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J. Phys. Chem. C 2008, 112, 5810-5815
Mechanism of Laser-Induced Size Reduction of Gold Nanoparticles As Studied by Single and Double Laser Pulse Excitation Hitomi Muto, Ken Miyajima, and Fumitaka Mafune´ * Department of Basic Science, Graduate School of Arts and Sciences, The UniVersity of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ReceiVed: December 1, 2007; In Final Form: January 18, 2008
Gold nanoparticles with an average diameter of ≈11 nm were prepared by laser ablation of a gold metal plate in an SDS aqueous solution. An 80-µm microdroplet of the solution in diameter was ejected in the atmosphere from a microdroplet nozzle. Structural changes of the nanoparticles in the microdroplet, after they are irradiated with a focused single nanosecond laser pulse at the wavelength of 532 nm, were studied by transmission electron microscopy (TEM) and optical absorption spectroscopy. It was revealed that the gold nanoparticles are fragmented into the smaller particles. In order to investigate the dynamics of fragmentation, the nanoparticles were irradiated with delayed double laser pulses, both of which have an identical wavelength and pulse energy (532 nm, 30 mJ). The average diameter of the product fragments was smallest when the two laser pulses simultaneously irradiated the nanoparticles. The diameter increased with an increase in the delay time from the first to the second laser pulse. The delay-time dependence of the particle size indicates that the fragmentation of the gold nanoparticles is caused by the Coulomb explosion of the multiply charged nanoparticles.
Introduction Nanometer-sized gold particles have been attracting much attention of scientists and engineers because they possess sizedependent physical and chemical properties which the bulk materials do not have.1-9 For instance, Haruta and co-workers discovered the catalytic activity of small gold nanoparticles on the metal oxide surface.10-11 The reactivity was found to increase with a decrease in the particle size. On the other hand, it is well-known that an optical absorption of the gold nanoparticles in the visible region relating to the surface plasmon resonance changes drastically with their size and shape.5,12-22 In order to elucidate the size-dependent physical and chemical properties of the nanoparticles, it is critically important to control the size and the shape of the nanoparticles. Gold nanoparticles are commonly prepared by the chemical reduction of gold ions in a solution in the presence of a stabilizing reagent. Brust and co-workers developed a one-pot method to synthesize gold nanoparticles stabilized by dodecanethiol molecules.23 The sulfur atom in the thiol group binds strongly to the gold atom on the particle surface. In this case, spherical gold nanoparticles with an average diameter of ∼5 nm are produced. On the other hand, citric acid is used to reduce the gold ions in water to synthesize gold nanoparticles.24-26 In this case, the citrate anions form the charged layer around the particle surface, which provides the particles with electric repulsive forces between them. Recently, there is growing interest in laser-based methods for the preparation nanoparticles in a solution:27-38 Gold nanoparticles are prepared by laser ablation of a gold metal plate in water even without a stabilizing reagent.39-43 The surfactantfree gold nanoparticles are quite important, because the given stabilizing reagent at the given concentration can be added in the solution after the synthesis. Note that the predetermined * Corresponding author. E-mail:
[email protected].
stabilizing reagent is indispensable when gold nanoparticles are prepared by the chemical synthesis. However, the larger particles tend to be formed, and the size of the nanoparticles is widely distributed by the laser-based method. In this relation, it is known that the size of the nanoparticles can be reduced in a controlled manner by the laser irradiation onto the nanoparticles (laser-induced size reduction).44-48 The mechanism of the size reduction is still not well-known. Kamat and co-workers concluded from picosecond photoabsorption spectroscopy that size reduction is a result of fragmentation caused by the Coulomb explosion of the photoionized metal nanoparticles.49 On the other hand, Koda and co-workers proposed a mechanism that the fragmentation of the photoexcited gold nanoparticles proceeds through melting and vaporization.45 Plech et al. observed the structural changes of nanoparticles and the water molecules in the vicinity of the nanoparticles by resolved X-ray scattering.50 They found that the particles undergo a melting transition within a time scale of 1 ns, and hence, they can be fragmented into small particles by the thermal process. In our previous study, we observed many electrons liberated from the gold nanoparticles into the solution when the nanoparticles are irradiated with a laser pulse so intense that the particles are size-reduced.51 We estimated the charge state of the particles from the number densities of the particles and the electrons, finding that the particles are so highly charged that they can be fragmented into the smaller ones by the Coulomb repulsive forces. Hence, we concluded that the size reduction of the gold nanoparticles is caused by the Coulomb explosion of the highly charged particles. The purpose of the present study is to obtain better evidence of the Coulomb explosion of gold nanoparticles under irradiation of the laser pulse. We used a liquid droplet nozzle which ejects a single liquid droplet with a diamaeter of 80 µm in the atmosphere. As the volume of the sample solution is made much
10.1021/jp711353m CCC: $40.75 © 2008 American Chemical Society Published on Web 03/18/2008
Laser-Induced Size Reduction of Gold Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5811
Figure 2. Schematic diagram of the experimental setup. A He-Ne laser was used for the precise alignment of the liquid droplet nozzle to the pulsed laser. Figure 1. Optical absorption spectra of gold nanoparticles with different average diameters prepared by the laser-based method.
