Here we present our experimental results in synthesizing Au-Ag bimetallic nanoalloys with tunable localized surface plasmon resonance frequency through nanosecond laser-induced heating in the presence of polyvinyl alcohol as a reducing and capping agent via three different procedures: (i) Mixture of HAuCl4 and AgNO3 precursors, (ii) Mixture of Au nanoparticles (NPs) and AgNO3 precursor and (iii) Mixture of both Au and Ag NPs. Presence of single absorption band and direct dependence of the Au/Ag molar ratio to the shift of the absorption peak and lack of core-shell structure in Transmission Electron Microscope images confirms that the formed NPs are homogeneous alloys.
©2012 Optical Society of America
During the past two decades investigations on nanoparticles (NPs) have become a paradigm for the promising discipline of nanotechnology. The intense research activity in the field of NPs that has been conducted by chemists, physicists, and material scientists is motivated by the search for new materials in order to further miniaturize electronic devices, as well as by the fundamental question of how molecular electronic properties evolve with increasing size in this intermediate region between molecular and solid-state physics . Such particles are now being employed in various technologies including improved catalysts , interfaces for SERS technology  and components in optical and electronic devices [4,5], whose properties depend on plasmonic oscillations. As the plasmonic coupling between these NPs is one of the most intriguing optical properties, its characteristic enhancement of local optical field at particle-particle junction is enormously useful for many sensing applications [6,7]. The synergistic control of various key parameters sensitive to the localized surface plasmon resonance (LSPR) frequency even broadens its potentials and at the same time allows us to tune the LSPR band, including particle size and shape . Further control of LSPR frequency in wider range has been achieved by bimetallic NPs fabricated in the form of either core-shell or alloy of two metals. The interaction of the two metallic components in either nanometer or atomic scale permits facile tuning of surface plasmon band mostly in solution phases. The resulting LSPR frequency in general lies in between that of the pure component, depending on the relative amounts of the two components.
In particular, Au/Ag metal nanocomposites, i.e.,(alloy and/or core-shell particles) have stimulated increasing interest because of their composition-dependent tunable electrical, optical and catalytic properties [8,9] and their potential use as taggants for security applications  which are different from those of corresponding individual metal NPs. So far, several types of bimetallic NPs have been synthesized [11–13]. Especially, the combinations of Au and Ag are interesting to investigate for their composition-sensitive optical properties, because monometallic NPs of Au have an intense surface-plasmon absorption peak in the visible region at a wavelength different from that of Ag NPs and because complete miscibility of Au and Ag can be obtained at any composition in both bulk materials and NPs . For example, Mallin et al  has demonstrated that LSPR frequency of Au/Ag alloy NPs either blue-shifted or red-shifted linearly with an increase in Ag or Au content, respectively. Interestingly, in the case of synthesis of Au core-Ag shell NPs by digestive ripening followed by annealing, which causes their conversion to Au-Ag alloy NPs  or by laser ablation  shows a single LSPR band tuned over the entire wavelength region in between that of pure Ag and Au. Formation of Au-Ag alloy under laser irradiation of mixture of monometallic colloidal solutions has been reported by F. Hajiesmaeilbaigi  et al. Though there are many reports in literature, study of most important feature of these bimetallic NPs is the tunability of LSPR frequency by the composition always remains of interest.
