Size Quantization in Layered HgI2 Colloids

Colloidal particles of HgI₂ with a particle size less than about 25 Å are formed by the reaction of HgCl₂ with NaI in acetonitrile. The particles are charged with Cl^- ions that fill normally empty tetrahedral coordination sites at the edges; therefore they have an empirical formula HgI₂Cl_0.6^0-6. About 40% of the mercury in the colloidal solution is in the form of the ionic complex HgI₂Cl^-. These conclusions are based on extensive spectroscopic studies of various Hg-I-Cl species and experiments involving ultracentrifugation, ultrafiltration, and treatment of the colloids with ion-exchange resins. The absorption spectrum of the colloidal HgI₂ consists of three peaks at 4.26, 4.94, and 6.04 eV, and is attributed to size quantization effects. This spectrum is consistent with the first three allowed transitions in a simple particle-in-a-box model with infinite potential barriers in which the colloidal particles have the usual tetrahedral, layered structure of red HgI₂, with dimensíons of 26.1 Å perpendicular to the layer plane (four layers thick) and 13.3 Å in the direction parallel to the layer plane.