Platinum Hybrid Nanocatalyst with Controlled

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J. Phys. Chem. C 2008, 112, 13510–13515

A Novel Urchinlike Gold/Platinum Hybrid Nanocatalyst with Controlled Size Shaojun Guo,†,‡ Liang Wang,† Shaojun Dong,*,†,‡ and Erkang Wang†,‡ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China, and Graduate School of the Chinese Academy of Sciences, Beijing, 100039, People’s Republic of China ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: June 27, 2008

In this paper, we have explored a simple and new strategy to obtain quasimonodisperse Au/Pt hybrid nanoparticles (NPs) with urchinlike morphology and controlled size and Pt shell thickness. Through changing the molar ratios of Au to Pt, the Pt shell thickness of urchinlike Au/Pt hybrid NPs could be easily controlled; through changing the size of Au NPs (the size was easily controlled from ∼3 to ∼70 nm via simple heating of HAuCl4-citrate aqueous solution), the size of urchinlike Au/Pt hybrid NPs could be facilely dominated. It should be noted that heating the solution (100 °C) was very necessary for obtaining three-dimensional (3D) urchinlike nanostructures while H2PtCl6 was added to gold NPs aqueous solution in the presence of reductant (ascorbic acid). The electrocatalytic oxygen reduction reaction (ORR, a reaction greatly pursued by scientists in view of its important application in fuel cells) and the electron-transfer reaction between hexacyanoferrate(III) ions and thiosulfate ions of urchinlike Au/Pt hybrid NPs were investigated. It is found that the as-prepared urchinlike Au/Pt hybrid NPs exhibited higher catalytic activities than that of ∼Pt NPs with similar size. Introduction Platinum has stimulated extensive research owing to its unusual physical and chemical properties and many technological applications such as chemical sensors and biosensors and as catalysts in the reduction of pollutant gases emitted from automobiles.1 Much experimental evidence has proven that both catalytic efficiency and selectivity are highly dependent on the shape, composition, and size of the platinum-based nanomaterials.2 Therefore, the synthesis of platinum-based nanostructured materials with specific morphology and controlled size is of great significance. To date, a great number of methods such as wet-chemical approach, organic-phase route, electrochemical approach, etc. have been employed to synthesize advanced platinum-based functional nanomaterials with specific morphology and catalytic properties.3 For example, Sun and co-workers3a developed a simple high-temperature organic-phase route to synthesize monodisperse Pt nanocubes and studied their catalysis for oxygen reduction. Wang and co-workers3b employed a facile electrochemical method for preparing unconventional tetrahexahedron Pt nanospheres with high-index surfaces for electrocatalytic applications. Although the maximization of high-index surfaces and abundant corner and edge sites should be the criteria for selection of an excellent nanocatalyst,4 changing the Pt nanocatalyst with anisotropic morphology to a more complex one such as a three-dimensional (3D) flower-like nanostructure5-9 will greatly reduce the amount of Pt and improve the efficient utility of Pt. Sun et al.5 have successfully demonstrated a facile, efficient, and economical route for the large-scale synthesis of 3D flower-like platinum nanostructures via a simple mild wetchemical method using neither template nor surfactant. ElSayed’s group6 described a simple method to prepare a novel multiarmed Pt nanostar with high yield, which possesses a * Corresponding author. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Science.

unique single-crystal nanostructure with high catalytic activity. To the best of our knowledge, a simple aqueous-phase approach to the synthesis of quasimonodisperse 3D urchinlike Au/Pt hybrid NPs with controlled sizes has not been reported so far. In this paper, we have explored a simple, facile, efficient, and economical route to rapidly prepare quasimonodisperse 3D urchinlike Au/Pt hybrid NPs. Note that heating the solution (100 °C) was very necessary for obtaining urchinlike nanostructures while H2PtCl6 was added to a gold NP aqueous solution in the presence of reductant (ascorbic acid). Through changing the molar ratio of H2PtCl6 to HAuCl4, the density of the urchinlike Pt shell on the surface of hybrid NPs could be easily dominated; through changing the diameter of gold NPs, the size of urchinlike Au/Pt hybrid NPs could be facilely dominated. Most importantly, the as-prepared urchinlike Au/Pt hybrid NPs obtained here exhibit high catalytic activities toward ORR and the electron-transfer reaction between hexacyanoferrate(III) ions and thiosulfate ions. It is found that this hybrid nanomaterial revealed higher electrocatalytic performance for oxygen reduction than that of ∼6 nm Pt NPs (the Pt loadings for two samples tested are the same). The higher peak currents associated with the hydrogen adsorption and desorption events provided by the urchinlike Au/Pt hybrid NPs indicate that the urchinlike Au/Pt hybrid NPs possess higher electroactive surface area than that of ∼6 nm Pt NPs. Rotating ring-disk electrode (RRDE) voltammetry further demonstrated that ∼6 nm urchinlike Pt NPs could catalyze the four-electron reduction of O2 to H2O in an air-saturated 0.5 M H2SO4 solution. And also, the rate constant values for the electron-transfer reaction between hexacyanoferrate(III) ions and thiosulfate ions in the presence of ∼6 nm urchinlike Pt NPs is higher than that of ∼6 nm Pt NPs. This work may open a general approach for designing other hybrid nanomaterials, which could potentially be applied in different catalytic reactions.

