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 k 1, dependent on eaq –; and growth rate k 2, 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 k 2 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.

Plasmas with dense concentrations of reactive species such as hydrated electrons and hydroxyl radicals are generated from focusing intense femtosecond laser pulses into aqueous media. These radical species can reduce metal ions such as Au3+ to form metal nanoparticles (NPs). However, the formation of H2O2 by the recombination of hydroxyl radicals inhibits the reduction of Ag+ through back-oxidation. This work has explored the control of hydroxyl radical chemistry in a femtosecond laser-generated plasma through the addition of liquid ammonia. The irradiation of liquid ammonia solutions resulted in a reaction between NH3 and OH·, forming peroxynitrite and ONOO−, and significantly reducing the amount of H2O2 generated. Varying the liquid ammonia concentration controlled the Ag+ reduction rate, forming 12.7 ± 4.9 nm silver nanoparticles at the optimal ammonia concentration. The photochemical mechanisms underlying peroxynitrite formation and Ag+ reduction are discussed.

Photochemical reduction of aqueous Ag+ and [AuCl4]− into alloy Au–Ag nanoparticles (Au–Ag NPs) with intense laser pulses is a green synthesis approach that requires no toxic chemical reducing agents or stabilizers; however size control without capping agents still remains a challenge. Hydrated electrons produced in the laser plasma can reduce both [AuCl4]− and Ag+ to form NPs, but hydroxyl radicals (OH·) in the plasma inhibit Ag NP formation by promoting the back-oxidation of Ag0 into Ag+. In this work, femtosecond laser reduction is used to synthesize Au–Ag NPs with controlled compositions by adding the OH· scavenger isopropyl alcohol (IPA) to precursor solutions containing KAuCl4 and AgClO4. With sufficient IPA concentration, varying the precursor ratio enabled control over the Au–Ag NP composition and produced alloy NPs with average sizes less than 10 nm and homogeneous molar compositions of Au and Ag. By investigating the kinetics of Ag+ and [AuCl4]− coreduction, we find that the reduction of [AuCl4]− into Au–Ag NPs occurs before most of the Ag+ is incorporated, giving us insight into the mechanism of Au–Ag NP formation.

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