DOI: 10.1021/cg901170j
Hierarchically Nanostructured Coordination Polymer: Facile and Rapid Fabrication and Tunable Morphologies
2010, Vol. 10 790–797
Kai Liu, Hongpeng You,* Guang Jia, Yuhua Zheng, Yeju Huang, Yanhua Song, Mei Yang, Lihui Zhang, and Hongjie Zhang* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China Received September 23, 2009; Revised Manuscript Received October 17, 2009
ABSTRACT: Hierarchically nanostructured coordination polymer Ce(1,3,5-BTC)(H2O)6 architectures with tunable morphologies have been successfully prepared on a large scale via a simple and rapid solution phase method at room temperature. The assynthesized products are characterized by X-ray diffraction, thermogravimetric analysis, energy dispersive X-ray (EDX) analysis, scanning electron microscopy, and transmission electron microscopy, respectively. A possible splitting formation mechanism for the interesting architectures has been proposed to interpret the growth process. By rationally adjusting the synthetic parameters such as concentration, reaction temperature, surfactant, solvent, static, and ultrasonic treatment, Ce(1,3, 5-BTC)(H2O)6 with straw-sheaflike, flowerlike, wheatearlike, strawlike, bundlelike, and urchinlike architectures, and nanorods can be selectively obtained. This facile, rapid, mild, cost-effective, reproducible, and environment-friendly growth strategy has considerable potential, and could readily be extended to the growth of other nano/microscaled metal-organic materials with novel morphologies and unique properties.
Introduction Nanostructured materials have attracted a great deal of attention because of their conspicuous physicochemical properties that differ markedly from those of bulk materials and potential for wide-ranging applications.1 The properties of nanocrystals depend not only on their chemical composition, phase, size, and shape, but their assemblies as well.2 Over the past several years, the synthesis of nanomaterials with welldefined architectures has attracted great attention due to the potential uses of the nanostructures as building blocks for various nanodevices and nanosystems.3 The research on nanostructures has rapidly extended from simple structures to the assembly of nanocrystals into ordered superstructures aiming to achieve increased structural complexity and functionality.4 However, the introduction of catalysts, high temperature, or pressure for the fabrication of hierarchical architectures induces heterogeneous impurities, increases the production cost, and leads to difficulty for scale-up production. Therefore, it is necessary to introduce an easy, rapid, inexpensive, environmentally friendly, and gram-scale onestep method to produce highly pure, hierarchical architectures assembled controllably from independent and discrete nanobuilding blocks in bulk quantities to meet the requirements for potential industrialization uses in nanodevices. In the past two decades, coordination polymers (CPs) or metal-organic frameworks (MOFs) have received intense interest due to their potential applications in gas storage, catalysis, optics, recognition, and separation, purification, and sensors.5-7 A structural study of macro-scaled crystalline samples is a main fundamental interest in metal-organic materials. On the other hand, miniaturizing the size of CPs (MOFs) crystals to the nanometer scale by functionalizing the *Corresponding author. E-mail:
[email protected] (H.Y.); hongjie@ciac. jl.cn (H.Z.). pubs.acs.org/crystal
Published on Web 11/10/2009
crystal interfaces will provide further opportunities to integrate novel functions into the materials without changing the characteristic features of the metal-organic crystal itself, and will allow the correlation between the chemical and physical properties and interfacial structures of nanocrystals to be investigated. Recently, with the development of nanoscience and nanotechnology, several micro- and nanometer-sized metal-organic complexes have been synthesized and their chemical and physical properties could be tailored according to the types of metal ions and organic building blocks used.8-10 Metal-organic materials containing lanthanide centers have been explored due to the ability of these elements to have larger coordination spheres (vs transition metals), as well as their unique electronic, optical, magnetic, and chemical characteristics resulting from the 4f electronic shells.11-13 Lanthanides in particular have an inherent affinity for oxygen-containing ligands over other functional groups and thus provide the potential for recognition and/or discrimination among a variety of linker and/or guest species. 