Metal-oxide nanostructures for energy applications

One of the most important challenges for our society is providing powerful devices for energy conversion and storage. The number of proposed technologies in today’s green and renewable energy science is large and still increasing: among all the dye-sensitized solar cells (for energy generation) and Li-ions batteries (for energy storage) have attracted a lot of interest thanks to the easy fabrication processes and the cheap materials involved. Great attention has been paid on the investigation of one-dimensional metal-oxide nanostructures for a new generation of power sources, because of their unique electronic properties, such as high electron mobility and low carrier recombination rate, high surface-to-volume ratio and excellent surface activity. Among the large number of semiconductive metal oxide nanostructures, TiO2 and ZnO are of particular interest due to the fact that they are the best candidates as active materials in electrochemical devices thanks to their chemical and electronic properties. Several approaches have been proposed for TiO2 nanostructure synthesis and among them anodic oxidation is now a well-established technique that can provide large area uniform nanotubular arrays on Ti foil with relatively high specific surface. Regarding zinc oxide, many papers report on the synthesis of ZnO nanostructures performed by means of different techniques. Most of them exploits high temperature processes often using catalyst particles, requires the presence of a sacrificial template, introduces chemical contamination or exhibit slow kinetics. This PhD thesis investigates the fabrication of different metal-oxide nanostructures and their integration as electrodes into DSCs and LiBs: in particular the work deals with TiO2 nanotube arrays obtained by anodic oxidation and with ZnO sponge-like films obtained by combined sputtering/thermal oxidation techniques. Vertically oriented TiO2 NTs were obtained by anodic oxidation of titanium foil and fully characterized in terms of stoichiometry, crystalline phase and morphology. TiO2 nanotubes were tested both in DSC and in LiBs showing improved charge transport properties due to the 1-dimensional structures and a reduced recombination rate (and a subsequent higher carrier lifetime value) that could be attributed to the reduced presence of defects and trap-sites in the nanotubes with respect to the nanoparticle-based electrodes. As competitive alternative to TiO2 nanotubes, porous ZnO films were obtained by a simple two-step method, involving the sputtering deposition of a sponge-like layer of metallic zinc, followed by a low-temperature treatment allowing for the complete oxidation of zinc, thus forming sponge-like ZnO films. Also in this case ZnO nanostructured films were fully characterized tested both in DSC and in LiBs showing interesting performance. Thanks to the its 3D nanostructuration, the superimposition of small branches able to grow in length almost isotropically and forming a complex topography, ZnO sponge-like can combine the fast transport properties of one dimensional material and the needed surface area usually provided by nanocrystalline electrodes.