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Composition-Controlled Synthesis of Bimetallic Gold-Silver Nanoparticles Nancy N. Kariuki,† Jin Luo,† Mathew M. Maye,† Syed A. Hassan,† Tanya Menard,† H. Richard Naslund,‡ Yuehe Lin,§ Chongmin Wang,§ Mark H. Engelhard,§ and Chuan-Jian Zhong*,† Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902, and Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352 Received June 24, 2004. In Final Form: September 14, 2004 This paper reports findings of an investigation of the synthesis of monolayer-capped binary gold-silver (AuAg) bimetallic nanoparticles that is aimed at understanding the control factors governing the formation of the bimetallic compositions. The synthesis of alkanethiolate-capped AuAg nanoparticles was carried out using two related synthetic protocols using aqueous sodium borohydride as a reducing agent. One involves a two-phase reduction of AuCl4-, which is dissolved in organic solution, and Ag+, which is dissolved in aqueous solution. The other protocol involves a two-phase reduction of AuCl4- and AgBr2-, both of which are dissolved in the same organic solution. AuAg nanoparticles of 2-3 nm core sizes with different compositions in the range of 0-100% Au have been synthesized. The two synthetic routes were compared in terms of bimetallic composition and size properties. Our new findings have allowed us to establish the correlation between synthetic feeding of metals and metal compositions in the bimetallic nanoparticles, which have important implications to the exploration of gold-based bimetallic nanoparticles for constructing sensing and catalytic nanomaterials.
Introduction Bimetallic nanoparticles exhibit interesting electronic, optical, and chemical or biological properties due to new bifunctional or synergistic effects.1-11 Gold nanoparticles12,13 have recently emerged as viable catalysts of a wide range of interests.1,2 The formation of oxide-supported * To whom correspondence should be addressed: cjzhong@ binghamton.edu. † Department of Chemistry, State University of New York at Binghamton. ‡ Department of Geological Sciences, State University of New York at Binghamton. § Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory. (1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (2) Zhong, C. J.; Luo, J.; Maye, M. M.; Han, L.; Kariuki, N. N. In Nanotechnology in Catalysis; B. Zhou, S., Hermans, G. A., Somorjai, K., Eds.; Academic/Plenum Publishers: New York, 2004; Vol. 1, Chapter 11, p 222. (3) (a) 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, 9396. (b) Mallin, M. P.; Murphy, C. J. Nano Lett. 2002, 2, 1235, (4) Srnova-Sloufova, I.; Vlckova, B.; Bastl, Z.; Hasslett, T. L. Langmuir 2004, 20, 3407. (5) Schmid, G.; West, H.; Mehles, H.; Lehnert, A. Inorg. Chem. 1997, 36, 891. (6) Shi, H. Z.; Zhang, L. D.; Cai, W. P. J. Appl. Phys. 2000, 87, 1572. (7) Shibata, T.; Bunker, B. A.; Zhang, Z.; Meisel, D.; Vardeman, C. F.; Gezelter, J. D. J. Am. Chem. Soc. 2002, 124, 11989. (8) Moskovits, M.; Srnova-Sloufova, I.; Vlckova, B. J. Chem. Phys. 2002, 116, 10435. (9) Zhong, C. J.; Maye, M. M. Adv. Mater. 2001, 13, 1507. (10) Bradley, J. S. In Clusters and Colloids; Schmid, G., Ed.; WileyVCH: Weinheim 1994; Chapter 6. (11) (a) Davis, R. J.; Boudart, M. J. Phys. Chem. 1994, 98, 5471. (b) Via, G. H.; Drake, K. F., Jr.; Meitzner, G.; Lytle, F. W.; Sinfelt, J. H. Catal. Lett. 1990, 5, 234. (12) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (13) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17.
