Anisotropic Overgrowth of Metal Heterostructures Induced by a Site‐Selective Silica Coating

Abstract

Anisotropic metal nanostructures exhibit rich morphologydependent properties. First, they possess intriguing plasmonic behaviors and are attractive for various plasmon-based applications. Second, their surfaces often provide high densities of atomic steps, ledges, and kinks, which can serve as catalytically active sites. Third, anisotropic metal heterostructures can exhibit better catalytic properties than monometallic ones, where the presence of one metal can enhance the stability and promote the catalytic performance of the others. Fourth, anisotropic metal nanostructures can function as building blocks for shape-directed formation of unconventional superlattices. Wet-chemistry approaches are most commonly employed for producing anisotropic metal homoand heteronanostructures. 5] Selective molecular capping plays an important role in most anisotropic growth systems. For example, during Au nanorod growth, cetyltrimethylammonium bromide (CTAB) molecules are thought to preferentially adsorb on the side facets. The preferential CTAB adsorption slows down the growth on the side facets and promotes the growth at the two ends, leading to the production of Au nanorods. During the overgrowth of Au/Pd core/shell nanostructures, CTAB molecules adsorb more strongly on the Pd {100} facets, giving cubic and cuboidal structures. In other examples, the coating of Au nanorods with polyaniline and adsorption of particular thiol molecules on Au nanospheres induce the production of bimetallic Au/Ag nanostructures with different geometries. Selective molecular adsorption can be regarded as a soft templating method. By this means, we have successfully realized transverse overgrowth on Au nanorods. Although the role played by soft templating molecules has been recognized in the growth of anisotropic metal nanostructures, soft templating processes are actually complicated by the delicate interplay among many thermodynamic and kinetic factors, including metal precursor diffusion, interfacial strain, facile equilibrium between adsorbed and free molecules in solutions, facet-dependent metal deposition rates, and molecular bonding strength. As a result, each type of soft template generally functions for a specific material system. To realize anisotropic overgrowth to obtain metal nanostructures with controlled morphologies, one must stabilize the templating effect. An alternative is to make use of solid materials such as silica as the template. Compared to soft capping molecules, solid materials can be regarded as hard templates. Ideally, hard templates should be deposited on starting metal nanocrystals at desired locations, leaving the uncoated part for further metal deposition. Moreover, hard templates should be robust enough during metal deposition. In this way, hard templating can offer a widely applicable route to anisotropic metal homoand heterostructures. To date, only one example can be found in this direction, where silica is partially coated on Au nanospheres and serves as a hard template to guide Ag overgrowth on the exposed Au surface. Herein, we describe a general route to the anisotropic overgrowth of metals on Au nanorods. The overgrowth is enabled by site-selective silica coating. Silica is first selectively deposited at the two ends of the nanorods owing to the higher curvature, or deposited on the side surface when the ends are blocked by thiol-terminated methoxypoly(ethylene glycol) (mPEG-SH). The coated silica layer thereafter guides the overgrowth of metals on the exposed Au surface. The method has been tested for the overgrowth of Au, Ag, Pd, and Pt. In previous experiments, the overgrowth of a second metal on colloidal metal nanocrystals typically ends up with core/shell bimetallic nanostructures. Preferential bonding of soft capping molecules, such as thiolated polymers and molecules, to the ends can induce the overgrowth of Ag and Au on the side surface of Au nanorods, but the deposition of Ag and Au at the ends has been unavoidable. In this work, with site-selective silica coating, eight types of unprecedented anisotropic metal nanostructures are produced. Our route therefore is a robust approach for the preparation of metal homoand heterostructures with designed morphologies and functions. The starting Au nanorods are stabilized with a CTAB bilayer in aqueous solution (Figure 1a). The CTAB bilayer is less ordered and dense at the ends than on the side surface of Au nanorods owing to the larger curvature at the ends. Hydrolysis of tetraethyl orthosilicate (TEOS) results in silica coating. In route 1, silica coating dynamics is finely controlled, so that silica is selectively coated at the nanorod ends (Figure 1b), as less energy is needed for removing CTAB molecules at the ends for silica coating. Subsequent overgrowth of a metal takes place at the exposed side surface, leading to transverse metal overgrowth (Figure 1c). In route 2, the ends of the Au nanorods are [*] Dr. F. Wang, Dr. S. Cheng, Dr. Z. H. Bao, Prof. J. F. Wang Department of Physics, The Chinese University of Hong Kong Shatin, Hong Kong SAR (China) E-mail: jfwang@phy.cuhk.edu.hk [] These authors contributed equally to this work.