Aqueous Solution Synthesis of Cd(OH)2 Hollow Microspheres via

Aug 26, 2008 - The controlled synthetic experiments indicate that the growth process of Cd(OH)2 hollow microspheres involves first the formation of Cd...
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J. Phys. Chem. C 2008, 112, 14360–14366

Aqueous Solution Synthesis of Cd(OH)2 Hollow Microspheres via Ostwald Ripening and Their Conversion to CdO Hollow Microspheres Wen-Shou Wang, Liang Zhen,* Cheng-Yan Xu, and Wen-Zhu Shao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, People’s Republic of China ReceiVed: May 26, 2008; ReVised Manuscript ReceiVed: July 11, 2008

In this work we demonstrate that Cd(OH)2 hollow microspheres could be prepared in high yield by a facile aqueous solution route from the mixture of aqueous solutions of CdCl2, Na2MoO4, and NaOH at room temperature. The synthesized products are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, selected-area electron diffraction, and the nitrogen adsorption-desorption isotherm technique. The Cd(OH)2 hollow microspheres have an average diameter of 3 µm and hollow interior of about 1.5 µm. The shell consists of numerous single-crystalline nanoplates with thickness of about 100 nm and sizes of 400-700 nm. The controlled synthetic experiments indicate that the growth process of Cd(OH)2 hollow microspheres involves first the formation of CdMoO4 solid microspheres and then the formation of Cd(OH)2 solid microspheres through the reaction between CdMoO4 and OH- ions controlled by the difference of solubility product for CdMoO4 and Cd(OH)2. The Ostwald ripening mechanism is proposed to account for the formation of Cd(OH)2 hollow microspheres on the basis of scanning electron microscopy observations of intermediate products at different precipitation stages. Furthermore, the Cd(OH)2 hollow microspheres can be easily converted to CdO semiconductors with similar morphology by calcining Cd(OH)2 in air at 350 °C for 4 h. 1. Introduction Inorganic hollow micro/nanostructures have aroused considerable attention during the past few years because of their unique properties and various promising applications in nanoscale chemical reactors with enhanced selectivity, efficient catalysts, drug-delivery carries, building blocks for photonic crystals, nanocapsules for hydrogen storage, and so forth.1-3 Until now, the template-directed approach is probably the most effective and general method for the synthesis of hollow micro/nanostructures in which hard templates, such as monodispersive silica,4 polymer latex spheres,5 and reducing metal nanoparticles,6 and soft templates, including emulsion droplets,7 vesicles,8 gas bubbles,9 etc., are employed. Although conceptually simple and versatile, it is well-known that hollow micro/ nanostructures prepared from the conventional template-directed approach usually suffer from disadvantages related to the prior templates’ surface modification, time-summing template removing, poor mechanical strength of the spherical shell, etc., which may prevent them from being used in large-scale applications. Apart from the template-assisted method, some novel templatefree approaches have been recently reported to construct many inorganic hollow micro/nanostructures based on the Kirkendall effect,10 Ostwald ripening,3,11 oriented attachment mechanism,12 and galvanic replacement.13 However, it still remains a major challenge to develop a facile one-step solution route for the preparation of inorganic hollow nano/microstructures. Recently, nanostructures of divalent metal hydroxides, such as Mg(OH)2,14 Ni(OH)2,15 Cu(OH)2,16 etc., have been extensively prepared because these hydroxides can be used as potential sacrificial templates or precursors to synthesize the corresponding metal oxides. Among the various divalent metal * To whom correspondence should be addressed. Fax: +86-451-86413922. Tel.: +86-451-8641-2133. E-mail: [email protected].

