Aging effect on vanadium oxide hybrid nanotubes: A XANES study

MARTÍN E. SALETA; DINA TOBIA; SANTIAGO J. A. FIGUEROA; LÓPEZ, CARLOS A; MARA GRANADA; RODOLFO SÁNCHEZ; MARCOS MALTA; ROBERTO TORRESI

Virtual

Congreso; 30th Annual Users´ Meeting (RAU) LNLS/CNPEM; 2020

Vanadium oxides present a rich magnetic phase diagram depending on the oxidation state of the V ions. In particular the vanadium oxide nanotubes (VOx NTs) are multiwall nanostructures constituted by alternating layers of VOx and a surfactant. The V-ions are in different oxidation states: +4 (3d1) and +5 (3d0). This coexistence of oxidation states, with different magnetic behavior, confers 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 x NTs. For this complete characterization, we used electron spin resonance (ESR) and dc-susceptibility techniques, which were supplemented with TEM microscopy. By X-ray absorption near edge spectroscopy (XANES) we calculated the amount of V in +4 and +5 oxidation state employing the Wong method. Wong and co-workers correlated the oxidation state and local environment of V compounds to spectral features of XANES signal [1]. In VOx NTs we observed that for aging in normal environmental conditions of pressure, temperature and humidity, the V4+ ions oxidize to V5+. 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 V4+/V5+ relationship in the VO x NTs, which may contribute to explain the significant dispersion of data reported in the bibliography [2]. [1] J Wong et al., Phys. Rev. B 30 (1984) 5596?610[2] M.E. Saleta et al., J. Phys.: Condens. Matter 31 (2019) 505701