Formation of an intermediate band in the energy gap of TiO2 by Cu-N-codoping: First principles study and experimental evidence

First principles calculations are performed to study the electronic and optical properties of Cu-doped, N-doped and (Cu+2N)-co-doped anatase TiO 2. Strong hybridization between Cu 3d and N 2p orbitals above the valence band leads to the formation of an isolated intermediate band (IB) deep in the band gap of pure TiO2. The new energetic features, predicted by DFT, have been experimentally confirmed using UV-vis-NIR spectroscopy. In particular, the diffuse reflectance spectrum of the co-doped TiO2 sample shows the presence of two edges which confirm the existence of IB in the band gap. This IB in the band gap of TiO2 is responsible for high visible light absorption through a two-step optical transition between valence and conduction bands via the IB. In mono-doped samples, only a reduction of the band gap is observed which is consistent with the first-principles calculations. X-ray photoelectron spectroscopy of mono- and co-doped TiO2 samples establishes the chemical states of several atomic elements and especially clarifies the key presence of O-vacancies leading to a new position of conduction band minima. The presence of broad IB and the absence of dopant energy levels close to the conduction band minimum in (Cu+2N)-co-doped TiO 2 qualify it to be an efficient material for photovoltaic conversion, photocatalytic water splitting, and photocatalytic degradation of pollutants.