New-phase VO2 micro/nanostructures: investigation of phase transformation and magnetic property

New-phase VO2 micro/nanostructures built by nanoflakes have been first synthesized by a hydrothermal method with NH4VO3 as precursor in the presence of poly(vinyl pyrrolidone) (PVP). The combined structural analysis of X-ray powder diffraction (XRD) and X-ray absorption fine structure (XAFS) spectroscopy determined the crystal structure as a new-phase vanadium dioxide, which is the isostructure of monoclinic NiWO4 and designated as VO2(D). In particular, electron spin resonance (ESR) measurements provides the direct evidence of vanadium ion in the four oxidation state. The formation energy of VO2(D) was estimated and showed a very close value to rutile-type VO2(R), which guided the preparation of VO2(R/M) by making use of the structural transformation from VO2(D) to VO2(R) at 320 °C, which was a comparatively lower temperature than other vanadium oxide, such as VO2(B). The obtained VO2(R) shows the reversible metal-to-insulator transition (MIT) near critical temperature (Tc) which is associated with clear changes in differential scanning calorimetry (DSC) curves. In addition, the temperature-dependent DC electrical conductivity of the new-phased VO2(D) exhibits Arrhenius-type behaviour, which reveals its semiconducting character with a band gap of 0.33 eV. ESR and temperature-dependent magnetic susceptibility measurements were employed to obtain information on the magnetic properties of VO2(D), which correspond to one-dimentional Heisenberg system.