Origin of the type-II band offset between rutile and anatase titanium dioxide: Classical and quantum-mechanical interactions between O ions

Abstract

Titanium dioxide is one of the most promising semiconductors for photocatalytic splitting of water for hydrogen. The mixed rutile/anatase system shows even more favorable photocatalytic properties than the pristine ones. Band offset is a key factor that determines the photocatalytic activity of the mixed phase. However, the type of band alignment and the value of the band offset are still under debate both experimentally and theoretically. The difficulty of determining the band offset by commonly used core-level alignment calculation lies in the different symmetry and large lattice mismatch between the two phases. Here, we adopt our recently developed three-step method, which can overcome the lattice mismatch problem, to study the band offset with high accuracy. In the calculation, we used an intermediate phase $\mathrm{Ti}{\mathrm{O}}_{2}\mathrm{II}$ to build superlattice models of rutile(101)//$\mathrm{Ti}{\mathrm{O}}_{2}\mathrm{II}(001)$ and $\mathrm{Ti}{\mathrm{O}}_{2}\mathrm{II}(100)$//anatase(112) to determine the core-level alignment. Our studies show a type-II, staggered band alignment, with the valence band maximum (VBM) of rutile 0.80 eV above that of anatase, in agreement with recent experimental results. We further analyzed the electronic structure of the two phases, and found that the band offsets of the VBM originate from both the electrostatic interaction and electronic hybridization in rutile and anatase, which contribute 0.36 eV and 0.44 eV, respectively.