Facile Hydrothermal Synthesis of Yttrium Hydroxide Nanowires Nan Li,† Kazumichi Yanagisawa,*,‡ and Nobuhiro Kumada§ College of Materials Science and Engineering, Jilin UniVersity, 2699 Qianjin Street, Changchun, 130012, P. R. China, and Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi UniVersity, 2-5-1 Akbono-cho, Kochi 780-8520, Japan, and Department of Research Interdisciplinary Graduate School of Medicine and Engineering, UniVersity of Yamanashi, 4-4-37 Takeda, Kofu 400-8510, Japan
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 978–981
ReceiVed July 18, 2008; ReVised Manuscript ReceiVed September 16, 2008
ABSTRACT: Single-crystalline Y(OH)3 nanowires were synthesized from Y2O3 powder by a simple acetic acid assisted hydrothermal method under near neutral conditions. The nanowires show unusual, bundle-like morphology with diameter of 20-50 nm and aspect ratio of 600-2000. It is found that the presence of acetic acid and its concentration play a crucial role in determining the product morphology. In order to understand the formation mechanism of the Y(OH)3 nanowires, the growth process was surveyed by analyzing the samples at different growth stages, and a growth mechanism was proposed.
1. Introduction Yttrium, one of the important rare earth elements, offers applications in a broad range of fields. For example, yttrium oxide is commonly used as a host material for phosphors. Europium doped yttrium oxide is considered to be the best red oxide phosphor because of its excellent luminescent characteristics, high stability in air and resistance to degradation under applied voltages.1 Taking advantage of its high melting point (2430 °C) and phase stability, yttrium oxide has been also used as bulk ceramics for refractory applications.2 Furthermore, yttrium oxide has found applications in a wide variety of catalytic reactions owing to its basic nature.3 In these applications, the morphologies of materials are often regarded as particularly important factors that influence the chemical or physical properties of products, especially when the products are in nanometer scale. As a result, many synthetic methods, such as coprecipitation, sol-gel processing and hydrothermal synthesis, have been explored to control the shape and dimension of yttrium hydroxide and oxide. Among these methods, hydrothermal synthesis is a promising way to prepare one-dimensional (1D) nanostructured material with high-crystallinity under mild conditions. Several reports have described the hydrothermal synthesis of 1D yttrium hydroxide and corresponding yttrium oxide nanostructures, like nanowires,4,5 nanotubes,4-11 and nanobelts.12,13 Most of these nanostructures were obtained by a two-step method, i.e. preparation of precursor colloids by adding base into solutions containing yttrium salt such as yttrium nitrate or yttrium chloride followed by hydrothermal treatments in strong basic media. In the present work, we exploited a one-step hydrothermal route to prepare yttrium hydroxide nanowires under near neutral condition, where yttrium oxide powder instead of soluble yttrium salts was directly used as a starting material.
2. Experimental Section Yttrium oxide (99.9%) and acetic acid (>99.0%) were purchased from Wako pure chemical industries, Ltd. (Japan) and used without * Corresponding author. Telephone: +81 88-844-8352. Fax: +81 88-8448362. E-mail:
[email protected]. † College of Materials Science and Engineering, Jilin University. ‡ Research Laboratory of Hydrothermal Chemistry, Faculty of Science, Kochi University. § Department of Research Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi.
Figure 1. XRD pattern of (a) Y(OH)3 nanowires obtained in 6.67 × 10-2 mol/L acetic acid solution at 200 °C for 24 h and (b) standard Y(OH)3 pattern (JCPDS No. 83-2042). further purification. In a typical synthesis process, 15 mL of 6.67 × 10-2 mol/L acetic acid aqueous solution and 0.45 g of yttrium oxide were put into a 25 mL Teflon-lined autoclave. The autoclave was sealed, heated in an electric oven to 200 °C at a heating ramp of 5 °C/min with agitation and maintained at this temperature for 24 h with rotation. Then it was cooled to room temperature by air quenching. The precipitate was collected by centrifuge, washed with distilled water and dried at room temperature. Powder X-ray diffractions (XRD) were measured on a Rigaku RTP300RC diffractometer operating at 40 kV and 100 mA using Cu KR radiation (λ ) 1.54056 µm). The patterns were collected in the range of 5-70 ° in 2θ/θ scanning mode with a 0.02 ° step and scanning speed of 4 °/min. Micrographs of field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were obtained using JEOL JSM-6500F electron microscope operating at 12 kV and JEOL 2010 electron microscope operating at 200 kV, respectively. Fourier transform infrared (FTIR) spectra were measured on Shimadzu FTIR-8200PC spectrophotometer at room temperature.
