Synergistic piezo-photocatalysis on carbon nitride nanotubes for organic pollution elimination

Piezo-photocatalytic systems, which harness solar and mechanical energy simultaneously, have promising applications in environmental remediation. However, most piezoelectric materials suffer from defects such as small mechanical energy capture area and weak piezoelectric polarization. Here, one-dimensional (1D) nitrogen-oxygen bi-defective carbon nitride (g-C3N4) nanotubes with high specific surface area were synthesised via a two-step hydrothermal calcination method, which were employed as piezo-photocatalyst for first time. The 1D structure endows g-C3N4 nanotubes with stronger mechanical energy harvesting capabilities and larger piezoelectric coefficients. Nitrogen-oxygen bi-defective endows g-C3N4 nanotubes with the optimal energy band structure. When co-stimulated by visible light and ultrasonic, the optimal porous g-C3N4 nanotubes demonstrated remarkable efficiency in the piezo-photocatalytic degradation of methyl orange (MO) with the largest apparent rate constant (k=0.0345 min-1), which was 19.17 and 2.28 times greater than ultrasonic alone and light illumination alone, respectively. The large piezoelectric potential and specific surface area, the optimal energy band structure, and the ideal optimization between piezo-response and visible light response by the generation of a one-dimensional structure all contribute to the increased catalytic activity of g-C3N4 nanotubes. This work elucidates effects of various factors such as catalyst morphology, defects and piezoelectric polarization on piezo-photocatalytic capacity, as well as provides new ideas for designing highly efficient piezo-photocatalysts based on microstructure and energy band engineering.