Tartrate-Induced Self-Assembly of Highly Positively Charged Three

Feb 4, 2013 - ABSTRACT: Novel three-dimensional (3D) spiral flowerlike Cd(OH)2 structures were prepared for the first time through a tartrate-induced...
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Tartrate-Induced Self-Assembly of Highly Positively Charged ThreeDimensional Spiral Cd(OH)2 Flowers with Largely Exposed (001) Plane Han-Qiao Shi,†,‡ Yu Liu,† Hong-Mei Xiao,† and Shao-Yun Fu*,† †

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100039, China

‡

S Supporting Information *

ABSTRACT: Novel three-dimensional (3D) spiral flowerlike Cd(OH)2 structures were prepared for the first time through a tartrate-induced hydrothermal self-assembly route. Scanning electron microscopy observation showed that the nanopetals were assembled in the way of spiraling sequential overlap on the side edge of early nanopetals to gradually form the 3D spiral flowerlike Cd(OH)2 structures. Time-dependent experiments with appropriate intervals have clearly disclosed that the self-assembly process of 3D spiral Cd(OH)2 flowers is governed by a nucleation−dissolution−recrystallization growth mechanism. The self-assembled 3D spiral Cd(OH)2 flowers are highly positively charged and possess superior adsorption and separation performance over other two morphological structures of hollow spheres and sub-microsized plates reported previously. The largely preferential exposure of the (001) plane with the highest surface charge density is the dominant cause for the excellent adsorption and separation performance. structures by a facile hydrothermal treatment. Wang et al.17 reported the aqueous solution synthesis of Cd(OH)2 hollow spheres. Cd(OH)2 nanorings were fabricated by Miao et al.18 via ultrasonic chiseling on Cd(OH)2 sub-microsized plates. Shinde et al.19 developed a solution chemistry approach for the selective formation of ultralong nanowire bundles of crystalline Cd(OH)2. However, to our best knowledge, little work has been reported on the self-assembly of 3D Cd(OH) 2 structures.14 In addition, due to its positively charged property, Cd(OH)2 can effectively adsorb negatively charged dyes on its surface, and it thus is a potential candidate for the separation of negatively charged molecules from matrices.13,15,20,21 The adsorption and separation performance of Cd(OH)2 has been reported for nanowires, nanostrands, and necklace-like nanostructures.13,15,22 Nonetheless, the influence of Cd(OH)2 structural features and exposed crystal plane on its adsorption and separation performance has not been discussed. Herein, we demonstrate a tartrate-induced hydrothermal route for the self-assembly of novel 3D spiral flowerlike Cd(OH)2 structures with a largely preferential exposure of the (001) plane at the pH value of 12.5. In the 3D spiral flowerlike Cd(OH)2 structures, the nanopetals are assembled in the way of spiraling sequential overlap on the side edge of early nanopetals. The self-assembly growth mechanism is disclosed by time-dependent experiments with appropriate intervals.

1. INTRODUCTION Owing to its promising applications in energy conversion, chemical sensing, adsorption and separation technique, photonic and electronic apparatus, drug-delivery carrier, catalyst support, etc., self-assembly of nanobuilding blocks into complex three-dimensional (3D) structures has received increasing interest in recent years.1−4 The self-assembled 3D structures provide promising complex functions and direct bridges between the nanoscale objects and the macroscale world.5,6 The self-assembled 3D structures not only inherit the selfproperties of nanobuilding units but also show collective properties (coupling effect, synergistic effect, etc.) coming from self-assembly of nanobuilding units.7 Therefore, the development of self-assembly synthesis for various complex 3D structures is of great significance. Recently, nanostructured metal hydroxides including Ni(OH)2, Mg(OH)2, Cu(OH)2, and Cd(OH)2 have been extensively studied.8−12 Among these nanostructured metal hydroxides, Cd(OH)2 is a wide band gap semiconductor with a number of possible applications including solar cells, photo transistors and diodes, transparent electrodes, sensors, cathode electrode materials of batteries, separation of negatively charged dyes, and so forth.13−15 Therefore, various morphological Cd(OH)2 structures have been prepared using different methods.13,15−19 Ichinose et al.15 reported the spontaneous formation of Cd(OH)2 nanostrands in water. Ye et al.13 fabricated ultralong Cd(OH)2 nanowires by a template-free hydrothermal method. Shi et al.16 demonstrated fabrication of hexagonal sub-microsized plates of Cd(OH)2 with nanoporous © 2013 American Chemical Society

