二氧化铈纳米材料的可控制备及其高压结构相变的研究
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摘要
当物质的尺寸减小到纳米尺度范围内,由于限域效应和表面效应,物质将出现许多新颖的物理和化学性质。研究发现:纳米材料的物理化学性质紧紧依赖其本身的尺寸和形貌,因此,实现纳米材料尺寸和形貌的可控制备成为当今材料领域具有重大意义的研究热点。
高压物理学是一门以材料物理学、地球物理学以及天文物理学为背景发展起来的交叉学科。在高压下,物质内部分子之间、原子之间距离会发生变化,导致宏观上物质的力学、热学、光学、电学和磁学性质发生改变。近年来,随着纳米技术的飞速发展,纳米材料与高压手段相结合,出现一些非常有趣的新现象、新结构和新规律,因此,纳米材料的高压研究吸引了研究者极大的研究热情。
CeO_2作为一种典型的功能型稀土氧化物,具有较高的抛光精度、高度的化学稳定性、卓越的紫外吸收能力、独特的储放氧功能、优异的离子导电特性和较高的介电常数,因而广泛地应用于微电子电路化学机械抛光、电化学、紫外涂层和催化等技术领域。纳米CeO_2由于具有许多优异于体材料的物理化学性质,近年来受到研究者的广泛关注,关于其尺寸和形貌可控制备的研究已经成为合成领域一个具有重大科学意义的研究课题。同时,具有不同尺寸、形貌的CeO_2纳米材料在高压下的结构相变也是全面认识CeO_2纳米材料物理化学性质的新课题。
本文以CeO_2纳米材料作为研究对象,利用水热法和溶剂热法成功制备出不同尺寸、形貌的CeO_2纳米材料,包括:零维纳米颗粒、一维纳米棒、二维纳米片、三维八面体形貌和自组装球形纳米结构。系统地研究了水热和溶剂热过程中,反应温度、反应时间、添加物的浓度、溶液的pH值和表面活性剂等因素对CeO_2纳米材料的尺寸、形貌和结晶度的重要影响,探索了不同尺寸、形貌的CeO_2纳米材料的生长机制,并对其光学性质进行了深入探索。同时,利用原位高压同步辐射X光衍射和高压Raman方法,对不同尺寸、形貌的CeO_2纳米材料的高压结构相变进行了系统的研究,揭示出CeO_2纳米材料本身的性质(包括尺寸和形貌)以及高压研究中静水性条件对其高压结构相变的重要影响。
系统地研究了水热法中反应温度、反应时间、溶液的pH值和表面活性剂对CeO_2纳米材料结晶度、形貌和尺寸的调制作用。成功制备了具有不同尺寸、形貌的CeO_2纳米晶体,包括:一维纳米棒,二维纳米片和三维纳米八面体以及自组装纳米球。在CeO_2一维纳米晶体的水热制备中,首次获得了[100]择优生长的一维纳米棒;研究发现,沉淀剂NaOH对CeO_2纳米晶体的形貌具有重要的调制作用,通过提高pH值,可以实现由零维颗粒状→三维立方体→一维棒状形貌的可控制备,碱性物质起到了结构导向的重要作用,抑制了其它晶面的生长。在CeO_2二维纳米晶体的制备中,首次合成了具有(110)暴露面的CeO_2纳米片;在CeO_2纳米片的制备过程中首次发现氨水对片状形貌的形成具有重要的影响:既作为矿化剂,同时又充当了结构导向剂。在CeO_2三维纳米晶体的制备中,揭示了具有(111)暴露面的CeO_2纳米八面体的形貌演化过程;此外,还利用表面活性剂PVP,成功获得了CeO_2自组装纳米球,阐明了PVP较大的空间位效应导致自组装纳米球的生成。探索了不同尺寸、形貌的CeO_2纳米晶体的光学性质,研究表明CeO_2纳米片、纳米八面体和自组装纳米球的光学带隙分别为3.45 eV,3.42 eV和3.51 eV,与体材料(3.19 eV)相比,紫外吸收边发生明显的蓝移,这是由于CeO_2纳米材料的量子尺寸效应。
利用简单的溶剂热法成功制备出单分散的CeO_2自组装纳米球,发现正丁醇对CeO_2纳米自组装球的形貌具有重要的调制作用,不仅充当了反应溶剂,而且起到了表面活性剂的重要作用。通过进一步添加碱性沉淀剂,还成功获得了10 nm以下的单分散CeO_2超精细纳米晶体。该方法不需要其他有机添加剂的辅助,避免了煅烧工艺,是一种简单、高效、节能的一步制备方法。CeO_2单分散自组装纳米球和超精细纳米颗粒的光学研究表明,它们具有更加优异的紫外吸收能力,与体材料相比,带隙宽度分别蓝移约10%和13%,这种较大的蓝移是由于最小组成晶粒的尺寸与CeO_2的玻尔激子半径十分接近。
在高达55GPa的压力范围内,利用原位高压同步辐射X光衍射和高压Raman方法系统地研究了尺寸为150 nm、具有(111)暴露面的CeO_2纳米八面体的高压相变行为,发现其发生了由立方萤石型结构向正交α-PbCl_2型的结构相变,相变压力和相变弛豫过程均高于体材料和12 nmCeO_2球形纳米颗粒,该相变是可逆相变,体弹模量略高于体材料。详细地分析了内部缺陷、表面能和内聚能对其相变行为的影响,阐明其新奇的高压相变行为归因于自身特殊的几何形貌。还进一步揭示出暴露的(111)晶面的难压缩性最终导致CeO_2纳米八面体表现出独特的高压相变行为。卸压后的样品仍然保持原始八面体形貌。
