Synthesis, Characterization, and Physicochemical Properties of Well

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Synthesis, Characterization, and Physicochemical Properties of Well-Coupled Y2O3 Nanobelt-Ag Nanocrystals Nanocomposites Min Han,† Xun Li,† Baojun Li,† Naien Shi,† Kunji Chen,‡ Jianming Zhu,‡ and Zheng Xu*,† State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, and State Key Laboratory of Solid State Microstructure, Nanjing UniVersity, Nanjing 210093, People’s Republic of China ReceiVed: July 10, 2008; ReVised Manuscript ReceiVed: September 17, 2008

In this paper, Y2O3 nanobelt-Ag nanocrystals multifunctional nanocomposites have been prepared by thermal decomposition of solid metal nitrate salts in dodecylamine and 1-octadecene mixed solvents from room temperature to 320 °C. For each synthesis, gram-scale nanocomposites can be easily produced. Microstructure characterization results demonstrate that Ag nanocrystals are well-dispersed and partially embedded in Y2O3 nanobelts to form well-coupled nanocomposites. Significant enhancement of dielectric performance and modification of optical absorption as well as the excellent catalytic hydrogenation performance are observed in the synthesized nanocomposites compared with those of bare Y2O3 nanobelts or pure Ag nanocrystals. Introduction The integration and hybridization of different functional nanostructures to fabricate multicomponent nanocrystals or heterostructural nanocomposites has been proved to be a promising strategy for creating novel nanomaterials with desired performance. They will not only exhibit the multifunctionalities of their components but also may generate new collective properties arising from the strong synergistic interaction of their components.1 Heterodimers of Au-Fe3O4 and CdS-FePt nanocrystals,2,3 dumbbell-shaped nanostructures composed of zero-dimensional (0D) nanospheres and one-dimensional (1D) nanorods,4 and complicated multicomponent nanostructures made up of tetrapod (CdTe)-tetrapod (CdS)-tetrapod (CdTe)5 are such good examples. As a class of important functional nanostructures, the quasi-1D nanobelt (NB) has been widely studied since its discovery in 2001,6 which has appropriate thickness, flat and well-defined surfaces, and can serve as an ideal miniature substrate for the integration and growth of other functional nanomaterials on it to form advanced nanocomposites. Unfortunately, up to now, NB-based nanocomposites have not been effectively synthesized and systematically investigated. To the best of our knowledge, only SnO2 (or ZnO) NB-ZnO (or SnO2) nanocomposites, SnO2 NB-Co0.05Ti0.95O2 nanotapes, and SnO2-Cu bilayer nanoribbons have been synthesized via the conventional thermal evaporation method or pulsed laser deposition technology.7 Therefore, exploring a simple chemical method to fabricate other NB-based nanocomposites under mild conditions and studying their properties are urgently needed and continue to be a key challenge issue in current chemistry and material science. As one of the rare earth sesquioxides, Y2O3 nanomaterials have been attracting much attention because they possess some unusual properties and have potential applications in many industries, such as high-performance ceramics, catalysis, electronic engineering, and phosphors.8 Recently, Y2O3 NBs have been successfully synthesized at gram scale by our research * Corresponding author. E-mail: [email protected]. Fax: +8625-83314502. Tel.: +86-25-83593133. † School of Chemistry and Chemical Engineering. ‡ State Key Laboratory of Solid State Microstructures.

