A new method for the synthesis of nanocrystalline anatase Cr-doped TiO2 colloids from nanotubes utilizing the hydrothermal ion-intercalation (HII) method has been devised using hydrothermal treatment in an acidic environment. To investigate the photoelectrochemical reaction that occurs when water is split into H-2 and O-2 under visible light, a Cr-doped TiO2 thin film served as a model photoelectrode. The photoelectrochemical activity of Cr-doped TiO2 was higher than that of the undoped sample. The photoelectrochemical activity of photoelectrodes increased drastically with increased chromium doping. A red shift in the band gap was induced by Cr doping of TiO2. It was revealed that the Cr-doped TiO2 photoelectrode was able to utilize a wide range of light in the visible region of the spectrum. At high Cr concentrations, the lower photoelectrochemical activity is attributed to the effect of Cr3+ ion recombination and excess Cr3+ ions forming secondary phase Cr2O3. (c) 2008 Elsevier B.V. All rights reserved.