Silver Nanoparticle Oxide Coating via a Surface-Initiated Reduction

J. Phys. Chem. C , 2009, 113 (5), pp 1758–1763. DOI: 10.1021/jp808860n. Publication Date (Web): January 13, 2009. Copyright © 2009 American Chemica...
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J. Phys. Chem. C 2009, 113, 1758–1763

Silver Nanoparticle Oxide Coating via a Surface-Initiated Reduction Process R. Brenier* UniVersite´ de Lyon, F-69000, UniVersite´ de Lyon 1, Laboratoire PMCN, CNRS, UMR 5586, F69622 Villeurbanne Cedex, France ReceiVed: October 7, 2008; ReVised Manuscript ReceiVed: NoVember 27, 2008

A simple chemical method is described for coating oxide substrates with silver nanoparticles. We report results about Pyrex and ZrO2-coated Pyrex substrates obtained by sol-gel elaboration. Atomic force microscopy reveals the irregular polyhedral shapes of the nanoparticles. The morphology of the layer and the amount of deposited silver hugely depend on the concentration of silver nitrate in the solution. After a reaction time of 48 h, the amount of deposited silver is maximum for silver nitrate concentration close to 5 mM. We show that the reduction of Ag+ ions by ethanol, in the mild conditions that we used, is sensitive to the nature of the oxide and is faster for ZrO2 than for Pyrex surfaces. The mechanism of this surface-initiated process, mainly governed by the partial negative charge on oxygen, is discussed. The optical properties of the silver nanoparticles are studied by transmission spectroscopy. The dipole plasmon resonance is red-shifted with respect to the value for particles in vacuum, according to the dielectric properties of the substrates. A sensitivity factor of about 67 nm per refractive index unit of substrate has been fitted. 1. Introduction The synthesis of metallic nanoparticles has been the topic of intensive research efforts for several decades. This interest lies in many potential applications, for example, in catalysis1,2 or optoelectronics.3 Silver nanoparticles of size between 20 and 200 nm have a high efficiency for interaction with light, and their tunable optical properties have prompted many studies for photonic applications, especially for surface-enhanced Raman spectroscopy (SERS).4,5 These properties are due to the collective oscillations of the nanoparticle′s conduction band electrons, known as the localized surface plasmon resonances. Indeed, the shape, size, dielectric environment, or supporting substrate determine the spectroscopic response of the nanoparticles.6-8 Then intensive work has been devoted to particle elaboration with the aim to control these crucial parameters with more or less success. Physical preparation methods9 have been proposed and lithography10 is very suitable for array arrangement of particles. Besides, chemical methods have been developed for a long time for preparing colloidal dispersion of metals in aqueous media.11 Afterward, various reducing agents have been used for producing rather spherical silver colloids more or less aggregated in solution.12-14 More exotic shapes such as triangular nanoplates have been obtained owing to a capping agent and stabilized by thiol surface modification.1516 Hirai et al. have demonstrated that simple alcohols such as methanol can reduce rhodium chloride and silver nitrate in the presence of poly(vinyl alcohol) (PVA) at temperatures around 100 °C. The protective role of PVA is successful for obtaining colloidal dispersion of Rh but fails for Ag, which forms a precipitate. Later, Rao et al.17 have shown that nice colloidal solutions of Ag nanoparticles are yielded from reduction of silver nitrate by dry ethanol at 87 °C, using poly(vinylpyrrolidone) (PVP) as a protective agent. At room temperature as well, monoalcohols can be efficient reducers.18 Huang et al. have cleared up the reasons why the media has to * Telephone number: 04 72 43 29 87. Fax number: 04 72 43 26 48. E-mail address: [email protected].

