【作者】 王金桂；

【导师】 陈铁红；

【作者基本信息】 南开大学 ， 物理化学， 2010， 博士

【摘要】 介孔材料是指孔径介于2-50 nm的一类多孔材料。由于其高比表面积、有序孔道、孔径分布连续可调等特点,在吸附、分离、催化、电极材料、光电器件、化学传感器、非线性光学材料等领域有重要的应用价值。多级结构介孔二氧化硅不仅在功能材料的制备和应用方面有重要的意义,而且与生物矿化、仿生材料化学研究密切相关,因此一直是介孔材料合成研究的热点。多级结构介孔材料的合成要求同时实现在纳米尺度上的介观结构、孔道排列、孔道尺度和微米尺度上的形态参数的控制,因此是材料合成中具有挑战性的研究课题。介孔材料的合成一般以双亲有机分子模板和无机物种相互作用组装为基础,但通常只能得到一些单一相态和简单形貌的介孔材料,难以实现不同空间领域的介观相态控制、多级孔结构和微米尺度上的复杂形貌控制。一些自然界中的生物矿物具有精巧的形貌和复杂的多级结构,而控制生物矿物沉积的过程往往是动态变化的有机质(种类和数量随矿化过程不断变化)调控的无机物矿化的过程。受此启发,本论文将有机模板的动态变化引入到介孔二氧化硅材料的制备中,探索新颖介观结构、复杂形貌和多级结构介孔二氧化硅的合成,并且利用氮气吸附,X-射线衍射(XRD),X-射线小角散射(SAXS),扫描电镜(SEM),透射电镜(TEM)等表征技术,研究了合成的介孔二氧化硅的介观结构、宏观形貌和形成机理。论文主要内容分为四部分：1)具有辐射孔道的纳米球形介孔二氧化硅(AMS)的合成以阴离子表面活性剂为模板,采用共结构导向剂方法,通过控制体系的pH值来控制阴离子表面活性剂的组装,成功地制备了具有辐射型孔道的介孔二氧化硅纳米球。球的尺寸和介观结构可以在一定pH值范围内(8.8-6.4)精细调节。调节合成体系的pH值,可以制备具有核-壳结构的各向异性形貌(椭球体,花生状,三叉形等)的介孔二氧化硅。核-壳结构的形成是由于核和壳内的介孔排列取向不同所导致的。当进一步降低pH值,形成洋葱状层状结构的介孔二氧化硅。实验证明,以阴离子表面活性剂为模板制备介孔二氧化硅体系中,形貌和结构受pH值影响很大。在这个合成体系中,有序结构的形成经历了长达数小时的有机-无机杂化体的自我调整,较阳离子表面活性剂制备介孔材料具有更长的调整时间。对于辐射孔形成机理,研究发现首先形成的是具有一定形貌的有机-无机杂化体,此阶段的杂化体为无序结构,随后表面活性剂-二氧化硅复合胶束经历自调整的过程,形成适应颗粒外部形貌的有序结构。自调整过程有利于新型有序介孔结构的形成,例如一维孔道的辐射型排列。2)具有辐射孔道的介孔二氧化硅空心球的超声辅助合成及其空化气泡模板机理通过在阴离子表面活性剂合成介孔二氧化硅体系引入超声波空化气泡,制备了分散的、形貌完整的空心结构介孔二氧化硅。介孔二氧化硅空心球直径在100-500nm,球壳很薄且厚度均匀,厚度为35-40 nm,球壳内的介孔为高度有序的辐射型排列。通过控制超声波的辐射时间、pH值及不同种类的添加剂可以有效的控制球壳的厚度和介观结构(六方、辐射或者无序)。超声空化气泡得到的介孔孔道大小均一、孔道短,这样有利于分子扩散和传输。同时由于合成中以氨基硅烷作为助表面活性剂,通过萃取去除模板机之后形成表面氨基功能化的介孔孔道。氨基功能化辐射孔道的空心球作为催化剂应用在Knoevenagel缩聚反应中,表现出很高的催化活性。对于空心结构介孔二氧化硅的形成,提出超声空化气泡为空心结构的模板,阴离子表面活性剂体系的有机-无机杂化体经历自调整形成有序辐射孔道的机理：首先超声空化气泡被阴离子表面活性剂体系的有机-无机杂化体包围,形成空心结构的有机-无机杂化体,随后经历杂化体内部胶束的自调整,形成了有序辐射型介孔孔道。超声空化气泡模板为空化气泡,较高分子、乳液、碳球等填充物为模板制备空心结构具有环境友好、步骤简单(不用除模板)和成本低的优点。3)放射虫形貌多级结构介孔二氧化硅的动态组装合成及其乳液模板机理设计了一种新的乳液体系来动态组装合成介孔二氧化硅多级结构空心球。为模拟生物矿化中有机质的动态产生,我们使用了一类长链烷基酰基氨基酸阴离子表面活性剂十二烷基氨酸钠。在十二烷基肌氨酸钠的水溶液中加入盐酸使得部分表面活性剂的羧酸根被质子化成为十二烷基肌氨酸(一种极性油),原来的溶液由于油滴的存在而形成乳液,这样表面活性剂先以油滴的方式被预存储。在加入氨基丙基三甲氧基硅烷(3-aminopropyltrimethoxysilane, APMS)和正硅酸乙酯(TEOS)后,由于在油滴的油水界面上发生羧酸与氨基的酸碱反应,表面活性剂从预存储的油相释放并输运扩散到水相,与硅源自组装为介观结构材料。在整个材料组装过程中,由于与氨基硅烷的作用表面活性剂从油相到水相释放,又作为模板与硅源自组装而被从溶液中耗散,因此表面活性剂从乳液油相的释放是一个动态的过程。通过这个动态的组装过程,我们得到了具有放射虫形态的多级结构介孔二氧化硅,球壳为蠕虫孔,脊刺为蠕虫孔壁的多泡室结构。辐射形脊刺的形成可能源于表面活性剂乳液油滴的自发乳化作用,即乳液油滴在外界作用下发生形变而分化为更小的油滴的过程。由于在我们的实验体系中,形成乳液的油滴不同于由非极性油形成的乳液油滴(如环己烷或三甲基苯),这些油滴实际上是质子化的阴离子表面活性剂,通过与有机胺的酸碱反应可以转化为离子化的表面活性剂分子而转入水相,从而动态地与硅源自组装成为多级结构,这个在油滴的油水界面上引起扩散和组装过程可能导致油滴界面形变而形成复杂的表面多级结构。以乳液为模版的动态组装方法,实现了表面活性剂的预存储和释放的动态过程,在表面活性剂不断扩散的动态过程中实现了多级结构和复杂形貌的合成。这种动态组装方法在制备多级结构材料方面具有潜在的应用,可以应用于其他多级结构材料的合成。4)以聚合物-表面活性剂介晶相为动态模板合成单晶状多级孔结构介孔二氧化硅SBA-1高分子聚电解质和带有相反电荷的表面活性剂可以自组装形成高度有序的介观结构的复合物(介晶)。我们采用聚丙烯酸(PAA)和阳离子表面活性剂(CTAB)形成的有序介观结构复合物介晶为模板制备了具有两种不同孔径大小的多级孔结构介孔二氧化硅SBA-1单晶颗粒。多级孔结构SBA-1单晶的形成源于聚合物一表面活性剂复合介晶的动态模板机理。与一般静态模板有所不同,PAA-CTA复合介晶模板与硅源组装过程中不仅仅引导了无机组分的介观有序结构,而在无机组分与有机相组装的同时,PAA从介晶模板中解离出来与硅源一胶束复合结构形成相分离。也就是说,在整个组装过程中,无机硅源与表面活性剂组装的动力学过程与PAA的相分离的动力学过程协同作用,这样初始状态PAA-CTA介晶颗粒的长程有序结构得以保留,并通过无机组分的引入而转变为Pm(?)n胛立方介观结构。而与此同时发生的聚合物相分离形成了PAA相域作为单晶颗粒中二次纳米孔的模板。因此PAA-CTA介晶在多级孔结构SBA-1单晶颗粒的合成中可以被称为动态模板。这种动态模板方法在其他多级结构单晶体的制备中将有潜在的应用,并且对复杂生物矿物的形成机理将有一定的指导意义。更多还原

【Abstract】 Mesoporous materials are a type of porous materials with ordered pores on the range of 2-50 nanometers, which have attracted great attention due to their wide applications to adsorption, separation, catalysis, electrode materials, optoelectronic devices, chemical sensors and nonlinear optical mater, and so on, due to their high surface area, ordered mesopores and tunable pore distribution. Hierarchically structured mesoporous silica has attracted great attention due to their potential application and their relationship to the biomineralization and biomimetic chemistry. To achieve the