Controllable Fabrication, Growth Mechanisms, and Photocatalytic

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Controllable Fabrication, Growth Mechanisms, and Photocatalytic Properties of Hematite Hollow Spindles Xun Li,† Xin Yu,‡ Jinghui He,† and Zheng Xu†,* State Key Laboratory of Coordination Chemistry and Laboratory of Solid State Microstructure, School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing, 210093, People’s Republic of China, and School of Chemistry and Chemical Engineering, Xuzhou Normal UniVersity, Xuzhou, 221116, People’s Republic of China ReceiVed: September 6, 2008; ReVised Manuscript ReceiVed: December 15, 2008

Hematite hollow spindles were successfully fabricated via an environmental friendly hydrothermal route at one step without employing any templates. The samples were characterized by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and N2 adsorption-desorption. Influencing factors such as reaction temperature, pH value, and the ethylene glycol were systematically investigated. A possible formation mechanism was proposed. Because of the stable hollow structure, the hematite hollow spindles exhibited good photocatalytic property for phenol degradation and reusable feature. 1. Introduction Hollow particles with typical curved inner cavities are of high importance because of their unique properties of low density, high surface area, and good permeation. As a result, they have potential applications in various fields, such as catalysis,1-3 sensors,4,5 drug delivery,6,7 Li-ion batteries,8,9 and water treatment.10-12 At the present time, template methods were extensively employed to prepare such hollow structures, including hard templates such as polymer latex particles,13 silica spheres,3 metal nanoparticles,14 and carbon spheres,15 and soft templates such as emulsion droplets,16,17 block copolymer,18,19 and gas bubbles.20 However, both of the hard and soft templateassisted synthesis methods usually require tedious procedures, including template modification, precursor attachment, and core removal. Although several template-free methods for fabrication of hollow particles have been developed based on physical processes, such as oriented attachment,21,22 Kirkendall effect,1,23,24 and Ostwald ripening,25,26 the kind of materials constituting hollow structure is still limited. So, searching for a facile approach to obtain various hollow structures is still urgently needed. Hematite (R-Fe2O3), an n-type semiconductor with the band gap of 2.2 eV, is the most stable phase of iron oxide and is usually used as a photocatalyst.27,28 Hematite hollow particles have attracted much attention recently for improving photocatalytic activity.11,12,29 Although hematite hollow particles with various shapes, such as spheres,30-32 urchins,33 flutes,34 peanuts,35 rings,36 tubes,37,38 and spindles,12,35,39 have already been synthesized, the methods to obtain hollow spindles need to prepare complex or precursor first, followed by a multistep process to obtain the final product. Moreover, toxic organic solvent and hardly washing surfactant or a high reaction temperature of 400 °C were involved in those methods. Therefore, a facile, low* Author to whom correspondence should be addressed. E-mail: [email protected]. Phone: +86-25-83593133. Fax: +86-2583314502. † Nanjing University. ‡ Xuzhou Normal University.

Figure 1. XRD patterns of hematite hollow spindles.

temperature, and environmentally friendly preparation method is still a challenging strategy. In this paper, we report on an environmental friendly hydrothermal route to fabricate hematite hollow spindles under relatively low temperature in one step. The morphology and microstructure of the hollow spindles were characterized in detail. The effects of the reaction temperature, pH value, and the ethylene glycol on the morphology of the product were studied, and the formation mechanism was proposed. Furthermore, by using phenol degradation as a probe reaction, the photocatalytic property of the hematite hollow spindles was investigated. 2. Experimental Section 2.1. Preparation. All the reagents are of analytical grade and were used without further purification. In a typical experimental procedure, 3 mmol of FeCl3 · 6H2O was dissolved into 10 mL of deionized water under magnetic stirring. The pH of the solution was adjusted with 10 mL of 1 M HCl solution. Then 12 mmol of urea and 2.5 mL of ethylene glycol were added to the solution under magnetic stirring. After 30 min of stirring, the solution was transferred into a 30 mL stainless

