Formation and Annealing of BaTiO 3 and SrTiO 3 Nanoparticles in KOH Solution

Transcription

1 J. Mater. Sci. Technol., 2010, 26(1), Formation and Annealing of BaTiO 3 and SrTiO 3 Nanoparticles in KOH Solution Tomoki Tsumura, Keiichi Matsuoka and Masahiro Toyoda Department of Applied Chemistry, Faculty of Engineering, Oita University, Dannoharu 700, Oita, , Japan [Manuscript received January 7, 2009, in revised form April 8, 2009] Barium titanate (BaTiO 3 ) and strontium titanate (SrTiO 3 ) nanoparticles were synthesized separately through hydrothermal reaction of crystalline TiO 2 particles and corresponding alkaline earth hydroxides, Ba(OH) 2 and Sr(OH) 2 respectively, in 50 mol dm 3 KOH solution at 150 C. Each structural evolution of BaTiO 3 and SrTiO 3 during the hydrothermal treatment was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscopy (FE-TEM) and thermogravimetrydifferential analysis (TG-DTA). In the BaTiO 3 system, round particles with cubic perovskite-type structure were obtained within 1 h. However, these particles were gradually dissolved and then were re-precipitated in the form of cube-shape BaTiO 3 particles with a smaller lattice constant a c than that of the former phase. After the BaTiO 3 particles formed firstly have disappeared completely, or the two phases coexistence stage with different lattice constant a c passed, lattice constant a c of BaTiO 3 phase re-precipitated continuously decreased with annealing time. In contrast, once SrTiO 3 particles are formed, the lattice constant a c decreased continuously throughout the reaction. The result indicates that SrTiO 3 particles were annealed without dissolution and precipitation process under the present condition. KEY WORDS: BaTiO 3; SrTiO 3; Nanoparticle; Hydrothermal reaction 1. Introduction Perovskite-type titanate has been studied and utilized as one of key materials for electronic devices. With surge in the use of mobile devices with multimedia functions such as camera, audio, computer and phone, demand for the downsizing of those devices has increased rapidly for several decades. Therefore hydrothermal method for obtaining of electro ceramics on a nanoscale has attracted attention, because size, morphology and composition of the particles are uniform and controllable [1 3]. It is generally well known that BaTiO 3 and SrTiO 3 nanoparticles could be synthesized hydrothermally in KOH solution. However, the particles obtained below 200 C have cubic phase with many defects, such as hy- Corresponding author. Prof. Ph.D.; Tel: ; Fax: ; address: ttsumura@cc.oitau.ac.jp (T. Tsumura). droxyl group, its dielectric properties of the device made from them have been reported to be poor [4 6]. So optimization of reaction condition, such as increasing temperature, pressure [7], concentration of KOH, and molar ratio of Ba to Ti [8], addition of sodium chloride [9], treating in NH 3 aqueous medium [10], applying ultra sonic during reaction process [11] etc., has been carried out and it is still important to understand formation and annealing process for perovskite nanoparticles under hydrothermal condition. Formation mechanism of BaTiO 3 nanoparticles under hydrothermal condition has been extensively studied and now two possible mechanisms, dissolution and precipitation and in situ reaction, were proposed [12 18]. The former includes two steps, dissolution of TiO 2 and nucleation/growth of BaTiO 3 crystal from liquid phase [13 15, 17, 18]. The latter is a diffusion mechanism, i.e. Ba species diffuse into TiO 2 bulk from aqueous medium and then BaTiO 3 domain is formed through topochemical reaction of the TiO 2