smaller than the focusing region of the laser beam, all the nanoparticles in the droplet are surely excited by each laser pulse. Size reduction of the nanoparticles induced by the irradiation of the single nanosecond laser pulse was examined by optical absorption spectroscopy and transmission electron microscopy (TEM). The ionization efficiency of the gold nanoparticles was measured as functions of the particle size and the pulse energy of the laser. In addition, the gold nanoparticles were irradiated with the delayed double laser pulses, both of which have an identical wavelength and pulse energy (532 nm, 30 mJ). The size of the particles was measured at different delay times between the first and the second laser pulses. We discussed the mechanism of the fragmentation which is induced by the laser irradiation onto the gold nanoparticles. Experimental Section Gold nanoparticles were prepared by laser ablation of a gold metal plate in an aqueous solution of sodium dodecyl sulfate (SDS).27-30,38,52-54 The metal plate was placed on the bottom of a glass vessel filled with 10 mL of a 1 × 10-4 M SDS aqueous solution. The fundamental output of a Nd:YAG pulsed laser (1064 nm) operating at 10 Hz with a pulse energy of 90 mJ was focused by a 250-mm focal length lens onto the metal plate. The spot size (typically 2 mm in diameter) of the laser on the surface of the metal plate was varied by changing the distance between the lens and the metal surface. The concentration of the gold atoms dispersed in the solution as gold nanoparticles was determined by the absorbance at 300 nm, which was typically 1.3 mM after 36000 laser shots.30 Hereafter, when the concentration of gold nanoparticles is given in this paper, it refers to the concentration of gold atoms dispersed as gold nanoparticles. The average size of the nanoparticles as prepared was 11.0 ( 5.0 nm (Au≈39000). The size of the particles can be varied by tuning the pulse energy of the laser. Figure 1 shows absorption spectra of sizeselected gold nanoparticles prepared by these laser-based methods. The spectra exhibit a surface plasmon band at 520 nm on the broad interband transition in the UV-vis region. Evidently, the peak height of the surface plasmon band decreases, while its width increases with a decrease in the particle size. This spectral change is consistent with the theoretical prediction.12 We used these solutions of the sizeselected nanoparticles as sample targets. Figure 2 shows the experimental setup used in the present study. A liquid droplet nozzle designed originally for biological use (the Nanoplotter nozzle) was diverted to our experiment. A pulse voltage with the height of +50-80 V and the width of 50-100 µs generated by a pulse power supply was applied to the nozzle, which was triggered by a delay generator (Stanford Research DG535). The nozzle ejected a microdroplet of
80-µm diameter whose velocity in the atmosphere was constant at 1.2 m s-1. The volume of the single droplet was 2.7 × 10-4 µL. In order to observe structural changes of gold nanoparticles after the irradiation of the single or double laser pulse(s), more than 104 droplets were collected in a 100-µL microtube at a distance of 20 mm downstream from the nozzle. An individual microdroplet was illuminated at a distance of 7 mm downstream from the nozzle by a second harmonic output of the Nd:YAG laser (532 nm; beam diameter ∼8 mm). To avoid evaporation of the droplet by the tightly focused laser pulse, the droplet was illuminated about 70 mm away from the 150-mm focal length lens. Here, the beam waist at the crossing region was estimated to be ∼4 mm. A flashlight operated at 10 Hz was used to visualize the tiny droplet. The position and angle of the liquid droplet nozzle was determined by monitoring the light of a continuous He-Ne laser scattered by the droplet. The pulse laser was synchronized to the droplet nozzle. In practice, the Q-switch of the pulse laser operating at 10 Hz was triggered 6 ms after the droplet nozzle by the delay