1,3,5-Benzenetricarboxylic acid has three carboxylic groups with multifarious coordination modes and could be regarded as a good candidate in the construction of high-dimensional lanthanide coordination complexes. Some metal-organic compounds based on lanthanide and 1,3,5-BTC have been constructed successfully.14-18 For example, Chen et al. reported the preparation of microporous MOFs based on Eu3þ (Tb3þ) and 1,3,5-BTC, which can be used for sensing small molecules and anions.15 Qiu’s group has been engaging in the assembly of novel frameworks with zeolite or zeolite-like topology.16 Our group recently has successfully prepared three-dimensional (3D) sheaflike, butterflylike, and flowerlike architectures and one-dimensional (1D) rodlike nanostructures of Eu(1,3,5-BTC)(H2O)6 on a large scale by a facile and rapid precipitation method under mild conditions.17 Herein, we present the first preparation of the coordination polymer r 2009 American Chemical Society
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hierarchical architectures (cerium benzene-1,3,5-tricarboxylate) on a large scale via one-step precipitation in solution phase under moderate conditions. The Ce(1,3,5-BTC)(H2O)6 can exhibit 1D ribbonlike structure. The influence of concentration, reaction temperature, surfactant, solvent, static, and ultrasonic treatment was investigated in detail. More interestingly, morphologies of the coordination polymer can be rationally tailored by varying these growth conditions. 2. Experimental Section 2.1. Preparation. Ce(NO3)3 aqueous solution (pH = 3.5) was obtained by dissolving Ce(NO3)3 3 6H2O in deionized water with agitation. In a typical synthesis of Ce(1,3,5-BTC)(H2O)6 strawsheaflike architectures, 1 mL of 0.5 M Ce(NO3)3 aqueous solution was added into 1,3,5-H3BTC (0.5 mmol) water-ethanol solution (40 mL, v/v = 1:1) under vigorous stirring at room temperature and a large amount of white precipitate occurred immediately. After stirring for 10 min, the precipitate was collected by centrifugation, washed several times with ethanol and water, and dried in air at room temperature for characterization. Time-dependent experiments were carried out by adjusting the reaction time (10-120 s) with other fixed reaction parameters. Ce(1,3,5-BTC)(H2O)6 with different forms of splitting such as straw-sheaflike, flowerlike, wheatearlike, strawlike, bundlelike, urchinlike architectures, and nanorods were carried out by adjusting concentrations of 1,3,5H3BTC and Ce(NO3)3 (0.05, 3 mmol each), reaction temperature (50, 70 °C), the amount of polyvinylpyrrolidone (PVP K30, M = 58 000, 1.0 g), water-ethanol ratio (40 mL, v/v = 3:1), ethanol as the solvent (40 mL), static reaction (0.5, 1 mmol each), and ultrasonic treatment (0.5, 1 mmol each) and while other reaction parameters were kept unchanged. 2.2. Characterization. Elemental analysis of C, H, and Ce in the solid samples was carried out on a VarioEL (Elementar Analysensysteme GmbH) and an inductive coupled plasma (ICP) atomic emission spectrometric analysis (POEMS, TJA), respectively. Powder X-ray diffraction (XRD) patterns were performed on a D8 Focus (Bruker) diffractometer (continuous, 40 kV, 40 mA, increment = 0.02°). Thermogravimetric analysis (TGA) data were recorded on a thermal analysis instrument (SDT2960, TA Instruments, New Castle, DE) with a heating rate of 10 °C min-1 in an air flow of 100 mL min-1. The morphology of the samples were inspected using a scanning electron microscope (SEM, S-4800, Hitachi) equipped with an energy dispersive X-ray spectrum (EDX, JEOL JXA-840). Transmission electron microscopy (TEM) images were obtained using a JEOL 2010 transmission electron microscope operating at 200 kV. All the measurements were performed at room temperature.