group via a solid-liquid phase chemical route.9 The obtained Y2O3 NBs possess high dielectric constant and can be viewed as an ideal medium material for integrating other functional nanounits to construct novel quasi-1D dielectric nanocomposites. Considering that noble-metal nanocrystals (NCs) possess unique optical, electrical, and catalytic properties,10 positioning and confining them on the surfaces of Y2O3 NBs to form wellcoupled metal-dielectric nanocomposites will not only modulate the performance of Y2O3 NBs and noble-metal NCs but also may generate novel physical and chemical properties due to the strong quantum confinement and synergistic effect, which will affect electronic communication across the interfaces between Y2O3 NBs and metal NCs and lead to drastic change of the local electronic structures of Y2O3 NBs and metal NCs. Moreover, such nanocomposites can be used as a “secondary platform” for epitaxial growth of some useful semiconductor nanomaterials to form more complicated heteronanostructures, offering a promising opportunity to realize a variety of functionalities. So, the combination of noble-metal NCs with Y2O3 NBs to form well-coupled nanocomposites and investigation of their physicochemical properties is carried out. It is not only important for fundamental research but also for exploring novel functional material to be applied in industrial fields. Here, we report the synthesis, characterization, and properties of well-coupled Y2O3 NB-Ag NCs nanocomposites. The typical synthesis of those nanocomposites is based on thermal decomposition of solid Y(NO3)3 · 6H2O and AgNO3 in the mixed solvent of dodecylamine and 1-octadecene. The adding sequence and amount of those solid precursors and the adding time of AgNO3 are the important factors that can affect the quality of the final formed Y2O3 NB-Ag NCs nanocomposites. In order to obtain well-coupled Y2O3 NB-Ag NCs nanocomposites, the preformation of highly crystallized Y2O3 NBs is not desirable because they have not enough defect sites on their surfaces for the nucleation and growth of Ag NCs. So, control of the crystallization degree of preformed Y2O3 NBs is vital and prerequisite, which can be realized by adjusting the reaction temperature and time. The optimized synthetic process is shown in the Experimental Section. For each synthesis, well-coupled Y2O3 NB-Ag NCs nanocomposites can be easily obtained at the gram scale. Microstructure analysis results demonstrate that

10.1021/jp806119w CCC: $40.75  2008 American Chemical Society Published on Web 10/25/2008

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Ag NCs are well-dispersed and partially embedded in Y2O3 NB matrix to form well-coupled nanocomposite, which inhibits the migration of Ag NCs and increases the mutual interactions of them with the Y2O3 NB. Significant enhancement of dielectric performance and modification of optical absorption as well as the excellent catalytic hydrogenation performance are observed in the synthesized Y2O3 NB-Ag NCs nanocomposites compared with those of bare Y2O3 NBs or pure Ag NCs. Experimental Section Synthesis of Y2O3 NB-Ag NCs Nanocomposites. A solid sample of Y(NO3)3 · 6H2O (2.05 g), dodecylamine (8 mL), and 1-octadecene (10 mL) were added into a clean and dry threeneck flask at room temperature. Then the reactor was heated to 320 °C at a rate of 8 °C · min-1. After 20 min, another solid sample of AgNO3 (0.1 g) was rapidly added into the reaction system, and the reactor was maintained at 320 °C for 10 min. Finally, the reactor was naturally cooled down to room temperature. The crude products were separated and redispersed in heptane or hexane. When 50 mL of absolute ethanol was added into it, the yellow-brown precipitates were obtained, which were separated by centrifugation at 12 000 rpm and washed with heptane and absolute ethanol several times to remove the solvent and byproduct. Finally, the precipitates were dried in vacuum at 60 °C for 4 h and used for further characterization. Synthesis of Bare Y2O3 NBs. A solid sample of Y(NO3)3 · 6H2O (2.05 g), dodecylamine (8 mL), and 1-octadecene (10 mL) were added into a 250 mL three-neck flask. Subsequently, the reactor was heated from room temperature to 320 °C at a rate of 8 °C · min-1 and maintained at 320 °C for 30 min. The posttreatment processes were the same as those for Y2O3 NB-Ag NCs nanocomposites. Synthesis of Pure Ag NCs (8 to ∼10 nm). A solid sample of AgNO3 (0.1 to ∼3.0 g), dodecylamine (8 mL), and 1-octadecene (10 mL) were added into a 250 mL three-neck flask. Subsequently, the reactor was heated from room temperature to 320 °C at a rate of 8 °C · min-1 and maintained at 320 °C for 10 min. The post-treatment processes were the same as those for Y2O3 NB-Ag NCs nanocomposites. Characterization. The powder X-ray diffraction (XRD) pattern was recorded on an X’Pert Pro MPD diffractometer (PANalytical) with Ni-filtered Cu KR radiation (λ ) 1.5418 Å) in 2θ ranging from 15° to 80°. The corresponding work voltage and current are 40 kV and 40 mA, respectively. X’Pert High Score software was used to deal with the acquired diffraction data. The X-ray photoelectron spectroscopy (XPS) spectrum was recorded on a VG ESCALAB MKII. The accelerating voltage and working current are 12.5 kV and 20 mA, respectively. The corresponding pressure is 2 × 10-8 mbar. To avoid charge effects, the sample was mixed with graphite powder. This method allowed the sample to be finely dispersed and intimately contacted with the conducting graphite. Transmission electron microscopy (TEM) images were taken on a JEM-200CX instrument (Japan), using an accelerating voltage of 200 kV. High-resolution TEM (HRTEM) images were obtained on a JEOL-2010 (Japan) at an accelerating voltage of 200 kV. The ac electrical properties of the samples were measured using an HP 4194A impedance analyzer at a frequency of 103 to 106 Hz at room temperature. Absorption spectra were collected at room temperature on a Shimadzu UV-vis absorption diode array spectrometer. Catalytic Hydrogenation of Nitro Compounds. The hydrogenation reaction was carried out in a 100 mL stainless steel

