Synthesis of Spherical Silver Nanoparticles with Controllable Sizes in

prepare spherical, almost monodisperse silver nanoparticles ranging in diameter from 10 to 80 nm ... particles in organic media,16 preparation of sphe...
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J. Phys. Chem. C 2007, 111, 7910-7917

Synthesis of Spherical Silver Nanoparticles with Controllable Sizes in Aqueous Solutions Alexander Pyatenko,* Munehiro Yamaguchi, and Masaaki Suzuki Research Institute of Genome-based Biofactory, National Institute of AdVanced Industrial Science and Technology (AIST), Sapporo 062-8517, Japan ReceiVed: February 7, 2007; In Final Form: April 1, 2007

A new method that combines the seed technique with laser treatment of a synthesized colloid was used to prepare spherical, almost monodisperse silver nanoparticles ranging in diameter from 10 to 80 nm ((10%). Particles in the 10-40 nm size range can easily be produced in a one-step synthesis. Preparation of particles in the 40-60 nm range requires a multistep synthesis that involves laser treatment of the synthesized colloid. Particles larger than 60 nm were produced using a multistep synthesis with laser treatment of intermediate products. Appropriate “soft” conditions were developed for the laser treatment, where “soft” laser fluence was defined as the maximum possible fluence that heats and melts but does not evaporate the spherical particles and, therefore, does not change their sizes. The numerical values for this fluence were calculated for different particle diameters using the particle heating-melting-evaporation model.35,21 The criteria for application of this model are discussed.

1. Introduction Due to their unique optical, electrical, and magnetic properties,1-3 silver nanoparticles have been the object of intensive studies during the past decade. Since their properties strongly depend on particle size and shape,4,5 it is important to be able to produce such nanoparticles in different shapes and desirable sizes. Among the different shapes, such as cubes and prisms,6,7 plates and disks,8,9 and rods and wires,10,11 spherical particles play a unique role as the only type for which the classical Mie’s theory1 provides an analytical solution of Maxwell equations. Spherical silver particles are needed for many applications, such as SERS,12,13 single-molecule labeling and recognition,14,15 and many others. While considerable progress has recently been made in the production of silver particles in organic media,16 preparation of spherical silver particles in aqueous solutions is still a subject of intensive study. Of the many various existing preparation schemes for such particles, the simplest and most commonly used synthesis is the chemical reduction of silver salt by one of two reduction agents: NaBH4 or citrate.17-20 Although borohydride can usually be used to produce small particles, synthesis of larger particles is difficult. While large silver particles can easily be synthesized using citrate, it is also difficult to control their size and shape. Using the citric reduction method and irradiation of the colloid with a laser beam, we succeeded earlier in preparing a silver colloid with a relatively high concentration of spherical and almost monodisperse particles with an average size of about 8 nm.21 That time, it was also suggested that these small particles could be used as seeds in further synthetic processes. The seed method has a long history.19,22-25 Recently, it was further developed by the groups of Murphy26-28 and ElSayed29,30 mainly for the synthesis of noble metal (particularly gold) nanorods. Jana, Gearheart, and Murphy have also used this seed technique in the size-controllable synthesis of spherical gold nanoparticles.31,32 By controlling the experiment conditions, they were able to produce spherical gold nanoparticles 20* Corresponding author. E-mail: [email protected].

100 nm in diameter with a relatively narrow size distribution ((20%). They pointed out two main difficulties in the production of almost monodisperse gold particles: bulk nucleation, leading to small particle formation, and the formation of nanorods. Since gold particles are more inert than silver, it is reasonable to expect that some additional efforts to minimize bulk nucleation and formation of nanorods will be necessary if this technique is applied to the synthesis of spherical silver nanoparticles. This paper proposes a new, simple method of synthesizing spherical silver nanoparticles of various desired sizes in an aqueous solution. The key to this method is a combination of the seed technique with laser treatment of colloids. 2. Experimental 2.1. Seed Particle Preparation. The procedure for preparing seed particle colloid has been described in detail21 and is only schematically repeated here. A silver nitrate solution (50 mL of 0.01 M AgNO3 in 450 mL of Milli-Q water) was heated to the boiling point while being stirred mechanically. Just as boiling began, 10 mL of a citrate solution (1 g of trisodium citrate, C6H5Na3O7, dissolved in 100 mL of water) was added. Boiling then continued for 1 h. A 10 mL quantity of this colloid was then placed in a small 20 mm diameter glass vial and irradiated by the second harmonic of Nd:YAG laser (λ ) 532 nm). The laser beam had a diameter of 7 mm, pulse duration time of about 10 ns, and pulse frequency of 10 Hz. The maximum laser power of 3 W (corresponding to J ) 0.78 J/cm2) was used.21 Using this procedure, almost monodisperse silver nanoparticles with an average size of 8-10 nm and size distribution of 1.5-2 nm were prepared in different experiments. Concentration of the particles in seed particle colloid was about 1013 cm-2. 2.2. One-Step Synthesis. The procedure was similar to the citric reduction of silver nitrate, but this time some amount of the seed particle colloid and 10 mL of the citrate solution (1 g of trisodium citrate dissolved in 100 mL of water) were added as the water solution of silver nitrate (50 mL of 0.01 M AgNO3 and 450 mL of water) started to boil. After 1 h of boiling with

