Formation and Size Control of Silver Nanoparticles by Laser Ablation

Silver nanoparticles were produced by laser ablation of a metal silver plate in an aqueous solution of sodium dodecyl sulfate, C12H25OSO3Na. The absor...
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J. Phys. Chem. B 2000, 104, 9111-9117

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Formation and Size Control of Silver Nanoparticles by Laser Ablation in Aqueous Solution Fumitaka Mafune´ , Jun-ya Kohno, Yoshihiro Takeda, and Tamotsu Kondow* Cluster Research Laboratory, Toyota Technological Institute, and East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan

Hisahiro Sawabe Central Technical Research Laboratory, Nippon Mitsubishi Oil Corporation, 8 Chidori-cho, Naka-ku, Yokohama 231-0815, Japan ReceiVed: April 6, 2000; In Final Form: June 26, 2000

Silver nanoparticles were produced by laser ablation of a metal silver plate in an aqueous solution of sodium dodecyl sulfate, C12H25OSO3Na. The absorption spectrum of the silver nanoparticles is found to be essentially the same as that of silver nanoparticles chemically prepared in a solution. The size distribution of the nanoparticles measured by an electron microscope shifts to a smaller size with increase in the concentration of sodium dodecyl sulfate and with a decrease in the irradiation laser power. These findings are explained by a scheme that the nanoparticles are formed via rapid formation of an embryonic silver particle and a consecutive slow particle growth in competition with termination of the growth due to SDS coating on the particle.

1. Introduction Nanoparticles suspended in a solution have attracted much attention because of their size-dependent optical properties, magnetic properties, catalytic properties, etc.1-5 For instance, gold nanoparticles show intense photoluminescence only when their sizes are much smaller than a certain value (ca. less than 5 nm in diameter).1 One of the most important tasks is to develop a simple and versatile method to prepare nanoparticles in solutions in a size-selected and -controlled manner.6-12 Chemical reduction of metal ions is most commonly employed in the preparation of metal nanoparticles in solutions. Specifically, a metal salt dissolved in water is confined inside reversed micelles in a nonpolar solvent, and metal nanoparticles are formed in the reversed micelles by chemical reduction. The size of the metal particles is determined by that of the reversed micelles, which is controlled by changing the amount of water inside the reversed micelles with changing a molar ratio of water to the nonpolar solvent.10-12 More recently, an electrochemical method has been developed for generating stabilized metal nanoparticles for a wide variety of metals.13 Alternatively, ablation of a metal surface immersed in a liquid could produce nanoparticles of the metal in the liquid.14-17 In fact, gas-phase metal clusters are most conveniently prepared by laser ablation.18,19 Metal atoms with a less amount of small metal clusters are ablated from a metal rod by laser irradiation and are aggregated into metal clusters with a sufficiently larger sizes. Self-aggregation of the nanoparticles suspended in the liquid should be prevented by hindering direct contact of the nanoparticles. For example, Takami and co-workers have prepared small metal clusters by laser ablation of a metal plate in liquid helium,20-22 where the clusters are encapsulated in helium bubbles. In the present paper, we described a method to prepare stable nanoparticles in a solution containing a surfactant by use of * Corresponding author. E-mail: [email protected].

Figure 1. Schematic diagram of the experimental apparatus.

laser ablation on a metal plate immersed in the solution. The surfactant which surrounds each nanoparticles prevents direct contact of the nanoparticles. As a benchmark experiment, the laser ablation against a silver plate was performed in an aqueous solution of sodium dodecyl sulfate, C12H25SO4Na (denoted as SDS hereafter). The formation mechanism of silver nanoparticles in the solution was examined by changing the concentration of SDS and the power of the ablation laser. 2. Experimental Section Silver nanoparticles were produced by laser ablation of a metal silver plate in an aqueous solution of SDS. As shown in Figure 1, the metal plate (>99.99%) was placed on the bottom of a glass vessel which was filled with a 10-mL aqueous solution of SDS. The metal plate was irradiated by the focused output of the second harmonic (532 nm) of Quanta-Ray GCR-170 Nd:YAG laser operating at 10 Hz with a lens having a focal length of 250 mm. The spot size of the laser beam on the surface of the metal plate was varied in the range of 1-3 mm in

