J. Phys. Chem. B 2003, 107, 12589-12596
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Formation of Gold Nanonetworks and Small Gold Nanoparticles by Irradiation of Intense Pulsed Laser onto Gold Nanoparticles 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 ReceiVed: February 10, 2003; In Final Form: June 13, 2003
Gold nanoparticles with an average diameter of ∼20 nm were prepared in water by laser ablation at 1064 nm against a gold metal plate in it. The gold nanoparticles thus prepared in water and those mixed with an aqueous solution of sodium dodecyl sulfate (SDS) were irradiated with an intense pulsed laser at 532 nm. The products in the solution were examined by transmission electron microscopy (TEM) and optical absorption spectroscopy. The TEM images of the products revealed that gold nanonetworks and much smaller gold nanoparticles were produced selectively by a proper choice of the laser fluence and the SDS concentration. The optical absorption spectra measured simultaneously showed that the gold nanonetworks have an optical absorption in the wavelength longer than 600 nm which is assignable to longitudinal plasma oscillation of the gold nanonetworks, while the smaller gold nanoparticles (clusters) have an absorption band in the visible to the UV region. Taking advantage of those characteristic absorption bands, we constructed a two-dimensional mapping which illustrates the formation of the gold nanonetworks and the smaller nanoparticles as functions of the laser fluence and the SDS concentration.
1. Introduction Metal nanoparticles including clusters exhibit size-dependent chemical and physical properties unique to themselves.1-8 Even gold which is considered to be a chemically inert material shows catalytic activity for a low-temperature CO oxidation, when it is pulverized into particles with diameters less than 10 nm and supported on oxides of magnesium and titanium.9,10 It is no doubt that the shape of nanoparticles should be considered in determining their properties. Furthermore, in multicomponent nanomaterials, the interactions between different components play an important role in determining their properties in addition to the unique size-dependency. In the last 10 years, much effort has been invested in synthesizing two-dimensional and three-dimensional wellarrayed structures, in which size-selected metal nanoparticles are bound to each other with ligands covering the metal nanoparticles (hydrophobic interactions between ligands). On the other hand, interconnected networks of copper metallic particles have been prepared by chemical reduction of a copper salt into metal nanoparticles in a micelle.11,12 Optical absorption spectra of the networks show red-shift in comparison with those of spherical copper particles. The red-shift is explained in such a manner that the networks consist of elongated metallic copper rods and wires. A method called “laser-assisted size reduction” has been developed so as to pulverize metal nanoparticles into smaller ones in a solution through their selective “heating” caused by resonant electronic excitation of the metal particles.13-17 For instance, the surface plasmon band of gold nanoparticles centering at 520 nm is excited by an intense pulsed laser (532* Corresponding author. † Present address: Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 1538902, Japan.
nm) in a solution. Gold nanoparticles “heated” by the plasmon excitation are fragmented into smaller nanoparticles. On the other hand, under irradiation of a 514-nm cw laser, gold nanoparticles with diameters of ∼10 nm in water aggregate into groups in which the gold nanoparticles are about to touch mutually, while maintaining their individual geometries,18 whereas gold nanoparticles in water and alcohol further coagulate each other.19-21 Evidently, the structures of the products are expected to change significantly with the fluence of the laser and the concentration of the surfactant employed for stabilization of the nanoparticles in a solution, in addition to the wavelength and the pulse width of the lasers. To develop this technique further, we performed a systematic study on how the shape of gold nanoparticles after the laser irradiation changes with the laser fluence and the surfactant concentration. We prepared, first, surfactant-free gold nanoparticles having an average diameter of ∼20 nm in pure water by laser ablation at 1064 nm of a gold metal plate. Note that gold nanoparticles thus produced hardly absorb photons at 1064 nm. Second, water containing surfactant-free gold nanoparticles was mixed with an aqueous solution of sodium dodecyl sulfate (SDS), and afterward the gold nanoparticles were irradiated with an intense pulsed 532-nm laser. It was found that nanonetworks and smaller nanoparticles of gold were produced in the solution by selecting a proper combination of the laser fluence and the SDS concentration. The mechanism of the network formation was discussed. 2. Experimental Section Surfactant-free gold nanoparticles were prepared by laser ablation of a gold metal plate (>99.99%) which was placed on the bottom of a glass vessel filled with 10 mL of commercially available ultrapure water (Kanto Chemicals Co., Inc.).22-27 The laser ablation was performed by using the fundamental
10.1021/jp030173l CCC: $25.00 © 2003 American Chemical Society Published on Web 10/23/2003
12590 J. Phys. Chem. B, Vol. 107, No. 46, 2003
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Figure 1. Optical absorption spectrum of gold nanoparticles prepared in pure water by laser ablation at 1064 nm (2.5 J/pulse cm2, 1 h) of a gold metal plate.
