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

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 I4(1)/amd space group. The average crystallite size as estimated by the Scherrer's equation varies in the range of similar to 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 (E-g) 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 E-g indicate inverse dependence with the Urbach energy (E-u) 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.