Abstract:
TiO 2 has many applications for example in opto-electronic devices and therefore complementary theoretical investigations are important to explore the full potential of this material. The structural properties and electronic band structures of TiO 2 (Rutile and Anatase), Cr:TiO 2 and Nb:TiO 2 have been investigated using ab initio methods. The structural properties were obtained using both local density approximation (LDA) and generalized gradient pproximation (GGA) employing pseudopotentials and plane wave basis sets. For the Rutile phase of TiO 2 , the calculated band structure, cohesive energy, equilibrium lattice constant and bulk modulus were found to be in good agreement with other recent calculations and also with experimental data. Compared with the Rutile phase, the Anatase had similar ground-state properties except for a large band gap of 2.28 eV while Rutile had a
band gap of 1.89 eV. It was found that Anatase had a bulk modulus of 171.4 GPa, which was smaller than that of Rutile by 18.9 Gpa and hence softer. The calculated O-Cr bonds lengths were between 3.81 Bohr and 3.946 Bohr, being slightly stretched with respect to the Ti-O bonds lengths in pure Rutile (3.749 Bohr and 3.81 Bohr) while for Anatase, the calculated O-Cr bonds were between 3.65 Bohr and 3.72 Bohr, which were slightly stretched with respect to the Ti-O bonds in pure Anatase (3.58 Bohr and 3.65 Bohr). On doping the Rutile structure with Cr and Nb atoms, there was introduction of new states within the band gap, principally between 8.67 eV and 10.56 eV. These new states were located between 6.663 eV and 8.939 eV in the Anatase structure. It was also realized that during the 2% doping with Cr and Nb, there were fewer new states in the band gap compared to many new states realized during the 4% doping and this happened in both phases, that is, Rutile and
Anatase. This shows that a higher doping concentration of 4% results in more
energy states and hence more carriers, than in 2% thus making TiO 2 a better
conductor than either 2% doping or pure TiO 2 . Also after doping TiO 2 (Anatase and Rutile) with either Cr or Nb at 2% and 4%, it was established that there was removal of the band gap implying improved conductivity rather significantly compared to pure TiO 2 . More donor bands were observed with the 4% than the 2% doping, which also implies improved electrical conductivity.
