Quasi‐Epitaxial Growth of Ni Nanoshells on Au Nanorods

Abstract

Gold nanorods have been extensively studied during the past decade because of their interesting optical properties, arising from localized surface–plasmon resonances (SPRs). Because of the shape anisotropy of nanorods, two well-defined SPRs can occur, namely, those parallel and transversal to their long axis, which offers a high degree of optical control when illuminated with polarized light. Additionally, nanorod assemblies can be constructed with various orientations, offering potential applications in waveguiding and construction of electronic devices. The technique of manipulating gold nanorods for exploitation of such properties is, however, not largely developed and, therefore, there is a need for novel ways to guide nanorod assembly. For example, gold nanorod alignment has been achieved by stretching in a polymer matrix, or applying an external electric field, which allowed detailed optical characterizations to be made. On the other hand, Au rod strings were made by bio-recognition-driven assembly, selective binding of chemical functionalities, and by assembly on modified carbon nanotubes. We present in this communication a first approach toward magnetic functionalization of gold nanorods, which is intended as a tool for magnetic manipulation and directed assembly. Additionally, the same technique can be exploited for the synthesis of magnetic nanoshells, which are rare but can be expected to display a large magnetic anisotropy, with potential uses in data storage. The surface modification of gold nanorods has often been accomplished through coating with various materials, such as Ag, Ag2S/Ag2Se, [17] SiO2, [18,19] Pd, or Pt. However, the synthesis of composite nanomaterials containing gold nanoparticles and a magnetic material (transition metals in particular) is complicated, especially in the case of gold nanorods, since they easily reshape at moderately high temperatures that are typically used for synthesis in organic solvents. Thus, for the coating of Au rods with magnetic metals, a low-temperature process is required that can simultaneously involve a slow reduction, so as to avoid nucleation in solution. For these reasons, we have chosen nickel as the coating material, because the synthesis of air-stable Ni colloids by reduction with hydrazine, at low temperature and in the presence of the surfactant cetyl-trimethylammonium bromide (CTAB, also used for Au nanorod synthesis) has been reported previously. As this process requires the use of a suitable catalyst, and gold was found to be inefficient, we used gold nanorods with platinum nanocrystals selectively grown at their tips, which were recently developed by us. The transmission electron microscopy (TEM) images in Figure 1 illustrate the various steps during the synthesis, including the starting Au rods, the growth of Pt tips, and finally the uniform coating with Ni shells of various thickness. Based on our previous work, we used gold nanorods coated with platinum at a Au/Pt molar ratio of 20, which leads to the growth of small Pt crystals on both tips of the Au rods, as shown in Figure 1B. The use of a minimum amount of Pt was intended to preserve as much as possible the optical response of the original Au rods and promote preferential Ni reduction at the tips. However, reduction of NiCl2 with hydrazine in the presence of Au/Pt rods stabilized with CTAB was found to lead to homogeneous growth of Ni shells around the rods, as indicated by the roughness and contrast observed in Figure 1C–E. It is remarkable that when using pure Au rods as seeds no Ni reduction was achieved, while just by including the Pt tips, the reduction took place invariably over the whole rod. Both energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) (see Supporting Information) have confirmed the presence of both nickel and gold in the final, composite nanorods (the Pt signal was too low in EDS because of its small relative amount, but Pt could be identified from XPS). The process allowed a fine tuning of the thickness of the Ni shell, through varying the molar ratio between Ni and Au, as shown in Figure 1C–E for molar ratios of 1, 5, and 10. By increasing the Ni/Au ratio, both the length and width increased similarly, resulting in a gradual decrease of the aspect ratio from 5.1 down to 2.6 (see Supporting Information, Fig. S1). Before proceeding with a more detailed analysis of the prepared nanoparticles, we carried out a qualitative demonstration of their magnetic character, by means of a hand-held permanent magnet. First of all, during the washing step of the synthesis (see Experimental) we used the magnet for separation of the particles from the solvent, observing complete separation (Fig. S2) over periods of few hours (longer for thinner C O M M U N IC A TI O N