Design, synthesis, catalytic application, and strategic redispersion of plasmonic silver nanoparticles in ionic liquid media

•Silver nanoparticles stabilized in tetraalkylphosphonium halide ionic liquids.•Particles used to catalyze the reduction of Eosin Y with lithium borohydride.•Catalytic activity is function of particle size.•Large nanoparticles (>15 nm) show no significant catalytic activity.•Large silver particles can be redispersed to recover catalytic activity.

Silver nanoparticles synthesized in tetraalkylphosphonium ionic liquids are found to be excellent catalysts for borohydride-induced reductive degeneration of Eosin-Y, a dye that has been classified as a Class 3 carcinogen by the International Agency for Research on Cancer. TEM images indicated that the size of the Ag nanoparticles was significantly influenced by heat-induced sintering. A strategy was devised to redisperse smaller Ag nanoparticles from their aggregated/sintered counterparts via a two-step protocol that involved oxidative etching of Ag nanoparticles, followed by a re-reduction step. This protocol led to a reduction in the sintered Ag nanoparticle size from 15.7 ± 6.1 nm to 3.7 ± 0.8 nm, which was consistent with the size of the as-synthesized nanoparticles. The as-synthesized and the redispersed Ag nanoparticles were found to catalyze the bleaching of Eosin-Y with comparable efficiencies; first order rate constants for Eosin Y reduction were ∼8 times higher for smaller Ag nanoparticles compared to their sintered counterparts. An examination of the kinetics of Ag nanoparticle etching was performed via temperature-controlled UV–vis spectroscopy. Changes in the oxidation state of Ag during this sequence of events were also followed by in situ X-ray absorption spectroscopy of Ag nanoparticles in the ionic liquid.

Graphical abstractAg NPs in tetraalkylphosphonium ionic liquids are prone to sintering at elevated temperatures, but it is possible to redisperse them as small catalytically active Ag NPs for Eosin Y degradation.Figure optionsDownload full-size imageDownload high-quality image (142 K)Download as PowerPoint slide