Hydrothermal synthesis and characterization of

Paper

Journal of the Ceramic Society of Japan 117 [3] 264-267 2009

Hydrothermal synthesis and characterization of Bi 4 Ti 3 O 12 powders Zhiwu CHEN, Ying YU,* Jianqiang HU,* Anze SHUI and Xinhua HE *

College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Department of Chemistry, College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China

The Bi4Ti3O12 powders were synthesized by a hydrothermal route. The influence of hydrothermal conditions on crystal structures and morphologies of Bi4Ti3O12 particles was studied. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), fourier transform infrared (FT–IR) spectroscopy and ultraviolet visible (UV-vis) spectroscopy. These results revealed that the Bi4Ti3O12 powders prepared thus possessed a plate-like shape, a typical bismuth layered perovskite structure, and a wide UV-vis absorption peak. Additionally, the formation mechanism of the Bi4Ti3O12 nanoplates was also discussed. ©2009 The Ceramic Society of Japan. All rights reserved.

Key-words : Bi4Ti3O12, Hydrothermal synthesis, Powder, Starting materials, Mineralizer [Received November 4, 2008; Accepted January 15, 2009]

1. Introduction Bismuth titanate (Bi4Ti3O12) that is constructed by the regular intergrowth of [Bi2O2]2+ layer and perovskite type layers [Bi2Ti3O10]2– has a layered perovskite structure, in which Bi ions occupy twelve-coordinated sites.1),2) As a promising ferroelectric piezoelectric and electro-optic material, Bi4Ti3O12 has relatively low coercive field, low dielectric constant, high Curie temperature (675°C), and high breakdown strength.3),4) These excellent properties have been utilized in many fields such as non-volatile memory, optical memory, piezoelectric, and electro-optic devices.5),6) Conventionally, Bi4Ti3O12 powders were usually synthesized by a solid state method7) or a molten salt method.8) However, regretfully, the method need high temperature (around 800–1000°C) and therefore results in high agglomeration and compositional inhomogeneity of powders. Several alternative chemical synthesis routes have been proposed including hydrolysis,9) co-precipitation method,10) sol–gel process11) and so on. However, a high calcined temperature is usually necessary for the crystallization of Bi4Ti3O12 powders, although the improved chemical methods can produce fine powders with compositional homogeneity. As a promising way to prepare bismuth titanate powders, the hydrothermal process offer many advantages compared with several other synthesis techniques, such as low processing temperature, high degree of crystallinity, well-controlled morphology, high purity and narrow particle size distribution.4),12),13) Currently, Bi4Ti3O12 powders have been successfully synthesized by hydrothermal processes.13)–15) Gu et al.13) synthesized the orthorhombic Bi4Ti3O12 nanoplates by polyethylene glycolassisted hydrothermal method at 200°C, in which bismuth nitrate (Bi(NO3)3·5H2O) and titanium butoxide (Ti(OC4H9)4) were used as the starting materials. The different precursors for the hydrothermal synthesis of tetragonal Bi4Ti3O12 powders have also been reported.14) For example, the larger particles with a plate-like †

Corresponding author: Z. Chen; E-mail: [email protected]

264

morphology were obtained using the Bi–Ti hydroxide (precursor A), while the Bi–Ti xerogel (precursor B) was used to form the smaller ones with a spherical morphology. In addition, Pookmanee et al.15) synthesized the orthorhombic Bi4Ti3O12 powders at 150°C by reacting titanium isopropoxide (Ti(O(C3H7)4)) with bismuth nitrate (Bi(NO3)3·5H2O) under alkaline conditions, and the powders had generally spherical shape. But above-mentioned results were only obtained with limited success, and hydrothermal synthesis conditions need to be optimized further. Up to now, to the best of our knowledge, the report on hydrothermal synthesis of regular Bi4Ti3O12 nanoplate by using Bi(NO3)3·5H2O and TiO2 is lacking, especially for the investigation of the UV-vis absorption spectrum. In this paper, the Bi4Ti3O12 nanoplates were fabricated by hydrothermal method from Bi(NO3)3·5H2O, TiO2 and NaOH solution. The influence of the mineralizers, reaction temperature and time on the crystal structure and the morphology of bismuth titanate powders were investigated in detail.

2.