Figure 1. XRD pattern of the as-synthesized Cd(OH)2 hollow spheres.

hydroxides, Cd(OH)2 is an important precursor that is potentially converted into cadmium oxide through dehydration or into other functional materials, such as CdS, CdSe, and so forth, by reaction with appropriate elements or compounds. Various Cd(OH)2 nanostructures have been prepared during the past few years. For example, Tang et al. synthesized ultralong singlecrystalline Cd(OH)2 nanowires by a hydrothermal method using alkali salts as mineralizers.17 Cd(OH)2 nanorings were obtained by irradiating Cd(OH)2 precipitate using a high intense ultrasonic horn.18 Shinde et al. developed a solution chemistry route for formation of crystalline Cd(OH)2 nanowire bundles on glass substrates at low temperature.19 Other Cd(OH)2 nanostructures, including nanodisks20 and nanostands,21 were also reported. However, there has been no research focused on the formation of Cd(OH)2 hollow microspheres. In addition, as an important

10.1021/jp8046483 CCC: $40.75  2008 American Chemical Society Published on Web 08/26/2008

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Figure 2. SEM images and EDS spectrum of the as-synthesized Cd(OH)2 hollow microspheres: (a) overall product morphology, (b) an enlarged SEM image, (c and d) SEM images of an individual microsphere, (e and f) SEM images of two individual microspheres with hollow interior, and (g) EDS spectrum taken from the microspheres.

Figure 3. TEM images of (a) three Cd(OH)2 hollow microspheres and (b) an individual nanoplate broken down from the hollow microspheres. Inset: corresponding SAED pattern taken on the selected nanoplate (shown in panel b).

II-VI semiconductor, the preparation of CdO nanostructures has received increasing attention because of its interesting properties.22 For example, CdO nanobelts and microsized sheets were synthesized by directly evaporating CdO powder at 1000 °C without the presence of catalyst.23 Kuo and Huang prepared ultralong CdO nanowires on gold-coated silicon substrates by using a vapor transport process.24 Wang et al. prepared dendritic CdO nanofilms on glassy carbon surfaces and studied their electrogenerated chemiluminescence behaviors in aqueous solution.22e However, to the best of our knowledge, rare research

Figure 4. Nitrogen adsorption-desorption isotherm and corresponding pore size distribution (inset) of the Cd(OH)2 hollow microspheres.

work has been reported on the synthesis of CdO hollow microspheres. In this work, we developed a simple aqueous solution for the synthesis of Cd(OH)2 hollow microspheres from the mixture of aqueous solutions of CdCl2, Na2MoO4, and NaOH at room temperature. The shells of the hollow microspheres consist of

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Figure 5. SEM images of typical products prepared under the controlled synthetic conditions: (a and b) the direct reaction between aqueous CdCl2 and Na2MoO4 solutions for 10 min without further adding NaOH aqueous solution at room temperature; (c and d) the direct reaction between aqueous CdCl2 and NaOH solutions for 5 days without using Na2MoO4 aqueous solution at room temperature.

numerous single-crystalline nanoplates. The controlled synthetic experiments indicate that the formation process of Cd(OH)2 hollow microspheres involves first the formation of CdMoO4 solid microspheres and then the growth of Cd(OH)2 solid microspheres through the reaction between CdMoO4 and OHions controlled by the difference of the solubility product (Ksp) for CdMoO4 and Cd(OH)2. The Ostwald ripening mechanism was proposed to account for the formation of hollow microspheres on the basis of scanning electron microscopy observations of intermediate products at different precipitation stages. Furthermore, the Cd(OH)2 hollow microspheres can be easily converted to CdO semiconductors with the similar morphology by calcination at 350 °C for 4 h in air. The present approach is typically characterized by a one-step process, which can lead to relatively pure products in comparison with the templateassisted route that requires additional postprocessing procedures to remove the templates. Another advantage is that massive products can principally be produced because of the mild and simple preparation conditions, which is important with respect to technical applications. 2. Experimental Section 2.1. Synthesis. All chemicals were analytical grade and used as received without further purification. The Cd(OH)2 hollow microspheres were prepared by an aqueous solution at room temperature, which is similar to our previous studies.25 In a typical procedure, 0.2 mol · L-1 aqueous CdCl2 solution (25 mL) was added into 25 mL of mixed aqueous solutions of Na2MoO4 (0.2 mol · L-1) and NaCl (10 mmol) under strong magnetic stirring at room temperature, resulting in the precipitation of white product, indicating the formation of CdMoO4. The mixture was kept under stirring for about 10 min, and then 25 mL of NaOH aqueous solution (10 mol · L-1) was added into 12.5 mL of the above mixture suspension, and it was stirred for another 10 min to form Cd(OH)2. The resulted suspension was placed