Received: September 28, 2012 Revised: January 6, 2013 Published: February 4, 2013 1091

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Figure 1. (a) Low-magnification SEM image, (b) medium-magnification SEM image of the 3D spiral flowerlike Cd(OH)2 structures, (c) highmagnification SEM image of an individual Cd(OH)2 spiral flower, (d) powder XRD pattern of the 3D spiral flowerlike Cd(OH)2 structures. (e) TEM image of the edge part of one single Cd(OH)2 spiral flower, (f) HRTEM image of the edge part of an individual Cd(OH)2 petal, and (g) enlarged image of the area marked by a rectangle in panel f. into a 40 mL Teflon-sealed autoclave and maintained at 130 °C for 120 min. After being cooled to room temperature, the product was separated from the solution by centrifugation, afterward was washed several times with alcohol and deionized water, and then was dried in air at 60 °C for 4 h. To investigate the growth mechanism of the 3D spiral flowerlike Cd(OH)2 structures, the corresponding Cd(OH)2 products were also collected at different reactive stages during the synthetic process. 2.3. Characterization. The phase purity of the product was characterized using an X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) observation was conducted using a HITACHI S-4300 scanning electron microscope (Japan). The samples were also observed with a JEOL JEM-2100 transmission electron microscopy (TEM) in bright field and a JEM-2100F high resolution transmission electron microscopy (HRTEM) at 200 kV. Fourier transform infrared (FTIR) spectrum was recorded with a Varian 3100 FT-IR spectrometer using a KBr wafer. Room temperature UV−vis adsorption spectra were recorded on a U-3900 spectrophotometer (HITACHI) in the wavelength range of 200−800 nm. The nitrogen adsorption and desorption isotherms were obtained at 77 K. The Brunauer−Emmett− Teller (BET) surface area was calculated from the linear part of the BET plot.

Control experiments are carried out to investigate the role of tartrate and pH value in the self-assembly of 3D spiral Cd(OH)2 flowers. Moreover, these self-assembled 3D spiral Cd(OH)2 flowers show superior adsorption and separation performance over Cd(OH)2 hollow spheres and sub-microsized plates reported previously.16−18 In addition, the relationship between its adsorption−separation performance and structural feature is discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. CdCl2·2.5H2O and tartaric acid (C4H6O6) were purchased from Tianjin Jinko Institute of Refined Chemical Engineering. NaOH was purchased from Beijing Chemical Works. All the chemicals were of analytical grade and utilized without further purification. 2.2. Synthesis. In a typical procedure, 0.032 g of CdCl2·2.5H2O and 0.158 g of tartaric acid were dissolved in 35 mL of deionized water. The solution was magnetically stirred for 30 min to ensure the full complexation. And an aqueous solution of 0.25 M NaOH was gradually added under constant stirring to adjust the pH value of solution to predetermined values. Then, the mixture was transferred 1092

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Figure 2. SEM images of the 3D spiral flowerlike Cd(OH)2 structures at different hydrothermal treatment stages: (a) 0 min, (b) 20 min, (c) 25 min, (d) 30 min, (e) 65 min, and (f) 120 min, (g) schematic illustration of the formation and morphological evolution of the 3D spiral flowerlike Cd(OH)2 structures in the whole synthetic process.

3. RESULTS AND DISCUSSION

Cd(OH)2 sheets, and the 2D Cd(OH)2 sheets do not show positive and negative polar ends. The phase identification of the 3D flowerlike Cd(OH)2 structures has been further corroborated by X-ray diffraction measurement using CuKα radiation (λ = 1.5406 Å). As shown in Figure 1d, the diffraction peaks in the XRD pattern of 3D flowerlike Cd(OH)2 structures can be indexed to the pure hexagonal phase of Cd(OH)2 with the lattice constants of a = 3.4947 Å and c = 4.7106 Å (JCPDS No. 31-0228). The strong and sharp diffraction peaks in the XRD pattern indicate that the obtained 3D flowerlike Cd(OH)2 structures are well crystallized. Moreover, the peak of (001) plane is much stronger in its intensity than the peaks of other planes, which reveals that the (001) plane is highly developed and largely exposed, showing significant preferred orientation. The structures of these 3D flowerlike Cd(OH)2 structures were further investigated by TEM. Figure 1e shows the fringe part of one Cd(OH)2 flower. The SAED pattern (right-handed top inset of Figure 1e) demonstrates its single-crystalline nature

