Unravelling the Impact of Ta Doping on the Electronic and Structural Properties of Titania: A Combined Theoretical and Experimental Approach

The introduction of new energy levels in the forbidden band through the doping of metal ions is an effective strategy to improve the thermal stability of TiO2. In the present study, the impact of Ta doping on the anatase to rutile transition, structural characteristics, and anion and cation vacancy formation were investigated in detail using density functional theory and experimental characterization, including X-ray diffraction, Raman, Brunauer-Emmett-Teller surface area, UV-vis diffuse reflectance spectroscopy, high-resolution transmission electron spectroscopy, and X-ray photoelectron spectroscopy. The average crystallite size of TiO2 decreases with an increase in the Ta concentration. At high temperatures, more oxygen atoms enter the crystal lattice and occupy the vacancies, leading to lattice expansion. Importantly, we find that Ta doping preserved the anatase content of TiO2 up to annealing temperatures of 850 °C which allows anatase stability to be maintained at typical ceramic processing temperatures. The substitution of Ti4+ by the Ta5+ ions increased the electron concentration in the crystal lattice through formation of Ti3+ defect states. Raman studies revealed the formation of new Ta bonds via disturbing the Ti-O-Ti bonds in the crystal lattice. It is concluded that under the oxidizing conditions the Ta5+ ions could be enhanced on the Ta-TiO2 surface due to slow diffusion kinetics.