Uniform Ln(OH)3 and Ln2O3 (Ln = Eu, Sm) Submicrospindles: Facile

Synopsis. Monodisperse hexagonal Ln(OH)3 (Ln = Eu, Sm) submicrospindles with highly uniform morphology and size have been successfully synthesized in ...
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DOI: 10.1021/cg9003559

Uniform Ln(OH)3 and Ln2O3 (Ln = Eu, Sm) Submicrospindles: Facile Synthesis and Characterization

2009, Vol. 9 4127–4135

Zhenhe Xu,†,‡ Chunxia Li,† Piaoping Yang,† Zhiyao Hou,† Cuimiao Zhang,†,‡ and Jun Lin*,† †

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and ‡Graduate University of the Chinese Academy of Sciences, Beijing 100049, P. R. China Received March 31, 2009; Revised Manuscript Received July 23, 2009

ABSTRACT: Monodisperse hexagonal Ln(OH)3 (Ln = Eu, Sm) submicrospindles with uniform morphology and size have been successfully synthesized in a large scale via a facile aqueous solution route from the mixture of aqueous solutions of LnCl3 and NaOH at 5 °C without using any surfactant or template. The as-synthesized products are characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), energy-dispersive X-ray (EDX) spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). The SEM and TEM images show that the as-formed Ln(OH)3 samples have a spindlelike shape with an equatorial diameter of 80-200 nm and a length of 500-900 nm, which are aggregates of even smaller nanoparticles. The self-assembly process via orientation attachment is proposed to interpret the growth process for the submicrospindles. During this process, the concentration of NaOH, synthesis temperature, and reaction time play crucial roles in confining the growth of these submicrospindles. Through calcining the Ln(OH)3 submicrospindles, the Ln2O3 particles with the same morphology could be obtained. 1. Introduction In modern chemistry and materials science, controlled synthesis of well-defined inorganic nanocrystals have been a focus of worldwide research work because they have potential applications in many fields including catalysis, luminescent devices, sensors, and nanoelectronics.1 Thus far, dramatic efforts have been dedicated to develop new methods for the fabrication of a range of high-quality inorganic nanostructures in different systems. From the perspective of application, nanomaterials are not only synthesized in large quantities with a desired composition, reproducible size, shape, and structure but also are prepared and assembled using simplicity, lowcosts, ease of scale-up, and relative greenness (aqueous solution) constitute the key trains of this method.2 Therefore, the development of a mild and more controlled method for creating such novel architectures will be of general interest. Rare earth compounds, such as oxides,3 phosphates,4 fluorides,5 and vanadates,6 have been extensively studied because of their potential applications in high-performance magnets, luminescent devices, catalysts, and other functional materials based on the electronic, optical, and chemical characteristics arising from the 4f electrons. Among them, rareearth hydroxides with various morphologies have been synthesized during past few years.7 For instance, Yada and coworkers described the synthesis of rare earth oxide nanotubes templated by dodecylsulfate assemblies.8 Li and co-workers developed a hydrothermal method for the preparation of 1D nanostructured lanthanide hydroxide Ln (OH)3 (Ln = Er, Tm, Yb, Dy, Ho, Y, etc.) and the corresponding nanostructured rare earth oxides were made by calcining the precursors.9 Xu et al. adopted a hydrothermal route to obtain Dy, Tb, and Y hydroxide nanotubes.10 Although great progress has been achieved on the synthesis for rare-earth hydroxides, they

usually require surfactants, and high reaction temperature. The introduction of surfactants into the synthetic process undoubtedly brings impurities into the final products, increases the production cost, and leads to difficulty for scaleup production. The synthetic strategy operating at hightemperature is also an energy consumption route and a disadvantage for procedure control.11 One can overcome these difficulties by developing the simple, one-step, and effective solution-phase methods for fabricating novel assemblies of the rare-earth hydroxides materials under a lowtemperature, surfactant-free conditions or without the aid of other techniques. Because of its easily controllable reaction conditions and the relatively abundant reactant sources, the so-called soft chemical route, based on a solution process, might provide an attractive option for large-scale production of nano- and micromaterials with special morphologies.12 In the present work, we developed a simple aqueous solution for the syntheses of highly uniform Ln(OH)3 (Ln = Eu, Sm) submicrospindles by a simple one-pot method from the mixture of aqueous solutions of LnCl3 and NaOH at 5 °C without surfactants or templates. We propose a possible formation process and preliminary growth mechanism for the submicrospindles. Various controlled synthetic experiments indicate that the growth process of submicrospindles involves the formation of nuclei, the nuclei then grow at the cost of the small ones, and these crystal nuclei in the particle aggregates grow along the same direction (oriented attachment) simultaneously to form submicrospindles. Through calcinating the Ln(OH)3 submicrospindles, the Ln2O3 with the same morphology have been obtained. 2. Experimental Section

