Preparation of supported metallic nanoparticles using supercritical fluids: A review

Supercritical fluids (SCFs) have been used to deposit thin metal films onto a wide range of surfaces and incorporate metallic particles into different inorganic and organic substrates for microelectronic, optical and catalytic applications. The technique involves the dissolution of a metallic precursor in a SCF and the exposure of a substrate to the solution. After incorporation of the precursor with the substrate, the metallic precursor is reduced to its metal form by a wide variety of methods resulting in films or particles. The reduction methods employed are chemical reduction in the SCF with a reducing agent, such as hydrogen and alcohols, thermal reduction in the SCF and thermal decomposition in an inert atmosphere or chemical conversion with hydrogen or air. Both highly dispersed and uniformly distributed metal crystallites (<10 nm) and agglomerated metal clusters (tens of nanometers) with a wide size distribution supported on a variety of substrates such as silicon wafer, metal foil, polymer membranes, organic and inorganic porous materials were produced by the different studies in the literature. The average particle size and size distribution are affected by the precursor reduction method and conditions, type and amount of precursor in the synthesis system, the surface properties of the substrate such as surface area and chemical nature of the surface.