CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 2971–2973
Communications Synthesis of Single-Crystalline Barium Tetratitanate Nanobelts via Self-Sacrificing Template Process Xianke Zhang, Shaolong Tang,* Jiangying Yu, Lin Zhai, Yangguang Shi, and Youwei Du Nanjing National Laboratory of Microstructures, Department of Physics and Jiangsu ProVincial Laboratory for NanoTechnology, Nanjing UniVersity, Nanjing 210093, China ReceiVed October 24, 2008; ReVised Manuscript ReceiVed May 14, 2009
ABSTRACT: Single-crystalline BaTi4O9 nanobelts 200-500 nm in width and 30-100 µm in length have been synthesized by ion exchange between Na2Ti6O13 nanobelts and BaCl2, a so-called self-sacrificing template process. The Na2Ti6O13 and BaTi4O9 nanobelts were characterized by a range of methods including powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy, and selected area electron diffraction. Furthermore, the formation mechanism of BaTi4O9 nanobelts has also been discussed in the viewpoint of the crystal structure. One-dimensional (1D) nanostructures, such as nanorods, nanowires, nanotubes, and nanobelts, have attracted a wide range of interests because of their fascinating size-dependent optical, electronic, magnetic, thermal, mechanical, and chemical properties.1 Binary nanowires and nanobelts, for example, binary oxides ZnO, TiO2, SnO2, Mn3O4,2-5 sulfides ZnS, CdS, Bi2S3,6-8 and other binary compounds Sb2Te3, SiC, MgB2, ZnSe,9-12 have been synthesized and studied. Comparatively little work has been performed on the fabrication of very important 1D ternary transition metal oxide nanostructures, which might exhibit novel sizedependent properties on nanoscale, compared with 1D nanostructures of elements and binary compounds. Barium titanate is an important kind of dielectric, ferroelectric, piezoelectric, and electrostrictive material. BaTiO3 nanowires and nanonods have been synthesized.13-15 Meanwhile, the physical properties of individual single-crystalline BaTiO3 nanowires have been discussed.16 Mao et al. proposed a wet-chemical, hydrothermal synthesis, using a titanium oxide (TiO2) nanotube as a precursor material to generate BaTiO3 nanotubes.17 Wang et al. have reported the synthesis of ferroelectric BaTi2O5 nanobelts at low temperature through a two-step hydrothermal process.18 In a recent paper,19 we presented use of BaTiO3 polycrystal powders as the precursor by a molten-salt medium process to prepare long single-crystalline BaTi2O5 nanobelts. However, the synthesis of different kinds of barium titanate 1D nanostructures still is a great challenge to materials researchers despite intensive experimental efforts. With the development of the microwave communication and the exploitation of the high-frequency wireless electric wave, BaTi4O9 microwave dielectric ceramics have attracted wide attention.20,21 What is more, BaTi4O9 combined with RuO2 is one of the important photocatalysts for water decomposition that has recently emerged as a new class of materials.22,23 In order to improve the functional properties such as photocatalytic activity, it is highly desirable to * Corresponding author. Tel: +86 25 83593817; e-mail:
[email protected].
control the size and morphology of this material, so the photocatalytic ability of BaTi4O9 evidently may be improved if it can be transformed into a 1D nanostructure. In this communication, we present a facile approach in molten salt via ion exchange to synthesize single-crystal BaTi4O9 nanobelts for the first time. It is worth noting that our method is low-cost, uses all inorganic raw materials, no organic dispersant or surfactant, and the most green chemistry route. In a typical procedure of fabricating BaTi4O9 nanobelts, first, 0.05 g of TiO2 (anatase) nanoparticles and 10 g of NaCl were mixed and ground for 20 min. The mixture was then placed in an alumina crucible and annealed at 950 °C for 2 h in a crucible furnace. Second, a mixture of ground 0.5 g of BaCl2 and 1.5 g of NaCl was added into the reactants at the end of 2 h. This process continued for 1.5 h, and the crucible was subsequently naturally cooled to room temperature. Samples were collected, washed several times with distilled water to remove the residual NaCl and BaCl2, and then dried at 90 °C in a drying oven. The X-ray diffraction (XRD) pattern was measured in a D/maxgrA diffractometer with Cu Ka radiation (λ ) 1.54 Å) at 40 kV and 100 mA. The observations of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) patterns were carried out in an S-3400N II instrument, operating at 20 kV. Transmission electron microphotographs (TEM) and selected-area electron diffraction (SAED) were obtained in a JEM-4000EX instrument. The samples for TEM observations were prepared by deposition of a drop of the colloidal dispersion of Na2Ti6O13/ BaTi4O9 onto 200 mesh Cu grids coated with a carbon layer. In our experiment, we found that anatase TiO2 could react with molten NaCl to form single-crystalline sodium titanate nanobelts, Na2Ti6O13 nanobelts. Teshima et al. first reported the fabrication of Na2Ti6O13 whiskers by calcination of TiO2 and NaCl mixtures.24 The phase purity of the products was examined on a Rigaku-D X-ray diffractometer. Figure 1b shows the XRD pattern of the sodium titanate nanobelts. All of the diffraction peaks can be
10.1021/cg801196u CCC: $40.75 2009 American Chemical Society Published on Web 05/27/2009
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Figure 1. XRD pattern of the obtained (a) BaTi4O9 and (b) Na2Ti6O13 nanobelts.
