ELECTROCHEMISTRY OF COLLOIDAL SILVER PARTICLES IN AQUEOUS-SOLUTION - DEPOSITION OF LEAD AND INDIUM AND ACCOMPANYING OPTICAL EFFECTS

Colloidal silver particles (ca. 8 nm in diameter) act as microelectrodes in aqueous solution. They are charged via electron transfer from free radicals which are generated radiolytically. The stored electrons reduce adsorbed Pb2+ and In3+ ions. - The first monolayer of lead atoms deposited on the silver particles does not absorb at 215 nm where metallic lead has an absorption band. The Pb atoms cause the 380 nm surface plasmon band of the silver particles to be shifted to shorter wavelengths. This is explained in terms of electron donation from chemisorbed Pb atoms to the silver carrier. The Pb atoms of a submonolayer (which are formed by "under-potential" deposition) do not react with methyl viologen, MV2+, whereas the lead atoms deposited in subsequent layers ("Nernst potential" deposition) are readily oxidized by MV2+. The absorption spectrum of colloidal lead in water is also discussed (epsilon(max) = 3.2 . 10(4) M-1 cm-1 at 215 nm). - Reduction of In3+ on the silver particles also leads to a blue-shift of the plasmon band. It is attributed to electron transfer from deposited In atoms. However, as In metal deposition proceeds, In+ is produced by the conproportionation of the indium metal with In3+. When almost all the In3+ ions have been reduced, indium metal is further deposited by In+ reduction. Silver particles carrying a tight indium shell absorb at 225 nm with epsilon = 3.8 - 10(4) M-1 cm 1. This epsilon-value is somewhat lower than the calculated one, and attributed to a reduction in mean free path for conduction electrons in small In colloids.