There have been impressive developments in the field of nanotechnology in the recent past, with numerous methodologies formulated to synthesize NPs of particular shape and size depending on specific requirements. The synthesis of these materials has been approached from numerous angles ranging from innovative laser ablation  microwave  sonochemical techniques , and purely chemical synthetic routes . Most recently, modified synthetic techniques have allowed for the formation of alloys and heterogeneous metal particles with sizes, shapes, and compositions similar to their single-elemental counterparts, yet with new and interesting chemical and physical properties [21–26]. Here we report a remarkable “Laser irradiation” technique which is a bottom-up approach in comparision to the approach of laser ablation of bulk materials in solution for producing NPs. Previously we were successful in synthesizing monodisperse metal NPs of silver  through this method with extremely good stability. Now, Laser irradiation process was adapted for synthesis of monodisperse bimetallic alloy NPs of gold and silver. These syntheses involved irradiation of metal salts in polyvinyl alcohol (PVA) matrix, where PVA acts as both reducing and stabilizing agent , thereby generating homogeneous distribution of the metal nanoparticles. As it has been theoretically and experimentally demonstrated that if NPs possess a strong absorption band whose energy coincide with the photon energy of a laser, the NPs can selectively be heated above their melting point into related liquid systems [29,30]. We have attempted in taking advantage of this localized heating process to prepare Au-Ag alloys with nanoscale structures which specifically uses intense nanosecond laser pulses to generate the optical breakdown of the containing medium following the generation of the free radicals responsible for the reduction process of metal ions. Moreover, this technique is one of the simple methods in synthesizing NPs which has the following interesting features: very simple and convenient, synthesized NPs have regular spherical shapes and crystallized NPs can easily be obtained in one-step procedures without subsequent heat-treatments, because of the high energetic state of irradiated species without formation of by-products. Resizing and reshaping of colloidal NPs synthesized by other methods are also possible through melting and fragmentation technique by laser irradiation . The synthesized NPs completely collected in solutions forming thus a colloidal solution make them very easy to handle as suspended or powdered (by centrifuge). Moreover, this method is free from reducing agents, which are potential impurities with no pollution and contamination. In addition, colloidal metallic/bimetallic NPs with controlled size and shape can be produced with no agglomerations for several weeks.
With all the above mentioned advantages, here we present a novel technique of synthesizing metal alloy NPs through laser induced heating of precursor solution by directly irradiating metal precursor solution to pulsed nanosecond laser in the presence of polymer matrix. For many applications like surface enhanced Raman scattering or fluorescence microscopy in which the optical field enhancement associated with surface plasmon excitation is exploited, tunability of this collective resonance over a wide energy range is required. Because of these wide range applications we have investigated synthesis of Au-Ag alloy NPs under nanosecond laser irradiation via three different methods (i) Irradiating mixture of HAuCl4 and AgNO3 precursors, (ii) Irradiating mixture of Au NPs and AgNO3 precursor and (iii) Irradiating mixture of both Au and Ag NPs.
2. Experimental details
Gold (III) chloride trihydrate (HAuCl4.3H2O), silver nitrate (AgNO3) and PVA were purchased from Aldrich and Acros Organics respectively and all the chemical materials were used without further purification. Deionized water was purified to a resistivity of 18.2 MΩ-cm for use during the synthesis. All glassware were cleaned with an aqua regia solution (3:1, HCl: HNO3), and then rinsed with nanopure water prior to use. In order to test the tunability of LSPR band of Au-Ag bimetallic NPs, we prepared four solutions namely 0.125 mM HAuCl4 (solution A), 0.125 mM AgNO3 (solution B), 0.187 mM AgNO3 (solution C) and 0.25 mM AgNO3 (solution D). These were used in preparing Au-Ag precursor solutions with different ratios as shown in Table 1 . Combination of solution A and B is labelled as concentration 1, solution A and C is labelled as concentration 2, and solution A and D is labelled as concentration 3. 1%wt PVA Polymer solution was used throughout the synthesis. Then these solutions were allowed to stir for 60 minutes and are exposed to ns laser.
A pulsed Nd:YAG laser (Spectra-Physics, USA) was used to irradiate the colloidal solutions at 532 nm of 6 ns pulse width, 8 mm beam diameter and a repetition rate of 10 Hz. Optical absorption spectra of the colloidal solutions were recorded by using a UV-Vis spectrophotometer (JASCO). Size and shape of the Au, Ag and their alloy NPs were investigated by FEI model TECNAI G220 S-Twin TEM instrument. TEM samples were prepared by mounting a droplet of the colloidal solution of interest on a carbon-coated copper grid.
