Effect of titanium dioxide solubility on the formation of BaTiO3 nanoparticles in supercritical water

Fluid Phase Equilibria 257 (2007) 233–237

Effect of titanium dioxide solubility on the formation of BaTiO3 nanoparticles in supercritical water Mehrnoosh Atashfaraz a,b , Mojtaba Shariaty-Niassar b , Satoshi Ohara a , Kimitaka Minami a , Mitsuo Umetsu a , Takashi Naka a , Tadafumi Adschiri a,∗ a

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan b Department of Chemical Engineering, Faculty of Engineering, University of Tehran, 16 Azar Street, Enghelab Avenue, Tehran, Iran Received 24 July 2006; received in revised form 9 March 2007; accepted 24 March 2007 Available online 30 March 2007

Abstract Fine BaTiO3 nanoparticles were prepared by hydrothermal synthesis under supercritical condition (400 ◦ C and 30 MPa) from mixture of barium hydroxide and titanium dioxide as starting precursors. First, conditions for synthesizing BaTiO3 were examined by using batch reactors. High pH condition, pH > 13, is necessary to obtain phase pure BaTiO3 . The reason was discussed based on the solubility of titanium dioxide, which that dissolution–recrystallization process is essential for the synthesis of BaTiO3 nanoparticles. Rapid heating of the starting precursors by mixing with high temperature water in a flow reactor is effective to synthesize smaller size and narrower particle size distribution for the BaTiO3 nanoparticles, compared with the case of slow heating with a batch reactor. © 2007 Elsevier B.V. All rights reserved. Keywords: Supercritical water; BaTiO3 nanoparticles; Solubility; Formation mechanism

1. Introduction Barium titanate is used for a variety of applications, including microwave devices, dynamic random access memories (DRAMs), and multilayer capacitors [1,2] due to its outstanding dielectric and ferroelectric properties [3,4]. In case of capacitors, for example, a high dielectric constant material allows more charge storage per unit area, which gives rise to fabricate smaller devices. Furthermore, due to the large polarizability and low current leakage, BaTiO3 is considered as suitable for thin film capacitors in applications such as high-density DRAM. For these applications, it is required to produce BaTiO3 particles with nanometer in size and with narrow particle size distribution. There are many methods for synthesizing fine BaTiO3 powders. BaTiO3 nanocrystals have been synthesized by using a hydrothermal method [5–21], sol–gel processing [22–27], the oxalate route [28], microwave heating [29], a micro-emulsion

∗

Corresponding author. Tel.: +81 22 2175629; fax: +81 22 2175631. E-mail address: [email protected] (T. Adschiri).

0378-3812/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2007.03.025

process [30,31], and a polymeric precursor method [32]. Lowtemperature synthesis has also provided possibility for high purity, homogeneous, and ultra-fine BaTiO3 nanoparticles [33]. The BaTiO3 particles produced by these methods are larger than several 10 nm in diameter. The crystal phase is cubic, and that BaTiO3 does not have any ferroelectric property. Several researchers have reported the reason for the formation of cubic BaTiO3 in these methods is due to the presence of the residual hydroxyl ions in the oxygen sublattice of BaTiO3 [16,34]. The high temperature (1000 ◦ C) is required to remove those residual hydroxyl groups. Recently, Hakuta et al. [35] has synthesized tetragonal structure BaTiO3 nanoparticles using barium hydroxide, titanium dioxide and NaOH as starting materials by supercritical hydrothermal synthesis. The objective of this study is to clarify the formation mechanism of BaTiO3 nanoparticles in supercritical hydrothermal synthesis from the standpoints of titanium dioxide solubility. In addition, effect of rapid heating of the starting precursors on the synthesis of BaTiO3 nanoparticles with smaller size and narrower particle size distribution was also discussed based on the titanium dioxide solubility.

