Synthesis of Gold Nanoparticles from Aqueous Solutions of Hydrochloroauric Acid under Multipulse Femtosecond Irradiation

The main modes of synthesis of gold nanoparticles in aqueous solutions of hydrochloroauric acid under multipulse (3 × 10⁶) femtosecond laser irradiation have been studied. UV–Vis spectroscopy, dynamic light scattering, and transmission electron microscopy (TEM) showed that there are two types of nanoparticles dominating in the investigated range of laser-pulse energies W = 1.6–200 µJ: ultrasmall (~1–5 nm) and plasmonic (~5–50 nm) nanoparticles. The particle size is found to be determined by two threshold pulse energies. The first corresponds to the development of avalanche water ionization, which initiates reduction of [AuCl₄]^– ions to neutral atoms, formation of nuclei for nanoparticles, and their subsequent growth. The second threshold is determined by the avalanche-plasma heating, which leads to an explosive rise in the liquid temperature with overheating, melting, and fragmentation of previously formed gold nanoparticles.