Role of Cr doping in tuning the optical and dielectric properties of TiO2 nanostructures

Abstract

A series of chromium doped titania samples having compound formula Ti1-xCrxO2 (0≤ x ≤ 0.2) were synthesized through cost-effective acid modified sol-gel process. These samples are characterized through several analytical techniques to explore their microstructural phase, optical and dielectric properties. Rietveld refined x-ray diffraction (XRD) patterns confirm the monophase nature with tetragonal crystal structure in anatase phase having I41/amd space group. The average crystallite size as estimated by the Scherrer's equation varies in the range of ~9–14 nm on doping of Cr ions in TiO2. Structural parameters such as lattice constants, bond length (Ti–O), bond angle <Ti–O–Ti> and x-ray density are found to change as a function of Cr doping. The infrared spectroscopy ensures bending/stretching vibrational modes associated with the functional groups at different wavenumbers. Energy bandgap (Eg) as determined using Tauc's relation varies from 3.10 to 3.75 eV with Cr ions substitution in TiO2 matrix that can be ascribed to the creation of oxygen vacancies/defects in the host lattice. However, a fall in the intensity of photoluminescence as observed on doping of Cr ions is associated with the alteration in oxygen vacancies which are directly responsible for the electron-hole recombination rate. The values of Eg indicate inverse dependence with the Urbach energy (Eu) with the increase in dopant ions. The dielectric measurements as a function of frequency exhibit enhancement in the real and imaginary parts of dielectric constant for the same compositions. A deviation from the universal dielectric response (UDR) mechanism is observed at the lower frequencies for all the samples. Additionally, Cole-Cole plots divulge the non-Debye type of relaxation mechanism and ac conductivity follows the Jonscher power law.