Optical Absorption Properties of Dispersed Gold ... - ACS Publications

Dec 19, 2008 - nanoparticles, first investigated scientifically by Michael Faraday in 1857. ... in films of gold, but rather a wine red hue that Farad...
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J. Phys. Chem. B 2009, 113, 2647–2656

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Optical Absorption Properties of Dispersed Gold and Silver Alloy Nanoparticles† Jess Wilcoxon* Nanoscale Physics Research Laboratory, UniVersity of Birmingham, Birmingham B15 2TT, United Kingdom ReceiVed: August 4, 2008; ReVised Manuscript ReceiVed: October 23, 2008

The oldest topic in nanoscience is the size-dependent optical properties of gold and silver colloids or nanoparticles, first investigated scientifically by Michael Faraday in 1857. In the modern era, advances in both synthesis and characterization have resulted in new insights into the size-dependent absorbance of Au and Ag nanoparticles with sizes below the classical limit for Mie theory. In this paper we discuss the synthesis and properties of core/shell and nanoalloy particles of Au and Ag, compare them to particles of pure gold and silver, and discuss how alloying affects nanoparticle chemical stability. We show that composition, size, and nanostructure (e.g., core/shell vs quasi-random nanoalloy) can all be employed to adjust the optical absorbance properties. The type of nanostructurescore/shell vs alloysis reflected in their optical absorbance features. I. Introduction The investigation of the dependence of the optical properties of dispersed metal colloids with nanometer scale dimensions on size, composition, and shape has a long and colorful history.1 Alchemists in early Roman periods utilized the unique optical properties of such noble metals as gold to make valuable objects such as the Lycurgus cup. This ornate goblet with embedded colloidal particles of gold and silver can be viewed in the British museum. Like thin films of gold later investigated by Michael Faraday,2 this antiquity has a green tint in reflected light but a red tint when viewed in transmission. The strikingly deep colors found in medieval stained glass are another example of the use of colloidal metals to provide inorganic pigments for color. The durability and lack of fading with time of such inorganic pigments compared to organic ones is a major advantage. Michael Faraday made the first scientific investigation of the color of colloidal gold with the goal of discovering why very thin films of gold have different colors when viewed in reflected or transmitted light and how these colors depend on thickness.2 His estimates of the nanometer scale thickness of such films were surprisingly good given the technique available at that time. His solutions of colloids also did not have the gold color found in films of gold, but rather a wine red hue that Faraday correctly attributed to a high state of dispersion and a small size he estimated in the 10 nm range. Modified versions of his synthetic approach are still utilized in modern studies.3-5 The Faraday synthesis as modified by Turkevich3 is the basis of most solution-based formation of nanoparticles of gold and silver in water or polar solvents. These colloids are stabilized by surface charge, typically in the form of citrate ion.3-7 Such methods usually produce small populations of triangular and rod-like structures in addition to spherical ones. Because the optical properties of metal nanoclusters are very sensitive to shape as well as to surface charge, these synthetic drawbacks complicate detailed theoretical analysis of the size-dependent optical properties of these nanoparticles. Also, it is difficult to synthesize metal clusters in aqueous solution with dimensions less than 5 nm. Nanoparticles of Ag and Au less than 5 nm are †

Part of the “J. Michael Schurr Special Section”. * E-mail: [email protected].

particularly interesting as they may exhibit both classical and quantum size effects in their absorbance spectra.1,6,7 Nanoalloys of gold and silver, metals that have essentially identical lattice constants and are completely miscible, present new opportunities to investigate the effect of nanostructure on optical properties. We expect their optical properties might depend not only on size, shape, and composition, but also on whether the silver and gold is randomly distributed throughout the nanoparticle or segregated into a core/shell nanostructure. For small nanoparticles with a size less than 5 nm, as investigated in the present study, this structural distinction can be very challenging to elucidate based upon conventional electron microscopy, so the optical properties may provide the best way to distinguish nanoalloys from core/shell nanoalloys. However, to accomplish this goal requires a synthesis and selection technique to identify particles of identical shape and size, making nanostructure and composition the only significant variables. Two nanostructures of AgAu nanoparticles have been studied previously, core/shell structures that require a sequential reduction process and nanoalloys that are typically synthesized by simultaneous coreduction of precursor metal salts.8-20 Nanostructures with a Ag core and Au shell, Ag/Au, were synthesized using Ag seed particles onto which Au was reduced and deposited either radiolytically8 or chemically.16,18 Other studies investigated Au/Ag nanostructures, made by depositing Ag on Au core particles.12,13,15,17,19 Finally, by coreduction of both metals, a nanoalloy type structure was investigated.9-11,14 A few studies investigated both nanoalloy and core-shell nanostuctures15,18,20 Even when one metal is deposited onto the seeds from another, it is often observed that interdiffusion of the shell atoms into the core or vice-versa to form a diffuse interface occurs.9 This typically occurs when a more noble metal such as Au is deposited onto a less noble one like Ag.11 When this occurs, the optical absorbance will change with time after synthesis as the nanostructure evolves. This structural evolution might involve annealing of defects and/or interdiffusion of segregated atoms. Because the time between synthesis and measurement of optical properties is rarely considered an important variable, comparison of results from various groups is complicated.

10.1021/jp806930t CCC: $40.75  2009 American Chemical Society Published on Web 12/19/2008

2648 J. Phys. Chem. B, Vol. 113, No. 9, 2009

Wilcoxon

TABLE 1: Synthesis Conditions sample Ag, d ) 4.0 Ag, d ) 3.2 Ag, d ) 2.4 Au, d ) 2.0 Au, d ) 4.0 Ag/Au, 1/1, d ) 4.4 Ag/Au, 1/2, d ) 3.1 Ag/Au, 1/2, d ) 4.8 Au/Ag, 1/1, d ) 3.5 Au/Ag, 1/1.9, d ) 3.5 Au/Ag, 1.9/1, d ) 3.1 AgAu, 2/1, d ) 4.5 AgAu, 1/1, d ) 5.0 AgAu, 1/2, d ) 4.3 AgAu, 2/1, d ) 3.2 AgAu, 1/1, d ) 3.1

metal salt(s) AgBF4 AgNO3 Ag(C6H4CO2) HAuCl4 HAuCl4 AuPPh3Cl1 AuPPh3Cl AuPPh3Cl Ag(C6H4CO2) Ag(C6H4CO2) Ag(C6H4CO2) HAuCl4, AgBF4 HAuCl4, AgBF4 HAuCl4, AgBF4 HAuCl4, AgBF4 HAuCl4, AgBF4

surfactant

reductant 2

TOAC (0.1 M) TOAC (0.1 M) None TOPB (0.1 M)3 C12E5 (0.2 M)4 N.A. N.A. N.A. N.A. N.A. N.A. TOAC (0.1 M) TOAC (0.1 M) TOAC (0.1 M) TOAC (0.1 M) TOAC (0.1 M)

