Controllable Synthesis Of CeO₂ Nanomaterials And Investigations Of Their High Pressure Phase Transition

It is well known naomaterials exhibit various novel physical and chemical properties compared with their bulk counterparts. The differences between namomaterials and bulk materials are usually attributed to the confinement effect and surface effect owing to the decrease of the particle size and the enlargement of surface-area/particle-size ratio. In order to clearly reveal the nano-effects, many works have been carried out around this topic and made effective progresses. And the researches confirm the properties of nanomaterials such as optical and electrical properties, mechanical stability and phase-transition mechanics are sensitive dependency on size, shape and structure. Thus, tailoring the size, shape and structure of nanostructure becomes a hot topic.High pressure physics is a novel cross-disciplinary, which bases on Material physics, Geophysics and Astronomy. High pressure can effectively change the distances between molecules and atoms. The changes of structure will do strong effects on the properties of materials. Measurements of the high-pressure properties not only directly characterize the physical properties of material, but also indirectly reveal its corresponding intrinsic characteristics. Thus, high-pressure technology is an important and effective route for us to understand the structures, properties and even their relationship. In recent years, nanotechnology develops rapidly. Nanotechnology combines with high-pressure technology, and it will reveal many attractive and novel phenomena, structures and mechanics.As a well-known functional rare earth material, CeO₂ has a wide range of applications including ultraviolet (UV) blockers, abrasives, catalysts and solid oxide fuel cells. Nanostructured CeO₂ possesses the superior physical and chemical properties compared with its bulk counterparts. Thus, many progresses have been made in the study of the synthesis of CeO₂ nanomaterials and in the investigation of their corresponding novel properties. In the past decades, extensive research efforts have been devoted to the design and preparation of CeO₂ nanomaterials with different shapes/sizes due to their particular shape/size-dependent properties. Up to now, well-defined CeO₂ nanostructures with various morphologies including nanoparticles, nanorods, nanoflowers, and hollow structures have been successfully fabricated. At the same time, high pressure study on CeO₂ nanomaterials with various sizes and shapes have attracted much energy, and it provides a new perspective to understand CeO₂ nanomaterials, comprehensively.CeO₂ nanomaterials with different morphologies were successfully synthesized by a facile hydrothermal/solvothermal process. We systematically researched the impacts of experimental environments including reaction time, reaction temperature, reactant concentration, pH value and surfactant on the size, shape and structure of CeO₂ nanomaterials. The growth mechanics of different morphologies were seriously investigated. Moreover, high pressure studies on CeO₂ nanomaterials with different size and shape were carried out through in-situ X-ray diffraction and Raman techniques. Additionally, the effects of experimental conditions'on high pressure behaviors of CeO₂ nanomaterials were also illustrated. We have found both the properties of nano-CeO₂ itself and hydrostatic condition do a strong impact on the high-pressure behaviors of CeO₂ nanomaterials.In a hydrothermal process, nano-CeO₂ with different morphologies were obtained. It was found (100)-terminated CeO₂ nanorods can be prepared by modified reactant concentration. The diameter of CeO₂ nanorods is about 10 nm, with the length of more than 100 nm. It was revealed the growth mechanism from nanoparticles to nanocubes and then to nanorods. In the formation of nanorods, NaOH plays an important role. (110)-dominated CeO₂ nanosheets was obtained for the first time using a facile one-step hydrothermal method. The average size of CeO₂ nanosheets is 15 nm. In the typical hydrothermal process, NH₃·H₂O plays a critical role in tailoring the surface structure of the CeO₂ nanocrystals without the assistance of any surfactant or template. The key function of the NH₃·H₂O on tailoring the morphology of CeO₂ nanocrystals was investigated. The results show that the NH3·H2O not only serves as the precipitant, but also acts as the structural direction agent in the formation of (110)-dominated CeO₂ nanosheets. In the synthesis process, NH3·H2O was used to tailor the surface structure of the sample without any surfactants or requiring any precursor. It is a one-step process without using organic additives. Hence, it is an environmentally friendly method for making advanced nanomaterials and devices. In order to further confirm the indispensable function of NH₃·H₂O on tailoring the morphology of CeO₂ nanocrystals, some