【作者】 张万忠；

【导师】 乔学亮；

【作者基本信息】 华中科技大学 ， 材料学， 2007， 博士

【摘要】 作为一种新兴的功能材料,纳米银广泛用作催化材料、防静电材料、低温超导材料、生物传感器材料、电子浆料和抗菌抑菌材料。纳米银的性能与其结构、形貌、尺寸和尺寸分布以及材料本身所处的化学物理环境密切相关,纳米银的形貌、尺寸和尺寸分布可以通过采用不同的合成技术和反应条件来调控。因此,研究银系纳米材料的可控制备技术、探索不同反应条件下纳米银的形核和生长机制具有重要意义。本文在大量阅读文献的基础上首先对纳米银的主要制备方法及材料的光谱特性进行了简要的回顾,分析了微乳液法可控合成纳米银粒子的影响因素,论述了化学法制备各向异性银纳米材料的研究进展,指出了银系纳米材料研究中存在的不足。然后,围绕球形纳米银和棒状纳米银的可控制备及形成机理等方面展开研究工作,研究了表面活性剂琥珀酸二异辛酯磺酸钠(AOT)的合成,探讨了微乳液体系特别是AOT微乳体系中纳米银制备的影响因素,研究了柠檬酸钠体系中纳米银粒子的制备。最后,利用该晶粒作晶种在溴化十六烷基三甲基铵(CTAB)体系中研究了银纳米棒(线)的生长和形成机制。主要内容包括以下几个方面:在氮气保护下,以对甲苯磺酸为催化剂,合成了高纯度表面活性剂AOT,研究了酯化温度、体系真空度和酯化料比对酯化反应的影响。结果表明,在130℃、29.33 KPa、n(异辛醇):n(顺丁烯二酸酐)=1.82:1的条件下反应2h,所得酯化产物无色透明,酯化率可达99.6%。对双酯的磺化反应研究表明,适宜的磺化条件为130℃, n(NaHSO3):n(顺丁烯二酸酐)=1.10:1, w(AOT)=10%,反应时间3.5h。经甲醇溶解、活性碳吸附和正己烷抽提等方法提纯、旋转蒸发后,得到白色蜡状固体,红外光谱分析显示产物的纯度较好,可用来替代进口产品。在AOT-十二烷-水三元微乳体系中制备了球形银的纳米粒子。紫外-可见光(UV-Vis)吸收光谱显示,在较低的水合肼浓度下,Ag+的还原反应是不完全的;在反应的初级阶段形成的产物有Ag4+等中间产物,随后这些纳米团簇逐渐生长或相互团聚形成较大的银纳米粒子。硝酸银的浓度越高,银晶粒的形核和生长速率越大;水对表面活性剂的摩尔比(W)越大,反应形成的粒子粒径越大,粒子的尺寸分布也越宽。透射电镜显微图像显示在该体系中制备的纳米银粒子为球形结构,所得粒子具有很窄的尺寸分布,同时,形成粒子的粒径也很小,最小的平均粒径仅为1.6nm。傅立叶变换红外光谱(FT-IR)分析表明,通过银原子和AOT分子中磺酸基团的配位作用,表面活性剂分子被吸附于银粒子的表面,由此赋予形成粒子在油性溶剂中良好的稳定性。此法制备的纳米银溶胶不仅无毒无味,而且具有很高的稳定性,即使保存几个月也不会有沉淀析出。在AOT、短链烷烃和水组成的三元微乳体系中也研究了球形纳米银粒子的制备。结果表明,W值增大,微乳液液滴的直径变大,形成的纳米银溶胶的UV-Vis特征光谱吸收强度增大,共振谱带的半峰宽明显减小,表明形成粒子的平均粒径随体系中的含水量增加而明显增大。水合肼比NaBH4在AOT反胶束中的溶解度大,能完全还原水核中增溶的硝酸银。硝酸银和水合肼的浓度在一定范围内增大,形成的纳米银粒子增多,但还原剂浓度过高会破坏纳米银溶胶的稳定性。连续相链烃分子的碳链变长,UV-Vis光谱的吸收强度变大并发生蓝移,表明体系中生成的纳米银数量增多,形成粒子的平均粒径也相对减小。在十二烷基硫酸钠(SDS)、环己烷、异戊醇和水的四元微乳体系研究了纳米银粒子的制备,该体系和AOT微乳体系一样也可实现对纳米银粒子尺寸和形貌的可控制备。考察了反应时间、硝酸银浓度和W值对形成粒子的粒径和尺寸分布的影响。在典型值W=5和W=20时,形成的是球形纳米银粒子,银粒子的平均粒径分别为6.5nm和12.1nm,粒子的尺寸分布也随W值的增加而明显增大。通过对纳米银溶胶的离心分离,获得的产物为片状银粉,这种片状银的尺寸较大,厚度也在纳米至亚微米之间。这种现象表明,SDS表面活性剂不仅在纳米粒子的形成过程中形成保护层阻止了纳米粒子之间的团聚,而且在由纳米粒子集聚生长形成片状银粉的过程中起着模板的导向作用。利用柠檬酸钠包覆化学还原法制备了纳米银晶粒,重点考察了的反应温度、还原剂的加入方式以及银溶胶的陈化时间等因素对晶粒形成的影响,还利用不同银晶种制备了含银纳米棒的溶胶,由此进一步证实反应条件对纳米银晶粒形成的影响。三种温度下形成的纳米银溶胶,其共振峰的半峰宽明显不同。UV-Vis光谱研究表明,0℃下反应形成的银溶胶,纳米银晶粒的粒径最小,升高温度,银晶粒的粒径存在一个极值,继续升高温度,反应生成的纳米银晶粒的平均粒径也相对较小。一次性加料方式获得的纳米银晶粒的平均粒径仅为2nm左右,连续加料时形成的纳米银粒子的平均粒径达到了10nm。适度的陈化有利于银晶粒完善晶形。纳米银粒径越小,陈化处理前后的变化越明显,陈化时间太长,纳米银晶粒会发生严重的团聚。然而,陈化处理对粒径较大的纳米银几乎没有影响。在由CTAB、抗坏血酸(Vc)、AgNO3和银晶种组成的反应体系中,加入NaOH制备了含纳米棒的银溶胶,主要考察了反应时间、反应温度、CTAB浓度以及种子浓度和NaOH加入量对银纳米棒制备的影响。反应时间越长,形成了银纳米棒长径比越大。较高的温度下反应生成的银纳米棒的长径比减小,纳米棒的尺寸和形状的单分散性越好;反应温度降低,Ag+的还原速率下降,有利于形成具有较大长径比的银纳米棒。晶种数量较少,晶种在CTAB胶束中的分布均匀,有利于生成长径比较大的纳米棒。胶束浓度不是反应的敏感因素,在一定程度上变化不会影响银纳米棒的形成。溶液的酸碱性在银纳米棒形成过程中具有重要的作用。NaOH较多时,Vc发生二级电离,形成的是银的纳米棒;如果控制溶液的酸碱度使Vc发生一级电离,UV-Vis光谱中会在350nm处产生银本体材料的特征光学吸收,但不产生银纳米棒的径向共振吸收,表明此时反应形成的是银的纳米线。更多还原

【Abstract】 As an advanced functional material, silver nanoparticles have extensive applications in civil and industrial areas. For example, they can be used as catalytic materials, cryo-genic superconducting materials, biosensor materials, microelectronic materials and bacte-riostatic materials, etc. The capability of silver nanoparticles is dependent on their struc-ture, shape, size, size distribution and the chemical-physical environment. Generally, the shape, size and size distribution of silver particles can be controlled by adjusting the reac-tion conditions such as reducing agent, stabilizer and so on or employing different syn-thetic techniques. Therefore, it is necessary to study the nucleation and growth mechanism of silver nanomaterials. In the past few years, nanoparticle production by a size-controlled or shaped-controlled procedure has become a new and interesting research focus. Based on the large number published literatures, the dissertation has firstly made a brief intro-duction on the synthetic methods as well as the Ultraviolet-visible (UV-Vis) spectral prop-erties of silver nanoparticles, and then it has evaluated the influencing factors of silver nanoparticles prepared in microemulsion method by a controlled