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2.1. Materials. The initial chemicals Eu2O3 and Sm2O3 (with purity >99.99%, Changchun Applied Chemistry Science and Technology Limited, China), and other chemicals were purchased from Beijing Chemical Company. All chemicals are analytical grade reagents and used directly without further purification.

r 2009 American Chemical Society

Published on Web 08/13/2009

*To whom any correspondence should be addressed. E-mail: [email protected].

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to pure hexagonal phase [space group P63/m], which are consistent with the values in the standard cards (JCPDS No. 83-2305 for Eu(OH)3, 83-2036 for Sm(OH)3). The lattice constants of the calcined products were calculated according to the equation 1=d 2 ¼ ðh2 þk2 þl 2 Þ=a2

Figure 1. XRD patterns of the as-prepared Eu(OH)3 and Sm(OH)3 samples. The standard datas for Eu(OH)3 (JCPDS card 83-2305) and Sm(OH)3 (JCPDS card 83-2036) are also presented in the figures for comparison. 2.2. Preparation. In a typical synthesis, 1 mmol of Eu2O3 was dissolved in dilute HCl, resulting in the formation of a colorless solution of EuCl3. After evaporation followed by drying at 100 °C in ambient atmosphere, a powder of EuCl3 was obtained. Ten milliliters of distilled water was added to the powder of EuCl3 to form a transparent solution and then 10 mL of NaOH aqueous solution (5 mol L-1) was added into the above solution with stirring, and it was stirred for 2 h to form Eu(OH)3. The resulting suspension was placed at 5 °C for 5 days without further stirring or shaking. The products were then collected by centrifugation, washed with distilled water, and absolute ethanol to remove any residuals, and finally dried in air at 60 °C for 4 h. The conversion of the as-obtained Eu(OH)3 submicrospindles to Eu2O3 submicrospindles was carried out in an oven at 800 °C for 4 h in air. The Sm(OH)3 and Sm2O3 submicrospindles were prepared in a similar way above. 2.3. Characterization. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer with Cu KR radiation (λ= 0.15405 nm). Thermogravimetric data were recorded with Thermal Analysis instrument (SDT 2960, TA Instruments, New Castle, DE) with the heating rate of 10 °C min-1 in an air flow of 100 mL min-1. The morphology and structure of the samples were inspected using a field emission scanning electron microscopy (FE-SEM, XL 30, Philips) equipped with energy-dispersive X-ray (EDX) spectrometer and a transmission electron microscope. Low- and high-resolution transmission electron microscopy (TEM) was performed by using an FEI Tecnai G2 S-Twin instrument with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiople CCD camera. The samples for TEM observations were prepared by dispersing some products in ethanol. This procedure was followed by ultrasonic vibration for 2 min and deposition of a drop of the dispersion onto a carbon-coated copper grid. The excess liquid was wicked away with a filter paper, and the grid was dried at 70 °C.

3. Results and Discussion 3.1. Structure and Morphology of Ln(OH)3 Submicrospindles. The composition and phase purity of the products were first examined by XRD, as shown in Figure 1. The diffraction patterns of the as-prepared two products can be indexed