在最高压力为34 GPa的压力范围内,利用原位高压Raman方法对由最小组成单元尺寸小于其玻尔激子半径、5 nm的颗粒组成的CeO_2自组装纳米球进行了高压结构相变的研究,发现:在整个压力范围内,样品没有发生结构相变,与体材料在31 GPa就发生结构相变的结果截然不同,这种反常的高压相变行为可能是由于其较小的晶粒尺寸导致表面能大大提高。
利用原位高压同步辐射X光衍射和Raman方法,系统地研究了具有相同尺寸(平均尺寸为12 nm)、不同相貌的CeO_2纳米材料(包括纳米颗粒、纳米棒和纳米片)在不同静水压条件下的高压结构相变行为。首次发现:在静水条件下,CeO_2纳米颗粒在51 GPa压力范围内、纳米棒在45.5 GPa压力范围内、纳米片在高达71.5 GPa压力范围内均保持立方萤石型结构,没有发生在12 nm和15 nmCeO_2纳米颗粒以及体材料中所经历的压致结构相变。然而,在非静水压条件下,这些纳米晶体高压相变行为与前人报道的体材料和纳米颗粒的高压相变十分相似。对卸压后的样品进行HRTEM分析发现,晶界处有正交α-PbCl_2相存在,表明在非静水压条件下,晶界处存在较大的应力,从而使CeO_2纳米晶体在晶界处极易形成高压相成核点,导致相变的发生。表明以往人们在小尺寸CeO_2纳米材料所观察到的压致结构相变是非静水压导致的,揭示出结构稳定性提高才是小尺寸纳米效应的本质特征。
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, CeO_2 has a wide range of applications including ultraviolet (UV) blockers, abrasives, catalysts and solid oxide fuel cells. Nanostructured CeO_2 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 CeO_2 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 CeO_2 nanomaterials with different shapes/sizes due to their particular shape/size-dependent properties. Up to now, well-defined CeO_2 nanostructures with various morphologies including nanoparticles, nanorods, nanoflowers, and hollow structures have been successfully fabricated. At the same time, high pressure study on CeO_2 nanomaterials with various sizes and shapes have attracted much energy, and it provides a new perspective to understand CeO_2 nanomaterials, comprehensively.
CeO_2 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 CeO_2 nanomaterials. The growth mechanics of different morphologies were seriously investigated. Moreover, high pressure studies on CeO_2 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 CeO_2 nanomaterials were also illustrated. We have found both the properties of nano-CeO_2 itself and hydrostatic condition do a strong impact on the high-pressure behaviors of CeO_2 nanomaterials.
