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 1972 launched four decades of intensive research into the underlying chemical and physical processes involved. Despite much collected evidence, a thoroughly convincing explanation of why mixed-phase samples of anatase and rutile outperform the individual polymorphs has remained elusive. One long-standing controversy is the energetic alignment of the band edges of the rutile and anatase polymorphs of TiO2 (ref.). 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.
Bibliographical noteFunding Information:
The work presented here made use of the UCL Legion HPC Facility, the IRIDIS cluster provided by the EPSRC-funded Centre for Innovation (EP/K000144/1 and EP/K000136/1), and the HECToR supercomputer through our membership of the UK’s HPC Materials Chemistry Consortium, which is funded by EPSRC grant EP/F067496. The work in Dublin was supported by SFI through the PI programme (PI grant numbers 06/IN.1/I92 and 06/IN.1/I92/EC07), and made use of the Kelvin supercomputer as maintained by TCHPC. A.W. acknowledges support from the Royal Society for a University Research Fellowship and EU-FP7 under grant agreement 316494. D.O.S. and C.W.D. are grateful to the Ramsay Memorial Trust and University College London for the provision of their Ramsay Fellowships. D.O.S., R.G.P. and A.W. acknowledge membership of the Materials Design Network.