TiO2 is a key material for photocatalytic water splitting, where it has been found that samples composed of mixtures of the anatase and rutile polymorphs outperform the pure phase samples. Several explanations for this observation were proposed with no consensus being reached until, using the ChemShell code to perform hybrid QM/MM embedded cluster calculations, we showed that it is a consequence of the fundamental band alignment between the two phases. Because of the relative positions of the valence and conduction band edges of both polymorphs, the effective band gap is reduced and charge separation occurs on photoexcitation that favours strongly the photocatalytic water splitting process. In a follow up study, we analysed the band alignment among the eight known polymorphs of TiO2, finding that the band edges can vary surprisingly widely, simply as a consequence of the difference in crystal structure among the phases. Our results helped explain many experimental observations that have consequences for energy storage, photocatalysis and optoelectronic applications.
D. O. Scanlon et al., “Band alignment of rutile and anatase TiO2“, Nature Materials, 2013, 12, 798; J. Buckeridge et al., “Polymorph Engineering of TiO2: Demonstrating How Absolute Reference Potentials Are Determined by Local Coordination“, Chem. Mater., 2015, 27, 3844.