Effects of charge transport on polarization of strained TiO2 : A first-principles investigation

Ferroelectric materials have potential applications in surface catalysis and photocatalysis due to the electron-hole separation induced by the built-in field. On the other hand, both the adsorption-induced charge transfer and the photogenerated free carriers influence the stability of the ferroelectric polarization. Therefore an in-depth understanding of the interaction between the charge transfer and the polarization is of the essence. In this paper, we take rutile TiO₂ under applied strain as an example to study the effects of charge transfer and charge redistribution on polarization since it is known as a high-performance photocatalytic material as well as an incipient ferroelectric material. We consider charge transfer or charge redistribution induced by surface adsorption and photoexcitation and as a function of on-site Coulomb interaction. We find that the polarization generally decreases if charge transfer occurs in the ferroelectric materials, independent of the origin of the charge transfer or charge redistribution. The pseudo-Jahn-Teller theory is successfully adopted to explain the interaction between the charge transfer and macroscopic polarization. These findings give a clear physical picture of the effect of charge transfer and charge redistribution on the stability of polarization, which could contribute to the understanding of the ferroelectric catalytic and photocatalytic materials.