Nanocomposite ionic conductors with one-dimensional conduction paths can in principle be constructed using nanotubular templates, which are filled with an ionic conducting phase. Besides the fundamental aspect of the investigation of highly anisotropic ion conduction properties, the potential applications of low-dimensional solid ion conductors are widespread, including solid state sensors or heterogeneous catalysis.
In this work, we use self-organized ZrO2 or TiO2 nanotubes as template for the deposition of a proton-conducting sulfonated aromatic polymer. Pristine ZrO2 or TiO2 nanotubes can be prepared by electrochemical anodization of zirconium or titanium metal films in a fluoride-ion containing electrolyte. Nanotubular ZrO2 is highly resistive at room temperature, whereas TiO2 nanotubes show n-type semiconducting behavior. The electrochemical reduction of zirconia nanotubes is much more difficult than that of TiO2 nanotubes, because Zr4+ ions are less easily reduced than Ti4+ ions. In that sense, zirconia nanotubes represent a model insulating matrix and titania nanotubes a model semiconducting matrix to infiltrate with a room temperature ion conductor.
A model solid ion conductor was used for infiltration: sulfonated poly-ether-ether-ketone (SPEEK) is a familiar proton-conducting ionomer at room temperature and most properties, including the proton conductivity, are very well known. The presence of water molecules in the channels of the nanophase-separated polymer structure provides a water-mediated pathway for ion transport.
The first decisive step for the realization of such highly anisotropic nanocomposites is to prepare a highly wetting ionomer solution allowing to completely fill the nanotubular templates. The crucial aspect is to find an appropriate solvent for the ionomer with excellent wetting capability of ZrO2 or TiO2 surfaces. The resulting nanocomposite structure presents a high aspect ratio of the ionic conductor and highly oriented conduction paths.
The resulting material was characterized using SEM, X-ray diffraction and FTIR showing the successful filling of the nanotubular matrix with the ionomer. We also study the water vapour pressure dependence of the ionic conductivity of these nanocomposites with semiconducting titania matrix in order to realize humidity sensitive detectors. The proton mobility opens also the possibility to realize model heterogeneous catalysts for proton-catalyzed reactions.