Optical properties of anatase and rutile titanium dioxide : ab initio calculations for pure and anion-doped material

The optical properties of rutile and anatase titanium dioxide (TiO2) are calculated from the imaginary part of the dielectric function using pseudopotential density functional method within its generalized gradient approximation (GGA) and a scissors approximation. The fundamental absorption edges calculated for the unit cell of both rutile and anatase are consistent with experimentally reported results of single crystal rutile and anatase TiO2 and with previous theoretical calculations. A significant optical anisotropy is observed in the anatase structure which holds promise for investigating the band gap modification with better visible-light response and provides a reliable foundation for addressing the effect of impurities on the fundamental absorption edge/band gap of anatase TiO2. Further calculations on the electronic structure and the optical properties of C-, N-, and S-doped anatase TiO2 are performed. The results are analyzed and discussed in terms of optical anisotropy and scissors approximations.