Abstract
Density functional theory simulations are used to investigate the formation and mobility of Ti interstitial ions, Tii, at the (110) surface of rutile TiO2. Interstitials were found to be favoured in the second layer below the surface plane, where they induce electron polaron states at surface and subsurface lattice Ti atoms. Reduction of the surface significantly lowers the barrier for Tii formation at the surface: the barrier for formation of Tii is reduced to just ∼0.5 eV for a Ti atom next to two bridging oxygen vacancies. However, the barrier to separate the interstitial from the surface oxygen vacancies is ∼2.5 eV. The bulk diffusion barrier is recovered after the interstitial is moved away from the vacancy complex. These results support an experimentally postulated mechanism of Tii formation and contribute to our understanding of the TiO2 surface reduction and reoxidation.
Highlights
Titanium dioxide (TiO2) nds applications in many different areas, such as paints and coatings,[1,2] catalysis,[3,4] optical instruments,[5] solar cells[6,7] and gas sensors.[8,9] Its high refractive index is exploited in sunscreen and to make white pigments
The results show that the barrier for formation of Ti interstitials (Tii) is reduced to just $0.5 eV for a Ti atom next to two bridging oxygen vacancies
As the interstitial is moved inside the slab, these states move closer in energy and become degenerate and recover the polaron state of Tii in the bulk
Summary
Titanium dioxide (TiO2) nds applications in many different areas, such as paints and coatings,[1,2] catalysis,[3,4] optical instruments,[5] solar cells[6,7] and gas sensors.[8,9] Its high refractive index is exploited in sunscreen and to make white pigments. Formation of Ti interstitials and their diffusion into the bulk crystal The inverse of this process has been studied experimentally: reoxidation studies show that surface adsorbed oxygen from the gas phase can react with Tii from the bulk to grow new strands,[32,39] islands[23] and complete 1 Â 1 layers of TiO2,16,32,38 indicating that the bulk crystal behaves as a defect reservoir. The results show that the barrier for formation of Tii is reduced to just $0.5 eV for a Ti atom next to two bridging oxygen vacancies In this con guration the energy of the interstitial site is lower than that of the Ti atom at its lattice site. Tii quickly recovers a bulk diffusion barrier as it moves away from the Ti vacancy (vTi) inside the sample
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