Controllable Hydrothermal Synthesis of KTa1-xNbxO3 Nanostructures with Various Morphologies and Their Growth Mechanisms Yongming Hu,†,‡ Haoshuang Gu,*,† Zhenglong Hu,† Wenning Di,† Ying Yuan,† Jing You,† Wanqiang Cao,† Yu Wang,‡ and H. L. W. Chan‡
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 3 832–837
State Key Lab of Ferro & Piezoelectric Materials and DeVices of Hubei ProVince, and Faculty of Physics & Electronic Technology, Hubei UniVersity, Wuhan 430062, People’s Republic of China, and Department of Applied Physics, The Hong Kong Polytechnic UniVersity, Hong Kong SAR, China ReceiVed March 9, 2007; ReVised Manuscript ReceiVed December 8, 2007
ABSTRACT: Various single-crystalline nanostructures of potassium tantalite-niobate (KTa1-xNbxO3, KTN) were synthesized by a hydrothermal method at 200 °C. Interesting structures including KTaO3 nano-octahedrons, KTa0.5Nb0.5O3, KTa0.35Nb0.65O3, and KTa0.77Nb0.23O3 nanocubes, KTa0.25Nb0.75O3 tower-like nanostructures, and KNbO3 truncated octahedrons were obtained. The amount of KOH and polyethylene glycol (PEG) is crucial for the formation of pure phase KTa1-xNbxO3 with specific morphologies. The dissolution-precipitation growth mechanism should be accountable for the formation of perfect KTN cubes, and the competition between the supersaturation ratio and the surface energies of nanoclusters may react to the growth of octahedron and truncated octahedron KTN. The experimental results show that the formation of high-quality single-crystalline KTN with tower-like nanostructures may be ascribed to the oriented attachment mechanism. Introduction Over the last several decades, complex oxides with perovskite structures (BaTiO3, PbZr1-xTixO3, KNbO3, etc.) have attracted great attention because of their interesting properties and extensive applications in science and engineering, such as transducers, infrared detectors, channel switchers, nonvolatility memory devices, and so on.1 With the miniaturization trend in device size, nanostructures of these oxides have been the focus of much research efforts due to their novel properties distinct from their bulk countparts, such as size-induced depression of the phase-transition temperature and the emergence of ferroelectricity in antiferroelectric thin films.2 Up to now, a large amount of nanomaterials with perovskite structures have been fabricated by a variety of approaches.2c,3 Specifically, a solutionbased method has been utilized for synthesizing BaTiO3, SrTiO3 nanowires/nanorods or nanotubes; an hydrothermal approach has been carried out for preparing PbZr1-xTixO3, PbTiO3, KNbO3 nanowires/nanorods or nanotubes, etc. It has been reported that the ability to specifically and controllably tune the chemical composition of nanoscale materials is critical for fundamental studies as well as for potential applications in practical devices.4 The control of particle size, shape, and crystalline structure presents some of the key issues in this area.5Most research has, so far, focused on the controllable synthesis of metal, semiconductor, and binary oxide nanostructures including Au, Ag, Cu, CeO2, SnO2, In2O3, ZnO, Al2O3, WO3/ZrO2, CdS, and ZnxCd1-xSe, etc.6 However, limited studies have been reported for the controlled synthesis of complex oxides nanostructures.4,7 Therefore, the controllable synthesis, growth mechanism, and corresponding physical properties of complex oxides with various morphologies are worth investigating. As an important ABO3-type perovskite solid solution, potassium tantalite-niobate (KTa1-xNbxO3, KTN) has attracted much * To whom correspondence should be addressed. E-mail: guhsh583@ yahoo.com.cn. † Hubei University. ‡ The Hong Kong Polytechnic University.
Figure 1. The perovskite-type crystals formed of potassium tantaliteniobate (primitive cell).
