High piezo–catalytic activity of ZnO/Al2O3 nanosheets utilizing ultrasonic energy for wastewater treatment

Abstract

Abstract Designing and developing integrated energy conversion system that can effectively harvest and transform various type of energy shows great prospect. Here, we synthesize ultrathin ZnO/Al2O3 composite by calcination from its Zn–Al layered double hydroxide (Zn–Al LDH) precursor, whereas the ultrathin ZnO acts as active species that can directly utilize mechanical energy to drive chemical reaction. The thickness of the obtained ultrathin ZnO/Al2O3 composite is about 4.5 nm and its piezo–catalytic activity is assessed under ultrasonication. As−prepared ultrathin ZnO/Al2O3 composite exhibits higher piezo−catalytic reaction rate (k) over methyl orange dye (MO) degradation as compared to that of ZnO/Al2O3 with the thickness about 14 nm (k: 1.19 vs. 0.23 h−1). The experimental and simulated results indicate U–ZnO/Al2O3 exhibits easy bending deformation, which can result in higher piezoelectric potential (1.42 vs. 1.23 V at 105 Pa of balanced side pressure and unbalanced top pressure). Furthermore, with the ultrathining process, abundant surface oxygen vacancies (VO) appear. As a result, the content of surface free charges is enhanced due to the presence of VO, which can be driven by piezoelectric potential and then involved in the generation of radical species during the piezo–catalytic reactions. The generated hydroxyl radical (∙OH) is the active species and it mainly comes from transformation of superoxide radicals (·O2−). This work not only gives clear understanding of piezo–catalytic process but also sheds light a promising strategy for constructing piezo–catalytic materials with optimized geometric or electronic properties.