Phenomenological study of Au and Pt nanowires grown in porous alumina scaffolds

Abstract

Porous anodic aluminum oxide, commonly known as AAO, has been widely used as a scaffold to synthesize nanowires and nanotubes. The porous alumina structure can be obtained from a simple electrochemical oxidation process, applying a positive voltage to an aluminum film placed in an electrolyte, and resulting in the formation of periodically arranged arrays of pores. It is possible to tune pore diameters and pore spacing by adjusting parameters such as the type of electrolyte, the pH, and the applied voltage. Once the barrier oxide is removed from the bottom of the pores, porous alumina that has been formed on conducting substrates can be used for growth of metal nanowires using electrodeposition. We synthesized Au and Pt nanowire arrays on Au or Pt substrates. During electrodeposition, Au nanowires that grew out of the pores developed a pyramidlike faceted shape. This was not observed for overgrown Pt nanowires. To understand this phenomenon, the microstructure and crystallographic characteristics of the overgrown Au and Pt nanowires were studied using SEM, TEM and XRD. It was found that the overgrown Au caps were single crystalline with (111) facets and textured along the [100] direction, while the Au nanowires in the pores were poly-crystalline with a [11 11] texture. Pt nanowires grown in pores were also polycrystalline and had a [111] texture, but the grain size was much smaller than that of the Au. In contrast with Au, no change of texture or microstructure was observed when Pt grew out of pores. The structure change observed for Au involves nucleation of a new crystal with a (100) texture. This is thought to be related to the changes in the overpotential that occur when the Au emerges from the pores.