9050
J. Phys. Chem. B 2001, 105, 9050-9056
Dissociation and Aggregation of Gold Nanoparticles under Laser Irradiation 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: March 28, 2001; In Final Form: June 26, 2001
Gold nanoparticles having an average diameter of ∼8 nm were prepared in an aqueous solution of sodium dodecyl sulfate by laser ablation at 1064 nm of a gold metal plate and were irradiated by a laser at 532 nm for size-reduction of the gold nanoparticles produced by laser ablation. The diameters of the gold nanoparticles thus produced were measured directly by electron microscopy, while the average diameter was estimated from the optical absorption spectrum of the solution containing the nanoparticles with the aid of the Drude theory of a conducting droplet. The average diameter of the nanoparticles was found to decrease toward the smallest possible diameter as the laser shot increases; in this condition, the resulting nanoparticles have comparable diameters. In addition, the smallest possible diameter was found to decrease with the laser fluence. It was also shown that aggregation of the nanoparticles is not negligible when the laser fluence is high. In conclusion, nanoparticles with an average diameter was pulverized into smaller nanoparticles with a desired average diameter and a narrow distribution by a proper selection of the laser fluence and the laser shots. The mechanism of the particle pulverization was proposed.
1. Introduction Metal nanoparticles have attracted much attention because of size-dependent physical and chemical properties.1-4 For instance, a gold nanoparticle shows a drastic decrease of the melting point in the size range less than 5 nm in diameter.5,6 The optical properties are also size-dependent;7 it is well-known that in a smaller size range they exhibit intense photoluminescence.8 Among the size-dependent chemical properties, catalytic activity of metal nanoparticles is the closest to a practical use. It has been shown recently that TiO2-supported gold nanoparticles (less than 10 nm in diameter) catalyze oxidation of CO even at a temperature as low as 200 K,9,10 although gold itself does not exhibit any catalytic activity. As described above, it is practically important to prepare metal nanoparticles with a desired size and distribution. Evidently, preparation of size-selected nanoparticles with diameters less than 10 nm in a solution is an urgent task in chemistry and physics of nanoscale materials.11-20 Over the past 10 years, many endeavors have been undertaken to invent methods of such preparation. Chemical reduction of a metal salt into metal nanoparticles in a micelle or a reversed micelle is one of the most successful methods to obtain metal nanoparticles stabilized in solutions.11,12 Smaller nanoparticles are produced in reverse micelles. In this case, the size of the nanoparticles depends on that of the reversed micelles, so that nanoparticles with a desired size can be obtained by introducing a proper amount of water inside the reversed micelles through adjusting a molar ratio of water to the nonpolar solvent. Laser ablation of a metal plate in a solution is found to be an alternative method to prepare metal nanoparticles in a solution.15-20 Several investigations have revealed that metal nanoparticles of interest are obtained by optimizing the concentration of the surfactant employed for stabilizing the nanoparticles and the power of the laser used for the ablation.18,19 The formation * Corresponding author. E-mail:
[email protected].
mechanism of the nanoparticles is taken into account in the optimization.18,19 There is no way to further change the size of the nanoparticles produced by the above-mentioned methods. If the nanoparticles possess a strong absorption band whose energy coincides with the energy of a laser, the nanoparticles are excited by irradiation of the laser, so that the size can be changed. Such a technique, “laser-assisted size-control”, could be a powerful tool to prepare desired nanoparticles.21-25 One can apply this size-control to changing the size of gold nanoparticles by using a 532-nm laser, because the gold nanoparticles have an intense surface plasmon peak centering at 520 nm. Actually, gold nanoparticles ∼10 nm in diameter are known to aggregate in an aqueous solution under irradiation of a 514-nm cw laser.21 Aggregation occurs similarly under irradiation of a pulsed Nd:YAG laser at 1064 nm.22 On the other hand, gold nanoparticles ∼40 nm in diameter are fragmented into those having 10 nm diameter by irradiation of a pulsed Nd:YAG laser at 532 nm. A similar phenomenon is observed in silver nanoparticles under irradiation of a pulsed laser.23 The feature of size-reduction is altered significantly by changing the fluence of an irradiation laser24,25 and is explained in terms of heat rising after excitation of the surface plasmon transition by the laser. In the present study, we investigated laser-assisted size reduction of gold nanoparticles
