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
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Abstract
The nucleation and growth of vanadium oxide nanotubes
(VO<sub><i>x</i></sub>-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 <sup>51</sup>V NMR and flame spectroscopy. For short durations of the
hydrothermal synthesis, the initial V<sub>2</sub>O<sub>5</sub>-surfactant
intercalate is progressively transformed into VO<sub><i>x</i></sub>-NT whose crystallization starts to be detected after a hydrothermal
treatment of 24 h. Upon heating from 24 h to 7 days, VO<sub><i>x</i></sub>-NT are obtained in larger amount and with an improved
crystallinity. The detection of soluble amines and cyclic metavanadate
[V<sub>4</sub>O<sub>12</sub>]<sup>4–</sup> in the supernatant
solution along the hydrothermal process suggests that VO<sub><i>x</i></sub>-NT result from a dissolution–precipitation
mechanism. Metavanadate species [V<sub>4</sub>O<sub>12</sub>]<sup>4–</sup> could behave as molecular precursors in the polymerization
reactions leading to VO<sub><i>x</i></sub>-NT