(1064 nm) of a Quanta-ray GCR-170 Nd:YAG pulsed laser operating at 10 Hz, which was focused by a lens having a focal length of 250 mm. 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 amount of gold atoms ejected into water per laser shot as a form of nanoparticles was estimated to be ∼1 nmol at the laser power of 60 mJ/pulse.28 A transmission electron microscope (JEOL JEM-100S ×50000) was employed to take electron micrographs of the nanoparticles thus produced. The diameters of more than 500 nanoparticles in sight on a given micrograph were measured to determine the distribution of the particle diameters (size-distribution). The optical absorption spectrum of the nanoparticle-containing water was measured by a Shimadzu UV-1200 spectrometer. An aqueous solution containing given concentrations of the gold nanoparticles and sodium dodecyl sulfate (SDS) was prepared by mixing the nanoparticle-containing water and an aqueous solution of SDS at a given SDS concentration,29 and was illuminated by the output of the second harmonic (532 nm) of a Quanta-ray GCR-170 Nd:YAG pulsed laser operating at 10 Hz. The laser was focused to a spot as large as 0.023 cm2 on the solution surface by a lens having a focal length of 250 mm. The solution was homogenized by a magnetic stirrer so as to obtain a solution in which gold nanoparticles and nanonetworks are homogeneously distributed in space. A Scientech AC2501 power meter was used to monitor the laser power. Sodium dodecyl sulfate (>99.9%) was purchased from Japan Surfactant Co. 3. Results Figure 1 shows a typical optical absorption spectrum of a solution containing surfactant-free gold nanoparticles, which is produced by laser ablation of a gold metal plate in ultrapure water at the wavelength of 1064 nm. The spectrum exhibits one intense peak on a tail part of a broad band, and is essentially the same as that of a solution obtained similarly by laser ablation of a gold metal plate in an aqueous solution of sodium dodecyl sulfate (SDS). These two spectra agree in feature with the spectrum of gold nanoparticles chemically produced from gold ions.25-28 The agreement shows that gold nanoparticles are really produced by the laser ablation of the gold metal plate at the wavelength of 1064 nm both in the ultrapure water and the aqueous solution of SDS.
Figure 2. Electron micrograph and size distribution of gold nanoparticles produced by 1064-nm laser ablation (2.5 J/pulse cm2, 1 h) in pure water. The average diameter, daverage, is 20.7 nm with the width of 13.1 nm in the diameter distribution.
