Rapid mixing of metal atoms ejected in water by pulsed laser ablation of the boundary between jointed Au and Ag target metal plates

Abstract

We prepared metal alloy nanoparticles by pulsed laser ablation of the boundary between jointed target metal plates of Au and Ag in purified water. When the spot size radius of the focused laser on the target was 2.5 mm, separate Au nanoparticles and Ag nanoparticles were mainly observed with slight alloying. In contrast, when the spot size radius was 0.25 mm, the ejected Au and Ag atoms were found to be more mixed, efficiently forming Au and Ag alloy nanoparticles. The mixing distance of Au and Ag atoms was estimated to be ∼0.5 mm. The dynamics of the plasma plume and the cavitation and their interaction with Au and Ag atoms were quite complex. Hence, we assumed that during plasma plume formation and cavitation formation, Au and Ag atoms diffuse in the plasma plume and cavitation as a uniform medium with a maximum temperature and minimum viscosity. It was suggested that the fast mixing of Au and Ag atoms occurred inside the plasma plume, taking the high temperature, several thousand K of the plasma plume into account. Moreover, the mixing was promoted by the overlapping of plasma plumes, which were generated on Au and Ag plates separately, owing to explosive plasma motions. Then, the alloy nanoparticles were formed in the cavitation bubble where Au and Ag atoms were spatially distributed by plasma plumes. The mixing of the atoms in the cavitation bubble was not likely based on the mixing distance.