Formation and Reduction of Semiconductor‐Like Aggregates of Silver‐Carboxy‐Alkane‐Thiolates in Aqueous Solution

In slightly acidic solutions of Ag + and carboxy‐methane‐thiol (CMT) or carboxy‐ethane‐thiol (CET), aggregates are formed whose absorption spectrum is significantly different from that of the monomeric silver‐carboxy‐alkane‐thiols. The build‐up of the aggregates occurs slowly. They develop a band at 335 nm (AgCMT) or 355 nm (AgCET), and simultaneously, acquire fluorescence. Silver is produced upon the photolysis of the aggregates. A very small amount of silver atoms deposited on the aggregates is sufficient to quench the fluorescence substantially. Both the absorption and fluorescence bands are blue‐shifted as the aggregates become smaller during photolysis or dissolution in alkaline solution. The aggregates are proposed to have semiconductor‐like ionic planes separated from each other by the organic groups. Electron microscopy shows that the aggregates are partly crystalline and have a size of about 25 Å when they are freshly prepared. The crystallinity increases and the structure changes upon aging for several days.—When Ag + is reduced by NaBH 4 in the presence of carboxy‐alkane‐thiols, stable silver aggregates of different sizes are formed. The absorption bands and chemical behaviour of some of the aggregates resemble those of oligomeric silver clusters investigated previously. The stability of the clusters is explained in terms of “pre‐complexation” of silver surface atoms by the carboxy‐thiols.