| Nanoscale materials have attracted much attention due to their unique physical and chemical properties as well as their potential applications in catalysts, gas sensors, and optoelectronic nanodevices and so on. One critical scientific and technological issue of nanoscale materials is to further improve their corresponding activity as well as utilization efficiency. Surface is a special and important component of solid state materials. Since many chemical and physical processes take place on the surface of materials, the surface structure determines many surface related physical and chemical properties of the materials. Therefore, controllable synthesis of nanomaterials with specific crystal structures and various morphologies with different surface structure becomes a hot research topic nowadays. According to our knowledge, however, most of naturally and artificially grown nanocrystals are exposed with low energy facets, as high-energy surfaces usually diminish quickly for minimization of surface energy during a crystal growth process. Yet we believe that with our knowledge of crystal growth, we can create some proper crystal growth environments where nanocrystals exposed with high energy facets could be fabricated.On the other hand, nanoscale heterostructured materials have been extensively studied due to the synergistic effects existed among the components of the heterostructured materials, which can significantly enhance the performances of materials in various fields such as catalysis, gas sensing, solar cells, etc. However, due to the difficulty of control synthesis of nanocrystals with specific surface, it is usually hard to study the heterostructures on specific crystal surfaces. According to our efforts on the control of surface structure of nanocrystals, we firmly believe that controllable synthesis of nanoscale heterostructure on specific surface of nanocrystals also plays a critical role in tuning the properties of the materials.In this thesis, we respectively synthesized Fe2O3and ZnO nanocrystals with specific surface structures and constructed noble metals/metal oxides nanoscale heterostructure on various crystal facets, and investigated their surface related properties. The thesis is composed by4chapters.Chapter1. Briefly review on the crystal surface structure and corresponding properties, crystal growth mechanism and properties/application of nanoscale heterostructure materials, as well as clarify my research significance and main contents.Chaper2. a-Fe2O3nanocrystals exposed with{113},{012} and{001}/{012} were successfully fabricated respectively by controlling supersaturation of growth species in the reaction. The effect of supersaturation on shape evolution of a-Fe2O3nanocrystals was studied, which showed that the facets with high surface energy, such as{113}, tended to be formed in the reaction solution of high supersaturation, while the relatively more stable facets, such as{012} or{001}, were favored in the reaction solution of lower supersaturation. In addition, the differences of the catalytic activity in CO oxidation and sensing performances of these specific faceted nanocrystals were ascribed to the deferences of the adsorption capability to CO and oxygen species on different crystal facets.Chapter3. Pt was loaded on the previously fabricated a-Fe2O3nanocrystals. From the characterization of Pt loaded samples, it was found that Pt mainly existed as the forms of PtO2and Pt(OH)2, respectively, which also varied according to different crystal facets. Moreover, the catalytic activity of Pt loaded samples in CO oxidation was co-determined by Pt as well as PtO2, where the latter one was proven to be more important. Gas sensing performances of these samples are all enhanced with existence of Pt, especially on the{012} facet.Chapter4. ZnO pyramid nanocrystals with (001),(001) and{101} polar crystal facets has been successfully synthesized, and the relationship between surface structures and migrations of photon-generated carriers has been studied by selective load of (Ag, Au)/MnO2-It was discovered that, under ultraviolet light, holes tended to migrate towards (001) facet exposed with O2-, turning (001) into an oxidizing facet, which led to the formation of MnO2; on the other hand, electrons tend to migrate towards (001) facet exposed with Zn2+, making (001) a reducing facet where noble metals (Ag, Au) were loaded. |
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