Nanoparticles in ionic liquids

Koen Binnemans, KU Leuven, Department of Chemistry, Laboratory of Coordination Chemistry

Ionic liquids are solvents that are entirely composed of ions. Typically they are organic salts with a melting point below 100 °C. Because of their low vapor pressure (low volatility), recycleability, high solvating power (less solvent required), ionic liquids are solvents which enable "greener" or more sustainable synthetic routes for the preparation of nanoparticles. Ionic liquids tend to stabilize nanoparticles. They allow to obtain stable dispersion of nanoparticles without the need of additional stabilizers They are structured solvents that can act as a template for the shape-selective synthesis of nanoparticles. It is possible to synthesize in ionic liquids nanoparticles with a shape which cannot be obtained easily by other synthetic methods. Ionic liquids are an excellent medium for microwave-assisted synthesis of nanoparticles, which generally leads to rod-like and wire-like nanoparticles. These solvents can control polymorphism of inorganic compounds. For instance, depending on ionic liquid composition, TiO2 will crystallize in the anatase or the rutile structure. Their large electrochemical window (up to 5V) allows fabrication of materials by electrochemical methods, including the electrodeposition of nanocrystalline metals. Nanocrystalline metals have a higher hardness and improved corrosion resistance in comparison with microcrystalline metal layers. For instance nanocrystalline cobalt layers could compete with chromium layers for corrosion protection. Ionic liquids have a very low vapor pressure so that they do not evaporate, even not at high temperatures under high vacuum conditions. Therefore, metallic nanoparticles can be made by sputtering of metals on the surface of an ionic liquid. Nanosized semiconductors like silicon and germanium can be electrochemically deposited in ionic liquids. Ionic liquids are useful solvents for biphasic catalysis with precious metal nanoparticles. After the reaction, the precious metal catalyst can be recovered because it is immobilized in the ionic liquid phase.