1748
J. Phys. Chem. C 2010, 114, 1748–1751
Synthesis and Characterization of Single-Crystalline BaTi2O5 Nanowires Zhao Deng,† Ying Dai,*,†,‡ Wen Chen,†,‡ and Xinmei Pei† State Key Laboratory of AdVanced Technology for Materials Synthesis and Processing, and School of Materials Science and Engineering, Wuhan UniVersity of Technology, Wuhan 430070, People’s Republic of China ReceiVed: April 8, 2009; ReVised Manuscript ReceiVed: NoVember 24, 2009
Large-scale single-crystalline BaTi2O5 nanowires were synthesized using BaC2O4 · H2O and TiO2 powders as precursors by a simple molten salt method. The as-synthesized BaTi2O5 nanowires were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The results show that the BaTi2O5 nanowires are single crystalline with the preferred growth direction of [011]. The phase evolution of the synthesis process has been investigated. Local piezoresponse force measurements indicate that the BaTi2O5 nanowires have switchable polarization at room temperature. 1. Introduction Ferroelectric materials constitute an important class of multifunctional compounds, attractive for numerous technological applications such as nonvolatile memory devices, sensors, multilayer capacitors, and microactuators. One-dimensional (1D) nanomaterials including ferroelectric nanowires are of fundamental importance for their potential applications as building blocks in nanoelectronics.1-3 The technologically most important ferroelectric material is PbZr1-xTixO3 (PZT). Recently, a significant effort has been directed toward the design of a new lead-free ferroelectric for applications, as the toxicity of Pb causes serious environmental problems. The newly found leadfree ferroelectric BaTi2O5 is a very promising material.4-7 It possesses a higher Curie temperature (703 ∼ 748 K), higher dielectric constant, and lower dielectric loss as compared to BaTiO3. The spontaneous polarization of the needle-shaped BaTi2O5 crystal is approximately 7 µC/cm2 at room temperature, and the dielectric constant along the b-axis is very high, reaching 30 000 at its TC ) 703 K.8 Obviously, the ferroelectric property of BaTi2O5 crystals has a relationship with the crystallographic orientations. And as a new ferroelectric, understanding its ferroelectricity at low dimensionality and at nanoscale is important for its application in nanoelectronics. All these features make the synthesis and characterization of BaTi2O5 1D nanostructures a very promising work. However, to the best of our knowledge, relatively few works about the synthesis of BaTi2O5 1D nanostructures are available in the literature. Wang et al.9 prepared BaTi2O5 nanobelts through a two-step hydrothermal reaction. Briefly, sodium titanate nanobelts were first synthesized. In the second step, the obtained nanobelts were ion-exchanged with barium ions under alkaline condition. The whole synthesis process is complex, and the obtained nanobelts are with poor crystallinity. Yu et al.10 reported that BaTi2O5 nanobelts were obtained just using BaTiO3 polycrystal powders as precursor by a molten salt method. However, they did not explain how the cubic phase BaTiO3 powders transformed to monoclinic phase BaTi2O5, and there is no description concerning the nanobelts yield. Moreover, little * Corresponding author. Tel.:+86-27 87887684. Fax.: +86-27-87864580. E-mail address:
[email protected]. † School of Materials Science and Engineering. ‡ State Key Laboratory of Advanced Technology for Materials Synthesis and Processing.