at room temperature for 5 days without further stirring or shaking. Then the products were collected by centrifugation, washed several times with distilled water and absolute ethanol to remove any residuals, and finally dried in air at 60 °C for 4 h. The conversion of the as-obtained Cd(OH)2 hollow microspheres to CdO hollow microspheres was carried out in an oven in air at 350 °C for 4 h. 2.2. Characterization. X-ray diffraction (XRD) patterns of the as-obtained samples were recorded on a Rigaku D/max-rA diffractometer with Cu KR radiation (λ ) 1.5406 Å). Scanning electron microscopy (SEM) images were taken on a Hitachi S-4700 field emission scanning electron microscope. Transmission electron microscopy (TEM) characterization and selectedarea electron diffraction (SAED) patterns were carried out on a Philips Tecnai 20 microscope at an accelerating voltage of 200 kV. For the TEM experiment, the as-synthesized powders were first dispersed in ethanol by ultrasonic treatment. Then a small drop of the dispersion was transferred to a holey carbon film supported on a copper grid. N2 adsorption-desorption isotherms were performed on a Micrometrics ASAP 2020 V3.00 H system with nitrogen as the analysis gas after the sample was degassed in a vacuum at 120 °C for overnight (note that some Cd(OH)2 was converted into CdO during the degassing process). The pore diameter and pore size distribution at the shell were determined by the Brunauer-Emmett-Teller (BET) method. 3. Results and Discussion Figure 1 shows the representative XRD pattern of the assynthesized Cd(OH)2 hollow microspheres. The strong and sharp diffraction peaks in the XRD pattern indicate that the obtained products are well crystallized. All the diffraction peaks can be perfectly indexed as the mixed phase of Cd(OH)2. One is a tetragonal phase with cell constants of a ) 7.46 Å and c ) 8.61 Å (JCPDS no: 13-0026), and the other is a monoclinic phase with cell constants of a ) 5.63 Å, b ) 10.18 Å, c )

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Figure 6. SEM images showing the evolution process of Cd(OH)2 microspheres obtained at different reaction periods: (a and b) 2 h, (c and d) 8 h, and (e and f) 5 days. Inset in panel a: EDS spectrum taken from the corresponding microspheres (in panel a).

Figure 7. Schematic illustration of the formation process of the Cd(OH)2 hollow microspheres via Ostwald ripening.

3.40 Å, and β ) 91° (JCPDS no: 20-0179). No peaks related to other impurity phases, such as CdMoO4, could be assigned, revealing that the as-synthesized Cd(OH)2 samples are of high purity by this facile method.

Figure 2 shows the SEM images of the as-synthesized Cd(OH)2 hollow microspheres. The low-magnification SEM image shown in Figure 2a displays that the samples are composed of a large scale of Cd(OH)2 with a well-preserved

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Figure 8. XRD pattern of the CdO hollow microspheres by calcining Cd(OH)2 hollow microspheres in air at 350 °C for 4 h.

Figure 9. (a) Low-magnification and (b) high-magnification SEM images of the CdO hollow microspheres by calcining Cd(OH)2 hollow microspheres in air at 350 °C for 4 h.

spherical morphology. The microspheres exhibit relatively good dispersion. The diameter of the microspheres typically ranges from 2.5 to 3.5 µm, with an average diameter of 3 µm. Careful observation reveals that the obtained microspheres show rather rough surfaces, as shown in Figure 2b. A novel hierarchical structure appeared when an individual Cd(OH)2 microsphere was magnified. Figure 2c shows a typical individual Cd(OH)2 microsphere with diameter of about 3 µm, exhibiting the detailed structure information of the microspheres. The peripheral surface of the hierarchical microsphere is not smooth, which is actually composed of numerous nanoplates. The enlarged SEM image (Figure 2d) indicates that the nanoplates have an average thickness of 100 nm and sizes of 400-700 nm with a smooth surface. Most of the nanoplates have a hexagonal shape. Interestingly, from the panels a and b in Figure 2, some Cd(OH)2 hierarchical microspheres display broken sites and expose their hollow interiors, providing direct evidence that the assynthesized Cd(OH)2 microspheres at room temperature in aqueous solution have a hollow structure. Although the proportion of the broken microspheres appears to be only about 10% from the SEM observation shown in Figure 2a, the actual proportion is expected to be larger as some microspheres may have no broken shells and others may have had their holes out of sight of the microscope. Parts e and f of Figure 2 show two individual Cd(OH)2 hollow microspheres with diameters of 2.5-3.5 µm. The hollow interior has diameters of 1-1.5 µm, and the shell is also composed of numerous nanoplates. The chemical composition analysis, performed by using energydispersive X-ray spectroscopy (EDS) equipped on the SEM (Figure 2g), indicates that the hollow microspheres are made

