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