Study On The Shell Aggregation Of Transition Metal Oxides Using PMMA Microspheres As Template

Researches on the morphology and size tailoring during the synthesis process of materials are quite necessary for the development of material. Nowadays, phase composition is not the only factor that should be considered during the application of materials. Microstructures, such as the shape, size and texture, are even more important. The oxides of transitional metal exhibit highly desirable optical, electronic, magnetic and catalytic properties, which have been widely applied in field such as solid catalysis, photocatalysis, shape-selective separation, micro electromagnetic device, photochromic materials, electrode materials and information storage device. In recent years, shaped aggregations by nanoparticles of transitional metal oxides attract more interests. Such shaped aggregations perform the properties of the bulk materials. What's more, the sub-building block units exhibit unique properties such as surface effect, little size effect and quantum measure effect, due to their nanoscaled size.However, anisotropic growth and structure collapse are two problems during the fabrication of the aggregations of transitional metal oxides. As a result, nanoparticles might grow into bulk particles which are hard to aggregate. In this thesis, spherical shell shaped aggregation of transitional metal oxides is supposed to be obtained using polymethylmethacrylate (PMMA) as core template. Particles of transitional metal oxides are induced to grow preferably on the surface of the PMMA microspheres, based on the electrostatic and stereochemical interactions at the inorganic-organic interface. Then, spherical shell shaped aggregation is supposed to finally be constructed by these building blocks through self- assembling. Such 3-D hierarchical organization with curved symmetry results in special surface properties.ZnO and CuO crystals were selected as the shell materials, since both of these two oxides are important semiconductor oxides. During our experimental process, PMMA microspheres were pretreated by NH3.H2O first. Solution route was then used to prepare the transitional metal oxides crystals. Precipitation agent in this step was crucial. After that, an aging reaction was carried out to obtain spherical shell shaped aggregation with well crystallinity. SEM, XRD, TEM, TGA, UV and IR measurements were used to investigate the morphology, size, texture and structure of the products. Furthermore, formation mechanism of such spherical shell shaped aggregation was explored.The results show that shape and size of the sub-crystallites, as well as the form of the aggregations can be tuned by many parameters, such as the zince/copper source, precipitation agent, aging temperature, aging time and pH value of the solution. The final core-shell composites contain integral spherical shell aggregated by ZnO crystals with the size of 100 nm, or CuO crystals with size of 200 nm-300 nm. The composites exhibit novel UV adsorption in the range of ultra-violet band. The decomposition temperature of the PMMA core increase for about 50℃-100℃with the protection of the oxide shell. High-resolution observation shows that the shell aggregation has porous character. More interestingly, porous ZnO aggregation with hollow interior are obtained after the removing of the PMMA core, implying that the shell aggregation depends on certain binding and interpenetrating among its building units.XPS analysis shows that a trace of N was detected on the surface of NH3.H2O treated PMMA microspheres. Ammonolysis is believed to take place at the surface of the PMMA microspheres. Since O and N are preferable ligands to Zn ions and Cu ions, the cations tended to bind to the surface of the PMMA cores. The crystal growth centers are thus supplied to induce the preferable crystallization of ZnO crystal on the surface of PMMA. It is found that NaOH is the optimal precipitation agent for ZnO crystal aggregated on the PMMA surface, and NH3.H2O for CuO crystal. Both the precipitation can react with the cation to form coordination structure, correspondingly. Formation of the complex restrains the crystal growth homogeneously, and the heterogeneous nucleation on the PMMA surface becomes preferable. With the growth of the nuclei during the aging reaction following a typical Ostwald ripening process, the constructional shell aggregations composed of transitional metal oxide particles are finally formed.