Synthesis of polyoxovanadates via “chimie douce”

Abstract

A large variety of new polyoxovanadates have been synthesized during the past few years by sol–gel chemistry or hydrothermal methods. These wet chemistry methods offer many advantages compared to the usual solid state syntheses. New open structures have been obtained from aqueous precursors. They result from the self-assembling of ionic species in the solution. Vanadium oxide gels and sols, V 2O 5· nH 2O, are formed around the point of zero charge (pH≈2). They have a ribbon-like structure and exhibit a liquid crystal behavior. These mesophases are similar to those currently observed with nematic polymers. Xerogel layers deposited from V 2O 5· nH 2O gels exhibit some preferred orientation and behave as versatile host structures for intercalation giving new hybrid organic–inorganic nanocomposites. Layered structures are formed around pH≈7 in the presence of large organic cations. They are built of mixed valence polyoxovanadate planes made of [VO 5] pyramids and [VO 4] tetrahedra. Organic cations lie between the oxide layers where they interact with the negative oxygen of the VO double bonds. Anions can behave as templating agents. Hollow cluster shells are formed around anions that remain encapsulated within the negatively charged polyvanadate cage. Large cations only behave as counter ions for the formation of a neutral crystalline network. It appears that the molecular structure of V V precursors depends mainly on pH, but the way they self-assemble may be governed by other ionic species in the solution.