Ionic Liquids for the Synthesis and Stabilization of Metal Nanoparticles

The synthesis and stabilization of metal nanoparticles (M-NPs) from metals, metal salts, metal complexes and metal carbonyls in ionic liquids (ILs) is reviewed. The electrostatic and steric properties of ionic liquids allow for the stabilization of M-NPs without the need of additional stabilizers, surfactants or capping ligands. The synthesis of M-NPs in ILs can be carried out by chemical or electroreduction, thermolysis and photochemical methods including decomposition by microwave or sono-/ultrasound irradiation. Gas-phase syntheses can use sputtering, plasma/glow-discharge electrolysis and physical vapor deposition or electron beam and γ-irradiation. Metal carbonyl precursors M_x(CO)_y contain the metal atoms already in the zero-valent oxidation state needed for M-NPs so that no extra reducing agent is necessary. Microwave-induced thermal decomposition of precursors in ILs is a rapid and energy-saving access to M-NPs because of the significant absorption efficiency of ILs for microwave energy due to their ionic charge, high polarity and high dielectric constant. M-NP/IL dispersions can be applied in catalytic reactions, e. g., in C–C coupling or hydrogenation catalysis.