有机—有机自组装合成新型碳纳米结构及其形貌控制和性质研究

【作者】 闫妍；

【导师】 赵东元；

【作者基本信息】 复旦大学 ， 无机化学， 2007， 博士

【摘要】 纳米材料因其特殊的光学、电学、磁学和化学性质,成为目前最活跃的研究领域之一。由碳元素组成的各类纳米结构,如碳纳米管、富勒烯及类富勒烯化合物、碳洋葱、碳纳米纤维等,自其发现以来就受到研究者的高度重视。碳元素在这些材料中通常以sp~2或sp~3的杂化形式存在,除了能形成共价键外,通常还存在大范围的不定域π键,这种独特的结构赋予材料丰富的物理化学性质,在电子器件、导热器件、抗摩擦剂、磁性存储介质等方面都有着巨大的实用性意义。与众不同的稳定性使得碳纳米结构的形成过程需要巨大的能量辅助,其合成方法往往局限于高温、高压或催化条件下的固相反应策略。固相合成过程虽然已有多年的研究基础,但其成本过高,对仪器和制备过程要求苛刻,所采用的前驱物通常也具有较强的毒性。此外,这种极端条件下的合成过程较难控制,所获产物通常纯度较低,其分离纯化过程也存在一定的难度。因此,开发温和、经济的绿色溶液合成过程,寻找碳纳米结构合成与组装规律的研究势在必行。在本论文中,我们以限制空间中碳纳米结构的合成为主要研究内容,成功地开发了碳纳米结构的液相合成路线。通过深入了解有机—有机组装过程,选择新的合成体系和不同的碳源物质,在较低温度下成功地合成了一系列新型碳纳米结构,丰富了碳纳米结构溶液合成过程的反应规律。同时我们还对一种新型碳纳米结构材料—介孔碳材料的形貌控制手段和高温热稳定性进行了研究,为其进一步实际应用打下坚实基础。在论文的第二章中,我们提出一种低温、无催化的溶剂热合成方法。采用糠醇为聚合单体、十二烷基苯磺酸钠为结构导向剂,通过控制有机分子的自组装过程,首次合成了高度晶化的糠醇聚合物纳米线结构。通过控制结构导向剂的用量和溶剂热反应的时间,一维聚合物纳米结构的尺寸可以在宽度为10-30nm之间,而长度则可以在30-800nm之间调控。值得一提的是,我们将该方法运用到了一系列结构相似的聚合物纳米结构的制备中,结果表明有机-有机导向下溶剂热合成法也是合成其他高分子聚合物纳米结构的一种普适方法。在第三章中,我们深化限制空间的合成概念,提出了一种低温溶剂热注入合成路线,采用中间相沥青或者糠醇为碳前驱物,通过控制芳环分子之间的自组装过程,首次在低温液相下合成得到了3-5nm超小尺寸的石墨纳米颗粒、尺寸在30-50nm之间的碳洋葱颗粒和直径小于10nm而长度可达数百纳米的石墨纳米绳。这些碳纳米结构的形貌均一,产率很高,有望在催化、导热器件、抗摩擦剂、润滑剂、结构复合材料添加剂等实际应用研究中发现新的性质。论文的第四章,重点阐述了介孔碳材料的形貌控制。提出了利用喷雾干燥诱导自组装的方法实现对介孔碳的形貌和结构的双重调控。实验中我们采用三嵌段共聚物P123或者F127为结构导向剂,低分子量酚醛树脂为碳前驱体,得到了直径在100nm-5μm的球形介孔碳材料。简单地改变反应物比例和反应温度等条件,可以实现对产物孔道结构和孔径的双重调控。在反应温度为200或250℃时均可以得到具有六方、层状、蠕虫状等多种结构的球形介孔碳材料。通过考察喷雾干燥自组装的方法合成介孔碳微球的形成过程,对有序结构的排列和组装方式给出了详细的解释。论文的第五章中我们以具有二维六方有序结构的介孔碳材料FDU-15为例,系统地考察了介孔碳材料在各种不同气氛中,不同的处理温度和时间条件下的热稳定性。在750℃的二氧化碳,350℃的氧气或者800℃的水蒸气中,FDU-15-900材料可以保持介观结构的稳定性超过3h。在一定的条件下,热处理过程能够简单且有效地打开FDU-15的介孔孔道,较大地提高介孔碳材料的比表面积和孔体积。实验结果表明,二氧化碳与碳骨架之间的反应较为温和,处理后的介孔碳材料孔径有所增加,同时介孔孔容明显增大;而水蒸气处理后的碳材料则表现为微孔孔容的显著增加。氧气气氛下,FDU-15的有序结构则迅速被破坏,其比表面积和孔容也没有明显增加。本论文以有机分子的自组装过程为主要研究内容,开发出一系列新型溶液相合成路线,通过嵌段共聚物或表面活性剂的结构导向作用,合成出以下三大类碳纳米结构;1.多种不同尺寸的一维有机聚合物纳米品;2.具有不同形貌的石墨化碳纳米结构,如碳洋葱、碳纳米线等;3.具有纳米尺度周期性与球形形貌的多孔碳纳米结构。通过这一系列的合成研究,我们期望能对有机—有机限制条件下纳米结构的合成规律、合成机理有更深的理解,对介孔碳材料的微观结构和宏观形貌有更精准的控制。在未来的工作中,将以此为依据,设计合成更多功能性的新型纳米结构,探索这些材料在诸多领域的实际应用。更多还原

