Direct Formation of Thin-Walled Palladium Nanotubes in Nanochannels under an Electrical Potential

In this work, we demonstrate a simple methodology for synthesizing vertical or free-standing palladium (Pd) nanotube arrays in a highly efficient fashion. The hydroxyl-terminated surface of the nanochannel in an alumina membrane is used to form an interfacial double layer by specifically adsorbing hydrogen ions (H + ) from an acidic solution and leaving counterions to predominate in the diffuse layer. Locally enriched H + ions at the wall of alumina nanochannel produce a higher content of adsorbed hydrogen on the electrochemically reduced metallic Pd surface under an appropriate electric potential. The higher Pd growth rate at peripheral positions rather than elsewhere in the nanochannel was due to the dual reductions of Pd(II) compound by hydrogen and electric potential. The disparity of growth rates in combination with diffusion-controlled growth efficiently gives rise to the formation of nanotubes with wall thicknesses as narrow as 6 nm.