LiBH4 (0.04 M) LiBH4 (0.04 M) NaBH4 (0.1 M) LiAlH4 (0.04M) Li(C2H5)3BH (0.04 M) NaBH4 (0.1 M) NaBH4 (0.1 M) NaBH4 (0.1 M) NaBH4 (0.1 M) NaBH4 (0.1 M) NaBH4 (0.1 M) LiBH4 (0.04 M) LiBH4 (0.04 M) LiBH4 (0.04 M) LiBH4 (0.04 M) LiBH4 (0.04 M)

stabilizer C12H25SH C12H25SH C12H25SH C12H25SH C12H25SH C12H25SH C12H25SH C12H25SH C12H25SH C12H25SH C12H25SH See text See text See text C12H25SH C12H25SH

(0.01 (0.01 (0.01 (0.01 (0.01 (0.01 (0.01 (0.01 (0.01 (0.01 (0.01

solvent M) M) M) M) M) M) M) M) M) M) M)

(0.01 M) (0.01 M)

toluene toluene toluene benzene octane toluene toluene benzene benzene benzene benzene toluene toluene toluene toluene toluene

1 AuPPh3Cl ) gold triphenylphosphine chloride ) (C6H5)3PAuCl. 2 TOAC ) tetraoctylammonium chloride ) (CH3(CH2)7)4NCl. 3 TOPB ) tetraoctylphosphosium bromide ) (CH3(CH2)7)4PBr. 4 C12E5 ) penta-ethyleneglycol-mono-n-dodecyl ether.

In the case of both large, D > 5 nm, nanoalloys, charge stabilized in water,10,13 and small, 2-3 nm nanoalloys, sterically stabilized by ligands such as alkanethiols in oils like toluene,9,11,15 only a single absorbance peak is usually observed in the optical absorbance spectra, whose position shifts continuously from that of pure Ag clusters, (∼420 nm), to that of pure Au clusters, (∼520 nm) as a function of composition. Interestingly, even for the case of core/shell nanostructures such as Ag/Au8,9,11,16 and Au/Ag,13,15 only a single plasmon absorbance peak is observed for clusters for sizes less than ∼12 nm, although simple Mie theory predicts that two peaks should be observed. For either charge or sterically stabilized clusters, the deposition of even small, monolayer amounts of Au onto Ag clusters causes very large broadening of the initially sharp Ag plasmon absorption followed by very significant red shifting of the resulting asymmetrical peak, just in the nanoalloy case. This observation is often rationalized by invoking an alloying effect in the shell or at the interface between Ag/Au.8,12,15,18 However, pure Ag or Au clusters with sizes of only 2-3 nm have such broad absorbance plasmons that it is doubtful whether one would be able to observe two distinct absorbance peaks even with a sharp interface between the metals. It is very difficult to verify this surface alloying effect by structural measurements such as electron microscopy since the lattice constant of Au and Ag are essentially identical.17 Recent high angle annular dark field microscopy studies of bimetallic Ag/Au and Au/Ag core/shell nanoparticles have shown that the core is predominantly Ag, and no evidence of interatomic diffusion at the interface could be observed.20 Thermodynamic considerations suggest that Ag would prefer to be in the shell of these nanostructures, but these calculations do not account for the metal ligand binding at the surface of surfactant stabilized clusters, and this favors the presence of Au at the surface. In this paper we describe the formation of nanosize, neutral bimetallic particles in low dielectric constant, nonpolar media using inverse micelles as reactors to solubilize ionic salt precursors and then reduce them chemically to form seed particles of both Ag and Au.21 A heterogeneous growth method described previously is then used to form core/shell, Ag/Au and Au/Ag particles, whereas coreduction of Ag and Au salts is used to form AgAu nanoalloys.24 These uncharged spherical nanoparticles are sterically stabilized against aggregation by the surfactant used to form the inverse micelle. Upon addition of strongly binding ligands, such as alkyl thiols, it is possible to

size-select and analyze their optical properties using analytical methods like size-exclusion chromatography, SEC.7,22,23 Nanoparticles prepared and analyzed by SEC can be studied in identical chemical environments, and SEC will separate nanoparticles with differing shape (e.g., rod-like, triangular) as well. We investigate the effect of Ag/Au ratio on the peak plasmon absorbance wavelength and the damping or absorbance line width of the absorbance for both alloy and core/shell nanostructures in identical chemical environments. We show that increasing the Au content of Ag/Au core/shell nanoparticles leads to a red shift and increased damping of the plasmon whereas an increase in Ag content in Au/Ag nanoparticles results in a blue shift and reduced damping. The nanoalloys also redshift with increasing Au content and exhibit increased line broadening (i.e., energy loss or damping) for smaller sizes. For a constant composition, each type of nanostructure can be distinguished and identified solely by its optical absorbance features. The ability to control size, composition, and nanostructure to create a unique absorbance signature should be useful in applications where the nanoparticles serve as chemical labels or taggants. II. Experimental Section Nanocluster Synthesis. Our synthesis of core/shell nanoparticles uses a heterogeneous seed growth technique that we have previously described in detail.24 In this process a solution of purified seed nanoparticles that forms the nanoparticle core is first synthesized using the inverse micelle nanoparticle growth method.5,6,21 The size and size dispersion of these seeds is verified using size-exclusion chromatography as described below and in previous publications.7,22,23,25 The commercially available metal salt precursors and concentrations of each inverse micelle solution used to generate the Ag, and Au seed particles and then deposit either Au or Ag on their surface to form Ag/Au or Au/Ag nanoparticles are shown in Table 1. The samples are referenced in the text and tables by their SEC diameter, d, and ratio of Ag/Au. For example, Ag/Au, 1/2, d ) 4.8 nm is a core/ shell particle with a Ag core/Au shell nanostructure, a composition of 1 atom Ag for 2 atoms of Au (66% Au), and a total particle diameter of 4.8 nm. Table 2 shows the diameters, d, as measured from size exclusion chromatography, SEC and transmission electron microscopy, TEM. Table 2 also summarizes optical absorbance properties such as the peak absor-

Dispersed Gold and Silver Alloy Nanoparticles

J. Phys. Chem. B, Vol. 113, No. 9, 2009 2649

TABLE 2: Cluster Size and Optical Properties sample

te (min)

d (nm, SEC)

Ag, d ) 4.0 Ag, d ) 3.2 Ag, d ) 2.4 Au, d ) 2.0 Au, d ) 4.0 Ag/Au, 1/1, d ) 4.4 Ag/Au, 1/2, d ) 3.1 Ag/Au, 1/2, d ) 4.8 Au/Ag, 1/1, d ) 3.5 Au/Ag, 1/1.9, d ) 3.5 Au/Ag, 1.9/1, d ) 3.1 AgAu, 2/1, d ) 5.0 AgAu, 1/1, d ) 4.8 AgAu, 1/2, d ) 4.3 AgAu, 2/1, d ) 3.0 AgAu, 1/1, d ) 3.2