In a hydrothermal process, nano-CeO_2 with different morphologies were obtained. It was found (100)-terminated CeO_2 nanorods can be prepared by modified reactant concentration. The diameter of CeO_2 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 CeO_2 nanosheets was obtained for the first time using a facile one-step hydrothermal method. The average size of CeO_2 nanosheets is 15 nm. In the typical hydrothermal process, NH_3·H_2O plays a critical role in tailoring the surface structure of the CeO_2 nanocrystals without the assistance of any surfactant or template. The key function of the NH_3·H_2O on tailoring the morphology of CeO_2 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 CeO_2 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 NH_3·H_2O on tailoring the morphology of CeO_2 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 CeO_2 octahedron with the size of 150 nm was obtained. On this basis, with the assistant of surfactant PVP, ultrafine CeO_2 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 CeO_2 nanosheets nano-octahedra and naospheres were 3.45 eV, 3.42 eV and 3.51 eV, respectively. Compared to bulk CeO_2 (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 CeO_2 nano-materials.
Monodisperse CeO_2 nanospheres with the average size of 40 nm, self-assembled by well-crystalline CeO_2 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 CeO_2 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 CeO_2 nanospheres. With the modification and steric effect of n-butanol, CeO_2 nanoparticles aggregated to highly compacted nanospheres. Interestingly, addition NH_3·H_2O or other Alkaline substances, ultrafine CeO_2 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 CeO_2 nano-octahedrons enclosed by eight (111) planes. The CeO_2 nano-octahedrons are shown to be more stable than their bulk counterparts and some other reported CeO_2 nanocrystals of smaller size. The transition pressure from cubic to orthorhombic phase is approximately 12 GPa higher than that of 12 nm CeO_2 nanocrystals even though they have similar volume expansion at ambient conditions. Additionally, the phase transition toα-PbCl_2 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 CeO_2 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 CeO_2 nanospheres self-assembled by 5 nm CeO_2 nanoparticles at room temperature. Surprisingly, as the pressure elevate to 34 GPa, the CeO_2 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 CeO_2 nanoparticles, which is less stable than the bulk materials. The enhancement of phase stability might be attributed to the increase of surface energy of CeO_2 nanospheres as the size of the building units decrease.
CeO_2 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 CeO_2 nanoparticles, nanorods and nanosheets are much higher than the bulk counterpart, which could transform toα-PbCl_2-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-CeO_2. TEM image of CeO_2 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 CeO_2 nanomaterials.
高压物理学是一门以材料物理学、地球物理学以及天文物理学为背景发展起来的交叉学科。在高压下,物质内部分子之间、原子之间距离会发生变化,导致宏观上物质的力学、热学、光学、电学和磁学性质发生改变。近年来,随着纳米技术的飞速发展,纳米材料与高压手段相结合,出现一些非常有趣的新现象、新结构和新规律,因此,纳米材料的高压研究吸引了研究者极大的研究热情。
