prepared at 45 °C and Ba(OAc)2 solid were examined as shown in Figure 3. In the spectrum of Figure 3B compared with that of Figure 3A, the two absorption bands at 1550 and 1450 cm-1 (due to the bidentate mode of acetate bonded to Ti) and an absorption band at 1050 cm-1 (due to the isopropyl group bonded to Ti atom by bridging mode) are still observed, while the relative intensities of the absorption bands at 1131.9 [ν(O-CC) of Ti-OiPr, monodentate mode], 1014.9 [ν(C-C) of Ti-OiPr, monodentate mode], 1600 (Ti-OH), and 1580 cm-1 (Ti-OAc, bridging mode) are shown to decrease with the addition of water into the Ti(OiPr)4-x(OAc)x solution. These results imply that water removes the Ti-OiPr bonding in which the type of isopropyl groups is monodentate and the Ti-OAc in which the acetate ligand is in the bridging mode. In Figure 3F, the two absorption bands at 1550 and 1405 cm-1 (∆ν ) 145

Figure 3. FT-IR spectra of (A) a AcOH/TIP mixture with a reaction time of 1 h, (B) solution A + H2O, (C) solution A + solution E, (D) a mixture of solution E and a KOH solution of 3 N, (E) a H2O/Ba(OAc)2 mixture, (F) Ba(OAc)2. Mole numbers of TIP, AcOH, Ba(OAc)2, and water in all of solutions are 0.1, 0.4, 0.1, and 5.56, respectively.

cm-1) can be attributed to the bridging mode of acetate bonded to a barium atom. Figure 3E shows the FT-IR spectrum for the solution of Ba(OAc)2 in deionized water. Along with absorption bands due to the bridging acetate mode, a new absorption band at around 1650 cm-1, which is not observed in Figure 3F, can be attributed to the monodentate acetate mode.22,26 These results indicate that some of the bidentate acetate groups bonded to barium atoms in an aqueous solution are converted to the monodentate mode. Figure 3D shows the FT-IR spectrum for the mixture of a Ba(OAc)2 aqueous solution with a KOH solution measured to investigate the behavior of Ba(OAc)2 in a strong alkaline solution (pH > 13), which is used for the synthesis of BaTiO3. In this figure, two absorption bands at around 1550 and 1405 cm-1 due to the bridging acetate mode bonded to barium atoms are observed, and the intensity of an absorption band at around 1650 cm-1 due to the monodentate acetate band is increased compared to Figure 3E. From those results, it is expected that Ba-OAc bonds are not readily removed even in a strong alkaline solution. Figure 3C shows the FT-IR spectrum for the mixture between solutions A and E with a reaction time of 1 h at 45 °C. When Figure 3C is compared with Figure 3E, it is seen that solution C has a more intense band than that of a Ba(OAc)2 aqueous solution for the monodentate COO- stretching mode bonded to barium at around 1650 cm-1. This result could be attributed to the increase of the solubility of Ba(OAc)2. Tredway and Risbug27 reported that the solubility of Ba(OAc)2 in water increases with a decrease in the pH by addition of acetic acid. Therefore, because of the unreacted AcOH with TIP in solution, the pH of a mixture of solution A with solution E is lower than that of solution E and the solubility of Ba(OAc)2 in solution C will be higher than that in solution E. Therefore, bidentate acetate groups of Ba(OAc)2 are expected to be converted to monodentate groups. 3.3. Characterization of As-Prepared BaTiO3 Powders. Figure 4 is the FT-IR spectra for BaTiO3 powders as-prepared and washed twice with deionized water. Similar absorption bands at around 1650 and

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Figure 4. FT-IR spectra of as-prepared BaTiO3 powders and twice-washed BaTiO3 powders.

Figure 5. XRD patterns of the as-prepared BaTiO3 powders, the twice-washed BaTiO3 powders, the pure Ba(OAc)2, and the PDF card for Ba(OAc)2 (PDF 26-0131).

