Pure metal nanoparticles formed using pulsed lasers
Nanometer-sized metal particles have attracted much attention from scientists and engineers because they display specific physical and chemical properties that the bulk materials do not possess. Thus, considerable effort has been focused on preparing metal nanoparticles. For instance, a simple chemical method has already been developed to synthesize gold nanoparticles stabilized by thiol molecules, which contain a long hydrocarbon chain and a sulfur group.1 The sulfur atom binds strongly to the gold atom on the particle's surface, from which the long carbon chain protrudes. A long chain is important because it prevents particles from aggregating in an organic solvent. As an alternative, citric acid is commonly used to reduce gold ions in water to synthesize gold nanoparticles.2 Here, the citrate anions form the charged layer around the particle surface and provide electrical repulsive forces between the particles. In both methods, reducing and stabilizing reagents are required to keep nanoparticles stable in the liquid phase.
Recently, interest has been growing in laser-based methods to prepare nanoparticles in water.3,4 We have demonstrated formation of chemical-free noble-metal nanoparticles by laser ablation of bulk materials in water. For instance, we prepared gold nanoparticles by ablation of a gold metal plate. The plate was placed on the bottom of a glass vessel filled with 10ml of distilled and deionized water. A nanosecond Nd:YAG (neodymium-doped yttrium aluminum garnet) pulsed laser (wavelength 1064nm) operating at 10Hz with a pulse energy of 80mJ was focused onto a metal plate using a 250nm lens. The concentration of the colloidal dispersion is typically 0.12mM of gold atoms after 36,000 laser shots (see Figure 1). Only commercially available bulk-metal rods and distilled and deionized water were purchased as starting materials. No reducing chemicals or stabilizing reagents (such as surfactants) were required. We can produce completely pure metal nanoparticles in water with the aid of a pulsed laser. Surprisingly, in water the nanoparticles are very stable against aggregation, even though they are not stabilized by surfactants.
For basic scientists, the main question to be answered is why the nanoparticles are stable in water. The answer is found in the negative charge on the particle's surface. The surface of gold nanoparticles produced by laser ablation in water has recently been investigated by x-ray photoelectron, IR, and mass spectrometry, and ζ-potential (i.e., the electrokinetic potential in colloidal systems) measurements.5 The outcome was that the nanoparticles are negatively charged because the particle surface is partially oxidized. We studied the surface of gold nanoparticles by examining the chemical interactions between the nanoparticles and positively or negatively charged surfactants. When positively charged cetyltrimethylammonium bromide (CTAB) is added gradually to the colloidal dispersion, gold nanoparticles aggregate, because the CTA+ ions neutralize the negative charge of the particles (see Figure 2). By taking advantage of this electrostatic interaction (or ‘titration’), we concluded that 3.3–6.6% of the surface atoms are negatively charged.6 We expect that these metal particles will be used in a variety of research fields because they have physical and chemical properties that their bulk materials do not, thus providing new research opportunities.
Fumitaka Mafuné is an associate professor of chemistry. He holds a BA in chemistry and a DSci from the University of Tokyo. He started preparing nanoparticles using pulsed lasers in 1999, when he was an assistant professor at the Toyota Technological Institute.
