Template-Free Hydrothermal Synthesis and Formation Mechanism of

Nov 21, 2008 - Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, Univer...
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J. Phys. Chem. C 2008, 112, 19916–19921

Template-Free Hydrothermal Synthesis and Formation Mechanism of Hematite Microrings Sheng-Liang Zhong,†,‡ Ji-Ming Song,† Sen Zhang,† Hongbin Yao,† An-Wu Xu,† Wei-Tang Yao,† and Shu-Hong Yu*,† DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, The People’s Republic of China, and College of Chemistry and Chemical Engineering, Jiangxi Normal UniVersity, Nanchang, Jiangxi 330022, The People’s Republic of China ReceiVed: July 27, 2008; ReVised Manuscript ReceiVed: October 26, 2008

Hematite microrings with an outer diameter ∼12 µm, inner diameter ∼4 µm, and thickness ∼500 nm have been successfully synthesized by a facile template-free hydrothermal method using K3Fe(CN)6 and NaOH as reagents through a redox reaction. The influence of the reaction time, reaction temperature, concentration of K3Fe(CN)6, and NaOH on the evolution of shape and structures has been studied in detail. The optimal condition for preparation of the microrings is 180 °C for 5 h, the NaOH concentration is 4 M, and the K3Fe(CN)6 concentration is 0.02 M. The possible hollowing growth mechanism for the hematite microrings has been discussed. Magnetic hysteresis measurement revealed that hematite microrings after calcination at 700 °C for 4 h display a magnetic behavior with remanent magnetization of 0.211 emu/g and coercivity of 2005 Oe at room temperature. 1. Introduction Owing to their unique structural features, ringlike objects exhibit novel properties with ring cavities, which may strongly enhance the functionality of nanomaterials.1 For instance, a remarkable uniform field enhancement effect was realized in the cavities of gold nanorings, which could serve as resonant nanocavities for holding or probing smaller nanostructures in sensing and spectroscopy application.2 To date, many ringlike objects have been prepared by various methods. Wang et al. reported that benzene-ring-like ZnO rings can be prepared by a microemulsion method.3 An ultrasonically assisted process was developed to prepare gold nanorings and Cd(OH)2 nanorings,4,5 and a polymer-assisted synthesis of ZnO hexagonal nanorings has been reported.6 Boron nitride (BN) nanorings were synthesized using a sulfur vapor-assisted solid-state method at 600 °C.7 Ringlike semiconductors were obtained through a selfassembling route in solution phase.8 Recently, Ag2V4O11 nanorings and microloops have been prepared by a facile hydrothermal method.9 Cobalt oxide nanorings were prepared by decomposition of the cobalt hydroxide nanosheets.10 Co-Ni magnetic flux-closure alloy nanorings have been fabricated by our group via a facile solvothermal method.11 CuInSe2 nanorings were also obtained through a solution method and were used to create solar cells.12 Gd2O3 nanorings have been synthesized by low-temperature (90 °C) hydrolysis of Gd(acac)3 and subsequent in situ thermal dehydration of the hydrolyzed precursor-surfactant aggregates at 320 °C.13 It is obvious that the fabrication of ringlike objects has been attracting more and more interest in recent years; however, it is still a big challenge to fabricate ringlike objects due to the demand of precision control in preparation. Under ambient conditions, hematite is the most stable iron oxide with n-type semiconducting properties (Eg ) 2.1 eV),14 * To whom correspondence should be addressed. Fax: + 86 551 3603040. E-mail: [email protected]. † University of Science and Technology of China. ‡ Jiangxi Normal University.

