Cr-doped TiO2 (Cr-TiO2) nanoparticles with anatase and rutile bicrystalline phases were synthesized by a simple one-step flame spray pyrolysis (FSP) technique. The influences of chromium doping on the phase composition, microstructures, and optical properties of Cr-TiO2 were analyzed by the methods of XRD, TEM, EPR, XPS, UV-vis diffuse reflectance spectroscopy, and nitrogen adsorption-desorption. It was revealed that Cr3+ doping not only extends the visible light response of TiO2 but also increases the content of rutile in the bicrystalline TiO2. XPS results confirmed that Cr3+ can effectively promote the formation of oxygen vacancy which is contributed to the anatase-to-rutile transformation. EPR analysis indicated that the existing state of Cr in Cr-TiO2 is closely relative to the Cr content: Cr is mainly incorporated into the crystal lattice of TiO2 when the Cr content is lower than 1 at.%, while higher Cr content is favorable to the formation of Cr2O3 clusters. The photocatalytic activities of different Cr-TiO2 photocatalytsts were evaluated in terms of the photocatalytic degradation of 2,4-dichlorophenol (2,4-DCP) under visible light irradiation. Appropriate Cr3+ doping can significantly enhance the visible-light photocatalytic activity of TiO2, which is attributed to the improvement of visible light response as well as the suitable anatase-to-rutile ratio. Excess Cr3+ doping is detrimental to the improvement of visible light photocatalytic activity, which is relative to the formation of Cr2O3 clusters as well as too high rutile content. (c) 2012 Elsevier B.V. All rights reserved.