Ex Situ X‑ray Diffraction, X‑ray Absorption Near Edge Structure, Electron Spin Resonance, and Transmission Electron Microscopy Study of the Hydrothermal Crystallization of Vanadium Oxide Nanotubes: An Insight into the Mechanism of Formation

The nucleation and growth of vanadium oxide nanotubes (VO_<i>x</i>-NT) have been followed by a combination of numerous ex situ techniques along the hydrothermal process. Intermediate solid phases extracted at different reaction times have been characterized by powder X-ray diffraction, scanning and transmission electron microscopy, electron spin resonance, and V–K edge X-ray absorption near-edge structure spectroscopy. The supernatant vanadate solutions extracted during the hydrothermal treatment have been studied by liquid ⁵¹V NMR and flame spectroscopy. For short durations of the hydrothermal synthesis, the initial V₂O₅-surfactant intercalate is progressively transformed into VO_<i>x</i>-NT whose crystallization starts to be detected after a hydrothermal treatment of 24 h. Upon heating from 24 h to 7 days, VO_<i>x</i>-NT are obtained in larger amount and with an improved crystallinity. The detection of soluble amines and cyclic metavanadate [V₄O₁₂]^4– in the supernatant solution along the hydrothermal process suggests that VO_<i>x</i>-NT result from a dissolution–precipitation mechanism. Metavanadate species [V₄O₁₂]^4– could behave as molecular precursors in the polymerization reactions leading to VO_<i>x</i>-NT