Electronic structure of defect states in hydroxylated and reduced rutile TiO2(110) surfaces

It has been experimentally observed that a bridging oxygen vacancy on the rutile TiO2(110) surface introduces localized Ti3+ 3d1 states about 1 eV below the conduction band which are not removed upon dissociation of a water molecule and formation of a pair of hydroxyl groups. Density functional calculations based on pure exchange-correlation functionals have not been able to satisfactorily reproduce and analyze these findings. Here we show that a correct description of the localized defect states on reduced and hydroxylated TiO2(110) is achieved only if proper geometry relaxation is accounted for using hybrid exchange functionals. We confirm the electron trapping nature of Ti(OH) groups but find no evidence that these defects should also act as hole traps by formation of Ti4+(OH)* radicals.