interest because of its piezoelectric, acousto-optic, nonlinear optical, and electro-optic properties, which are highly relevant for applications in band filters, holographic gratings, and electrooptic (EO) modulators, etc.8 Figure 1 gives the perovskite-type crystal form of KTa1-xNbxO3, which exhibits cubic phase structure as x < 0.4, tetragonal structure as 0.4 e x e 0.57, and orthorhombic structure as x > 0.57.9 In addition, KTN is a promising candidate for lead-free and biocompatible transducers with tunable piezoelectric responses, which have very large quadratic EO coefficients around the phase transition temperature (Tc) during the transition from the paraelectric cubic phase to the ferroelectric tetragonal phase, and the Tc from cubic to tetragonal phase can be varied from 0 K (x ) 0) to 705 K (x ) 1) by only adjusting the ratio of Ta/Nb in the KTN composition.8f,10 Hence, synthesis of KTN nanostructures with specific morphologies is required to meet different scientific and technological needs. In this article, we present a systematical synthesis of the KTa1-xNbxO3 nanostructures with different levels of niobium substitution (x ) 0, 0.23, 0.5, 0.65, 0.75, and 1) and various morphologies by a hydrothermal method at 200 °C for 24 h. The composition of the products are tunable by adjusting the
10.1021/cg070230q CCC: $40.75 2008 American Chemical Society Published on Web 01/30/2008
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ratio of Ta/Nb depending on the stoichiometric amount of KTa1-xNbxO3. On the basis of the experimental results, some possible formation mechanisms of these nanostructures are identified. Experimental Section The KTa1-xNbxO3 nanostructures with different levels of niobium substitution (0 e x e 1) and various morphologies were synthesized by controlling the hydrothermal conditions, using tantalum pentoxide (Ta2O5, g99%), niobium pentoxide (Nb2O5, g99%), and potassium hydroxide (KOH, g82%) as the precursors in a homemade stainlesssteel autoclave with a 50 mL Teflon (poly[tetrafluoroethylene]) lining. Distilled water was used in the preparation of all aqueous solutions. The representative KTa1-xNbxO3 with niobium substitution of x ) 0, 0.23, 0.5, 0.65, 0.75, and 1 were synthesized as follows. First, various amounts of KOH (20 g for KTa0.77Nb0.23O3, 24 g for KTa0.25Nb0.75O3, and KNbO3, 32 g for KTaO3, KTa0.5Nb0.5O3 and KTa0.35Nb0.65O3, respectively) were dissolved in 10 mL of distilled water. Then based on the nominal composition of KTa1-xNbxO3, stoichiometric amounts of analytical-grade Ta2O5 and/or Nb2O5 (calculated according to 0.01 mol of KTN) were added to the KOH solutions prepared above. After the sample was stirred violently for 1 h, the feedstock was introduced into the 50 mL Teflon vessel, and filled up with different additives [0.8 g of polyethylene glycol (PEG) for KTaO3, 0.16 g of PEG for KTa0.25Nb0.75O3, 1 g of PEG for KNbO3, and 30 mL of distilled water for the remaining] until it reached 80% of its volume so as to obtain the specific morphologies. Subsequently, the vessel was sealed in the autoclave and placed in a stainless steel tank to perform a hydrothermal reaction at 200 °C for 24 h. After the autoclave was cooled down naturally to room temperature in air, the obtained products were washed several times by centrifugation with distilled water and ethanol, respectively, and dried at 60 °C for 8 h. Finally, some white powder was obtained. X-ray diffraction (XRD) was performed on a Bruker AXS-D8ADVANCE X-ray diffractometer system using Cu KR (λ ) 1.5406 Å) as the radiation source. Field emission scanning electron microscopy (FESEM) measurements were carried out by JEOL JSM-6700F. Transmission electron microscopy (TEM), including high-resolution TEM (HRTEM) and selective area electron diffraction (SAED) characterizations, were taken with an FEI TECnai G2 TEM using an acceleration voltage of 200 kV.
Results and Characterization Judging from the XRD patterns in the 2θ range from 20 to 60°, all six samples display pure phase as shown in Figure 2a. The spectra in 1 and 2 can be indexed to the cubic lattices of KTaO3 and KTa0.77Nb0.23O3, which agree with the Joint Committee on Powder Diffraction Standards (JCPDS) Nos. 38-1470 and 70-2011 with space group Pm3jm (221) with lattice constants a ) 4.0046 and 4.0011 Å, respectively. The spectrum of 3 shows the single-phase tetragonal perovskite structure of KTa0.5Nb0.5O3 with lattice constants a ) b ) 3.9914 Å, and c ) 4.0452 Å. However, the spectra of 4-6 exhibit the orthorhombic perovskite structures of KTa0.35Nb0.65O3, KTa0.25Nb0.75O3, and KNbO3 (JCPDS: 32-0822), which can be validated from the asymmetric diffraction peaks at 2θ ≈ 45.5° (Figure 2b). The lattice constants for KTa0.35Nb0.65O3 are a ) 5.7087 Å, b ) 5.6741 Å, and c ) 4.0038 Å; for KTa0.25Nb0.75O3 are a ) 5.6993 Å, b ) 5.6668 Å and c ) 3.9955 Å; and for KNbO3 are a ) 5.6968 Å, b ) 5.6523 Å, and c ) 3.9954 Å. This demonstrates for orthorhombic structures that the lattice constants decrease with increasing levels of Nb. Previous studies have shown that the radius of Nb5+ ions (0.69 Å) is very close to the that of Ta5+ (0.60 Å), and the Nb5+ ions in KTN are symmetry-breaking defects residing at the Ta sites which hop among several equivalent positions.11,12 Therefore, the structure transitions from cubic, tetragonal to orthorhombic are evident from the results mentioned above.
