TiO2 mesocrystal (TMC) with {001} facet exposed is uniformly codoped with nitrogen (N) and fluorine (F) to form the TMC with N and F codoping (NFT) during the topochemical transformation. The visible light induced reduction of Cr(VI) to Cr(III) was evaluated with a clear correlation to their optical absorption due to the N dopant with the localized midgap state. The comparison of methylene blue (MB) degradation under the visible or UV light irradiation indicates that the F codoping with the role of water dissociation, is discerned to associate with N doping based on the comparison between NFT and NFT-NaOH with the treatment of NFT by NaOH aqueous solution for removing F. The unambiguously effect of surface adsorbed F as the assistant role of N in the localized midgap state is further elucidated from monitoring of charge separation and trapping processes using the femtosecond time-resolved diffused reflectance (TDR) measurement. (C) 2015 Elsevier B.V. All rights reserved.