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2. Experimental method 2.1. Batch experiment Batch type hydrothermal synthesis of BaTiO3 nanoparticles was performed using a pressure resistant tube reactor (SUS 316) with inner volume of 5.0 mL. The reactor was loaded with 0.01 M barium hydroxide octahydrate (Wako pure chemical industries) and 0.01 M titanium dioxide (30 wt%, Ishihara sangyo kaisha) water suspension. The solution pH was changed from 13 to 4 by dropwise addition of HNO3 . Thereafter, the reactor was tightly capped and put in an electric furnace whose temperature was maintained at 400 ◦ C. Heating rate up to the temperature was around 3 min. According to the density of loaded reactant solution (1.79 mL) at reaction temperature, pressure of reaction media of 30 MPa during residence time was expected. The reaction was performed for 10 min and terminated by quenching the reactor in a water bath at room temperature. After recovering the products by filtration, they were rinsed by distilled water. Then, the products were dried at 60 ◦ C during over night. 2.2. Flow experiment Fig. 1 shows a schematic illustration of a flow-type apparatus used in this study. The reactor was made of SUS 316 stainless steel tube. Barium hydroxide octahydrate (Wako pure chemical industries) and titanium dioxide (30 wt%, Ishihara sangyo kaisha) were the starting materials and they were dispersed in water with a concentration of 0.01 M. The starting solution was fed into the reactor by a highpressure pump at a flow rate of 2 mL/min and mixed with supercritical water of 450 ◦ C with flow rate of 10 mL/min and thus the solution was rapidly heated to the reaction temperature. The temperature and pressure in the reactor were maintained at 400 ◦ C and 30 MPa, respectively. The residence time of the solution in the reactor was about 18 s, which was evaluated by total flow rate (12 mL/min), reactor volume (10 mL) and density of pure water at the reaction temperature and pressure (0.358 g/cm3 ). At the exit of the reactor, the fluid was rapidly quenched by using an external water jacket. Then

Fig. 1. Schematic diagram of the supercritical water flow apparatus.

the produced particles were collected by using an upstream in-line filter. During the experiment, pressure was controlled by using a back-pressure regulator positioned after a cooling unit. 3. Analyses To investigate crystal structure of the BaTiO3 powders, X-ray diffraction method (XRD) with Cu K␣ radiation was used. Crystalline size was also evaluated from the XRD peaks by Scherrer’s equation. Morphologies and size of particles were observed by electron microscopy (TEM, JEM-1200EXII (JEOL, Ltd.)). Raman spectroscopy was also used for confirming tetragonality of the product. 4. Solubility estimation In order to estimate of titanium dioxide solubility, chemical equilibria of BaTiO3 system were written as follows: Ba(OH)2 (s) ↔ Ba2+ + 2OH− , BaOH+ ↔ Ba2+ + OH− ,

K1 = [Ba2+ ][OH− ]2

K2 =

[Ba2+ ][OH− ] [BaOH+ ] [Ti(OH)3 + ] [H+ ]

TiO2 (s)+H+ +H2 O ↔ Ti(OH)3 + ,

K3 =

TiO2 (s) + 2H2 O ↔ Ti(OH)4 (aq),

K4 = [Ti(OH)4 ]

(1) (2) (3) (4)

TiO2 (s) + 3H2 O ↔ Ti(OH)5 − + H+ , K5 = [Ti(OH)5 − ][H+ ]

(5)

HNO3 − ↔ H+ + NO3− , H2 O ↔ H+ + OH− ,

KHNO3 =

[H+ ][NO3− ] [HNO3 ]

Kw = [H+ ][OH− ]

(6) (7)