3. Results and Discussion 3.1. Composition and Crystal Phase of the Ce(1,3,5-BTC)(H2O)6. Elemental analysis was first used to investigate the composition of the as-obtained products. The experimental results of C, H, and Ce are shown to be 23.25, 3.17, and 31.26%, respectively, which are basically in agreement with theoretical values of C (23.74%), H (3.30%), and Ce (30.76%), confirming the molecular formula Ce(1,3,5-BTC)(H2O)6. The chemical composition and crystal structure of the samples were also determined by powder XRD measurements. The XRD pattern of the as-synthesized Ce(1,3,5BTC)(H2O)6 straw-sheaves is shown in Figure 1. The peaks are strong and narrow, indicating the good crystallinity of the as-prepared sample. All observed peaks can be indexed to the known bulk phase of La(1,3,5-BTC)(H2O)6 and Sm(1,3,5-BTC)(H2O)6.18 No peaks of any other phases or impurities were detected. In addition, XRD patterns of the obtained Ce(1,3,5-BTC)(H2O)6 with other morphologies are pure phase and show no peaks of impurities (Figure S1 in the
Figure 1. Simulated XRD pattern using the X-ray structure of La(1,3,5-BTC)(H2O)6 single crystal (a) and XRD patterns of the straw-sheaflike structures of the Ce(1,3,5-BTC)(H2O)6 (b).
Supporting Information), indicating that structures of the coordination polymer prepared under different experimental conditions are the same. On the basis of the crystal structure of the bulk phase La(1,3,5-BTC)(H2O)6 and Sm(1,3,5-BTC)(H2O)6,18 the crystal structure of the Ce(1,3,5-BTC)(H2O)6 can thus be of monoclinic, space group Cc. The structure unit of the product is shown in Figure 2a. The center Ce atom is nine-coordinated by six oxygen atoms from water molecules as well as three oxygen atoms from three carboxylate groups of 1,3,5-H3BTC ligands to form a tricapped trigonal prismatic geometry. Three carboxylate groups of the ligand represent different coordination modes (Figure 2b). One bonding carboxylate group is unidentate, one is bidentate, and each ligand also presents a free carboxylate group. As shown in Figure 2c, the structure consists of parallel 1D ribbonlike molecular motifs extending along the a axis. These 1D molecular motifs are further stacked in such a way that phenyl groups of the 1,3,5-H3BTC ligands superimpose along the c axis. In addition, the combination of noncovalent interactions (hydrogen-bonding and π-π stacking) leads to the formation of a 3D network structure (Figure 2d). The thermal gravimetric analysis (TGA) curve taken in air (Figure S2, Supporting Information) exhibits two major stages of rapid weight loss in the temperature range from 80 to 430 °C. The weight loss for the two stages was measured to be 22.84 and 39.97%, respectively. This result is basically in agreement with the theoretical weight loss of the six water molecules (23.74%) and the organic ligand (38.46%) of assumed structure Ce(1,3,5-BTC)(H2O)6, further supporting that the products are composed of cerium benzene-1,3, 5-tricarboxylate. 3.2. Morphology Characterization and Possible Formation Mechanism of the Ce(1,3,5-BTC)(H2O)6 Straw-sheaves. The details of the morphological features of the Ce(1,3,5BTC) 3 6H2O were studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 3 shows typical images of the as-obtained, welldispersed straw-sheaves by introducing the solution of Ce(NO3)3 (0.5 mmol) into the 1,3,5-H3BTC solution (0.5 mmol) under stirring at room temperature for 10 min. Figure 3a,b shows typical SEM images of the uniform straw-sheaflike Ce(1,3,5-BTC)(H2O)6 on a large scale. The product looks like straw-sheaf with two fantails consisting of a bundle of outspread nanorods, which are closely bonded to each other in the middle, so we call it a “straw-sheaf structure”. Careful
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Figure 2. The asymmetric unit (a), coordination modes of the carboxylate groups (b), 1D ribbonlike structure along the a axis (c), perspective view of the packing along the c axis of the Ce(1,3,5-BTC)(H2O)6 (d). All the figures were drawn using the CIF file of La(1,3,5-BTC)(H2O)6. The hydrogen atoms were omitted for clarity. Ce blue (shown as polyhedra), O red, C black.