preparation procedure. The research progresses in silver anisotropic nanomaterials synthesized by chemical re-duction are discussed. The shortages in the relevant research fields are also analyzed. Subsequently, the studies on the controlled synthesis and formation mechanism of spheri-cal and rod-like silver nanoparticles have been performed in detail.Sodium di(2-ethylhexyl) sulfosuccinate (AOT) has been synthesized by purging with N2 and catalyzing with p-toluenesulfonic acid. Effects of reaction temperature, vacuum and molar ratio of 2-ethyl hexanol to maleic anhydride on esterification are studied. The esterification product is colorless and transparent and the esterification conversion is up to 99.6% at the condition of 130℃, 29.33 KPa, n(2-ethyl hexanol):n(maleic anhydride) = 1.82:1 and reaction time 2h. Optimal sulfonation conditions are 130℃, molar ratio of NaHSO3 to di(2-ethylhexyl)maleate) = 1.10:1, mass fraction of AOT = 10% and reaction time 3.5h. After purification by methanol dissolution, activated carbon adsorption, hexane extraction and rotatory evaporation, the obtained product is a kind of white waxy solid. Fourier transform-infrared spectrometry (FT-IR) analysis shows that the product has a higher quality and can substitute for imported product.Spherical silver nanoparticles have been prepared in AOT-dodecane-H2O ternary mi-croemulsion system. The UV-Vis absorption spectra and transmission electron micros-copy (TEM) have been used to trace the growth process and elucidate the structure of the silver nanoparticles. UV-Vis spectra show that the reaction is incomplete at low N2H4 concentration and the Ag4+intermediates form at early stages of the reaction and then the clusters grow or aggregate to larger nanoparticles. The higher the AgNO3 concentration is, the faster the growth rate of the silver particles is. The higher molar ratios of water to AOT (W) value has been found to give larger particle size and broader size distribution. TEM micrographs confirm that the silver nanoparticles are all spherical. The resulting particles have a very narrow size distribution. Meanwhile, the diameter size of the particles is so small that the smallest mean diameter is only 1.6 nm. FT-IR results show that the surfac-tant molecules are strongly adsorbed on the surface of silver particles through a coordina-tion bond between the silver atom and the sulfonic group of AOT molecules, which en-dowed the particles with a good stability in oil solvents. Thus, the silver colloidal has a high stabilization and can be preserved for a long time without precipitation. As dodecane is used as oil solvent to prepare silver nanoparticles, the formed nano-silver sol is almost nontoxic. As a result, the silver nanoparticles need not be separated from the reaction solu-tion and the silver sol may be directly used in antibacterial fields.Spherical silver nanoparticles are also synthesized in AOT microemulsion system using short carbon chain alkanes as oil phase. Effects of various factors on the formation of silver nanoparticles have been studied by UV-Vis spectra technology. As the increase of the molar ratio of water to surfactant, the absorption intensity of UV-Vis spectra increase and the half band width decrease obviously. It means that the diameter of microemulsion as well as the mean diameter of the formed particles increases with the water content in the system. Hydrazine hydrate can be reduced the silver nitrate solubilized in the water core of another microemulsion completely because of its higher solubility in AOT mi-croemulsions. In some degree, the amount of silver nanoparticles obviously increases with the concentration of AgNO3 and hydrazine hydrate. However, the stability of silver col-loidal solution