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.