A straightforward NH4F-fluorination approach to modify BiVO4 and synergistic effect of fluorine substitution and adsorption on charge transfer behavior and visible-light activity

Herein, a straightforward fluorination method is employed to enhance the photocatalytic performances of BiVO4. Compared with untreated BiVO4, the visible light activity of treated BiVO4 sample (0.15F-BiVO4, obtained with 0.15g NH4F) has increased by about 1.4 and 4 times for degrading methylene blue (MB) and Rhodamine B (RhB), respectively. The activity enhancement is mainly due to the promoted charge transfer and separation by fluorine substitution for oxygen and surface fluorine adsorption. On one hand, fluorine substitution nearly does not change valence band (VB, 2.71 eV), but upshift conduction band (CB) position (0.23 eV/F-BiVO4 vs. 0.36 eV/BiVO4), resulting in an obvious blue shift for light absorption (525 nm/F-BiVO4 vs. 545 nm/ BiVO4). On the other hand, fluorine substitution results in the formation of oxygen vacancy. Because of the high electron affinity, surface adsorbed fluorine, as well as surface oxygen vacancies as electron traps, have improved charge separation effectively. Moreover, oxygen can easily be captured by surface oxygen vacancies to form center dot O2- species, thus changing reaction pathway. In addition, surface adsorbed fluorine facilitates interfacial charge transfer between dye molecules and photocatalyst.