J. Phys. Chem. C 2008, 112, 1321-1329
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ARTICLES Examination of Silver Nanoparticle Fabrication by Pulsed-Laser Ablation of Flakes in Primary Alcohols Daniel Werner,† Shuichi Hashimoto,*,† Takuro Tomita,† Shigeki Matsuo,† and Yoji Makita†,‡ Department of Ecosystem Engineering, The UniVersity of Tokushima, Tokushima 770-8506, Japan, and Hearth Technology Research Center, AIST, Takamatsu 761-0395, Japan ReceiVed: July 11, 2007; In Final Form: October 5, 2007
This paper describes the formation and subsequent behavior of silver nanoparticles (NPs) observed by optical spectra, atomic force microscopy, and transmission electron microscopy in primary alcohols by nanosecond pulsed-laser irradiation (1064 and 532 nm, typically at 1 J/(cm2 pulse)) of silver flakes. Effects of the carbon chain length of the solvents and the irradiation atmosphere of the solutions on the Ag NP formation were investigated. The effect of alcohol chain length in aerated solutions can be described as follows: (1) in shortchain alcohols such as methanol and ethanol, the NPs are extremely unstable and easily settled down to form precipitates by centrifugation treatment; (2) very stable NPs are formed with an appreciably smaller particle size distribution in alcohols with chain lengths from C-3 to C-5 than in alcohols with longer chain length than C-5; (3) the yield of NPs is dependent on the alcohol chain length. On the other hand, the yield of NPs is greater in Ar- and N2-saturated solutions than in aerated solutions. Additionally, the yield is similar regardless of the chain length, with smaller size distributions than those in air-equilibrated solutions. Oxygen molecules dissolved in the solvents are responsible for these observations. The oxygen effect consists of two parts: (1) the scavenging of electrons generated by the plasma formation and thermionic emission due to extremely high temperature under the ablation condition; (2) the formation of an oxide layer on the surface of particles that hampers further growth processes to form NPs. Furthermore, we observed the formation of string segments in evacuated ethanol due to coagulation and coalescence of bare metal particles, giving rise to the splitting of the plasmon band. Thus we demonstrated that the systematic change in the solvent and irradiation atmosphere can control the particle size and size distribution. The present findings may add a new aspect to better manipulate NP fabrication based upon the laser ablation method.
Introduction Nanosized materials have attracted a great deal of attention in recent years because of a fundamental interest in their physical and chemical properties distinct from both bulk materials as well as atoms and molecules. At the same time, nanomaterials are promising for possible applications such as optical and magnetic devices, catalysts, and biomedical diagnoses.1-4 From the production point of view, for both the fundamental research and the applications, it is important to examine the way to control the particle sizes, crystallinity, and shapes for outstanding performances. Additionally, it is important to prevent coagulation and coalescence of nanoparticles (NPs) once generated, especially for an application such as the building block of superstructures, which is a subject of keen interest in fabricating a new generation of advanced electronic materials.5-7 Among the various methods to prepare NPs, laser ablation technique in liquids has been a choice actively explored for the facile production of metal NPs since more than a decade ago.8,9 The advantage of NPs prepared by the ablation method has been * Corresponding author. E-mail:
[email protected]. † The University of Tokushima. ‡ AIST.
