Defect generation, d-d transition, and band gap reduction in Cu-doped TiO2 nanoparticles

TiO 2 doped with Cu 2+ initiates the formation of brookite phase along with anatase. Doping of Cu 2+ introduces structural defects into TiO 2 . The direct evidence is the low intense and broad diffraction peaks. Raman peaks of doped TiO 2 are also broad and are blueshifted. Pure TiO 2 exhibits an absorption in the UV region, the position of which is shifted towards the visible region on incorporation of Cu into it. The visible absorption peaks arise due to the d – d transition of Cu 2+ in the crystalline environment of TiO 2 . Incorporation of Cu 2+ distorts the local structure of TiO 2 , resulting in the loss of octahedral symmetry surrounding Cu 2+ . The Jahn-Teller distortion splits the 2 E g and 2 T 2g state of Cu 2+ into several d states. Interaction of light excites the electron from ground to several of the excited states and gives the visible absorption peaks in the framework of TiO 2 . These Cu 2+ d states and oxygen defects create band states, thereby favoring electronic transition to these levels and resulting in lowering of band gap of TiO 2 . A direct confirmation is the increase in the magnitude of Urbach energy with the reduction in the band gap of doped TiO 2 .