Figure 2 shows a typical transmission electron micrograph of the products obtained by the laser ablation in the ultrapure water and the size-distribution histogram of the products obtained from the micrograph. The gold nanoparticles which are the most dominant products in sight have an average diameter of 20.7 nm with one standard deviation of 13.1 nm. A 10-6 M SDS solution containing gold nanoparticles with an average diameter of 20.7 nm was irradiated with a pulsed laser having the wavelength of 532 nm and the fluence of 5 J/pulse cm2. Figure 3 shows typical electron micrographs of photoproducts (see panel (a)), and the optical absorption spectra of the virgin and the laser irradiated solutions (see panel (b)). As shown in panel (a), the photoproducts are constructed of gold nanoparticles having ∼10 nm in diameter which are connected by gold wires and extend in the range of 10-500 nm in length. Note that gold nanoparticles dispersed in ultrapure water are networked by the laser irradiation, as well. As shown in panel (b), the optical absorption spectrum of the virgin solution (dotted curve) has an intense peak at the wavelength of 520 nm on a tail part of a broad band, while that of the laserirradiated solution has a broad structure (solid curve) on the tail part of the broad band. As discussed in section 4, the peak at 520 nm is assigned to the surface plasmon of the spherical part of the nanonetworks, the broad structure emerged in the visible to the IR region by the laser irradiation is assigned to the surface plasmon of wires or rods, and the broad band tailed toward the UV region is assigned to an interband transition. Figure 4 shows optical absorption spectra of the laserirradiated solutions obtained at different numbers of the laser
Formation of Au Nanonetworks and Nanoparticles
Figure 3. (a) Electron micrographs of gold nanonetworks produced after parent gold nanoparticles (average diameter of 20.7 nm) in a 10-6 M SDS solution were irradiated with a pulsed laser (532 nm, 5 J/pulse cm2) for 20 min. (b) Optical absorption spectra of a solution containing gold nanonetworks thus produced (solid line) and the parent gold nanoparticles in a 10-6 M SDS solution before laser irradiation (dotted line).
shots, where the laser fluence and the wavelength of the laser are set to be 1 J/pulse cm2 and 532 nm, respectively. As shown in Figure 4, the intensity of the broad structure increases gradually with the number of the laser shots. In addition, a small peak appears at the wavelength of 700 nm when the number of the laser shot exceeds 4800, and shifts to a longer wavelength with a further increase in the number of the laser shots. Electron micrographs in panels (a), (b), and (c) of Figure 5 illustrate the photoproducts in the solutions which give spectra b, c and f of Figure 4, respectively. Spherical gold nanoparticles are still observed even after 2400 laser shots (see panel (a) of Figure 5a). With a further increase in the number of the laser shots, the nanoparticles start to adhere and finally form a network (see panels (b) and (c) of Figure 5). Figure 6 shows an electron micrograph (panel (a)) and the spectrum (panel (b)) of gold nanoparticles in a 3 × 10-4 M SDS solution obtained by the laser irradiation (532 nm, 5 J/pulse cm2, 12000 laser shots). Parent gold nanoparticles (average diameter of 20 nm) which are networked in a dilute SDS solution tend to be size-reduced even under the similar condition of the laser irradiation, as the SDS concentration increases. This tendency is also manifested in panel (b) of Figure 6 that the height of the plasmon peak decreases and the width of the plasmon peak increases after the laser irradiation. Namely, the network formation and the size reduction are determined by the concentration of SDS, at least.
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Figure 4. Optical absorption spectra of gold photoproducts (gold nanoparticles and nanonetworks) produced by laser irradiation (532 nm, 1 J/pulse cm2) onto gold nanoparticles in pure water with different numbers of the laser shots. The peak centering at 520 nm and the broad tail in the wavelength region of 700-1100 nm originate mainly from spherical and wire parts of nanonetworks, respectively. The spectra show that the protoproducts are networked at a greater extent by a larger number of the laser shots.