ð1Þ

where d is the interplanar distance, h, k, l are the crystal indices (Miller indices), and a is the lattice constant. On the basis of the (110) crystal planes (d = 0.317 nm in Figure 2D, d = 0.318 nm in Figure 3D), the lattice constant is a = 0.6363 and 0.6349 nm for Eu(OH)3 and Sm(OH)3, which is well-compatible with the literature value of a = 0.6368 and 0.6352 nm (JCPDS No. 83-2305 for Eu(OH)3, 83-2036 for Sm(OH)3), respectively. According to previous investigation on the precipitation of rare-earth chloride in hydrochloric solutions by various alkalis, it is known that the fresh precipitates exist as an amorphous rare-earth hydroxide Ln(OH)3.13a The as-prepared samples in this work upon precipitation and aging at 5 °C using NaOH exhibit a wellcrystallized Ln(OH)3 structure, which indicated there is complete transformation of the amorphous precursor to crystallized structure in the present synthesis conditions. The morphology and microstructure details of the asprepared Ln(OH)3 pruducts were studied by SEM, TEM, and HRTEM techniques. Images a and b in Figure 2 show the low- and high-magnification SEM images of the asprepared Eu(OH)3 samples. From Figure 2A, it can be clearly seen that the Eu(OH)3 samples are composed of a large scale of uniform and monodisperse submicrospindles, suggesting the high yield achieved with this approach. It can be calculated that the equatorial diameters and lengths of these submicrospindles are about 100-200 nm and 500900 nm, respectively. More careful examination of the highmagnification SEM image (Figure 2B) shows that the surfaces of sample are very rough, indicating that the submicrospindles consist of many even smaller nanoparticles, which are organized into ordered chains that are aligned approximately parallel to the spindle long axis. The chemical composition of the Eu(OH)3 submicrospindles was further investigated with EDX, which indicates that the submicrospindles are made of Eu and O and the molar ratio of Eu to O is approximately equal to 1: 3 except the Si and Au peaks from measurement (see the Supporting Information, Figure S1). To further study the fine structure of the above submicrospindles, we performed TEM. A representative TEM micrograph for Eu(OH)3 submicrospindles is shown in Figure 2C, clearly showing that the products are entirely composed of uniform submicrospindles with equatorial diameters of about 100-200 nm and lengths of about 500-900 nm, consistent with the values shown in the SEM image (Figure 2B). The typical HRTEM image (Figure 2D) of Eu(OH)3 submicrospindles clearly shows lattice fringes with interplanar spacing of 0.317 nm that correspond to the (110) plane of Eu(OH)3 phase. Under similar reaction conditions as those for preparing Eu(OH)3 samples, we also obtain Sm(OH)3 samples. The chemical composition of the Sm(OH)3 sample was investigated with EDX, which indicates that the samples are made of Sm and O and the molar ratio of Sm to O is approximately equal to 1:3 except the Si and Au peaks from measurement (Supporting Information, Figure S2). SEM and TEM images of the as-prepared Sm(OH)3 samples are depicted

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Figure 2. (A) Lower-magnification, (B) higher-magnification, (C) TEM, and (D) HRTEM images of the as-prepared Eu(OH)3 submicrospindles.

Figure 3. (A) Lower-magnification, (B) higher-magnification, (C) TEM, and (D) HRTEM images of the as-prepared Sm(OH)3 submicrospindles.

in Figure 3. The low-magnification SEM image (Figure 3A) shows the general view, clearly indicating that the products consist of large-scale and monodisperse submicrospindles. More careful examination of the magnified SEM image (Figure 3B) shows that the surfaces of the submicrospindles are also rough, with equatorial diameters of about 80-150 nm and lengths of about 500-800 nm. Figure 3C is a representative TEM image of the Sm(OH)3 sample, clearly showing that the products are entirely composed of uniform submicrospindles with equatorial diameters of about 80-150 nm and lengths of about 500-800 nm, consistent with the values

shown in the SEM image (Figure 3B). The HRTEM image in Figure 3D reveals that the samples are highly crystallized with the interplanar spacing of 0.318 nm corresponding to the (110) crystal plane of Sm(OH)3 submicrospindles. 3.2. Phase Identification and Morphology of Ln2O3 Submicrospindles. Well-dispersed and uniform Ln(OH)3 spindles of hierarchical structures were used as precursor to fabricate Ln2O3 crystals. On the basis of TGA data (Figure 4), the Eu(OH)3 and Sm(OH)3 samples were calcined in air at 800 °C for 4 h to ensure their complete decomposition, and phase-pure Eu2O3 and Sm2O3 were obtained, respectively.

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Figure 4. TGA curves of the Eu(OH)3 and Sm(OH)3 samples.

