ZnO-Based Hetero-Nano/Microstructure Photocatalysts:Controllable Synthesis And Their Photocatalytic Performances

Semiconductor-assisted photocatalysis has attracted considerable attention because it is one of the promising tools to solve environmental problems through the photochemical degradation of organic contaminants. ZnO, a wide band-gap semiconductor, attracts extensive attention and has been regarded as one of the most promising photocatalyst due to its high redox potential, superior chemical stability, low-cost, and nontoxicity. However, its quick recombination of charge carriers, poor response to visible light and critical drawback of photocorrosion greatly restricts its practical application range. In recent years, lots of works, including developing ZnO-based heterostructures or composites with electron scavenging agents such as metal ion, metal oxides and organic molecules, have been carried out for modification of this semiconductor photocatalyst to obtain broadening of responsive-light range, efficiency improvement and photostability of ZnO. Based on these works, this dissertation is focused on the synthesis and photocatalytic application of novel ZnO-based hetero-nano/microstructures. Furthermore, photocurrent transient response (PTR) and electrochemical impedance spectroscopy (EIS) are employed to investigate their photoelectric properties, in order to reveal the effect of different components on the photogenerated charges behaviors. Besides, the relationships between photoelectric properties and photocatalytic activities of modified photocatalysts were discussed. The dissertation mainly concerns with the following three aspects::1. Hierarchical nanostructures of nickel-doped Zinc Oxide:morphology controlled synthesis and enhanced visible-light photocatalytic activityA series of nickel-doped ZnO (Ni-ZnO) nanostructures have been synthesized using a simple solvothermal approach in different solvents. It is confirmed that Ni2+ substitute for Zn2+ in ZnO matrix, leading to an extended visible-light absorption and enhanced photocatalytic activity under visible light irradiation. According to the result obtained from the photocurrent transient response and electrochemical impedance spectroscopy, the higher photodegradation efficiency could be ascribed to the enhanced charge separation and transfer efficiency of photogenerated charge carriers causing by the incorporation of Ni2+ in ZnO. The photocatalytic results demonstrate that hexagonal-based polyhedrons obtained in EG-water (v/v,1:1) mixed solvent shows the best photodegradation efficiency among all the samples, standing on higher fraction of exposed polar facets and larger specific surface area. The effect of Ni doping concentration on morphology and photocatalytic activity of the doped ZnO nanocrystals prepared in EG-H2O mixed solvent system is also studied in detail.2.α-Fe2O3 decorated ZnO nanorod-assembled hollow microspheres:Synthesis and enhanced visible-light photocatalysisα-Fe2O3 nanoparticles were homogeneously decorated onto the surface of ZnO nanorods-assembled hollow spheres by the acetone-assisted impregnation approach followed by high-temperature calcination. The obtained heterostructure shows an obvious red-shift to the visible light region.The photocatalyst of 7.5 wt% α-Fe203/Zn0 (FZ-3) calcinated at 400℃ processes the highest visible-light photocatalytic activity for degradation of Rh B. Photocurrent transient response and electrochemical impedance spectroscopy provided that the photoinduced charge-transfer ability of ZnO could be efficiently improved by coupling with α-Fe2O3.3. Synthesis of carbon coated Ni-doped ZnO core-shell nanorods with improved photocatalytic activities and photostabilityNi-ZnO@Carbon core-shell nanorods (NZO@C NRs) were successfully prepared by two-step solvothermal method. The results indicated that Ni2+ substituted for Zn2+ in the lattice of ZnO, whereas all the carbon was uniformly coated on the surface of Ni-ZnO NRs. By changing the amount of glucose, the thickness of carbon layer could be easily controlled from 2.5 to 4.7 nm. Furthermore, the light absorption ability of NZO@C NRs had greatly enhanced in the visible region. The photocatalytic studies reveal that NZO@C NRs exhibit a higher activity compared with the pure ZnOs and Ni-ZnO NRs under visible light irradiation, which might be attributed to the effectively separated photogenerated charge carriers based on the synergistic effect of nickel and carbon with ZnO. Significantly, the photocatalytic activity of NZO@C-3 NRs is virtually unchanged after reused for 6 cycles, which suggests that the photostability of the photocatalyst could be improved greatly by carbon coating.