10.1021/jp8079217 CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009

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Figure 2. EDS spectra of hematite hollow spindles.

autoclave, then the autoclave was sealed and heated at 170 °C for 12 h. After that, the autoclave was cooled to room temperature naturally and the final pH of the solution was recorded. The products were centrifuged and washed with deionized water three times, followed by absolution ethanol, then dried in air at 80 °C for 3 h. 2.2. Characterization. All products were imaged with a HITACHI S-4800II field emission scanning electron microscope (FE-SEM) equipped with an AMETEK energy dispersive X-ray spectroscope to obtain FESEM images. The TEM images and the selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-200CX transmission electron micro-

Li et al. scope (TEM). High-resolution microscopy images were recorded on a JEOL-2100 high-resolution transmission electron microscope (HRTEM). The phase structures were characterized by powder X-ray diffractometry (SHIMADZU XRD-6000 X-ray diffractometer with graphite-monochromatized Cu KR radiation, λ ) 0.15406 nm). The Fourier transform infrared (FTIR) spectra were taken on a VECTOR 22 spectrometer (BRUKER). The Brunauer-Emmett-Teller (BET) tests were performed on a Micromeritics ASAP-2020 nitrogen adsorption apparatus. 2.3. Photocatalysis Experiments. The products were used as photocatalyst for the phenol degradation. In all experiment, 0.1 g of sample was added to 150 mL of a 0.1 g/L phenol aqueous solution and then the solution was magnetically stirred for 15 min in the dark. The suspension was then exposed to UV irradiation from a high-pressure Hg lamp (500 W, Nanjing Stonetech) at room temperature. The main wavelength of the high-pressure Hg lamp is 365 nm after filtering. One milliliter of solution was withdrawn by a syringe every 30 min and 20 µL of solution was injected into the HPLC after being filtered with a membrane. The concentration of phenol left in the filtrate was analyzed by high performance liquid chromatograph (Agilent HPLC 1100, Alltima C18 reverse column, methanol/water (7:3) eluent set at 1.0 mL/min). For recycling of the catalyst, the used sample was centrifuged and washed with absolution ethanol three times, then dried in air at 80 °C for 3 h. 3. Result and Discussion 3.1. Characterizations of the Hematite Hollow Spindles. The XRD pattern in Figure 1 shows that all the peaks of the as-prepared product can be unambiguously indexed to the

Figure 3. FE-SEM (a, b), TEM (c, d), and HRTEM (e, f) images of as-prepared hematite hollow spindles.

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Figure 4. FE-SEM images of the products obtained after hydrothermal reaction at 170 °C for (a) 0.5, (c) 1, (d) 2, (e) 3, and (f) 6 h; (b, g) TEM images of panels a and f, respectively; (h) XRD patterns of the products above.

Figure 5. FE-SEM images of the products obtained at reaction temperature of (a) 150 and (b) 130 °C. (c) XRD patterns of the products above.

rhombohedral phase of hematite, which is in good accordance with PDF No. 84-0307. The composition of the hematite hollow spindles was further confirmed by the EDS result (Figure 2), in which the signal of Al came from the substrate of Al foil. The

atom ratio of Fe/O is 44:56 for the product, which agrees well with the value calculated from the formula of hematite. Figure 3a shows a low-magnification FESEM image of the asprepared hematite hollow spindles. It is obvious that these spindle-

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Figure 6. (a) The relationship of calculated and measured final pH value to the usage of 1 M HCl. FE-SEM images of the products obtained from the solution with different volume of 1 M HCl: (b) 0, (c) 5, (d) 10, and (e) 12 mL.