of Cd and O only and the molar ratio of Cd to O is approximately equal to 1:2. The morphology and structure of the Cd(OH)2 hollow microspheres are further characterized using TEM and the SAED pattern. Figure 3a shows the TEM image of Cd(OH)2 microspheres. The microspheres have an average diameter of about 3 µm, and their surface is not smooth, which is in agreement with the SEM observations. Although the diameters and shell thicknesses of the microspheres are rather larger, the light pale color regions in the central parts can be seen in contrast to the dark edges, which is evidence of the hollow nature of the microspheres. Figure 3b shows the TEM image of an individual Cd(OH)2 nanoplate broken down from the hollow microspheres. The nanoplate is very thin with size of 400-700 nm. The SAED pattern taken from the selected nanoplate (inset in Figure 3b) clearly demonstrates that the nanoplate is single-crystalline. Nitrogen adsorption-desorption isotherms are measured to determine the specific surface area and pore volume of the Cd(OH)2 hollow microspheres, and the corresponding results are presented in Figure 4. The isotherms are typical type IVlike with a distinct H3 hysteretic loop in the range of 0.8-1.0 P/P0, which indicates the presence of mesoporous and macroporous materials according to IUPAC classification. The plot of the pore size distribution (inset in Figure 4) was determined by using the Barrett-Joyner-Halenda (BJH) method from the desorption branch of the isotherm. The average pore diameter of the Cd(OH)2 hollow microspheres is 21.8 nm. The BET surface area and pore volume of the hollow microspheres are 41.7 m2/g and 0.23 cm3/g, respectively.

Synthesis of Cd(OH)2 Hollow Microspheres To explore the formation mechanism of the Cd(OH)2 hollow spheres, two controlled experiments were carried out. The first experiment was carried out in the absence of NaOH aqueous solution. Without the addition of aqueous NaOH solution into the reaction system, monodispersive CdMoO4 solid microspheres with diameters ranging from 1.5 to 3.5 µm were obtained when aqueous CdCl2 solution was added into the aqueous Na2MoO4 solution without further adding NaOH aqueous solution at room temperature after 10 min, as shown in Figure 5, parts a and b. The other experiment was carried out by the direct reaction between aqueous CdCl2 and NaOH solutions at room temperature for 5 days without using Na2MoO4 (the concentration of aqueous CdCl2 and NaOH solutions are both 0.2 M, respectively). The SEM image shown in Figure 5c displays that the products consist of beltlike microstructures together with some flocculence-like aggregates. The beltlike microstructures have an average width of 15 µm and length of several to tens of micrometers. The highmagnification SEM image shows that the flocculence-like aggregates are composed of tiny wirelike nanostructures. These controlled experimental results indicate the formation process of Cd(OH)2 in our study contains two steps: the first is formation of CdMoO4 solid microspheres, and then they are converted to Cd(OH)2. The first formation of CdMoO4 solid microspheres from the reaction between aqueous CdCl2 and Na2MoO4 solutions plays important roles in the final growth of Cd(OH)2 microspheres, where CdMoO4 microspheres might act as both the cadmium source to release Cd2+ ions and structure director in the formation of Cd(OH)2 microspheres in the reaction system. In the reaction system, the following three reactions are believed to be responsible for the formation of Cd(OH)2 microspheres: (1) Cd2+ + MoO42- ) CdMoO4, (2) CdMoO4 ) Cd2+ + MoO42-, and (3) Cd2+ + 2 OH- ) Cd(OH)2. CdMoO4 microspheres are first obtained through the direct reaction between CdCl2 and Na2MoO4 aqueous solution, as shown in reaction 1. When NaOH aqueous solution is introduced, the conversion from CdMoO4 to Cd(OH)2 is possible because of their Ksp difference for CdMoO4 (2.2 × 10-9) and Cd(OH)2 (3 × 10-14) at room temperature. Therefore, Cd(OH)2 is more thermodynamically stable than CdMoO4 due to its lower Ksp at room temperature. The CdMoO4 microspheres can be in situ dissociated slowly to release the Cd2+ ions in the reaction system when OH- ions are introduced (reaction 2). Thus, the anion-exchange reaction between MoO42- and OH- ions is spontaneously taken place to form Cd(OH)2 when the Cd2+ ions meet with OH- ions (reaction 3). It is well-known that complex morphologies of inorganic materials are usually difficult to be produced by directly mixing two aqueous solutions of metal salts because of a rapid decrease in supersaturation and further depletion of reaction nutrients in a short period of time.26 However, in the present reaction system, using the first formed CdMoO4 microspheres as the precursor might slowly release Cd2+ ions in the solution and lead to a suitable reaction rate for generation of Cd(OH)2, which might provide a favorable chemical environment for the growth of Cd(OH)2 microspheres composed of nanoplates. In comparison with the direct reaction between aqueous CdCl2 and NaOH solutions, reaction velocity in our reaction system can be adjusted through the CdMoO4 slow-release Cd2+ method, which can regulate the kinetics of nucleation and growth of the products and further efficiently control the morphology and structure of the final products. In order to reveal the evolution process of Cd(OH)2 hollow microspheres in more detail, experiments were carried out at

