Theory of Carbon Doping of Titanium Dioxide

Recent experimental studies have determined that carbon doping dramatically improves the photocatalytic activity of TiO2 in the visible-light region. Using density functional theory (DFT) calculations within the generalized gradient corrected approximation, we investigate various structural models of carbon impurities in both the anatase and rutile polymorphs of TiO2 and analyze the associated modifications of the electronic band structure. We compare the stability of all these diverse species on the basis of their energy of formation as a function of the oxygen chemical potential, which determines whether the system is in an oxidizing or reducing environment. At low carbon concentrations, we find that, under oxygen-poor conditions, substitutional (to oxygen) carbon and oxygen vacancies are favored, whereas, under oxygen-rich conditions, interstitial and substitutional (to Ti) C atoms are preferred. Higher carbon concentrations undergo an unexpected stabilization caused by multidoping effects, interpreted as inter-species redox processes. Carbon impurities result in modest variations of the band gap but induce several localized occupied states in the gap, which may account for the experimentally observed red shift of the absorption edge toward the visible. Our results also indicate that carbon doping may favor the formation of oxygen vacancies in bulk TiO2.