10.1021/jp071080x CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007

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intensive mechanical stirring, the colloid was cooled to room temperature and analyzed. As the amount of seed particles was changed in different syntheses, the concentration of finally synthesized nanoparticles in colloid varied from1013 to 1011 cm-2 when the average size of the synthesized particles was changed from 10 to 40 nm. 2.3. Multistep Synthesis. The first step was similar to the one-step procedure: 10 mL of the seed particle colloid and 2 mL of the citrate solution were added to a water solution of silver nitrate (10 mL of 0.01 M AgNO3 in 100 mL of water) just as it started to boil. After 1 h of boiling with stirring, the next step was carried out. Another 10 mL of 0.01 M AgNO3 in 100 mL of water was heated and added to the boiling colloid together with 2 mL of the citrate solution. Boiling with intensive stirring continued for another hour. The same procedure was then repeated. When the colloid volume became too large (larger than about 500 mL), the colloid was cooled and divided into equal parts. Only one of these parts was used in the next step, and the amounts of silver nitrate and citrate solutions added to the boiling colloid were also reduced proportionally. A small amount of the boiling colloid (0.2-0.4 mL) was removed for TEM (transmission emission microscopy) and UV-vis (ultraviolet-visible spectrophotometry) absorption analyses at 1-h intervals. 2.4. Multistep Synthesis with Laser Irradiation. The same multistep synthesis was conducted, as described above. After a few steps (usually five), the colloid was cooled and irradiated for 1 h by the second harmonic of an Nd:YAG laser. The choice of laser power is described below. For irradiation, a 20 mL portion of the colloid was placed in a 35 mm diameter glass vial. 2.5. Apparatus. The TEM sample was prepared by placing one droplet of the prepared colloid onto a micro grid. A Hitachi H-800 model (voltage ) 200 kV) was employed for TEM observation. More than five microphotographs of different areas of the mesh were used to measure the particle size distribution (PSD). Over 100 particles (usually more than 400) were counted to plot a histogram and measure the PSD. UV-vis absorption spectra were measured by a Shimadzu UV-1200 spectrometer. 3. Results and Discussion 3.1. One-Step Synthesis. Assuming there is no bulk nucleation and the reduction of silver ions occurs only on the surface of seed particles, that is, the total number of silver particles in colloid is constant and only the homogeneous growth of seed spherical particles takes place, the final particle diameter, dp, can be calculated as

dp/dp0 ) (1 + n+/ns)1/3

(1)

where dp0 is the diameter of seed particles, and ns and n+ are the concentrations of seed particles and silver ions added to the colloid (in the form of AgNO3). This equation is well-known and often used in the seed technique applying to spherical particle synthesis.23,31 Several syntheses were achieved by changing the n+/ns ratio in eq 1. The results of TEM observation for four different syntheses are presented in Figure 1. Most of the particles were spherical, and the average size of a spherical particle increased with the n+/ns ratio. When this ratio exceeded 20, however, the synthesis of spherical particles was accompanied by the synthesis of nanorods. The relative concentration and aspect ratio of these nanorods increased with the n+/ns ratio. Table 1 compares the average sizes of spherical particles measured from

