Electrons and protons : a synergy in cation chemistry and in the development and reactivity of suspended nanostructures

This seminar illustrates the coupling between acid-base and redox exchanges involved in the formation of oxide nanoparticles in solution, as well as in structural and/or morphological transformations initiated by reactions at the oxide-solution interface. These phenomena are of prime importance not only in materials chemistry, but also in the environment, through biogeochemical processes involved, for example, in the mineralogy of aqueous media, in the cycling of various elements, and in water treatment. In these various fields, iron and manganese are among the major elements involved. We briefly reviewed the mechanism of electron exchange reactions between soluble complexes and the influence of acidity on the kinetics and thermodynamics of electron exchange (potential-pH diagram). The combination of the two reactions was illustrated by various examples concerning the synthesis of manganese oxide nanoparticles, with control of crystal structure and particle morphology. We then turned our attention to acid-base reactions on the surface of iron oxide nanoparticles that induce electronic and/or ionic transfer across the solid-solution interface. Three very different cases were examined: the role of ferrous ions in the crystallization of ferrihydrite into spinel, the structural and partially reversible transformation of magnetite into maghemite under various circumstances, and finally the morphological transformation of hematite by ferrous ion adsorption. The difference in the behavior of magnetite and hematite is essentially due to the electronic nature of the solid: conductor or semiconductor.