Photochemical Strategies for the Facile Synthesis of Gold− Silver Alloy

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Photochemical Strategies for the Facile Synthesis of Gold-Silver Alloy and Core-Shell Bimetallic Nanoparticles† Carlos Miguel Gonzalez, Yun Liu, and J. C. Scaiano* Centre for Catalysis Research and InnoVation and Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie, Ottawa K1N 6N5, Canada ReceiVed: March 6, 2009; ReVised Manuscript ReceiVed: May 10, 2009

The reduction of HAuCl4 and AgNO3 in aqueous surfactant solutions by 2-hydroxy-2-propyl radical generated by the photochemical cleavage of 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-1-propane-1-one, commercialized as Irgacure-2959 (I-2959), produces Au-Ag bimetallic nanoparticles with different composition, and architecture,, alloys, and core-shells. The nanoparticle architecture depends on the surfactant employed. The use of hexadecyltrimethylammonium chloride (CTAC) produces Au(core)-Ag(shell) nanoparticles, as expected on the basis of the redox properties of the two metals, whereas when the surfactant is sodium dodecyl sulfate (SDS), it promotes the formation of Au-Ag alloy nanoparticles; the effect is attributed to silver’s Coulombic advantage that compensates for the fact that gold is a “more noble” metal. Both structures are characterized by UV-vis spectroscopy, transmission electron microscopy (TEM) and high-resolution transmission spectroscopy (HR-TEM). Nanoparticles are gradually finding applications in novel devices, catalysts, and numerous applications in biology and the health sciences.1,2 While most synthetic strategies rely on thermal methods, an alternative is to employ photochemical techniques. This approach, however, remains one of the least explored and sometimes poorly understood. Photochemical methods for the synthesis of noble metal nanoparticles are well established, but long irradiation times and modest yields are common. Recently, we reported that by using transient species to produce Ag nanoparticles (NP), such as the hydroxydiphenylmethyl radical, the irradiation time required to reduce Ag+ to AgNP was a few minutes.3 We also reported the synthesis of AuNP using the water-soluble benzoin, Irgacure-2959 (I2959), which produces photochemically 2-hydroxy-2-propyl radicals, Scheme 1.4,5 This radical reduces Au(III) (as AuCl4-) to AuNP, with a photolysis time of just minutes. Similarly, Au(I) is also readily reduced by I-2959.5,6 The preparation and optical properties of gold and silver nanoparticles have been extensively studied. These metallic nanoparticles have been produced with different sizes and shapes.8 The most commonly used method is the thermal reduction of ionic species of these metals with reducing agents such as NaBH4, N2H2, sodium citrate, acid ascorbic, and alcohols.6,9,10 Another important source of information arises from the study of the optical properties of metal nanoparticles in solution.11 For example, both Au and Ag nanoparticles show a strong absorption band in the UV-visible region leading to their intense color.8,12 The current interest in the analytical applications of AuNP and AgNP is largely related to their high molar absorptivity12 and the intrinsic capacity to respond to a diversity of chemical environments and physical stimuli through a change of their optical properties.13 Solutions of silver spherical nanoparticles are usually yellow, whereas for gold the shades vary from pink to intense blue depending on the nanoparticle size, shape, and their degree of aggregation. The origin of color †

Part of the “Hiroshi Masuhara Festschrift”. * Corresponding author. E-mail: [email protected].

SCHEME 1: Production of AuNP Following Norrish Type I Photocleavage of I-2959a

a

The 3-electron reduction of Au(III) involves several reductive and disproportionation steps. The efficiency of particle generation is determined by the production of ketyl radicals that occurs with a quantum yield of 0.29.7