generator. The pulse energy of the laser was typically 30 mJ (pulse width 10 ns). In order to elucidate the fragmentation dynamics of gold nanoparticles, we examined the size of the gold nanoparticles after they were illuminated by the delayed double laser pulses, which were generated by the two Gaussian Nd:YAG pulsed lasers. The delay of the two pulses was controlled by the delay generator. As the velocity of the droplet in the atmosphere is 1.2 m s-1, it moves only 1.2 µm in 1 µs. Hence, the droplet which is much smaller than the beam waist of the pulse laser is considered to stay still in space. We measured the size of the gold nanoparticles as a function of the delay time from the first to the second laser pulse. The optical absorption spectrum of the solution in the UVvis region was observed by the Shimadzu UV-1200 spectrometer. A sample for the transmission electron microscopy (TEM) was prepared using the same solution. The TEM images of the nanoparticles were observed by JEOL JEM-2000 EX at the magnification of 200000×. The size distribution of nanoparticles was obtained by measuring diameters of more than 1000 nanoparticles in sight. Results Figure 3a shows absorption spectra of gold nanoparticles in a 1 × 10-2 M SDS aqueous solution before and after irradiation of a single laser pulse (30 mJ/pulse). Figure 3, parts b and c, shows the size distributions of the nanoparticles together with the TEM images. The nanoparticles before the laser irradiation are almost spherical with an average diameter of 11.0 nm, whereas the particles after irradiation of a single laser pulse are size-reduced to an average diameter of 3.7 nm. The size distribution of the nanoparticles before the laser irradiation scarcely overlaps with that after the laser irradiation. Hence, we are able to assert that almost all the nanoparticles in the
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Muto et al.
Figure 4. Average diameters of gold nanoparticles after fragmentation induced by the irradiation of the single laser pulse (30 mJ/pulse) as a function of the initial diameter of parent gold nanoparticles. Error bars show the standard deviation of the mean diameters measured in the several experiments.
Figure 3. (a) Optical absorption spectra of gold nanoparticles before and after the pulse laser irradiation at the pulse energy of 30 mJ in a 1 × 10-2 M SDS aqueous solution. (b) and (c) Histograms of the diameter of gold nanoparticles before and after the pulse laser irradiation, respectively. Insets in panels (b) and (c) show the electron micrographs of the gold nanoparticles.
liquid droplet are excited and are subjected to fragmentation by the single laser pulse in this experimental setup. Now, we prepared aqueous solutions of size-selected gold nanoparticles. After irradiation of a single laser pulse at 30 mJ/pulse onto the nanoparticles, the size of the particles was measured. Average diameters are found to be almost the same regardless of the initial size and distribution (Figure 4). This finding ensures that we are able to discuss the fragmentation dynamics without taking the size distribution of the parent nanoparticles into account. On the other hand, we measured the size of the gold nanoparticles after irradiation of a single laser pulse as a function of the pulse energy. As shown in Figure 5a, the size decreases with increasing the pulse energy. This arises from the fact that the initial charge state of the gold nanoparticles right after the laser irradiation increases with the pulse energy as plotted in Figure 5b, as discussed later.
Figure 5. (a) Average diameter of the gold nanoparticles after fragmentation induced by the single laser pulse as a function of the pulse energy. (b) Initial charge state of the gold nanoparticles right after the laser irradiation as a function of the laser pulse energy. The charge state is estimated using eqs 3 and 4. The broken lines are obtained by the least-square fittings, assuming that the initial charge state is proportional to the pulse energy.