3. Results and discussion
3.1 Method I - mixture of HAuCl4 and AgNO3 precursors
Alloy NPs with various Au/Ag mole ratios (1:0, 0.75:0.25, 0.5:0.5, 0.25:0.75, 0:1) are synthesized by using pre-determined initial mole ratios of Au and Ag ions in solutions and there by irradiating these solutions directly with ns laser of 532 nm wavelength at 185 mJ/cm2 fluence for 40 minutes duration of exposure. The resulting NP dispersions exhibiting different colors caused by the reduction of metal salts were found to be dependent on the concentrations of AgNO3 and HAuCl4 in solution, indicating the formation of bimetallic Au-Ag NPs. The dispersions containing pure Au particles turned from colorless to wine color while that of solution containing pure Ag particles turned to yellow color, the intermediate compositions resulted in colors varying from wine to yellow. For example, 0.25Au/0.75Ag solution turned yellowish wine and as the Au/Ag molar ratio increased, the intensity of the wine color increased.
Figure 1(a) shows UV-Vis absorption spectra of the resultant NPs corresponding to concentration-1 with varying initial Au/Ag molar ratios together with those of pure Au and Ag NPs from which several aspects can be understood. First, single surface plasmon absorption band is observed and the wavelength of maximum absorption is found to be red-shifted from 410 nm (Ag NPs) to 526 nm (Au NPs) with increase in the Au mole fractions. These results are inconsistent with UV-Vis analysis of core-shell NPs because both physical mixture of monometallic NPs and formation of core/shell NPs exhibit two absorption peaks . It can be seen from Fig. 1(a) that the spectrum of pure Au and Ag NPs are having LSPR bands centered at 526 nm and 410 nm respectively, whereas LSPR bands are centered at 514 nm, 497 nm and 455 nm for Au-Ag alloy NPs of compositions Au0.75Ag0.25, Au0.5Ag0.5, and Au0.25Ag0.75 respectively, which are clearly located at intermediate positions between the intrinsic Au and Ag LSPR bands. Furthermore, the peak position shifted to longer wavelength region as the Au/Ag molar ratio changed from 0.25Au/0.75Ag to 0.75Au/0.25Ag. Similar surface Plasmon absorption studies were also carried out for concentrations-2 and 3 of similar compositions by simply varying Au/Ag mol ratios.
The UV-Vis absorption spectra of Au-Ag alloy nanoparticles corresponding to concentration-3 are shown in Fig. 1(b). LSPR bands are centered at 526 nm and 410 nm for pure Au and Ag NPs same as in the earlier case but LSPR bands of Au-Ag alloy NPs of compositions Au0.75Ag0.25, Au0.5Ag0.5, and Au0.25Ag0.75 are centered at 498 nm, 458 nm, and 426 nm respectively which are different from that of same compositions corresponding to concentration-1. Similar results were observed for concentration-2 also.
Based on Drude model and quasi-static theory Jian Zhu  developed numerical calculations to describe absorpton properties of Au-Ag alloy nanospheres. With the increase in gold content, the absorption peaks of Au-Ag alloy nanospheres red shifted from 402 nm to 526 nm. Our experimental results match well with their theoretical calculations.
Figure 1(c) shows tunable characteristics of LSPR frequency of Au-Ag alloy NPs of all the three concentrations under study, in which the UV-Vis absorption peak position is plotted against the percentage of HAuCl4 in the precursor solutions. These experimental results suggest that the LSPR frequency of the laser-induced synthesized alloys can be tuned over the entire wavelength range between pure Au and Ag by adjusting their relative concentrations. This finding also suggests that bimetallic NPs formed were homogeneous alloy NPs and thus, the specific absorption bands correspond to the LSPR bands of Au/Ag alloy NPs of a specific composition. It can also been seen from the Fig. 1(c) that the LSPR wavelengths of Au-Ag alloy NPs are red-shifted from that of a monometallic Ag NPs in proportion to the increase in the mol fraction of the Au content.
