COMPOSITE PD-AG PARTICLES IN AQUEOUS-SOLUTION

Colloidal composite particles consisting of a nucleus of palladium and a mantle of silver are prepared via the reduction of silver ions on t he surface of Pd particles (mean radius: 4.6 nm) with formaldehyde. Th e absorption spectra of the particles coated with Ag shells of differe nt thicknesses are compared to the calculated ones (using Mie theory a nd the dielectric data of the bulk materials). The particles possess a surface plasmon absorption band close to 380 nm when more than 10 mon olayers are deposited. Where the shell thickness is less than 10 monol ayers, the absorption band is located at shorter wavelengths. The band disappears below about three monolayers. The calculated position of t he band agrees with the observed one, although the intensity of the ba nd is smaller than expected from theory. The electron mean free path i n the shell is much smaller than the value predicted by Granqvist and Hunderi. Chemisorbed SH- ions damp the plasmon band, even when the she ll is only four monolayers thick. This damping effect is less pronounc ed than for pure silver particles. Damping also takes place when the i odide ion adsorbs to the colloidal particles.