【作者】 张继辉；

【导师】 夏海兵；

【作者基本信息】 山东大学 ， 材料物理与化学， 2014， 博士

【摘要】 纳米材料因其独特的表面效应、量子尺寸效应等在力学、热学、电学、磁学、光学等方面表现出优良的性能,因此在包括高新医药生物、新型能源开发、新信息技术等新兴领域具有极大的潜在应用价值,近些年已经成为人们重点研究的热门课题。尤其是以金、银为代表的贵金属纳米材料,由于其具有独特的光学、电学、催化等性质以及良好的生物相容性,在光学、电子学、生物学、催化以及构建具有二维和三维结构新材料等诸多领域具有重要的应用价值。随着现代科学技术的飞速发展,社会对纳米材料性能的要求越来越高并且日趋多样化,单一组成的物质难以满足不同形式的需求,纳米复合材料应运而生并得到迅速的发展。特别是对于金、银这样的贵金属,将它们制备成金核银壳或者银核金壳这样的壳层结构,还可提高贵金属的利用率。以金为核或壳的核壳纳米粒子通过调节内核直径和壳层厚度的比值,能实现等离子体共振在很宽波段范围内的可调节性,因而此类核壳纳米材料能广泛应用于光学传感器、生物传感器。金、银纳米晶以及二者复合的纳米晶所表现出的许多独特性质与其形貌和尺寸及组成是紧密相关的。因此致力于纳米晶形貌及尺寸等方面的研究工作一直没有停过。经过人们近几十年来的不断研究和探索,已经成功的合成出各种形貌的金、银纳米晶以及金、银复合的纳米晶,如类球形、棒状、立方体、凹形立方体、二十四面体、类球形核壳结构,立方体核壳结构等。伴随着这些不同形貌纳米晶的合成,同时出现了很多不同的合成方法,目前比较常用的是种子生长法制备各种形貌各种尺寸的金、银纳米晶。我们正是利用种子生长法,在CTAC体系中合成了不同形貌、不同尺寸的类球形金纳米晶,产率很高,尺寸分布均一性也较好。并且以此为种子,加入到CTAC的生长溶液中继续生长,通过变化类球形金纳米晶的加入量,得到了不同银壳厚度的金核银壳结构纳米晶,为了分析金核银壳结构所表现出来的表面等离子共振性质随银壳厚度变化的规律,我们分别以粒径为9.8nm,15.2nm,22.6nm,28.2nm的类球形金纳米晶作为种子,合成金核银壳结构纳米晶。我们以金核表面等离子共振吸收峰的变化为参照,随着银壳厚度的增加,相对于类球形金纳米晶本身的吸收峰,金核的峰逐渐蓝移,且强度逐渐的减小。然而银壳所表现出来的吸收峰逐渐的增强,且总体呈现一个逐渐红移的趋势,我们在使用了四种不同粒径的球形纳米晶做种子生长银壳后,发现核壳结构所表现出的表面等离子性质以银的特征为主导时,银壳厚度是不同的。因此我们认为,核壳结构中银的吸收峰占主导时的临界范围与银壳厚度关系不大,主要与银壳厚度与核壳结构纳米晶本身半径的比有关。根据我们的分析,我们推测这个比值≥0.28时,核壳结构主要表现银的特征吸收峰。种子生长法是在合成纳米晶常用的一种的方法,最近,两步种子生长法简化到向生长溶液中加入NaBH4,而不是加入事先制备好的金晶种,众所周知的是,NaBH4是一种很强的还原剂,可以将Au3+直接还原成Au0,生长溶液中加入NaBH4,可以形成很小的金纳米晶,可以起到种子的作用,从而可以生长成更大的金纳米晶。然而使用无种子生长法合成的许多形貌的纳米晶产率和质量都不尽理想,不如传统种子生长法合成的好。为了解决这些难题,本文全面细致的研究了通过向生长溶液中加入NaBH4这样的无种子生长法来制备不同形貌的金纳米晶。在无种子生长法中,直接被NaBH4还原的金种子的大小约为1.5nm,有利于合成各向异性的金纳米晶。我们发现,在生长溶液中加入冰水中新配的NaBH4后,反应溶液静置生长前搅拌25-30s时,得到的纳米晶的产率和形貌均一性都较好。我们利用这个改良的无种子生长法成功合成了凹形立方体、立方体、三八二十四面体、棒状、类球形的金纳米晶,并且通过改变NaBH4在反应溶液中的浓度,得到了不同尺寸的金纳米晶,对其所表现出的不同表面等离子共振性质进行了研究。重点研究了AgNO3、AA浓度的变化对合成金纳米棒的影响,与传统种子生长法中得到结论是一致的,通过改变生长溶液中反应物的浓度,合成了具有相同长径比,不同尺寸(长×宽)的金纳米棒。在生物医学的实际应用中,相同长径比的金纳米棒用于在近红外照射下进行光热治疗肿瘤,纳米棒的尺寸,即长度和宽度,会对活体内的半衰期循环造成明显的影响。因此,合成出具有相同长径比但是不同长度和宽度的金纳米棒就很有意义。我们还比较了具有类似尺寸的凹形立方体和立方体金纳米晶在表面等离子共振性质上表现出的不同,凹形立方体金纳米晶显示出比具有类似尺寸的立方体金纳米晶更宽化且红移的表面等离子吸收峰,这与理论模拟是一致的,即具有凹形面的立方体金纳米晶的表面等离子共振吸收峰比具有平面的立方体的要红移,这都是由于凹形立方体纳米晶具有更尖的顶端的缘故。循环伏安测试常常用来表征不同形貌金纳米晶的表面活性。我们利用循环伏安测试法我们对所得到的不同形貌的金纳米晶进行了电化学性能测试,证明了像凹形立方体和三八二十四面体这样具有高指数晶面的纳米晶具有较高的化学活性,而像立方体,类球形和棒状这些具有低指数面的纳米晶的化学活性较低,同时对这几种不同形貌的纳米晶在碱性条件下进行了对甲醇的催化氧化,结果是一致的,也就是具有高指数面的纳米晶具有更好的催化性能,反之,催化性能较低。我们将无种子生长法扩展到银纳米晶的合成中,成功的合成出了不同尺寸的类球形银纳米晶,以及小于18nm的立方体金核银壳纳米晶,并对其表面等离子共振性质进行了表征。各种形貌,不同尺寸的金、银纳米晶的合成,证明了经过我们改良的无种子生长法的普遍适用性,是非常值得推广的一种方法。使用这种简单、高效、易掌握的方法来合成不同形貌的金、银纳米晶,必然会对金纳米晶在不同技术应用的研究上,提供更大的方便,具有重要的意义。更多还原

【Abstract】 Nanomaterials show excellent property in mechanics, thermology, electrology, magnetic and optics due to the unique surface effect, quantum size effect and so on, which has important promised applications in new scientific and technological undertakings, such as biomedicine, new energy development, new information technology, has been one of the hottest topic. Nobel metals nanostructured materials such as gold, silver, which has important applications in photonics, electronics, biology, catalysis and new material with two-dimensional and three-dimensional structure owing to their unique optics, electrology, catalytic properties and biocompatibility. Because of the rapid development of modern science and technology, the requirement for the properties of nanomaterials is more and more high and variegated. It is hardly for the material with single component to satisfy various requirements, therefore, nanocomposites emerge as the times require and get rapidly developed. Especially for the gold