Second, dependence of the position of LSPR band by varying laser parameters such as irradiation fluence and duration of exposure was studied. At a constant irradiation fluence of 185 mJ/cm2 and for different exposure times of 10, 20 and 30 minutes, we observed that the LSPR band of Au-Ag alloy composition of Au0.5Ag0.5 corresponding to concentration-3 was found to be located at 470 nm for 10 min but for 20 and 30 min duration of irradiation it was observed at 458 nm apart having variation in absorption intensity as shown in Fig. 2 . The increase in the intensity of LSPR band with time indicates continuous reduction of the metal ions. We did not observe any noticable shift in the λmax after 20 min of irradiation time indicating the size selectivity of the Au-Ag alloy NPs. It can also be observed that for same exposure time of 20 minutes and for different irradiation fluences of 185, 139, 99 mJ/cm2 LSPR wavelengths were found to be at 458, 478 and 496 nm respectively. We observed that at lower fluence precursor solution under irradiation requires longer irradiation time in forming alloy NPs. This suggests that the position of LSPR band of Au-Ag alloy system is dependent on relative concentrations of Au and Ag along with irradiation energy and duration of exposure.
Finally, In order to examine whether synthesized sample solution by laser irradiation was homogeneous and consisted of alloy NPs, instead of core/shell or monometallic Au and Ag NPs, we have carried a second set of UV-Vis absorption spectroscopic studies. It is well known that the surface plasmon bands are characteristic for the metal and NPs . Core/shell NPs will give rise to two surface plasmon absorption bands and the individual band intensities should depend on the initial compositions of the metal ions. A similar situation will arise from a dispersion containing monometallic Au and Ag NPs instead of homogeneous alloy NPs as discussed earlier . Figure 3 shows UV-Vis absorption spectra of a solution of 0.5Au/0.5Ag NPs and a physical mixture of pure Au and Ag monometallic NPs. In the case of 0.5Au/0.5Ag molar ratio alloy NPs, single peak was observed at 458 nm. Conversely, in the case of the mixture of pure monometallic NPs, two separate peaks were obtained at 410 nm and 526 nm, corresponding to the peak absorbance of pure Au and Ag, respectively. Hence, this result suggests that the irradiated solution consists of alloy NPs and is neither a mixture nor a core/shell which will be further supported by TEM analysis. From TEM analysis, the synthesized bimetallic NPs are spherical with an average size less than 20 nm. Figure 4(a) shows TEM image and Fig. 4(b) shows corresponding size distribution of Au-Ag alloy synthesized with concentration-2 for Au0.5Ag0.5 molar ratio. Particles are spherical in shape with an average size of ~13 nm. As explained from UV-Vis absorption spectroscopy, presence of homogeneous alloy nanoparticles indicates the absence of core–shell particles.
Under laser irradiation, bimetallic mixture of Au and Ag, the solution containing two metal precursors at low concentrations are thus stable in the presence of each other without AgCl precipitation. The strong reducing agents which are solvated electrons and free radicals produced by ns laser pulse readily react with both precursors. Even though the initial probabilities of being reduced by photolytic radicals and solvated electrons are same for both the metal precursors, formation of Au atoms may prevail because of higher reduction potential for AuCl4- species (0.93 V) than that of Ag+ ions (0.77 V). So Au ions might be initially consumed by the reducing species and then Ag ions are reduced at the surface of Au. Here
one would expect more chances of forming core/shell structure than alloy formation. However, both UV-Vis absorption spectra and TEM images indicate the formation of Au-Ag alloy NPs. This might be due to the high intensity delivered by ns laser pulses may allow an efficient competition in favour of reduction at which alloying could be achieved in a very short time shorter than any other processes to be encountered, thus suppressing the possibility of metal segregation. Another expectation is that the core/shell structures might have been encountered at a certain stage during irradiation, and thus, the alloying process may be due to the spontaneous interdiffusion between the two atoms promptly after core/shell formation because of the similar atomic size of Au and Ag atoms. However, in order to conclude further that the formed NPs are alloys rather than core/shell NPs, we have attempted in synthesizing bimetallic NPs with Au NPs-Ag precursor and with both Au-Ag NPs solutions by assessing mechanism involved in alloying under the intense ns laser field.