1551 cm-1, observed in the spectrum of Figure 3D, appear in the spectrum of Figure 4A but not in the spectrum of Figure 4B. This indicates clearly that acetate groups bonded to barium atoms by the monodentate and bidentate modes still exist in the asprepared BaTiO3 powders. The XRD patterns of the as-prepared BaTiO3 powders, the washed BaTiO3 powders, the pure Ba(OAc)2, and the PDF card corresponding to Ba(OAc)2 (PDF 260131) are shown in Figure 5. From Figure 5A-C, we observe a crystalline structure similar to Ba(OAc)2 in as-prepared BaTiO3. However, the XRD pattern of the washed powder (Figure 5D) shows the cubic phase BaTiO3 with a trace amount of BaCO3. It is concluded that the crystalline BaTiO3 can be synthesized in a solution reaction at 45 °C and the pseudo-Ba(OAc)2 phase, which is removed easily by washing, remains in the as-prepared powders. In general, the utilization of Ba salts in the sol-gel process is limited because of the low solubility of most Ba compounds in water and the insolubility of all Ba salts in alcohol and other organic solvents. Nevertheless, Ba(OAc)2 with a high water solubility of 720 g/L28 has been employed successfully in sol-gel processing of BaTiO3 ceramics and glasses.29-32 However, it is well-

Figure 6. FT-IR spectra of the as-prepared BaTiO3 powders and calcined BaTiO3 powders.

known that the production of stoichiometric BaTiO3 particles by the sol-gel process is not easy even if Ba(OAc)2 is used as a starting material, and thus an excess of Ba(OAc)2 is often used to obtain the stoichiometric BaTiO3 powders.33,34 Although formation of nonstoichiometric BaTiO3 powders ([Ba2+]/[Ti4+] < 1) has been reported to be caused by the incomplete mixing and leaching of Ba2+ ions from the surface near layers during the washing and the wet milling of calcined BaTiO3 powders,35,36 no detailed chemistry has yet been reported. From Figures 3D, 4A, and 5C, we observe a crystalline structure similar to that of Ba(OAc)2 even in the strong alkaline condition (pH > 13). This indicates that a Ba(OAc)2 solid is not perfectly dissociated into Ba2+ and CH3COO- ions. From these results, it is concluded that the low dissociation of Ba(OAc)2 in the strong alkaline aqueous solution may be the major reason of the low reactivity of barium atoms and may lead to the formation of nonstoichiometric BaTiO3 particles even when the molar ratio of Ba(OAc)2 to TIP is unity in reaction mixtures. 3.4. Characterization of Calcined BaTiO3 Powders. To characterize the formation of the intermediate and final product phases, as-prepared powders were calcined up to 900 °C. Figure 6 shows the FT-IR spectra for as-prepared and calcined powders over temperatures from 200 to 900 °C. The FT-IR spectrum of as-prepared powders shows absorption bands at 1375 cm-1 due to δ(CH3) and at 1655 and 1345 cm-1 due to the stretching vibrations νas(COO-) and νs(COO-), respectively, of an acetate group coordinated to the barium atoms by the monodentate mode. An absorption band at 1551 cm-1 is due to the bidentate acetate mode bonded to barium. The intensity of those bands is shown to decrease as the calcination temperature increases to 300 °C. Three new absorption bands, at 1605, 1311, and 773 cm-1, appear in the calcined sample at 200 °C and disappear at temperatures above 400 °C. The bands at 1610 and 1310 cm-1 are due to the stretching vibrations νas(COO-) and νs(COO-), respectively, of a carboxylic acidic salt, and the FT-IR spectrum of barium oxalate (Ba(C2O4)) has been reported to have bands at 1603, 1314, and 775 cm-1.37 Therefore, it is clear that Ba(C2O4) is generated at temperatures lower than 400 °C by thermal decomposition of unreacted pseudo Ba(OAc)2 [having the Ba(OOC2H3) bond], leading to the evolution of methyl groups (CH3). At temperatures higher than 300 °C, the

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Figure 7. TGA curve of pure Ba(OAc)2 (heating rate ) 5 °C/min). Figure 9. XRD patterns of the BaTiO3 powders calcined with different temperatures and the PDF card for K2CO3 (PDF 160820).

