First-principles plane-wave pseudopotential calculations have been performed to study the formation energy, electronic structure and optical properties of V-, N- and V/N-doped TiO2. The calculated results show that V:N defect pairs tend to bind to each other, which facilitates the enhancement of the concentration of N dopant compared with N-doped case. In addition, the incorporation of V into N-doped TiO2 lattice induces the appearance of passivated shallow acceptor and donor levels near the VBM and CBM, which would act as capture traps for photoexcited holes or electrons. Furthermore, the optical absorption coefficient spectra show that V/N codoping can enhance the optical absorption region, which can be attributed to lower excitation energy from the N 2p orbitals to V t(2g) orbitals rather than a band gap narrowing. These findings give a reasonable explanation of recent experimental results. Crown Copyright
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