【作者】 祝文霞；

【导师】 孙德军；

【作者基本信息】 山东大学 ， 物理化学， 2007， 博士

【摘要】 层状双氢氧化物(Layered double hydroxides,简称LDHs),是由二价和三价金属离子组成的具有类水滑石层状结构的混合金属氢氧化物,是一类具有广阔应用前景的典型的新型无机材料。由于同晶置换使得LDH层片带永久正电荷,层间可交换的阴离子和水分子补偿其正电性,层间距因阴离子不同而变化。LDH独特的结构特征和电性质,使得这类材料在催化,吸附,纳米复合材料,药物载体,以及生物传感等众多领域都具有广泛的应用。LDH插层纳米复合材料的应用更是引起了众多研究者的兴趣,因为,LDH层间插入功能性分子,不仅可以改变LDH的层间距,同时还可以得到具备特殊性能的新材料。近年来,LDH胶体分散体系在制备薄膜,细胞基因/药物传输,稳定乳液,形成液晶等方面又有许多新应用。这些新应用反过来又要求得到颗粒分散度低,稳定的LDH胶体分散体系。制备LDH使用最普遍的方法是共沉淀法,有研究者认为,采用非稳态液相共沉淀法制备LDH时,其晶体结构在共沉淀初期已经形成,之后的胶溶过程只是起到了使粒子逐渐长大以及改善其结晶度的作用。传统结晶理论认为,晶体生长是原子,分子或离子以特定方式在晶核上沉积的过程。近年来,研究者发现生物有机体中存在的非经典结晶过程(Non-classical crystallization),其中纳米粒子(而非原子,分子或离子)作为基本构筑单元进行晶体生长。这一概念已成功用于无机纳米材料制备中,促使我们对LDH晶体的形成机理进行深一步的探索,并为我们得到具有可控粒径及形貌的LDH粒子及其稳定的胶体分散体系提供新方法。本课题组已经发现片状LDH粒子胶体分散体系可以形成液晶相,这无疑是对无机溶致液晶领域的一个很大的贡献,对LDH液晶相变的研究更是会不断验证和丰富Onsager液晶相变理论。调节胶体分散体系中粒子间的相互作用,可以有效的控制体系的相行为。可控的液晶相行为则有利于提高液晶的理论及实际应用价值。我们知道,液晶在显示工业中的巨大应用价值仍然是液晶领域研究的主要推动力,但目前液晶显示器中使用的大部分都是有机小分子液晶。与有机分子相比,无机物质具有稳定,价格低廉,以及富电子等特点,如果能使无机溶致液晶应用于显示器件上,液晶显示工业将会产生新的飞跃。为了检测无机溶致液晶是否能应用于显示技术上,其电光效应的测定就显得十分必要。本文系统研究了粒子间有效吸引力的引入以及电场的作用对LDH胶体分散体系液晶相行为的影响,以求实现液晶相变的人工调控,为无机溶致液晶的应用奠定基础。本文通过一步快速沉淀,充分洗涤沉淀,以及随后的水热处理过程,得到了稳定的LDH胶体分散体系。该方法具有简单易操作,所得样品数量多,以及得到的分散体系稳定性高等优点。采用XRD,SEM,TEM和HRTEM,以及AFM等对样品进行了表征。结果表明,在共沉淀过程中形成了无定形纳米粒子,而不是晶体LDH粒子。之后的水热处理过程,促进了无定形沉淀的结晶。晶体生长的初期,观察到无定形纳米粒子通过界面成核相互结合。我们为晶体LDH纳米片状粒子提出了新的结晶机理,即,晶体LDH粒子的形成包括基于粒子的非经典晶体生长以及基于离子的奥氏熟化两种机制。LDH粒子的尺寸可以被有效的调节。延长水热处理的时间以及提高水热处理的温度,都能使LDH粒子的直径增大。粒子的厚度在特定温度下几乎不受水热处理时间的影响,而当升高温度时粒子的厚度会有轻微的增大。电解质NaCl的加入,则不利于LDH粒子的晶体生长。LDH胶体分散体系中,当粒子浓度足够大时会出现各向同性相到向列相液晶相转变。本文研究了LDH/PEG混合体系的液晶相行为。通过AFM观察,红外吸收光谱测定,以及晶体结构分析对比了纯LDH体系和PEG/LDH混合体系。结果表明,加入PEG分子以后,体系中LDH粒子的形貌,粒径,厚度,红外吸收光谱,以及晶体结构都基本没有发生变化。PEG分子在颗粒表面没有发生吸附,并且也没有改变颗粒的性质,只是改变了体系溶剂的性质,诱导颗粒间产生了一种有效的吸引力。与纯LDH胶体分散体系相比,PEG/LDH混合体系发生了更为复杂的相行为,出现了四相甚至五相共存。包括一个各向同性的上相,一个底部沉积相,以及中部两个或三个双折射相。其中必定有一相是向列相,且其形成机理为成核与生长。五相共存的体系中由底部沉积相之上长出的液晶相,具有一定的位置有序性,可能是柱型相。二元体系中出现多相共存,看似与吉布斯相律相矛盾,而粒子的多分散性以及在重力场作用下粒子将寻求更大的密度范围很好的解释了其合理性。PEG/LDH混合体系中的多相共存现象,是PEG分子诱导LDH颗粒之间产生的一种十分有效的吸引力,颗粒多分散度,以及粒子在重力场中的沉降共同作用的结果。本文采用偏光显微镜以及光学检测系统研究了外施加电场对LDH胶体分散体系液晶相行为的影响。研究表明,LDH的向列相及各向同性相分散体系都表现出很强的电光效应。向列相体系中,在电场的作用下,粒子排列更加有序,体系表现出更强的双折射性,使粒子重新定向排列的最小电压值为1V,当电压值低于1V时,电场对样品不起作用。而各向同性相的体系中,电场则诱导了液晶相的形成。两种体系对电场的最初响应时间均在1s之内,随着电场的移除,样品双折射性迅速减弱,之后逐渐恢复到样品的初始状态,这是一个可逆的过程,可以对样品顺序施加方波电场,几十秒之内就可以完成一个周期。而向列相凝胶以及浓度非常小的各向同性相体系则对外施加电场没有响应。此外,本文采用不同于传统插层方法的新方法合成了LDH插层纳米复合材料。首先用共沉淀法制备了无定形纳米粒子沉淀,然后使新鲜沉淀在SDS,SDBS,柠檬酸钠,硬脂酸钠等溶液中生长,水热条件下处理一定时间,即可得到LDH纳米复合材料。对样品进行了XRD,TEM和TGA表征。结果表明,SDS,柠檬酸根,硬脂酸根离子都成功插入了LDH的层间,形成了与水中生长得到的六角片状结构的LDH形貌不同的LDH复合纳米粒子。在SDBS溶液中生长的无定形纳米粒子得到的是针状的纳米粒子,不过未能发生插层反应。由无定形纳米粒子生长成LDH复合纳米粒子的生长机理涉及粒子接触和奥氏熟化。另外还采用传统的离子交换法合成了LDH插层纳米复合材料。首先制备出稳定的LDH胶体分散体系,再用SDBS进行改性,使SDBS的阴离子置换出LDH层间的cl离子,增大层间距。然后在改性的LDH层间插入较常用的若丹明,香豆素等荧光染料分子,形成了形貌为不规则多边形的LDH复合纳米粒子。总之,本文的研究实现了以无定形纳米粒子为前驱体水热合成稳定的LDH胶体分散体系,以及水热合成LDH插层纳米复合材料。并通过向LDH胶体分散体系中引入有效的吸引力,以及通过对体系施加外电场,实现了体系液晶相行为的有效调控。更多还原

