  Electronic structures and optical properties of C-N-codoped anatase TiO2 were
calculated by using GGA+U method based on the density functional theory. The
calculated results showed that the N-doped, C-doped, and C-N-codoped TiO2
produced 2p states in band gap, and the band gaps of the three doped systems
decreased compared with the pure TiO2. According to the optical results, the
band edges of the three doped systems shifted to the long wavelength region,
and the visible optical absorption from 450 to 800 nm was observed. Moreover,
the visible light response of C-N-codoped TiO2 was better than the C or N
single doped TiO2, indicating that there was a synergistic effect for the
C-N-codoped TiO2, which offseted the deficiencies of C or N-doped TiO2.