be basic in this case. The most popular chemical way for producing silver (and other metals) nanoparticles in solution is the so-called polyol method19,20 pioneered by Fievet et al.21 In this method, the precursor salt is dissolved in ethylene glycol (essentially) and the reduction is processed at a refluxing temperature of 85 °C at least. The synthesis of highly anisotropic silver structures such as nanowires and nanocubes has been possible owing to adjustment of the molar ratio between silver nitrate and the capping agent (PVP) in the polyol process.22 Presently, for SERS applications, a great challenge with chemical methods resides in the transfer and organization on a substrate of the nanoparticles elaborated in solution. Some protocols have been proposed, for example, dripping and evaporation of a small volume of solution on the substrate,23 dipping the substrate into the solution during19 or after24 particle formation, self-assembling of the nanoparticles on a functionalized substrate.25 These methods can be difficult to control and even introduce a modification of the particle organization.26 An interesting alternative can be the direct nucleation and growth of the nanoparticles on the soaked substrate. Very few researchers have investigated this way. Liz-Marzan et al. have reported Ag deposition on the glass walls of the vessel when silver nitrate is reduced by N,N-dimethylformamide without a stabilizing agent.1427 Kim et al. have shown that glass substrates can be silver coated in ethanol containing 5 mM of silver nitrate at a temperature of 45 °C, using butylamine as a reducer. These conditions are mild; nevertheless, the glass surface has to be negatively charged in a basic solution, prior to silver formation. In this paper, we present our first results on the formation of silver nanoparticles on the substrate itself, via the reduction of silver nitrate in ethanol. Pyrex and ZrO2-coated Pyrex substrates were used. The nature of the silver layer was identified from the determination of its crystal structure by X-ray diffraction. The shape of the nanoparticles and the morphology of the silver layers were imaged by atomic force microscopy. The optical properties of the nanoparticles were determined by transmission spectroscopy. The effect of the silver nitrate concentration in the solution is first studied for a reaction time of 48 h. Then

10.1021/jp808860n CCC: $40.75  2009 American Chemical Society Published on Web 01/13/2009

Silver Nanoparticle Oxide Coating

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the progressive coverage of a ZrO2 surface by the silver nanoparticles is shown as a function of the reaction time. Finally, the influence of the nature of the oxide on the reduction process is tested and discussed. The effect of the dielectric properties of the supporting substrates on the optical properties of the nanoparticles is emphasized. 2. Experimental Section Pyrex substrates of dimensions 7.5 (or 3.5) × 2.5 × 0.2 cm3 were used. After the substrates were cleaned with ethanol, some of them were coated with zirconium oxide films via sol-gel elaboration.28 For this purpose, the substrates were dipped in the sol and withdrawn at the speed of 20 cm/min. After drying at 100 °C in air for 5 min, the coated substrates were annealed in a furnace under oxygen atmosphere. Some of them were directly introduced for 30 min into the furnace preheated at 250 °C and then cooled down. Others were heated in the furnace at 350 °C for 30 min and then at 600 °C for 30 min again and, finally, cooled down. The thicknesses of the ZrO2 films were determined by profilometry and were 170 and 100 nm for the films annealed at 250 and 600 °C, respectively. The ZrO2 sol was obtained29 by mixing an equimolar quantity of zirconium n-propoxide with acecylacetone. Zr n-propoxide 70% solution in propanol (3 cm3) was used, and acetylacetone served as a stabilizer. Finally, the sol was diluted with 15 cm3 of isopropanol. Hydrolysis reaction developed in ambient atmosphere, without additional liquid water. The formation of the silver nanoparticles on the two opposite faces of the different substrates was achieved after the following process. First, solutions (50 or 80 cm3) of ethanol, distilled water, and silver nitrate were prepared in Pyrex bottles. Silver nitrate 0.1 or 1 M solutions in water were used. The concentrations of ethanol and water were fixed at 14.3 and 9.25 M, respectively, in all the experiments. The Ag+ concentration was the only one to be varied between 2.5 and 170 mM. Second, the substrate was soaked in a bottle of solution which was carefully corked. The bottle was placed in an electric oven at the fixed temperature of 32 °C. The duration of the process was measured from this latter operation, up to 48 h. Finally, the treated substrates were rinsed with ethanol and dried in Ar flow at room temperature. X-ray diffraction measurements were performed in a Bruker D8 Advance instrument operating with the Cu KR radiation at the wavelength of 0.154 nm. The θ-2θ geometry was used with rotating sample. The Ag nanoparticles on the different surfaces were imaged by atomic force microscopy (AFM). An Asylum instrument was used in tapping mode with a driven frequency of about 300 kHz and a scan rate of 1 Hz. Transmission optical absorption spectra were measured in a Lambda900 spectrometer from Perkin-Elmer. 3. Results and Discussion A. Influence of Ag+ Concentration. After immersion of Pyrex substrates in the silver nitrate solutions for 48 h, the substrates appear dark yellow to the naked eye, indicating the formation of a layer, whereas the liquid remains uncolored. A typical X-ray diffraction pattern of the coated substrate, after immersion in a solution of 5 mM AgNO3, is shown in Figure 1. This spectrum contains a broad and strong peak around 2θ ) 22.2° due to the amorphous structure of Pyrex. The fine and intense peak located at 2θ ) 38.18° well corresponds to the (111) planes of the usual fcc crystal structure of silver (PDF card no. 04-007-7997). The two other weak and broad peaks at 2θ ) 44.3° and 64.4° can be attributed to the (200) and (220) plane families of silver, respectively. According to Scherrer’s