considered essentially contamination-free when prepared from pure metals in aqueous/nonaqueous solutions. The laser ablationbased chemically pure metal NPs are distinct from those produced by other chemical methods, in which the presence of residual ions is a serious disadvantage that hinders applications such as surface enhanced Raman scattering (SERS) spectroscopy in chemical analysis. The ablation method, however, suffers from low production yields: it is difficult to accumulate sufficient concentrations for various applications. Another disadvantage is that the size distribution of the NPs tends to be wide because the aggregation of ablated atoms and clusters is difficult to control. Furthermore, long-term stability is questionable, and, to overcome this problem, surfactants have been used as a stabilizer for remedy.10-12 Thus further refinement of the method is a keen subject of study along with the mechanistic aspect of NP formation on the basis of the interaction of an intense pulsedlaser light with the material. Previously, most of the laser ablation studies in liquids were carried out by the irradiation of a focused nanosecond laser beam on a metal target such as plates, foils, and rods; only a few studies have been carried out for metal powders or flakes suspended in liquids.13-15 In particular, Kawasaki and co-
10.1021/jp075401g CCC: $40.75 © 2008 American Chemical Society Published on Web 01/16/2008
1322 J. Phys. Chem. C, Vol. 112, No. 5, 2008 workers demonstrated that the laser irradiation of the flakes is a highly efficient way to produce NPs with high yields.14,15 As for the medium of laser ablation-based NP formation, water has been the primary choice, possibly because of its inert and nonflammable nature. Nevertheless, acetone has been claimed to bear superior ability to stabilize NPs among a few organic solvents studied,15,16 and this result prompted us to investigate better nonaqueous solvents for the ablation-based formation of NPs with extended stability and with a narrow size distribution. Although it is against the criterion that the ablation is a contamination-free method for NP production, the previous studies have been carried out to reveal that the presence of chlorides in the aqueous medium during ablation provides reduction of the average particle size, prevents the formation of large particles, and increases the efficiency of the ablation process.17-19 In this regard, laser ablation in liquids saturated with inert or even reactive gas molecules might be interesting because the former can remove the effect of dissolved oxygen that may act to oxidize the surface of metals at a high temperature under ablation conditions, while the latter can scavenge electrons and radicals produced during the process of laser ablation, possibly affecting the yield and stability of NPs. In spite of the numerous studies on ablation-based metal NP production in liquids, there still remain issues to be investigated to improve the shortcomings inherent in the method. For instance, no systematic study has been carried out on the effect of solvent polarity, viscosity, and thermodynamic property for the production yield, size distribution, and long-term stability of the NPs. Additionally, the effect of irradiation atmosphere was rarely investigated previously. Although Kawasaki’s group carried out the laser irradiation of flakes suspended in N2bubbled solutions, it is still not clear whether inert atmosphere or atmospheric oxygen can really affect the yield of NP formation.14 It might be interesting to see the effects of a pressure of inert gas and evacuation on the NP formation. Presently we have carried out the production of NPs for Ag and Au by the nanosecond laser ablation of flakes in primary alcohols of various chain lengths in order to reveal the effect of solvents more systematically. We also investigated the effect of irradiation atmosphere: air-equilibrated, N2- and Ar-saturated, and evacuated solutions were employed. We found that the yield of Ag NPs is quite sensitively dependent on the alcohols employed and the irradiation atmosphere, while that of Au NP was insensitive. Here we report on the yields and size distributions of Ag NPs distinctly dependent on the carbon number of primary alcohols in aerobic solutions and the dramatically increased yields regardless of the solvents in anaerobic solutions. We also report the formation of pearl necklace-like strings in evacuated ethanol due to coagulation and coalescence. Experimental Section Ag (99.9%) and Au (99.9%) flakes with nominal thicknesses of 0.1-0.2 µm were obtained from HORIKIN, Inc. Reagentgrade alcohols (98.0-99.8%) were dehydrated by adding 4A molecular sieves before use. Laser irradiation was carried out with a nanosecond Nd:YAG laser (Continuum, NY61-10), which delivers 6 ns full width at half-maximum (fwhm) pulses (wavelength: 1064 and 532 nm; repetition rate: 10 Hz; beam diameter: 6 mm). The laser beam was focused by a convex lens with a focal length of 100 mm onto a sample solution (4 mg/10 mL) contained in a 18 mm inner-diameter cylindrical vessel under continuous stirring of the suspension; the laser beam just in front of the vessel had a diameter of 3.0-3.2 mm. The laser irradiation was carried out