3.1. Morphology and Structure Characterization. Figure 1a−c shows the SEM results of the as-synthesized Cd(OH)2 samples at low, medium, and high magnifications, respectively. Figure 1a clearly demonstrates that the products are 3D flowerlike Cd(OH)2 structures with a diameter ranging from 2 to 3 μm. Further observation (Figure 1b,c) shows that each flower is made up of thin petals. The Cd(OH)2 petals are about 60 nm in thickness (Figure 1b). In a single assembly, all the 2D Cd(OH)2 petals grow along one crystallographic axis, showing a feature of concentric assemblies, which is similar to the flowerlike ZnO structures.23 The flowerlike ZnO structures are composed of 1D ZnO rods. The 1D ZnO rods have a Znterminated positive end and an O-terminated negative end. All rods grow along either positive or negative polar axis, namely, concentric unipolar assemblies. Nonetheless, in our work, the 3D spiral flowerlike Cd(OH)2 structures are composed of 2D 1093

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Figure 3. (a) Low-magnification SEM image of the samples prepared in the absence of tartaric acid at a pH value of 12.5, (b) enlarged SEM image of the sample in panel a, (c) SEM image of the sample prepared in the presence of tartaric acid at the pH value of 10, and (d) HRTEM image of an individual Cd(OH)2 structure in panel c.

and can be well indexed to hexagonal Cd(OH)2 along the [001] zone axis. Figure 1f shows the typical HRTEM image of the fringe part of a Cd(OH)2 nanopetal. The enlarged image (Figure 1g) of the area labeled by a rectangle in Figure 1f reveals the clear lattice fringes, indicating that the Cd(OH)2 nanopetal has a crystalline structure, which is in accordance with the XRD result in Figure 1d. The interplanar spacing is measured to be about 0.301 nm, which corresponds to the (100) plane of the hexagonal Cd(OH)2. In addition, the 3D flowerlike Cd(OH)2 structures were still stable at room temperature after aging for one month as shown in Figure S1, Supporting Information. 3.2. Self-Assembled Growth Mechanism. In order to understand the formation process of the 3D flowerlike Cd(OH)2 structures, the samples obtained at different growth stages were carefully examined by SEM observation. When the 0.25 M NaOH solution was gradually added to the 4 mM CdCl2 solution combined with tartaric acid to adjust the pH value to the typical value of 12.5, white Cd(OH)2 precipitates were formed. The corresponding products were collected before they were transferred into a Teflon-sealed autoclave for hydrothermal treatment, and the small Cd(OH)2 nanoparticles were observed as shown in Figure 2a. When the reaction was prolonged to 20 min, the products were only small Cd(OH)2 nanoparticles (Figure 2b). As the reaction proceeded to 25 min, a small amount of Cd(OH)2 nanopetals emerged but the major products were still small Cd(OH)2 nanoparticles (Figure 2c). At the reaction time of 30 min, the amount of Cd(OH)2

nanopetals increased while the amount of Cd(OH)2 nanoparticles decreased as exhibited in Figure 2d. When the reaction time was exceeded to 65 min, Cd(OH)2 nanopetals continued to grow and 3D flowerlike morphological structures began to appear (Figure 2e). It was interestingly noticed that the petals were assembled in the way of spiraling sequential overlap on the side edge of early petals in the 3D flowerlike Cd(OH)2 structures. Finally, when the reaction time was further prolonged to 120 min, well-defined 3D spiral Cd(OH)2 flowers were produced and small Cd(OH)2 nanoparticles almost completely disappeared (Figure 2f). It deserves to be mentioned that in solutions it is very complicated to successfully account for the formation mechanism of self-assembled complex 3D structures.24 In our experiment, the detailed self-assembly process of 3D spiral flowerlike Cd(OH)2 structures has been clearly demonstrated through collecting the samples at appropriate intervals. From the above results, it is reasonable to presume that the formation of 3D spiral flowers of Cd(OH)2 is based on the nucleation− dissolution−recrystallization self-assembly growth mechanism, which is similar to that in the synthesis of t-selenium nanotubes.25 The whole evolution process for the as-prepared 3D spiral flowerlike Cd(OH)2 structures is illustrated in Figure 2g. First, the small Cd(OH)2 nanoparticles were precipitated from the supersaturated solution before being transferred into the Teflon-sealed autoclave for hydrothermal treatment. Second, under hydrothermal treatment the small Cd(OH)2 nanoparticles started to dissolve, and initial Cd(OH)2 nano1094

