Controlled Synthesis of Three-Fold Dendrites of Ce(OH)CO3 with

Article pubs.acs.org/crystal

Controlled Synthesis of Three-Fold Dendrites of Ce(OH)CO3 with Multilayer Caltrop and Their Thermal Conversion to CeO2 Li-Wu Qian,†,‡ Xin Wang,‡ and He-Gen Zheng*,† †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructure, Nanjing University, Nanjing, 210093, P. R. China ‡ Department of Chemistry, Chizhou University, Chizhou 200240, P. R. China S Supporting Information *

ABSTRACT: Three-fold dendrites of Ce(OH)CO3 with multilayer caltrop have been synthesized on a large scale by means of a complexing agent assisted solution route. The shape of these as-prepared architectures can be tuned effectively by controlling the reaction conditions, such as reaction time and the molar ratio of complexing agent/Ce3+. As a typical morphology, the growth process of 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop has been examined, and a possible mechanism is discussed. The phase and morphology of the as-prepared product have been characterized by means of powder X-ray diffraction (XRD), (high resolution) transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM). The morphology of dendrites is sustained after thermal decomposition−oxidation Ce(OH)CO3 to CeO2. To extend this method, La(OH)CO3 and Eu3+/Ce(OH)CO3 dendritic structures can also be achieved at similar reaction conditions. After similar heat treatment in an oven, their dendritic structures are also retained. La(OH)CO3 completely transforms into La2O2(CO3), and Eu3+/Ce(OH)CO3 changes to Eu3+/CeO2. The photoluminescent properties of Eu3+/Ce(OH)CO3 and Eu3+/CeO2 dendrites have also been investigated. This work sheds some light on the design of welldefined complex nanostructures.

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INTRODUCTION In recent years, there has been growing interest in the controlled synthesis of higher ordered inorganic crystals with specific dimensions and well-defined shapes because of their widespread potential applications in catalysis, drug delivery, acoustic insulation, photonic crystals, and other areas.1 The controlled synthesis of nanoparticles with ordered superstructures or complex architectures is a key for the success of “bottom-up” approaches toward future nanodevice fabrication.2 Moreover, hierarchical nanostructures with particular structural features have large surface areas and allow for heterostructures, which can be applied in photovoltaics and multifunctional nanoelectronics.3 A dendrite is a kind of material that has a main stem from which many side branches grow out and a hierarchical structure with primary, secondary, tertiary, and even higher order branches.4 The formation of dendrites is generally related to fractal growth and hierarchical self-assembly under nonequilibrium conditions.5−8 To date, various dendrites have been synthesized through fractal growth and selfassembly.6−8 In these fabrications, the growth habit of crystals always plays an important role in determining the final morphology; meanwhile, a complexing agent is applied to modify the growth behaviors and orientations of crystals.7 The growth of hierarchical dendrites is normally explained by either anisotropic growth in a preferred orientation from a core or oriented attachment assisted by surface-capped agents.7,8 The © 2011 American Chemical Society

complexing agent could tune the driving forces for liberation of precursors ions, nucleation, and growth modes in the reaction process.8 Therefore, to extend the synthesis of inorganic micro-/nanostructures morphologies and to deepen the comprehension of crystal growth behavior, it is necessary to choose suitable complexing agents or surfactants for preparing inorganic hierarchical dendrites with uniform size. Herein, sodium tartrate (Na2tar) is chosen to control the synthesis of Ce(OH)CO3 hierarchical dendrites, and Na2tar proved to be useful for preparing hierarchical dendrites. Rare earth carbonate hydroxides have been applied as a novel group optical material.9 It has been extensively studied and synthesized with different microstructures, and the obtained architectures further lead to varied optical properties.10 The cerium carbonate hydroxide has also been used widely in the preparation of cerium oxide (CeO2).11 The morphology and crystal size of CeO2 can be easily controlled by using Ce(OH)CO3 as decomposition precursors. As an important functional inorganic material, Ceria has been widely applied in catalysts,12 fuel cell,13 ultraviolet absorbers,14 hydrogen storage materials,15 oxygen sensors,16 optical devices,17 polishing materials.18 It is well-known that the new properties and Received: August 10, 2011 Revised: November 25, 2011 Published: December 2, 2011 271

