Presence of Gap States at Cu/TiO<sub>2</sub> Anatase Surfaces: Consequences for the Photocatalytic Activity

Copper-modified titania is a system of interest for its potential for photocatalytic applications in the production of solar fuels. Still, the role of copper in the process is unclear. In this work, small copper clusters on the (101) and (100) surfaces of anatase have been investigated by first-principles simulations based on density functional theory, to shed light on their atomic and electronic structure, and to understand their effect on the photocatalytic process. The main effects of copper on the electronic structure are to provide states above the edge of the valence band of titania and to lead to the formation of midgap states. There are two types of midgap states, respectively, associated with direct Cu–Ti bonds and to Ti<sup>3+</sup> polarons. The latter are the result of charge donation from copper and lie in the vicinity of the surface. Moreover, the copper tetramer (Cu<sub>4</sub>) displays empty states at the bottom of the conduction band that play a key role in accommodating excess electrons. We discuss how these features should enhance the photoresponse of TiO<sub>2</sub>, contribute to increase the lifetime of the photogenerated electron–hole pairs and contribute to increase the activity of this material for CO<sub>2</sub> reduction, a key step in the photoproduction of hydrocarbons.