This report describes the pulsed laser-induced process of noble metal flakes and nanoparticles (NPs) in liquids studied by an in-situ extinction spectroscopy which allowed detection of time-dependent events such as pulverization of flakes concomitantly with growth of nanoparticles (NPs) and splitting of NPs into smaller particles. Two key observations were made in the present study. First, we observed a remarkable spectral modification ascribable to a size reduction during the 532 and 1064 nm laser ablation of the flakes from NPs with a characteristic surface plasmon (SP) band to much smaller NPs (<2-3 nm diameter) devoid of the SP band in neat and highly concentrated dodecanethiol solutions at moderate laser fluences of 0.7-1.3 J/(cm2 pulse). Previously production of such small particles was not feasible by the 1064 nm laser light. Only in solvents that thermally decompose by laser irradiation to give graphite shells to cover NPs or by two-step size reduction procedures, i.e., initial formation of NPs followed by further splitting due to absorption of the 532 nm laser light, have such observations been made. Thus, we demonstrated the superior nature of dodecanethiol as a solvent to protect the NPs from growing over other solvents such as acetone and water. Second, we observed the laser ablation-induced agglomeration of Au NPs leading to immediate flocculation in pure water. The observation taking place in the time scale of minutes is conceptually different from the previously observed photoinduced coagulation (J. Phys. Chem. 1994, 98, 2143-2147 and J. Phys. Chem. 1994, 98, 11997-12002) in which a photoenhanced van der Waals attractive force acting on excitation of the SP band is responsible for the coagulation phenomena taking place over many hours. In the present case, small positively charged particles produced by laser ablation of Au NPs by a 532 nm nanosecond pulsed laser irradiation are responsible for immediate combination reactions with intact negatively charged species giving rise to unstable bigger particles susceptible to developing flocculations. This deduction is based on the retardation effect of the agglomeration observed in various irradiation atmosphere such as Ar and O2. Thus, the present study sheds light on another aspect of laser ablation in liquids relevant to the time evolution of NP formation and its growth dynamics in seconds and minutes, which received less attention before.
ASJC Scopus subject areas