TEM photographs with the sizes calculated by eq 1. The good agreement between experimental and calculated values of dp proves that the main process was the reduction of silver ions on the surface of seed particles and that the main products were the spherical particles, at least while the n+/ns ratio did not exceed 100. Very small particles with sizes of a few nanometers can be seen in Figure 1d. These particles can be identified as the result of bulk nucleation, which can already start at a n+/ns ratio about 50. However, the total amount of these very small particles is still rather small, and those particles were not observed in most of the other TEM photographs made for n+/ ns ) 45. UV-vis extinction spectra of different colloids are presented in Figure 2a. The positions of all plasmon peaks correspond well to Mie’s theory for spherical particles:1 the maxima were close to 400 nm and shifted to red as the particles became larger. However, a red tail appeared when the n+/ns ratio exceeded 20. Small peaks can be distinguished in the tail region for curves 2-5 in the insert of Figure 2a. The maxima of these peaks shifted from 600 to 700 nm as the n+/ns ratio became larger. These peaks can be identified as the longitudinal peaks of the plasmon absorption of nanorods.33 The low intensity of these peaks also proves the small relative amount of nanorods. To eliminate nanorods from the colloids so that only spherical particles remained, the colloids were treated with soft laser irradiation. 3.2. Soft Laser Irradiation. Size reduction of nanoparticles after laser irradiation is a well-known phenomenon.34,35 The reshaping of gold nanorods into spherical particles was studied experimentally by El-Sayed et al.36,37 and theoretically by Wang and Dellago.38 Two different mechanisms of size reduction have been proposed by two research groups. Kamat and co-workers34 concluded from their picosecond photoabsorption spectroscopic measurements that the photoejection of electrons from a particle into a solution caused ionization and Coulomb explosion of the ionized particle. On the other hand, Takami et al.35 have studied the fragmentation of gold nanoparticles by nanosecond laser pulses and proposed a simple explanation for their size reduction through the heating-melting-evaporation mechanism. Recently, Mafune and co-workers39 have detected the photoelectrons ejected from silver nanoparticles by nanosecond laser pulses, and therefore, seemed to confirm the mechanism proposed by Kamat et al.34 Discussing their results, Mafune et al.39 speculated about two possible mechanisms of photoejection: thermionic emission and two-photon ionization. Let us estimate the possibility of both processes taking place under our experimental conditions, as well as for the parameters used by Mafune et al.39 First, we have calculated the photon density in the laser beam. For our experimental parameters: λ ) 532 nm (2.33 eV), pulse duration τ0 ) 10 ns, beam diameter 7 mm, and pulse energy 60 mJ pulse-1 (maximum energy used in these experiments), the photon density was about 4 × 1029 m-2 s-1.To estimate how many photons can interact with a particle’s electrons simultaneously, we have to define the interaction time. Because a photon interacts with electrons of a nanoparticle through the surface plasmon, collective oscillation of all conductive electrons, this time, could be estimated through the dephasing time, T2, which is the characteristic time of exponential relaxation of the plasmon after one-photon excitation.40 According to the literature,41 the dephasing time for silver is about 10 fs. Thus, we can say that the interaction time of a laser photon with a nanoparticle’s electron is no longer than 10 fs. By using the absorption cross sections, calculated for particles of different diameters using the Mie theory,1 σabs532(dp), we have

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Figure 1. TEM photographs for four colloids prepared in one-step syntheses with different n+/ns ratios: (a) n+/ns ) 3.6; (b) n+/ns ) 18.9; (c) n+/ns ) 32.4; (d) n+/ns ) 45. All photographs have the same magnification. The inserts are the histograms of particle size distribution calculated only for spherical particles.

TABLE 1: Average Size of the Spherical Particles Synthesized in One-Step Experiments dp, nm measured n+/ns

before soft laser treatment

after soft laser treatment

calculated by eq 1a

3.6 18.9 32.4 45 90

14.2 20.2 27.1 29.6 34.0

14.0 19.8 28.7 28.8 33.5

13.3 21.7 25.8 28.7 36.0

a

dp0 ) 8.0 nm.