10.1021/jp804347q CCC: $40.75  2008 American Chemical Society Published on Web 08/12/2008

A Novel Urchinlike Gold/Platinum Hybrid Nanocatalyst Experimental Section Synthesis of Gold NPs. Twenty-five nanometer gold NPs were prepared by reduction of HAuCl4 aqueous solution with trisodium citrate according to ref 10. Briefly, 50 mL of 0.01% HAuCl4 solution was heated to boiling, and 0.75 mL of 1% trisodium citrate was added. The solution was kept boiling for 15 min, then allowed to cool at room temperature. Thirteen nanometer gold NPs were synthesized according to the ref 11. Next 100 mL of 1 mM HAuCl4 was brought to a reflux while stirring and then 10 mL of a 38.8 mM trisodium citrate solution was added quickly, which resulted in a color change of the solution from pale yellow to deep red. After the color changed, the solution was refluxed for an additional 15 min. The gold NPs with a diameter of ∼3 nm were synthesized according to the literature.12 Synthesis of ∼6 nm Pt NPs. Six nanometer Pt NPs were prepared by reduction of H2PtCl6 aqueous solution with trisodium citrate according to ref 13. Briefly, 1 mL of 1% H2PtCl6 aqueous solution was added into 100 mL of water and then the solution was heated to boiling. Aging of the H2PtCl6 solution was not necessary in this synthetic procedure. Then, 3 mL of 1% sodium citrate aqueous solution was added rapidly, and the mixture was kept at a boiling temperature for certain times. TEM was used to examine particle size distribution, and an average diameter of 6 nm of Pt colloids was confirmed (data not shown). Apparatus. Transmission electron microscopy (TEM) measurements were made on a HITACHI H-8100 EM with an accelerating voltage of 200 kV. The sample for TEM characterization was prepared by placing a drop of prepared solution on a carbon-coated copper grid that was then dried at room temperature. XPS measurement was performed on an ESCALAB-MKII spectrometer (United Kingdom) with Al KR X-ray radiation as the X-ray source for excitation. UV-vis spectra were collected on a CARY 500 Scan UV-vis-nearinfrared (UV-vis-NIR) spectrophotometer. Electrochemical experiments were performed with a CHI 832 electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China). A conventional three-electrode cell was used, including a Ag/AgCl (saturated KCl) electrode as reference electrode, a platinum wire as counter electrode, and a bare or nanomaterial modified gold as working electrode. An EG&G PARC Model 366 bipotentiostat was used for RRDE experiments. A rotating GC (5 mm) disk-platinum ring electrode was used as a working electrode. The collection efficiency (N) of the ring electrode obtained by reducing ferricyanide at a disk electrode was 0.139. The Preparation of Modified Electrode. The electrocatalytic dioxygen reduction measurements were carried out in a 0.5 M H2SO4 solution at the scan rate of 50 mV/s in the presence of oxygen or air. The working electrode was a gold disk with a diameter of 2 mm, polished with Al2O3 paste and washed ultrasonically in Millipore water. The electrode was loaded with 5 µL of urchinlike Au/Pt hybrid NPs (the concentration of Pt is 0.804 mM). For comparing the active surface of nanomaterial modified electrode, 10 µL of urchinlike Au/Pt hybrid NPs or Pt NPs solution (the concentration of Pt is 1.93 mM; the same Pt loadings for two nanomaterials) was dropped on the GC electrode (5 mm) and allowed to dry at room temperature. Then 5 µL of Nafion (0.2%) was placed on the surface of the above nanomaterial modified GC electrodes. Catalytic Properties of Urchinlike Au/Pt Hybrid NPs. For the electron-transfer reaction between potassium ferricyanide and sodium thiosulfate, 0.2 mL of 0.1 M potassium ferricyanide and 0.8 mL of urchinlike Au/Pt hybrid NPs solution (the concentration of Pt is 1.93 mM) were added to 8.8 mL of water

