Understanding photochemical pathways of laser-induced metal ion reduction through byproduct analysis

Laser-induced reduction of metal ions is attracting increasing attention as a sustainable route to ligand-free metal nanoparticles. In this work, we investigate the photochemical reactions involved in reduction of Ag⁺ and [AuCl₄]^- upon interaction with lasers with nanosecond and femtosecond pulse duration, using strong-field ionization mass spectrometry and spectroscopic assays to identify stable molecular byproducts. Whereas Ag⁺ in aqueous isopropyl alcohol (IPA) is reduced through plasma-mediated mechanisms upon femtosecond laser excitation, low-fluence nanosecond laser excitation induces electron transfer from IPA to Ag⁺. Both nanosecond and femtosecond laser excitation of aqueous [AuCl₄]^- produce reactive chlorine species by Au-Cl bond homolysis. Formation of numerous volatile products by IPA decomposition during both femtosecond and nanosecond laser excitation of [AuCl₄]^- is attributed to enhanced optical breakdown by the Au nanoparticle products of [AuCl₄]^- reduction. These mechanistic insights can inform the design of laser synthesis procedures to improve control over metal nanoparticle properties and enhance byproduct yields.