like products have a narrow size distribution 1.2-2.0 µm in length and 600-800 nm in width. Almost every spindle has a hole on the tip and its hollow structure can be seen. As shown in the highmagnification FESEM image in Figure 3b, the shell of this hollow spindle is rough and consists of very small nanoparticles about 20 nm in size. A top-down view of a spindle was showed in the inset of Figure 3b. From the hole on the tip, an ca. 60 nm shell thickness can be estimated. Panels c and d of Figure 3 are typical TEM images of the hematite hollow spindles, which further demonstrate the hollow feature of the spindle. The shell thickness observed from the TEM images was in accordance with the SEM result. The selected area electron diffraction (SAED) pattern of a single hollow spindle reveals that the long axis of the spindle is along the [001] direction, which is in good agreement with other groups’ results.37,40 Figure 3e shows the HRTEM image taken from the tip part of a hollow spindle, and the 0.36 nm lattice spacing corresponds to the (012) planes of hematite. The angle between the (012) planes and the [001] direction of hematite is ca. 32°, which is in agreement with the calculated value of 32.4°.35 The HRTEM image of the edge of another hollow spindle was shown in Figure 3f. The marked lattice spacing is 0.251 nm, which is in good accordance with the value for the (110) planes of hematite. It is also clearly shown that the (110) planes paralleled the [001] direction of hematite. 3.2. Growth Mechanism of the Hematite Hollow Spindles. 3.2.1. Morphology EWolution with the Reaction Time. To investigate the growth mechanism of hematite hollow spindles,

time-dependent experiments were carried out. The hydrothermal reaction was conducted at 170 °C for 0.5 h, and the yellow products were formed. As shown in SEM and TEM images, many rod-like particles ca. 400 nm in length and 100 nm in width were found (Figure 4a,b). The XRD pattern identified them as the tetragonal phase of FeOOH (β-FeOOH) (Figure 4h). By increasing the reaction time to 1 h, the rod-like β-FeOOH was self-organized to flower-like particles ca. 800 nm in diameter (Figure 4c). After being reacted for 2 h, some spindles were grown from the β-FeOOH flower-like particles (Figure 4d) and the XRD pattern confirmed that they were the hematite phase; at the same time, the β-FeOOH flower-like particles disappeared gradually. Almost all products were transferred into hematite spindles with a red brown color after being reacted for 3 h (Figure 4e). As shown in Figure 4f, some of the hematite spindles had a hole on the tip and several small particles presented near the hole when the reaction was conducted for 6 h. The TEM image of the product reacted for 6 h demonstrates that the spindle is hollow with a small remainder inside (Figure 4g), whereas all spindles become completely hollow after being reacted for 12 h (Figure 3). 3.2.2. Effect of the Reaction Temperature. The effect of the reaction temperature was studied. The experimental results showed that the reaction temperature had a remarkable influence on the phase structure and the morphology of the product. Decreasing the reaction temperature to 150 °C, the products was a mixture of R-Fe2O3 spindles and rod-like β-FeOOH

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Figure 7. FE-SEM images of the products obtained when the dosage of ethylene glycol is (a) 0, (b) 1, (c) 4, and (e) 8 mL; (d, f) TEM image of panels c and e, respectively.

TABLE 1: The Percents of the Three Hydroxyl Modes on the Different Facets of Hematite facet

singly coordinated

(001) (100) (110) (012)

66.7 33.3 50

doubly coordinated

triply coordinated

100.0 33.3 33.3

33.3 50

down the increasing rate of the pH value (eq 2). Then, β-FeOOH was formed via the hydrolysis of Fe3+ (eq 3). Finally, as shown in eq 4, R-Fe2O3 was formed from the dehydration of β-FeOOH. When the HCl was excessive, some R-Fe2O3 will be dissolved (eq 5). Figure 8. FT-IR spectra of as-obtained hematite hollow spindles.

particles (Figure 5a). Further decreasing the reaction temperature to 130 °C, the products were all the rod-like β-FeOOH (Figure 5b). The transformation of β-FeOOH to R-Fe2O3 takes place at 170 °C.41 The Corresponding XRD pattern in Figure 5c showed the variance of the phase structure of the products with temperature. 3.2.3. Effect of the pH Value. In addition to the reaction temperature, the pH value also played an important role in the formation of such hollow spindles. The pH value of the solution increased gradually with the production of ammonia by the decomposition of urea, as described in eq 1. Meanwhile, the HCl in the solution may consume some of the ammonia to slow

