Optimized Control And Properties Of Cerium Oxide Micro-/Nanostructures

Due to easy shuttle between Ce4+and Ce3+, cerium oxide with the nature of non-toxicity and environmental benign has been ultilized in many fields of energy and environment. During these, cerium oxide can not only serve as active components, but also the promising supports. In this dissertation, multi-shell ceria, porous microcube-like ceria and CeO2/CePO4co-crystallized composites are suceessfully fabricated via the facile hydrothermal route. In combination with various characterization methods, microscopic structures and the formation mechnisms of these hierarchical structures are explored in detail and the relationships between the morphologies and their corresponding properties are aslo established. The main research contents are listed as follows:(1) The hydrothermal treatment is adopted to enhance the capture of cerium species in the stage of adsorption of carbonaceous spheres (CS). When the obtained adsorbed-CS containing enough cerium species are calcined at the heat ramp of1℃min-1,2℃min-1and10℃min-1respectively, single-shell, double-shell and triple-shell ceria are harvested in order. It is obvious that shell numbers increase with the heat ramp. The law is confirmed when some quadruple shell appeared at the heat ramp of15℃min-1. Take triple-shell ceria for example, they possess hierarchical featured size:the exterior, middle and interior shells are about300nm,180nm and80nm, respectively, and the average shell thickness is30nm.(2) These multi-shell ceria have higher specific area of～90m2/g and the pore distribution is in the range of1～4nm, which might allow them to serve as good supports for noble metal nanoparticles. Here, Au nanoparticles are selected and deposited to construct the heterogeneous catalyst Au/CeO2. The catalyst shows superior catalytic performance for the reduction of4-nitrophenol.(3) The hydrothermal-enhanced approach for the fabrication of smaller sized multi-shell structures has certain generalization and has been extended to synthesize the multi-shell structures of various metal oxides (La2O3,NdO3Co3O4, NiO), doped ceria (Co-CeO2) and ceria-based composites (Al2O/CeO2C/CeO2). Among them, and have the potential application in photocatalysis due to their improved absorption for UV-vis light. And the approach opens the door for synthesis of smaller sized multi-shell functional materials and has good application future.(4) In the glucose-oxalate system, a common hydrothermal method was utilized to obtain microcube-like ceria precursors with the size of～20μm. And the precursors possess lots of pore channels. From the XRD and IR analysis, it is clear that these precursors are orgnic-inorganic crystals with high crystallinity and have many hydrophilic functional groups. The results from TG-DSC analysis indicate the orgnic-inorganic crystals have good thermal stability and start to decompose until335℃. The weight loss reaches to～51%, which aslo shows the crystals contain lots of orgnic component. Through calcination to the as-formed precursors, hierarchically porous microcube-like ceria were successfully obtained. The porous structures are made up of nanoparticles and have higher specific area of～53.1m2/g.(5) CeO2/CePO4co-crystallized composites are fabricated by adjusting the molar ratio of Ce/P in the hydrothermal environment. It is found that the photoluminescience of the composites is dependent on the molar ratio of Ce/P and the strongest PL emission can be achieved with initial Ce/P of4. It is proposed that the proper coatings of CeO2to CePO4might be responsible for the good PL properties.(6) Uniform CePO4nanowires have been synthesized in a low phosphate concentration system through a single-step hydrothermal process. The CePO4nanowires are shown to go through phase evolution from pure monoclinic to mixed hexagonal and monoclinic phase by only increasing the initial molar ratio of Ce/P. Interestingly, the strongest photoluminescence was observed in the CePO4nanowires synthesized with the initial Ce/P of4:1, which proved to be the critical phase evolution point between the hexagonal and monoclinic CePO4. The better PL properties might be related to the existence of the structure-sensitive energy level in this particular structure of the critical point.