A series of nitrogen/Ti3+ co-doped titanium dioxides (N-TiO2-x) were successfully synthesized through a calcination-vacuum activation method by using low-cost ammonium hydroxide as the nitrogen source. Interestingly, the nitrogen could be further doped into TiO2 lattice via the substitution of oxygen vacancies after the vacuum activation treatment. The as-prepared catalysts exhibited high visible light activity by improved substitution of nitrogen concentration with no affecting the UV-light photocatalytic activities. The samples were characterized by the XRD, UV-vis DRS, ESR, XPS and TEM analyzes. When the temperature of vacuum activation was 300 C, N-TiO2-x catalyst showed the highest photocatalytic activity for the degradation of methyl orange (MO) under the visible light irradiation. The increase in the visible-light photocatalytic activity was attributed to the synergistic effect between Ti3+ and nitrogen doping, which contributed to the increase of absorption in the visible light region. It can be claimed that N-TiO2-x (300 degrees C-3 h) contained the highest concentration of Ti3+. (C) 2015 Elsevier B.V. All rights reserved.