found that the number of photons that can simultaneously interact with the electrons of one particle changes from 0.1 to 20 when particle diameter changed from 10 to 100 nm. Because the photon energy was transferred to collective oscillation of all conductive electrons in the particle, to estimate the energy transfer from photon to one electron, the number of photons has to be divided by the number of conductive electrons, which for silver, as well as for gold, is equal to the number of atoms in a nanoparticle. Therefore, the number of photons that can interact simultaneously with one conductive electron changes from 3 × 10-7 to 7 × 10-7 when particle diameter changed

from 10 to 100 nm. These values are far too small to speak of any possibility for two-photon ionization or photoelectric emission in our experiments. While in their experiments Mafune et al.39 used nearly the same laser pulse energy, 50 mJ pulse-1, the pulse was focused by a lens onto the solution. This focusing can increase the photon density near the focus location by 4 orders of magnitude. Thus, it is possible to expect that in their experiments the number of photons per one conductive electron was about 10-3-10-2, that is one photon transferred from 0.1 to 1% of this energy (3.49 eV for λ ) 355 nm) to one electron. Taking into account the value of the work function for silver (4.26 eV1), this is still too low to speak of two-photon ionization. To estimate the probability of thermionic emission, it is necessary to know the time interval during which the electrons are heated by absorbing the laser photons, but are not yet exchanging this heat with the particle lattice. From literature, this e-p coupling time can be estimated at 1-3 ps.36,37 In the minimum time interval of 1 ps, the number of incident photons per one conductive electron is equal to (3-7) × 10-5, depending on particle diameter. The energies associated with these values are still too low compared to the work function value. Therefore, we can declare that under our experimental conditions, both the two-photon ionization and thermionic electron emission are negligible.

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J. Phys. Chem. C, Vol. 111, No. 22, 2007 7913 sizes will not be changed. This is called “soft” irradiation, compared to the “hard” irradiation needed for particle heating, melting, and complete evaporation by one laser pulse, defined in our previous work.21 To quantitatively define this soft laser fluence, that is, the maximum possible fluence before the sizes of the spherical particles are changed, the Mie theory1 was used to calculate the absorption cross section at a wavelength of 532 nm, σabs532, for particles of different diameters, dp, and then the energy absorbed by particle irradiated by a single laser pulse

Qabs(dp) ) J σabs532(dp)

(2)

where J is the laser fluence. Takami et al.35 have estimated possible heat losses and showed that they are negligible compared to the energy absorbed by the particles from a laser pulse. We made the same estimations for our experimental conditions: laser fluence 0.1 J/cm2, pulse duration 10 ns, and silver nanoparticles with diameters from 10 to 100 nm. The results are shown in Supporting Information, Figure S1, where the radiation and boiling heat fluxes from one particle for the pulse duration time are plotted against particle diameter together with the energy absorbed by the particle from the pulse. Radiation heat flux was estimated assuming that  ) 1, Tp ) Tb(Ag) ) 2485 K (the maximum possible particle temperature). Boiling heat transfer flux was estimated assuming the heat transfer coefficient equal to 5 × 103 W cm-2 K-1 (maximum possible value42), and ∆T ) 2 × 103 K. It can easily be seen from the Supporting Information, Figure S1, that both heat fluxes are smaller than 1% of the energy absorbed by the particle. Thus, we are justified in our assumption that all the energy absorbed by the particle will be spent on its heating and melting: Figure 2. UV-vis extinction spectra of colloids produced in a onestep synthesis, (a) before, and (b) after soft laser treatment. The inserts display details in the 400-800 nm region. Curves 1-5 correspond to n+/ns ratios increasing from 3.6 to 90 (see Table 1).

The situation was different in experiments of Mafune et al.39 Due to focusing of the laser beam, photon density could be higher by 4 orders of magnitude. Thus, the number of incident photons per one electron in 1 ps could be as high as 0.1-1, allowing thermionic emission of an electron to take place. The photon density can also be increased by using pico- or femtosecond laser pulses. However, for a nanosecond laser (Nd: YAG, in particular) used without beam focusing, the probability of electron emission is too low for a wide range of laser pulse energies. In this case, it is reasonable to use the particle heatingmelting-evaporation model proposed by Takami et al.35 and developed further in our previous work.21 Spherical silver particles have one absorption peak, with the maximum located at about 400 nm for small particles (dp less than about 20 nm) and shifted to the red for larger particles.1 Silver nanorods have two peaks on the absorption curve: a transverse peak located at about 400 nm and a longitudinal peak at 500-800 nm, or even shifted to the IR region depending on particle size and aspect ratios.10,33 As mentioned above, small longitudinal peaks were also observed at 600-700 nm for colloids synthesized at n+/ns ratios greater than 20. Due to the existence of the longitudinal peak, light absorption at a wavelength of 532 nm for nanorods must be higher than for spherical particles. In such a case, the laser fluence high enough to heat and melt nanorods without evaporating the spherical particles needs to be identified. At such soft laser power, the spherical particles will be heated and melted by a single laser pulse, but will not be evaporated (even partially); therefore, their

