A novel heterojunction composite of CoO_x/Bi₄Ti₃O₁₂ was synthesized through a combination of molten salt and photodeposition methods. The optimal sample exhibited superior performance in the piezocatalytic degradation of methyl orange (MO) dye with a degradation rate of 1.09 h^-1, which was 2.4 times higher than that of pristine Bi₄Ti₃O₁₂. Various characterizations were conducted to reveal the fundamental nature accountable for the outstanding piezocatalytic performance of CoO_x/Bi₄Ti₃O₁₂. The investigation of the band structure indicated that the CoO_x/Bi₄Ti₃O₁₂ composite formed a type-I p-n heterojunction structure, with CoO_x acting as a hole trapper to effectively separate and transfer piezogenerated carriers. Significantly, the MO degradation rate of the best CoO_x/Bi₄Ti₃O₁₂ sample further increased to 2.96 h^-1 under combined ultrasonic vibration and simulated sunlight. The synergy between piezocatalysis and photocatalysis can be ascribed to the following factors. The photoexcitation process ensures the sufficient generation of charge carriers in the CoO_x/Bi₄Ti₃O₁₂, while the piezoelectric field within Bi₄Ti₃O₁₂ promotes the separation of electron-hole pairs in the bulk phase. Furthermore, the heterojunction structure between Bi₄Ti₃O₁₂ and CoO_x significantly facilitates the surface separation of charge carriers. This increased involvement of free electrons and holes in the reaction leads to a remarkable enhancement in catalytic MO degradation. This work contributes to the understanding of the coupling mechanism between the piezoelectric effect and photocatalysis, and also provides a promising strategy for the development of efficient catalysts for wastewater treatment.

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