Structure and Band Alignment of InP Photocatalysts Passivated by TiO2Thin Films

Composite materials made by a low-band-gap semiconductor covered by a thin oxide layer are increasingly used in solar cells and photocatalysis. InP/TiO2 heterojunctions show high efficiency because of visible light absorption by the InP component and an efficient migration of photogenerated electrons toward TiO2. The passivating TiO2 film also guarantees stability in humid environments. In this work, we investigate the nature of InP/TiO2 by means of density functional theory calculations, showing that due to strong interface chemical bonds, the formation of an InP/TiO2 junction is a favorable process. Irrespective of the anatase TiO2 surface in contact with InP, (101) or (001), the system behaves as a type II junction, where the TiO2 band edges are lower in energy than InP ones, providing a rationalization for the observed electron-hole separation. We also considered explicitly the role of the TiO2 thin layer thickness, finding that a favorable junction is retained for every thickness of the titania passivating film. The highest efficiency is predicted for film thicknesses of 2-3 nm as this provides a good compromise between favorable band edge alignment and efficient transfer of the photogenerated electrons to the surface.