Size Quenching during Laser Synthesis of Colloids Happens Already in the Vapor Phase of the Cavitation Bubble

Although nanoparticle synthesis by pulsed laser ablation in liquids (PLAL) is gaining wide applicability, the mechanism of particle formation, in particular size-quenching effects by dissolved anions, is not fully understood yet. It is well-known that the size of small primary particles (d <= 10 nrn), secondary particles (spherical particles d > 10 nm), and agglomerates observed ex situ is effectively reduced by the addition of small amounts of monovalent electrolyte to the liquid prior to laser ablation. In this study, we focus on the particle formation and evolution inside the vapor filled cavitation bubble. This vapor phase is enriched with ions from the afore added electrolyte. By probing the cavitation bubbles' interior by means of small-angle X-ray scattering (SAXS), we are able to examine whether the size quenching reaction between nanoparticles and ions starts already during cavitation bubble confinement or if these reactions are subjected to the liquid phase. We find that particle size quenching occurs already within the first bubble oscillation (approximately 100 its after laser impact), still inside the vapor phase. Thereby we demonstrate that nanoparticle ion interactions during PLAL are in fact a gas phase phenomenon. These interactions include size reduction of both primary and secondary particles and a decreased abundance of the latter as shown by in situ SAXS and confirmed by ex situ particle analysis (e.g., static SAXS and TEM).