Formation Mechanisms of Uniform Colloid Particles*

The relationship of the shape and structure of uniform colloidal particles to different mechanisms of formation by precipitation in homogeneous solutions is reviewed. Specifically, conditions leading to (a) amorphous spheres, (b) nonspherical crystals of different shapes, and (c) polycrystalline particles of different morphologies including spheres are distinguished. The last case is observed when larger particles are formed by aggregation of nanosized precursors. It is now recognized that the latter process is more common than previously understood. A model explaining the size selection of monodispersed spheres by aggregation is illustrated in the case of silver particles. It was also demonstrated that uniform polycrystalline particles of other shapes are generated by the aggregation process. Challenges faced in the effort to develop at least a semi-quantitative explanation of the shape factor in the latter case are outlined.