hierarchically structured mesoporous silica, simultaneous control of the structural parameters on the nanometer scale and the morphology on the micrometer scale are required. It is still a challenge to synthesize the hierarchically structured mesoporous silica for the materials science. In the past decades, on the basis of the interaction between the amphiphilic organic molecules and the inorganic species in the static hydrothermal system, only simple structure and morphology could be obtained, because it was difficult to control the mesophase at the different region and the morphology on the micrometer scale. Therefore, the hierarchical structure and complex morphology were difficult to prepare and rarely reported in the static hydrothermal system. Inspired by the biomineralization of the natural organism, which behaves dynamic process of time and space, we brought the idea of "dynamic process" into the synthesis of mesoporous silica to fabricate silica-based novel meso-structures, complex morphology and hierarchical structure by exploring the new synthetic system. The meso-structured silica were characterized by nitrogen sorption analyzer, power X-ray diffraction (XRD), small angle X-ray scattering (SAXS), scanning electron microscope (SEM), transmission electron microscope (TEM), etc. There are four main parts in this thesis.1) Anionic surfactant-templated mesoporous silica (AMS) spheres with radially oriented mesoporesMesoporous silica spheres with radially oriented mesopores were synthesized using an anionic surfactant as the template through the co-structure directing route. The spherical size and mesostructure can be finely tuned by changing the pH value of the synthetic system in the range of 8.8 to 6.4. In addition, when the pH value was decreased to 5.8, instead of spheres, anisotropic morphologies such as elliptical, peanutlike and trifurcate particles were obtained, exhibiting core/shell structure due to the different orientations of the mesopores in the core and the shell of the particles. When the pH value was further decreased, lamellar mesophase would be form. It is proposed that the evolution of the morphologies and mesostructures of the products templated by anionic surfactants strongly depend on the pH value of the synthetic system, and the formation process undergoes a re-alignment self-assembly mechanism (several hours). Firstly, clusters of organic-inorganic composite aggregate to form larger organic/inorganic hybrid with a special shape. Subsequently, the hybrid re-alignments to form preferential ordered structure (for example, radial mesochannel) to minimize its micelle free energy dependent on its shape.2) Synthesis of silica hollow spheres with ordered and radially oriented mesochannels by cavitation bubble templated methodUsing anionic surfactant as templates, ordered, robust and dispersed mesoporous silica hollow spheres with radially oriented mesochannels were synthesized with the aid of ultrasonic irradiation. The diameter of the hollow spheres was mostly in the range of 100-500 nm. The hollow spheres possessed thin and uniform shell with the thickness of 35-40 nm, and the shell with radially oriented mesopores exhibited well-ordered structure. By controlling the sonochemical processing time, pH value and additional reagent, the shell thickness, mesostructure (hexagonal, radial, or disordered), and shape of the inner cavity (hexagonal or spherical shape) of the hollow spheres could be facilely tuned. The obtained mesoporous silica possessed uniform and short meso-channels, which were