A nitrogen-doped TiO2 (N-TiO2) photocatalyst was prepared by calcination of the hydrolysis precipitate of Ti(SO4)(2) with aqueous ammonia. The prepared N-TiO2 was treated with NH4F (F-N-TiO2) by an impregnation-calcination method. The photocatalyst (F-N-TiO2) was characterized by X-ray diffraction (XRD), Fourier Transform Infrared (FT-IR), UV-vis diffusive reflectance spectroscopy (DRS), BET and X-ray photoelectron spectroscopy (XPS). With blue light-emitting diode (LED) as the light source, its photocatalytic activity for the degradation of formaldehyde was investigated. NH4F treatment enhances markedly photocatalytic activity of N-TiO2. The treatment increases the visible absorption of N-TiO2, decreases its specific surface area and influences the concentration of oxygen vacancies in N-TiO2. Photocatalytic activity of F-N-TiO2 depends on the visible absorption, the specific surface area, and the concentration of oxygen vacancies. The preparation conditions, such as the calcination temperature and the initial molar ratio of NH4F to N-TiO2, have a significant influence on the photocatalytic activity. The doping mechanism of NH4F was investigated. (C) 2010 Elsevier B.V. All rights reserved.