2 34 T. Tsumura et al.: J. Mater. Sci. Technol., 2010, 26(1), bulk and Ba species inserted [12,13,16]. Moreover combination of the two mechanisms was also proposed depending on the synthesis conditions [13]. Although there are no so many reports about formation mechanism for SrTiO 3, it also can be formed in the same mechanism as BaTiO [19,20] 3. Meanwhile, there are several studies for structural evolution during annealing process under hydrothermal condition, which is supposed to be dependent on synthesis conditions [21 23]. It is also essential to understand how the crystal is annealed during hydrothermal reaction, or how the defects in the crystal decrease. In the present paper, formation and annealing process for BaTiO 3 under hydrothermal condition in 50 mol dm 3 KOH solution at 150 C was investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), field emission transmission electron microscopy (FE- TEM) and thermogravimetry-differential analysis (TG-DTA), comparing with that for SrTiO Experimental Procedure BaTiO 3 nano particles were synthesized through hydrothermal reaction in 50 mol dm 3 KOH solution. KOH (special grade chemicals, Wako Pure Chemical Inst. Ltd.) and Ba(OH) 2-8H 2 O (special grade chemicals, Wako Pure Chemical Inst. Ltd.) were dissolved in purified water and then 3.5 g of TiO 2 powders (P25, Degussa Co. Ltd.) were dispersed in the solution, molar ratio of K to Ti and Ti to Ba being 10 to 1. The mixture obtained was heated at 150 C in heated-air drier for different heating time in the lidded PTFE vessel of 100 cm 3 set in a stainless-steel autoclave to get autogenous hydrothermal condition. After the hydrothermal treatment the product was naturally cooled down to room temperature in the vessel and was taken out to a polypropylene cup to mix with purified water and rinse by decantation. The decantation was repeated until ph of the supernatant liquid down to less than 8. SrTiO 3 nano particles were also synthesized in a same manner as BaTiO 3. Crystalline structure of the samples prepared was investigated by XRD measurement with Ni-filtered CuKα radiation (Rigaku, RINT UltimaOT). Lattice parameter a c of the samples prepared was calculated from the XRD pattern by referring internal standard MgO powders mixed into the samples. Morphology of sample powders was investigated under FE-SEM (Hitachi Ltd. S-800) and FE-TEM (Hitachi Ltd. HF- 2000). Weight loss behavior during heating with a constant heating rate of 10 C min 1 in static air was studied by using a TG-DTA apparatus (Rigaku Co. Ltd, Thermo plus TG 8120). 3. Results and Discussion Change in XRD patterns of BaTiO 3 synthesized at 150 C in 50 mol dm 3 KOH solution with different reaction time is shown in Fig. 1(a) and (b), compared with that of SrTiO 3 synthesized at 150 C in 50 mol dm 3 KOH (Figs. 1(c) and (d)). After heating for 1 h, diffraction pattern of cubic BaTiO 3 showing perovskite-type structure is observed with impurity phase of BaCO 3 denoted by closed circle and no diffraction lines assigned to starting materials, TiO 2 (two crystalline phases of anatase and rutile type structure) and Ba(OH) 2 are detected (Fig. 1(a)). Since XRD lines assigned to BaTiO 3 become sharper with the increase of heating time, 111, 200, 210 and 211 lines for BaTiO 3 phase are magnified in Fig. 1(b) to make their change more visible. During a period from 1 to 20 h, position of those diffraction lines discontinuously shifts toward higher angle in 2θ with the increase of heating time. Each diffraction line of 1 h sample is an asymmetrical peak profile with a tail on the higher angle side, which may indicate the presence of small amount of BaTiO 3 phase with smaller lattice constant. For two samples reacted for 5 and 10 h, all XRD lines of them obviously split. With the increase of reaction time, a peak split on lower angle side gets smaller but another on higher angle side gets bigger for all diffractions 111, 200, 210, and 211. After heating for 20 h, diffraction lines on lower angle side disappear completely and then diffraction lines on higher angle side still shift to higher angle side continuously with the increase of reaction time. As in the case of SrTiO 3, SrTiO 3 showing cubic perovskite-type phase was formed after 1 h reaction with small amount of by-product SrCO 3 denoted by open circle (Fig. 1(c)). But unlike BaTiO 3 system, there is no change in peak profile of diffraction lines and then these lines obviously shift continuously toward higher angle with the reaction time (Fig. 1(d)). These results indicate that structural evolution with the reaction time at 150 C in 50 mol dm 3 KOH solution is quite different between BaTiO 3 and SrTiO 3 system. This difference may agree with the result reported by Lencka et al. [19]. They calculated stability diagrams for Ba-Ti and Sr-Ti hydrothermal systems at 363 and 433 K, which indicate that ph limit for the formation of BaTiO 3 is higher than that of SrTiO 3 and thus SrTiO 3 is more easily formed than BaTiO 3 under the same condition. Lattice constant a c calculated from XRD data for BaTiO 3 and SrTiO 3 synthesized is plotted against reaction time in logarithmic scale as shown in Fig. 2. For BaTiO 3 samples (Fig. 2(a)), discrete shrinkage of a c from to nm is observed as mentioned in the results of XRD pattern, the reduction of lattice volume between 1 h-sample and 120 h-sample calculated being 2.718%. On the other hand, continuous shrinkage of a c from to nm is observed for SrTiO 3 samples, the reduction of lattice volume between 1 h-sample and 120 h-sample calculated being %. Since the volume change of BaTiO 3 lattice is about 2.7 times larger than that of SrTiO 3, BaTiO 3 may have higher concentration of defects in