NH2CONH2 + 3H2O f 2NH3 · H2O + CO2

(1)

HCl + NH3 · H2O f NH4Cl + H2O

(2)

FeCl3 + 3NH3 · H2O f FeOOH + 3NH4Cl + H2O (3) 2FeOOH f Fe2O3 + H2O

(4)

Fe2O3 + 6HCl f 2FeCl3 + 3H2O

(5)

So, the solutes in the final solution depended on the amount of HCl solution and urea. If the urea is excessive, the final solutes are NH3 · H2O and NH4Cl. The concentration of H+ ([H+]) can be calculated approximately as [H+] ) Ka(NH3){[NH4+]/ [NH3 · H2O]}, where Ka(NH3) ) 5.70 × 10-10.42 If the ammonia and HCl are equivalent, the final solute is only NH4Cl and the

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Figure 9. Surface hydroxyl configuration on the hematite (001), (100), (110), and (012) facets (H atom in the hydroxyl is omitted). Singly, doubly, and triply coordinated hydroxyls are indicated as S, D, and T, respectively. The distances between two O atoms of contiguous singly coordinated hydroxyls in (100), (110), and (012) facets are also marked.

Figure 10. Schematic illustration of the growth of the hematite hollow spindles.

concentration of H+ can be calculated directly as [H+] ) {Ka(NH3)[NH4+]}1/2. When the HCl is excessive by a little, the final solutes are NH4Cl and FeCl3, in which the concentration of H+ mainly depends on FeCl3 and can be calculated as [H+] ) Kw{Ksp(Fe(OH)3)/[Fe3+]}-1/3, where Kw ) 1.0 × 10-14 and Ksp(Fe(OH)3) ) 2.79 × 10-39,43 while the HCl is excessive by a lot, the final solutes are NH4Cl, FeCl3, and HCl, in which H+ is mainly supplied by HCl directly. The hematite hollow spindles shown in Figure 3 were fabricated according to the conditions described in the preparation section. Keeping the amount of FeCl3 · 6H2O, urea, ethylene glycol, and the total volume of the aqueous solution constant, the pH value was adjusted by adding different volumes of 1 M HCl solution. The measured pH values are shown in Figure 6a. As a comparison, the calculated curve is drawn in the same figure. It can be seen that the measured pH values were lower than the calculated one, which may be ascribed to the CO2 released during the decomposition of urea. FE-SEM images of the products obtained from the solution with different pH values were shown in Figure 6. At a final pH of 8.24, the products were completely solid spindles (Figure 6b), while the tip of each solid spindle became concave at pH 7.77 (Figure 6c). When the final pH was 7.23, the product was almost hollow spindles (Figure 6d). Whereas when the pH value decreased to 4.87, some broken hollow spindles, as well as some very small irregular particles, were formed (Figure 6e). No product was obtained when the pH value fell below 1.

The amount of urea also can significantly change the morphology of the products via changing the pH value, which is similar to that by HCl. The relationship between the final pH value and the amount of urea and the corresponding FE-SEM images of the products was shown in Figure S1 in the Supporting Information. These results showed that the pH value played a key role in the formation of hollow spindles. The situation might be similar to the preparation of hematite nanotubes in a slight acidic solution.37 The sharp tips of the spindles, due to high surface energy and low coverage (showed in the text below), were easily attacked by the protons. Therefore, the tip of the spindles will be dissolved gradually and the etching degree depends on the pH value. The optimum pH value is ca. 7.23. When the pH value is less than 7.23, the spindles will be broken into small fragments until all dissolved. 3.2.4. Effect of Ethylene Glycol. The effect of the amount of ethylene glycol on the morphology of the products was investigated. In those experiments, a total volume of 12.5 mL of deionized water and ethylene glycol and 10 mL of 1 M HCl solution were kept constant. Figure 7 shows the FESEM and TEM images of the products at various amounts of ethylene glycol. Without ethylene glycol, only hematite pseudocubics can be formed (Figure 7a). When 1 mL of ethylene glycol was added, ellipsoids 1.2 µm in length and 800 nm in diameter were obtained (Figure 7b). By increasing the volume of ethylene glycol to 2.5 mL, the products were hollow spindles 1.5 µm in