J. Phys. Chem. C, Vol. 112, No. 37, 2008 14365 room temperature for various reaction times of 2 h, 8 h, and 5 days. The representative SEM images of the products prepared at certain reaction time intervals are shown in Figure 6. In part a of Figure 6, it can be clearly seen that only solid microspheres, with diameters in range of 2-3.5 µm, are prepared after reaction time of 2 h. The enlarged SEM image shows that the solid microsphere is composed of numerous nanoplates, as shown in Figure 6b. The EDS spectrum taken from these microspheres shows the presence of Cd and O peaks together with a Au signal coming from the Au coating used for mounting the sample for SEM observation. No Mo signal is detected in the sample. The EDS result also implies that all the CdMoO4 has been converted to Cd(OH)2 after reaction time of 2 h. When the reaction time is prolonged to 8 h, some Cd(OH)2 core-shell microspheres are observed in this stage (Figure 6c). Figure 6d shows an individual core-shell microsphere with diameter of 3 µm. When the reaction time is further prolonged to 5 days, the cores in the center of the core-shell microspheres are evacuated completely, resulting in the hollow structures, as demonstrated in part e of Figure 6. Figure 6f shows that an open void evolved from the hollow interior space is clearly present in the center of the microsphere. On the basis of the above experimental results, the formation mechanism of the Cd(OH)2 hollow microspheres could be attributed to the Ostwald ripening process. This ripening process, a classic phenomenon in general crystal growth, involves “the growth of larger crystals forms those of smaller size which have a higher solubility than the larger ones”.27 Recently, the Ostwald ripening process has been discussed in detail for preparation of TiO2 and Cu2O hollow nanospheres in Zeng and co-worker’s previous work28 and is further demonstrated as a general mechanism for the synthesis of many other compounds with hollow interiors in the literature.29 According to the evolution of time-dependent experiments, this Ostwald ripening process for Cd(OH)2 hollow microspheres was associated with a progressive redistribution of matter from the interior to the exterior of the microspheres. A schematic representation for the formation of Cd(OH)2 hollow microspheres is shown in Figure 7. In the initial stage, CdMoO4 solid microspheres are formed through the reaction between aqueous solutions of CdCl2 and Na2MoO4 (steps 1 and 2). When aqueous NaOH solution is added into the reaction system, CdMoO4 microspheres might act as both the cadmium source to release Cd2+ ions and structure director in the formation of Cd(OH)2 solid microspheres (step 3). At this stage, the exteriors of the newly formed Cd(OH)2 solid microspheres are packed much looser than the interior, indicating the intrinsic density variations inside these newly formed microspheres.29 The exterior crystallites packed loosely would serve as starting growth sites on attracting the smaller particles underneath for the subsequent recrystallization. As the mass is transported with the reaction continuing, the void space between the exterior loosely packed areas and the interior closely packed ones is generated mainly through the Ostwald ripening process, which divided most of the solid microspheres into discrete regions, resulting in the formation of core-shell microspheres (step 4). Essentially, the nanocrystallites located in the inner cores have higher solubilities due to the higher surface energies associated with their larger curvatures, compared with the outer nanocrystallites. Thus, it is obvious that the cores in the center of the microspheres could be completely consumed with further increasing the reaction time, so the final hollow microspheres are created (step 5). Therefore, the timedependent experiments indicate that the Ostwald ripening process is an underlying mechanism in this hollowing process

