Fig. 5. Optical absorption curves of undoped and S-doped TiO2. Schematic illustrations of the band structure of undoped and S-doped anatase TiO2 were indicated at the insets. Energies are not in scale. Eg, Eg_S, and , represent the band gap of TiO2, S-doped TiO2, and the band gap narrowing of S-doped TiO2, respectively.