CeO_2作为一种典型的功能型稀土氧化物,具有较高的抛光精度、高度的化学稳定性、卓越的紫外吸收能力、独特的储放氧功能、优异的离子导电特性和较高的介电常数,因而广泛地应用于微电子电路化学机械抛光、电化学、紫外涂层和催化等技术领域。纳米CeO_2由于具有许多优异于体材料的物理化学性质,近年来受到研究者的广泛关注,关于其尺寸和形貌可控制备的研究已经成为合成领域一个具有重大科学意义的研究课题。同时,具有不同尺寸、形貌的CeO_2纳米材料在高压下的结构相变也是全面认识CeO_2纳米材料物理化学性质的新课题。
本文以CeO_2纳米材料作为研究对象,利用水热法和溶剂热法成功制备出不同尺寸、形貌的CeO_2纳米材料,包括:零维纳米颗粒、一维纳米棒、二维纳米片、三维八面体形貌和自组装球形纳米结构。系统地研究了水热和溶剂热过程中,反应温度、反应时间、添加物的浓度、溶液的pH值和表面活性剂等因素对CeO_2纳米材料的尺寸、形貌和结晶度的重要影响,探索了不同尺寸、形貌的CeO_2纳米材料的生长机制,并对其光学性质进行了深入探索。同时,利用原位高压同步辐射X光衍射和高压Raman方法,对不同尺寸、形貌的CeO_2纳米材料的高压结构相变进行了系统的研究,揭示出CeO_2纳米材料本身的性质(包括尺寸和形貌)以及高压研究中静水性条件对其高压结构相变的重要影响。
系统地研究了水热法中反应温度、反应时间、溶液的pH值和表面活性剂对CeO_2纳米材料结晶度、形貌和尺寸的调制作用。成功制备了具有不同尺寸、形貌的CeO_2纳米晶体,包括:一维纳米棒,二维纳米片和三维纳米八面体以及自组装纳米球。在CeO_2一维纳米晶体的水热制备中,首次获得了[100]择优生长的一维纳米棒;研究发现,沉淀剂NaOH对CeO_2纳米晶体的形貌具有重要的调制作用,通过提高pH值,可以实现由零维颗粒状→三维立方体→一维棒状形貌的可控制备,碱性物质起到了结构导向的重要作用,抑制了其它晶面的生长。在CeO_2二维纳米晶体的制备中,首次合成了具有(110)暴露面的CeO_2纳米片;在CeO_2纳米片的制备过程中首次发现氨水对片状形貌的形成具有重要的影响:既作为矿化剂,同时又充当了结构导向剂。在CeO_2三维纳米晶体的制备中,揭示了具有(111)暴露面的CeO_2纳米八面体的形貌演化过程;此外,还利用表面活性剂PVP,成功获得了CeO_2自组装纳米球,阐明了PVP较大的空间位效应导致自组装纳米球的生成。探索了不同尺寸、形貌的CeO_2纳米晶体的光学性质,研究表明CeO_2纳米片、纳米八面体和自组装纳米球的光学带隙分别为3.45 eV,3.42 eV和3.51 eV,与体材料(3.19 eV)相比,紫外吸收边发生明显的蓝移,这是由于CeO_2纳米材料的量子尺寸效应。
利用简单的溶剂热法成功制备出单分散的CeO_2自组装纳米球,发现正丁醇对CeO_2纳米自组装球的形貌具有重要的调制作用,不仅充当了反应溶剂,而且起到了表面活性剂的重要作用。通过进一步添加碱性沉淀剂,还成功获得了10 nm以下的单分散CeO_2超精细纳米晶体。该方法不需要其他有机添加剂的辅助,避免了煅烧工艺,是一种简单、高效、节能的一步制备方法。CeO_2单分散自组装纳米球和超精细纳米颗粒的光学研究表明,它们具有更加优异的紫外吸收能力,与体材料相比,带隙宽度分别蓝移约10%和13%,这种较大的蓝移是由于最小组成晶粒的尺寸与CeO_2的玻尔激子半径十分接近。
在高达55GPa的压力范围内,利用原位高压同步辐射X光衍射和高压Raman方法系统地研究了尺寸为150 nm、具有(111)暴露面的CeO_2纳米八面体的高压相变行为,发现其发生了由立方萤石型结构向正交α-PbCl_2型的结构相变,相变压力和相变弛豫过程均高于体材料和12 nmCeO_2球形纳米颗粒,该相变是可逆相变,体弹模量略高于体材料。详细地分析了内部缺陷、表面能和内聚能对其相变行为的影响,阐明其新奇的高压相变行为归因于自身特殊的几何形貌。还进一步揭示出暴露的(111)晶面的难压缩性最终导致CeO_2纳米八面体表现出独特的高压相变行为。卸压后的样品仍然保持原始八面体形貌。
在最高压力为34 GPa的压力范围内,利用原位高压Raman方法对由最小组成单元尺寸小于其玻尔激子半径、5 nm的颗粒组成的CeO_2自组装纳米球进行了高压结构相变的研究,发现:在整个压力范围内,样品没有发生结构相变,与体材料在31 GPa就发生结构相变的结果截然不同,这种反常的高压相变行为可能是由于其较小的晶粒尺寸导致表面能大大提高。
利用原位高压同步辐射X光衍射和Raman方法,系统地研究了具有相同尺寸(平均尺寸为12 nm)、不同相貌的CeO_2纳米材料(包括纳米颗粒、纳米棒和纳米片)在不同静水压条件下的高压结构相变行为。首次发现:在静水条件下,CeO_2纳米颗粒在51 GPa压力范围内、纳米棒在45.5 GPa压力范围内、纳米片在高达71.5 GPa压力范围内均保持立方萤石型结构,没有发生在12 nm和15 nmCeO_2纳米颗粒以及体材料中所经历的压致结构相变。然而,在非静水压条件下,这些纳米晶体高压相变行为与前人报道的体材料和纳米颗粒的高压相变十分相似。对卸压后的样品进行HRTEM分析发现,晶界处有正交α-PbCl_2相存在,表明在非静水压条件下,晶界处存在较大的应力,从而使CeO_2纳米晶体在晶界处极易形成高压相成核点,导致相变的发生。表明以往人们在小尺寸CeO_2纳米材料所观察到的压致结构相变是非静水压导致的,揭示出结构稳定性提高才是小尺寸纳米效应的本质特征。
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, CeO_2 has a wide range of applications including ultraviolet (UV) blockers, abrasives, catalysts and solid oxide fuel cells. Nanostructured CeO_2 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 CeO_2 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 CeO_2 nanomaterials with different shapes/sizes due to their particular shape/size-dependent properties. Up to now, well-defined CeO_2 nanostructures with various morphologies including nanoparticles, nanorods, nanoflowers, and hollow structures have been successfully fabricated. At the same time, high pressure study on CeO_2 nanomaterials with various sizes and shapes have attracted much energy, and it provides a new perspective to understand CeO_2 nanomaterials, comprehensively.
CeO_2 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 CeO_2 nanomaterials. The growth mechanics of different morphologies were seriously investigated. Moreover, high pressure studies on CeO_2 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 CeO_2 nanomaterials were also illustrated. We have found both the properties of nano-CeO_2 itself and hydrostatic condition do a strong impact on the high-pressure behaviors of CeO_2 nanomaterials.
