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 i on reduction in aqueous solution, are redetermined using pulse radioly sis. 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 i n the excited state with the aqueous solvent. The absorption spectrum of Ag2+ has two peaks (at 310 and 265 nm). The equilibrium Ag0 + Ag+ r eversible 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 ap pear. However, ionic strength studies indicate that Ag4(2+) is not the immediate precursor of these clusters, as an additional step of growt h occurs in between. At high doses in the pulse, practically all the s ilver ions are reduced, when the solution contains formate. Neutral in termediates of the growth of the particles, such as Ag2 and Ag4, can b e 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.