A Generic Approach to Desired Metallic Nanowires Inside Native Porous Alumina Template via Redox Reaction

We report a facile, economic, and generic way to mono- and multisegment metallic nanowires (MNWs) of various pure metals (e.g., Au, Pt, Pd, Cu, Ni, and Co) and their alloys with both linear and branched topologies, by merely infiltrating aqueous solutions of metal chloride salts into Au-coated native porous anodic aluminum oxide template with Al foil on its outside edge. Redox reactions of two galvanic cells where the Al foil acts as the anode are responsible for the formation of the MNWs. Redox reaction of the top galvanic cells on the surrounding Al foil leads to the formation of metal atoms on the Al foil surface, which subsequently diffuse away from the Al foil and into the nanochannels. Simultaneously, redox reaction of the bottom galvanic cell where the Au layer serves as a cathode results in the formation of metal atoms on the top surface of the bottom Au layer, followed by crystal nucleus formation and growth upward the channels to form short MNWs. With the elongation of the infiltration duration, the diffusing metal atoms coming from the top galvanic cells reach the tips of the growing MNWs, and combine with those on the MNW tips coming from the bottom galvanic cell, resulting in longer MNWs under the nanochannel geometrical confinement. The approach enables excellent control over the composition, location, length, and diameter of the individual segments and the topology of the overall NWs that are promising for many applications in nanotechnology.