gold nanoparticles or gold-containing bimetallic nanoparticles is expected to enhance the catalytic activity and selectivity.14-17 Several types of gold-based bimetallic nanoparticles for catalytic reactions have recently been studied in our laboratory.2,18 Gold-silver (AuAg) nanoparticles and their assemblies have shown interesting electronic and structural properties. Recently, AuAg nanoparticles of certain bimetallic compositions have been synthesized using two-phase (toluene-water)3a and onephase (water)3b methods. While there are several other reported methods,19-24 the synthesis of the bimetallic nanoparticle system with both composition and size controllability in a wide range has not been established. This situation is in part due to the fact that there is the propensity for Ag+ to form precipitation with halogen ions in aqueous solutions when it is used as a precursor in combination with AuCl4- precursor. In this report, we describe the synthesis and characterization of alkanethiolate-capped AuAg nanoparticles (Scheme 1) with both size and composition controllability. Our synthetic approaches have involved two related routes. One route involves a two-phase reduction of AuCl4(14) Bismas, P. C.; Nodasaka, Y.; Enyo, M.; Haruta M. J. Electroanal. Chem. 1995, 381, 167. (15) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (16) Bond, G. C.; Thompson, D. T. Gold Bull. 2000, 33, 41; Catal. Rev. 1999, 41, 319. (17) Bond, G. C. Catal. Today 2002, 72, 5. (18) Luo, J.; Lou, Y. B.; Maye, M. M.; Zhong,C. J.; Hepel, M. Electrochem. Commun. 2001 3, 172. (19) Kim, M. J.; Na, H. J.; Lee, K. C.; Yoo, E. A.; Lee, M. Y. J. Mater. Chem. 2003, 13, 1789. (20) Fan, C.; Jiang, L. Langmuir 1997, 13, 3059. (21) Rodriguez-Gonzalez, B.; Sanchez-Iglesias, A.; Giersig, M.; LizMarzan, L. M. Faraday Discuss. 2004, 125, 133. (22) (a) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (b) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (23) Shon, Y. S.; Dawson, G. B.; Porter, M.; Murray, R. W. Langmuir 2002, 18, 3880. (24) Chen, D. H.; Chen, C. J. J. Mater. Chem. 2002, 12, 1557.
10.1021/la048438q CCC: $27.50 © 2004 American Chemical Society Published on Web 11/04/2004
Bimetallic Gold-Silver Nanoparticles Scheme 1. A Schematic Illustration of Monolayer-Capped Binary Nanoparticlesa
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chemical adsorption are important in achieving high activity and selectivity. The general concept has in fact been demonstrated for fuel cell catalysts involving other bimetallic nanoparticle catalysts.1,2,32 Experimental Section
a Closed and open circles represent Au and Ag atoms, respectively.
dissolved in organic solution and Ag+ dissolved in aqueous solution. This route follows largely the protocol reported earlier,3 in which AuCl4- ions are phase-transferred to organic solvent while Ag+ remains in aqueous solution before the two-phase reduction. We provide detailed characterization data to establish the correlation between synthetic feeding of metals and metal composition in the bimetallic nanoparticles. The other route involves a twophase reduction of both AuCl4- and AgBr2- dissolved in the same organic solution. In this route, both AuCl4- and AgBr2- are transferred to organic phase before the twophase reduction. In both routes, the reducing agent is in the aqueous phase. The second route to establish the correlation between the synthetic feeding and the bimetallic composition in the nanoparticles is to our knowledge reported for the first time. The major advantages of the second route over the first route include the elimination of precipitation of Ag+ and the transfer of it to organic phase before the two-phase reduction.25 While these two synthetic routes produce bimetallic AuAg nanoparticles, the effectiveness of each in manipulating composition, size, and yield is different. A detailed correlation of them should provide viabilities in fine-tuning the bimetallic nanoparticles. Such a fine-tuning is important because there are intriguing opportunities in exploring the interfacial chemistry of AuAg nanoparticles in terms of the manipulation of surface binding affinities at Ag vs Au sites. The basis for the surface binding chemistry stems from the extensive studies of selfassembled monolayers of thiols and alkanoic acids on planar silver and gold surfaces.26-28 The exploitation of such interfacial chemistry is therefore increasingly important because multicomponent surface compositions are expected to produce synergistic effects in electronic, optical, catalytic, and magnetic properties.1,2,29-31 The design and preparation of catalysts with bifunctional properties constitute one example where the suppression of adsorbed poisonous species and the modification in electronic band structure in terms of the strength of (25) Lahtinen, R. M.; Mertens, S. F. L.; East, E.; Kiely, C. J.; Schiffrin, D. J. Langmuir 2004, 20, 3289. (26) Lin, S. Y.; Tsai, T. K.; Lin, C. M.; Chen, C. H.; Chan, Y. C.; Chen, H. W. Langmuir 2002, 18, 5473. (27) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R.; Yang, G.; Liu, G. Y. Langmuir 2001, 17, 95. (28) Tao, Y. T.; Wu, C. C.; Eu, J. Y.; Lin, W. L. Langmuir 1997, 13, 4018. (29) Whetten, R. L.; Shafigulin, M. N.; Khoury, J. T.; Schaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (30) (a) Schwank, J. Gold Bull. 1983, 16, 98. (b) Turkevich, J. Gold Bull. 1985, 18, 86. (31) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927.