comparative experiments were performed. In the comparative experiments, NH3·H2O was substituted by NaOH or Na2CO3, while other synthetic parameters were kept the same as those in the typical synthesis. When NaOH or Na2CO3 was used instead of NH3·H2O, no regular morphologies could be observed and the resulting products agglomerate randomly. Thus, NH3·H2O is irreplaceable. With addition of Na3PO4, (111)-dominated CeO₂ octahedron with the size of 150 nm was obtained. On this basis, with the assistant of surfactant PVP, ultrafine CeO₂ nanoparticles were successfully synthesized. The size of nanoparticles is 80-100 nm. It was found that the larger steric effect of PVP led to the formation of self-assembled nanospheres. Optical research revealed the optical band gap for CeO₂ nanosheets nano-octahedra and naospheres were 3.45 eV, 3.42 eV and 3.51 eV, respectively. Compared to bulk CeO₂ (3.19 eV), they exhibited lager blue shift and their blue-shifting exceeded 7.5%, 8% and 13 %. This large blue shift is due to quantum size effect of CeO₂ nano-materials.Monodisperse CeO₂ nanospheres with the average size of 40 nm, self-assembled by well-crystalline CeO₂ ultrafine nanoparticles were synthesized through a facile solvothermal route using n-butanol as solvent in the absence of any surfactant or template. The formation process of the monodisperse self-assembly CeO₂ nanospheres is briefly discussed. The key function of n-butanol is investigated by comparative experiments. It is found that n-butanol not only serves as solvent, but also acts as surfactant in the formation of monodisperse self-assembly CeO₂ nanospheres. With the modification and steric effect of n-butanol, CeO₂ nanoparticles aggregated to highly compacted nanospheres. Interestingly, addition NH₃·H₂O or other Alkaline substances, ultrafine CeO₂ nanoparticles with size of 5-8 nm were prepared. We investigated their optical properties, and obtained the optical band gap were 3.51 eV and 3.60 eV. Compared with bulk counterpart, the blue-shifting was about 10% and 13%, respectively.In situ high-pressure X-ray diffraction and Raman spectroscopy have been performed on well-shaped CeO₂ nano-octahedrons enclosed by eight (111) planes. The CeO₂ nano-octahedrons are shown to be more stable than their bulk counterparts and some other reported CeO₂ nanocrystals of smaller size. The transition pressure from cubic to orthorhombic phase is approximately 12 GPa higher than that of 12 nm CeO₂ nanocrystals even though they have similar volume expansion at ambient conditions. Additionally, the phase transition toα-PbCl₂ phase is very sluggish and uncompleted even up to 55 GPa. TEM image of the sample after decompression from 55 GPa clearly shows that the nano-octahedrons preserve the starting shape. Such distinct high-pressure behaviors in CeO₂ nano-octahedrons have been discussed in terms of their special exposure surface. Further analysis shows that the lower compressibility of the exposed (111) planes in the nano-octahedrons is believed to be the major factor to the elevation of phase-transition pressure and the sluggishness of the transition.High-pressure Raman study under quasi-hydrostatic condition has been performed on CeO₂ nanospheres self-assembled by 5 nm CeO₂ nanoparticles at room temperature. Surprisingly, as the pressure elevate to 34 GPa, the CeO₂ nanospheres still retain the cubic fluorite-type structure, indicating the sample is more stable than the bulk counterpart. Whereas, previous high-pressure studies show the phase transition at 22.3/26.5 GPa for 12 nm CeO₂ nanoparticles, which is less stable than the bulk materials. The enhancement of phase stability might be attributed to the increase of surface energy of CeO₂ nanospheres as the size of the building units decrease.CeO₂ nanocrystals with similar grain size which have particle, rod and sheet morphologies have been studied by in situ high-pressure X-ray diffraction studies under quasi-hydrostatic condition and non-hydrostatic condition. It is found under quasi-hydrostatic condition, all the samples maintain their fluorite-type structure in the whole compression processes. This indicates phase transition pressures for CeO₂ nanoparticles, nanorods and nanosheets are much higher than the bulk counterpart, which could transform toα-PbCl₂-type structure at about 31 GPa. However, all samples exhibited phase transition pressures at 31 GPa, while compressed at non-hydrostatic condition, which similar to bulk-CeO₂. TEM image of CeO₂ nanosheets after decompression from 58 GPa under non-hydrostatic condition clearly shows that obvious regrowth occurred in the interface of grains. This indicates larger strain exists in grain boundaries, which is much higher than that applied by anvils. The larger strain could be stronger enough to lead to occurrence of phase transitions. Thus, hydrostatic condition has an important effect on the high-pressure behaviors of CeO₂ nanomaterials.