Figure 3. SEM (a-c), TEM (d) images at different magnifications, and the EDX spectrum (e) of the Ce(1,3,5-BTC)(H2O)6 strawsheaflike architectures (0.5:0.5 mmol, R.T., vigorous stirring).
observations (Figures 3c,d, and S3) reveal that the individual straw-sheaf has a length in the range of 6-8 μm and a middle diameter in the range of 1-2 μm. Interestingly, the individual nanorods composing the two fantails of these superstructures are compressed, which have widths of 100-200 nm, thicknesses of 30-50 nm, respectively. TEM image of a typical straw-sheaf (Figure 3d) indicates that numerous nanorods with very high density are radially arranged from
the center of the sheaflike structures. Furthermore, the chemical composition of the straw-sheaves was further investigated with EDX, which indicates that the architectures are made of Ce, C, and O except the Si and Pt peaks from measurement (Figure 3e), confirming that these strawsheaves are formed from cerium and benzene-1,3,5-tricarboxylic acid. The formation process of hierarchical architectures is a complex process, which is affected by both crystal growth environments and crystal structures, including the degree of supersaturation, diffusion of the reaction, surface energy, crystal structures, and so forth.19 Generally, well-controlled complex morphologies are usually difficult to acquire by directly mixing two incompatible solutions because of the rapid decrease in supersaturation and further depletion of reaction nutrients in a short period of time. However, we readily obtained well-organized, assembled nanorod strawsheaflike architectures via rapid and direct precipitation reaction. Time-dependent experiments were carried out to gain insight into the formation mechanism of Ce(1,3,5-BTC)(H2O)6 superstructures. The morphological evolutions of the intermediate products collected after different reaction times (10-120 s) were recorded by SEM (Supporting Information, Figure S4). From these SEM images, we can clearly see that well-organized sheaflike structures have already been formed at the very early stage, which are composed of numerous nanorods with lengths of 6-8 μm. As the reaction time increases, we can only obtain similar and much denser straw-sheaves with two fantails based on the 1D rodlike structure. Generally, the growth process of crystals can be classified into two steps: an initial nucleating stage and a crystal growth stage. At the initial nucleation stage, the formation of the seeds is crucial for further growth of the crystals. The subsequent crystal growth stage is a kinetically and thermodynamically controlled process. Our experiments indicate that the nucleation and growth of the Ce(1,3,5-BTC)(H2O)6 take place in a very short time. This growth behavior is quite similar to those of inorganic nanomaterials, such as Bi2S3,20 LnVO4,21 carbonates
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Table 1. Summary of the Experimental Conditions and the Corresponding Morphologies of the Ce(1,3,5-BTC)(H2O)6 Samples sample
T (°C)
s1 s2 s3 s4 s5 s6 s7 s8 s9 s10 s11 s12
R.T. R.T. R.T. 50 70 R.T. R.T. R.T. R.T. R.T. R.T. R.T.
concentration (mmol)
PVP (g)
water/ ethanol
treament
morphology
0.05 0.5 3 0.5 0.5 0.5 0.5 0.5 0.5 1 0.5 1
0 0 0 0 0 1.0 0 0 0 0 0 0
1:1 1:1 1:1 1:1 1:1 1:1 3:1 pure ethanol 1:1 1:1 1:1 1:1
stirring stirring stirring stirring stirring stirring stirring stirring static static ultrasonic ultrasonic
flower straw-sheaf wheatear straw-sheaf straw bundle straw-sheaf straw-sheaf urchin nanorod bundle bundle
length (μm) 5 6-8 2-3 10-12 3-6 5 5-7 12 10-15 10 3-4 2-4
Figure 4. SEM images (a-d) of the Ce(1,3,5-BTC)(H2O)6 flowerlike architectures at different magnifications (0.05:0.05 mmol, R.T., vigorous stirring).
Figure 5. SEM images (a-d) of the Ce(1,3,5-BTC)(H2O)6 wheatearlike architectures at different magnifications (3:3 mmol, R.T., vigorous stirring).