is due to the surface plasmon absorption originating from the coherent motion of the outermost electrons of the nanoparticle when they interact with an external electromagnetic field. Mie explained the origin of this behavior for the first time; by solving the Maxwell equation with the corresponding boundary conditions, Mie found that the optical absorption spectra for metal nanoparticles in solutions depend on the size of the nanoparticles and the electronic properties of both the nanoparticles and the surrounding medium.14 Gold and silver nanoparticles have been investigated in great detail; bimetallic nanoparticles on the other hand have received less attention.12,15,16 Because of the low catalytic power of gold in comparison to other noble metals such as palladium and platinum, the investigation of the properties of gold containing nanoparticles such as alloys, core-shells, and supported17 structures deserves attention because of the potential enhancement in their catalytic properties.2,17-19 We now report the photochemical preparation of Au-Ag alloys in SDS micelles and Au(core)-Ag(shell) nanoparticles in hexadecyltrimethylammonium chloride (CTAC) micelles. We also discuss the mechanism of formation of these two different Au-Ag nano-

10.1021/jp902061v CCC: $40.75  2009 American Chemical Society Published on Web 05/27/2009

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particles using data obtained through the UV-vis spectra and imaging techniques. The optical properties of these species are also reported in detail. Our method allows for the formation of small Au-Ag alloy nanoparticles with reproducible average size. We also report two methods for the preparation of Au-Ag core-shell nanoparticles using the ketyl radical from I-2959 as a reducing agent and the surfactants CTAC or SDS as tools to steer the formation of specific nanostructures. Throughout this manuscript we reserve the word “alloy” for bimetallic systems where we have evidence of a true mixed composition, while we describe as core-shell those particles where the two metals occupy different regions of the nanoparticle. Experimental Section Materials. Chloroauric acid hydrate, HAuCl4 · 3H2O, was used as received (Sigma-Aldrich). Silver nitrate (Sigma-Aldrich) was recrystallized from water, and the photoreducing ketone I-2959 (Irgacure-2959 from Ciba Speciaty Chemicals) was recrystallized from analytical reagent grade methanol. Sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (CTAC), both from Sigma-Aldrich, were used as received. Nitric acid (from Fisher) was used as received. Photochemical and Spectroscopic Instrumentation. For all the experiments a Luzchem photoreactor equipped with eight 8 W UVA lamps was used. This corresponds to an irradiance of about 54 W/m2 with approximately 4% spectral contamination, composed of visible and UVB light. Absorption spectra were recorded at room temperature with 1 nm resolution using a Cary-1 UV-vis spectrometer. The nanoparticles were also characterized by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM). TEM specimens were prepared by placing small drops of the nanoparticle solution onto a copper grid coated with carbon film (300 mesh, obtained from Electron Microscopy Sciences). The size of the nanoparticles was determined by TEM, while the structures of the nanoparticles were examined by HR-TEM. Both TEM and HR-TEM studies were performed using a JEOL JEM-2100F field emission transmission electron microscope equipped with an ultrahigh resolution pole-piece operating at 200 kV. An Oxford energy dispersive X-ray spectrometer (EDS) attached to the JEM-2100F microscope was used to determine the average elemental composition of the nanoparticles. Synthesis of Au-Ag Alloy Nanoparticles. The alloys were prepared by irradiating a solution of I-2959, HAuCl4 · 3H2O and AgNO3 in SDS micelles, with Milli-Q ultrapure water as the solvent, using stoichiometric amounts of I-2959. For all the experiments, the total combined concentration of Au and Ag was 0.2 mM, and for SDS, 0.01 M (slightly above the surfactant cmc). The photoreduction was carried out in 1 cm × 1 cm × 3 cm fused silica cuvettes. The solutions were purged with argon and irradiated with eight UVA lamps. The nanoparticles were prepared with different molar ratios of metals from xAu ) 1.0 to xAg ) 1.0. For the TEM images, the remaining SDS was eliminated from the copper grids by washing three times with 10 µL of isopropanol. The reactions took between 4 and 8 min to be completed. The course of the reaction was monitored by UV-vis spectroscopy. Depending on the Au-to-Ag ratio, the solutions showed different tones of yellow with a dominant reddish tone at high molar ratio of gold. Synthesis of Self-Assembled Au-Ag Core-Shell Nanoparticles. For this synthesis, we used the same reagents as those used for the preparation of Au-Ag alloy NP except that for this experiment CTAC was used as the surfactant at a

