Photoinduced Synthesis of Anisotropic Gold Nanoparticles in Room

J. Phys. Chem. C 2007, 111, 7629-7633

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Photoinduced Synthesis of Anisotropic Gold Nanoparticles in Room-Temperature Ionic Liquid Jinmiao Zhu, Yuhua Shen,* Anjian Xie,* Lingguang Qiu, Qiang Zhang, and Shengyi Zhang School of Chemistry and Chemical Engineering, Anhui UniVersity, Hefei 230039, People’s Republic of China ReceiVed: February 12, 2007; In Final Form: April 1, 2007

Anisotropic gold nanoparticles have been prepared by the photochemical reduction in the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) without any additional capping agent. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and Fourier transform infrared spectroscopy have been used to characterize the as-prepared products. The results show that especial shape gold particles can be controlled by [BMIM][BF4] and reaction conditions such as reaction time and reagent concentration. The [BMIM][BF4] is a reaction medium, template, and capping agent. Under different reaction conditions, various morphologies such as sheet (triangle and hexagon) polyhedron of gold nanoparticles can be obtained. The mechanisms of photochemical reduction reaction and controlled growth of gold nanoparticles have also been discussed.

Introduction Room-temperature ionic liquids (RTILs) are attracting much interest in many fields of chemistry and industry due to their potential as a “green” recyclable alternative to the traditional organic solvents.1-4 Because of its unique physical and chemical properties, such as large electrochemical window, polar but low interface tension, low interface energies, high thermal stability, and extended hydrogenbond systems,5-11 the latest developments of RTILs as a reaction medium for inorganic nanomaterials have received much attention and could offer many opportunities and challenges for the synthesis of nanoparticles with unique shape and structures.12-13 Novel nanostructures can be produced by selecting a suitable RTILs reaction medium; TiO2,14 metal nanoparticles,1,13,15-18 Te nanorods,19 Si,20 CoPt,21 M2S3 (M ) Bi, Sb) nanorods,22 CuCl nanoplatlets,23 Bi2S3 flowers,24 and porous silica25-27 have been synthesized, as reported recently. Metal nanoparticles play important roles in many different areas such as optical, electronic, catalysis, medical, magnetic, information storage, and surface-enhanced Raman scattering (SERS). The intrinsic properties of a metal nanoparticle are mainly determined by its size, shape, composition, crystallinity, structure, etc.28 The control of nanoparticle size and a better understanding of their chemical behavior have attracted considerable interest because of their size- and shape-dependent physicochemical properties.29 Anisotropic metal nanoparticles exhibit optical properties of significant technological interest including enhanced fluorescence,30 nonlinear optical properties, optical resonances in the near-infrared (NIR), and orientationdependent plasmon excitation.31 In addition, unlike spherical nanoparticles anisotropic nanoparticles often possess different atomic planes and/or increased surface area that can be exploited for enhanced or selective catalysis of a variety of chemicals.32 Thus, the development of bulk solution synthetic methods that offer shape control is of paramount importance if the full * To whom correspondence should be addressed. E-mail: (Y.S.) [email protected]; (A.X.) [email protected]. Tel.: +86-551-5108090. Fax: +86-551-5107342.

potential of these materials is to be realized. Some methods of solution synthesis use thermal reduction of metal ion salts, and they yield relatively small quantities of the desired particle shape.33 Some electrochemical methods have been developed to produce metal nanoparticles.34-39 Light has also been used to modify the shape of nanoparticles.40 Jin et al. demonstrate the previously described photoinduced method for converting silver nanospheres into triangular silver nanocrystals-nanoprisms.41 One classic metal nanoparticles system is gold. Many methods have been introduced to prepare Au nanoparticles. However, a major drawback is that they use an external reducing agent, an additional capping agent, and the condition of reaction is very rigorous. Herein, we report a UV-assisted method, which does not require the use of capping reagents, for Au nanosheets and polyhedrons under mild reaction conditions. The synthesis method is simple but very effective. In this technology, RTILs are used as reaction medium and capping reagent. We demonstrate that continuous wave UV irradiation is able to control the growth of gold nanoparticles in RTILs. The large structural anisotropy of gold particles has been gained, and these particles should substantially influence their optical properties, including light absorption, scattering, and SERS. Experimental Section Chemicals. Tetrachloroauric acid tetrahydrate (HAuCl4‚ 4H2O), acetone, and anhydrous ethanol were of analytical grade and were obtained from Shanghai Chemical Reagent Co. Ltd. (China). 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]) was purchased from Kemo Chemical Reagent Co. Ltd. (China). The chemical structure of [BMIM][BF4] is shown in Scheme 1. All chemicals were used without further purification. Doubled distilled water was used in our experiments. Synthesis of Gold Nanoparticles. In a typical synthesis, 0.1 g HAuCl4‚4H2O was dissolved in a mixed solution of [BMIM][BF4] (1 mL) and acetone (0.1 mL) in a 1 cm × 1 cm × 3 cm quartz cuvette. The cuvette was displayed in a darkroom and the distance between the side face of cuvette and light source

