Mesoporous composite films of titania (TiO2) and antimony-doped tin oxide (ATO) were prepared on titanium plate substrates from pastes by the doctor-blade method. The photoelectrochemical properties of the samples with different ATO contents were investigated by the degradation of methylene blue under UV light irradiation. It was found that the photocatalytic activity of TiO2 was significantly improved with the help of the introduction of ATO nanocrystals and a forward bias. The synergetic effect of ATO and bias was analyzed by electrochemical impedance spectra (EIS). The EIS analysis results demonstrated that the separation of photogenerated electron–hole pairs can be efficiently enhanced by introducing ATO nanoparticles and applying an external bias.