and silver nanocrystal, the availability was raised by combining Au and Ag into a core-shell configuration. The surface plasmon resonance can be controllable in a broad range by tuning the ratio of core diameter and shell thickness, as a result, the core-shell configuration materials have promised applications in optical sensor and biosensor.Gold and silver nanocrystals and their composite exhibit many unique characteristic depend on their shape, size, and composition. The work in researching the shape and size of the nanocrystal always is in progress. Under the constant researching and exploring, gold and silver nanocrystals with various shape have been synthesized, such as quasi-spherical, rod-like, cubic, concave cubic, trisoctahedral, Quasi-Spherical core-shell, cubic core-shell and so on. A lot of different methods have been used for synthesized nanocrystals with various shapes. Up to date, the seed-mediated growth method is the dominant workhorse for synthesis of Au and Ag NCs with different shapes.We successfully synthesize high yield and uniform quasi-spherical gold nanocrystals with different shapes and sizes by the seed-mediate growth method in the CTAC solution. Next, the prepared quasi-spherical gold nanocrystals were used as seeds for the overgrowth of thin shell of silver in the CTAC solution. The Au@Ag nanocrystal with different shell thicknesss were synthesized by varying the amount of quasi-spherical gold nanocrystals. In order to analyse the relation between the surface plasmon resonance and the varied thickness of shell, the prepared quasi-spherical gold nanocrystals with the diameter of9.8,15.2,22.6, and28.2nm were used as seeds for synthesizing Au@Ag nanocrystal. The varied surface plasmon resonance of Au core as the reference, compared with the quasi-spherical gold nanocrystals, which blue shifted as the thickness of the Ag shell increased, and the intensity of peak decreased. Howerve, the intensity of peak for Ag shell is increased and red-shifted. We use the quasi-spherical gold nanocrystals with four different diameters as the seed for the overgrowth of Ag shell, and we find that when the SPR of Ag shell dominated in the SPR of core-shell nanocrystals, the thickness of the Ag shell is different. Therefore, we believe that the critical range of the the SPR of Ag shell dominated in the SPR of core-shell nanocrystals has nothing to do with the thickness of Ag shell, but associates with the ratio of the thickness of Ag shell to the radius of the core-shell nanocrystal. Accordingly our analysis, the ratio is equal or greater than0.28.A broadly used strategy for preparing gold nanocrystals is a seed-mediated approach, the two-step seed-mediated growth process has been recently simplified into a seedless by adding the trace amount of NaBH4instead of preformed Au seeds into the growth solution. It is well-known that NaBH4is a very strong reducing agent and can directly reduce Au3+into Au0. The addition of NaBH4into the growth solution is expected to form very small Au NCs, which can act as seeds for growth of large Au NCs. However, the quality and production yield of Au NCs with varied shapes obtained via currently available seedless growth methods are relatively poor as compared to those obtained via conventional seed-mediated growth method. Thus, to address this challenge, the present work aims to meticulously explore the seedless growth of Au NCs via