10.1021/jp0711850 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/08/2007

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approached to 2 cm. The mixture was irradiated by UV light (Pyrex filter, ∼254 nm, 450 W Hanovia medium-pressure lamp) for 8 h. When chloroauric acid was added into [BMIM][BF4], the color of solution varied from slightly yellow to pink immediately. With the reaction processing, the color of the solution changed to dark red gradually. The final mixture was diluted with ethanol and the precipitates obtained were centrifuged and were washed several times using doubly distilled water and absolute ethanol. Further experiments were also conducted under different conditions, using procedures similar to those presented above. Characterization. Samples for Fourier transform infrared spectroscopy (FTIR) spectroscopy were prepared by dropcoating films on a KBr pellet. FTIR spectroscopy measurements were recorded with a Nexus 870 FTIR spectrophotometer with a resolution of 4 cm-1 (America Nicolet Co.). Scanning electron microscopy (SEM) measurements were taken with a Leica Stereoscan-440 scanning electron microscope. Transmission electron microscopy (TEM) measurements were performed on a JEM model 100SX electron microscope instrument (Japan Electron Co.) operated at an accelerating voltage at 80 kV. Selected area electron diffraction (SAED) on the Au nanoparticles was also obtained. X-ray photoelectron spectroscopy (XPS) was carried out on an Escalab M KII X-ray photoelectron spectrometer with Mg KR X-rays as the excitation source (photon energy ) 1253.6 eV). The phase structure and phase purity of the as-synthesized products were examined by X-ray diffraction (XRD) using a MAC Science Co. Ltd. MXP 18 AHF X-ray diffractometer with monochromatized Cu KR radiation (λ ) 1.54056 Å).

Figure 1. X-ray photoelectron spectra of the Au nanoparticles prepared via photoinducing 0.1 g/mL HAuCl4‚4H2O/[BMIM][BF4] solution for 8 h. (a) XPS survey scan of the [BMIM][BF4]-capped Au nanoparticles. The expected Au, C, N, B, and F signals are obtained. (b) The highresolution XPS spectrum of the informative gold element for the structure determination of the [BMIM][BF4]-capped Au nanoparticles.

Results and Discussion The photochemical reduction reaction of Au3+ has been studied by several authors42-45 in colloidal solutions. In our experiments, the formation mechanisms of Au nanoparticles could be explained as follows: At first, Au3+ and acetone is photoexcited to the excited-state by the UV light; then hydrogen atom is abstracted from [BMIM][BF4] by the excited acetone to form (CH3)2C•OH radical. Second, [Au3+Cl4-]* is reduced to Au2+ by the (CH3)2C•OH radical, which is very unstable. Au2+ forms Au3+ and Au+ via dismutation reaction.33,46-48 Au+ also disproportionates to form Au0 and Au2+. Aum+ (m ) 3, 2, 1) could be reduced to Au0 by (CH3)2C•OH radical. The gold atoms then further aggregate into nanoparticles. The overall process can be formulated by the following equations: hV

Au3+Cl4- 98 [Au3+Cl4-]* hV

(CH3)2CO 98 (CH3)2CdO* hV

(CH3)2CdO* + RH 98 (CH3)2C•OH + R• (CH3)2C•OH

(1) (2) (3)

[Au3+Cl4-]* 98 Au2+Cl3- + Cl

(4)

2Au2+Cl3- f Au3+Cl4- + Au+Cl2-

(5)

2Au+Cl2- f Au2+Cl3- + Au0 + Cl-

(6)

(CH3)2C•OH

Aum+ 98 Au0(m ) 3, 2, 1)

(7)

nAu0 f (Au)n

(8)

To confirm the presence of Au (0) and the interaction between [BMIM][BF4] and the Au particles, XPS of the purified products

Figure 2. FT-IR spectra of (a) the pure [BMIM][BF4] and (b) the solution of 1.0 g/mL HAuCl4‚4H2O/[BMIM][BF4].

