Galvanic Replacement Reaction: A Route to Highly Ordered Bimetallic Nanotubes

Hollow bimetallic nanostructures are of great importance for various applications. Understanding the fundamental mechanisms occurring during the synthesis of such nanomaterials by wet chemistry remains very challenging. This Article reports a mechanistic study on the galvanic replacement reaction between planar arrays of silver nanowires grown site-specifically on tall silicon nanogratings and HAuCl4 in lack of any stabilizing or capping agent, which might complicate and alter the conversion process of silver nanowires into silver-gold nanotubes. The direct contact of the silver nanowires with the substrate is found to modify the reaction as compared to nanowires in suspension. We show that when using diluted HAuCl4, AgCl nanoclusters precipitate on the surface during the process resulting in an increased surface roughness of the nanotubes. Overcoming this drawback requires saturating the HAuCl4 solution with NaCl; this allows distributing the anodic and cathodic sites over the surface of the nano-objects in a homogeneous manner, allowing in turn obtaining nanotubes with a smooth surface. For both protocols (i.e., pure HAuCl4 or HAuCl4 saturated with NaCl), the conversion rate was found to increase with the concentration of HAuCl4 in the solution. We further show that the kinetic of the reaction and the surface roughness of the nanotubes become more important when raising the temperature from 0 to 100 degrees C. Furthermore, we show that by using the proposed approach, one can synthesize double-walled bimetallic nanotubes.