Figure 2. (a) The XRD patterns of as-prepared typical KTN nanostructures in the 2θ range of 20° to 60°: (1) KTaO3, (2) KTa0.77Nb0.23O3, (3) KTa0.50Nb0.50O3, (4) KTa0.35Nb0.65O3, (5) KTa0.25Nb0.75O3, (6) KNbO3; (b) the 2θ diffraction peak (narrow scan) around 45.5° for the samples of all six compositions.
Figure 3 shows the FESEM images of KTN nanostructures with different levels of Nb substitution. One can see that the amount of KOH and/or additives play an important role in the formation of pure phase KTN with specific morphologies. For example, as can be seen from Figure 3a,b, KTa0.77Nb0.23O3 nanocubes with uniform size distribution (100-200 nm) can be formed if 20 g (∼7.3 M) of KOH is introduced, while KTaO3 octahedrons composed of almost perfect equilateral triangle facets with an edge size of about 500 nm can be obtained if 32 g (∼11.5 M) of KOH is introduced with 0.8 g of PEG added at the same time. When using only 32 g of KOH, KTa0.5Nb0.5O3and KTa0.35Nb0.65O3 nanocubes can be made as shown in Figure 3c,d, and the sizes of cubes vary from 50 to 500 nm. Figure 3e illustrates that tower-like KTa0.25Nb0.75O3 nanostructures can be created if 24 g (∼8.8 M) of KOH is added together with 0.16 g of PEG, which has a tip size of about 60-200 nm and a length reaching up to 3 µm. For comparison, Goh et al. have also synthesized tower-like nanostructures of KNbO3 by a hydrothermal method at 150 °C for 16 h in 15 M KOH solution.13 Figure 3f reveals that KNbO3 truncated octahedron structures can be developed if 24 g of KOH and 1 g of PEG are added. Figure 4a-c present the TEM images of the KTaO3 octahedrons with various orientations when the electron beams are parallel to the , , and , respectively, which can be noted from the scheme (only the directions are shown) in Figure 4d. It is evident that the morphologies are very regular. Figure 4e shows an HRTEM image of KTaO3 taken from one corner of a single octahedron with electron beams parallel to the directions, and the corresponding
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Figure 3. FESEM images of KTN nanostructures: (a) KTaO3 octahedron; (b) KTa0.77Nb0.23O3 nanocubes; (c) KTa0.50Nb0.50O3 nanocubes with different size distribution; (d) KTa0.35Nb0.65O3 nanocubes with different size distributions; (e) KTa0.25Nb0.75O3 tower-like nanostructures; (f) KNbO3 truncated octahedron nanostructures.
Figure 4. TEM images of KTaO3 octahedrons: (a-c) with electron beams parallel to , , and , respectively; (d) the scheme of octahedron; (e) the HRTEM image of KTaO3 octahedrons with an electron beam parallel to the direction; the inset shows the twodimensional Fourier transform patterns of the corresponding HRTEM image.
