A diffusion Monte Carlo study of point defect diffusion in rutile TiO₂ bulk

TiO₂ is one of the most popular photocatalysts. The diffusion of point defects (Ti interstitial/O vacancy) is responsible for charge transport. We investigated the diffusion mechanism of the point defects in rutile TiO₂ bulk using diffusion Monte Carlo (DMC). There are two issues related to the point defect diffusion: (a) The diffusion of positively charged defects from the bulk inside to the (101) surface promotes the surface oxidation reaction. Then, which type of defect transfers positive charges faster? (b) There are two known diffusion paths for Ti interstitial defects, one parallel to the c-axis and the other perpendicular. In which direction are the defects easier to diffuse? Our DMC calculations established that (a) Ti interstitial defects transfer a larger amount of positive charges to the surface and (b) the primary diffusion direction of Ti interstitial defects changes from `perpendicular to c-axis' to `parallel', along with the change of defect charge, 0 to +4. Both conclusions qualitatively support the previous GGA-DFT work. Yet, the barrier energy prediction is quantitatively much different by ~2eV at the maximum. We will discuss this huge difference and justify the barrier energies predicted by DMC, based on Bader charge analysis and electronic density prediction.