Mechanistic insights into MnOx-cocatalyzed piezophotocatalytic dye degradation over S-scheme MnOx/BiFeO3 heterojunctions

Abstract

MnO_x is commonly used as an oxidative cocatalyst to promote charge carrier separation, yet, its underlying mechanism remains incompletely understood. In this study, MnO_x nanoparticles were deposited onto the surface of BiFeO₃ nanosheets via a photodeposition method, and their promoting effects in piezocatalytic and piezo-photocatalytic reactions were systematically investigated. The synthesized MnO_x/BiFeO₃ composites exhibited enhanced catalytic performance in RhB degradation. In the piezocatalytic system, the optimized MnO_x/BiFeO₃ catalyst achieved a degradation rate constant of 0.78 h⁻¹, approximately 4.6 times that of pure BiFeO₃. Under simultaneous ultrasonic vibration and light irradiation, the rate constant further increased to 1.4 h⁻¹, representing 1.8 times and 1.4 times the rates observed under individual piezocatalytic and photocatalytic conditions, respectively. Comprehensive characterization techniques were employed to elucidate the mechanism behind the enhanced performance. The results reveal that MnO_x modification induces interfacial stress, enhancing the piezoelectric response of BiFeO₃. Moreover, an S-scheme heterojunction is formed at the MnO_x/BiFeO₃ interface, wherein holes in the valence band of BiFeO₃ recombine with electrons from MnO_x, effectively promoting charge separation and transport while preserving the strong redox capability of both charge carriers. The synergistic piezo-photocatalytic effect of MnO_x/BiFeO₃ is attributed to the global piezoelectric potential of BiFeO₃, which extends the spatial range of interfacial charge separation within the S-scheme heterojunction. Additionally, the high-density photogenerated electron–hole pairs produced under light irradiation effectively supplement the intrinsic charge carriers. This study not only clarifies the potential mechanism by which MnO_x functions as a hole-trapping cocatalyst, but also highlights the unique advantages of S-scheme heterojunctions in the field of piezo-photocatalysis, offering valuable insights for the design of efficient piezocatalytic materials.