Unveiling Step-by-Step the Bottom-Up Kinetics and Growth Mechanism of Laser-Induced Colloidal Gold Nanoparticles

Laser-induced metal nanoparticle generation is a synthesis process that has been widely used, even on an industrial scale. However, their bottom-up growth kinetics and underlying mechanisms are not yet well understood. Hence, we described these step-by-step kinetic processes by using in situ UV–vis spectroscopy to determine the temporal evolution of gold nanoparticles (AuNPs) produced via nanosecond laser irradiation of aqueous AuCl4 – precursor solution. We monitored the size, shape, and concentration of spheroidal AuNPs over the synthesis time, applying a Mie-Gans-based model to the measured optical absorption spectra. Transmission electron microscopy supported the theoretical size analysis. The proposed experimental and theoretical approaches were used to provide quantitative kinetic data for AuNPs synthesized under different experimental conditions, which was achieved by modifying the initial metal precursor concentration in the reaction medium. The reported methodology allowed the identification of dominant physicochemical subprocesses occurring at specific time intervals during the synthesis experiment, including nucleation, polynuclear surface reaction-mediated growth, particle fragmentation, and intraparticle ripening. The elementary mechanism responsible for the observed kinetic steps was described in terms of the Finke-Watzky model, which consists of two simultaneous processes: slow, continuous nucleation, and fast autocatalytic surface growth. Finally, the initial precursor concentration adopted in the synthetic procedures effectively controlled the determining kinetic parameters such as rate constants for nucleation and growth. These insights indicate a high level of kinetic control over the AuNP genesis, physical properties, and optical response, which is potentially achievable in other metal nanoparticles.