and it is also a candidate for applications in catalysts,15 sensors,16 and semiconductors.17 Its nontoxicity, biodegradability, low cost, and relatively good stability are definitely very attractive features for applications. Controlled synthesis of hematite with various structures has drawn much attention in recent years. Until now, one-dimensional,14,18 particle-like,19 plate-shaped,20 acicular,21 shuttlelike,22 dendritic, flowerlike, urchinlike, and cantaloupe-like superstructures have been synthesized.23 Hematite nanocubes,24 tube-in-tube nanostructures,25 and hollow spheres26 have also been reported. Chu et al. synthesized ringlike R-Fe2O3 particles by a hydrothermal process using a redox reaction of Fe2+ and S2O82- in solution in the presence of polyethylene glycol.27 Compared with the urchinlike R-Fe2O3, higher coercivity was realized in the ringlike products. Furthermore, the ringlike form exhibits a photocatalytic property for the degradation of salicylic acid. Recently, a rapid microwaveassisted hydrothermal approach has been developed to synthesize R-Fe2O3 nanorings in aqueous solution,28 which showed that the prepared R-Fe2O3 nanorings with a high surface-to-volume ratio are ideal candidates for applications in chemical sensors and nanocavities, as well as in advanced optical/electric nanodevices. In this paper, hematite microrings have been synthesized by a facile template-free hydrothermal method using K3Fe(CN)6 and NaOH as precursors. The formation of such hematite microrings has been found to be controlled by a synergistic effect, including reaction time, concentrations of the precursors, reaction temperature, and pH value. A hollowing growth mechanism of the microrings is proposed. 2. Experimental Section All reagents were analytical grade from Sinopharm Chemical Reagent Co. Ltd. and were used without further purification. Manipulations and reactions were carried out in air without the protection of nitrogen or inert gas. In a typical synthesis, 0.2634 g of K3Fe(CN)6 (0.8 mM) was put into a 53 mL Teflon-lined autoclave and dissolved with 20 mL of deionized water under

10.1021/jp806665b CCC: $40.75  2008 American Chemical Society Published on Web 11/21/2008

Synthesis and Formation of Hematite Microrings

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19917

Figure 1. XRD pattern of the product synthesized at 180 °C for 5 h. [K3Fe(CN)6] ) 0.02 M, [NaOH] ) 4 M.

stirring. A 6.4 g NaOH (0.16 M) was put into a beaker and dissolved with 20 mL deionized water, then the NaOH solution was added dropwise into the above K3Fe(CN)6 solution. After that, the mixture was stirred for another 10 min. The autoclave was sealed and heated to 120-200 °C for different periods of time. After being cooled down to room temperature naturally, the products were collected by centrifugation and washed with deionized water and absolute ethanol several times. Finally, the products were dried under vacuum at 60 °C for 6 h for further characterization. X-ray powder diffraction (XRD) patterns of the products were obtained on a Philips X′ Pert Pro Super X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.541 78 Å). Field scanning electron microscope (SEM) images were explored on a field emission scanning electron microanalyzer (JEOL-6700F) and Quantan 2000 environmental scanning microscope. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) was performed on a JEOL-2010 transmission electron microscope. The magnetic properties of samples were studied using a superconducting quantum interface device magnetometer (Quantum Design MPMS XL).

Figure 2. (a-c) SEM images of the product synthesized at 180 °C for 5 h. (d) HRTEM of a region of part c. Insert is the electron diffraction pattern. [K3Fe(CN)6] ) 0.02 M, [NaOH] ) 4 M.

Figure 3. (a) XRD and (b-d) SEM images of the sample after thermal treatment at 700 °C for 4 h of the product prepared at 180 °C for 5 h.

3. Results and Discussion The crystal phase and chemical composition of the synthesized samples were examined by the X-ray powder diffraction method. Figure 1 shows that the product prepared at 180 °C for 5 h is composed of cubic Fe3O4 (JCPDS file No. 75-0033) and hexagonal R-Fe2O3 (JCPDS file No. 84-0307). The typical SEM images of the products are shown in Figure 2. A lowmagnification image in Figure 2a shows that the number of the microrings is ∼40% among all particles. The microrings have an average outer diameter of ∼12 µm, inner diameter of ∼4 µm, and thickness of ∼500 nm (Figure 2b). A high-magnification SEM image shows that each of the rings is constructed of many small plates, and the plates seem to overlap through a helical growth route (Figure 2c). Furthermore, the plates are not closely stacked. The fringes in a typical high-resolution transmission electron microscopy image (Figure 2d) are separated by ∼2.7 Å, which agrees well with the interplanar distance of the (104) plane of R-Fe2O3. The SAED pattern taken from the edge of the rings indicated high crystallinity of the microring (inset in Figure 2d). From these results, it can be concluded that the microrings correspond to the hematite structure. Flowerlike, platelike, and octahedral products, in addition to microrings, were also found in the products (see Supporting Information Figure S1).

Figure 4. The room temperature hysteresis loops for the sample prepared at 180 °C for 5 h after thermal treatment at 700 °C for 4 h.

