Photocatalytic Properties of Bismuth Oxyfluoride Thin Films Deposited by Reactive Magnetron Sputtering in Ar/O2/CF4 Atmosphere
Résumé
Bismuth oxyfluoride films were successfully deposited by reactive magnetron sputtering in different Ar/O2/CF4 gas mixtures. Films deposited with a reactive gas ratio (Rf), which is the O2 to (O2 + CF4) flow rates ratio, ranging between 0 and 0.5 consist of two phases, BiO0.5F2 and metallic bismuth (Bi o); whereas for higher Rf the metallic phase disappears. The content of metallic Bi into oxyfluoride matrix can be tuned by Rf, but also by the target applied power or by varying only one reactive gas flow rate. By increasing the film thickness, the composition stays stable but the phase crystallization is enhanced. Hence, we succeed to form in one-step Bi/BiO0.5F2 heterojunctions, where the metal presence improves the light absorption, the photogenerated carrier separation, and thus the photocatalytic efficiency for MO degradation in water. For all these conditions, the film obtained with Rf = 0.3 and td = 20 min seems to correspond to an optimal composition. Indeed, 90 % of MO was degraded after 2 hours of irradiation in the presence of this film, which is stable after 3 photocatalytic cycles (≈ 9 hours). This heterojunction is also efficient to photoreduce CO2 with a photoprodcution rate of 25 micro-mol/h/g and a selectivity for CO at 70%.
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