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Figure 11. (a) Chromatograms of phenol solution which is degraded by hematite hollow spindles analyzed every 30 min. (b) Degradation rate of phenol versus irradiation time on different samples. (c) Degradation rate of phenol on hematite hollow spindles after 3 h of irradiation for different cycles. (d) FESEM image of the hollow spindles collected after 5 cycles used.

length and 600 nm in diameter (Figure 3). With a further increase of the volume of ethylene glycol to 4 mL, the hollow spindles became shorter and smaller, 600 nm in length and 400 nm in diameter (Figure 7c). The TEM image shows their hollow structure with a shell thickness of about 50 nm (Figure 7d). If 8 mL of ethylene glycol was added, hematite nanoparticles 60 nm in diameter with some very small holes were obtained (Figure 7e,f). The FT-IR spectrum of the hematite obtained here (Figure 8) showed a broad peak centered at 3446 cm-1 and a medium peak at ca.1635 cm-1, which can be ascribed to the O-H vibration on the hydroxylated hematite surfaces.39 There are three types of surface hydroxyls based on their different coordination modes: singly, doubly, and triply coordinated hydroxyl; and the surface hydroxyl configuration on the various crystal faces of hematite is quite different.44-46 As shown in Figure 9, singly coordinated hydroxyls are present on the (100), (110), and (012) facets of hematite, whereas all are doubly coordinated on the (001) facet. The percents of the three hydroxyl modes on the different facets of hematite could be easily calculated (Table 1), and were in good agreement with Barron’s results.44 Hydroxyls with different coordination number have quite different reactivity. The doubly coordinated surface hydroxyls were nearly inert whereas the singly coordinated hydroxyls were reactive, because the former present as uncharged species whereas the later are charged either positively or negatively.46-48 This result has been evidenced in the fabrication of hematite spindles, in which the phosphate or sulfate was adsorbed on the hematite by reaction only with the singly coordinated hydroxyls to form a bidentate binuclear surface complex, due to its higher reactivity than the doubly coordinated one.35,37,49-51 As is well-known, ethylene glycol is a common bidentate ligand, which can also coordinate with metal ions to form the metal-glycolate complex.52-55 FT-IR spectra of the products

evidence the presence of ethylene glycol (Figure 8). Three small peaks at 2971, 1049, and 880 cm-1 could be assigned to the vibrational bands of CH2-, the stretching band of C-OH, and the stretching band of C-C, respectively.56 Moreover, based on the simple calculation, the distance between two O atoms of the ethylene glycol is ca. 0.25 nm in the metal-glycolate complex, which matches that of contiguous singly coordinated hydroxyls well. Therefore, it is easy to form the surface complex between the ethylene glycol and Fe atoms at the sites of the contiguous singly coordinated hydroxyls on (100), (110), and (012) facets, while the adsorption capacities and affinities for ethylene glycol are much lower for the (001) facet, where the surface hydroxyls are all inert doubly coordinated. So, it is reasonable that the preferential growth direction is along the c-axis. The amount of ethylene glycol has a remarkable influence on the morphology of the hematite. The aspect ratio of hematite particles increased as the amount of ethylene glycol increased from 0 to 2.5 mL, due to preferential adsorption on (100), (110), and (012) facets and preferential growth along the [001] direction. By further increasing the amount of ethylene glycol to 4 mL, the growth rate along the [001] direction obviously slows down and the aspect ratio of the spindles decreases, because the excess ethylene glycol starts to adsorb on the (001) facet. When the volume of ethylene glycol increases to 10 mL, a large number of monodispersion nanoparticles are formed, and because every crystal plane of hematite was covered with excessive ethylene glycol, at the same time, the growth rate of the nanoparticles is very slow. On the basis of the above experiment results, the growth mechanism of the hematite hollow spindles is proposed as illustrated in Figure 10. The pH value of the solution increased gradually with the decomposition of urea above 90 °C.57 Rod-like β-FeOOH was formed first due to the hydrolysis of Fe3+ in the weaker basic solution,40,58 then