SCHEME 1: The Chemical Structure of [BMIM][BF4]

is recorded from a drop-cast film of the gold nanoparticles solution on a Si (111) substrate (shown in Figure 2). An XPS survey scan is shown in Figure 1a, and it can be noted that the main elements presented in the [BMIM][BF4]-capped Au nanoparticles can be ascertained. The existence of carbon, nitrogen, oxygen, fluorine, and boron signals verifies the presence of [BMIM][BF4] molecules on the surface of the Au particles. Au-binding energy peaks with various core levels could be observed. The Au 4f core level spectra recorded from the Au nanoparticles are shown in Figure 1b. The Au 4f7/2 and 4f5/2 peaks occur at a binding energy of 83.34 and 87.05 eV, respectively, and are assigned to metallic Au.49-51 In comparison with conventional Au0 (84.0 and 87.7 eV), the peaks shift down to a lower binding energy, indicating that the chemical environment around Au atoms is changed.52 In general, the Au 4f core-level energy is very sensitive to the chemical environment around the Au core and particularly to the electrondonating ability of the ligand and to the strength of the interaction between the Au and the ligand. The shift shows that electrons are inclined to transfer from the [BMIM][BF4] to the Au atoms on the nanocrystal surface, further implying a stronger interaction between them, which maybe affected by the aggregation of reduced Au atoms,53 arrangement of Au atoms on the surfaces of the Au particles, and further induced anisotropic growth of created small Au clusters.52

Photoinduced Synthesis of Anisotropic Gold Nanoparticles

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TABLE 1: Frequencies of FTIR Absorption Bands for the Pure [BMIM][BF4] and HAuCl4‚4H2O/ [BMIM][BF4] Solution frequencies of absorption bands (cm-1) [BMIM][BF4] HAuCl4‚4H2O/[BMIM][BF4] 3159, 3117

3148, 3100

1630

1631

1575, 1459

1575, 1462

1171

1058

1055

851, 758

840, 878, 750-772

assignments C-H of imidazole ring stretching vibration CdC stretching vibration imidazole ring skeleton stretching vibration C-H of imidazole ringin-plane deformation vibration BF4- stretching vibration m-substituted imidazole ring

FTIR spectra of pure [BMIM][BF4] and HAuCl4‚4H2O/ [BMIM][BF4] solution are presented in Figure 2, and the main frequencies of peaks are listed in Table 1. In Figure 2a, the bands at 3159 and 3117 cm-1 are assigned to the C-H of imidazole ring stretching vibration. The bands at 2965-2875 and 1630 cm-1 in pure [BMIM][BF4] are due to the stretching vibration for the C-H bonds of alkyl chains and CdC group, respectively. The bands at 1575 and 1459 cm-1 are due to the imidazole ring skeleton stretching vibration. The bands at 1171 and 1058 cm-1 are due to the C-H of the imidazole ring inplane deformation vibration and the BF4- stretching vibration, respectively. The bands at 851 and 758 cm-1 probably originate from m-substituted imidazole ring. Compared with the pure [BMIM][BF4], several significant changes (Figure 2b) are observed in the FTIR spectrum of the HAuCl4‚4H2O/ [BMIM][BF4] solution: (1) Two C-H of imidazole ring stretching vibration bands are down-shifted by 11 and 17 cm-1. (2) The relative intensity of imidazole ring skeleton stretching vibration bands is weakened compared with methyl symmetrical deformation vibration band at 1384 cm-1 whose position and intensity have not changed obviously before and after [AuCl4]- is added. The value of I1575/I1384 and I1459/I1384 for RTILs is about 3:2 and 4:3, but for RTILs/[BMIM][BF4] is ca. 1:6 and 1:3, respectively. (3) C-H of imidazole ring in-plane deformation vibration band (∼1171 cm-1) disappears. (4) The shape of the peaks in the region of 500-900 cm-1 has varied due to the change of the relative intensity and position of these bands. The characteristic absorption bands at 851 cm-1 is split into 840 and 878 cm-1, the band at 758 cm-1 is broadened to 750-772 cm-1, and they become weaker compared with the band at 1384 cm-1. The above changes of bands demonstrate that [AuCl4]has an effect on the electron cloud density of imidazole ring. Based on the analysis of FTIR spectra, we can conclude that there are strong interactions between RTILs and HAuCl4‚4H2O, and the interactions focuses on the imidazole ring of RTILs. The [BMIM][BF4] acted as a template to provide effective sites for the nucleation of gold nanoparticles through combining with [AuCl4]-. Powder X-ray diffraction (XRD) was used to characterize the phase structure of the obtained products. The XRD pattern of the products prepared is displayed in Figure 3, and the peaks are assigned to diffraction from the {111}, {200}, {220}, {311}, and {222} planes of face-centered cubic (fcc) gold, respectively. This indicates that the product is composed of pure crystalline Au, and the interaction between [BMIM][BF4] with Au nanoparticles is not only adsorption but a covalent bond. It is worth noting that the relative diffraction intensities of (111)/(200) and

