Energetics and optical properties of carbon impurities in rutile TiO 2 .

Titanium dioxide is one of the most promising materials for many applications such as photovoltaics and photocatalysis. Non-metal doping of TiO 2 is widely used to improve the photoconversion efficiency by shifting the absorption edge from the UV to visible-light region. Here, we employ hybrid density-functional calculations to investigate the energetics and optical properties of carbon (C) impurities in rutile TiO 2 . The predominant configurations of the C impurities are identified through the calculated formation energies under O-poor and O-rich growth conditions. Under the O-poor condition, we find that C occupying the oxygen site (C O ) is energetically favorable for Fermi-level values near the conduction band minimum (n-type TiO 2 ), and acts as a double acceptor. Under the O-rich condition, the C i -V Ti complex is energetically favorable, and is exclusively stable in the neutral charge state. We also find that interstitial hydrogen (H i ) can bind to C O , forming a C O -H i complex. Our results suggest that C O and C O -H i are a cause of visible-light absorption under oxygen deficient growth conditions.