2844 J. Phys. Chem. C, Vol. 113, No. 7, 2009 aggregated to form flower-like particles. As the reaction proceeded, β-FeOOH particles gradually turned to R-Fe2O3 particles at a temperature of 170 °C and grew up to form spindles under ethylene glycol assistance. This transformation was similar to the dissolution-recrystallization mechanism.39,41,59 After the hematite spindles were formed, a typical Ostwald ripening occurred under the hydrothermal condition. Due to the low coverage of ethylene glycol on the (001) facet and high surface energy of the tip, the proton etching takes place on the (001) facet first to form a hole on the tip of the spindle and then the inner part of the spindle gradually transfers to the outer surface through the hole and the spindle forms a hollow structure finally. Figure 4g provided further evidence from which we could see a small remainding particle in the hollow cave as an intermediate in the reaction midway. Meanwhile, the sharp tips of the hematite spindles were gradually dissolved from outer toward the interior, to form a hole on the tip of the spindle (Figure 4f). By prolonging the reaction time to 12 h, hollow hematite spindles were finally formed. The high coverage of the ethylene glycol prevented the outer surface from the acid etching and made sure it forms the shell of the hollow spindles. 3.3. Photocatalytic Property of the Hematite Hollow Spindles. To evaluate the potential application in water treatment of as-obtained hematite hollow spindles, the degradation performance for the organic pollutants was investigated. Phenol, a high toxic pollutant in the wastewater of coal distillation, was chosen as the organic pollutant. For comparison, commercial hematite (BET: 4.4 m2/g) and hematite solid spindles (BET: 14.1m2/g) shown in Figure 6b were used as the control samples. Figure 11a was a typical chromatogram of the phenol solution analyzed by the HPLC every 30 min, in which the hematite hollow spindles (BET: 20.8 m2/g (Supporting Information)) were used as the photocatalyst. The peaks at retention time ca. 2, 8, and 15 min belong to carboxylic acid, multihydroxybenzene, and phenol, respectively. It is clear to see that the phenol can be degraded to multihydroxybenzne first and carboxylic acid finally by the hematite hollow spindles under UV irradiation. Figure 11b shows the concentration of phenol changes with time, from which we can see clearly that hematite hollow spindles exhibited better photocatalytic activities than that of the commercial or solid one. In the case of hematite hollow spindles as phtocatalyst, the concentration of phenol decreased to half-value of the initial in only 30 min and to below 10% after 3 h. To evaluate the amount of phenol adsorbed on the surface of hematite, a control experiment was conducted in the dark. As shown in Figure 11b, the slight decrease of phenol in the first 30 min could be attributed to the surface adsorption of phenol, which is ca. 4% of the initial phenol. After that, the concentration of phenol was almost constant without UV irradiation, which gave the evidence that the phenol was degraded in the photocatalysis experiment but adsorbed. Although the BET surface area of our hematite hollow spindle is less than that of other porous iron oxide due to its pore size,34,60 the catalytic activity is quite good, which might be attributed to the stable hollow structure. Also, it is possible that the hollow internal cavity enables storage of more molecules to increase the reaction chance and the pores on the tip of spindles also improve the transportation of the reactant and product, which are of benefit to phenol degradation. Furthermore, the used hematite hollow spindles can be regenerated by simply washing with ethanol three times and drying in air at 80 °C for 3 h. The catalytic

