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 TiO 2 to facilitate O 2 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 O 2 π z * molecular orbital enable O 2 adsorption and reduction. Sites that create unoccupied band gap states with energies below that of the O 2 π z * orbital reduce TiO 2 ’s reduction ability as these states result in photoexcited electrons with insufficient reduction potential to reduce O 2 . B dopants in interstitial and relaxed substitutional sites, whose gap states lie >1.5 eV above the valence band maximum (VBM) and hence above O 2 ’s π z * level, facilitate the reduction of O 2 to the peroxide state with adsorption energies on TiO 2 of −1.22 to −2.77 eV. However, N dopants, whose gap states lie less than ∼1 eV above the VBM impede O 2 adsorption and reduction; O 2 on N-doped (101) anatase relaxes away from the surface. Interstitial and substitutional N dopants require two photoexcited electrons to enable O 2 adsorption. C doping, which introduces gap states between those introduced by N and B, aids O 2 adsorption as a peroxide for interstitial doping, although substitutional C does not facilitate O 2 adsorption. Dopants for enhancing the photocatalytic reduction of O 2 in order of predicted effectiveness are interstitial B, relaxed substitutional B, and interstitial C. In contrast, substitutional C and interstitial and substitutional N hinder O 2 reduction despite increasing visible light absorption. Dopants within the surface layer likely deactivate quickly due to the high exothermicity of O 2 reacting with them to form BO 2, CO 2, and NO 2 .