7.16 7.37 7.62 7.7 7.15 7.08 7.41 6.99 7.3 7.28 7.41 6.92 6.97 7.13 7.45 7.4

4.0 3.2 2.4 2.0 4.0 4.4 3.1 4.8 3.5 3.5 3.1 5.0 4.8 4.3 3.0 3.2

bance wavelength, λp, and the half-width at half-height to the red of this peak, ∆λ1/2. In most cases, the absorbance to the blue of the asymmetrical absorbance peak is too large to allow determination of the half-width to the blue of the peak. The cationic surfactants listed in the footnotes of Table 1, tetraoctylammonium chloride (TOAC) and tetraoctylphosphosium bromide (TOPB), were purchased from Fluka chemicals. The nonionic surfactant, penta-ethyleneglycol-mono-n-dodecyl ether (C12E5) was obtained in highly purified form from Nikko Chemicals, Japan. The metal salts, HAuCl4 and AgBF4; metalorganics, gold triphenylphosphine chloride, (C6H5)3PAuCl, and silver benzoate, Ag(C6H4CO2); and solvents, toluene and benzene, were purchased from Aldrich chemical and used as received. A feed stock source of Ag or Au atoms that is easily reduced by either LiBH4 or NaBH4 and is soluble in the same solvent as the seed nanoparticles is required for the heterogeneous growth approach.24 We have determined optimal organometallic sources to be gold triphenylphospine chloride and silver benzoate, since both of these metal-organics are soluble in benzene or toluene. In each synthesis we select a core particle with a size that will result in a core/shell nanoparticle with a desired Ag/Au ratio and final size. However, in certain cases, the final size and Ag/Au ratio differs from that calculated due to incomplete deposition. We now provide a description of a specific synthesis of each nanostructure with a final size of ∼4-5 nm and a 1:1 Ag/Au ratio. Ag and Au Seed Nanoparticles. Following the heterogeneous growth method previously described,24 we prepared Ag core particles with sizes of 2.4, 3.2, and 4.0 nm. These pure Ag nanoparticle solutions also provide a reference for comparison of optical absorbance to that of Ag/Au and AgAu nanostructures. An inert gas glovebox was used due to the air sensitivity of the reducing agents employed. For example, (see Table 1), a 2.4 nm Ag nanoparticle seed solution was grown by NaBH4 reduction of 0.01 M silver benzoate in toluene containing the stabilizer dodecanethiol. Larger, 4.0 nm seed Ag particles solutions were formed by LiBH4 reduction of 0.01 M AgBF4 in an inverse micelle solution of the cationic surfactant, TOAC (0.1 M) in toluene. AgBF4 dissolved completely, forming a transparent solution, following magnetic stirring for several hours. Dodecanethiol at 0.01 M was added prior to reduction by a 1 M stock solution LiBH4 dissolved in THF. Vigorous evolution of hydrogen and formation of a dark yellow/red solution occurs rapidly, less than 1 min after addition of the reductant.

d (nm, TEM) 4.3 N.A. 2.5 2.0 4.2 4.6 3.4 5.0 3.7 N.A. 3.2 na 5.5 4.5 3.2 3.7

λp (nm)

∆λ1/2 (nm)

449 460 493 none 513 501 511 491 511 487 501 487 492 511 491 506

59 75 96 N.A. 91 74 111 98 82 78 103 59 62 76 90 82

Similarly, 0.01 M of HAuCl4 was dissolved via magnetic agitation in an inverse micelle solution of TOPB (0.1 M) in toluene. Upon addition of the stock 1 M solution LiAlH4 in THF to a final concentration of 0.04 M, vigorous bubbling of H2 is observed and a change in color to dark orange/red occurred in less than a minute, producing 1.8-2 nm clusters for use as seeds for the Ag/Au nanostructures described in Tables 1 and 2. Deposition of additional gold atoms as described previously24 was used to grow larger Au particles for use as seeds to grow Au/Ag nanostructures. Au, d ) 4 nm seeds could also be grown directly using the recipe of Table 1. Ag/Au Nanoparticles. A 4 mL, 0.01 M solution of Ag, d ) 3.2 nm core particles in benzene containing 0.002 M of the alkyl thiol, dodecanethiol, and a reducing agent, NaBH4 (0.1M, final concentration), is placed in a vial containing a magnetic stir bar. The vial has a septum-sealed cap, and the sample is prepared and sealed under argon. A gas-tight syringe is filled with a 0.01 M solution of AuPPh3Cl in benzene and connected to the vial containing the seed particles with a 1/16” I.D. Teflon tube terminated in a needle that penetrates the septum of the vial. A syringe pump delivers the AuPPh3Cl in benzene solution into the magnetically stirred vial at a rate of 4 mL/hr. Reduction of the Au(I) by NaBH4 and release of hydrogen gas is observed. A solution color change from yellow to reddish/yellow also occurs as Au is deposited onto the 3.2 nm Ag seeds. After 4 mL is delivered into the solution, a 1:1 Ag:Au nanoparticle forms, sample Ag/Au, 1/1, d ) 4.4 nm, Table 1. Au/Ag Nanoparticles. A series of Au core particles with sizes of 2, 2.4, 2.8, 3.2, and 4.0 nm are prepared as described previously.24 Two specific recipes for Au, d ) 2 nm and Au, d ) 4.0 nm seeds are shown in Table 1. A 4 mL solution of 0.01 M solution of 2.4 nm Au core particles in benzene or toluene containing 0.002 M of the alkyl thiol dodecanethiol, and a reducing agent, 0.1 M NaBH4, is placed in a vial containing a magnetic stir bar. The vial has a septum-sealed cap and the sample is prepared and sealed under argon. A gas-tight syringe is filled with a 0.01 M solution of silver benzoate, Ag(C6H4CO2), in benzene and connected to the vial containing the seed particles with a 1/16” I.D. Teflon tube terminated in a needle that penetrates the septum of the vial. A syringe pump delivers the 0.01 M Ag(C6H4CO2) feedstock solution into the magnetically stirred vial at a rate of 4 mL/hr. After 4 mL is delivered into the 0.01 M Au seed solution, a 1:1 Ag:Au nanoparticle is produced, Au/Ag, 1/1, d ) 3.5 nm, Table 1. AgAu Nanoalloys. An inverse micelle solution containing 0.01 M HAuCl4 and 0.01 M AgBF4 is formed by dissolving

