Aging effects on magnetic vanadium oxide nanotubes

MARTÍN E. SALETA; SANTIAGO J. A. FIGUEROA; MARA GRANADA; HORACIO E. TROIANI; RODOLFO D. SÁNCHEZ; MARCOS MALTA; ROBERTO M. TORRESI

Barcelona

Conferencia; Internarional Conference on Magnetism; 2015

The vanadium oxide (VOx) nanotubes (NTs) are multiwall nano-structures constituted by alternating layers of VOx and a surfactant. The V-ions are in different oxidation states: the paramagnetic +4 (3d1) and the diamagnetic +5 (3d0). This coexistence of V oxidation states, with different magnetic behavior, gives rise to interesting and promising physical and chemical properties. Due to these characteristics the doped with other transition metal ions and undoped VOx-NTs, have demonstrated to be very versatile for possible technological-industrial applications such as cathode Li-ion and/or catalysis. In both applications the performance of the material is strongly associated with the ratio V4+/V5+ present in the nanostructure. Consequently, it is important to quantify the fraction of V ions and study different alternatives to vary these proportions. This allows to "tune" the relationship V4+/V5+ suitable for each application. The possible use of these nanostructures as a technological component, such as batteries cathode, implies that the physico-chemical properties do not change across the time. The study of aging effects results primordial for possible applications and consequently the stability of the initial fraction V4+/V5+ in time.In this work we describe the results of a systematic study of the time evolution of the magnetic properties of VOx-NTs. For this completely characterization, we used electron spin resonance and dc-susceptibility techniques, which were supplemented with TEM microscopy and XANES.We observed that aging in normal environment conditions of pressure, temperature and humidity, that the +4 ions oxidize to +5. Although the multiwall tubular structure is maintained, this oxidation process produces a markedly change in its magnetic properties.Finally, this work allowed us to bring light to the question about the spread dispersion of data reported in VOx-NTs. In the scientific literature some authors reported in their quantifications around 50% of V4+, while others informed values close to 14%.