【Abstract】 Layered double hydroxides （LDHs）, also known as anionic clays or hydrotalcite-like compounds, are a class of inorganic materials with potential applications in wide areas. They are structurally related to brucite Mg（OH）₂ with some certain divalent ions replaced by trivalent ones, and the net positive charges in the layers due to the replacement are compensated by exchangeable anions and water molecules in the interlayer. The distance between the layers is changeable with different anions. LDHs can be used in catalysis, adsorption, nanocomposites, drug carriers and biosensors due to their unique structural and electrical properties. More and more researchers have interests in the application of LDHs nanocomposites. By intercalating functional molecules into the gallery between the sheets, the distance between the layers can be changed and in the meantime we can obtain new materials with special capabilities. Recently, efforts have been focused on preparation of stable colloidal dispersions of monodispersed LDHs for their merits in preparation of thin films, delivery of cellar genes/drugs, stabilization of Pickering emulsions and formation of mineral liquid crystals. A typical and widely used method for preparing LDHs is coprecipitaion. It has been reported in literatures that when using non-steady coprecipitaion method to prepare LDHs, the crystalline structure of LDHs has formed at an early stage of the coprecipitaion and the following process of peptization only was used to make the particles grow up and develop their crystallinity. For a long time, crystal growth has been considered a process involving deposition of individual atoms or ions onto an existing crystal. However, a different picture about the growth of crystalline structures has emerged, in which nanoparticles serve as building blocks for construction of single crystals. This is referred as non-classical crystallization. Non-classical crystallization in organism has recently been found in crystallization of inorganic materials, which promotes us anew realizing the crystallization mechanism of LDHs and provides a new method for us to obtain stable dispersions of LDHs particles with controlled size and shapes. In our group, it has been found that it can form mineral liquid crystalline phases in colloidal dispersions of LDH plates, which has largely contributed to the field of mineral lyotropic liquid crystals. Any researches for liquid crystalline properties of colloidal LDH dispersions can contribute to the typical Onsager’s theory of liquid crystalline phase transition, and eventually it can well validate and enrich the Onsager’s theory. As we all known that the phase behavior of colloidal dispersions can be well controlled through adjusting the interactions between colloidal particles in the dispersions. In turn, the controlled liquid crystalline phase behavior can well develop the liquid crystalline phase transition theory and improve the application values of mineral liquid crystal materials. It has been known that, the application of liquid crystals in display techniques is still the major impetus of the researches in the liquid crystal field. However, what now have been used in displays mostly are small organic molecule liquid crystals. In comparison to organic molecules, the inorganic substances may have enhanced optical, electrical, and magnetic properties, and they are probably more stable and cheaper. If the mineral liquid crystals can be used in displays, a great revolution would be induced in the liquid crystal display industries. In order to examine the practicability of their application in display techniques, it is very important to investigate the electrooptic effects of these mineral liquid crystal materials. In this thesis, the