A first principles investigation of Bi₂O₃-modified TiO₂ for visible light Activated photocatalysis: The role of TiO ₂ crystal form and the Bi³⁺ stereochemical lone pair

Modification of TiO₂ with metal oxide nanoclusters is a novel strategy for the design of new photocatalysts with visible light activity. This paper presents a first principles density functional theory (DFT) analysis of the effect of modifying TiO₂ rutile (110) and anatase (101) and (001) surfaces with Bi₂O₃ nanoclusters on the band gap and the nature of the photoexcited state. We show that band gap modifications over unmodified TiO₂ depend on the crystal form: modifying rutile (110) results in new Bi₂O₃ derived states that shift the valence band upwards. On anatase surfaces, there is little effect due to modification with Bi₂O₃ nanoclusters, but an enhanced UV activity would be expected. Analysis of electron and hole localisation in a model photoexcited state shows enhanced charge separation in Bi₂O₃-modified rutile (110) but not in Bi₂O₃-modified anatase. The effect of the Bi³⁺ lone-pair on the properties of Bi₂O ₃-modified TiO₂ contrasts with SnO-modified TiO ₂, consistent with the weaker lone pair in Bi₂O ₃ compared with SnO.