CRYSTAL GROWTH & DESIGN
Synthesis of BaTiO3 Nanowires at Low Temperature
2007 VOL. 7, NO. 12 2713–2715
Changlong Jiang, Katagiri Kiyofumi, Yifeng Wang, and Kunihito Koumoto* Nagoya UniVersity, Graduate School of Engineering, Nagoya, 464-8603 Japan ReceiVed October 27, 2006; ReVised Manuscript ReceiVed September 28, 2007
ABSTRACT: In this paper, we report the facile synthesis of single-crystalline BaTiO3 nanowires at low temperature (50 °C) in ethanol solution by using inorganic raw materials. The nanowires have a uniform diameter of 100 nm and a length up to 10 µm. X-ray diffraction patterns confirmed that it is the tetragonal phase of BaTiO3, transmission electron microscopy studies proved that the nanowires grew perpendicular to the [110] axis. This process involves low-cost, all inorganic raw materials, unlike sol–gel or hydrothermal syntheses that employ organometallic precursors or tradition preparations that need high temperature. Some experimental factors such as reaction temperature and aged time were found to be vital to the final formation of the nanowires.
1. Introduction During the past decade one-dimensional (1D) nanostructures, such as nanowires,1–3 nanorods,4–6 and nanotubes,7–9 have received considerable attention from the scientific community. Much effort has been employed to probe the electronic,7–9 optical,10 and magnetic11 properties of these nanostructures because they exhibit physical and chemical properties different from their bulk counterparts.12 BaTiO3 (BT) is among the important electroceramic dielectric materials of major interest for ferroelectric applications.13–15 It exhibits large nonlinear optical coefficients and large dielectric constants, which can be found in widespread applications in the manufacture of multilayer capacitors, thermistors, and electro-optical devices,16 electromechanical devices, transducers, capacitors, actuators, high-k dielectrics, dynamic random-access memory, and fieldeffect transistors.17,18 Because these effects depend on structure and finite size, considerable efforts have been expended for the controllable synthesis of crystalline materials and thin films of these ferroelectric materials.19 The traditional techniques for the preparation of BT nanostructures rely on high pressure, high temperature, surfacecapping agents, or an organometallic precursor-mediated growth process.20–25 Therefore, seeking a simple route for the low-cost, lower-temperature, large-scale, controlled growth of the nanostructures is highly desired and is important for exploring 1D nanomaterials for applications in nanosystems and nanodevices. The development of soft-chemistry routes for the synthesis of BT nanocrystals has also started recently.26–29 Herein we report the facile synthesis of BT nanowires at low temperature (50 °C) in ethanol solution. This process involves low-cost, all inorganic raw materials, unlike sol–gel or hydrothermal syntheses that employ organometallic precursors.
2. Experimental Section In a typical reaction, 0.0055 mol of Ba(OH)2 · 8H2O and 0.005 mol of H2TiO3 powders were added to a beaker containing 60 mL of ethanol solution, and then the beaker was put into a 50 °C water bath for 4 h under magnetic stirring. After the reaction was completed, the beaker was taken out, cooled to room temperature, and aged for 12 h. The obtained product was washed with 0.1 M formic acid, ethanol, and distilled water, respectively. The final sample was dried at 50 °C for 6 h in a vacuum for further characterization. The phase of the as-prepared products was determined by X-ray diffraction (XRD) using a Philips X’Pert PRO SUPER X-ray diffrac* To whom correspondence should be addressed. E-mail: koumoto@ apchem.nagoya-u.ac.jp.
Figure 1. XRD pattern of BT nanowires; all reflections can be assigned to the tetragonal phase without other crystalline byproducts. tometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.541874 Å). Scanning electron microscopy (SEM) and fieldemission scanning electron microscopy (FE-SEM) were applied to investigate the morphology, which were carried out with an X-650 scanning electron microanalyzer and a field-emission scanning electron microanalyzer (JEOL-6300F, 15 kV) respectively. Transmission electron microscopy (TEM) images were taken on a Hitachi model H-800 instrument with an accelerating voltage of 200 kV.