effects of the introduction of an effective attraction between the colloidal LDH particles and the application of an electric field on the liquid crystalline phase behavior of LDH dispersions have been explored. I hope that it can provide an effective theoretical fundamental for the potential applications of mineral lyotropic liquid crystals and achive artificial adjustment of liquid crystalline phase transition.In this thesis, stable colloidal LDH dispersions have been prepared by a fast coprecipitaion, thoroughly washing precipitates, and a following hydrothermal treatment of the filter cake. This is a simple method which can be easily manipulated in laboratories and can obtain LDH products in large amount. The XRD, SEM, TEM, HRTEM, and AFM characterization results demonstrated that amorphous nanoparticles instead of crystalline LDH particles formed in the coprecipitation process, and the following hydrothermal treatment resulted in crystallization of the amorphous precipitates. Attachment of amorphous nanoparticles with interface nucleation was observed in the early stage of crystal growth. Thus, we propose a new crystallization mechanism for LDH nanoplates, in which both particle-mediated non-classical crystal growth and ion-mediated Ostwald ripening are involved in formation of crystalline LDH particles. The size of LDH particles can be effectively adjusted. Increasing hydrothermal treatment time and temperature resulted in increase of LDH particle diameters. The particle thicknesses were not influenced by treatment time at a certain temperature and increased slightly by treatment at higher temperature. Addition of electrolyte NaCl is unfavorable for crystal growth of LDH particles.In colloidal LDH dispersions, liquid crystalline phase transition will occur when the particles exceed a certain concentration. In this thesis, the liquid crystalline phase behavior of mixed LDH/PEG dispersions has been explored. The AFM, IR, and XRD characterization results indicated that the addition of PEG molecules has not changed the shape, diameter, thickness, IR spectroscopy, and crystalline structure of the LDH particles. PEG molecules were not adsorbed on the LDH particles, and they only changed the property of the solvent which induced an effective attraction among the LDH particles. In comparison to the pure LDH dispersions, more complicated phase behavior in the mixed PEG/LDH dispersions was observed. At certain concentrations of LDH and PEG, a four-phase equilibrium and even a five-phase equilibrium appeared, including an isotropic upper phase, a sedimentation bottom phase, and two or three birefringent middle phases. One of the liquid crystalline phases must be nematic, and the phase separation proceeds via nucleation and growth. In the five-phase coexistence system, the new phase grown upon the sedimentation phase has a positional order, and it may be a columnar phase due to the Bragg reflections. It seems paradoxical with the Gibbs’ phase rule for the multiphase coexistence in a binary system. The particles polydispersity and gravity reconcile the contradiction. We explain the experimental phenomena by the interplay between a PEG-induced effective attraction, LDH particles polydispersity, and the sedimentation in the gravitational field. The effects of an