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Vanadium oxides present a rich magnetic phase diagram depending on the oxidation state of the V ions. In particular the vanadium oxide nanotubes (VO<sub>x</sub> NTs) present several promising perspectives for different technological applications for which it is essential to know the oxidation state of V ions, as well as to evaluate the stability with the aging time of the tubes. In this work we present a systematic study of the time evolution of the magnetic properties of VO<sub>x</sub>NTs. For this complete characterization, we used electron spin resonance (ESR) and dc-susceptibility techniques, which were supplemented with TEM microscopy and XANES. We observed that for aging in normal environmental conditions of pressure, temperature and humidity, the V<sup>4+</sup>ions oxidize to V<sup>5+</sup>. Although the multiwall tubular structure is maintained, this oxidation process produces a marked change in the magnetic properties. We conclude that the aging of the samples affects the V<sup>4+</sup>/ V<sup>5+</sup>relationship in the VO<sub>x</sub> NTs, which may contribute to explain the significant dispersion of data reported in the bibliography.&#13.
PMID: 31469102 [PubMed - as supplied by publisher]
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