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Figure 1. (a) UV-vis absorption spectra of Au-Ag alloy nanoparticles with increasing gold molar fraction xAu shown in the numbers by each curve. In all cases the total cation concentration was 0.2 mM. Samples were subject to a single UVA exposure of 5-7 min. For comparison purposes, solutions of gold nanoparticles and silver nanoparticles were prepared by the same methods described above (xAu equal 0 and 1).

concentration of 0.01 M. After photolysis the resulting solutions of nanoparticles showed a variety of tones, from dark red to yellow. Bimetallic core-shell nanoparticles were prepared by varying the molar fraction of metal nanoparticle precursor, with a total metal ion concentration of 0.2 mM. The synthesis of these bilayer nanoparticles was done in one step. Tuning the desired structure was done by mixing first HAuCl4 · 3H2O with CTAC (0.01 M) and the required amount of I-2959 in ultrapure Milli-Q water, followed by the addition of AgNO3, followed immediately by irradiation of the sample. This procedure allows for the temporal isolation of AuCl4- and I-2959 from Ag+, delays the precipitation of AgCl, allowing the initial formation of a gold core followed by the deposition of Ag atoms. The experiments resulted in the formation of two clearly distinct absorption bands originated by the presence of unmixed gold and silver domains distributed in different regions of the nanoparticles. Synthesis of Au-Ag Core-Shell Nanoparticles by Stepwise Deposition of Layers of Ag onto Au Nanoparticles. Gold nanoparticles were prepared in surfactant solutions of CTAC and SDS employing I-2959 in stoichiometric concentration using the methods reported earlier.5 The silver shell was deposited onto the surface of AuNP (prepared from 0.2 mM HAuCl4) by using mixtures of different molar ratios of AgNO3 with the corresponding amount of I-2959 followed by UVA irradiation. In the case of CTAC micelles, the procedure resulted again in the formation of two distinct absorption bands. The same experiment in SDS micelles resulted in agglomeration of nanoparticles. At low Ag concentration (xAg ) 0.2), the UV spectrum shows two bands corresponding to the formation of core-shell nanoparticles but a large scatter background was also present. It was not possible to prepare inverse core-shell nanoparticles, Ag (core)-Au (shell) by this method, since the higher reduction potential of AuCl4- compared to Ag+ makes this process unfavorable. Addition of HAuCl4 to solutions of AgNP resulted in an initial loss of color, followed by the appearance of a pink color, indicative of the formation of AuNP by the reduction of AuCl4- by Ag(0). Results Au-Ag Alloy Nanoparticles. As reported in previous studies,12,20 the formation of alloy structures is indicated by the presence of only one plasmon band that shifts to the blue as the molar ratio of Ag increases,16 Figure 1. For the formation of bimetallic core-shell structures, two bands are expected in the UV spectra.

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Figure 4. UV-vis spectra of self-assembled Au-Ag core-shell nanoparticles with varying gold:silver ratio for a 0.2 mM solution (total metal ions) containing CTAC and I-2959 irradiated for 6-8 min.

Figure 2. (a) TEM image of Au-Ag alloy nanoparticles prepared using a mixture of HAuCl4:AgNO3 80:20 and I-2959 in SDS solution (size bar ) 20 nm). (b) HR-TEM image of the same sample. (c) Histogram (346 particles), where the mean size is calculated to be 7.4 ( 1.2 nm.

Figure 3. Change of UV-vis spectra for the formation of nanoparticles from a 0.2 mM solution (xAu ) 0.5) containing SDS and I-2959. UVA cumulative irradiation time in seconds given by the numbers next to each curve, and the UVA irradiance in these experiments was about 50 W/m2.