Figure 3. X-ray diffraction pattern of the product.

Figure 4. SEM image (a) and TEM image (b) of the gold nanosheets. The inset in (a) shows that the thickness of a single nanosheet is about 60 nm; the inset in (b) shows an SAED pattern from one gold nanohexagonal.

(111)/(220) are much higher than the standard file (JCPDS 040784; 30 versus 1.89 and 30 versus 3, respectively). These results confirm that our products are mainly dominated by {111} facets, and thus their (111) planes tend to be preferentially oriented parallel to the surface of the supporting substrate. The morphologies of the samples are investigated by SEM and TEM. When the concentration of HAuCl4 is turned to 0.1 g/mL, we gain some regularly shaped Au nanosheets such as triangular and hexagonal (shown in Figure 4). Figure 4a shows scanning electron micrographs. Nanosheets of gold with sharp fringe but rough surface are observed. The maximal dimension exceeds 4 µm and the thickness is ca. 60 nm. Figure 4b presents a TEM image for Au nanosheets, triangular and hexagonal, and it is consistent with the view of SEM. Some striplike veins are observed on the surface of Au nanosheets. It indicates that electron density of the nanosheets is not even. The inset (shown in Figure 4b) provides the corresponding SAED pattern, which is of hexagonal symmetry, and the spots can be indexed based on the fcc structure of gold. The strongest spots (boxed) could be indexed to the {220} reflections (corresponding to the lattice spacing of 1.44 Å), the circled spots corresponded to the formally forbidden 1/3/{422} reflections (corresponding to the lattice spacing of 2.49 Å), and the weakest intensity spots (triangled) could be assigned to the {311} reflections (corresponding to the lattice spacing of 1.23 Å). The inner set of spots in the [111] pattern are believed to originate from the 1/3 {422} plane. The presence of the 1/3 {422} reflections indicates that the surface of the gold thin sheet was flat.54-55 Pileni et al. suggested in their paper that Au existed in the platelike structure, which might be the reason for the occurrence of the 1/3 (422) forbidden reflections.56 Time-dependent SEM of Au nanosheets are also detected, and the results indicate that the nanosheets become larger with prolonged reaction time (shown in Figure 5). The average size of sheets increases from several microns to hundreds of microns. Compared with Figure 4a, these particle brims are not as sharp as samples via photoinducing for 8 h. More triangular nano-

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Figure 6. SEM images of the gold particles prepared via photoinducing for 16 h at different concentrations of HAuCl4‚4H2O: (a) 0.05 g/mL; (b) 0.025 g/mL. The inset in (b) shows an enlarged view of a polyhedral Au particle.

Figure 5. SEM images recorded from the gold particles as a function of time of reaction (a) 16 h; (b) 24 h; (c) and (d) 168 h, low and high magnification SEM images, respectively. The inset in (c) shows an enlarged view of a caplike Au nanosheet.