3. Results and Discussion A representative XRD pattern of the as-synthesized product is given in Figure 1. All the diffraction peaks can be assigned to the tetragonal phase of BT without any indication of crystalline byproducts such as BaCO3 or TiO2, in good agreement with the reported data (JCPDS, 81-2203). Representative SEM images of BT nanowires are shown in Figure 2. An overview image (Figure 2a) at low magnification illustrates that the sample consists entirely of barium titanate nanowires on a large scale. The length of the nanowires is up to 10 µm. In Figure 2b, the pattern with high magnification demonstrates that the as-obtained nanowires have a uniform diameter of about 100 nm. TEM images of the BT nanowires obtained from the syntheses are presented in Figure 3. Analysis of the pattern in Figure 3a reveals that the diameters of the BT nanowires range from 50 to 120 nm. Nanowire lengths vary from 1 µm to a few micrometers; some short nanorods might be formed during the
10.1021/cg0607607 CCC: $37.00 2007 American Chemical Society Published on Web 11/03/2007
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Figure 2. Representative SEM images of as-synthesized BT nanowires. (a) An overview image proves the exclusive presence of the BT nanowires on large scale; (b) high-magnification photo of the nanowires.
Figure 3. (a) TEM image of as-synthesized BT nanowires; (b) TEM pattern of a 100-nm diameter BT nanowire; (c) a high-resolution TEM image of the nanowire that shows lattice fringes perpendicular to the [110] direction, and the inset is the electron diffraction pattern, showing its singlecrystal structure.
Figure 4. (a) SEM image of the sample obtained at 70 °C for 4 h; (b) SEM pattern of the sample prepared without aging treatment; (c) SEM pattern of the sample prepared with aging for 6 h.
ultrasonic dispersing of the sample in alcohol for TEM characterization. Figure 3b shows a typical single BT nanowire 100 nm in diameter and 8 µm in length. The TEM results are in accordance with the SEM observations as shown in Figure 2. Analysis of the electron diffraction pattern (inset in Figure 3c) demonstrates that the nanowires are composed of crystalline BT with a tetragonal structure. A representative high-resolution TEM (HRTEM) image of the nanowire in Figure 3c also confirms the single-crystalline nature of the nanowires. Figure
3c clearly shows the lattice fringes of the BT nanowires. The calculation of the lattice spacing and the analysis of its orientation firmly indicated that the nanowires grew perpendicular to the [110] axis. To probe the formation mechanism of the single-crystalline BT nanowires under mild condition, a series of experiments were performed. The reaction temperature and the aged time after the reaction might play important roles in the growth of the final nanowires. On the basis of the experiment results, the
Synthesis of BaTiO3 Nanowires
possible formation mechanism of the BT nanowires might be dominated by the Ostwald ripening process.30,31 When the reaction was heated at 70 °C for 4 h, the obtained sample consisted of BT microrods of about 1 µm in diameter and over 10 µm in length, since the crystal growth rate is large at higher temperature. In addition, the product obtained without aging treatment only contains BT nanoparticles not nanowires or nanorods, whereas when the the sample was aged for a short time (6 h), the final sample consisted of short nanorods with nanoparticles. However the specific formation mechanism of the singlecrystalline BT nanowires is not yet clear and warrants further investigation, and the related study is still underway.
4. Conclusions The present research demonstrates that single-crystalline BT nanowires with a tetragonal structure can be fabricated at low temperature using inorganic raw materials in an ethanol solution. This process involves low-cost, all inorganic raw materials, unlike sol–gel or hydrothermal syntheses that employ organometallic precursors or tradition preparation that needs high temperature. These nanowires should serve as an ideal candidate for fundamental studies of nanoscale ferroelectricity and piezoelectricity. The synthetic strategy presented here may be extended to other functional inorganic nanowires with different chemical composition by choosing appropriate synthetic conditions. Acknowledgment. This work is supported by the 21st Century COE program “Nature-Guided Materials Processing” of Japan.
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