10.1021/jp001336y CCC: $19.00 © 2000 American Chemical Society Published on Web 09/07/2000

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Mafune´ et al. A transmission electron microscope (JEOL JEM-100S ×50000) was employed to take the electron micrographs of the solutions studied. Practically, a drop of the sample solution of interest was placed on a copper mesh coated with collodion and sputtered by gold ions in advance. The drop on the copper mesh was dried by heating to 320 K. After repeating this procedure three times, the mesh was washed with water to remove free SDS. Each size distribution was obtained by measuring the diameters of more than 1000 particles in sight on the given micrograph. 3. Results

Figure 2. Absorption spectrum of the silver nanoparticles produced by laser ablation of a silver metal plate in a 0.01 M aqueous solution of SDS (solid line). The dotted line shows the absorption spectrum calculated on the basis of Drude model.

diameter by changing the distance between the focusing lens and the metal plate. A Scientech power meter monitored the output of the 532 nm laser with the maximum output of about 90 mJ/pulse. Upon irradiation of the laser beam, the solution gradually turns brownish- yellow. The absorption spectrum of the solution was measured by a Shimadzu UV-1200 spectrometer. At least five different runs were accumulated on an NEC computer to obtain one spectrum.

Figure 2 shows an absorption spectrum of a 0.01 M aqueous solution of SDS in which a silver plate was irradiated with the 532-nm laser having the output power of 40 mJ/pulse. The spectrum exhibits a characteristic peak at 400 nm and a tail of a broad band extending toward the UV wavelength range. The absorption spectrum is essentially the same as that for silver nanoparticles prepared chemically by reduction of a silver salt in the reversed micelles.10-12 The agreement of the two spectra shows that silver nanoparticles are formed by the laser ablation in the solution. The formation of the silver nanoparticles in the solution is also verified by the electron micrograph of a specimen prepared from the solution as described above. Figure 3 shows electron micrographs and corresponding size distributions of silver nanoparticles produced by laser ablation (the wavelength of 532 nm and 90 mJ/pulse) of a silver plate immersed in aqueous solutions of SDS having different SDS concentrations. The nanoparticles thus produced were calculated

Figure 3. Electron micrographs and size distributions of the silver nanoparticles produced by laser ablation at 90 mJ/pulse in a SDS aqueous solution at the various SDS concentrations. The concentrations of the solution in panels a-c are 0.003, 0.01, and 0.05 M, respectively. The average size decreases with an increase in the SDS concentration.

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Figure 4. Electron micrographs and size distributions of the silver nanoparticles produced by laser ablation in a 0.01 M SDS aqueous solution at the various laser powers. The laser powers in panels (a)-(c) are 40, 55, and 70 mJ/pulse, respectively. The average size increases and the distribution broadens with an increase in the laser power.

