Enhancing dye degradation in darkness: The role of SnO2 nanorod clusters in flexocatalysis

Flexoelectricity is present in all dielectric materials and exhibits spontaneous electrical polarization when subjected to a strain gradient. In the present study, low-dimensional, pinecone-like tin oxide (SnO₂) nanorod clusters demonstrated favorable flexocatalytic properties during a dye degradation experiment in which light irradiation was excluded; the catalytic activity was driven solely by mechanical force. Notably, the SnO₂ lattice did not have a non-centrosymmetrical structure, indicating the absence of a piezoelectric effect. However, the nanoscale structure exhibited notable flexocatalysis under ultrasonic vibration. During the aforementioned dye degradation experiment, the concentration of dye molecules underwent nearly complete degradation under ultrasonic treatment. The corresponding rate constant was 0.484 hr^-1, indicating a 28-fold improvement compared with commercially available SnO₂ nanoparticles. Further characterization revealed that the as-synthesized SnO₂ exhibited abundant oxygen vacancies. Through heat treatment at multiple temperatures, the concentration of oxygen vacancies was systematically reduced, and this reduction demonstrated a negative correlation with dye degradation efficiency. Within this system, oxygen vacancies played a pivotal role in modifying the band structure, thereby hindering the recombination rate of electron-hole pairs. This behavior indicates a notable enhancement in flexocatalysis efficiency with the elevation of defect concentration through thermal treatment. The present study demonstrated the viability of flexocatalysis for SnO₂ with a wide band gap and highlighted the synergistic effect of defects, even in the absence of light, to present a promising strategy for polluted water treatment applications.