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We report a simple and one-step synthesis of gold-silver alloy nanoparticles NPs with various mole ratios performed by direct irradiation using a highly intense femtosecond laser pulse. Gold-silver alloy NPs of 2 - 3 nm with reasonably narrow size distribution were formed in the plasma reaction zone during femtosecond laser-assisted reduction of the mixed aqueous solutions of chloroauric acid (HAuCl4) and silver nitrate (AgNO3) in the presence of polyvinylpyrrolidone (PVP). Solution concentrations were adjusted to avoid the generation of AgCl precipitates. The resulting NPs were characterized by using UV-visible spectrometer and a high resolution transmission electron microscope (HRTEM) for investigating the alloy formation and compositions. The alloy formation was confirmed from the fact that the optical absorption spectroscopy of all samples after irradiation showed a single peak of surface plasmon resonance (SPR). Additionally, it was found that the absorption maximum of the plasmon bands shift linearly to the red with increasing Au content. Transmission electron micrographs indicated that nanoparticle as small as 2 nm could be feasibly fabricated by using this simple technique, regardless of the low formation yield. Further indication of alloy formation was shown by the EDX analysis which confirmed the presence of two elements homogenously distributed throughout the volume of a particle. The formation process for bimetallic alloy NPs was also discussed.
Femtosecond laser, liquid, gold-silver alloy nanoparticles, photoionization and fragmentation.
Bimetallic alloy nanoparticles have attracted much attention over the past decades and much effort has been devoted to their fabrication (including homogenous alloys, core-shell and mixed particles) due to their synergetic and unique catalytic, electronic and optical properties as a function of the composition, which are different from those of the monometallic constituents. For instance, absorption peak of surface plasmon resonance (SPR) for AuAg alloy NPs can be tuned systematically from 520 to 400 nm by changing the alloy composition whereas plasmonic absorption for spherical Au and Ag NPs are generally restricted to be around 520 and 400 nm, respectively. In addition to the optical properties, AuAg alloy NPs also exhibit a better and synergistic performance in their catalytic activity for CO oxidation as compared to that of either Au or Ag NPs (Liu et al. 2005; Yen et al. 2009). Efforts for the fabrication of AuAg bimetallic NPs have been also conducted by various techniques ranging from chemical methods using various reagents (Chen and Chen 2002; Pal et al. 2007 and 2008), to physical approaches using various energies such as γ-ray (Treguer et al. 1998), microwave (Raveendran et al. 2006) and laser (Compagnini et al. 2008). Many attempts were done due to the understanding that the preparation conditions of the nanoparticle have a direct effect on the size, structure, and composition and consequently, in the resulting optical and electrochemical properties. Particularly for the structures, synthesis method determined whether the resulting bimetallic particles may exhibit alloy behavior (Raveendran et al. 2006), core-shell (Compagnini et al. 2008) system or any other phase segregation behavior.
In the last few years, radiation-assisted processes have been used to obtain metal colloids, including the techniques such as UV-visible light (Sakamoto et al. 2009) or laser (Besner et al. 2009), γ-rays (Li et al. 2007), and X-ray (Wang et al. 2009). These techniques were regarded as versatile to produce small NPs but they were time consuming. Of all the methodologies developed, direct irradiation of metal salts using femtosecond laser pulse is a novel technique for metal NPs preparation and has not been extensively reported, especially for multimetallic system. In our previous works, gold (Nakamura et al. 2008), platinum (Nakamura et al. 2009) and silver NPs have been successfully produced by direct laser irradiation of metal salt solution containing no reducing agent with pulse femtosecond lasers delivering high intensity pulses. The addition of dispersant agent like polyvynilpyrrolidone, this approach allows the formation of NPs as small as 2 nm with a standard deviation of about 14%. Succeeding the fabrication of monometallic NPs with narrow size distribution, we proceed to use femtosecond laser pulse irradiation of metal salt solution as a novel technique that produces perfectly homogeneous alloy NPs.