Qabs ) Fp(πdp3/6){Cps(Tm - T0) + ∆Hm + Cpl(Tb - Tm)} (3) Thus, for particles of different size, dp, it is possible to determine the required laser fluence, J*, that will heat a particle up to the melting point, Tm ) 1234 K, complete melting, and heat the liquid particle up to the boiling point, Tb ) 2485 K, by one laser pulse. All physical and thermodynamic constants used in eq 3, that is, silver density, Fp, heat capacities, Cps for solid and Cpl for liquid, and melting heat, ∆Hm, were adopted from Perry.43 Critical laser fluence, J*, calculated in accordance with the described procedure, is presented in Figure 4 as a function of particle diameter. One laser pulse with fluence higher than J* will start the particle evaporation. To apply the J* value calculated for a single laser pulse to the entire laser treatment process, one must be sure that the particle will cool down completely in the time between two consecutive pulses. For this purpose, we have calculated the process of particle cooling via two possible mechanisms: radiation and boiling conductive/convective cooling. The details of these calculations are shown as Supporting Information, S2. According to these results, the characteristic times ranged from 2 × 10-6 s to 2 × 10-5 s for radiation cooling, and from 10-5 s to 10-4 s for boiling, when the particle diameter varied between 10 and 100 nm. Both characteristic times are negligible compared to the 100 ms interval between two pulses. Thus, J* is the maximum possible fluence that heats and melts but does not evaporate the spherical particles and, therefore, does not change their sizes. On the other hand, as mentioned above, this laser fluence can be large enough to melt the nanorods. This critical laser fluence, J*, was used for the definition of soft irradiation.

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Figure 3. Soft laser fluence, J*, as a function of particle diameter. J* was calculated as the maximum possible laser fluence that would heat and melt the particle, without evaporating it, in one laser pulse.

Figure 4. Cube of average particle diameter, dp3, for colloids synthesized in multistep synthesis with (n+/ns)i ) 9 as a function of n+/ns ratio.

The TEM observation results show that in all the colloids synthesized by the one-step method with n+/ns ratios less than 100 and treated with soft laser irradiation for 1 h, the nanorods disappeared completely and the average size of spherical particles remained the same (see Table 1). The UV-vis extinction spectra for different colloids after irradiation displayed in Figure 2b are in good agreement with the TEM observation results. All the red tails, as well as the small red peaks, have disappeared. At the same time, small red shifts of the primary peaks and their widening corresponding to increase in particle size can still be observed. Plasmon peaks for colloids with an average particle size of 14.2 nm remained the same. These results strongly support our definition of critical laser fluence, as well as our assumption that “soft” laser irradiation is strong enough to melt the nanorods. Thus, by applying one-step synthesis and subsequent treatment of the synthesized colloid with soft laser irradiation, we obtained stable colloids containing only spherical and almost