fast Fourier transform is also shown in the inset in Figure 4e. The HRTEM image clearly displays an interplanar spacing of 0.285 nm, corresponding to the (101j) or (11j0) crystal planes. In addition, it can also be seen that the corners of the octahedrons are “cut” slightly, and a small amount of defects or dislocations on the surface of KTaO3 octahedrons are visible from the twodimensional lattice fringes, which may be attributed to the environmental disturbance in the hydrothermal system. Similar results have also been observed in other octahedrons, such as Pt, In2O3 octahedrons, etc.6h,14 Figure 5 shows the TEM and HRTEM images of KTa0.25Nb0.75O3 with tower-like structures. As shown in Figure 5a, the terrace steps are clearly visible from the larger end (∼500 nm) to the small one (∼200 nm), which agree with the results in the inset of Figure 3e. Figure 5b presents the two-dimensional
lattice fringes, which indicate that the KTa0.25Nb0.75O3 is structurally uniform and single crystalline. The SAED pattern in the inset can be indexed to the [11j0] zone of the orthorhombic structure. The interplanar distances between the adjacent lattice fringes are 3.99 and 4.04 Å, corresponding to the (001) and (110) lattice spacings of 3.9955 and 4.0294 Å, respectively. The results confirm that is the preferred growth direction of the KTa0.25Nb0.75O3 tower-like structures. Mechanism Discussion As previously reported, KNbO3 nanostructures were synthesized in solution via a dissolution-precipitation process.3k Nb2O5 dissolved into Nb6O198- ion, and forms single octahedron NbO67- anions by complex transforms, which act as elementary
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Figure 5. (a) The TEM image of KTa0.25Nb0.75O3 tower-like nanostructures; (b) the HRTEM image of the KTa0.25Nb0.75O3 nanostructures with an electron beam parallel to the direction; the inset shows the SAED patterns along the [001] zone axis.
Figure 6. A schematic illustration of the possible formation mechanisms of KTN nanostructures with various morphologies synthesized by hydrothermal treatment; Ta and/or Nb atoms are located in the polyhedrons of oxygen atoms. (a) KTaO3 octahedrons; (b) KTa0.77Nb0.23O3, KTa0.50Nb0.50O3 and KTa0.35Nb0.65O3 cube; (c) KTa0.25Nb0.75O3 tower-like nanostructures; (d) KNbO3 truncated octahedron nanostructures.
species for KNbO3 perovskite that leads to KNbO3 precipitation. Similarly, the reactions for the KTN nanostructures can be formulated as follows: 3(1 - x)Ta2O5 + 3xNb2O5 + 8OH- f (1 - x)Ta6O819 + xNb6O819 + 4H2O (with 0 e x e 1)
(1)
78(1 - x)Ta6O819 + xNb6O19 + 34OH f 6(1 - x)TaO6 +
6xNbO76 + 17H2O
(2)
7+ (1 - x)TaO76 + xNbO6 + K + 3H2O f KTa1-xNbxO3 +
6OH- (3) Figure 6 illustrates schematically the formation of KTN with different levels of Nb substitutions and various morphologies under the hydrothermal environment. In this case, Ta2O5 and Nb2O5 should be dissolved simultaneously into Ta6O198- and Nb6O198- ions under the same condition (reaction 1) at the initial stage of the hydrothermal reaction. Then complex transforms occur under higher alkaline conditions which after reaction 2 result in single octahedron TaO67- and NbO67- anions. Reaction 3 is a very important step for the formation of KTN with various morphologies during the hydrothermal process, which refers to the tiny crystalline nucleation in a supersaturated medium followed by crystal growth of KTN under drastic conditions. It can be seen that nanocubes were formed exclusively only when the KOH is used, as indicated by the step of Figure 6b. In fact, only nanocubes were detected for all composition (0 e x e 1) of KTN when KOH was used (please refer to Figure 3b-d for the representative SEM images with the phase from cubic,
tetragonal to orthorhombic). This may be due to the inherent perovskite structure of KTa1-xNbxO3 materials as presented in Figure 1 and their chemical potential in the solution. To learn more about the growth mechanisms, we have conducted experiments with several typical chemical compositions (x ) 0, 0.23, 0.5, 0.65, 0.75, and 1). Vessel temperature, pressure, supersaturation ratios, and surface energies are four crucial factors for the control of different morphologies. Because the same reaction medium (distilled water), vessel volume (50 mL, 80% filling ratios), and temperature (200 °C) were applied to all the compositions during the entire hydrothermal process, the effects of pressure on the morphologies with different compositions should be coherent. Therefore, the supersaturation ratios and surface energies