Figure 2a shows a TEM image of Au-Ag alloy nanoparticles prepared with a solution of Au:Ag molar ratio equal to 80:20. The mean size was 7.4 nm. The corresponding histogram for the particle diameter is given in Figure 2c. Figure 2b shows HR-TEM images of the same Au:Ag nanoparticles, which reveals the presence lattice defects, i.e., including twins and stacking faults. The calculated molar absortivity for gold nanoparticles (per total gold amount, ε ) absorbance/[HAuCl4]) was found to be similar to values previously reported.12 For the AuNP prepared for this study, the calculated εmax ) 3970 M-1 cm-1.12 To understand the dynamics of the formation of alloy nanoparticles, UV-vis spectra were recorded at different irradiation times for a solution with equal concentrations of both metal ions, Figure 3. The UV-vis spectra indicate that the first ionic species to be reduced is AuCl4- as the UV spectra at the early stages of the process shows a band at about 520 nm, and the solution at this time was pink. As the irradiation proceeded, the UV spectra maximum shifts to shorter wavelengths and the solution becomes orange as Ag+ is coreduced along with AuCl4-. Even if the tendency of ionic gold to reduce is greater

than that of the Ag+, AuCl4- needs to interact with more than 1 equiv of ketyl radical before the full reduction of AuCl4- is achieved, whereas Ag+ only needs one electron. Under our experimental conditions I-2950 is the dominant UVA absorber. Au-Ag Core-Shell Nanoparticles. Bilayer core-shell structures were found for the synthesis of Au-Ag nanoparticle using CTAC as the surfactant. Figure 4 shows the UV-vis spectra of core-shell nanoparticles with different metal composition. The UV-vis spectra for the Au-Ag core-shell bilayer nanoparticles show two plasmon resonance bands, Figure 4. The plasmon resonance energies depend on nanoparticle composition, shape, size and surroundings; as the Ag shell thickness increases, the plasmon band corresponding to gold shifts slightly to the blue. The appearance of two resonance16,21-25 plasmon bands and the consistent change of absorption band wavelength and molar absorptivity with irradiation time, Figure 4, strongly suggest the formation of core-shell structures rather than true alloys. The electronic spectra of our Au-Ag core-shell samples reproduce the electronic profiles of Au-Ag core-shell nanoparticles reported by other authors.21,22,24 The UV spectra for the solution of the bilayer nanoparticles, Figure 4, do not show an absorption maxima at 520 nm, indicating that pure gold nanoparticles were not present. Because of the most complex pattern absorption of the region around 400 nm, the same conclusion cannot be drawn directly for the formation of silver nanoparticles, but we can assume that similarly to gold, silver nanoparticles were not formed given the lower reduction potential for Ag+/Ag(0) in comparison to that for AuCl4-. Because of the interaction of Ag+ with Cl- from both CTAC and AuCl4-, AgNP are formed in presence of an excess of solid AgCl, which explain the unusual wide band for the AgNP centered at 400 nm; solid AgCl does not appear to undergo significant direct photodecomposition in the exposure times required in our work. As the reaction proceeds, AgCl gradually dissolves and contributes to the final composition of the particles, along with some larger Ag structures in the micrometer scale are also formed as evidenced by TEM imaging and EDS analysis. Figure 5 shows TEM images of core-shell nanoparticles prepared from an equimolar solution of Au and Ag. The HRTEM image, Figure 5c, supports the formation of core- shell structures. Figure 5b shows a micrograph for self-assembled core-shell nanoparticle, the particle clearly display a “core-shell” contrast. Most of the particles detected under TEM imaging have a darker central part (Au) and a clearer outer part, (Ag) or a dark line between the central (core) and the outer (shell). This is consistent with the contrast observed for other crystalline particles, core-shell or multilayer materials.21,26 The nanoparticles are rather polydisperse and for particles prepared with a ratio Au: Ag ) 1:1, the mean diameter is 12.7 ( 2.6 nm, a size for which good interlayer separation is common.25 Particles with

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Figure 7. UV-vis spectrum of a mixture of AuNP and AgNP solutions, both 0.1 mM, using as surfactant CTAC and generated from I-2959. The plasmon band corresponding to AgNP is rather broad as a result of the formation of AgCl when AgNO3 was dissolved in an aqueous solution of CTAC. Initial precipitation of AgCl promotes the formation of large particles.