3 T. Tsumura et al.: J. Mater. Sci. Technol., 2010, 26(1), Fig. 1 XRD patterns of BaTiO 3 (a), (b) and SrTiO 3 (c), (d) synthesized at 150 C in 50 mol/l KOH with different heating time: (a) 1 h, (b) 120 h, (c) 1 h, (d) 120 h Fig. 2 Change in lattice constant a c of BaTiO 3 (a) and SrTiO 3 (b) synthesized at 150 C crystal than SrTiO 3. SEM micrographs of BaTiO 3 and SrTiO 3 particles reacted for 1 and 120 h are shown in Fig. 3 to investigate morphological change of perovskite particles during reaction. BaTiO 3 particles heated for 1 h are round with the size below 50 nm (Fig. 3(a)), and

4 36 T. Tsumura et al.: J. Mater. Sci. Technol., 2010, 26(1), Fig. 3 FE-SEM photos of perovskite-type particles synthesized. BaTiO 3 particles reacted for 1 h (a) and 120 h (b) and SrTiO 3 particles reacted for 1 h (c) and 120 h (d) Fig. 4 TEM image of BaTiO 3 particles reacted for 5 h in 50 mol dm 3 KOH at 150 C then they change into cube one with the size of around 50 nm after 120 h (Fig. 3(b)). TEM image of BaTiO 3 particles reacted for 5 h which contains two perovskite phase with different lattice constant a c is shown in Fig. 4. The BaTiO 3 particles obviously consist of two different kinds of particles. One has a round shape with particle size ranging from 20 to 30 nm and other has an angular shape with particle size around 50 nm, which are denoted by circles and arrows respectively in Fig. 4. These results may support the process of dissolution and precipitation for BaTiO 3 particle under hydrothermal conditions in 50 mol dm 3 KOH solution, which was confirmed as a discontinuous shift of XRD lines of BaTiO 3 with reaction time between 1 and 20 h. However there is no morphological change for SrTiO 3 particles with reaction time (Fig. 3(c) and (d)). They are cube one with the size of around 50 nm for both samples heated for 1 and 120 h. Figure 5 shows thermal gravimetric curves of BaTiO 3 and SrTiO 3 particles reacted for 1 and 120 h. For BaTiO 3 samples (Fig. 5(a)), weight loss below 150 C could be attributed to the release of adsorbed water. Above 150 C weight of sample gradually losses with increasing heating temperature, the weight loss up to 800 C of sample reacted for 1 h being about

5 T. Tsumura et al.: J. Mater. Sci. Technol., 2010, 26(1), Fig. 5 Thermo-gravimetric curves of BaTiO 3(a) and SrTiO 3 (b) powders synthesized for 1 and 120 h. Heating rate is 10 C/min and atmosphere is static air Fig. 6 Scheme of formation and annealing process for BaTiO 3 and SrTiO 3 in 50 mol dm 3 KOH solution at 150 C 2.2% and 120 h-sample being about 0.8%. For SrTiO 3 samples, the behavior of weight change under heating is the same as that of BaTiO 3, but weight loss up to 800 C of SrTiO 3 sample is about two times larger than that of BaTiO 3 sample with the same reaction time, 1 h-sample being 4.0%, 120 h-sample 2.3%. Since the weight loss from 150 to 800 C could be attributed to the release of H 2 O from lattice defects, such as hydroxyl group and water in lattice as reported by several groups [4 7], the present result indicates that either SrTiO 3 may contain larger amount of defects than BaTiO 3 or SrTiO 3 may be easily annealed than BaTiO 3 by heating in air. Since difference in weight loss between 1 h-sample and 120 h-sample is about 1.4 and about 1.7% for BaTiO 3 and SrTiO 3 respectively, effect of annealing through the present hydrothermal treatment is supposed not to be so different between BaTiO 3 system and SrTiO 3 one. 4. Conclusion BaTiO 3 and SrTiO 3 nanoparticles were synthesized by reaction of TiO 2 and corresponding alkaline earth hydroxide under hydrothermal condition in 50 mol dm 3 KOH solution. Schematic formation and annealing process of BaTiO 3 and SrTiO 3 under the present hydrothermal condition are illustrated in Fig. 6. For BaTiO 3 system, the formation and annealing reaction proceed in three steps. At the first step, BaTiO 3 nano-particles were formed by dissolution and precipitation mechanism as reported before [13 18]. BaTiO 3 particles formed were dissolved and re-precipitated at the second step and then BaTiO 3 particles re-precipitated were annealed at the last step. In contrast, those of SrTiO 3 particles proceed in two steps; the formation step through the dissolution and precipitation mechanism and the annealing step without dissolution of SrTiO 3. REFERENCES [1 ] R. Roy: J. Solid State Chem., 1994, 111, 11. [2 ] M.Z.C. Hu, V. Kurian, E.A. Payzant, C.J. Rawn and R.D. Hunt: Powder Tech., 2000, 110, 2.