Figure 5 shows the XRD patterns of Eu2O3 and Sm2O3 after calcination. All of the diffraction peaks of the two samples in the pattern can be perfectly indexed to a cubic phase [space group: Ia3(206)], which are consistent with the values in the standard cards [JCPDS No. 34-0392 (Eu2O3), 65-3183 (Sm2O3)], respectively. No additional peaks for other phases have been found, indicating that Eu(OH)3 and Sm(OH)3 were completely converted to Eu2O3 and Sm2O3 at 800 °C for 4 h, respectively. The morphologies of the obtained Eu2O3 and Sm2O3 have been investigated and shown in Figure 6 and Figure 7, respectively. The low-magnification (Figure 6A) and highmagnification (Figure 6B) SEM images of Eu2O3 samples clearly indicate that calcination at temperature of 800 °C did not cause any significant changes in the spindle morphology, and the products consist of large-scale, monodisperse submicrospindles. The Eu2O3 submicrospindles inherited their parents’ morphology, but their size is slightly shrunk in comparison with Eu(OH)3 submicrospindles in that the density of the former is higher than that of the latter. Closer observation reveals that there are cracks on the surface of the submicrospindles, which may be attributed to the removal of H2O from the constituent OH- groups in the precursor during the calcination process. The result indicates that Eu2O3 submicrospindles were successfully obtained and that the morphologies were well-inherited from Eu(OH)3 submicrospindles except for the cracks on the surface. Nevertheless, the conversion did not lead to the change in the morphology and such a transformation was common for rare earth hydroxide compounds decomposition.13 The morphologies were maintained perhaps because of the higher activation energies needed for the collapse of these structures.14 Thus, high-quality Ln2O3 spindles with a hierarchical nanostructure can be fabricated in a large scale using this method. The EDX was used to further characterize the chemical composition of the as-prepared Eu2O3 samples. The EDX spectrum (Supporting Information, Figure S3)

Figure 5. XRD patterns of the as-prepared Eu2O3 and Sm2O3 samples. The standard datas for Eu2O3 (JCPDS card 34-0392) and Sm2O3 (JCPDS card 65-3183) are also presented in the figures for comparison.

shows the presence of Eu and O with an atomic ratio of Eu to O is approximately equal to 2:3 except the Si and Au peaks from measurement, and the EDX result gives further support for the XRD analysis above. To further study the fine structure of the above Eu2O3 submicrospindles, TEM was performed. Figure 6C shows a typical TEM image of the Eu2O3 submicrospindles. The crystalline structures of asprepared Eu2O3 submicrospindles were further explored by HRTEM. The typical HRTEM image (Figure 6D) of one Eu2O3 submicrospindles clearly shows lattice fringes with interplanar spacing of 0.313 nm that correspond to the (222) plane of Eu2O3. These results further confirm the presence of highly crystalline Eu2O3 submicrospindles after annealing at high temperature, agreeing well with the XRD results. Imagse A and B in Figure 7 show the low- and highmagnification SEM images of Sm2O3 samples, clearly indicating that they also consist of large-scale, monodisperse submicrospindles. It can be seen also that the Sm2O3 submicrospindles inherited their parents’ morphology, but their size is slightly shrunk in comparison with Sm(OH)3 submicrospindles in that the density of the former is higher than that of the latter. Closer observation reveals that there are also cracks on the surface of the submicrospindles. The EDX was used to further characterize the chemical composition of the as-prepared Sm2O3 submicrospindles. The EDX spectrum (Supporting Information, Figure S4) shows the presence of Sm and O with an atomic ratio of Sm to O is approximately equal to 2:3, and the EDX result gives further support for the XRD analysis above. Figure 7C shows a typical TEM image of Sm2O3 submicrospindles. Figure 7D is the typical HRTEM image of one Sm2O3 submicrospindles clearly shows lattice fringes with interplanar spacing of 0.315 nm that correspond to the (222) plane of Sm2O3. 3.3. Growth Mechanism. The morphologies and crystal phase purity of the as-prepared submicrospindles are mainly

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Figure 6. (A) Lower-magnification, (B) higher-magnification, (C) TEM, and (D) HRTEM images of the as-prepared Eu2O3 submicrospindles.

Figure 7. (A) Lower-magnification, (B) higherkmagnification, (C) TEM and (D) HRTEM images of the as-prepared Sm2O3 submicrospindles.

affected by the following experimental conditions, such as the concentration of NaOH, synthesis temperature, and reaction time, which have been discussed in detail as follows. (a) Effects of the Concentration of NaOH. It has been reported that pH is an important factor in controlling morphological and dimensional, particularly for low dimensional nanocrystals.15 In this work, the concentration of NaOH is found to be an important synthetic parameter to

influence the final morphology of the Eu(OH)3 products. Figure 8 shows the SEM images of the as-prepared products with the concentration of NaOH from 5 to 12.5 mol L-1 (10 mL) with the other synthetic parameters keeping constant. Figure 8A indicates that when the concentration of NaOH is 5 mol L-1 (10 mL), irregular spindlelike crystals and amorphous agglomerated nanoparticles formed. When the concentration of NaOH increases to 7.5 mol L-1 (10 mL),

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Figure 8. SEM images of Eu(OH)3 samples obtained at different concentrations of NaOH: (A) 5, (b) 7.5, (C) 10, and (D) 12.5 mol L-1.

