Kinetic Control of [AuCl4]- Photochemical Reduction and Gold Nanoparticle Size with Hydroxyl Radical Scavengers.

Laser-induced photochemical reduction of aqueous [AuCl4]- is a green synthesis approach requiring no chemical reducing agents or stabilizers; but size control over the resulting gold nanoparticles remains a challenge. Under optical breakdown conditions producing hydrated electrons (eaq-) and hydroxyl radicals (OH•) through decomposition of water, [AuCl4]- reduction kinetics follow an autocatalytic rate law, which is governed by rate constants: nucleation rate k1, dependent on eaq-; and growth rate k2, dependent on the OH• recombination product, H2O2. In this work, we add the hydroxyl radical scavengers isopropyl alcohol and sodium acetate to limit H2O2 formation. Higher scavenger concentrations both lowered k2 values and produced smaller gold nanoparticles with Gaussian size distributions and remarkably narrow mass-weighted size distributions. With sufficiently high scavenger concentrations, the mean nanoparticle size could be tuned from 3.8 to 6.1 nm with polydispersity indices below 0.08. Both the higher surface area-normalized catalytic activity of the gold nanoparticles synthesized in the presence of scavengers, and FTIR measurements, indicate no capping ligands on the nanoparticle surfaces. These results demonstrate that the size distributions of "naked" gold nanoparticles produced by photochemical [AuCl4]- reduction can be effectively tuned by controlling the reaction kinetics.