(CaCO3, BaCO3, MnCO3, CdCO3, SrCO3),22 nickel phosphate (Ni11(HPO3)8(OH)6),23 fluoroapatites (Ca5(PO4)3F),24 and so forth. The formation of these hierarchical architectures is considered to be a crystal splitting process. Thus, we assume that a splitting growth mechanism is most likely responsible for the formation of the straw-sheaflike Ce(1,3,5-BTC)(H2O)6 superstructures. To further understand the splitting growth mechanism of Ce(1,3,5-BTC)(H2O)6 system, controlled experiments have been conducted and tunable morphologies such as 3D strawsheaflike, flowerlike, wheatearlike, strawlike, bundlelike, urchinlike architectures, and 1D nanorods can be obtained. The results of morphology evolution under different reaction conditions are summarized in Table 1. 3.3. Influences of Different Reaction Parameters (Concentration, Temperature, Surfactant, Solvent, Static, and Ultrasonic Treatment) on the Morphologies of Ce(1,3,5-BTC)(H2O)6. 3.3.1. Effects of Concentration. Generally speaking, crystal splitting is associated with fast crystal growth, which depends strongly on the oversaturation of the solution.20 Variation in the concentration of reactants resulted in successive splitting of the Ce(1,3,5-BTC)(H2O)6 architectures. Figure 4 presents typical SEM images of the Ce(1,3,5-BTC)(H2O)6 hierarchical architectures obtained at low concentrations of 1,3,5-H3BTC and Ce(NO3)3 (0.05 mmol each) while keeping other reaction conditions the same. The low-magnification SEM image (Figure 4a) shows that the sample mainly consists of a large quantity of flowerlike architectures with diameters of about 5 μm. From the enlarged SEM image shown in Figure 4b, the
Ce(1,3,5-BTC)(H2O)6 flowers have 4-6 petals and a central nucleus, which are composed of numerous nanorods pointing toward the center of flowerlike structures. High magnification SEM images of the sample (Figure 4c,d) indicate that the width and thickness of these nanorods are in the range of 100-200 nm and 40-60 nm, respectively. As the concentrations of 1,3,5H3BTC and Ce(NO3)3 was increased to 0.5 mmol, uniform hierarchical straw-sheaflike architectures made of abundant nanorods with very high density were obtained on a large scale (Figure 3). When the concentrations reached 3 mmol, more interestingly, a large quantity of wheatearlike structures comprising several nanorods were obtained (Figure 5a,b). Close observations indicate that that these Ce(1,3,5-BTC)(H2O)6 microstructures look like natural wheatears with lengths of 2-3 μm. The nanorods have widths of 50-100 nm and thicknesses of 10-30 nm, respectively (Figure 5c,d). On the basis of the above results, we would like to point out that these flowerlike and wheatearlike hierarchical architectures show some analogy to the straw-sheaf structures and look like their counterparts. However, we can also clearly see that the degree of fractal splitting decreases as the concentration of the reactants increases from 0.05 to 3 mmol. We know that splitting is only possible if the oversaturation exceeds a certain “critical” level, which is specific for each mineral and the given conditions.25 However, according to the nucleation and growth theory of nanocrystals, when the concentration is higher, more nuclei will form in a shorter time. And a large number of nuclei will then grow slowly, which will prohibit the occurrence of the splitting growth process.20
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Figure 7. SEM images of the Ce(1,3,5-BTC)(H2O)6 bundlelike architectures assisted by PVP at different magnifications (a-d) (0.5:0.5 mmol, R.T., vigorous stirring, 1.0 g PVP).
Figure 6. SEM images of the Ce(1,3,5-BTC)(H2O)6 sheaflike (a, b) (0.5:0.5 mmol, 50 °C, vigorous stirring) and strawlike (c-f) (0.5:0.5 mmol, 70 °C, vigorous stirring) architectures at different magnifications.