acted as the key factors for determining the morphologies of products. It has been reported that the surface energies associated with different crystallographic planes are different, which should follow a normal sequence of γ{111} < γ{100} < γ{110} for the bcc structure.14 A perfect singlecrystalline cube corresponds to R ≈ 0.58 (R is the growth rate in the with respect to that of ), perfect octahedrons correspond to R ≈ 1.73, and the truncated octahedrons have the {100} and {111} facets with 0.87 < R < 1.73. When only KOH was used during the entire hydrothermal process, dissolution-precipitation may be the major reaction path in a supersaturated medium as summarized by the reactions above, which becomes the main growth mechanism for the formation of perfect KTN nanocubes (Figure 6b). Studies show that the surface energies difference among several low-index crystallographic planes can be altered by manipulating certain external conditions, which then affect the final crystal morphologies.4 A higher concentration of KOH and the introduction of surfactant in the solution not only enhance the dissolvability of Ta2O5 or Nb2O5 but also influence the supersaturation of TaO67- or NbO67- anions and surface energies of nanoclusters. In our case, the surfactant PEG molecules act as the most important component being introduced into the reaction system. It is known that the nonionized surfactant PEG molecules can form chainlike structures and be encapsulated in the closed shells on the surface of nanomaterials,7a,15 which could decrease the surface energies and increase the supersaturation ratio of nanoclusters. However, it is worth noting that the shape of the KTN particle has been very sensitive to the overall contents of PEG in our studies. For example, towerlike KTa0.25Nb0.75O3 can be formed when 0.16 g PEG is used (Figure 6d), whereas KTaO3 octahedra can be formed when 0.8 g of PEG is used (Figure 6a), and truncated octahedrons of KNbO3 can be formed when 1 g of PEG is used (Figure 6c). When a small amount of PEG is used, the growth of KTN is limited by the chainlike structures of PEG molecules, and the exposed (011) crystalline plane has the largest surface energy. Thus, the growth units may preferentially land on the KTN crystal facet which is composed of exposed (011) crystalline planes to form tower-like structures. With increasing PEG content, the surfactant is adsorbed on the KTN particle facets by hydrogen bonding, minimizing the surface energy. This, in turn, further increases the difference of surface energy between (001) and (011) planes of the KTN particles. Although the difference of surface energies among {110}, {100}, {111} facets can lead to their different growth rates, the high supersaturation ratio makes the effect of the surface energies difference on the growth to be relatively small. Therefore, the growth rates perpendicular to {110}, {100}, and {111} facets become quite close, so that the octahedron and truncated octahedron growth model can be realized easily.
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various chemical compositions and morphologies. Pure phase cubic KTaO3 and KTa0.77Nb0.23O3, tetragonal KTa0.5Nb0.5O3, and orthorhombic KTa0.35Nb0.65O3, KTa0.25Nb0.75O3 and KNbO3 with perovskite structure can be fabricated by a simple hydrothermal process. The amount of KOH and nonionized surfactant PEG plays a significant role in the formation of pure phase KTa1-xNbxO3 with specific morphologies, such as KTaO3 octahedrons, KTa0.5Nb0.5O3, KTa0.35Nb0.65O3, and KTa0.77Nb0.23O3 nanocubes, KTa0.25Nb0.75O3 tower-like nanostructures, and KNbO3 truncated octahedron structures. It can be concluded that dissolution-precipitation may be the major growth mechanism for the formation of perfect KTN cubes, whereas the octahedron and truncated octahedron growth may be associated with the competition between the supersaturation ratio and surface energies of nanoclusters. In addition, the formation of high-quality single crystalline KTN with towerlike nanostructures can be ascribed to the oriented attachment mechanism. The investigation of controllable fabrication and growth mechanism provides valuable insights into the studies of fundamental properties of complex perovskite oxides with different size and various morphologies.
Figure 7. The TEM image of the tower-like structures KTa0.25Nb0.75O3 assembled by small nanoparticles under the hydrothermal process at an early stage. Inset: HRTEM images of KTa0.25Nb0.75O3 nanostructures at different areas indicated by “A” and “B”, respectively.