Figure 1. XRD pattern of the synthesized Y2O3 NB-Ag NCs nanocomposites. The black solid dots denote the Y2O3 component while the hollow triangles stand for the Ag component.

autoclave. For each reaction, aromatic nitro compound (0.5 g) in 25 mL of ethanol was placed into the autoclave together with 0.10 g of Y2O3 NB-Ag NCs nanocomposites. o-Xylene was always used as internal standard for the posterior determination of conversion level and yields. After sealing the autoclave, its air content was purged by flushing three times with hydrogen. Next, the autoclave was pressurized at 2.0 MPa of hydrogen and finally heated up to 140 °C for 4-6 h. During the experiment, the stirring rate was fixed at 800 rpm (mechanical stirring). At the end of the reaction, the autoclave was naturally cooled down to ambient temperature and flushed two times with nitrogen. The nanocomposites were separated by centrifugation at 12 000 rpm, recovered, and washed with ethanol (3 × 5 mL) for reuse. The combined organic phase was dried over anhydrous Na2SO4 and evaporated under reduced pressure. The identification of the reaction products and intermediates in catalytic hydrogenation experiments was performed on a GC/MS-QP2010 (Shimadzu) instrument equipped with a DB-ms capillary column by comparing with the authentic samples. Results and Discussion The components and phase structure of the as-synthesized Y2O3 NB-Ag NCs nanocomposites are examined by powder XRD (Figure 1). Nine obvious and broad diffraction peaks are observed. In comparison with the Joint Committee on Powder Diffraction standard cards (JCPDS-74-1828 and JCPDS-040783), those diffraction peaks can be indexed as (200), (211), (220), (222), (420), (333), (026), (046), and (831) planes of the body-centered cubic phase (bcc) of Y2O3 and (111), (200), (220), and (311) planes of the face-centered cubic phase (fcc) of Ag. The results demonstrate that the obtained nanocomposites are composed of Y2O3 and Ag with bcc and fcc structure, respectively. The XPS results shown in Figure 2 further confirm the components of the nanocomposites and also provide some useful information on the electron configurations of the nanocomposites. The binding energies of Y3d3/2 and Y3d5/2 in the nanocomposites are 158.52 and 156.64 eV, respectively (red plot in Figure 2A). In comparison with those of bare Y2O3 NBs (black plot in Figure 2A), the binding energies of Y3d3/2 and Y3d5/2 in Y2O3 NB-Ag NCs nanocomposites increase 0.31 and 0.20 eV, respectively, and the relative intensities of them have an obvious change, implying that the electron configurations of Y2O3 NBs have been altered at a certain degree when they are hybridized with Ag NCs. At the same time, the binding energies of Ag3d3/2 and Ag3d5/2 (blue plot in Figure 2B) in the

Y2O3 Nanobelt-Ag Nanocrystals Nanocomposites

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Figure 2. XPS spectra of the synthesized Y2O3 NB-Ag NCs nanocomposites and the control samples: (A) magnified analysis of Y3d in the nanocomposites (red plot) and bare Y2O3 NBs (black plot) at the energy range from 164 to 151 eV; (B) magnified analysis of Ag3d in the nanocomposites (blue plot) and pure Ag NCs (black plot) at the energy range from 378 to 362 eV.