Thus, the concentration of reactants can significantly affect the size and shape of the products. 3.3.2. Effects of Reaction 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 has significant influence on the morphology of the assynthesized Ce(1,3,5-BTC)(H2O)6 samples. At room temperature, the as-prepared samples are entirely composed of straw-sheaves with obvious splitting, perfect uniformity, and monodispersity, as shown in Figure 3. With increasing the reaction temperature, the degree of splitting decreases. At the reaction temperature of 50 °C, sheaflike structures with simple splitting assembled from well-aligned nanorods appeared (Figure 6a). A high magnification SEM image (Figure 6b) shows that these sheaves are much longer (10-12 μm) than those prepared at room temperature, indicating that the trend of anisotropic growth is strengthened with increasing reaction temperature. When the reaction was carried out at 70 °C, splitting was strongly inhibited and only slight splitting occurred. A large quantity of strawlike nanostructures were produced, as shown in Figure 6c,d. More careful observations of the typical strawlike structure indicate that each straw is composed of several compressed nanorods (Figure 6e,f). Moreover, the flat nanorods have widths of 60-100 nm, thicknesses of 20-50 nm, and lengths in the range of 3-6 μm, respectively. The above results indicate that the products have obvious changes in morphology including the sizes and the decrease of the splitting extent with increasing reaction temperature. These observations may be explained according to both the nucleation and growth theory of nanocrystals and the crystal splitting theory. The formation of nanocrystals with nonthermodynamically equilibrium shapes is driven kinetically, which includes the initial formation of nuclei just
after supersaturation and subsequent growth of the nuclei. Meanwhile, the formation of these hierarchical architectures requires fast crystal growth. At higher temperature, on the one hand, more nuclei will form just after supersaturation. On the other hand, a large number of nuclei will lead to slow growth, which decreases the splitting extent.20 Therefore, sheaflike and strawlike structures can be selectively prepared by tuning the reaction temperature rationally. 3.3.3. Effects of Surfactant. Recently, various types of surfactants have been widely used in most solution routes in the synthesis of well-structured materials with controlled morphology due to their efficient self-assembly properties.21,23,26,27 These surfactants can control the nucleation and crystallization of nanomaterials with special orientation and morphology. Most importantly, the crystal splitting extent could be interrupted in the presence of surfactants. Therefore, we investigated the influence of PVP on the morphological evolution under the typical conditions for straw-sheaflike structures formation (R.T., 0.5:0.5 mmol). When PVP was not used, the product looks like straw-sheaf with two fantails, exhibiting a radiating form, as shown in Figure 3. This morphology character is that a sheaf of rodlike nanocrystals has been bandaged in the middle, with the top and bottom fanning out while the middle remains thin. However, if some amount of PVP (1.0 g) was added, bundlelike superstructures with no radiating fantails could be obtained on a large scale (Figure 7a). These microbundles are nearly monodispersed with lengths of about 5 μm (Figure 7b). Detailed SEM observations reveal that dual fanlike patterns disappeared and the splitting extent was held back (Figure 7c,d). The individual nanorods composing the microbundles are 100-200 nm in width and 30-50 nm in thickness, respectively. On the basis of the observation, we can reasonably assume that PVP plays an important role in the formation of the hierarchical bundlelike architectures. First, PVP may act as potential crystal face inhibitors in the system, which is preferentially absorbed onto some specific planes and benefits the formation of oriented nucleation, leading to anisotropic growth of the nanorods. Second, PVP may be a very potent surface stabilizer, which could interfere with the growth of the Ce(1,3,5BTC)(H2O)6 crystals, cap the crystal surfaces, and lower the energy cost for creating new surfaces, thus affecting the
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Figure 8. SEM images (a-d) of the Ce(1,3,5-BTC)(H2O)6 strawsheaflike architectures composed of nanowires (0.5:0.5 mmol, water-ethanol (40 mL, v/v = 3:1), R.T., vigorous stirring) at different magnifications.
