TEMPERATURE-MEDIATED PHASE TRANSFORMATION AND OPTICAL PROPERTIES OF TUNGSTEN OXIDE NANOSTRUCTURES PREPARED BY FACILE HYDROTHERMAL METHOD

Abstract

Different tungsten oxide nanocrystals were synthesized via a facile hydrothermal process – one step and free of additives-at different reaction temperatures and in a highly acidic environment. The phase transformation of samples, followed by the change of morphology and optical band gap, was observed as the reaction temperature varied from room temperature (23˚C) to 220˚C. The crystal phase transformed from monoclinic WO3 ·2H2O to orthorhombic WO3 ·H2O, then to monoclinic WO3 when the reaction temperature increased from room temperature to 100˚C, then to 220˚C, respectively. Corresponding to the phase transformation, the optical band gap increased from 2.43 eV to 2.71 eV, and the morphology varied from nanoplate to nanocuboid. The effect of the reaction temperature on the phase transformation was assigned to the dehydration process, which became stronger as the reaction temperature increased. These results gave an insight into the phase transformation and implied a simple method for manipulating the crystal phase and morphology of tungsten oxide nanostructures for various applications.