Figure 3. TEM images of Y2O3 NB-Ag NCs nanocomposites: (A) low-magnification TEM image of the nanocomposites; (B) high-magnification TEM image of the single nanocomposite; (C) HRTEM image of an individual nanocomposite recorded at the middle section of a Y2O3 NB; the white arrows show the interface regions between the Y2O3 NB and Ag NC; (D) HRTEM image of an individual nanocomposite recorded at one side edge of a Y2O3 NB; the white arrow with a labeled ellipse clearly shows the concave interface between the Ag NC and Y2O3 NB. Corresponding magnified HRTEM images are shown in the Supporting Information, Figure S1. For comparison, the TEM images of bare Y2O3 NBs and pure Ag NCs samples are also given, shown in the Supporting Information, Figure S2.

nanocomposites decrease 0.28 and 0.22 eV compared with those of pure Ag NCs (black plot in Figure 2B), indicating that hybridization with Y2O3 NBs will also affect the electron configurations of Ag NCs. These results reveal that Y2O3 NBs and Ag NCs have been well-coupled in the nanocomposites. By integral calculation, the peak area ratio of Ag and Y2O3 is ∼1:22, revealing that the weight percent of Ag NCs in the nanocomposites is ∼2.20%. The microstructure features of Y2O3 NB-Ag NCs nanocomposites are characterized by TEM and HRTEM (Figure 3). The low-magnified TEM image shown in Figure 3A exhibits largearea Y2O3 NB-Ag NCs nanocomposites. The corresponding

magnified TEM image of an individual composite nanostructure (Figure 3B) reveals that Ag NCs are dispersed on a Y2O3 NB and no obvious aggregates are observed. Statistical analysis results show that the size distribution of the Ag NCs (inset of Figure 3B) is narrow, about 8 to ∼10 nm. The HRTEM image (Figure 3C) of the individual nanocomposite taken at the middle section of the Y2O3 NB demonstrates that there are clear interfaces between crystalline Y2O3 and Ag NCs on the surface of the Y2O3 NB, forming a nice heterojunction array. Moreover, from the HRTEM image recorded at one side edge of the Y2O3 NB (Figure 3D), we can see that a Ag NC is partially embedded in the Y2O3 NB, which inhibits the migration and aggregation

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Figure 4. Dielectric feature of the samples measured at the frequency ranging from 103 to 106 Hz at room temperature: (A) dielectric constant of Y2O3 NB-Ag NCs nanocomposites (red line) and bare Y2O3 NBs (black line); (B) dielectric loss of the nanocomposites (red line) and the bare Y2O3 NBs (black line).

Figure 5. UV-vis absorption spectra of the samples: (A) Y2O3 NB-Ag NCs nanocomposites; (B) pure Ag NCs.

