Preparation of platinum nanoparticles by sonochemical reduction of the Pt(IV) ions: role of surfactants.

Sonochemical reduction processes of Pt(IV) ions in water have been investigated in the presence of various kinds of surfactants such as sodium dodecylsulfate (SDS) and sodium dodecylbenzenesulfonate (DBS) as anionic surfactants, and polyethylene glycol monostearate (PEG-MS) as non-ionic, dodecyltrimethylammonium chloride (DTAC) and bromide (DTAB) as cationic surfactants. An improved colorimetric determination reveals that Pt(IV) ion is reduced to zero valent metal in two steps: step (1)--Pt(IV) ion to Pt(II) ion, and step (2)--Pt(II) ion to Pt(0), and after the completion of step (1), step (2) sets in. It appears that rapid scrambling reactions among platinum ions and/or atoms, that is, Pt(I) + Pt(IV)-->Pt(II) + Pt(III), etc. take place. In the sonolysis of aqueous solutions of SDS, DBS or PEG-MS, two kinds of organic reducing radicals, R(ab) and R(py), are proposed to contribute to the reduction. Radical R(ab) is formed from the reaction of the surfactants with primary radicals such as hydroxyl radicals and hydrogen atoms originated from the sonolysis of water, and radical R(py) is formed from the direct thermal decomposition of surfactants in the interfacial region between the collapsing cavities and the bulk water. R(ab) is effective for both the reduction steps, whereas R(py) is involved only in the reduction step (1). This fact coincides with the previous reported sonochemical reduction of Pt(II) ions. Hydrogen atoms themselves scarcely participate in the reduction. The average diameter (1.0 nm) of platinum particles prepared from the system of PEG-MS is smaller than those from the aqueous solution of anionic surfactant SDS (3.0 nm) and DBS (3.0 nm).