| 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. |
PDF Full Text Request