Conclusion

Metal oxide nanostructures are of major interest in technology. It is therefore es­sential to have a full understanding of the phenomena involved in the aqueous synthesis of nanoparticles, so that their properties can be adjusted to a desired application. Understanding these phenomena is also important in other fields, for instance, in geology and environmental sciences, enabling us to explain the presence and formation of a given mineral. The precipitation of metal oxy(hydroxi)des is a complex phenomenon initiated by hydroxylation of the cations in solution and resulting from condensation of the hydroxylated species. Therefore, the acidity of the cations is the main charac­teristic of their reactivity. Three main parameters are essential in predicting and rationalizing the behavior of metal cations in water: the formal charge (the oxida­tion degree), size, and electronegativity, which determine the degree of polariza­tion of the oxygenated ligands. One may thus define five classes of cations: . . . The too weakly polarizing cations that form only aquocomplexes unable to condense and to precipitate; for instance, the alkaline cations M+, The cations that condense by olation and form polycations and hydroxides, typically, the divalent cations and also Al3+. The cations that condense by olation and oxolation and form oxyhydroxides and oxides (such as Cr3+, Fe3+, and Mn3+). The cations that condense essentially by oxolation and form oxides that are more or less hydrated (Ti4+, Mn4+, V5+). The cations that form anionic oxocomplexes and exhibit no trend toward condensation, typically, MnVII. . . . This series thus includes cations of increasing polarizing power, that is cations of increasing oxidation degree and electronegativity. Precipitation usually generates nanosized particles. In a system that is not con­tinuously fed, in which a limited amount of matter is available in the reactor, the nucleation step is always sudden and easy enough, allowing lower supersaturation and creating nuclei that have stopped growing because of the too low concentra­tion in soluble precursor. That does not, however, exclude an intense dynamics of dissolution–crystallization because of the evolution of the criticality of the particle size during the decrease in supersaturation.