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.

Laser-assisted metallic nanoparticle synthesis is a versatile “green” method that has become a topic of active research. This chapter discusses the photochemical reaction mechanisms driving AuCl4− reduction using femtosecond-laser irradiation, and reviews recent advances in Au nanoparticle size-control. We begin by describing the physical processes underlying the interactions between laser pulses and the condensed media, including optical breakdown and supercontinuum emission. These processes produce a highly reactive plasma containing free electrons, which reduce AuCl4−, and radical species producing H2O2 that cause autocatalytic growth of Au nanoparticles. Then, we discuss the reduction kinetics of AuCl4−, which follow an autocatalytic rate law in which the first- and second-order rate constants depend on free electrons and H2O2 availability. Finally, we explain strategies to control the size of gold nanoparticles as they are synthesized; including modifications of laser parameters and solution compositions.

We report the synthesis of silica-gold nanoparticles (silica-Au NPs) using a one-step femtosecond-reactive laser ablation in liquid (fs-RLAL) technique by focusing femtosecond laser pulses onto a silicon wafer immersed in an aqueous KAuCl4 solution. Characterization of the silica-Au NPs revealed two populations of Au NPs: (i) larger, isolated Au NPs with diameter 7.3±2.1 nm, and (ii) smaller Au NPs (3.4±0.8 nm) embedded in an amorphous silica matrix, along with new species of silicon observed from XPS analysis. The catalytic activity of the silica-Au NPs towards the reduction of para-nitrophenol by NaBH4 is significantly higher compared to the control Au NPs synthesized in the absence of a silicon wafer and other Au NPs recently reported in literature. The formation of the two populations of silica-Au NPs is ascribed to reaction dynamics occurring on two distinct timescales. First, the dense electron plasma formed within tens of femtoseconds of the laser pulse initiates reduction of the [AuCl4]– complex, leading to the formation of larger isolated Au NPs. Second, silicon species ejected from the wafer surface hundreds of picoseconds or later after the initial laser pulse reduce the remaining [AuCl4 ]– and encapsulate the growing clusters, forming ultrasmall Au NPs embedded in the silica matrix. The morphologies of the silica-Au NPs generated from fs-RLAL are distinct from those reported in recent RLAL experiments with nanosecond lasers, reflecting distinct mechanisms occurring on the different pulse duration timescales.

Synthesis of copper-silica nanocomposites with controllable morphology and composition were produced with a one-step femtosecond reactive laser ablation in liquid (fs-RLAL) technique. The composite nanomaterials were generated by focusing femtosecond near-IR laser pulses onto a silicon wafer immersed in an aqueous copper(II) nitrate solution, with the solution pH adjusted using nitric acid or potassium hydroxide. Under acidic conditions (pH 3.0 and 5.4), little copper was incorporated in the predominantly silica product (1.4 and 1.5 wt.%). These acidic conditions yielded large ~30-80 nm silica particles, with some particles consisting of copper core/silica shell. In contrast, increasing the solution pH to 10.4 resulted in extremely high Cu loading of 31.5 wt.% and a composite product consisting of 1.5 nm copper clusters distributed throughout a matrix of amorphous silica and copper phyllosilicate. The relationship between the precursor solution pH and the product morphology and copper loading is attributed to the point of zero charge (PZC) of silica, in which the high solution pH allows for electrostatic adsorption to occur between the deprotonated silica clusters from the ablated silicon wafer and the copper hydroxide dimer formed in solution.