3. Results and Discussion 3.1. Structural Features and Morphology. Figure 1 shows XRD patterns of the products obtained in 6.67 × 10-2 mol/L acetic acid solution by hydrothermal reaction at 200 °C for 24 h. The XRD patterns can be indexed to a pure hexagonal phase of Y(OH)3, in agreement with the reported data (JCPDS 83-2042) with lattice constants a ) 6.2610 Å and c ) 3.5440 Å. Notably, the (110) reflection is much more pronounced
10.1021/cg8007798 CCC: $40.75 2009 American Chemical Society Published on Web 01/15/2009
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Figure 2. SEM images of Y(OH)3 nanowires obtained in 6.67 × 10-2 mol/L acetic acid solution at 200 °C for 24 h at low (a) and high (b) magnification.
Figure 3. TEM (a) and HRTEM (b) image of Y(OH)3 nanowires obtained in 6.67 × 10-2 mol/L acetic acid solution at 200 °C for 24 h. Inset: electron diffraction pattern of corresponding nanowires.
compared with the literature data, suggesting the preferred growth is along c-axis. FTIR spectrum of this product shows it to be free of organic byproducts. SEM observation indicates that the sample consists of bundles of nanowires, as shown in Figure 2. These bundles are relatively monodispersed with length of 30-40 µm. TEM and HRTEM images (Figure 3) show needle-like tips of these nanowires. The diameter of these nanowires ranges from 20 to 50 nanometers. Assuming each nanowire spans the whole length of the bundle, the nanowires have a high aspect ratio of 600-2000. The inset of Figure 3b shows the SAED (selective area electron diffraction) pattern of one nanowire, revealing its single-crystalline nature. 3.2. Effect of Acetic Acid Concentration on Product Morphology. It is found that the concentration of acetic acid (HAc) plays a key role in determining the product morphology. Y(OH)3 could be obtained at concentration between 1.0 × 10-3
Figure 4. SEM images Y(OH)3 synthesized at 200 °C for 24 h with HAc concentration of 1.30 × 10-3 mol/L (a), 6.60 × 10-3 mol/L (b) and 1.30 × 10-2 mol/L (c). Insets show corresponding SEM images with high magnification.
mol/L to 9.5 × 10-2 mol/L. In this range, the morphology of the products changed from microrods to microtubes eventually to nanowires with increasing concentration. The SEM images of three typical samples with distinct morphologies are presented in Figure 4 a-c. At low HAc concentration, the obtained product is composed of microrods with diameter of 1-5 µm and length up to 10 µm. These rods aggregated together, forming a spherelike shape. A slight increase in concentration gave rise to morphological change to dispersed microrods with hexagonal cross-section. As HAc concentration was further increased, tubelike structures were obtained. These structures are found to be in a wide scale of size, ranging in diameter from less than 0.5 µm to more than 4 µm. A close observation reveals that the tubes are composed of numerous small rods. When HAc concentration was controlled to (6.0-7.0) × 10-2 mol/L, bundles of nanowires were obtained. If the concentration was
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Figure 5. SEM images of Y2O3-Y(OH)3 mixture hydrothermally synthesized in pure water at 200 °C for 72 h.
