Boron-Doped Anatase TiO2: Pure and Hybrid DFT Calculations

We report the results of density functional theory calculations on the electronic structure of boron-doped anatase TiO2. For the calculations we used both standard and hybrid exchange-correlation functionals. On the basis of a close comparison of computed and measured observable properties, in particular hyperfine coupling constants from electron paramagnetic resonance experiments and core level binding energies from X-ray photoelectron spectroscopy spectra, we propose the existence of both substitutional to oxygen and interstitial boron atoms in the bulk of anatase. Boron substitional to oxygen results in a paramagnetic defect, [BTi3](center dot), which introduces new states in the midgap of the material. The analysis of the stability of this center, however, shows that it should not be the dominant species and should convert into interstitial boron after annealing at high temperature. Various possible sites for interstitial boron have been considered where the dopant is coordinated to three, [BO3], or to four, [BO4], oxygen atoms. The electronic characteristics of interstitial boron are rather independent of the site where the atom is incorporated. Boron in interstitial positions behaves as a three-electron donor with formation of B3+ and reduction of Ti4+ to Ti3+. The possible copresence of substitutional and interstitial boron could also result in internal charge transfer leading to more complex situations.