Stability of uncapped gold nanoparticles produced via laser reduction in liquid

Abstract

Nanoparticles are excellent catalysts due to their high surface area to volume ratio that results in an increased number of catalytically active sites per unit mass. This plethora of highly reactive sites is often in contrast with the catalyst stability. Capping agents are used to maintain stability against aggregation; they can however have adverse effects on the catalytic activity of the nanoparticles by obstructing reactant access to active sites. A unique set of synthesis methods that leverage lasers, such as Laser Reduction in Liquid (LRL), are capable of producing catalytic nanoparticles that remain stable despite their lack of capping agent. The mode of stabilization for uncapped laser nanoparticles must arise from electrostatic repulsion. There are three main hypotheses that seek to describe the source of this electrostatic repulsion: oxidized species at the particle surface, adsorbed anions at the particle surface, or trapped electrons within the particle. Two types of LRL particles were analyzed: particles produced using a laser pulse duration on the order of ns, and particles produced using a laser pulse duration on the order of fs. The primary difference between these two synthesis conditions arose from the presence (fs) or absence (ns) of an electron rich plasma during nanoparticle synthesis. The stability of LRL nanoparticles was investigated using zeta potential measurements. It was found that the ns particles were significantly less stable than their fs counterparts. Aggregation kinetics were investigated using time resolved dynamic light scattering (TR-DLS). Using the aggregation kinetics measurements, the surface potential of the fs nanoparticles were determined using DLVO theory. The surface potential allowed us to calculate the theoretical electron density of the fs particles as a function of laser focus. The particle electron densities are similar to the electron density of the plasma in which they were synthesized. These results support the trapped electron hypothesis as being a key mode of stabilization for LRL nanoparticles.