has been reported on the properties of 1D BaTi2O5 nanostructures. In this paper, large-scale single-crystalline BaTi2O5 nanowires were successfully synthesized using barium oxalate and TiO2 as precursors in a molten mixed salt medium. The phase evolution of the synthesis process has been investigated. Piezoresponse force microscope measurement has been employed to characterize the BaTi2O5 nanowires’ property. 2. Experimental Section The experiments were carried out similar to the one described in ref 11. Barium oxalate (commercial, CR), TiO2 (commercial, anatase, AR), NaCl (commercial, AR), and KCl (commercial, AR) were mixed (molar ratio 1:2:20:20) and ground for 25 min in an agate mortar. The mixture was then placed in a corundum crucible and annealed at 870 °C for different holding times (0.5, 1, 3, 6, and 9 h). The product was collected after naturally cooling the furnace to room temperature and then washed and centrifugated several times with distilled water until no free chloride ions were detected by a silver nitrate solution. The product was finally dried at 80 °C for 12 h. The as-prepared products were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy measurements. The XRD analyses were carried out on a Philips vertical X-ray diffractometer (PW3050/60, MPSS) using Cu KR radiation. The morphologies were directly examined by SEM using JSM-5610LV and HITACHI s-4800. For TEM observations, the nanowires were ultrasonically dispersed in ethanol and then dropped onto carbon-coated copper grids. TEM observations were carried out on a JEM-2100F. An energydispersive X-ray spectrometer (EDS) was attached to the JEM2100F. Raman spectroscopy was performed at room temperature in a Renishaw RM-1000 Raman spectrometer, employing an Ar+ laser for excitation (λ ) 514 nm). The local polarization switching behaviors of the nanowires were characterized using high sensitivity piezoresponse force microscopy (PFM).12,13 The characterization was conducted on a scanning probe microscope (SEIKO SPI4000N). A silicon tip coated with Ru (Micro cantilever, SI-DF3-R) was used. The spring constant of the cantilever was 1.6 N/m, and the free resonance frequency was 27 kHz. For sample preparation, BaTi2O5 nanowires dispersed in water were drop-coated directly onto a highly oriented pyrolytic graphite (HOPG) substrate.
10.1021/jp909898z 2010 American Chemical Society Published on Web 01/07/2010
Single-Crystalline BaTi2O5 Nanowires
J. Phys. Chem. C, Vol. 114, No. 4, 2010 1749
Figure 3. SEM micrographs of the synthesized BaTi2O5 nanowires (870 °C, 9 h).
Figure 1. XRD pattern of the synthesized BaTi2O5 products (870 °C, 9 h) and the standard pattern of JCPDS No.85-0476.
Figure 2. Raman spectrum of the synthesized BaTi2O5 products (870 °C, 9 h).
3. Results and Discussion
Figure 4. (A) TEM images of the as-synthesized BaTi2O5 nanowires. (B), (C) HRTEM of a single BaTi2O5 nanowire; inset of (B) is the corresponding SAED pattern. (D) EDS spectra of the BaTi2O5 nanowire.
The XRD pattern of the prepared BaTi2O5 sample (870 °C, 9 h) is shown in Figure 1, which could be readily indexed to the monoclinic phase of BaTi2O5 with the unit-cell parameters a ) 9.410 Å, b ) 3.930 Å, c ) 16.892 Å, and β ) 103.03° (JCPDS No. 85-0476). All the peaks in the pattern can be well indexed with the JCPDS card, which demonstrates that the sample is pure phase BaTi2O5. In ref 10, the XRD result of the BaTi2O5 nanobelts was also indexed to the JCPDS card 850476, but the major peaks at 25.6°, 28.3°, and 35.2° did not appear and the intensity of the appeared peaks was not in accord with JCPDS card 85-0476. Raman spectroscopy analysis as a supplementary method was chosen to identify the phase and purity of the BaTi2O5 products. A representative Raman spectrum of BaTi2O5 nanowires at room temperature is shown in Figure 2. All the observed peaks including two strong bands at 341, 589 cm-1 and other bands at 97, 154, 181, 217, 243, 280, 309, 369, 412, 438, 485, 529, 645, 706, 780, and 879 cm-1 can be indexed to a pure BaTi2O5 phase according to the previous literature.14,15 It confirms that the prepared product (870 °C, 9 h) is pure phase BaTi2O5. Figure 3 shows the SEM micrographs of the as-prepared BaTi2O5 nanowires with high yield. The low-magnification SEM image (Figure 3A) illustrates that the BaTi2O5 products are quite uniform and mainly consisted of well-defined straight nanowires with a length reaching up to tens of micrometers. A closer examination of these BaTi2O5 nanowires in Figure 3B (inset of Figure 3B shows the end of a single nanowire) indicates that
the nanowires are clearly faced and have a rectangular cross section with widths between 80 and 200 nm and thicknesses between 70 and 150 nm. TEM and HRTEM results provide further insight into the microstructural details of the nanowires. Figure 4A is a TEM image of the as-synthesized BaTi2O5 nanowires. It can be seen that the nanowires have widths ranging from 80 to 200 nm and lengths reaching up to several micrometers, in good agreement with the SEM results. The HRTEM image of a single BaTi2O5 nanowire is shown in Figure 4B. The enlargement of a portion of the nanowire in Figure 4C exhibits clear lattice fringes, indicating a high crystallinity of the nanowire. The lattice spacing of 9.41 Å is recognized and can be ascribed to the (100) crystalline plane of BaTi2O5. The corresponding SAED pattern taken from the edge of the nanowire (inset of Figure 4B) can be indexed to the reflection of a monoclinic BaTi2O5 crystal recorded from the [101j] zone axis and suggests that the observed BaTi2O5 nanowires grow in the [011] direction. Energydispersive X-ray spectroscopy analysis confirms that the body of the nanowires is exclusively composed of Ba, Ti, and O except for the Cu peak arising from the copper grid of the TEM sample holder (Figure 4D). The above results illustrate that high yields of single-crystalline BaTi2O5 nanowires were successfully synthesized by the molten salt method. The whole process can be easily adjusted to prepare pure BaTi2O5 nanowires on a large scale (gram quantity).