Pyatenko et al. monodisperse particles. The average particle size can be controlled well by varying the n+/ns ratio and can easily be changed from 8 to 40 nm. A further increase in the n+/ns ratio leads to a further increase in nanorod concentration, diameter, and aspect ratio. The treatment of such colloids with soft laser irradiation cannot melt those nanorods completely. Increase in laser power starts the spherical particle evaporation and produces many very small particles. The following multistep procedure was designed to produce larger particles. 3.3. Multistep Synthesis. As demonstrated by Figure 1 and Table 1, the nanorods were not synthesized and the average size of spherical particles was controlled well by eq 1 when the n+/ns ratio was less than 20. This means that the probability of bulk reduction was very low, and practically all silver ions were reduced only on the surface of seed particles. Therefore, it was reasonable to expect that by repeating this process several times while controlling the amount of silver nitrate added to each step so that the n+/ns ratio remained below 20, it would be possible to synthesize colloids with larger spherical particles. The results of this multistep synthesis with constant ratio (n+/ ns)i ) 9 are presented in Figure 4. The cube of average particle diameter, dp3, is plotted as a function of the n+/ns ratio, which, in turn, is proportional to the number of steps. Experimental results could be fitted well by a straight line, as demonstrated in Figure 4. In accordance with eq 1, the slope of this line gives the value dp0 ) 10.0 nm for the average diameter of initial seed particles, which is in good agreement with the experimentally measured value, 8.6 ( 1.5 nm. Results of TEM observation demonstrated that only spherical particles were synthesized in the first five to six steps. However, after six to seven steps, the first nonspherical particles, ellipsoids, or nanorods with a very small aspect ratio appeared. The relative amount of these nonspherical particles was very small and even after 16 steps, ((n+/ns)Σ ) 144), did not exceed a small percentage, as seen in Figure 5a. For further purification, the colloid synthesized in 16 steps underwent the soft laser irradiation. After such treatment, all nonspherical particles disappeared (see Figure 5b), while the average size of spherical particles remained the same. Thus, colloids with almost monodisperse spherical particles of average size less than 60 nm can easily be produced using the described technique. However, it is difficult to apply this approach to larger particle synthesis because the number of steps increases approximately as [dp/dp0]3/g. Thus, the production of particles with an average diameter of 80 nm requires about 60 steps, and more than 100 steps are needed for dp ) 100 nm. To reduce the number of steps, the (n+/ns)i ratio was doubled from 9 to 18. This, however, caused more intensive nanorod formation, and the colloid had to be treated by soft irradiation after only five to seven steps. On the basis of these results, a combination of the multistep synthesis procedure and laser treatment of intermediate products was used for synthesis of spherical particles larger than 60 nm. 3.4. Multistep Synthesis with Intermediate Treatment by Soft Laser Irradiation. The following experimental scheme was used. First, step-by-step synthesis with (n+/ns)i ) 18 was continued for five steps. The soft laser treatment was then applied to the intermediate colloid. After that, multistep synthesis was continued for another five steps. Using this initial scheme, spherical particles with dp ) 80 nm would be synthesized in six series of five steps each. After the first two series, however, it was obvious that the intensity of nanorod formation decreased when particle size increased. As the average size of the particles in the colloid increases with each step, their total surface area

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Figure 6. Cube of average particle diameter, dp3, as a function of n+/ ns ratio for colloids synthesized in multistep synthesis with intermediate treatment by soft laser irradiation. Different symbols are used for different series of consistent steps made between two laser treatments.

Figure 5. TEM photographs for colloid synthesized in multistep synthesis after 16 steps with (n+/ns)i ) 9, (a) before and (b) after soft laser treatment.

increases as size squared. Since these are used as the seed particles in the next step, the probability of silver ion reduction on the surface of seed particles increases and the probability of bulk reduction decreases with each step. This increase in the average size of particles in the colloid made it possible to increase either the number of consistent steps in one series or the (n+/ns)i ratio. Accordingly, after the first two five-step series of synthesis, or after (n+/ns)Σ ) 180, the number of consistent steps before laser treatment was increased from five to seven, keeping the (n+/ns)i ratio equal to 18. In the final series, the (n+/ns)i ratio was doubled from 18 to 36. This approach substantially decreased the experiment time. The results of this multistep synthesis with intermediate laser treatment are presented in Figure 6, where the cubed average particle diameter, dp3, is plotted as a function of the n+/ns ratio. Individual symbols are used for the different series. After each series, the colloid underwent the soft laser treatment. Similar to the previous results (Figure 4), all points on this plot could be fitted well by a straight line, the slope of which corresponds to the value dp0 ) 9.8 nm for the average diameter of initial seed particles. The results of TEM observation are presented in Figure 7 for the final synthesized colloid ((n+/ns)Σ ) 594) before and after soft laser treatment. As seen in Figure 7a, no nanorods were formed for these large seed particles, and all the particles

Figure 7. TEM photographs of the final colloid synthesized in multistep synthesis with intermediate treatment by soft laser irradiation (corresponds to (n+/ns)Σ ) 594) (a) before and (b) after soft laser treatment.

had similar sizes. However, instead of nanorods or ellipsoids, nonspherical particles of some other shapes, like cubes or

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Figure 8. Average particle diameter measured in all synthesized colloids before and after soft laser treatment.