Figure 5. (a), (b) TEM images of self-assembled Au-Ag core-shell nanoparticles prepared using an equimolar mixture of HAuCl4:AgNO3. (c) HR-TEM image of the same sample as in (b). (d) Histogram (296 particles), mean size 12.7 ( 2.6 nm. EDS analysis reveals metallic particles free of chlorine (i.e, particles do not contain AgCl).

Figure 6. Change in UV-vis spectra during the irradiation of an equimolar solution leading to Au-Ag core-shell nanoparticles. The core-shell nanoparticles were prepared using an equimolar solution of HAuCl4 and AgNO3 containing I-2959 and CTAC. The UVA irradiation time in minutes is given by the numbers next to each curve.

higher silver composition tend to show better contrast (vide infra; see Figure 8 later). The self-assembled Au-Ag core-shell nanoparticles were made possible by the temporal isolation of AuCl4- and I-2959 from Ag+ in this micellar system. Thus, I-2959 will reside largely in the micelle, while AuCl4- is expected to act as a counterion to the positive CTAC micelle, while repulsive interactions will keep Ag+ away from the positive surface. Figure 6 shows the UV-vis spectra for the formation of selfassembled Au-Ag core-shell structures at different irradiation times for an equimolecular solution of both Ag+ and AuCl4-. Initially, the spectra show the appearance of gold nanoparticles with a band maximum at 520 nm that consistently shifts to the blue with irradiation time, along with the formation of a second resonance plasmon band at ∼415 nm. The blue shift of the plasmon band for gold has been related to the increase in the thickness of the silver layer.13,22 Such behavior has been recently termed as plasmon hybridization.27 The silver nuclei will exert a greater attraction over the outermost electron of gold than ligands in an aqueous solution. Consequently, interlayer electronic coupling leads to an increase in the energy required for oscillation of the Au valence electrons.

For the nanoparticle batch represented in Figure 5, prepared using a solution of 50% of both AuCl4- and Ag+, energy dispersive X-ray spectroscopy (EDS) revealed a molar ratio of Au:Ag ) 55.2-to-44.8, in close agreement to the equimolar ratio used in their preparation. For comparison purposes, the UV-vis spectrum of a mixture of AgNP and AuNP, both prepared independently, each at 0.2 mM, was also recorded, Figure 7. The resulting spectrum shows the typical band for AuNP located around 520 nm and for silver NP around 400 nm. Based in Figure 7, it is reasonable to conclude that a simple mixture of these two species does not promote significant change in the optical properties of the pure nanoparticles, indicating that no chemical change occurred when both solutions were mixed. Moreover, the nanoparticles resulting from the experiments described above are core-shell structures and not a mixture of Au and Ag nanoparticles. Another experiment was performed to confirm the formation of core-shell structures. A fresh solution of Au-Ag core-shell nanoparticles (xAg ) 0.8) was mixed with different aliquots of concentrated HNO3 (30, 70, 100, and 150 µL of acid added to 3 mL of nanoparticle solution). Figure 8 shows the UV-vis spectra for the solutions resulting from the mixture of Au-Ag core-shell nanoparticles with concentrated HNO3. As the amount of HNO3 added increased, the solution changed color from yellow to fairly pink. This indicates that the Ag shell dissolved as the concentration of HNO3 increased. Addition of more aliquots of concentrated HNO3 (up to 150 µL) did not dissolve the Au nanoparticles. Before addition of HNO3, a broad band with a maximum at 400 nm and a small shoulder at 540 nm was present, Figure 8; as HNO3 was added, the plasmon band at 400 nm disappeared and a band with a maximum 520 nm, consistent with AuNP, became evident and persisted when higher amounts of concentrated HNO3 were added. Spectroscopic studies confirm the results of the simple observation of Figure 8; thus the initial synthesis mix contained 20% Au, the EDS analysis of the original particles revealed 15% Au, but after acid treatment the gold content was 96%, consistent with the dissolution of the external silver shell. In contrast with core-shell particles, nitric acid treatment of alloys dissolves the nanoparticles and results in a solution with a gray appearance. Stepwise Assembly of Au-Ag Core-shell Nanoparticles. Figure 9 shows the UV-vis spectra of bilayer nanoparticles prepared in CTAC micelles by depositing a layer of silver on the surface of AuNP seeds previously prepared in CTAC. Both reduction processes were carried out by photolysis of I-2959 as described in the Experimental Section. Seeding is a common method for thermal nanoparticle synthesis.10,28 The UV-vis