14366 J. Phys. Chem. C, Vol. 112, No. 37, 2008 for Cd(OH)2 microspheres, which is essentially similar to what has been known in the preparation of anatase TiO2,28a MnS,29a CdMoO4,25c and Co-copolymer hybrid hollow spheres.29f After the Cd(OH)2 hollow microspheres are calcined in air at 350 °C for 4 h, phase-pure CdO hollow microsphere are formed. Figure 8 shows the XRD pattern of CdO obtained after calcination. All of the diffraction peaks in the pattern can be perfectly indexed to a cubic phase of CdO with cell constants of a ) 4.695 Å (JCPDS 05-064). No peaks related to other impurity phases, such as Cd(OH)2, could be assigned, revealing the high phase purity of CdO. Figure 9a shows the low-magnification SEM image of the CdO products, from which a number of uniform microspheres with an average diameter of 3 µm are clearly observed. Many Cd(OH)2 microspheres display broken sites and expose their hollow interiors, providing direct evidence that the assynthesized CdO microspheres have a hollow structure. Figure 9b shows an individual CdO hollow microsphere with diameter of 3.5 µm, whose shell is composed of nanoplates and nanoparticles, because some nanoplates collapsed due to shrinkage during the calcination. The SEM observations demonstrate that the hollow structure is well retained by calcining Cd(OH)2 hollow microspheres in air at 350 °C for 4 h. 4. Conclusions In summary, we have demonstrated a simple aqueous solution route for large-scale preparation of Cd(OH)2 hollow microspheres at room temperature. The first formation of CdMoO4 solid microspheres plays important roles in the final growth of Cd(OH)2 microspheres, where CdMoO4 solid microspheres might act as both the cadmium source to release Cd2+ ions and structure director in the formation of Cd(OH)2 microspheres in the reaction system. The formation mechanism of Cd(OH)2 hollow microspheres is believed to the Ostwald ripening process based on time-dependent experiments. Furthermore, by calcination of the Cd(OH)2 hollow microspheres in air at 350 °C for 4 h, CdO semiconductors with similar morphology can be obtained. It is expected that these materials can find potential applications in multiple fields as absorbents, catalyst supports, and functional host materials. Acknowledgment. This work was supported by the Natural Scientific Research Innovation Foundation of HIT (HIT. NSRIF. 2008.32). The authors thank Miaomiao Ye at Harbin Institute of Technology for skillful measurement on the BET gas-sorption experiment. References and Notes (1) Caruso, F.; Caruso, R. A.; Mohwald, H. Science 1998, 282, 1111. (2) Im, H. S.; Jeong, U.; Xia, Y. N. Nat. Mater. 2005, 4, 671. (3) Zeng, H. C. J. Mater. Chem. 2006, 16, 649. (4) (a) Jachson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2743. (b) Velikov, K. P.; Blaaderen, A. V. Langmuir 2001, 17, 4779. (c) Wang, Y.; Su, F. B.; Lee, J. Y.; Zhao, X. S. Chem. Mater. 2006, 18, 1347. (5) (a) Caruso, F.; Shi, X.; Caruso, R. A.; Susha, A. AdV. Mater. 2001, 13, 740. (b) Lu, Y.; McLellan, J.; Xia, Y. N. Langmuir 2004, 20, 3464. (6) (a) Gao, J.; Zhang, B.; Zhang, X.; Xu, B. Angew. Chem., Int. Ed. 2006, 45, 1220. (b) Sun, Y.; Mayers, B.; Xia, Y. N. AdV. Mater. 2003, 15, 641.

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