Chemicals. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4‚3H2O, 99%), tetraoctylammonium bromide (TOABr, 99%), decanethiol (DT, 96%), sodium borohydride (NaBH4, 99%), silver nitrate (AgNO3, 99+%), and potassium bromide (KBr, 99+%) were purchased from Aldrich. Other chemicals included toluene (Tl), hexane (Hx), and ethanol (EtOH). Water was purified with a Millipore Milli-Q water system. Synthesis of Nanoparticles. Two methods were used for the synthesis of AuAg nanoparticles encapsulated with alkanethiolate monolayer shells. Two-Phase Reduction of AuCl4- (in Tl)-Ag+ (aq). This is a two-phase reduction of AuCl4- in organic solution and Ag+ in aqueous solution for the preparation of decanethiolate-capped AuAg alloy nanoparticles, which followed a previous protocol.3 Briefly, in the synthesis of AuAg (1:4), 0.1222 g (3.10 × 10-4 mol) of HAuCl4 and 0.2356 g (1.39 × 10-3 mol) of AgNO3 were dissolved in 100 mL of deionized water. Then 0.9852 g (1.80 × 10-3 mol) of TOABr was dissolved in 100 mL of Tl, and 240 µL (1.16 µmol) of decanethiol was added as capping agent. Sodium borohydride (0.51 g, 0.01 mol) was dissolved in 25 mL of water and added (dropwise) as reducing agent. The reaction was stirred for 4 h, yielding decanethiolate-encapsulated AuAg alloy nanoparticles in the toluene phase. The solvent was removed, and the particles were suspended and washed three times using ethanol. The particles were then dried and dissolved in hexane. Different compositions of the AuAg nanoparticles were synthesized by controlling the feed ratios of the two metal precursors. Two-Phase Reduction of AuCl4- (in Tl)-AgBr2- (in Tl). The preparation of AuAg monolayer-protected nanoparticles followed a modified two-phase protocol, which involved the preparation of a stable negatively charged silver bromide sol (AgBr2-)25 as Ag precursor, which can be transferred into the organic phase. The synthesis was carried out by separately transferring of AgBr2- sol and AuCl4- from the aqueous phase to the organic phase using TOABr as the phase transfer reagent. For the preparation of silver nanoparticles, a negatively charged AgBr2hydrosol was prepared by adding (dropwise) AgNO3 (0.116 g, 6.83 × 10-4 mol, in 50 mL of water) to a KBr solution [0.886 g, 7.44 × 10-3 mol (10 times molar ratio) in 50 mL of water] under vigorous stirring. After 20 min, 0.488 g (8.92 mol) of TOABr dissolved in 50 mL of toluene was added to the hydrosol, forming a two-phase system. The mixture was stirred overnight to yield transparent solutions in both phases. For the preparation of goldsilver (1:4 molar ratio) nanoparticles, 0.064 g (1.63 × 10-4 mol) of HAuCl4 was dissolved in 25 mL of water followed by phase transfer using 0.490 g (8.96 × 10-4 mol) of TOABr in toluene (50 mL). AgNO3 (0.116 g, 6.83 × 10-4 mol) was dissolved in 50 mL of water and added (dropwise) to an aqueous solution (50 mL) of KBr [0.884 g, 7.43 × 10-3 mol (10 times molar ratio)] under vigorous stirring, followed by phase transfer using 0.496 g (9.07 × 10-4 mol) of TOABr in toluene (50 mL). The above two toluene phases, containing AgBr2- sol and AuCl4-, respectively, were then combined and stirred for 30 min. Decanethiol (115 µL, 0.55 µmol) was added as capping agent, and 0.246 g (6.50 × 10-3 mol) of NaBH4 (in 12 mL water) was added (dropwise) as reducing agent. The reaction was stirred for 4 h, yielding decanethiolateencapsulated AuAg (1:4 mole ratio) alloy nanoparticles in the toluene phase. The solvent was removed, and the particles were suspended and washed three times using ethanol. The particles were then dried and dissolved in hexane. Different compositions of alloy nanoparticles were synthesized by controlling the feed ratios of the two metal precursors. There is a major difference between these two synthetic routes. The first route cannot convert Ag+ into AgBr2-, because the formation of AgBr2- requires excess of Br-. Thus, Ag+ was not (32) (a) Xu, Y.; Ruban, A. V.; Mavrikakis, M. J. Am. Chem. Soc. 2004, 126, 4717. (b) Yang H.; Alonso-Vante, N.; Leger, J. M. Lamy, C. J. Phys. Chem. B 2004, 108, 1938. (c) Deivaraj, T. C.; Chen, W. X.; Lee, J. Y. J. Mater. Chem. 2003, 13, 2555.