However, the formation of high-quality single crystalline KTN with tower-like structures should be taken into account with reference to the oriented attachment mechanism brought forward by Penn and Banfield.16 They suggested that oxide nanoparticles were favorable for oriented attachment only for organic ligands and crystalline fusion of correctly attached particles. Herein, further evidence of oriented attachment mechanism for the formation of tower-like structures can be confirmed by the high-resolution TEM investigations at an early hydrothermal stage of about 4 h as shown in Figure 7. One can see that the tower-like structures are composed of a large amount of small particles (nanoplates), which should possess high surface energy and aggregate easily at the early stage of the hydrothermal process (see inset of Figure 7). In the so-formed aggregates, the crystalline lattice planes may be almost perfectly aligned or dislocations at the contact areas between the adjacent particles, which lead to defects in the final formed bulk crystals.16In some cases, it can be observed how individual nanoplates are assembled together and/or grow side by side in certain directions of or (see inset (A) in Figure 7), and the orientations of lattice planes are the same for the adjacent particles as shown in the inset (B), Figure 7. The observed lattice planes in these images of single nanoplate are (110) and (001), respectively, which are perpendicular or parallel to the orientation of tower-like structures ( direction). It is also believed that rod formation requires anisotropic crystal growth which is usually realized when the free surface energies of various crystallographic planes differ significantly. Therefore, oriented attachment seems to be the prerequisite for the condensation step leading to crystalline fusion under our experimental conditions. Similar results were also found for the fabrication of ZnO nanorods in solution conditions.16 Conclusion The current report describes the controllable synthesis of a series of KTa1-xNbxO3 single crystalline nanostructures with
Acknowledgment. The authors thank the National Science Foundation of China (NSFC) (Grant No. 50572026) and the Natural Science Foundation Creative Team Project of Hubei Province (Grant No. 2005ABC008) for their support of this work. The authors would also like to thank the Transmission Electron Microscopy Center of Hubei University for their Technical Assistance with microscopy.
References (1) (a) Kamalasanan, M. N.; Chandra, S.; Joshi, P. C.; Mansingh, A. Appl. Phys. Lett. 1991, 59, 3547–3549. (b) Shimomura, K.; Tsurumi, T.; Ohba, Y.; Daimon, M. Jpn. J. Appl. Phys. 1991, 30, 2174–2177. (c) Scott, J. F. Ferroelectr. ReV. 1998, 1, 1–130. (d) Weber, I. T.; Rousseau, A.; Viry, M. G.; Bouquet, V.; Perrin, A. Solid State Sci. 2005, 7, 1317–1323. (2) (a) Shih, W. Y.; Shih, W. H.; Aksay, I. A. Phys. ReV. B 1994, 50, 15575–15585. (b) Ayyub, P.; Chattopadhyay, S.; Pinto, R.; Multani, M. S. Phys. ReV. B 1998, 57, R5559–R5562. (c) Urban, J. J.; Spanier, J. E.; Ouyang, L.; Yun, W. S.; Park, H. AdV. Mater. 2003, 15, 423– 426. (3) (a) Cho, S. B.; Oledzka, M.; Riman, R. E. J. Cryst. Growth 2001, 226, 313–326. (b) Mao, Y.; Banerjee, S.; Wong, S. S. J. Am. Chem. Soc. 2003, 125, 15718–15719. (c) Luo, Y.; Szafraniak, I.; Nagarajan, V.; Wehrspohn, R. B.; Steinhart, M.; Wendorff, J. H.; Zakharov, N. D.; Ramesh, R.; Alexe, M. Integr. Ferroelectr. 2003, 59, 1513–1520. (d) Liu, J. F.; Li, X. L.; Li, Y. D. J. Cryst. Growth 2003, 247, 419–424. (e) Limmer, S. J.; Cao, G. AdV. Mater. 