dx.doi.org/10.1021/cg201040c | Cryst. Growth Des. 2012, 12, 271−280

Crystal Growth & Design

Article

application of materials are related to their shapes and sizes. Stimulated by both the promising applications and the interesting properties, much attention has been directed to the controlled synthesis of some novel Ce(OH)CO3 and CeO2 nanostructures. In the past few years, remarkable progress has been made in the synthesis of Ce(OH)CO3 nanostructures. Different morphologies of Ce(OH)CO3 crystals have been prepared by different procedures, such as nanorods,19 prisms,20 shuttles,21 dendrites,22 microplates,23 and other morphological structures.24 Nevertheless, to the best of our knowledge, a few Ce(OH)CO3 structures with hierarchically fractal architectures have been obtained; on the other hand, it is essential to understand the relationship between the intrinsic crystal structure of Ce(OH)CO3 and the kinetic factors employed during the synthetic course for realizing morphology-controlled synthesis. In this paper, 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop have been successfully synthesized by a complexing agent assisted hydrothermal approach. The influence of Na2tar on the formation of dendrites is discussed. The morphology of CeO2 has been pretty much the same as 3fold dendrites of Ce(OH)CO3 with multilayer caltrop after thermal decomposition. It is interesting to find that hierarchical dendrites also have been prepared in the synthesis of La(OH)CO3 and Eu3+/Ce(OH)CO3 at similar reaction conditions, whereas for the as-obtained Pr(OH)CO3 and Nd(OH)CO3, the complexing agents have no obvious effects on their morphology, and only nanoparticles are obtained. In particular, Ln(OH)CO3 is completely transformed into Ln2O2(CO3) after similar heat treatment in an oven, and the hierarchical morphology of Ln(OH)CO3 was sustained. Eu3+/ Ce(OH)CO3 undergoes thermal conversion to Eu3+/CeO2 with similar hierarchical dendrites. The photoluminescence of Eu3+/Ce(OH)CO3 and Eu3+/CeO2 spectra have also been studied. The motivation of this research is to develop a facile method to prepare dendritic inorganic nanostructures and understand the formation mechanism of fractal structure nanocrystals for exploring physical and chemical properties of inorganic dendrites.

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Table 1. Summary of the Reaction Parameters and Corresponding Resultsa sample

Na2tar (M)

time (h)

compounds

1 2

0 0.01

24 24

CeO2 Ce(OH)CO3

3 4

0.02 0.02

4 6

Ce(OH)CO3 Ce(OH)CO3

5

0.02

12

Ce(OH)CO3

6

0.02

24

Ce(OH)CO3

7

0.02

10

CeO2

8 9 10

0.02 0.02 0.02

24 10 24

11 12 13

0.02 0.02 0.02

10 24 24

La(OH)CO3 La2O2(CO3) Eu3+/Ce(OH) CO3 Eu3+/CeO2 Pr(OH)CO3 Nd(OH)CO3

morphology of product nanocubes 3-fold dendrites with multilayer caltrop nanoparticles nanoparticles and 3-fold dendrites with multilayer caltrop nanoplates and 3-fold dendrites with multilayer caltrop 3-fold dendrites with multilayer caltrop 3-fold dendrites with multilayer caltrop micropine dendrites micropine dendrites microplume dendrites microplume dendrites nanoparticles nanoparticles

a All samples were obtained with LnCl3·6H2O (Ln = Ce, La, Eu, Pr, Nd) as precursors.

XRD-6000) equipped with a Cu Kα radiation source (λ = 1.5418 Å) at a scanning rate of 6°/min (2θ from 10° to 80°). X-ray tube voltage and current were set at 40 kV and 30 mA, respectively. The morphology and crystal lattice of the samples were characterized by transmission electron microscopy (TEM; JEOL, JEM-100CX, with an accelerating voltage of 100 kV), high-resolution transmission electron microscopy (HRTEM; JEOL, JEM-2100F, with an accelerating voltage of 200 kV) and field-emission scanning electron microscopy (FESEM; JEOL, JSM-6700F with an accelerating voltage of 5 kV), respectively. Fourier Transform infrared spectroscopy (FTIR) was recorded on Perkin-Elmer Paragon 1000FT-IR spectrometer. The photoluminescence (PL) was investigated on Perkin-Elmer LS50B photoluminescence spectrophotometer by dispersing samples in alcohol.