of the Ag NC and enhances the interaction of the Ag NC and the Y2O3 NB.11 The concave interface between the Y2O3 NB and the Ag NC is observed, and its thickness is measured to be ∼3 nm. The well-crystallized region of the Y2O3 NB shows clear fringes with lattice spacing of 3.1 Å, which corresponds to the interplanar separation between the (222) plane of bcc Y2O3. Clear lattice fringes of Ag NCs are also observed, giving lattice spacing of 2.9 Å corresponding to interplanar separation between the (110) plane of fcc Ag. The frequency-dependent dielectric constant and dielectric loss (tangent δ) curves of Y2O3 NB-Ag NCs nanocomposites are shown in Figure 4. The red line shown in Figure 4A gives the dielectric constant of the nanocomposites measured at the frequency ranging from 103 to 106 Hz at room temperature. In comparison with that of bare Y2O3 NBs (black line in Figure 4A) and bulk powder Y2O3 (∼18),12 the dielectric constant of the nanocomposites is significantly increased at the measured frequency range. Two mechanisms are usually used to explain the frequency-dependent nature of the enhanced dielectric permittivity in metal/metal oxide nanocomposite dielectrics. One is space charge polarization, and the other is a dipole polarization.13 Considering that the size of Ag NCs in our nanocomposites is well above the conductor/insulator transition threshold of Ag (∼3 nm),14 the dipole polarization mechanism cannot play the dominant role, and the increased dielectric constant may be mainly attributed to the space charge polarization near the interfaces between Ag NCs and Y2O3 NBs. Normally, the piling of charges at the extended interface will result in increased conductivity and high dielectric loss. However, the dielectric loss of our nanocomposites (red line in Figure 4B) is less than that of bare Y2O3 NBs (black line in Figure 4B) measured at the same frequency range. A similar phenomenon has been

observed in Ag NPs/PbTiO3 composite films15 and Ag/carbon black/epoxy nanocomposites.16 The abnormal dielectric loss may be attributed to the Coulomb blockade effect, the well-known quantum effect of metal nanoparticles, which reduces the electron tunneling.16 As such, it reduces the conduction loss part from the total dielectric loss of the dielectric composite system. Our experimental results demonstrate that the dielectric performance of Y2O3 NBs can be modulated and optimized by hybridization of them with Ag NCs. Moreover, we found that the loading level and manner of Ag NCs have a great impact on the dielectric behavior of the nanocomposites, which will be discussed elsewhere. The UV-vis absorption spectrum of Y2O3 NB-Ag NCs nanocomposites exhibits one broad absorption peak, shown in Figure 5A. The broad peak, maximum value at about 430 nm, arises from the surface plasma resonance (SPR) absorption of the Ag NCs in the nanocomposites. In comparison with that of pure Ag NCs (Figure 5B), the SPR band of the Ag NCs in the nanocomposites broadens and extends toward the longwavelength region. This is reasonable because the SPR bands of noble-metal NCs are sensitive to their size, size distribution, and shape, as well as to the nature of the surrounding media.10a,17 Here, the size and shape of Ag NCs in the nanocomposites and in the pure Ag NCs sample are nearly the same, and only their media environment has been greatly altered. Therefore, the change of optical spectrum of Ag NCs in the nanocomposites is mainly attributed to the change of the local electrical field by hybridization of them with the Y2O3 NB, exhibiting strong dielectric confinement effect.18 Furthermore, the catalytic performance of Y2O3 NB-Ag NCs nanocomposites is also examined. Catalytic hydrogenation of aromatic nitro compounds is selected as the model reaction to

Y2O3 Nanobelt-Ag Nanocrystals Nanocomposites SCHEME 1: Chemical Reaction Formula for Hydrogenation of Various Aromatic Nitro Compounds Using Y2O3 NB-Ag NCs Nanocomposites as Catalyst

