Interactions of intense ultrashort laser pulses with liquid media and at solid-liquid interfaces produce plasmas with high densities of reactive chemical species that promote the formation of nanomaterials. This work discusses the current understanding of laser-induced chemical reactions in aqueous medium involving hydrated electrons and hydroxyl radicals, and the effects of chemical additives on these reactions. Two chemical strategies are presented for controlling the composition of the laser-induced plasma and nanomaterial properties. First, adding chemical scavengers of hydroxyl radicals will be shown to inhibit metal oxidation, enabling the synthesis of ligand-free Ag and Au-Ag alloy nanoparticles by reduction of Au and Ag salts. Second, ablating a silicon wafer in a solution containing two metal salts with different reduction potentials will be shown to enhance the deposition of the high-reduction potential metal onto the ablated silicon surface, producing high densities of metal nanostructures on silicon laser-induced periodic surface structures (LIPSS).
Uncapped “naked” Au, Ag, and Pd nanoparticles were synthesized using femtosecond laser-induced reduction of salt precursors in aqueous solution. Focusing femtosecond laser pulses into water induces optical breakdown of the medium, producing a dense plasma containing reactive electrons and radicals that can reduce metal ions to neutral metal atoms, which coalesce into nanoparticles. Manipulating the plasma composition by changing the solution pH and adding radical scavengers was found to enable improved control over the nanoparticle size distributions. The synthesized Au and Pd nanoparticles are catalytically active towards the hydrogenation of 4-nitrophenol to 4-aminophenol and could potentially be used in further catalysis applications.
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