Photogenerated carrier dynamics at the anatase/rutile TiO2 interface

$\mathrm{Ti}{\mathrm{O}}_{2}$ is an intensively studied photocatalytic material owing to its low cost and high activity. The anatase/rutile (A/R) mixed-phase $\mathrm{Ti}{\mathrm{O}}_{2}$ is recognized as an effective strategy to achieve high photocatalytic efficiency by the type-II band alignment favorable to spatial charge separation. However, the atomic structure, as well as the exact band alignment of the A/R mixed-phase $\mathrm{Ti}{\mathrm{O}}_{2}$, is very difficult to identify either in experimental measurements or theoretical simulations. Moreover, the time-dependent photogenerated carrier dynamics, which can determine the photocatalytic efficiency, has not been studied at the atomic scale. In this paper, we use an adaptive genetic algorithm to search the stable interface structures. We find that the band alignment is determined by the interfacial atomic structures. Especially, with oxygen vacancy $({\mathrm{O}}_{\mathrm{V}})$ at the interface, band alignment can be reversed as compared to that of the stoichiometric interface. Then, we select one stoichiometric and one defective structure to study the photogenerated carrier dynamics using the time-dependent ab initio nonadiabatic molecule dynamics. We find that in the stoichiometric system, for both the electron and the hole, the charge transfer happens within 400 fs, which is much shorter than the electron-hole recombination timescale at nanosecond-to-microsecond magnitude, which suggests that the charge transfer can occur efficiently at the interface before they recombine. For the defective A/R system with ${\mathrm{O}}_{\mathrm{V}}$, we find that the electron will be trapped by the defect state within 1 ps, while the hole dynamics is not affected. Our study provides atomic insights into the understanding of the band alignment and photogenerated carrier dynamics at the mixed A/R $\mathrm{Ti}{\mathrm{O}}_{2}$ interface, which provides valuable guidance for functional material design for solar energy conversion.