Optical properties of anatase TiO₂: synergy between transition metal doping and oxygen vacancies

Charge carriers (electrons and holes) are generated on the TiO₂ using UV radiation; this excitation energy can be reduced by modifying the material electronic structure, for example, by doping or creating oxygen vacancies. Here, the electronic structure of a transition metal-doped anatase, bulk and surface, and their interaction with oxygen vacancies are studied using density functional theory. The visible light response of metal-doped TiO₂ (101) is also determined. Transition metals generate intra-band gap states, which reduce the excitation energy but may also act as charge recombination sites. Dopants Fe, Co, and Ni remarkably enhance the visible light response due to the states in the middle of the gap. However, Co and Ni create heavier charge carriers. Our results show that Pd and Pt-doped TiO₂ generate states near the valence and conduction band with a "clean" band gap (without states in the middle of the gap). Moreover, Pt-doped TiO₂ maintains the charge mobility because it presents a small charge carriers mass. Hence, Pt-doped TiO₂ represents the best alternative to activate TiO₂ under visible light. The optical response of transition metal-doped TiO₂ follows the order 3d > 4d > 5d. The oxygen vacancies reduce the response of metal-doped TiO₂ to visible light because the unpaired electrons generated occupy the empty states of metal-doping. Graphical Abstract Density of states of TiO₂ (101) surface doped with transition metals and oxygen vacancies.