Monodisperse colloids of transition metal and lanthanide compounds
Monodisperse colloids are regularly shaped particles, often spheres, all of nearly the same size. Monodispersity is achieved by creating a short period of nucleation, after which no new particles are formed, but the particles continue to grow. Examples which have been known for a long time include gold sols, sulfur sols, and polymer latices. Recently, conditions have been found to prepare monodisperse colloids of metal hydroxides and oxides. Monodisperse powders are not necessarily good ceramic precursors, because the packing density of uniform spheres is low. The correct proportion of large and small particles to create a high packing density could be preferable. Hence it is desirable to control the growth of monodisperse colloids so that particles of a particular size can be produced. To control the size of the particles it is necessary to control the length of the growth time relative to the nucleation period. Monodisperse hydroxides of trivalent transition metal ions and of lanthanide ions have been produced by thermally forced hydrolysis of aqueous solutions. Calcination of these colloids can lead to oxide particles of the same spherical shape. This approach is limited, however, because the only controllable variables are the temperature, time, and initial conditions. More flexible is the generation of a critical reagent by a chemical reaction. Various monodisperse hydroxides and carbonates have been formed by the thermal hydrolysis of urea. This is an example of homogeneous precipitation, a procedure widely used in analytical chemistry to obtain precipitates of desirable properties. By introducing this additional homogeneous chemical reaction, further control of the systems can be obtained. Some examples include coprecipitation to obtain potential catalyst precursors of precious metals on alumina, coprecipitation to generate powders for high-temperature superconducting materials, and preparation of monodisperse lanthanide oxides for various applications.