It has been reported that the octahedral shape is the favorable morphology of Fe3O4 in strong basic conditions;29 thus, the octahedrons with smooth surfaces may be assigned as Fe3O4. This assumption can be confirmed by the subsequent sintering

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Figure 7. Structural characterstics of R-Fe2O3 viewed along [010]. (The model was made by Materials Studio software). Figure 5. (a) XRD pattern of the product prepared at 180 °C for 1.5 h. [K3Fe(CN)6] ) 0.02 M, [NaOH] ) 4 M. (b-d) SEM images of the product.

experiment. After sintering at 700 °C for 4 h the product prepared at 180 °C for 5 h, pure phase R-Fe2O3 (JCPDS card 84-0307) was obtained. The XRD pattern is shown in Figure 3a. The SEM results reveal that the rings still keep their shape (Figure 3b), which suggests that the microrings are stable even at high temperature. From Figure 3c, we can see that the roughness of the ring plates increased and some cracks were found on the surface. Furthermore, no polyhedron-like particles were found in the product (Figure 3d). This is because Fe3O4 was oxidized to R-Fe2O3. It is well-known that the physical and chemical properties of materials strongly depend on the size, size distribution, defect structure, and dimensions. To eliminate the effect of Fe3O4 on the magnetic properties of the microrings, the products prepared at 180 °C for 5 h were first calcined at 700 °C for 4 h. Figure 4 displays the hysteresis loop of the as-prepared product. The remanent magnetization and coercivity of the product are 0.2110

Figure 8. XRD patterns of the as-synthesized products obtained at 180 °C with different reaction times: (a) 2, (b) 4, (c) 12, and (d) 24 h. [NaOH] ) 4 M, [K3Fe(CN)6] ) 0.02 M.

emu/g and 2005 Oe, respectively, at room temperature. The remanent magnetization of the microrings is similar to that of

Figure 6. SEM images of the samples prepared at 180 °C for different reaction times: (a, b) 2, (c, d) 4, (e, f) 12, and (g-i) 24 h. [K3Fe(CN)6] ) 0.02 M, [NaOH] ) 4 M.

Synthesis and Formation of Hematite Microrings

Figure 9. SEM images of the products prepared at 180 °C for 5 h using different NaOH concentrations: (a) 2 and (b) 8 M. [K3Fe(CN)6] ) 0.02 M.

Figure 10. SEM images of the products prepared at 180 °C for 5 h with different concentration of K3Fe(CN)6: (a) 0.005 and (b) 0.04 M. [NaOH] ) 4 M.

Figure 11. SEM images of the products prepared at different reaction temperatures for 5 h: (a, b) 120, (c) 140, and (d) 200 °C. [NaOH] ) 4 M, [K3Fe(CN)6] ) 0.02 M.

the R-Fe2O3 nanorings reported.27 However, the coercivity is much higher than that of the nanorings. Usually, the coercivity of R-Fe2O3 with other morphologies is lower than 1700 Oe at room temperature.18b,23a,c,30 Thus, the R-Fe2O3 microrings after treatment at higher temperature display higher coercivity. To further understand the growth mechanism, the initial stage of experiments carried out at 180 °C for 1.5 h was investigated in detail. The XRD pattern shows that the obtained product is hexagonal R-Fe2O3 (JCPDS card 84-0307) (Figure 5a). Compared with the standard diffraction pattern, no peaks from other phases are found, indicating the high purity of the as-synthesized product. Flowerlike products were also found in the products (Figure 5b). Interestingly, each plate has a nanoflower with many nanopetals in the center part of the plate. Each plate has a diameter of about 10 µm and thickness of about 500 nm. Highmagnification SEM image shows that the outsides of the plates are constructed by overlapping of small hexagonal plates (Figure