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RESULTS AND DISCUSSION The Ln(OH)CO3 (Ln = Ce, La, Eu3+/Ce3+ Pr, Nd) have been synthesized through the complexing agent assisted solution route. The detailed reaction parameters and corresponding

EXPERIMENTAL SECTION

All reagents (analytical grade) were purchased from Shanghai Chemical Reagent Company and used without further purification. Synthesis of Rare Earth Carbonate Hydroxide (Ln = Ce, La, Eu3+/Ce3+, Pr, Nd). Ce(OH)CO3 was synthesized by a hydrothermal process. In a typical case, cerium(III) chloride heptahydrate (CeCl3·7H2O) (0.104 g, 0.28 mmol) was first dissolved into 0.024 M solution of Na2tar (C4H4Na2O6·2H2O) (23 mL, 0.56 mmol). The solution was stirred at room temperature for 10 min to form a cerium tartrate complex. Then, sodium hydroxide (NaOH) (0.168 M, 5 mL) was added into the solution. The concentration of Na2tar decreased from 0.024 to 0.02 M after the addition of 5 mL of (0.168 M) NaOH. After being stirred for 10 min, a clear and taupe solution was transferred into a 35 mL Teflon-lined autoclave (filled up to 80% of its total volume), and the autoclave was heated at 200 °C for 24 h. Then the autoclave was allowed to cool to room temperature. The obtained samples were collected after being centrifugally separated at 3500 rpm for 20 min, washed with deionized water, and dried at 60 °C in air. The as-synthesized Ce(OH)CO3 was calcined to produce straw-yellow CeO2 in air at 500 °C for 10 h. The sample La(OH)CO3 and Eu3+/ Ce(OH)CO3 (molar ratio Eu3+/Ce3+ = 1:9), La2O2(CO3) and Eu3+/ CeO2 were prepared at similar procedures. The detailed reaction parameters and corresponding results are summarized in Table 1. Characterization. The phase of as-prepared products was characterized on powder X-ray diffractometer (XRD, Shimadzu

Figure 1. XRD pattern of the Ce(OH)CO3 obtained with a 2:1 molar ratio of tar2−/Ce3+ at 200 °C for 24 h.

results are summarized in Table 1. The typical XRD pattern of the sample 6 is shown in Figure 1. It can be seen that the pattern fits well with the hexagonal phase Ce(OH)CO3 [space 272



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Figure 2. Images of Ce(OH)CO3 crystals obtained with a 2:1 molar ratio of tar2−/Ce3+ at 200 °C for 24 h: (a, b) low and enlarged magnification SEM image; (c) TEM image of individual Ce(OH)CO3 crystal; the inset gives the corresponding SAED; (d) HRTEM image of the Ce(OH)CO3 crystal.

Figure 3. Typical SEM images of Ce(OH)CO3 prepared with a 2:1 molar ratio of tar2−/Ce3+ at 200 °C for different times: (a) 4 h; (b) 6 h; (c) 12 h; (d) 24 h.

group: p-6̅2c] with lattice constants a = b = 7.2382 and c = 9. 9596 Å (JCPDS Card No. 32-189). No peaks of any other phases or impurities can be detected, indicating its pure phase. To further illustrate our synthesis strategy, the product was also characterized by FESEM and HRTEM. The SEM image of the sample 6 is shown in Figure 2 and uniform3-fold dendrites

of Ce(OH)CO3 with multilayer caltrop were obtained on a large scale (Figure 2a). The as-obtained Ce(OH)CO3 possesses regular and uniform 3-fold dendrites with multilayer caltrop. The length range of dendrites with multilayer caltrop is from 2 to 2.7 μm, and its thickness is about 1 μm. It can be seen that each caltrop has smooth surfaces (Figure 2b). All the SEM 273



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Figure 4. Typical images of the products prepared at 200 °C for 24 h with different molar ratios of tar2−/Ce3+: (a) 0, TEM image of CeO2; (b) 1, SEM image of Ce(OH)CO3.

Figure 5. A schematic illustration of the formation of three-fold dendrites of Ce (OH)CO3 with multilayer caltrop.