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Figure 10. (a) TEM image of Au-Ag core-shell nanoparticles prepared using a mixture of AuNP: AgNO3 equal to 50:50. (b) Histogram (399 particles), mean diameter 10.9 ( 2.0 nm.

Figure 8. Bottom: UV-vis spectra for a mixture of self-assembled core-shell nanoparticles (xAg) 0.8) with different volumes of HNO3 (see text). Volumes (in microliters) given in the figure were added to 3 mL of nanoparticle solution (100 µL equals 1.6 × 10-3 mol) and the spectra recorded within 10 min after acid addition. Top: TEM data for the initial particles, clearly revealing the lighter shell. The size bar at the bottom left corresponds to 5 nm.

Figure 11. UV-vis spectra following an attempted synthesis of Au-Ag core-shell nanoparticles with varying gold mole fraction. We attempted to deposit silver on the surface of gold seed nanoparticles by photochemical reduction in SDS (0.01 M) using I-2959. The numbers next to each trace indicate the mole fraction of gold, and the heavy trace corresponds to pure AuNP seeds. The total metal composition was 0.2 mM.

of the plasmon band and by extensive light scattering, Figure 11, where scattering signals are particularly evident around 700 nm. Discussion

Figure 9. UV-vis spectra of Au-Ag core-shell nanoparticles with varying Ag mole fraction. Silver was deposited onto the surface of gold seed nanoparticles in CTAC solution by photoreduction using I-2959. The numbers next to each trace indicate the mole fraction of gold, and the two heavy traces correspond to the monometallic nanoparticles.

spectra show the two expected absorption bands corresponding to the presence of core-shell bilayer structures. As for the selfassembly method, the plasmon band corresponding to the Au core blue shifts as the thickness of the Ag layer increases. The resonance plasmon absorption of pure gold nanoparticles, Figure 9, shows a band at 518 nm. This last comparison indicates that the shift of the gold plasmon band is related again to the influence of the Ag shell over the outermost electrons of the Au core. Figure 10 shows a representative TEM image and the corresponding histogram, the latter based on 399 particles. Seeding experiments in SDS micelles resulted primarily in agglomeration of nanoparticles and coalescence. At low Ag concentration (xAg ) 0.2), the UV spectrum shows a hint of two bands corresponding to the formation of core-shell nanoparticles but large scattering is also observed, Figure 11. At concentrations with molar fractions of silver above 0.2, aggregation of the nanoparticles was evidenced by the absence

