Alloying and Dealloying Processes Involved in the Preparation of Metal Nanoshells through a Galvanic Replacement Reaction

The galvanic replacement reaction between Ag nanoparticles and an aqueous HAuCl4 solution has recently been demonstrated as a simple and convenient route to metal nanostructures with hollow interiors and highly crystalline walls (see, for example, Sun, Y.; Mayers, B. T.; Xia, Y. Nano Lett. 2002, 2, 481. Sun, Y.; Xia, Y. Science, 2002, 298, 2176). However, the details of morphological, compositional, structural, and spectral changes involved in the entire process of this template-engaged reaction is yet to be elucidated. The experimental results described in this letter indicate that the templating process proceeded through two distinctive steps: (i) formation of pinhole-free nanoshells with homogeneous, uniform walls of Au/Ag alloys via a combination of the replacement reaction and alloying; and (ii) formation of porous nanoshells (nanocages) through a dealloying process, in which Ag was selectively dissolved from the walls made of Au/Ag alloys. As alloying and dealloying proceeded, the surface plasmon resonance peaks of resultant metal nanostructures could be continuously tuned from ∼425 to ∼1030 nm.