2650 J. Phys. Chem. B, Vol. 113, No. 9, 2009 these metal salts in a 0.1 M solution of the cationic surfactant tetraoctylammonium chloride (TOAC) in toluene using vigorous stirring overnight in a glovebox. The transparent precursor solution is placed in a vial with a magnetic stir bar, and a stock solution of 1 M LiBH4 in tetrahydrofuran (THF) is rapidly injected, resulting in a final reducing agent concentration of 0.04M. The LiBH4 reductant rapidly reduces the light yellow precursor solution to a dark orange/red color with the evolution of hydrogen gas bubbles. The reduction is done in a glovebox because of the very reactive and moisture-sensitive nature of this reducing agent. The nanoalloys are not air sensitive and can be handled outside the glovebox. The ratio of the Au and Ag precursor salts is used to change the final Ag/Au ratio in the nanoparticles. The alloy nanoparticles studied range in size from 3.1 to 5.5 nm. An example is AgAu, 1/1, d ) 5.0 nm in Table 1. To permit purification and chromatographic analysis of the alloy clusters, an alkyl thiol is added a day after reduction. The smaller 3 nm AgAu shown are made in an identical method, but the alkyl thiol is added prior to reduction. By-product salt and excess surfactant were removed by addition of a 10-fold volumetric excess of either methanol or acetone to the solution. These nonsolvents result in precipitation of the alkyl thiol passivated alloy nanoclusters from the solution. Mild centrifugation for 30-60 min at 1000g compacts the precipitate and allows the supernatant to be decanted and the nanoalloys washed in more nonsolvent and then redissolved in the solvent, toluene, used for chromatographic, TEM, and XRF analysis. Size-exclusion Chromatography (SEC). We use a commercial Water’s Corporation Delta-prep high pressure liquid chromatograph to size fractionate and study the absorbance properties of the alloy nanoparticles. The system consists of an autosampler (model 717), used to inject 10 µL of the 0.01 M nanoparticle solutions into a mobile phase; a solvent pump that delivers a steady flow of the mobile phase, toluene, through a degasser; a solvent filter; a guard column; and a SEC column. The clusters are separated by size and shape using a Polymer Laboratories type PL1000 column with dimensions of 7.8 mm (diameter) × 250 mm (length) packed with 1000 Å pore size polystyrene microgel particles with an average size of 5 µm. This high resolution column yields an elution peak full width at half-height of 0.25 min, which determines our ability to resolve separate peaks corresponding to clusters of different size. The elution of the nanoparticles and nonabsorbing chemicals is detected online using a photodiode array, PDA, (model 996) and a differential refractive index detector (model 410) to monitor nonabsorbing chemicals such as surfactants. A mobile phase flow of 1.0 mL/min was used that, for the column pore volume of the PL1000 column, means that clusters elute between 5 and 12 min, corresponding to a hydrodynamic size range of 1-10 nm. Further details regarding the size calibration and other instrumental details were given previously.22,23 As in previous work, a mobile phase additive of dodecane thiol at 0.01 M in toluene was used to minimize chemical interactions between the clusters and the column. We use the 400 nm wavelength channel of the PDA to detect the visible absorbance associated with elution of the nanoparticles. The complete absorbance spectra from 290 to 800 at 4 nm bandwidth is also collected every 2 s. This corresponds to about 15 complete spectra in a resolution-limited 30 s elution peak. In previous work we demonstrated that the elution time te is related to the cluster hydrodynamic diameter, Dh by te ≈ log Dh. A best-fit to a series of linear alkanes and polystyrene polymer standards was used to obtain a calibration of Dh as

Wilcoxon

Figure 1. Chromatograms, solid lines, showing absorbance at 400 nm vs elution time for three Ag/Au samples. Open circles label the Ag, d ) 2.4 nm seed, green curve; open squares are the Ag/Au, 1/2, d ) 3.1 nm, blue curve; and open triangles are the Ag/Au, 1/2, d ) 4.8 nm, red curve.

measured by dynamic light scattering with te. The metal nanocluster core diameter is obtained from Dh by subtracting the organic passivating thickness. This thickness can be estimated by addition of a several different alkyl thiols to a cluster solution and measuring the value of Dh. Using the core size from TEM then yields the organic passivating thickness. For dodecanethiol used in the present study the organic passivating thickness was found to be 1.2 ( 0.1 nm.22,24,25 Transmission Electron Microscopy (TEM). In these studies transmission electron microscopy was used primarily to confirm the hydrodynamic size obtained by SEC and to calibrate the column using a series of Au cluster size standards. The TEM samples were prepared as in our previous work by depositing about 2 µL of a 0.01 M nanoalloy solution onto a holey carbon grid resting on a piece of filter paper. The wicking action of the filter paper rapidly draws the solvent through the holes in the grid and spreads the clusters over a large region of the grid, avoiding cluster pile up during the drying process. Since the clusters have an alkyl thiol on their surface regions, hexagonally packed nanoparticles often form as shown in Figures 3, 8 and 9. X-ray Fluorescence (XRF). The Au and Ag concentrations in the purified core/shell and alloy nanoparticles that were analyzed using a QuantXTM XRF instrument with a Rh X-ray tube providing X-rays with energies from 1-45 KeV and thermoelectrically cooled X-ray detector with an energy resolution of 0.25 KeV. The response was calibrated using known concentrations of stock solutions of AuPPh3Cl and Ag(C6H4CO2) in benzene. The stock solutions were mixed in ratios appropriate for the Ag:Au ratios investigated in the samples. The Ag:Au ratios listed in Tables 1 and 2 were obtained by comparison of the Ag (K line at 22.104 KeV) and Au (L line at 9.711 KeV) XRF peak areas from the core/shell and alloy nanoparticles to the standard samples. III. Results and Discussion Figure 1 shows the absorbance signal at 400 nm from the online PDA spectrometer vs elution time for three Ag/Au metal nanocluster solutions described in Tables 1 and 2. Each sample is stabilized with an organic shell of dodecanethiol. The total thickness of the organic shell has been determined previously

Dispersed Gold and Silver Alloy Nanoparticles

Figure 2. Bright-field TEM of a hexagonally ordered region of the Ag/Au, 1/2, d ) 4.8 nm sample whose chromatogram and absorbance is shown in Figures 1 and 4.

