Formation of Au@Pd@Cu core–shell nanorods from Au@Pd nanorods through a new stepwise growth mode

Abstract

Au@Pd@Cu core–shell nanorods (NRs) were prepared using Au@Pd NRs as seeds. The resultant crystal structures were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and X-ray diffraction (XRD) measurements. Au@Pd seeds were prepared by reducing H₂PdCl₄ with cetyl trimethyl ammonium bromide (CTAB) and ascorbic acid in an aqueous solution. Dumbbell-type Au@Pd particles were formed at low Pd/Au molar ratios of 0.5–2.5, whereas rectangular Au@Pd NRs with {100} facets were produced at high Pd/Au molar ratios of 5 and 10. When Cu shells were further grown over rectangular Au@Pd NRs as seeds, Au@Pd@Cu nanocrystals with {100} facets were grown epitaxially through a new single island-growth mechanism designated as the Tsuji–Ikedo mechanism. In this mechanism, crystal growth of Cu shells over Au@Pd cores starts from the formation of single semispherical nuclei on a wide side facet followed by growth to one rectangular rod shell, further growth of neighboring rectangular rod shells, and eventual full covering by large rectangular Cu shells with {100} facets. In many cases, the growth rates of Cu shells over respective surfaces of Au@Pd NRs differ, so that Au@Pd@Cu particles with different thickness of Cu shells are prepared. Similar Au@Pd@Cu NRs with Au@Pd cores deviated from the center were also grown using dumbbell-type Au@Pd NRs, which indicates that flat Pd surfaces are unnecessary for the formation of rectangular Cu shells over Au@Pd NRs. The optical properties of Au@Pd and Au@Pd@Cu NRs were examined by observing ultraviolet (UV)-visible (Vis)-near infrared (NIR) extinction spectra.