SURFACE-CHEMISTRY OF COLLOIDAL GOLD - DEPOSITION AND REOXIDATION OF PB, CD, AND TL

Lead, cadmium and thallium are deposited on colloidal gold particles i n aqueous solution. The deposition occurs through the reduction of the metal ions by excess electrons which are donated to the gold particle s by free organic radicals generated radiolytically. The absorption sp ectra of the composite particles are measured and compared to calculat ed ones (using Mie theory and the optical constants of the pure metals ). - Au-Pb particles are stable and well separated, and there is reaso nable agreement between experimental and calculated spectra. Reoxidati on of the lead mantle by Ag+ ions leads to a silver mantle, although a bout 5% of the deposited lead is not oxidized; this is attributed to t he strong binding of Pb adatoms to the gold surface. The reoxidation o f Pb adatoms by Ag+ occurs much more slowly than the oxidation of indi vidual Pb(n) particles, which is attributed to the more electropositiv e nature of the deposited metal. - Au-Cd particles tend to agglomerate , especially when the Cd mantle is reoxidized by Ag+ ions. The resulti ng Au-Ag particles contain a large percentage of non-oxidized cadmium; alloy formation seems to be a possible explanation. - The spectra of Au-Tl particles are not in good agreement with the calculated ones; it seems that the reduced Tl does not form an even mantle around the Au( n) particles.