DFT Insights into the Role of Relative Positions of Fe and N Dopants on the Structure and Properties of TiO2.

The location and nature of the doped elements strongly affect the structural, electronic and optical properties of TiO2. To tailor the band structure and modify the photoelectrochemical properties of TiO2, a pair of dopants is selected. Fe and N atoms are inserted in the TiO2 network at substitutional and interstitial sites with different relative distances. The main objective behind the different locations and sites of the doped elements is to banish the isolated unoccupied states from the forbidden region that normally annihilates the photogenerated carriers. Fe at the Ti site and N at the O site doped in the TiO2 network separated at a distance of 7.805 Å provided a suitable configuration of dopant atoms in terms of geometry and band structure. Moreover, the optical properties showed a notable shift to the visible regime. Individual dopants either introduced isolated unoccupied states in the band gap or disturbed the fermi level and structural properties. Furthermore, the other co-doped configurations showed no remarkable band shift, as well as exhibiting a suitable band structure. Resultantly, comparing the band structure and optical properties, it is argued that Fe (at Ti) and N (at O) doped at a distance of 7.805 Å would strongly improve the photoelectrochemical properties of TiO2.

Electronic Band Structure Variations in the Ceria Doped Zirconia: A First Principles Study.

Using first principle calculations, the effect of Ce with different doping concentrations in the network of Zirconium dioxide (ZrO2) is studied. The ZrO2 cell volume linearly increases with the increasing Ce doping concentration. The intrinsic band gap of ZrO2 of 5.70 eV reduces to 4.67 eV with the 2.08% Ce doping. In 4.16% cerium doped ZrO2, the valence band maximum and conduction band minimum come closer to each other, about 1.1 eV, compared to ZrO2. The maximum band gap reduction of ZrO2 is observed at 6.25% Ce doping concentration, having the value of 4.38 eV. No considerable shift in the band structure is found with further increase in the doping level. The photo-response of the ZrO2 is modulated with Ce insertion, and two distinct modifications are observed in the absorption coefficient: an imaginary part of the dielectric function and conductivity. A 2.08% Ce-doped ZrO2 modeled system reduces the intensities of peaks in the optical spectra while keeping the peaks of intrinsic ZrO2. However, the intrinsic peaks related to ZrO2 completely vanish in 4.16%, 6.25%, 8.33%, and 12.5% Ce doped ZrO2, and a new absorption hump is created.