SURFACE-CHEMISTRY OF COLLOIDAL GOLD - DEPOSITION OF LEAD AND ACCOMPANYING OPTICAL EFFECTS

Lead ions were reduced on the surface of colloidal gold particles (190 angstrom in diameter) and the resulting bimetallic particles were investigated using spectrophotometry and electron microscopy. Deposition of lead adatoms causes the plasmon absorption band of gold to be shifted to shorter wavelengths, which is explained by Pb --> Au electron donation and double layer charging. Three monolayers of lead are sufficient to produce the plasmon band of lead. Mie theory calculations of the optical spectrum (using the bulk optical constants of Au and Pb) yielded excellent agreement with the observed spectra for lead mantles thicker than 1 monolayer. The deposited lead is oxidized by oxygen, except for the first monolayer, which is produced by underpotential deposition. Similarly, lead atoms in the first layer are not oxidized by methyl viologen, whereas the atoms in the subsequent layers readily react. Experiments on the preparation of trimetallic particles (gold nucleus, lead layer, and outer cadmium layer) are also reported.