SCHEME 2: Illustration of the Formation Mechanism of Gold Sheets in [BMIM][BF4]

particles become hexagonal nanoparticles. Some of sheetlike particles are not flat but scalariform, and the brims of plates are slant. As the particles formed or the reaction progressed, the Au (III) concentration in the bulk solution is gradually decreased. The crystal growth at the periphery of the sheet will hence be restrained, producing a heterogeneous texture with an outward-oriented decreasing Au (III) concentration gradient. As shown in Figure 4a, as a result of different local concentration, the lacunose surface formed. The defect location of the crystal surface could be used as the step sources of crystal growth. As the nucleation position point of secondary crystal growth, these defects contribute to forming scalariform-stacking fault structure. There are some hexagonal caplike Au sheets in the aging solution for a week shown in Figure 5c. The size of sheets even approaches 100 µm. Some cracked cap crests can be also observed (Figure 5d), and we find that the cracked position is thinner than the edge of the Au sheet. It shows that the Au sheets are brittle. We believe that the surface tension will act as a role to the sheetlike crystal brim curling and the fracture when the size of crystal arrives at a certain dimension. The sheetlike crystal brims auto-curl and the center position of the sheet is pulled thinner because of the tension action. Therefore, some sheets happen to dehisce in the washing and desiccation process. RTILs form extended hydrogenbond systems in the liquid state9 and are therefore supramolecular solvents that have ordered structures.12,57-58 Thus, it can be concluded that the formation of large-scale gold nanosheets is directly related to the special properties and structures of RTILs. It is reasonable

to deduce that the organized structure of the RTILs has a template effect for the formation of Au sheets. On the other hand, RTILs act as a capping agent that is selectively adsorbed on a specific crystal surface of Au to effectively lower the energy. This prevents further growth and aggregation of that facet relative to others. Fast-growing facets will eventually disappear during growth, resulting in a crystal terminated by slower-growing facets.53 The whole formation process of Au nanosheets in the RTILs can be proposed as illustrated in Scheme 2. With the addition of the reactant HAuCl4‚4H2O, the solution will become pink immediately, indicating that there exists the interaction between [BMIM]+ with [AuCl4]-. [AuCl4]- is reduced to a Au cluster via photoinducing. Then [BMIM][BF4] selectively adsorbs on specific crystal planes of Au to induce the gold nanoclusters epitaxy and assembly. The Au nanosheets are formed finally. The as-prepared nanosheets are single-crystalline with {111} lattice planes as the basal planes, while the {110} planes are the sidewalls of Au nanosheets as the terminated planes.56 Many metals and a variety of other materials with cubic structure have an equilibrium shape dominated by {111} faces and {110} faces indicating that these faces have the lowest energies. As the higher surface energy face, the {110} side faces are prone to arrest the gold atoms, and the smaller sheets could assemble together along the {110} boundary. In accordance with a previous publication39,59 and based on the results obtained above, two possible further growth mechanisms of Au nanosheets are also pointed out. (1) Atom-oriented attachment mechanism: The small nanosheets continue to grow mainly along the direction, bounded by the {111} planes, forming larger nanosheets, and the process occurs such as Ostwald ripening. It can be confirmed through the unequirotal, rough, or slant edge of sheets (Figure 5). (2) Edge-selective particle fusion mechanism: Some small nanosheets will be connected together along the {110} lateral planes, which are of relatively high surface energy, leading to the formation of very large, but thin, triangular, hexagonal nanosheets. Some regular thin sheets (Figure 4) are sufficient to indicate the rationality of such a connection mechanism. It is found that the Au nanoparticle shapes were highly sensitive to the concentration of the gold precursor used in the experiments. When the concentration of HAuCl4‚4H2O is 0.05 g/mL, it can be observed that some polyhedrons are fabricated (Figure 6a); these particles are unequirotal with size of 1-5 µm. Reducing this concentration of HAuCl4‚4H2O to 0.025 g/mL, we are also able to produce nanocrystals with polyhedral shapes (Figure 6b). The size of particles is 1-3 µm, and the number of planes becomes fewer when the concentration of HAuCl4‚4H2O reduced. Some little sphere-like particles can be

