Mechanistic Insights into MnOx-Cocatalyzed Piezophotocatalytic Dye Degradation over S-Scheme MnOx/BiFeO3 Heterojunctions

Abstract

MnOx is commonly used as an oxidative cocatalyst to promote charge carrier separation, yet, its underlying mechanism remains incompletely understood. In this study, MnOx 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 MnOx/BiFeO3 composites exhibited enhanced catalytic performance in RhB degradation. In the piezocatalytic system, the optimized MnOx/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 MnOx modification induces interfacial stress, enhancing the piezoelectric response of BiFeO3. Moreover, an S-scheme heterojunction is formed at the MnOx/BiFeO3 interface, wherein holes in the valence band of BiFeO3 recombine with electrons from MnOx, effectively promoting charge separation and transport while preserving the strong redox capability of both charge carriers. The synergistic piezo-photocatalytic effect of MnOx/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 MnOx 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.