Unveiling the Potential of BiNbO₄ as a Piezo-Photocatalyst toward Environmental and Energy Applications

Solar-driven photocatalysis is pointed as a promising approach for environmental remediation; however, typical high recombination rates of the photogenerated electron–hole (e–/h+) pairs observed in semiconductors still hinder practical applications. Alternatively, the use of piezoelectric materials as photocatalysts can significantly mitigate these recombination processes, enhancing charge separation. In this work, the piezo-photocatalytic properties of bismuth niobate (BiNbO4) are investigated for both environmental remediation and H2 evolution. By controlling the hydrothermal synthetic conditions, a α-BiNbO4/Bi5Nb3O15 heterojunction could be engineered with enhanced structural distortions of the nanocrystalline planes. As a result, both the piezoelectric and charge separation properties were improved in relation to the pristine phases, resulting in improved photocatalytic performance in the presence of both light irradiation and ultrasound stimuli. Piezo-photocatalytic assays reveal improvements on the methylene blue degradation (MB) and methanol oxidation of 1.96 and 8.5 times, respectively, in relation to pure photocatalysis. Additionally, the α-BiNbO4/Bi5Nb3O15 piezo-photocatalyst was also tested toward visible-light H2 evolution, reaching 130.64 μmol H2 g–1 h–1 using methanol as a sacrificial reagent. The piezo-photocatalytic degradation of Fipronil, an emerging pharmaceutical contaminant, was also evaluated, resulting in the complete oxidation of the substrate under 2 h of combined illumination and ultrasound, which is 4.2 times faster than solely photocatalysis. Operando Electron Paramagnetic Resonance spectroscopy (EPR) was employed to evaluate the effect of ultrasound cavitation on the interfacial charge transfer of the material, allowing the rationalization of the piezo-photocatalytic behavior in the above-mentioned applications.