to have the average diameters of 16.2 ( 4.0, 14.9 ( 8.4, and 11.7 ( 5.3 nm in the 0.003, 0.01, and 0.05 M solutions, respectively. The result shows that the average diameter decreases with an increase in the SDS concentration. A similar measurement was performed in a 0.01 M solution with changing the laser power (see Figure 4). The average diameters of the nanoparticles were obtained to be 7.9 ( 3.3, 10.7 ( 5.8, and 12.8 ( 4.1 nm for runs with the laser powers of 40, 55, and 70 mJ/pulse, respectively. Figure 5 shows the change in the average diameter of the nanoparticles as functions of the SDS concentration (panel a) and the laser power (panel b), where the error bars represent one standard deviation of the average diameters determined from several runs of measurements. In the absorption spectrum of silver nanoparticles, the broad band in the UV region originates from an interband transition of the silver nanoparticles. Its line shape does not change appreciably with the particle size, but its intensity is proportional to the number of silver atoms contained in the nanoparticle.10 In other words, the absorbance of the solution at the wavelength of the interband transition (250 nm) is proportional to the number of silver atoms in the solution. It follows that the number density of the nanoparticles can be derived from the absorbance if the size distribution of the nanoparticles is known. In reality, all the size distributions of the nanoparticles produced under the various conditions are not known. In this regards, let us adopt “relative abundance” of the nanoparticles from the absorbance at 250 nm. Evidently, the abundance is approximately proportional to the number density of the silver nanoparticle as far as the size distribution does not change significantly. Figure 6a shows the relative abundance of the

silver nanoparticles as a function of the laser power under different focusing conditions. When the laser is tightly focused with its spot diameter, a, of 1.2 mm, the relative abundance increases up to the laser power of 40 mJ/pulse and then levels off, as the laser power increases. At the conditions with a ) 1.6 and 2.4 mm, the relative abundance starts to increase at a threshold laser power, and then levels off, as the laser power increases. To examine the influence of the spot diameter on the relative abundance of the silver nanoparticles, the relative abundance was measured at different laser-spot diameters under the same laser fluence; the laser power increased linearly with the laser-spot area. The quadratic increase in the relative abundance with the spot diameter shows that formation of each silver nanoparticle takes places in a sufficiently smaller region than the laser spot, and hence, the number of sites for nanoparticle formation increases linearly with the laser-spot area. Figure 7 shows the relative abundance as a function of the number of the laser shots. The abundance increases almost linearly with the laser shots in the initial stage, and its slope is reduced above a laser shot of about 50000. Laser ablation of the silver metal plate was performed in a dodecanethiol solution in heptane, which turned to be slightly brown after 10 h. The electron micrograph showed that silver nanoparticles with 5-30 nm in diameter were produced. The comparison of the present absorption spectrum with that of the nanoparticles produced in the dodecanethiol solution in heptane shows that a much less amount of silver nanoparticles are produced in the heptane solution than that in the aqueous solution. In conclusion, the relative abundance of silver nano-

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Mafune´ et al.

Figure 5. Average radius of the nanoparticles (solid circle) and average dispersion of the radius distribution (open circle) are plotted as a function of the concentration of SDS in panel (a) and as a function of the laser power in panel (b). The error bars represent one standard deviation determined from several runs of measurements. The solid curves in panel (a) show the calculated radius on the basis of the dynamic formation model (see in Section 4).

particles depends on the concentration of a surfactant and naturally on the properties of the surfactant and the solvent employed.

Figure 6. (a) Relative abundance of the silver nanoparticles as a function of the laser power under the various focusing conditions. The spot sizes of the laser were 1.2 nm (solid circle), 1.6 nm (open circle), and 2.4 mm (solid square) in diameter, respectively. The solid curves show the eye guide. (b) Relative abundance of the silver nanoparticles at different diameters of the laser spot on the metal surface under the same laser fluence.

4. Discussion 4.1. Optical Absorption of Silver Nanoparticles Produced in Solution. The absorption spectrum of the silver nanoparticles exhibits the surface plasmon peak at ∼400 nm on the broad band due to an interband transition (see Section 3 for interband transition).23-33 The presence of the single surface-plasmon peak implies that the silver nanoparticles are spherical. Note that ellipsoidal particles would have two plasmon peaks in the absorption spectrum.10,33 An optical absorption of particles having diameters much smaller than the wavelength of incoming light is well reproduced by the calculation based on the Mie theory.23-33 When N particles of volume V are suspended in a medium with the dielectric constant, 0, the extinction coefficient, κ, of the particles is given by

κ ) 18πNV03/22/λ[(1 + 20)2 + 22]