Figure 8. XRD patterns of the pure and calcined Ba(OAc)2.

intensities of bands at 1605, 1311, and 773 cm-1 gradually decrease, and three new intense bands at 1470, 890, and 710 cm-1 due to potassium carbonate (K2CO3)38 and two bands at 843 and 690 cm-1 due to BaCO337 are observed up to 800 °C. Those results indicate that barium carbonate (BaCO3) is generated by Ba(C2O4) decomposition, leading to evolution of CO, and subsequently K2CO3 is generated by the reaction of potassium cations in particles with CO32-. CO32- is assumed to be produced by the reaction of O2 in air with CO and/or CO2 evolved by the decomposition of Ba(C2O4) and/or BaCO3. To confirm the above-mentioned decomposition process of pseudo Ba(OAc)2 remaining in as-prepared powders, the thermal decomposition behavior of pure Ba(OAc)2 was investigated. TGA for pure Ba(OAc)2 solid at a heating rate of 5 °C/min shows the three-step decomposition (∼300, 330-430, and ∼430 °C) as can be seen from Figure 7. To confirm the crystal structure at each step, XRD patterns of pure Ba(OAc)2 and of samples calcined at 300, 400, and 500 °C were obtained as given in Figure 8. The XRD pattern of the sample fired at 300 °C shows that intensities of the Ba(OAc)2 peaks were decreased considerably and that Ba(C2O4) was formed by the evolution of methyl groups (CH3). The XRD pattern at 400 °C shows that Ba(C2O4) disappeared and crystalline BaCO3 was formed with a trace of BaO, and the pattern at 500 °C shows that the crystallinity of BaO was considerably increased. From

Figure 10. XRD patterns of the BaTiO3 powders synthesized with a [AcOH]/[TIP] ratio and calcined at 800 °C.

these results, we propose the following decomposition mechanism for the pure Ba(OAc)2: vCH3

vCO

vCO2

Ba(OOCCH3)2 98 Ba(OOC)2 98 BaCO3 98 BaO (5) Those results support the variation of FT-IR spectra for the calcined BaTiO3 powder due to the thermal decomposition of pseudo Ba(OAc)2 as described above. In Figure 6, the bands due to K2CO3 and BaCO3 are shown to have disappeared, and only the FT-IR spectrum corresponding to pure BaTiO3 is obtained at 900 °C. Figure 9 shows the XRD patterns of the calcined BaTiO3 powders with different temperatures. The XRD peaks due to the pseudo-Ba(OAc)2 phase, discussed previously with Figure 5D, are observed in the sample calcined at 200 °C, but most of them are absent in the sample calcined at 300 °C. At calcination temperatures from 400 to 700 °C, the XRD patterns of the samples show that crystalline BaTiO3 and K2CO3 coexist. Finally, well-crystallized cubic BaTiO3 powders without any second phase are obtained at temperatures above 800 °C, as seen in Figure 9. XRD patterns for samples synthesized with a [AcOH]/ [TIP] ratio and calcined at 800 °C are shown in Figure

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Figure 11. SEM micrographs of (A) as-prepared powders synthesized with the [AcOH]/[TIP] ratio of 4 at 45 °C for 3 h and (B) the twice-washed powders.

10. Obviously, the larger the [AcOH]/[TIP] ratio, the lower the intensity of the XRD peaks. The level of modification of the TIP by acetate groups is quite important because the reaction of water with the alkoxy groups is much faster than that with acetate groups. For [AcOH]/[TIP] ratios larger than 3, unreacted AcOH exists in the reaction mixture as discussed with Figure 2. It is expected that unreacted AcOH can participate in the reaction given by eq 1 and the direct esterification reaction with iPrOH generated by the chemical modification reaction between TIP and AcOH as given by eq 6, leading to the formation of a Ti-OH bond and the generation of water, respectively. Water produced by i

