Catalytic Activity of Piezoelectric and Paraelectric BaTiO₃ Nanoparticles

Ultrasonically assisted catalysis-also called sonocatalysis-is one of the advanced oxidation processes used for wastewater treatment. It has been shown that using nanoparticles of piezoelectric materials as nanocatalysts in sonocatalysis greatly enhances the catalytic reaction rates, a phenomenon that has been dubbed piezocatalysis. Since both sonocatalysis and piezocatalysis are excited by ultrasonic waves and occur simultaneously, it is challenging to discriminate between these two processes and to quantify the contribution of the material's piezoelectric properties to the overall catalytic activity. It has been previously reported that the piezoelectric properties of the catalyst nanoparticles can improve their catalytic activities by up to one order of magnitude. In this study, we compare the catalytic activity of nanoparticles of both ferroelectric and paraelectric BaTiO₃, hence piezoelectric and non-piezoelectric BaTiO₃ nanoparticles. BaTiO₃ nanoparticles of two different sizes were synthesized using a microwave-assisted hydrothermal method. After a full characterization by transmission electron microscopy (TEM), X-ray diffraction (XRD), and temperature-dependent Raman spectroscopy, the catalytic activities of the BaTiO₃ nanoparticles were determined by monitoring the time dependence of the optical absorption of a solution containing the model pollutant methyl orange, to which the dispersed piezoelectric BaTiO₃ particles were added as catalysts. The 50 nm nanoparticles were found to have a tetragonal crystal structure and symmetry and to be piezoelectric, while the 10 nm nanoparticles had a cubic crystal structure and symmetry and exhibited no piezoelectricity. This study reveals that non-piezoelectric BaTiO₃ nanoparticles exhibit a moderate catalytic activity for the degradation of methyl orange, similar to that of non-piezoelectric TiO₂ nanoparticles. Furthermore, it also shows that, at room temperature, 90% of the overall catalytic activity of piezoelectric BaTiO₃ nanoparticles is due to piezocatalysis, while the remaining 10% is related to sonocatalysis. Using liquid chromatography coupled to mass spectrometry (LC-MS), possible chemical decomposition pathways of the methyl orange dye have also been suggested.