Ionic Liquid-Assisted Electron Transfer in Micelles for Simultaneous Deposition of Uniform Ag Nanoparticles at Electrode Surface and in Bulk Microemulsions

Abstract

The synthesis of metal nanoparticles (NPs) with controllable size from aqueous or organic solutions has long been a scientific and technological challenge. Here we show a unique electrodeposition approach from ionic liquid microemulsions (ILM) which enables simultaneous fabrication of uniform controllable Ag NPs at both electrode substrate and in bulk solution. In the ILM system containing cetyltrimethylammonium bromide (CTAB), butanol, 1-butyl-3-methyl-imidazolium chloride ([BMIM][Cl]), and an aqueous solution of silver nitrate, uniform Ag NPs with a narrow size distribution of 20 ~ 50 nm could be electrodeposited on the cathode surface at low deposition current (0.2~1.5 mA·cm-2). The size and size distribution of Ag NPs can be adjusted by tuning the deposition current, time, temperature and IL concentrations. Remarkably, at high deposition current (>1.50 mA·cm-2) the electrochemical reduction of Ag NPs takes place not only on the electrode surface but also beyond into the bulk microemulsion phase. Uniform Ag NPs of diameters 2 ~ 13 nm thus can be obtained from the bulk ILMs. It is found that the ionic liquid plays a quaternary role as a soft template and co-surfactant for the formation of micro-reactors, an electrolyte for enhancing conductivity, and an electron carrier facilitating electron transport between nanodropletsfor Ag reduction. The electrocatalytic activity of the electrodeposited Ag NPs has been investigated for oxidation of glycerol for potential applications in direct alcohol fuel cells.