In order to investigate the dynamics of fragmentation of gold nanoparticles, we used delayed double laser pulses, both of which have an identical wavelength and pulse energy (532 nm, 30 mJ). Figure 6 shows the TEM images of the gold nanoparticles after the irradiation of the delayed double laser pulses in a 1 × 10-2 M SDS aqueous solution at different delay times from the first to the second laser pulse, ∆t. Evidently, the gold nanoparticles are smallest at ∆t ) 0 µs. Figure 7 shows the average diameter of the nanoparticles as a function of ∆t. The average diameter is 3.3 nm at ∆t ) 0 µs, increases readily until 30 ns (see inset of Figure 7), then gradually with an increase in ∆t, and levels off in ∆t ) 1 µs. The size of the gold
Laser-Induced Size Reduction of Gold Nanoparticles
Figure 6. Electron micrographs of gold nanoparticles after irradiation of the delayed double laser pulses (532 nm, 30 mJ/pulse) (a) ∆t ) 0 ns, (b) ∆t ) 10 ns, (c) ∆t ) 100 ns, and (d) ∆t ) 1 µs, respectively.
J. Phys. Chem. C, Vol. 112, No. 15, 2008 5813 excited particles undergo a rapid relaxation to their electronic ground state through the strong electron-electron and electronphonon coupling.49 As the absorption cross section of the particle is sufficiently large and the laser pulse is much longer than this relaxation time, the photoexcitation and relaxation are repeated within the duration of each laser pulse. The photon energy absorbed by the nanoparticles in each photoexcitationrelaxation cycle is converted to heat, causing the their temperature to rise. This conjecture is supported by the study of Koda and co-workers where the electron temperature of gold nanoparticles was measured by observing the blackbody radiation from the particles excited by the nanosecond laser pulse.45 Their electron temperature was found to reach as high as the boiling point of gold. Under the circumstances, the gold nanoparticles are ionized by the thermionic emission.55 The electrons emitted from the particles are solvated temporally by water molecules (solvated electrons). We observed the transient absorption spectrum of the solvated electrons in the aqueous solution, measuring the their number density in the previous study.51 It was found that the gold nanoparticles are multiply charged to Au35000+520. The formation of highly charged gold nanoparticles is supported by the theoretical calculation.55 Coulomb Explosion of Gold Nanoparticles. The multiply charged gold nanoparticles are unstable. According to the liquid drop model for the Coulomb explosion, a multiply charged particle becomes unstable when the disruptive Coulomb force exceeds the attractive cohesive force.56-58 As a result, multiply charged gold nanoparticles can break up by the Coulomb explosion. The produced small particles are stable enough, although they are still charged. In the liquid drop model, the degree of the instability is expressed by the fissility defined as X ) Ec/2Es, where Ec and Es are the Coulomb energy and the surface energy of the particle, respectively. The fissility of gold nanoparticles is given by
X ) 0.9 Figure 7. The average diameters of the gold nanoparticles after irradiation of the delayed double laser pulses as a function of the delay time from the first laser pulse to the second one, ∆t. Open circles show the diameter after irradiation of the delayed double laser pulses of 30 mJ/pulse. Solid circle at ∆t ) 0 ns indicates the diameter after irradiation of the single laser pulse of 60 mJ/pulse. Inset shows the enlarged view in the short delay-time region.
nanoparticles at ∆t ) 0 µs is almost the same as that of the nanoparticles size-reduced by the single laser pulse at 60 mJ/pulse. On the other hand, the size of the gold nanoparticles in ∆t > 1 µs is almost the same as that of the nanoparticles size-reduced by the single laser pulse at 30 mJ/pulse. Hence, the second laser pulse in ∆t > 1 µs does not induce essentially the fragmentation of the gold nanoparticles. Note that error bars of the particle size in Figures 4, 5, 7, and 8 show the standard deviation of the mean sizes measured in the several experiments.