evaluate the performance of the nanocomposites. For comparison, the catalytic performances of both bare Y2O3 NBs and pure Ag NCs prepared under the same conditions as that for Y2O3 NB-Ag NCs nanocomposites are also measured. The results reveal that both bare Y2O3 NBs and pure Ag NCs have no obvious catalytic activity. However, Y2O3 NB-Ag NCs nanocomposites exhibit unprecedented catalytic performance under the same reaction conditions. It can catalyze hydrogenation of various aromatic nitro compounds to corresponding amines with complete conversion (100%) at 140 °C for 4-6 h, which is similar to or better than that of the recently reported Pt/γ-Fe2O3 nanocomposite,19 Ru/SnO2 nanocomposite,20 and Ag NPs/SiO221 catalysts. The experimental results are summarized in Supporting Information Table S1, and the corresponding chemical formula is shown in Scheme 1. It should be mentioned that hydrodehalogenation is an undesired side reaction, which usually takes place during hydrogenation of halo nitro aromatics to the corresponding amines.22 However, with the use of Y2O3 NB-Ag NCs nanocomposites as the catalyst, the hydrodehalogenation phenomenon can be completely restrained. When other reducible groups like CC or CN multiple bonds, -COOH, and -CHO coexist with the nitro group, only the nitro group is reduced except for the case of hydrogenation of m-nitrobenzaldehyde, in which partial -CHO (∼12.6%) is reduced to -CH2OH. Moreover, catalytic dynamic experiments demonstrate that there is no accumulation of hydroxylamine intermediates observed in our catalytic process, which resolves the stubborn problems encountered in the current nitro compounds hydrogenation industrial field.22 On the basis of the current experimental results, the origin of the excellent catalytic performance of the nanocomposites may be attributed to the strong synergistic interaction of Ag NCs with Y2O3 NBs. The detailed catalytic hydrogenation process or mechanism is not fully clear at present, which requires in situ characterization means under high pressure to provide more convincing information. Further work on this is underway. Conclusion In conclusion, Y2O3 NB-Ag NCs multifunctional nanocomposites have been successfully prepared by direct thermal decomposition of solid metal nitrate in 1-octadecene and dodecylamine mixed solvents from room temperature to 320 °C. The fabrication method is quite simple, and gram-scale nanocomposites can be produced for each batch. TEM and HRTEM results reveal that Ag NCs are well-dispersed and partially embedded in the Y2O3 NB to form well-coupled nanocomposites. In comparison with those of bare Y2O3 NBs or pure Ag NCs, the obtained Y2O3 NB-Ag NCs nanocomposites exhibit enhanced dielectric performance and broadened optical absorption, as well as excellent catalytic hydrogenation performance. This work not only promotes the development of current composite nanostructure research in nonhydrolytic systems but also paves the way for the fabrication of a nanobelt-metal nanocrystals based functional nanodevice. In addition, it also provides some useful information on the rational design of high-performance nanocatalysts.