Experimental procedure

In this experiment, Bi(NO3)3·5H2O and TiO2 were used as starting materials, and NaOH was used as mineralizer. All chemical reagents were of analytical grade. First, the starting material (Bi:Ti = 4:3) and mineralizer were mixed uniformly according to a designed molar ratio. The mixture was then sealed in a Teflonlined stainless steel autoclave with a filling capacity of 80%. The hydrothermal synthesis of Bi4Ti3O12 was conducted under different temperatures (from 100 to 180°C) and times (form 10 to 72 h). After cooling, the products were filtered, washed with distilled water, and dried at room temperature. Crystalline structures of the Bi4Ti3O12 nanoparticles were examined using an X-ray diffractometer (XRD, D/Max-3C, Japan) with Cu Ka radiation. The morphologies of the synthesized Bi4Ti3O12 particles were observed by a Scanning electron microscopy (LEO 1530 VP, SEM). Infrared spectra were collected by a Nicolet-Nexus 670 FTIR spectrometer from 400 to 4000 cm–1. Ultraviolet visible (UV-vis) absorption spectrum of ©2009 The Ceramic Society of Japan


the as-prepared Bi4Ti3O12 powders was recorded in the wavelength range of 300–850 nm using a U–3010 spectrophotometer (Tokyo, Japan). Samples for UV-vis absorption characterization were prepared by dispersing the as-prepared powders in the ethanol by ultrasonic, and the pure ethanol was used as a reference sample.

3.

Results and discussion

The influences of the concentration of mineralizers on the formation of Bi4Ti3O12 were summarized in Table 1. It can be seen from Table 1 that the structure of bismuth titanate are strongly dependent on the contentration of NaOH. If the contentration was less than 1.0 M or larger than 4.0 M, the phase structures of the powders synthesized were unknown (multi-phases). While pure Bi4Ti3O12 can be synthesized hydrothermally using Bi(NO3)3·5H2O and TiO2 in a 1.0–4.0 M NaOH aqueous solution at temperatures of 180°C for 48 h. To study the effects of reaction temperature on product, the hydrothermal synthesis of Bi4Ti3O12 powder was carried out in a 2 M NaOH aqueous solution at temperatures of 100, 120, 140, 160, 180°C for 48 h. The XRD patterns of prepared particles are shown in Fig. 1. If the powder was synthesized at hydrothermal temperature at 100°C, only three small diffraction peaks of Bi4Ti3O12 were observed, which indicted that the product contained mostly amorphous precipitate and only a little of Bi4Ti3O12 formed. With an increase of the reaction temperature, the diffraction peaks of Bi4Ti3O12 became clear. When reaction temperature increased to 180°C, the heights of the diffraction peaks belonging to Bi4Ti3O12 increased further. It revealed that

Table 1. Influence of the Mineralizer Concentrations on Crystal Structures of the Bi4Ti3O12 Nanoplates Synthesized by the Present Method NaOH (M)

Temperature (°C)

Time (h)

Crystalline structure

none

180

48

unknown phase

0.5

180

48

unknown phase

1.0

180

48

Bi4Ti3O12

2.0

180

48

Bi4Ti3O12

3.0

180

48

Bi4Ti3O12

4.0

180

48

Bi4Ti3O12

5.0

180

48

unknown phase

Fig. 1. XRD patterns of the Bi4Ti3O12 powders synthesized using different temperatures.

JCS-Japan

bismuth titanate particles with well-crystallinity were synthesized at higher temperature. All diffraction peaks of the XRD patterns are assigned to Bi4Ti3O12 as reported in JCPDS file(JCPDS card number 35–0795), indicating that the synthesized powders are monophasic bismuth titanate and have a orthorhombic structure. The Jade analytic program was used to refine the crystal structure. The results show that the Bi4Ti3O12 powders were synthesized at 180°C with lattice constants of a = 0.5429 nm, b = 3.2167 nm, c = 0.5395 nm, matching well the literature values of a = 0.5449 nm, b = 3.2185 nm, c = 0.5410 nm (JCPDS card number 35–0795). Compared with the previous solution process,10) the hydrothermal process developed herein successfully decreased the synthesis temperature of Bi4Ti3O12 from 300 to 140°C. It is evident that the hydrothermal environment remarkably accelerates the reaction kinetics of the formation of Bi4Ti3O12.The formation of Bi4Ti3O12 powders is further confirmed by FT–IR spectra. Figure 2 shows FT–IR spectra of Bi4Ti3O12 powders synthesized in a 2 M NaOH aqueous solution at temperatures of 120, 140, 180°C for 48 h. The peaks around 795 and 578 cm–1 correspond to the stretching vibrations of Bi– O and Ti–O respectively.11),16) If the powder was synthesized at hydrothermal temperature at 100°C, these two peaks are almost nonexistence. As the reaction temperature increased to 140°C, these peaks became clear. When reaction temperature increased to 180°C, it can be seen that the intensity of the peaks belonging to Bi–O and Ti–O increased further, which indicate that the products are well crystallized. The absorption peaks of Ti–O and Bi– O bonds become stronger and sharper with the increment of reaction temperature, which indicates that the crystallization of the Bi4Ti3O12 powders improves with increasing reaction temperature, consistent with the XRD results. In order to understand the influences of reaction time on the synthesis of Bi4Ti3O12, the reaction temperature was fixed at 180°C, and the reaction time was prolonged from 10 h to 72 h. Figure 3 shows the XRD patterns of hydrothermal synthesized Bi4Ti3O12 particles at various reaction time. It was found that the Bi4Ti3O12 crystallites began to be formed at 10 h, but only three small diffraction peaks of Bi4Ti3O12 were observed, which indicted that only a little of Bi4Ti3O12 formed, the product contained mostly amorphous precipitate. With increasing of reaction time, the peak intensity of obtained Bi4Ti3O12 particles tends to increase. It revealed that well crystallized Bi4Ti3O12 particles can be synthesized between 30 h and 72 h (Fig. 3). The structure and

