Size-Dependent Level Alignment between Rutile and Anatase TiO2 Nanoparticles: Implications for Photocatalysis.

Motivated by the enormous importance of nanoscale TiO2 in a wide range of photocatalytic applications and the ill-understood high activity of the commercial nano-TiO2 photocatalysts, we provide a predictive map of how the anatase-rutile level alignment varies from the smallest nanoparticles (NPs) to the bulk. Specifically, we compute the size dependence of the energies of vacuum-referenced electronic levels in a range of realistically structured rutile and anatase TiO2 NPs employing accurate all-electron density functional calculations. In agreement with most recent work, a staggered type II anatase level alignment is predicted for the bulk phases, which we further find to persist into the regime of large NPs. We predict that other level alignments will emerge when the diameter of the TiO2 NPs is reduced below ∼15 nm. Our results suggest how experiment could test the widely debated importance of the bulk-like type II anatase level alignment for enhanced photoactivity of nano-TiO2.