Figure 6. The crystal structure of the hexagonal Ce (OH)CO3; the hydrogen atoms are omitted for clarity.

shown in Figure 2 is a result of growth along the ± [11̅00], ± [011̅0], and ± [1̅010] directions.28 In the complexing agent assisted synthetic process, the molar ratio of tar2−/Ce3+, the reaction time, and temperature have a great effect on the chemical composition and morphology of the final product. A series of condition-dependent experiments have been carried out to understand the characteristic effects of complexing agent on the nucleation and crystal growth processes and further to uncover the underlying mechanism for the controllable composition and shape. It is found that the size of the obtained monodisperse Ce(OH)CO3 hierarchical dendrites can be easily tuned by the reaction time. No product appeared before 4 h. When the reaction time was prolonged up to 4 h, a large amount of nanoparticles was obtained (sample 3; Figure 3a). When the reaction time was up to 6 h, the product mainly consisted of nanoparticles, and a certain amount of 3fold dendrites with multilayer caltrop emerged. The SEM image of the sample shows that 3-fold dendrites with multilayer caltrop are symmetric and holds the tendency of growing along three equivalent directions (sample 4; Figure 3b). When the reaction time was extended to 12 h, the amount of 3-fold dendrites with multilayer caltrop was increased and a little particles still existed (sample 5; Figure 3c). After further increasing the reaction time to 24 h, the particles vanished and

images reveal a clear and well-defined dendritic fractal structure with 3-fold symmetry. Each 3-fold dendrite has many layers structure, and the angle between these folds is 120°. It is worth mentioning that these Ce(OH)CO3 hierarchical dendrites are sufficiently stable even though they are ultrasonic for a long time. The high symmetry and single crystallinity were also revealed by TEM observations, and the TEM image of individual Ce(OH)CO3 dendrite is shown in Figure 2c, It is clear to see that the Ce(OH)CO3 dendrite also displays a 3-fold symmetric structure like a caltrop. The morphology of Ce(OH)CO 3 architecture was further investigated by HRTEM. The diffraction spots are indexed to the (11̅00), (011̅0), and (101̅0) plane of hexagonal Ce(OH)CO3, respectively, suggesting that the nanoplate is a well-developed single crystal (inset of Figure 2c). The HRTEM image in Figure 2d further reveals the single-crystal characteristic structure of nanoplates. The 3.64 Å spacing of crystallographic planes corresponds to the (1120) lattice fringe of Ce(OH)CO3, indicating the growth along [0001] direction was confined, and it grew preferentially along the [110̅ 0], [0110̅ ], and [1010̅ ] directions; 3-fold symmetric nanoplates were then obtained. Thus, it can be concluded that Ce(OH)CO3 dendrites are stacked through “oriented attachment” of small caltrops along the [0001] direction.25,26 The 3-fold symmetric structure 274



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Figure 7. (a, b) Low and enlarged magnification SEM image of micropine dendrites of La(OH)CO3 obtained with a 2:1 molar ratio of tar2−/La3+ at 200 °C for 24 h; (c, d) TEM and HRTEM image of La(OH)CO3 crystals, the inset gives the corresponding SAED; (e, f) low and enlarged magnification SEM image of microplume dendrites of Eu3+/Ce(OH)CO3 obtained with a 2:1 molar ratio of tar2−/Eu3+:Ce3+ (1:9) at 200 °C for 24 h; (g, h) TEM and HRTEM images of Eu3+/Ce(OH)CO3 crystals; the inset gives the corresponding SAED.

the size of Ce(OH)CO3 hierarchical structures increases with extending the reaction time. The result shows that the dendrites are evolved by hierarchical self-assembly under nonequilibrium conditions.5 In the solution synthesis, surfactants or additives are commonly used to regulate the size and shape of inorganic crystals. In the current experiment, the addition of Na2tar to the reaction solution obviously influences the morphology of the as-obtained Ce(OH)CO3 microcrystals. The as-obtained products were not hierarchical dendrites of Ce(OH)CO3 in the absence of Na2tar and exhibited irregular nanocubes of CeO2 morphology with a size of 5−80 nm (sample 1; Figure 4a). Three-fold dendrites of Ce(OH)CO3 with multilayer caltrop were obtained if the molar ratio of tar2−/Ce3+ was increased to 1. The length of 3-fold dendrites of Ce(OH)CO3

Figure 8. Schematic representation of the growth directions in (a) 3fold dendrites with multilayer caltrop, (b) micropine dendrites, and (c) microplume dendrites.

