Effect of structural and thermal disorder on the optical band gap energy of Cr₂O₃ nanoparticles

Chromium oxide (Cr₂O₃) nanoparticles have been synthesized at different pH values by using co-precipitation technique. As prepared nanoparticles are then structurally characterized using x-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscope (HR-TEM), fourier transform infrared (FT-IR) and micro-Raman spectroscopy. XRD, micro-Raman, HRTEM and fourier transform infrared (FT-IR) spectra confirmed the formation of single phase rhombohedral structure of Cr₂O₃ nanoparticles. Optical properties of Cr₂O₃ nanoparticles have been studied using UV–vis absorption spectroscopy. Tauc's relation was used to determine the optical band gap energy of Cr₂O₃ nanoparticles and found to be decreasing with the increase of crystallite size. This phenomenon is explained in terms of Urbach energy. Urbach energy has been calculated to account for disorder and found to be increasing from 569 to 934 meV as pH value increased from 6 to 10, respectively. Debye Waller factor and electron-phonon interactions of Cr₂O₃ nanoparticles have also been investigated in this present study, to explain observed discrepancy between experimental and theoretical determined optical band gap values. (paper)