Figure 9. SEM images of Eu(OH)3 samples at different synthesis temperature: (A) 5 and (B) 30 °C.

the products are consists of nearly monodispersity submicrospindles and some irregular agglomerated nanoparticles, as shown in Figure 8B. Figure 8C shows the morphology of products derived from 10 mol L-1 NaOH (10 mL), indicating that the product is composed of monodisperse submicrospindles, and the morphology quality is greatly improved. When the concentration of NaOH is 12.5 mol L-1 (10 mL), the as-prepared sample is almost entirely composed of submicrospindles with perfect uniformity and monodispersity (Figure 8D). As the concentration of NaOH increases to 15 mol L-1 (10 mL), there is no further change in morphology and size. From the analysis above, we can conclude that with the amount of NaOH increasing, the morphologies of the submicrospindles transfer to perfect uniformity, monodispersity and the optimal addition of NaOH for the formation of Eu(OH)3 hexagonal submicrospindles is determined to be 15 mol L-1 (10 mL). Effects of the Synthesis Temperature. By fixing other reaction conditions, the effect of the synthesis temperature on the morphology and size of the as-prepared products was investigated. It is found that the reaction temperature also

Figure 10. XRD patterns of Eu(OH)3 nanostructures synthesized at 5 °C with different reaction time.

has significant influence on the morphology of the assynthesized Eu(OH)3 samples. At the reaction temperature of 5 °C, the as-prepared samples are almost entirely composed of submicrospindles with perfect uniformity, monodispersity, as shown in Figure 9A. When the reaction temperature is increased to 25 °C, the samples also mainly

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Figure 11. SEM images of Eu(OH)3 samples at different reaction time: (A) 2, (B) 12, (C) 48, and (D) 120 h.

contain submicrospindles, but their morphology is not uniform (Figure 9B). This result indicates that appropriate reaction temperature is important for the preparation of Eu(OH)3 submicrospindles, which might be related to the reaction rate decreased by lower temperature in the present reaction system. It is well-known that the slow reaction rate in the solution phase is favorable for the nucleation and crystallization for nanocrystals. Therefore, the slow reaction rate controlled by fall of temperature would offer a favorable chemical environment for the formation of the Eu(OH)3 submicrospindles. (b) Effects of the Reaction Time. To gain a better understanding on the evolution mechanism of these uniform Eu(OH)3 submicrospindles, the growing process of the Eu(OH)3 submicrospindles was analyzed by XRD and observed the images of Eu(OH)3 sample at different stages, as shown in Figures 10 and 11. When the reaction time was reduced to 2 h, the products obtained consisted of aggregates of amorphous particles and the precipitates were ill-crystallized, as confirmed by the SEM image (Figure 11A) and a few weak peaks in the XRD pattern (Figure 10). When the reaction time was prolonged to 12 h, a few spindlelike microparticles 400800 nm in length and 100-150 nm in width formed, as shown in Figure 11B. After 48 h, more spindles appear in the products at the expense of the amorphous particles (Figure 11C). At the same time, the size of the spindles has grown larger in all directions and the size uniformity is greatly improved. Figure 11D shows a SEM image of the as-prepared Eu(OH)3 submicrospindles at 5 °C for 120 h, the uniform and monodispersity submicrospindles with equatorial diameters of about 100-200 nm and lengths of about 500-900 nm were produced. Formation Mechanism for the Ln(OH)3 Submicrospindles. According to these time-dependent studies, the spindlelike structure Ln(OH)3 form very slowly after the reaction of Ln3þ and OH- in the solution. A possible schematic illustration of

Scheme 1. Illustration for the Formation Process of the Spindlelike Structures

formation mechanism of this spindlelike structure is shown in Scheme 1. On the basis of the series of experimental data, we believe that, in this case, the formation of the Ln(OH)3 submicrospindles belongs to a self-assembly process via oriented attachment. In the initial stage, solutes are produced to form a supersaturated solution, leading to nucleation. The nuclei then grow by a diffusive mechanism to form polycrystalline subunits, which, in turn, aggregate to form the large polycrystalline assemblages. First, Ln(OH)3 nanoparticles formed by reaction between Ln3þ and OH- ions. The nanoparticles then grow at the cost of the small ones, and these crystal nuclei in the particle aggregates to grow along the same direction simultaneously to form submicrospindles in this concentration of NaOH. It appears that such a selfassembly process proceeds once the nanoparticles are formed based on the observation of very few growing spindles coexisting with amorphous nanoparticles. Although in this process, spindles are observed with different lengths, crystals of highly uniform sized spindles result from the oriented attachment growth process.16 However, general hydrothermal synthesis of rare earth hydroxide nanowires/nanorods is governed by a solution-solid (SS) process7,17 and the overall growth rate is fast, the growth rate of the anisotopic material is generally faster along c-axis, raising the growth rate by a larger margin, and a rodlike structure is obtained. In the

