The structure of (Au) bipyramids, very promising for their specific plasmonic properties, is fully analyzed for gradual stages of growth. From combined use of HRTEM (high resolution transmission electronic microscopy) in specific conditions and SAED (selected area electronic diffraction) analysis, the atomic structure of these elongated objects of different sizes is extracted (from 2 to 60 nm). In the case of silver(I)-assisted growth from citrate capped seeds, a clear pentatwinned structure, made by the superimposition of specific pairs of crystallographic zones is found for the different size nanoparticles representing the different steps of the growing bipyramids. The experimental results were analyzed in the framework of a recent atomistic approach developed for metal−environment interactions to account for the stability of multitwinned nanorods or bipyramids in a complex environment. Efficient in describing the multitwinned nanoparticles and the drastic effects of environment on the stabilization of elongated multitwinned shapes, this model supports the experimental features of bipyramids and nanorods formation from citrate seeds as a function of the nature of the surface chemical ligands: citrate seeds in a growth solution prepared without silver(I) yields the formation of a few pentatwinned nanorods, while the silver(I)-assisted growth process mainly leads to pentatwinned bipyramids. The conservation of a bulk pentatwinned structure inherited from isotropic decahedral (i-Dh) seeds all along the growth is retrieved by this approach for two different environments.

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The influence of initial seed structure on final nanoparticle geometry has been investigated by an original "competitive approach" using SAXS, UV-Vis Spectroscopy and TEM analysis. Herein by using the seed mediated gold nanoparticle growth, seeds with different size and crystalline structure were synthesized and injected into the same growth media. The seeds were chosen due to their different growth evolution into two different morphologies. CTAB coated single crystalline seeds grow into rod like shapes whereas citrate coated multi-crystalline seeds mainly grow into wheat shape polycrystalline particles with small elongation called nanobeans in coexistence with a large quantity of spheres. When seeds are added into the same growth media for competition, a mixture of the different morphologies is obtained. By controlling the number of added competing seeds in the solution, it was found that the multi-crystalline seeds have a higher growth rate than the single crystalline seeds. This has a direct impact on the final distribution of size and morphologies of the nanoparticles. The consequence is a large tendency towards nanobean and sphere structure formation even when a small number of multi-twinned seeds is added in the solution. This method demonstrates the importance of the nature of defects hidden in the initial seed and proves that these defaults are inherited to the final nanocrystal throughout the growth stage from the beginning of the growth. This competitive seeded growth approach is an easy way to identify the influence of seeds morphologies in synthetic pathway of nanoparticles formation.