Li et al. performance for phenol degradation of the used hematite hollow spindles is almost invariable after five cycles (Figure 11c). Figure 11d shows a typical FESEM image of the hematite hollow spindles used for five cycles, from which we can see that the hollow structure does not change any. The stable hollow structure might be important for the stable catalytic activity of hematite hollow spindles. 4. Conclusions In summary, hematite hollow spindles were successfully fabricated via a facile and environmentally friendly hydrothermal route without any templates. On the basis of the experimental results, a possible formation mechanism was proposed. Experimental results show that the reaction temperature, pH value, and the dosage of ethylene glycol play important roles in the formation of these hollow spindles. By simply changing the pH value, hollow or solid spindles can be selectively fabricated. The size of the hollow spindles can be adjusted from 1 µm to less than 100 nm by using different dosages of ethylene glycol. Moreover, the hematite hollow spindles exhibited a good photocatalytic activity for phenol degradation, which is higher than that of the commercial or solid one. Easy regeneration and near-constant photocatalytic activities suggest that these hematite hollow spindles may be applicable in water treatment in the future. Acknowledgment. We thank the National Natural Science Foundation of China (NNSFC) for financial support under major project No. 90606005 and surface project No. 20571040. Supporting Information Available: FESEM images of the products from different amounts of urea and BET test of the hematite hollow spindles. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Yin, Y.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorja, G. A.; Alivisatos, A. P. Science 2004, 304, 711. (2) Kidambi, S.; Dai, J. H.; Li, J.; Bruening, M. L. J. Am. Chem. Soc. 2004, 126, 2658. (3) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (4) Li, X.; Lou, T.; Sun, X.; Li, Y. Inorg. Chem. 2004, 43, 5442. (5) Li, B.; Xie, Y.; Jing, M.; Rong, G.; Tang, Y.; Zhang, G. Langmuir 2006, 22, 9380. (6) Bae, Y.; Fukushima, S.; Harada, A.; Kataoka, K. Angew. Chem., Int. Ed. 2003, 42, 4640. (7) Cai, Y.; Pan, H.; Xu, X.; Hu, Q.; Li, L.; Tang, R. Chem. Mater. 2007, 19, 3081. (8) Cao, A. M.; Hu, J. S.; Liang, H. P.; Wan, L. J. Angew. Chem., Int. Ed. 2005, 44, 4391. (9) Li, B.; Rong, G.; Xie, Y.; Huang, L.; Feng, C. Inorg. Chem. 2006, 45, 6404. (10) Yu, H.; Yu, J.; Cheng, B.; Liu, S. Nanotechnology 2007, 18, 065604. (11) Li, L.; Chu, Y.; Liu, Y.; Dong, L. J. Phys. Chem. C 2007, 111, 2123. (12) Zeng, S.; Tang, K.; Li, T.; Liang, Z.; Wang, D.; Wang, Y.; Zhou, W. J. Phys. Chem. C 2007, 111, 10217. (13) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (14) Liang, H. P.; Zhang, H. M.; Hu, J. S.; Guo, Y. G.; Wan, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2004, 43, 1540. (15) Sun, X.; Li, Y. Angew. Chem., Int. Ed. 2004, 43, 3827. (16) Putlitz, B. Z.; Landfester, K.; Fischer, H.; Antonietti, M. AdV. Mater. 2001, 13, 500. (17) Bao, J.; Liang, Y.; Xu, Z.; Si, L. AdV. Mater. 2003, 15, 1832. (18) Huang, H.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805. (19) Zhang, D.; Qi, L.; Ma, J.; Cheng, H. AdV. Mater. 2002, 14, 1499. (20) Peng, Q.; Dong, Y.; Li, Y. Angew. Chem., Int. Ed. 2003, 42, 3027. (21) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (22) Penn, R. L. J. Phys. Chem. B 2004, 108, 12707.

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