BiFeO3: the “Holy Grail” of piezo-photo-catalysts?

Because of their intrinsic polarization and related properties, ferroelectrics have been considered as good and promising catalysts to address current energy and environmental issues. As photocatalysts or piezocatalysts, they can use the free and inexhaustible sunlight source or waste mechanical energies, respectively, as stimulus to produce at their surface reactive species, which in turn decompose organic molecules into harmless byproducts in wastewater. Here, we investigated the photocatalytic, piezocatalytic and combined piezo-photo-catalytic activity of pure and single-phase 60 nm-size BiFeO3 (BFO) nanoparticles, we fabricated by a simple, low temperature and, cost-effective chemical reaction route. We show that beyond its usual multiferroic properties, BFO is an extremely efficient and very versatile photo- and/or piezo-catalyst making it a very competitive and, even better material compared to other catalysts reported in the literature. We show e.g., that BFO can efficiently photo- or piezo-degrade various dyes (Eosin B (EB), Methyl blue (MB), Methyl Orange (MO), Rhodamine B (RhB), Methylene blue (MLB)) or antibiotics (Norfloxacin (NOR), Tetracycline (TC)). After only 1 h of natural sunlight irradiation, the photodegradation of EB, MB, MLB, NOR and TC is found to be 99 %, 93 %, 83 %, 78 %, and 96 %, respectively. As piezocatalyst, BFO is also remarkably efficient as the piezodegradation ratio of the RhB reached 93 % after 10 min subjected to an ultrasonic source and kept in the dark, while the photodegradation ratio is only 54 % after 10 min under sunlight illumination. More strikingly, when the sunlight and the ultrasonic wave are combined, the RhB dye is completely decomposed within 5 min. As a result, the piezo-photo-catalytic rate constant is found to be 41746 L mol-1 min-1, which is the highest value ever reported. Finally, to avoid dispersing the particulate-based catalysts in the polluted solution, which makes the nanoparticles difficult to be collected and reused and, potentially causes secondary pollution in water bodies, we designed BFO/P(VDF-TrFE) composite films as free-standing, non-toxic, reusable and flexible catalyst system we also studied. These exciting findings will be presented in line with the physical properties at play in photo-piezo-catalysis mechanisms.