J. Phys. Chem. C, Vol. 112, No. 35, 2008 13511 SCHEME 1: Procedure To Design the Urchinlike Hybrid Au/Pt NPs

and the solution was kept at ∼20 °C. The catalytic reaction started by adding 0.2 mL of 1 M sodium thiosulfate solution. The reaction was monitored by in situ UV-vis-NIR absorption spectroscopy. Results and Discussion The whole preparation strategy is shown in Scheme 1. A twostep process was employed to synthesize quasimonodisperse urchinlike Au/Pt hybrid NPs. First, 0.5 mL of 1 wt % HAuCl4 solution was added to 50 mL of aqueous solution and the solution was heated to boiling with stirring. Then 0.75 mL of 1 wt % sodium citrate was quickly introduced to the above solution. After heating for several minutes, the golden-red solution appeared, indicating the formation of gold NPs. Then 1 mL of 0.1 M ascorbic acid (excess) was subsequently added to the gold NP boiling solution, followed by adding 1.25 mL of 1 wt % H2PtCl6. In this process, the heat-treatment could obviously accelerate the kinetics of reduction of Pt salt by ascorbic acid on the colloidal gold surface. After 20 min of heating, the urchinlike Au/Pt hybrid NPs (sample 1) were obtained (30 mL). The structure and morphology of the resulting product were characterized by using TEM. Figure 1A,B shows the typical TEM images of the as-synthesized Au/Pt hybrid NPs at different magnifications. It is observed that quasimonodisperse urchinlike nanospheres exist in the resulting product. The structural details

Figure 1. Typical TEM images (A, B) of the as-synthesized urchinlike Au/Pt hybrid NPs at different magnifications (Pt:Au at 2:1); typical TEM image (C) of the as-synthesized urchinlike Au/Pt hybrid NPs (Pt: Au at 1:1).

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Figure 2. Typical TEM images of the as-synthesized urchinlike Au/ Pt hybrid NPs, using ∼13 nm gold NP as a seed (A), and Au/Pt core/ shell nanostructure, using ∼25 nm gold NP as a seed (B), without heating the solution (Pt:Au at 2:1).

Figure 4. EDX image (A) and XPS spectra of the as-synthesized urchinlike Au/Pt hybrid NPs (Pt:Au at 2:1): Pt4f orbital (B), and Au4f orbital (C).

Figure 3. Typical TEM images of the as-synthesized urchinlike Au/ Pt hybrid NPs, using ∼3 nm gold NP as a seed at different magnifications.

are revealed in the magnified TEM image (Figure 1B). The average diameter of the hybrid NPs was statistically calculated to be about 35 nm. In addition, the density of the urchinlike Pt shell on the surface of hybrid NPs could be controlled via changing the molar ratio of H2PtCl6 to HAuCl4. When the molar ratio was at 1:1, a low-density urchinlike Pt shell was obtained (Figure 1C). Furthermore, if the size of the gold seed was reduced to ∼13 or ∼3 nm, urchinlike Au/Pt hybrid NPs with a smaller size of about 20-25 (Figure 2A) or 6 nm (Figure 3) were obtained. It should be noted that less Pt could be coated on the surface of the gold seed (Figure 2B) without heating the aqueous solution of the gold seed (∼24 nm), H2PtCl6, and ascorbic acid, indicating that the heat treatment could obviously accelerate the kinetics of reduction of Pt salt by ascorbic acid on the colloidal gold surface. We also prepared one sample, under identical conditions used for preparing sample 1 (see above), except not adding gold seeds. It is found that more than 1 min was needed to reduce Pt salt (the solution color will change to gray-dark) in this case. The corresponding TEM images of the obtained Pt spheres are shown in Figure S1, Supporting Information. From the magnified image (Figure S1B, Supporting Information), it is observed that the surface of Pt nanospheres is relatively smooth. This indicates that Pt salt could probably preconcentrate on the surface of Au seeds during the reduction process and also the produced Pt by reduction could further self-catalyze the reaction between Pt salt and ascorbic acid, thus leading to well-defined Au/Pt hybrid NPs with urchinlike morphology. The chemical composition of Au/Pt hybrid NPs was determined by energy-dispersive X-ray spectroscopy (EDX) (Figure 4A). The EDX spectrum with two main peaks (Au and Pt) was

