Direct Experimental Evidence of Nucleation and Kinetics Driven Two-Dimensional Growth of Core-Shell Structures

We report the appearance of different anisotropic one-dimensional (1D) and two-dimensional (2D) gold (Au) core-silver (Ag) shell nanostructures in a single reaction environment. High-resolution transmission electron microscopy (HRTEM) images taken from the core region of a core shell nanostnicture revealed the prominent presence of the hexagonal-shaped gold seed leading to formation of a triangular final particle. The HRTEM studies provide the direct experimental evidence of the "silver halide" model proposed by Sigmund et al. (Lofton, C.; Sigmund, W. Adv. Funct Mater. 2005, IS, 1197-1208) to explain the kinetic growth mechanism behind their formation. It is important that this information cannot be identified from a single composite nanoparticle due to the lack of atomic number (Z) contrast difference. We have studied energy-dispersive X-ray line profile spectra and elemental mapping using the high-angle annular dark-field scanning/transmission electron microscopy (STEM-HAADF) technique corresponding to the Au-M and Ag-K energies from the similar core shell structures. This confirms the composition of the core to be made of gold and that of the shell of silver. The line profile along the relative thickness map of a hexagonal and triangular nanoplate obtained using energy-filtered TEM indicates the formation of nearly uniform 2D structures. The nearly equal thicknesses of the core and outside shell of a core shell nanoplate measured using electron energy loss spectroscopy in STEM mode also confirm the 2D growth of a gold seed forming a triangular core shell nanoplate.