Self-powered piezoelectric NaNbO3 induced band position rearrangement and electrocatalysis in MoS2/NaNbO3 heterojunction for generation of reactive oxygen species for organic pollutant removal

Piezocatalytic heterojunction was synthesized using sodium niobate (NaNbO3, NBO) and molybdenum disulfide sheets (MoS2, MS), and characterized via SEM, XRD, XPS, UPS, EPR, PFM, and EIS. Metronidazole (MET) removal using MS(x)/NBO heterojunction was tested via batch experiments in an ultrasonicator (40 kHz and 150 W) for 160 min. MET removal obtained under optimized MS(30)/NBO (30 wt% of MS) was 82.8%, and the removal mechanism was governed by .OH formed due to reduction of H2O2, electrocatalytic oxidation at MS anode, and oxidation by holes in depletion region at MS-water and MS-NBO interface. Scavenging and quantification studies confirmed the formation of H2O2 (∼27 µM) due to piezoelectric effect induced electrocatalysis. In the absence of piezoelectric effect, MET removal and .OH formation were insignificant. Moreover, COMSOL simulation revealed the role of orthorhombic NBO as parallel plate capacitor in the heterojunction that created electrocatalysis and formation of depletion and accumulation regions at MS-NBO and MS-water interface. Finally, effects of pH (2–12), competitive anions (Cl−, NO3–, SO42−, CO32–, and PO43−) and working temperature (28–63 ℃) on piezocatalytic MET removal were investigated through experiments and theoretical understanding. The increase in temperature due to ultrasonication improved efficiency of MET removal due to improvement in interfacial charge transfer, and increase in thermodynamic feasibility of .OH generation. Overall, MS(30)/NBO activity could be attributed to generation of charge carriers due to mechanical excitation and formation of piezoelectric effect due to cavitation under ultrasonic waves.