further increased to above 0.1 mol/L, an unknown phase was detected in XRD pattern, which is probably an yttrium acetate compound. It should be noted that although HAc has a great influence on the morphology of product, it is not necessary for the conversion from Y2O3 to Y(OH)3. According to the phase diagram of Y2O3-H2O system,14 Y2O3 is not stable under hydrothermal conditions at temperature lower than 550 °C. We also proved that in pure water, Y2O3 can be converted into Y(OH)3 through hydrothermal reaction. The conversion speed, however, is very slow. Our experiments indicate that after 72 h of hydrothermal reaction, some Y2O3 is still remained in the product. When HAc was introduced, even though the HAc concentration was as low as 1.0 × 10-3 mol/L, the conversion was completed within several hours, suggesting that HAc accelerates hydrothermal reaction. The SEM image of the Y2O3-Y(OH)3 mixture obtained in pure water is shown in Figure 5. The morphology of the starting material consisting of micrometer-sized irregular particles was maintained on the whole and a few needles with diameter of 80-200 nm were observed. This morphology is totally different from that of Y(OH)3 synthesized in the presence of HAc. 3.3. Growth Process. In order to understand the formation mechanism of the Y(OH)3 nanowires, the growth process was surveyed by analyzing the samples obtained at different growth stages. Figure 6. andFigure 7 shows the XRD results and SEM images of samples received after 0 h (just when the oven reached 200 °C at a heating ramp of 5 °C /min), 0.5 h, 1 h, and 2 h of hydrothermal reaction at 200 °C, respectively. When the oven reached 200 °C, the characteristic peaks of hexagonal Y(OH)3 appeared in the XRD pattern, as shown in Figure 6 a. This sample is composed of irregular blocks and a small quantity of microrods with quasi-hexagonal geometry (Figure 7a). It is apparent that these irregular blocks are unreacted Y2O3, while the microrods are Y(OH)3. A magnified SEM image of a typical microrod clearly shows that it is constructed by nanorods aggregated in the same orientation. After 30 min of hydrothermal reaction, more microrods appeared in the SEM image, which is in agreement with the XRD result. Compared with the 0 h sample, these rods are less faceted, and the diameter is not uniform along the long axis. The central part of the rod possesses the biggest diameter and the diameter tapers toward the tips, forming a spindle-like structure. It is interesting to note that the tips of the rod are in the shape of the mouth of a volcano and the rod looks like a tube. As the reaction proceeded, the microrods increased in size. The aspect ratio increased accordingly, indicating that the growth along the long-axis was faster
Figure 6. XRD patterns of products synthesized at 200 °C in 6.67 × 10-2 mol/L acetic acid for 0 h (a), 0.5 h (b), 1.0 h (c), and 2.0 h (d).
Figure 7. SEM images of products synthesized at 200 °C in 6.67 × 10-2 mol/L acetic acid for 0 h (a), 0.5 h (b), 1.0 h (c) and 2.0 h (d).
than that along the short-axis. This result is also supported by XRD results that the intensity of (110) diffraction of Y(OH)3 increased remarkably in comparison with that of (101) diffraction (Figure 6). The conversion from oxide into hydroxide was completed in 2 h, as indicated in XRD result. At that time, the tips of these rods began to split into wires. Prolonging the reaction time to 24 h resulted in no significant change in XRD patterns. However, the whole rod developed into a bundle of nanowires (Figure 2). In order to check the stability of these nanowires, reaction was allowed to proceed to 72 h. The product morphology remained unchanged. 3.4. Growth Mechanism. On the basis of the above results, the growth mechanism of the yttrium hydroxide nanowires can be proposed as follows. Under hydrothermal conditions, yttrium oxide gradually dissolves into the solution and then yttrium hydroxide nucleates. As a consequence of the high anisotropic structure along c-axis of hexagonal phase of yttrium hydroxide, the nuclei evolve into 1D nanocrystals. These nanocrystals tend to align side by side and fuse together spontaneously by a socalled oriented attachment,15-17 where the thermodynamic driving force is the substantial reduction of surface energy
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and growing preferable along circumference. Finally, the microrods are dissolved from the defect sites and split into nanowires.
4. Conclusion
Figure 8. Sketch for the growth mechanism of Y(OH)3 nanowires.
