The gel route to transition metal oxides

Abstract

The so-called “sol-gel” process offers new approaches to the synthesis of transition metal oxides. Based on inorganic polymerization from molecular precursors, it leads to highly condensed species or colloids. These colloids are actually two-phase systems in which small oxide particles are dispersed in a liquid medium. A very large interface separates both phases and interfacial phenomena, at the oxide-water interface, lead to new features in the physics and chemistry of transition metal oxides. Ordered aggregation of oxide particles may occur, giving rise to colloidal crystals or anisotropic tactoids in which the mean distance between particles can be of about 0, 1 μm. This distance can be decreased leading to ordered solid aggregates. Transition metal oxide gels exhibit the physical properties of both phases, i.e., electronic properties arising from electron hopping through the mixed valence oxide network and ionic properties arising from proton diffusion through the liquid phase. Electronic and ionic properties appear to be strongly related through the very large interface. Large coatings can be easily deposited from colloidal solutions and transition metal oxide gels should be very useful for making microionic devices.