Increasing the optical response of TiO₂ and extending it into the visible region through surface activation with highly stable Cu₅ clusters

The decoration of semiconductors with subnanometer-sized clusters of metal atoms can have a strong impact on the optical properties of the support. The changes induced differ greatly from effects known for their well-studied, metallic counterparts in the nanometer range. In this work, we study the deposition of Cu₅ clusters on a TiO₂ surface and investigate their influence on the photon-absorption properties of TiO₂ nanoparticles via the computational modeling of a decorated rutile TiO₂ (110) surface. Our findings are further supported by selected experiments using diffuse reflectance and X-ray absorption spectroscopy. The Cu₅ cluster donates an electron to TiO₂, leading to the formation of a small polaron Ti³⁺ 3d¹ state and depopulation of Cu(3d) orbitals, successfully explaining the absorption spectroscopy measurements at the K-edge of copper. A monolayer of highly stable and well fixated Cu₅ clusters is formed, which not only enhances the overall absorption, but also extends the absorption profile into the visible region of the solar spectrum via direct photo-induced electron transfer and formation of a charge-separated state.