3.2 Method II – mixture of Au nanoparticles and Ag precursor
Here, we report the synthesis of Au-Ag bimetallic alloy NPs by irradiating a mixture of solutions containing Au NPs and Ag+ ions in different ratios. Au NPs having an average diameter less than 20 nm and a characteristic LSPR frequency at 526 nm were synthesized by irradiating pure Au precursor solution (Au:Ag→ 1:0) with ns laser of 532 nm at 185 mJ/cm2 fluence for 30 minutes duration of exposure. Now, these Au NPs solution was mixed with Ag+ ionic solution in different ratios (Au: Ag → 1:1, 1:2, 2:1). Figure 5(a) shows the UV-Vis absorption spectra of Au-Ag alloy NPs obtained from a mixed solution of Au colloids and Ag+ ions with a molar ratio of 1:1, before and after laser irradiation. It can be seen that before laser irradiation the surface plasmon band of the mixture is at 525 nm which arises due to the Au NPs in the mixture. After laser irradiation at 532 nm, it was found that the characteristic plasmon peak of the Au NPs gradually shifts towards blue, diminishes and finally becomes unobservable as the irradiation time increased up to 15 minutes. Instead, a new LSPR band centered at 458 nm is formed and intensified. As the irradiation (>15 min) time increased, we did not observe any shift of the plasmon band apart from the increase in intensity which implies more number of particles are getting synthesized. From these spectral changes, it can be inferred that Ag+ ions are reduced on the surface of laser-heated Au NPs and that the formed bimetallic structures are simultaneously melted to Au-Ag alloy NPs. Without alloying the Au NPs would give a sharp plasmon peak at around 520nm, or if the Ag NPs are formed it would give a surface plasmon peak around 410nm in the absorption spectrum. Neither pure Ag NPs nor Au-Ag core-shell structures were observed in solution after 532 nm laser irradiation as seen from the absence of any surface plasmon peak around 410 nm and from TEM images.
Figure 5(b) shows tunable characteristics of LSPR band of mixed solution of Au colloids and Ag+ ions with different molar ratios (Au: Ag → 1:1, 1:2, 2:1) before and after laser irradiation as a function of irradiation time, in which the UV-Vis absorption peak position is plotted against duration of laser irradiation. As the irradiation time increases the LSPR frequency gradually shifts towards blue up to certain extent and thereafter no spectral shift was observed with respect to irradiation time. It can also been observed that, after laser irradiation of initial solutions with different Au:Ag molar ratios, the new absorption band shifts towards blue as the molar fraction of Ag increases. Thus by simply varying the Ag content in the solution, LSPR frequency of alloy NPs can be tuned in between the surface plasmon bands of gold and silver. Figure 6(a) shows TEM image and Fig. 6(b) shows corresponding size distribution of Au-Ag alloy NPs obtained after irradiation of mixture of Au NPs and Ag precursor in 1:1 ratio for 40 minutes duration of exposure. Homogeneous alloy nanoparticles with an average size of ~16 nm are observed through this method also.
Under irradiation with an intense pulsed laser at 532 nm, a gold NP is able to absorb many photons during a single laser pulse . A parent Au NP in the colloidal solution will experience multiphoton absorption during a single laser pulse, and the NP will be heated to its boiling point in a few picoseconds via electron-phonon interactions .These hot gold NPs fragment into smaller NPs by releasing atoms and clusters. Besides, gold NPs will be efficiently ionized, and the charge density produced may be sufficient to cause explosive repulsion [31,35,36]. Ag+ ions, which are in the close vicinity of Au NPs are reduced at the surface of Au through transfer of heat and thus, alloying which could be due to the spontaneous interdiffusion between the two atoms. This may be attributed to the fact that Au (2.35 Å) and Ag (2.36 Å) are miscible in all proportions due to their similar lattice constants.