Figure 3. (a) SEM image of the as-synthesized BaTi4O9 nanobelts. (Inset) EDS analysis of the BaTi4O9 nanobelts. (b) TEM image of the BaTi4O9 nanobelts. (c) An individual single crystalline BaTi4O9 nanobelts. Inset of (c) is the SAED pattern recorded from the [1, 0, 2j] zone axis.
Figure 2. (a) SEM image of the as-synthesized Na2Ti6O13 nanobelts. (Inset) Energy-dispersive X-ray spectroscopy analysis of the as-prepared Na2Ti6O13 nanobelts. The Al peak originates from the substrate. (b) TEM image of the Na2Ti6O13 nanobelts, indicating the width of nanobelts is about several hundred nanometers. (c) A typical single Na2Ti6O13 nanobelts. Inset of (c) is the selected area electron diffraction (SAED) pattern recorded from the [1, 0, 0] zone axis.
indexed to Na2Ti6O13 (JCPDS 73-1398) with the monoclinic phase. The morphology of sodium titanate nanobelts was characterized by scanning electron microscopy. Figure 2a gives low-magnification scanning image of sodium titanate nanobelts. Energy dispersive X-ray spectroscopy analysis (inset of Figure 2a) shows that the chemical components of the nanobelts are the elements Na, Ti, and O. Figure 2b presents the transmission electron microscopy image of Na2Ti6O13 nanobelts. From the SEM and TEM images, the nanobelts are 200-500 nm in width, and their lengths range from 30 µm to even longer than 100 µm. A typical single nanobelt is shown in Figure 2c. The SAED (inset of Figure 2c) pattern indicates that the nanobelt is single crystalline. The nanobelt grows along its [010] crystallographic direction, which is consistent with other related reports.25 Figure 1a presents the XRD pattern of the ultimate product. All of the peaks can be readily indexed to a pure orthorhombic structure of BaTi4O9 (JCPDS 77-1565). As shown in Figure 3a, the diameter of BaTi4O9 nanobelts is about 200-500 nm, and few nanoparticles also can be seen. EDS data indicate that these nanobelts are
Figure 4. (a) Structure of the Na2Ti6O13 crystal. (b) Schematic illustration of an individual Na2Ti6O13 nanobelt.
composed of Ba, Ti, and O (inset of Figure 3a). Figure 3b is the image of low-magnification TEM micrograph of BaTi4O9 nanobelts. An individual nanobelt is also shown in Figure 3c. SAED pattern (inset of Figure 3c) shows the diffraction spots of (201) and (211) planes, which is in agreement with the X-ray diffraction patterns shown in Figure 1a. Furthermore, the SAED patterns validated that the observed BaTi4O9 nanobelts are single crystalline. The growth direction of the nanobelt was determined to be its [010] crystallographic direction. From the viewpoint of the crystal structure, Na2Ti6O13 has a rectangular tunnel structure (Figure 4a), and the tunnel is along the [010] direction. Figure 4b presents the schematic diagram of a single Na2Ti6O13 nanobelt and the surfaces index deduced from Figure 2c. When ion-exchange between barium ions of molten BaCl2 and sodium ions of Na2Ti6O13 arises, barium ions may in principle diffuse into the Na2Ti6O13 nanobelts via the top surface (100), bottom surface (1j00), side surfaces (001) and (001j), end surfaces (010) and (01j0), whereas barium ions can hardly enter the Na2Ti6O13 nanobelts via the top and bottom surfaces because
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of the TiO6 octahedrons layers of the Na2Ti6O13 crystal in the [100] direction. The Na2Ti6O13 structure does not have an open channel along the [001] direction for the barium ions replacement of sodium ions. However, the channels are open along the [010] direction. Therefore, the ion exchange proceeded mainly along the [010] direction,26 which agrees with the growth direction of the BaTi4O9 nanobelts (as shown in Figure 3c). Furthermore, the lattice mismatch between Na2Ti6O13 and BaTi4O9 is very small (1.3%) along the [010] direction, which may easily lead to 1D structure in this direction. In conclusion, single-crystalline BaTi4O9 nanobelts have been synthesized. In the process of preparing BaTi4O9 nanobelts, it was suggested that 1D Na2Ti6O13 nanobelts served as the precursors, and molten NaCl provided an apt solvent environment for ionexchange between Na2Ti6O13 and molten BaCl2. The final products still kept the 1D nanostructure of the precursor, which confirmed the effect of the Na2Ti6O13 precursors template on BaTi4O9 nanostructures. The proposed synthetic route is thought to be a socalled self-sacrificing template process.27,28 Furthermore, the width distribution of both Na2Ti6O13 and BaTi4O9 nanobelts is almost identical (about 200-500 nm), which may give further support to the proposed mechanism. The synthetic strategy presented here may be extended to fabricate other 1D titanate nanostructures with different chemical compositions.
Acknowledgment. This work was supported by the National Key Project of 2005CB623605).
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