A new metastable phase of needle-like nanocrystalline VO2 center dot H2O and phase transformation

Needle-like nanocrystalline VO2 . H2O, crystallized in a new metastable pha se with orthorhombic symmetry (a=6.015, b = 4.642, and c = 3.763 Angstrom), was synthesized by a long hydrothermal reaction from NH4VO3 and hydrazine. The structure and composition of these nanocrystallites were characterized by X-ray powder diffraction, transmission electron microscopy, selected ar ea electron diffraction, infrared absorbency spectroscopy, and thermogravim etric analysis. The influence of chemical reaction conditions on the featur es of the final products was investigated, and the optimum conditions were found to be 170 degreesC for 15 days with a N2H4 concentration of 8.0%. Dif ferential scanning calorimetry was conducted to study the phase transformat ion from as-prepared metastable VO2 . H2O to the more stable (B)VO2 and rut ile-tvpe VO2. (C) 2001 Academic Press.