We investigate the electronic structures and optical properties of Cu-doped TiO2-x, using density functional theory (DFT) calculations. We find that Cu doping creates an empty impurity band lying above the valence band of TiO2, which bridges most of the gap between the valence band maximum and conduction band minimum in undoped TiO2. The results also show that oxygen vacancies produce a similar effect and that Cu-doping can increase the stability of oxygen vacancies. We propose that Cu-doped TiO2-x improves light harvesting and could be a promising visible-light photocatalyst. (C) 2014 Elsevier B.V. All rights reserved.