J. Phys. Chem. C, Vol. 112, No. 50, 2008 19919 5c). Furthermore, it seems that the small hexagonal plates packed densely through a highly parallel fashion at the outside but in a random fashion in the center part (Figure 5c, d). Time-dependent experiments were also carried out at 180 °C. The representative SEM images of the products prepared at certain reaction time intervals are shown in Figure 6. When the reaction time was 2 h, hexagonal plates with a cavity appeared at the center of the hexagonal plates (Figure 6a). A magnified SEM image shows that the hollow part at the center of the hexagonal plates is loose and coarse and that many nanoparticles are found (Figure 6b). Thus, we can speculate that the center part is the most reactive part of the plate. The reason could be that the center part is constructed of freshly formed Fe2O3 particles smaller sizes. Compared with the aged Fe2O3, the freshly formed smaller Fe2O3 particles have a higher activity and are much easier to react with CN- ions. On the other hand, the center part of the big plates has more grain boundaries than the other places. By careful observation, it was also found that the edges of the small hexagonal plates are easier to be reacted than other places. Compared with the results obtained at 180 °C for 1.5 h (Figure 5c), the edges of the small, hexagonal plates became blurry. When the reaction time was increased to 4 h, a hollow cavity was finally formed in the middle of the plates (Figure 6c, d). However, the number ratio of microrings among the particles is still lower than that of the product prepared after reaction for 5 h. It has to be noted that octahedral particles were also found in the product (Figure 6c). When the reaction time was prolonged to 12 h, ringlike particles with similar diameter were still found in the product (Figure 6e, f), indicating that the rings are stable in the solution. Similar results were observed in the product prepared after 24 h, but some cavities at the center of the plates became larger (Figure 6g, h, i). According to the Ostwald ripening process, large crystals grow as small crystals gradually dissolve. Herein, the hexagonal plates were obtained at the expense of the nanopetals of the flowers, as shown in Figure 5b-d. It is believed that the relative growth rate of different crystal facets and the differences in the growth rates of various crystal facets result in different outlooks of the crystallites.3,31 In this case, the side facets having higher surface energy grow faster than those of the mostly exposed (top/down) facets and finally lead to the formation of hexagonal plates. According to the structural characteristics of R-Fe2O3 shown in Figure 7, the view along [010] indicates that there are more Fe atoms appearing on the (100) plane, whereas there are no Fe atoms on the (001) plane. Thus, the CN- anions have more chances to attact the Fe atoms on the (100) plane than the (001) plane; that is, the (100) plane will be more reactive that the (001) plane. This structural modeling result may explain why the edges of the plates (the (100) plane) tend to be more reactive and be dissolved further, especially when the tiny plates formed at the initial stages will tend to dissolve due to the driven force based on the Ostwald ripening process. The XRD patterns were taken for the products prepared after different reaction times, which are used to determine the phase evolution process (Figure 8). When the reaction time was 2 h, in addition to the dominant R-Fe2O3 phase (JCPDS card 840307), a very weak diffraction peak for the Fe3O4 phase (JCPDS card 75-0033) was detected. However, pure R-Fe2O3 was obtained when the reaction time was 1.5 h. It means that at this stage, the Fe3+ begins to be reduced to Fe2+ and reacts with the Fe3+ in the solution to form Fe3O4. The SEM images show that a round cavity appeared at the center of the disks (Figure 6a, b). With the reaction time prolonged to 4 h, the intensities for Fe3O4 diffraction peaks increased, the intensities for the R-Fe2O3

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phase decreased correspondingly, and ringlike particles were obtained. When the reaction time was 12 and 24 h, respectively, the main crystalline phase was Fe3O4, and only weak diffraction peaks for R-Fe2O3 were observed in the final product. It can be concluded from the above results that the iron ions in the products existed as Fe3+ ions at the early stage of the reaction, and some of them were reduced to Fe2+ with a prolonged reaction time. The chemical reactions concerned in the formation of R-Fe2O3 can be proposed as the following:

[Fe(CN)6]3- T Fe3+ + 6CN3+

Fe

-

+ 3OH T Fe(OH)3

2Fe(OH)3 T Fe2O3 + 3H2O -

Fe2O3 + CN

+ 4H2O f 2Fe(OH)2 + CO32- + NH4+

Fe(OH)2 + 2Fe(OH)3 f Fe3O4 + 4H2O NH4+ + OH- f NH3 v

+ H2O

SCHEME 1: Schematic Illustration for Shape Evolution of r-Fe2O3 Microrings in the Whole Synthetic Process

(1) (2) (3) (4) (5) (6)

