新型多孔材料及自组装杂化材料的合成与表征

【作者】 周旭峰；

【导师】 余承忠；

【作者基本信息】 复旦大学 ， 无机化学， 2008， 博士

【摘要】 有机-无机自组装的有序介孔材料是化学、物理、材料等多学科交叉研究的热点领域之一。十几年来,人们通过多种方法合成了一系列具有不同组分、形貌、孔结构、孔径大小和用途的介孔材料,同时也对介孔材料的形成机理进行了细致的探索。但到目前为止,介孔材料实际应用的例子还很罕见。因此,深入理解其制备过程中的自组装机制,进而设计合成新型功能介孔材料并拓展其应用仍有很大的发展空间。介孔二氧化硅是人们最熟悉的介孔材料,但无定形二氧化硅材料本身性质不活泼,因此选择其它功能组分取代二氧化硅,制备具有不同性能的介孔材料一直是人们的追求目标。介孔二氧化硅本身是一个很好的基底材料,既可以对它进行功能化的修饰或改性,也可以用作硬模板来反相复制合成其它的介孔材料,同时介孔二氧化硅的合成经验也为其它组分介孔材料的制备提供了很好的指导和借鉴。因此,在本论文中我们以介孔二氧化硅作为出发点,从反应前驱体的选择,到二氧化硅墙壁的化学转化,再到以多孔氧化硅为硬模板的反相复制和进一步的功能化,合成了多种具有不同组成和性能的非二氧化硅介孔材料、多孔材料以及自组装的晶体材料,对有机.无机自组装的机理进行了详细的考察。第二章中,我们以乙基桥联的有机无机杂化硅源取代无机硅源,通过低温合成策略,在使用非离子型嵌段共聚物为结构导向剂和添加有机扩孔剂的条件下,成功制备得到了具有超大孔径的立方相介孔有机硅材料,其约15 nm的孔径是已报道的同类材料中最大的。通过详细考察反应温度和酸度对结构的影响,发现随着合成温度的降低,材料在保持高度有序介观结构的基础上孔径逐渐增大,而在更低温度时,酸度的降低又有利于有序介观结构的形成。在使用有机硅源和无机硅源的混合物作为前驱体时,观察到随着有机硅源比例的增加,产物的晶胞参数和孔径均随之减小。通过调变有机硅源和结构导向剂的比例,发现随着硅源量的增加,产物从最初的单分散有机硅纳米空心小球逐步向有序有机硅介观结构转化。结合原位表征结果,我们提出了以预先形成的球状复合胶束为结构单元进一步堆积形成有序大孔径介孔有机硅材料的合成机理。第三章中,我们选用带有更长烷基(辛基和己基)为桥联基团的有机硅氧烷作为硅源,考察其自组装行为,得到了有机硅物种自身水解和缩聚形成的杂化晶体材料。详细的结构表征结合理论计算表明,该晶体的基本结构单元为由两分子的前驱物完全水解并通过一步缩聚形成的大环状二聚体,二聚体结构单元通过硅羟基间的氢键和碳链间的范德华力进一步搭建成为分子晶体材料,其中碳链在产物中采取了稳定的全反式构型。前人报道必须使用含有较强相互作用力或刚性桥联基团的有机硅氧烷才能制备出有机硅杂化晶体材料,而桥基为柔性的长链烷基的有机硅氧烷只能形成无定形的产物。通过对合成条件的详细研究,发现采用添加表面活性剂,加入有机溶剂降低反应物浓度,以及降低合成温度等实验手段以减缓有机硅前驱物的溶胶凝胶反应速率是获得烷基链桥联的有机硅杂化晶体材料的关键。该工作为硅基有机无机杂化材料的溶胶凝胶过程提供了新的认识,同时也为合成具有其它官能团的杂化硅晶体提供了一定的指导。第四章中,我们以有序的介孔二氧化硅SBA-15为前驱物,通过镁蒸气还原二氧化硅形成硅的方法成功地将材料转化成具有高比表面积和高孔容的介孔多晶硅材料。结果表明,还原后硅和氧化镁的复合材料具有高度有序的介观结构,其中镁取代了二氧化硅骨架中硅的位置,而硅则迁移到模板的介孔孔道中。除去氧化镁后产生大量的介孔,硅和少量未反应或者后处理过程中氧化产生的二氧化硅共同形成了无机骨架,其中硅以5 nm左右的纳米晶形式存在。该介孔多晶硅材料在550 nm处具有较强的发光性能。在此基础上还尝试用其它不同结构的二氧化硅材料作为反应前驱物,得到一系列具有特殊形貌和孔性质的多晶硅材料。第五章中,我们以多孔二氧化硅为模板复制得到多孔碳材料,并开展了应用研究。第一部分中,以大孔二氧化硅泡沫材料作为硬模板,以蔗糖为碳源,反相复制得到了大孔、介孔和微孔复合的多级孔碳泡沫材料。产物既保留了二氧化硅模板中的大孔,又因氧化硅墙壁的除去产生了尺寸均一且贯穿材料整体的介孔孔道,同时碳墙壁本身又提供了微孔,因而是一种可容纳不同客体分子的优良载体。第二部分中,通过在棒状介孔碳孔道中负载二氧化钛,合成了介孔碳/二氧化钛复合的光催化剂。它同时集合了介孔碳对有机物的强吸附能力和二氧化钛纳米晶的优良光催化性能,对高浓度有机染料的处理能力要远高于商品化光催化剂P25。更多还原

