© Copyright 2000 by the American Chemical Society
VOLUME 104, NUMBER 35, SEPTEMBER 7, 2000
ARTICLES Structure and Stability of Silver Nanoparticles in Aqueous Solution Produced by Laser Ablation 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: May 17, 2000; In Final Form: June 26, 2000
Silver nanoparticles were produced by laser ablation of a metal silver plate in aqueous solutions of surfactants, CnH2n+1SO4Na (n ) 8, 10, 12, 16). The nanoparticles thus produced were characterized by electron microscopy and UV-visible absorption spectroscopy. The abundances of the nanoparticles before and after centrifugation were measured as a function of the surfactant concentration. The concentration dependence of the abundance implies that the surfactant coverage and the charge state on the nanoparticle surface are closely related to the stability of the nanoparticles in the solutions. The nanoparticles tend to be aggregated when the coverage is less than unity, while they are very stable when the surface is covered with a double layer of the surfactant molecules.
1. Introduction Intensive investigations on metal nanoparticles suspended in solutions have been undertaken extensively because of their sizedependent characteristic properties.1-5 In these studies, one of the key issues is to develop a practical method for preparation of nanoparticles suspended in liquid. Pileni and co-workers have synthesized metal nanoparticles in reversed micelles by chemical reduction of metal ions in them.6,7 The size of the nanoparticles thus produced is controlled by the volume of the reversed micelles, because the volume depends critically on the molar ratio of the surfactant to water inside the reversed micelles. ElSayed and co-workers have reported a reductive synthesis of platinum nanoparticles in a solution containing platinum ions and capping polymer molecules. They have shown that the shape of platinum nanoparticles can be varied by changing the molar fraction of the capping polymer in the solution.8,9 Tichelaar and * Corresponding author. E-mail:
[email protected].
co-workers have prepared subnanometer-sized silver particles, which is free of any stabilizing polymer, by anodic dispersion of a silver electrode in an aqueous NaOH solution.10 Apart from these chemical syntheses, new methodology has been developed to produce nanoparticles by laser ablation of a metal plate in a solution by Henglein et al.11,12 and others.13-15 We have shown that the size distribution of the silver nanoparticles produced by laser ablation in an aqueous solution containing sodium dodecyl sulfate (SDS) as a surfactant is determined by particle growth in a plume by the laser ablation and its termination by SDS coating.16 Evidently, SDS plays an important role in determining stability and size of the nanoparticles, because the termination of the nanoparticle growth is controlled by the diffusion and the attachment rates of SDS on the nanoparticle. It is expected, therefore, that the size distribution and the stability of the nanoparticles depend critically on the properties of the surfactant employed. In the present study, we examined nanoparticle formation in
10.1021/jp001803b CCC: $19.00 © 2000 American Chemical Society Published on Web 08/17/2000
8334 J. Phys. Chem. B, Vol. 104, No. 35, 2000
Mafune´ et al. repeatedly at least five times by a Shimadzu UV-1200 spectrometer, and the spectra obtained by these different runs were accumulated in an NEC computer so as to obtain the spectrum with a good signal-to-noise ratio. An electron micrograph was observed by a transmission electron microscope (JEOL JEM-100S ×50000). A drop of the clear portion was placed on a copper grid, which was coated with collodion and subsequently hydrophilic-sputtered in advance, and was dried by heating to 320 K. After this procedure was repeated three times, the grid was washed in water in order to remove free surfactant molecules. The size distributions of the nanoparticles produced were obtained by measuring the diameter of more than 1000 particles viewed in the micrographs. 3. Results
Figure 1. Electron micrograph (panel a) and an absorption spectrum (panel b) of silver nanoparticles produced by 532 nm laser ablation of a silver metal plate in a 0.01 M aqueous solution of SDS.
solutions containing surfactants, CnH2n+1SO4Na, with different lengths of the hydrocarbon chain. The size and the stability of the nanoparticles produced in the different solutions were observed by electron microscopy and optical absorption spectroscopy, in combination with a centrifugal precipitation method. 2. Experimental Section Silver nanoparticles were produced by laser ablation of a silver metal plate in an aqueous solution of a different surfactant; CnH2n+1SO4Na with n ) 8, 10 (Kanto Chemical Co., Inc.), that with n ) 12 (Nacalai Tesque, Inc. >95%; denoted as SDS hereafter), and that with n ) 14 (Tokyo Kasei Kogyo Co., Ltd. containing ca. 40% sodium stearyl sulfate). The metal plate (>99.99 in purity) was placed on the bottom of a glass vessel filled with a 10 mL aqueous solution of a surfactant and was irradiated with the second harmonic (532 nm) output of a Quanta-ray GCR-170 Nd:YAG laser focused by a lens having the focal length of 250 mm. A typical pulse width and a repetition rate were ∼10 ns and 10 Hz, respectively. A spot size of the laser beam was adjusted within 1-3 mm in diameter by changing the distance between the lens and the metal plate. The output of the 532 nm laser (