3-fold dendrites of Ce(OH)CO3 with multilayer caltrop were formed (sample 6; Figure 3d). These images clearly reveal that the Ce(OH)CO3 hierarchical structures are formed from nanoparticles to 3-fold dendrites with multilayer caltrop and 275



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The mechanism of the formation Ce(OH)CO3 dendrites is probably related to the degree of supersaturation and the limited regime of diffusion.27 A seed particle of Ce(OH)CO3 is formed by nucleation under supersaturation and the growth of dendrites proceeds via an oriented attachment of diffusing particles at certain favorable sites, determined by the crystal structure.27 In this paper, Na2tar is used as organic molecules to control the crystal nucleation and growth in the reaction system. At the beginning of the hydrothermal reaction, tar2− anions could coordinate with Ce3+ cations, forming a relatively stable complex, which sharply decreases the free Ce 3+ concentration in the solution and slows the rate of the formation of Ce(OH)CO3 crystals.8 Upon heating to a certain temperature, the complex will release tar2− and Ce3+ ions, and tar2− will partially decompose to release CO32− anions slowly. The newly formed CO32− and the addition of OH− anions react with Ce3+ cations to produce Ce(OH)CO3 anisotropic nuclei. Tar2− anions preferentially bind to the crystal faces of the growing particle to kinetically control the growth rates of various facets of a seed.26 As evidenced by TEM and SAED results, the anisotropic growth of Ce(OH)CO3 along the direction is substantially suppressed, and it would grow preferentially along six crystallographically equivalent directions . Little 3-fold symmetric “caltrop” nanoplates will gradually appear, and tar2− anions again possibly fasten onto the (0001) surface of Ce(OH)CO3 nanoplates through the −COO− and −OH groups and stop the growth of Ce(OH)CO3 nanoplates along the direction. Then, these initially formed nanoplates assemble in layer-by-layer stacking style with the help of the hydrogen bond and electrostatic effects of adsorbed tar2− anions.25,26 This interaction can also be confirmed by IR spectrum (see the Supporting Information, Figure S1). With the reaction extending, the crystal growth are

Figure 9. XRD pattern of the CeO2 obtained through thermal decomposition−oxidation process at 500 °C for 10 h.

with multilayer caltrop is in the range of 2−2.5 μm and its thickness is in the range of 0.6−1 μm, respectively. The number of layers of obtained Ce(OH)CO3 dendrites is fewer than that of Ce(OH)CO3 dendrites obtained at the tar2−/Ce3+ molar ratio of 2 (sample 2; Figure 4b). No product could be obtained when the molar ratio of tar2−/Ce3+ was increased to 3. Other complexing agents such as trisodium citrate (Na3cit) and sodium malate (Na2mal) at an equal molar concentration were used. It was found that no Ce(OH)CO3 or hierarchical dendrites of Ce(OH)CO3 could be obtained, suggesting that these complexing agents could not play the same role as did Na2tar in the synthesis process. The above experiment demonstrates that Na2tar acts as both a participating and a coordinating agent. The result also suggests that the presence of an appropriate amount of Na2tar plays a crucial role in the growth of 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop.

Figure 10. Images of CeO2 crystals calcined at 500 °C for 10 h: (a, b) low and enlarged magnification SEM image; (c) TEM image of individual CeO2 crystal; the inset gives the corresponding SAED; (d) HRTEM image of the CeO2 crystal. 276



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Figure 11. (a, b) Low and enlarge magnification SEM image of La2O2(CO3) crystals calcined at 500 °C for 10 h; (c, d) low and enlarge magnification SEM image of Eu3+/CeO2 crystals calcined at 500 °C for 10 h.

numbers of tar2− anions chelation with Ce3+ cation. This slows the nucleation process of Ce(OH)CO3 and could not achieve the complexing agent assisted growth of Ce(OH)CO 3 dendrites. When the reaction system is low to 200 °C, a relatively stable complex could not dissociate to release Ce3+ and CO32− ions, resulting in no product formation. On the basis of the above experiment results, it is natural to deduce that generation of branched or dendritic structures is the result of kinetically and/or thermodynamically controlling the initial nucleating stage and subsequent crystal growth stage through changing reaction parameters, such as the tar2−/Ce3+ molar ratio, reaction temperature and inherent electronic nature of complexing agent, and so on. To further explore the reason for Na2tar affecting the formation of the Ce(OH)CO3 dendrites, the hexagonal structure of Ce(OH)CO3 is shown in Figure 6 (left-right). The C−O triangles (CO32−), Ce3+ and OH− ions are stacked alternately along its axes. As a kind of ligand, it is well-known that Na2tar could easily coordinate with Ce3+ ions. The interplanar lattice of (101̅0), (011̅0), and (0001) is 7.2382, 7.2382, and 9.9596 Å, respectively. In comparison with [101̅0] and [0110̅ ] directions, the [0001] shows relatively smaller steric repulsion for the coordination of tar2− anions. So, the crystal structure growth is limited in the [0001] direction and predominantly occurs along the six energetically equivalent directions. The diffraction pattern also demonstrates that dendrite is oriented along the [0001] direction with three branches along [11̅00], [101̅0], and [011̅0] (Figure 2c). Thus, the formation of 3-fold symmetric dendrites with multilayer caltrop is a result of growth along ± [11̅00], ± [1̅010] and ± [011̅0] direction.28 The complexing agent assisting the generation of dendrites is closely related to the crystal structure; it has also been observed in the synthesis of other inorganic compounds.7f In order to further explore the real factors affecting the dendrites and extend this method, La(OH)CO3, Eu3+/Ce-