Werner et al. for air-equilibrated, N2-bubbled, and Ar-bubbled samples and a sample that was evacuated and sealed at 0.4 Pa. The output energy of the laser was varied between 40 and 130 mJ/pulse: the laser fluence employed under our experimental conditions is estimated as 1.3-1.4 J/(cm2 pulse) for 100 mJ/pulse of the laser output. Clear yellowish-brown color developed on the laser ablation for C-1 to C-5 alcohol solutions because Ag flakes and large particles settled down immediately, but this is not the case for alcohols with chain lengths longer than C-5, which give rise to turbid solutions, presumably because of greater viscosity. These large particles were removed by centrifugation for 5 min at 4000 rpm. However, the problem arises for methanol and ethanol because the color of the solutions is dramatically reduced after centrifugation. Especially for methanol, the solution after centrifugation was almost colorless, although, without centrifugation, the clear colored solution remained at least 5-6 h at ambient temperature. In contrast, the centrifugation scarcely affected the extinction spectra of alcohol solutions with C-3 to C-5 chain lengths. Since the remarkably low stability of Ag NPs in methanol and ethanol hampered the same pretreatment protocol for optical measurements, we measured the extinction spectra both with and without centrifugation for the two solvents. The extinction spectra without centrifugation may represent true figures because the centrifugation enhances the agglomeration of Ag NPs in these two solvents. The extinction spectra of the solutions were recorded on a UV-vis spectrophotometer (Hitachi, U-2010). The NPs were examined by a JEOL JEM-3010 transmission electron microscope (TEM) operated at 300 keV in a high-resolution mode. The specimen for the TEM measurements were prepared by placing a drop of a sample solution onto a support made of Cu mesh coated with evaporated carbon film of 13 nm thickness; the support was dried in a vacuum desiccator. Atomic force microscope (AFM) images were recorded on a SEIKO SPI3700 for a conventional examination of the NPs. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Shimadzu ESCA-3400. Results 1. Effect of Alcohol Chain Length in Aerobic Solution. In the first place, laser ablation was carried out for Ag flakes in primary alcohols with various chain lengths in the atmospheric air at ambient temperature. Yellowish-brown color developed in the irradiated solutions, depending on both the period and the fluence per pulse of the laser. Figure 1A shows normalized extinction (absorption and scattering) spectra for the irradiation period of 5 min at 1064 nm. The spectra with a peak approximately at 400 nm with various peak intensities and spectral bandwidths are essentially ascribable to the surface plasmon band of Ag NPs.20 Remarkably, the peak intensity, spectral bandwidth, and the peak position of the spectra (Figure 1B) are strongly dependent on the alcohols employed as solvents. We also carried out the irradiation at 532 nm. Qualitatively speaking, the optical spectral changes dependent on the alcohol chain length for the 532 nm irradiation follow a trend similar to that for the 1064 nm irradiation, although broader spectral envelopes and weaker intensities were noted for the irradiation at 532 nm (Figure 1C). The weaker intensity for 532 nm excitation is due to a weaker absorption of flakes at this wavelength.15 Here we describe how the systematic change in the solvent molecules affects the yield and size distribution. Figure 1D shows the peak intensity of the plasmon band for irradiation at 1064 nm as a function of the number of carbon
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Figure 1. (A) UV-vis extinction spectra (normalized) of the Ag NPs in primary alcohols under aerobic conditions for samples irradiated at 1064 nm for 5 min with a pulse energy of (95.7 ( 3) mJ/pulse. (B) The peak wavelength of the extinction spectra and a calculated extinction cross section curve as a function of the refractive index (n) of the solvents. Curve 1 shows the calculated curve based on the Mie theory with dipole oscillation approximation (ref 31), curve 2 is the experimental curve observed in Ar-saturated solution, and curve 3 is that observed in aerated solution. Data points are for different primary alcohols from methanol (far left) to 1-octanol (far right). (C) UV-vis extinction spectra (normalized) of the Ag NPs in primary alcohols under aerobic conditions for samples irradiated at 532 nm for 5 min with a pulse energy of (95.2 ( 2.7) mJ/pulse. (D) Extinction peak values as a function of carbon number of alcohols for irradiation at 1064 nm. Curve 1 represents the observations in aerated solutions, and curve 2 represents those in Ar-saturated solutions. Dashed curves are drawn on the basis of the spectra without centrifugation, while solid curves are drawn using the spectra after centrifugation.
