Light-induced formation of silver particles and clusters in crosslinked PVA/PAA films

Thin films able to sustain an efficient photoreduction of Ag+ ions with 350 nm photons in air were prepared by crosslinking poly(vinyl alcohol) with glutaraldehyde in the presence of poly(acrylic acid). Standard colloid techniques served as screening methods for the selection of polymer compositions yielding films with desired properties. When present at high concentrations, Ag+ ions were reduced at room temperature in the films by poly(vinyl alcohol) but poly(acrylic acid) inhibited the slow dark reaction. Optical signals with maxima above 400 nm resulted from both reduction processes. The available evidence confirmed that they originated from nanometer-sized metal particles and is inconsistent with results for other proposed chromophores. An additional absorption centered at 280 nm that formed only under illumination was assigned to Ag-3(+) clusters. Small Ag crystallites with similar size distributions and with an average diameter of 5 nm were the main product of the photoreduction in non-crosslinked or crosslinked films. Larger particles were detected less frequently, and in the former films they consisted predominantly of crystallite aggregates. These results along with the longterm stability of the photogenerated Ag-3(+) clusters are consistent with a particle nucleation process based on diffusion and coalescence of mobile metal atoms in the films.