Tuning the surface states of TiO2 using Cu5 atomic clusters

• In the ambient environment, the adsorbed oxygen changes the most stable Cu5 atomic quantum cluster (AQC) isomer from a 2-dimensional trapezoidal structure to a 3-dimensional bipyramidal structure. • The bipyramidal Cu5 AQC creates two ferromagnetically coupled polarons in the TiO2 while the trapezoidal Cu5 AQC only creates a single polaron. • In the presence of oxygen, these polaronic states associated with the bare cluster are joined by further mid-gap energy levels, formed from hybrid states located on copper and oxygen atoms. The ability of Cu5 AQCs to create states within the bulk gap of TiO2 under ambient conditions demonstrates that the absorption spectrum of the AQC-TiO2 complex is more closely matched to the peak of the solar spectrum, than either the AQC or TiO2 alone. If the ability of Cu 5 atomic quantum clusters (AQCs) to create states within the bulk gap of TiO 2 persists in the presence of oxygen, then the absorption spectrum of the AQC-TiO 2 complex could be matched to the peak of the solar spectrum, thereby enhancing its photocatalytic activity under ambient conditions. Here we demonstrate that this is indeed the case, by examining the electronic structure of Cu 5 AQCs adsorbed on a rutile (110) TiO 2 surface in the presence of oxygen. We find that adsorbed oxygen changes the most stable AQC isomer from a 2-dimensional trapezoidal structure to a 3-dimensional bipyramidal structure. Furthermore, the bipyramidal AQC creates two ferromagnetically coupled polarons in the TiO 2 . In contrast, the trapezoidal AQC only creates a single polaron. In the presence of oxygen, these polaronic states associated with the bare cluster are joined by further mid-gap energy levels, formed from hybrid states located on copper and oxygen atoms. Dissociated oxygen-dimer adsorption on Cu 5 generally leads to significant deformation of the whole cluster, which accounts for the dramatic drop of the total energy compared to molecular oxygen dimer adsorption. The ability of Cu 5 AQCs to create states within the bulk gap of TiO 2 under ambient conditions demonstrates that the absorption spectrum of the AQC-TiO 2 complex is more closely matched to the peak of the solar spectrum, than either the AQC or TiO 2 alone.