Catalytic Activity of BaTiO3 Nanoparticles for Wastewater Treatment: Piezo- or Sono-Driven?

Piezocatalytic reactions are generally excited by ultrasonication and, as such, should occur simultaneously with other catalytic reactions caused by different mechanisms such as sonocatalysis or tribocatalysis. One of the main challenges is how to discriminate between these effects in a catalysis experiment and quantify each contribution. In order to distinguish these effects, we use nano- and micro-particles of piezoelectric barium titanate (BaTiO3) and then compare their catalytic properties below and close to the piezoelectric to non-piezoelectric phase transition temperature at which the crystalline structure changes from the low-temperature tetragonal, piezoelectric phase to the higher-temperature cubic, non-piezoelectric phase. All other parameters, such as particle size, surface termination, etc., remain unaltered. The phase transition in bulk occurs at about 120 °C, but the transition temperature decreases as a function of particle size and reaches room temperature for particle sizes of about 20 nm. Transmission electron microscopy and X-ray diffraction were used to characterize the morphology and crystalline phase of the nanoparticles, respectively. Since the piezoelectric properties and the lattice dynamics are closely related, temperature-dependent Raman spectroscopy provides an even better insight into the nano- and micro-structural properties, allowing the ferroelectric phase transition temperature to be estimated. The catalytic activities of the BaTiO3 nanoparticles were determined by monitoring the optical absorption of a solution containing the model pollutant methyl orange, after ultrasonication of the solution to which were added the dispersed piezoelectric BaTiO3 particles as catalysts. An exponential-like correlation has been found between the catalytic reaction rates and the tetragonal distortion of the crystal structure of the BaTiO3 particles. Our study has also established that while 10% of the catalytic reaction of BaTiO3 is related to either sonocatalysis or tribocatalysis, the remaining 90% of the overall catalytic activity is ascribed to the piezocatalytic contribution.