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Figure 1. TEM micrographs for AuAg (a) 1:4 and (b) (4:1) alloy nanoparticles the two-phase reduction of AuCl4- (Tl)-Ag+ (aq). transferred to the organic phase. The limited quantity of Br- as a result of the exchange of TOA+Br- with HAuCl4 (3.1 mM) is capable of forming AgBr but is not sufficient to form AgBr2-. Indeed, we observed the formation of a white precipitate in the solution even before the reduction step. Ag was thus not transferred to the organic phase. In contrast, the second route involved an excess of Br- in the aqueous phase to form a stable negatively charged AgBr2- first, which was then transferred into the organic phase. Analysis and Characterization. The alloy nanoparticles have been characterized using direct current plasma-atomic emission spectrometry (DCP-AES), UV-visible spectrophotometry (UV-vis), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). DCP Analysis. The composition analysis was performed using an ARL Fisons SS-7 DCP-AES. Measurements were made on emission peaks at 267.59, 328.07, and 589.59 nm for Au, Ag, and Na, respectively. The dissolution of the nanoparticles (digestion) for the DCP-AES analysis was achieved by following a previous protocol.33 Briefly, the alloy nanoparticle solution was dried (hot water bath) and treated with 50% nitric acid (6 mL) followed by treatment with aqua regia (6 mL). The solution was finally diluted with 25% hydrochloric acid (100 mL). The dilution factor was 10 before the DCP analysis. Na+ was added into the sample solution as an internal standard. Calibration curves were made from dissolved standards with concentrations from 0 to 50 ppm in the same acid matrix as the unknowns. Detection limits, based on three standard deviations of the background intensity, are 0.008, 0.004, and 0.003 ppm for Au, Ag, and Na, respectively. Standards and unknowns were analyzed 10 times each for 3-s counts. Instrument reproducibility, for concentrations greater than 100 times the detection limit, results in 60% Au composition shows a peak shape that closely resembles that for the monometallic gold nanoparticles (∼2.0 nm) prepared similarly.12,13 The SP band for AuAg nanoparticles with 40% Au composition appears between these two extremes. We further note that a noticeable amount of precipitation was observed for the synthesis of AuAg nanoparticles with lower %Au (e.g., 17% and 24%Au) compositions, which was due to precipitation of AgBr from the solution.
binding energy Au
Ag
S
% Au from DCP-AES
4f7/2
4f5/2
3d5/2
3d3/2
2p3/2
2p1/2
24 68
84.0 84.2
87.6 88.0
368.0 368.2
374.0 374.2
161.8 161.9a 162.6a
163.0 163.1a 163.9a
a Determined from spectral deconvolution of the XPS data in Figure 5.