observed (other peaks originated from ITO glass substrate), indicating that the hybrid nanostructure was made up of metallic gold and platinum. It is known that X-ray photoelectron spectroscopy (XPS) is valuable for detecting the surface composition of samples, and depending on the experimental setup and elements presented, this technique has a detectable depth of 2-10 nm. XPS was further employed to investigate the composition of hybrid NPs. XPS patterns of these hybrid nanomaterials (Pt:Au at 2:1) show significant Pt4f signal corresponding to the binding energy of metallic Pt (Figure 4B) and weak Au4f signal characteristic of metallic Au (Figure 4C). This indicates that gold NPs inside the hybrid NPs (Pt:Au at 2:1) are hard to detect in this case due to the high-density urchinlike Pt shell existing on the surface of gold NPs (the detectable depth of X-ray is less than 10 nm). Thus, we can affirm that the Au/Pt hybrid NPs probably exist in the form of core/shell nanostructure. In addition, the above statement can further be supported by recent reports.3d,e For instance, Tian et al.3d recently reported a three-dimensional heterogeneous nucleation and growth of Pt on the surface of Au nanooctahedra. A bimetallic core-shell Au/Pt NP was obtained. The growth of the urchinlike Pt shell on the hybrid NPs surface could be readily monitored by UV-visible spectroscopy (UV-vis). Figure 5 shows the UV-vis spectra of the asprepared Au NPs and Au/Pt hybrid NPs. Before Pt growth, the solution displays a characteristic surface plasmon absorption (SPR) peak at 522 nm (Figure 5a). Upon Pt growth, the SPR of the hybrid nanostructure shows wide adsorption in the visible region. It is noted that the high-density urchinlike Pt shell on the surface of gold NPs (Figure 5c) exhibits a wider visible absorption peak than that of the low-density urchinlike Pt shell (Figure 5b). This may be attributed to the following facts. The urchinlike Pt shell has no characteristic absorption in the visible-near-infrared (Vis-NIR) spectroscopy, which is of importance in changing the optical absorption of the Au/Pt hybrid nanostructure. Second, the change in the dielectric that surrounds the gold may be an important factor as well as the scattering by the Pt shell.14 It should be noted that the result

A Novel Urchinlike Gold/Platinum Hybrid Nanocatalyst

Figure 5. UV-vis spectra of as-prepared gold NPs (a) and urchinlike Au/Pt hybrid NPs (b, c): Pt:Au at 1:1 (b) and Pt:Au at 2:1 (c) (∼25 nm gold NP as a seed).

Figure 6. CVs of O2 reduction at Pt:Au 1:1 hybrid NPs (a, b); Pt:Au 2:1 hybrid NPs (c); ∼25 nm gold NPs (e) modified gold electrode; and bare gold electrode (d) in air-saturated (line a, d and e) and in O2-saturated (lines b and c) 0.5 M H2SO4 solution. Scan rate: 50 mV/s.

from vis-NIR spectroscopy is in good accord with that from XPS because the urchinlike Pt shell on the surface of gold NPs did not homogeneously cover the gold particles. Several groups15,16 have revealed that Pt NPs could be used as a good building block for an alternate assembling tunable electrocatalyst for ORR with its special catalytic characteristics. The novel Au/Pt hybrid NPs mentioned here for the fabrication of electrochemical devices are an extremely promising prospect. Of central interest here is the electrocatalytic application of the hybrid NPs. The electrocatalytic activity of the as-prepared Au/ Pt hybrid NPs has been investigated for ORR. Figure 6a,b shows the typical cyclic voltammograms (CVs) of oxygen reduction at the Au/Pt hybrid NPs (low-density urchinlike Pt shell, 25 nm gold NPs as a seed) modified gold electrode in a 0.5 M H2SO4 solution in the presence of air and saturated oxygen. In the presence of air, a remarkable catalytic reduction current occurs at 0.46 V (Figure 6a) at a scan rate of 50 mV/s. Higher catalytic current for dioxygen reduction is observed at 0.40 V in the presence of saturated oxygen (Figure 6b), while no catalytic reduction current can be observed at the bare gold (Figure 6d) and gold NPs (Figure 6e) modified electrode in the potential range employed. It should be noted that the hybrid NPs with high-density urchinlike Pt shell modified gold electrode exhibits higher electrocatalytic current for dioxygen reduction (Figure 6c) than that of hybrid NPs with the low-density one (Figure 6b). In addition, the oxygen reduction potential (0.4 V) observed at the urchinlike hybrid NPs modified electrode is more positive than that of other Pt-based electrodes.17,18 For instance, our group

