Synthesis and characterization of bimetallic nanorods

Abstract

Anisotropic metal nanoparticles (NPs), and especially nanorods (NRs) exhibit interesting optical properties, which arise from their localized surface plasmon. Unlike nanospheres, gold NRs have a longitudinal surface plasmon resonance (LSPR) in the visible or near‐infrared range of the spectrum. By altering e.g. the shape or the dimensions (aspect ratio) of the NRs, the LSPR can be tuned, which makes them interesting materials for a broad range of light based applications, such as photocatalysis [1], data storage [2] and photothermal applications. The optical properties of gold nanoparticles can be extended even further by introducing a second metal. However, synthesizing bimetallic NRs with a good control over the metal composition and distribution while retaining the rod shape is challenging. In this study we present bimetallic systems composed of gold‐based NRs coated with a protective mesoporous silica layer. We show that it is possible to synthesize bimetallic core‐shell nanorods within a mesoporous silica shell, by etching away part of the gold and overgrowing the remaining Au‐core with a second metal while precisely controlling the core‐size, metal‐shell thickness and thus the metal‐to‐metal ratio [3]. Depending on the choice of metal, different growth behavior was observed. Overgrowth with Ag resulted in a smooth shell whereas the Pt and Pd metal shells had a rough morphology (Figure 1). The different types of bimetallic NRs were characterized in detail with advanced electron microscopy techniques such as Energy‐dispersive X‐ray spectroscopy (EDX), high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) and electron tomography. Subsequently, we used these silica coated bimetallic core‐shell structured rods as a starting material to make fully alloyed NRs, whereby the two metals were mixed via thermal treatment without loss of anisotropy [4]. The alloying process was followed in detail with in‐situ HAADF‐STEM heating and EDX measurements by making use of a special heating holder (Figure 2).