Defect-Tailored ZnO Nanoflowers Enable Efficient, Metal-Free Ammonia Synthesis through Coupled Piezoelectric and Photocatalytic Activation

Abstract

As a sustainable and promising alternative, the nitrogen reduction reaction (NRR) has attracted growing attention for environmentally friendly ammonia production. This study presents an innovative strategy employing ZnO nanoflowers (NFs) as a catalyst for NRR, activated through a piezo-photocatalytic mechanism. This approach enables energy harvesting from mechanical stimuli in natural environments. The intentional introduction of oxygen vacancies contributes to the generation of additional trapping centers and active sites, thereby prolonging carrier lifetime and further boosting NRR performance. Time-resolved photoluminescence (TrPL) spectroscopy reveals that ZnO samples annealed at 300°C (denoted as ZnO-300) exhibit an extended carrier lifetime of 6 ns—three-fold higher than that of pristine ZnO. The NRR activity of ZnO-300 under the synergistic action of piezo- and photocatalysis reaches 1765.3 μg gcat–1h-1, corresponding to a 220% increase relative to the pristine material. Complementary density functional theory (DFT) calculations corroborate the experimental findings by demonstrating that oxygen vacancies facilitate nitrogen adsorption and effectively lower the reaction energy barrier. These results underscore the potential of piezo-photocatalytic ZnO with optimized defect structures as a high-performance, metal-free catalyst for nitrogen fixation. The capability to simultaneously harvest mechanical and solar energy makes this system particularly appealing for sustainable ammonia synthesis.