Boron Environments in B-Doped and (B, N)-Codoped TiO2 Photocatalysts: A Combined Solid-State NMR and Theoretical Calculation Study

The structures and local environments of boron species in B-doped and (B, N)-codoped TiO2 photocatalysts have been investigated by solid-state 11B NMR spectroscopy in conjunction with density functional theory (DFT) calculations. Up to seven different boron sites were identified in the B-doped anatase TiO2, which may be classified into three categories, including interstitial, bulk BO3/2 polymer, and surface boron species, and has been supported by results obtained from FT-IR and XPS spectroscopy as well as from DFT calculations. Two types of interstitial borons, namely the tricoordinated (T*)- and pseudotetrahedral-coordinated (Q*) borons, were observed in addition to the two types of bulk BO3/2 polymer and three types of surface B, in good agreement with experimental data. Further density of state analyses revealed that, compared to undoped TiO2, the T* species in boron-doped TiO2 are solely responsible for the observed increase in energy band gap, whereas the presence of Q* species tend to lead to a decrease in band gap and hence are more favorable for the absorption in the visible-light region. In comparison with B- and N-doped TiO2, (B, N)-codoped TiO2 tends to exhibit a much higher visible-light photocatalytic activity for the oxidation of rhodamine B. Accordingly, a photochemical mechanism of the (B, N)-codoped TiO2 under visible-light irradiation is proposed.