[Fe(CN)6]3- ions are very stable in aqueous solution at room temperature, even under strong basic conditions. No precipitate was produced with the addition of sodium hydroxide solution. Under hydrothermal conditions, however, at first, [Fe(CN)6]3ions dissociated slowly into Fe3+ ions and CN- (eq 1).23a Second, the Fe3+ ions were precipitated by the OH- anions to form Fe(OH)3 (eq 2), and then the Fe(OH)3 dehydrated to R-Fe2O3 under hydrothermal conditions. At the same time, the reaction as described in eq 4 occurred with an increase in CNconcentration, and Fe(OH)2 was obtained. This is why the redox reaction does not happen at the early stage when the CNconcentration is low. Simultaneously, the Fe(OH)2 and Fe(OH)3 reacted to form Fe3O4 under strong basic conditions. Thus, the Fe3O4 phase was not found at the early stage. Among these six possible reactions, the reaction described in eq 4 plays a crucial role in the formation of R-Fe2O3 rings and Fe3O4 particles. The resulting CO32- and NH4+ produced in eq 4 had been verified by chemical methods after the reaction. The effect of the concentration of NaOH on the synthesis was investigated in detail. The SEM images of the products prepared at 180 °C for 5 h using different NaOH concentrations are shown in Figure 9. When the concentration of NaOH was 2 M, hexagonal plates constructed with small plates were obtained (Figure 9a). Shallow holes were found at the center of some plates, but some plates have no cavities at their center parts. This result suggested that holes are not easily formed at low NaOH concentration. When the concentration of NaOH was increased to 8 M, again, no cavities were found at the center of the plates. Instead, hexagonal plates with diverse sizes and thickness were obtained, and the surfaces of the plates were smooth. It has to be noted that the plates are not constructed by small plates as the products prepared when the NaOH concentration is 4 M (Figure 9b). From above, it can be seen that the reduction ability of CN- increases with the concentration of the NaOH. When the NaOH concentration is low, the reaction rate of eq 4 is very slow, even difficult, to take place. It is wellknown that the freshly prepared Fe(OH)3 can be soluble in hot alkaline solution, and it is difficult to dissolve after aging. Thus, when the NaOH concentration is too high, the freshly formed Fe(OH)3 can be dissolved quickly and form highly stacked hexagonal Fe2O3 plates in a short time, which are difficult to react with CN- after being aged in the concentrated NaOH solution. Interestingly, it has been found that the concentration of K3Fe(CN)6 is another important factor for the formation of the

microrings. The plates are formed when the concentration of K3Fe(CN)6 is 5 mM. The SEM image reveals that the surfaces of the plates are smooth, and no cavity was found in the middle part of the plates (Figure 10a). However, when the K3Fe(CN)6 concentration was increased to 0.04 M, holes were found on these plates, which is similar to the results when the concentration of K3Fe(CN)6 is 0.02 M (Figure 10b). This may be because the CN- concentration is too low to allow the reaction in eq 4 to happen. It also supports the reason why eq 4 does not happen when the reaction time is shorter than 2 h if the concentration of K3Fe(CN)6 is 0.02 M. Because the [Fe(CN)6]3- ions are very stable, they dissociate very slowly, and the involved CNconcentration increases slowly. The reaction in eq 4 can happen only when the concentration of the CN- anions reached a certain value. In addition to the reaction time and the concentrations of K3Fe(CN)6 and NaOH, the temperature is also an important factor for the formation of the microrings. The plates constructed by smaller plates with a flower in the center part were found when the reaction temperature was 120 °C (Figure 11a, b). The plates with cavities at their centers were found in the product, but most of the holes were not hollow when the reaction temperature was 140 °C (Figure 11c). It is worth noting that the rings are not well-formed when the reaction temperature goes up to 200 °C, and most of the microrings are broken (Figure 11d). Thus, it can be concluded that the redox reaction rate increases with an increase in the reaction temperature. A possible growth process is proposed on the basis of the above results as schematically illustrated in Scheme 1. At first, [Fe(CN)6]3- ions dissociate into Fe3+ ions, which react with OH- anions to form Fe(OH)3 nanopetals. Second, these nanopetals connect to each other through the center to form 3D flowerlike structures. Third, the freshly formed Fe(OH)3 flowers are dissolved to form hematite hexagonal plates under the strong basic conditions through the Ostwald ripening process. At the same time, the hexagonal plates overlap together and compact densely in a highly directed manner by taking advantage of a flowerlike particle as a growing center to form bigger plates. Onsager et al. proposed that such side-by-side gathering occurs to maximize the entropy of the self-assembled structure by minimizing the excluded volume per particle.32 The process repeats, and thus, thicker plates are obtained. Then the central part of the plate reacts further with CN- anions to produce Fe(OH)2, which further preferentially reacts with Fe(OH)3 to form Fe3O4 particles. Finally, the central part of the plate is consumed, and a hollow hole is formed at the center. 4. Conclusions In summary, R-Fe2O3 microrings can be produced by a facile hydrothermal process using K3Fe(CN)6 and NaOH as reagents through a redox reaction. The formation of such hematite microrings is controlled by a synergistic effect, including

