A novel method for preparing monosized ultrafine particles of gold utilizing a technique of the reductive stripping of Au(III)-loaded organics has been developed. Au(III)-loaded organics were prepared by extracting HAuCl4 dissolved in an aqueous HCl or HNO3 solution with MIBK, DBC or TBP, and an aqueous solution containing suitable reductive agents of Au(III), such as HCOOH, H2C2O4, CH3OH, etc., were used for stripping purpose.
Organic loaded with Au(III) loose their yellow color through photochemical reaction. Both yellow and colorless solutions of Au in MIBK or DBC can be reduced to a metallic state to form monosized ultrafine particulates at ambient temperature by stripping Au-loaded organics with an aqueous solution containing suitable reductant. The required amount of reductant is far less than that of equivalent molar ratio, indicating that water plays a role of the reductant of gold in organics. In the case of TBP, however, Na2SO3, Na2C2O4 and HCOONa, instead of their acid form, are a more effective reductant for Au. However, the yield of gold particulates from Au(III)-loaded TBP is low compared to those from the other organic even though the excess amount of Na salts was used. Gold particulates thus formed are smaller than 0.3 μm in diameter. When NaPP was used with H2C2O4, the particle size becomes finer.