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Figure 7. CVs of ∼6 nm urchinlike Au/Pt hybrid NPs compared to that of ∼6 nm Pt NPs. CVs were run in a N2 sparged 0.5 M H2SO4 electrolyte at a scan rate of 100 mV/s. The loading is the same for each sample.

Figure 8. Current-potential curves for the reduction of air-saturated O2 at a rotating platinum ring-GC electrode with urchinlike Au/Pt hybrid NPs adsorbed on the disk electrode. The potential of the ring electrode was maintained at 1.0 V. Rotation rate: 100 rpm. Scan rate: 50 mV/s. The supporting electrolyte was 0.5 M H2SO4.

studied the electrocatalytic reduction of oxygen at the Pt-coated Au particle and they observed the reduction of oxygen at ∼0.1 V (Ag/AgCl).17 Crooks et al.18 reported that the dendrimer-encapsulated Pt NPs modified electrode showed the reduction peak at ∼0.25 V. Comparison of the results obtained at our electrode with the existing literature reveals that the urchinlike Au/Pt hybrid NPs modified electrode shows higher electrocatalytic activity. Furthermore, the ∼6 nm urchinlike Au/Pt hybrid NPs possesses higher electrochemical active area than that of ∼6 nm Pt NPs, as evidenced by the current associated with the hydrogen adsorption and desorption events (Figure 7). The above results indicate that the Au/Pt hybrid NPs have good electrocatalytic activity for ORR. Also, the ORR was probed through a RRDE experiment to demonstrate the ORR process of the urchinlike Au/Pt hybrid NPs modified electrode. It is found that the as-prepared hybrid NPs modified GC electrode reduces O2 by almost four electrons to H2O, as confirmed by the RRDE technique. Figure 8 shows the voltammetric curves for oxygen reduction, recorded at the RRDE with the Au/Pt hybrid NPs immobilized on the GC disk electrode. In this experiment, disk potential was scanned from +0.85 to 0.1 V, while the ring potential was kept at +1.0 V to detect any H2O2 evolved at the disk. A large disk current was obtained, whereas almost no ring current was observed, indicating that the as-prepared Au/Pt hybrid NPs reduce O2 by almost four electrons to H2O. From the ratio of the ring-disk current, the electron-transfer number (n) is calculated to be about 4 according to the equation n ) 4 - 2(IR/IDN).19 The electron-transfer reaction between Fe(CN)63- and S2O32was also employed as an example to further demonstrate the

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Figure 10. Pseudo-first-order plot of the kind used to calculate kinetic rates without Pt catalyst (a) and 6 nm Pt NPs (b) and urchinlike Au/Pt hybrid NPs (c).

which was reported to be of vital importance to high electrocatalytic performance. Thus, it is expected that this hybrid nanomaterial can probably be used for new functional building blocks to assemble new three-dimensional (3D) complex mutilcomponent nanostructures, e.g. multiwalled carbon nanotube/hybrid Au/Pt nanoarchitectures, which are believed to greatly enhance the electrocatalytic activity of Pt and be useful for electrochemical nanodevices and fuel cells, etc. Figure 9. UV-vis spectra change of Fe(CN)63- during the reaction between Fe(CN)63- and S2O32- at 20 °C in the presence of ∼6 nm urchinlike Au/Pt hybrid NPs (A) and ∼6 nm Pt NPs (B). The inset of part A shows the uncatalyzed reaction in the absence of Pt catalyst.