Hence, as a test case for bimetallic nanoparticle synthesis, here we use silver and gold because of their convenient optical properties to distinguish core-shell growth from alloy growth. Using direct irradiation with femtosecond laser pulses, AuAg alloy NPs with sizes approaching 5 nm (or below), which are most desirable for catalytic purposes, are expected to form in aqueous phase by adjusting the solution concentration to avoid AgCl formation.
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Colloidal dispersion of Au-Ag alloy NPs were synthesized by direct irradiation of a mixed solution of gold and silver salts using highly intense femtosecond laser pulses at room temperature. All chemicals were used without further purification. Firstly, the solutions of gold and silver ions with the concentration of 1.32-10-5 M were prepared separately by dissolving HAuCl4½¥3H2O (Sigma-Aldrich, +99.999%) and AgNO3 (Sigma-Aldrich, +99.998%) in extra-pure water containing 0.01wt% of polyvinylpyrrolidone (PVP) as a dispersant. The concentrations were fixed low to ensure the reaction quotient (Q) to be 1.74-10-10, safely below the Ksp of AgCl(s) 1.8-10-10 and hence, ensuring the complete solubility of Ag+ in the presence of Cl- from the gold salt. The fresh aqueous solutions containing gold or silver ions were then mixed in appropriate amounts to obtain solutions with Au/Ag molar ratio of 90/10, 75/25, 50/50, 25/50 and 10/90 in a total volume of 3.0 ml. No AgCl precipitation was observed in the bottom of cuvette just after the mixing. In the laser irradiation experiment, the mixed solution was introduced into a rectangular quartz glass cuvette and irradiated for 9 minutes by highly intense femtosecond pulse laser at the wavelength of 800 nm generated by a chirped pulse amplification system (Spitfire, Spectra-physics Co.). The typical pulse width, repetition rate and maximum pulse energy per pulse were 100 fs, 100 Hz and 6 mJ, respectively. The laser beam was tightly focused by using an aspheric lens with a focusing length of 8 mm (NA = 0.5) and directed perpendicularly to the surface of the cuvette into the solutions as shown in Fig. 1. Considering the diameter of laser beam before focusing was approximately 3.2 mm, the theoretical laser intensity is about 3.2 x 1018 W/cm2 at the focal spot without account for the aberration. This condition also ensures that the optical breakdown of any solvent was exceeded.
After irradiation, each sample was characterized by a UV-visible spectrometer (V630iRM, JASCO Co. Tokyo, Japan) to observe an SPR spectrum in 200 - 800 nm range and then confirm the NPs formation. TEM experiments were performed on a JEOL2000-EXII (200kV, JEOL Ltd., Tokyo) to obtain the electron micrographs for all samples. We also used a high-resolution transmission electron microscopy (HRTEM, TITANTM, FEI Co. London, UK) operating at 300 kV accelerating voltage along with energy dispersive spectroscopy (EDS) analysis to feature the morphological structures and the composition of the particles fabricated in the solutions with the Au/Ag concentration of 25/75, 50/50 and 75/25. For the TEM observatios, several drops of samples were placed on a carbon coated microgrid and left to dry in room temperature. The bright field image of each sample was then processed using image analysis software to measure the size of each NPs. At least 500 particles were counted for each sample, taken from three separate areas on the microgrid.
