ABSTRACT
Direct interactions of aqueous metal salt solutions with bases failed, as a rule, to yield uniform particles; thus, it was necessary to design some other approaches to achieve such dispersions. The fundamental ideas were based on the fact that seldom, if ever, can a metal ion in an aqueous solution be directly converted to an oxide. Instead, the initial steps involve the formation of metal hydroxides or oxyhydroxides. Either these products remain stable as precipitated, or they convert to oxides on aging by processes that may take different lengths of time but that can be accelerated by various dehydration methods. Consequently, metal ion hydrolysis
To produce uniform colloids by precipitation, it is necessary to control several stages in the process, including nucleation and particle growth. In principle, a short burst of nuclei, followed by the capture of constituent solute species, should yield monodispersed particles. This appealing concept was originally proposed by LaMer to describe the formation ofmonodispersed sulfur hydrosols (6), and it was generally accepted by the scientific community active in the field. More recently, it has been recognized in numerous cases that the formation of monodispersed colloids is a more involved process; that is, the nuclei grow to nanosized particles, which then aggregate in a controlled manner into the final uniform colloids (7). Recently, a model that accounts for such a mechanism was developed (8), and is described later in this chapter.