In a hydrothermal process, nano-CeO_2 with different morphologies were obtained. It was found (100)-terminated CeO_2 nanorods can be prepared by modified reactant concentration. The diameter of CeO_2 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 CeO_2 nanosheets was obtained for the first time using a facile one-step hydrothermal method. The average size of CeO_2 nanosheets is 15 nm. In the typical hydrothermal process, NH_3·H_2O plays a critical role in tailoring the surface structure of the CeO_2 nanocrystals without the assistance of any surfactant or template. The key function of the NH_3·H_2O on tailoring the morphology of CeO_2 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 CeO_2 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 NH_3·H_2O on tailoring the morphology of CeO_2 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 CeO_2 octahedron with the size of 150 nm was obtained. On this basis, with the assistant of surfactant PVP, ultrafine CeO_2 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 CeO_2 nanosheets nano-octahedra and naospheres were 3.45 eV, 3.42 eV and 3.51 eV, respectively. Compared to bulk CeO_2 (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 CeO_2 nano-materials.
Monodisperse CeO_2 nanospheres with the average size of 40 nm, self-assembled by well-crystalline CeO_2 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 CeO_2 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 CeO_2 nanospheres. With the modification and steric effect of n-butanol, CeO_2 nanoparticles aggregated to highly compacted nanospheres. Interestingly, addition NH_3·H_2O or other Alkaline substances, ultrafine CeO_2 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 CeO_2 nano-octahedrons enclosed by eight (111) planes. The CeO_2 nano-octahedrons are shown to be more stable than their bulk counterparts and some other reported CeO_2 nanocrystals of smaller size. The transition pressure from cubic to orthorhombic phase is approximately 12 GPa higher than that of 12 nm CeO_2 nanocrystals even though they have similar volume expansion at ambient conditions. Additionally, the phase transition toα-PbCl_2 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 CeO_2 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 CeO_2 nanospheres self-assembled by 5 nm CeO_2 nanoparticles at room temperature. Surprisingly, as the pressure elevate to 34 GPa, the CeO_2 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 CeO_2 nanoparticles, which is less stable than the bulk materials. The enhancement of phase stability might be attributed to the increase of surface energy of CeO_2 nanospheres as the size of the building units decrease.
CeO_2 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 CeO_2 nanoparticles, nanorods and nanosheets are much higher than the bulk counterpart, which could transform toα-PbCl_2-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-CeO_2. TEM image of CeO_2 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 CeO_2 nanomaterials.
引文
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