| Layered Double Hydroxides (LDHs) are an important class of lamellar functionalmaterials, which consist of Hydrotalcite (HT) and Hydrotalcite-Like Compounds (HTLCs).They have a wide range of applications in many important industry fields, such as medicine,catalysis, adsorption, biology and environmental protection and so on, because of theirsuper-molecule structure, the anion-exchange ability and the controllable size. However, thenaturally occurring example of this class of materials is the mineral hydrotalcite,Mg6Al2(OH)16CO3·4H2O, containing carbonate ions in between the layers and the amount isvery limited. The above drawbacks of natural LDHs impose restrictions on the use of someaspects. Developing artificial methods to design novel LDHs is the key to make full use ofthem, which not only has important academic significance but also has practical value. Inthis dissertation, with the characteristics of exchangeable cations in the layers, weintroduced Zn2+into LDHs since its size matches with that of Mg2+. So, Mg2+can bepartially substituted by Zn2+in the process of coprecipitation, and the ternary threedimensional (3D) ZnMgAl-LDHs microspheres were prepared. In consideration of the largesurface area of the3D LDHs microspheres, the adsorption properties of the preparedsamples were characterized. Meanwhile, with the “edge effect”, the lattice matches betweenZnO and ZnMgAl-LDHs, the heterostructures consist of uniform ZnO nanoparticles orderlystanding at the edges of two-dimensional (2D) surfaces of ZnMgAl-LDHs nanoplatelets.Their application for degradation of organic dyes was also studied. The main points of thisdissertation are summarized as follows:Firstly,3D ZnMgAl-LDHs microspheres consisting of nanoplates were synthesized atlow temperature using urea as a precipitator. This preparation method does not use anyorganic solvent as reaction media. The as-prepared LDHs were characterized by XRD, TG,AAS, SEM, TEM and BET. The possible formation mechanism for the architectures wasdiscussed. It is proposed that the introduction of Zn2+played a key role in the formation ofthe microsphere structure. The adsorption properties of the as-prepared sample wereevaluated by adsorbing methyl orange. The3D LDHs microspheres show high adsorptioncapacity. The optimum pH value for the adsorption of MO onto LDHs is3.0. The adsorption process can be well described by the pseudo-second-order kinetic model and the adsorptionisotherm matches Langmuir model well. In addition, the LDHs microspheres can berecovered and used repeatedly.Secondly, modification of LDHs microspheres by ZnO nanoparticles withheterostructure was successfully conducted using glycol-water mixed solvents as reactionmedia and NH3·H2O as a pH adjustor. The effect of solvent and the pH value on theheterostructures were studied. The photocatalytic activity of the as-prepared composites wasevaluated by degradation of phenol in water under ultraviolet light irradiation. TheZnO/LDHs show enhanced photocatalytic activity when the molar ratio of the two differentcomponents were optimized, which can be attributed to the large surface area of LDHs andthe better dispersity of ZnO nanoparticles. With increasing the ZnO content continuously,the photocatalytic activity of the ZnO/LDHs composite photocatalyst decreases reverselybecause the aggregation of ZnO nanoparticles is prejudicial to separation of thephotoinduced carriers efficiently.The investigation indicates that degree of oxide coverage over other materials plays animportant role on photocatalytic performance of the catalysts. The coupling ofsemiconductors with LDHs can overcome its weakness effectively, and enhance itsphotocatalytic activity. The results reported in this study provide insight to construct othercomplex metal oxides with LDHs to form composite photocatalysts. The as-preparedphotocatalysts exhibit excellent photocatalytic activity for the degradation of the organicdyes. Therefore, it is promising to degrade organic pollutants in the field of wastepurification. |
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