Preparation of metal nanoparticles in water-in-oil (w/o) microemulsions

The use of an inorganic phase in water-in-oil microemulsions has received considerable attention for preparing metal particles. This is a new technique, which allows preparation of ultrafine metal particles within the size range 5 nm<particle diameter<50 nm. This article presents a review of the current literature in the field of particle preparation of several metals such as silver, copper, cadmium, cobalt, nickel, cadmium and gold in the inverse microemulsion systems. The reactant metal salts and reducing agents are mostly soluble in water and therefore the nucleation of metal particles proceeds in the water pools of the microemulsion. The rate of particle nucleation is a function of the percolation degree of microemulsion droplets. Besides a short introduction into some aspects of the microemulsion types and formation, we mainly focus on the kinetics of metallic particle formation. Effects of stabilizer (emulsifier) type and concentration and the type of continuous phase, reducing agent and additive on the particle formation are summarized and evaluated. The influence of several other parameters such as temperature, the incident light, the nature of metal salts and reaction conditions are also reviewed. These results indicate that the nature of the stabilizer emulsifier, the surface activity of additives and the colloidal stability of microemulsion droplets play decisive role on the particle size and distribution during the preparation of metal particles.