Figure 3. Absorbance spectra collected online during the chromatography of Ag/Au, 1/2, d ) 4.8 nm clusters for the two elution times given in the legend.

for Au nanoclusters and is assumed to contribute 2 × 1.2 ) 2.4 nm to the measured hydrodynamic diameter of the nanoalloys as well. This total shell thickness was subtracted from the hydrodynamic diameter as measured by SEC to obtain the core diameter sizes indicated in this figure and Table 2. The absorbance chromatograms of Figure 1 show that the smallest Ag clusters elute at a time that corresponds to a metal core diameter of 2.4 ( 0.2 nm. The smallest clusters spend the greatest amount of time exploring the pore structure of the SEC column and thus have the longest retention time of the three samples shown. These Ag, d ) 2.4 nm clusters are used as seeds to grow the Ag/Au, 1/2, d ) 3.1 nm cluster sample whose chromatogram is also shown in this figure. The larger size of the resulting core/shell particles gives a single elution peak whose faster elution corresponds to a core size of 3.1 ( 0.3 nm. Furthermore, the elution line width, or size dispersion, is

J. Phys. Chem. B, Vol. 113, No. 9, 2009 2651

Figure 4. Normalized absorbance spectra obtained at the elution peaks in figure 1 vs wavelength. Open circles, red curve is a reference Ag, d ) 4 nm spectrum; open triangles, green curve, is the Ag/Au,1/2, d ) 3.1 nm sample; open squares, blue curve, is the Ag/Au, 1/2, d ) 4.8 nm sample; and filled circles, black curve, is a reference Au, d ) 4.0 nm, absorbance spectrum.

Figure 5. Absorbance spectra normalized at their peak, collected at the peak elution times noted in Table 2 for the following samples: Ag/ Au, 1/2, d ) 4.8, open circles, red curve; Ag/Au, 1/2, d ) 3.1, open squares, blue curve; Au, d ) 4.3, open triangles, green curve; and Ag, d ) 4.0, solid circles, black curve.

as good as the seeds. The elution line width of 0.25-0.3 min is very close to that of a monodisperse molecule of a solvent like octane, and indicates negligible size (and shape) dispersion in these two samples. No peak at the original seed elution time is observed, demonstrating that the Au is deposited entirely onto the seeds. If a larger 3.2 nm Ag seed is used, and the same amount of Au is deposited to form a Ag/Au nanostructure, then the resulting Ag/Au, 1/2, d ) 4.8 nm sample elutes at the shortest time as expected for a SEC separation mechanism. The elution peak of this sample shown in Figure 1 corresponds to a size of 4.8 ( 0.4 nm, in good agreement with the core size of 5 ( 0.5 nm estimated from the TEM shown in Figure 3. The ordering of the particles in this TEM shows that the dodecanethiol ligand is still present on the nanoparticle surface following the Au deposition. The line width of the elution peak for this Ag/Au, 1/2, d ) 4.8 sample is larger than the other two samples shown,

2652 J. Phys. Chem. B, Vol. 113, No. 9, 2009 which may indicate more size and/or shape polydispersity compared to the other two samples. A small elution peak near t ) 8.6 min in Figure 1 could indicate the formation of a small population of very small Au clusters, and this long retention time corresponds to a size of only ∼1.0 nm. However, if one compares the spectra at the majority peak at t ) 7.0 min with that at t ) 8.6 min, Figure 4, one observes no significant difference in peak absorbance wavelength or line width. This means that the cluster subpopulation eluting at this delayed time must interact chemically with the column material instead of following a strict size exclusion mechanism. So, there are two distinct surface structures with nearly identical average size. In previous work we have shown how one can learn about the strength of binding of the passivating ligand to a cluster of a given size and composition.25 In an ideal SEC elution process there should be no specific chemical interaction of the metal cluster with the column. We try to achieve this by passivation of the cluster surface with alkyl thiols. If complete passivation is successful, then one should observe a nearly symmetrical elution profile with the excess line width, (excess line width ) observed line width-instrumental line width), determined solely by the sample polydispersity. This line width can be as small as observed in a pure solvent for the best Au preparations.22 However, if the stabilizing cluster ligand binds only weakly to the cluster surface, then a significant fraction of the injected clusters may bind to the column and fail to elute. This can be quantified by removing the column and measuring the elution peak area using the same detector conditions and comparing it to the area when the column is used for size fractionation. For pure Au clusters stabilized by alkyl thiols, the ratio of these areas, a measure of the SEC column elution efficiency, is 100% within experimental error. For Ag clusters it is often lower, 40-80%, showing the binding to be weaker between Ag and dodecanethiol. Not surprisingly, coating the Ag seeds with Au increases this efficiency, showing that the presence of Au atoms at the cluster surface enhances thiol binding, whereas coating Au clusters with Ag results in lower column efficiency and more elution peak asymmetry. The elution time at the peak of the chromatogram shown in Figure 1 represents the average size of the clusters from that sample, and the absorbance spectrum collected at this time represents the average absorbance profile. Figure 5 shows the peak elution absorbance spectra from two sizes of the Ag/Au, 1/2 samples. For comparison, absorbance spectra from 4 nm pure Ag and Au clusters are also shown. The effect of the Au shell on the absorbance is to blue shift the peak relative to a pure Ag cluster of the same size while broadening the absorbance line width. The optical absorbance line width is similar to that of the pure Au reference sample and is quite asymmetric around the peak. As in the case of pure Ag clusters,7 decreasing the Ag/Au nanoparticle size from 4.8 to 3.1 nm red shifts the absorbance peak. So, the red shift of the peak with decreasing size for Ag/Au nanoparticles indicates that the Ag core controls the direction of the peak shift, since pure Au clusters blue shift with decreasing size. Meanwhile, the Au shell leads to major broadening of the peak. This shows that the interband transitions of the Au shell, which dominates the higher energy, shorter wavelength region of the absorbance is a strong component of the total absorbance oscillator strength. This is the reason why the symmetry of the pure Ag absorbance peak is lost in the Ag/Au structures. The long wavelength absorbance line width of the Ag/Au samples is quite similar to the pure Au, d ) 4.0 nm cluster sample shown. So, the energy loss and damping