q2 n
(1)
where q and n are the charge state and the number of atoms included in the particle. It is known that the multiply charged particles break up into the fragments (multifragmentation) when X g 1, both the evaporation and fission occur in 0.3 < X < 1, and only evaporation of atoms takes place when X < 0.3.56-58 The fissility of the fragments must be less than 0.3. Otherwise, the fragments would dissociate further into the smaller ones by the fission. Now, let us assume that the fissility of the fragment is equal to 0.3. This assumption is supported by the fact that Au39000+520 was actually fragmented into Au1900+25, whose fissility is X ) 0.3.56-58 It follows that the charge state of the fragment, qf, is given by
qf2 ) 0.33nf
(2)
where the size of the fragment is defined as nf. Hence, the initial charge state, qi, is given by
Discussion Ionization of Gold Nanoparticles by the Nanosecond Laser Pulse. The photoexcitation dynamics of gold nanoparticles has been studied intensively.12-14,49,55 The absorption spectrum of gold nanoparticles exhibits a peak at 520 nm characteristic of the surface plasmon band on the broad interband transition extending from the ultraviolet to the visible region (see Figure 3a).12-15 When the nanoparticles are irradiated with the laser pulse at 532 nm, they are electronically excited. However, the
q i ) qf
ni ni ) x0.33nf ∝ ninf -1/2 nf nf
(3)
or
nf ∝
ni2 qi2
(4)
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Figure 8. The average diameter (open circles) and the charge state, qf′, (solid circles) of the gold nanoparticles after irradiation of the double laser pulses as a function of the delay time from the first laser pulse to the second one, ∆t. The broken line of X ) 0.3 shows the threshold level of qf′ for evaporation and/or fission, while the line of X ) 1 exhibits the threshold level of qf′ for multi-fragmentation. The upwardpointing arrows indicate the change of the charge state by the irradiation of the second laser pulse at each ∆t (see text).
where ni stands for the initial size of a parent particle. Here, eq 4 suggests that the size of the fragments should decrease with an increase in the initial charge states. Figure 5a shows the laser pulse-energy dependence of the diameter of the fragments. The size of the fragments decreases with an increase in the laser pulse energy, suggesting evidently that the parent nanoparticles should be more charged as the pulse energy increases. As ni and nf are known in this experiment, we estimated qi using eqs 2 and 3 and plotted as a function of the pulse energy (see Figure 5b). It was revealed that the initial charge state increases with an increase in the pulse energy. The broken lines in the both panels are obtained by the least-square fittings, assuming that the initial charge state is proportional to the pulse energy. On the other hand, we prepared solutions of size-selected gold nanoparticles. We measured the size of the fragments when irradiated with the single laser pulse at 30 mJ/pulse (Figure 4), finding that the average size of the fragments does not change regardless of the initial size and distribution of the parent nanoparticles. As nf is constant in eq 4, this finding indicates that the initial charge state, qi, is proportional to the initial size of the nanoparticles, ni. This conjecture is consistent with the fact that absorption cross section of the nanoparticles increases with an increase in the particle size. Hence, we can conclude that the initial charge state of the nanoparticles increases with an increase in the particle size. Fragmentation of Gold Nanoparticles by the Delayed Double Laser Pulses. In the present study, gold nanoparticles were irradiated with delayed double laser pulses (532 nm, 30 mJ). When the nanoparticles are irradiated with the first laser pulse, they become so highly charged that they are fragmented into smaller particles with size nf. Although the fragments are still multiply charged, whose charge state is +qf, they remain stable, as discussed in the previous section. The fragment particles are to recombine with the electrons in the solution, which are gradually neutralized. Hence, the charge state of the fragments decreases from +qf to zero with time. When the fragment particles are illuminated by the second laser pulse in the course of neutralization, the charge state rapidly increases (see Figure 8): The fragment particles attain additional +qf by the second laser pulse. In other words, the fragment particles can possess qf′ from +2qf to +qf, depending on ∆t. When qf′ exceeds the threshold value for the Coulomb explosion, the fragments are considered to dissociate further into the smaller ones. Here, the threshold value is given by qf′ ) 1.83qf, because X ) 0.9qf′2/nf > 1 and qf2 ) 0.33nf.
Muto et al. According to the previous study, the lifetime of the solvated electrons was 144 ns in a 2 × 10-2 M SDS solution.51 As the lifetime increases with an increase in the concentration of SDS, which provides the negative charge on the particle surface by covering the particle surface, the predominant decay channel of the solvated electrons must be recombination of electrons with the positively charged gold nanoparticles. It follows that the charge state decreases with the lifetime of ∼144 ns. In this case, the fragments can obtain qf′ g 1.83qf by the second laser pulse within the limited delay time (