(1)

where λ is a wavelength of the incoming light in the medium, and 1 and 2 represent the real and imaginary parts of the dielectric function, , respectively ( ) 1 + i2). Suppose that

the Drude model is applicable to describe the dielectric function of the silver particle, one obtains the real and imaginary parts of the dielectric function. In addition, when the diameter of the particle is smaller than the electron mean free path (∼52 nm in a silver crystal), 1 and 2 become size dependent due to additional electron scattering at the particle boundary.26 The optical absorption spectrum thus calculated reproduces the experimental one, when the plasma frequency is reduced to 90% of the plasma frequency of a silver crystal, and the collision frequency and the Fermi velocity of the silver crystal are adopted. The reduction of the plasmon frequency may result from attachment of surfactant on the silver nanoparticles (see the dotted line in Figure 2).10 In the range of the particle radius studied (5-7.5 nm), no appreciable change of the spectral features (peak width and shift) with the particle radius was observed, as expected from the theory. In this range, an intrinsic size effect competes with an extrinsic size effect, which acts toward the opposite direction.25,33 Actually, the surface plasmon peak of silver nanoparticles with the radius of 1-5 nm is reported to change its width and height as the radius changes, and the width of the surface plasmon peak increases linearly

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rp ) (3NpVa/4π)1/3

(2)

In the derivation of eq 2, the particle is regarded as spherical because the absorption spectrum exhibits a single plasmon peak. The cross section for attachment of a silver atom to the particle should be proportional to the geometrical cross section of the particle (πrp2) (see eq 2). Suppose that a silver atom diffuses toward the particle with a velocity of Va. Then, the rate of increase in the particle volume is given by

Va dNp(t)/dt ) kπrp2VadaVa

Figure 7. Relative abundance of the silver nanoparticles as a function of the laser shot. The abundance increases almost linearly with the laser shot below 50000 shots.

with the reciprocal of the radius (intrinsic size effect).10 On the other hand, the width of the surface plasmon peak increases with increase in the radius in the range larger than >10 nm, because of inhomogeneous polarization of the particles in the electromagnetic field of the incoming light: Excitation of a different multipole mode makes a peak in the spectrum at a different energy (extrinsic size effect). The alternative explanation for the invariable peak width in the present study can be given by the relatively broad size distribution of the nanoparticles. The change of the size dependence of the spectral profile is observed in the optical absorption spectra of gold nanoparticles.33 4.2. Dynamic Formation Mechanism. Description of Dynamic Formation Mechanism. It is shown in the present experiment that (1) the nanoparticles are produced above a threshold laser power, (2) their number density increases proportionally with the laser power, and (3) their average size increases with an increase in the laser power and decreases with increase in the SDS concentration. These findings are explained in terms of rapid formation of an embryonic silver particle and a consecutive particle growth in competition with termination of the growth due to SDS coating on the particle. Let us consider the particle formation in a chronological order. Immediately after the laser ablation, a dense cloud of silver atoms is built over the laser spot of the metal plate. As the interatomic interaction is much stronger than the interaction between a silver atom and a SDS molecule or a solvent molecule, silver atoms are aggregated as much as silver atoms are supplied. This initial rapid aggregation continues until silver atoms in the close vicinity are consumed almost completely. As a result, an embryonic silver particle forms in a region void of silver atoms (cavity). However, the supply of silver atoms outside the region through diffusion causes the particle to grow slowly even after the rapid growth ceases. In competition, the slow growth tends to be terminated by coating the particle surface with SDS molecules, which diffuse through the solution toward the particle. Calculation of AVerage Particle Size. Let us define Np0 as the number of silver atoms in an embryonic particle and Np(t) as that at a time, t, after the slow growth started, and Va as the effective volume of a silver atom. The radius of the particle, rp, is then given by the particle volume, NpVa, as