Wilcoxon mechanism reflected in the line width is similar to pure Au nanoclusters. This suggests that the energy loss due to inelastic scattering at the Au/solvent interface is more important than the loss at the Ag/Au interface. However, evidence for Ag in the core is still reflected in the weaker absorbance to the blue of the peak for both Ag/Au nanostructures compared to the pure Au, d ) 4 nm reference spectra. If more Au is deposited onto the Ag core, then a small additional red shift from 520 to 530 nm is observed. However, the most significant peak red shift and broadening occurs with small amounts of deposited Au as has been noted in the earliest work in this area.8 As can be observed in Figure 5 and Table 2, Ag/Au nanoparticles with an average total size of around 4-5 nm have absorbance spectra that red-shift and broaden with increasing amount of deposited Au. Indeed, the Ag/Au, 1/2, d ) 4.8 nanoparticle solution has a spectrum nearly identical to a pure Au nanocluster of the same size (see Table 2). The symmetry of the absorbance profile for 4 nm size Ag nanoparticles around the peak absorbance position is lost rapidly as Au is deposited on the surface, indicating the absorbance mechanism at shorter wavelengths is strongly influenced by the interband transitions from d-type orbitals to sp-type conduction band of the Au on the cluster surface. The observation of a red shift and increase in line width with increasing Au content for Ag/Au nanostructures is common to samples prepared in both aqueous and nonaqueous solution. In the first study of Ag/Au nanostructured particles by Mulvaney and co-workers, 7.6 nm Ag nanoparticles were used as seeds for radiolytically reduction and deposition of Au.8 As noted in the original work, one might expect these larger particles, based upon Mie theory, to exhibit two absorbance peaks corresponding to the Ag and Au plasmon resonance energies if a sharp interface between Ag core and Au shell is maintained during the growth process. As in the present experiments, only a small amount of Au deposited on the Ag nanoparticle, less than a monolayer, was sufficient to cause a broadening and disappearance of the sharp, symmetrical Ag plasmon absorbance. This rapid damping of the Ag absorbance was attributed to formation of a surface alloy of AgAu due to diffusion of the Au atoms into the nanoparticles. For smaller clusters, as in the present work, broadening of the absorbance spectra simply due to increased electron scattering at the cluster/solvent interface could also obscure the distinct absorbance peaks from Ag and Au. A similar spontaneous alloying attributed to interdiffusion of Ag and Au was reported by Shibata and co-workers for Au/ Ag nanostructured particles.12 However, later measurements by Chen and co-workers using X-ray absorption fine structure, XAFS, to study both nanoalloys and Ag/Au nanostructures could not distinguish the two types of structure based upon the optical spectra alone.18 In both cases, 10 nm nanostructured particles showed a red-shift and broadening occurs with increasing Au content. XAFS did show distinct core/shell and alloy nanostructures depending on the synthesis method. Because the Mulvaney experiments took place in aqueous solution and the particles were likely charge stabilized, their observations could reasonably differ from the present study. Later observations on Ag/Au nanostructures by Shon and coworkers are more directly comparable to the present work since the nanoparticles were sterically stabilized, prepared, and studied in nonpolar solvents.9,11 In these experiments, gold was deposited onto preformed 4.2 nm Ag nanoparticles stabilized by dodecanethiol. The plasmon absorbance from the Ag core was broadened and red-shifted as more gold was deposited. A most interesting observation was that Au was incorporated into the

Dispersed Gold and Silver Alloy Nanoparticles core via diffusion from the surface. In fact, X-ray photoelectron spectroscopy9 (XPS) showed that for Ag/Au, 2.5/1 nanoparticles about 60% of the surface atoms were Ag whereas in Ag/Au, 1/2.5, over 86% of the surface sites were Ag. This result would not be expected based upon the stronger binding of Au with dodecanethiol compared to Ag, which would favor Au at the surface.7 However, not all the cluster surface atoms are ligated to the dodecanethiol, so migration of the unbound atoms into the interior is a possible explanation. The fraction of Ag and Au at the nanoparticle surface was inferred from the relative XPS S2p doublet peaks areas whose binding energies were known to correspond to Au-thiol or Ag-thiol surface sites. XPS measurements are a global composition average over the entire sample and thus cannot give the composition of an individual 4.0 nm nanoparticle. Another a surprising observation in these studies11 was an apparent shrinkage of the final nanoalloy particle size from seed, Ag, d ) 4.2, to 3.0 nm. A similar shrinkage of the Ag core was noted by J. Yang and co-workers when forming Ag/Au nanoparticles in toluene.16 A possible explanation of this core shrinkage is that some of the Au was deposited on the 2 nm Ag seed subpopulation observed in their electron microscopy and that these smaller nanoparticles grew at the expense of the large 4.2 nm Ag seeds via a etching process where the larger nanoparticles lose surface atoms to the smaller clusters in the initially bimodal Ag nanoparticle distribution. Etching and aging of Au nanoparticles in the presence of alkyl thiols has been established by previous work.25 This process can narrow the size distribution as well as leading to a smaller average size. The time scale for the etching depends on the chain length of the alkyl thiol, being faster for shorter chain thiols but requiring many weeks for dodecanethiol, the thiol used in the present work. Because our optical measurements and SEC were obtained only a few days postsynthesis, we do not believe ongoing redistribution of atoms between clusters plays a significant role in the optical properties reported. Figure 2, a TEM of the Ag/Au, 1/2, d ) 4.8 sample confirms that size estimated by the chromatography is approximately 5 ( 0.5 nm and that no major etching of the core/shell nanoparticles has occurred. The lighter contrast of the lower Z Ag core of many of the particles shown is consistent, but does prove a Ag/Au nanostructure since particle orientation on the grid affects diffraction contrast as well. We have previously examined the nanostructure of Ag/Au, 1/2 and Ag/Au, 2/1 nanoparticles with a total size of around 4 nm using high angular dark field imaging microscopy, HAADF.17 By modeling the HAADF intensity line profiles through the Ag/Au nanoparticles and comparing to experimental data, it was established that Au atoms were concentrated in the shell and that Ag atoms were mainly in the core for the 1:2 composition of Figure 4. However, interdiffusion at the interface could not be ruled out, especially for the 2/1 composition. This is likely to remain a difficult issue to settle by structural methods alone because of the high spatial resolution required. If interdiffusion of atoms at the Ag/Au or Au/Ag interface destroys the core shell structure, than we expect the optical absorbance of all three nanostructures to be identical. In Figure 6 we show absorbance profiles normalized to their peaks obtained during SEC of core/shell and alloy nanoparticles with sizes ranging from 3.5 to 4.8 nm. The absorbance profile of each has a unique shape and position depending on whether a sequential or coreduction was used to form the clusters. For example, AgAu, 1/1, d ) 4.8 alloy nanoclusters have a blueshifted and narrower peak compared to either Ag/Au, 1/1 or

J. Phys. Chem. B, Vol. 113, No. 9, 2009 2653

Figure 6. The effect of nanoparticle structure on absorbance is shown by absorbance spectra of three samples of nearly similar composition but different spatial distribution of Ag and Au atoms: Ag/Au, 1/1, d ) 4.4, open circles, red curve; Au/Ag, 1/1, d ) 3.5, open squares, blue curve; and AgAu, 1/1, d ) 4.8, open triangles, green curve. Spectra were collected online at the elution peak during chromatography.

