Photocatalytic properties of bismuth oxyfluoride thin films deposited by reactive magnetron sputtering in Ar/O2/CF4 atmosphere

Bismuth oxyfluoride films were successfully deposited by reactive magnetron sputtering using different Ar/O2/CF4 gas mixtures. For a reactive gas ratio (Rf), defined as the O2/(O2 + CF4) flow rate ratio, ranging from 0 to 0.6, the films consist of two phase-system composed of BiO0.5F2 and metallic bismuth (Bi0); whereas for Rf = 0, a single phase BiF3 is observed when Rf > 0.5. The amount of metallic Bismuth within the oxyfluoride matrix can be tuned not only by Rf, but also by adjusting the target power or by varying a single reactive gas flow rate. Increasing the film thickness does not significantly affect the composition but enhances crystallization of the phases. Hence, we succeed in forming one-step Bi/BiO0.5F2 heterojunctions, where the metal presence improves the light absorption, the photogenerated carrier separation, and thus the photocatalytic efficiency for Methylene Orange, MO, degradation in water. Among all the conditions investigated, the film obtained with Rf = 0.3 and a deposition time, td = 20 min exhibits the best overall performance. Indeed, 90 % of MO is degraded after 2 hours of irradiation, and the catalyst remains stable over 3 photocatalytic cycles (approximately 9 hours of total use). Moreover, this material selectively photoconverts CO2 in CO, with a higher efficiency to create useful electrons (rē ≈ 29 mmol/molcata) than pure BiO0.5F2 thin film, and even nanostructured TiO2 P25 (rē ≈ 11 mmol/molcata).