the synthetic feeding molar ratio is likely due to differences in nucleation and growth rates for individual metals. Although X-ray can penetrate a depth of about 5 nm into the surface, there is an attenuation effect in terms of the relative intensity of elements in the surface depth profile. As such, XPS probes relatively more surface composition than the composition in the layers under the surface. The overall XPS data are quite comparable with the DCPAES data. The subtle difference reflects the relative difference surface composition vs bulk composition. Additional information on the core-shell binding properties was provided by an analysis of the S(2p) binding energy (BE) for decanethiolates on the nanocrystal surface and other surface species. A set of XPS spectra in the S(2p) region is shown in Figure 5. The peak positions are shown in Table 2. The doublet feature is characteristic of the thiolate species adsorbed on Au and Ag. We have applied spectral deconvolution with a Lorentzian profile to analyze the S(2p) bands. The fitting constraint is a doublet of peaks that have a width of ∼0.95 (0.10 eV at half of the peak-height. The difference (1.2 eV) between 2p3/2 and 2p1/ 2 peaks for the S(2p) band is the same in all cases. Earlier work3 revealed that there were two sulfur species in terms of relative BEs and ratios, which may provide a way to determine (at least qualitatively) the relative concentration of the metals on the surface of the particles. It showed a predominantly lower BE component for S(2p) for silver-gold nanoparticles, which, in comparison with the lower BE characteristic of S(2p) on silver nanoparticles, was attributed to the possibility of silver atoms residing preferentially on the surface of the particles. Interestingly, in comparison with the S(2p) for AuAg nanoparticles with a lower Au % (e.g., 24% Au, 161.9 and 163.1 eV), the data for those with a higher Au % (e.g., 68% Au) showed an additional set of S(2p) components at a higher binding energy (162.6 and 163.9 eV). In fact, the S(2p) spectra show two sets of S(2p) doublets (161.9 and
(36) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy 1995, Chastain, J., King, R. C., Jr., Eds.; Physical Electronics: Eden Prairie, MN.
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Figure 7. TEM micrographs (top panel) and size analyses (bottom panel) for AuAg nanoparticles of different compositions, prepared by two-phase reduction of AuCl4-(Tl)-AgBr2-(Tl) (Au %): (a) 11, (b) 23, (c) 34, (d) 41, and (e) 64.
Figure 8. Composition data for AuAg nanoparticles derived from DCP analysis. The plot shows the relationship between the Au % from the synthetic feeding and Au % in the nanoparticles (slope ) 0.98. r2 ) 0.99) (NPs, nanoparticles; SF, synthetic feeding).
The precipitation was increasingly more significant as the Au % was decreased in the synthetic feeding solution. 2. AuAg Nanoparticles from Two-Phase Reduction of AuCl4-(Tl)-AgBr2-(Tl). Nanoparticle Morphology. Figure 7a-e shows a representative set of TEM micrographs for AuAg nanoparticles of different compositions synthesized from the twophase reduction of AuCl4-(Tl)-AgBr2-(Tl). Similar to the above results, the individual nanoparticles appear to be well-isolated, which is consistent with intershell interdigitation between DT molecules capped on the nanoparticles.34 The average particle size and size distribution from analysis of the TEM images (Figure 7) yielded 3.0 ( 0.6, 3.2 ( 0.4, 2.9 ( 0.4, 2.70 ( 0.4, and 2.5 ( 0.3 nm for 11%, 23%, 34%, 41%, and 64% of Au in the synthetic feeding, respectively. This set of data showed a gradual decrease of size with the increase of Au % in the synthetic feeding. It is important to emphasize that this synthesis route is clearly better controlled than the previous synthetic route, because there is no precipitation in the product solution. The average particle sizes are slightly larger than those synthesized by the previous route of two-phase reduction of AuCl4- (Tl)-Ag+ (aq). In general, the route of the two-phase reduction of AuCl4- (Tl)-AgBr2(Tl) offers fine-tunability in size control. Analysis of Bimetallic Composition. The bimetallic compositions of the Au-Ag nanoparticles were obtained from DCP analysis. Figure 8 presents a representative set of results, showing the relationship between the mole
Figure 9. (A) UV-vis spectra for AuAg nanoparticles with different composition (Au % in nanoparticles): (a) 0, (b) 11, (c) 23, (d) 34, (e) 41, (f) 64, (g) 75, and (h) 100. (B) Plot of peak position (λAbs in nm) vs Au % in nanoparticles. (AU stands for arbitrary unit for the absorbance.)
