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

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 ZneAl layered double hydroxide (ZneAl 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 piezoecatalytic 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 UeZnO/Al2O3 exhibits easy bending deformation, which can result in higher piezoelectric potential (1.42 vs. 1.23 V at 10(5) Pa of balanced side pressure and unbalanced top pressure). Furthermore, with the ultra-thining 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 piezoecatalytic reactions. The generated hydroxyl radical (center dot OH) is the active species and it mainly comes from transformation of superoxide radicals (center dot O-2(-)). This work not only gives clear understanding of piezoecatalytic process but also sheds light a promising strategy for constructing piezoecatalytic materials with optimized geometric or electronic properties. (c) 2019 Elsevier Ltd. All rights reserved.