There are several methods to estimate density, temperature and pressure dependence of K, over a wide range of temperature and pressure including critical point, such as Helgeson–Kirkham–Flowers (HKF) estimation approach [36], Sue and Adschiri’s method [37], density model [38] and so on. In this study we employed density model. Activity coefficient can be estimated by Pitzer’s equation, however, under the present dilute conditions and relative lower temperature region, it was confirmed almost unity (0.7–1) [39]. Then, for the solubility estimation, it was assumed to be unity. For the calculation of equilibrium constant for Eqs. (1)–(7), thermodynamic data such as Gibbs free energy, enthalpy, and heat capacity were used in the literatures [40–42]. The equation was as follows:   1 H 1 ln K = ln Kr − − R T Tr   CP 1 ρr AP + (8) ln − (T − Tr ) RTr (∂AP /∂T )Pr T ρ T By using Eqs. (1)–(7) and mass balance and charge balance, solution pH has been calculated at different temperatures.

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Fig. 3. XRD patterns of the synthesized nanoparticles ((a) pH 4, (b) pH 7, (c) pH 9 and (d) pH 13). Fig. 2. Plot of calculated solubility of TiO2 ((a) with addition HNO3 , starting pH 4, 7 and 9 and (b) without addition HNO3 , starting pH 13).

By the combination of Eqs. (3)–(5) for K3 –K5 and calculated pH, the solubility of titanium dioxide in high temperature water is estimated by the following equation: [TiO2 ]total dissolved = [Ti(OH)3 + ] + [Ti(OH)4 ] + [Ti(OH)5 − ] (9) Fig. 2 shows solubility of TiO2 with changing temperature. When we added HNO3 and starting pH is 9, 7 or 4, the solubility of titanium dioxide is very lower than the case without adding HNO3 . At high pH condition, the solubility of titanium dioxide is very high.

5. Results and discussion In order to determine the produced nanoparticles crystal structure, X-ray diffraction method (XRD) was used. Fig. 3 shows the XRD pattern of products at pH 4, 7, 9 and 13. It was confirmed that only at a high pH condition, the products were in single phase BaTiO3 and in other cases, titanium dioxide was obtained. It is important that even without NaOH, the initial pH of the solution is as high as 13 and tetragonal BaTiO3 could be synthesized at this condition. Next, the reason why TiO2 was formed at the lower pH range will be discussed based on the solubility of TiO2 . Fig. 2 shows that at pH 4, 7 and 9, the solubility of titanium dioxide is very low and only at high pH environment, pH > 13, the solubility of titanium dioxide is high and BaTiO3 was obtained. This means that only at high pH environment, pH > 13, titanium dioxide can be dissolved in water and after that titanium ions can react with barium ions to synthesize

Fig. 4. TEM images of the BaTiO3 nanoparticles at 400 ◦ C and 30 MPa ((a) batch type reactor and (b) flow type reactor).

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Table 1 Size of the BaTiO3 nanoparticles TEM Batch (nm) Flow (nm)

16–89 18–35

Scherrer’s equation Batch (nm) Flow (nm)

22 19

BaTiO3 . This dissolution–precipitation mechanism is considered to be critical in the supercritical hydrothermal synthesis of BaTiO3 nanoparticles using barium hydroxide and titanium dioxide as precursors. Fig. 4 shows TEM images of the products. For the case of the flow experiment (Fig. 4(b)), small particles were obtained than that in batch experiment (Fig. 4(a)). Also, particle size distribution was narrower for the flow experiment. From the full widths at half maximum of XRD peaks, crystalline size of the products was calculated by Scherrer’s equation. Table 1 summarizes the size of nanoparticles evaluated by both TEM and XRD. In both cases of the flow experiment and the batch experiment, the crystalline sizes obtained from Scherrer’s equation are similar to the particle size in TEM images. This suggests that the synthesized nanoparticles are of single crystal. The reasons why the size of nanoparticles from the flow type method is smaller and the size distribution is narrower are discussed below based on the solubility of titanium dioxide. In general, solubility of metal oxides drastically decreases around the critical point [43]. In a batch type reactor, because of the slower heating rate, titanium dioxide dissolution and transformation to BaTiO3 gradually takes place, while the BaTiO3 particle grows. Some of the titanium dioxides that dissolve in a higher temperature range will precipitate in a later stage above the critical point due to the drastic decrease of the solubility which forms smaller BaTiO3 particles. On the other hand, for the case of the flow type experiment, rapid heating of titanium dioxide and barium hydroxide leads to the homogenous formation of BaTiO3 without its gradual crystal growth. That leads to the smaller particle formation and narrower particle size distribution. Therefore

