Recently, researchers at Texas A&M University (TAMU) have demonstrated a series of deposition processes for fabricating organized arrays of nanowire/nanotube coaxial hybrids. The teams approach incorporates template-based sol-gel processes and electroplating. The templates are anodic aluminum oxide (AAO) membranes with an organized array of columnar pores, which are fabricated by anodizing aluminum sheets in acids. In principle, such AAO membranes can also be fabricated using aluminum films coated on Si wafers or other flexible supporting sheets. The sol-gel process is a wet-chemical technique primarily for the fabrication of metal oxides from a chemical precursor solution. Since the deposition is uniform on the entire surface of a substrate in a sol-gel process, desirable complex structure can be obtained by using a proper substrate. For example, nanotubes of a large variability of materials can be deposited into the pores of AAO membranes. On the other hand, electroplating is a process that uses electrical current to reduce the cations of a desired material from a solution and coat the material onto a conductive object.

In order to fabricate the hybrid structure, researchers at TAMU first deposited TiO2 into the AAO template with the sol-gel process. The TiO2 coating is controlled to a proper thickness so that the TiO2 tubes are formed in the pores while the pores remain open. A gold layer is then deposited on one surface of the TiO2 coated AAO membrane to serve as the cathode for electroplating. Cobalt is then electroplated to fill the TiO2 nanotubes and form an organized array of nanowire/nanotubes core-shell structures. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) measurements show a high nanowire filling factor and a clean interface between the nanowire and the nanotube materials.

This is a versatile method for well-controlled fabrication of hybrid structures of a wide variety of materials, such as metals, metal oxides, semiconductors, magnetic materials, superconductors, etc. This low-cost approach is carried out in ambient conditions and is in principle suitable for large-area applications on either rigid or flexible supporting substrates. Moreover, the microstructures of the nanowires and nanotubes are controllable by tuning the conditions of the related chemical processes.

The researchers presented their results in Nanotechnology.