Aqueous methods for the synthesis of colloidal metal oxide nanoparticles at ambient pressure

Abstract Colloidal metal oxide nanocrystals are important building blocks for the fabrication of nanostructured metal oxides for applications in biology, chemistry, energy, the environment, and electronics. Water as a solvent has many benefits. It is chemically stable, abundant, safe, and green. The formation of a complex metal cation precursor is generally the onset for the growth of metal oxide nanoparticles in aqueous solutions. Once the precursors are present and reach the supersaturated level, the condensation reaction takes place rapidly and forms nuclei that are the seed material for the subsequent growth of nanoparticles. The particle size and morphology depend on the stability and solubility of the nanoparticles. The secondary growth process that occurs during the aging process through the dissolution-crystallization mechanism can control the nanoparticle size and size distribution. The nanoparticles could also aggregate and subsequently proceed into the gel status via the aging process or, on the other hand, could remain as a stable sol status, which depends on the physicochemical condition of the solution. In this chapter, we discuss the nucleation, growth, stabilization, and the effect of fluid flow on the synthesis of metal oxide nanoparticles. An example for the synthesis and assembly of ZnO using a microreactor-assisted chemical process is included at the end of the chapter as an example of scalable manufacturing of metal oxide nanocrystals, nanostructures, and nanostructured thin films.