AuAg alloy NPs were obtained through the simultaneous reduction of gold and silver ions at low concentrations using tightly focused femtosecond laser pulse in aqueous phase. Figure 2 shows UV-visible absorption spectra of the bimetallic colloids prepared with different Au/Ag molar ratio. In general, only one plasmon absorption band was observed for each bimetallic system and the plasmon absorption intensity decreases with increasing Ag contents. In this case, while the monometallic dispersions of Au NPs have plasmon absorption band at about 508 nm, there is no plasmon absorption maxima for monometallic Ag NPs. This fact is in agreement with the previous study where monometallic Ag can only be formed at high concentration of about 300 mM (data is not shown here). For mixed solutions, only a single absorption band was observed for each composition and was located between those for pure gold and silver NPs. It clearly indicates that homogenous alloy NPs were formed because such absorption cannot be obtained either by the simple physical mixture of monometallic Au and Ag colloidal dispersion, or the formation of core-shell Au/Ag NPs where there appeared two absorption peaks as suggested by Mie theory (Chen et al. 2006). It is also expected that in an ideal AuAg alloy system the surface plasmon absorption maximum should be linearly correlated to the alloy compositions. As shown in Fig. 3, the positions of the absorption peaks are plotted against Au molar fraction in the reaction mixtures. In the figure, a monotonic variation of SPR peak positions with Au molar fraction is obtained (note that no SPR data for Ag sample was plotted on the graph) and it may infer that the compositions of the alloy colloids correlate to the concentration of Au and Ag ions in the initial mixture. The peak maximum shifted to the longer wavelength range as the Au/Ag molar ratio changed from 10/90 to 90/10. This observation also suggested that bimetallic NPs formed were homogeneous alloys and thus, the specific absorption bands are attributed to the SPR bands of AuAg alloy NPs of a specific composition. However, due to a low concentration, the color of the AuAg nanaoparticle solutions were very weak for being observed by naked eye as confirmed by a weak absorption intensity for each composition (Figure 2).
In order to determine the size and size distribution of the NPs, TEM analysis has been performed. Figure 4 shows a representative set of TEM images for AuAg NPs prepared in this work and their respective size distributions are also presented, except for pure Ag sample. As shown in this figure, it was quite hard to find Ag NPs all over the microgrid, so the size distribution was not shown here. For Au and AuAg NPs, on the other hand, the individual NPs appear to be well isolated. The size distribution analysis based on the TEM images yielded the particle size gradually ranging from 2.2 ± 0.6 nm for AuAg = 10/90 sample to 3.4 ± 1.9 nm for pure Au NPs. While there is only a small difference in mean diameter of nanoparticles as Au content decreases, in general, the particle sizes show a tendency to decrease as the Ag content increases. This fact is in a good agreement with the result observed in UV-visible spectrum that the plasmon intensity gradually decrease as the increase in Ag contents. Although in low concentration pure silver NPs were hardly formed, the presence of gold in the mixed solution helps the formation of silver atoms and hence leads to the formation of AuAg alloy NPs. Further support for the homogenously alloy structure of the AuAg NPs have been provided by HRTEM images. Figure 5 shows the high-resolution micrograph of the nanoparticles prepared for initial Au/Ag ratio of 25/75, 50/50 and 75/25. The fast-Fourier transform (FFT) images are presented in each inset. The round NPs appeared homogenous in composition. The presence of fringed facet implies that synthesized NPs were highly crystalline. A brief morphological study of the small particles in our AuAg colloidal dispersion through the analysis of FFT images revealed that there is no trend of the class of the structures present in our samples. Due to the resolution limit of the instrument, most of particles appeared to have undefined structures (Fig. 5c) rather than simple face-centered-cubic (Fig. 5a) or twinned structures (Fig. 5b). We believe that particle size and morphology mainly depends on the local condition of clustering process for individual particles, not on the composition of elemental component only.