Fig. 5. Raman spectra of BaTiO3 nanoparticles at 400 ◦ C, 30 MPa (flow reactor).

rapid heating in flow reactor is effective to synthesize BaTiO3 nanoparticles with the smaller size and narrower particle size distribution. Fig. 5 shows the Raman spectra for synthesized BaTiO3 nanoparticles from flow reactor. In Fig. 5, the bands around 515 and 260 cm−1 are assigned to the transverse optical modes of A1 symmetry, whereas the band around 300 cm−1 , which is characteristic of the tetragonal phase, is assigned to the B1 mode. The sharp peak at 300 cm−1 suggests that the tetragonal phase is the dominant phase within the synthesized nanoparticles. 6. Conclusion We could prepare fine BaTiO3 nanoparticles with tetragonal phase from barium hydroxide and titanium dioxide as starting materials at high pH condition by the supercritical hydrothermal synthesis. We have proposed a dissolution–precipitation mechanism for supercritical hydrothermal synthesis of BaTiO3 nanoparticles based on the solubility of titanium dioxide. We demonstrated that rapid heating in a flow reactor is effective to synthesize smaller size and narrower particle size distribution for the BaTiO3 nanoparticles. Acknowledgements This work was supported by a Scientific Research Grant from the Ministry of Education, Science, Sports, and Culture of Japan. This research was also partly supported by a Grant-in-Aid for the COE project, Giant Molecules and Complex Systems, 2002. References [1] J.F. Scott, High Ann. Rev. Mater. Sci. 28 (1998) 79–100. [2] D.M. Tahan, A. Safari, L.C. Klein, J. Am. Ceram. Soc. 79 (1996) 1593– 1598. [3] L.E. Cross, Am. Ceram. Soc. Bull. 63 (1984) 586–590. [4] D. Hennings, Int. J. High Technol. Ceram. 3 (1987) 91. [5] P.P. Phule, S.H. Risbud, J. Mater. Sci. 25 (1990) 1169–1183. [6] D. Hennings, G. Rosenstein, S. Scheinemacher, J. Eur. Ceram. Soc. 8 (1991) 107–115. [7] M.M. Wu, R.R. Xu, S.H. Feng, L. Li, D.H. Chen, Y.J. Luo, J. Mater. Sci. 31 (1996) 6201–6205. [8] C.T. Xia, E.W. Shi, W.Z. Zhong, J.K. Guo, J. Eur. Ceram. Soc. 15 (1995) 1171–1176. [9] J.O. Eckert Jr., C.C. Hung-Houston, B.L. Gersten, M.M. Lencka, R.E. Riman, J. Am. Ceram. Soc. 79 (1996) 2929–2939. [10] E.B. Slamovich, I.A. Aksay, J. Am. Ceram. Soc. 79 (1996) 239–247. [11] A.N. Christensen, Acta Chem. Scand. 24 (1970) 2447. [12] L. Zhao, A.T. Chien, F.F. Lapse, J.S. Speck, J. Mater. Res. 11 (1996) 1325–1328. [13] R. Vivekanandan, T.R.N. Kutty, Powder Technol. 57 (1989) 181–192. [14] G. Busca, V. Buscaglia, M. Leoni, P. Nanni, Chem. Mater. 6 (1994) 955–961. [15] T. Noma, S. Wada, M. Yano, T. Suzuki, J. Appl. Phys. 80 (1996) 5223–5233. [16] R. Asiaie, W.D. Zhu, S.A. Akbar, P.K. Dutta, Chem. Mater. 8 (1996) 226–234. [17] I.J. Clark, T. Takeuchi, N. Ohtori, D.C. Sinclair, J. Mater. Chem. 9 (1999) 83–91. [18] D. Hennings, S. Schreinemacher, J. Eur. Ceram. Soc. 9 (1992) 41–46. [19] J. Menashi, R.C. Reid, L. Wagner, Cabot Corporation, U.S. Pat No. 4,829,033, 1989.

