Band alignment of rutile and anatase TiO 2

The most widely used oxide for photocatalytic applications owing to its low cost and high activity is TiO2. The discovery of the photolysis of water on the surface of TiO2 in 19721 launched four decades of intensive research into the underlying chemical and physical processes involved2–5. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive6. One longstanding controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO2 (ref. 7). We demonstrate, through a combination of state-of-the-art materials simulation techniques and X-ray photoemission experiments, that a type-II, staggered, band alignment of ∼0.4 eV exists between anatase and rutile with anatase possessing the higher electron affinity, or work function. Our results help to explain the robust separation of photoexcited charge carriers between the two phases and highlight a route to improved photocatalysts. A general consensus places the bandgaps of rutile and anatase TiO2 at 3.03 and 3.20 eV, respectively. In 1996, electrochemical impedance analysis established that the flatband potential of anatase is ∼0.2 eV more negative than that of rutile, indicating that the conduction band of anatase lies 0.2 eV above that of rutile8. This band alignment, illustrated in Fig. 1a, would favour the transfer of photogenerated electrons from anatase to rutile, and the transfer of holes from rutile to anatase at a clean interface (although the valence band positions in this alignment are very similar) and was supported by several experiments9–11. Alternatively, recent photoemissionmeasurements have reported that the work function of rutile is 0.2 eV lower than that of anatase, placing the conduction band of anatase 0.2 eV below that of rutile12 (Fig. 1b). Electron paramagnetic resonance experiments focusing on mixed rutile/anatase samples have demonstrated that electrons flow from rutile into anatase, with holes moving in the opposite direction13–16. These studies have provided information on the interface (for example, a newly discovered interfacial trapping site, lattice and surface electron trapping sites, and surface hole trapping sites) and on recombination in these mixed samples14,15. The fundamental band alignment between anatase and rutile, which is necessarily the driving force for the kinetics of both ionic and electronic charge carriers, however, is still not understood. The intrinsic band alignment will always act as the boundary conditions imposed on a particular interface, and will be a dominant factor in any photocatalytic activity.