A New Metastable Phase of Needle-like Nanocrystalline VO2·H2O and Phase Transformation

Needle-like nanocrystalline VO2·H2O, crystallized in a new metastable phase with orthorhombic symmetry (a=6.015, b=4.642, and c=3.763 Å), was synthesized by a long hydrothermal reaction from NH4VO3and hydrazine. The structure and composition of these nanocrystallites were characterized by X-ray powder diffraction, transmission electron microscopy, selected area electron diffraction, infrared absorbency spectroscopy, and thermogravimetric analysis. The influence of chemical reaction conditions on the features of the final products was investigated, and the optimum conditions were found to be 170°C for 15 days with a N2H4concentration of 8.0%. Differential scanning calorimetry was conducted to study the phase transformation from as-prepared metastable VO2·H2O to the more stable (B)VO2and rutile-type VO2.