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present condition, the growth direction of the Ln(OH)3 submicrospindles is also confined to the c-axis direction. But, the overall growth rate is much slower than hydrothermal condition, and oriented attachment played a dominant role in this crystal growth evolution process. Oriented attachment means in this mechanism that the bigger crystals are grown from small primary building blocks along with the c-axis. As time goes by, the nanoparticles then grow at the cost of the small ones and these crystal nuclei in the particle aggregates to grow along the same direction simultaneously to form submicrospindles. Ln(OH)3 submicrospindles have a hexagonal crystal structure, similar to that of ZnO17 and LnPO4,18 which are wellknown to exhibit anisotropic growth. Studying the crystal structure character of lanthanide hydroxides can help us to understand the growth mechanism of lanthanide hydroxides submicrospindles. The crystal structure of the representative hexagonal Eu(OH)3 is shown in Figure S5 (Supporting Information). The structure of Eu(OH)3 is hexagonal P63/ m, the europium atoms were placed in positional set 2(c) which is a special site (1/3, 2/3, 1/4), and the oxygen atoms were placed in positional set 6(h), x, y, 1/4. Each Eu3þ ion prefers 9 oxygen coordination numbers because of its large radius, six at distance of 2.473(9) A˚ (Eu-O2) and three at 2.466(6) A˚ (Eu-O1).19 The coordination polyhedron is close to bring a trigonal prism with the europium atom located at its center (Figure S5A, Supporting Information). The overall structure of this compound may be described as columns built up of alternate europium and oxygen atoms, extending along the c-axis (Figure S5B, Supporting Information), in which the distance between each of the two neighboring Eu atoms are the same with the unit-cell parameter c. The packing structure of hexagonal Eu(OH)3 viewing along the b-axis can be described as infinite linear chains, parallel to the c-axis (Figure S5C, Supporting Information). From a structural point of view, hexagonal Eu(OH)3 consists of infinite linear chains extending along the c-axis. From a thermodynamic perspective, the activation energy for the c-axis direction of growth of hexagonal Eu(OH)3 is lower than that of growth perpendicular to the c-axis.18 Therefore, the atomic interactions along the a- and b-directions are much weaker than that along the c-axis, this means that the growth direction of the Eu(OH)3 submicrospindles is largely confined to the [001] direction, which are well in agreement with HRTEM observation (Figure 2D). Hexagonal Eu(OH)3 is isostructural with hexagonal Sm(OH)3; therefore, the formation of Sm(OH)3 submicrospindles can also be explained based on this way. 4. Conclusions In summary, a facile, low-temperature, and surfactant-free aqueous solution route has been developed for the synthesis of Eu(OH)3 and Sm(OH)3 submicrospindles. The study of the influences of experimental parameters shows that the morphology and size of the final products can be perfectly manipulated by controlling the concentration of NaOH, the synthesis temperature and reaction time. Eu2O3 and Sm2O3 submicrospindles were also obtained by a thermal decomposition method using Eu(OH)3 and Sm(OH)3 submicrospindles as the precursor, respectively. Although a detailed investigation concerning the formation mechanism is still in progress, we suggest that the submicrospindles are formed by a self-assembly process via oriented attachment. Our study might open a novel, facile and

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environmentally friendly solution-phase chemical route to the large-scale synthesis of other architectures for advanced applications at low temperature. Acknowledgment. This project is financially supported by National Basic Research Program of China (2007CB935502, 2010CB327704), and the National Natural Science Foundation of China (NSFC 50702057, 50872131). Supporting Information Available: The energy-dispersive X-ray (EDX) spectroscopic analyses of the Eu(OH)3, Sm(OH)3, Eu2O3, Sm2O3. Crystal structure of hexagonal-phased Eu(OH)3 (A). Crystal structure of Eu(OH)3 viewed from the c-axis (B) and the b-axis (C) (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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