TUNABLE FEATURES OF TIO2 NANOPARTICLES FOR PHOTOCATALYTIC AND PHOTOVOLTAIC APPLICATIONS: A FUNDAMENTAL STUDY OF ELECTRONIC AND STRUCTURAL ASPECTS.

Nanosized titanium dioxide has been widely explored as both pristine and doped semiconductor. TiO2 nanocrystals were successfully synthesized by three different procedures. Two of them are based on a controlled sol–gel reaction, implemented either by doping or by a hydrothermal growth step with the use of surfactants, exploiting three-dimensional micelles. The latter one is a microemulsion-mediate route, to accurately control the growth of Ti precursor in nanoreactors by the confinement in aqueous droplets. As for the doping, both metal and non-metal elements have been chosen, namely praseodymium, silver and nitrogen. In this latter case, both organic and inorganic sources, in turn, were used to modulate the N content of doped-titania. All samples were characterized from the morphological, structural, optical, and electronic points of view. Moreover, for selected N-TiO2 samples, the role of the aging time after the calcinations of the powders (from “freshly prepared” to “old” samples) both on the intensity of the optical features and on the bulk paramagnetic nitrogen concentration was also considered. Two main fields of applications for titania powders and films were investigated: photocatalysis and photovoltaics. Thus, the photocatalytic activity of several samples was tested using either UV or visible light irradiation with regard to different pollutants, ranging from ethanol (both in aqueous and gaseous media) to the more complex methylene blue molecule (deposited onto the oxide film). Besides, advanced oxidation processes were successfully applied to the degradation and final mineralization of bisphenol A and 4-cumylphenol. The use of TiO2 immobilized in thin films is of paramount importance for the plant-scale applicability of the process, especially if a low intensity irradiation source, such as solar light, is to be exploited. Then, photocatalytic reduction of hexavalent chromium was conducted with the use of electrodeposited TiO2 layers. Even though (doped) titanium dioxide is one of the most commonly adopted semiconductors in photocatalysis, it suffers from a relatively high bandgap – hence the doping to increase visible light absorption – and from recombination of photogenerated charge carriers. Furthermore, univocal conclusions on the Fermi energy levels, chemical nature and location of the doping centers, and related charge transfer processes versus dopant concentration are absent in the literature. Therefore, electronic effects induced by the presence of the heteroatom in the TiO2 nanocrystals were investigated by a synergistic combination of electrochemical experiments (Mott-Schottky plots, photovoltage and photocurrent measurements) and theoretical DFT calculations. Results on the doped materials point towards a reduced tendency to charge carriers’ recombination and different effects on the (quasi-)Fermi energy location of the final material. Recombination is a central issue also in solar cell devices, resulting in possible low efficiencies. The ability of spin-coated and spray-pyrolyzed TiO2 blocking layers in preventing or reducing losses arising from electron transfer via the transparent conductive substrate has been examined in bilayer hybrid dye-sensitized solar cell (DSC) devices. Then, different types of both solid-state dye-sensitized solar cells and DSCs with a liquid electrolyte were tested making use of the more effective spray-pyrolyzed TiO2 as “blocking layer” and optimizing home-made nanostructured titania pastes.