3.3 Method III- mixture of both Au and Ag nanoparticles
In the earlier method, we have demonstrated that laser-induced heating can promote reduction and deposition of Ag+ ions and alloying of Au and Ag. Laser-induced heating can also alloy Au and Ag when the Ag source is in the form of dispersed NPs. In this method, we initially prepared Au and Ag NPs of particle size less than 20 nm by irradiating their precursor solutions with ns laser at 532 nm and at 185 mJ/cm2 fluence of energy. Figure 7 shows the UV-Vis optical absorption spectrum of the 1:1 molar ratio of Au-Ag colloidal suspensions before and after laser irradiation at 532 nm. Prior to the proceedings with irradiation colloidal solution exhibited two distinct absorbance peaks at 410 nm and 526 nm, corresponding to Ag and Au surface plasmon bands respectively, which indicate segregated Au and Ag particles in the mixed suspension. For solutions exposed to the laser the absorption spectra changed as the irradiation time was increased. The Au surface plasmon band increases in magnitude and shifts to lower wavelength while the Ag plasmon band decreases in intensity without obvious spectral shift. This variation in UV-Vis absorption spectra clearly implies changes in the colloidal properties. Further irradiation leads to a single absorption peak at 458 nm, located at an intermediate position between the Au and Ag plasmon bands. As the irradiation (>30 min) time increased we didn’t observe any shift of the plasmon band apart from the increase in intensity which implies more number of particles are getting synthesized. This surface plasmon band is likely due to the formation of the Au-Ag alloy. Two plasmon bands (Au and Ag) would be expected if the colloids consisted of individual Au and Ag particles or of bimetallic composites with a core-shell structure. Figure 8(a) shows TEM image of resultant AuAg alloy NPs obtained by irradiating mixture of individual Au and Ag NPs for 30 min duration of exposure and the corresponding size distribution is shown in Fig. 8(b) with an average size of ~13 nm.
Upon laser irradiation at 532 nm, as discussed in the earlier method, parent Au NPs in the mixed colloidal solution get excited and will be heated to their boiling point. With increase in irradiation time, some Ag NPs might be heated as they also have absorption at 532 nm. With lower melting temperature, there is probability that some Ag NPs reach melting point even before Au NPs. When such Ag NPs contact heated Au NPs, homogeneous Au-Ag alloy NPs are observed. This mechanism might be responsible for the conversion of physical mixture of Au-Ag NPs to homogeneous Au-Ag alloy NPs with increase in duration of exposure of intense ns laser pulse. After centrifuging we estimated the yield of Au-Ag nanoparticle powder as 6 mg for an irradiated solution of 7 ml at 183 mJ/cm2 fluence energy in a timeperiod of 40 minutes.
In conclusion, simple, innovative and convenient approach in synthesizing stable Au-Ag bimetallic nanometer-sized particles with tunable LSPR frequency in PVA matrix under pulsed laser irradiation of a mixed solution of Au ions (or nanoparticles) and Ag ions (or nanoparticles) has been demonstrated. Formation of homogeneous alloyed particles was clearly observed by UV-Vis absorption spectra and TEM images. UV-Vis absorption spectroscopic studies confirmed that the tunability of LSPR frequency can be achieved anywhere in between LSPR bands of monometallic Au and Ag NPs just by varying Au/Ag molar ratios. Detailed studies on the change in the position of LSPR band as a function of laser fluence and duration of irradiation were also presented. Sizes of Au-Ag alloy nanoparticles less than 20 nm were confirmed through TEM images. It is expected that this selective heating strategy can be extended to prepare other bi- or multi-metal nanoparticles.
R. Kuladeep acknowledges University Grants Commission (UGC), India, for financial assistance. D. Narayana Rao acknowledges financial support from the Department of Science and Technology, India. This research is performed in the framework of ITPAR Phase II FaStFal 2007-2010 project.