J. Phys. Chem. C, Vol. 112, No. 46, 2008 17897 Acknowledgment. We acknowledge the financial support from the National Natural Science Foundation of China (NSFC) under the major research program of nanoscience and nanotechnology no. 90606005, and major project no. 20490210, project no. 20571040, and Postdoctoral Research Start-up Foundation of Nanjing University (no. 0210 0030 25). Moreover, we greatly appreciate Professor Shuyuan Zhang (University of Science & Technology of China, Structure Research Laboratory, Hefei, Anhui 230026, People’s Republic of China) for his help with HRTEM characterization and analysis. Supporting Information Available: HRTEM images of the Y2O3 NB-Ag NCs nanocomposites (Figure S1), TEM images of bare Y2O3 NBs and pure Ag NCs (Figure S2), catalytic experimental data (Table S1), and the dynamic experiment results. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Jun, Y. W.; Choi, J. S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (b) Zeng, H.; Sun, S. H. AdV. Funct. Mater. 2008, 18, 391. (2) (a) Yu, K.; Chen, M.; Rice, P. M.; Wang, S. X.; White, R. L.; Sun, S. H. Nano Lett. 2005, 5, 379. (b) Li, Y.; Zhang, Q.; Nurmikko, A. N.; Sun, S. H. Nano Lett. 2005, 5, 1689. (c) Xu, C. J.; Xie, J.; Ho, D.; Wang, C.; Kohler, N.; Walsh, E. G.; Morgan, J. R.; Chin, Y. E.; Sun, S. H. Angew. Chem., Int. Ed. 2008, 47, 173. (d) Glaser, N.; Adams, D. J.; Bo¨ker, A.; Krausch, G. Langmuir 2006, 22, 5227. (3) Gu, H. W.; Zheng, R. K.; Zhang, X. X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664. (4) (a) Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787. (b) Mokari, T.; Sztrum, C. G.; Salant, A.; Rabani, E.; Banin, U. Nat. Mater. 2005, 4, 855. (c) Kudera, S.; Carbone, L.; Casula, M. F.; Cingolani, R.; Falqui, A.; Snoeck, E.; Parak, W. J.; Manna, L. Nano Lett. 2005, 5, 445. (5) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190. (6) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (b) Shi, W. S.; Peng, H. Y.; Wang, N.; Li, C. P.; Xu, L.; Lee, C. S.; Kalish, R.; Lee, S.-T. J. Am. Chem. Soc. 2001, 123, 11095. (7) (a) Sun, S. H.; Meng, G. W.; Zhang, G. X.; Zhang, L. D. Cryst. Growth Des. 2007, 7, 1988. (b) Zhao, J. W.; Ye, C. H.; Fang, X. S.; Qin, L. R.; Zhang, L. D. Cryst. Growth Des. 2006, 6, 2643. (c) He, R.; Law, M.; Fan, R.; Kim, F.; Yang, P. Nano Lett. 2002, 2, 1109. (d) Law, M.; Zhang, X. F.; Yu, R.; Kuykendall, T.; Yang, P. Small 2005, 1, 858. (8) (a) Fang, Y. P.; Xu, A. W.; You, L. P.; Song, R. Q.; Yu, J. C.; Zhang, H. X.; Li, Q.; Liu, H. Q. AdV. Funct. Mater. 2003, 13, 955. (b) Bazzi, R.; Flores, M. A.; Louis, C.; Lebbou, K.; Zhang, W.; Dujardin, C.; Roux, S.; Mercier, B.; Ledoux, G.; Bernstein, E.; Perriat, P.; Tillement, O. J. Colloid Interface Sci. 2004, 273, 191. (c) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256. (d) Guzman, J.; Corma, A. Chem. Commun. 2005, 743. (e) Alarcon-Flores, G.; AguilarFrutis, M.; Falcony, C.; Garcia-Hipolito, M.; Araiza-Lbarra, J. J.; HerreraSuarez, H. J. J. Vac. Sci. Technol., B 2006, 24, 1873. (f) Yang, Z. K.; Lee, W. C.; Lee, Y. J.; Chang, P.; Huang, M. L.; Hong, M.; Hsu, C. H.; Kwo, J. Appl. Phys. Lett. 2007, 90, 152908. (g) Wang, H.; Uehara, M.; Nakamura, H.; Miyazaki, M.; Maeda, H. AdV. Mater. 2005, 17, 2506. (9) Han, M.; Shi, N. E.; Zhang, W. L.; Li, B. J.; Sun, J. H.; Chen, K. J.; Zhu, J. M.; Wang, X.; Xu, Z. Chem. Eur. J. 2008, 14, 1615. (10) (a) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) Feldheim, D. L.; Foss, C. A. Metal Nanoparticles: Synthesis, Characterization, and Applications; Marcel Dekker, Inc.: New York, 2002. (c) Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44, 7852. (d) Lee, S. J.; Morrill, A. R.; Moskovits, M. J. Am. Chem. Soc. 2006, 128, 2200. (e) Shen, Z.; Duan, H. W.; Frey, H. AdV. Mater. 2007, 19, 349. (f) Jana, N. R.; Pal, T. AdV. Mater. 2007, 19, 1761. (g) Chen, J.; Wiley, B.; Mcllellan, J.; Xiong, Y.; Li, Z. Y.; Xia, Y. Nano Lett. 2005, 5, 2058. (11) Yoon, B.; Hakkinen, H.; Landman, U.; Worz, A. S.; Antonietti, J. M.; Abbet, S.; Judai, K.; Heiz, U. Science 2005, 307, 403. (12) Leskela, M.; Kukli, K.; Ritala, M. J. Alloys Compd. 2006, 418, 27. (13) Ravindran, R.; Gangopadhyay, K.; Gangopadhyay, S.; Mehta, N.; Biswas, N. Appl. Phys. Lett. 2006, 89, 263511. (14) Banerjee, S.; Chakravorty, D. Appl. Phys. Lett. 1998, 72, 1027. (15) Tang, L.; Du, P.; Han, G.; Weng, W.; Zhao, G.; Shen, G. Surf. Coat. Technol. 2003, 167, 177.

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