(3)

where da is the number density of silver atoms in the cloud of the silver atoms and kπrp2 is the attachment cross section. Let us assume that da does not change appreciably in the time scale of our interest for the first-order approximation, although da might slightly decrease with time. Then, the volume and the radius of the particle at a time t are given, respectively, by

Np(t)Va ) ((Np0Va)1/3 + (π/48)1/3kVadaVat)3

(4)

rp(t) ) r0 + (1/4)kVadaVat

(5)

where r0 is the radius of the embryonic particle. As shown in eq 5, the radius of the particle increases linearly with t. On the other hand, the attachment cross section of a SDS molecule should be proportional to k′πrp2. Then, the rate of increase in the number of the SDS molecules, Ns, which attaches to the particle is

dNs(t)/dt ) k′πrp2dsVs

(6)

where ds is a density, and Vs is a velocity of a surfactant molecule in the solution. Then, Ns is given by

Ns(t) ) (4π/3)(k′dsVs/kVadaVa)[(r0 + (1/4)kVadaVat)3 - r03] (7) the total area that Ns SDS molecules occupy is NsS, when the surface area occupied by one SDS molecule on the particle is defined as S. The radius of the spherical particle, rs(t), which has the same surface area as NsS is given by

rs(t) ) (NsS/4π)1/2 ) (S/3)1/2 (k′dsVs/kVadaVa)1/2 [(r0 + (1/4)kVadaVat)3 - r03]1/2 (8) In other words, rs(t) is the maximum radius of the spherical particle that the Ns SDS molecules completely cover. Evidently, the radius, rs(t), increases hyper-linearly with t. It should be noted that the radius of the embryonic particle, r0, depends on the number density of silver atoms (da) in the silver cloud. The da-dependence of r0 is obtained on the assumption that the cavity radius does not change with the silver concentration in the silver cloud. This assumption is rationalized because an effective region within which silver atoms attract strongly is determined by the interatomic interaction, but not by the density of silver atoms. Figure 8 illustrates the change of the calculated particle radius with a reduced time, kt, where Va is calculated to be 1.25 × 10-29 m3 from the atomic radius of a silver atom in a silver crystal, and S of 6.2 × 1019 m2 is taken from a literature.34 In addition, the number density of the SDS molecules, ds, is

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Figure 8. Time evolution of the particle radius, rp (solid line) and rs (dotted line), calculated on the basis of the dynamic formation model. The radius of the particle is calculated to be 7.6 nm when r0 ) 5.5 nm and k′/k ) 0.1.

obtained from the SDS concentration, on the assumption that the SDS molecules are homogeneously distributed in the solution. The number of the silver atoms liberated into the solution per single laser shot is estimated to be 1011-1012 from the absorbance of the silver nanoparticles in the region of the interband transition. The number density of silver atoms in the silver cloud, da, is estimated to be 1023-1024 m-3 on the assumption that silver atoms ablated into the solution are thermalized immediately within a ∼1 µm diffusion length. The velocities of a silver atom and an SDS molecule in the solution are estimated from each diffusion constant, and the ratio, k′/k, is left as a variable parameter. As shown in Figure 8, the radius of the particle (rp) increases linearly with the reduced time. On the other hand, the maximum radius (rs) of the particle, which can be fully covered by Ns SDS molecules, increases more rapidly with the reduced time and exceeds at the reduced time where the two lines crosses. This crossing point determines the average radius of the nanoparticles observed. 4.3. Average Size of Nanoparticles. Dependence of AVerage Size on SDS Concentration. The average size of nanoparticles produced at different SDS concentrations by a given laser power is obtained from the crossing point of the two lines given by eqs 5 and 8 (see Figure 8). The calculation predicts that smaller nanoparticles tend to be produced in a more highly concentrated SDS solution because a slope of the rs - kt curve increases as the SDS concentration increases and hence the crossing point appears at an earlier reduced-time. Evidently, this prediction is consistent with the observed concentration dependence of the average particle size. The solid curve in Figure 5a depicts the calculated average radius by using eqs 5 and 8 with k′/k ) 0.1 and r0 ) 5.5 nm. This dynamic formation mechanism also predicts that the embryonic particle continues to grow until it consumes all of the available silver atoms without any effective termination. Actually, the optical absorption spectrum shows that particles produced in pure water continue to grow in the absence of any surfactant and finally precipitate within a day. Dependence of AVerage Size on Laser Power. The observed laser-power dependence of the average size originates mainly from the da dependence of r0 in the framework of this model; both rp and rs similarly depend on da, and hence the average size determined from the crossing point of the rs - kt and rp kt curves does not change with da, if r0 is constant. The behavior accords well with the physical picture that the attachment rate