Figure 1. X-ray diffraction pattern of the silver nanoparticule layer elaborated by soaking a Pyrex substrate in ethanol solution containing 5 mM of silver nitrate for 48 h.

relation,30 the coherent domain sizes of the (111) and (200) planes are about 45 and 1.7 nm, respectively. Thus, the layer appears highly textured. Most silver grains have an average size of 45 nm and have grown with (111) planes parallel to the substrate. Very small grains of size lower than 2 nm are also present with a more random orientation. A more detailed view of the morphology of the silver layers is obtained from atomic force microscopy. The images of the films prepared with the solutions containing 2.5 and 5 mM AgNO3 are displayed in panels a and b of Figure 2, respectively. In both figures, the layers appear composed of roughly spherical and joining nanoparticles. Indeed, the top views show irregular polyhedral shapes with three, four, or five sides and more or less smooth corners. The in-plane average size of the nanoparticles is 50 nm. Because of the particle contact, the AFM tip cannot reach the substrate and the measurement of the particle heights is necessarily underestimated by AFM. Therefore, the average particle height appears to be at least of 30 nm for both AgNO3 concentrations, which is compatible with the coherent domain size of 45 nm measured by X-ray diffraction technique. Therefore, the nanoparticles have roughly spherical shapes of average diameter 50 nm. However, the nanoparticles are less close together and larger voids exist for 2.5 mM AgNO3 than for 5 mM AgNO3. Indeed, the amount of deposited silver greatly depends on the Ag+ concentration c of the solution. This effect can be conveniently studied by optical absorption measurements. The absorbance of the nanoparticles is depicted in Figure 3 for the different concentrations c. For concentrations between 2.5 and 50 mM, the spectra contain a main peak between 405 and 420 nm with a strong shoulder around 370 nm which can be attributed to the dipole and quadrupole plasmon resonances of the silver nanoparticles, respectively.7,31 A very broad shoulder also is present around 550 nm showing particle agglomeration. For c ) 170 mM, the absorbance is low and weakly depends on the wavelength. Let’s analyze the peak location λm of the dipole resonance with respect to the particle morphology. λm depends on the size and shape of the particles and on the refractive index ns of the substrate.7 For spherical particles of diameter 50 nm in vacuum, the dipole plasmon resonance is close to λm ) 375 nm and the quadrupole resonance is overwhelmed.31 For polyhedral shapes, the presence of corners induces multimodal resonances appearing as shoulders on the high energy side of the main plasmon band, which is red-shifted. In our case, the corners are rather rounded, so these effects are expected to be weak, as they are for truncated nanocubes.8 According to Malinsky et al.,32 the

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Figure 2. AFM images of the silver nanoparticule layers elaborated from solutions containing (a) 2.5 mM and (b) 5 mM of silver nitrate.

Figure 3. Optical absorption spectra (after subtraction of the substrate absorbance) of Ag nanoparticles formed on Pyrex substrates, for the different silver nitrate concentrations of the solutions, after a reaction time of 48 h.

presence of a substrate produces a red shift of the dipole resonance with a sensitivity factor ∆λm/∆ns of 87 nm per refractive index unit (RIU). Assuming this value, although the shape and density of the silver particles are different in our work, the supporting Pyrex (ns ) 1.47) could shift λm at about 415 nm. This value is in rather good agreement with our results, considering that the presence of shoulders on both sides of the main resonance disturbs the determination of λm (see section 3D). The red shift of λm slightly restores the visibility of the quadrupole plasmon resonance at about 370 nm, showing that this latter one is poorly affected by the substrate effect. Let′s take a closer look at the dependence of the spectra on Ag+ concentration c of the solution. Since the peak absorbance is proportional to the volume of silver deposited as nanoparticles, optical absorption measurements clearly show that this volume is largely maximized with AgNO3 concentrations close to 5 mM. For the highest concentration of 170 mM, the volume is even rather weak. By comparison of the shapes of the different spectra in Figure 1, one can realize that the relative contribution of particle agglomeration much increases when c exceeds 10 mM. B. Influence of the Reaction Time. The influence of the reaction time on Ag particle formation was studied during immersion of the substrates in solutions containing 5 mM AgNO3 in order to maximize the amount of deposited silver. Pyrex substrates coated with ZrO2 films annealed at 600 °C were chosen for their high efficiency for promoting silver deposition (see next section). The optical properties and the morphology of the formed silver nanoparticles were studied for reaction times of 6, 12, and 24 h. The corresponding optical absorption spectra are given

