Single-Crystal Ferroelectric-Based (K,Na)NbO₃ Microcuboid/CuO Nanodot Heterostructures with Enhanced Photo–Piezocatalytic Activity

Developing single-crystal-based heterostructured ferroelectrics with high-performance photo–piezocatalytic activity is highly desirable to utilize large piezopotentials and more reactive charges that can trigger the desired redox reactions. To that end, a single-crystal-based (K,Na)NbO₃ (KNN) microcuboid/CuO nanodot heterostructure with enhanced photo–piezocataytic activity, prepared using a facile strategy that leveraged the synergy between heterojunction formation and an intense single-crystal-based piezoelectric effect, is reported herein. The catalytic rhodamine B degrading activity of KNN/CuO is investigated under light irradiation, ultrasonication, or co-excitation with both stimulations. Compared to polycrystalline KNN powders and bare KNN single-crystals, single-crystal-based KNN/CuO exhibits a higher piezocurrent density and an optimal energy band structure, resulting in 5.23 and 2.37 times higher piezocatalytic degradation activities, respectively. Furthermore, the maximum photo–piezocatalytic rate constant (≈0.093 min⁻¹) of KNN/CuO under 25 min ultrasonication and light irradiation is superior to that of other KNN-based catalysts, and 1.6 and 48.6 times higher than individual piezocatalytic and photocatalytic reaction rate constants, respectively. The excellent photo–piezocatalytic activity is attributed to the enhanced charge-carrier separation and proper alignment of band structure to the required redox levels by the appropriate p–n heterojunction and high piezoelectric potential. This report provides useful insight into the relationships between heterojunctions, piezoelectric responses, and catalytic mechanisms for single-crystal-based heterostructured catalysts.