SILVER ATOMS AND CLUSTERS IN AQUEOUS-SOLUTION - ABSORPTION-SPECTRA AND THE PARTICLE GROWTH IN THE ABSENCE OF STABILIZING AG+ IONS

The absorption spectra of Ag0 and Ag2+, the first products of silver ion reduction in aqueous solution, are redetermined using pulse radiolysis. Ag0 absorbs at 360 nm (1.6 X 10(4) M-1 cm -1), i.e., at a longer wavelength than the free atom in vacuo. The great width of 0.91 eV of the absorption band is explained by a strong interaction of the atom in the excited state with the aqueous solvent. The absorption spectrum of Ag2+ has two peaks (at 310 and 265 nm). The equilibrium Ag0 + Ag+ reversible Ag2+ has a free enthalpy < -0.36 eV, and from this value the hydration free energy of Ag2+ is calculated to be more negative than -3.6 eV. Approximate values for electrochemical standard potentials of Ag2+ are also given. The reaction 2Ag2+ --> Ag4(2+) occurs with 2k = 2.6 X 10(9) M-1 s-1. As the absorption band of Ag4(2+) disappears, the 355- and 325-nm absorption bands of long-lived oligomeric clusters appear. However, ionic strength studies indicate that Ag4(2+) is not the immediate precursor of these clusters, as an additional step of growth occurs in between. At high doses in the pulse, practically all the silver ions are reduced, when the solution contains formate. Neutral intermediates of the growth of the particles, such as Ag2 and Ag4, can be detected, and the rate of particle growth is not influenced by added electrolytes. The oligomeric clusters are not long-lived under these conditions, and the large silver particles formed tend to agglomerate rapidly.