Effective ways to enhance photocatalytic activity of ZnO nanopowders：high crystalline degree, more oxygen vacancies, and preferential growth
The practical application of ZnO nanopowders (NPs) in purifying wastewater would be determined by the balance of some issues, such as low-cost and scalable preparation, high photocatalytic activity, antiphotocorrosion, and convenient recycle. In this contribution, a series of ZnO NPs was scalably prepared in the 220 oC, 250 oC and 280 oC autoclaves, respectively. The structural, morphological and surface information of the samples were systematically examined by XRD, SEM, FT-IR, TEM, Raman, PL, and UV-visible spectroscopy. Along with the increase of reaction temperature, the mean diameter of ZnO NPs became larger in the range of 110-130 nm, and the color of products showed lighter, resulting in the less surface area but a stronger UV absorbance in the region of 200-400 nm. Moreover, the preferential growth along (002), the crystalline degree and the green emission intensity for ZnO 280 became more overt than those of ZnO 250 and ZnO 220. Some other parameters including stretching vibration and E2(high) mode of Zn-O bonding or diffraction peaks for ZnO 280, also shifted to lower wavenumbers or lower angles as a comparison to ZnO 220, revealing more oxygen vacancies exiting in ZnO 280. As expected, ZnO 280 showed the best photocatalytic activity and antiphotocorrosion among three samples on the degradation of rhodamine B due to plenty of oxygen vacancies, preferential growth along (002) direction and the high crystalline degree. It might suggest that, along with the increase of preparation temperature, the obtained ZnO nanopowders would have better photocatalytic activity. Therefore this work would designate a route to synthesize well crystallized ZnO nanopowders, and pave an avenue to commercial application of ZnO NPs in the treatment of organic wastewater.