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

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

A Comprehensive Study of the Mechanism of Formation of Polyol-Made Hausmannite Nanoparticles: From Molecular Species to Solid Precipitation

This study aims at achieving a better understanding of the mechanisms of formation of Mn₃O₄ nanoparticles prepared by the polyol process. The role of each reactant is studied, and a possible scheme of reaction is proposed, involving the activation of dioxygen by Mn­(II) species. The growth of the particles (evolution of the size and concentration of particles) has been followed in solution by SAXS, and the results have been compared to those obtained by other techniques on dried powders. The results indicate a decrease of the number of particles in solution with time together with their enlargement. A stabilization of the size and number of particles is reached after a few hours. The shape of the particles then evolves into a truncated ditetragonal-dipyramidal polyhedron

Storage of Visible Light for Long-Lasting Phosphorescence in Chromium-Doped Zinc Gallate

ZnGa₂O₄:Cr³⁺ presents near-infrared long-lasting phosphorescence (LLP) suitable for in vivo bioimaging. It is a bright LLP material showing a main thermally stimulated luminescence (TSL) peak around 318 K. The TSL peak can be excited virtually by all visible wavelengths from 1.8 eV (680 nm) via d–d excitation of Cr³⁺ to above ZnGa₂O₄ band gap (4.5 eV–275 nm). The mechanism of LLP induced by visible light excitation is entirely localized around Cr_N2 ion that is a Cr³⁺ ion with an antisite defect as first cationic neighbor. The charging process involves trapping of an electron–hole pair at antisite defects of opposite charges, one of them being first cationic neighbor to Cr_N2. We propose that the driving force for charge separation in the excited states of chromium is the local electric field created by the neighboring pair of antisite defects. The cluster of defects formed by Cr_N2 ion and the complementary antisite defects is therefore able to store visible light. This unique property enables repeated excitation of LLP through living tissues in ZnGa₂O₄:Cr³⁺ biomarkers used for in vivo imaging. Upon excitation of ZnGa₂O₄:Cr³⁺ above 3.1 eV, LLP efficiency is amplified by band-assistance because of the position of Cr³⁺⁴T₁ (⁴F) state inside ZnGa₂O₄ conduction band. Additional TSL peaks emitted by all types of Cr³⁺ including defect-free Cr_R then appear at low temperature, showing that shallower trapping at defects located far away from Cr³⁺ occurs through band excitation