Controlled Synthesis Of Gold Nanorods And Quasi-spherical Silver Nanocrystals And Study On Their Properties

Due to their excellent property in mechanics, thermology, electrology, optics and magnetic, noble metal (such as gold, silver and so on) nanocrystals (NCs) have been widely used in many applications such as catalyst, new energy development, biosensors, biomedicine and so on. Among them, gold nanocrystals (Au NCs) have attracted much attention because of their potential applications in many fields such as optical materials, microelectrode reactions, catalyst, biological, medicine, and microelectronics. Further, the synthesis of gold nanorods (Au NRs) in the past decades has been extensively studied due to their special size/shape-dependent optical/electric properties, which has been found in many applications such as cell imaging, cancer diagnostic marker, and optical detection, etc.Seed-mediated growth method is the dominant strategy for the synthesis of Au NRs. Many research groups have focused on the optimization of this method in order to control the sizes and aspect ratios of Au NRs. One of the successful methods is that the formation of mixed micelles by addition of additives into the growth solution can improve the numbers and stability of micelles, thus achieving the controlled synthesis of Au NRs in size and morphology. However, these optimized methods reported recently still have some limitations, such as difficulties in controlling size and length at the same time, the complicated preparation process, and difficulty in continuously size control (diameter and length) of AuNRs of small aspect ratio (< 3).On the basis of current challenges, in this thesis, we aim to achieve controlled synthesis of Au NRs by introduction of alcohol additives (such as ethanol, n-butanol, n-pentanol, n-heptanol, n-nonanol, and n-dodecanol) into growth solution based on the well-known seed-mediated growth method. We mainly investigated the effect of chain lengths of the alcohols used on the synthesis of the Au NRs. Further, we studied the preparation of Au NRs assisted by n-heptanol in detail and the catalytic performance of those Au NRs with high-index facets. In addition, a simply seedless method was developed to directly prepare monodisperse, quasi-spherical silver nanocrystals (AgNCs).The main content of the thesis is as follows:(1) Effect of addition of alcohols on the synthesis of Au NRs. The addition of alcohols (ethanol, n-butanol, n-pentanol, n-heptanol, n-nonanol, and n-dodecanol) into the growth solution can control the preparation of Au NRs by influencing the CTAB micelles. Our results indicated that:(ⅰ) When the length of the carbon chain of n-heptanol (n-nonanol) is appropriate, they can penetrate into the hydrophobic alkyl chain of the CTAB micelles, and further form the compact n-heptanol-CTAB mixed micelles, thus achieving the controlled synthesis of Au NRs. By adjusting the concentration of the n-heptanol (and n-nonanol), we successfully prepared Au NRs with controlled sizes and aspect ratios, whose diameters and lengths can be tuned from 9 nm to 150 nm, and from 30 nm to 185 nm, respectively, (ⅱ) Even when the concentration of CTAB was reduced to about 50 mM, uniform Au NRs with controllable sizes with low aspect ratio can be obtained by addition of the n-heptanol/n-nonanol into the growth solution. (ⅲ) The yield and quality of Au NRs with different sizes can be improved by using n-heptanol-C14TAB (n-heptanol-C12TAB) mixed micelles. (ⅳ) When n-pentanol (ethanol, n-butanol) was added to the growth solution, few of them can penetrate into the hydrophobic alkyl chain of the CTAB micelles due to the short lengths of their carbon chains. Thus, the sizes of the Au NRs can’t be tuned by their addition into the growth solution; a lot of byproducts would be produced. (ⅴ) The hydrophobic interaction between n-dodecanol and CTAB is rather strong due to the long length of the carbon chain of n-dodecanol, thus the sizes of the Au NRs cannot be tuned in the presence of the n-dodecanol.(2) Effect of other reaction conditions on the synthesis of Au NRs in the presence of appropriate amount of n-heptanol. It was found that that the sizes and aspect ratios of Au NRs also can be tuned by adjusting the concentration of Auseed, AgNO3 and ascorbic acid (AA), whose diameters and lengths can be tuned from 6 nm to 94 nm, and from 21 nm to 130 nm, respectively. Moreover, the Au NRs with high-index facets were obtained. Cyclic voltammetry (CV) was used to test the electrochemical properties of these as-prepared AuNRs with high-index facets. It is found that the Au NRs with high-index facets exhibit higher catalytic activity. The electrocatalytic activity and stability test of AuNRs with different index facets for methanol oxidation in as electrocatalysts were further compared. The results suggested that Au NRs with higher index facets are more active but more unstable than Au NRs with low index facets; vice versa.(3) A simply seedless method for synthesis of monodisperse, quasi-spherical sliver nanocrystals (Ag NCs). The uniform Ag NCs in size and morphology were produced in high yield, whose sizes were tuned via addition of different amount of NaBH4 into the mixtures of AgNO3, AA, and CTAC. Optimal pH and CTAC concentration both are important for the synthesis of quasi-spherical Ag NCs. As-prepared 50 nm AgNCs show a higher enhancement factor for 4-ATP molecules. Our results indicate that these 50 nm AgNCs can be used as promising candidates for SERS applications.In summary, Au NRs with varied sizes with low aspect ratio were successfully prepared by the assistances of alcohols in the growth solution. In addition, a simply seedless method was developed to achieve controlled preparation of quasi-spherical Ag NCs with varied sizes. These methods reported in this thesis for the production of Au NRs and Ag NCs are facile, efficient, and easily accessible, which should be important for further exploitation of AuNRs and AgNCs in different applications.