层状溶致液晶体系中金银纳米材料的制备

【作者】 孙振文；

【导师】 陈晓；

【作者基本信息】 山东大学 ， 物理化学， 2007， 硕士

【摘要】 层状溶致液晶具有各向异性的特征以及易于调控的结构性质使其在模板合成纳米材料的领域中逐渐受到重视。本文试图利用这种模板的软物质特性和长程有序性对产物结构形貌进行调控，研究内容主要包括两部分：第一部分，利用生物分子大豆卵磷脂构建层状生物液晶，用以模拟生物膜，并以此生物液晶为仿生模板，通过在水相中还原HAuCl₄和AgNO₃溶液制备金银纳米材料。实验过程中，首先利用红外光谱确定了卵磷脂中α-维生素E的还原性，通过偏光显微镜（POM）、小角X射线散射（SAXS）对液晶结构进行了表征，然后利用透射电镜（TEM）和紫外-可见吸收光谱（UV-vis）等测试技术系统研究了液晶组成、包覆剂的加入以及反应温度等条件对产物形貌的影响。利用卵磷脂构建的生物液晶为模板还原金盐可得到球形粒子与二维平面盘形粒子的混合产物。卵磷脂头基与AuCl₄^-的络合及在金表面的吸附导致了盘形金纳米结构形成。但由于这种作用较弱，因此生成的盘形产物不规则。此外由于还原反应进行较快，导致产物中有大量球形粒子的存在。在金纳米产物的制备过程中，加入CTAB作为包覆剂后，CTAB与AuCl₄^-可形成络合物，从而减慢反应速度，这更加有利于盘状产物的形成。而CTAB在金表面具有强的吸附作用，导致生成的片状产物较规则，主要为三角形或六边形形状。以同样的液晶模板制备银纳米材料，同样得到了球形粒子与不规则二维平面盘形粒子的混合产物，但银盘形粒子的粒径远小于金盘形粒子。这可能是由于硝酸银体系反应速率较慢，因此液晶的有序结构对银纳米粒子的生长所起的作用较大，致使银产物的粒径较小。而不规则片状产物的生成也表明卵磷脂头基对银表面的弱吸附作用同样存在。这种制备贵金属纳米材料的方法简单、易于调控。该工作将生物模板与液晶模板有机结合，同时也将生物、纳米材料与液晶结合，利用接近实际体系的生物液晶做仿生模板，率先进行制备各向异性贵金属纳米材料的探索，将有利于人们对各向异性纳米材料仿生合成机理的认识。论文第二部分，利用聚氧乙烯醚类非离子表面活性剂C₁₂E₄所构建的层状液晶为模板，直接通过化学还原来制备二维平面金微米和纳米盘与银纳米粒子。在液晶模板中C₁₂E₄作为还原剂，原位还原氯金酸与硝酸银溶液，并通过加入聚合物分子PVP（聚乙烯吡咯烷酮）或改变其它反应条件对产物的形貌进行调控。论文系统研究了模板组成、包覆剂的加入、金属盐浓度、反应温度、pH值等条件对产物形貌的影响。通过偏光显微镜（POM）、小角X射线散射（SAXS）、透射电镜（TEM）、扫描电镜（SEM）和紫外-可见吸收光谱（UV-vis）等表征手段对反应前后溶致液晶模板的结构与性质变化、所得产物的形貌与性质进行考察，并对不同形貌产物的形成机理进行了探讨。以氯金酸水溶液取代水与C₁₂E₄形成层状液晶相，改变液晶体系的组成，可调控纳米粒子的形貌并得到高产率的三角形及六边形的金纳米或微米单晶薄片。所得金单晶盘具有面心立方结构和（111）取向的晶面，边长最长可达10μm以上。反应过程中，液晶的层状结构对反应物及晶核的排布起到的限制作用对单晶的生长极为重要，是二维平面金盘状单晶生成的关键因素。通过加入包覆剂PVP，可以提高反应速度并对单晶盘的尺寸进行调控。以硝酸银水溶液取代水与C₁₂E₄形成的液晶体系中，通过还原反应可得到球形银纳米粒子。比较在同一反应体系中形成的Ag与Au纳米材料，由于C₁₂E₄的EO头基带有部分正电性，因此EO头基对Ag⁺有一定排斥作用，从而将Ag⁺定位于水相区域的中间，最终得到球形银纳米粒子；而带负电性的AuCl₄^-可以有序的吸附在EO头基表面，晶核间有序接触并与相邻晶核聚集，最终导致了二维平面金纳米盘的形成。在具有不同层间距体系中所得金盘状产物的厚度不同进一步说明层状液晶所起的模板作用较明显；而包覆剂PVP的加入不仅显著提高了反应速度，而且其在Au表面强的吸附作用使产物的粒径分布更加均匀。该工作首次利用层状液晶做模板，建立了大量合成二维平面金单晶纳米盘的简易而有效的新途径，通过改变反应条件可以加快反应速度并对产物形貌进行调控，从而大大增强了这种方法在实际中应用的可行性；同时，有助于人们进一步认识层状液晶模板的作用，并为各向异性纳米材料的获得提供了良好的思路。更多还原