M. Atashfaraz et al. / Fluid Phase Equilibria 257 (2007) 233–237 [20] E.W. Shi, C.T. Xia, W.E. Zhong, B.G. Wang, C.D. Feng, J. Am. Ceram. Soc. 80 (1997) 1567–1572. [21] B.D. Begg, E.R. Vance, J. Nowotny, J. Am. Ceram. Soc. 77 (1994) 3186–3192. [22] P.K. Dutta, J.R. Gregg, Chem. Mater. 4 (1992) 843–846. [23] M.H. Frey, D.A. Payne, Phys. Rev. B 54 (1996) 3158–3168. [24] S. Schlag, H.F. Eicke, Solid State Commun. 91 (1994) 883–887. [25] R.N. Viswanath, S. Ramasamy, Nanostruct. Mater. 8 (1997) 155–162. [26] H. Shimooka, M. Kuwabara, J. Am. Ceram. Soc. 79 (1996) 2983–2985. [27] G. Pfaff, J. Mater. Chem. 2 (1992) 591–594. [28] T. Takeuchi, M. Tabuchi, K. Ado, K. Honjo, O. Nakamura, H. Kageyama, Y. Suyama, N. Ohtori, M. Nagasawa, J. Mater. Sci. 32 (1997) 4053– 4060. [29] H.I. Hsiang, F.S. Yen, J. Am. Ceram. Soc. 79 (1996) 1053–1060. [30] Y. Ma, E. Vileno, S. Suib, P.K. Dutta, Chem. Mater. 9 (1997) 3023–3031. [31] C. Beck, W. Hartl, R. Hempelmann, J. Mater. Res. 13 (1998) 3174–3180. [32] J. Wang, J. Fang, S.C. Ng, M. Gan, C.H. Chew, X. Wang, Z. Shen, J. Am. Ceram. Soc. 82 (1999) 873–881.

237

[33] W.S. Cho, J. Phys. Chem. Solid 59 (1998) 659–666. [34] S.W. Lu, B.I. Lee, Z.L.W.D. Wang, J. Cryst. Growth 219 (2000) 269–276. [35] Y. Hakuta, H. Ura, H. Hayashi, K. Arai, J. Mater. Lett. 59 (2005) 1387–1390. [36] H.C. Helgeson, D.H. Kirkham, G.C. Flowers, Am. J. Sci. 281 (1981) 1249–1516. [37] K. Sue, T. Adschiri, K. Arai, Ind. Eng. Chem. Res. 41 (2002) 3298–3306. [38] G.M. Anderson, S. Castet, J. Schott, R.E. Mesmer, Geochim. Comsochim. Acta (1991) 1769–1779. [39] K. Sue, K. Murata, Y. Matsuura, M. Tsukagoshi, T. Adschiri, K. Arai, Fluid Phase Equilib. 194–197 (2002) 1097–1106. [40] E.L. Shock, D.C. Sassani, M. Willis, D.A. Sverjensky, Geochim. Cosmochim. Acta 61 (1997) 907–950. [41] A. Stefansson, Chem. Geol. 172 (2001) 225–250. [42] K.G. Knauss, M.J. Dibley, W.L. Bourcier, H.F. Shaw, Appl. Geochem. 16 (2001) 1115–1128. [43] K. Sue, Y. Hakuta, R.L. Smith Jr., T. Adschiri, K. Aria, J. Chem. Eng. Data 44 (1999) 1422–1426.