An extension of the template process for membranes and fibers is the use of physical molds/jigs prepared by electrochemical oxidation processes (e.g. Al2O3 membranes), or by electrodeposition or electrophoresis (nano-architectured substrates), or by electro-assisted extrusion (electrospinning). Before illustrating these techniques with a number of examples, we'll first define the basic principle. Columnar-porosity Al2O3 membranes are among the most widely used molds/templates. We take advantage of the columns to fill them by injection of metals and polymers under pressure, or even by electroplating, in order to obtain metal nanofibers (Bi, Te, Cu, carbons) after dissolution of the template by chemical etching. In the case of rod-shaped compounds with dipole moment and surface charges (e.g. ZnO), nanoarchitected films are obtained by applying an electric field. The interaction of the dipoles and surface charges with the field produces and rotates the rods, respectively.
Finally, we report on the development of nanofibers/nanotubes using theelectrospinning technique, which can be seen as an imitation of nature for the artificial synthesis of fibers. More specifically, a polymeric fluid is extruded from the orifice of a needle to form a droplet subjected to an electric field. Under the influence of the latter, and after optimization of key parameters such as potential, distance and feed rate, the droplet will stretch to create a myriad of non-oriented fibers deposited on a substrate in the form of mats. We can thus obtain core-crown fibers, composite nanofibers, or even porous C nanotubes that can trap catalysts (Pd, Sn) in their core. Such objects are of vital importance for many potential applications (energy, medicine, aeronautics, environment), which justifies the unbridled research in this field. As usual, this is illustrated by examples from electrochemical energy storage.