To verify the composition of the NPs, energy dispersive spectroscopy (EDS) analysis of the single isolated particles was carried out (Fig. 6). Since there was only a small number of nanoparticles found in the microgrid, about 3 particles were chosen to analyze their average composition. It is found that the composition of individual particles varied from particle to particle for a given sample and thus the average composition was used for discussion. For samples with initial composition mixture of Au/Ag ions = 25/75, 50/50 and 75/25 According to the EDX analysis (Table 1), the Au/Ag elemental ratios of bimetallic nanoparticles were determined to be 67.2/32.8, 74/26, and 87.7/12.3, respectively. This result suggested that the final compositions in the alloy particles were not the same as the initial feeding ratio in the solution mixtures even though the samples showed bimetallic nature or consists of the two metal elements. It is indeed intriguing that all samples predominantly contained Au atoms (Au-rich alloy particles) even for the samples with large amount of Ag ions in the initial mixture. Assuming that Au ions were all consumed by the laser during the given irradiation time, if there were 25 gold ions and 75 silver ions in the reaction mixture (initial Au/Ag = 25/75), only about 16% of Ag ions has been converted to Ag atoms and alloyed with Au atoms in the resulted NPs (Au/Ag = 67.2/32.8). Additionally, in the sample with initial ratio of 50/50 and 75/25, the percentages of Ag ions that have been converted into Ag atoms were about 35% and 42%, respectively. Thus, it seems that the presence of Au ions in the reaction mixture of low concentration advances the formation of Ag atoms in the alloy, clarifying the previous prediction deducted from UV-visible spectra analysis of the samples (Fig. 2).
Metal NPs are commonly synthesized by direct chemical reduction of ion precursors of soluble agents in solution due to the relative difference in the reduction potential. In the present work, metal atoms are produced by femtosecond laser pulse irradiation-induced reduction of metal ion precursors. Theoretically, the light intensity of the tightly focused laser beam used in this study is in the order of 1018 W/cm2, which is high enough for the atoms or molecules or condensed matter to absorb several photons simultaneously and induces the bound-free transition thus releasing free electrons (multiphoton ionization). The process would easily provide a few free electrons with low initial kinetic energy created from molecules with low ionization potential in the focal volume at the front part of the pulse. The energetic electrons were accelerated further in the strong laser field and collided with atoms or molecules in the solution inducing an optical breakdown of the bulk solution indicated by the presence of plasma and bubbles. At this point, water molecules which represent the most abundant species in the solution might be dissociated by an intense femtosecond laser field, giving rise to the formation of free radicals such as HË™, OË™ and OHË™, and the ejected electrons (Nikogosyan and Angels 1980; Chin and Lagace 1996). It should be pointed out that plasma and tiny bubbles have been experimentally observed. The energetic radicals and solvated electrons that were contained in the plasma might be then caught by H+ or OH- ions to form the bubbles confirmed as H2 and O2 gases by chromatography test in the previous works (Nakamura et al. 2008 and 2009), and/or they can be trapped by metal ions, resulting in formation of metal atoms during the femtosecond laser irradiation. It has been reported elsewhere that the mean free time of the ejected electrons were very short (about 1 fs) in condense matter (Chin 2009). This would mean that if a whole 100 fs pulse focuses into a small focal zone, there would be a few tens of cycles of collisional ionization resulting in optical breakdown and allowing an efficient reduction process. Owing to the presence of plasma and bubbles, the laser energy can be delivered and deposited throughout the solution without stirring even though the irradiation point was on a fixed position in the solution. However, the precipitation mechanism for both monometallic gold (Zhao et al. 2003; Nakamura et al. 2008) and silver (Abid et al. 2002) from their corresponding metal salt using laser irradiation pulses have been proposed elsewhere, and now we are interested in revealing the mechanism for bimetallic system.
