Formation and Optical Properties of Cylindrical Gold Nanoshells on Silica and Titania Nanorods

Metal−insulator core−shell structures have been demonstrated to have interesting and tunable optical properties. Systems previously investigated include silica-capped gold particles and gold shells surrounding silica particles. However, many of the systems studied so far have been spherical (or zero-dimensional). Thus, it would be of interest to look at the synthesis and optical properties of one-dimensional (i.e., rodlike) nanostructures. In this paper, the authors present and discuss the formation and properties of silica and titania nanorods encapsulated with a thin gold shell. Nanorods of silica and titania ∼10 μm in length and with diameters ∼ 90−200 nm are made by combining sol−gel electrophoresis with a suitable template. After removing the template at high temperature, the surface of the rods is re-hydrolyzed by heating in water. 3-Aminopropyltrimethoxysilane is reacted with the surface hydroxyl groups, self-assembling amine functionality on the surface of the rods. These groups act as anchoring sites for the gold, which forms a thin shell around the oxide nanorod. UV−vis absorbance spectra of these samples are analyzed to determine the relationships between shell thickness, core size, core material, and properties.