crystal splitting and prompting the formation of hierarchical bundlelike architectures from individual nanorods. 3.3.4. Effects of Solvent. The solvent was an important factor in the morphology evolution of the Ce(1,3,5-BTC)(H2O)6. As clearly presented in Figure 3, the as-obtained straw-sheaves are composed of compressed and straight nanorods when volume ratio of water/ethanol at 1 (40 mL, v/v = 1:1) and the other reaction conditions are kept the same. As the volume ratio of water/ethanol was increased to 3 (40 mL, v/v = 3:1), straw-sheaflike structures with obvious splitting were also obtained on a large scale. The first impression is that the Ce(1,3,5-BTC)(H2O)6 straw-sheaves with lengths of 5-7 μm have opened their dual fantails to a large extent, but again display a remarkable uniformity (Figure 8a,b). However, a careful observation of the detailed structure reveals that straw-sheaves are assembled from wellaligned nanowires instead of nanorods (Figure 8c). All the nanowires are radially oriented to its center and self-organized into a straw-sheaflike assembly. The high-magnification SEM image of the straw-sheaf fringe clearly displays that the fantails are composed of abundant nanowires with a diameter of about 30 nm (Figure 8d). Furthermore, we also used ethanol instead of water as the single solvent (40 mL) to study the solvent effect on the morphology evolution of the sample. It was found that the as-obtained product mainly consists of some typical straw-sheaflike structures (Figure S5, Supporting Information). The straw-sheaves are much longer than those prepared under water-ethanol mixed solvents, which can reach up to 12 μm in length. In addition, some detached sheaflike structures with split ends were also observable. A more detailed study on the crystal structure indicates that these straw-sheaves are also assembled from well-aligned nanowires instead of nanorods (Figure S5, Supporting Information). This observation demonstrates that the change of solution polarity can lead to the different dipole-dipole interactions, which would change the initial speciation, interfere with the nanocrystal growth, kinetically control the progress of splitting, and induce the assembly and stacking of Ce(1,3,5-BTC)(H2O)6 molecules in different modes. 3.3.5. Effects of Static Reaction. We also found that the same solution that would produce straw-sheaf superstructures under vigorous stirring would yield yet other striking
Figure 9. SEM images of the Ce(1,3,5-BTC)(H2O)6 urchinlike architectures (a, b) (0.5:0.5 mmol, R.T., static reaction) and nanorods (c-f) (1:1 mmol, R.T., static reaction) at different magnifications.
morphologies under static conditions. When 1,3,5-H3BTC (0.5 mmol) and Ce(NO3)3 (0.5 mmol) reacted without stirring at room temperature, surprisingly, the Ce(1,3,5-BTC)(H2O)6 product gradually changed to an urchinlike morphology. As shown in Figure 9a, uniform hierarchical urchinlike architectures made of nanorods with very high density were obtained, which had an overall size in the range of 10-15 μm. A more careful observation of the typical structures, as shown in Figure 9b, indicates that abundant nanorods with very high density grow pointing toward the center of the sphere. In this case, the fractal splitting was almost completely compared with the straw-sheave structures obtained under vigorous stirring. Similarly, we investigated the morphology evolution of the Ce(1,3,5-BTC)(H2O)6 by increasing concentrations of reactants to 1 mmol without stirring. The overall morphology of the sample indicates that there exists a great deal of well-dispersed nanorods with lengths of about 10 μm (Figure 9c,d). No other 3D hierarchical architectures can be detected, indicating that splitting was strongly inhibited. The higher magnification SEM images in Figure 9e,f exposes some details of the 1D rodlike structures. It can be clearly seen that these nanorods exhibit a smooth surface and flat tip, which are 100-200 nm in width and 40-60 nm in thickness. Therefore, the morphology transformation between 3D hierarchical urchinlike architectures and 1D rodlike nanostructures can be realized easily by controlling reactants concentrations under static conditions rationally. 3.3.6. Effects of Ultrasonic Reaction. We also extended our efforts to the synthesis of the Ce(1,3,5-BTC)(H2O)6 under ultrasonic conditions. Figure 10 shows typical images of the as-obtained architectures by introducing the solution of Ce(NO3)3 (0.5 mmol) into the 1,3,5-H3BTC solution (0.5 mmol) under ultrasonic treatment at room temperature. As shown in Figure 10a,b, all the Ce(1,3,5-BTC)(H2O)6 products have a bundlelike morphology assembled from
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structure by coordination interaction, the 1,3,5-BTC ligands bridge adjacent Ce3þ ions to form ribbonlike molecular motifs extending along the a axis. The occurrence of 1D ribbon in the compound is attributed to the steric orientation of the carboxylate groups of 1,3,5-BTC.14-18 Thus, this observation suggests that the flexible coordination mode of the 1,3,5-BTC ligand in combination with the high coordination number of the Ce3þ ions favors the formation of 1D ribbonlike structure, which may benefit for the anisotropic nucleation and splitting growth process of the Ce(1,3,5BTC)(H2O)6. In fact, as already shown by our previous work,17 isostructural Eu(1,3,5-BTC)(H2O)6 tends to form 3D sheaflike, butterflylike, and flowerlike architectures and 1D rodlike nanostructures. 4. Conclusions
Figure 10. SEM images (a-d) (0.5:0.5 mmol, R.T., ultrasonic reaction) and (e, f) (1:1 mmol, R.T., ultrasonic reaction) of the different splitting Ce(1,3,5-BTC)(H2O)6 bundlelike architectures at different magnifications.