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petals began to nucleate and grow, which is called dissolution− recrystallization. As the reaction process continued, Cd(OH)2 nanopetals gradually grew around the centers of initial nanopetals in the way of spiraling sequential overlap on the side edge of early petals, and at the same time initial Cd(OH)2 precursors were gradually dissolved into the solution, which provided a material source for the growth of Cd(OH)2 nanopetals. Third, as the tartrate-assisted growth of Cd(OH)2 nanopetals continued, the number of nanopetals increased. In the meantime, the size of flowers became larger as the reaction was prolonged. In other words, the size of flowerlike Cd(OH)2 structures could be controlled by altering the time of hydrothermal treatment. Finally, the 3D spiral flowerlike Cd(OH)2 structures were successfully prepared by selfassembly. Control experimental work demonstrates that the final morphologies of the as-prepared products are strongly affected by the reactive condition as shown in Figure 3. In the absence of tartaric acid at the pH value of 12.5, only irregular Cd(OH)2 plates with a thickness ranging from 150 to 250 nm appeared in the final products (Figure 3a,b). It seems that tartrate is essential in the self-assembly growth of 3D spiral flowerlike Cd(OH)2 structures. It is known that tartrate is an important complexing ligand for metal ions. It has served as shapecontrolling or structure-directing agent in the synthesis of micro-/nanostructures with various morphologies.26−30 In our synthetic system, tartrate ions could coordinate with cadmium ions to form complexes. The release of Cd2+ from this complex takes place in a controlled manner, and the concentration of free Cd2+ in the solution is decreased. Reaction velocity could be adjusted through the complexation slow-release method,27,28 which could regulate the kinetics of nucleation and growth and further efficiently control the subsequent growth of Cd(OH)2 nanopetals along the specific direction and position of earlier nanopetals with the lowest growth energy. This complexation slow-release method of tartrate for self-assembled growth has been reported in the hydrothermal self-assembly of hierarchical Co hyperbranches.28 The pH value also plays an important role in determining the morphology of the final Cd(OH)2 product. In the presence of tartaric acid, when the pH of the mixed solution was higher than 12, 3D spiral flowerlike Cd(OH)2 structures were generated as shown in Figure 1. When the pH of the mixed solution was smaller than 12, Cd(OH)2 spheres were prepared. Figure 3c is a typical SEM image of the asprepared product at the pH value of 10, from which numerous nanospheres with relatively uniform size can be clearly observed. The diameter of the spheres typically ranges from 30 to 45 nm. The HRTEM image of the Cd(OH)2 spheres is shown in Figure 3d. A clear contrast between the dark edge and the pale center can be observed, which indicates that the Cd(OH)2 spheres have a hollow interior. In fact, the pH of the system can influence the coordinating interaction, the chemical potential, and the rate of the ionic motion.31,32 So, at different pH values the different crystal faces will be preferential to grow or suppress, leading to the different morphological products. The FT-IR spectrum of the 3D spiral flowerlike Cd(OH)2 structures (Figure 4) exhibits strong characteristic absorptions for the carbonyl group of the tartrate carboxylate ligands in the asymmetric and symmetric vibration regions. The asymmetric stretching vibrations υas‑COO− appear at 1597 cm−1, and the symmetric stretching vibrations υs‑COO− are observed at 1406 cm−1. The bands are shifted to lower frequencies compared to those of free tartaric acid, which suggests the interaction

Figure 4. FT-IR spectrum of the 3D spiral flowerlike Cd(OH)2 structures.

