Nanorods versus nanospheres: a bifurcation mechanism revealed by principal component TEM analysis.

A quantitative analysis of object populations obtained by TEM images is performed for the classical scheme of aqueous seedless synthesis of nanorods. Using an effective way to represent nanoparticle size distributions, we unravel that spheres, usually considered to be a side-product, are in fact coming from a competing route during nanorod formation. The differentiation between spheres and rods appears above a critical size of 5 nm and is due to different growth rates between faces. The initial repartition of faces on nuclei or on the nanoparticles at the critical size can be the source for the final differentiation between globules and rods. The efficiency of the selection is strongly influenced by the production of the initial seeds and, in particular, by the amount of borohydride added in the present scheme.