Mafune´ et al. of a silver atom to the particle should increase with da and simultaneously that of a surfactant molecule also increases with the geometrical cross section of the growing particle, resulting in the formation of particles having the same radius. The radius of the embryonic particle, r0, should increase linearly with da1/3, as the radius of the cavity does not change with da. The solid curve in Figure 5b is the prediction given by this mechanism, where da is supposed to increase linearly with the laser power above the threshold value (see Figure 6). The radius of the cavity, in which an embryonic silver particle is present, is estimated to be in the order of ∼100 nm. Dependence of AVerage Size on Number of Laser Shots. Figure 7 shows the relative abundance of the silver nanoparticles as a function of the number of the laser shots. The abundance increases almost linearly with the laser shots in the initial stage, and its slope is slightly reduced above a laser shot of about 50000. The finding indicates that the nanoparticle formation takes place within each laser shot. According to our preliminary experiment on gold nanoparticles which has an appreciable optical absorption at 532 nm, the abundance of the nanoparticles by the laser ablation at 532 nm increases and then levels off immediately as the number of the laser shot increases. This immediate leveling off is considered to arise from absorption of the incoming ablation laser by gold atom cloud over the metal plate. It is highly likely that the gold nanoparticles are dissociated into smaller particles by the ablation laser. Koda and co-workers have reported that chemically prepared gold nanoparticles in a solution are dissociated into smaller particles (photoinduced reshaping) under irradiation of a 532 nm laser.35 Conversely, the linear increase in the abundance of the silver nanoparticles with the laser shot implies that no appreciable dissociation of the silver nanoparticles occurs under irradiation of the ablation laser. The reduction of the slope above 50000 shots should originate from a weak absorption at 532 nm by the silver nanoparticles which attenuate the laser beam penetrating through the solution above the metal plate and hence cause a less efficient nanoparticle formation. Note that all the experiments have been performed in a region where the relative abundance increases linearly with the laser shot. 4.4. Average Size vs Static Properties of SDS. In a diluted SDS solution whose SDS concentration is less than the critical micelle concentration (CMC), SDS molecules are unimolecularly dissolved. On the other hand, the SDS molecules form micelles at a concentration higher than CMC; 8.2 mM at 298 K.36 If the static properties of the SDS molecules govern the average size of the nanoparticles in the solution, the average size should change significantly at the CMC. Nevertheless, no appreciable change in average size is observed at the CMC when the SDS concentration is changed. It is concluded, therefore, that the static dissolved form of the SDS molecules in the solution has no direct influence on the determination of the average size. This finding is consistent with the dynamic formation mechanism proposed. 5. Conclusions Laser ablation of a silver plate in an aqueous solution of sodium dodecyl sulfate was employed to prepare size-selected and size-controlled silver nanoparticles in the solution. We characterized the nanoparticles by using optical absorption spectroscopy and electron microscopy. We proposed a mechanism based on a rapid growth of an embryonic silver particle and a consecutive slow growth in competition with termination of the growth due to SDS coating on the particle. Seemingly, this method is applicable to preparing metal nanoparticles of any kind including multicomponent ones.

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