Synthesis And Properties Of Ultrathin CeO₂ Nanowires And Its Composite Materials

Recent years, one-dimensional nanostructure has been the focus of investigation due to its anisotropy and unique application in mesoscopic physics and manufacture of nano-device. As an important rare earth oxide, cerium oxide (CeO2 or ceria) has been applied in the fields including catalysis, optics, electrochemistry and environmental chemistry due to its unique physical and chemical properties. The surface defects of CeO2 mainly attributed to oxygen vacancy. In the oxygen-lacked environment, CeO2 can form a large amount of oxygen vacancies, releasing oxygen to the environment; while in oxygen-enriched environment, CeO2 can store oxygen by the oxygen vacancy, so the CeO2 possesses a superior ability to release oxygen and store oxygen. Oxygen vacancy on the surface of CeO2 plays an important role during the reaction of CO catalytic oxidation and the enhancement of catalytic activity usually due to the oxygen storage capacity (OSC) of CeO2. With the decrease of the size of materials to nanoscale, defects in nanocrystalline oxides could be drastically reduced, resulting in the degree of stoichiometric intensified and generation of electron carriers increased. With the decrease of size of particles, CeO2 particles obtain larger ratio of surface area and volume, generating more oxygen vacancies. When the size of CeO2 reduced into nanoscale, CeO2 not only retains the unique electronic layer structure of rare earth elements, but also possesses the features caused by size effect, making CeO2 nanocrystalline have many unique properties superior to bulk materials. In this thesis, we take one-dimensional CeO2 nanowires as research object, and develop the technology of chemical preparation and property of the composite materials. The results obtained are as follows:1. We established a new technology of preparation of ultrathin CeO2 nanowires by one-step method. CeO2 ultrathin nanowires with a diameter of 5 nm and a length over 500 nm have been synthesized by a one-step refluxing approach in a water/ethanol mixed solvent without any templates or surfactants. The formation mechanism of the ultrathin nanowires and the effect of the ratio of mixed solvent to the shape of CeO2 has been investigated. The CeO2 ultrathin nanowires possess a high surface area as large as 125.31 m2 g-1, with high adsorption capacities of organic dyes and heavy metal ions. The adsorption period of Congo red dye is only 30 min, but the adsorption capacity is as high as 2382.75 mg g-1. Besides, the CeO2 ultrathin nanowires could adsorb the Congo red dye selectively from the mixed solution of several organic dyes.2. We implemented effective composition of CeO2 ultrathin nanowires and noble metal. We dispersed the networks composed by CeO2 ultrathin nanowires in the HAuCl4 aqueous solution, and used NaBH4 as reductant, distributing Au nanoparticles with a size of 2-3 nm onto the CeO2 ultrathin nanowires uniformly. Noble metal-cerium oxide composite materials could complete the catalytic reduction of p-nitrophenol to p-aminophenol within 10 s. The ceria networks with large surface area not only stabilize the noble metal nanoparticles against aggregation but also enhance synergistic effect between substrate and noble metal nanoparticles, leading to extremely high activity and stability for catalytic application.3. We established a preparation technology for a series of transition metal doped CeO2 ultrathin nanowires. A series of Ce1-xCuxO2(x=0,0.02,0.05,0.10,0.20 and 0.40), Ce0.90M0.10O2 (M= Cu, Fe, Co, and Ni) ultrathin nanowires and CeO2 nanowires co-doped by double transiton metal ions could be prepared by a one-step refluxing method in a water/ethanol mixed solvent. We tested the catalytic property of these materials in heterogeneous reaction with CO oxidation as probe reaction. The result reveals that Ce0.90Cu0.10O2 doped with 10 at.% copper element obtained the highest catalytic activity and the T100 reduced to as low as 135℃. In the catalytic processes, both of the Ce4+/Ce3+ and Cu2+/Cu+redox cycles contributed to the promotion of CO oxidation by forming oxygen vacancies and the catalytic performance is strongly dependent on the choice of substrate.