Gold and silver nanoparticles have been investigated in detail;1,8 alloy nanoparticles have also been studied but overall have received less attention.12,21,26,29 Because of the low catalytic power of gold in comparison to other noble metals such as palladium and platinum, the investigation of the properties of gold containing nanoparticles such as alloys and core-shell structures deserves more attention because of the possibility of an enhancement in the catalytic properties of these nanostructures in comparison to the bulk materials. Other approaches to enhance AuNP catalytic properties include the use of active supports, which have shown considerable success in recent reports.2,17,18 The photoinitiated preparation of Au-Ag alloys in SDS micelles and Au(core)-Ag(shell) nanoparticles in CTAC micelles using I-2959 as an initiator is fast, clean, and efficient. Our method allows for the formation of small Au-Ag alloy nanoparticles with reproducible mean size in the presence of modest concentrations of SDS. We also report two methods (stepwise or one-step) for the preparation of Au-Ag core-shell nanoparticles that use the ketyl radical (1) and CTAC as a means to tune the desired structure. Gold is a better noble metal than silver, a characteristic that is of course a reflection of the corresponding redox potentials; from this perspective, one would expect gold to be reduced first in any system that contains Ag(I) and either Au(I) or Au(III). In fact, there are examples of galvanic exchange,30 in which silver particles can be used to reduce Au(III) to Au(0) and AuNP from them. Thus, the

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observations for the Au-Ag (core-shell) systems are not surprising, as they reflect directly redox properties. Similarly, failure to construct reverse particles, Ag-Au (core-shell), simply reflects that these nanomaterials would be counter to the electrochemical properties of these metals and require different experimental approaches.31 We were initially surprised by the formation of actual alloys in the presence of SDS, in contrast with expectations based on redox potentials, as explained above. We interpret this observation in terms of an electrostatically assisted formation of true Au-Ag alloys. That is, with the initiator predominantly in the micelle, the reducing radicals, even if hydrophilic, will originate within the micelle. The two sources of metal atoms, Ag+ and AuCl4-, are positive and negative, respectively. Thus, in the case of negative micelles (such as SDS) we expect Ag+ to be attracted to the micellar surface, effectively acting as a counterion, ready to accept the electron from the ketyl radical. In contrast, AuCl4- will be repelled by the micellar surface and thus the probability of at least the first reductive step [Au(III) f Au(I)] will decrease. In other words, while gold will have the redox advantage under all conditions, in the case of SDS silver acquires an electrostatic or Coulombic advantage that partly compensates for its redox handicap. Fortuitously, this compensation seems to be just right to form alloys, as illustrated in Figure 1. To get further insight into the dynamics of formation of the alloy nanoparticles, UV-vis spectra at different irradiation times were recorded for a solution with a Ag+:AuCl4- equimolar solution with a total metal nanoparticle precursor concentration of 0.2 mM, Figure 3. The UV-vis spectra indicate that for the first 60 s of photolysis no plasmon band is detected and there is only a minor indication of increased scattering. Several hypotheses can explain this result; for example, we already know that the Au(III) f Au(I) conversion gives no absorptions in this region. Further, very small nanoparticles do not show a plasmon resonance band. After 90 s we observe a shoulder at ∼525 nm (the solution was fairly pink), suggestive of gold plasmon absorptions; these may be formed by either direct reduction of gold by ketyl radicals or possibly by galvanic exchange30 with silver atoms or nascent AgNP, i.e., +

Au(III) + Ag f Ag + Au(II) f f Au 0

0

As the reaction proceeds, the alloy plasmon band becomes well-defined and after about 150 s shows a maximum around 480 nm (the solution turned orange), with Ag+ being successfully coreduced along with AuCl4-, Figure 3. Note how short the irradiation times are a clear indication of how efficiently our method yields nanoparticles. As for the self-assembly method (i.e., in CTAC micelles), the plasmon band corresponding to the Au core blue-shifts as the thickness of the Ag layer increases. Note that the resonance plasmon absorption of unmixed gold nanoparticles, Figure 9, shows a band at 518 nm. This last comparison indicates that the shift of the gold resonance band is related again to the influence of the thickness of the Ag shell over the outermost electrons of the Au core.16,24 A mixture HAuCl4 and AgNO3 produces the insoluble salt AgCl (Ksp ) 1.77 × 10-10). This is undesirable since we have observed that photoreduction in the presence of AgCl promotes the formation of large particles with increased polydispersity, and also silver crystals on the micrometer scale. When these two reagents are mixed in the presence of SDS, the precipitation of AgCl does not take place. This probably reflects that the SDS

