Piezocatalytic activation of persulfate (PS) has great application potential in environmental remediation; however, the relationship between piezocatalyst thickness and catalytic activity is not clear, limiting the further improvement of catalytic activity and application of the technology. Herein, the Bi₂Fe₄O₉ (BFO) piezocatalysts with tunable thickness were prepared through a facile hydrothermal method by tuning the molar ratio of Bi(NO₃)·5H₂O and FeCl₃·6H₂O for piezocatalytic activation of peroxydisulfate (PDS). The BFO with the smallest thickness exhibits excellent catalytic activity, and the \({\text{SO}}_{{4}}^{ \cdot - }\) and \(\cdot {\text{OH}}\) are the major reactive oxygen species for degrading organic pollutants. Further XPS investigations and finite element analyses demonstrate that the decreased thickness of BFO not only exposes more Fe²⁺ sites for PDS activation, but also improve the piezoelectric effect to accelerate the regeneration of Fe²⁺, thus enabling an enhanced synergy effect between PDS activation and piezocatalysis for outstanding catalytic activity. This work provides an understanding of the relationship between thickness of piezocatalysts and its catalytic activity over PDS activation, facilitating the development of more efficient piezocatalysts and PS-based advanced oxidation processes.

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