TiO2-β-Bi2O3 junction as a leverage for the visible-light activity of TiO2 based catalyst used for environmental applications

Solution combustion synthesis was used to produce a junction between TiO2 and β-Bi2O3, which increases the photocatalytic activity of TiO2-β-Bi2O3 composites under visible and UV-light for the photocatalytic oxidation of aqueous Bisphenol A (BPA) solution. Surface (XPS, UV–vis DR analysis), electronic (EIS, EPR) and structural (SEM, TEM, XRD, N2 physisorption analysis) properties of the composites and BPA degradation performance were analyzed and correlated to the amount of β-Bi2O3 present in the composites (weight ratio Ti:Bi was varied between 1:0.2 and 1:0.8). The results show that (i) the narrow band gap semiconductor β-Bi2O3 acts as a visible-light photosensitizer for TiO2, (ii) a heterojunction between TiO2 and β-Bi2O3 enables the transfer of visible-light generated h+ from the valence band (VB) of β-Bi2O3 to the upper lying VB of TiO2, (iii) a p-n junction enables the transfer of visible-light photo-generated e− in the conduction band (CB) of β-Bi2O3 to the CB of TiO2, and (iv) under UV-light illumination both semiconductors act as scavengers for charge carriers generated by the other component. EPR measurements show that the amount of generated charge carriers and reactive oxygen species (ROS) was higher under UV-light illumination; besides, the amount of generated ROS increased with the increasing amount of β-Bi2O3 phase. On the other hand, BPA degradation runs and EIS measurements revealed that if the weight ratio Ti:Bi is over 1:0.4, the catalytic activity and the amount of generated charge carriers dropped due to excessive formation of a segregated β-Bi2O3 phase. The discrepancy between EPR measurements and catalytic BPA degradation tests can be contributed to the fact that BPA degradation proceeds either directly with photo-generated charge carriers (oxidizing organic species directly by photo-generated holes), or indirectly with ROS that include also EPR silent species (hydrogen peroxide and singlet oxygen).