Figure 4. Optical absorption spectra of Ag nanoparticles formed on ZrO2-coated Pyrex annealed at 600 °C, for the different reaction times in the solution containing 5 mM of silver nitrate.

in Figure 4. The AFM images are displayed in Figure 5a-c. After a reaction time of 6 h, most of the nanoparticles that spread on the surface are isolated (Figure 5a). Their shape is roughly spherical and irregular. Their height and in-plane sizes are largely distributed. A line scan across some of the biggest nanoparticles (Figure 5d) indicates that the heights do not exceed 40 nm. The maximum in-plane size seems slightly higher (60 nm), but the convolution by the tip leads to overestimate the value. Nevertheless, assuming a maximum particle size of about 40-50 nm can satisfactorily explain that the corresponding optical spectrum of Figure 4 contains a symmetric peak centered at 454 nm, characteristic of the dipole plasmon resonance. For the reaction time of 12 h, the AFM image (Figure 5b) shows that the density of particles has increased and many of them touch each other. This feature induces a tail in the optical spectrum of Figure 4, for wavelengths greater than 500 nm. Particles of maximum size 50-60 nm become more numerous, and a quadrupole plasmon resonance starts to appear in the optical spectrum at 385 nm as a shoulder of the dipole at 450 nm. After a reaction time of 24 h, the particle density is so high (Figure 5c) that all particles are touching without much great increase of the maximum particle size. Then in the optical spectra every contribution (dipole and quadrupole resonances and tail above 500 nm) goes on increasing. C. Influence of the Substrate Nature and Chemical Mechanisms. For the study of the influence of the substrate nature on Ag nanoparticle formation, the Ag+ solution concentration of 5 mM has been chosen for its efficiency demonstrated in section 1. Figure 6 shows the optical absorption spectra obtained with the different substrates. For the particles on Pyrex,

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Figure 5. AFM images of the silver nanoparticule layers elaborated from a solution containing 5 mM of silver nitrate, during the reaction times of (a) 6 h, (b) 12 h, and (c) 24 h. (d) Line scan in image (a).

Figure 6. Optical absorption spectra (after subtraction of the substrate absorbance) of Ag nanoparticles formed on the different substrates, after a reaction time of 48 h in the solution containing 5 mM silver nitrate.

the absorbance has a main dipolar plasmon resonance at 414 nm with a quadrupole shoulder at 370 nm, according to the description in section 1. These two resonances are located at 442 and about 355 nm (weak), respectively, for the nanoparticles on ZrO2 films annealed at 250 °C (size of about 35 nm). As for the ZrO2 films annealed at 600 °C, the absorbance of the nanoparticles (size of about 50 nm) follows the evolution described up to 24 h of substrate immersion in section 2. The absorbance of the main peak is located at around 439 nm and what was a shoulder is now a peak at 382 nm, developed as much as the dipole contribution.

In order to discuss the mechanisms of Ag particle formation, we focus on the values of the absorbance Amax of the main dipole plasmon resonance as a measure of the amount of deposited silver. These values are 0.39 for Pyrex, 0.64 for ZrO2 films annealed at 250 °C, and 1.32 for ZrO2 films annealed at 600 °C. Then, ZrO2 surfaces are much more efficient than Pyrex ones for promoting Ag nanoparticle formation. Which chemical mechanisms operate? In the polyol method, the general mechanism of salt reduction involves alcohol oxidation giving aldehyde or cetone and protons.18,1621 Hirai et al. report that the reduction of rhodium chloride in ethanol produces acetaldehyde and protons. Besides, according to the same authors, Ag precipitation is produced by refluxing a methanol-water solution of silver nitrate (in the presence of poly(vinyl alcohol)) on a boiling water bath. Then reduction of silver nitrate in ethanol can reasonably be supposed to occur under the same conditions

