Enhanced visible-light absorption of TiO2 with different concentrations of N, Co, or Na monodopants is explored with the use of density functional theory with Hubbard U on-site correction. The effects of dopant concentrations on the absorption are determined. The results show that the absorption efficacy of TiO2 can be tuned into the visible-light range by substituting the Ti atoms with N atoms, in which a strong peak of red-light absorption is found at the concentration of 4.38%. Substituting Co or Na at Ti atom site can also enhance visible-light absorption, and significant effect is achieved when the Co and Na concentrations reach 18.45% and 7.20%, respectively. The mechanism is elucidated on the basis of the electronic structure. We used Na, when we investigated the effects of doping methods on the absorption coefficient and electronic properties. The results proved helpful information for designing and preparing solar energy materials based on doped TiO2. (C) 2014 Elsevier B.V. All rights reserved.