contributed by the interface elimination. As a result, microrods as observed in the 0 h sample are formed. The nucleation and attachment of nanoparticles proceed very quickly, so that it is difficult to catch the stage. Oriented attachment can give rise to a formation of quasi-single crystals with some characteristics of a single crystal, which can explain the hexagonal geometry of the as-observed microrods. During the procedure of their attachment, some defects like dislocation are introduced into the microrods as a consequence of imperfect attachment.18 The dislocation of the attached crystal accumulates during the crystal growth. Therefore, the microrod gradually loses its anisotropy and its shape changes into cylinder. The formation of tube-like structure can be attributed to the predominant reactant feeding on the circumference of a microrod, making the growth rate of the circumference higher than that of the central part of a microrod. The growth of microrods proceeds in this way at a high speed, until the starting yttrium oxide is depleted. At this stage, the reactant concentration in the solution begins to drop. When the concentration drops to a certain level, the particle stops its growth on the whole. However, because the site where defect exists has a higher solubility than those free of defects, there is an internal concentration gradient. Consequently, the microrods dissolve from the defects and grow onto other facet to maintain the solubility equilibrium, resulting in bundles of nanowires. This process is similar as the intraparticle ripening mechanism proposed by Peng et al.19 HAc plays a key role in the formation of the unusual nanowires. It probably acts in two ways. First, the addition of HAc changes the pH value of solution, and accelerates the conversion from yttrium oxide to hydroxide. Second, acetic acid molecules or ions adsorb to the surface of yttrium hydroxide crystallites, introducing defects and blocking the attachment mechanism. Consequently, microrods are formed in dilute HAc solutions, while nanowires are received at high concentration due to the presence of considerable defects. On the basis of the above analysis, the growth mechanism of the nanowires is schematically shown in Figure 8. The growth of the yttrium hydroxide nanowires begins from 1D nanocrystals. These crystals align in the same direction and fuse together via oriented attachment process, leading to microrods. These microrods grow into tube-like structures by loss of anisotropy
In summary, we have developed a facile hydrothermal synthetic method to prepare single-crystalline yttrium hydroxide nanowires. Unlike the monodispersed nanowires synthesized under strong basic conditions, the nanowires obtained under present conditions form regular bundles. The growth of nanowires involves the oriented attachment of 1D nanocrystals to form microrods and selective dissolution from defect sites to form bundles of nanowires. Y(OH)3 nanowires could be converted into Y2O3 by calcination at 500 °C in air. The morphology was maintained except for slight shrinkage in size resulting from the higher density of the oxide. By using mixture of Eu2O3 and Y2O3 powder as staring material, Eu-doped Y(OH)3 nanowires could be prepared. This unusual structure may lead to new opportunities in yttrium chemistry.
References (1) Bhargava, R. N. J. Lumin. 1996, 70, 85–94. (2) Micheli, A. L.; Dungan, D. F.; Mantese, J. V. J. Am. Ceram. Soc. 1992, 75, 709–711. (3) Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. In New Solid Acids and Bases; Elsevier: New York, 1989; p 41. (4) Bai, X.; Song, H.; Yu, L.; Yang, L.; Liu, Z.; Pan, G.; Lu, S.; Ren, X.; Lei, Y.; Fan, L. J. Phys. Chem. B 2005, 109, 15236–15242. (5) Wang, X.; Li, Y. Chem. Eur. J. 2003, 9, 5627–5635. (6) Tang, Q.; Liu, Z.; Li, S.; Zhang, S.; Liu, X.; Qian, Y. J. Cryst. Growth 2003, 259, 208–214. (7) Wang, X.; Sun, X.; Yu, D.; Zou, B.; Li, Y. AdV. Mater. 2003, 15, 1442–1445. (8) Fang, Y. P.; Xu, A. W.; You, L. P.; Song, R. Q.; Yu, J. C.; Zhang, H. X.; Li, Q.; Liu, H. Q. AdV. Funct. Mater. 2003, 13, 955–960. (9) Li, W.; Wang, X.; Li, Y. Chem. Commun. 2004, 164–165. (10) De, G.; Qin, W.; Zhang, J.; Zhang, J.; Wang, Y.; Cao, C.; Cui, Y. Solid State Commun. 2006, 137, 483–487. (11) Wan, J.; Wang, Z.; Chen, X.; Mu, L.; Qian, Y. J. Cryst. Growth 2005, 284, 538–543. (12) Wu, X.; Tao, Y.; Gao, F.; Dong, L.; Hu, Z. J. Cryst. Growth 2005, 277, 643–649. (13) He, Y.; Tian, Y.; Zhu, Y. Chem. Lett. 2003, 32, 862–863. (14) Shafer, M. W.; Roy, R. J. Am. Ceram. Soc. 1959, 42, 563–570. (15) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63, 1549– 1557. (16) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751–754. (17) Penn, R. L.; Oskam, G.; Strathmann, T. J.; Searson, P. C.; Stone, A. T.; Veblen, D. R. J. Phys. Chem. B 2001, 105, 2177–2182. (18) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969–971. (19) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2001, 123, 1389–1395.
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