【Abstract】 Organic-inorganic self-assembled ordered mesoporous materials have become a hot interdisciplinary spot of chemistry, physics and materials science. In the last 16 years, mesoporous materials with various compositions, morphologies, pore structures, pore sizes and functions have been successfully synthesized, and their formation mechanism has been carefully investigated. However, the practical application of mesoporous materials is seldom reported. Therefore, there are still big challenges in the in-depth understanding of their self-assembly mechanisms and further design and synthesis of novel functional mesoporous materials, as well as their application research.Mesoporous silica is the most well known among all mesoporous materials. Due to the plain physical and chemical properties of amorphous silica itself, the fabrication of mesoporous materials with functional compositions is a main task in this area. Mesoporous silica can serve as a good substrate for further modification, or a hard template to produce mesoporous replicas. Meanwhile, the experience accumulated in the synthesis of mesoporous silica is beneficial to guide the preparation of mesoporous materials with other compositions. In this dissertation, by taking mesoporous silica as our starting point, we have prepared several novel non-silica mesoporous materials and Si-based organic/inorganic hybrid crystals via changing inorganic precursors, chemical transformation of silica frameworks, or replication of porous silica templates, and carried out careful investigation on the organic-inorganic self-assembly mechanisms.In chapter 2, periodic mesoporous organosilica (PMO) materials with ultra large mesopores and face centered cubic mesostructures were prepared via a low temperature synthesis, using ethylene bridged organosilica precursors and organic swelling agents. The pore size of～15 nm is the largest among PMO materials ever reported. The influence of reaction temperature and acidity was carefully investigated. The pore size was expanded by decreasing temperature, and low acidity benefited the formation of ordered mesostructure at relatively low temperatures. In the case of using mixture of organosilica and silica precursors, the shrinkage of unit cell and pore size was found when the ratio of organosilica ones was increased. A continuous structural transformation from mono-dispersed organosilica hollow nano-spheres to PMO materials was observed when the ratio of organosilica precursors to organic templates was increased, and a packing of preformed spherical composite micelles was proposed for the formation mechanism of PMO materials with ultra large cage-like mesopores.In chapter 3, organosilica precursors with longer alkyl bridging groups, octylene and hexylene groups, were applied and non-porous, self-assembled bridged organosilica crystals were obtained. The careful characterization and theoretical calculation revealed that cyclic dimers from completely hydrolyzed precursors served as building blocks to construct the crystals via hydrogen bonds between silanol groups and van der Waals interactions among carbon chains. Alkyl chains adopted all-trans conformation in the crystals. In previous literature reports, such bridged organosilica crystals were obtained in the case that organic groups had strong interactions, while only amorphous hybrids were formed when flexible long alkyl chains were used as bridging groups. It was found in our case that the decreasing of the sol-gel reaction rate through addition of surfactants, or decreasing of precursor concentrations or reaction temperatures, was critical to the formation of alkyl bridged hybrid crystals. This research adds new understandings to the sol-gel process of organosilica materials and may provide guidance to the synthesis of Si-based hybrid crystals with other functional bridging groups.In chapter 4, mesoporous polycrystalline silicon materials with high surface area and pore volume were prepared by a magnesiothermic reduction of mesoporous silica precursor SBA-15. It was found that the Si-MgO composites after reduction constituted an ordered mesostructure in which Mg substituted Si in the original silica frameworks, while Si nanocrystals were expelled into and filled original mesopores. The mesopores were generated by the removal of MgO. Si nanocrystals of～5nm in diameter, as well as little silicon oxides due to incomplete reaction or post oxidation formed the inorganic framework. The mesoporous silicon had a relatively strong photoluminescent band centered at 550 run. Silica precursors with other structures were also applied in the same reaction, and porous silicon materials having different morphologies and pore structures were obtained.In chapter 5, two different kinds of porous carbon based materials were synthesized by replication of porous silica precursors. In the first part, carbon foam materials with hierarchical pores, including macro-, meso and micropores, were prepared using macroporous silica foams as templates and sucrose as carbon source. The macropores of silica foams were well preserved. The removal of silica frameworks generated uniform mesopores of～5 nm in width, and the carbon frameworks provided micropores. Such hierarchically porous carbon with interconnected pores at different length scales may become good substrates for the loading of different guest molecules. In the second part, a novel mesoporous carbon/titania composite photocatalyst was produced by loading of titania nanoparticles into the pores of rod-like mesoporous carbon materials. It had a much better performance in the degradation of organic dyes with high concentration compared to commercial titania photocatalyst P25, due to the combination of strong adsorption capacity of mesoporous carbon to organic molecules and excellent photocatalytic activity of titania nanocrystal.更多还原