Fig. 2. FT–IR spectra of the Bi4Ti3O12 powders synthesized using different temperatures.

265

JCS-Japan

Chen et al.: Hydrothermal synthesis and characterization of Bi4Ti3O12 powders

Fig. 3. XRD patterns of the Bi4Ti3O12 powders synthesized using different times.

composition of Bi4Ti3O12 particles prepared at different reaction time were also studied with FT–IR spectra. It is found that the absorption peaks of Ti–O and Bi–O bonds become stronger and sharper with the increment of reaction time, which indicates that the crystallization of the Bi4Ti3O12 particles improves with increasing reaction time, consistent with the XRD results too. In conclusion, it was found that increasing both reaction temperature and time had a positive effect on the formation of Bi4Ti3O12. Figure 4 showed SEM micrographs of the Bi4Ti3O12 powders prepared in 2 M NaOH solution at different reaction temperature for 48 h. The prepared powders at 100°C were agglomerated and irregular in shape(Fig. 4(A)), which should be the amorphous phase according to the XRD result. As the reaction temperature increased to 140°C, the crystallinity was improved, and particles with regular plate shape obviously increased in numbers. The Bi4Ti3O12 powders had a large amount of regular nanoplate in addition to a few of irregular particles, as shown in Fig. 4(B). When reaction temperature increased to 180°C, the particles have a significant increase in size. The Bi4Ti3O12 powders consisted of only perfect regular plate single crystals, and irregular particles disappeared nearly (Fig. 4(C)). It implied that a high reaction temperature favored the growth of regular plate single crystals in the hydrothermal process. In addition, it can be seen in Fig. 4(C) that most of the Bi4Ti3O12 nanoplates have uniform distribution in both rim size and thickness. The average rim size of an individual nanoplate is about 200 nm and thickness of 15– 30 nm. It can be observed that the nanoplates stack in disorder and aggregate, some interlude one another and some parallel one another, which may be due to the high surface energy of nanoplates. The growth mechanism is useful in predicting the growth and evolution of the single crystals. There have been two formation mechanisms proposed for the hydrothermal reaction.17) One of them is dissolution and recrystallization mechanism, the other formation mechanism is in situ transformation process. The formation mechanism of the Bi4Ti3O12 single crystals may be belongs to dissolution and recrystallization process. The precursor used in the present study was a coprecipitation containing titanium and bismuth cations. When NaOH was used as a mineralizer, the precursor was dissolved in an alkali solution under the conditions of high temperature and high pressure. Under the suitable processing condition of hydrothermal treatment, the amorphous phase formed firstly. The amorphous was an intermediate phase to crystalline Bi4Ti3O12 phase. It is well known that 266

Fig. 4. SEM images of the Bi4Ti3O12 powders prepared using reaction temperatures. (A) 100°C, (B) 140°C, (C) 180°C.

the amorphous phase, in general, should have higher free energy than the crystalline phase in the case of the same chemical composition. As the reaction temperature and/or time increased, the crystalline Bi4Ti3O12 particles were formed by the processing of nucleation, precipitation, dehydration and growth, in the expense of the redissolution of the amorphous owing to their higher free energy than the crystalline phase of the Bi4Ti3O12, corresponding to higher solubility in the same media. In addition, the crystal structure of bismuth titanate can be described as a sequence of alternating bismuth oxide and perovskite-like layers stacked along the c-axis.1),2),4) The two layer are linked by the bond between a Bi ion in the BiO layer and an apex oxygen in the perovskite-like layer. This bond is the weakest in the lattice. Under the hydrothermal homogeneous system, this factor make the growth rate of a axis and c axis much faster than that of b axis, resulting in a platelet morphology with platelet faces composed of (010) planes. Therefore, there were only pure regular plate shape single crystals ultimately. The UV-vis absorption spectrum of the Bi4Ti3O12 nanoplates prepared in 2 M NaOH solution at 180°C for 48 h is given in Fig. 5. The shown spectrum is corrected for the solvent contribution. The absorption spectrum of the as-prepared Bi4Ti3O12 nanoplates shows a well-defined exciton band at 361 nm and red-

JCS-Japan


Acknowledgements The project was funded by National Natural Science Foundation of China (No. 50702022).