【Abstract】 Synthesis of noble metal nanostructures has been an active research area for many decades, because of the importance of these materials for catalysis, photography, electronics, photonics, information storage, optoelectronics, biological labeling, imaging and sensing. A lot of efforts have been focused on controlling of the shape, size, crystallinity and structures. Particular emphasis has recently been placed on the control of shape, because in many cases it allows one to finely tune the properties with a greater versatility than can be achieved otherwise. In this dissertation lyotropic liquid crystals are used as templates to obtain noble metal nanostructures with controllable shape and size. The experimental research includes two parts.In the first part, using LLC lamellar phase made of lecithin and water to form mimic biomembrane, we synthesize gold and silver nanoparticles. During this progress, the vitamin E in lecithin acts as the reducing agent.The first step of our experiments is to define Vitamin E’s reducibility with FT-IR, then POM and SAXS are used to determine the LLC phase structure. At last, The effects of LLC composition, capping agent, reaction temperature, and pH value are studied by TEM and UV-vis, etc.Templated by AuCV-containing lecithin lamellar phases, a mixture of Au nanoplates and nanoparticles is prepared. The selective adsorption of lecithin headgroups on metal surfaces plays an important role in the plate-like nanoparticle formation, but the adsorption is such a relatively weak force that the nanoplates are formed to irregular shape. Due to the rapid reduction rate, plenty of spherical nanoparticles are formed. Adding CTAB to LLC as capping agent, we got regular and larger plates, which show hexagonal and triangular geometries with smooth edges. The reaction rate decreased due to the formation of complex compound between CTAB and AuCl₄^-. It is helpful to the formation of consequent growth of plates. The strong adsorption of CTAB on surface of Au nuclei results in regular and larger plate-like products. For AgNO₃-containing system, irregular silver nanoplates and nanoparticles are also obtained, which are smaller than Au products obtained from AuCl₄^--containing system. This may be due to the slower reaction rate, which makes the templating effect more important. The formation of irregular silver plates also suggests the headgroup adsorption weakly on silver.Our effort for using biological LLC as template to synthesize anisotropic noble metal nanoparticles will be helpful to understand the formation mechanism of nanoparticles with controllable shape and size in biomimic system.In the second part of dissertation, anisotropic gold and silver nanostructures are prepared from LLC lamellar phase templates made of nonionic surfactant C₁₂E₄ and water. Single-crystalline gold nanoplates, with triangular or hexagonal shapes, are successfully synthesized for the first time with a large-scale amount from lamellar LLC templates. And spherical silver nanoparticles are also obtained. POM （polarized optical microscope）, SAXS （small angle X ray scattering）, TEM （transmission electron microscope）, SEM （Scanning electron microscope ） and UV-vis spectrum measurements are used to investigate effect of different conditions on the product, including LLC composition, reaction temperature, pH value, concentration of capping agents poly （N-vinyl-2- pyrrolidone） （PVP） and metal salts.In HAuCl₄-containing lamellar phase, the morphology of obtained nanoparticles can be tuned by changing the LLC composition, and the yield of the triangular or hexagonal gold nanoplates is high. And gold nanoplates are single crystals with a preferential growth direction along the Au（111） plane. The longest edge is up to more than ten microns in size. During the reaction process, the lamellar structure of LLC restricts the reactant and nuclei in water domain. This is very important for the following single-crystal growth of nanoplates. By adding capping agent PVP, the reaction rate can be increased and the size of nanoplate be tuned. In AgNO₃-containing LLC, we get spherical Ag nanoparticles by reduction.Comparing the Au nanoplates and Ag nanoparticles obtained in the same LLC system, we think the difference between products is caused by the positive electrical charge on the EO groups. In the AgNO₃-containing system, the Ag⁺ is repeled by the positive electrical charge and restrained in the middle of water domain and this leads to the formation of nanoparticles. And in the HAuCl₄-containing system, the positive electrical charge attracts the AuCl₄^- to the surface of EO groups and this leads to the formation of Au nanoplates. The thickness of nanoplates can be varied by changing the repeated distance. And this indicates that the LLC template have an obvious effect on the products.Obtained results suggest a novel, simple and effective way to produce particles with controllable shape and size, as well as a clue to understand the mechanism for crystal growth.更多还原