Ammonium hexadeca-oxo-heptavanadate microsquares. A new member in the family of the V7O16 mixed-valence nanostructures

Abstract

This paper presents a new mixed valence heptavanadate (NH₄)₂V₇O₁₆, obtained from a two-stage treatment in a single container of ammonium metavanadate with hexadecylamine in an acetic medium. The hydrolysis of the precursor under normal conditions leads to an intermediate, layered hybrid nanocomposite, V₂O₅/alkylamine, which after an in situ hydrothermal treatment is transformed almost quantitatively into an amine-free microcrystalline phase constituted by perfect square microparticles of (NH₄)₂V₇O₁₆. The analysis of composition, structure and morphology of the product points to a mixed valence vanadium oxide with a high content of V(IV) (approximately 73%). The microsquares have a tetragonal structure similar to that of BaV₇O₁₆, as well as to those proposed for the VO_x/amine hybrid nanocomposite series: nanotubes, nano urchins and the compound (en)V₇O₁₆. The results suggest that all these phases belong to the V₇O₁₆ family, but that they differ in the amine content, the degree of reduction and the curvature of the network. The feasibility of obtaining flat nanostructures based on V₇O₁₆ without templates, beyond demonstrating the robustness of the structural unit V₇O₁₆ in networks with different degrees of reduction and curvatures, clarifies the role of alkylamines in this type of anti-entropic supramolecular process. First, the amine provides a stable platform that allows for an orderly reduction of the network under hydrothermal conditions and, when the medium favours the stability of the V–amine bond, the alkylamine contributes to the driving force that leads to the curving of the V–O network.