well-aligned nanorods. The close-up view of SEM images (Figures 10c,d and S6a,b) indicates that these microbundle structures are different from the typical straw-sheaves as depicted in Figure 3. First, the top and bottom of the bundles are not fanning out although they are also assembled by a sheaf of nanorod. Radiating fantails could not be found in these superstructures. Second, these microstructures are much smaller than the straw-sheaves. The individual microbundles have a length in the range of 3-4 μm, a diameter of 0.5-1 μm, and the individual nanorods composing the bundles are 50-100 nm in width, 10-20 nm in thickness. A further increase of the concentrations of 1,3,5-H3BTC and Ce(NO3)3 to 1 mmol led to much smaller bundles, which have a diameter ranging from 300 to 500 nm (Figure 10e). This is seen more clearly in the SEM image of an individual bundlelike structure in Figure 10f, where splitting was strongly inhibited and only slight splitting occurred at the tips. The higher magnification SEM images (Figures 10f and S6c,d) expose some details of the smaller bundlelike nanostructure of Ce(1,3,50BTC)(H2O)6; it is actually built from several nanorods with lengths ranging from 2 to 4 μm, widths of 50-150 nm, and thicknesses of 20-40 nm, respectively. The above observations indicate that the presence of ultrasonic could interrupt the crystal growth and crystal splitting, and most importantly result in the morphology evolution and size modulation. In general, crystal splitting often occurs in a crystal with structural anisotropy, which, for example, prefers 1D growth and has relatively small lateral adhesion energy.12,13 Our synthesis suggests that Ce(1,3,5-BTC)(H2O)6 has a strong splitting ability, and this is largely determined by the anisotropic nature of the coordination polymer. It can be explained in terms of the typical Ce(1,3,5-BTC)(H2O)6 crystal structure, as shown in Figure 2c. One can see that the interesting feature is the presence of parallel 1D ribbonlike
We have presented a simple, mild, rapid, low-cost, environmentally friendly, and effective one-step solution route for the selectively preparing 3D straw-sheaflike, flowerlike, wheatearlike, strawlike, bundlelike, urchinlike architectures and 1D rodlike nanostructures of Ce(1,3,5-BTC)(H2O)6. The shape and size of the coordination polymer can be perfectly manipulated by controlling the concentration, reaction temperature, surfactant, solvent, ultrasonic, and static treatment. The formation of these nano/microstructures may be elucidated via the fractal splitting growth mechanism, as observed in some natural minerals. Most importantly, the special 1D ribbonlike structure of the Ce(1,3,5-BTC)(H2O)6 may be responsible for the formation of these hierarchical architectures and nanorods. This facile methodology could be extended for the controlled synthesis of other functional coordination compound nanostructures, and the obtained 3D architectures and 1D nanorods could be introduced as the building block for novel minioptoelectronic devices. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (Grant No. 20771098) and the National Basic Research Program of China (973 Program, Grant Nos. 2007CB935502 and 2006CB601103). Supporting Information Available: XRD patterns, TGA curve, and SEM images (Figures S1-S6). This material is available free of charge via the Internet at http://pubs.acs.org.
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