Study On Preparation And Photocatalytic Properties Of Hierarchical ZnO Micro-/Nanocrystals

In recent years, the water pollution problem caused by organic pollutants has aroused great attention. ZnO is an important semiconductor material. Because of its excellent photocatalytic performance, ZnO becomes a research focus in the field of photo-degradation of organic pollutants. In this paper, hierarchical ZnO micro-/nanopowders were prepared using zinc salt and sodium hydroxide as raw materials. The structure and morphology of the samples were characterized by XRD, FESEM, PL, FT-IR, UV-vis and BET. The degradation behaviors of organic pollutants, such as methyl orange, methylene blue and rhodamine B, were studied using ZnO as photocatalysts under various conditions.The main contents of this dissertation are as follows:(1) Preparation and photocatalytic properties of wheatear-like ZnO micro-/nanostructuresWheatear-like ZnO with hierarchical structures were successfully prepared via a one-step hydrothermal method using Zn(Ac)2 and NaOH as raw materials. Using wheatear-like ZnO as photocatalyst, the degradation behaviors of methyl orange(MO), methylene blue(MB) and rhodamine B(RhB) were studied with the changing factors, including initial concentrations of pollutant, solution pH values, using level of ZnO, temperature and light intensity. Thus, the optimal photodegradation conditions for pollutants were obtained. The optimal conditions for MO: initial concentration of MO is 10 mg/L, pH=13, the using level of ZnO is 5 g/L, T=298 K, 250 W high pressure mercury lamp. The optimal conditions for MB: initial concentration of MB is 10 mg/L, natural pH, the using level of ZnO is 0.3 g/L, T=298 K. The optimal conditions of RhB: initial concentration of RhB is 10 mg/L, pH=7, the using level of ZnO is 0.6 g/L, T=298 K.(2) Preparation and photocatalytic properties of flower-like ZnO micro-/nanostructuresFlower-like ZnO with hierarchical structures were successfully prepared via a one-step hydrothermal method using ZnCl2 and NaOH as raw materials. Using flower-like ZnO as photocatalyst, the degradation behaviors of MO, MB and RhB were studied with the changing factors, including initial concentrations of pollutant, solution pH values, using level of ZnO. Thus, the optimal photodegradation conditions for pollutants were obtained. The optimal conditions for MO: initial concentration of MO is 5 mg/L, pH = 13, the using level of ZnO is 0.6 g/L. The optimal conditions for MB: initial concentration of MB is 10 mg/L, natural pH, the using level of ZnO is 0.6 g/L. The optimal conditions for RhB: initial concentration of RhB is 10 mg/L, pH=7, the using level of ZnO is 0.8 g/L.(3)Preliminary study of photocatalytic mechanism of ZnOWhen ZnO crystal are irradiated by high pressure mercury lamp(365 nm), the photo-generated electron and hole pairs appear. These electron–hole pairs can migrate into the catalyst surface and react with adsorbed OH-、O2 and H2 O to form hydroxyl radicals ·OH. These radicals are powerful oxidants which can decompose the chromophores of pollutant as well as the intermediate products, and thus accelerate the degradation of organic pollutants. In this paper, the wheatear-like and flower-like ZnO possess special 3-D hierarchical structures and expose positive crystal face(0001), which is beneficial to improving the photocatalytic activity of ZnO.