6 38 T. Tsumura et al.: J. Mater. Sci. Technol., 2010, 26(1), [3 ] M. Yoshomura and W. Suchanek: Solid State Ionics, 1997, 98, 197. [4 ] L. Qi, B.I. Lee, P. Badheka, L.Q. Wang, P. Gilmour, W.D. Samuels and G.J. Exarhos: Mater. Lett., 2005, 59, [5 ] T. Yan, X.L. Liu, N.R. Wang and J.F. Chen: J. Cryst. Growth, 2005, 281, 669. [6 ] H. Jena, K.V. G. Kutty and T.R.N. Kutty: Mater. Res. Bull., 2004, 39, 489. [7 ] K. Matsui, T. Noguchi, N. M. Islam, Y. Hakuta and H. Hayashi: J. Cryst. Growth, 2008, 310, [8 ] S.K. Lee, T.J. Park, G.J. Choi, K.K. Koo and S.W. Kim: Mater. Chem. Phys., 2003, 82, 742. [9 ] W. Sun, W. Liu and J. Li: Powder Tech., 2006, 166, 55. [10] S.K. Tripathy, T. Sahoo, M. Mohapatra, S. Anand and R.P. Das: Mater. Lett., 2005, 59, [11] S.H. Jhung, J.H. Lee, J.W. Yoon, Y.K. Hwang, J.S. Hwang, S.E. Park and J.S. Chang: Mater. Lett., 2004, 58, [12] W. Hertle: J. Am. Ceram. Soc., 1988, 71, 879. [13] J.J. O. Eckert, C.C. Hung-H., B.L. Gestern, M.M. Lencka and R.E. Riman: J. Am. Ceram. Soc., 1996, 79, [14] P. Pinceloup, C. Courtois, J. Vincens, A. Leriche and B. Thierry: J. Euro. Ceram. Soc., 1999, 19, 937. [15] H. Xu and L. Gao: Mater. Lett., 2002, 57, 490. [16] X. Wang, B.I. Lee, M.Z. Hu, E.A. Payzant and D.A. Blom: J. Mater. Sci.: Mater. Electronics, 2003, 14, 495. [17] J. Moon, E. Suvaci, A. Morrone, S. A. Costantino and J. H. Adair: J. Eur. Ceram. Soc., 2003, 23, [18] L. Qi, B.I. Lee, P. Badheka, D.H. Yoon, W.D. Samuels and G.J. Exarhos: J. Eur. Ceram. Soc., 2004, 24, [19] M.M. Lencka and R.E. Riman: Ferroelectrics, 1994, 151, 159. [20] S. Zhang, J. Liu, Y. Han, B. Chen and X. Li: Mater. Sci. Eng. B, 2004, 110, 11. [21] R.I. Walton, A. Norquist, R.I. Smith and D. O Hare: Faraday Discuss., 2002, 122, 331. [22] J. Moon, E. Suvaci, T. Li, S.A. Costantino and J.H. Adair: J. Eur. Ceram. Soc., 2002, 22, 809. [23] T.I. Kim, J.K. Jung and K.S. Ryu: Mater. Res. Bull., 2004, 39, 1045.