PrOH + AcOH f iPrOAcE + H2O

(6)

reaction (6) would be able to hydrolyze Ti-bound isopropyl groups, leaving terminal hydroxyl groups, which can condense and form oxo bridging (Ti-O-Ti). The amount of the Ti-OH bond and H2O generated by reactions (1) and (6), respectively, increases with an increase in the molar ratio of [AcOH]/[TIP]. Subsequently, much Ti-O-Ti oxo-bridging bonding, according to reaction (3), is created before the addition of an aqueous Ba(OAc)2 solution. Therefore, the homogeneity of the Ba and Ti components, i.e., the degree of BaO-Ti bond formation in the precursor system, tends to be low with an increase in the molar ratio of [AcOH]/ [TIP]. Hence, the low crystallinity of specimens synthesized at high [AcOH]/[TIP] ratios is explained by the inhomogeneous molecular-scale mixing of the two reactants. Figure 11 shows the SEM micrographs of as-prepared and washed powders synthesized with a [AcOH]/[TIP] ratio of 4 for 3 h at 45 °C. The needlelike particles coexist with spherical particles in the as-prepared powders, while near-spherical fine BaTiO3 particles with a size of 100 nm are shown to be obtained by washing. Therefore, the needlelike powders in Figure 11 are the pseudo-Ba(OAc)2 phase that is washable. This result also clearly substantiates the earlier discussion that an undesirable pseudo-Ba(OAc)2 phase is obtained along with the crystalline BaTiO3 in as-prepared powders. 4. Conclusions The reaction mechanisms of Ba(OAc)2 and TIP during the formation of BaTiO3 powders were investigated with

respect to (1) the effect of the molar ratio of [AcOH]/ [TIP] on the chemical modification of TIP and the crystallinity of the final BaTiO3 powders, (2) the reaction of Ti(OiPr)4-x(OAc)x with water, (3) the behavior of Ba(OAc)2 in water and strong alkaline solution, and (4) the characterization of the conversion process of the asprepared powders into pure crystalline BaTiO3 by calcination. The modification of TIP by acetate was found to saturate at about two acetate groups per titanium atom and the titanium oxo bridge (Ti-O-Ti) to be generally formed by reactions (3) and (4) in the text. Unreacted AcOH in the reaction mixtures with a [AcOH]/[TIP] ratio larger than 3 was found to participate in reactions (1) and (6) and promote the formation of a Ti-O-Ti bond. As the [AcOH]/[TIP] ratio increased, the amount of the Ti-O-Ti oxo-bridging bond increased, but the crystallinity of the final BaTiO3 powders became lower. The Ti-OiPr (monodentate mode of isopropyl groups) and Ti-OAc (bridging mode of acetate) bonding in Ti(OiPr)4-x(OAc)x was found to be removed by the addition of water. Some Ba-OAc bonds remained in aqueous and strong alkaline solutions, and the low reactivity of Ba(OAc)2 in a strong alkaline solution may be the main reason for the formation of nonstoichiometric BaTiO3 powders. Crystalline BaTiO3 powders and pseudo-Ba(OAc)2 phases were found to coexist in as-prepared powders. Ba(C2O4), BaCO3, and K2CO3 intermediate phases formed during the thermal treatment of as-prepared powders as a result of decomposition of the pseudo-Ba(OAc)2 phases. Finally, pure crystalline BaTiO3 powders were obtained by thermal treatment at 900 °C for 1 h with a heating rate of 5 °C/min. Acknowledgment This work was supported by Sogang University Research Grant 2003. Literature Cited (1) Slamovich, E. B.; Aksay I. A. Structure Evolution in Hydrothermally Processed BaTiO3 Films. J. Am. Ceram. Soc. 1996, 79, 239. (2) Her, Y. S.; Matijevic, E.; Chon, M. C. Controlled DoubleJet Precipitation of Uniform Colloidal Cryatalline Sr- and Zr-doped Barium Titanates. J. Mater. Res. 1996, 11, 3121.

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Received for review April 3, 2003 Revised manuscript received October 20, 2003 Accepted November 5, 2003 IE030276Q