Photoinduced Synthesis of Anisotropic Gold Nanoparticles also seen as byproducts or initial products. These nanoparticles are prone to aggregate and deposit so that we can see some thin transparent sheetlike things on the bottom of the vessel by the naked eye. It is difficult to disperse for these small particles are assembled very tightly and we have even attempted to destroy the tight sheets by supersonic treatment, but we have not almost gained any dispersed particles. These small particles come together in stepwise fashion to form a big sheet. On the other hand, the high surface energy of microcrystal accelerates the particles to absorb together. The facts suggest that [BMIM][BF4] and the concentration of gold precursor play a key action to the growth of Au nanoparticles. We believe that lowering overall gold precursor concentration results in the selective growth of Au nanoparticles, and it suggests that subtle differences in the gold embryonic seed formation and their subsequent growth might lead to this shape selection.60 Conclusions In summary, we have succeeded in the fabrication of anisotropic Au nanoparticles in RTIL ([BMIM][BF4]). The influence of reaction surroundings is further studied, and the mechanism of reaction and crystal growth are preliminarily clarified. For practical purposes, this procedure opens a simple way to tailor the properties of chemically prepared metal nanoparticles to the needs of a specific use, by their size and shape. This method can be also extended to the controlled synthesis of other inorganic materials. Acknowledgment. This work is supported by the National Science Foundation of China (20471001, 20671001, and 20501001), the Important Project of Anhui Provincial Education Department (ZD2007004-1), the Specific Project for Talents of Science and Technology of Universities of Anhui Province (2005hbz03), and the Foundation of Key Laboratory of Environment-friendly Polymer Materials of Anhui Province. References and Notes (1) Itoch, H.; Naka, K.; Chujo, Y. J. Am. Chem. Soc. 2004, 126, 30263027. (2) Welton, T. Chem. ReV. 1999, 99, 2071-2083. (3) Robin, D. R.; Kenneth, R. S. Science 2003, 302, 792-793. (4) Emily, R. C.; Christopher, D. A.; Paul, S. W.; Paul, B. W; Philip, W.; Russell, E. M. Nature 2004, 430, 1012-1015. (5) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247-14254. (6) Huddleston, J. G.; Visser, A. E.; Rerchert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156-164. (7) Dai, L. Y.; Yu, S. Y.; Shan, Y. K.; He, M. Y. Eur. J. Inorg. Chem. 2004, 237-241. (8) Markus, A.; Kuang, D. B.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988-4992. (9) Elaiwi, A.; Hitchcock, S. B.; Seddon, K. R.; Srinivasan, N.; Tan, Y. M.; Welton, T.; Zora, J. A. J. Chem. Soc., Dalton Trans. 1995, 21, 34673472. (10) Dupont, J.; Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667-3692. (11) Seddon, K. R. Nat. Mater. 2003, 2, 363-365. (12) Antonietti, M.; Kuang, D. B.; Smarsly, B.; Zhou, Y. Angew. Chem., Int. Ed. 2004, 43, 4988-4992. (13) Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R.; Dupont, J. Chem.sEur. J 2003, 9, 3263-3269. (14) Nakashima, T.; Kimizuka, N. J. Am. Chem. Soc. 2003, 125, 63866387. (15) Scheeren, C. W.; Machado, G.; Teixeira, S. R.; Morais, J.; Domingos, J. B.; Dupont, J. J. Phys. Chem. B 2006, 110, 13011-13020. (16) Li, Z. H.; Liu, Z. M.; Zhang, J. L.; Han, B. X.; Du, J. M.; Gao, Y. N.; Jiang, T. J. Phys. Chem. B 2005, 109, 14445-14448.