between the 3D spiral flowerlike Cd(OH)2 structures and tartrate ligand.33,34 The interaction might force the nuclei to grow along specific directions and positions.34,35 Moreover, because Cd(OH)2 has plenty of surface-located hydroxyl groups, many hydrogen bonds are formed between the hydroxyl groups of tartrate ions and the surface hydroxyl groups of Cd(OH)2. Thus, its nuclei growth may be afforded via hydrogen bonds along a preferential direction and position with the lowest growth energy.35−37 This hydrogen bondinduced orientation growth mechanism has also been previously proposed in the sodium tartrate-mediated selfassembly of boehmite hollow core/shell and hollow microspheres.29 In addition, tartrate might bind to certain crystal faces of the nuclei and control the overall growth kinetics. Combined with Figure 1, it is inferred that in the presence of tartaric acid, the (001) plane should be selectively absorbed by tartrate through its COO− and −OH functions. The (001) plane would be eventually largely exposed. On the basis of the above discussion, it is thus suggested that the growth of 3D spiral Cd(OH)2 flowers is dominated by the nucleation−dissolution−recrystallization growth mechanism. The sequentially spiraling assembly of Cd(OH)2 nanopetals in 3D flowerlike structures results from the complexation slowrelease growth along the direction induced by tartrate. The surface of a growing crystal that acts as a substrate for the next layer evolves constantly in its characteristics such as surface roughness and morphology, etc.23 Therefore, after the growth of an early petal, a certain direction on the surface of this early petal may become the growth orientation with the lowest growth energy. Thus, with continued growth of Cd(OH)2 nanopetals and spiral development of the growth direction for nucleation of new nanopetals under the induction of tartrate, the 3D spiral Cd(OH)2 flowers composed of 2D nanopetals are finally obtained via self-assembly. 3.3. Adsorption and Separation Performance. The previous works revealed that Cd(OH)2 nanomaterials are extremely positively charged and have wide applications in the adsorption and separation of negatively charged organic molecules or DNA, etc.13,15,20,21 In the present work, Congo Red (a kind of anionic dye13,38) was chosen to investigate the adsorption and separation ability of the obtained Cd(OH)2 materials of different morphologies with UV−vis absorption spectra. The same amount of Cd(OH)2 samples with three different morphologies were respectively added to three 1095

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adsorbed by 3D spiral Cd(OH)2 flowers is larger than that by Cd(OH)2 hollow spheres and sub-microsized plates,16−18 and the adsorption intensity of Congo Red solution almost disappears. The contrastive photograph in Figure 5b confirms this result. It is worth noting that their adsorption and separation performance is inconsistent with the result of nitrogen adsorption−desorption measurements. The BET surface areas of Cd(OH)2 hollow spheres, sub-microsized plates, and 3D spiral flowers are 50.9 m2/g, 11.3 m2/g, and 4.0 m2/g, respectively (Figures S2−S4). Combination of Figure 5 and the BET results clearly indicates that specific surface area of Cd(OH) 2 is not the dominant factor influencing the adsorption−desorption performance of Congo Red. This may be because nitrogen and Congo Red have different molecular sizes and different adsorption mechanisms. Because of the much smaller molecular size of nitrogen (Schemes S1 and S2), nitrogen molecules could more easily penetrate into the interstitial voids within the aggregation of the obtained samples than Congo Red, and thus the effective surface area of Cd(OH)2 for adsorption of Congo Red is lower than the measured BET result. Moreover, the adsorption mechanism is physical adsorption (depending on effective surface area) for nitrogen, while it is chemical adsorption (depending on effective surface area and positive charge density here) for Congo Red. As a result, the specific surface area results via the BET measurements cannot accurately account for the adsorption and separation performance of the obtained samples. To explore the relationship between the adsorption and separation performance and the structures of the products, the XRD patterns of these Cd(OH)2 structures are compared (see Figures 1d and 6). The Cd(OH)2 hollow nanospheres have nano-polycrystalline structures (Figures 3d and 6a), and thus they have no preferentially exposed crystal plane. The diffraction peaks in the XRD pattern of Cd(OH)2 submicrosized plates can also be indexed to the pure hexagonal phase of Cd(OH)2 with the lattice constants of a = 3.4947 Å and c = 4.7106 Å (JCPDS No. 31-0228), which has the same crystal structure with the 3D spiral Cd(OH)2 flowers. The regular hexagonal crystal arrangement of the Cd(OH)2 submicrosized plates leads to the preferred exposure of low-index planes (Figures 3a and 6b).39 Nonetheless, the (001) plane of the 3D spiral Cd(OH)2 flowers is so much exposed that other

separated 30 mL Congo Red solutions (0.04 mM) under vigorous stirring for 24 h to ensure electrostatic adsorption equilibrium. Among them, hollow spheres17 and sub-microsized plates16,18 have also been synthesized by other routes, and the novel 3D spiral flowerlike structures are reported for the first time here. Then, the solutions were centrifuged and monitored by UV−vis spectroscopy. As shown in Figure 5a, the adsorption

Figure 5. (a) UV−vis absorption spectra and (b) corresponding photograph of (A) initial Congo Red solution and (B−D) Congo Red solution after removing the electrostatic composites combined with (B) Cd(OH)2 hollow nanospheres, (C) sub-microsized Cd(OH)2 plates, and (D) 3D spiral Cd(OH)2 flowers by centrifugation.