Figure 12. Emission spectra of the products (a) Eu3+/Ce(OH)CO3 and (b) Eu3+/CeO2.

mainly governed by the diffusion of reaction species to the surfaces of the crystals and the Ostwald ripening process at the cost of the smaller nanoparticles, and the size of the as-obtained 3-fold dendrites with multilayer caltrop increases with the reaction time. On the basis of the above results, it is reasonable to believe that Na2tar plays double roles in the reaction. One is to serve as a chelating ligand to form a stable complex with Ce3+ and further kinetically control the reaction rate, and the other is to act as capping agent to affect the facet growth and their assembly. The formation of 3-fold dendrites with multilayer caltrop can be described with the help of a simple schematic illustration shown in Figure 5. However, driven by a relatively low hydrogen bond and electrostatic effects, a few layers of Ce(OH)CO3 caltrop are self-assembled into a dendritic structure with a tar2−/Ce3+ molar ratio of 1. If the molar ratio of tar2−/Ce3+ is increased to 3:1, the Ce3+ cation is hardly freed from the complex because of high coordination 277



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branches on both sides (Figure 8b).28 With the decrease of the radius of atoms, the increment rate of the motion of Ce(OH)CO3 nuclei break the limit of spatial confinement and diffusion regime, the tendency of growth along the [1̅010] and [0110̅ ] would be increased to the same rate as the growth of [11̅00] and form a compact 3-fold symmetric dendritic structure with multilayer caltrop (Figure 8a). With adulteration of the little radius of europium, the growth of ± [1̅010] and ± [011̅0] directions is relatively faster than that of [11̅00] due to the increasing rate of the motion of Eu3+/Ce(OH)CO3 nuclei. This leads to the formation of microplume dendrites with long nanoribbons on both sides (Figure 8c). When the precursor is Pr(OH)CO3 or Nd(OH)CO3, the motion of Pr(OH)CO3 or Nd(OH)CO3 in the reaction system is high owning to its little radius, resulting in an increased diffusion. In these conditions, the high rate of diffusion leads to the formation of particles. These studies suggest that fractal growth of crystals might arise under conditions far from equilibrium where the rate of diffusion of the reactive species is limited. As to the formation of Ce(OH)CO3 dendrites with multilayer, La(OH)CO3 and Eu3+/Ce(OH)CO3 dendrites with a single layer, it is speculated that the different frameworks formed by the ligand coordination with the Ln3+ ion plays an important role in determining the final shape of the product. Of course, these factors need to be further studied in detail. Because CeO2 is cubic in structure, it is difficult to prepare CeO2 hierarchical dendrites directly. In this paper, it is natural to use 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop as a template precursor for the synthesis of CeO2 with the same dendrite structure. When 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop were calcined in air at 500 °C for 10 h,19,21,22a,b,24b CeO2 were obtained. Figure 9 (sample 7) presents the typical XRD pattern of as-prepared CeO2 samples. The strong and sharp reflection peaks suggest that the asprepared products are well crystallized. All the peaks could be indexed to the cubic phase of ceria with fluorite structure (JCPDS Card No. 34-0394). As shown in Figure 10, SEM images show that 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop are kept after thermal decomposition−oxidation to CeO2. 3-fold dendrites of CeO2 with multilayer caltrop are synthesized on a large scale and in high purity. The low-magnification image shows that the product consists almost entirely of 3-fold symmetric structures with multilayer caltrop, but its size is smaller than that of Ce(OH)CO3 template precursor. It is evident that the high yield and good uniformity of CeO2 dendrites is obtained with this approach (Figure 10a,b). Figure10c also displays typical TEM images of as-prepared CeO2 with 3-fold symmetric caltrop. It can also be seen that the sample does not break this nanostructure into discrete particles after sonication for 20 min. The HRTEM image in Figure 10d further reveals the singlecrystal characteristic structure of nanoplates. The 1.38 Å spacing of crystallographic planes corresponds to the (400) lattice fringe of CeO2, indicating it grows preferentially along the [100]. The SAED pattern taken from discretional nanoplate further reveals the single crystalline structure of nanoplate (inset of Figure 10c). It is obvious that this method is effective and convenient to obtain cerium oxides dendrites in considerably little treatment time. The obtained La(OH)CO3 and Eu3+/Ce(OH)CO3 dendrites were also quenched at 500 °C,19,21,22a,b,24b Eu3+/Ce(OH)CO3 decomposed and formed Eu3+/CeO2 (sample 11), whereas La(OH)CO3 decomposed and formed La2O2(CO3) (sample 9,

