VO₂(B) nanorods: solvothermal preparation, electrical properties, and conversion to rutile VO₂ and V₂O₃

The solvothermal reduction of V₂O₅ by formaldehyde or isopropanol yields nanorods of the metastable, monoclinic VO₂(B) phase. The structural transition in VO₂(B), which occurs near room temperature, has been monitored using electrical resistivity measurements, performed both on pressed pellets of the nanorods as well as on nanorods dispersed on patterned contacts. A sudden, 10⁵ increase in the electrical resistivity upon cooling below 290 K is seen in measurements on VO₂(B) samples. Such a transition in the electrical resistivity has not previously been reported in this material. The transition is reminiscent of the metal-to-insulator transition observed in the case of pressed pellets of polycrystalline rutile VO₂ upon cooling below 340 K. The metastable VO₂(B) nanorods are converted to rutile VO₂ by heating in argon, and to corundum V₂O₃ by reducing in 5%H₂:95%N₂. In both transformations, the structural integrity of the nanorods is compromised, with large, dense, rutile VO₂ crystallites and less well-defined nanorods of V₂O₃ being formed.