2003, 15, 427–431. (f) Zhang, T.; Jin, C. G.; Qian, T.; Lu, X. L.; Bai, J. M.; Li, X. G. J. Mater. Chem. 2004, 14, 2787–2789. (g) Xu, G.; Ren, Z.; Du, P.; Weng, W.; Shen, G.; Han, G. AdV. Mater. 2005, 17, 907–910. (h) Hu, Y.; Gu, H.; Sun, X.; You, J.; Wang, J. Appl. Phys. Lett. 2006, 88, 193120. (i) Zheng, H.; Zhan, Q.; Zavaliche, F.; Sherburne, M.; Straub, F.; Cruz, M. P.; Chen, L. Q.; Dahmen, U.; Ramesh, R. Nano Lett. 2006, 6, 1401–1407. (j) Magrez, A.; Vasco, E.; Seo, J. W.; Dieker, C.; Setter, N.; Forró, L. J. Phys. Chem. B 2006, 110, 58–61. (k) Liu, H.; Hu, C.; Wang, Z. L. Nano Lett. 2006, 6, 1535–1540. (4) Mao, Y.; Wong, S. S. AdV. Mater. 2005, 17, 2194–2199. (5) Pacholski, C.; Kornowski, A.; Weller, H. Angew. Chem. Int. Ed. 2002, 41, 1188–1191. (6) (a) Yu, S. H.; Yang, J.; Han, Z. H.; Zhou, Y.; Yang, R. Y.; Qian, Y. T.; Zhang, Y. H. J. Mater. Chem. 1999, 9, 1283–1287. (b) Sun, Y.; Xia, Y. Science 2002, 298, 2176–2179. (c) Sun, W.; Xu, L.; Chu, Y.; Shi, W. J. Colloid Interface Sci. 2003, 266, 99–106. (d) Li, Y.; Li, X.; Yang, C.; Li, Y. J. Mater. Chem. 2003, 13, 2641–2648. (e) Zhong, X.; Han, M.; Dong, Z.; White, T. J.; Knoll, W. J. Am. Chem. Soc. 2003, 125, 8589–8594. (f) Ho, C.; Yu, J. C.; Kwong, T.; Mak, A. C.; Lai, S. Chem. Mater. 2005, 17, 4514–4522. (g) Jiang, L.; Sun, G.; Zhou, Z.; Sun, S.; Wang, Q.; Yan, S.; Li, H.; Tian, J.; Guo, J.; Zhou, B.; Xin, Q. J. Phys. Chem. B 2005, 109, 8774–8778. (h) Hao, Y.; Meng, G.; Ye, C.; Zhang, L. Cryst. Grow. Design 2005, 5, 1617–
Hydrothermal Synthesis of KTa1-xNbxO3 Nanostructures 1621. (i) Wang, D.; Song, C. J. Phys. Chem. B 2005, 109, 12697– 12700. (j) Tang, B.; Ge, J.; Zhuo, L.; Wang, G.; Niu, J.; Shi, Z.; Dong, Y. Eur. J. Inorg. Chem. 2005, 4366–4369. (k) Zhang, L.; Ai, Z.; Jia, F.; Liu, L.; Hu, X.; Yu, J. C. Chem. Eur. J. 2006, 12, 4185–4190. (7) (a) Kang, Z.; Wang, E.; Jiang, M.; Lian, S.; Li, Y.; Hu, C. Eur. J. Inorg. Chem. 2003, 370–376. (b) Han, J. T.; Huang, Y. H.; Wu, X. J.; Wu, C. L.; Wei, W.; Peng, B.; Huang, W.; Goodenough, J. B. AdV. Mater. 2006, 18, 2145–2148. (8) (a) Gulmann, R.; Hulliger, J.; Hauert, R.; Moser, E. M. J. Appl. Phys. 1991, 70, 2648–2653. (b) Kuang, A. X.; Lu, C. J.; Huang, G. Y.; Wang, S. M. J. Cryst. Growth 1995, 149, 80–86. (c) Imai, T.; Yagi, S.; Sugiyama, Y.; Hatakeyama, I. J. Cryst. Growth 1995, 147, 350– 354. (d) Bao, D. H.; Kuang, A. X. J. Cryst. Growth 1997, 171, 314– 317. (e) Wu, H. G.; Wang, S. M.; Xu, Z. X.; Fu, J. Mater. Lett. 2003, 57, 2742–2745. (f) Toyoda, S.; Fujiura, K.; Sasaura, M.; Enbutsu, K.; Tate, A.; Shimokozono, M.; Fushimi, H.; Imai, T.; Manabe, K.; Matsuura, T.; Kurihara, T. Jpn. J. Appl. Phys. 2004, 43, 5862–5866.
Crystal Growth & Design, Vol. 8, No. 3, 2008 837 (9) Yang, Z.; Guan, Q.; Wei, J.; Wang, J. J. Therm. Anal. Calorim. 1995, 45, 297–302. (10) (a) Rytz, D.; Scheel, H. J. J. Cryst. Growth 1982, 59, 468–484. (b) Guan, Q.; Wang, J.; Lian, Y.; Yang, C.; Ye, P. Appl. Phys. Lett. 1993, 63, 2186–2188. (11) Huang, C. L.; Weng, M. H. Jpn. J. Appl. Phys. 1999, 38, 5949–5952. (12) Chou, H.; Shapiro, S. M.; Lyons, K. B.; Kjems, J.; Rytz, D. Phys. ReV. B 1990, 41, 7231–7234. (13) Goh, G. K. L.; Levi, C. G.; Choi, J. H.; Lange, F. F. J. Cryst. Growth 2006, 286, 457–464. (14) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153–1175. (15) Wang, W. Z.; Zhan, Y. J.; Wang, G. H. Chem. Commun. 2001, 727– 728. (16) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969–971.
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