Au/Ag, 1/1 samples. Comparing the Ag/Au, 1/1 to the Au/Ag, 1/1 spectra, we observe that the Ag/Au sample is slightly blueshifted compared to the Au/Ag sample. The broadening to the red of the peak is the same within experimental error, but the presence of Au in the shell of the Ag/Au sample results in a more asymmetrical absorbance profile, with stronger short wavelength absorbance, nearly identical to a pure Au sample of the same size. This shows that most of the energy dissipation due to electron scattering is occurring at the shell/solvent interface. The more random distribution of Ag and Au atoms in the AgAu, 1/1 nanoparticle sample produces a spectrum that differs significantly from a pure Ag or Au sample of the same size (see Figure 5) and whose peak is blue-shifted and narrower than either the Ag/Au or the Au/Ag nanostructure. The results shown in Figure 6 demonstrate that even if one were to analyze each of these samples for average size and composition using TEM and XRF one would still not be able to predict the optical absorbance of a dispersion of these nanoparticles. The absorbance depends on the distribution of atoms within the cluster, determined by the sequence of reduction of the two metals. However, the data of Figure 6 allows one to prepare samples having unique optical tags and subsequently identify any sample labeled with a chosen tag from its optical absorbance profile alone. Three Au/Ag samples with compositions of 1.9/1, 1/1, and 1/1.9 were investigated. The optical properties of these samples are shown in Table 2. The Au/Ag, 1.9/1, d ) 3.1 nm sample shows very little effect of the Ag deposited on the Au on the absorbance peak at 501 nm or the line width. The presence of the Ag shell does not affect the optical properties to the same degree as the deposition of Au on Ag nanoparticles. As the amount of Ag deposited increases to 1/1, the line width reflecting the amount of damping of the electron oscillation in the Au/Ag nanoparticle is reduced. As the amount of Ag is increased further to 1/2, a blue shift in the peak position is finally observed with a small decrease in line width. Comparing this behavior to the Ag/Au samples in Table 2, the relative effect of the Ag/Au ratio on both line width and peak position is smaller for the Au/Ag nanostructures. The mild changes in peak position and line width we observed in 4 nm Au/Ag nanoparticles differ from that reported for 12

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Wilcoxon

Figure 7. Absorbance spectra obtained at the elution peak for three AgAu nanoalloy samples and one reference Au sample. Labels on the spectra are as follows: AgAu, 2/1, d ) 5.0, open circles, red curve; AgAu, 1/1, d ) 4.8, open squares, blue curve; AgAu, 1/2, d ) 4.3, open triangles, green curve; and Au, d ) 4.0, solid circles, black curve.

nm Au seed nanoparticles with Ag deposited on the surface by Lu and co-workers.13 That study reported a large shift in the peak absorbance from 520 nm (12 nm gold seed) to 420-430 nm (27 nm Au/Ag nanoparticle) with significant narrowing of the line width and a much more symmetrical absorbance profile than we observe. Upon the basis of the total Au/Ag particle size of 27 nm, the silver shell thickness was about 7 nm, much larger than the total particle size of our largest Au/Ag samples. The larger shell thickness may provide more effective electromagnetic screening of the Au core and likely a spatially sharper Au/Ag interface relative to the total particle size. This does not rule out some interdiffusion of atoms at the Au/Ag interface for both small and large nanoparticles as was suggested in the work of Shibata and co-workers.12 They found that atomic interdiffusion was greatest for the smallest Au-core particles studied, 4.6 nm, and thus of direct relevance to the even smaller Au-core particles of the present study. Shibata et al. reported that for particles such as the 12 nm Au-core particles of Lu et al. that the Au/Ag interface is sharp to within a monolayer. The amount of Au/Ag atomic interdiffusion was found to depend strongly on the presence of defects such as vacancies at the interface in their model. If such vacancies are annealed naturally during the cluster deposition/growth process, then a sharper interface would be predicted, even for smaller particles. The optical absorbance features of AgAu alloy nanoparticles are shown in Table 2 and in Figure 7 for three compositions of similar sizes between 4.3 and 5 nm. The absorbance spectrum of a reference Au, d ) 4 nm particle is also plotted. Optical properties for smaller 3 nm AgAu nanoparticles are also presented in Table 2. The spectra in Figure 7 have been offset vertically for easier comparison. As the ratio of Ag/Au increases from 2:1 to 1:2 the peak red-shifts from 487 to 511 nm. The latter is within experimental error of the peak of the Au, d ) 4.0 nm sample shown in Table 2. The half-width to the red of the peak broadens from 59 to 86 nm as the Au content increases. The 3 nm AgAu nanoparticles also red-shift and broaden with Au content, but for each composition have more broadening, as is common for smaller particles of both Au and Ag.7 Chromatography and TEM indicate that all these AgAu nanoalloy samples have low size dispersion of between 5 and

Figure 8. Bright-field TEM of a nanoalloy sample, AgAu, 1/2, d ) 4.3 nm. The nanoparticles are stabilized with dodecanethiol on the cluster surface and form ordered hexagonal arrays on the holey carbon grid.

10% of the average size and a spherical shape. Figure 8 shows on highly ordered region of the AgAu, 1/2, d ) 4.3 nm sample. Facets on individual nanoparticles can be seen in this image. Comparing the nanoparticle electron diffraction contrast of Figure 8 to that of Figure 3 of the Ag/Au, 1/2, d ) 4.8 nm sample, we can see that the center region of many of the nanoparticles in Figure 3 appear to be lighter or lower contrast, as might be expected if the lower Z metal Ag is concentrated in the core. However, because all nanoparticles on the grid have a wide range of orientation relative to the incident electron beam, the diffraction contrast varies considerably from particle to particle in both TEMs. When combined with the data of Table 2, showing the latter sample to have more line broadening, the core/shell structure is suggested, but it not possible to rule out the minority presence of Au atoms in the core nor Ag in the shell. Figure 9a, left panel, shows a TEM of the Au, d ) 4 nm sample, and the right panel (b) is a TEM of the Au/Ag, 1/1, d ) 3.7 nm sample, whose optical properties are given in Table 2. There is no evidence from panel b that Ag atoms are concentrated in the shell, and the differences in diffraction contrast between nanoparticles in each TEM due to particle orientation effects are as great as between panels. So, one would not be able to identify the nanostructure from these TEMs, although the optical absorbance is significantly different, as noted previously. For example, the Au/Ag, 1/1, d ) 3.7 nm sample has less absorbance on the short wavelength side of the peak due the presence of the Ag in the shell. The identical gaps between particles in Figures 3, 8, and 9 are due to the presence of dodecanethiol on the cluster surface. Without a surfactant on the cluster surface, cluster aggregation will occur. The effect of such aggregation on the optical absorbance is a significant red shift of the absorbance peak to the 600-700 nm range and broadening of the absorbance in the case of Au nanoclusters.26 There is no evidence from either TEM or SEC of any of the samples of Table 2 that aggregation plays a role in the optical absorbance properties.

Dispersed Gold and Silver Alloy Nanoparticles

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Figure 9. Bright-field TEM of the (a) Au, d ) 4 nm and (b) Au/Ag, 1/1, d ) 3.7 nm core/shell sample, both stabilized with dodecanethiol and imaged with the same magnification and resolution.

