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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 [AuCl 4 ] − 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 [AuCl 4 ] − 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 [AuCl 4 ] − is attributed to enhanced optical breakdown by the Au nanoparticle products of [AuCl 4 ] − reduction. These mechanistic insights can inform the design of laser synthesis procedures to improve control over metal nanoparticle properties and enhance byproduct yields. 

Laser photoreduction of metal ions onto graphene oxide (GO) is a facile, environmentally friendly method to produce functional metal–GO nanocomposites for a variety of applications. This work compares Au–GO nanocomposites prepared by photoreduction of [AuCl 4 ] − in aqueous GO solution using laser pulses of nanosecond (ns) and femtosecond (fs) duration. The presence of GO significantly accelerates the [AuCl 4 ] − photoreduction rate, with a more pronounced effect using ns laser pulses. This difference is rationalized in terms of the stronger interaction of the 532 nm laser wavelength and long pulse duration with the GO. Both the ns and fs lasers produce significant yields of sub-4 nm Au nanoparticles attached to GO, albeit with different size distributions: a broad 5.8 ± 1.9 nm distribution for the ns laser and two distinct distributions of 3.5 ± 0.8 and 10.1 ± 1.4 nm for the fs laser. Despite these differences, both Au–GO nanocomposites had the same high catalytic activity towards p -nitrophenol reduction as compared to unsupported 4–5 nm Au nanoparticles. These results point to the key role of GO photoexcitation in catalyzing metal ion reduction and indicate that both ns and fs lasers are suitable for producing functional metal–GO nanocomposites.