would be destroyed, if the concentration of hydrazine hydrate were too high. The plasma resonance intensity of silver colloid increase and the most absorption position happen to blue-shift clearly. The results mean that the quantity of silver nanopar-ticles has increased and the mean diameter of the particles has also decreased.Size and shape controlled silver nanoparticles have been synthesized in sodium do-decyl sulfate (SDS) quaternary microemulsion system. The effects of reaction time, AgNO3 concentration and the molar ratios of water to SDS (W) on the particle diameter and size distribution are investigated. TEM micrographs confirm that the mean diameters of spherical silver nanoparticles obtained at two typical values of W=5, 20 are 6.5nm, 12.1nm, respectively, and the size distribution of the particles increases obviously with the W values. Flake-like silver powders can be obtained by centrifugation of the particles. The result indicates that SDS surfactant molecules not only can form a protection layer on the surface to prevent the particles from aggregation, but also may act as a template to orient the particle growth to form flake-like particles, which may be important in the fabrication of flake-like silver powders.In the capping action of citrate, silver nanocrystals have been prepared by chemical reduction. The effects of reaction temperature, charging mode of reductant and the aged time of silver colloid on the formation of silver nanocrystals are mainly investigated. The effects of reaction conditions can be further confirmed by synthesizing silver nanorods under the induction of different silver nanocrystals. The silver colloid is prepared under three temperatures, the half-band width of resonance peaks are clearly different. The re-sults show that the diameter of nanocrystals prepared at 0℃is smallest and the particle diameter increase with the temperature, but has a maximum. The mean diameter of resul-tant nanocrystals is about 2 nm in one-time mode and the mean diameter of them will at-tain 10 nm in continuous mode, when NaBH4 is added into the solution. Appropriate aged treatment is favorable to perfect the crystalline form. The aged time has obvious influence on the smaller nanocrystals and has little influence on the larger nanoparticles. However, the nanoparticles will aggregate into larger nanoparticles if the solution is aged for a long time.Silver nanorods and nanowires of controllable aspect ratio have been synthesized in the mixed solution composed of cetyltrimethylammoniun bromide (CTAB), ascorbic acid (Vc), AgNO3 and silver seeds by adding NaOH. The longer reaction time, the greater the aspect ratio of the formed nanorods is. The aspect ratio of silver nanorods decrease with the increase of reaction temperature, meanwhile, the monodispersity of size and shape of the nanorods turn better. On the contrary, it is advantageous to the formation of silver nanorods with higher aspect ratio. As fewer seeds are easily distributed into rod-like mi-celles and its nucleation and growth can be controlled by the micelles, the silver rods in-crease in average aspect ratio as the seed concentration decrease. The micelle concentra-tion is not the main factor to influence the formation of silver nanorods. The relative amount of NaOH in solution has an important role in the fabrication process of silver nanorods and nanowires. Ascorbic acid will ionize to form the ascorbate dianion when the amount of NaOH is higher. By contraries, ascorbic acid ionizes to form the monoanion of ascorbic acid at a low concentration of NaOH. Two different forms of ascorbic acid have different complexation capabilities with CTAB and silver seed in solution, which is im-portant in nanorod and nanowire formation.更多还原