Several groups have investigated AgAu nanoalloys with a more-or-less random distribution of Ag and Au atoms. Larger, charge-stabilized clusters in water, as well as smaller, sterically stabilized clusters in organic solution were investigated. A common finding is that only a single absorbance peak is measured and that its position shifts nearly linearly with composition from that of a pure Ag cluster absorbance at ∼420 nm to pure Au absorbance at ∼520 nm. Mallin and Murphy, for example, reported that AgAu clusters with sizes between 5 and 6 nm had absorbance peaks in water at 430, 465, and 505 nm for compositions of Ag/Au ) 3/1, 1/1, and 1/3, respectively.10 Their spectra also show very significant broadening, (e.g., >100 nm for AgAu, 3/1 composition), exceeding that observed in our AgAu, 2/1 sample (59 nm). It is possible that their growth process in water produces more defects at the interface, leading to more damping and electron scattering energy loss and thus additional line broadening. Link and coworkers, synthesized larger, ∼10 nm AgAu nanoparticles in water and also observed only a single plasmon absorbance peak whose position depended linearly on composition.4 Perhaps due to the larger overall cluster size, their linewidths were considerably narrower, about 50 nm for their AgAu, 1/1 sample compared to the Mallin studies. This reduced broadening indicates reduced energy loss at the nanoparticle/water interface. The actual peak positions for a given composition were nearly identical, showing a negligible size dependence for AgAu nanoparticles in water. We observe only a slight blue shift for the AgAu nanostructure with decreasing size, similar in magnitude to that reported previously for pure Au nanoclusters.7 Charge stabilized AgAu clusters in water may have optical spectra that differ from clusters of similar size and composition that are neutral and sterically stabilized in organic solvents such as toluene or hexane. A study by Kariuki and co-workers of 2-3 nm AgAu nanoclusters synthesized by a two phase reduction of AuCl4 and AgBr2 and stabilized by dodecanethiol can give insight into this question and allow comparison to our dodecanethiol-stabilized 3 nm and 4-5 nm AgAu clusters.14 As in the studies of larger AgAu clusters in water, a red-shift,

loss of peak symmetry, and broadening was observed with increasing Au content. Instead of a linear dependence of peak position on Au content over the full range of Au, the peak absorbance first red-shifted for compositions less than Ag/Au ) 1:2 (∼66% Au) and then saturated at 520 nm for higher Au content. This is similar to our observations obtained during SEC, and the observed peak position of our AgAu, 1/1, d ) 3.2 nm of 491 nm is identical to that reported by that group. The peak of 506 nm for our AgAu, 1/2, d ) 3.1 nm is also close to the value of 520 nm found in their experiments. A pure 3 nm Au cluster has an absorbance maximum at 500 nm.7 IV. Conclusions We used SEC to size separate and analyze nanoparticles with two types of nanostructure, core/shell and alloy, as a function of composition and size. We find a very weak dependence of plasmon energy and damping in nanoalloys compared to core/ shell structures of equal size. The nanoalloys show a blue shift with decreasing size, comparable to that observed for pure Au clusters. We also observed that increasing the Au content of Ag/Au core/shell nanoparticles leads to a red shift and increased broadening of the plasmon absorbance, whereas an increase in Ag content in Au/Ag nanoparticles results in a blue shift and reduced peak width. In both cases, the larger amounts of Au in the nanoparticle increases the absorbance at wavelengths to the blue of the peak possibly due to the importance of interband transitions in Au compared to Ag. For a constant composition and size, each type of nanostructure can be distinguished and identified solely by its optical absorbance features. Acknowledgment. This work was partially performed at Sandia National Laboratories and supported by the Division of Materials Sciences, Office of Basic Energy Research of the U.S. Department of Energy under Contract No. DE-AC04-94AL8500. References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer-Verlag: Berlin, 1995.

2656 J. Phys. Chem. B, Vol. 113, No. 9, 2009 (2) Faraday, M. Philos. Trans. R. Soc. London 1857, 147, 145. (3) Turkevich, J.; Garton, G.; Stevenson, P. C. J. Colloid Sci. Suppl. 1 1954, 26. (4) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3533. (5) Wilcoxon, J. P.; Abrams, B. L. Chem. Soc. ReV. 2006, 1194. (6) Wilcoxon, J. P.; Williamson, R. L.; Baughman, R. J. J. Chem. Phys. 1993, 98, 9950. (7) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. J. Chem. Phys. 2001, 115, 998. (8) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7064. (9) Hostetler, M. J.; Zhong, C.-J.; Yen, B. K. H.; Anderegg, J.; Gross, S. M.; Evans, N. D.; Porter, M.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 9397. (10) Mallin, M. P.; Murphy, C. J. Nano. Lett. 2002, 2, 1237. (11) Shon, Y.-S.; Dawson, G. B.; Porter, M.; Murray, R. W. Langmuir 2002, 18, 3880. (12) Shibata, T.; Bunker, B. A.; Zhang, Z.; Meisel, D.; Vardeman II, C. F.; Gezelter, J. D. J. Am. Chem. Soc. 2002, 124, 11989. (13) Lu, L.; Wang, H.; Zhou, Y.; Xi, S.; Zhang, H.; Hu, J.; Zhao, B. Chem. Commun. 2002, 144. (14) Kariuki, N. N.; Luo, J.; Maye, M. M.; Hassan, S. A.; Menard, T.; Naslund, H. R.; Lin, Y.; Wang, C.; Engelhard, M. H.; Zhong, C. Langmuir 2004, 20, 11240.

Wilcoxon (15) Wilson, O. M.; Scott, R. W. J.; Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 1015. (16) Yang, J.; Lee, J. Y.; Too, H. J. Phys. Chem. B 2005, 109, 19208. (17) Li, Z. Y.; Yuan, J.; Chen, Y.; Palmer, R. E.; Wilcoxon, J. P. Appl. Phys. Lett. 2005, 87, 243013. (18) Chen, H. M.; Liu, R.-S.; Jang, L.-Y.; Lee, J.-F.; Hu, S. F. Chem. Phys. Lett. 2006, 421, 118. (19) Chandran, S. P.; Ghatakb, J.; Satyamb, P. V.; Sastry, M. J. Colloid Interface Sci. 2007, 312, 498. (20) Li, Z. Y.; Wilcoxon, J. P.; Yin, F.; Chen, Y.; Palmer, R. E.; Johnson, R. L. Faraday Discuss. 2008, 138, 363. (21) Wilcoxon, J. P. U.S. Patent No. 5,147,841, Sep. 15, 1992. (22) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. Langmuir 2000, 16, 9912. (23) Wilcoxon, J. P.; Craft, S. A. Liquid Chromatographic Analysis and Characterization of Inorganic Nanoclusters. In Nanostructured Materials; Elesvier Science Ltd.: 1997, 9, 85. (24) Wilcoxon, J. P.; Provencio, P. P. J. Am. Chem. Soc. 2004, 126, 6402. (25) Wilcoxon, J. P.; Provencio, P. J. Phys. Chem. B 2003, 107, 12949. (26) Wilcoxon, J. P.; Martin, J. E.; Schaefer, D. W. Phys. ReV. A 1989, 39, 2675.

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