addition of NaBH4into the growth solution. Seedless method with size of about1.5nm is in favor for the synthesis of anisotropic Au NCs. We have found that after addition of freshly prepared ice-cold aqueous solution of NaBH4into growth solutions, the time of stirring the reaction mixture prior to aging at room temperature is optimized to be25-30s for producing high yield and well-defined Au NCs.Concave cubic, cubic, trisoctahedral, rod-like, quasi-spherical Au NCs have been synthesized by the modified seedless growth method, and the size of the resulting Au NCs increased with the decrease of NaBH4concentration in the final reaction media. The surface plasmon resonance of the resulting Au NCs has been studied meticulously. Also the influence of the concentration of AgNO3and AA on the synthesis of Au NRs has been analysed, and the result is agreement with the conventional seed mediated growth method. The Au NRs with the same aspect ratio but with the different dimensions (length×width) have been synthesized via varied the concentration of the reactant in the growth solution. In practical applications in biomedicine, for instance, when Au NRs with the same aspect ratios are utilized for photothermal tumor therapy under irradiation of near-infrared light, the dimension of the NRs, length and width, is expected to considerably affect their in vivo circulation half-life. Thus, it becomes desirable to synthesize Au NRs with varied length and width but fixed aspect ratios. Concave cubic Au NCs show a broader SPR band in comparison with those of cubic Au NCs with similar edge length. The result is in good agreement with theoretical simulation that the SPR band for cubic Au-NCs with concave surfaces is expected to red-shift into the long wavelength range as compared to that of cubic Au NCs with flat surfaces because the concave cubic NCs have sharper tips.Cyclic voltammetry (CV) has been commonly used to characterize the activity of the surface facets of as-prepared Au NCs with different shapes. We use the cyclic voltammetry to test the electrochemical property of the as-prepared Au NCs with different shapes. It is discovered that the concave cubic and trisoctahedral Au NCs with high-index facets have higher activity the cubic, quasi-spherical and rod-like Au NCs with the low-index facets, we further studied the electrocatalytic activity of quasi-spherical Au NCs, cubic Au NCs, Au NRs, concave cubic Au NCs, and TOH Au NCs, and by utilizing them as electrocatalysts for methanol oxidation. The test results suggests that the TOH or concave cubic Au NCs are more active for electrocatalysis of methanol oxidation, which can be easily rationalized as a result of the presence of high-index facets in the former two NCs, which are in good agreement with previous results.We expand the seedless growth method to the synthesis of Ag NCs, and quasi-spherical Ag NCs and cubic Au@Ag NCs with the size below18nm have been prepared. The surface plasmon resonance has been characterized.The synthesis of gold and silver nanocrystals with varied shape and size via our modified seedless growth method demonstrates its universality, which deserves to be popularized. Thus, our work opens a general, facile, efficient, and, more importantly, easily accessible strategy for the production of uniform Au NCs directly in water, which should be important for further exploitation in different technical applications.更多还原