  Using density-functional theory calculations within the generalized gradient
corrected approximation, the models that nonmetallic impurities X substituted
for O or Ti sites in anatase TiO2 were investigated. The conclusion of the
valence states of nonmetallic impurities X substituted for O or Ti sites in
TiO2 is also supported by the results of optimized cell parameters and the
local structures. Furthermore, an effective nonmetallic passivated codoping
approach to modify the band edges of TiO2 is proposed.
