| Because of the unique optical and electronic properties, gold nanocrystals have attracted a great deal of attention. Seeding approach has been proven to be a conventional synthetic strategy to prepare gold nanocrystals since the nucleation and growth processes can be separated effectively in this approach. Along this direction, a variety of gold nanocrystals with different size and shape have been successfully prepared during the past decades. For examples, Natan et al. synthesized large-size colloidal Au nanocrystals through a NH2OH/Au3+ seeding method. The size of the nanocrystals was tunable in the range of 30-100 nm by simply changing the initial particle diameter and the amount of Au3+. Murphy et al. extended the seed-mediated method to the synthesis of anisotropic Au nanocrystals including stars, tetrapods, triangles, cubes, and rods. CTAB molecules, considered to bind more strongly to the (100) than the (111) faces, were used as the surfactant. Thus, both the concentration of CTAB and the formation rate of Au0 contributed to the various shapes of Au nanocrystals. Anyway, to prepare gold nanocrystals with anisotropic shape, generally appropriate capping agents which can be absorbed predominantly on definite facets of gold are necessary to direct the preferential growth of the nanocrystals along one or two facets of gold. It is still a great challenge to control the shape of the gold nanocrystals in the seeding approach without the help of special capping agents up till now. Recently, it is more intriguing to note Gu et al. reported the synthesis of tadpole-shaped gold nanocrystals through a simple temperature-reducing approach, in the absence of additional capping agent or surfactant. The formation mechanism is proposed as an aggregation-based growth process, but this approach can hardly be extended to the preparation of other shaped gold nanocrystals.In our previous work, we investigated the formation and stability of gold nanoflowers prepared by the seeding approach by using the mixture of tetrachloroauric acid and hydroxylamine as the growth solution. It is identified that both the pH of the reaction system and the chlorine ions released from tetrachloroauric acid affect the intraparticle ripening of the gold nanoflowers. However, rapid consumption of the gold salt by the strong reducing agent, hydroxylamine, made it difficult to understand the combined effect of growth and ripening processes in the seeding approach on shape evolution of the nanocrystals.In this dissertation, in the seeding approach, we used sodium citrate, a kind of wake reducing agent, to reduce tetrachloroauric acid. The role of growth and intraparticle ripening during the growth of gold nanocrystals can be studied carefully in this slower process. At the same time, by controlling the effect of growth and ripening, a variety of anisotropic gold nanocrystals can be prepared.This dissertation includes two parts:Chapter 2, we employed 25 nm spherical gold nanocrystals as the seeds and the mixture of tetrachloroauric acid and trisodium citrate as the growth solution. The combined effect of the growth and ripening on shape evolution of the nanocrystals was investigated under different pH since it is known that both the activity of the gold salt and the ripening of the nanocrystals are tunable by adjusting the pH. It was found that the shape evolution of the nanocrystals is dependent on the pH of the reaction, attributed to the combined effect of growth and intraparticle ripening.Chapter 3, a good understanding of the effect of growth and ripening can serve as a guide to the synthesis of gold nanocrytals. When controlling the gowth and ripening in different balance, a variety of anisotropic gold nanocrystals such as gourd-shaped, tadpole-like, dumbbell-shaped and rod-like, were observable without the presence of special capping agents. This work demonstrated the possibility to control the shape of the gold nanocrystals in seeding approach by simply controlling the balance of the growth and intraparticle ripening.
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