chain length. In short-chain alcohols such as methanol and ethanol, Ag NPs are very unstable. For instance, in methanol, yellow color once observed after irradiation disappeared completely by centrifugation, suggesting that Ag particles grow so quickly that they instantaneously settle down. This result is consistent with the previous observation of the laser ablation of a Ag plate in methanol.9 In ethanol, only a slight improvement was observed in terms of the stability compared with methanol because the color was weaker but still remained after centrifugation. Contrary to this observation, the peak intensity increased markedly in alcohols from C-3 to C-5, with a significantly narrow bandwidth comparable to that in acetone, which has been demonstrated by Kawasaki’s group to bear a superior ability to stabilize Au and Ag NPs.15 In sharp contrast to the short-chain alcohols, we observed a very broad envelope of the plasmon band for long-chain alcohols from C-6 to C-12. The previous ablation studies did not address these chain-length-dependent drastic changes in the Ag plasmon bands, which may worth noting and scrutinizing. Since the various bandwidths of the plasmon band hampered the estimation of the production yields of Ag NPs from the optical spectral measurement, AFM images was recorded for the particles produced by the ablation, and the particle size and number density (number of particles in unit volume) were
examined. As a prototypical example, Figure 2 A-C compares the AFM images of Ag NPs placed on a Si substrate after evaporation of the same volume of solvents (A: ethanol; B: 1-pentanol; C: 1-octanol) produced by 532 nm irradiation. First, the number density of particles is remarkably dependent on the solvent, which is in accord with the intensity of the solventdependent extinction spectra. Second, the distribution of the particle size is closely associated with the spectral bandwidth: a larger size distribution corresponds to a wider spectral bandwidth. These observations are well explicable within the framework of the Mie scattering theory because the plasmon band undergoes red-shift accompanied by a broader envelope with increasing particle diameter.21 Inspection of the extinction spectra coupled with the AFM images reveals that the yield of Ag particles increases with increasing carbon number of the primary alcohols. This statement is more clearly made if we plot the integrated spectral area as a function of the number of carbons in primary alcohols, as will be shown later in Figure 3D. A small concern can be generated, however, about the shape of the curve in Figure 3D, especially for aerated long-chain alcohols, because the overestimation of a measure can take place by representing the yields by the spectral area because the spectral bandwidth tends to be
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Figure 2. Particle size distributions (left) based on AFM images (right) for Ag NPs produced in three alcohols upon 532 nm laser irradiation. AFM images were obtained for NPs placed on a silicon substrate: (A) in ethanol, (for 60 particles, average diameter 33 nm), (B) in 1-pentanol (for 80 particles, average diameter 26 nm), and (C) in 1-octanol (for 50 particles, average diameter 52 nm). The measurements were carried out for samples after centrifugation.
greater for particles with a larger diameter. Thus we should keep in mind that the actual yields can be smaller in these cases. 2. Effect of Irradiation Atmosphere. Previous ablation studies of metal targets to produce NPs have been carried out mostly in aerobic solvents, possibly because of experimental constraint: tightly focusing the laser beam to the targets placed inside the vessels frequently causes damage to the front wall of the glass vessels, and hence irradiation was carried out with a configuration where the laser beam was introduced from the top of the vessels in open air. Since we employed suspended Ag flakes following Kawasaki’s adaptation,14 we exploited the advantage of a suspension that can be subjected to N2- and Arbubbling and also to the evacuation treatment. Thus the effect of irradiation atmosphere was studied. Under the circumstances, the observation of Ag NP formation was made dramatically
different from that irradiated in aerobic solutions (Figure 3) despite the fact that no discernible effect of the atmosphere was observed for Au NPs formation. First, a remarkably efficient production of Ag NPs was observed in terms of extinction spectra in N2- and Ar-saturated solvents such as methanol and ethanol (Figure 3A) in which the yield of Ag NPs were very low in aerobic solutions. Second, in the anaerobic solutions, the bandwidth of the surface plasmon band for long-chain alcohols such as 1-octanol (Figure 3B) is no longer broad as observed in the aerobic solution and is instead close to that in 1-pentanol. Compared with these striking differences, the spectral change in 1-pentanol is not so astonishing: the extinction peak intensity increased slightly in N2- and Ar-saturated solutions with a slightly narrower bandwidth (Figure 3C). The observed effect of irradiation atmosphere is
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Figure 3. Effect of irradiation atmosphere on Ag NP formation in alcohols: (A) extinction spectra in aerated, N2-saturated, and Ar-saturated ethanol (1064 nm laser irradiation for 5 min, 95.2 mJ/pulse), (B) extinction spectra in aerated and Ar-saturated 1-octanol (1064 nm laser irradiation for 5 min, 96.9 mJ/pulse), (C) extinction spectra in O2-saturated, aerated, and Ar-saturated 1-pentanol (1064 nm laser irradiation for 5 min, 95.9 mJ/pulse), (D) integrated spectral area as a function of carbon number of primary alcohols both in aerated and Ar-saturated solutions. All the spectra were measured after centrifugation, but in Figure 3D, the integrated spectral areas for methanol and ethanol are calculated based on the spectra without centrifugation (see the text).