【Abstract】 Nanostructured materials have attracted extensive attention of researchers due to their great potential applications. Carbon nanostructures, such as fullerenes, carbon nanotubes and carbon onions, are materials of increasing interest due to their excellent electronic properties as well as their good physical and chemical properties. Conventionally, the solid phase interaction, such as chemical vapor deposition, arc-discharge, laser ablation and plasma radiation, is extensively used in the synthesis of carbon nanostructures because it allows relatively high energy assisted conditions. These synthesis methods have limitations in terms of large-scale and economical production because of their harsh synthetic conditions and low production yields. However, carbon nanostructures have seldom been reported by solution methods which is well suited for studying growth mechanisms of colloidal nanocrystals, especially for nucleation. Therefore, solution method could promisingly be applied in the synthesis of carbon nanostructures which may solve the present synthetic challenges and contribute to the further understanding of the growth mechanisms of carbon nanostructures.The scientific issues of this thesis are based on the concept of synthesis and organic-organic self-assembly of carbon nanostructures in a confined environment. First, we have developed new synthesis strategies of polymer/carbon nanostructures with alternative composites by choosing several kinds of carbon precursors. Secondly, the morphology control and thermal stability of novel mesostructured polymer/carbon materials were studied by using couples of modern analytic techniques. The content of this thesis includes synthesis and characteration of nanostructured and nanoporous carbons, reaction mechanism research, and exploration of the applicable properties.In Chapter 2, a surfactant-templated polymerization method was engaged in fabricating PFA nanostrutures under a solvothermal condition. Single crystalline PFA nanostructures with wire-like morphology were successfully synthesized for the first time under an organic-organic assisted assembly. Both the size and shape of PFA nanostructures can be controlled by the species of organic templates and the reaction time. Furthermore, the organic-organic assembly under the solvothermal process is compatible in fabricating 1-D polymeric nanostructures with different components. In Chapter 3, we demonstrate a low-temperature solution-phase synthesis of graphitic nanostructures by using furfural alcohol and mesophase pitches as carbon precursors through a hot injection method. The kinetics of the nucleation and growth has been sucessfully controlled during the carbon nanostructure evolutions. As a result, three kinds of graphite nanostructures could be obtained by varying the reaction time （5～30 min） or the concentration of H₂SO₄ (0.038～0.061 mol L^-1) in the oleic acid solution. The hot injection synthesis offers a broad synthetic strategy which could be applied into various aromatic molecular polymerization systems in synthesizing high-quality graphite nanostructures. Those materials will surely provide more interesting properties for advanced researches. The assemblies of the aromatic molecules in the solution may be useful for studying the mechanisms of the graphite structural organizations in nanoscale.In Chapter 4, one-step EISA method was engaged in the synthesis of spherical mesoporous polymers via an aerosol-assisted process in which amphiphilic triblock copolymers （PEO-PPO-PEO） were used as templates and a soluble low-molecular weight resol were used as carbon precursors. The block copolymer plays a very important role in directing the organic-organic assemblies. The pore structure of the particles with no visible defects could be simply controlled by adjusting concentrations of surfactants in the reactions or by choosing block copolymers with different EO-block lengths. This approach provides an efficient and productive route to synthesize mesoporous polymer/carbon particles with controllable particle size and pore structures.In Chapter 5, the thermal stabilities of ordered mesoporous carbon FDU-15 are demonstrated by using CO₂, O₂ or water vapor as the activating gases. The effects of activation temperature and treatment time on the thermal behaviors of FDU-15 are systematically investigated for a better understanding of the pore structure and carbon pore wall textures under these conditions. The activation process can simply increase the specific surface area and pore volume of the mesoporous carbons, and open the mesopore channels randomly. It is found that FDU-15 after carbonization at 900℃is thermally stable under CO₂ at 750℃, O₂ at 350℃and water vapor at 800℃for at least 3 h. CO₂ gas is a mild atmosphere to produce great mesopore volumes, while water vapor activation renders a continuous increase of micropore volumes and O₂ gas can rapidly destroy the carbon mesostructures. The residues from activation procedure can provide ordered mesoporous structures with high surface area and adjustable micro- and mesoporosity which are highly desirable for potential applications such as selective heterogeneous catalysis and adsorption.By using the concept of organic-organic self-assembly, three kinds of carbon nanostructures have been successfully developed under solution reactions: 1. 1D crystalline polymer nanostructures with controllable sizes; 2. graphitic carbon nanostructures with different morphologies; 3. mesoporous carbon spheres. Through these serial synthesis investigations, we hope to go deep into the understanding of the principles for the organic-organic assembly, and furthermore, rationally design and preparation of novel materials with better functionality.更多还原