hexagons, can be observed. Soft laser irradiation was also useful in melting these particles. After re-crystallization, all the particles in Figure 7b became spherical. Nonspherical particles were still observed when the laser power used for colloid irradiation was less than the soft power (about 20%) calculated for 80 nm particles. On the other hand, when the laser power exceeded the 20% soft power, a relatively large number of small particles was immediately observed, changing the particle size distribution, that is, shifting the average diameter to the smaller sizes and, more importantly, broadening their distribution. The correct choice for the soft irradiation is clearly evident in Figure 8, where the average particle diameter measured in the colloid after soft irradiation is plotted against the average particle diameter measured in the same colloid before irradiation. These data show that the average particle diameter remains the same after 1 h of soft irradiation, proving the definition of soft laser fluence as the maximum possible fluence for heating and melting that does not evaporate the spherical particles. This in turn proves that the heating-melting-evaporation model35,21 can be successfully applied for nanoparticle interaction with a laser beam, at least for the following experimental conditions: aqueous colloids prepared with citrate without any surfactants or capping agents and using the second harmonic of an Nd: YAG nanosecond pulse laser. Finally, the standard deviations in average particle diameters, std(dp), are plotted in Figure 9 against those diameters, dp, for all colloids synthesized in these experiments. Figure 9 demonstrates that the average standard deviation can be accurately estimated as 10% and that all the colloids from these experiments contained almost monodisperse nanoparticles whose sizes changed predictably from 10 to 80 nm. Taking into account this particle monodispersion and the wide range of diameters, it is possible to claim that our method is superior to that of Mafune et al.,44-46 which produces nanoparticles by laser ablation of a metal plate in liquid. 4. Conclusion A new method has been developed that combines the seed technique with laser treatment of synthesized colloids and can be used to produce spherical, almost monodisperse silver nanoparticles in the size range of 10-80 nm ((10%) in an

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Figure 9. Standard deviations of average particle diameter compared to those diameters for all synthesized colloids.

aqueous solution. Spherical particles of different sizes can be prepared in a one-step or multistep synthesis, with soft laser treatment of the final product or the intermediate colloids. The size of the particles is predictable and can easily be calculated using the main equation that governs the seed method (eq 1). Soft laser fluence utilized in this method is defined as the maximum possible fluence for heating and melting that does not evaporate the spherical particles and, therefore, does not change their sizes. This soft laser fluence was calculated using the particle heating-melting-evaporation model,35,21 the applicability of which was verified by the experiment results. Supporting Information Available: Figure S1, shows the radiation and boiling heat fluxes from one particle for the pulse duration time plotted against particle diameter together with the energy absorbed by the particle from the laser pulse. The details of the calculations of the process of particle cooling via two possible mechanisms, radiation and boiling conductive/convective cooling, are shown in S2. This material is available free of charge via the Internet at http:/pubs.acs.org. References and Notes (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer Series in Material Science 25; Springer: Berlin, 1995. (2) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729. (3) Metal Nanoparticles. Synthesis, Characterization, and Application; Feldheim, D. L., Foss, C. A., Jr., Eds.; Marcel Dekker: New York, 2002. (4) Mock, J. J.; Barbic, M.; Smith, D. R.; Schultz, D. A.; Schultz, S. J. Chem. Phys. 2002, 116, 6755. (5) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269. (6) Sherry, L. J.; Chang, S. H.; Schatz, G. C.; Van Duyne, R. P.; Wiley, B. J.; Xia, Y. Nano Lett. 2005, 5, 2034. (7) Wilei, B. J.; Im, S. H.; Li, Z. Y.; McLellan, J.; Siekkinen, A.; Xia, Y. J. Phys. Chem. B 2006, 110, 15666. (8) Lu, L.; Kobayashi, A.; Tawa, K.; Ozaki, Y. Chem. Mater. 2006, 18, 4894. (9) Maillard, M.; Huang, P.; Brus, L. Nano Lett. 2003, 11, 1611. (10) Hu, J.; Chen, Q.; Xie, Z.; Han, G.; Wang, R.; Ren, B.; Zhang, Y.; Yang, Z.; Tian, Z. AdV. Funct. Mater. 2004, 14, 183. (11) Ni, C.; Hassan, P. A.; Kaler, E. W. Langmuir 2005, 21, 3334. (12) Wang, D. S.; Kerker, M. Phys. ReV. B 1981, 24, 1777. (13) Canamares, M.; Garcia-Ramos, J.; Gomez-Varga, J.; Domingo, C.; Sanchez-Cortes, S. Langmuir 2005, 18, 8546. (14) McFarland, A.; Van Duyne, R. Nano Lett. 2003, 3, 1057. (15) Zhang, J.; Malicka, J.; Gryczynski, I.; Lakowicz, J. J. Phys. Chem. B 2005, 109, 7643.

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