electric field on the liquid crystalline phase behavior of the colloidal nematic and isotropic LDH dispersions were studied with polarized light microscopy and optical detection system observations. The results demonstrated that these dispersions showed strong electrooptic effects. In the nematic dispersions, the LDH plates can align more orderly and show stronger birefringence by the action of an electric field. In a nematic dispersion with concentration of 20 wt%, we observed a threshold voltage 1 V, below which there was no optical response. Electric field can induce forming liquid crystalline phase in the isotropic dispersions with concentration lower than the I-N phase transition threshold. The initial response times of these dispersions are all less than 1 s. The induced birefringence rapidly decreases after removing the electric field, and then gradually decreases to the original state of these dispersions. The process is reversible. A square-pulse electric field can be successively applied on these dispersions, and the reversible switching can proceed in a few tens of seconds. There is no optical response in very dilute LDH dispersions and gel samples. The results of the electrooptic effects in LDH dispersions clearly demonstrated that this family of mineral liquid crystals may have the potential applications in simple on-off switching and low-frequency display technology.Moreover, in this thesis, intercalated LDH nanocomposites have been prepared using a new method different from the traditional ones. Amorphous precipitates were first synthesized by a non-steady coprecipitation method, and then the fresh precipitates were separately added into the aqueous solutions of SDS, SDBS, citrate sodium, and stearate sodium. We can eventually obtain LDH nanocomposites after some certain hydrothermal treatment time. The XRD, TEM and TGA characterization results demonstrated that the anions of SDS, citrate sodium, and stearate sodium have successfully intercalated into the interlayer of the LDH, which produced LDH nanocomposite particles with shapes different from the regular hexagonal plate-like LDH particles grown up in water. Needle-like LDH nanocomposite particles were obtained from the amorphous particles hydrothermal treated in the SDBS solution, but the anions have not intercalated into the interlayer of the LDH. The growth mechanism of the LDH nanocomposite particles from amorphous particles involved particle attachment and Ostwald ripening. Furthermore, intercalated LDH nanocomposites have also been prepared using a traditional ion-exchange method. We first synthesized stable LDH dispersions, and then modified the LDH particles with SDBS. The distance between the layers of the LDH increased because the chlorine anions have been replaced by the long SDBS ones. The increased distance between the layers was favorable for intercalating fluorescent dye molecules such as rhodamine and coumarin.In summary, stable LDH dispersions in large amount and intercalated LDH nanocomposites have been successfully synthesized from amorphous precursors using hydrothermal treatment method; and the liquid crystalline phase behavior of the LDH dispersions has been successfully adjusted through the introduction of an effective attraction and the application of an electric field.更多还原