Ag+ + 3⁄2CH3-CH2-OH f Ag + 3⁄2CH3-CHO + 3H+

(1) This assumption can explain our own results. Indeed, we have not observed such a oxido-reduction in the liquid phase but on the substrate itself, unlike Kurihara et al. in the polyol process.19 The reason may be that the processing temperature that we used (32 °C) is mild compared to Hirai′s experiments. So a surfaceassisted mechanism does exist. However, we never observed Ag particle formation on surfaces such as polypropylene which do not contain oxygen. Indeed, Huang et al.18 have already

1762 J. Phys. Chem. C, Vol. 113, No. 5, 2009 reported that the reduction of silver nitrate in 2-propanol was initiated at room temperature by adsorption of Ag+ ions on Ag2O particles. Thus, in our case, such an adsorption is likely to occur on oxygen atoms of the substrates bearing a partial negative charge. The difference of Pauling electronegativities ∆N between the cation and oxygen is a simple but a pertinent parameter for scaling the partial charge. For ZrO2 and SiO2 the values of ∆N are 2.1 and 1.7, respectively, and the partial negative charge on oxygen is greater in ZrO2 than in SiO2. Thus, the initiating step of Ag+ reduction is faster and reaction 1 has a higher rate on the former than on the latter oxide, in accordance with our experimental results. To understand the intermediate behavior of the ZrO2 films annealed at 250 °C, it is necessary to remember the chemical evolution of films during annealing. In the sol-gel process, acetylacetone reacts with Zr-propoxide to form acetylacetonate groups in the sol. These groups transform into acetate groups in the film during annealing at 200 °C (about 1 per Zr atom according to the reactant proportion), and the latter ones vanish at 350 °C,33 leading to an amorphous oxide. Crystallization in the metastable tetragonal phase occurs at 400 °C.29 Whatever the annealing temperature may be, hydroxyl groups Zr-OH cover internal (pores) and external surfaces. Nevertheless, the proportion of Zr-O-Zr oxo bridges over hydroxyl is all the more elevated as the annealing temperature is high, because condensation reactions are thermally activated. Therefore, in our present experiments, both hydroxyl and acetate groups lie on the surface of the ZrO2 films annealed at 250 °C. Their presence significantly decreases the surface density of oxo bridges and the rate of Ag+ adsorption. Then, much fewer Ag particles form on ZrO2 films annealed at 250 °C than on ZrO2 films annealed at 600 °C, within the same conditions. The efficiency of silver nanoparticle formation also can be influence by the roughness of the surface. On rough surfaces, the surface area of Ag+ adsorption and the density of nucleation sites are increased. In order to estimate such a contribution, the different surfaces free of nanoparticles have been characterized by AFM. The roughness ratio R defined as the ratio of the true to the projected areas of the surface has been calculated for each image. The obtained values are 1.014, 1.023, and 1.027, respectively, for the naked Pyrex, for ZrO2-coated Pyrex annealed at 250 °C, and for ZrO2-coated Pyrex annealed at 600 °C. Undoubtedly, the absorbance Amax increases with the roughness ratio R of the corresponding surface. Nevertheless, the ratio Amax/R does not have the same value for the different surfaces as one would expect if the roughness was the dominant effect. For example, the ratio Amax/R is more than 3 times that for the nanoparticles on the ZrO2-coated pyrex annealed at 600 °C than it is for the nanoparticles on Pyrex substrate. Therefore, it is concluded that the roughness of the surface may have a small contribution to the efficiency of silver nanoparticle formation, but the main factor is the chemical nature of the oxide. D. Influence of the Substrate on the Optical Properties. The peak location λm of the dipole plasmon resonance in the optical spectra of the silver nanoparticles on the different substrates is disturbed by particle agglomeration. This effect can be minimized with samples elaborated with short reaction times, but not entirely canceled, conversely to elaboration methods based on nanolithography.32 For example, most of the nanoparticles obtained on the ZrO2 film after the shortest reaction time of 6 h are isolated, but some of them are joining

Brenier

Figure 7. Peak position of the dipole plasmon resonance λm of the silver nanoparticles against the refractive index of the supporting substrate ns, for samples of type 1 (full circles) and for samples of type 2 (empty squares).

