Effect of calcination temperature on phase transformation, structural and optical properties of sol–gel derived ZrO2 nanostructures

Abstract

Zirconia (ZrO2) nanostructures of various sizes have been synthesized using sol–gel method followed by calcination of the samples from 500 to 700°C. The calcined ZrO2 powder samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infra-red spectroscopy (FT-IR), UV–visible spectroscopy (UV–vis.), Raman spectroscopy (RS) and thermogravimetric analysis (TGA). The phase transformation from tetragonal (t) to monoclinic (m) was observed. The average diameter of the ZrO2 nanostructures calcined at 500, 600 and 700°C was calculated to be 8, 17 and 10nm, respectively. The ZrO2 sample calcined at 500°C with tetragonal phase shows a direct optical band gap of 5.1eV. The value of optical band gap is decreased to 4.3eV for the ZrO2 calcined at 600°C, which contains both tetragonal (73%) and monoclinic (27%) phases. On further calcination at 700°C, where the ZrO2 nanostructures have 36% tetragonal and 64% monoclinic phases, the optical band gap is calculated to be 4.8eV. The enhancement in optical band gap for ZrO2 calcined at 700°C may be due to the rod like shape of ZrO2 nanostructures. The tetragonal to monoclinic phase transformation was also confirmed by analyzing Raman spectroscopic data. The TG analysis revealed that the ZrO2 nanostructure with dominance of monoclinic phase is found to be more stable over the tetragonal phase. In order to confirm the phase stability of the two phases of ZrO2, single point energy is calculated corresponding to its monoclinic and tetragonal structures using density functional theory (DFT) calculations. The results obtained by theoretical calculations are in good agreement with the experimental findings.