beneficial to the diffusion of the molecules in the mesopores. It was proved that the high reaction velocity in the Knoevenagel condensation.The formation process of the hollow spheres with radially mesostructure was proposed of the cavitation bubble as the template of the hollow structure. Subsequently, a relatively slow cooperative realignment process of the silica/surfactant hybrid mesophase in this anionic surfactant templating system formed the ordered radial mesopores. Firstly, the bubbles generated by ultrasonic irradiation were surrounded by the silica/surfactant hybrid to form hollow structure. During the succedent reaction, the initially formed disordered silica/surfactant hybrid mesophases in the shell underwent a structural transformation and finally formed radially oriented mesochannels. The cavitation bubbles were in situ templates for hollow structure, which possessed environment-benign, process-facile and low cost advantages compared with polymer spheres, emulsion and carbon spheres as the consumed templates.3) Synthesis of radiolaria-like, hierarchically structured mesoporous silica hollow spheres by emulsion-based kinetic self-assemblyRadiolaria-like, hierarchically structured mesoporous silica hollow spheres with long, radially protruding and multicellular structured spines were fabricated through a kinetic self-organization process. This hierarchically structured silica is self-assembled by a unique emulsion-templating method, in which the acidified anionic surfactant N-lauroylsarcosine sodium (Sar-Na) acts as an oil phase. In this way, the surfactants can be pre-stored as the oil phase. Under the succedent experimental procedure, the pre-stored surfactants were gradually released and meanwhile were involved in the co-assembly with silica precursors to form mesostructured silica. The kinetic processes of deformation of oil droplets, release of surfactants, and co-assembly of surfactants and silica precursors result in the complex organization of the hierarchically mesostructured silica. This kinetic method achieved the pre-store and release process of the surfactant, which was related to the growth mechanism of morphological control of unicellular organisms, for instance, the radiolaria. This method would be expected to produce other complex morphologies and hierarchical structures.4) Synthesis of hierarchically nanoporous single crystal mesoporous silica from kinetic template of mesomorphous PAA-CTA complexes Hierarchically nanoporous single crystal mesoporous silica was fabricated for the first time by using mesomorphous polyelectrolyte-surfactant complexes (PAA-CTA) as kinetic template. Through the synergy of two kinetic self-assembly processes of silica-surfactant micelles co-assembly and polymer phase separation, the long range order through the mesomorphous polyelectrolyte-surfactant complexes was reserved and evolved into single crystal Pm3n mesostructured silica, and the domains of phase-separated polyelectrolyte chains served as the template for secondary nanopores within the single crystal SBA-1. Interestingly and importantly, the presence of large amount of secondary nanopores did not disturb the long-range order of mesostructure of the mesoporous silica particles, which would possess both the functions of crystal-like regularity and high diffusion efficiency of hierarchical pores. This kinetic templating mechanism would be generally applicable in fabrication of other hierarchically structured single crystal materials and may be related to the formation mechanism of complicated biominerals.更多还原