Synthesis and Formation of Hematite Microrings reaction time, concentrations of the precursors, reaction temperature, and pH value. The formation mechanism of hematite microrings has been proposed. Under strong basic conditions, [Fe(CN)6]3- ions dissociate slowly into Fe3+ and CN-; the freshly formed Fe3+ ions combine with OH- to obtain flowerlike Fe(OH)3 particles, which dehydrate into hematite plates; and these plates overlap through a highly directed way to form bigger plates. The flowerlike particles can also act as the nucleation center, which was consumed due to the redox reaction between Fe2O3 and CN- ions, and at last, hematite microrings can be formed. This hollowing mechanism, which is required by a synergistic effect of experimental parameters, could be helpful for further understanding the formation process of numerous hollow micro- or nanostructures in material synthesis. Acknowledgment. S.H.Y. acknowledges funding support from the National Natural Science Foundation of China (Nos. 50732006, 20671085, 20701035, 20621061); 2005CB623601, Anhui Development Fund for Talent Personnel and Anhui Education Committee (2006Z027, ZD2007004-1); the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry; and the PartnerGroup of the Chinese Academy of Sciences, the Max Planck Society. Supporting Information Available: SEM image of the product synthesized at 180 °C for 5 h. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Kong, X. Y.; Ding, Y.; Yang, R. S.; Wang, Z. L. Science 2004, 303, 1348. (b) Sano, M.; Kamino, A.; Okamura, J.; Shinkai, S. Science 2001, 293, 1299. (c) Martel, R.; Shea, H. R.; Avouris, P. Nature 1999, 398, 299. (d) Liu, J.; Dai, H.; Hafner, J. H.; Colbert, D. T.; Smalley, R. E.; Tans, S. J.; Dekker, C. Nature 1997, 385, 780. (e) Sun, Y. G.; Xia, Y. N. AdV. Mater. 2003, 15, 695. (f) Yada, M.; Sakai, S.; Torikai, T.; Watari, T.; Furuta, S.; Katsuki, H. AdV. Mater. 2004, 16, 1222. (g) Yan, F.; Goedel, W. A. Angew. Chem., Int. Ed. 2005, 44, 2084. (h) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625. (i) Hobbs, K. L.; Larson, P. R.; Lian, G. D.; Keay, J. C.; Johnson, M. B. Nano Lett. 2004, 4, 167. (j) Larsson, E. M.; Alegret, J.; Ka¨ll, M.; Sutherland, D. S. Nano Lett. 2007, 7, 1256. (k) MarinAlmazo, M.; Garcia-Gutierrez, D.; Gao, X.; Elechiguerra, J. L.; Kusuma, V. A.; Sampson, W. M.; Miki-Yoshida, M.; Dalton, A. B.; Escudero, R.; Jose-Yacaman, M. Nano Lett. 2004, 4, 1365. (l) Yi, D. K.; Kim, D. Y. Nano Lett. 2003, 3, 207. (2) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Kall, M.; Bryant, G. W.; de Abajo, F. J. G. Phys. ReV. Lett. 2003, 90, 57401. (3) Li, F.; Ding, Y.; Gao, P. X.; Xin, X. Q.; Wang, Z. L. Angew. Chem., Int. Ed. 2004, 43, 5238. (4) Jiang, L. P.; Xu, S.; Zhu, J. M.; Zhang, J. R.; Zhu, J. J.; Chen, H. Y. Inorg. Chem. 2004, 43, 5877. (5) Miao, J. J.; Fu, R. L.; Zhu, J. M.; Xu, K.; Zhu, J. J.; Chen, H. Y. Chem. Commun. 2006, 3013. (6) Peng, Y.; Xu, A. W.; Deng, B.; Antonietti, M.; Co¨lfen, H. J. Phys. Chem. 2006, 110, 2988. (7) Hao, X. P.; Wu, Y. Z.; Zhan, J.; Yang, J. X.; Xu, X. G.; Jiang, M. H. J. Phys. Chem. B 2005, 109, 19188. (8) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2005, 127, 18262. (9) Shen, G. Z.; Chen, D. J. Am. Chem. Soc. 2006, 128, 11762.

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