feeding of the two metals in the synthetic solution and the resulting composition (Au %) in the nanoparticles from this synthesis protocol. Clearly, the result shows a very good linear relationship. The linear correlation (r2 ) 0.99) is much better than that for the AuAg nanoparticles synthesized by the two-phase reduction of AuCl4- (Tl)-Ag+ (aq) (r2 ) 0.94). Importantly, the fact that the slope (0.98) is essentially 1 is indicative of an effective conversion of the two metals. This is a major improvement in comparison with the slope obtained for the previous route (0.79). The finding thus demonstrates that the bimetallic composition can be precisely controlled by the synthetic feeding of the two metals. Surface Plasmon Resonance Band. Figure 9A shows a representative set of UV-visible spectra characterizing the surface plasmon (SP) resonance band for the different AuAg nanoparticles in hexane solution. The UV-vis spectra for monometallic nanoparticles are included for comparison, which show SP band at 436 and 520 nm for Ag and Au, respectively. In comparison with the previous data for the two-phase reduction of AuCl4- (Tl)-Ag+ (aq) (Figure 6a), the data here clearly showed a much better defined trend in the red shift of the SP band with increasing Au % composition in the nanoparticles. Again, the spectra show only a single SP band, ruling out the possibility of a mixture of monometallic silver and gold nanopartices. The red shift is also consistent with the observation of color of the
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nanoparticle solution, which was brown for nanoparticles with 11% Au and red for those with 75% Au. A plot of the wavelength of the maximum absorption versus Au % in the nanoparticles is shown in Figure 9B. This trend is consistent with change in the concentration of Au or Ag in the nanoparticle. The SP band feature for the AuAg nanoparticles with the higher Au % composition (e.g., 75% and 64% Au) shows a trend approaching that for monometallic gold nanoparticles (∼2.0 nm), whereas those with the lower Au % composition (e.g., 11% and 23% Au) approach that for monometallic silver nanoparticles (∼3.0 nm). The trend of the SP band intensity is consistent with the fact that the extinction coefficient of silver is higher than that of Au. This is true in both Figures 6 and 9. In both cases, we observed a general trend that the absorption intensity of nanoparticles with higher Ag content is higher. There are one or two data points that seem to be slightly off the trend due to either a slight difference in concentration or a change in spectral baseline. We emphasize that the gradual and systematic variations of the bimetallic compositions and optical properties obtained here for the synthetic route of two-phase reduction of AuCl4- (Tl)-AgBr2-(Tl) are associated with the fact that both metal precursors are in the organic phase and the synthesis solution is free of precipitation. This route thus shows major improvements in comparison with the synthetic route of the two-phase reduction of AuCl4(Tl)-Ag+ (aq) where the Ag precursor is in the aqueous solution and precipitation materials are present in the synthesis solution. Conclusions We have studied two simple synthetic protocols toward the preparation of monolayer-capped AuAg bimetallic particles with controllable composition and sizes. These protocols, upon further modification, should be also applicable for the preparation of other bimetallic or multicomponent nanoparticles. These two synthetic protocols exhibited differences in terms of controlling composition and size properties of the nanoparticle product. The experimental results have shown that the two-phase reduction of both AuCl4- and AgBr2- in the same organic
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solution yields a better control of the composition by controlling the synthetic feeding ratios of the two metal precursors. There is almost one-to-one conversion of the two metals from the synthetic solution to the bimetallic nanoparticle product. In comparison, the two-phase reduction of AuCl4- (organic)-Ag+ (aqueous) solution shows the ability of controlling the composition by synthetic feeding ratios of the two metal precursors, but the controllability is somewhat lower because the conversion of metals from synthetic feeding to the AuAg nanoparticles is about 1:0.8. The size of the nanoparticles obtained from the two-phase reduction of AuCl4- (Tl)AgBr2- (Tl) is better controlled than those from the twophase reduction of AuCl4- (Tl)-Ag+ (aq) system. This assessment reflects the fact that the two metal precursors in the latter route are largely in the two different phases and bimetallic nanoparticle formation is more likely to be occurring at the interface. Moreover, the SP band displays a trend of red shift with the increase of Au % in the bimetallic nanoparticles, which is much better defined for nanoparticles from the two-phase reduction of AuCl4- (Tl)-AgBr2- (Tl) than from the two-phase reduction of AuCl4- (Tl)-Ag+ (Tl). The XPS data of for S species revealed subtle shifts associated with the relative surface bimetallic composition. The experimental data, along with the fact that there is an insignificant precipitation of nanoparticles prepared using the route of two-phase reduction of AuCl4- (Tl)-AgBr2(Tl), suggest the presence of homogeneity for Au and Ag metals in the bimetallic nanoparticles following the reduction of the metal precursors in the same organic phase. Further work to delineate the optical properties with the nanoparticle’s composition and size and to develop strategies for the assembly of the bimetallic nanoparticles for catalytic or sensory applications is in progress. Acknowledgment. Financial support of this work in part from National Science Foundation (0316322, 0349040), the Petroleum Research Fund administered by the American Chemical Society (40253-AC5M), and 3M Corp. is gratefully acknowledged. LA048438Q
