Synthesis and Phase Stability of Metastable Bixbyite V₂O₃ Colloidal Nanocrystals

We have recently developed a colloidal route to vanadium sesquioxide (V 2 O 3 ) nanocrystals with a metastable bixbyite crystal structure. In addition to being one of the first reported observations of the bixbyite phase in V 2 O 3, it is also one of the first successful colloidal syntheses of any of the vanadium oxides. The nanocrystals, measuring 5 to 30 nm in diameter, possess a flower-like morphology which densify into a more spherical shape as the reaction temperature is increased. The bixbyite structure was examined by X-ray diffraction and an aminolysis reaction pathway was determined by Fourier transform infrared spectroscopy. A direct band gap of 1.29 eV was calculated from optical data. Under ambient conditions, the structure was found to expand and become less distorted, as evidenced by XRD. This is thought to be due to the filling of structural oxygen vacancies in the bixbyite lattice. The onset of the irreversible transformation to the thermodynamically stable rhombohedral phase of V 2 O 3 occurred just under 500 °C in an inert atmosphere, accompanied by slight particle coarsening. A critical size of transformation between 27 and 42 nm was estimated by applying the Scherrer formula to analyze XRD peak widths during the course of the transformation. The slow kinetics of transformation and large critical size reveal the remarkable stability of the bixbyite phase over the rhombohedral phase in our nanocrystal system.