In the case of bimetallic mixture of gold and silver, the solution to be irradiated contains the two metal precursors which are at low concentrations and thus stable in the presence of each other with no AgCl precipitation. The strong reducing species, solvated electrons and free radicals provided by femtosecond laser pulse, readily react with both precursors. The respective initial amounts of metal atoms resulted from the competition between the Au and Ag reduction processes. Even though the initial probabilities of being reduced by photolytic radicals and electrons are the same for both metal precursors, the formation of gold atoms may be prevailed because reduction potential for AuCl4- species (E0NHE = +0.93 V) is greater than Ag+ ions (E0NHE = +0.799 V). Rather to form core-shell structures, however, AuAg alloy NPs were formed in our case (Figure 2). It is well known that atomic size of Ag is similar to that of Au, so interdiffusion between Au and Ag atoms in the solution is easy, but high intensity delivered by femtosecond laser pulse may allow an efficient interdifussion which could be achieved in a very short time, shorter than any other chemical or electrochemical processes, thus preventing the possibility of the inter-metallic electron exchange that would cause metal segregation. Recognizing this, the formation of alloy NPs can be demonstrated in brief as follows:
H2O + nhn ® eaq-, H+, HË™, OË™, OHË™, H2, O2 (1)
[AuCl4]- + 3eaq- ® Au0 (s) + 4Cl- (2)
(Au0 - Ag+) + eaq- ® (Au Ag)* (3)
(Au - Ag)* ® (Au Ag)0 (interdifussion) (4)
As shown in the previous section, the formation of AuAg alloy nanoparticle has been confirmed by a linear variation of absorption band with respect to the initial feeding ratio of the components in the reaction mixture. However, the monometallic silver nanoparticles cannot be formed as confirmed by the absence of the absorption peak around 400 nm. Only when silver ions are mixed with gold ions, the absorption band originated from the alloys can be observed. It is most likely that either Au ions or nanoparticle plays a key role in the formation of silver NPs in low concentration solution. Because a high reduction potential, Au atoms were formed first and might acted as a host for Ag+ ions adsorbed on the surface (Eq. 2). The reduction potential of Ag+ may become higher by adsorbing on the Au NPs (more accessible to photochemically generated radicals and electrons) and thus the formation of the zero-valent Ag atoms at low concentration could be possible. Once Ag atoms were formed in the surface of Au atoms, interdifussion between the two atoms were pronounced, leading to the formation of homogenous AuAg alloy particles. The similar feature was also reported by Zeng, et. al. (2007) using the same irradiation technique, in which the formation of Ag atoms at low concentration (5 mM) could be possible only if the negatively charged TiO2 NPs were co-existed with Ag+ ions in the solution. They argued that during the excitation by the femtosecond laser irradiation, TiO2 NPs transfer the excited electrons from the valance band to the adsorbed Ag+ cations, thus enhancing the likelihood of Ag0 formation resulting in the formation of Ag-TiO2 composite NPs as a final product. Nevertheless, the adsorption Ag+ cations on Au atoms prior to alloy formation in our case is most likely explanation for the formation of Au-rich alloy NPs as confirmed by EDX analysis (Table 2) and for the reason why their corresponding particle size gradually decreases with an increase in Ag+ ratio in the solution mixture.
Although the genuine growth mechanism of alloy NPs is not very well understood at present and needs further verification, at least the evidences of alloy nanoparticle formation can be well supported by our experimental results. The spectral evidence came from the appearance of a single plasmon band in the visible range as shown in Fig. 2 and the linear blue shift in the plasmon peak with the increase in Ag mole fraction (Fig. 3). Also the lack of apparent core-shell structure in HRTEM images (Fig. 5) further supported the formation of gold-silver alloy nanoparticle.
The AuAg alloy NPs can be successively prepared by femtosecond laser irradiation of the corresponding mixture ions at low concentration due to AgCl restriction. Despite that the production yield is very low, we found the evidence of the alloy formation from the facts that UV-visible absorption spectra showed continuous shift with varying gold mole fraction. TEM images also exhibited the lack of core-shell structures. Furthermore, EDS analysis confirmed the particles are Au-rich alloy NPs even for the samples with large amount of Ag+ ions feeding in the solution mixture. The experimental results also revealed that monometallic Ag NPs could not be produced in this case, and its formation at low concentration requires either Au ions or atoms to be co-existed in the solution. This result is consistent with our previous work of silver synthesis using the same method. Mechanism of the alloy formation has also been proposed, and the experimental study to confirm the mechanism is underway.