J. Phys. Chem. C, Vol. 111, No. 21, 2007 7633 (17) Dupont, J.; Fonseca, G. S.; Umpierre, A. P.; Fichtner, P. F. P.; Teixeira, S. R. J. Am. Chem. Soc. 2002, 124, 4228-4229. (18) Firestione, M. A.; Dietz, M. L.; Seifert, S.; Trasobares, S.; Miller, D. J.; Zaluzec, N. J. Small 2005, 1, 754-760. (19) Zhu, Y. J.; Wang, W. W.; Qi, R. J.; Hu, X. L. Angew. Chem., Int. Ed. 2004, 43, 1410-1414. (20) EL Abedin, S. Z.; Borissenko, N.; Endres, F. Electrochem. Commun. 2004, 6, 510-514. (21) Wang, Y.; Yang, H. J. Am. Chem. Soc. 2005, 127, 5316-5317. (22) Jiang, Y.; Zhu, Y. J. J. Phys. Chem. B 2005, 109, 4361-4364. (23) Taubert, A. Angew. Chem., Int. Ed. 2004, 43, 5380-5382. (24) Jiang, J.; Yu, S. H.; Yao, W. T.; Ge, H.; Zhang, G. Z. Chem. Mater. 2005, 17, 6094-6100. (25) Zhou, Y.; Schattka, J. H.; Antonietti, M. Nano Lett. 2004, 4, 477481. (26) Zhou, Y.; Antonietti, M. Chem. Mater. 2004, 16, 544-550. (27) Zhou, Y.; Antonietti, M. AdV. Mater. 2003, 15, 1452-1455. (28) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176-2179. (29) Chen, S. M.; Liu, Y. D.; Wu, G. Z. Nanotechnology 2005, 16, 2360-2364. (30) Mohamed, M. B.; Volkov, V.; Link, S.; El-Sayed, M. A. Chem. Phys. Lett. 2000, 317, 517-523. (31) Schider, G.; Krenn, J. R.; Gotschy, W.; Lamprecht, B.; Ditlbacher, H.; Lertner, A.; Aussenegg, F. R. J. Appl. Phys. 2001, 90, 3825-3830. (32) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 16471650. (33) Eustis, S.; Hsu, H. Y.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811-4815. (34) Huang, S. X.; Ma, H. Y.; Zhang, X. K.; Yong, F. F.; Feng, X. L.; Pan, W.; Wang, X. N.; Wang, Y.; Chen, S. H. J. Phys. Chem. B 2005, 109, 19823-19830. (35) Sander, M. S.; Tan, L. S. AdV. Funct. Mater. 2003, 13, 393-397. (36) Dryfe, R. A. W.; Walter, E. C.; Penner, R. M. ChemPhysChem 2004, 5, 1879-1884. (37) Faure, C.; Guillot, S.; Weisbecker, P.; Saadaoui, H. AdV. Mater. 2006, 18, 1141-1146. (38) Dobbs, W.; Suisse, J. M.; Douce, L.; Welter, R. Angew. Chem. 2006, 118, 4285-4288. (39) Sioss, J. A.; Keating, C. D. Nano Lett. 2005, 5, 1779-1783. (40) Callegari, A.; Tonti, D.; Chergui, M. Nano. Lett. 2003, 3, 15651568. (41) Jin, R. C.; Cao, Y. C.; Hao, E. C.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A. Nature 2003, 425, 487-490. (42) Han, M. Y.; Quek, C. H. Langmuir 2000, 16, 362-367. (43) Eustis, S.; Krylova, G.; Smirnova, N.; Eremenko, A.; Tabor, C.; Huang, W. Y.; El-Sayed, M. A. J. Photochem. Photobiol., A 2006, 181, 385-393. (44) Korchev, A. S.; Bozack, M. J.; Slaten, B. L.; Mills, G. J. Am. Chem. Soc. 2004, 126, 10-11. (45) Henglein, A. Chem. Mater. 1998, 10, 444-450. (46) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574-2579. (47) Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 1401414019. (48) Bronstein, L.; Chernyshov, D.; Valetsky, P. Langmuir 1999, 15, 83-91. (49) Senapati, S.; Ahmad, A.; Khan, M. I.; Sastry, M.; Kumar, R. Small 2005, 5, 517-520. (50) Stoycheva, S.; Himmelhaus, M.; Fick, J.; Kornviakov, A.; Grunze, M.; Ulman, A. Langmuir 2006, 22, 4170-4178. (51) Mandal, S.; Mandale, A. B.; Sastry, M. J. Mater. Chem. 2004, 14, 2868-2871. (52) Felten, A.; Bittencourt, C.; Pireaux, J. J. Nanotechnology 2006, 17, 1954-1959. (53) Yin, Y. D.; Alivisatos, A. P. Nature 2005, 437, 664-670. (54) Jin, R. C.; Cao, Y. W.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901-1903. (55) Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482-488. (56) Germani, V.; Li, J.; Ingert, D.; Wang, Z. L.; Pileni, P. J. Phys. Chem. B 2003, 107, 8717-8720. (57) Mele, A.; Tran, C. D.; Lacerda, S. H. D. Angew. Chem. 2003, 115, 4500-4502. (58) Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H. Chem. Lett. 2003, 32, 740-741. (59) Li, C. C.; Cai, W. P.; Cao, B. Q.; Sun, F. Q.; Li, Y.; Kan, C. X.; Zhang, L. D. AdV. Funct. Mater. 2006, 16, 83-90. (60) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673-3677.