Size evolution of laser-generated nanoparticles before and after cavitation bubble confinement
Nanoparticle synthesis by pulsed laser ablation in liquids (PLAL) is a method to obtain particles with clean surfaces, free of any organic residuals. A drawback to this method is the particle size distribution, which exhibits an intrinsic bi- or multi-modality. One known strategy to narrow the particle size distribution in a single step is to use electrolytes or macromolecules as additives to the liquid phase. The laser impact on the ablation target is followed by a hemispherical cavitation bubble, which acts as a container for the ablated mass for the first tens to hundreds of microseconds, depending on the laser parameters, and significantly affects the genesis of the final particle sizes. Clarifying the role of this cavitation bubble and identifying the time and location of additive-particle interactions are the main objectives of this dissertation. Laser percussion drilling was used to demonstrate that neither the shape nor the lifetime of the cavitation bubble significantly affects the size of the resulting silver nanoparticles due to the large amounts of nanoclusters (size ≤ 3 nm) that form during PLAL. These nanoclusters add to the final nanoparticle sizes by subsequent growth processes on a macroscopic time scale. The interior of the gaseous cavitation bubble in the presence of micromolar concentrations of additives (sodium chloride and polyvinylpyrrolidone (PVP)) was probed by analytical X-ray methods that exploited the brilliant beams generated from synchrotrons. Using spatio-temporally resolved small-angle X-ray scattering (SAXS), it was shown that dissolved NaCl does start to interact with the ablated species of a gold target already inside the gaseous phase of the cavitation bubble as both the size and abundance of large nanoparticles decreased with respect to the ablation in pure water. X-ray Hartmann mask imaging (XHI) was used for the analysis of the macromolecular ligand PVP for reducing the size of gold nanoparticles. In contrast to SAXS, XHI allows one to record the entire cavitation bubble in a single-shot experiment. While an effect already inside the cavitation bubble was not excluded, PVP mostly affected the nanoparticles in the liquid phase, after the collapse of the bubble, by hindering the ripening of the nanoparticles. The time and location of the interaction of different classes of size-reducing additives were identified as well as the vast generation of nanoclusters. These new findings present starting points for additional studies on the origin of the bimodal size distribution that is obvious before the evolution of the laser-induced cavitation bubble.
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