Resolving the Controversy about the Band Alignment between Rutile and Anatase: The Role of OH-/H+ Adsorption

The synergic effect of mixing ruffle (R) and anatase (A) crystals in photo catalysis is often attributed to a staggered alignment of the band structures, but it is widely disputed whether the conduction band edge of ruffle or that of anatase is higher. Photoelectron spectroscopy (PES) supports the former, and flat-band potential (FBP) measurements the latter. Theoretical alignment of the bulk band structures as well as calculated offsets across actual interfaces support the PES data. The theoretical study presented here shows that the FBP data can be explained by taking into account the adsorption of OH- and H+ ions in the electrolyte solution. We conclude that, in the case of nanopowders applied in photocatalysis, the dipole layer created by surface adsorption and the one at the R/A interface both have an influence on the alignment, and the end result may change from experiment to experiment.