by accident (see AFM image of Figure 5a). Therefore, the absorbance spectra of two types of samples are analyzed: samples of type 1 made of quasi-isolated nanoparticles as in Figure 5a and samples of type 2 in which most of the nanoparticles are in close proximity as in Figure 5b. In every case, solutions containing 5 mM of silver nitrate were used for elaboration. Let′s notice that the thicknesses of the ZrO2 films are at least twice the silver particle size, which is thick enough for considering these films as bulk substrates. The peak locations λm of the dipole plasmon resonance of the nanoparticles have been found at 425 and 414 nm, respectively, for samples of types 1 and 2 on Pyrex substrates (ns ) 1.47), at 448 and 442 nm, respectively, for samples of types 1 and 2 on ZrO2 substrates annealed at 250 °C (ns ) 1.833), at 455 and 448.5 nm, respectively, for samples of types 1 and 2 on ZrO2 substrates annealed at 600 °C (ns ) 1.9229). In Figure 7, λm is depicted versus ns, showing that the variation is linear for both types of samples. The best fits give sensitivity factors ∆λm/∆ns of 67 and 77.3 nm per RIU of substrate for samples of types 1 and 2, respectively. Therefore, the expected red shift of the dipole resonance with increasing refractive index of the substrate is significant whatever the particle density. The most accurate value of the sensitivity factor of 67 nm per RIU of substrate measured for quasi-isolated nanoparticles is lower than the value of 87 nm per RIU fitted by Malinsky et al.32 The reason for this difference could be that the contact areas between nanoparticles and substrates are greater in the work of Malinsky et al. than in the present work. Conclusion It has been shown in this paper that layers of silver nanoparticles can be formed on oxide substrates by reduction of silver nitrate by ethanol at 32 °C. The concentration of silver nitrate in the solution has a huge influence on the morphology of the layers. The amount of deposited silver on Pyrex is maximum around 5 mM silver nitrate, for a reaction time of 48 h. The nanoparticles have irregular polyhedral shapes. Their optical properties depend on the supporting substrate and on particle density. The dipole plasmon resonance is red-shifted with a sensitivity factor of 67 nm per refractive index unit of the substrate for quasi-isolated nanoparticles. The mild elaboration conditions we used avoid bulk reduction of silver nitrate. Indeed, the reduction of Ag+ ions by ethanol is initiated on the substrate, explaining that the rate of nanoparticle formation is greater on ZrO2 than on Pyrex surfaces, owing to larger partial charges on oxygen in the former than in the latter oxide. This

Silver Nanoparticle Oxide Coating feature opens promising perspectives for particle organization on substrates, considering the versatility of sol-gel chemistry for oxide combination. Acknowledgment. The author is grateful to A. Piednoir for her efficient help with atomic force microscopy imaging. References and Notes (1) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El Sayed, M. A. Science 1996, 272, 1924–1926. (2) Haruta, M.; Date, M. Appl. Catal., A 2001, 222, 427–437. (3) Gotschy, W.; Vonmetz, K.; Leitner, A.; Aussenegg, F. R. Appl. Phys. B: Lasers Opt. 1996, 63, 381–384. (4) Moskovits, M. J. Raman Spectrosc. 2005, 36, 485–496. (5) Anderson, D. J.; Moskovits, M. J. Phys. Chem. B 2006, 110, 13722– 13727. (6) Mulvaney, P. Langmuir 1996, 12, 788–800. (7) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (8) Wiley, B. J.; Im, S. H.; Li, Z. Y.; McLellan, J.; Siekkinen, A.; Xia, Y. J. Phys. Chem. B 2006, 110, 15666–15675. (9) Malynych, S. Z.; Chumanov, G. J. Vac. Sci. Technol., A 2003, 21, 723–727. (10) Jensen, T. R.; Duval, M. L.; Kelly, K. L.; Lazarides, A. A.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 1999, 103, 9846–9853. (11) Hernandez, L.; Nord, F. F. J. Colloid. Sci. 1948, 3, 363–368. (12) Creighton, J. A.; Blatchford, G. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790–798. (13) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391–3395. (14) Pastoriza-Santos, I.; Liz-Marzan, L. M. Langmuir 1999, 15, 948– 951. (15) Jiang, X.; Zeng, Q.; Yu, A. Langmuir 2007, 23, 2218–2223.

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