similar for Ar and N2. Contrastingly, the laser ablation in O2saturated solutions gave a spectrum with a much broader envelope and decreased peak intensity than in aerated solutions (see, for instance, Figure 3C). Overall, the chain-lengthdependent profile of NP formation (Figure 3D) reveals that, while the yield of Ag NPs is dependent on the chain length of alcohols (also remember the overestimation problem in Figure 3D described in the previous section), the replacement of the irradiation atmosphere by Ar and N2 brought about nearly constant yields of NPs regardless of the chain length of alcohols with a remarkably narrow particle size distribution, particularly for alcohols with chain lengths over C-6. 3. Formation of Nanonetworks in Evacuated Ethanol. When the 1064 nm laser irradiation of Ag flakes was carried out in evacuated ethanol for 2-5 min at 100 mJ/pulse (1.31.4 J/(cm2 pulse)), the color of the solution turned green within a few minutes after terminating the irradiation. The particles thus formed were eventually settled down if the solution was left under the original evacuated conditions; however, the green solution remained unsettled for at least 2 months by exposing it to the air after the irradiation. The extinction spectrum of the green solution was recorded and depicted in Figure 4A, showing the splitting of the plasmon band with the appearance of an additional weak band centered at 600 nm in addition to the typical 400 nm band of spherical Ag NPs. This spectrum is
quite reminiscent of the one observed for network-type aggregates of Au NPs in which two bands were attributed to excitations with the electric field parallel (low-energy mode) and normal (high-energy mode) to the chain axis.22 The spectrum is also similar to the one observed for nanorods where the low-energy band was assigned to a longitudinal plasmon mode, while the high-energy band (the same as that observed for spherical particles) is assigned to a transverse mode originating from the rod.23 It seems difficult to distinguish spectroscopically coagulated (particles are almost touching but separated) and coalesced (neighboring particles grow together by building a common grain boundary) strings. However, the TEM observation revealed that approximately 80% of the NPs in the green solution are aggregated to form the chain-like segments and that both coagulated and coalesced moieties are formed within in the strings (Figure 4B). Discussion Effects of Alcohol Chain Length and Oxygen on Ag NP Yield. The laser ablation by a nanosecond pulsed laser to produce NPs inherently involves complicated processes, including melting and vaporization, plasma formation, and shock wave generation.24 NPs thus produced could be temporarily heated to several thousand Kelvin due to multiphoton absorption of
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Figure 4. Extinction spectrum (A) and TEM image (B) of Ag NPs generated in evacuated ethanol upon irradiation of 1064 nm laser light for 5 min (95.2 mJ/pulse). The image on the left is a magnified image of that on the right. Both coalesced (a) and coagulated (b) linkages are observed in the magnified image.
the laser pulses.25 Electron ejection as a form of solvated electrons was observed by exciting the plasmon band by a few groups,26-29 and the mechanism was ascribed to the thermionic emission.28,29 Traditionally, the NP formation has been considered to follow the nucleation and growth from evaporated atomic particles produced by the ablation of metal targets.11 Additionally, Kawasaki’s group has recently proposed a new mechanism in which the laser ablation causes explosive splitting of the metal flakes into submicron particles and, consecutively, the submicron particles into NPs.15 Our present investigation is based on the product analyses by UV-vis, AFM, and TEM, and thus, it is difficult to look into the mechanism of early events in NP formation as studied by in situ and transient absorption spectroscopic methods. Here we tentatively follow a traditional view of the “nucleation and growth model” on the following grounds: (1) we observed the Ag NP formation by UV-vis and AFM measurements of samples irradiated by 1064 nm laser pulses for a period of 1 min and centrifuged, and this result suggests that the formation of fine NPs takes place simultaneously with that of submicron
particles even at early stages; (2) we observed the plasma emission when the laser beam with well less than 1 J/(cm2 pulse) hits the Ag flakes, suggesting the occurrence of an extremely high temperature leading to immediate vaporization of atoms and ionization. The yield of ablation-induced NPs appeared to be remarkably affected by the irradiation atmosphere; the yields were always larger in N2- and Ar-saturated alcohols than in air-equilibrated solutions (see both Figures 1D and 3D). This striking difference in the yield of NP is primarily ascribed to the effect of oxygen dissolved in aerated alcohols. Here we point out two possible effects of oxygen as (1) the oxide layer formation on the surface of Ag particles heated to a very high temperature above the melting point, and (2) the scavenging of electrons generated in the ablation due to plasma formation and ionization. The formation of the oxide layer was actually confirmed by the measurement of XPS spectra of Ag NPs produced by the laser ablation under aerobic conditions (see Supporting Information, Figure S1) and thus the Ag NPs can be regarded as covered with Ag2O. As for the mechanism of the ionization, thermionic