To elucidate the SDS-concentration dependence of the network formation, the absorbance of the solution related to the gold nanonetworks was measured as a function of the concentration of SDS. The absorbance at 800 nm is employed as a measure of the network formation, because the absorbance at 800 nm is almost zero when the nanoparticles are size-reduced (no nanonetworks are formed), while it reaches about 0.2 when the gold nanonetworks are formed. Figure 7 shows the absorbance at 800 nm after the laser irradiation (532 nm, 4.3 J/pulse cm2, 12000 laser shots) as a function of the concentration of SDS. The absorbance at 800 nm reaches about 0.2 below the SDS concentration of 4 × 10-5 M, and is approximately zero above 5 × 10-5 M. In other words, gold nanoparticles are jointed into nanonetworks at SDS concentrations lower than 4 × 10-5 M, whereas they are fragmented into smaller nanoparticles at concentrations higher than 5 × 10-5 M. Similar SDS-concentration dependences are observed at the laser fluences of 1 and 2 J/pulse cm2 (see Figure 8a and b, respectively). The critical concentration for the network formation and size reduction was found to increase with an increase in the laser fluence. In summary, the network formation and the size reduction are determined by the concentration of SDS as well as the condition of the laser irradiation. 4. Discussion 4.1. Formation of Three-Dimensional Gold Nanonetworks by Laser Irradiation. Gold nanoparticles with the average diameter of 20.7 nm dispersed in pure water or in a diluted SDS solution are networked by the laser irradiation at 532 nm as proved by high-resolution electron transmission microscopy
12592 J. Phys. Chem. B, Vol. 107, No. 46, 2003
Figure 5. Electron micrographs of photoproducts (gold nanoparticles and nanonetworks), which correspond to Figure 4b, c ,and f shown in panels (a), (b), and (c), respectively.
(TEM) (see Figures 3 and 5). The optical absorption spectra of the corresponding laser-irradiated solutions also manifest the network formation (see Figures 3b and 4). As clarified in Figures 3-5, the sizes of the gold nanonetworks increase with the number of the laser shots. These findings indicate that the spherical particles and the wires in the gold nanonetworks are melted together, but are not bound weakly. Actually, under the electron microscope used, the gold nanonetworks are found to translate readily on a substrate, collodion-coated copper grid, while maintaining their whole geometry; the nanonetworks bond together tightly and extend in the three dimensions are weakly held on the collodion-coated copper grid. Therefore, it is highly unlikely that the gold nanonetworks are accidentally formed during the sampling for the TEM measurements. In our recent study, we have demonstrated that platinum nanoparticles are jointed by gold wires, when a mixture of spherical gold and platinum nanoparticles dispersed in pure water is irradiated with a pulsed laser at 532 nm; gold nanoparticles are selectively heated under irradiation of the 532 nm laser, and adhere to platinum nanoparticles. This finding provides a further support that the network formation proceeds by the laser irradiation on gold nanoparticles in pure water or in a diluted SDS solution.30 4.2. Optical Absorption Spectrum of Nanonetworks. The peak and the broad structure of the optical absorption spectrum of the gold nanonetworks are assignable on an approximation that the spectrum is constructed of the optical absorption of the spherical part and the elongated parts (wires and rods). It is well-known that a spherical gold particle has surface plasmon
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Figure 6. (a) Electron micrographs of small gold nanoparticles produced after gold nanoparticles are irradiated with the pulsed laser (532 nm, 5 J/pulse cm2) in a 3 × 10-4 M SDS solution. (b) Optical absorption spectra of the small gold nanoparticles after laser irradiation (solid line) and gold nanoparticles before laser irradiation in a 3 × 10-4 M SDS solution.
which gives an intense peak in the wavelength of 520 nm. On the other hand, a gold nanorod has longitudinal plasmon absorption in the range of 650-1000 nm and transverse plasmon absorption at 520 nm. In the framework of the approximation described above, the spectrum of a gold nanonetwork has an intense peak at 520 nm consisting of the contributions from the surface plasmon of the spherical parts and the transverse plasmon of the elongated parts, and has a broad structure in the range from 650 to 1000 nm which originates from the longitudinal plasmon of the elongated parts. In comparison with the observed spectrum shown in panel (b) of Figure 3, one can conclude that the broad structure extending from 650 nm toward the IR region is assignable to the longitudinal plasmon of the elongated parts of the gold nanonetworks produced. Mie theory gives an optical absorption coefficient of gold particles in a solution with diameters smaller than the wavelength of an incoming light (
