Increasing the Photocatalytic Activity of Anatase TiO2 through B, C, and N Doping

We utilized density functional theory (DFT) to systematically investigate the ability of B, C, and N interstitial and O substitutional surface and near-surface dopants in TiO2 to facilitate O2 reduction and adsorption. Periodic boundary condition calculations based on the PBE+U DFT functional show that dopants that create filled band gap states with energies higher than that of the near surface O2 π z * molecular orbital enable O2 adsorption and reduction. Sites that create unoccupied band gap states with energies below that of the O2 π z * orbital reduce TiO2’s reduction ability as these states result in photoexcited electrons with insufficient reduction potential to reduce O2. B dopants in interstitial and relaxed substitutional sites, whose gap states lie >1.5 eV above the valence band maximum (VBM) and hence above O2’s π z * level, facilitate the reduction of O2 to the peroxide state with adsorption energies on TiO2 of −1.22 to −2.77 eV. However, N dopants, whose gap states lie less than ∼1 eV above the VBM impede O2 adsorption and reduction; O2 on N-doped (101) anatase relaxes away from the surface. Interstitial and substitutional N dopants require two photoexcited electrons to enable O2 adsorption. C doping, which introduces gap states between those introduced by N and B, aids O2 adsorption as a peroxide for interstitial doping, although substitutional C does not facilitate O2 adsorption. Dopants for enhancing the photocatalytic reduction of O2 in order of predicted effectiveness are interstitial B, relaxed substitutional B, and interstitial C. In contrast, substitutional C and interstitial and substitutional N hinder O2 reduction despite increasing visible light absorption. Dopants within the surface layer likely deactivate quickly due to the high exothermicity of O2 reacting with them to form BO2, CO2, and NO2.