1750
J. Phys. Chem. C, Vol. 114, No. 4, 2010
Deng et al.
Figure 6. SEM and TEM images and SAED pattern of sample prepared at 870 °C holding for 1 h: (A) SEM image; (B) TEM image. Inset of (B) is the SAED pattern of the rod indicated by the arrows.
Figure 5. XRD patterns of the samples prepared at 870 °C for different holding times (0.5, 1, 3, 6, and 9 h) and the standard patterns of JCPDS No 085-0476, 071-2394, 076-0319, 079-2265, and 084-2213.
In order to understand more fundamental issues of BaTi2O5 nanowires’ formation, the precursors were heated in air at a temperature of 870 °C for different holding times (0.5, 1, 3, 6, and 9 h). The precursors consisted of BaC2O4 and TiO2 in molten salt (NaCl, KCl). The evolution of the samples’ phase composition is presented in Figure 5. With the synthesis carried through, barium oxalate (0, the major peaks at 2θ ) 25.94° and 2θ ) 29.84°) gradually dehydrated and transformed to BaCO3 (], the major peaks at 2θ ) 23.89°) and then vanished at about 3 h. The BaCO3 phase disappeared at about 6 h. In fact, various phases exist in our molten flux system at the synthesized temperature. For example, the product obtained at 870 °C holding for 0.5 h is a mixture of BaC2O4, TiO2, BaCO3, Ba4Ti13O30, BaTiO3, and a relatively small amount of BaTi2O5. Complex chemical reactions would happen among the various phases. Although the exact reactions are difficult to be figured out at present, the trend of phase evolution in the reaction system is clear. As the holding time increased, the TiO2 (2, the major peak at 2θ ) 35.91°) gradually decreased. BaTi2O5 (b, the major peak at 2θ ) 25.62°) gradually increased, and single-phase BaTi2O5 was obtained at 870 °C for 9 h. It is interesting to note that Ba4Ti13O30 existed as an intermediate phase during the synthesis process. Even at the very beginning (0.5 h), a large amount of Ba4Ti13O30 (9, the major peak at 2θ ) 19.07°) appeared in the products. Then the Ba4Ti13O30 phase gradually decreased while the BaTi2O5 phase increased as the holding time increased, which is shown in Figure 5 (the enlarged patterns of the dashed line area, Ba4Ti13O30 peak at 2θ ) 19.07° and BaTi2O5 peak at 2θ ) 19.34°). To further identify Ba4Ti13O30 intermediate phase, SEM and TEM results of the product (870 °C for 1 h) are shown in Figure 6. The product consisted of nanowires and particles. The SAED pattern of the nanowire in Figure 6B can be indexed to Ba4Ti13O30 (JCPDS 084-2213). Furthermore, the SAED patterns taken from different nanowires are identical. It reveals that intermediate phase Ba4Ti13O30 indeed exists as 1D nanostructures during the synthesis of BaTi2O5 nanowires. Although the exact process of the intermediate
Figure 7. (A) Topography of BaTi2O5 nanowires. (B) Local piezoelectric displacement-voltage loop and piezoelectric hysteresis loop of the BaTi2O5 nanowire.
