Er-doped TiO2 thin films have been prepared by dual-frequency magnetron co-sputtering with two targets composed of TiO2 and Er2O3. The structural and optical properties of these thin films were investigated using various characterization techniques. X-ray photoelectron spectroscopy measurements show that the content of Er increases when the power on Er2O3 target increases. X-ray diffraction patterns indicate that the anatase TiO2 peaks become weaker with the increasing power on Er2O3 target and the films tend to form amorphous structure. Scanning electron microscopy results show that the grain size decreases with the rise in Er content. UV-vis spectrophotometric analyses reveal that the values of band gap decrease from 335 13% eV to 3.02 13% eV with the increasing amount of Er from 0 at% to 32 at%. The red shift in the optical adsorption edge may contribute to.a higher photocatalytic activity of TiO2 thin films. (C) 2017 Elsevier B.V. All rights reserved.