intensity of Congo Red solution obviously decreases after removing the electrostatic composites combined with hollow spheres, sub-microsized plates, and 3D spiral flowers of Cd(OH)2. The decrease of adsorption intensity of Congo Red solution after removing the electrostatic composites

Figure 6. Powder XRD patterns of (a) Cd(OH)2 hollow nanospheres and (b) sub-microsized Cd(OH)2 plates. 1096

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excellent adsorption and separation performance for the asprepared 3D spiral flowerlike Cd(OH)2 structures. This is similar to the previous reports that the exposed crystal plane in ZnO nanostructures is closely related to adsorption− desorption performance;44 also, protein adsorption on nanozeolite is crystal plane-dependent.45

exposed planes can be ignored (Figure 1d). As shown in Table S1 and Figure 7, the atomic plane density of low-index planes is

4. CONCLUSIONS In summary, the self-assembled 3D spiral Cd(OH)2 flowers have been successfully synthesized via a facile tartrate-assisted hydrothermal route. The nucleation−dissolution−recrystallization growth mechanism is proposed for the self-assembly of 3D spiral flowerlike Cd(OH)2 structures. Control experiments revealed that tartrate and pH value are key control factors in the self-assembly of 3D spiral flowerlike Cd(OH)2 structures. The largely preferential exposure of the (001) plane with the highest surface charge density plays a dominant role for the excellent adsorption−separation performance of 3D spiral Cd(OH)2 flowers. The self-assembled growth mechanism and structural features presented herein may offer an elegant route for the design of other advanced 3D structures with novel properties.

Figure 7. Schematic illustration for the space lattice of hexagonal Cd(OH)2 with the lattice constants of a = b = 3.4947 Å and c = 4.7106 Å (JCPDS No. 31-0228), in which Cd(OH)2 is colored in green.

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ASSOCIATED CONTENT

S Supporting Information *

higher than that of high-index planes, and the (001) plane of the hexagonal Cd(OH)2 crystal has the highest atomic plane density. As the crystal plane index increases, the atomic plane density is drastically reduced. The largely preferential exposure of the (001) plane with the highest Cd atomic density ([Cdn(OH)m(H2O)np‑m](2n−m)+)13,15 would lead to the greatly increased surface charge density. The influence of the morphology of the products is discussed here on the adsorption and separation performance. A high specific surface area of the products has a beneficial effect on the adsorption and separation performance. The Cd(OH)2 hollow nanospheres have the highest specific surface area, while the 3D Cd(OH)2 spiral flowers have the lowest specific surface area. However, the adsorption and separation performance follows the order of 3D Cd(OH)2 spiral flowers, sub-microsized plates, and hollow nanospheres. Thus, there is no correlation between the surface areas and the adsorption and separation performance for our materials, demonstrating that there are other more important factors that govern the performance. It is well-known that the smaller the particle size, the more severe the aggregation of nanoparticles.41−43 The product of Cd(OH) 2 hollow nanospheres emerges in the severe aggregation form due to their smallest particle size among the three samples as shown in Figure 3c. Sub-microsized plates are also difficult to prevent from aggregation because of their small particle size and close packing as shown in Figure 3a−b. The interstitial voids within the aggregation of the obtained samples cannot be effectively used for the adsorption− separation. On the other hand, as shown in Figure 1, the microsized 3D spiral flowerlike structures show a loosely packed structure by fixing nanopetals along different spatial directions, thus decreasing the aggregation of the nanopetals. Therefore, the difference in effective surface areas would become much smaller than the BET results for the 3D spiral flowerlike structures, the sub-microsized plates, and the hollow nanospheres. The total charge amount by the production of surface charge density and effective surface area determines the adsorption and separation performance.40 Consequently, the largely preferential exposure of the (001) plane with the highest surface charge density should be the dominant cause for the

Some SEM images; nitrogen adsorption−desorption isotherms of the samples; molecular structure of Congo Red and N2; and Cd atomic plane density of different crystal planes. This information is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-10-82543752. E-mail: [email protected]. Web: http://sourcedb.ipc.cas.cn/cn/lhsrck/200904/ t20090415_49499.html. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Basic Research Program of China (No. 2010CB934500) and the National Natural Science Foundation of China (No. 11002142). The kind help in discussion from Prof. R. K. Li of Technical Institute of Physics and Chemistry of Chinese Academy of Sciences is highly appreciated.

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dx.doi.org/10.1021/cg301423c | Cryst. Growth Des. 2013, 13, 1091−1098