Nanostructured Photoanodes for Solar Cells

During the last decades, a considerable interest in renewable sources has ocurred, aiming to replace the exhaustible fossil fuels with renewable energy. Solar energy is a great answer to all the needs that humanity currently faces. Apart from being clean and renewable, it is readily available, so we just need to have e cient ways of harvesting it. Since the 70s, solar cells have been developed, following the great technological advances in science. In fact, since then, great e ciency responses have been reached, regarding solar harvesting and conversion into electrical energy. From the several known solar cell generations, the photoelectrochemicals cells (PEC); (semiconductor with an electrolyte) have emerged, which can be used for the generation of both electricity (dye-sensitized solar cells DSCs) and fuel (H2); (Photoelectrochmeical cells for water splitting). Given this advantage, we should develop strategies to improve these cells quality: higher e ciency but at lower cost. The great advances in nanotechnology and nanoscience during the last decades have assisted this demand. In particular, nanostructured materials, which exhibit very di erent physical properties when compared with the materials' bulk state, have been a awless response for this purpose. Moreover, various nanomaterials with di erent geometries (e.g. thin lms, nanoparticles, nanopores, dendrites, nanowires, nanotubes, etc.) have been explored in order to obtain an enhanced e ciency in the photoresponse. Recently, the nanotubes (NTs) have been widely exploited for applications in PECs (for both, DSCs and to split water). Displaying a one dimentional architecture, they have shown to be quite advantageous for an e cient charge transport (electrons and holes) in a semiconductor. The charge transport has been proven to perform a role of utmost importance in the PEC photoresponse. With the developments made in the 90s, in particular on the Al anodization that leads to an anodic nanoporous oxide, it is now possible to obtain self-organized nanotubes and nanopores of di erent types of semiconductors. This thesis focuses on the development of semiconductors with a tubular structure (NTs) by anodizing metals, in order to apply them as photoanodes. We developed nanostructured photoanodes for solar cells, in particular for DSCs and PECs for H2 fuel generation by water splitting. We describe the synthesis and characterization of anodic hexagonally ordered NT arrays of TiO2, HfO2 (for DSCs applications) and α-Fe2O3 (PECs for water splitting). The optimization of these NTs was made with great detail and thoroughness in order to contribute to the scienti c community in this topic. Indeed, we have explored crucial points during the synthesis process that, until now, no one has perched. We present a description of the work developed during the PhD. Highly ordered TiO2 NTs were synthesized by electrochemical anodization of Ti foils. We investigated the e ect of the Ti surface roughness (applying di erent pre-treatments, such as the electropolishing, chemical etching and mechanical polishing) prior to the anodization, on the length, growth rate and degree of self-organization of the obtained NT arrays. The mechanisms related to the TiO2 NT formation and growth were correlated not only with the corresponding anodization curves but also with their appropriate derivatives (1st order) and suitable integrated and/or obtained parameters, to reveal the onset and end of di erent electrochemical regimes. This enables an in-depth