Ultrasound-enhanced photocatalytic nitrogen fixation by ZrFe-MOF: Piezoelectric-photocatalytic coupling for boosting charge separation

To develop a low-carbon and highly efficient alternative to the energy-intensive Haber-Bosch process for ammonia synthesis, this study designed a Zr, Fe bimetallic organic framework (ZrFe-MOF) using 1,4-Benzenedioic acid (H 2 BDC) as the organic ligand and Zr 4+ /Fe 3+ as dual metal centers via a hydrothermal synthesis method. The resulting material was systematically investigated for its performance in photocatalytic and piezoelectric (under ultrasonic conditions) synergistic nitrogen reduction reaction (PPNRR).The synergistic coordination of trivalent and tetravalent metal ions engenders an augmented density of coordination defects and active sites within the material framework. Structural characterization confirmed that Fe species were uniformly dispersed within the Zr-BDC matrix, forming a stable coordination structure . The optimal ZrFeBDC-55 exhibited distinct piezoelectric properties and achieved an ammonia production rate of 7026.85 μmol·L −1 ·h −1 ·g −1 , under the synergistic effect of light and ultrasound, significantly outperforming monometallic Zr-BDC and Fe-BDC, respectively. Mechanistic analysis revealed that the bimetallic synergy introduced sub-bandgap energy levels that effectively suppressed recombination of photogenerated electron-hole pairs, while the more negative conduction band potential enhanced the catalyst’s reduction capability. Furthermore, the piezoelectric effect promoted efficient charge carrier separation and migration under ultrasonic conditions. The catalyst demonstrated excellent recyclability, maintaining both structural integrity and catalytic activity over four cycles. Notably, irrigation of rice seedlings with the synthesized ammonia solution resulted in a 32% increase in plant height and optimized root development, verifying agricultural application potential. This work presents a novel strategy for high-performance photopiezoelectric catalysts for nitrogen reduction and lays a foundation for low-carbon agricultural nitrogen supply technologies.