(OH)CO3, Pr(OH)CO3, Nd(OH)CO3 (sample 8, 10, 12, 13) were also hydrothermally synthesized at the tar2−/Ln3+ molar ratio of 2 under similar conditions. The obtained Pr(OH)CO3 and Nd(OH)CO3 are irregular nanoparticles (see the Supporting Information, Figure S2). The SEM of La(OH)CO3 and Eu3+/Ce(OH)CO3 with hierarchical dendrites are shown in Figure 7 (see the Supporting Information, Figure S3), the La(OH)CO3 crystal exhibits fine three-dimensional (3D) hierarchical nanostructures (Figure 7a). Further observation displays that individual micropine dendrite is composed of a long central trunk and highly ordered branches distributed on both sides of the trunk (Figure 7b). The lengths of the dendrite trunks are 2−5 μm, and those of the branch range from 500 nm to 1 μm. There is a striking periodic corrugated structure on the trunk and branch of a single dendrite. The TEM and HRTEM images of the La(OH)CO3 micropine dendrites and the corresponding SAED pattern are shown in Figure 7c,d. The SAED pattern of micropine dendrites shows the single crystalline nature of the samples. The lattice spacing from lattice structure was found to be 3.66 Å, which corresponds to the distance between the adjacent (1120) planes. The growth along the [0001] direction was confined; the micropine dendritic structure is the result of growth along ± [11̅00], ± [011̅0] and ± [1̅010] directions. It is clear that the dendritic nanostructures are symmetric, and the angle between the trunk and the branches are mostly about 60° for their hexagonal structure. The microplume dendrites of Eu3+/Ce(OH)CO3 is shown in Figure 7e,f. The uniform hierarchical structures consisted of nanoribbons with widths of 500 nm and lengths of up to several micrometers. The overview morphology of Eu3+/ Ce(OH)CO3 is that many highly aligned nanoribbons flock from one central stem toward outside to form microplume hierarchical structures. Figure 7g is the TEM image of the junction of the nanoribbon and central stem. The SAED taken on the nanoribbon also confirm the single crystalline nature of the sample. The corresponding HRTEM image is shown in Figure 7h. The fringe spacing is determined to 3.62 Å, which is close to the (1120) lattice spacing of the hexagonal structure Eu3+/Ce(OH)CO3, indicating the crystal growth is preferential in the [11̅00], [011̅0], and [101̅0] directions. It is clear that the choice of Na2tar as a complexing agent proves to be effective for preparing hierarchical dendrites. On the basis of the above experiments, the radius of the metal atom probably plays a crucial role in the variation of the obtained Ln(OH)CO3 dendritic structure. The ionic radius of rare earth decreases from atomic number 57 to 71 because of lanthanide contraction, and the ionic radius of La3+, Ce3+, Pr3+, Nd3+, Eu3+ is 1.061, 1.034, 1.013, 0.995, 0.950 Å, respectively.29 In principle, fractal and dendritic growth are affected by the diffusion of the reaction species to the surface of the crystals, interfacial energy and the structural anisotropy of the crystals.27 It is anchored by tartaric acid molecules that attachment and growth of branches are facilitated synchronously and symmetrically along six equivalent directions, resulting in the formation of the dendritic structures (Figure 8).8 Fractal and dendritic growth are expected in the diffusion-limited regime, away from the equilibrium conditions.27 The are six equivalent directions.28,29 La(OH)CO3 nuclei has a slow rate of motion to the fractal structure due to large radius, [11̅00] grows much faster than the other directions for spatial confinement, subsequent growth along the other two crystallographically equivalent directions [101̅0] and [01̅10], resulting in the formation of micropine dendrites with central and symmetric 278