micelles are negatively charged and can sequester the Ag+ cations as micelle counterions, thus minimizing interaction between Ag+ and Cl-. This is evidenced by the absence of turbidity that normally indicates the formation of insoluble AgCl. As mentioned before, the use of CTAC as surfactant does not allow the formation of Au-Ag alloy nanoparticles but rather core-shell bilayer nanoparticles. The order of addition of the reagents is very important to obtain the desired bilayer nanostructure and if this addition order is altered, the process results in a mixture of particles of different architectures in both the nano- and micrometer scales. The formation of Au-Ag core-shell structures can be explained by a temporal separation of AuCl4- from Ag+ around the positively charged CTAC micelles. It is possible that a strong electrostatic attraction keeps AuCl4- confined within positive CTAC micelles, and I-2959, while water-soluble, probably has a preference for partition in the micelle or near its interface. Conclusion The photochemical generation of bimetallic nanoparticles is extremely efficient when they involve the generation of strongly reducing ketyl radicals from excited state precursors with short lifetimes, as is the case for the benzoin I-2959,7 a strategy similar to that reported recently for monometallic nanoparticles.4,5 In the case of Au/Ag nanoparticles, these can be prepared as true alloys, or as core-shell materials, with variable size. In particular, in the case of alloy nanoparticles, it may be worth exploring their catalytic applications,2 although this may require somemethodoptimizationtoproduceslightlysmallernanoparticles. Interestingly, while in all cases redox properties favor the early deposition of gold,32 in the case of SDS micelles silver has an electrostatic advantage that leads to a delicate balance and the formation of true alloys. Acknowledgment. We acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), CFI, and the Government of Ontario. References and Notes (1) Ozin, G. A.; Arsenault, A. C. Nanochemistry: A Chemical Approach to Nanomaterials; Royal Society of Chemistry: London, U.K., 2005. (2) Corma, A.; Garcia, H. Chem. Soc. ReV. 2008, 37, 2096–2126. (3) Scaiano, J. C.; Aliaga, C.; Maguire, S.; Wang, D. J. Phys. Chem. B 2006, 110, 12856–12859. (4) McGilvray, K. L.; Decan, M. R.; Wang, D.; Scaiano, J. C. J. Am. Chem. Soc. 2006, 128, 15980–15981. (5) Marin, M. L.; McGilvray, K. L.; Scaiano, J. C. J. Am. Chem. Soc. 2008, 130, 16572–16584. (6) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801–802. Turkevich, J.; Stevenson, P. C.; Hiller, J. Discuss. Faraday Soc. 1951, 11, 55–75. (7) Jockusch, S.; Landis, M. S.; Freiermuth, B.; Turro, N. J. Macromolecules 2001, 34, 1619–1626. (8) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025–1102. Eustis, S.; El-Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209–217. (9) Henglein, A. Chem. ReV. 1989, 89, 1861–1873. Mirkhalaf, F.; Paprotny, J.; Schiffrin, D. J. J. Am. Chem. Soc. 2006, 128, 7400–7401. Lee, Y. W.; Kim, N. H.; Lee, K. Y.; Kwon, K.; Kim, M.; Han, S. W. J. Phys. Chem. C 2008, 112, 6717–6722. (10) Henglein, A.; Meisel, D. Langmuir 1998, 14, 7392–7396. (11) Henglein, A.; Tausch-Treml, R. J. Colloid Interface Sci. 1981, 80, 84–93. Henglein, A. J. Phys. Chem. 1993, 97, 5457–5471. (12) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529–3533. (13) Mulvaney, P. Langmuir 1996, 12, 788–800. (14) Mie, G. Ann. Phys. 1908, 25, 377. (15) Esumi, K.; Matsumoto, T.; Seto, Y.; Yoshimura, T. J. Colloid Interface Sci. 2005, 284, 199–203. Zhou, M. C., S.; Zhao, S.; Ma, H. Physica E 2006, 33, 28–34. Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410–8426.

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