catalytic activity of the obtained urchinlike Au/Pt hybrid NPs.20 The decline of the Fe(CN)63- peaks with and without Au/Pt hybrid NPs as a function of time was monitored by the absorption at ∼420 nm (Figure 9). The absorption was monitored every 10 min for 60 min and the kinetic experiments were carried out at 20 °C. Without Au/Pt hybrid NPs, a small spectral decline was observed in the inset of Figure 9A. For urchinlike Au/Pt hybrid NPs (Figure 9A), the absorption spectra of Fe(CN)63- suffered a significant change in the band peak intensity, indicating the good catalytic activity of present hybrid NPs. As a comparison, the catalytic reaction between Fe(CN)63and S2O32- with ∼6 nm Pt NPs as a catalyst (the same amounts of Pt for ∼6 nm Au/Pt hybrid NPs and Pt NPs) was demonstrated, as shown in Figure 9B. A smaller spectral decline is observed than that of Au/Pt hybrid NPs, indicating that the present urchinlike Au/Pt hybrid NPs own higher catalytic activity than that of ∼6 nm Pt NPs. Figure 10 shows pseudofirst-order plots of -ln(A420 - A500) against time for the uncatalyzed and catalyzed reactions. The rate constant values for the reaction in the absence (a) and presence of ∼6 nm Pt NPs (b) and Au/Pt hybrid NPs (c) were facilely obtained from the slope of the straight lines to be 0.0008, 0.0115, and 0.0133 min-1, respectively. The catalytic efficiency in the presence of urchinlike Au/Pt hybrid NPs is 16-fold larger than that in the absence of Pt catalyst, which further indicates the as-prepared Au/Pt hybrid NPs own good catalytic efficiency. Considering the particular structure of urchinlike Au/Pt hybrid NPs, two important factors should be responsible for the above high electrocatalytic activity. First, the Pt shell on the surface of gold NPs is in the form of an urchinlike structure. These urchinlike structures (Figure 1A,B) own porosity, which will increase the surface-to-volume ratios and utilization efficiency of Pt. Second, the size of urchinlike Pt NPs is less than 10 nm,

Conclusions In summary, we have reported a novel approach to rapidly synthesize quasimonodisperse urchinlike Au/Pt hybrid NPs with controlled sizes. Catalytic experiments for oxygen electrocatalytic reduction and electron-transfer reaction between hexacyanoferrate(III) ions and thiosulfate ions indicate that the present urchinlike Au/Pt hybrid NPs own high catalytic activity. It is expected that the as-prepared Au/Pt hybrid NPs, combined with supporting materials such as carbon nanotubes and mesoporous carbon, could be used as advanced cathode material for fuel cells in the future. Acknowledgment. This work was supported by the National Science Foundation of China (Nos. 20575063, 20575064, and 20675076) and the 973 Project 2007CB714500. Supporting Information Available: Typical TEM images of as-prepared Pt spheres in the absence of gold seeds at different magnification. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Service, R. F. Science 1999, 285, 682. (b) Kordesch, K. V.; Simader, G. R. Chem. ReV. 1995, 95, 191. (c) Hrapovic, S.; Liu, Y. L.; Male, K. B.; Luong, J. H. T. Anal. Chem. 2004, 76, 1083. (d) Rouxoux, A.; Schulz, J.; Patin, H. Chem. ReV. 2002, 102, 3757. (2) (a) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A. Science 1996, 272, 1924. (b) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 12663. (3) (a) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974. (b) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732. (c) Maksimuk, S.; Yang, S.; Peng, Z.; Yang, H. J. Am. Chem. Soc. 2007, 129, 8684. (d) Fan, F.-R.; Liu, D.-Y.; Wu, Y.-F.; Duan, S.; Xie, Z.-X.; Jiang, Z.-Y.; Tian, Z.-Q. J. Am. Chem. Soc. 2008, 130, 6949. (e) Song, J. H.; Kim, F.; Kim, D.; Yang, P. Chem. Eur. J. 2005, 11, 910. (f) Guo, S.; Fang, Y.; Dong, S.; Wang, E. J. Phys. Chem. C 2007, 111, 17104. (g) Guo, S.; Dong, S.; Wang, E. Chem. Eur. J. 2008, 14, 4689– 4695. (4) Xiong, Y.; Wiley, B. J.; Xia, Y. Angew. Chem., Int. Ed. 2007, 46, 7157. (5) Sun, S. H.; Yang, D. Q.; Villers, D.; Zhang, G. X.; Sacher, E.; Dodelet, J. P. AdV. Mater. 2008, 20, 571.

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