Ag NP Synthesis by Pulsed-Laser Ablation of Flakes emission of electrons is postulated but not proved experimentally in the present investigation. The solubility of O2 (moL dm-3) in primary alcohols at ambient temperature is similar but, strictly speaking, is the largest for methanol and ethanol with a slightly decreasing trend with increasing number of carbons (see Supporting Information, Table S1). The replacement of O2 by Ar (N2) in the solvents will give rise to a diminished electron scavenging efficiency of the solvents, leading to an increased yield of NPs by enhancing the recombination of ejected electrons with ablated fragments (cations). More decreased plasmon band intensity in O2-saturated solution than in air-equilibrated solution is consistent with the assumption of electron scavenging by oxygen molecules at the higher O2 concentration. Since the rate of formation of the surface oxide layer that may hamper the growth of particles also depends on the concentration of dissolved O2, oxygen molecules act to reduce the yield of NPs. Another remarkable effect of the introduction of N2 and Ar is the spectral change from a broad one to a significantly sharp one typically observed in long-chain alcohols such as 1-octanol (Figure 3B). This observation corresponds to a reduction in the particle size distribution from a polydisperse to a much narrower size distribution. The polydisperse distribution may arise from the growth process of the nuclei if the nucleation takes place homogeneously in solution. This is not necessarily proved in the case of laser ablation, but we assume this is the case because the ablation takes place instantaneously with the ejection of atomic species ubiquitously in the irradiated area in solutions. Our common understanding of the growth is that a diffusionlimited growth would result in a uniform size distribution of NPs, while that by the growth-limited process may give rise to polydisperse distribution.30 The present experimental result of inhomogeneous particle size distribution in long-chain alcohols suggests that the growth is controlled by the rate of the surface process rather than that of diffusion. The reason for the slow rate of the surface process is ascribed to the oxidation of the surface of Ag particles, precursors of Ag NPs, to form Ag2O that may act as an obstacle to their growth. Therefore, if the oxide layer formation on the surface of the Ag particles is minimal, the rate of surface reaction to grow should be faster because bare metal surfaces can contribute more easily to the growth. This accounts for the observed transition of the particle size distribution upon the introduction of Ar or N2. It should be kept in mind that, even in short-chain alcohols, this reduction in the size distribution takes place accompanied by the slight sharpening of spectral bandwidth, as observed in TEM images, although not so obviously observed (Supporting Information, Figure S2). The third point that needs to be mentioned is the effect of oxygen on the spectral peak position in various alcohols (Figure 1B). For Ag particles of 10-100 nm in diameter as in the present case, the peak position of the plasmon band depends on both the refractive index or dielectric constant of the medium and the particle diameter.20,21 Curve 1 in Figure 1B represents the peak position of the extinction cross section as a function of the refractive index for primary alcohols from methanol (far left) to 1-octanol (far right) calculated by a simple dipole oscillator approximation.31 This calculation cannot take into account the spectral change dependent on particle size nor particle size distribution, and thus we can only see the effect of the solvent medium on the relative (not absolute) value of the peak position. The inspection of the experimental data curves (curves 2 and 3) reveals that the peak position is located at relatively longer wavelengths in aerobic solution than in Ar-/
J. Phys. Chem. C, Vol. 112, No. 5, 2008 1327 N2-saturated alcohols. This is again because of larger particle sizes and size distributions in aerobic solutions and the reduction in both size and size distribution contributes to the blue-shift of the peak position observed in Ar-saturated solutions. It is noteworthy that, for the alcohols from C-3 to C-5, the peak position increases with a trend similar to that of the calculated curve even in aerobic solutions, indicating a similar particle size distribution in these alcohols. Additionally, curve 2 is closer in shape to curve 1 because of the similar narrow size distribution attained in Ar-/N2-saturated alcohols. Thus the peak wavelength as well as the spectral bandwidth of the plasmon band can be a good measure for predicting particle size and size distribution. Observation of Chain-like Segments. The observation of chain-like segments similar to the present result has been reported in previous