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.