Origins of Electronic Band Gap Reduction inCr/NCodopedTiO2

Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase ${\mathrm{TiO}}_{2}$ significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large ($$\sim${}1\text{ }\mathrm{eV}$) band gap reduction of noncompensated codoped rutile ${\mathrm{TiO}}_{2}$. It is shown that $\mathrm{Cr}/\mathrm{N}$ codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr $3{d}^{3}$ levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr $3d$ and delocalized N $2p$ states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.