Long-lived nonmetallic silver clusters in aqueous solution: Preparation and photolysis

Radiolytic reduction of Ag+ ions in aqueous solution and reduction by sodium borohydride in the presence of polyphosphate leads to long-lived nonmetallic silver clusters besides particles of metallic silver (having a narrow plasmon absorption band at 380 nm) and quasi-metallic silver (having a blue-shifted and broadened plasmon absorption band). The best conditions for the preparation of cluster solutions are described, and a mechanism of cluster formation is discussed. The Ag42+ cluster formed in the first stages of Ag+ ion reduction lives for many hours even in the presence of air. It is oxidized by H2O2 and K2S2O8. Its great stability is explained on the basis of the oscillatory behavior of the standard redox potential of silver microelectrodes at very small agglomeration numbers as described previously. Larger clusters with sharp absorption bands at 300, 330, and 345 nm (absorption coefficients of more than 104 M-1 cm-1) are formed in the later stages of Ag+ reduction. They are very sensitive toward air and thermally less stable than Ag42+. The absorptions of these clusters are tentatively attributed to CTTS transitions