Ba4Ti13O30 transformed to BaTi2O5 needs further investigation, we believe that the 1D nanostructures of Ba4Ti13O30 intermediate phase play an important role for the synthesis of BaTi2O5 nanowires. In ref 10, monoclinic phase BaTi2O5 nanobelts were obtained from cubic BaTiO3 polycrystal powders directly. The authors did not explain how the Ba/Ti ratio changed from 1:1 (BaTiO3 polycrystal powders) to 1:2 (BaTi2O5 nanobelts). Different from their work, the method used in this study employs the BaC2O4 · H2O and TiO2 powders as precursors (with the Ba/ Ti ratio of 1:2), and the BaTi2O5 nanowires are obtained after a series of reactions at synthesis temperature. The local polarization switching behavior and effective piezoelectric coefficients d*33 of the BaTi2O5 nanowires were characterized using PFM. Figure 7A gives the topography of the BaTi2O5 nanowires. The measurement was achieved by keeping the PFM tip fixed above the nanowire and applying a dc voltage from -9 to 9 V while recording the piezoresponse signal.16 The local piezoelectric displacement-voltage loop and polarization hysteresis loop of the BaTi2O5 nanowire are shown in Figure 7B. As the figure shows, a typical well-shaped “butterfly” loop is observed. The maximum effective d*33value is estimated to be about 300 pm/V for the BaTi2O5 nanowire. The result shows clearly that polarizations are switchable in the BaTi2O5 nanowires. 4. Conclusions In summary, large-scale single-crystalline BaTi2O5 nanowires have been synthesized through a molten salt route using barium oxalate and TiO2 powders as the precursors. The as-synthesized BaTi2O5 nanowires all have a uniform structure with rectangular cross section (about 80-200 nm in width, 70-150 nm in thickness) and lengths reaching up to tens of micrometers. The phase evolution of the precursors heated at 870 °C for different holding times has been investigated. Pure phase BaTi2O5 nanowires were obtained after heating at 870 °C for 9 h. The intermediate phase Ba4Ti13O30 exists as 1D nanostructures and plays an important role in the synthesis of BaTi2O5 nanowires. Piezoresponce force microscope measurements indicated that
Single-Crystalline BaTi2O5 Nanowires the local polarization is switchable at room temperature. The method in this study provides a simple and convenient route for preparing low-dimensional nanostructures of this kind of ternary important oxides. As there is increasing demand for leadfree ferroelectrics for applications, this new ferroelectric 1D nanomaterial will have unique applications as building blocks in nanoelectronics. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 50672072, 50972115, 50932004) and the Ph.D. Programs Foundation of Ministry of Education of China (No. 20090143110002). References and Notes (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (2) Wang, Z. Y.; Hu, J.; Yu, M. F. Appl. Phys. Lett. 2006, 89, 263119. (3) Wang, Z. Y.; Hu, J.; Yu, M. F. Nanotechnology 2007, 18, 235203.
J. Phys. Chem. C, Vol. 114, No. 4, 2010 1751 (4) Xu, J.; Akishige, Y. Appl. Phys. Lett. 2008, 92, 052902. (5) Beltran, H.; Gomez, B.; Maso, N.; Cordoncillo, E.; Escribano, P.; West, A. R. J. Appl. Phys. 2005, 97, 084104. (6) Xu, J.; Akishige, Y. Ferroelectrics 2007, 356, 316. (7) Akishige, Y. Jpn. J. Appl. Phys., Part 1 2005, 44, 7144. (8) Akishige, Y.; Fukano, K.; Shigematsu, H. Jpn. J. Appl. Phys., Part 2 2003, 42, 946. (9) Wang, L.; Li, G. C.; Zhang, Z. K. Mater. Res. Bull. 2006, 41, 842. (10) Yu, J. Y.; Tang, S. L.; Wang, R. L.; Shi, Y. G.; Nie, B.; Zhai, L.; Zhang, X. K.; Du, Y. W. Cryst. Growth Des. 2008, 8, 1481. (11) Mao, Y. B.; Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2003, 125, 15718. (12) Deng, X. Y.; Wang, X. H.; Wen, H.; Chen, L. L.; Chen, L.; Li, L. T. Appl. Phys. Lett. 2006, 88, 252905. (13) Wang, X. H.; Deng, X. Y.; Wen, H.; Li, L. T. Appl. Phys. Lett. 2006, 89, 162902. (14) Javadpour, J.; Eror, N. G. J. Am. Ceram. Soc. 1988, 71, 206. (15) Hushur, A.; Kojima, S.; Shigematsu, H.; Akishige, Y. J. Korean Phys. Soc. 2005, 46, 86. (16) Wen, H.; Wang, X. H.; Deng, X. Y.; Li, L. T. Appl. Phys. Lett. 2006, 88, 222904.
JP909898Z