References and links
1. G. Schmid, Clusters and Colloids: From Theory to Application (VCH, Weinheim, 1994).
2. G. Schmid, H. West, H. Mehles, and A. Lehnert, “Hydrosilation reactions catalyzed by supported bimetallic colloids,” Inorg. Chem. 36(5), 891–895 (1997). [CrossRef]
3. G. Upender, R. Satyavathi, B. Raju, K. Shadak Alee, D. Narayana Rao, and C. Bansal, “Silver nanocluster films as novel SERS substrates for ultrasensitive detection of molecules,” Chem. Phys. Lett. 511(4-6), 309–314 (2011). [CrossRef]
4. M. Valden, X. Lai, and D. W. Goodman, “Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties,” Science 281(5383), 1647–1650 (1998). [CrossRef] [PubMed]
6. K.-S. Lee and M. A. El-Sayed, “Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition,” J. Phys. Chem. B 110(39), 19220–19225 (2006). [CrossRef] [PubMed]
7. L. Wang, X. Shi, N. N. Kariuki, M. Schadt, G. R. Wang, Q. Rendeng, J. Choi, J. Luo, S. Lu, and C.-J. Zhong, “Array of molecularly mediated thin film assemblies of nanoparticles: correlation of vapor sensing with interparticle spatial properties,” J. Am. Chem. Soc. 129(7), 2161–2170 (2007). [CrossRef] [PubMed]
8. A.-Q. Wang, C.-M. Chang, and C.-Y. Mou, “Evolution of catalytic activity of Au-Ag bimetallic nanoparticles on mesoporous support for CO oxidation,” J. Phys. Chem. B 109(40), 18860–18867 (2005). [CrossRef] [PubMed]
9. A.-Q. Wang, J.-H. Liu, S. D. Lin, T.-S. Lin, and C.-Y. Mou, “A novel efficient Au–Ag alloy catalyst system: preparation, activity, and characterization,” J. Catal. 233(1), 186–197 (2005). [CrossRef]
11. M. P. Mallin and C. J. Murphy, “Solution-phase synthesis of sub-10 nm Au-Ag alloy nanoparticles,” Nano Lett. 2(11), 1235–1237 (2002). [CrossRef]
12. X. Liu, A. Wang, X. Wang, C.-Y. Mou, and T. Zhang, “Au-Cu Alloy nanoparticles confined in SBA-15 as a highly efficient catalyst for CO oxidation,” Chem. Commun. (Camb.) (27), 3187–3189 (2008). [CrossRef] [PubMed]
13. Y.-H. Chen, Y.-H. Tseng, and C.-S. Yeh, “Laser-induced alloying Au–Pd and Ag–Pd colloidal mixtures: the formation of dispersed Au/Pd and Ag/Pd nanoparticles,” J. Mater. Chem. 12(5), 1419–1422 (2002). [CrossRef]
14. N. N. Kariuki, J. Luo, M. M. Maye, S. A. Hassan, T. Menard, H. R. Naslund, Y. Lin, C. Wang, M. H. Engelhard, and C. J. Zhong, “Composition-controlled synthesis of bimetallic gold-silver nanoparticles,” Langmuir 20(25), 11240–11246 (2004). [CrossRef] [PubMed]
15. M. S. Shore, J. Wang, A. C. Johnston-Peck, A. L. Oldenburg, and J. B. Tracy, “Synthesis of Au(Core)/Ag(Shell) nanoparticles and their conversion to AuAg alloy nanoparticles,” Small 7(2), 230–234 (2011). [CrossRef] [PubMed]
16. H. Han, Y. Fang, Z. Li, and H. Xu, “Tunable surface plasma resonance frequency in Ag core/Au shell nanoparticles system prepared by laser ablation,” Appl. Phys. Lett. 92(2), 023116 (2008). [CrossRef]
17. F. Hajiesmaeilbaigi and A. Motamedi, “Synthesis of Au/Ag alloy nanoparticles by Nd:YAG laser irradiation,” Laser Phys. Lett. 4(2), 133–137 (2007). [CrossRef]
18. F. Hajiesmaeilbaigi, A. Mohammadalipour, J. Sabbaghzadeh, S. Hoseinkhani, and H. R. Fallah, “Preparation of silver nanoparticles by laser ablation and fragmentation in pure water,” Laser Phys. Lett. 3(5), 252–256 (2006). [CrossRef]
