Synthesis of gold, platinum, and gold-platinum alloy nanoparticle colloids with high-power megahertz-repetition-rate lasers: the importance of the beam guidance method

Nanoparticles of noble metals and their alloys are of particular interest for biomedicine and catalysis applications. The method of laser ablation of bulk metals in liquids gives facile access to such particles as high-purity colloids and is already used in industrial research. However, the method still lacks sufficient productivity for industrial implementation into series production. The use of innovative laser technology may help to further disseminate this colloid synthesis method in the near future. Ultrashort-pulsed lasers with high powers and megahertz-repetition-rates became available recently, but place high demands on the accurate optical laser pulse delivery on the target. Full lateral pulse separation is necessary to avoid a reduction of nanoparticle productivity due to pulse shielding. In this study, we compare flexible but rather slow galvanometer scanning with much faster but more expensive polygon-wheel scanning in their performance in the production of colloidal nanoparticles by laser ablation in liquid. Both beam guidance technologies are applied in the laser ablation of gold, platinum, and a gold-rich platinum alloy in micromolar saline water. We found that the dimensions of the scan pattern are crucial. A threshold pattern length exists, at which one scan technology becomes more productive than the other one. In addition, a much lower productivity was found for the ablation of gold compared to that of platinum. Alloying gold with only 10 at.% of platinum improved the productivity nearly to the level of platinum, reaching 8.3 g/h.