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see the Supporting Information, Figure S4). As Figure 11 shown, the hierarchical dendrites of the precursors are all retained after reaction. It can also be found that the morphology of obtained La2O2(CO3) and Eu3+/CeO2 are entirely the same as the La(OH)CO3 and Eu3+/Ce(OH)CO3 dendrites, but its size is smaller than that of template precursors. It is further demonstrated that Eu3+/Ce(OH)CO3 and La(OH)CO3 dendritic structures are sustained in the thermal conversion process to Eu3+/CeO2 and La2O2(CO3). Figure 12 shows the photoluminescence (PL) spectra of microplume dendrites of Eu3+/Ce(OH)CO3 and Eu3+/CeO2 under 258 nm excitation. The PL spectra show five strong emission peaks at 592, 617, 635, 651, and 706 nm. It is wellknown that the emission peak at 592 nm is ascribed to the 5 D0−7F1 transition, and the emission peak at 612 and 632 nm can be assigned to 5D0−7F2.30 It is found that electric-dipole 5 D0−7F2 transition is a paramount position in the PL spectra of the Eu3+/Ce(OH)CO3 and Eu3+/CeO2. The fluorite-type CeO2 structure has the high inversion symmetry of Ce sites, and thereby, a forbidden electric-dipole 5D0−7F2 transition cannot be observed.31 It is Eu3+ cations replacing Ce4+ cations that induce the symmetry distortion of the local environment around the Eu3+ ions. The symmetry of sites occupied by Eu3+ cations degrades with a high Eu3+ doping concentration in CeO2/Eu3+ powder phosphors. Eu3+ doping of CeO2 and Ce(OH)CO3 also generates symmetry distortion of local environment around the Eu3+ ions because of the different ion sizes between Ce4+/Ce3+ and Eu3+ cations. This is the reason why the emission 5D0−7F2 at 612 and 632 nm is observed and becomes dominant for the high Eu3+ cations doping content.31,32 The emissions at 651 and 706 nm can be assigned to 5D0−7F3 and 5D0−7F4, respectively. It can be found that PL emission intensity for Eu3+/CeO2 sample is stronger than that of the Eu3+/Ce(OH)CO3 sample. The emission intensity of the samples is sensitive to their crystallinity, and Eu3+/CeO2 from thermal conversion of Eu3+/Ce(OH)CO3 has a high degree of crystallinity, resulting in strong emission intensity.

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Article

ASSOCIATED CONTENT

S Supporting Information *

FTIR spectrum, XRD patterns, and SEM images. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: 86-25-83314502.

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ACKNOWLEDGMENTS The work described here was supported by the National Science Foundation of China (Nos. 20971065 and 21021062), National Basic Research Program of China (2010CB923303), Anhui Provincial University Natural Science Foundation (No. KJ2010B130 and KJ2011A208), Anhui Provincial Natural Science Foundation (No. 090414190), Key Subject of Chizhou Unversity (2011XK04), and State Key Laboratory for Modification of Chemical Fibers and Polymer Material, Dong Hua University, Key Lab of Novel Thin Film Solar Cells (KF200901), CAS.

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REFERENCES

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CONCLUSIONS

In summary, 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop have been prepared in the presence of the complexing agent Na2tar by hydrothermal treatment. It is effective and convenient that 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop have been used for the synthesis of CeO2 with the same morphology as the template precursor. Na2tar as a complexing agent plays a very important role in the formation of 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop. The possible growth mechanism of 3-fold dendrites of Ce(OH)CO3 with multilayer caltrop is proposed. Furthermore, La(OH)CO3 and Eu3+/Ce(OH)CO3 dendritic structures can also be obtained with this method. La(OH)CO3 and Eu3+/Ce(OH)CO3 are transformed into Ln2O2(CO3) and Eu3+/CeO2, respectively, and their dendritic structures are sustained through heat treatment in an oven. The photoluminescent properties of Eu3+/Ce(OH)CO3 and Eu3+/CeO2 microplume dendrites have also been investigated. This work may present a way for the synthesis of other dendrites structure of inorganic materials. The simple synthesis of cerium carbonate hydroxide dendrites holds promise for application in photoluminescence devices. 279



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Controlled Synthesis of Three-Fold Dendrites of Ce(OH)CO3 with

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