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•Ligand-free Pd nanoparticles synthesized by laser reduction in liquid (LRL).•Stabilization of Pd precursors with acid produces ultrasmall Pd nanoparticles.•Nanoflower and nanopopcorn Pd structures were observed.•Laser-Pd nanoparticles outperform conventional Pd catalysts in Suzuki cross-coupling.
We report the synthesis of uncapped ‘naked’ palladium nanoparticles (PdNPs) through the facile and green route of femtosecond laser reduction in liquid using two different precursors: potassium tetrachloropalladate (K2PdCl4) and palladium nitrate (Pd(NO3)2). Stabilization of the Pd precursor by lowering the pH enabled the synthesis of PdNPs in the sub-5 nm range. Laser synthesis using both precursors generated ultrasmall spherical NPs with average diameters of 1.2 nm and 3.1 nm for the nitrate and chloride precursors, respectively. An additional population of anisotropic large particles with nanopopcorn and nanoflower morphologies observed in electron microscopy images appears to result from the aggregation of the ultrasmall PdNPs. The PdNPs are efficient catalysts for Suzuki cross-coupling reactions and for the reduction of para-nitrophenol by sodium borohydride in water. The excellent catalytic activity of our laser-generated PdNPs is attributed to the ligand-free structure and small particle sizes generated through laser reduction in liquid.