studies. For instance, Creighton observed the formation of strings of Au and Ag NPs once produced by citrate reduction in aqueous solution upon the addition of neutral pyridine molecules, which displace the adsorbed citrate anions.22 Another example is the formation of aggregates upon addition of CuSO4 up to a concentration of 1.5 × 10-4 M to chemically prepared Au hydrosols.32 An example of the laser ablation method is given by Mafune and co-workers, who observed that Au nanonetworks consisting of spherical particles and wires interconnected with each other were formed in a less concentrated aqueous sodium dodecyl sulfate (SDS) solution (10-6 M) far below the CMC (8 × 10-3 M) under irradiation of an intense (5 J/(cm2 pulse)) laser of 532 nm.33 Their explanation is that photofragments are not well stabilized by low concentrations of SDS molecules and grow into network structures by encounter and coalescence as long as they are melted by lasers. We should point out the difference in the mechanism of the formation of the strings and networks in these cases from ours. In the former case, we can imagine that the surface of the NPs produced by the chemical reduction is only minutely oxidized because of the mild nature of the method so that they can easily coalesce when the coagulation takes place upon addition of pyridine or CuSO4. In our laser ablation study, however, the surface of the Ag particles is covered with an oxide layer, as was confirmed by the XPS study when irradiated in aerated solutions, and in this case, the particles are resistant to aggregation to form immediate precipitation. Only when the irradiation was performed in evacuated state were the particles produced assumed to undergo minimal oxidation, and thus are susceptible to coalescence because only the solvent molecules cannot prevent the particles from agglomeration in long term. The important difference from the above latter case by Mafune’s group is that we demonstrated the coagulation and coalescence of Ag NPs without excitation to undergo melting if the covering oxide layer is absent, while they showed that the melted Au particles resulting from intense laser excitation of the plasmon band can give rise to the network structures. Long-Term Stability in Aerobic Solutions. Long-term stability of colloidal gold has been investigated previously. For instance, Klabunde and co-workers have investigated the colloidal gold formed by clustering of metal atoms in organic media.34 They showed that polar solvents, especially acetone and ethanol, yielded stable colloids, but that nonpolar organics such as toluene, pentane, and diethylether, and water yielded large gold particles that precipitated. The lack of stabilization in nonpolar solvents is ascribed to the absence of surface charge and lower dielectric constant. The dispersibility of a few metal NPs including Au and Ag was investigated by Kimura and coworkers also in nonaqueous solutions produced by a gas flowsolution trap method in which the fine particles formed in the
1328 J. Phys. Chem. C, Vol. 112, No. 5, 2008 gas phase are carried away with a stream of gas before being trapped in the organic solvents contained in a separate vessel.35 The dispersibility was found to correlate with the dielectric constant () of the solvents: the particles are more stable in solvents with a greater value of . Thus, the particles in hexane coagulate to form sediment deposit. They also found that the σ potential of Au particles in acetone and dichloromethane is slightly negative, and this negative charge is responsible for the stabilization in these solvents with relatively large values. Thus, for the stabilization of bare metal colloids, was shown as a good measure for the choice of solvent. Although laser ablation-based preparation of Ag and Au NPs has been made in various organic solvents, the long-term stability is a different story. For instance, Au NPs prepared in dimethyl sulfoxide (DMSO), tetrahydrofuran (THF) and acetonitrile are stable only for several hours at room temperature after preparation according to a previous study.36 In this respect, Ag NPs prepared in aerobic primary alcohols in the present study are found to be more stable: they last in 1-pentanol for at least 4 months and in 1-octanol for at least 2 months at room temperature under room light for solutions with a peak extinction value of 4-5. Note that it was found that the coagulation of Au and Ag NPs is accelerated under visible light irradiation.37,38 It is also pertinent to note that the samples we used are relatively high in concentration and that the long-term stability depends largely on concentration since flocculation takes place more easily at higher concentrations. As described above, Ag NPs prepared presently are most likely to be covered with an oxide layer, especially when irradiated in aerobic alcohols, and the role of the surface oxide layer seems to be very important for the stabilization of Ag NPs. We also note that the Ag NPs are more stable in C-3 to C-5 alcohols than in others investigated here. Additionally, we found that Ag NPs are very unstable in methanol and ethanol, in which even the treatment of centrifugation largely affected the