References 1) 2) 3) 4) 5)

Fig. 5. UV-vis absorption spectrum of the Bi4Ti3O12 powders prepared using 2M NaOH solution, 180°C, and 48 h served as mineralizer, reaction temperature, and reaction time, respectively.

6) 7) 8)

shifted with respect to those from the microemulsion method (350 nm).18) The band edge absorption begins with the wavelength at ~800 nm. The interesting light absorption in the visible spectral range suggests that more absorption states or defect energy bands exist in the Bi4Ti3O12 nanoplates, which maybe related to the specific hydrothermal condition in the synthesis of Bi4Ti3O12 nanoplates. The as-prepared Bi4Ti3O12 nanoplates may be good candidate for visible-light photocatalysis materials from the viewpoint of practical applications as a result of their good light absorption in visible light as well as partial ultraviolet light.

4. Conclusions In summary, Bi4Ti3O12 nanoplates were successfully synthesized through the present hydrothermal method. The crystallinity of the Bi4Ti3O12 nanoparticles gradually increased with increasing reaction temperatures and times. Our XRD, SEM, and UV-vis spectrum studies revealed that the Bi4Ti3O12 had a typical bismuth layered perovskite structure, a plate-like shape with the average sizes of about 200 nm and thickness of 15–30 nm, and novel optical property. The unique structure and optical property of the Bi4Ti3O12 nanoplates are expected to find applications in catalyst.

9) 10) 11) 12) 13)

14) 15) 16) 17) 18)

J. F. Dorrian, R. E. Newnham and D. K. Smith, Ferroelectrics, 3, 17–27 (1971). X. F. Du and I. W. Chen, J. Am. Ceram. Soc., 81, 3253–3259 (1998). K. Shoji, Y. Uehara and K. Sakata, Jpn. J. Appl. Phys., 35B, 5126–5128 (1996). Q. B. Yang, Y. X. Li, Q. R. Yin, P. L. Wang and Y. B. Cheng, J. Eur. Ceram. Soc., 23,161–166 (2003). Z. C. Ling, H. R. Xia, W. L. Liu, H. Han, X. Q. Wang, S. Q. Sun, D. G. Ran and L. L. Yu, Mater. Sci. Eng. B, 128, 156– 160 (2006). S. H. Ng, J. Xue and J. Wang, J. Am. Ceram. Soc., 85, 2660– 2665 (2002). L. G. Van Uitert and L. Egerton, J. Appl. Phys., 32, 959–963 (1961). T. G. Lupeiko, S. S. Lopatin and L. A. Lisutine, Izv. Akad. Nauk SSSR, 22, 801–805 (1986). Y. Osamu, M. Noboru and H. Ken, Br. Ceram. Trans. J., 90, 111–113 (1991). P. H. Xiang, Y. Kinemuchi, T. Nagaoka and K. Watari, Mater. Lett., 59, 3590–3594 (2005). H. S. Gu, A. X. Kuang and X. J. Li, Ferroelectrics, 211, 271– 280 (1998). Q. B. Yang, Y. X. Li, Q. R. Yin, P. L. Wang and Y. B. Cheng, Mater. Lett., 55, 46–49 (2002). H. S. Gu, Z. L. Hu, Y. M. Hu, Y. Yuan, J. You and W. D. Zou, Colloid. Surface. A: Physicochem. Eng. Aspects., 315, 294– 298(2008). D. Chen and X. Jiao, Mater. Res. Bull., 36, 355–363 (2001). P. Pookmanee, P. Uriwilast and S. Phanichpant, Ceram. Int., 30, 1913–1915 (2004). R. A. Nyquist and R. O. Kagel, “Infrared Spectra of Inorganic Compound.” Academic Press (1971) Fig. 105. R. Gut, E. Schmid and J. Serralach, Helv. Chim. Acta., 54, 609–613 (1971). L. J. Xie, J. F. Ma, Z. Q. Zhao, H. Tian, J. Zhou, Y. G. Wang, J. T. Tao and X. Y. Zhu, Colloid. Surface. A., 280, 232–236 (2006).

267