19. M. Tsuji, N. Miyamae, S. Lim, K. Kimura, X. Zhang, S. Hikino, and M. Nishio, “Crystal structures and growth mechanisms of Au/Ag core-shell nanoparticles prepared by the microwave-polyol method,” Cryst. Growth Des. 6(8), 1801–1807 (2006). [CrossRef]
20. F. Gao, Q. Lu, and S. Komarneni, “Interface reaction for the self-assembly of silver nanocrystals under microwave-assisted solvothermal conditions,” Chem. Mater. 17(4), 856–860 (2005). [CrossRef]
21. G. S. Métraux, Y. C. Cao, R. Jin, and C. A. Mirkin, “Triangular nanoframes made of gold and silver,” Nano Lett. 3(4), 519–522 (2003). [CrossRef]
22. J.-I. Park, M. G. Kim, Y.-w. Jun, J. S. Lee, W.-r. Lee, and J. Cheon, “Characterization of superparamagnetic ‘Core-Shell’ nanoparticles and monitoring their anisotropic phase transition to ferromagnetic ‘Solid Solution’,” Nanoalloys. J. Am. Chem. Soc. 126(29), 9072–9078 (2004). [CrossRef]
23. Q. Zhang, J. Y. Lee, J. Yang, C. Boothroyd, and J. Zhang, “Size and composition tunable Ag–Au alloy nanoparticles by replacement reactions,” Nanotechnology 18(24), 245605 (2007). [CrossRef]
24. Z. Peng, B. Spliethoff, B. Tesche, T. Walther, and K. Kleinermanns, “Laser-assisted synthesis of Au-Ag alloy nanoparticles in solution,” J. Phys. Chem. B 110(6), 2549–2554 (2006). [CrossRef] [PubMed]
25. S. Liu, G. Chen, P. N. Prasad, and M. T. Swihart, “Synthesis of monodisperse Au, Ag, and Au-Ag alloy nanoparticles with tunable size and surface plasmon resonance frequency,” Chem. Mater. 23, 4098–4101 (2011).
26. L. Xu, L. S. Tan, and M. H. Hong, “Tuning of localized surface plasmon resonance of well-ordered Ag/Au bimetallic nanodot arrays by laser interference lithography and thermal annealing,” Appl. Opt. 50(31), G74–G79 (2011). [CrossRef] [PubMed]
27. K. L. N. Deepak, R. Kuladeep, K. Shadak Alee, and D. Narayana Rao, “Synthesis of silver nanoparticles in poly (vinyl alcohol) matrix in solution and thin films through laser irradiation,” J. Nanosci. Nanotechnol. 11, 1–8 (2011). [PubMed]
28. S. Porel, S. Singh, S. S. Harsha, D. N. Rao, and T. P. Radhakrishnan, “Nanoparticle-Embedded polymer: In situ synthesis, Free-standing films with highly monodisperse silver nanoparticles and optical limiting,” Chem. Mater. 17(1), 9–12 (2005). [CrossRef]
29. P. V. Kamat, “Photophysical, photochemical and photocatalytic aspects of metal nanoparticles,” J. Phys. Chem. B 106(32), 7729–7744 (2002). [CrossRef]
31. A. Takami, H. Kurita, and S. Koda, “Laser-induced size reduction of noble metal particles,” J. Phys. Chem. B 103(8), 1226–1232 (1999). [CrossRef]
32. C. S. Ah, S. D. Hong, and D.-J. Jang, “Preparation of Au core Ag shell nanorods and characterization of their surface Plasmon resonances,” J. Phys. Chem. B 105(33), 7871–7873 (2001). [CrossRef]
33. J. Zhu, “Theoritical study of the optical absorption properties of Au-Ag bimetallic nanospheres,” Physica E 27(1-2), 296–301 (2005). [CrossRef]
34. F. Mafuné, J.-y. Kohno, Y. Takeda, and T. Kondow, “Dissociation and aggregation of Gold nanoparticles under laser irradiation,” J. Phys. Chem. B 105(38), 9050–9056 (2001). [CrossRef]
35. P. V. Kamat, M. Flumiani, and G. V. Hartland, “Picosecond dynamics of silver nanoclusters: Photoejection of electrons and fragmentation,” J. Phys. Chem. B 102(17), 3123–3128 (1998). [CrossRef]
36. H. Kurita, A. Takami, and S. Koda, “Size reduction of gold particles in aqueous solution by pulsed laser irradiation,” Appl. Phys. Lett. 72(7), 789–791 (1998). [CrossRef]