stability. These results suggest that the stability is not necessarily correlated with the of the solvents for Ag particles produced by laser ablation. It is worth noting that we found that Ag NPs formed in evacuated acetone are actually unstable, although Kawasaki’s group demonstrated that Ag and Au NPs generated in N2bubbled acetone (and later exposed to air) are extremely stable.15 Our result has indicated that Ag NPs formed by irradiation in aerated alcohols or in Ar-/N2-saturated alcohols and then exposed to the air are generally stable but not stable if formed in evacuated alcohols. This may suggest that the surface oxidation of Ag NPs can proceed gradually during the period of exposure to the air even at ambient temperature. Therefore the mechanism of stabilization by acetone proposed by Kawasaki, a strong interaction between the acetone carbonyl group and metal NP surface such as charge transfer between the metal surface and the carbonyl oxygen atom, needs to be reconsidered. Alternatively, the interaction of the surface oxide layer with solvents such as acetone and 1-pentanol is important for the stabilization. The formation of oxide on the surface of NPs may induce dense packing of solvent molecules such as acetone and alcohols that can prevent the particles from coagulating. This point should be cleared up in future investigations. Summary We investigated a way to further improve the procedure for metal NP production based on the well-established laser ablation technique in liquid without undermining its advantages such as a concise and contamination-free preparation. Thus no stabilizer is employed; instead, primary alcohols with various chain
Werner et al. lengths are employed to examine how the systematic change in the solvent molecules affects the yield, size distribution, and stability of produced NPs. We also looked at the effect of irradiation atmosphere. Under aerated conditions, although the NPs are extremely unstable and easily settled down to form precipitates by centrifugation treatment in short-chain alcohols such as methanol and ethanol, very stable NPs are formed with an appreciably smaller particle size distribution in alcohols with chain lengths from C-3 to C-5 than in alcohols with chain length longer than C-5. Additionally, the yield of NPs is larger for long-chain alcohols. On the other hand, the yield of NPs is similar regardless of the chain length and is greater in Ar- and N2-saturated solutions than in air-equilibrated solutions with appreciably smaller size distributions. This is ascribed to the effect of oxygen dissolved in the solvents. The oxygen effect consists of two parts: the scavenging of electrons generated by the plasma formation and thermionic emission of electrons due to extremely high temperature under the ablation condition, and the formation of an oxide layer on the surface of particles that hampers further growth processes to form NPs. Furthermore, we observed the formation of string segments in evacuated ethanol due to coagulation and coalescence of the particles due to the absence of the surface oxide acting as a protective layer. Thus we demonstrated that the systematic change in the solvent and irradiation atmosphere can control the particle size and size distribution. Although ample studies have been carried out on the laser ablation-based fabrication of noble metal NPs in the past, the present result may add some new aspect to better manipulate NP fabrication. Acknowledgment. Financial supports by the Japan Society for the Promotion of Science, Grant-in-Aid for Scientific Research (#19655069), and Suzuki Foundation (#H018-20) are gratefully acknowledged. We are grateful to Prof. T. Hirotsu of AIST for his generous help with the measurement of TEM images. Supporting Information Available: XPS spectra of Ag NPs formed in aerated acetone and ethanol; histogram of particle size distribution observed by TEM images upon irradiation in aerobic ethanol and Ar-saturated ethanol; concentration of oxygen in various alcohols at 20 °C. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293-346 and references therein. (2) Haruta, M. Catal. Today 1997, 36, 153-166. (3) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078-1081. (4) Debouttiere, P.-J.; Roux, S.; Vocanson, F.; Billotey, C.; Beuf, O.; Favre-Reguillon, A.; Lin, Y.; Pellet-Rostaing, S.; Lamartine, R.; Perriat, P.; Tillement, O. AdV. Funct. Mater. 2006, 16, 2330-2339. (5) Wang, Z. L. AdV. Mater. 1998, 10, 13-30. (6) Gutierrez-Wing, C.; Santiago, P.; Ascencio, J. A.; Camacho, A.; Jose-Yacaman, M. Appl. Phys. A 2000, 71, 237-243. (7) Whetten, R. L.; Shafigullin, M. N.; Khourt, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. M.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397406. (8) Fojtik, A.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 252-254. (9) Neddersen, J.; Chumanov, G.; Cotton, T. M. Appl. Spectrosc. 1993, 47, 1959-1964. (10) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B 2000, 104, 8333-8337. (11) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B, 2000, 104, 9111-9117. (12) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B 2001, 105, 5114-5120.
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