Arrested precipitation in solution
Controlled precipitation reactions can yield dilute suspensions of quasi monodispersed particles. This happens with the use of seeds of very small particles for the subsequent growth of larger ones. Small crystallites are less stable than larger ones as the stability of the initially small crystallites formed is influenced by the dynamic equilibrium and tend to dissolve into their respective ions. Subsequently, the dissolved ions recrystallize on larger crystallites, which are thermodynamically more stable which process is termed as Ostwald ripening. The use of acetonitrile, as a solvent, or the addition of styrene/maleic anhydride copolymer allows the preparation of stable CdS nanoparticles, with an average size of 34 and 43 Å, respectively. Cubic ZnS and CdS nanocrystallites can be synthesized in aqueous and methanolic solutions without organic surfactant (capping agent).
SiO2 synthesis
SiO2 nanoparticles can be formed by the hydrolysis of TEOS in ethanol containing ammonia after 24 h reaction time. Silica powders are prepared by precipitation in ethanol by a precipitation technique based on controlled hydrolysis of a silicon alkoxide (tetraethylorthosilicate, TEOS) in a mixture of ethanol, aqueous ammonia (21%) and water. The porogen used 3-aminopropyltriethoxysilane (APTES) can be mixed to the TEOS prior to adding to the solvent mixture.